JPS62124510A - Grating optical coupler - Google Patents

Abstract

PURPOSE:To permit conversion of a light wave near a spot light source to slab guided light collimated with small-sized constitution and high efficiency and the efficient focusing of the slab guided light to channel guided light by forming a channel waveguide to part of a slab waveguide and providing grating structure to the channel waveguide. CONSTITUTION:The channel waveguide 6 is formed to part of the slab waveguide structure. The channel waveguide extends to the end face of a substrate 2 and a semiconductor laser 8 which is a light source is coupled to one end thereof. The grating 10 is formed to part of the channel waveguide 6. The stage for such formation consists in forming the slab waveguide 4 by sputtering on the quartz substrate 2, forming a stripe pattern 32 of a resist and using the same as a mask for etching. The channel waveguide 6 is so formed as to have a desired propagation constant by controlling the width and height thereof. A grating pattern is formed by electron beam exposure to form a resist mask 34 and dry etching is executed again to form a relief-like grating coupling part 10 on the channel waveguide 6.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical coupler using a grating structure, and particularly to an optical coupler for coupling light waves propagating through an optical waveguide.

[Conventional technology]

When performing various calculation processes using light propagating in an optical waveguide, it is advantageous to use a slab optical waveguide as the optical waveguide. For example, light propagating in a slab optical waveguide is made to interact with surface acoustic waves.

In this case, the guided light is preferably parallel light. However, the light of a semiconductor laser generally used as a light source is diverging light, and therefore it is necessary to collimate the diverging light from the light source.

Therefore, conventionally, a collimating optical waveguide lens is disposed in the slab optical waveguide.

FIG. 13 is a schematic perspective view showing an example of such an optical system. In the figure, 2 is an optical waveguide substrate, and 4 is a slab optical waveguide formed on the surface thereof.

An optical waveguide lens 36 is formed in the slab optical waveguide. Examples of the 8-lens include a geodesic lens, a Luneburg lens, and a Fresnel lens. A semiconductor laser 8 is coupled to the end face of the substrate 2. Since the semiconductor laser is placed at the focal point of the optical waveguide lens 36, the diverging light 38 introduced from the semiconductor laser into the slab optical waveguide 4 passes through the optical waveguide lens 36 and is collimated into parallel beams. This becomes guided light 40.

[Problem that the invention seeks to solve]

The conventional collimating optical waveguide optical system as described above has the following problems.

(1) Since the divergence angle of the diverging light from the semiconductor laser is constant, in order to obtain a wide range of parallel light, the distance between the semiconductor laser and the optical waveguide lens should be large (i.e., use a lens with a long focal length) Therefore, the device cannot be miniaturized.

(2) In order to improve the accuracy of the parallelism of the collimated light beam, it is necessary to position the waveguide lens in the optical waveguide and the semiconductor laser coupling end face with extremely high precision, which makes device fabrication difficult. It is not easy and the yield is low.

(3) Various waveguide lenses have been proposed, but none of them have a great degree of freedom in design in terms of focal length, aperture, etc., and device specifications are limited.

On the other hand, H, M, 5toll “old gh-bright
tnesslasers using integra
ted optics,” SP I E139
, pp. 113-116 (197B) discloses the use of grating optical coupling to obtain a wide parallel beam in an optical waveguide.

This paper describes a configuration in which a semiconductor laser and a grating optical coupler are monolithically integrated also in an optical waveguide. According to this, it is possible to downsize the device. However, in the configuration specifically described in the paper, the grating optical coupler appears to be a grating structure formed on a slab optical waveguide, and in such a configuration, the light beam from the semiconductor laser is a divergent beam. Therefore, there is a problem that the diffraction efficiency decreases and it is difficult to convert the beam width with sufficient efficiency.

[Means for solving problems]

According to the present invention, in order to solve the problems of the prior art as described above, a channel optical waveguide is formed in a part of a slab optical waveguide structure, and the channel optical waveguide has a grating structure in at least a part. A grating optical coupler is provided.

〔Example〕

Hereinafter, specific embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic perspective view showing a first embodiment of the present invention.

In the figure, 2 is an optical waveguide substrate, and 4 is a slab optical waveguide formed on the surface of the substrate.

A channel optical waveguide 6 is formed in a part of this slab optical waveguide structure.
The channel optical waveguide extends to the end surface of the substrate 2, and a semiconductor laser 8 serving as a light source is coupled to one end of the channel optical waveguide. A grating 10 is formed in a part of the channel optical waveguide 6.

Light 12 from semiconductor laser 8 can propagate through channel optical waveguide 6 without divergence. The propagation constant is determined by the waveguide mode propagating through the channel waveguide 6.

FIG. 2 is an enlarged plan view of a grating optical coupler having the above configuration. The guided light 12 propagating through the channel waveguide 6 undergoes Bragg diffraction by the grating structure 10 formed in a part of the channel waveguide, and is converted into the guided light 14 propagating through the slab waveguide 4.

In order to cause Bragg diffraction, the following Bragg conditions must be approximately satisfied.

βi"+K"-2β: Kcos(□-φ)=βd2 Here, βiβd are the propagation constants of the incident waveguide light 12 and the diffracted waveguide light 14, respectively, and K is the grating constant. Given. Further, φ is the angle formed between the direction of the grating equal phase plane and the direction of the channel waveguide 6 (see FIG. 2). FIG. 3 shows a vector diagram showing the Bragg conditions.

For example, Corning #705 as channel waveguide 6
Considering that a rib structure is formed on the slab waveguide using 9 glass, the propagation constant is βi/k (~βd/k)
=N~1.5; becomes =2π/λ. Here, λ is the wavelength of light in vacuum. If we approximate that there is not much difference in the propagation constants of the slab waveguide light 14 and the channel waveguide light 12, then (1
Equation 1 is 2βsinφ=K (2
1λ ・°・Δ=
+3) Rewritten as 2Nsin φ, for example, if the optical wavelength is λ=0.78μm,
If the deflection angle 2φ = 90°, the grating period is A
=0.368 μm is preferable.

The light wave that has undergone Bragg diffraction in this way is output as the waveguide light 14 of the slab waveguide 4 which has a confinement effect only in one dimension, and is outputted as the waveguide light 14 of the slab waveguide 4 which has a confinement effect only in one dimension, depending on the length of the grating structure 10 along the length direction of the channel waveguide 6. This makes it possible to obtain a beam width that increases the width of the beam.

In addition, in Bragg diffraction used in this example, the incident light wave is guided light propagating in a channel waveguide, and the diffracted light wave is guided light propagating in a slab waveguide (strictly speaking, it has confinement only in one-dimensional direction in the channel waveguide). The condition is that the beam is guided light in slab mode.

In this example, since beam coupling is performed within the channel optical waveguide, it is possible to prevent the incident light from spreading.
Beam width conversion can be performed efficiently.

The degree of coupling between the diffracted wave and the incident wave can be adjusted by the coupling coefficient of the grating. The coupling coefficient of the grating is a value determined by parameters such as the amount of refractive index modulation or the amount of relief depth modulation in the grating section.

In order for the output guided light 14 to be outputted efficiently, an appropriate grating coupling coefficient must be achieved for the desired beam width. When the grating has a uniform coupling coefficient, the spatial distribution of the power of the output guided light 14 is represented by a function that monotonically decreases from the coupling end. Applying coupled wave theory, the efficiency η(L) of the output optical power to the input optical power from the coupling end to the length is 77 (
L) = tan h2(L) (represented by 41, which is the grating coupling coefficient here. FIG. 4 schematically shows the output light power distribution.

The partial optical power up to the length explained above is the integral value of the power density in the shaded area.

FIG. 5 shows the relationship between coupling length and coupling efficiency η(L).

Equation (4) shows that as L increases, the coupling efficiency η becomes 100%.
It shows that it approaches. approximately 100 when L=π or more
% can be obtained. Therefore, it is desirable that the desired output beam width W be set approximately within the range of the following equation, taking into account the overall efficiency and the homogeneity of the distribution. That is, when the coupling is weak and W<□, the total efficiency η(W) decreases, and when the coupling is strong and W>π, the distribution of the output light becomes non-uniform.
This is because in either case, the output beam is not desirable.

As described above, when the coupling coefficient of the grating is uniform, the spatial distribution of output light becomes non-uniform in principle. Therefore,
When it is necessary to obtain a particularly uniform beam for the application,
When forming the grating structure, the distribution of refractive index modulation or the distribution of relief depth may be appropriately weighted.

FIG. 6 is a schematic perspective view showing a second embodiment of the invention. In this embodiment, contrary to the first embodiment, the input light from the slab waveguide is output to the channel waveguide.

In the figure, 16 is a photodetector formed in a part of the slab optical waveguide, and one end of the channel optical waveguide 6 is coupled to the photodetector 16.

The guided light 18 incident from the slab waveguide 4 undergoes Bragg diffraction at the grating 10 formed in the channel waveguide 6, is converted into guided light 20 that propagates through the channel waveguide 6, and is efficiently detected by the photodetector 16. Is possible.

In the first embodiment, it was possible to make the total efficiency close to 100% by making the coupling length sufficiently long, but in this embodiment, the light coupled to the channel waveguide 6 is recirculated into the slab waveguide light 22. It has the possibility of being coupled to a transmitted wave.

Therefore, it is preferable to optimize the coupling coefficient of the grating for the desired beam width W in order to achieve high efficiency. For this purpose, an optimal design can be performed using a method similar to the method of distributing coupling coefficients of gratings in order to maximize input coupling efficiency in a grating coupler. Regarding the efficiency of grating couplers, see for example R0 Ulrich "Efficiency of op.
tical-gratingcouplers,” J
, Opt, Sac, 8m, 6 3
．． 1 1. pp.

1419-1431 (1973), etc., have detailed descriptions.

In this embodiment, the wide slab waveguide light 18 can be converted into the channel waveguide light 20 and the optical power can be focused on the minute photodetector element 16, and the response speed can be improved by downsizing the photodetector element. There is an effect.

Further, by adjusting the coupling coefficient of the grating, it is also possible to realize a control knob function that couples part of the optical power of the slab waveguide light 18.

FIG. 7 is a schematic perspective view showing a third embodiment of the present invention. In this embodiment, a plurality of grating coupling portions 11a and 11b are formed on one channel waveguide 10, thereby providing a plurality of output beams 14a and 14.
This embodiment differs from the first embodiment only in that b can be obtained.

FIG. 8 is a schematic perspective view showing a fourth embodiment of the present invention. In this embodiment, a plurality of channel waveguides 6a to
6c are arranged in parallel, and the grating coupling parts 10a to 10c formed on them are arranged in a state shifted in the width direction of the incident beam 18, thereby controlling the spatial distribution of the incident slab waveguide light 18. Each channel waveguide 6
Independent photodetector 16a-1 coupled to one end of 3-6C
The only difference from the second embodiment is that it can be detected at 6C.

FIG. 9 is a schematic perspective view showing a fifth embodiment of the present invention. In this embodiment, the channel width of the channel waveguide 6 is the same as that of the substrate end surface input portion side and the grating coupling portion 1 side.
0 side, and differs from the first embodiment only in that it has a horn-shaped channel width transition region 24 in the middle thereof.

As explained in the first embodiment, only light waves that substantially satisfy the Bragg condition participate in coupling at the grating coupling portion. When the incident light has multiple modes, that is, it has a discrete spectrum with different propagation constants, only the modes with propagation constants that approximately satisfy the Bragg condition participate in coupling in the grating, Other modes will be transparent. Therefore, in order to achieve maximum power transfer, it is necessary that the incident light have a single propagation constant, that is, a single mode, in the grating coupling section 10. This requirement does not necessarily require that the grating coupling portion 10 be a single-mode channel waveguide, but only that the incident light be selectively excited in a single-mode state.

Generally, in order to make the channel waveguide into a single mode, the width of the rib-type structure shown in Figure 9 must be several μm.
It is necessary to set the degree of When this channel width becomes several times the wavelength used, the grating constant K becomes ambiguous in size and vector direction in the grating formed on it, and the selectivity of the incident light becomes wider. Become. Furthermore, the propagation constant of the diffracted light, that is, the output light, is broadened, resulting in poor collimation accuracy. From the above considerations, it can be seen that in terms of characteristics, it is preferable for light waves to be coupled in the Bragg region in the grating coupling section 10.

The above requirements, namely (1) single mode propagation in the crating coupling region (2)
In order to satisfy optical coupling in the region of the Bragg condition, it is necessary to propagate a single mode in a relatively wide channel waveguide.

However, on the other hand, in order to achieve highly efficient input coupling, it is desirable that the field distribution of the laser emission region match the field distribution of the channel waveguide 6 at the end-coupled portion with the semiconductor laser 8; The channel waveguide width of the side end face coupling portion and the channel waveguide width of the grating coupling portion have different optimum values.

Therefore, in this embodiment, a horn-shaped transition region 24 shown in FIG. 9 is formed.

In particular, when the channel waveguide has a single mode in the end face filling part on the input side and a multimode in the grating coupling part, a horn-shaped channel waveguide is used to ensure single mode propagation in the grating coupling part. It is desirable to have a sufficiently slow channel width transition in the transition region 24.

By designing as described above and optimizing the grating coupling section and the input side end surface coupling section, it is possible to optimize the characteristics of both the end surface input coupling and the grating coupling section in this embodiment.

FIG. 10 is a schematic perspective view showing a sixth embodiment of the present invention. In this embodiment, an optical fiber 26 is used as a source of input light, and in order to match the field distribution between the core of the optical fiber and the channel waveguide 6, the channel waveguide 6 near the input side end face coupling part is thickness is increasing. 28 is a transition region where the thickness of the channel waveguide changes.

FIG. 11 is a schematic perspective view showing a seventh embodiment of the present invention.

In the grating coupler of the present invention, substantially satisfying the Bragg condition leads to high efficiency of the device. However, it is conceivable that the propagation constant may change due to an error in the dimensions of the channel waveguide 6 during device fabrication, or that the grating period Δ and the tilt angle φ include errors. Further, the wavelength shift of the semiconductor laser 8, which is the input light source, is also a big problem.

Therefore, in this embodiment, a photodetector 16 is coupled to one end of the channel waveguide 6, and the semiconductor laser 8 is controlled according to the output of the detector. That is, if the Bragg condition is not satisfied, the light wave 30 transmitted through the grating coupling section 10 increases, so the degree of coupling is monitored by detecting it with the photodetector 16 and the semiconductor laser 8 is controlled. I can do it. The oscillation wavelength of a semiconductor laser can be controlled by adjusting the temperature, injection current, etc.

In this embodiment, as described above, by monitoring the amount of deviation of the Bragg condition using the transmitted wave 30, the degree of coupling is not only maximized, but also a desired degree of coupling is provided to the output beam 14.
It is also possible to control the intensity, distribution width, etc.

In this example, we have described a method of changing the wavelength of the light source as a method to satisfy the phase matching of the Bragg condition, but there are also various phase matching methods used in optical directional couplers, etc. The propagation constant may be changed using a method such as loading a variable thin film.

Next, an example of a manufacturing means for realizing the grating optical coupler of the present invention will be briefly described. FIG. 12 shows the manufacturing process.

First, Corning #7 was formed on the quartz substrate 2 by sputtering.
059 glass is deposited to form the slab waveguide 4.

Next, a resist stripe pattern 32 is formed by photolithography to serve as an etching mask (FIG. 12(a)).

Aeratin can be produced by a dry process such as an ion beam. The channel waveguide 6 is manufactured by controlling its width and height to obtain a desired propagation constant [Fig. 12(b)]
]. Furthermore, a grating pattern is formed by electron beam exposure to form a resist mask 34 [12th
Figure (C)]. Next, dry etching is performed again to form a relief grating coupling portion 10 on the channel waveguide 6.

In order to couple the semiconductor laser, the end of the channel waveguide is further polished. In order to construct a photodetector as shown in the second embodiment, it is advantageous to use a Si substrate having an SiO□ layer as a haphazard layer as the substrate 2. GaA
It goes without saying that a monolithic device that integrates a light source and a photodetector can be realized by using a substrate made of S, InP, or the like.

In the above-mentioned manufacturing example, an example was shown in which a relief-type grating coupling portion 10 is formed in a lip-type channel waveguide 6, but in the present invention, a strip waveguide, a buried waveguide, etc. may be used as the channel waveguide. You can also
Furthermore, a refractive index distribution type grating or the like may be used as the grating.

〔Effect of the invention〕

In the present invention as described above, a channel waveguide is formed in a part of the slab waveguide and a grating structure is provided in the channel waveguide, so that various effects shown below can be achieved.

■ A light wave close to a point light source can be converted into collimated slab-guided light with high efficiency in an extremely compact configuration. Also,
Conversely, slab waveguide light can be effectively focused into channel waveguide light.

■ The intensity distribution and coupling efficiency of the output guided light can be controlled by adjusting the grating coupling coefficient and the degree of matching of Bragg conditions, allowing a large degree of freedom in application form and design specifications.

[Brief explanation of drawings]

1 and 6 to 11 are perspective views of the optical coupler of the present invention. Figure 2 is an enlarged plan view of the optical coupler of the present invention, Figure 3 is a vector diagram showing Bragg conditions, Figure 4 is a power distribution diagram of output light, and Figure 5 is a coupling length and coupling diagram. It is a figure showing the relationship with efficiency. FIGS. 12(a) to 12(d) are process diagrams for manufacturing the optical coupler of the present invention. FIG. 13 is a perspective view showing a conventional guided light collimating optical system. 2: Waveguide substrate, 4 Nislab waveguide, 6: Channel waveguide, 8: Semiconductor laser, 10 Nigel rating, 16
: Photodetector, 24.28: Transition region, 26: Optical fiber. Agent Patent Attorney Minoru Yamashita Child 1 Figure 2 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 12 District procedure city council) 1985 2)] 51 ] Commissioner of the Japan Patent Office Michibe Uga 1 Indication of the underlying matter Patent Application No. 1982-262655 2 Name of the invention Grating optical coupler 3 Person making the amendment/Relationship with the 19 cases Name of patent applicant (100) Canon Co., Ltd. 4 Agent Address: 40 Toranomon, 13-1 Toranomon 5-chome Exit, Minato-ku, Tokyo
Column 6 for detailed explanation of the invention in the Mori Building III specification: Contents of amendment (1) Specification, I? Page 6, Company 1: “2β! Corrected as K CO 3 J.

Claims (3)

[Claims] (1) A channel optical waveguide is formed in a part of the slab optical waveguide structure, and the channel optical waveguide has a grating structure in at least a part,
Grating optical coupler. (2) The channel optical waveguide has a portion other than the grating structure that has a different light propagation state from the grating structure, and the portion that connects these portions with different light propagation states undergoes mode conversion as light propagates. 2. A grating optical coupler as claimed in claim 1, having a horn-shaped transition region that does not occur. (3) The grating optical coupler according to claim 1, wherein the grating structure of the channel optical waveguide is capable of realizing a single mode optical propagation state.