JP3266632B2 - Waveguide diffraction grating - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an arrayed waveguide diffraction grating having improved polarization dependence.

[0002]

2. Description of the Related Art In the field of optical communication, a method (wavelength division multiplex transmission system) of expanding a communication capacity by mounting a plurality of signals on light of different wavelengths and transmitting the signals through one optical fiber has been studied. I have. In this method, an optical multiplexer / demultiplexer that multiplexes or demultiplexes light of different wavelengths plays an important role, but an optical multiplexer / demultiplexer using an array waveguide diffraction grating has a communication capacity at a narrow wavelength interval. It is promising because the number of multiplexes can be increased.

In recent years, attempts have been made in a wavelength division multiplex transmission system to increase the degree of multiplexing and dramatically increase the amount of transmission. To achieve this, a multiplexer / demultiplexer capable of multiplexing / demultiplexing a plurality of signal lights having a wavelength interval of about 1 nm or less is required. However, in a conventional optical multiplexer / demultiplexer using a diffraction grating, the available diffraction orders are limited and sufficient dispersion cannot be obtained, so that the wavelength interval cannot be reduced to 1 nm or less.

[0004] As an effective way to solve the above problem,
A method using an arrayed waveguide type diffraction grating is known.
('Arrayed-waveguide grating for wavelength divisi
on multi / demultiplexer with nanometer resolution ';
See Electronics Letters, vol. 26, pp. 87-88, 1990. and Japanese Patent Application No. 1-65588. )

FIG. 5 is a schematic diagram showing a configuration of a conventional optical multiplexer / demultiplexer 1 using an array waveguide 2. As shown in FIG. This optical multiplexer / demultiplexer 1
Is formed on the substrate 3 and has an input waveguide 4
, Slab waveguides 51 and 52 on the input side and output side, array waveguide 2 and output waveguide 6. The wavelength-division multiplexed light is introduced into the optical multiplexer / demultiplexer 1 from the input waveguide 4 and spread by a diffraction effect in an input-side slab waveguide 51 following the input waveguide 4, and further spread to the input-side slab waveguide 51. The light is then introduced into the array waveguide 2. The light introduced into the array waveguide 2 is propagated into a plurality of channel waveguides 21 constituting the array waveguide 2 and introduced into an output slab waveguide 52 connected to the array waveguide 2. You. Here, the light introduced into the array waveguide 2 has different lengths of the respective channel waveguides 21. Therefore, the position where light is condensed at the end of the output side slab waveguide 52 differs depending on the wavelength, and signal light is extracted from the output waveguide 6 at a different position for each wavelength.

Here, in the array waveguide diffraction grating 2,

[0007]

(Equation 2)

The following equation is satisfied: In equation (2),
n _s is the effective refractive index in the slab waveguide 5, n _c is the channel waveguides 21 ... effective refractive index of constituting the arrayed waveguide 2, d is the arrayed waveguide 2 at the connecting portion between the output-side slab waveguide 52 The period (pitch) of the waveguide interval, θ is the diffraction angle of the diffracted light in the output side slab waveguide 52, ΔL
Represents the optical path length difference between the channel waveguides 21 constituting the array waveguide 2, m (integer) represents the diffraction order, and λ represents the wavelength. Near the center wavelength λ ₀ , the diffraction order m is

[0009]

(Equation 3)

## EQU1 ## At this time, sin θ ≒ θ, co
Within the range where sθ ≒ 1, the variance is given by

[0011]

(Equation 4)

## EQU1 ## In an arrayed waveguide grating, the dispersion is proportional to the optical path length difference ΔL. Therefore, Δ
There is a feature that a large dispersion can be obtained by making L large, which is incomparable with the conventional diffraction grating.

In addition, since all the optical waveguide circuits can be manufactured on a substrate of about 3 inches at a time, mass productivity, characteristic stability, and low performance can be achieved as compared with a bulk type manufactured by assembling lenses and diffraction gratings. It is also advantageous in terms of price and the like. As described above, the array waveguide type diffraction grating has characteristics superior to other conventional diffraction gratings in producing an optical multiplexer / demultiplexer for wavelength division multiplexing transmission having a narrow wavelength interval.

[0014]

However, the optical multiplexer / demultiplexer 1 has a drawback that it has polarization dependence. This is because the channel waveguides 21 constituting the array waveguide 2 are arranged in the same manner.
Has birefringence. That is, in the above equation (2), the effective refractive index n _TE of the channel waveguides 21 with respect to the horizontally polarized (TE) light and the vertically polarized (T
M) Since the effective refractive index n _TM for the light is different, the center wavelength λ ₀ does not match between the TE light and the TM light. For example, in the case of a quartz-based waveguide manufactured by the flame deposition method, the birefringence value B = n _TM −n T _E = 4 × 10 ⁻⁴ . At this time, the deviation of the center wavelength is about 0.4 nm from the equation (2). 0.4n is required to split light multiplexed at a wavelength interval of 1nm.
The wavelength shift of m is too large. The birefringence of the channel waveguides 21 depends on the material used and the manufacturing method. In the case of anisotropic crystals, of course, they have birefringence,
Even a glass-based waveguide that should be isotropic in nature may have birefringence. For example, in the case of a quartz-based waveguide fabricated on a silicon substrate by a flame deposition method, the birefringence is caused by the fact that the values of the thermal expansion coefficients of silicon and quartz glass differ by one digit.

On the other hand, as a method of controlling the birefringence value of the waveguide, a method of loading a film having residual stress such as amorphous silicon on the waveguide is known. (IEEE J
OURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL.8,
No.6, PP.1128-1131, AUG.1990) The birefringence value of the waveguide changes due to the stress applied to the waveguide by the amorphous silicon film. FIG. 6A is a diagram illustrating an example of a waveguide loaded with a film having a residual stress. This waveguide is formed by forming a waveguide core 9 surrounded by a cladding glass 8 on a silicon substrate 7, and has a width W of a width W formed by a sputtering method on the cladding glass 8. It is formed by stacking amorphous silicon films 10.
FIG. 6B shows the relationship between the birefringence value B and the width W in the waveguide having the configuration shown in FIG. 6A, and shows the relationship between the birefringence index when the thickness of the stress applying film is 6 μm. It is a graph which shows a change. (IEEE JOURNAL ON SELECTED AREAS IN COMMUNICAT
IONS. VOL.8, No.6, PP.1128-1131, AUG.1990)

In this case, the amorphous silicon film 1
The birefringence value B can be changed by changing the width W of 0, but cannot be set to 0. That is, the polarization dependency cannot be eliminated only by applying this method to the arrayed waveguide diffraction grating and loading the amorphous silicon film along the plurality of channel waveguides 21 constituting the arrayed waveguide 2. . The present invention has been made in view of the above prior art, and has been applied to an array waveguide diffraction grating.

It is an object of the present invention to solve the problem of polarization dependence, and to realize a more practical optical multiplexer / demultiplexer for wavelength division multiplexing transmission.

[0018]

A waveguide diffraction grating according to the present invention is manufactured on a substrate and has a plurality of channels having a birefringence.
The above-mentioned channel is placed on an array waveguide diffraction grating composed of a tunnel waveguide.
Fewer stress applying parts to change the birefringence value of the channel waveguide
A waveguide grating loaded with at least one;
Each of the stress applying portions is formed of the plurality of channel waveguides.
The solution was to partially cover everything.

Further, the waveguide diffraction grating of the present invention is characterized in that
The birefringence value of the number of channel waveguides is B, and the optical path length is Li (i
= 1, 2, 3,...)
The birefringence value of the portion of the path covered by the stress applying portion is B ′,
When the length is li (i = 1, 2, 3,...),

[0020]

(Equation 5) That the relationship was satisfied was taken as the solution.

[0021]

According to the present invention, a film which has a residual stress and changes the birefringence value of the waveguide (hereinafter referred to as a stress applying film).
Is loaded on at least a part of the waveguide to eliminate the polarization dependence. A feature of the present invention is to totally eliminate polarization dependence by utilizing a difference in birefringence value between a portion where a stress applying film is loaded and a portion where a stress applying film is not loaded in a channel waveguide. Therefore, there is no need to remove the birefringence of the channel waveguide, and a waveguide diffraction grating that does not depend on the polarization state of input light can be easily manufactured.

FIG. 1 is a view for explaining the present invention in detail, and shows the channel waveguides 2 _i ... Of the array waveguide 2 in the vicinity where the stress applying film 11 is loaded. Channel waveguides 2 _i (i = 1, 2) constituting the array waveguide 2
2,3, ...) and the length of each L _i, the difference between them is constant, it is _{ΔL = L i + 1 -L i} . The effective refractive index of the waveguide where the stress applying film 11 is not loaded is n _TE , n _TM for TE light and TM light, respectively.
It is. The length of the portion where the stress applying film 11 is loaded is l _i (i = 1, 2, 3,...), And the effective refractive indices are n ′ _TE and n ′ _TM , respectively.

The effective optical path length difference between adjacent channel waveguides 2i...

[0024]

(Equation 6)

Similarly, in the case of TM light,

[0026]

(Equation 7)

## EQU1 ## Therefore, the difference between the effective optical path length of the TE light and the effective optical path difference of the TM light is given by the equation (6)-(5).

[0028]

(Equation 8)

## EQU1 ## However, in equation (7), B = n
_TM -n _TE, and _{B '= n' TM -n '} TE.

If the length l _{i of the} portion on which the stress applying film 11 is loaded is set so that the value of the equation (7) becomes 0, there is no difference in the effective optical path length difference from the TE light, and The wavelength shift is eliminated, that is, the polarization dependence can be eliminated.

[0031]

【Example】

(Embodiment 1) FIG. 2 shows the configuration of a wavelength division multiplexing optical multiplexer / demultiplexer using an array waveguide diffraction grating as a first embodiment of the present invention. This wavelength division multiplexing optical multiplexer / demultiplexer includes an input waveguide 4, an input side slab waveguide 51,
The array waveguide diffraction grating 2, the stress applying film 11, the output side slab waveguide 52, and the output waveguide 6 are sequentially arranged. The stress applying film 11 is made of an amorphous silicon film, and the waveguide is made of a quartz-based waveguide manufactured by a flame deposition method. The waveguide core was 7 μm × 7 μm, and the relative refractive index difference was 0.75%. The optical waveguide length difference of the array waveguide diffraction grating 2 was 122.93 μm, the pitch was 25 μm, the focal length (radius of the fan-shaped slab waveguide) was 7766 μm, and the interval between the output waveguides 6 was 25 μm. These design values are designed so that light having a wavelength multiplexing interval of 1 nm can be multiplexed / demultiplexed.

In the waveguide diffraction grating of the present invention, signal light from the optical communication network enters the multiplexer / demultiplexer of the present invention via an optical fiber (not shown) connected to the input waveguide 4. The signal light is spread by diffraction in the input side slab waveguide 51 and is incident on a plurality of channel waveguides 2 _i forming the array waveguide diffraction grating 2. Array waveguide diffraction grating 2
The signal light that has passed there converges at the end of the output waveguide 6. The position is different for each wavelength due to the dispersion action of the array waveguide diffraction grating 2, and the multiplexed signal light is extracted from separate output waveguides 6.

Before explaining the method of eliminating the polarization dependence by the stress applying film 11, a case where the stress applying film 11 is not loaded (corresponding to W = 0) will be described first. At this time, the effective refractive index n _c of the channel waveguide 2 _i is n _TE = 1.4500 for TE light, and n _TM = 1.45 for TM light.
04, and the birefringence value that is the difference is B = 4 × 10 ⁻⁴ . From equation (2), the center wavelength of the TE light is 1.5500μ.
m, 1.5004 μm for TM light, 0.4 nm
Deviation has occurred.

FIG. 3 (a) shows a non-polarized LED light having a wavelength of 1.55 μm incident from the input waveguide 4 of the conventional waveguide diffraction grating shown in FIG. 5, and an output waveguide 6 having a diffraction angle of 0 °. 7 is a graph showing the results of measuring the wavelength characteristics of the loss of light emitted from the central output waveguide 6 using a spectrum analyzer. Power of input light is TE component and TM
Component, the center wavelength is shifted by 0.4 nm, and an excess loss of 3 dB is generated.
There is a problem as an optical multiplexer / demultiplexer.

Next, consider the case where the waveguide diffraction grating of the present invention loaded with the stress applying film 11 is used. Stress applying film 11
Consists of amorphous silicon, the shape, the length of the portion covering the channel waveguide 2 _i as shown in FIG. 1 as it goes in the drawings below, it was increased such triangles by 327.8Myuemu. That is, l _{i + 1} −l _i = 327.8 μm. At this time, since the width W of the stress applying film 11 covering the waveguide can be considered to be infinite, the birefringence value is B ′ = 2.5 × 10 ⁻⁴ from the graph of FIG. 6B. These values are (7)
Substituting into the expression,

[0036] BΔL + (B'-B) ( l i + 1 -l i) = 4 × 10 -4 × 122.93 × 10 -6 + (2.5 × 10 -4 -4 × 10 -4) × 327.8 × 10 - ⁶ = 0

As is clear from the above, the optical path length difference between the TE light and the TM light becomes equal, and the polarization dependence is eliminated. FIG. 3B is a graph showing the loss at this time measured under the same conditions as in FIG. 3A. The peak in this graph is 1
It was confirmed that there was no excess loss and the polarization dependency was eliminated.

(Embodiment 2) FIG. 4 shows a second embodiment of the present invention. 4 is different from that shown in FIG. 2 in that a plurality of stress applying films 11 made of an amorphous silicon thin film are loaded on the arrayed waveguide diffraction grating 2. In this example, the yield can be improved by trimming the amorphous silicon film to be the stress applying films 11. In the example of FIG. 4, three amorphous silicon stress applying films 1 are provided on the array waveguide 2.
1, 12, and 13 are loaded. Before the trimming, the stress applying films 12 and 13 are both rectangular, and the lengths covering the channel waveguides 2 _i ... Are the same in any of the waveguides. . The shape of the stress applying film 11 in the central portion is designed in the same manner as in the first embodiment, and the polarization dependence can be almost eliminated by the same effect as that of the stress applying film 11 of the first embodiment. However, since the birefringence value of the waveguide and the birefringence change amount due to the stress imparting film 11 generally have manufacturing variations, the polarization dependency may not be able to be exactly eliminated by the stress imparting film 11 alone. Therefore, while connecting optical fibers to the input waveguide 4 and the output waveguide 6 and measuring the wavelength characteristics, the two stress applying films 12 and 13 adjacent to the slab waveguide 5 are obtained until the characteristics shown in FIG. Gradually trim. The broken lines in FIG.
3 represents the respective trimmed portions. The trimming shape is a triangle as shown in the figure, and (l _{i + 1} −l _i ) is set so that the value of the expression (7) is strictly zero when the three stress applying films 11, 12, and 13 are combined. Has been adjusted. From the point of this trimming method, the stress applying film 1
The number 1 is not necessarily three, and the number can be selected as appropriate. However, considering the re-trimming and the symmetry of the optical circuit, about three including the spare is appropriate.
In consideration of trimming while measuring, it is optimal to use a YAG laser or the like for trimming, but of course, the operation of performing measurement after trimming by dry etching, wet etching or the like may be repeated.

In both the first and second embodiments, the loading of the stress applying film 11 changes the birefringence value of the waveguide and also changes the effective refractive index of the waveguide. The center wavelength shifts due to this effect. To compensate for this, the optical path length difference ΔL of the array waveguide 2 is set in consideration of the wavelength shift. The input or output waveguides are shifted by an amount corresponding to the wavelength shift. Can be dealt with at the design stage. Further, by using the fact that the refractive index of glass depends on temperature, it is also possible to adjust the temperature of the entire waveguide and adjust the center wavelength.

In the first and second embodiments, a quartz-based waveguide manufactured by a flame deposition method is used as the waveguide, but the present invention is not limited to this material system.
It is clear that the present invention can be applied to any waveguide material such as a crystal waveguide such as lithium niobate and an organic material-based waveguide such as PMMA. Furthermore, although an amorphous silicon film is used as the stress applying film 11, the present invention is not limited to this, and any film that can change the birefringence value obtained by applying stress to the waveguide may be used. A film of any material or manufacturing method can be applied.

[0041]

Generally, in a waveguide type optical device using interference, the birefringence of the waveguide causes polarization dependency. Conventionally, removing birefringence requires a great deal of effort,
This has been a factor in lowering the productivity and increasing the price, and it has been difficult to realize an optical device having no polarization dependence suitable for practical use.

As described above, the waveguide diffraction grating of the present invention does not eliminate the birefringence of the channel waveguide, but uses the subtle change in the birefringence value to make use of the array waveguide diffraction grating. Is totally eliminated. In addition, the manufacturing process simply involves loading a stress applying film such as an amorphous silicon film on a waveguide manufactured by a conventional procedure by a sputtering method, and can be realized with small labor and low cost.

According to the waveguide diffraction grating of the present invention, the polarization-independent optical multiplexer / demultiplexer is a waveguide type suitable for mass production and has a narrow wavelength interval. This makes it possible to realize a high-density wavelength division multiplexing transmission system with a wavelength interval of 1 nanometer or less, and is expected to have an incalculable effect in increasing the capacity of an optical communication system.

[Brief description of the drawings]

FIG. 1 is an enlarged view of a main part of a waveguide diffraction grating of the present invention.

FIG. 2 is a schematic configuration diagram showing one embodiment of a waveguide diffraction grating of the present invention.

3A is a graph showing the wavelength characteristic of a loss near the center wavelength of the conventional waveguide grating, and FIG. 3B is a graph showing the wavelength characteristic of a loss near the center wavelength of the waveguide grating of the present invention; It is.

FIG. 4 is a schematic configuration diagram showing another embodiment of the waveguide diffraction grating of the present invention.

FIG. 5 is a schematic configuration diagram showing an example of a conventional waveguide diffraction grating.

6A is a schematic configuration diagram of a conventional stress applying film, and FIG. 6B is a graph showing a change in a birefringence value of a channel waveguide caused by the stress applying film shown in FIG. .

[Explanation of symbols]

 Reference Signs List 2 array waveguide 21 channel waveguide 3 substrate 11 stress applying film 12 stress applying film 13 stress applying film

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Isao Nishi 1-6-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A 64-77002 (JP, A) 2-244105 (JP, A) IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS Vol. 6, P1128-1131, AUG. 1990

Claims (4)

(57) [Claims] 1. A plurality of birefringent elements formed on a substrate and having a birefringence.
Array waveguide consisting of multiple channel waveguides on diffraction grating
Stress that changes the birefringence value of the channel waveguide
A waveguide grating in which at least one application portion is loaded.
What Each of the stress applying sections is provided with the plurality of channel waveguides.
Waveguide diffraction grating characterized by partially covering the entire path
Child. 2. The waveguide diffraction grating according to claim 1, wherein
Te, B birefringence values of said plurality of channel waveguides, the optical path length L
i (i = 1, 2, 3,...) and the plurality of channels
The birefringence value of the portion of the waveguide covered with the stress applying part is
B ′, and when the length is li (i = 1, 2, 3,...), The following relationship is satisfied:
Folded lattice. [Equation 1 ] 3. A process according to claim 1 or claim 2 waveguide grating according stress applying portion is characterized by a trimmable film. Wherein said plurality of channel waveguides are silica-based waveguides, waveguide grating according to claim 1, wherein said stress applying portion is amorphous silicon.