JPH08184719A - Wavelength selective detector and optical communication system using same - Google Patents

Abstract

PURPOSE: To provide the wavelength selective detector which has high quantum efficiency and a high wavelength selection rate and the optical communication system which uses it. CONSTITUTION: The wavelength selective detector has a wavelength selection area consisting of two waveguides 11 and 12 which perform mode coupling through a grating 13 and an optical detection area consisting of two waveguides 11 and 12 except the grating and an optical detection layer 14. In the optical detection area, a gap layer 15 which has a lower refractive index than any of the layer of the waveguide 12 and the optical detection layer 14 is inserted between the waveguide 12 which is adjacent to the optical detection layer 14 between the two waveguides 11 and 12 and the optical detection layer 14. Consequently, light having wavelength selected by the wavelength selection area is efficiently absorbed by the optical detection layer 14 and light having unselected wavelength is hardly absorbed. Therefore, the wavelength selective detector which has the high quantum efficiency and high wavelength selection rate can be obtained as a single compact element.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication system based on a wavelength selective photodetector and wavelength division multiplexing for demultiplexing detection.

[0002]

2. Description of the Related Art In optical fiber communication, a wavelength division multiplexing system (hereinafter referred to as WDM system) has been actively developed as a method capable of effectively utilizing a wide wavelength band and dramatically increasing the transmission capacity. There is. WD like this
In the M system, a wavelength selective photodetector that spatially separates an arbitrary wavelength signal light and then detects it as an electric signal and can tune its wavelength over a wide range plays an important role. And it is expected that the performance will be improved. At present, as a wavelength selective photodetector, a device having a photodetection function unit integrated with a grating compensation type demultiplexing coupler is proposed, as proposed in Japanese Patent Laid-Open No. 2-239209. is there.

[0003]

In the above-mentioned known example, since the photo-detecting layer is formed directly on the demultiplexing coupler, the quantum efficiency is high and the size is small. On the other hand, depending on the system, the cross from the signal light of the wavelength which is not selected is selected. Sometimes the talk fell into a situation where it could not be ignored, and the transmission characteristics deteriorated.

Therefore, an object of the present invention is to achieve high quantum efficiency,
An object of the present invention is to provide a wavelength selective photodetector having a high wavelength selective ratio and an optical communication system using the same.

[0005]

According to the present invention, a wavelength selection region consisting of two waveguides for performing intermode coupling through coupling compensation means (typically a grating) and a coupling compensation means are not included. In a wavelength-selective photodetector having two waveguides and a photodetection region formed of a photodetection layer, in the photodetection region, a waveguide adjacent to the photodetection layer of the two waveguides. A gap layer having a lower refractive index than both the waveguide layer and the photodetection layer is inserted between the photodetection layer and the photodetection layer. With this configuration, the light of the wavelength selected in the wavelength selection region is efficiently absorbed in the photodetection layer, and the light of the non-selection wavelength is hardly absorbed. Therefore, wavelength selection with high quantum efficiency and high wavelength selection ratio is achieved. The photodetector can be realized compactly with a single element.

[0006]

[First Embodiment] FIGS. 1 to 5 are views for explaining a first embodiment of the present invention. FIG. 1 shows an example of the basic structure of a wavelength selective photodetector according to the present invention. Before describing the first embodiment in detail, the principle of the present invention will be described with reference to FIGS. In FIG. 1, 11 is a first waveguide through which light is input and transmitted, and 12 is a second waveguide for branching light of a selected wavelength. The grating 13, which is a coupling compensating means, is formed so that the two waveguide modes existing in the two-layer waveguides 11 and 12 are forward-directionally coupled to each other. Due to the existence of the grating 13, the first waveguide 11
And the second waveguide 12 forms a directional coupler. In this example, as shown in FIG. 1, the waveguide mode 1 propagating in the first waveguide 11 is subjected to forward directional coupling in the wavelength selection region by the grating 13, and the waveguide mode 2 propagating in the second waveguide 12. Is converted to. The converted light of mode 2 is absorbed by the photodetection layer 14 formed on the second waveguide 12 in the photodetection region to which the reverse voltage is applied. The light absorbed by the light detection layer 14 is detected as a photocurrent.

The tuning wavelength is controlled by the current injected into the wavelength selection area. The period of the grating 13 is 10
It is about −20 μm. The full width at half maximum of the detected wavelength is 1
Wide design is possible from nm to 10 nm or more. on the other hand,
Non-selected light does not undergo mode conversion by the grating 13 and passes through and propagates through the first waveguide 13. At this time, unless there is a large difference in quantum efficiency in the photodetection layer 14 when propagating in the photodetection region between the waveguide mode 1 and the waveguide mode 2, crosstalk between wavelength channels occurs.

Therefore, in the present invention, the quantum efficiency ratio between the two guided modes is improved by inserting a gap layer 15 shown in FIG. This will be described in detail.

FIG. 2A shows the distribution of two modes (here, referred to as an even (or 0th) mode and an odd (or 1st) mode) in the wavelength selection region. Figure 2
In (a), the grating is actually formed in a portion having a portion where the even mode and the odd mode overlap,
Here, it is symbolically depicted as being formed between two waveguides. FIG. 2B shows the guided mode in the light detection region. In this example, three modes of 0th order, 1st order, and 2nd order are established in the light detection region. The light input from the first waveguide 11 (the waveguide on the left side of FIGS. 2A and 2B) becomes an even mode, and becomes an odd mode after conversion by the grating. The odd-mode light is strongly coupled mainly with the 0th-order and second-order modes in the photodetection region, and two modes have a distribution in the photodetection layer, and thus are strongly absorbed. That is, quantum efficiency or absorption efficiency (mA
/ MW) becomes higher. On the other hand, even mode light is
It becomes the first-order mode light in the light detection region. According to the present invention, by providing a gap layer having a lower refractive index than both of the photodetection layer and the second waveguide (the waveguide on the right side of FIGS. 2A and 2B), total reflection is achieved. The light of the even mode (or the primary mode of the photo detection region) does not reach the photo detection layer, and the quantum efficiency of the even mode light can be suppressed. FIG.
Is when the refractive index of the gap layer is the same as that of the cladding layer,
The ratio of even-mode light transmitted through the gap layer and reaching the photodetection layer is shown. The amount of transmitted light due to the evanescent wave is attenuated as the gap layer becomes thicker. Gap layer thickness 0.7
The transmittance is almost 0 at μm.

4 and 5 show the effect of the interstitial layer according to the invention. FIG. 4 shows the quantum efficiencies for the odd mode (corresponding to the selected wavelength) and the even mode (corresponding to the non-selected wavelength) when the thickness of the gap layer is changed. In the even mode, the quantum efficiency decreases as the gap layer becomes thicker.
At a thickness of 0.7 μm, the quantum efficiency is 20 dB or more,
Go down. This tendency is as expected from the result of FIG. For the odd mode, the quantum efficiency is improved by 1 dB.
FIG. 5 shows the quantum efficiency ratio between the odd mode and the even mode. In this example, it can be seen that the mode selection ratio can be improved by 25 dB as compared with the case where there is nothing by the effect of inserting the gap layer having a thickness of 0.7 to 0.8 μm.

The first embodiment will be described in detail below. The manufacturing process of this embodiment will be described. For the device, the grating 13 is etched on the InP clad substrate 16.
First, the grating 13 is etched by reactive ion beam etching after writing a pattern with a photomask using a resist. The length of the grating wavelength selection region is 1900 μm, and the length of the light detection region is 100 μm. Subsequently, the first waveguide 11 made of InGaAsP (wavelength λg = 1.33 μm corresponding to the band gap) has a thickness of 0.5 μm, the InP intermediate cladding layer 17 has a thickness of 1.5 μm, and InGaAsP (λg
= 1.11 μm) and the second waveguide 12 is 0.2 μm
To a thickness of 0.7 μm, and the InP interstitial layer 15 has a thickness of I
The nGaAs photodetection layer 14 is grown to a thickness of 0.5 μm. The photodetection layer 14 and the gap layer 15 are etched except for the photodetection region, and then the InP clad layer 18 and InGa
The AsP cap layer 19 is regrown. At this time, the input end of the second waveguide 12 is removed so that the input light is transmitted through the first waveguide 11.
It is designed to selectively bind to. An AuZn / Au electrode 20 is provided on the upper portion of the cap layer 19, and an AZn / Au electrode 20 is provided on the rear surface of the substrate 16.
The uGeNi / Au electrode 21 is formed into a film, and etching for separating the light detection region and the wavelength selection region is performed. Further, an antireflection film 23 is formed on the end face.

The light from the tunable laser is fed to the first waveguide 11
The wavelength characteristics of the photocurrent in the photodetection region and the first
The wavelength characteristic of the intensity of light emitted from the waveguide 11 was measured. The full width at half maximum of the detected wavelength was about 2 nm. The detection current from the unselected wavelengths is sufficiently small, and the crosstalk between the wavelengths is at a negligible level. By controlling the injection current I to the wavelength selection region, the wavelength of the light transferred between the two waveguides 11 and 12 is changed by the grating forward coupling, and as a result, the peak wavelength of the detection current is tuned. Be done. The response spectrum or wavelength characteristic of the light emitted from the first waveguide 11 has a notch filter characteristic in which only the detected wavelength is missing. If you don't select all wavelengths,
By removing the wavelength to be grating-coupled from the transmission wavelength range, all wavelength light is transmitted through the first waveguide 11 and output. In addition, in the present invention, the layer position of the grating 13 may be anywhere as long as the two modes interact with each other, for example, the upper part of the first waveguide,
It may be the lower part of the second waveguide or the like. However, the first waveguide 1
In the present embodiment in which 1 is used as the input waveguide, FIG.
As can be seen from FIG. 1, the position shown in FIG. 1 is the best place where the two modes interact.

[0013]

[Second Embodiment] A second embodiment will be described with reference to FIG.
The same functional parts as those in FIG. 1 are designated by the same reference numerals. The difference from the first embodiment is that the wavelength selection is performed by the backward coupling (reflection coupling). The grating 13 is a two-layer waveguide 11,
The two guided modes existing in 12 are formed so as to be inversely coupled to each other. The length of the grating wavelength selection region is 500 μm, and the length of the light detection region is 100 μm. The wavelength-multiplexed optical signal is input from the first waveguide 11 and proceeds to the wavelength selection region without being absorbed by the photodetection layer 14 in the photodetection region. Only the optical signal of the selected wavelength is reverse-mode converted by the grating 13 in the wavelength selection region, travels through the second waveguide 12 toward the incident end, and is absorbed by the photodetection layer 14. The incident ends of the second waveguide 12, the gap layer 15, and the photodetection layer 14 are etched so that the incident light does not directly enter, and the light shielding film 51 is coated. Then, the entrance end and the exit end of the first waveguide 11 are coated with antireflection coating. The full width at half maximum of the detected wavelength is narrower than that of the forward coupling structure of the first embodiment, and 1
It is just a little less than nm. The other points are the same as in the first embodiment.

[0014]

[Third Embodiment] A third embodiment of the present invention will be described with reference to FIG. The same functional parts as those in FIG. 1 are designated by the same reference numerals. The difference from the above-described embodiment is that wavelength selection is performed by a backward coupling and a resonator structure. Therefore, the grating period is the same as in the second embodiment. Both ends of the second waveguide 12 are removed so that the input light is selectively coupled to the first waveguide 11. In this embodiment, as shown in FIG. 7, the waveguide mode 1 propagating in the first waveguide 11
Is subjected to reverse directional coupling in the wavelength selection region 2 by the grating 13 and is converted into a waveguide mode 2 propagating in the second waveguide 12. The converted light of mode 2 is absorbed by the photodetection layer 14 formed on the second waveguide 12 in the photodetection region via the gap layer 15. And again, the wavelength selection region 1
In, the light is received in the backward direction coupling by the grating 13 and converted into the guided mode 1, and becomes light in the same direction as the input light. As described above, the light of the selected wavelength is transmitted through the grating 13
While being resonated between the two waveguides 11 and 12 via, the light is absorbed by the photodetection layer 14. The light absorbed by the light detection layer 14 is detected as a photocurrent. The tuning wavelength is controlled by the currents I ₁ and I ₂ injected into the wavelength selection regions 1 and ₂ .

Due to the resonator structure, the full width at half maximum of the detected wavelength is narrower than that of the second embodiment which is formed by one backward coupling and is about 0.03 nm. By controlling the injection currents I ₁ and I ₂ to the wavelength selection regions 1 and 2, the wavelength resonated between the two waveguides 11 and 12 is changed by the grating backward coupling, and as a result, the peak of the detection current is obtained. Wavelength tuning is performed.

[0016]

[Fourth Embodiment] FIG. 8 shows an example of using the wavelength selective photodetector of the present invention in a bus type communication network. In the figure, 71 is an optical fiber bus, 72 and 73 are optical repeaters, and 74-7.
Reference numeral 7 is an optical node, and 78, 79, 701 and 702 are terminals. Optical nodes 74-77 include wavelength selective photodetectors according to the present invention. Further, the optical nodes 74 to 77 may include an E / O converter such as a laser. Optical repeater 7
Reference numerals 2 and 73 are provided to compensate for transmission loss and branch loss. Of the signals / data that come through the transmission line,
By tuning the wavelength selective photodetector in the optical nodes 74 to 77 to the desired wavelength channel, only the desired light is split into electrical signals for the terminals 78, 79, 701, 702. In this example, no deterioration in transmission characteristics due to crosstalk between wavelength channels was observed.

[0017]

Fifth Embodiment FIG. 9 shows an example in which the wavelength selective photodetector of the present invention is used in a loop network. In the figure, 81 is an optical transmission line, 82 to 87 are optical nodes, and 88 and 89 are optical nodes for transmission with a loop. 801-80
6 is a terminal. Optical nodes 82-87 include wavelength selective photodetectors according to the present invention. Optical nodes 82-8
7 or the terminals 801 to 806 include an E / O converter. The role of the wavelength selective photodetector in receiving the wavelength signals from the terminals 801 to 806 is the same as in the fourth embodiment.

[0018]

As described above, according to the present invention, a wavelength selection region composed of two waveguides for coupling between modes via a coupling compensating means such as a grating and a coupling compensating means such as a grating are included. In a wavelength-selective photodetector including two waveguides and a photodetection region including a photodetection layer, the waveguide and the light in the photodetection region that is adjacent to the photodetection layer Since a gap layer having a refractive index lower than that of either the waveguide layer or the photodetection layer is inserted between the detection layer and the detection layer, crosstalk between wavelength channels can be sufficiently suppressed. Therefore, it is possible to provide a wavelength selective photodetector that can be applied to an optical communication system having excellent transmission characteristics.

[Brief description of drawings]

FIG. 1 is a sectional view illustrating a wavelength selective photodetector according to the present invention.

FIG. 2 is a diagram for explaining the principle of the present invention using the wavelength selective photodetector of FIG.

FIG. 3 is a graph illustrating the principle of the present invention using the wavelength selective photodetector of FIG.

FIG. 4 is a graph illustrating the principle of the present invention using the wavelength selective photodetector of FIG.

5 is a graph illustrating the principle of the present invention using the wavelength selective photodetector of FIG.

FIG. 6 is a sectional view illustrating a second embodiment of the wavelength selective photodetector according to the present invention.

FIG. 7 is a sectional view illustrating a third embodiment of the wavelength selective photodetector according to the present invention.

FIG. 8 is a diagram showing an example of a bus type optical communication system using the wavelength selective photodetector according to the present invention.

FIG. 9 is a diagram showing an example of a loop type optical communication system using a wavelength selective photodetector according to the present invention.

[Explanation of symbols]

 11 1st waveguide 12 2nd waveguide 13 Grating 14 Photodetection layer 15 Gap layer 16 Cladding substrate 17, 18 Cladding layer, 19 Cap layer 20, 21 Electrode 23 Antireflection film 51 Light-shielding film 71, 81 Optical transmission line 74- 77, 82-89 Optical node 78, 79, 701, 702, 801-806 Terminal 72, 73 Optical repeater

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication H04J 14/00 14/02 // H01L 31/10 H04B 9/00 E H01L 31/10 A

Claims (9)

[Claims] 1. A wavelength having a wavelength selection region formed of two waveguides for performing intermode coupling through a coupling compensation unit, and a light detection region formed of two waveguides not including the coupling compensation unit and a photodetection layer. In the selective detector, the waveguide layer and the photodetection layer are provided between the photodetection layer and the waveguide of the two waveguides that is adjacent to the photodetection layer in the photodetection region. A wavelength selective photodetector, wherein a gap layer having a refractive index lower than any of the above is inserted. 2. The wavelength selective photodetector according to claim 1, wherein the coupling compensation means is a grating. 3. The wavelength selective photodetector according to claim 2, wherein intermode coupling due to grating in the wavelength selective region is forward directional coupling. 4. The wavelength selective photodetector according to claim 2, wherein the intermode coupling due to the grating in the wavelength selective region is reverse directional coupling. 5. The wavelength-selective photodetection according to claim 2, wherein the intermode coupling due to grating in the wavelength selective region is a reverse directional coupling, and the intermode coupling has a resonator structure. vessel. 6. The wavelength selective photodetector according to claim 2, wherein the grating in the wavelength selective region is formed at a position where two modes interact with each other. 7. The wavelength selective photodetector according to claim 6, wherein the grating in the wavelength selective region is formed on the substrate side of the input waveguide. 8. The wavelength selective photodetector according to claim 1, wherein a current is injected into the wavelength selective region so that a tuning wavelength is controlled. 9. A plurality of wavelength selective photodetectors according to any one of claims 1 to 8 are provided, and an optical transmission / reception device including the wavelength selective photodetectors is connected by an optical fiber transmission line. Optical communication system.