JP2016180867A - Optical element, mach-zehnder wavelength filter and ring resonator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical element applicable as a coupler in a wavelength filter or the like and less likely influenced by fabrication errors, in which a branch ratio can be arbitrarily set.SOLUTION: The optical element includes an optical waveguide core 30 in which a first coupling part 40 having a 3 dB branch ratio, a phase shift part 50, and a second coupling part 60 having a 3 dB branch ratio are connected in series in this order. The first coupling part 40 branches input light into two beams and transmits the light to the phase shift part 50. The phase shift part 50 imparts a phase difference to the light branched into two beams by the first coupling part 40 and transmits the light to the second coupling part 60. The second coupling part 60 allows the light from the phase shift part 50 to interfere and branches the interfered light in a branch ratio depending on the phase difference to output.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical element that can be used as a coupler in, for example, a wavelength filter.

  In recent years, Si wire waveguides that use Si (silicon) as a material for thin wire waveguides have attracted attention in developing optical devices that are advantageous for miniaturization and mass productivity.

  In the Si thin wire waveguide, an optical waveguide core that substantially becomes a light transmission path is formed using Si as a material. Then, the periphery of the optical waveguide core is covered with a clad made of, for example, silica having a refractive index lower than that of Si. With such a configuration, the refractive index difference between the optical waveguide core and the clad becomes extremely large, so that light can be strongly confined in the optical waveguide core. As a result, a small curved waveguide having a bending radius reduced to, for example, about 1 μm can be realized. Therefore, it is possible to create an optical circuit having a size comparable to that of an electronic circuit, which is advantageous for downsizing the entire optical device.

  In addition, in the Si wire waveguide, it is possible to divert the manufacturing process of a semiconductor device such as a CMOS (Complementary Metal Oxide Semiconductor). Therefore, realization of photoelectric fusion (silicon photonics) in which electronic functional circuits and optical functional circuits are collectively formed on a chip is expected.

  By the way, in a passive optical network (PON) using a wavelength division multiplexing (WDM) system, different reception wavelengths are assigned to each subscriber side device (ONU: Optical Network Unit). . A station side device (OLT: Optical Line Terminal) generates a downstream optical signal to each ONU at a transmission wavelength corresponding to a reception wavelength of a destination, and multiplexes and transmits them. Each ONU selectively receives an optical signal having a reception wavelength allocated to itself from downstream optical signals multiplexed at a plurality of wavelengths. In the ONU, in order to selectively receive the downstream optical signal of each reception wavelength, for example, an optical element (hereinafter also referred to as a wavelength filter) provided with a function as a wavelength filter for separating a specific wavelength is used. . And the technique which comprises this wavelength filter by the Si thin wire | line waveguide mentioned above is implement | achieved.

  As a wavelength filter using a Si thin wire waveguide, for example, there is a wavelength filter using an optical interference phenomenon (see, for example, Patent Documents 1 and 2 and Non-Patent Documents 1 and 2). Among wavelength filters that use the interference phenomenon, Mach-Zehnder type wavelength filters are widely used.

  The Mach-Zehnder type wavelength filter includes, for example, a coupler and two arms connected to the coupler and having different optical path lengths. In the Mach-Zehnder type wavelength filter, light branched into two at the coupler is input to two arms. Then, the lights output from these two arms are combined and interfered. As a result, light having a wavelength corresponding to the difference in optical path length between the two arms can be extracted. Furthermore, by connecting a plurality of such Mach-Zehnder type wavelength filters, the wavelength band of the extracted light can be widened.

  As the above-described coupler, for example, a directional coupler or a multimode interference (MMI) coupler can be used.

JP 2009-198914 A JP 2003-149472 A

Technical digest OFC / NFOEC 2010, paper OWJ3, March 2010 Journal of Selected Areas in Quantum Electronics, vol. 16, p. 33, 2010

  The directional coupler includes two optical waveguide cores that are spaced apart from each other and arranged side by side. In the directional coupler, an arbitrary branching ratio can be set by appropriately changing the coupling length of these two optical waveguide cores. Therefore, the directional coupler has an advantage that the branching ratio can be easily set to a desired value.

  However, the directional coupler has a drawback that it is easily affected by manufacturing errors. Therefore, in the directional coupler, deviation from a desired branching ratio is likely to occur due to manufacturing errors. Therefore, when a directional coupler is used as a coupler of a Mach-Zehnder type wavelength filter, it is difficult to extract light having a desired wavelength with a high extinction ratio.

  On the other hand, the MMI coupler has an advantage that it is less susceptible to manufacturing errors than the directional coupler. However, with an MMI coupler, it is difficult to design an arbitrary branching ratio except when the branching ratio is 3 dB.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an optical element that can be used as a coupler in, for example, a Mach-Zehnder type wavelength filter, and that is hardly affected by manufacturing errors and that can arbitrarily set a branching ratio. There is.

  As a result of intensive research, the inventor has constructed an optical element with two coupling portions each configured as the above-described MMI coupler or directional coupler and a phase shift portion connecting between them, thereby reducing manufacturing errors. It has been found that an optical element that is not easily affected and that can arbitrarily set the branching ratio can be configured. In order to solve the above-described problems, the optical element according to the present invention has the following features.

  The optical element of the present invention includes an optical waveguide core in which a first coupling portion having a branch ratio of 3 dB, a phase shift portion, and a second coupling portion having a branch ratio of 3 dB are connected in series in this order. The first coupling unit splits the input light into two and sends it to the phase shift unit. The phase shift unit gives a phase difference to the light branched into two at the first coupling unit and sends the light to the second coupling unit. The second coupling unit causes the light transmitted from the phase shift unit to interfere, and branches the interfered light at a branching ratio corresponding to the phase difference and outputs the branched light.

  In the optical element of the present invention, a phase difference is given to the light branched by the first coupling portion. As a result, the light interfered by the second coupling unit is output at a branching ratio corresponding to the phase difference given by the phase shift unit. Therefore, in the optical element of the present invention, the branching ratio of the output light from the second coupling unit can be arbitrarily set by appropriately adjusting the phase difference given by the phase shift unit.

  Further, in the optical element of the present invention, the influence of manufacturing errors can be reduced. The effect of this manufacturing error will be described in detail later.

(A) And (B) is the schematic of the optical element of this invention. It is the schematic of the Mach-Zehnder type | mold wavelength filter using the optical element of this invention. It is the schematic of the Mach-Zehnder type | mold wavelength filter using the optical element of this invention. It is a figure which evaluates the characteristic of the Mach-Zehnder type | mold wavelength filter using the optical element of this invention. It is the schematic of the ring resonator using the optical element of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shape, size, and arrangement relationship of each component are merely schematically shown to the extent that the present invention can be understood. In the following, a preferred configuration example of the present invention will be described. However, the material and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many changes or modifications that can achieve the effects of the present invention can be made without departing from the scope of the configuration of the present invention.

(Optical element)
An optical element according to an embodiment of the present invention will be described with reference to FIG. FIG. 1A is a schematic plan view showing an optical element. FIG. 1B is a schematic end view of the optical element shown in FIG. In FIG. 1A, only the optical waveguide core described later is shown, and the cladding layer and the support substrate are omitted.

  Note that, in FIG. 1A, a rough propagation direction of light is indicated by an arrow R. Since the reverse process holds for light, the light propagation direction is not limited to the arrow R. In the following description, the direction along the thickness of the support substrate is defined as the thickness direction. The direction along the light propagation direction is the length direction. Moreover, let the direction orthogonal to a length direction and a thickness direction be a width direction.

  The optical element 100 includes a support substrate 10, a clad layer 20, and an optical waveguide core 30.

  The support substrate 10 is composed of a flat plate made of, for example, single crystal Si.

The clad layer 20 is formed on the support substrate 10 so as to cover the upper surface 10 a of the support substrate 10 and include the optical waveguide core 30. The clad layer 20 is formed using, for example, SiO ₂ as a material.

  The optical waveguide core 30 is made of, for example, Si having a refractive index higher than that of the cladding layer 20. As a result, the optical waveguide core 30 functions as a substantial light transmission path, and the input light propagates in the propagation direction according to the planar shape of the optical waveguide core. Further, the optical waveguide core 30 is preferably formed away from the support substrate 10 by a distance in the range of, for example, at least about 1 to 3 μm in order to prevent the propagating light from escaping to the support substrate 10. .

  Further, the optical waveguide core 30 is configured by connecting the first coupling portion 40, the phase shift portion 50, and the second coupling portion 60 in series in this order.

  The first coupling unit 40 is configured as an MMI coupler having a branch ratio of 3 dB. The first coupling unit 40 includes an MMI unit 45, first and second input / output units 41 and 42 provided in parallel to one end 45a of the MMI unit 45, and a second input unit 45b provided in parallel to the other end 45b of the MMI unit 45. 3 and fourth input / output units 43 and 44. Here, the first and second input / output units 41 and 42 are used as input ports, and the third and fourth input / output units 43 and 44 are used as output ports.

  Here, a case where light input from the first or second input / output unit 41 or 42 of the first coupling unit 40 propagates in the order of the first coupling unit 40, the phase shift unit 50, and the second coupling unit 60 will be described. To do.

  The MMI unit 45 is configured as a multimode waveguide that propagates a plurality of order modes. The MMI unit 45 splits the light input from the first or second input / output unit 41 or 42 into two by causing intermode interference. Then, the MMI unit 45 sends one of the two branched lights to the third input / output unit 43 and sends the other to the fourth input / output unit 44. The light branching ratio in the MMI unit 45 can be set according to the length of the MMI unit 45.

  However, it is difficult to design the MMI unit 45 to an arbitrary branching ratio except when the branching ratio is 3 dB. Therefore, in this embodiment, the length of the MMI unit 45 is designed so that the branching ratio that can be easily designed is 3 dB (that is, the light intensity ratio is 1: 1).

  In addition, the width of the MMI unit 45 is designed so that the equivalent refractive index difference between the modes to be interfered matches between the TE (Transverse Electric) polarization and the TM (Transverse Magnetic) polarization. As a result, the first coupling unit 40 can branch the input light without depending on the polarization.

  The first and second input / output units 41 and 42 are designed so that the width continuously increases toward the one end 45a of the MMI unit 45. The first and second input / output units 41 and 42 excite a plurality of order modes of input light and input them to the MMI unit 45. The mode excited by the first and second input / output units 41 and 42 is set corresponding to the mode interfered by the MMI unit 45.

  Further, the third and fourth input / output units 43 and 44 are designed such that the width continuously decreases as the distance from the other end 45b of the MMI unit 45 increases. The third and fourth input / output units 43 and 44 send the light branched into two by the MMI unit 45 to the phase shift unit 50, respectively.

  The phase shift unit 50 includes a first arm 51 and a second arm 52. The 1st arm 51 and the 2nd arm 52 connect between the 1st coupling part 40 and the 2nd coupling part 60 mentioned below. In the configuration example of FIG. 1, the first arm 51 includes a third input / output unit 43 of the first coupling unit 40 on one end 51a side, and a first coupling waveguide unit 61 of the second coupling unit 60 on the other end 51b side. Connected with. The second arm 52 is connected to the fourth input / output unit 44 of the first coupling unit 40 on the one end 52a side and to the second coupling waveguide unit 62 of the second coupling unit 60 on the other end 52b side. . Then, the first arm 51 sends the light sent from the third input / output unit 43 to the first coupling waveguide unit 61. Further, the second arm 52 sends the light sent from the fourth input / output unit 44 to the second coupled waveguide unit 62.

  The first arm 51 and the second arm 52 have different optical path lengths. Therefore, a phase difference corresponding to the optical path length between the first arm 51 and the second arm 52 is generated between the light propagating through the first arm 51 and the light propagating through the second arm 52.

  The widths of the first arm 51 and the second arm 52 are designed so that the equivalent refractive indexes of the propagating TE polarization and the TM polarization coincide. As a result, the phase shifter 50 can give a common phase difference to the TE polarized wave and the TM polarized wave.

  The second coupling unit 60 includes a first coupling waveguide unit 61 and a second coupling waveguide unit 62.

  The first coupling waveguide section 61 and the second coupling waveguide section 62 are arranged apart from each other and side by side. The first coupled waveguide section 61 and the second coupled waveguide section 62 are formed with an equal width and length. The surface positions of the one end 61a of the first coupling waveguide section 61 and the one end 62a of the second coupling waveguide section 62 coincide, and the other end 61b of the first coupling waveguide section 61 and the second coupling waveguide section 62 are aligned. The other end 62b is designed so that the surface positions thereof coincide.

  The lengths of the first coupling waveguide section 61 and the second coupling waveguide section 62 are designed to be ½ of the complete coupling length. As a result, the first coupling waveguide section 61 and the second coupling waveguide section 62 constitute a directional coupler having a branching ratio of 3 dB. The complete coupling length is a coupling length at which light input to one of the first coupling waveguide section 61 and the second coupling waveguide section 62 moves to the other 100%.

  In addition, the width of the first coupled waveguide section 61 and the second coupled waveguide section 62 and the separation distance between the first coupled waveguide section 61 and the second coupled waveguide section 62 are determined by the propagating TE polarization and TM polarization. It is designed so that the equivalent refractive indices of the waves match. As a result, the second coupling unit 60 can give a common branching ratio to the TE polarization and the TM polarization.

  In the optical element 100, the light input from the first or second input / output unit 41 or 42 is branched into two at the first coupling unit 40 with an intensity ratio of 1: 1. One of the branched lights is input to the first arm 51 of the phase shift unit 50, and the other is input to the second arm 52. The light that has passed through the first arm 51 and the second arm 52 is input to the first coupling waveguide section 61 and the second coupling waveguide section 62 of the second coupling section 60 and interferes therewith.

  As described above, the second coupling portion 60 has a branching ratio of 3 dB as a single unit. However, in this optical element 100, the phase shift unit converts the light input from the first arm 51 into the first coupling waveguide unit 61 and the light input from the second arm 52 into the second coupling waveguide unit 62. At 50, a phase difference is given. As a result, the light interfered by the second coupling unit 60 is output from the first coupling waveguide unit 61 and the second coupling waveguide unit 62 with a branching ratio corresponding to the phase difference given by the phase shift unit 50. . In the optical element 100, the branching ratio of the output light from the second coupling unit 60 is arbitrarily adjusted by appropriately adjusting the phase difference given to the light propagating through the first arm 51 and the light propagating through the second arm 52. Can be set to

  In the optical element 100, even if a directional coupler that is easily affected by manufacturing errors is used as the second coupling unit 60, the influence of manufacturing errors on the entire optical element 100 can be reduced. The influence of manufacturing errors on the optical element 100 will be described below.

Transfer matrix _{M 1} of the first coupling portion 40 is given by the following equation (1). As described above, since the branching ratio of the first coupling unit 40 is set to 3 dB, the amplitude branching ratio is 1 / √2.

Transfer matrix _{M 2} of the phase shift unit 50 is given by the following equation (2). φ represents ½ of the phase difference generated between the first arm 51 and the second arm 52.

Transfer matrix _{M 3} of the second coupling portion 60 is given by the following equation (3). As described above, the branching ratio of the second coupling unit 60 is set to 3 dB. However, since a directional coupler is used as the second coupling unit 60, the branching ratio is likely to vary due to the influence of manufacturing errors. Therefore, in the following equation (3), the amplitude branching ratio of the second coupling unit 60 is a ratio in which variation due to manufacturing error occurs, and is represented by a and b as a ratio with respect to input light. J represents an imaginary unit.

  Therefore, the transfer matrix M of the optical element 100 is given by the following equation (4).

 [V1AHB06115P-E]

  Then, using the above equation (4), the transmission matrix of the optical element 100 is expressed as the equation indicating the light intensity, and the following equation (5) is obtained.

  If the ideal amplitude branching ratio of the second coupling unit 60 when the fluctuation due to the manufacturing error does not occur is A and B as the ratio to the input light, ab in the above equation (5) is expressed by the following equation (6) Given in. Here, δ represents the amount of variation from the ideal amplitude branching ratio due to the influence of manufacturing errors in terms of light intensity.

  When the above equation (6) is approximated with δ being a minute value, the following equation (7) is derived.

  Since the second coupling unit 60 is set so that the branching ratio is 3 dB, the ideal amplitude branching ratios A and B are A = B = 1 / √2. By substituting this into the above equation (7), ab can be expressed by the following equation (8).

  Then, ab expressed by the above equation (8) is substituted into the above equation (5) to derive the following equation (9).

In the optical element 100, as shown in the above equation (9), the terms including the fluctuation amount δ of the amplitude branching ratio are δ ² cos (2φ) and −δ ² cos (2φ). Since the fluctuation amount δ is a minute value of δ <1, δ ² cos (2φ) and −δ ² cos (2φ) are minute values. Therefore, in the optical element 100, even if a directional coupler that is easily affected by manufacturing errors is used as the second coupling unit 60, the influence of manufacturing errors on the entire optical element 100 is reduced.

  Here, as a comparative example, the influence of manufacturing errors in the second coupling unit 60 (that is, the directional coupler) alone will be described. Using the above equation (3), the transmission matrix of the second coupling portion 60 is expressed as an equation indicating the light intensity, and the following equation (10) is obtained.

  In the second coupling unit 60 alone, as shown in the above equation (10), the fluctuation amount δ directly affects the amplitude branching ratio. Compared with the optical element 100 represented by the above equation (9) and the transmission matrix of the second coupling unit 60 represented by the above equation (10), it is clear that the optical element 100 has a small influence of the variation δ on the amplitude branching ratio. It is.

  As another comparative example, when the first coupling unit 40 of the optical element 100 is changed from an MMI coupler to a directional coupler (that is, the first coupling unit 40 and the second coupling unit 60 are both directional couplers). The influence of the manufacturing error in the case of When the transmission matrix in this case is expressed as an expression indicating the light intensity, the following expression (11) is obtained.

  Similar to the transmission matrix of the optical element 100 described above, A = B = 1 / √2, and ab expressed by the above equation (8) is substituted into the above equation (11), so that the following equation (12) is obtained. Led.

In the configuration in which the first coupling unit 40 and the second coupling unit 60 are both directional couplers, as shown in the above equation (12), the term including the variation amount δ of the amplitude branching ratio is 2δ ² cos (1 + 2φ ) And −2δ ² cos (1 + 2φ). Therefore, the influence of the fluctuation amount δ on the amplitude branching ratio is smaller than that of the second coupling unit 60 (directional coupler) alone. However, the influence of the fluctuation amount δ on the amplitude branching ratio is greater than that of the optical element 100.

  As described above, the optical element 100 can reduce the influence of manufacturing errors by including the two coupling portions 40 and 60 and the phase shift portion 50 that connects the two coupling portions 40 and 60.

  In this embodiment, the configuration example using the MMI coupler as the first coupling unit 40 and the directional coupler as the second coupling unit 60 has been described. However, the configuration of the optical element 100 is not limited to this.

  For example, both the first coupling unit 40 and the second coupling unit 60 may be MMI couplers. The MMI coupler has a drawback that the loss of light is larger than that of the directional coupler. Therefore, when both the first coupling unit 40 and the second coupling unit 60 are MMI couplers, it is disadvantageous from the viewpoint of reducing light loss. However, when both the first coupling unit 40 and the second coupling unit 60 are MMI couplers, the optical element 100 can be configured without including a directional coupler that is easily affected by manufacturing errors. Therefore, the variation of the branching ratio due to the manufacturing error can be further reduced as compared with the optical element 100 of this embodiment.

  Moreover, both the 1st coupling | bond part 40 and the 2nd coupling | bond part 60 can also be made into a directional coupler. As already described, in the case where both the first coupling unit 40 and the second coupling unit 60 are directional couplers, from the viewpoint of reducing the fluctuation of the branching ratio, the optical element 100 of this embodiment is used. Is also disadvantageous. However, since an MMI coupler with a large light loss is not included, it is advantageous from the viewpoint of reducing the light loss.

  In the optical element 100 shown in FIG. 1 using an MMI coupler as one of the first coupling unit 40 and the second coupling unit 60 and using a directional coupler as the other, the first coupling unit 40 and the second coupling unit 60 are used. Compared with the case where both of them are MMI couplers, the loss of light is small, and the branching ratio fluctuations due to manufacturing errors are compared with the case where both the first coupling unit 40 and the second coupling unit 60 are directional couplers. Is small. Therefore, it is advantageous both from the viewpoint of reducing optical loss and from the viewpoint of reducing branching ratio fluctuations due to manufacturing errors.

  The combination of the MMI coupler and the directional coupler used as the first coupling unit 40 and the second coupling unit 60 can be determined as appropriate according to the application.

(Mach-Zehnder type wavelength filter)
A Mach-Zehnder type wavelength filter using the above-described optical element will be described with reference to FIG. FIG. 2 is a schematic plan view showing a Mach-Zehnder type wavelength filter. In FIG. 2, the clad layer and the support substrate are omitted. In addition, the same reference numerals are given to components common to the above-described optical element 100 (see FIG. 1), and description thereof is omitted.

  The Mach-Zehnder type wavelength filter 300 is configured using the optical element 100 described above as a coupler.

  The Mach-Zehnder type wavelength filter 300 includes three optical elements (first optical element 100-1, intermediate optical element 320, and second optical element 100-2) and two phase adjustment units 310 (first phase adjustment unit 310-1). And a second phase adjustment unit 310-2). The first optical element 100-1, the first phase adjustment unit 310-1, the intermediate optical element 320, the second phase adjustment unit 310-2, and the second optical element 100-2 are connected in series in this order.

  Each of the phase adjustment units 310 includes a pair of arm cores (first arm core 311 and second arm core 312) having different optical path lengths. In the configuration example of FIG. 2, the optical path length of the first arm core 311 is designed to be longer than the second arm core 312.

  The first arm core 311 is connected to the first coupled waveguide portion 61 of the optical element 100 on one end 311a side, and is connected to the intermediate optical element 320 on the other end 311b side. The second arm core 312 is connected to the second coupling waveguide section 62 of the optical element 100 on the one end 312a side, and connected to the intermediate optical element 320 on the other end 312b side. Accordingly, the first arm core 311 and the second arm core 312 connect the optical element 100 and the intermediate optical element 320 in parallel. Here, the first arm core 311 and the second arm core 312 of the first phase adjustment unit 310-1 connect between the first optical element 100-1 and the intermediate optical element 320. In addition, the first arm core 311 and the second arm core 312 of the second phase adjustment unit 310-2 connect the second optical element 100-2 and the intermediate optical element 320.

  The first phase adjustment unit 310-1 and the second phase adjustment unit 310-2 are arranged so that the first arm core 311 and the second arm core 312 are antisymmetric with respect to the intermediate optical element 320. The By arranging in this way, the first optical element 100-1 and the second optical element 100-2 are input from the first or second input / output unit 41 or 42 of the first coupling unit 40, and the first optical element 100-1 and the second optical element 100-2 The wavelength band of the light extracted from the first and second input / output units 41 and 42 of the first coupling unit 40 on the other side of the optical element 100-1 and the second optical element 100-2 can be widened. That is, a flat-top characteristic can be realized for both the bar state and cross state output lights.

  The intermediate optical element 320 is a multiplexer / demultiplexer having a branching ratio of 3 dB. As the intermediate optical element 320, for example, an MMI coupler or a directional coupler can be used. FIG. 2 shows a configuration example when the intermediate optical element 320 is configured as an MMI coupler. Note that the optical element 100 of this embodiment can also be used for the intermediate optical element 320.

  In the Mach-Zehnder type wavelength filter 300, light is input from the first or second input / output unit 41 or 42 of the first optical element 100-1. The input light is branched by the first optical element 100-1. The branched light is input to the first arm core 311 and the second arm core 312 of the first phase adjustment unit 310-1. In the first phase adjusting unit 310-1, a phase difference corresponding to an optical path length difference between the first arm core 311 and the second arm core 312 is generated between the light propagating through the first arm core 311 and the light propagating through the second arm core 312. Given. The light propagating through the first arm core 311 and the second arm core 312 is interfered by the intermediate optical element 320 and branched again. The light branched by the intermediate optical element 320 is input to the first arm core 311 and the second arm core 312 of the second phase adjustment unit 310-2. In the second phase adjustment unit 310-2, similarly to the first phase adjustment unit 310-1, a phase difference is given to the light propagating through the first arm core 311 and the light propagating through the second arm core 312. The light that has passed through the second phase adjustment unit 310-2 is interfered by the second optical element 100-2. The interfered light is output from the first and second input / output units 41 and 42 of the second optical element 100-2, respectively.

  In the Mach-Zehnder type wavelength filter 300, light having a wavelength corresponding to the phase difference provided by the first phase adjustment unit 310-1 and the second phase adjustment unit 310-2 is converted into the first and second light of the second optical element 100-2. It can be taken out from the input / output units 41 and 42.

  Here, the Mach-Zehnder type wavelength filter using the optical element 100 is not limited to the configuration example including the two phase adjustment units 310-1 and 310-2. A Mach-Zehnder type wavelength filter that connects 2N phase adjustment units can be configured by using 2N + 1 optical elements.

  A Mach-Zehnder type wavelength filter including 2N phase adjustment units will be described with reference to FIG. FIG. 3 is a schematic plan view showing a Mach-Zehnder type wavelength filter. In FIG. 3, the clad layer and the support substrate are omitted. In addition, the same components as those of the optical element 100 (see FIG. 1) and the phase adjusting unit 310 (see FIG. 3) described above are denoted by the same reference numerals, and description thereof is omitted.

  In the configuration example of FIG. 3, the Mach-Zehnder type wavelength filter 400 includes five optical elements (first, second, fourth, and fifth optical elements 100-1, 2, 4, and 5 and intermediate) as 2N + 1 optical elements. An optical element (third optical element) 420) and four phase adjusting units 310 (first to fourth phase adjusting units 310-1 to 310-4) as 2N phase adjusting units.

  The first and second optical elements 100-1 and 1002, the intermediate optical element 420, and the fourth and fifth optical elements 100-4 and 5 are arranged in this order. The first to fourth phase adjusting units 310-1 to 310-4 connect these five optical elements 100-1 and 2, 420, 100-4, and 5. The first arm core 311 and the second arm core 312 of each phase adjustment unit 310 connect the adjacent optical elements 100-1, 2, 4 and 5, and 420 in parallel.

  In the Mach-Zehnder type wavelength filter 400, the intermediate optical element 420 is arranged as the (N + 1) th (herein, third) optical element. The intermediate optical element 420 is a multiplexer / demultiplexer having a branching ratio of 3 dB. As the intermediate optical element 420, for example, an MMI coupler or a directional coupler can be used. FIG. 3 shows a configuration example when the intermediate optical element 420 is configured as an MMI coupler. Note that the optical element 100 of this embodiment can also be used for the intermediate optical element 420.

  In the Mach-Zehnder type wavelength filter 400, a plurality of pairs of phase adjustment units 310 can be combined. In each of these sets of phase adjustment units 310, the first arm core 311 and the second arm core 312 are arranged to be antisymmetric with respect to the intermediate optical element 420. By arranging in this way, it is possible to realize a flat-top characteristic for both output light in the bar state and the cross state.

  Thus, in the Mach-Zehnder type wavelength filter 400, the wavelength band of the output light can be made wider by adjusting the branching ratio of each optical element 100.

(Characteristic evaluation)
The inventor performed a simulation for evaluating the characteristics of the optical element 300 shown in FIG. In this simulation, in the optical element 300 shown in FIG. 2, from the first or second input / output unit 41 or 42 of the first coupling unit 40, one of the first optical element 100-1 and the second optical element 100-2. The intensity of the light input and output from the first or second input / output unit 41 or 42 of the first coupling unit 40 on the other side of the first optical element 100-1 and the second optical element 100-2 was confirmed. Here, the branching ratio of the second coupling unit 60 alone configured as a directional coupler was changed, and the intensity of each output light was confirmed. The phase difference 2φ provided by the phase shift unit 50 was 2π / 7.

  The simulation results are shown in FIG. FIG. 4 is a diagram for evaluating the characteristics of the Mach-Zehnder type wavelength filter 300. In FIG. 4, the vertical axis represents the intensity in arbitrary units, and the horizontal axis represents the phase generated in the first phase adjusting unit 310-1 and the second phase adjusting unit 310-2 corresponding to the wavelength of the input light in arbitrary units. It is shown in

  In FIG. 4, a curve 201 indicates that a branching ratio of 3 is set when the branching ratio of the first coupling element 60 of the first optical element 100-1 and the second optical element 100-2 is designed to be 3: 7. The output light intensity on the other side is shown. A curve 203 indicates the strength when the branch ratio of the first coupling element 60 of the first optical element 100-1 and the second optical element 100-2 is designed to be 1: 1 (that is, 3 dB). A curve 205 indicates the output on the side where the branching ratio 7 is set when the branching ratio of the first coupling element 60 of the first optical element 100-1 and the second optical element 100-2 is designed to be 7: 3. It shows the intensity of light.

  As shown in FIG. 4, even if the branching ratio of the second coupling unit 60 alone is changed, the intensity of the output light is almost the same. Therefore, even when the branching ratio of the second coupling unit 60 alone varies from 3 dB due to manufacturing errors, it was confirmed that the influence of the variation hardly affects the branching ratio of the entire optical element 100. .

(Ring resonator)
A ring resonator using the above-described optical element will be described with reference to FIG. FIG. 5 is a schematic plan view showing the ring resonator. In FIG. 5, the clad layer and the support substrate are omitted. In addition, the same reference numerals are given to components common to the above-described optical element 100 (see FIG. 1), and description thereof is omitted.

  The ring resonator 500 is configured using the optical element 100 described above as a coupler.

  The ring resonator 500 is provided between the two optical elements 100 (the first optical element 100-1 and the second optical element 100-2) and the first optical element 100-1 and the second optical element 100-2. A pair of curved waveguide cores (first curved waveguide core 510 and second curved waveguide core 520).

  The first curved waveguide core 510 connects between the second coupling waveguide section 62 of the first optical element 100-1 and the second input / output section 42 of the second optical element 100-2. The second curved waveguide core 520 connects the second coupling waveguide portion 62 of the second optical element 100-2 and the second input / output portion 42 of the first optical element 100-1. Accordingly, the first optical element 100-1, the first curved waveguide core 510, the second optical element 100-2, and the second curved waveguide core 520 are connected in an annular shape in this order. The first optical element 100-1, the first curved waveguide core 510, the second optical element 100-2, and the second curved waveguide core 520 constitute a ring waveguide core 550.

  In the ring resonator 500, light is input from the first input / output unit 41 of the first optical element 100-1. The first optical element 100-1 branches the input light. One of the branched lights is input to the second optical element 100-2 via the first curved waveguide core 510. The second optical element 100-2 branches the input light. One of the branched lights is input to the first optical element 100-1 via the second curved waveguide core 520. In the ring resonator 500, light having a wavelength that resonates with the ring waveguide core 550 with respect to the light input from the first input / output unit 41 is converted into the first optical element 100-1 and the second optical element 100-. 2 from the first coupling waveguide section 61. The wavelength of the output light can be set according to the optical path lengths of the first curved waveguide core 510 and the second curved waveguide core 520.

10: support substrate 20: clad layer 30: optical waveguide core 40: first coupling unit 50: phase shift unit 60: second coupling unit 100: optical element 300, 400: Mach-Zehnder wavelength filter 310: phase adjustment unit 320, 420 : Intermediate optical element 500: Ring resonator

The optical element of the present invention includes an optical waveguide core in which a first coupling portion having a branch ratio of 3 dB, a phase shift portion, and a second coupling portion having a branch ratio of 3 dB are connected in series in this order. The first coupling unit splits the input light into two and sends it to the phase shift unit. The phase shift unit gives a phase difference to the light branched into two at the first coupling unit and sends the light to the second coupling unit. The second coupling unit causes the light transmitted from the phase shift unit to interfere, and branches the interfered light at a branching ratio corresponding to the phase difference and outputs the branched light. One of the first coupling unit and the second coupling unit is configured as a multimode interference coupler, and the other is configured as a directional coupler. The optical element of the present invention is used as a multiplexer / demultiplexer of a Mach-Zehnder type wavelength filter or a ring resonator coupler.

Claims (6)

A first coupling part having a branching ratio of 3 dB, a phase shift part, and a second coupling part having a branching ratio of 3 dB are provided with an optical waveguide core connected in series in this order,
The first coupling unit splits the input light into two and sends it to the phase shift unit,
The phase shift unit gives a phase difference to the bifurcated light and sends it to the second coupling unit.
The optical element characterized in that the second coupling unit causes the light transmitted from the phase shift unit to interfere, and branches the interfered light at a branching ratio corresponding to the phase difference and outputs the branched light. The optical element according to claim 1, wherein the first coupling unit and the second coupling unit are each configured as a multimode interference coupler or a directional coupler. The optical element according to claim 2, wherein one of the first coupling unit and the second coupling unit is configured as a multimode interference coupler, and the other is configured as a directional coupler. The first optical element, the first phase adjustment unit, the intermediate optical element, the second phase adjustment unit, and the second optical element are connected in series in this order,
Each of the first phase adjustment unit and the second phase adjustment unit has a first arm core and a second arm core,
The first arm core and the second arm core of the first phase adjustment unit connect the first optical element and the intermediate optical element in parallel,
The first arm core and the second arm core of the second phase adjustment unit connect the second optical element and the intermediate optical element in parallel,
The first arm core and the second arm core have different optical path lengths from each other,
The Mach-Zehnder type wavelength filter, wherein the first optical element and the second optical element are the optical elements according to claim 1. Comprising first to second N + 1 (N is a positive integer) optical elements, and first to second N phase adjustment units,
The first to second N + 1 (N is a positive integer) optical elements are arranged in this order,
The first to second N phase adjusting units connect between the first to second N + 1 optical elements,
Each of the first to second N phase adjustment units has a first arm core and a second arm core,
The first arm core and the second arm core connect the first to second N + 1 optical elements adjacent to each other in parallel,
The first arm core and the second arm core have different optical path lengths from each other,
The Mach-Zehnder type wavelength filter, wherein the first to Nth and N + 2 to 2N + 1 optical elements are optical elements according to any one of claims 1 to 3. A first optical element and a second optical element; a first curved waveguide core and a second curved waveguide core;
The first optical element and the second optical element are the optical elements according to any one of claims 1 to 3,
A ring waveguide core in which the first optical element, the first curved waveguide core, the second optical element, and the second curved waveguide core are annularly connected in this order is configured;
Light having a wavelength that is input from the first coupling unit included in the first optical element and resonates with the ring waveguide core is transmitted from the second coupling unit included in each of the first optical element and the second optical element. A ring resonator characterized by being output.

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