JP3348431B2 - Dispersion compensating optical circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an optical communication system,
In particular, the present invention relates to a dispersion compensating optical circuit used in a wavelength division multiplexing communication system or an ultra-high speed optical communication system, and more particularly to a dispersion compensating optical circuit that performs wideband dispersion compensation with small insertion loss.

[0002]

2. Description of the Related Art A conventional dispersion compensating element mainly used in an optical communication system, in particular, a wavelength division multiplexing communication system or an ultra-high speed optical communication system, includes a dispersion compensating fiber and a fiber grating. Some are used. Further, a waveguide type dispersion compensating optical circuit has been proposed as a dispersion compensating element having another configuration to solve the problems of the dispersion compensating element using the dispersion compensating fiber and the fiber grating. These will be described below in order.

A dispersion compensating element using a dispersion compensating fiber is intended to compensate the dispersion by intentionally shifting the amount of dispersion from a normal fiber by adding impurities to a fiber core. This dispersion compensating fiber is described, for example, in Optics letters, Vol. 5 (1980), pages 476 to 478. In this example, GeO
By adding ₂ to the core, an effect of dispersion compensation is obtained. For example, to compensate for the dispersion of a 100 km normal fiber, a fiber length of about 10 km is required.

On the other hand, a dispersion compensating element using a fiber grating, by intentionally shifting the core diameter of a fiber provided with a diffraction grating, effectively changes the spacing between the diffraction gratings and reflects light in accordance with the wavelength. In this case, the position to be changed is changed, and the dispersion is compensated by the wavelength dependence of the reflection time. A dispersion compensating element using this fiber grating is, for example, the Journal of Lightwave Technol.
1 from page 1325 of Ogy, Volume 11 (1993)
It is described on page 330.

Referring to FIG. 2, the configuration and function of a dispersion compensating element using a fiber grating will be described. FIG.
FIG. 2 is a diagram schematically showing a configuration of a fiber grating. Light incident on the present fiber enters from the left side of FIG. While the refractive index of the core portion and the distance between the diffraction gratings are fixed, the diameter of the core portion is reduced so as to gradually decrease toward the end. As a result, the effect of confining light in the core part gradually weakens, and the light seeps into the cladding part, so that the effective refractive index gradually decreases toward the end. As a result, an effect is obtained in which the distance between the diffraction gratings becomes shorter toward the end. Accordingly, incident light having a longer wavelength is reflected at the near end, and light having a shorter wavelength is reflected at the far end, and a delay effect according to the wavelength is obtained. By controlling the length of the fiber, any dispersion compensation can be performed. For example, a signal having a wavelength distribution of 1 nm in a wavelength range of 1.5 μm
To compensate for the distance of km, a fiber length of about 1 m is required.

Both of the above two types of dispersion compensating elements have problems in terms of element dimensions, insertion loss, band, and the like. The details will be described later.

As a device for overcoming these problems,
A waveguide-type dispersion compensation optical circuit has been proposed. This optical circuit is disclosed in Japanese Patent Application No. 5-167645 or Electr.
onics Letters, Vol. 31 (1995), pages 2192 to 2194. The configuration of the waveguide type dispersion compensating optical circuit will be briefly described below with reference to FIG. On a substrate 32 made of silicon, a plurality of curved optical waveguides 31 forming an asymmetric Mach-Chehner type interference system are formed. These optical waveguides 31 are connected in series by a variable coupler 33 whose coupling intensity can be controlled by temperature and a coupler 34 which couples by half the intensity. In each optical waveguide section 31, a phase difference depending on the wavelength occurs due to a Mach-Cheander type interference effect. By connecting a plurality of these, a kind of grating is formed, but due to the asymmetric structure, a difference occurs in the propagation path depending on the wavelength, and it is possible to compensate for dispersion. Element size is 7
6x88 mm.

[0008]

The above-described conventional dispersion compensating elements have the following problems. First, the dispersion compensating element using the dispersion compensating fiber has a problem that the element length is long and the insertion loss is large. For example, the element length required to compensate for the dispersion of a normal fiber of 100 km is as long as about 10 km. The insertion loss at that time is as large as about 7 dB. Furthermore, this fiber has a problem that it is unstable against vibration and temperature and cannot be used as a transmission line.

Next, a dispersion compensating element using a fiber grating has a problem that the element length is long and the compensation band is narrow. For example, in the wavelength range of 1.5 μm band, 1
A long element length of about 1 m is required to compensate a signal having a wavelength distribution of nm by 100 km of the fiber. Further, since the wavelength range used in a normal wavelength division multiplexing communication system is at least 30 nm, it is necessary to prepare a total length of 30 m or 30 fiber gratings in order to cover the entire range. Is increased.

The waveguide type dispersion compensating optical circuit does not have the above-mentioned problems associated with the dispersion compensating element using the dispersion compensating fiber or the fiber grating, but still has a large element length of about 8 cm at present. . The main reason is that a waveguide having a large curvature is required to cause a delay of an optical signal by the optical waveguide, but the curvature radius of the waveguide is suppressed to about 1 cm in order to suppress deterioration of insertion loss. It is necessary. Further, the fact that only an effect equivalent to that of a diffraction grating can be basically obtained is also a reason why a large optical path length dependency cannot be obtained.

As described above, each of the conventional dispersion compensating elements has a problem and has not reached a satisfactory level. Accordingly, it is an object of the present invention to provide a dispersion compensating element having an element size of about cm or less, having a small insertion loss and a wide compensation band, overcoming the above problems.

[0012]

In order to achieve the above object, a dispersion compensating optical element according to the present invention (hereinafter referred to as a first invention) has a function of reducing the wavelength dependence of the propagation speed of signal light in an optical fiber. A dispersion compensating optical circuit for compensating, having a function of strongly refracting incident light depending on the wavelength, comprising a plurality of photonic crystals arranged in series in the light traveling direction,
Two adjacent photonic crystals of the plurality of photonic crystals are formed symmetrically with respect to a symmetry axis whose structure passes through an intermediate point of each of the two photonic crystals and intersects with the traveling direction of light, and The signal light is arranged symmetrically with respect to the axis of symmetry, and when the signal light propagates through the first photonic crystal,
It is characterized in that the light is incident on the leading photonic crystal at an incident angle set so as to be largely refracted according to the wavelength of the signal light.

According to the present invention, due to the effect of the change in the optical path length due to the wavelength in the photonic crystal and the effect of the slow propagation speed in the photonic crystal, the dispersion having a small size that stably performs a large dispersion compensation over a wide band with a small insertion loss. A compensating optical circuit can be realized. In the present invention, a large refraction phenomenon and a spectral phenomenon characteristic of a photonic crystal are used.
That is, according to the present invention, the property of a photonic crystal that light propagates in a crystal with a very large refraction angle at a certain incident angle, and that the refraction angle changes greatly depending on the wavelength of the incident light. Utilizes the properties of photonic crystals. In the present invention, the substrate is formed of a high-resistance silicon single crystal substrate having a low impurity concentration in order to avoid the influence of free carrier absorption.

FIG. 4A shows a photograph of an example of this phenomenon observed by a CCD camera. FIG. 4B shows a photograph when similar light is incident on a normal silicon crystal for comparison. When light having two wavelengths of 0.99 μm and 1.0 μm is incident on the photonic crystal from the same angle and from the same position, as shown in FIG. An angle difference of 50 ° results. This phenomenon changes variously when the wavelength or the incident angle is changed. As an example, FIG. 5 shows a change in the angle of the wavelength dependence of the propagation angle. It can be seen that the angular dispersion is the largest at an incident angle of 15 °, and 50 ° /% is obtained. Also, it can be seen that the angle change of each propagation occurs almost linearly according to the wavelength change.

By utilizing this phenomenon, propagating light is greatly refracted, and the propagation angle changes greatly according to the wavelength. This makes it possible to cause an optical path length difference in the photonic crystal according to the wavelength. It is also known that the group velocity in a photonic crystal is extremely lower than the light velocity in a vacuum, and this optical path length difference appears as a large difference in effective propagation time.

The spectral effect used here has an optical path difference enhancing effect about 50 times that of a normal diffraction grating, and about 500 times that of a regular prism. In this respect, it is greatly different from the conventional waveguide type dispersion compensation optical circuit which uses the same effect as the diffraction grating. Preferably, the wavelength dependence of the signal propagation velocity in the optical fiber is compensated for by utilizing the synergistic effect with the large group velocity dispersion of the photonic crystal. As a result, the same dispersion compensation as that of a large-sized dispersion compensation optical circuit can be performed by a small-sized dispersion compensation optical circuit.

In the present invention, the signal light is largely refracted by using the above-described properties of the photonic crystal, and a large curved propagation optical path is generated by arranging two photonic crystals symmetrically. it can. Since the refraction angle in the photonic crystal has the property of being sensitive to the wavelength of the propagating light, the degree of curvature of the optical path changes greatly according to the wavelength, resulting in a large optical path difference. Can be done. Further, since the propagating light in the photonic crystal can be slowed down by about two orders of magnitude with respect to the speed of light in a vacuum, even if the element size is about 6 mm,
An optical path difference sufficient to compensate for chromatic dispersion of a fiber length of about 00 km is obtained.

The present dispersion compensating optical circuit is most suitable as a dispersion compensating optical circuit used in an optical communication system, particularly a wavelength division multiplexing communication system or an ultra-high speed optical communication system. For example, when used in the field of wavelength division multiplexing communication, the dispersion compensating optical circuit according to the present invention (hereinafter referred to as the second invention) is used for dividing a signal into a plurality of wavelength channels in a wavelength division multiplexing communication system for transmission. A dispersion compensating optical circuit for compensating chromatic dispersion of a signal for each channel, and has a function of strongly refracting incident light depending strongly on a wavelength, and a plurality of optical circuits arranged in series in the traveling direction of light. Equipped with a photonic crystal,
Two adjacent photonic crystals of the plurality of photonic crystals are formed symmetrically with respect to a symmetry axis whose structure passes through an intermediate point of each of the two photonic crystals and intersects with the traveling direction of light, and The signal light is arranged symmetrically with respect to the axis of symmetry, and when the signal light propagates through the first photonic crystal,
At the incident angle set so as to be largely refracted according to the wavelength of the signal light, the light is incident on the first photonic crystal, and the two adjacent photonic crystals, each of which is closer to the incident end, A third photonic crystal disposed in series in the light traveling direction for each channel between the first photonic crystal and the second photonic crystal closer to the emission end; The third photonic crystal has a function of branching an optical signal and is arranged on the light emission side of the first photonic crystal, and the fourth photonic crystal has a function of merging the optical signal. It is arranged on the light incident side of the second photonic crystal, and is characterized by simultaneously performing a dispersion compensation function in wavelength division multiplexing communication and a signal extraction and insertion (add-drop) function.

[0019]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. Embodiment 1 This embodiment is an example of the embodiment of the dispersion compensating optical circuit according to the first invention, and FIG. 1 is a schematic diagram showing the structure of the dispersion compensating optical circuit of this embodiment. Two photonic crystals that strongly refract incident light depending on the wavelength, that is, a photonic crystal 11 and a photonic crystal 12,
It is formed symmetrically on the silicon semiconductor substrate 13 and arranged. The substrate 13 is made of high-resistance silicon having a low impurity concentration in order to avoid the influence of free carrier absorption. Photonic crystal 11 and photonic crystal 12
In each case, a cylinder of silicon having a diameter of 0.3 μm is periodically arranged in a triangular lattice at a pitch of 0.6 μm in silicon oxide.

This photonic crystal, as described above,
The property of light propagation in photonic crystals exhibiting a refraction phenomenon and a spectroscopic phenomenon in which light propagates with a very large refraction angle at a certain incident angle, and the refraction angle changes greatly depending on the wavelength of the incident light. As shown in FIG. 1, the propagation angle in the photonic crystal greatly changes according to the wavelength of the signal light. In FIG. 1, four optical paths are indicated by arrows for easy understanding, but the wavelengths actually change continuously. The photonic crystal 11 and the photonic crystal 12
It has exactly the same structure, and is designed to be symmetrical with respect to the axis of symmetry passing through the intermediate point between the photonic crystals 11 and 12 in FIG. Therefore, the incident light 14 from the left direction in FIG. 1 is bent more upward at the photonic crystal 11 as the wavelength becomes shorter, and is returned again downward at the same angle by the photonic crystal 12 to become the output light 15 on the right. Output.

From the wavelength dependence of the propagation angle obtained in FIG.
When the incident angle of the incident light 14 is set to be 15 ° with respect to the end face of the photonic crystal 11, an angular dispersion of 50 ° /% is obtained. The wavelength distribution of the incident light is 1 in the 1.5 μm band.
If there is nm, a difference in propagation angle of 3 ° can be obtained from this angular dispersion. Photonic crystals 11 and 12
Is 3 mm, and the propagation angle of the signal light at the center is 70 °, the propagation angle difference of 3 ° appears as an optical path length difference of 3 mm.

On the other hand, since the group velocity in the photonic crystal can be reduced to about 1/100 of the light velocity in vacuum, the 3 mm optical path length difference obtained here is accompanied by a 1 ns signal-to-signal time difference. The light is emitted as the light 15. Since the time difference of 1 ns usually corresponds to a time difference between wavelengths after propagation of about 100 km in the fiber, dispersion compensation of a fiber length of 100 km is possible.

Embodiment 2 This embodiment is another example of the embodiment of the dispersion compensating optical circuit according to the first invention. In the first embodiment, it has been assumed that the group velocities of the photonic crystals 11 and 12 are constant. However, in fact, the group velocities of the photonic crystals 11 and 12 have a greater or less dispersion. The group velocity changes according to the wavelength. Even if this effect is used, it is possible to equivalently make a difference in the delay time for each wavelength,
It is difficult to make the size of the dispersion compensating element sufficiently small only by this. That is, if the group velocity dispersion is to be made extremely large, the group velocity dispersion itself has further dispersion, that is, it is difficult to obtain a constant rate of group velocity change over the entire wide band. Therefore, by using the effect of the group velocity dispersion to some extent and simultaneously using the wavelength dependence effect of the propagation angle described in the first embodiment, the element size can be reduced synergistically from the dispersion compensation optical circuit of the first embodiment. In addition, a dispersion compensation effect with good wavelength uniformity can be obtained.

Embodiment 3 This embodiment is an example of the embodiment of the dispersion compensating optical circuit of the second invention, and FIG. 6 is a schematic diagram showing the configuration of the dispersion compensating optical circuit of this embodiment. It is a block diagram. Photonic crystals 61 and 62 similar to the two photonic crystals 11 and 12 constituting the first embodiment, respectively, are formed on a substrate 63.
And another photonic crystal 68 (only four are shown in FIG. 6) between the photonic crystal 61 and the photonic crystal 62, and a photonic crystal 69
(Only four are shown in FIG. 6) are formed on the emission side.

The photonic crystal 68 has a function of splitting one incident light into two waves. In addition, the photonic crystal 6
Reference numeral 9 denotes an arrangement in which the same thing as the photonic crystal 68 is symmetrically arranged, and has a function of merging two waves. One of the signal lights branched into two waves by the photonic crystal 68 is introduced into the light receiving unit 66, and the other is transmitted. The signal light emitted from the light emitting section 67 is combined with the transmitted light by the photonic crystal 69 and introduced into the photonic crystal 62.

With these configurations, the incident light 64 having a plurality of wavelength channels is incident on another photonic crystal 68 for each channel by the photonic crystal 61,
The light is converted into an electric signal by another light receiving unit 66. Similarly, the transmitted light is also separately signal-inserted by the light emitting unit 67, and the optical signals from all the channels are combined by the photonic crystal 62 to be emitted as one emitted light 65 to the outside.

In the current wavelength division multiplexing communication, the standard wavelength interval between channels is 1 nm. If the interval between the light receiving unit 66 and the light emitting unit 67 is 160 μm, the lateral width of the photonic crystal 61 or the photonic crystal 62 is 3 mm. Become.

In the present embodiment, the effect of dispersion compensation described in the first embodiment is at the same time. In FIG. 6, the light beam interval in the photonic crystal 61 from the wavelengths λ1 to λ4 is shown large for easy understanding.
° interval. Therefore, if all the channel lights are set to the vicinity of the inclination angle of 70 degrees, dispersion compensation for the wavelength width of 1 nm at the maximum can be performed on the optical signal of each channel.
Actually, since the wavelength difference between channels is 1 nm, the wavelength width is suppressed to 0.5 nm or less, and the light width of the optical signal propagating in the photonic crystal 68 or 69 is 80 nm.
μm or less, and the size of the light receiving portion 66 or the light emitting portion 67 may be 80 μm or more.

The light receiving section 66 or the light emitting section 67
In this case, a light receiving element or a semiconductor laser element may be integrated on the substrate 63, or a light receiving element or a semiconductor laser element may be disposed outside by providing a mechanism for oblique reflection.

In these embodiments, two photonic crystals are used, but the same effect can be obtained by using three or more photonic crystals or inserting another crystal between the photonic crystals. Can be expected. Although the center wavelength of the incident light is set to 1.5 μm, this wavelength can be easily changed depending on the design of the photonic crystal. In the embodiment, the substrate is made of silicon. However, the substrate can be made of a compound semiconductor material such as indium phosphide or gallium phosphide. Further, here, the configuration is such that the shorter the wavelength, the longer the propagation delay occurs. However, it is easily possible to design such that the longer the wavelength, the longer the propagation delay.

[0031]

According to the present invention, in a dispersion compensating element for compensating the wavelength dependence of the signal propagation velocity in an optical fiber, a photonic crystal having a strong propagation angle dependence on the wavelength is symmetrically arranged, and the optical path length according to the wavelength is determined. Due to the change and the effect of the slow group velocity of the photonic crystal, it is possible to achieve wideband dispersion compensation with small dimensions and insertion loss.
Thereby, the following effects can be obtained. The first effect is that the dispersion compensation optical circuit can be reduced in size and weight, and can be highly integrated. The reason is that there is no limitation on the radius of curvature required for the optical waveguide, and the lithography technology used in the current LSI manufacturing process can be applied. The second effect is an improvement in productivity. The reason is that the first effect reduces the element size and increases the number of elements that can be produced from a wafer having the same area. In addition, productivity is improved by eliminating the need for a positioning step involved in mounting individual optical components such as fibers. A third effect is improvement in characteristic stability. The reason for this is that the effect of mechanical vibration can be reduced and the control of the element temperature can be facilitated by reducing the element size.
By applying the dispersion compensating optical circuit according to the present invention,
A good dispersion compensating element can be realized at low production cost.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a dispersion compensation optical circuit using a photonic crystal according to a first embodiment.

FIG. 2 is a schematic diagram showing a configuration of a conventional fiber grating type dispersion compensation element.

FIG. 3 is a schematic diagram showing a configuration of a conventional waveguide type dispersion compensation optical circuit.

FIGS. 4A and 4B are diagrams showing CCD observation images of light propagated inside a photonic crystal and a silicon semiconductor, respectively.

FIG. 5 is a diagram showing the wavelength dependence of the propagation angle of light propagating inside a photonic crystal.

FIG. 6 is a block diagram illustrating a configuration of a dispersion compensating optical circuit that enables wavelength division multiplexing communication having a signal extraction and insertion function using a photonic crystal according to a third embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Photonic crystal 12 Photonic crystal 13 Substrate 14 Incident light 15 Outgoing light 21 Cladding 22 Core 23 Diffraction grating 24 Incident light 25 Outgoing light 31 Optical waveguide 32 Substrate 33 Variable coupler 34 Coupler 35 Incident light 36 Outgoing light 41 Incident light 42 Inside Propagating light 61 Photonic crystal 62 Photonic crystal 63 Substrate 64 Incident light 65 Outgoing light 66 Light receiving section 67 Light emitting section 68 Photonic crystal 69 Photonic crystal

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-9-133826 (JP, A) JP-A-9-36834 (JP, A) Hideo Kosaka et al. information and communication Engineers elect Russia Nix Society Conference Proceedings 1, Japan, the Institute of Electronics, information and communication Society, September 7, 1998, 1998, P420-421 (58) investigated the field (Int.Cl. ^7, DB name) H04B 10/00-10/28 H04J 14/00-14/08 G02B 1/02 G02B 6/293

Claims (8)

(57) [Claims] 1. A dispersion compensating optical circuit for compensating a wavelength dependence of a propagation speed of a signal light in an optical fiber, the circuit having a function of strongly refracting an incident light strongly depending on a wavelength, and It comprises a plurality of photonic crystals arranged in series, and each of two adjacent photonic crystals among the plurality of photonic crystals has a structure passing through an intermediate point of each of the two photonic crystals. The signal light is formed symmetrically with respect to the axis of symmetry intersecting the traveling direction, and is arranged symmetrically with respect to the axis of symmetry. When the signal light propagates through the leading photonic crystal, the signal light is largely refracted according to the wavelength of the signal light. A dispersion-compensating optical circuit, which is incident on the leading photonic crystal at an incident angle set as described above. 2. The dispersion according to claim 1, wherein the wavelength dependence of the propagation speed of the signal light in the optical fiber is compensated for by utilizing a synergistic effect with the large group velocity dispersion of the photonic crystal. Compensation light circuit. 3. A dispersion compensating optical circuit for compensating a chromatic dispersion of a signal for each channel when a signal is divided and transmitted to a plurality of wavelength channels in a wavelength division multiplexing communication system. It has a function of refracting incident light greatly, and includes a plurality of photonic crystals arranged in series in the light traveling direction, and two adjacent photonic crystals among the plurality of photonic crystals are The structure is formed symmetrically with respect to the axis of symmetry passing through the midpoint of each of the two photonic crystals and intersecting the direction of light propagation, and is arranged symmetrically with respect to the axis of symmetry. When propagating through the inside, the light is incident on the first photonic crystal at an incident angle set so as to be greatly refracted according to the wavelength of the signal light. The near morphism end towards the first photonic second crystals closer to the exit end
And third and fourth photonic crystals arranged in series in the light traveling direction for each channel between the photonic crystals of the above. The third photonic crystal has a function of branching an optical signal. The fourth photonic crystal is disposed on the light incident side of the second photonic crystal and has a function of merging optical signals. A dispersion compensating optical circuit characterized by simultaneously performing a dispersion compensating function in multiplex communication and a signal extraction and insertion (add-drop) function. 4. The dispersion compensation optical circuit according to claim 3, further comprising a light receiving unit and a light emitting unit between the third photonic crystal and the fourth photonic crystal. 5. The dispersion compensating optical circuit according to claim 4, wherein the light receiving section is a light receiving element, and the light emitting section is a semiconductor laser element. 6. The dispersion compensating optical circuit according to claim 1, wherein the wavelength distribution of the incident light is in a 1.5 μm band. 7. The photonic crystal according to claim 1, wherein a column made of silicon is periodically arranged in a triangular lattice at a predetermined pitch in silicon oxide.
The dispersion compensating optical circuit according to any one of the above. 8. The substrate according to claim 1, wherein the substrate is formed of a high-resistance silicon single crystal substrate having a low impurity concentration.
8. The dispersion compensating optical circuit according to any one of items 1 to 7.