JPH11174268A - Optical functional element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical isolator or an optical wavelength demultiplexer excellent in reliability and safety with a simple configuration by forming a diffraction grating by the interference of first and second light waves propagated mutually in opposite directions inside a medium. SOLUTION: To an optical fiber or an optical wavehuide composed of the medium for changing a refractive index by the intensity of light, that is the one for indicating a Pockels effect or a Kerr effect, the two light waves of different frequencies ω1 and ω2 propagated mutually in the opposite directions are introduced. In this case, for electric fireld distribution inside the optical fiber or the optical waveguide, the wave of cyclic refractive index distribution advancing in a certain fixed direction by the interference of the light waves, that is a dynamic diffraction grating, is formed. Since a desired light wave is reflected or transmitted by the diffraction grating, the optical isolator or the wavelength demultiplexer is realized with a simple configuration. Also, monolithic integration is possible as the optical device of a semiconductor laser, a manufacture process is simplified and the reliability is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical functional device,
In particular, the present invention relates to an optical functional element that can be applied to an optical isolator or a wavelength demultiplexer used in an optical fiber communication system and can be integrated.

[0002]

2. Description of the Related Art Recent optical fiber communication systems,
Above all, with the development of optical multiplex communication technology, demands for integration of optical isolators and wavelength demultiplexers for light waves are increasing. Conventional optical isolators use a magneto-optical crystal exhibiting the Faraday effect as a 45-degree polarization rotator, and sandwich this polarization rotator between two polarizers whose polarization directions of light transmission are shifted by 45 degrees. In general, the light is transmitted only in one direction by the action of the polarization rotator.
Further, as shown in FIG. 9, the conventional wavelength demultiplexer converts multi-wavelength light incident from an optical fiber or the like into a parallel light beam by a lens 60 and a reflecting mirror 61 and makes the parallel light beam incident on a diffraction grating 62. Wavelength light is diffracted in different directions depending on the wavelength.
The diffracted light is converged by a lens 63 for each wavelength, and is guided to an output optical fiber 64 selectively disposed at a corresponding position.

[0003]

However, in these conventional structures, optical isolators and optical wavelength demultiplexers are formed by combining optical elements formed independently of each other. The man-hours required to process the material when assembling and the man-hours required to assemble these are necessary.
There is a problem that many man-hours are required as a whole. Further, it is necessary to control the processing accuracy and the assembly accuracy with high accuracy, and there is a problem that reliability and stability are poor. Further, it is not suitable for integration with an optical semiconductor element.

For this reason, in the prior art, Japanese Patent Laid-Open No.
As described in Japanese Patent Publication No. 26, a dynamic diffraction grating using a voltage traveling wave has been proposed, and it has been proposed to configure an optical isolator and a wavelength demultiplexer with the dynamic diffraction grating.
However, in this technique, a signal source for generating a high frequency is required to realize a dynamic diffraction grating, and as a result, the above-mentioned problem has not been solved.

The present invention eliminates such problems of the conventional optical isolator or optical wavelength demultiplexer, can be realized with a simple configuration, has a simple structure and assembly, and has excellent reliability and stability. Another object is to provide an optical functional device capable of realizing an optical wavelength demultiplexer.

[0006]

According to the present invention, there is provided a medium whose refractive index changes in accordance with the intensity of light, first and second light waves propagated in the medium in opposite directions to each other. It is characterized by comprising a diffraction grating formed by waves having a refractive index distribution formed in the medium by interference of light waves.

Here, when the optical functional device of the present invention is configured as an optical isolator, the first and second light waves have different frequencies ω ₁ and ω ₂ propagating in the medium in opposite directions. The diffraction grating is composed of first and second light waves, and the diffraction grating is configured as a dynamic diffraction grating formed by waves having a periodic refractive index distribution and traveling in a certain direction formed by interference of the light waves.

[0008] When the optical functional device of the present invention is configured as a wavelength demultiplexer, the first and second lightwaves have the same frequency and propagate in the medium in opposite directions. The diffraction grating includes two light waves, and the diffraction grating is configured as a diffraction grating formed by standing waves generated by interference of the light waves.

The principle of operation of the optical isolator will be described. In FIG. 1, different frequencies ω ₁ and ω ₂ propagating in opposite directions are applied to an optical fiber or an optical waveguide made of a medium exhibiting the Pockels effect or the Kerr effect, in which the refractive index changes according to the intensity of light. Introduce two light waves. In this case, the electric field distribution in the optical fiber or the optical waveguide causes the velocity v ′ = (ω ₁
+ Omega ₂₎ and wave wavelength 4πc / (ω ₁ -ω ₂₎ to proceed at _{c / (ω 1 + ω 2} ) to the right (in the case of ω _1> ω _2), the speed _{v = (ω 1 -ω 2)} c / (Ω ₁ + ω ₂ ) to the left (ω
Wavelength to proceed in the case of _{1> ω 2) 4πc / (} ω 1 +
ω ₂ ). Here, c is the speed of light. The speed v 'is usually about 10 times or more the speed of light and is extremely high. Cause no change. On the other hand, the light wave propagating at the velocity v is about 1/20 of the light velocity, and is sufficiently slow. That is, a dynamic diffraction grating is formed. Note that such a dynamic diffraction grating can be realized by, for example, a semiconductor laser.

Therefore, when a light wave of frequency ω ₀ is incident on the diffraction grating moving at such a velocity v, FIG.
As shown in (a), when the diffraction grating moves in the same direction as the propagation direction of the light wave, Doppler
The effect, frequency of a light wave as seen from the diffraction grating is lower than ω ₀ ω ₀ ^- = becomes _{ω 0 (1-v / c} ). On the other hand, FIG.
(B), when the lightwave propagates in the direction opposite to the moving direction of the diffraction grating, the frequency of a light wave as seen from the diffraction grating is higher than ^{_{ω 0 ω 0 + = ω (}} 1 + v / c) . Accordingly, the pitch of the diffraction grating frequency omega ₀ ^- if designed to transmit only light waves, but in the case of FIG. (A) light waves can pass, light waves can not pass in the case of FIG. (B) Therefore, it is possible to function as an optical non-reciprocal element. For example, in the configuration of FIG. 1, ω ₀ = [ω] ² / ω
_When a frequency of ₁ (where [ω] = (ω ₁ + ω ₂ ) / 2) is incident, the light of ω ₀ can propagate in the direction of B → A, but in the direction of A → B The light cannot be propagated by being reflected by the dynamic diffraction grating. Conversely, a light wave of ω ₀ = [ω] ² / ω ₂ can propagate in the A → B direction, but cannot propagate in the B → A direction. Therefore, it can function as an optical isolator.

On the other hand, the principle of the wavelength demultiplexer will be described. FIG.
As shown in FIG. 7, on one optical fiber or optical waveguide 10 having a plurality of light entrance / exit ports, multi-wavelength light having _a frequency of ω ₁ , ω ₂ , ω ₃ ,. Is incident. Further, light having a frequency ω ₁ is incident on the optical waveguide 10 from the incident end P 12 in a direction opposite to the propagation direction of the multi-wavelength incident light, and interferes with a light wave having a frequency ω ₁ in the multi-wavelength light wave to be fixed. It is a standing wave. The diffraction grating formed by the standing wave modulating the refractive index of the optical fiber or the waveguide 10 diffracts and reflects only the light wave of the frequency ω ₁ and emits the light wave from the incident end (outgoing end) P13. it can be removed only light components of the frequency omega ₁ in. Therefore, other wavelength components can be separated by the same method, and it becomes possible to function as a wavelength demultiplexer.

[0012]

Next, embodiments of the present invention will be described with reference to the drawings. 4 to 6 show an embodiment in which the present invention is applied to an optical isolator. First, FIG.
, The optical fiber 20 is connected to two Y-branch couplers 21,
22 and two light waves having different frequencies ω ₁ = 194 THz (wavelength 1.55 μm) and a frequency ω ₂ = 224 THz (wavelength 1.34 μm) propagating in opposite directions to each other from the input ends P22 and P24. Optical fiber 20
To be introduced. The optical fiber 20 is preferably an Er-doped fiber having a nonlinear optical effect, unlike an ordinary optical fiber. Therefore, the distribution of the refractive index in the optical fiber 20 due to the interference of the light waves is
A wave (dynamic diffraction grating) having a period traveling at a velocity v = 2.15 × 10 ⁷ m / s is obtained. In such a dynamic diffraction grating, the frequency ω ′ ₁ = 225 THz from the incident end P21.
(Wavelength 1.32 μm), or a frequency ω ′ ₂ = 195 THz (wavelength 1.32 μm) from the incident end P23.
(538 μm), the light is reflected by the Doppler effect described above. Therefore, the wavelength 1.32μ
The light wave of m can propagate in the direction from the incident end P23 to the incident end P21, but cannot be propagated in the direction from the incident end P21 to the incident end P23 by being reflected by the dynamic diffraction grating. Conversely, a light wave having a wavelength of 1.538 μm is converted from the incident end P21 to the incident end P23.
Although it can propagate in the direction, the incident end P23 → the incident end P2
It cannot propagate in one direction. As a result, anisotropy occurs in the propagation characteristics depending on the propagation direction of the light wave, and the light wave can be used as an optical isolator.

FIG. 5 shows an embodiment in which the optical isolator of the present invention is configured as an optical waveguide. In the figure,
After a light guide layer made of an InGaAsP semiconductor is grown on the InP substrate 31, a core portion of a Y-branch waveguide having a width of about 1 μm is formed by ordinary photolithography and etching. Then, by growing an InP layer,
The whole core is buried with InP to form a Y-branch optical waveguide 30 composed of an InGaAsP core. And frequency 1
Two light waves of 94 THz and 224 THz are transmitted to the input end P3.
2 and P34, a dynamic diffraction grating is formed in the optical waveguide 30 traveling at a speed v = 2.15 × 10 ⁷ m / s. Therefore, similarly to the embodiment of FIG. 4, by injecting a light wave of a certain frequency from the incident end P31 or P33, this light wave can be transmitted only in one direction, and can function as an optical non-reciprocal element. .

FIG. 6 shows an optical isolator of the present invention, in particular, FIG.
FIG. 11 is a diagram showing an example in which an optical isolator having the same configuration as that shown in FIG. 1 is monolithically integrated with a semiconductor laser. That is, a well-known semiconductor laser 32 is formed by laminating a semiconductor layer on an InP substrate 31, forming an active layer 33 and forming an electrode 34, and using the semiconductor laser 32 to form an optical waveguide 30 constituting the optical isolator. It is integrated with one incident end, here the incident end P33. Here, for example, the wavelength of the semiconductor laser 32 is 1.32 μm, and the wavelengths of the interference light waves of _two frequencies ω ₁ and ω ₂ incident from the incident ends P32 and P34 of the optical waveguide 30 are λ ₁ = 1. If they are set to 55 μm and λ ₂ = 1.34 μm, the light emitted from the semiconductor laser 32 passes through the optical waveguide 30 and can be emitted from the incident end P31 to an external optical element. On the other hand, the reflected light having the same wavelength of 1.3 μm reflected from the external optical element to the incident end P31 is reflected toward the incident end P31 by the dynamic diffraction grating formed from the interference light wave, and thus the semiconductor laser It is not propagated by 32. Therefore, it is possible to block the reflected return light to the semiconductor laser 32. In this case, the incident end P33
Wavelength of omega ₁ of the light emitted is so 1.55 .mu.m, filter provided on the entrance end P33, for example, can be removed by the diffraction grating wavelength filter 35.

Next, an embodiment of a wavelength demultiplexer according to the present invention will be described with reference to FIGS. First, in FIG.
The two Y-branch couplers 41 to 43 are cascaded to form an optical fiber wavelength demultiplexing circuit composed of continuous optical fibers 40. At the incident end P41 of the first Y-branch coupler 41, multi-wavelength light waves of four wavelengths of frequencies ω ₁ , ω ₂ , ω ₃ and ω ₄ are incident. Then, an interference wave having a frequency ω ₁ propagating in the opposite direction through the Y-branch coupler 41 is introduced from the incident end P42 on the opposite side of the first Y-branch coupler 41. Thereby, the first Y
Within branch coupler 41, and the corresponding frequency omega ₁ of the light wave in the incident light and the interference wave frequency omega ₁ is interference, a diffraction grating is formed. Therefore, the frequency omega ₁ of the light wave of the light wave of multiple wavelengths is reflected by the first Y-branch coupler 41 is emitted from the emission end P43, it is branched from the light wave of multiple wavelengths. Hereinafter, similarly, the light wave of the frequency ω ₂ is branched in the second Y branch coupler 42, the light wave of the frequency ω ₃ is branched in the third branch coupler 43, and finally the frequency ω ₄
Is emitted from the emission end P44. Thereby, a wavelength demultiplexer is configured.

FIG. 8 shows an embodiment in which the wavelength demultiplexer of the present invention is configured as an optical waveguide. Similarly to the optical isolator shown in FIG.
After growing a light guide layer made of an aAsP semiconductor, it is about 1 μm thick by ordinary photolithography and etching.
The core portion of the Y-branch waveguide having a width is formed. Then InP
By growing the layer, the whole core is buried with InP, and the optical waveguide 50 in which four Y-branch optical waveguides 51 to 54 composed of an InGaAsP core are cascaded is formed.
Then, multi-wavelength light waves of four wavelengths ω ₁ , ω ₂ , ω ₃ , and ω ₄ enter from the incident end P51 of the first Y-branch optical waveguide 51. In addition, each of the first to fourth Y-branch optical waveguides 5
Interference waves having frequencies ω ₁ , ω ₂ , ω ₃ , and ω ₄ are incident from incidence ends P 52 to P 55 facing the opposite sides of ₁ to 54, respectively. Thus, in each Y branch optical waveguide 51 to 54, the frequency _{ω 1, ω 2, ω 3} , is a diffraction grating that reflects light waves omega ₄ is formed, sequentially frequency omega ₁ at the Y branch optical waveguide 51 to 54, Each of the light waves ω ₂ , ω ₃ , and ω ₄ is branched from the multi-wavelength light wave and emitted from the output terminals P56 to P59, and can function as a wavelength demultiplexer.

The present invention is not limited to the configuration of the above-described embodiment, but may be applied to various embodiments as long as the configuration uses an optical function element including a diffraction grating formed by interference of two light waves. Needless to say, this is possible.

[0018]

As described above, according to the present invention, the first and second light waves are propagated in opposite directions to a medium whose refractive index changes according to the intensity of light, and the first and second light waves are caused to interfere with each other by the interference of these light waves. A diffraction grating formed by a wave having a refractive index distribution formed in the medium is provided. The diffraction grating is configured to reflect or transmit a desired light wave. Since optical devices can be realized and monolithic integration with semiconductor laser optical devices is also possible, they are extremely useful as optical communication devices, simplifying the manufacturing process, improving reliability,
It can be provided at low cost.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining the principle of forming a diffraction grating in the present invention.

FIG. 2 is a diagram for explaining the principle that the present invention is configured as an optical isolator.

FIG. 3 is a diagram for explaining the principle that the present invention is configured as a wavelength demultiplexer.

FIG. 4 is a configuration diagram of a first embodiment in which the present invention is configured as an optical isolator.

FIG. 5 is a configuration diagram of a second embodiment in which the present invention is configured as an optical isolator.

FIG. 6 is a configuration diagram of a third embodiment in which the present invention is configured as an optical isolator.

FIG. 7 is a configuration diagram of a first embodiment in which the present invention is configured as a wavelength demultiplexer.

FIG. 8 is a configuration diagram of a second embodiment in which the present invention is configured as a wavelength demultiplexer.

FIG. 9 is a configuration diagram of an example of a conventional wavelength demultiplexer.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Optical fiber or optical waveguide 20 Optical fiber 21, 22 Y branch coupler 30 Optical waveguide 31 Inp substrate 32 Semiconductor laser 35 Diffraction grating wavelength filter 40 Optical fiber 41-43 Y branch coupler 50 Optical waveguide 51-54 Y branch optical waveguide 55 InP substrate

Claims (7)

[Claims] 1. A medium whose refractive index changes in accordance with the intensity of light, first and second light waves propagating in opposite directions in the medium, and interference in the medium by the light waves. An optical functional device comprising a diffraction grating formed by waves having a refractive index distribution. 2. The method according to claim 1, wherein the first and second light waves are composed of first and second light waves having different frequencies ω ₁ and ω ₂ propagating in the medium in directions opposite to each other. 2. The optical functional device according to claim 1, wherein the optical functional device is configured as a dynamic diffraction grating formed by a wave having a periodic refractive index distribution which travels in a certain direction and is formed by interference of light waves. 3. A third optical wave frequency omega ₃ is propagated in said medium, this frequency omega _3, omega ₃ = [omega] ² / omega ₁ or omega ₃ = [omega] _² / ω ² (provided that , [Ω] = (ω ₁
The optical functional element according to claim 2, wherein the optical functional element is set to + ω ₂ ) / 2) and is configured as an isolator for the third light wave. 4. The medium comprises an optical fiber or an optical waveguide made of a medium whose refractive index changes according to the intensity of light, and the optical fiber or the optical waveguide is provided with the first and second light waves. 4. The optical functional device according to claim 3, wherein the incident first and second incident ends and the third and fourth incident ends from which the third light wave enters and exits are branched. 5. The first and second light waves comprise first and second light waves of the same frequency propagating in the medium in opposite directions to each other, and the diffraction grating is generated by interference of the light waves. The optical functional device according to claim 1, wherein the optical functional device is configured as a diffraction grating formed by standing waves. 6. A multi-wavelength light including the first light wave is incident on the medium, and the diffraction grating is configured as a wavelength demultiplexer that reflects a first light wave from the multi-wavelength light. Item 6. The optical functional element according to item 5. 7. The medium comprises an optical fiber or an optical waveguide made of a medium whose refractive index changes according to the intensity of light, and the optical fiber or the optical waveguide has a multi-wavelength including the first light wave. A first incident end for receiving light,
A second input end for receiving the second lightwave, a first output end for outputting the reflected first lightwave, and a light obtained by splitting the first lightwave from the multi-wavelength light; The optical functional device according to claim 6, wherein the second emission end is formed to be branched.