JPH02226776A - Wavelength selection photodetector - Google Patents

Abstract

PURPOSE:To realize light detection with high efficiency by arranging a plurality of distributed feedback region constituted of a diffraction grating along the travelling direction of incident light, and inserting a light receiving region between the above distributed feedback regions. CONSTITUTION:A wavelength selection detector has a structure wherein a light receiving region A is sandwiched by two distributed feedback regions B formed by distributed Bragg reflectors (DBR). In the case where the wavelength of an input guided light is sufficiently separated from the Bragg wavelength lambdaB, the guided light is transmitted without generating diffraction. In the case where the wavelength of the input guided light is close to the wavelength lambdaB, the light is reflected by Bragg diffraction. In this case, when the phase shift between the two regions B satisfies a specified condition, Fabry-Perot etalon resonates at the Bragg wavelength of the DBR in the region B, and the light intensity in the region B is increased. When absorption is not present in the region A, only the Bragg wavelength is in the transmission state. In the case where absorption is present in the region A, the light is sufficiently absorbed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a photodetector that selectively detects wavelength-multiplexed signals for each wavelength.

[Prior art 1] Conventionally, when detecting an arbitrary optical signal from a wavelength multiplexed optical signal, an optical demultiplexer is used to select only the optical signal having the desired wavelength, and then a so-called optical signal having a photoelectric conversion function is used. It was detected using a detector.

Optical demultiplexers include directional coupler-type optical demultiplexers that utilize the wavelength selectivity of the coupling characteristics between two optical waveguides, optical demultiplexers that use multilayer interference film bandpass filters, or Examples include those using diffraction grating dispersion and Bragg reflection. However, optical demultiplexers using the above principle have long element lengths and large insertion losses, are difficult to integrate into optical waveguides, and are difficult to tune the center wavelength. However, none of them were satisfactory. Furthermore, the wavelength selectivity, expressed as a separable center wavelength interval, is 100 to 200 for the directional coupler type.
It has been extremely difficult to apply this method to high-density wavelength multiplexing or optical frequency multiplexing in the future era of coherent communications.

In contrast to the above-mentioned optical demultiplexers, optical wavelength filters that utilize a distributed feedback structure consisting of a diffraction grating have recently been attracting attention due to their high wavelength selectivity, tunability, and suitability for integration. (Unexamined Japanese Patent Publication No. 1983-2213.62-
241387.63-133105, etc.).

[Problems to be Solved by the Invention] However, the optical wavelength filter element using the distributed feedback structure of the diffraction grating described above has several drawbacks in application.

For example, the usable wavelength range or spacing is limited. In other words, effective wavelength-selective transmission occurs only in the wavelength range where the distributed feedback of the diffraction grating occurs, that is, around the Bragg wavelength of the diffraction grating.In other wavelength ranges, light is always transmitted, and light It no longer functions as a wavelength filter. Another problem is that it is unsuitable for detecting multiple wavelengths. In other words, when trying to detect multiple wavelengths using an optical wavelength filter,
First, the light to be detected must be separated by a branch path, discriminated by each optical wavelength filter, and then detected using a photodetector connected to the subsequent stage. By using this branch path, the optical loss can be reduced by the number of branches. increased accordingly and was undesirable.

The present invention was made in view of the drawbacks of the above-mentioned conventional techniques, and an object of the present invention is to provide a wavelength selective photodetector that has an adjustable wavelength selection function and enables efficient light detection. do.

[Means for Solving the Problems] The wavelength-selective photodetector of the present invention has a plurality of distributed feedback regions each made of a diffraction grating provided along the traveling direction of incident light, and between the distributed feedback regions there is a light-receiving region. In addition, a plurality of distributed feedback regions made of diffraction gratings are provided along the traveling direction of the incident light, and the light receiving region and the phase control region are inserted between the distributed feedback regions. Is the distributed feedback region provided with an electrode for injecting current, or is the distributed feedback region formed of a first-order diffraction grating, or is it composed of a diffraction grating? A number of distributed feedback regions are provided along the traveling direction of incident light, and a light receiving region and a gain region are inserted between the distributed feedback regions.

A plurality of distributed feedback regions made of diffraction gratings are provided along the traveling direction of incident light, and a phase control region and a gain region of the light receiving region 1 are inserted between the distributed feedback regions.

[Operation 1] The present invention detects an optical signal of a desired wavelength by utilizing resonance in a distributed feedback region and light absorption in a light receiving region. Furthermore, if the gain region is provided between the distributed feedback regions,
Radiation loss when the optical signal passes through the distributed feedback region can be compensated for. In addition, by providing a phase control region together with a light receiving region between one distributed feedback region, and injecting a current into the distributed feedback region via an electrode, it is possible to compensate for the radiation loss described above, and to compensate for the distributed feedback region by current injection. It is possible to respond to changes in the refractive index of the wavelength and select a desired wavelength. Further, by providing a gain region and a phase control region together with a light receiving region between the distributed feedback regions, the radiation loss can be compensated for and the detection wavelength can be selected.

〔Example〕

Next, two embodiments of the present invention will be described with reference to the drawings.

The basic configuration of an embodiment of a wavelength selective photodetector according to the present invention is shown in FIG.

On the right side of FIG. 1, this wavelength selective photodetector has a light receiving area A.
has a structure sandwiched between two distributed feedback regions B formed by DBRs (Distributed Bragg Reflectors).

Here, the state of transmission and reflection for input guided light in this example will be explained.

First, when the input guided light has a wavelength that is sufficiently different from the Bragg wavelength λ, the guided light is transmitted without diffraction, as shown in FIG. 2(a). However, if the diffraction grating constituting the DBR in distributed feedback region B is not a first-order diffraction grating, radiated diffracted light will be generated in the vertical direction of the diffraction grating, but in any case, at this time, the wavelength selective photodetector will be in the transmission state. It is. In addition, when the input waveguide light has a wavelength near the black wavelength, as shown in Fig. 2 (bl
As shown in FIG. At this time, the phase shift φ between the two distributed feedback regions 8
is (mho, 1. 2. . . .) or (m: o, 1, 2. . . .) where no: effective refractive index of the waveguide.

LA: When the length of the light receiving area A is satisfied, the Fabry-Perot etalon becomes resonant at the Bragg wavelength of the DBH in the distributed feedback area B, and the light intensity inside the distributed feedback area B becomes high. And light receiving area A
If there is no absorption in the light-receiving region A, only the Bragg wavelength will be transmitted in the reflected state as shown in Figure 2(a), but if there is absorption in the light-receiving region A, the guided light in the resonant state will be As shown in FIG. 3(b), the light is strongly absorbed in the light receiving area A. This is utilized in the wavelength selective photodetector according to the present invention.

In this example, the detection wavelength λ. 0.8 μm, and the effective refractive index n0 of the optical waveguide 11 is 3.4, then the grating constant A of the diffraction grating constituting the DBR of the distributed feedback region B is expressed as follows.

=mx0.117Ei (μII+) In equation (2), when m = 1, that is, when a first-order diffraction grating is used, the optical loss due to radiation diffraction disappears and the DBR reflection becomes highly efficient, but technology is required. Therefore, m=2. In this case, a second-order diffraction grating was used.

Next, a second embodiment of the present invention will be described.

In the first embodiment described above, a second-order diffraction grating is used in the distributed feedback region B, but in this case, the first-order diffraction light is emitted out of the waveguide as radiation diffraction light, resulting in a loss. Therefore, in FIG. 1, the feedback efficiency from the distributed feedback region B becomes low,
Since sufficient characteristics cannot be obtained, in this embodiment, a gain that compensates for the radiation loss is given to the distributed feedback system, and as shown in FIG. and provided between the two distributed feedback regions B. This wavelength selective photodetector is made of n-type GaAs in FIG.
n''-GaA epitaxially grown on the substrate 41'
A buffer layer 42 consisting of s. Cladding layer 43 made of n-AlGaAs. Optical waveguide layer 44 made of n-AlGaAs, 1-GaAs or 1-GaAs/1-Al
Active layer 50 made of GaAs MQW. p-AlGa
It is composed of a cladding layer 45 made of As, an electrode formation layer 46 made of GaAs, an n-side electrode 47, and a ρ-side electrode 48.

The distributed feedback region B is formed by exposing the optical waveguide layer 44 by etching, and then forming a relief diffraction grating with a period of 0.235 μI by three-beam interferometry and ion etching.
R49 and its length is between 300 μm and 500 μm. The light receiving region A, the gain region G, and the distributed feedback region B are separated by a high resistance region H doped with Fe.

A reverse bias voltage -■ is applied to the light-receiving region A, which becomes a pin-type photodiode, and a constant current I is applied to the gain region G to compensate for radiation loss due to the DBR 49. is injected into the optical amplifier. Here, the quadratic coupling coefficient of DBR49 is o, oi (μ1-1), and its length is 30
The transmitted light intensity of this photodetector when changed to 0 μm and 500 μm, the reflected light intensity, and the absorbed light intensity in the light receiving area are the fifth
As shown in the figure. However, in this case, the amount of absorbed light α generated when the guided light passes through the light receiving area A once is changed depending on the length of the DBR 49, and at any length, the amount of absorbed light α between the two DBRs is If the phase shift is expressed by the above equation (1), that is, (m+), as shown in Figure 5 (a), when the length of the DBR49 is 300 μI, strong absorption occurs at the Bragg wavelength of 0.8 μI, and absorption occurs around the Bragg wavelength. At wavelengths far from the Bragg wavelength, absorption occurs due to one passage through the light-receiving region A.Furthermore, D shown in FIG. 5(b)
When the length of BR49 is 500 μl, the wavelength selectivity of the distributed reflectance becomes higher than when the length is 300 μl, and as a result, the absorption spectrum width becomes narrower.

In this way, the absorption spectrum width for wavelength selection changes depending on the DBH length, but in this embodiment, it can be adjusted between 0.1 and 2.

Next, a third embodiment of the present invention will be described.

In the second embodiment described above, the gain region G was placed between the distributed feedback regions B, but in this embodiment, a configuration is adopted in which gain can be applied to the distributed feedback region, that is, DBH. However, when current is injected to give a gain to the DBH, the refractive index changes at the same time, so as can be seen from equation (2) above, a shift in the Bragg wavelength occurs. Therefore, it is necessary to provide a region for controlling the phase according to the wavelength to be selected. FIG. 6 shows a configuration diagram of a wavelength selective photodetector based on the above idea.

In this embodiment, as shown in FIG. 6, a phase control region P is provided adjacent to the light receiving region A and between two distributed feedback regions B, and the detection wavelength is 0.83μ.
Similarly, the DBR 49 of the distributed feedback region B was formed using a relief-like second-order diffraction grating. The layer structure is the same as the above-mentioned second layer structure.
Similar to the example, but after fabrication of the relief-like diffraction grating,
By regrowth p-^lGaAslGaAs Clara p”
- A GaAs electrode forming layer 46 is formed to allow current injection.

The phase control region P performs refractive index control using the plasma effect of carrier electron gas caused by current injection. Also,
The light receiving area A1, the phase control area P, and the distributed feedback area B are all separated by a high resistance area H.

Here, sticking to this example, the length of DBR49 is 100
Figure 7 shows the detected light intensity in the light-receiving area A when the amount of light is varied between μl and 500 μl and the amount of absorbed light in one pass is set in the same manner as in the second embodiment described above. .

As is clear from FIG. 7, as in the case of the second embodiment, as the DBH length becomes longer, the wavelength selectivity becomes higher and the detected light spectrum width, that is, the absorption spectrum width becomes narrower. Further, in this embodiment as well, as in the second embodiment, the absorption spectrum width can be adjusted from 0.1 to 2 depending on the DBH length.

Next, in this example, the DBR length is set to 300μ, and 2-
Figure 8 shows the received light intensity when the intensity is changed to .).

As is clear from FIG. 8, in the case of phase shift mπ,
Although a peaky light reception spectrum is not generated, a peaky light reception spectrum is generated as the phase shift increases, and the peak wavelength can be controlled by the shift phase.

Therefore, the number of channels of distinguishable wavelengths can be controlled by adjusting the half-width of the spectrum according to the wavelength multiplexing degree. In addition, the refractive index of the length of the phase control region P is 5 x
10-' has changed. As described above, according to this embodiment, by controlling the current injected into the distributed feedback region B and the current injected into the phase control region P, the photodetector can change the selected wavelength by controlling the gain and the selected wavelength. Become.

Next, a fourth embodiment of the present invention will be described.

In this embodiment, in contrast to the second embodiment described above, as shown in FIG. This makes it possible to change the detection wavelength.

A current I is injected into this phase control region P' via an electrode 47, and the refractive index is controlled by utilizing the plasma effect of the carrier electron gas. Also,
A constant current 6 is injected into the gain region G to compensate for the loss of light emitted by the DBR 49 in the distributed feedback region B. A reverse bias voltage -■ is applied to the light receiving area A, and the absorbed light is detected as a current. In this example,
By controlling the injection current IP to the phase control region P, the refractive index is changed to control the phase shift between both DBRs,
The resonance of the Fabry-Perot etalon generated here changes the wavelength absorbed in the light receiving area A.

The relationship between the detected light intensity, the detected wavelength, and the phase shift in this example was the same as that in FIG. 8 shown in the third example described above.

Next, a fifth embodiment of the present invention will be described.

In this embodiment, an example is shown in which a first-order diffraction grating with a grating constant of 0.122 μm is formed as a diffraction grating constituting the DBR of the distributed feedback region B.

When using a first-order diffraction grating, loss due to radiation is reduced, so there is no need to provide a gain region between the distributed feedback regions 8, and only the light receiving region A and phase control region P are inserted.

This first-order diffraction grating was fabricated using a two-beam interference method using a high refractive index prism 101, as shown in FIG. 1O.

In FIG. 10, after coating a photoresist 102 on the leading waveguide surface of a substrate 105, a prism with a refractive index of 1.8 is closely attached via an index matching liquid 103, and a He-fed laser beam with a wavelength of 3250 nm +04 The interference fringes were exposed.

Here, the detection wavelength λ. If 0.83 μm and the effective refractive index n0 of the waveguide is 3.4, then the lattice constant Δ of the first-order diffraction grating constituting the DBR is: =0.122 (μm).

Therefore, the incident angle θ of the two-beam interference light on the substrate 105 to obtain a lattice constant of 0.122 (μm) is 1°, since the refractive index of the prism (index matching liquid) is 1.8. θ=47.7°.

As described above, a first-order diffraction grating with a grating constant of 0.122 is obtained, and the detection wavelength can be shifted by injecting current into the phase control region. The relationship between the detected light intensity, the detected wavelength, and the phase shift in this case was the same as that shown in FIG. 8 described above.

Next, a sixth embodiment of the present invention will be described.

In this example, the lattice constants of the distributed feedback regions included in the wavelength-selective photodetector used in the third example described above are individually set, and the wavelength-selective photodetector is configured as shown in FIG. , three wavelengths λ1 . λ2. Optical signals of λ3 can be detected simultaneously. Here, the lattice constants of the distributed feedback region of each wavelength selective photodetector 110 are 0.235 μm and 0.238 μm, respectively.
μm, 0.241 μm, respectively 0.8 μm,
Optical signals with wavelengths of 0.81 μm and 0.82 μm were detected.

In the wavelength-selective photodetector used in this example, light with a wavelength that is optically separated from the Bragg wavelength is not transmitted and detected, so that the three wavelength-selective photodetectors can independently perform photodetection.

In the above description, the end faces of the photodetector were not described, but when this photodetector is used alone, it is necessary to provide a non-reflective structure on both end faces. This is because if the end face is optically coupled as a reflective surface, undesired wavelengths may be detected or oscillation may occur, which is not convenient. There are anti-reflective coatings.

Furthermore, the above example deals with wavelengths in the 0.8 μm band, so an example using a GaAs-based semiconductor has been described. For example, for wavelengths in the 1.3.1.5 μm band, InP-based semiconductors
It is clear that similar wavelength-selective photodetectors can be obtained, such as in the InGaAsP system.

〔Effect of the invention〕

As explained above, according to the present invention, it has a wavelength selection function that can adjust the wavelength selection width and center wavelength, and also gives gain to the optical signal, so that the diffraction that occurs when the light passes through the distributed feedback region Efficient light detection that compensates for light loss becomes possible. Furthermore, since light with wavelengths far enough away from the Bragg wavelength is transmitted and not absorbed, multiple optical signals can be detected independently without the need for branching paths, making it possible to support wavelength selection for optical multiplexed signals. There is an effect.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing the basic configuration of a wavelength selector according to the present invention. FIG. 2 is a diagram showing the transmission and reflection characteristics of a Fabry-Perot etalon using distributed feedback. FIG. 3 is a diagram showing the wavelength selector according to the present invention. 4, 6, 9 and 11 are cross-sectional views showing other embodiments of the wavelength selective photodetector according to the present invention, and FIG. , FIG. 7 is a diagram showing element characteristics in another embodiment of the wavelength selective photodetector according to the present invention, and FIG. 8 is a diagram showing the detection when the selected wavelength is made variable in the wavelength selective photodetector according to the present invention. FIG. 10 is a diagram showing a method of manufacturing a diffraction grating. DESCRIPTION OF SYMBOLS 11... Optical waveguide, 12... Electrode, 4l---n-GaAs substrate, 42---n''-GaAs buffer layer, 43...
n-^lGaAs cladding layer, 44--...'n-Al
GaAs optical waveguide layer, 45----p-AIGaAs cladding layer, 46...p"-GaAs electrode formation layer, 4
7...n-side electrode, 48...p-side electrode, 49...DBR. 50-...-1-GaAs or 1-GaAs/1-A
IGaAs MQvI active layer, 101... Prism, 102... Photoresist, +03... Index matching liquid, 104...
・He-Cd laser beam, 105... Substrate, 110... Wavelength selective photodetector, 8... Light receiving area, B... Distribution transition area, G... Gain Area, P... Phase control area. Patent applicant Canon Co., Ltd.

Claims (6)

[Claims] (1) A wavelength selective photodetector characterized in that a plurality of distributed feedback regions each consisting of a diffraction grating are provided along the traveling direction of incident light, and a light receiving region is inserted between the distributed feedback regions. (2) Wavelength selection characterized in that a plurality of distributed feedback regions composed of diffraction gratings are provided along the traveling direction of incident light, and the light receiving region and the phase control region are inserted between the distributed feedback regions. Photodetector. (3) The wavelength selective photodetector according to claim 2, further comprising an electrode for injecting current into the distributed feedback region. (4) The wavelength selective photodetector according to claim 2, wherein the distributed feedback region is formed by a first-order diffraction grating. (5) Wavelength-selective photodetection characterized in that a plurality of distributed feedback regions composed of diffraction gratings are provided along the traveling direction of incident light, and a light receiving region and a gain region are inserted between the distributed feedback regions. vessel. (6) A plurality of distributed feedback regions composed of diffraction gratings are provided along the traveling direction of incident light, and a light receiving region, a phase control region, and a gain region are inserted between the distributed feedback regions. Wavelength selective photodetector.