JPS58149025A - Optical heterodyne-homodyne detector - Google Patents

Abstract

PURPOSE:To make the branching loss of a coupling circuit substantially small with respect to signal light and to make the higher outputs of local oscillating light sources less necessary by incorporating the 1st, the 2nd photodetecting parts which detect the 1st, the 2nd coupled beams of light obtained from the 1st, the 2nd exit ends to convert said beams to electrical signals, respectively and a coupling part which couples the electrical signals from the 1st, the 2nd photodetecting parts. CONSTITUTION:When the beam diameters and optical paths of signal light 4 and local oscillation light 9 are adjusted, the 1st signal light 7 and the 1st local oscillation light 13, the 2nd signal light 8 and the 2nd local oscillation light 14 are coupled to form the 1st, the 2nd coupled beams 23, 24 of light. The 1st, the 2nd beams 23, 24 are made incident to the 1st, the 2nd photodetecting parts 17, 18 respectively past the 1st, the 2nd exit ends 15, 16. The 1st, the 2nd parts 17, 18 are constituted of photodiodes, amplifiers, etc., and output the 1st, the 2nd intermediate frequency outputs 19, 20 of a frequency f0 by heterodyne detection from the respective coupled beams of light in the form of electrical signals. The 1st, the 2nd intermediate frequency outputs 19, 20 are electrically coupled in a coupling part 21 to an intermediate frequency output 22.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical multiplexing and detection device in an optical heterodyne detection or optical homodyne detection optical communication system.

Optical heterodyne in optical communication, especially optical fiber communication,
The optical homodyne detection method is attracting attention as a method that enables long repeat interval transmission because of its high optical reception sensitivity. One of the problems with the receiving section of this system is that branching loss occurs in the multiplexing circuit that combines the signal light and the local oscillation light. For example, when a half mirror is used as this multiplexing circuit to combine both parties, the half mirror divides the light beam into two, so the light beam is at least 3 dB (50 dB) for both parties.
%), and these branching losses directly lead to S/NO deterioration, that is, deterioration of optical reception sensitivity.

Conventionally, the solution to this problem was to increase the transmittance of Mi2- to reduce the branching loss for the signal light, and at the same time compensate for the increased branching loss for the locally oscillated light by increasing the output of the locally oscillated light source. has been considered. However, in this case, increasing the output of the local oscillation light source has the disadvantage that it increases the size of the device and reduces the reliability of the light source. Especially when using a semiconductor laser as a local oscillation light source, 20 mW
Although the above output is required, it is not easy to guarantee reliability with this output in industry.

The purpose of the present invention is to eliminate such drawbacks, effectively reduce branching loss for signal light in a multiplexing circuit, and
An object of the present invention is to provide an optical heterodyne/homodyne detection device in which a high output of a local oscillation light source is not a must.

According to this invention, the invention has first and second input ends and first and second output ends, and the incident light from the first and second input ends is transmitted to the first and second input ends, respectively. A directional coupler that is branched at a ratio of approximately 1:1 to the output end, and a signal light is input from the first input end, and a local oscillation light is input from the second input end. 1g1 and a second light receiving section that receive the combined light of l/Ii1 and Al2 obtained from the output ends of the Ml and jlE2, respectively, and convert them into electrical signals; and the electricity from the first and second light receiving sections. An optical heterodyne/homodyne detection device including a combining unit that combines signals is an advantage.The optical heterodyne/homodyne detection device of the invention detects the signal light and local oscillation light that are multiplexed by a directional coupler (multiplexing circuit). Optical components that were conventionally treated as losses can also be combined.
It receives light and synthesizes the resulting electrical signal in addition to the electrical signal obtained from the light component, which has conventionally been considered an effective component. In this case, if both electrical signals are in phase, the voltage additive law holds true for the signal component, and the power additive law holds true for the noise generated in the light receiving section, so 87N improves by up to 3 dB compared to the case where they are not combined. be done. Therefore, it is possible to effectively reduce the amount t&loss that the signal light receives in the multiplexing circuit to zero, and to obtain an optical heterodyne-homodyne detection device that does not require high output of the local oscillation light source. Next, the present invention will be explained in detail using the drawings.

Figure Al1 is a configuration diagram for explaining the @1 embodiment of the present invention. In this embodiment, the directional coupler 1 is composed of a half mirror 2. The signal light 4 emitted from the transmission line 3 is converted into a substantially parallel tapered beam by the first lens 5, and enters the 71-fumirror 2 from the first input end 6, and is reflected by xi, Jli
2O (divided into two parts of I light 7.8. On the other hand, signal light 4 is f
Local oscillation light 9ti having a frequency different by o is emitted from a local oscillation light source 10 made of a semiconductor laser, and is converted into substantially parallel light by a JII2 lens 11. The light then enters the half mirror 2 from the input end 12 of the II20 and is divided into two parts, the first and second local oscillation lights 13 and 14. By setting the beam diameters and optical paths of the signal light 4 and the local oscillation light 9 to p4!1, the first
The signal light 7 and the first local oscillation light xa, the signal light 8 of the irises 2 and the local oscillation light 14 of Al2 are combined to form the first combined light 23 and 24 of Al2. 11J1, Second Goyouko 23.
24 passes through the first and second output ends 15 and 16 and enters the JIll and second light receiving sections 17 and 18, respectively. 1st.
Second light receiving part 17.18 #'i7 Otodiode medium increase m
The first frequency f0 is detected by heterodyne detection from the combined beams of each party. The intermediate frequency output 19.20 of I12 is output in the form of an electrical signal. First,
The second intermediate frequency outputs 19 and 20 are matched in phase and electrically combined in a combining section 21 to become an intermediate frequency output 22.

Incidentally, the phase adjustment was carried out by changing the length of the conducting wire connecting the building 1, the second light receiving section 17, 18, and the combining section 21.

Next, the effects of this embodiment will be explained. First, in the first embodiment, the optical power of the signal light 4 is assumed to be Ps.

The first step required to perform effective heterodyne detection is
, it is assumed that the original power of the second local oscillation light 13 and 14 is PL, and therefore the original power of the local oscillation light 90 is 2Pt. P
t is approximately 5 mW in the case of optical fiber communication with a transmission speed of several hundred Mb/s. Note that the directional coupler 1. 1st, 2nd
In the lenses 5, 11, etc., excess loss caused by absorption, scattering, etc. is small and therefore ignored. Also, assuming a multiplexing circuit with no branching loss for the signal light, and when the PsO signal light and the PL local oscillation light are incident on the light receiving section, the S/N of the intermediate frequency output becomes a. do. However, in the 10th embodiment, the first and second received light ml? , 18 due to branch loss! -The first and second signal lights 7.8 which have become Ps,
Px, $1. Since the second local oscillation light 1-3° 14 is respectively combined and input, the S/N of the intermediate frequency output 19.20 of the first and second cells is a/2. However, in the combining section 21, the 1% second intermediate frequency output is 19.20
When the signals are combined in phase, the principle of voltage addition is applied to the signal component, and the principle of power addition is applied to the noise component, so 8/N is improved by two times, resulting in a rough result.

In the end, just by increasing the optical power of the local oscillation light 5 to twice the required amount PL, 8/N, which is the same as when the branching loss of the multiplexing circuit for the signal light 4 is zero, can be obtained.

In the conventional example, when using a half mirror, the S/N is a/Z.
I could only do it. Also, let us compare the transmittance of a mirror with FiS.
When trying to set the S/N to 0.8a at 0%, the branching loss of the mirror for the local oscillation light becomes large, so in order to obtain the local oscillation light input to the light receiving section by Pt, 5・PL is required.
O local local source light source output required - 511PL is approximately 2
This corresponds to 5rnW, and it is not easy to stably obtain this value with current semiconductor lasers. On the other hand, in the present invention, the S/N can be set to 1, and the output of the local oscillation light source can be 2@PL, about 10 mW, resulting in a significant improvement.

FIG. 2 is a configuration diagram for explaining a second embodiment of the present invention. In the second embodiment, the directional coupler 1
The feature is that the first and second light receiving sections 17, 18, and the combining section 21 are made of electric ICs that are made of carbon.

The directional coupler l includes a glass substrate 30 and the glass substrate 3.
GL made by diffusing titanium on 0, x2 4ff road 3
1.32. Signal light 4 (not shown) from the transmission line 3 is directly coupled to the first waveguide 31 from the 1g10 input end 6. The combined signal 4 is branched to a second waveguide with an optical power of approximately 1 at the proximal portion 3'3, becomes a second signal light 8 (not shown), and heads toward the second output end 16. The optical power of 1/2 is directed to the first output end 15 as a first signal light 7 (not shown). - Local oscillation light 9 from a local oscillation light source 10 such as a male conductor laser (not shown) ) is transmitted through the optical fiber 34 and coupled from the second input end 12 to the second waveguide 32.The locally oscillated light 9 is transmitted to the first and second waveguides in the proximal portion 33 as in the case of the signal light 4. Local oscillation light 13.1
4 (not shown). The first local oscillation light 13 is transmitted in the direction from the first waveguide 31tl to the first output end 15, and is combined with the signal light 7 of jll. Similarly, the second local oscillation light 14 also passes through the second waveguide 32 to the second output end 1.
While being transmitted in the direction of signal light 6 , it is combined with second signal light 8 . 1st. The second light receiving section 17.18 and the composite s21 are hybrid ICs 35, and the light receiving portions 17.18 of #41 and Tsuru 2 are connected to the first and second output ends 15.16, respectively.
It is close to. As in the case of the I@1 embodiment, the intermediate frequency output 22! from the synthesis section 21! is obtained.

In addition, transmission lol! 3 and the optical fiber 34 are preferably linear polarization preserving original fibers.

The effects of the irises 2 embodiment are also similar to those of the first embodiment.

In addition to the embodiments described above, various modifications can be made to the present invention. As the directional coupler 1, a half mirror 2 is used.
In addition to those using waveguides, various types of two-manufactured, two-output, four-terminal directional couplers can be used. First and second output ends 15 and 16 of directional coupler 1
and the eleventh and second light receiving sections 17 and 18 may be coupled by an optical fiber or the like. In addition, the first and second light receiving sections 17 and 18
An appropriate delay line of an ultrasonic delay line ring may be inserted between the 11th and 2nd intermediate frequency outputs 19 and 21 to perform phase matching of the eleventh and second intermediate frequency outputs 19 and 20. Naturally, a laser light source other than a semiconductor laser may be used as the local oscillation light source 10. The branching ratio of the directional coupler 1 is basically preferably 1:1, but it may deviate somewhat from 1:1. Further, the synthesis may be performed in the baseband band after detecting the eleventh and second intermediate frequency outputs 19.20. Note that the present invention uses local oscillation light 9
It can be similarly applied to a homodyne detection method in which the frequency of the signal light 4 is made equal to the frequency of the signal light 4.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a first embodiment of the present invention, and FIG. 2 is a block diagram showing a second embodiment. In the figure, 1 directional coupler, 6, 12...
First, input end of directional coupler 1 @2, 15°16
- First and second output ends of directional coupler l, 4 signal light, 9... local oscillation light, 23, 24... river drop 1,
Second combined light, 17, 18 - Light receiving section of 11th building 2,
21...Synthesis section. 140- Figure Z

Claims (1)

[Claims] It has a straw 1 and a second input end and a first and irises 2 output end, and the incident light from the first and the input ends of the rattan 2 reaches the ml, respectively.
, 112, and a directional coupler that branches and outputs the signal light at a ratio of approximately 1:1 to the output end of the first input end.
By inputting the local oscillation light from the second input end, the first light obtained from the first and jlI2 output ends is
, an optical heterodyne system including first and second light receiving sections that respectively receive the second combined light and converting them into electrical signals, and a combining section that synthesizes the electrical signals from the first and @2 light receiving sections.
Homodyne detection device.