JPH02239209A - Optical wavelength filter and photodetector - Google Patents

Abstract

PURPOSE:To obtain the small-sized optical wavelength filter having a sharp wavelength selecting characteristic and good coupling efficiency by forming a diffraction grating in a region where the respective waveguide modes centering at 1st and 2nd waveguide layers overlap on each other. CONSTITUTION:The optical wavelength filter is laminated with two layers of the waveguide layers (1st waveguide layer 101 and 2nd waveguide layer 102) in the layer direction and a directional coupler by these layers is constituted. Since the respective waveguide layers 101, 102 are so formed as to vary in thickness and compsn., the propagation constants of the respective guided light rays vary. The diffraction grating 107 formed in the 2nd waveguide layer 102 is for selection of the wavelength to be optically coupled and the wavelength region to be selected is determined by the pitch thereof. The shift of the light power generated between the two waveguide layers 101 and 102 by the diffraction grating 107 becomes dominant so that the sharp wavelength selectivity is obtd. Since the shift of the light power is executed in the direction perpendicular to a substrate 103, the coupling length is shortened and the element is miniaturized.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to an optical wavelength filter for demultiplexing or multiplexing light waves in a wavelength multiplexing optical information transmission system, etc., and in particular to a directional coupler type optical guide. This paper concerns optical wavelength filters and photodetectors using wave paths. [Prior Art] A conventional optical wavelength filter using a directional coupler type optical waveguide is, for example, the R. C. Alferness et al.: A
ppliedPhysics Letters, 33,
P16t (t978), JP-A-61-250607
Or Miki et al., Institute of Electronics and Communication Engineers Research Report OQE81-1
Two optical waveguides constituting a directional coupler were formed on the same substrate, as described in 29. FIG. 8 is a diagram showing the configuration of such a conventional optical wavelength filter.

Since the two optical waveguides 81 and 82 are formed to have different line widths, heights, etc. as shown in the figure, the propagation constants of the guided light propagating through each optical waveguide are also different. At this time, by forming a diffraction grating 83 that compensates for the difference in propagation constant in one of the regions where the electric field distribution of the two guided lights exists, the two guided lights can be Optical coupling occurs between the wave paths. In other words, it is possible to select only light in a specific wavelength range and transfer optical power between waveguides.

Conventionally, optical wavelength filters have been used that utilize this optical power shift to perform multiplexing/demultiplexing between signal light and light waves of specific wavelengths, and photodetectors that receive light waves of specific wavelengths. It was being done. [Problems to be Solved by the Invention] However, in the conventional example described above, since the waveguides are formed in the same plane, the difference in propagation constant between two waveguides can only be determined by the line width, height, etc. of the waveguides. This could not be controlled and a large difference in propagation constant could not be obtained. Therefore, the operation of optical power transfer between waveguides is not only caused by the diffraction grating, but also includes interference effects between the two waveguides, making it difficult to obtain sharp wavelength selection characteristics. Furthermore, in conventional optical wavelength filters, the direction in which power transition occurs is the direction that strongly confines the guided light, that is,
Since this occurs not in a direction perpendicular to the substrate but in a plane direction where confinement is relatively loose, it was necessary to set the bond length of the bond region as long as 3 to 15 mm. This long coupling length has the disadvantage of increasing the size of the entire device, and also makes it difficult to create devices using waveguides made of materials with non-negligible absorption. Furthermore, in order to change the width and height of the waveguide, photolithography technology is required;
It is extremely difficult to achieve an accuracy of less than m, which results in poor reproducibility of device fabrication and causes deterioration of device characteristics.

An object of the present invention is to provide a compact optical wavelength filter and photodetector that have sharp wavelength selection characteristics and good coupling efficiency. [Means for Solving the Problems] The optical wavelength filter of the present invention demultiplexes a specific light wave from a wavelength-multiplexed optical signal,
In addition, there is an optical wavelength filter in which semiconductor layers are laminated to perform multiplexing, which is the opposite operation, and a first waveguide layer into which an optical signal is incident, and a first waveguide layer for performing demultiplexing. , outputs the demultiplexed specific light waves, and when performing multiplexing, selectively separates the optical signal and the specific light waves into a second waveguide layer into which the specific light waves to be multiplexed are input. The first waveguide layer and the second waveguide layer have different refractive indexes, film thicknesses, etc. so that the waveguide modes centered on each layer are different. The diffraction grating is formed in a region where each waveguide mode centered on the first and second waveguide layers overlaps. The photodetector is a photodetector that demultiplexes and detects a specific light wave from a wavelength-multiplexed optical signal, and includes a first waveguide layer into which the optical signal is incident, and a demultiplexer. a second waveguide layer for guiding the specific light wave; a diffraction grating for selectively separating the optical signal and the specific light wave; and a photoelectric conversion of the propagating light in the second waveguide layer. The first waveguide layer and the second waveguide layer have different refractive indexes, film thicknesses, etc. so that the waveguide modes centered on each layer are different; The diffraction grating is formed in a region where each waveguide mode centered on the first and second waveguide layers overlaps.

[Function] Since the first and second waveguide layers are in a laminated form, their propagation constants can be made to differ depending on their materials, and the difference in propagation constants that occurs between them can be made large. can. Therefore, the optical power transfer occurring between the two waveguide layers is dominated by the diffraction grating,
Sharp wavelength selectivity can be obtained. This manufacturing process is almost the same as that for general semiconductor devices, and its reproducibility is also good. Furthermore, since the optical power transfer occurs in the direction in which the light propagating in the first and second waveguide layers is strongly confined, that is, in the direction perpendicular to the substrate, the coupling length can be shortened. This allows the device to be made smaller.

[Example] FIG. 1 is a diagram showing the configuration of a first embodiment of the present invention.

First, the structure of this embodiment will be explained.

In this example, a substrate 1 having a thickness of 0.5 μm is formed on a substrate 1 made of GaAs.
Buffer 104 made of GaAs, 1.5 μm thick
Ala. sGao. Clad M10 which is aAs
5. GaAs and Alo. aGao. sAs are alternately stacked to form a multiple quantum well (MQW) with a thickness of 0.
2 μm first waveguide N101, 0.7 μm thick Alo.
sGao. The cladding is sAs [1 06, Ga
As and Aio4Gao. A second waveguide layer 102 having a thickness of 0.45 .mu.m and having a thickness of 0.45 .mu.m formed by alternately laminating layers of aAs and MQW is sequentially grown by the molecular beam epitaxial (MBE) method. Next, after fabricating a resist mask by photolithography using photoresist, the second waveguide layer 1 is etched by reactive ion beam etching (RIBE).
A diffraction grating 107 consisting of corrugations with a depth of 0.2 μm was formed on a part of the upper surface of the 02. Subsequently, Alo. @Gao. s
The cladding layer 108 made of As was regrown. In this way, in the optical wavelength filter of this embodiment, two waveguide layers (first waveguide layer 101, second waveguide layer 102) are laminated in the layer direction, and a directional coupler is constructed by these layers. There is. Since the waveguide layers 101 and 102 are formed to have different thicknesses and compositions, the propagation constants of light guided through each layer will be different. Diffraction grating 10 formed on second waveguide layer 102
7 is for selecting a wavelength to be optically coupled, and the wavelength range to be selected is determined by the pitch thereof.

Figure 2 is a diagram showing the optical electric field distribution in the waveguide mode of this example. The vertical axis represents the light intensity, and the horizontal axis represents the distance with respect to the first waveguide layer 101. As described above, the waveguide modes of this embodiment include an odd mode 32 that is established around the first waveguide layer l01 and an even mode 31 that is established around the second waveguide layer 102, and the diffraction grating 107 is This even mode 31
and odd mode 32 overlap. The operation of this embodiment in which two waveguide layers are provided in this manner will be described below.

An input optical device 09 consisting of a plurality of laser beams with wavelengths from 0.8 μm to 0.86 μm in steps of 0.01 μm was input coupled to the first waveguide layer 101 . As mentioned above, the modes established in these two waveguide layers include the even mode 31 and the odd mode 32, but the input light input to the first waveguide layer 101 is centered around the first waveguide layer 101. It combines with the odd mode 32 and propagates. In this case, in the region where the diffraction grating 107 does not exist, the odd mode 32 and the even mode 31 have different propagation constants, so they are hardly coupled and are almost independent.
Propagate. However, in the region where the diffraction grating is present, the propagation constant β of the odd mode 32. and the propagation constant β1 of even mode 3l
If the following function holds during this period, a shift in optical power will occur. Here, λ is the optical wavelength and r is the pitch of the diffraction grating.

When the optical power shift as described above occurs, the guided light of the odd mode 32 coupled with the input light 109 is converted to the guided light of the even mode 3l. Therefore, the input light finally becomes a light wave propagating through the second waveguide layer and is output as the selected output light 110. Lights of other wavelengths remain unselected output light 11
Output as 1. The region length β of the diffraction grating for complete coupling to occur is determined by:

However, 2εI, ε. are even mode 31 and odd mode 3, respectively.
2, and A+(x) is a component corresponding to the first-order diffracted light of the Fourier series expansion of the diffraction grating. In order to perform wavelength filtering with light having a center wavelength of 0.83 μm, r was set to 9 μm according to formula (2), and the complete bond length β was set to 250 μm according to formula (2).

As a result, of the light input to the first waveguide layer 101, the second
The wavelength characteristics of the selected output light 110 output from the waveguide N102 are as shown in FIG. It can be seen that wavelength filtering is being performed for approximately 170 people with a full width at half maximum. Naturally, the same method is used for combining waves. Incidentally, an anti-reflection coating was applied to the input and output end faces in order to reduce end face reflection.

FIG. 4 is a diagram showing the configuration of a second embodiment of the present invention. This embodiment aims to reduce optical loss by confining light in the lateral direction, whereas the first embodiment was of a slab type that did not confine light in the lateral direction. .

To explain the structure of this embodiment, in order to perform lateral optical confinement, an impurity such as Zn (or Si) is thermally diffused onto the upper surface of the cladding layer 108 to form the first waveguide layer 101 and the second waveguide layer. 102 was disordered to form a low refractive region 41 with a low refractive index. The other configurations are similar to those in the first embodiment and are given the same numbers. The first waveguide layer 1 is formed by the low refractive regions 41 formed on both sides.
The guided light in 01 and the second waveguide layer 102 was confined in the lateral direction, and loss due to diffraction spread of the guided light was reduced, resulting in a highly efficient optical wavelength filter.

In addition to the methods described above, various methods of lateral confinement can be applied, such as a method of forming a ridge, an embedding method, and a loading method.

FIG. 5 is a diagram showing the configuration of a third embodiment of the present invention.

In the first example, a diffraction grating for correcting the propagation constants of the two waveguide layers was formed in the second waveguide @I02, but in this example, it was formed in the cladding layer between the two waveguide layers. be.

That is, as shown in FIG. 5, after forming a cladding layer 51 made of AIGaAs, a corrugated diffraction grating is formed by photolithography, and a second waveguide layer 52 made of AIGaAs and a cladding layer 10 made of AIGaAs are formed.
8 was regrown.

The other configurations are the same as those in the first embodiment, and are given the same numbers. The performance of the device was not much different from that shown in the first example. When combining or integrating the optical wavelength filter according to the present invention with other elements, a convenient element shape can be selected. Further, as described above, the position where the diffraction grating is manufactured may be any position where both the optical electric field distributions (even mode 31 and odd mode 32) of the guided light exist. However, since the coupling efficiency varies accordingly, the coupling length needs to be adjusted.

FIG. 6 is a diagram showing the configuration of a fourth embodiment of the present invention. This example is realized by integrating a photodetector having wavelength selectivity by using the structure of the present invention.

First, the structure of this embodiment will be explained.

On the substrate 601 which is n''-GaAs, the thickness is 0.5 μm.
buffer layer 602 of n-GaAs with a thickness of 1.5μ
m's n-Alo. A cladding layer 603 made of sGa6, sAs, and a 0.2 μm thick n-Ale. sGao.
The first waveguide layer 604 is tAs, and the thickness is 0. I3 μm n-Alo. sGao. Cladding layer 6 made of sAs
05, i-GaAs and Alo. 4Gao. A multi-quantum well formed by stacking aAs alternately and having a thickness of 0.4 μm
The second waveguide layer 606 is subjected to molecular beam epitaxial (MBE)
They were grown sequentially using the method. After that, using a photolithography method, a depth of 0.05 μm and a pitch of 7.7 μm were formed.
A diffraction grating 607 consisting of corrugations was formed on the upper surface of the second waveguide layer 606 over a length of 1.277 mm. Next, f-Alo. iGao. A cladding layer 608 made of aAs and a cap layer 615 made of t-GaAs with a thickness of 0.5 μm were regrown. After that, the diffraction grating 607
cladding layer 608 and second waveguide layer 6 in an area adjacent to
06 was removed by etching. Next, an absorption layer 60 of i-GaAa with a thickness of 0.1 μm is placed on this removed portion.
9, 1.2 μm thick P-Ale. sGao. sA
8, a cladding layer 610 with a thickness of 0.5 μm, P”-
The cap layer 611 made of GaAs is regrown by the LPE method, and then an electrode 612 made of Cr/Au is formed on the cap layer 611, and AuG is grown on the back surface of the substrate 601.
An electrode 613 made of e/^U was formed.

In this embodiment, out of the light 614 input to the first waveguide layer 604, only the light having the wavelength selected by the optical wavelength filter is coupled to the second waveguide layer 606 and is detected by the photodetector. It is absorbed by a certain absorption layer 609. The photodetector is p
-i-n structure, and a reverse bias is applied between the electrodes 612 and 613. Therefore, carriers generated by absorption are detected as a current signal.

FIG. 7 is a diagram showing the wavelength characteristics of the signal light extracted as electricity in this example. In this example, since the depth of the corrugation is shallow and the interval between the waveguide layers is set wide, the coupling length is long, but the wavelength selection width is narrow by about 30 people.

As mentioned above, the optical wavelength filter according to the present invention can have a narrower or wider wavelength selection range depending on the application. It is possible to Note that all of the above embodiments are made of GaAs/A
It was composed of IGaAs-based materials, but of course, rr+
Other compound semiconductors such as GaAs/InGaP or glass materials such as SiO/Tilt, LiNbOs
, LiTaOs. It is obvious that it is also possible to construct it from optical crystals such as BSO. [Effects of the Invention] Since the present invention is configured as described above, it produces effects as described below.

According to the first aspect of the present invention, there is an effect that a small optical wavelength filter having sharp wavelength selectivity can be manufactured with good reproducibility. This manufacturing process is based on film formation and is suitable for integration, so optical ICs. OEIC
It is suitable for (optoelectronic IC) and can be widely applied to optical information transmission, optical communication, optical LAN, and optical computing. According to the second aspect of the present invention, there is an effect that a photodetector having the above-mentioned effects can be realized.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the configuration of the first embodiment of the present invention, Fig. 2 is a diagram showing the optical electric field distribution of the waveguide mode of the first embodiment, and Fig. 3 is a diagram showing the optical electric field distribution of the waveguide mode of the first embodiment. A diagram showing the wavelength characteristics of light emitted from the waveguide layer 102, FIGS. 4 to 6 are diagrams showing the configurations of second to fourth embodiments of the present invention, respectively, and FIG. 7 is a diagram showing the configuration of the fourth embodiment. In the example, FIG. 8 is a diagram showing the wavelength characteristics of the signal light extracted as a current, and is a diagram showing the configuration of the conventional example. 01,604...First waveguide layer, 02,606...Second waveguide layer, Q3, 601...Substrate, 04, 602...Buffer layer, 05, 108, 51, 603, 605 , 60
8, 610... cladding layer, 07, 607...
Diffraction grating, 09,614...Input light, 1O...Selected output light, II...Unselected output light, 3l...Even mode, 32...Odd mode, 4l...Low refractive region , 609...Absorption layer, 611,615...Cat 612.613...Electrode. Blue layer,

Claims (1)

[Claims] 1. Optical wavelength in a form in which semiconductor layers are stacked for demultiplexing a specific light wave from a wavelength-multiplexed optical signal and for performing multiplexing, which is the reverse operation. A filter, which includes a first waveguide layer into which the optical signal is incident, and when performing demultiplexing, outputs the demultiplexed specific light wave, and when performing multiplexing, a second waveguide layer into which the specific light wave to be combined is incident; and a diffraction grating for selectively separating or combining the optical signal and the specific light wave; The wave layer and the second waveguide layer have different refractive indexes, film thicknesses, etc. so that the waveguide modes centered on each layer are different, and the diffraction grating is different from the first and second waveguide layers. An optical wavelength filter is formed in an area where each waveguide mode overlaps around the waveguide layer. 2. A photodetector that demultiplexes and detects a specific light wave from a wavelength-multiplexed optical signal, comprising: a first waveguide layer into which the optical signal is incident; and the demultiplexed specific light wave. a second waveguide layer for guiding the light wave; a diffraction grating for selectively separating the optical signal and the specific light wave; and photoelectric conversion of the light propagating in the second waveguide layer. The first waveguide layer and the second waveguide layer have different refractive indexes, film thicknesses, etc. so that the waveguide modes centered on each layer are different. . A photodetector, wherein the diffraction grating is formed in a region where each waveguide mode centered on the first and second waveguide layers overlaps.