JPH0272335A - Dual balance type light receiving device - Google Patents

Abstract

PURPOSE:To decrease the delay time difference of mixed light reaching a light receiver and to suppress intensity noises of local oscillation light effectively by providing an optical path length varying means in the propagation path of the mixed light inputted to at least one light receiver. CONSTITUTION:Received light 1 and the local oscillation light 2 are mixed by a mixer 3 and distributed to two propagation paths. One mixed light beam 4 is made incident on the light receiver 7 and converted photoelectrically and the other mixed light beam 5 is made incident on a light receiver 8 through the optical path length varying means 6 and converted photoelectrically. The output signals of the receivers 7 and 8 are inputted to a subtracter 9 composed of a differential couple, etc., to obtain the difference signal. Thus, the optical path length varying means 6 is provided in the propagation path of the mixed light 5 inputted to the receiver 8 and then the delay time difference between the mixed light beams 4 and 5 reaching the receivers 7 and 8 can be minimized, so that the intensity noises of the local oscillation light can be suppressed effectively.

Description

[Detailed description of the invention] Table of Contents Overview Industrial Application Fields Prior Art Pages 3 Pages 4 Pages 5 Problems to be Solved by the Invention Page 9 Means for Solving the Problems Page 10 for · ·
・ ・ ・ ・ ・ ・ ・ ・ ・ ・ 11 Truth Examples ・ ・ ・ ・ ・ ・ ・ ・ ・
・ ・ Effects of the invention on page 11 ・・・・・・・・・・・
Summary on page 21 Concerning a dual-balanced light receiving device used in the receiver of a coherent optical communication system, the dual-balanced light receiving device is capable of effectively suppressing the intensity noise of local light by reducing the delay time difference of mixed light that reaches the light receiving device. In a dual-balance type light receiving device, the received light and local light are mixed and optical-to-electrical conversion is performed in two light receivers to obtain a difference signal. It is constructed by providing a variable optical path length semi-submersion in the propagation path of the input mixed light.

INDUSTRIAL APPLICATION FIELD The present invention relates to a dual-balanced light receiving device used in a receiving section of a coherent optical communication system.

In the field of optical communications, an intensity modulation/direct detection method is common in which intensity modulated light transmitted through an optical transmission line is directly received by a light receiving element and converted into an electrical signal. In recent years, however, due to demands for improved optical frequency usage efficiency and longer transmission distances, laser light sources with high spectral purity are used as carrier light for transmission and local light sources for reception.
Research on coherent optical communication systems that mix received light and local light on the receiving side to perform heterogain detection, homodyne detection, etc. is intensifying. According to this method, it is expected that the receiving sensitivity will be improved compared to the intensity modulation/direct detection method, so it will be easy to increase the repeating interval, reduce the number of repeaters, or increase the number of branches in the optical transmission line, and It becomes possible to construct transmission lines economically. There is a dual-balanced light receiving device that contributes to further improvement of reception sensitivity in coherent optical communication systems, and optimization of its configuration is being explored.

BACKGROUND ART FIG. 8 is an explanatory diagram of a coherent optical communication system. The optical signal from the transmitting section 71 is transmitted to the receiving section through an optical transmission path 72 made of a single mode optical fiber or the like. In the receiving section, the received light and local oscillation light (local light) from the local oscillation laser 73 are added to an optical mixer 74 and mixed,
The mixed output light is made incident on a light receiver 75 such as a photodiode, and the optical signal is converted into an electrical signal and applied to an amplifier 76. The signal component of the received light is transmitted to the optical receiver 7.
An intermediate frequency signal (for example, several Gl (
z) or as a baseband signal, this is demodulated by an appropriate demodulation circuit (not shown).

According to this method, a signal having an amplitude proportional to the product of the amplitude of the received light and the amplitude of the local light can be obtained as the output of the light receiver 75, so by using the local light with an appropriate intensity, High reception sensitivity can be achieved.

FIG. 9 is a CNR characteristic curve diagram showing the relationship between CNR (carrier-to-noise power ratio) and the intensity of the local light, using the intensity noise of the local light as a parameter. In the configuration shown in FIG. 8, when the intensity noise of the local light is small, the CNR or the minimum reception level is improved as the intensity of the local light is increased. However, based on the stability of the local oscillation laser 73 (if the intensity noise of the local oscillation light is large), even if the intensity of the local oscillation light is increased, the shot noise limit that can originally be reached will not be reached and the CNR will increase. Therefore, it is necessary to allow some degree of CNH deterioration by using local light with low intensity noise, or to suppress this intensity noise.

Therefore, a dual balance type light receiving device was proposed. In this method, as shown in FIG. 10, when the received light 81 and the local light from the local oscillation laser 82 are added to the optical mixer 83 and mixed, two output lights are obtained. The output lights are input to 84 and 85 and converted into electrical signals, amplified by amplifiers 86 and 87, and then sent to subtracter 88.
It is added to. The number of received lights incident on the light receivers 84 and 85 from the light mixer 83 such as a half mirror is 18.
On the other hand, the intensity noise components of the local light are in phase, so by calculating the difference between the output signals of the amplifiers 86 and 87 using the subtracter 88, the signal components are added, and the intensity noise is This cancels out the intensity noise of the local light, making it possible to suppress the intensity noise of the local light.

FIG. 11 is a diagram for explaining the function of the optical mixer 83 and also for explaining another example of the light receiver. The optical mixer 83 is
It is constructed by forming a reflective film 83b on a transparent vi 83a that has been subjected to anti-reflection treatment, and the refractive index of the transparent plate 83a is larger than the refractive index of the surrounding medium, and the refractive index of the reflective film 83b is as follows.
The refractive index of the surrounding medium is larger than that of the transparent plate 83a.
The refractive index is smaller than that of . The received light S passes through the reflective film 8
When reflected at 3b, this corresponds to reflection at the fixed end, and the phase of the reflected light is shifted by 180° to become S(π), while the transmitted light has no phase shift and becomes 5(0). Local light reflection film 83b
When reflected at the free end, the reflected light corresponds to reflection at the free end, and becomes reflected light L(0) without any phase shift, and when transmitted, the reflected light becomes L(0) without any phase shift. The mixed light distributed into the two optical paths is incident on the series-connected light receivers 91 and 92.
The signal from connection point 192 is input to amplifier 93 . By connecting the light receivers 91 and 92 in series in this manner, the subtracter 88 in FIG. 10 operates to add signal components and cancel intensity noise components.

FIG. 12 is an explanatory diagram of a dual-balanced reception system using a 3 dB optical coupler 101 as an optical mixer. Received light and local light are input to the 3 dB optical coupler 101, and the mixed output lights are connected in series. The light is incident on the light receivers 91 and 92 of . With this configuration, the first
Similar to the configuration shown in FIG. 1, intensity noise can be suppressed.

FIG. 13 is a diagram showing a specific example of the configuration of the optical coupling section to the light receiver.

(a) The ends of the optical fibers 111 and 112 that respectively propagate the mixed light distributed in the optical mixer are cut, and the cut surfaces are directly opposed to the light receiving sections of the light receivers 91 and 92, respectively.

(b) The ends of the optical fibers 113 and 114 through which the mixed light propagates are processed into a spherical shape with a tapered tip, and coupled to the light receivers 91 and 92 with a focusing effect.

(C) - The mixed light emitted from the end faces of the optical fibers 115 and 116 is focused by lenses 117 and 118 and coupled to a light receiver 91 and 92.

Problems to be Solved by the Invention In order to effectively suppress intensity noise in a dual-balanced reception system, it is required that the optical path lengths from the optical mixer to each optical receiver be the same. By the way, at Gb/s class transmission speeds, in order to reduce the capacity of the optical receiver, it is necessary to use an optical receiver with a small optical receiving diameter of several tens of micrometers. With this configuration, the distance between the optical fiber and the light receiver cannot be freely adjusted in order to maintain optical coupling efficiency. Therefore, if there are variations in the length of the optical fiber that serves as the optical transmission path between the optical mixer and the optical receiver, this will directly cause a delay time difference in the mixed light, making it difficult to effectively suppress intensity noise. .

The present invention was created in view of these circumstances, and provides a dual-balanced light receiving device that can effectively suppress the intensity noise of local light by reducing the delay time difference of mixed light reaching the light receiver. intended to provide.

Means to solve problems FIG. 1 is a diagram showing the principle of the present invention.

The received light 1 and the local light 2 are mixed in a mixer 3, divided into two propagation paths, and output.

One of the mixed lights 4 is incident on the light receiver 7 and undergoes optical-to-electrical conversion.

The other mixed light 5 is sent to a light receiver 8 via an optical path length variable means 6.
The light is incident on the light source and undergoes light-to-electrical conversion.

The output signal of the photoreceiver 7.8 is sent to a subtracter 9 consisting of a differential pair.
A difference signal is obtained.

Function Generally, the optical path length between two points in a light propagation medium is expressed as the product of the distance between the two points and the refractive index of the medium.If the optical path lengths are the same, the optical path length between the two points is The propagation time is the same. Therefore, the optical path length variable means is specifically realized by varying the length of the optical path or by varying the refractive index of part or all of the propagation medium.

In the present invention, since the optical path length variable means is provided in the propagation path of the mixed light manually input to at least one of the light receivers, the delay time difference of the mixed light reaching each light receiver is adjusted to be minimized. Therefore, it becomes possible to effectively suppress the intensity noise of local light.

Example Embodiments of the present invention will be described below based on the drawings.

FIG. 2 is a configuration diagram of a dual balance type light receiving device showing a first embodiment of the present invention. 11 is an optical fiber that propagates received light, and 12 is an optical fiber that propagates local light. 1
3 is an optical coupler formed by, for example, fusing and stretching optical fibers, and functions as an optical mixer. 1
4 is an optical fiber that propagates half the power of the mixed light mixed by the optical coupler 13, and 15 is an optical fiber that propagates the other half of the power of the mixed light. The light emitted from the end face of the optical fiber 14 is made into a substantially parallel light beam by the collimating lens 16, and this parallel light beam is
The light is focused by 7 and coupled to a light receiving portion of a light receiver 18 .

The light emitted from the end face of the optical fiber 15 is made into a parallel light beam by a collimating lens 20, and this parallel light beam is condensed by a condensing lens 21 and coupled to a light receiving portion of a light receiver 22. The vicinity of the end of the optical fiber 15 and the collimating lens 20 are fixedly held by a holder 19, and the holder 19 is movable in the direction of the optical axis connecting the output end of the optical fiber 15 and the light receiving part of the light receiver 22. is supported by

The heterogain or homodyne signals generated by the optical-to-electrical conversion in the photoreceivers 18 and 22 are transmitted to amplifiers 23 and 23, respectively.
．． 24 and added to a subtracter 25, where the intensity noise of the local light is canceled out, and a homodyne or heterogain signal is added and output.

As described above, in this embodiment, the collimating lens 20 is integrated with the optical fiber 15 and this integrated object is supported movably in the optical axis direction. The length of the optical path from the optical coupler 13 to the light receiver 22 can be varied by adjusting the distance of the propagation path of the incident light, and even if the length of the optical fibers 14 and 15 varies, the above adjustment can be made. By doing
It becomes possible to reduce the delay time difference of the mixed light reaching the light receivers 18 and 22, and effectively suppress the intensity noise of the local light.

In this embodiment, in order to balance the optical coupling efficiency, etc., the optical fiber 14 and the optical receiver 18 and the optical fiber 15
A parallel light beam system is constructed both between the optical fiber 14 and the light receiver 22, but it is not necessarily necessary to construct a parallel light beam system between the optical fiber 14 and the light receiver 18. It may also be a structure.

FIG. 3 is a configuration diagram of a dual balance type light receiving device showing a second embodiment of the present invention. Parts that are substantially the same as those in the previous embodiment are designated by the same reference numerals, and some explanations thereof may be omitted (the same applies hereinafter). In this embodiment, the light emitted from the optical fiber 15 is coupled to the light receiving section of the light receiver 22 via the collimating lens 30, the detour optical system 31, and the condensing lens 36.

The detour optical system 31 includes a reflecting mirror 3 that reflects the parallel light beam produced by the collimating lens 30 and takes it out in a direction perpendicular to the optical axis.
2, a reflecting mirror 33 that reflects the light extracted in a direction perpendicular to the optical axis in a direction parallel to the optical axis, and a reflecting mirror 34 that reflects the reflected light from the reflecting mirror 33 again in a direction perpendicular to the optical axis. , a reflecting mirror 35 that reflects the reflected light from the reflecting mirror 34 so that it coincides with the original optical axis, and a reflecting mirror 33.3.
4 is supported so as to be integrally movable in a direction perpendicular to the optical axis. With this configuration as well, by adjusting the position of the reflecting mirror 3334, the optical path length of the propagation path of the mixed light can be varied, making it possible to effectively suppress the intensity noise of the local light.

In the first and second embodiments described above, the reason why a parallel light beam system is adopted on the side where the optical path length is varied is as follows. That is, even if the length of the light propagation path in the portion that is a parallel light beam is changed, the optical coupling efficiency does not change significantly. This is explained based on the following fact.

FIG. 4 is a diagram for explaining the effect of using a parallel light beam system. As shown in FIG. 4A, an LD assembly 43 is composed of an LD (semiconductor laser) and a collimating lens 42, and a condensing lens 44 and an optical fiber 45.
The fiber assembly 46 is constructed from the fiber assembly 46, and both assemblies 1J
The relationship between the coupling loss and the lens is shown in figure (b) by measuring the change in the coupling loss when changing the separation distance between the lenses 43 and 46. This corresponds to the content disclosed in ``Study of Coupling System of Dispersion Fiber and LD'', 1986 IEICE Semiconductor/Materials Division National Conference, 398. - In general, the aperture angle of an LD is relatively large and the aperture is not uniform in the plane perpendicular to the optical axis, and even in such cases the coupling loss hardly depends on the lens spacing. In a light-receiving system in which beam parameters can be easily matched as in the embodiment, even if the length of the optical path of the parallel light beam section is changed, the coupling loss or coupling efficiency is large (there is no fear that it will change).

In the second embodiment, the detour optical system 31 is composed of only a reflecting mirror, but all or a negative part of the reflecting mirror may be replaced with a prism. In this case, if a movably supported corner cube prism is used in place of the reflecting mirrors 33, 34, the tolerance for the arrangement angle will be increased, so that manufacturing workability will be improved. Furthermore, when constructing a detour optical system from a reflecting mirror or prism, phase inversion of the reflected light on each reflecting surface becomes a problem, but by setting an even number of reflecting surfaces, the phase inversion of the detour optical system as a whole can be avoided. will disappear.

FIG. 5 is a configuration diagram of a dual balance type light receiving device showing a third embodiment of the present invention. In this embodiment, a frame member 51 having a through hole passing through the parallel light beam portion is provided between the collimating lens 30 and the condensing lens 36, and an optical medium selected from a group of optical media 52 having different refractive indexes is selected. It is configured so that it can be inserted into the frame member 51. When the thickness of the optical medium is 1 and its refractive index is n, the optical path length E in the optical medium is expressed as f=tn, so by adjusting n, the optical path length of the optical medium can be varied. In this way, the optical path length of the propagation path of the mixed light can be varied. Also with this configuration, by selecting an optimal optical medium, it is possible to effectively suppress the intensity noise of the local light.

The effect when the optical path length variable means is provided in the propagation path of the mixed light as in the first to third embodiments and the optical path length is adjusted optimally will be explained with reference to FIGS. 6 and 7. FIG. 6 is a model diagram of a linearly balanced photodetector, in which the intensity noise generated in the local light from the local oscillation laser 61 is NL.
The power transmission coefficient of the optical mixer 62 is ε, the quantum efficiency of the optical receiver 63 in the first branch is η2, and the gain of the amplifier (front end circuit) 64 in the first branch is H+H) [=A+
exp(Jφ,), the quantum efficiency of the photodetector 65 in the second branch is η2, and the gain of the amplifier 66 in the second branch is H
2(f) C=A2exp(jφ2)], the shot noise and thermal noise generated in the amplifier etc. are each Ns
, Nth is shown. Regarding shot noise N and thermal noise N and h among the noise sources,
Since there is no correlation between the noise generated in the first branch and the second branch,
There is no noise suppression effect due to differential synthesis. On the other hand, since the source of the local oscillation intensity noise N is a single local oscillation laser, there is a correlation between the noises in the first branch and the second branch. Therefore, by differentially combining, the intensity noise of the local light can be suppressed. This is the principle of the dual balanced reception system.

The intensity noise NL of the local light is NL-ξD' [(1-E)2+ E"θ2 a 22
E (1-E) θaMcosΔφ]PL2BIP
...(1) It is expressed as follows. where ξ is the true value of relative intensity noise (RIN), ξ= l QRIII/l.

PL is the intensity of the local light, and BIF is the intermediate frequency band. Also, D ” 77 + e / h u , θ=η2/
ηhΔφ=φ2−φ1=2πF1FΔτ where PIF is the intermediate frequency and Δτ is the delay time difference.

On the other hand, the intensity noise NL' of the local light when a single receiver is used without a dual balance type is found by setting a=Q in equation (1), and NL'=ξD2(
1-ε)2PL'BIF (2).

Therefore, from equations (1) and (2), the amount of improvement in RIN ξ by adopting a dual-balanced type is =ξ [1+
S'-2S McosΔφ]=ξ [(1-S)'+
25(1−McosΔφ)] (3). Here, S is a parameter representing a gain given by the product of branching ratio, gain ratio, and quantum efficiency ratio, and is represented by ε.

According to the result of equation (3), Δφ−0. It is clear that the amount of improvement is greatest when S = 1, and for the optical system, it is important to minimize Δφ with S = 1, that is, to change the optical coupling efficiency to each receiver. ZuniΔφ
A technical challenge is to bring it close to -0 (Δτ=0).

Figure 7 shows that S in equation (3) is S=1 (S=OdB
) and Δτ = 1.10, 100.1000 picoseconds, the amount of improvement in RIN and the data bit rate F
, is a graph showing the relationship between .

Here, PIF=2FAI, BIF=2×0. 7X
I'm F-ya. In a normal semiconductor laser, RIN=-
Since it is 150 to -160 dB/Hz, in order to ignore the influence of the intensity noise of the local light, an RI of 10 to 30 dB is required.
Improvement of N is required. In order to obtain an improvement of 30 dB in Gb/s class transmission, from equation (3) or Figure 7, it is necessary to suppress Δτ to about 1 picosecond, and this can be achieved by using a light source in the 1.55 μm band. When converted to length, it is 0.2 mm in optical fiber and 0.3 m in air.
corresponds to the length of In this way, in order to effectively suppress the intensity noise of local light at high bit rates, it is necessary to adjust the optical path length of the mixed light propagation path with an accuracy of 1 mm or less. Providing an optical path length variable means therein is extremely effective for the above adjustment.

Effects of the Invention As detailed above, according to the present invention, in a dual-balanced light receiving device, the optical path length variable means is provided in the propagation path of the mixed light input to at least one of the light receivers, so that the light receiving This has the effect that it becomes possible to effectively suppress the intensity noise of the local light by reducing the delay time difference of the mixed light that reaches the device.

Furthermore, since it is not necessary to strictly align the lengths of the optical fibers between the optical mixer and the light receiver when configuring the device, there is also the advantage that the device manufacturing workability is improved.

[Brief explanation of the drawing]

FIG. 1 is a diagram of the principle of the present invention. FIG. 2 is a configuration diagram of a dual-balanced light receiving device showing a first embodiment of the present invention. FIG. 3 is a dual-balanced light receiving device showing a second embodiment of the present invention. FIG. 4 is an explanatory diagram of the operation of the first and second embodiments of the present invention. FIG. 5 is a diagram of the configuration of a dual balance type light receiving device showing the third embodiment of the present invention. A model diagram of a dual-balanced reception system for explaining the effects of the embodiments of the invention; FIG. 7 is a graph showing the relationship between the amount of improvement in RIN and the data bit rate for explaining the effects of the embodiments of the invention; Fig. 8 is an explanatory diagram of the coherent optical communication system, Fig. 9 is a CNR characteristic curve diagram in the coherent optical communication system, Figs. It is a figure which shows the conventional example of the optical coupling part to the optical receiver in a receiving system. 1... Received light, 2... Local light, 3... Optical mixer, 4.5... Mixed light, 6... Optical path length variable means, 7.8.18, 22° 63° 5... Light receiver. Recruitment 2 Mei 1 Function 1 Figure 3 Lens ゛j-Electric descent mountain L (mm) (b) $1. Figure 4: Explanation diagram of $2×1 example effect! 18 row effect light model flash Figure 6 Bait 3 row Oshi row 60 Figure 5 10M 00M G +OG Thetahi, 7 sheets m (b/s) Many meetings also have a row length of 1ξ - Moon Figure 7 Figure 0 Healent Light 1 I Kanta Be 0 Explanatory Diagram Figure 8 - Arno Malathos Type Ukiyoshi τ In° Kano Akira Figure 10 CNRt Hayabusa; [-)) 1 Song *i. Figure 9: A clear view of the T1 Altitude type receiver 11, 0, V, and 12
Diagram (G) Diagram 13 of the row of light and elegance to the light receiving sign with water.

Claims (4)

[Claims] (1) In a dual-balanced light receiving device that mixes received light (1) and local light (2) and performs optical-to-electrical conversion in two light receivers (7, 8) to obtain a difference signal, at least Mixed light (5) input to one photoreceiver (8)
) A dual-balanced light receiving device characterized in that an optical path length variable means (6) is provided in the propagation path of the light receiving device. (2) A collimating lens (20) and a condensing lens (21) are provided on the optical axis connecting the output end of the optical fiber (15) through which the mixed light is guided and the light receiving part of the light receiver (22) to parallel A light beam system is configured, and the collimating lens (20) is connected to the optical fiber (15).
2. The dual-balance type light receiving device according to claim 1, wherein the integrated body is made movable in the direction of the optical axis. (3) A collimating lens (30) and a condensing lens (36) are provided on the optical axis connecting the output end of the optical fiber (15) through which the mixed light is guided and the light receiving part of the light receiver (22) to parallel the light. A light beam system was constructed, a detour optical system (31) formed by combining a reflecting mirror and/or a prism was provided to this parallel light beam system, and components of the detour optical system (31) were movably supported. The dual balance type light receiving device according to claim 1, characterized in that: (4) A dual-balanced light receiving system according to claim 1, characterized in that an optical medium selected from the optical medium group (52) having different refractive indexes is selectively inserted into the propagation path of the mixed light. Device.