JP4719135B2 - Optical waveguide device and optical device - Google Patents

Description

  The present invention relates to an optical waveguide device and an optical device (optical modulator) used for optical communication.

In recent years, expectations for an external modulation type optical modulator (external modulator) are increasing in order to realize an ultra-high-speed and wide-band optical communication network system.
In particular, in order to enable long-distance transmission of optical signals, a Mach-Zehnder type (MZ) using LiNbO ₃ (lithium niobate, lithium niobate; LN), which is excellent in high-speed modulation characteristics and dispersion resistance characteristics in a wide band. ) Optical modulators (MZ type LN optical modulators) are being developed.

  In this MZ type LN optical modulator, since the operating point fluctuates due to temperature drift, DC drift, etc., a bias voltage is applied to compensate for this. In general, a monitor PD (photodetector, light detector) is provided on the output side of the optical modulator, and the radiated light emitted from the branch portion of the Y branch optical waveguide on the output side of the MZ type optical waveguide is detected as monitor light. Based on this, feedback control for controlling the bias voltage is performed.

However, as described above, when the radiated light is used as the monitor light, the intensity of the radiated light is weak. Therefore, it is necessary to use a monitor PD having a high sensitivity as the light intensity detection. The width is narrow. In addition, processing of signals detected by the monitor PD is also limited.
Therefore, as shown in FIG. 18, for example, a 3 dB directional coupler 111 is provided on the output side of the MZ type optical waveguide 110, and the monitoring optical waveguide 112 is connected to one port on the output side of the 3 dB directional coupler 111. It is also conceivable that the intensity of the monitor light guided through the monitor optical waveguide 112 is detected and used for feedback control of the bias voltage.

However, if the monitor light is extracted by such a method, the intensity of the monitor light becomes the same as the intensity of the output light (signal light) as shown in FIG. The strength of the monitor is too high, and the monitor PD may be broken.
Recently, there has been proposed an RZ optical modulator that generates a RZ (Return to Zero) signal by using a two-stage Mach-Zehnder type optical modulator in which modulation by a clock signal and modulation by a data signal are connected in series. Yes.

  In such an RZ optical modulator, for example, as shown in FIGS. 20A and 20B, out of the two stages of the Mach-Zehnder type optical modulators 113 and 114, the output of the Mach-Zehnder type optical modulator 114 provided in the subsequent stage. The monitor PD 116 detects only the radiated light emitted from the branch portion of the Y branch optical waveguide 115 on the side as monitor light, and based on this, bias control of both the two-stage Mach-Zehnder type optical modulators 113 and 114 is performed. It is possible to do it.

However, in such a method, it is difficult to perform accurate bias control, and the control is complicated.
The present invention has been devised in view of such a problem, and allows a wider range of selection of components that can be employed as a light detection unit, enables accurate and reliable detection of monitor light, and further provides a bias. An object of the present invention is to provide an optical waveguide device and an optical device capable of obtaining monitor light having an appropriate intensity for use in control.

  It is another object of the present invention to enable easy and accurate bias control.

For this reason, the optical device according to the first aspect of the present invention is an optical device in which a Mach-Zehnder interferometer having first and second optical waveguides on the output side is formed on a substrate having an electro-optic effect. The second optical waveguide has a curved portion that warps in a direction away from the first optical waveguide , and is emitted from the curved portion into the substrate at a position shifted from the output end of the second optical waveguide having the curved portion. It is characterized by having a monitor unit for detecting radiated light as monitor light.

  An optical device according to a second aspect of the present invention is an optical device in which a Mach-Zehnder interferometer having first and second optical waveguides on the output side is formed on a substrate having an electro-optic effect. The optical waveguide is configured to be branched, and an optical waveguide other than the one of the branched optical waveguides is provided with a curved portion curved in a direction away from the first optical waveguide, and is an end of the substrate. It is provided so as to be interrupted in the middle so as not to reach the point, and is characterized in that a monitor unit is provided at the output end of one optical waveguide.

According to a third aspect of the present invention, there is provided an optical device in which a Mach-Zehnder interferometer having first and second optical waveguides on the output side is formed on a substrate having an electro-optic effect . The optical waveguide is configured to be branched, and an optical waveguide other than one of the branched optical waveguides includes a curved portion curved in a direction away from the first optical waveguide, and is an end of the substrate. The end of one optical waveguide is formed at a predetermined distance from the end surface or side surface of the substrate, and is provided on the end surface or side surface of the substrate or the end surface of the substrate. Or provided at a position away from the side surface, provided with a monitor unit for receiving monitor light emitted from the end of one optical waveguide and having a larger beam diameter through the inside of the device where the optical waveguide is not formed, That features It is.

The optical device of the present invention 請 Motomeko 4 wherein the first output side, a Mach-Zehnder interferometer having the second of the two optical waveguides, an optical device formed on a substrate having an electro-optical effect, the The second optical waveguide has a beam expanding portion on the end side of the substrate, and the beam expanding portion has the second optical waveguide at the end side. It is characterized by being composed of branches.

As described above in detail, according to the optical waveguide device and the optical device of the present invention, the monitor light is extracted through the monitor optical waveguide, so that the monitor light can be detected accurately and reliably, and is employed as a monitor unit. There is an advantage that the range of selection of possible parts is widened, and monitor light having an appropriate intensity for use in bias control can be obtained.
Further, according to the optical waveguide device and the optical device of the present invention, each monitor unit provided for a plurality of Mach-Zehnder interferometers independently detects monitor light, and performs bias control based on this. There is an advantage that the bias control can be easily and accurately performed.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(Description of the first embodiment)
First, an optical waveguide device according to a first embodiment of the present invention will be described with reference to FIGS.
Hereinafter, an optical modulator configured by applying the optical waveguide device of the present invention will be described with reference to FIG.

This optical modulator is a Mach-Zehnder type optical modulator (MZ type LN optical modulator) using lithium niobate (lithium niobate, LiNbO ₃ ; LN). Such an optical modulator is provided in an optical transmitter, for example.
That is, as shown in FIG. 1, the MZ type LN optical modulator 1 includes a lithium niobate crystal substrate (LN substrate) cut out by cutting a lithium niobate crystal in the Z-axis direction of the crystal orientation (Z-axis cut). ) 2 is formed with an optical waveguide 3 including a Mach-Zehnder interferometer type (Mach-Zehnder type) optical waveguide (MZ type optical waveguide) 3A, and a data signal (main signal) is input in the vicinity of the MZ type optical waveguide 3A. The main electrode 4 and the bias electrode 5 for applying a bias voltage are formed.

Here, lithium niobate crystal is used as the substrate 2, but the substrate 2 only needs to have an electro-optic effect, and other ferroelectrics such as lithium tantalate (LiTaO ₂ ; LT) crystal, for example. Crystals can also be used.
Here, MZ type LN optical modulator 1 with a substrate having an electro-optical effect such as LiNbO ₃ crystal or LiTaO ₂ crystal, for example, a metal film on a part of the substrate made of LiNbO ₃ crystal or LiTaO ₂ crystal and the like By forming and thermally diffusing to form an optical waveguide, or after patterning a metal film, forming an optical waveguide by proton exchange in benzoic acid, etc., and then providing an electrode in the vicinity of the optical waveguide It is formed.

Specifically, for example, when LiNbO ₃ crystal is used for a substrate, a titanium film (Ti film) is patterned on the substrate in accordance with a desired optical waveguide shape, and in such a patterned state, 7 to 1050 ° C. An optical waveguide is formed by heating and thermal diffusion for 10 hours.
In the present embodiment, as shown in FIG. 1, the optical waveguide 3 includes two parallel linear optical waveguides 3c and 3d in which an input-side optical waveguide 3a is branched through a Y-branch optical waveguide (branch optical waveguide) 3b. Further, these linear optical waveguides 3c and 3d are connected to the output side optical waveguide 3e and the monitoring optical waveguide 3f via a 3 dB coupler (waveguide type coupler) 6, respectively. The Y-branch optical waveguide 3b and the linear optical waveguides 3c, 3d, 3dB coupler 6 constitute an MZ type optical waveguide 3A.

  Here, the 3 dB coupler 6 is formed as a waveguide type optical coupler (waveguide type coupler). As this 3 dB coupler 6, for example, a symmetric type is used, and generally a waveguide type directional coupler type coupler (3 dB directional coupler) is used. More preferably, a cross-waveguide coupler (3 dB cross-waveguide coupler) with less wavelength dependency and good yield is used.

As described above, the optical modulator 1 according to this embodiment has a structure in which one MZ type LN optical modulator 3A and one 3 dB coupler 6 are integrated in one chip.
Further, as shown in FIG. 1, the main electrode 4 includes a signal electrode 4a provided so as to partially overlap one linear optical waveguide 3c constituting the MZ type optical waveguide 3A, and a part of the MZ type optical waveguide. The ground electrode 4b is provided so as to overlap with the other linear optical waveguide 3d constituting the waveguide 3A. A data signal supply unit (signal supply unit) 7 for supplying a data signal (main signal) is connected to the signal electrode 4a constituting the main electrode 4.

  Then, by supplying a data signal from the data signal supply unit 7 to the main electrode 4 and applying a voltage (data signal voltage) corresponding to the data signal to the linear optical waveguides 3c and 3d, the linear optical waveguide 3c, An electric field is generated in 3d to change the refractive index of the linear optical waveguides 3c and 3d. As a result, a phase difference is generated in the light propagating through each of the linear optical waveguides 3c and 3d, and the light having the phase difference is combined and interfered by the 3 dB coupler 6, thereby passing through the output-side optical waveguide 3e. Thus, output light (modulated signal) modulated according to the data signal is output.

  In the present embodiment, as shown in FIG. 1, one end (output side end, termination) of the signal electrode 4a and one end (output side end, termination) of the ground electrode 4b are connected by resistance. A data signal connected to the other end (input side end) of the signal electrode 4a and the other end (input side end) of the ground electrode 4b by making a traveling wave electrode by connecting with the (terminator). A microwave (microwave signal, high frequency, high frequency signal) as a data signal is supplied via a supply unit 7 (including a power supply circuit and a drive circuit), and a voltage corresponding to the microwave is supplied to the linear optical waveguides 3c and 3d. It is made to be applied. Thus, the present optical modulator 1 can be driven at a high speed.

In particular, the effective refractive index of the linear optical waveguides 3 c and 3 d is adjusted by changing the cross-sectional shape of the main electrode 4, and the light propagating through the linear optical waveguides 3 c and 3 d and the microwave supplied to the main electrode 4 are changed. By matching the speed, a broadband optical response characteristic can be obtained.
The bias electrode 5 includes an electrode 5a provided so as to partially overlap one linear optical waveguide 3c constituting the MZ type optical waveguide 3A, and the other straight line partially constituting the MZ type optical waveguide 3A. The ground electrode 5b is provided so as to overlap the waveguide 3d. A bias controller (for example, a bias control circuit) 8 is generally connected to the bias electrode 5.

By supplying a bias voltage (DC voltage) to the bias electrode 5 through the bias controller 8, a bias voltage (DC voltage) is applied to the optical waveguides 3c and 3d.
Here, as will be described later, the bias control unit 8 performs feedback control for controlling the bias voltage based on the intensity of the monitor light, thereby compensating for fluctuations in the operating point voltage of the MZ type LN optical modulator 1.

In the present embodiment, the Z-axis cut lithium niobate crystal substrate 2 is used, and since the refractive index change due to the electric field in the Z-axis direction is used, the main electrode 4 and the bias electrode 5 are linear optical waveguides 3c and 3d. It is formed directly above.
Thus, in this embodiment, since the main electrode 4 and the bias electrode 5 are patterned directly above the linear optical waveguides 3c and 3d, the light propagating in the linear optical waveguides 3c and 3d 5 may be absorbed. In order to prevent this, in this embodiment, a buffer layer is formed between the LN substrate 2 and the electrodes 4 and 5. The buffer layer may be formed as, for example, a SiO ₂ film, and the thickness may be about 0.2 to 1 μm.

For this reason, the present optical modulator 1 is configured by stacking a buffer layer having a thickness smaller than that of the substrate 2 between the main electrode 4 and the bias electrode 5 and the substrate 2.
In the present embodiment, each of the main electrode 4 and the bias electrode 5 is a single electrode having one signal electrode, and the optical modulator 1 is configured as a single drive type optical modulator. Is not something For example, in order to reduce the drive voltage, a dual electrode having two signal electrodes may be used to form a dual drive type optical modulator.

By the way, in the optical modulator 1 according to the present embodiment, as shown in FIG. 1, a 3 dB coupler 6 is provided on the output side of the MZ type optical waveguide 3 A, and one port on the output side of the 3 dB coupler 6 is provided. An output side optical waveguide (signal optical waveguide) 3e is connected to the first and second optical waveguides.
On the other hand, in the optical modulator 1 according to the present embodiment, as shown in FIG. 1, a monitor optical waveguide 3f is connected to another port on the output side of the 3 dB coupler 6, and the monitor optical waveguide 3f is The optical modulator (chip) 1 extends to the output end. The monitor optical waveguide 3f is provided with an attenuation section 9 as will be described later. A monitor PD (photodetector, bias control monitor PD, light detection unit) 10 is provided at the end of the monitor optical waveguide 3f. As a result, of the output light modulated by the MZ type optical modulator 1, after being guided to the monitoring optical waveguide 3f by the 3dB coupler 6 as a complementary signal of the output-side optical waveguide 3e and attenuated by the attenuation unit 9, The intensity of the monitor light is detected by the monitor PD10.

  As shown in FIG. 1, the monitor PD 10 is connected to a bias control unit 8 for controlling the bias voltage, and an intensity signal of the monitor light detected by the monitor PD 10 is sent to the bias control unit 8. It is like that. The bias control unit 8 performs feedback control for controlling the bias voltage (DC bias voltage) based on the intensity of the monitor light. That is, the bias control unit 8 performs feedback control of the bias voltage so that the detection value detected by the monitor PD 10 approaches the target value. As a result, fluctuations in the operating point voltage of the optical modulator 1 are compensated.

  By the way, as described above, when the 3 dB coupler 6 is provided on the output side of the MZ type optical waveguide 3A and the monitor light is extracted through the monitor optical waveguide 3f connected to the 3 dB coupler 6, the intensity of the monitor light is output. Since it becomes the same as the intensity of light (signal light), the intensity of the monitor light is too large. For example, when the intensity of the input light is large, an excessive current flows through the monitor PD 10 and may be broken. is there.

  Therefore, in the present embodiment, as shown in FIG. 1, an attenuation unit 9 for attenuating the output (power) of the monitor light is provided in the monitor optical waveguide 3f so that the intensity of the monitor light can be adjusted appropriately. It has been. As a result, the output of the monitor light is attenuated by the attenuator 9, and the output of the monitor light can be made appropriate according to the sensitivity of the monitor PD 10. For this reason, in the monitor PD 10, the intensity of the monitor light guided through the monitor optical waveguide 3 f and attenuated by the attenuation unit 9 is detected.

Hereinafter, a specific configuration example of the attenuation unit 9 will be described with reference to FIGS.
(1) As shown in FIG. 2, the attenuating section 9 is constituted by a bent waveguide 9a (waveguide type attenuator) having a reduced curvature radius.
Specifically, as shown in FIG. 2, an optical waveguide 3g extending linearly from the port of the 3 dB coupler 6 toward the end face 2A of the substrate 2 is formed, and linearly connected to the linear optical waveguide 3g. A curved waveguide 9a having a radius of curvature smaller than the optical waveguide 3g by a desired size may be formed. In this case, the monitoring optical waveguide 3f is composed of the linear optical waveguide 3g and the bent waveguide 9a.

Thus, if the monitoring optical waveguide 9 is constituted by the curved waveguide 9a having a small radius of curvature, a bending loss occurs when the monitor light propagates through the monitoring optical waveguide 9, so that the monitoring optical waveguide 9 is guided. The output of the monitored light is attenuated.
Here, when the attenuating section 9 is constituted by a bent waveguide 9a having a smaller radius of curvature, as shown in FIG. 2, the monitor PD 10 detects the guided light emitted from the end of the bent waveguide 9a as monitor light. Alternatively, as shown in FIG. 3, radiated light emitted from the bent portion of the bent waveguide 9a may be detected by the monitor PD 10 as monitor light.

In particular, as shown in FIG. 2, when the guided light guided through the curved waveguide 9a is used as the monitor light, the monitor PD 10 can be used with low sensitivity, and the cost can be kept low. is there. On the other hand, as shown in FIG. 3, when radiated light is used as the monitor light, the monitor PD 10 needs to have a high sensitivity, but there is an advantage that the positioning at the time of attachment is easy.
(2) As shown in FIG. 4, the attenuating unit 9 is constituted by two or more branch optical waveguides (a plurality of branch optical waveguides, multi-branch optical waveguides, waveguide type attenuators) 9b.

  Specifically, as shown in FIG. 4, an optical waveguide 3g extending linearly from the port of the 3 dB coupler 6 toward the end surface 2A of the substrate 2 is formed, and two optical waveguides 3g are connected to the linear optical waveguide 3g. What is necessary is just to form the above branched optical waveguide 9b. In this case, the monitoring optical waveguide 3f is constituted by the linear optical waveguide 3g and one of the two or more branched optical waveguides 9b.

As described above, if the monitoring optical waveguide 3f is configured to include two or more branching optical waveguides 9b, a part of the monitoring light is branched by each of the branching optical waveguides 9b. The output of the monitor light guided through one branching optical waveguide 9ba that constitutes is attenuated.
(3) As shown in FIG. 5, the attenuating unit 9 is configured by a 1: N coupler (waveguide coupler, waveguide attenuator) 9c that branches at a predetermined branch ratio of 1: N.

  Specifically, as shown in FIG. 5, an optical waveguide 3g extending linearly from the port of the 3 dB coupler 6 toward the end face 2A of the substrate 2 is formed, and the waveguide is connected to the linear optical waveguide 3g. A type 1: N coupler (waveguide coupler) 9c may be formed. In this case, the monitoring optical waveguide 3f is composed of the linear optical waveguide 3g and the optical waveguide 9ca connected to one port of the 1: N coupler 9c.

As described above, if the monitoring optical waveguide 3f is configured to include the 1: N coupler 9c, a part of the monitoring light is branched by the 1: N coupler 9c. : The output of the monitor light guided through the optical waveguide 9ca connected to one port of the N coupler 9c is attenuated.
(4) As shown in FIGS. 6 and 7, the attenuating section 9 is constituted by a beam expanding section (waveguide type attenuator) 9d provided near the end of the monitoring optical waveguide 3f.

  Here, for example, as shown in FIG. 6, the beam expanding portion 9 d is formed at a position a predetermined distance from the end surface 2 </ b> A of the substrate 2 (or the side surface of the substrate when the monitor light is emitted from the side surface of the substrate 2). What is necessary is just to comprise by the edge part of the optical waveguide 3f for a monitor. The predetermined distance may be set in consideration of the sensitivity of the monitor PD 10 and the like. Here, the monitor PD 10 is provided at a position away from the end surface 2A of the substrate 2, but may be attached to the end surface 2A of the substrate 2.

  In this way, if the beam expanding portion 9d is formed by disconnecting the monitoring optical waveguide 3f in the middle, the monitoring light is emitted from the end of the monitoring optical waveguide 3f and emitted from the end surface 2A of the substrate 2. The beam diameter of the monitor light increases and the output of the monitor light is attenuated. Further, since the beam diameter is increased, the tolerance of the position where the monitor PD 10 is provided can be increased.

  Further, the beam expanding portion 9d may be configured by bifurcating the end portion of the monitoring optical waveguide 3f, for example, as shown in FIG. That is, the optical waveguide 3g extending linearly from the port of the 3 dB coupler 6 toward the end surface 2A of the substrate 2 may be formed, and the two-branch optical waveguide 3h may be formed so as to be continuous with the linear optical waveguide 3g. In this case, the monitoring optical waveguide 3f is composed of the linear optical waveguide 3g and the two-branch optical waveguide 3h. Here, the monitor PD 10 is provided at a position away from the end surface 2A of the substrate 2, but may be attached to the end surface 2A of the substrate 2.

In this way, if the beam expanding portion 9d is configured by bifurcating the end portion of the monitoring optical waveguide 3f, the monitor light is branched by the two-branching optical waveguide 3h and monitored from the branched optical waveguides. Since the light is emitted, the emission area of the monitor light is expanded and the output of the monitor light is attenuated. Further, since the emission area of the monitor light is widened, the tolerance of the position where the monitor PD 10 is provided can be increased.
(5) The attenuation unit 9 is configured as an arbitrary combination of the above-described attenuation methods.

  That is, for example, as shown in FIG. 8, a curved waveguide 9a having a small radius of curvature is provided, and the end of the monitoring optical waveguide 3f is disconnected halfway so as to be at a predetermined distance from the end surface 2A of the substrate 2. In this way, the attenuation unit 9 is configured. For example, as shown in FIG. 9, two or more branching optical waveguides 9b are provided, and the end of the monitoring optical waveguide 3f is disconnected halfway along the predetermined distance from the end surface 2A of the substrate 2. Thus, the attenuation unit 9 is configured. Thus, by combining the above-described attenuation methods, even when the output of the monitor light is large, the attenuation can be reliably performed.

Thus, when the attenuation unit 9 is provided in the monitoring optical waveguide 3f of the optical modulator 1, as shown in FIG. 10, the output of the monitor light is made lower than the output of the output light emitted from the output side optical waveguide 3e. It will be possible to suppress.
Therefore, according to the optical modulator 1 as the optical waveguide device according to the present embodiment, the output side of the Mach-Zehnder type optical waveguide 3A is a 3 dB coupler 6 and the optical signal connected to one port on the output side of the 3 dB coupler 6 is used. When monitoring light is taken out with the waveguide as the monitoring optical waveguide 3f, the monitoring optical waveguide 3f is provided with the attenuator 9, so that the intensity of the monitoring light is adjusted to an appropriate intensity according to the sensitivity of the monitor PD10. be able to.

Therefore, according to the present optical waveguide device (optical modulator), the monitor light can be detected accurately and reliably, the range of selection of the monitor PD 10 that can be adopted is widened, and the intensity suitable for use in bias control is increased. There is an advantage that the monitor light can be obtained.
In the above-described embodiment, the optical waveguide device according to the present invention is applied to the Mach-Zehnder type optical modulator 1. However, the present invention is not limited to this. For example, optical modulation including a directional coupler type optical waveguide is provided. The present invention can also be applied to other optical modulators having a waveguide structure such as a resonator.

In the above-described embodiment, the case where the optical waveguide device according to the present invention is applied to an optical modulator has been described. However, the optical waveguide device is not limited to this, and the present invention may be applied to other optical switches, for example. It can be applied to an optical waveguide device.
In the above-described embodiment, a part of the output light output from the optical modulator is extracted as monitor light, and the intensity of this monitor light is detected to perform feedback voltage bias control. For example, the intensity of the detected monitor light may be used for feedback control of a voltage applied by a signal supply unit (including a power supply circuit and a drive circuit).

Furthermore, in the above-described embodiment, the optical modulator is configured as including the main electrode 4 and the bias electrode 5, but is not limited thereto, and is configured as including only the main electrode 4, for example. A data signal may be supplied to the main electrode 4 and a bias voltage may be applied.
(Description of Second Embodiment)
Next, an optical waveguide device according to a second embodiment of the present invention will be described.

Hereinafter, an RZ optical modulator configured by applying the optical waveguide device of the present invention [an optical modulator that generates a RZ (Return to Zero) signal by giving a clock signal and a data signal to input light (clock modulation type) The optical modulator)] will be described with reference to FIGS.
The RZ optical modulator according to the present embodiment is used as, for example, an optical transmitter in a long-distance optical transmission system.

In the present embodiment, as shown in FIG. 11, the RZ optical modulator 20 is a two-stage Mach-Zehnder optical modulator (MZ type LN optical modulator) using lithium niobate (lithium niobate, LiNbO ₃ ; LN). ) 21 and 41.
That is, as shown in FIG. 11, the RZ optical modulator 20 cuts the lithium niobate crystal in the Z-axis direction of the crystal orientation (Z-axis cut) on the lithium niobate crystal substrate (LN substrate) 22 cut out. In addition, an optical waveguide including a Mach-Zehnder type first optical waveguide (hereinafter referred to as a first optical waveguide) 23 and a Mach-Zehnder type second optical waveguide (hereinafter referred to as a second optical waveguide) 24 is formed. The first electrode (main electrode) 25 and the first bias electrode 26 are formed in the vicinity of the waveguide 23, and the second electrode (main electrode) 27 and the second bias electrode 28 are formed in the vicinity of the second optical waveguide 24. Composed.

  It is assumed that the first optical waveguide 23, the first electrode 25, and the first bias electrode 26 provided on the front stage side of the RZ optical modulator 20 are provided. Mach-Zehnder type second optical modulation comprising a second optical waveguide 24, a second electrode 27, and a second bias electrode 28 provided on the rear stage side of the RZ optical modulator 20. 41 (also referred to as an NRZ modulator for modulating an NRZ (Non Return to Zero) signal).

Here, lithium niobate crystal is used as the substrate. However, the substrate may be any substrate as long as it has an electro-optic effect. For example, another ferroelectric crystal such as lithium tantalate (LiTaO ₂ ; LT) crystal may be used. It can also be used.
Here, RZ optical modulator 1 with a substrate having an electro-optical effect such as LiNbO ₃ crystal or LiTaO ₂ crystal, for example, a metal film is formed on a portion of the substrate made of LiNbO ₃ crystal or LiTaO ₂ crystal and the like It is formed by forming an optical waveguide by thermal diffusion, or by patterning a metal film and then forming an optical waveguide by proton exchange in benzoic acid, and then providing an electrode in the vicinity of the optical waveguide. The

Specifically, for example, when LiNbO ₃ crystal is used for a substrate, a titanium film (Ti film) is patterned on the substrate in accordance with a desired optical waveguide shape, and in such a patterned state, 7 to 1050 ° C. An optical waveguide is formed by heating and thermal diffusion for 10 hours.
In the present embodiment, as shown in FIG. 11, the optical waveguide is divided into two parallel linear optical waveguides in which the input-side optical waveguide 30 is branched via a symmetric 3 dB coupler (optical coupler, waveguide coupler) 29a. The linear optical waveguides 23b and 23c are connected to the intermediate optical waveguide 31 and the monitoring optical waveguide 23d through symmetrical 3 dB couplers (optical couplers and waveguide couplers) 29b, respectively. It is formed to be. Here, the 3 dB couplers 29a and 29b have the same shape. The Mach-Zehnder type first optical waveguide 23 is constituted by the 3 dB coupler 29a, the linear optical waveguides 23b and 23c, and the 3 dB coupler 29b.

  Further, as shown in FIG. 11, the intermediate optical waveguide 31 is branched via a Y branch optical waveguide (branch optical waveguide) 24a and connected to two parallel linear optical waveguides 24b and 24c. The optical waveguides 24b and 24c are formed so as to be connected to the output-side optical waveguide 32 and the monitor optical waveguide 24d via a symmetric 3 dB coupler (optical coupler, waveguide coupler) 29c, respectively. The Y-branch optical waveguide 24a, the linear optical waveguides 24b, 24c, and the 3 dB coupler 29c constitute a Mach-Zehnder type second optical waveguide 24.

  Here, the 3 dB couplers 29a to 29c constituting the first optical waveguide 23 and the second optical waveguide 24 are formed as waveguide type optical couplers (waveguide type couplers). As the 3 dB couplers 29a to 29c, directional coupler type couplers (3 dB directional couplers) are generally used. More preferably, however, the cross waveguide type couplers (less wavelength dependency and good yield) 3 dB crossed waveguide coupler).

As described above, the RZ optical modulator 20 according to the present embodiment has a structure in which two MZ type LN optical modulators 21 and 41 and three 3 dB couplers 29a to 29c are integrated in one chip. Yes.
Further, as shown in FIG. 11, the first electrode 25 has a signal electrode 25a provided so that a part thereof overlaps one linear optical waveguide 23b constituting the first optical waveguide 23, and a part thereof is a first part. A ground electrode 25b provided so as to overlap the other linear optical waveguide 23c constituting the optical waveguide 23 is provided. A clock signal supply unit (signal supply unit) 33 that supplies a sine wave electric signal (microwave signal, high frequency signal) of, for example, a frequency of 20 GHz as a clock signal is connected to the signal electrode 25a constituting the first electrode 25. ing.

  Then, a clock signal is supplied from the clock signal supply unit 33 to the first electrode 25, and a voltage (clock signal voltage, signal voltage) corresponding to the clock signal is applied to the linear optical waveguides 23b and 23c, thereby forming a linear shape. An electric field is generated in the optical waveguides 23b and 23c, and the refractive indexes of the linear optical waveguides 23b and 23c are changed as + Δn and −Δn. As a result, a phase difference is generated in the light propagating through each of the linear optical waveguides 23b and 23c, and the light having the phase difference is combined and interfered by the 3 dB coupler 29b. For example, a 40 GHz optical clock signal [that is, a 40 Gb / s RZ data signal (optical RZ signal) in a data array of “1”, “1”, “1”,... Modulated signal] is output.

  In the present embodiment, one end (output side end, termination) of the signal electrode 25a and one end (output side end, termination) of the ground electrode 25b are connected by a resistor (terminator). Thus, the clock signal supply unit 33 (power supply circuit or the like) is connected to the other end (input side end) of the signal electrode 25a and the other end (input side end) of the ground electrode 25b. A clock signal is supplied via a drive circuit), and a voltage corresponding to the clock signal is applied to the linear optical waveguides 23b and 23c. Thus, the first optical modulator 21 can be driven at a high speed.

In particular, the effective refractive index of the linear optical waveguides 23 b and 23 c is controlled by changing the cross-sectional shape of the first electrode 25, and the light propagating through the linear optical waveguides 23 b and 23 c and the microscopic power supplied to the first electrode 25 are controlled. By matching the wave speed, a broadband optical response characteristic can be obtained.
Similarly, as shown in FIG. 11, the second electrode 27 also includes a signal electrode 27 a provided so as to partially overlap one linear optical waveguide 24 b constituting the second optical waveguide 24, and a part of the second electrode 27. And a ground electrode 27b provided so as to overlap the other linear optical waveguide 24c constituting the two optical waveguides 24. For example, a NRZ data signal (electric signal) of 40 Gb / s is supplied to the signal electrode 27 a constituting the second electrode 27 at a timing synchronized with, for example, a 40 GHz optical clock signal from the first optical modulator 21. A data signal supply unit (signal supply unit) 34 is connected.

  Then, an NRZ data signal is supplied from the NRZ data signal supply unit 34 to the second electrode 27, and a voltage (NRZ data signal voltage, signal voltage) corresponding to the NRZ data signal is applied to the linear optical waveguides 24b and 24c. Thus, an electric field is generated in the linear optical waveguides 24b and 24c, and the refractive indexes of the linear optical waveguides 24b and 24c are changed to + Δm and −Δm. As a result, a phase difference is generated in the light propagating through each of the linear optical waveguides 24b and 24c, and the light having these phase differences is combined and interfered by the 3 dB coupler 29c. Thus, for example, an optical RZ data signal (modulation signal) of 40 Gb / s is output.

  In the present embodiment, one end (output side end, termination) of the signal electrode 27a and one end (output side end, termination) of the ground electrode 27b are connected by a resistor (terminator). As a traveling wave electrode, an NRZ data signal supply unit 34 (power circuit) connected to the other end (input side end) of the signal electrode 27a and the other end (input side end) of the ground electrode 27b. NRZ data signal is supplied via a drive circuit (including a drive circuit), and a voltage corresponding to the NRZ data signal is applied to the linear optical waveguides 24b and 24c. Thus, the second optical modulator 41 can be driven at a high speed.

In particular, the effective refractive index of the linear optical waveguides 24 b and 24 c is controlled by changing the cross-sectional shape of the second electrode 27, and the light propagating through the linear optical waveguides 24 b and 24 c and the microscopic power supplied to the second electrode 27. By matching the wave speed, a broadband optical response characteristic can be obtained.
Further, as shown in FIG. 11, the first bias electrode 26 includes an electrode 26 a provided so that a part thereof overlaps one linear waveguide 23 b constituting the first optical waveguide 23, and a part thereof is a first part. A ground electrode 26b provided so as to overlap the other linear waveguide 23c constituting the optical waveguide 23 is provided. A first bias control unit (for example, a bias control circuit) 35 is connected to the first bias electrode 26.

A bias voltage (DC voltage) is applied to the first optical waveguide 23 by supplying a bias voltage (DC voltage) to the first bias electrode 26 through the first bias controller 35. .
Here, as will be described later, the first bias control unit 35 performs feedback control for controlling the bias voltage based on the intensity of the monitor light, thereby compensating for fluctuations in the operating point voltage of the first optical modulator 21. ing.

  Similarly, as shown in FIG. 11, the second bias electrode 28 includes an electrode 28 a provided so that a part thereof overlaps one linear waveguide 24 b constituting the second optical waveguide 24, and a part thereof is the first bias electrode 28 a. And a ground electrode 28b provided so as to overlap the other linear waveguide 24c constituting the two optical waveguides 24. A second bias control unit (bias control circuit) 36 is connected to the second bias electrode 28.

A bias voltage (DC voltage) is applied to the second optical waveguide 24 by supplying a bias voltage (DC voltage) to the second bias electrode 28 through the second bias controller 36. .
Here, as will be described later, the second bias control unit 36 performs feedback control for controlling the bias voltage based on the intensity of the monitor light, thereby compensating for fluctuations in the operating point voltage of the second optical modulator 41. is doing.

In the present embodiment, a Z-axis cut lithium niobate crystal substrate 22 is used, and a change in refractive index due to an electric field in the Z-axis direction is used. Therefore, the first electrode 25 and the first bias electrode 26 are the first optical waveguide. The second electrode 27 and the second bias electrode 28 are formed right above the straight waveguides 24 b and 24 c of the second optical waveguide 24.
Thus, in the present embodiment, the first electrode 25, the first bias electrode 26, the second electrode 27, and the second bias electrode 28 are patterned directly above the linear waveguides 23b, 23c, 24b, and 24c. The light propagating through the linear optical waveguides 23b, 23c, 24b, and 24c may be absorbed by the electrodes 25, 26, 27, and 28. In order to prevent this, in this embodiment, a buffer layer is formed between the LN substrate 22 and the electrodes 25, 26, 27, 28. The buffer layer may be formed as, for example, a SiO ₂ film, and the thickness may be about 0.2 to 1 μm.

For this reason, the present RZ optical modulator 20 is thinner than the substrate 22 between the first and second electrodes 25 and 27 and the first and second bias electrodes 26 and 28 and the substrate 22. A buffer layer is laminated.
In the present embodiment, each of the first electrode 25, the second electrode 27, the first bias electrode 26, and the second bias electrode 28 is a single electrode having one signal electrode, and the RZ light modulator 20 is single-driven. However, the present invention is not limited to this. For example, in order to reduce the drive voltage, a dual electrode having two signal electrodes may be used to form a dual drive type optical modulator.

  In the present embodiment, as shown in FIG. 11, a 3 dB coupler 29b is provided on the output side of the first optical waveguide 23, and the intermediate optical waveguide 31 is connected to one port on the output side of the 3 dB coupler 29b. The input side of the second optical waveguide 24, that is, the Y-branch optical waveguide 24 a of the second optical waveguide 24 is connected via the second optical waveguide 24. That is, the first optical modulator 21 and the second optical modulator 41 are connected in series along the optical waveguide direction.

Here, two optical modulators 21 and 41 are connected in series on the front and rear sides, but the number of optical modulators is not limited to this. That is, a plurality of optical modulators may be connected in series. In this case, a plurality of monitor PDs and signal supply units are also provided.
As a result, input light from a light source (semiconductor laser) (not shown) is introduced into the first optical waveguide 23 via the input-side optical waveguide 30 and the 3 dB coupler 29a, and desired when propagating through the first optical waveguide 23. After modulation based on a clock signal (electrical signal), a 3 dB coupler 29 b provided on the output side of the first optical waveguide 23 and an intermediate optical waveguide 31 connected to one port on the output side of the 3 dB coupler 29 b. Output to the second optical waveguide 24, modulated based on the NRZ data signal (electrical signal) in the second optical waveguide 24, and connected to one port on the output side of the 3dB coupler 29c and the 3dB coupler 29c. Modulated output light (signal light, optical NR signal) is output via the side optical waveguide 32.

  In this embodiment, as shown in FIG. 11, the first monitor optical waveguide 23d is connected to the other port of the 3 dB coupler 29b provided on the output side of the first optical waveguide 23. The optical waveguide 23 d extends to the output end of the RZ optical modulator (chip) 20. A first monitor PD (photodetector, first bias control monitor PD, light detector, photodetector) 37 is provided at the end of the first monitor optical waveguide 23d. Thereby, a part of the light modulated by the first optical modulator 21 is branched as monitor light by the 3 dB coupler 29b and guided to the first monitor optical waveguide 23d, and the intensity of the monitor light is increased by the first monitor PD 37. It is to be detected.

  Further, as shown in FIG. 11, the first monitor PD 37 is connected to a first bias control unit 35 for controlling the bias voltage, and the intensity signal of the monitor light detected by the first monitor PD 37 is the first. 1 bias is sent to the control unit 35. The first bias control unit 35 performs feedback control for controlling the bias voltage (DC bias voltage) based on the intensity of the monitor light. That is, the first bias control unit 35 performs feedback control of the bias voltage so that the detection value detected by the first monitor PD 37 approaches the target value. Thereby, the fluctuation | variation of the operating point voltage of the 1st optical modulator 21 is compensated.

  On the other hand, a second monitor optical waveguide 24d is connected to the other port of the 3 dB coupler 29c provided on the output side of the second optical waveguide 24. The second monitor optical waveguide 24d is an RZ optical modulator ( Chip) 20 extends to the output end. A second monitor PD (photodetector, second bias control monitor PD, photodetection unit, photodetector) 38 is provided at the end of the second monitor optical waveguide 24d. As a result, part of the light modulated by the second optical modulator 41 is branched as monitor light by the 3 dB coupler 29c and guided to the second monitor optical waveguide 24d, and the intensity of the monitor light is increased by the second monitor PD 38. It is to be detected.

  As shown in FIG. 11, the second monitor PD 38 is connected to a second bias control unit 36 for controlling the bias voltage, and the intensity signal of the monitor light detected by the second monitor PD 38 is the first monitor PD 38. It is sent to the 2-bias control unit 36. Then, the second bias control unit 36 performs feedback control of the bias voltage (DC bias voltage) based on the intensity of the monitor light. That is, the second bias control unit 36 performs feedback control of the bias voltage so that the detection value detected by the second monitor PD 38 approaches the target value. Thereby, the fluctuation | variation of the operating point voltage of the 2nd optical modulator 41 is compensated.

  Thus, in this embodiment, a part of the light (signal light) modulated by the first optical modulator 21 is extracted as monitor light, and the intensity of this monitor light is detected by the first monitor PD 37, Independently, a part of the light (signal light) modulated by the second optical modulator 41 is taken out as monitor light, and the intensity of this monitor light is detected by the second monitor PD 38.

The first monitor PD 37 and the second monitor PD 38 are connected to separate bias control units 35 and 36, respectively. That is, the first monitor PD 37 is connected to the first bias control unit 35 for performing bias control of the first optical modulator 21, and the second monitor PD 38 is for performing bias control of the second optical modulator 41. The second bias control unit 36 is connected.
Thereby, feedback control of the bias voltage performed to compensate for fluctuations in the operating point voltage of the first optical modulator 21, and bias voltage performed to compensate for fluctuations in the operating point voltage of the second optical modulator 41. The feedback control is performed independently of each other.

  Therefore, according to the RZ optical modulator 20 as the optical waveguide device according to the present embodiment, each of the optical modulators 21 is provided by the first monitor PD 37 and the second monitor PD 38 provided for each of the optical modulators 21 and 41. , 41 are independently monitored, there is an advantage that bias control of each of the optical modulators 21, 41 can be easily and accurately performed.

In the second embodiment, each of the first optical modulator 21 and the second optical modulator 41 is configured as a Mach-Zehnder type optical modulator. However, the present invention is not limited to this. You may comprise as a coupler-type optical modulator.
In the second embodiment, the input side of the second optical modulator 21 is the Y-branch optical waveguide 24a. However, the present invention is not limited to this. For example, it may be configured as a 3 dB coupler.

Further, in the second embodiment, 3 dB couplers 29 a and 29 b are provided on the incident side and the emission side of the first optical modulator 21, and further, a 3 dB coupler 29 c is provided on the emission side of the second optical modulator 41. However, the present invention is not limited to this. For example, it may be configured as a Y-branch optical waveguide.
(Description of First Modification of Second Embodiment)
Next, an RZ optical modulator as an optical waveguide device according to a first modification of the second embodiment of the present invention will be described with reference to FIG.

The RZ optical modulator according to the first modified example has a configuration of a first monitor optical waveguide coupled to a 3 dB coupler 29b provided on the output side of the first optical waveguide 23, as compared with the above-described second embodiment. The position at which the first monitor PD 37 used for bias control of the first optical modulator 21 is provided is different.
That is, in the first modification, as shown in FIG. 12, the first monitoring optical waveguide 23d connected to the other port on the output side of the 3 dB coupler 29b provided on the output side of the first optical waveguide 23 is A first monitor PD (light detector) 37 is provided at the end of the first monitor optical waveguide 23d that extends to the side surface of the RZ light modulator 50 (chip) and extends to the side surface of the RZ light modulator 50. ing.

Since other configurations are the same as those of the second embodiment described above, description thereof is omitted here.
As described above, according to the first modification, in addition to the effects of the configuration of the second embodiment described above, the first monitor PD 37 can be provided on the side surface of the RZ light modulator 20, so the first monitor PD 37, There is an advantage that the degree of freedom of the arrangement position of the second monitor PD 38 is increased.
(Description of Second Modification of Second Embodiment)
Next, an RZ optical modulator as an optical waveguide device according to a second modification of the second embodiment of the present invention will be described with reference to FIG.

The RZ optical modulator according to the second modification is different from the above-described second embodiment in the method of monitoring the light output from the second optical modulator 41. Therefore, the configuration on the output side of the second optical waveguide 24 is different, and the position where the second monitor PD (light detection unit) 38 used for bias control of the second optical waveguide 24 is provided is different.
That is, as shown in FIG. 13, in the RZ optical modulator 60 according to the second modification, the output side of the second optical waveguide 24 is a Y-branch optical waveguide 61, and this Y-branch optical waveguide 61 is used as the output-side optical waveguide. The output light from the second optical waveguide 24 is output via the Y branch optical waveguide 61 and the output side optical waveguide 32. And the radiated light radiated | emitted from the branch part of the Y branch optical waveguide 61 in an OFF state is used as monitor light. For this reason, the second monitor PD 38 is provided on the output side end face of the RZ optical modulator (chip) 60 at a position where the intensity of the emitted light from the branch portion of the Y branch optical waveguide 61 can be detected. .

Since other configurations are the same as those of the second embodiment described above, description thereof is omitted here.
Thus, according to the second modified example, in addition to the effects of the configuration of the second embodiment described above, the tolerance of the arrangement position of the second monitor PD 38 can be increased. (Description of the third modification of the second embodiment)
Next, an RZ optical modulator as an optical waveguide device according to a third modification of the second embodiment of the present invention will be described with reference to FIGS. 14 (A) and 14 (B). 14A is a schematic plan view, and FIG. 14B is a schematic cross-sectional view.

  The RZ optical modulator according to the third modification is different from the above-described second embodiment in the method of monitoring the light output from the first optical modulator 21 and the second optical modulator 41. For this reason, the configurations on the output side of the first optical waveguide 23 and the second optical waveguide 24 are different, and the position where the first monitor PD (light detection unit) 37 used for bias control of the first optical waveguide 23 is provided and The position where the second monitor PD (light detection unit) 38 used for bias control of the two optical waveguides 24 is provided is different.

  That is, as shown in FIGS. 14A and 14B, in the RZ optical modulator 70 according to the third modification, the output side of the first optical waveguide 23 is a Y-branch optical waveguide 71, and this Y-branch light The waveguide 71 is connected to the second optical waveguide 24 via the intermediate optical waveguide 31, and the output light from the first optical waveguide 23 is directed to the second optical waveguide 24 via the Y branch optical waveguide 71 and the intermediate optical waveguide 31. Output. And the radiated light radiated | emitted from the branch part of the Y branch optical waveguide 71 in an OFF state is used as monitor light. Therefore, the first monitor PD 37 is provided on the output side end face of the RZ optical modulator (chip) 70 at a position where the intensity of the radiated light from the branch portion of the Y branch optical waveguide 71 can be detected. .

  14A and 14B, the output side of the second optical waveguide 24 is a Y-branch optical waveguide 72, and the Y-branch optical waveguide 72 is connected to the output-side optical waveguide 32, so that The output light from the two optical waveguides 24 is output via the Y branch optical waveguide 72 and the output side optical waveguide 32. And the radiated light radiated | emitted from the branch part of the Y branch optical waveguide 72 in an OFF state is used as monitor light. Therefore, the second monitor PD 38 is provided on the output side end face of the RZ light modulator (chip) 70 at a position where the intensity of the radiated light from the branch portion of the Y branch optical waveguide 72 can be detected. .

  In particular, as shown in FIG. 14B, the radiated light emitted from the branch portion of the Y-branch optical waveguide 71 of the first optical waveguide 23 in the OFF state is in the thickness direction of the RZ optical modulator (chip) 70. Since the light propagates obliquely away from the chip surface, the light is emitted from a position close to the bottom surface of the substrate 22 at the output side end face of the RZ light modulator 70. For this reason, the first monitor PD 37 is provided at a position close to the bottom surface of the substrate 22 on the output side end surface of the RZ light modulator 70. The arrangement position of the first monitor PD 37 is determined according to the distance from the position of the branch portion of the Y-branch optical waveguide 71 of the first optical waveguide 23 to the end face on the output side of the RZ optical modulator 70.

  On the other hand, as shown in FIG. 14B, the radiated light emitted from the branch portion of the Y branch optical waveguide 72 of the second optical waveguide 24 in the OFF state is transmitted from the branch portion of the Y branch optical waveguide 72 to the RZ optical modulator. Since the distance to the end face on the output side of the (chip) 70 is short, the light is emitted from a position close to the surface of the substrate 22 on the end face on the output side of the RZ optical modulator 70. Therefore, the second monitor PD 38 is provided at a position close to the surface of the substrate 22 on the output side end face of the RZ light modulator 70. The arrangement position of the second monitor PD 38 is determined according to the distance from the position of the branch portion of the Y branch optical waveguide 72 of the second optical waveguide 24 to the end face on the output side of the RZ optical modulator 70.

  As described above, the second monitor PD 38 is provided at a position closer to the surface of the substrate 22 on the output-side end face of the RZ light modulator 70, and is located farther from the surface of the substrate 22 than the second monitor PD 38 (that is, The first monitor PD 37 is provided at a position close to the bottom surface of the substrate. That is, the two monitor PDs 37 and 38 are positioned in the thickness direction of the output side end face of the substrate 22 so that the intensity of the radiated light emitted from the branch portions of the corresponding Y branch optical waveguides 71 and 72 can be detected. They are offset.

  In the third modification, the configuration on the input side of the first optical waveguide 23 is also different. That is, in the third modification, the input side of the first optical waveguide 23 is a Y-branch optical waveguide 73, and the input-side optical waveguide 30 is connected to the first optical waveguide 23 via the Y-branch optical waveguide 73. ing. Since other configurations are the same as those of the second embodiment described above, description thereof is omitted here.

Thus, according to the third modification, in addition to the effects of the configuration of the second embodiment described above, the tolerance of the arrangement position of the second monitor PD 38 can be increased. In addition, since it is not necessary to form a monitor optical waveguide for extracting the monitor light, the overall size of the RZ light modulator 70 can be made compact.
(Explanation of a fourth modification of the second embodiment)
Next, an RZ optical modulator as an optical waveguide device according to a fourth modification of the second embodiment of the present invention will be described with reference to FIG.

The RZ optical modulator according to the fourth modification is different in the configuration on the output side of the first optical modulator 23 and the configuration on the output side of the second optical modulator 24 as compared with the second embodiment described above.
That is, as shown in FIG. 15, in the RZ optical modulator 80 according to the fourth modification, the output side of the first optical waveguide 23 is a Y-branch optical waveguide 81, and 1: N A waveguide type 1: N coupler (for example, a 1:10 coupler, a waveguide type coupler) 82 that branches at a predetermined branching ratio is connected. The first monitor optical waveguide 23d formed to extend to the output side end face of the RZ optical modulator (chip) 80 is connected to one port (smaller output) of the 1: N coupler 82. The first monitor PD (light detection unit) 37 is disposed at the end of the first monitor optical waveguide 23d.

On the other hand, as shown in FIG. 15, the Y-branch optical waveguide 24 a on the input side of the second optical waveguide 24 is connected to the other port (which has a larger output) of the 1: N coupler 82, so that the first optical The output light from the waveguide 23 is output toward the second optical waveguide 24 through the Y branch optical waveguide 81, 1: N coupler 82.
Further, as shown in FIG. 15, the output side of the second optical waveguide 24 is also a Y-branch optical waveguide 83, and a waveguide-type 1: branch that branches into the Y-branch optical waveguide 83 at a predetermined branch ratio of 1: N. N coupler (for example, 1:10 coupler, waveguide coupler) 84 is connected. Then, a second monitor optical waveguide 24d formed so as to extend to an output side end face of the RZ optical modulator (chip) 80 is connected to one port (smaller output) of the 1: N coupler 84. A second monitor PD (light detection unit) 38 is disposed at the end of the second monitor optical waveguide 24d.

On the other hand, as shown in FIG. 15, the output-side optical waveguide 32 is connected to the other port (the one with the larger output) of the 1: N coupler 84, and the output light from the second optical waveguide 24 is Y-branched. The light is output through the optical waveguide 83, 1: N coupler 84, and output side optical waveguide 32.
In the fourth modification, the configuration on the input side of the first optical waveguide 23 is also different. That is, in the fourth modification, the input side of the first optical waveguide 23 is a Y-branch optical waveguide 85, and the input-side optical waveguide 30 is connected to the first optical waveguide 23 via the Y-branch optical waveguide 85. ing. Since other configurations are the same as those of the second embodiment described above, description thereof is omitted here.

As described above, according to the fourth modification, the output light becomes weak at a ratio of 1: N. Therefore, by appropriately designing the branch ratio of the 1: N coupler, the monitor optical waveguides 23d and 24d after branching. There is no need to devise the shape of the waveguide in order to attenuate the monitor light propagating the light, and there is an advantage that the design of the waveguide structure is easy.
(Description of Fifth Modification of Second Embodiment)
Next, an RZ optical modulator as an optical waveguide device according to a fifth modification of the second embodiment of the present invention will be described with reference to FIG.

  The RZ optical modulator according to the fifth modification is different in the configuration on the output side of the first optical modulator 21 and the configuration on the output side of the second optical modulator 41 from the above-described second embodiment. Furthermore, a difference is that a Mach-Zehnder variable attenuator is provided as a third-stage Mach-Zehnder optical modulator after the second optical modulator 41. Such an RZ optical modulator is called a variable attenuator integrated RZ optical modulator.

  That is, as shown in FIG. 16, in the RZ optical modulator 90 according to the fifth modification, the output side of the first optical waveguide 23 is a Y-branch optical waveguide 91, and 1: N A waveguide type 1: N coupler (for example, 1:10 coupler, waveguide type coupler) 92 that branches at a predetermined branching ratio is connected. The first monitor optical waveguide 23d formed to extend to the output side end face of the RZ optical modulator (chip) 90 is connected to one port (the smaller output) of the 1: N coupler 92. The first monitor PD (light detection unit) 37 is disposed at the end of the first monitor optical waveguide 23d.

On the other hand, as shown in FIG. 16, the Y branch optical waveguide 24a on the input side of the second optical waveguide 24 is connected to the other port (the one with the larger output) of the 1: N coupler 92, and the first optical The output light from the waveguide 23 is output toward the second optical waveguide 24 via the Y-branch optical waveguide 91, 1: N coupler 92.
Further, as shown in FIG. 16, the output side of the second optical waveguide 24 is also a Y-branch optical waveguide 93, and a waveguide-type 1: branch that branches into the Y-branch optical waveguide 93 at a predetermined branch ratio of 1: N. An N coupler (for example, 1:10 coupler, waveguide coupler) 94 is connected. Then, a second monitor optical waveguide 24d formed so as to extend to an output side end face of the RZ optical modulator (chip) 90 is connected to one port (smaller output) of the 1: N coupler 94. A second monitor PD (light detection unit) 38 is disposed at the end of the second monitor optical waveguide 24d.

  On the other hand, a Mach-Zehnder type variable attenuator 95 is connected to the other port of the 1: N coupler 94 (which has a larger output) as shown in FIG. That is, the Mach-Zehnder variable attenuator 95 is connected to the subsequent stage of the second optical modulator 41. For this reason, the output light from the second optical waveguide 24 is input to the variable attenuator 95 via the Y-branch optical waveguide 93, 1: N coupler 94.

  Here, as shown in FIG. 16, the variable attenuator 95 is a Mach-Zehnder type comprising an input Y branch optical waveguide 96a, two parallel linear optical waveguides 96b and 96c, and an output Y branch optical waveguide 96d. An optical waveguide 96, an electrode 97, and a bias electrode 98 are provided. Then, a predetermined direct-current voltage (DC voltage) is applied to the linear optical waveguides 96b and 96c via the electrode 97, and the refractive indexes of the linear optical waveguides 96b and 96c are changed. The transmittance of 96c is changed to attenuate the output (power) of light (signal light) propagating through the linear optical waveguides 96b and 96c. For this reason, a voltage supply unit (including a power supply circuit and a drive circuit) is connected to the electrode 97.

  The Y branch optical waveguide 96d on the output side of the variable attenuator 95 has a waveguide type 1: N coupler (for example, 1:10 coupler, waveguide type coupler) 99 that branches at a predetermined branch ratio of 1: N. Are connected. A third monitor optical waveguide 100 formed so as to extend to the output side end face of the RZ optical modulator (chip) 90 is connected to one port (the smaller output) of the 1: N coupler 99. The third monitor PD (light detection unit) 101 is disposed at the end of the third monitor optical waveguide 100.

  Based on the intensity of the monitor light detected by the third monitor PD 101, feedback control of the bias voltage (DC bias voltage) is performed. That is, feedback control of the bias voltage is performed so that the detection value detected by the second monitor PD 101 approaches the target value. Therefore, a third bias control unit (bias control circuit) is connected to the second monitor PD101.

In this manner, by applying a predetermined DC voltage to the electrode 97 and applying a bias voltage so as to approach a target value that can be arbitrarily set, the DC voltage applied to the linear optical waveguides 96b and 96c can be varied. It is controlled so that the power of the output light can be adjusted.
On the other hand, as shown in FIG. 16, the output side optical waveguide 32 is connected to the other port (the one with the larger output) of the 1: N coupler 99, and the output light from the variable attenuator 95 is 1: N. The light is output through the coupler 99 and the output side optical waveguide 32.

  In the fifth modification, the configuration on the input side of the first optical waveguide 23 is also different. That is, in the fifth modification, the input side of the first optical waveguide 23 is the Y-branch optical waveguide 102, and the input-side optical waveguide 30 is connected to the first optical waveguide 23 via the Y-branch optical waveguide 102. ing. Since other configurations are the same as those of the second embodiment described above, description thereof is omitted here.

As described above, according to the fifth modification, as in the case of the fourth modification described above, the output light becomes weak at a ratio of 1: N, and therefore the branch ratio of the 1: N coupler is appropriately designed. As a result, there is no need to devise the waveguide shape in order to attenuate the monitor light propagating through the branched monitor optical waveguides 23d and 24d, and there is an advantage that the design of the waveguide structure is easy.
As shown in FIG. 17, the monitoring optical waveguide of the first embodiment is propagated to the monitoring optical waveguides 23d and 24d of the RZ optical modulator 20 as the optical waveguide device according to the second embodiment. It is also preferable to provide an attenuator 9 for attenuating the monitor light. Thereby, in addition to the effects in the configuration of the above-described second embodiment and each modification thereof, the monitor light can be detected accurately and reliably, the range of selection of the monitor PD 10 that can be adopted is widened, and used for bias control. Therefore, monitor light having an appropriate intensity can be obtained. In each modification of the second embodiment, when the monitor light propagating through the monitoring optical waveguide needs to be attenuated, the attenuation unit 9 may be provided in the monitoring optical waveguide in the same manner.

In each of the above-described embodiments and modifications, there is one in which the monitor PD is attached to the end surface of the substrate when the monitor light is extracted through the monitor optical waveguide. However, the present invention is not limited to this. Depending on the intensity of the monitor light guided through the monitor optical waveguide, the monitor optical waveguide may be attached to a position (for example, a housing) separated from the end face of the substrate by a predetermined distance.
(Supplementary note 1) a substrate having an electro-optic effect;
A plurality of Mach-Zehnder type or directional coupler type optical waveguides formed in series on the substrate;
A plurality of electrodes provided independently for each of the plurality of optical waveguides;
An optical waveguide device comprising: a plurality of light detection units that independently detect the intensity of light emitted from the plurality of optical waveguides.

(Supplementary note 2) A plurality of waveguide couplers provided on the respective output sides of the plurality of optical waveguides;
A plurality of monitoring optical waveguides coupled to one port of the plurality of waveguide couplers;
The optical waveguide device according to appendix 1, wherein the plurality of light detection units detect the intensity of the monitor light guided through the corresponding monitoring optical waveguide.

(Supplementary Note 3) The waveguide coupler is a 3 dB coupler,
The optical waveguide device according to appendix 2, wherein the monitoring optical waveguide is connected to one port of the 3 dB coupler.
(Supplementary Note 4) The waveguide coupler is a 1: N coupler,
The optical waveguide device according to appendix 2, wherein the monitoring optical waveguide is connected to one port of the 1: N coupler.

(Supplementary Note 5) The optical waveguide device according to Supplementary Note 4, wherein the 1: N coupler is a 1:10 coupler.
(Appendix 6) One or more waveguide couplers provided on the output side of one of the plurality of optical waveguides;
One or more Y-branch optical waveguides provided on the output side of the other optical waveguides among the plurality of optical waveguides;
One or a plurality of monitoring optical waveguides connected to one port of the waveguide coupler,
One of the plurality of photodetectors detects the intensity of the monitor light guided through the corresponding monitor optical waveguide, and the other photodetector detects from the branch portion of the corresponding Y branch optical waveguide. The optical waveguide device according to appendix 1, wherein the intensity of the emitted radiation is detected.

(Appendix 7) The plurality of optical waveguides are Mach-Zehnder type optical waveguides each having a Y-branch optical waveguide on the output side,
The plurality of light detectors are arranged with their positions shifted in the thickness direction of the output side end face of the substrate so that the intensity of the radiated light emitted from the corresponding branch portion of the Y branch optical waveguide can be detected. 2. The optical waveguide device according to appendix 1, wherein

(Appendix 8) One of the plurality of monitoring optical waveguides is formed to extend to the side surface of the substrate,
Supplementary note 2, wherein one of the plurality of light detection units is provided on a side surface of the substrate so as to detect light output from the one monitoring optical waveguide. The optical waveguide device according to any one of -5.

(Additional remark 9) The said board | substrate is comprised with lithium niobate, The optical waveguide device of any one of additional marks 1-8 characterized by the above-mentioned.
(Appendix 10) a substrate having an electro-optic effect;
First and second Mach-Zehnder type optical waveguides formed in series on the substrate;
A first signal electrode provided in the first Mach-Zehnder optical waveguide and supplied with a clock signal;
A second signal electrode provided in the second Mach-Zehnder optical waveguide and supplied with a data signal;
A first bias electrode provided in the first Mach-Zehnder type optical waveguide and applied with a bias voltage;
A second bias electrode provided in the second Mach-Zehnder optical waveguide and applied with a bias voltage;
A first monitor PD for detecting the intensity of light output from the first Mach-Zehnder type optical waveguide;
An optical modulator comprising: a second monitor PD for detecting the intensity of light output from the second Mach-Zehnder type optical waveguide.

(Supplementary Note 11) A 3 dB coupler provided on the output side of the first Mach-Zehnder optical waveguide or the second Mach-Zehnder optical waveguide;
The monitoring light that guides the light output from the first Mach-Zehnder type optical waveguide or the second Mach-Zehnder type optical waveguide to the first monitor PD or the second monitor PD as monitor light. A waveguide,
The optical modulator according to appendix 10, further comprising an attenuation unit that attenuates the monitor light propagating through the monitor optical waveguide.

(Supplementary note 12) a substrate having an electro-optic effect;
An optical waveguide formed on the substrate;
An electrode provided in the optical waveguide;
A monitoring optical waveguide for guiding a part of the light output from the optical waveguide as monitoring light;
An attenuator provided in the optical waveguide for monitoring, for attenuating the monitor light;
An optical waveguide device comprising: a light detection unit that detects the intensity of the monitor light guided through the monitoring optical waveguide and attenuated by the attenuation unit.

(Supplementary note 13) The optical waveguide is a Mach-Zehnder optical waveguide,
13. The optical waveguide device according to appendix 12, wherein a 3 dB coupler is provided on the output side of the Mach-Zehnder optical waveguide.
(Supplementary note 14) The optical waveguide device according to Supplementary note 12 or 13, wherein the attenuating portion is constituted by a curved waveguide having a small radius of curvature constituting the monitoring optical waveguide.

(Supplementary note 15) The optical waveguide device according to Supplementary note 12 or 13, wherein the attenuating portion is constituted by two or more branch portions constituting the monitoring optical waveguide.
(Additional remark 16) The said attenuation | damping part is comprised by the 1: N coupler which comprises the said optical waveguide for a monitor, The optical waveguide device of Additional remark 12 or 13 characterized by the above-mentioned.
(Additional remark 17) The said attenuation | damping part is comprised by the beam expansion part provided in the edge part vicinity of the said optical waveguide for a monitor, The optical waveguide device of any one of Additional remarks 12-16 characterized by the above-mentioned.

(Supplementary note 18) The optical waveguide device according to any one of supplementary notes 12 to 16, wherein a beam expanding part is provided in the vicinity of an end of the optical waveguide for monitoring.
(Additional remark 19) The said beam expansion part is comprised by making the edge part of the said optical waveguide for monitoring into the position of predetermined distance from the end surface or side surface of the said board | substrate, The light guide of Additional remark 17 or 18 characterized by the above-mentioned. Waveguide device.

(Supplementary note 20) The optical waveguide device according to supplementary note 17 or 18, wherein the beam expanding section is configured by bifurcating an end portion of the monitoring optical waveguide.
(Supplementary note 21) a substrate having an electro-optic effect;
A plurality of Mach-Zehnder type or directional coupler type optical waveguides formed in series on the substrate;
A plurality of electrodes provided independently for each of the plurality of optical waveguides;
A plurality of light detection units for independently detecting the intensity of light emitted from the plurality of optical waveguides;
Based on the intensity of light detected by one of the plurality of light detection units, the bias voltage applied to the electrode provided in the optical waveguide corresponding to the one light detection unit is controlled. An optical waveguide device, comprising: a bias control unit that performs the operation.

1 is a schematic diagram showing an overall configuration of an optical modulator as an optical waveguide device according to a first embodiment of the present invention. It is an enlarged view showing typically an example of composition of an attenuation part with which an optical modulator as an optical waveguide device concerning a 1st embodiment of the present invention is equipped. It is an enlarged view which shows typically the other structural example of the attenuation | damping part with which the optical modulator as an optical waveguide device concerning 1st Embodiment of this invention is equipped. It is an enlarged view which shows typically the other structural example of the attenuation | damping part with which the optical modulator as an optical waveguide device concerning 1st Embodiment of this invention is equipped. It is an enlarged view which shows typically the other structural example of the attenuation | damping part with which the optical modulator as an optical waveguide device concerning 1st Embodiment of this invention is equipped. It is an enlarged view which shows typically the other structural example of the attenuation | damping part with which the optical modulator as an optical waveguide device concerning 1st Embodiment of this invention is equipped. It is an enlarged view which shows typically the other structural example of the attenuation | damping part with which the optical modulator as an optical waveguide device concerning 1st Embodiment of this invention is equipped. It is an enlarged view which shows typically the other structural example of the attenuation | damping part with which the optical modulator as an optical waveguide device concerning 1st Embodiment of this invention is equipped. It is an enlarged view which shows typically the other structural example of the attenuation | damping part with which the optical modulator as an optical waveguide device concerning 1st Embodiment of this invention is equipped. It is a figure which shows the optical output of the signal beam | light and the monitor light in the optical modulator as an optical waveguide device concerning 1st Embodiment of this invention. It is a schematic diagram which shows the whole structure of the RZ optical modulator as an optical waveguide device concerning 2nd Embodiment of this invention. It is a schematic diagram which shows the whole structure of the RZ optical modulator as an optical waveguide device concerning the 1st modification of 2nd Embodiment of this invention. It is a schematic diagram which shows the whole structure of the RZ optical modulator as an optical waveguide device concerning the 2nd modification of 2nd Embodiment of this invention. It is a schematic diagram which shows the whole structure of the RZ optical modulator as an optical waveguide device concerning the 3rd modification of 2nd Embodiment of this invention. It is a schematic diagram which shows the whole structure of the RZ optical modulator as an optical waveguide device concerning the 4th modification of 2nd Embodiment of this invention. It is a schematic diagram which shows the whole structure of the RZ optical modulator as an optical waveguide device concerning the 5th modification of 2nd Embodiment of this invention. It is a schematic diagram which shows the whole structure of the RZ optical modulator as an optical waveguide device concerning the other modification of 2nd Embodiment of this invention. It is a figure for demonstrating the method to take out the monitor light used for bias control using the optical waveguide for a monitor. It is a figure for demonstrating the subject in the case of taking out monitor light using the optical waveguide for monitors, Comprising: It is a figure which shows the optical output of signal light and monitor light. It is a figure for demonstrating the bias control in a RZ optical modulator.

Explanation of symbols

1 MZ-type LN optical modulator 2 Substrate 2A End face 3A Optical waveguide (MZ-type optical waveguide)
3 optical waveguide 3a input-side optical waveguide 3b Y-branch optical waveguide 3c, 3d linear optical waveguide 3e output-side optical waveguide 3f monitoring optical waveguide 3g linear optical waveguide 3h two-branch optical waveguide 4 main electrode 4a signal electrode 4b ground electrode 5 Bias electrode 5a Electrode 5b Ground electrode 6 3dB coupler (waveguide coupler)
7 Data signal supply unit (signal supply unit)
8 Bias control unit 9 Attenuation unit 9a Curved waveguide 9b Branch optical waveguide (multiple branch optical waveguide, multi-branch optical waveguide)
9c 1: N coupler 9d Beam expansion unit 10 Monitor PD (photo detector, bias control monitor PD, light detection unit)
20 RZ optical modulator 21, 41 Mach-Zehnder type optical modulator (MZ type LN optical modulator)
22 Substrate 23 First optical waveguide 23b, 23c Linear optical waveguide 23d Monitor optical waveguide 24 Second optical waveguide 24a Y branch optical waveguide (branch optical waveguide)
24b, 24c Linear optical waveguide 24d Optical waveguide for monitoring 25 First electrode (main electrode)
25a Signal electrode 25b Ground electrode 26 First bias electrode 26a Electrode 26b Ground electrode 27 Second electrode (main electrode)
27a Signal electrode 27b Ground electrode 28 Second bias electrode 28a Electrode 28b Ground electrode 29a 3 dB coupler (waveguide coupler)
29b 3dB coupler (waveguide coupler)
29c 3dB coupler (waveguide coupler)
30 Input-side optical waveguide 31 Intermediate optical waveguide 32 Output-side optical waveguide 33 Clock signal supply unit (signal supply unit)
34 NRZ data signal supply unit (signal supply unit)
35 1st bias control part 36 2nd bias control part 37 1st monitor PD (light detection part)
38 Second monitor PD (light detector)
60, 70, 80, 90 RZ optical modulator 61, 71, 72, 73, 81, 83, 91, 93 Y-branch optical waveguide 82, 84, 92, 94 1: N coupler (for example, 1:10 coupler, waveguide) Type optical coupler)
DESCRIPTION OF SYMBOLS 95 Variable attenuator 96 Optical waveguide 96a, 96d Y branch optical waveguide 96b, 96c Linear optical waveguide 97 Electrode 98 Bias electrode 99 1: N coupler (For example, 1:10 coupler, waveguide type optical coupler)
100 Third monitor optical waveguide 101 Third monitor PD (light detection unit)
102 Y-branch optical waveguide

Claims (4)

In an optical device in which a Mach-Zehnder interferometer having first and second optical waveguides on the output side is formed on a substrate having an electro-optic effect,
The second optical waveguide includes a curved portion that warps in a direction away from the first optical waveguide,
A monitor unit that detects radiated light emitted from the curved part into the substrate as a monitor light at a position shifted from the output end of the second optical waveguide provided with the curved part;
An optical device characterized by that. In an optical device in which a Mach-Zehnder interferometer having first and second optical waveguides on the output side is formed on a substrate having an electro-optic effect,
The second optical waveguide is constituted by branching,
Among the branched optical waveguides, the optical waveguides other than the one optical waveguide have a curved portion bent in a direction away from the first optical waveguide, and are interrupted halfway so as not to reach the end of the substrate. Is provided as
A monitor unit is provided at the output end of the one optical waveguide.
An optical device characterized by that. In an optical device in which a Mach-Zehnder interferometer having first and second optical waveguides on the output side is formed on a substrate having an electro-optic effect,
The second optical waveguide is constituted by branching,
Among the branched optical waveguides, the optical waveguides other than the one optical waveguide have a curved portion bent in a direction away from the first optical waveguide, and are interrupted halfway so as not to reach the end of the substrate. Is provided as
The end of the one optical waveguide is formed at a predetermined distance from the end surface or side surface of the substrate,
The beam diameter is large on the end face or side face of the substrate or at a position away from the end face or side face of the substrate, emitted from the end of the one optical waveguide, and through the inside of the device where the optical waveguide is not formed. Equipped with a monitor to receive the monitor light
An optical device characterized by that. In an optical device in which a Mach-Zehnder interferometer having first and second optical waveguides on the output side is formed on a substrate having an electro-optic effect,
A monitor unit for receiving output light from the second optical waveguide;
The second optical waveguide includes a beam expanding portion on the end side of the substrate,
The beam expanding portion is configured by branching the second optical waveguide on the end side,
An optical device characterized by that.

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