JPS63205611A - Dual balance type photodetector - Google Patents

Abstract

PURPOSE:To stabilize signal receiving sensitivity by reflecting the output light of a photocoupler by a reflecting mirror part formed by molding the side face of a semiconductor substrate to a slant face and receiving said light by photodetectors provided on the semiconductor substrate. CONSTITUTION:The end face on one side in the longitudinal direction of the semiconductor substrate 1 is formed aslant and a metallic film 5 is provided on the slant end face to constitute the reflecting mirror part 3. The signal light and locally emitted light from optical fibers 13, 14 are synthesized by the photocoupler 8 and are then approximately equally divided. The divided light rays are detected by the photodetectors 4, 4 via the mirror part 3. The optical distance from the output port of the photocoupler 8 to the photodetector 4 is determined by the extended length of waveguide 2, 2 and the reflection position of the mirror part 3 and, therefore, the deviation of the phases of the synthesized light branched to the two rays on the photodetection surface is prevented if the waveguides 2, 2 are so formed that the optical distances between both coincide at the time of forming the waveguides on the substrate 1. The decrease in the electrical outputs taken out of the photodetectors 4, 4 is thus prevented. The good signal receiving sensitivity is thereby obtd.

Description

[Detailed Description of the Invention] Outline The output light of an optical coupler consisting of waveguides arranged in parallel on or inside a substrate made of a semiconductor or ferroelectric is reflected by a reflecting mirror formed by obliquely molding the side surface of the semiconductor substrate. Reflect it,
A dual balance type light receiver is configured to receive light with a light receiving element provided on a semiconductor substrate. This light receiver operates reliably without requiring complicated adjustments, and is advantageous in terms of implementation.

FIELD OF THE INVENTION The present invention relates to the structure of a dual-balanced optical receiver used on the receiving side of coherent optical communications.

In recent years, in the field of optical communications, research into coherent optical communication technology that directly controls the frequency or phase of light has become active due to demands for improved frequency usage efficiency and faster modulation speed. There is.

Generally, in a coherent optical communication system as shown in FIG. 2, the output light of the light source 11, which has a stable transmission frequency and high spectral purity, is frequency-modulated directly or by an external modulator (not shown), and then transmitted to the receiving side. Hetero gain detection is performed to restore the original information. The light receiving side includes a signal light sent from an optical fiber 13 as a transmission path and a local oscillation light (hereinafter referred to as local oscillation light) sent from an LD 12 with a constant output frequency slightly shifted from the signal light via an optical fiber 14. It is composed of an optical coupler 10 that combines the combined light, a light receiving element 15.16 that receives the combined light, an optical fiber 21.22 that interconnects these, and electric circuits such as a mixer 17 and an amplifier 18. It is required to be able to receive combined light with low loss and to have an excellent mounting surface.

Dual-balanced photodetectors are attracting attention as a device that may meet these requirements.

In the dual-balanced optical receiver, the output ports of the optical coupler 10, which input the signal light from the optical fiber 13 and the local light from the optical fiber 14, are branched into two, and the phase of the output light of each port is shifted by π. Focusing on this fact, we used photodetector pairs 15 and 16 that had the same characteristics and were connected in a so-called push-pull configuration, and configured the optical fibers 21 and 22 to have the same optical connection conditions, so that both ports The output light, that is, the combined light of the signal light and the local light, is received without loss, and in particular, the in-phase intensity noise of the local light is canceled out. The received combined light is sent to the light receiving element 1
5.16 and the mixer 17, it is converted into a beat signal (intermediate frequency signal) having a frequency that is the difference between the frequency of the signal light and the frequency of the local light, and this signal is amplified by the amplifier 18 to remove the common mode intensity noise. Obtaining electrical signals.

FIG. 3 shows a specific example of the configuration of a conventional dual-balanced photodetector. Reference numerals 21 and 22 denote optical fibers whose one ends are connected to an optical coupler (not shown), and the other ends of these optical fibers 21 and 22 have a tapered tip 21a.
, 22a are formed. Reference numerals 15 and 16 denote light receiving elements having the same characteristics provided on the substrate 26, and these light receiving elements 15 and 16 are connected to the amplifier 18 via the mixer 17. Tapered tip spherical portion 21 of optical fiber 21, 22
a and 22a face the light receiving surfaces of the light receiving elements 15 and 16, respectively. In the dual-balanced optical receiver configured in this way, by increasing the intensity of the local light input together with the signal light, the signal light can be detected with good sensitivity and S/N ratio due to the above-mentioned effect. can.

Problems to be Solved by the Invention However, in the dual-balanced photoreceiver described above, it is necessary to make the gain difference and the delay time difference of the optical paths zero, which requires advanced mounting technology and optical coupling technology. That is,
If there is a gain difference between the two optical paths, there will be a difference in the received light intensity at the light receiving elements 15 and 16, and the removal of in-phase intensity noise of the local light in the mixer 17 will be insufficient, and if there is a delay time difference, the received light will be Since the phase difference of light deviates from π and the intensity of the obtained intermediate frequency signal decreases, the optical fiber 2 serving as a light guide between the optical coupler 10 and the light receiving element 15.16 is
1.22 requires that the coupling state between the optical coupler 10 and the light receiving element 15 and 16 at both ends be exactly the same, and the optical lengths must be the same at the wavelength level of the intermediate frequency signal. There is. Therefore, if an attempt is made to actually create a dual-balance type photoreceiver as shown in FIG. 3, there is a problem in that extremely difficult mounting work is required. Furthermore, since the optical fibers 21 and 22 are disposed perpendicularly to the substrate 26, there is a problem in that the mounting volume increases and the device inevitably becomes larger.

The present invention was created in view of these problems, and its purpose is to provide a dual-balanced light receiver that can obtain good sensitivity and S/N ratio without requiring difficult mounting work, and can be miniaturized. It is about providing the equipment.

The problem with the above-mentioned conventional technology for solving the above problem is that when constructing a dual-balanced photodetector, optical couplers are formed by arranging waveguides in parallel on a semiconductor or ferroelectric substrate. This problem can be solved by forming a reflecting mirror section on the side surface of the substrate facing the waveguide end portion at an angle, and providing a light receiving element on the substrate to receive the guided light reflected by this reflecting mirror section. .

Function: In the dual-balanced light receiver of the present invention, since the optical coupler formed by the waveguides arranged in parallel and the light receiving element are coupled via the waveguide and the reflecting mirror, the two optical paths are Delay time differences are less likely to occur, and since the photodetector can be formed on the substrate according to the position of the waveguide and reflecting mirror, stable and high coupling efficiency can be achieved without the need for complicated optical axis adjustments, etc. Since the optical fiber does not need to be connected perpendicularly to the substrate, miniaturization is achieved.

衷-JLJ! Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.

Referring to FIG. 1(a), there is shown a partially cutaway perspective view of a dual-balanced light receiver configured to apply the present invention. Further, FIG. 1(b) shows a cross-sectional view of the substrate 1 along the waveguide 2 of FIG. 1(a). Reference numeral 1 denotes a semiconductor substrate made of, for example, silicon or InP, and two waveguides 2.2 are arranged symmetrically in parallel in the longitudinal direction on this semiconductor substrate 1 in a conventional manner. The approximately central portion of the waveguide 2.2 is arranged close to each other so that the predetermined length portions have a predetermined gap, and is configured to function as a two-power, two-output type 3 dB optical coupler 8.

7.7 is an electrode loaded on the optical coupler 8, and by changing the voltage applied between the electrodes 7.7, the output intensity distribution ratio of the optical coupler 8 can be finely adjusted. be.

One end face in the longitudinal direction of the semiconductor substrate 1 is formed obliquely, and a gold film 15 that reflects light well is provided on this inclined end face, for example, by metallizing treatment to form a reflecting mirror portion 3. The waveguide 2.2 extends from this inclined end face to the other end face.

13 and 14 are optical fibers for signal light and local light that are attached to the end surface opposite to the inclined end surface of the semiconductor substrate 1;
The optical axis is adjusted so that the propagating light is efficiently guided to the waveguide 2.2.

4.4 is placed on the semiconductor substrate 1 using the usual method so that the light receiving surface is located at a position where the axis of the light beam propagating through the waveguide 2.2 and reflected by the reflecting mirror section 3 intersects with the top surface of the semiconductor substrate 1. This is a light receiving element formed.

The signal light and the local light from the optical fibers 13 and 14 are linearly polarized light whose polarization state is uniform by a polarization control means (not shown), and after being combined by an optical coupler 8, the signal light and the local light are divided into approximately equal parts and sent to a reflecting mirror section. The light is received by the light receiving element 4.4 via the light receiving element 4.4. At this time, from the output port of the optical coupler 8 to the light receiving element 4
The optical distance from the semiconductor substrate 1 to
If these factors are controlled so that the optical distances match when forming the waveguide 2.2, it is possible to easily prevent the phase of the combined light split into two from shifting on the light receiving surface. , the electrical output extracted from the light receiving element 4.4 does not become small.

Variations in the loss at the connection between the optical fiber 13.14 and the waveguide 2.2 and variations in the 4°4 sensitivity of the light-receiving elements directly affect the removal characteristics of the common-mode intensity noise of the local light, so The gain in each optical path is 4.4 for both light receiving elements.
It is desirable that they match. If it is unavoidable that they cannot be matched, an appropriate voltage may be applied between the electrodes 7 and 7 to adjust the intensity distribution ratio between the output ports of the optical coupler 8.

In this embodiment, a metal 115 is provided on the inclined end face of the semiconductor substrate in order to form the reflecting mirror portion 3, but the end face inclination angle propagates through the waveguide 2.2 without providing a metal film. It may be set so that the total reflection condition for light is satisfied.

and tm Presentation As described in detail above, according to the present invention, the incident light is guided to the light receiving element via the waveguide and the reflecting mirror, so that the local light can be adjusted without the need for complicated adjustment work. It is possible to provide a dual-balanced optical receiver that can reduce intensity noise and have stable and high reception sensitivity.

Furthermore, since the optical fiber can be connected in the longitudinal direction of the substrate on which the light-receiving element is formed, there is also the effect that the device can be made smaller.

[Brief explanation of the drawing]

1(a) and (b) are partially cutaway perspective views of a dual-balanced photodetector showing a preferred embodiment of the present invention, and a cross-sectional view of the substrate along the waveguide. FIG. 2 is a conventional coherent photodetector. Block Configuration Diagram of Optical Communication System FIG. 3 is a schematic perspective view showing a specific configuration of a conventional dual-balanced light receiver. DESCRIPTION OF SYMBOLS 1... (semiconductor) substrate, 2... Waveguide, 3... Reflector part, 4.15
．． 16... Light receiving element, 5... Metal film, 13.14.21.22... Optical fiber, 7... Electrode, 8.10... Optical coupler. +4 (b) The present invention/) Cross country I sequence diagram Figure 1 I Asakusa sword with Albausos type, received error - Figure 3

Claims (3)

[Claims] (1) Waveguides (2) are arranged in parallel on a substrate (1) made of a semiconductor or ferroelectric material to form an optical coupler.
) A reflecting mirror part (3) is formed on the side surface side of the substrate (1) whose ends face each other by molding the side face diagonally, and this reflecting mirror part (3)
The light receiving element (4) that receives the guided light reflected by the substrate (1
) A dual-balanced optical receiver. (2) The dual balance type light receiver according to claim 1, wherein a metal film (5) is formed on the reflecting mirror portion (3). (3) The dual-balanced light receiver according to claim 1, wherein the reflecting mirror portion (3) is obliquely polished so as to satisfy a condition for total reflection of the guided light.