JPH11167133A - Photodetecting circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photodetecting circuit which has low loss and makes it easy to adjust the detection current of a balance type by using a multimode optical fiber only on the output side of an optical coupler. SOLUTION: The photodetecting circuit is equipped with an optical multiplexer 1 which multiplexes signal light and local oscillation light, multimode optical fibers 4 and 5, lenses 2 and 3 which couple the output light of the optical multiplexer with the multimode optical fibers, detectors 6 and 7 which detect the output light of the multimode optical fibers 4 and 5, and a differential amplifier 8.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical detection circuit, and more particularly to an optical homodyne detection circuit, an optical heterodyne detection circuit, and a detection circuit of an optical interferometer.

[0002]

2. Description of the Related Art Conventionally, in an optical homodyne detection circuit, an optical heterodyne detection circuit, and a detection circuit of an optical interferometer using a spatial light beam, an optical multiplexer is used to combine signal light and local light. If the spot size and the wavefront of each light beam are different on the multiplexing surface, the signal light and the local light do not interfere with each other, so that sufficient detection efficiency or visibility cannot be obtained.

[0003] If a single mode optical fiber is used at the output side of the optical multiplexer, the visibility of the interferometer can be set to 1, but a large loss occurs in the coupling between the spatial beam and the single mode optical fiber. Accordingly, the detection efficiency is reduced. Further, since the core system of the single mode optical fiber is 10 μm or less, precision is required for the adjustment of the coupling circuit, and it becomes unstable with time.

On the other hand, in the case of balanced detection in which two outputs of an optical multiplexer are detected and differentially combined, if the detection currents of the two optical detectors are made equal, the amplitude noise of the local light is suppressed. It can operate at the shot noise limit. Machida and Y. Yamamoto
o: “Quantum-Limited Operat
ion of Balanced Mixer Home
odyne and Heterodyne Rece
verses ", IEEE J. QuantumEle
ctron. 22, 5, 617-624 (198
6)].

[0005]

To this end, it is necessary to correct the difference between the branching ratio of the optical multiplexer and the quantum efficiency of the two optical detectors so that the detection currents of the two optical detectors are exactly equal. is there. Therefore, an optical directional coupler composed of a single mode optical fiber and having an adjustable coupling ratio is used, but the configuration of the optical directional coupler is complicated. At this time, as described above,
Coupling loss when coupling a spatial beam into a single mode optical fiber reduces detection efficiency. Further, when detecting an optical pulse having a very narrow pulse width, there is a disadvantage that the pulse characteristic of the optical pulse is deteriorated due to the dispersion characteristic of the optical fiber connected to the input side of the optical directional coupler. Furthermore, there is also a method of adjusting the branching ratio by using a polarizing beam splitter and a half-wave plate on the input side, but in this case, since the polarization direction of the signal light of the beam splitter output and the local light is orthogonal, the output is Although a polarizer is inserted on the side to make the polarization directions coincide, there is a disadvantage that the loss due to the polarizer increases by 3 dB and the detection efficiency decreases.

An optical detection circuit using a spatial beam is
In order to be affected by the background light, the background light needs to be shielded. The present invention provides an optical detection circuit that uses a multimode optical fiber only on the output side of an optical multiplexer and has a low loss and that can easily adjust a detection current in a balanced type in order to solve the above-described problem. The purpose is to do.

[0007]

In order to achieve the above object, the present invention provides: (1) an optical detector, an optical multiplexer for multiplexing signal light and local light, a multimode optical fiber, The multi-mode optical fiber includes means for coupling the output light of the optical multiplexer to the multi-mode optical fiber, and means for detecting the output light of the multi-mode optical fiber.

[2] In the optical detection circuit according to the above [1], the optical multiplexer is a half mirror, a non-polarization beam splitter or a polarization beam splitter. [3] In the optical detection circuit according to [1], means for coupling the output light of the optical multiplexer to the multimode optical fiber is a lens. [4] In the optical detection circuit according to the above [1], the means for detecting the output light of the multimode optical fiber is an optical detector and a differential amplifier.

With such a configuration, in the optical homodyne detection circuit, the optical heterodyne detection circuit, and the detection circuit of the optical interferometer, the multimode optical fiber is used for each of the signal light output from the optical multiplexer and the local light. By adjusting the coupling circuit to the optical multiplexer input beam, the optical multiplexer and the optical fiber so as to maximize the coupling efficiency to the optical coupler, the spot size and the wavefront of the signal light and the local oscillation light at the coupling surface of the optical multiplexer are adjusted. It is almost equal, and the detection efficiency or the visibility of the interferometer can be maximized.

At this time, the coupling loss to the multimode optical fiber is much smaller than the coupling loss to the single mode optical fiber, so that the loss of the detection circuit can be reduced. The change in the visibility of the interferometer due to the use of the multi-mode optical fiber is due to the difference between the wavefronts of the signal light and the local light. When a normal spatial beam is used, the deterioration is about 10%. Can improve the detection efficiency.

In the balanced detection circuit, the detection current of the two optical detectors can be easily equalized by adjusting the coupling circuit to the optical fiber. It is possible to operate at the noise limit. Further, since the multimode optical fiber is used only on the output side of the optical multiplexer converted into the amplitude information by the interference effect, there is no influence of the deterioration of the optical pulse characteristic due to the dispersion characteristic of the optical fiber. Further, since the core diameter of the multimode optical fiber is 50 μm or more, the adjustment of the coupling circuit is easy, and the multimode optical fiber is stable even with time.

Since a multimode optical fiber is used for the input section of the optical detector, the ratio of coupling of the background light to the multimode optical fiber is small even in the configuration of the optical detection circuit using a spatial beam, and the influence of the background light is reduced. The feature is that it can be removed.

[0013]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a configuration diagram of a balanced optical homodyne detection circuit using a spatial beam, an optical heterodyne detection circuit, and a detection circuit of an optical interferometer according to an embodiment of the present invention. As shown in this figure, A is a signal light input terminal, B is a local light input terminal, and is an optical multiplexer 1, lenses 2 and 3, multimode optical fibers 4 and 5, optical detectors 6 and 7, a differential amplifier. 8.

Here, the lenses 2 and 3 correspond to means for coupling to the multimode optical fibers 4 and 5. In addition, as the coupling circuit to the multi-mode optical fiber, an optical fiber with a processed end, an optical fiber with a lens, or the like can be used. The optical detectors 6 and 7 and the differential amplifier 8 correspond to means for detecting output light of an optical fiber.

The optical multiplexer 1 multiplexes a signal light component and a local light component, and a half mirror, a non-polarization beam splitter, or the like can be used. In FIG. 1, the signal light component of the output light of the optical multiplexer 1 is input to the optical multiplexer of the signal light A and the local oscillation light B using the optical multiplexer 1 and the optical fiber coupling lenses 2 and 3. And the local light components are adjusted so that the coupling efficiency to the multimode optical fibers 4 and 5 is maximized.

With this adjustment, the spot size and the wavefront at the optical fiber input end of the signal light A and the local light B become substantially equal, and as a result, the interference effect at the coupling surface of the optical coupler is optimized. The detection efficiency and the visibility of the interferometer are maximized, and high sensitivity can be achieved. Further, there is a method of using one optical detector as a detection circuit for the output light of the optical multiplexer 1, but as shown in this embodiment, two optical detectors 6 are used.
And 7 and the differential amplifier 8 combine to suppress the amplitude noise of the signal light and the local light and reduce the noise of the measurement system to the shot noise level, thereby improving the SN ratio and improving the sensitivity. Obviously.

For this purpose, it is necessary to equalize the detection currents of the two optical detectors. This can be easily performed by adjusting the coupling circuit of the optical fiber. There is no need to adjust the ratio. Also, since the multimode optical fiber is used only on the output side of the optical multiplexer and propagates the interference signal converted into the amplitude change, even if an optical pulse with a narrow pulse width is used, the dispersion of the optical fiber is reduced. Deterioration of pulse characteristics due to characteristics can be avoided. In addition, since the amount of the background light to be coupled to the optical fiber is very small, the background light is not affected even in a bright place.

It should be noted that the present invention is not limited to the above embodiment, and various modifications are possible based on the spirit of the present invention, and these are not excluded from the scope of the present invention.

[0019]

As described above, according to the present invention, the following effects can be obtained. (A) By adjusting the coupling efficiency to the multimode optical fiber to the maximum, it is possible to maximize the detection sensitivity in the optical homodyne detection circuit, the optical heterodyne detection circuit, and the detection circuit of the optical interferometer using the spatial light beam. .

(B) In the balanced detection circuit, the SN ratio at the shot noise limit can be achieved by adjusting the coupling efficiency with respect to the local light. (C) By using a multi-mode optical fiber only for the output of the optical multiplexer, it is possible to increase the sensitivity by reducing the coupling loss of the optical fiber, to avoid the deterioration of the pulse width due to the dispersion characteristics, and to remove the influence of the background light. And the configuration of the optical fiber coupling circuit is simple.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a balanced optical homodyne detection circuit, an optical heterodyne detection circuit, and a detection circuit of an optical interferometer using a spatial beam according to an embodiment of the present invention.

[Explanation of symbols]

 A signal light input terminal B local light input terminal 1 optical multiplexer 2,3 lens 4,5 multimode optical fiber 6,7 optical detector 8 differential amplifier

Claims (1)

[Claims] 1. An optical multiplexer for multiplexing signal light and local light, (b) a multimode optical fiber, and (c) an output light of the optical multiplexer is coupled to the multimode optical fiber. Means,
(D) means for detecting output light of the multimode optical fiber.