JPS59165536A - Optical heterodyne/homodyne detecting method - Google Patents

Abstract

PURPOSE:To improve the receiving sensitivity by controlling the intensity of the locally oscillated light so that the S/N of a demodulated output signal is set at the maximum level. CONSTITUTION:The output light 2 of the relative noise intensity K which is delivered from a local oscillator 1 receives the control of light intensity through an optical variable attenuator 3 and is turned into the locally oscillated light 4. This light 4 is multiplexed with the faint signal light 5 through an optical multiplexer 6. This multiplexed light 7 is detected by a light receiving part 9 having a photodetector with the equivalent input current noise set at i<2> and turned into a demodulated signal output 10. An output current 11 of the photodetector is virtually produced from the light 4. Therefore the current 11 is monitored by an ammeter 12, and the attenuator 3 is controlled so that the value of the current 11 satisfies I=(i<2>t/K)<1/2>.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to optical heterodyne and optical homodyne detection methods used in optical communication systems, original information processing systems, and the like.

In general, optical heterodyne and optical homodyne detection methods have the great advantage of increasing reception sensitivity by 10 to 100 times more than conventional optical direct detection methods.
It is expected to be an effective optical detection method for long-distance optical communication trunk systems and various high-sensitivity optical sensors.

This optical detection method achieves high reception sensitivity because it uses sufficiently large locally oscillated light, so that the influence of electrical noise in the optical receiver can be ignored compared to the shot noise generated in the photodetector. This is because detection can be performed with shot noise limited. If the local oscillation light is not large enough, the influence of electrical noise in the optical receiver will appear, causing 'O! The signal-to-noise ratio of the d signal output becomes poor.

Conventionally, it has been thought that when optical heterodyne/optical homodyne detection is performed, the higher the power of the locally oscillated light, the better. However, since locally oscillated light generally includes optical intensity noise, increasing the intensity of locally oscillated light does not necessarily improve the signal-to-noise ratio of the demodulated signal; The inventor has found that the signal-to-noise ratio is degraded. That is,
Conventional optical heterogain/optical homodyne detection methods
There was a drawback that the signal-to-noise ratio of the r4 signal output was not maximized.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an optical heterodyne/optical homodyne detection method that maximizes the signal-to-noise ratio of demodulated signal output under such circumstances.

The optical heterodyne/original homogain detection method of the present invention is an optical heterodyne method in which a signal light and a local oscillation light are combined, and the combined light is sent to a photodetector in a 16-inch front section to extract a demodulated signal output.・In the original homodyne detection method, the intensity of the locally oscillated light incident on the photodetector is determined by the intensity A from the optical receiver.
The point of zero is that the signal output is controlled so that the signal-to-noise ratio is maximized.

Next, the principle of the present invention will be simply explained using an example in which a photodiode is used as a photodetector. When performing optical heterodyne detection or original homodyne detection using a photodiode, the signal-to-noise ratio per unit frequency in the demodulated signal output is given by the following equation.

Here ■ s: Output current of the photodetector caused by the signal light ■L Output of the photodetector caused by the two locally oscillated lights e: Noise light intensity of the three locally oscillated lights added to the electron charge Relative noise intensity <i%> expressed as the square ratio of: Equivalent input terminal noise where the total noise generated in the optical receiver is expressed isometrically by the input current of the optical receiver. Here, differentiating equation (1) The optimal value of IL that maximizes the signal-to-noise ratio is determined by ■. , t, the following relational expression is obtained.

<1>=K” I opt
(2) Therefore, by adjusting the local oscillation light intensity using an optical attenuator or the like so as to satisfy the above relational expression, optical heterodyne/original homodyne detection can be performed where the signal-to-noise ratio is maximized.

Next, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is a block diagram for explaining a first embodiment of the present invention. First, the output light 2 having a relative noise intensity emitted from the local oscillator is controlled in optical intensity by a variable optical attenuator 3, and becomes locally oscillated light 4. Next, the local oscillation light 4 is multiplexed with the weak signal light 5 by the optical multiplexer 6. This combined light 7 is detected by an optical receiver 9 which has a photodetector and whose input signal tone is <+'t>, and a demodulated signal output 10 is obtained. At this time, since the output current 11 of the photodetector is mostly caused by the local oscillation light 4, this output current 11 is monitored by the current h112, and the optical variable attenuator 3 is adjusted so that the current value does not satisfy equation (3). In particular, the signal-to-noise ratio of the bale-like signal output 10 is maximized.

In this embodiment, the local oscillator 1 is He −N
e Laser was used. The relative noise intensity of the output light of this He-Ne laser is -1.50 dB/Hz "C"
there were.

As the variable attenuator 3, a glass plate having a variable transmittance was used. In addition, the optical multiplexer 6 has a transmittance of about 7 (1
, local oscillation light 4 is generated using Schiller 13 with a reflectance of about 30 meters.
, the two lights were combined at an angle of 45° with respect to the signal light 5. A high-speed photodiode 8 is used as a photodetector.
was used. The optical receiver 9 is composed of this high-speed photodiode 8, a preamplifier, a main amplifier, etc. The equivalent human power output of this optical receiver 9 was 10 pA4m.

6 - From this front input current noise and the relative noise intensity of the local oscillation light 4, calculate the optimum value of the output current 11 of the high speed photodiode 8 generated by the local oscillation light 4. ,.

According to equation (3), l. p t"" o, 32
mA is obtained. We actually measured the signal-to-noise ratio of the demodulated signal output 10 while adjusting the Motoshi variable attenuator 3 and varying the output current 11 of the high-speed photodiode 8 around 0.32 mA.
It was confirmed that the maximum signal-to-noise ratio can be obtained when is 0.32 mA. FIG. 2 shows how the signal-to-noise ratio of the demodulated signal output 10 changes with changes in the intensity of the local oscillation light 40, and a maximum value exists. Although the signal-to-noise ratio of the demodulated signal output 10 changes depending on the intensity of the local oscillation light 40, the high-speed photodiode 8 generated by the local oscillation light 4
If the output current 11 is within a range that satisfies equation (4), the signal-to-noise ratio of the ataxic signal output 10 can be set to a value within approximately 3 dB of its maximum value.

0.25 Iopt < IL < 4 Iopt
(4) FIG. 3 is a block diagram of a second embodiment of the present invention. In this embodiment, the local oscillator 1
The magnitude of the output light is adjusted by changing the magnitude of the driving M'ltt current 15 output from the drive circuit 14 of the local oscillator 1. Other parts are the same as in the first embodiment. In this embodiment, a semi-integrated laser was used as the local oscillator 1, and the magnitude of the local oscillation light 4 was adjusted by changing the drive current 15 of this semiconductor laser. At this time, the relative noise intensity of the local oscillation light 4 is the drive current 15
However, when the drive current value is 1.5 times or more than the oscillation threshold it6tff value, t'
i became constant 11 & -15 oaa/Hz. In this example as well, the relative noise intensity of the semiconductor laser is
Assuming 50 dB/H2, by adjusting the output type of high-speed photodiode 8 so that 11 satisfies the pre-occupation equation (3), optical heterodyne/optical homodyne detection is performed with the signal-to-noise ratio almost at its maximum. It was possible to realize this.

FIG. 4 is a block diagram for explaining a third embodiment of the present invention. In this example, an avalanche photodiode (APD) 16 was used as the photodetector. The output current 17 of this APD 16 is compared with a reference value by a controller 18 and a control signal 19 is output. The attenuation amount of the optical variable attenuator 3 is controlled by this control number 19, and the output current 17 of the APD 16 is always controlled.
is kept constant. The rest is the same as the first embodiment.

When using APD16 as a photodetector, APD16
By optimizing the multiplication factor and the intensity of the local oscillation light 40, it is possible to maximize the signal-to-noise ratio of the after-tone signal output 10. In this embodiment in which optical heterodyne detection is performed using the APD 16, the signal-to-noise ratio per unit frequency in the demodulated signal output 10 is expressed by the following equation.

However, M: APD16 multiplication factor I8. :M=1
The output current 17 of the APD 16 generated by the signal light 5 in the case of L1:A generated by the local oscillation light 4 when M=1
Output type of PD16 9- Current 17 x: Excess multiplication noise figure of APD16 where (
5) The following relational expression is obtained by finding the optimum value for IL+ by differentiating the expression.

Therefore, even when using APD16, demodulation 1 signal output 10
The output current of the APD 16, which maximizes the signal-to-noise ratio of
7 optimal i = I. , t are given by the same equation as in the case of the photodiode, as shown in equation (7).

Note that when using the APD16, it is desirable to further optimize M, and the optimum multiplication factor M. , given by the side equation.

In this example, the APD16 is St of x-02.
-APD was used.

From equations (7) and (8), in this case, the local excavation light 4
10- The optimum value of the output current 17 of the APD 16 caused by o
pt and the optimal APDO multiplication factor Mop, respectively, are I
o, t: 0.32 mA, Mop (= 6.7 is calculated. Output current 17 of APD 16 and APD 16
After adjusting the multiplication factor to these values, the demodulated signal output 1
A signal-to-noise ratio of 0 was measured, and it was confirmed that a signal-to-noise ratio that was approximately 2 dB higher than that of the first embodiment was achieved. In this embodiment as well, the output current 17 of the APD 16 is controlled within the range shown by equation (4), and
Demodulated signal output IO by increasing the multiplication factor of DI6
The signal-to-noise ratio can be brought to within approximately 3 dB of its maximum value.

In addition to the above-described embodiments, various modifications can be made to the present invention. For example, in the case of the second embodiment using a semiconductor laser, instead of assuming that the relative noise intensity is constant, the local oscillation light intensity dependence is also considered to find the conditions that maximize the signal-to-noise ratio of the demodulated signal output. The magnitude of the locally oscillated light may be adjusted accordingly. In addition, when using a light source such as a semiconductor laser whose relative noise intensity depends on the magnitude of its output as a local oscillator, the magnitude of the local oscillator output is adjusted to minimize the relative noise strength, and the demodulated signal output Adjustment of the local oscillation light intensity to optimize the signal-to-noise ratio may be performed at the source of the attenuator.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a first embodiment of the present invention. FIG. 2 is a diagram showing that the signal-to-noise ratio of the demodulated signal output changes with changes in the local oscillation light intensity, and that there is a maximum value, which is a basic characteristic that is the background of the present invention. FIG. 3 is a block diagram of a second embodiment of the invention, and FIG. 4 is a block diagram of a third embodiment of the invention. In the figure, I...Local oscillator, 2... Output light, 3
... Optical variable attenuator, 4... Local oscillation light, 5... Signal light, 6... Optical multiplexer, 7... Multiplexing jt, 8...
High-speed photodiode, 9... Optical receiver, 10...
Demodulated signal output, 11.17... Output current, 12...
Ammeter, 13... Mirror, 14... Drive circuit, 15
- Drive current, 16... Avalanche photodiode (APD), 18... Controller, 19...
It is a control signal. Substitute r8i 1 person burial inside M station 13- octopus/n 3rd mouth

Claims (1)

[Claims] (1) An optical heterodyne system that combines signal light and local oscillation light, receives this combined light in an optical receiver, and extracts a demodulated signal output from the optical receiver. In the homodyne detection method, an optical heterodyne is characterized in that the intensity of the locally oscillated light incident on the optical receiver is controlled so that the signal-to-noise ratio of the demodulated signal output from the optical receiver is maximized.・
Original homodyne detection method. (2) The output current value of the photodetector caused by the local oscillation light ■1 is 0.25 Io, <I, <4 I. ,. (However, <ij>: equivalent input current noise of the optical receiver, K
2. The optical heterogain/optical homodyne detection method according to claim 1, characterized in that the local oscillation light intensity is controlled so that: : relative noise intensity of local oscillation light).