JPH02236477A - Light beam tracking device - Google Patents

Abstract

PURPOSE:To eliminate background light noise and to obtain an accurate tracking error signal having high S/N by respectively multiplying the outputs of respective photodetectors by the outputs of respective variable phase shifters and giving the multiplied result to a tracking error signal arithmetic operation circuit. CONSTITUTION:An addition circuit 12 adds the outputs of the respective photodetectors 2a - 2d. Next, a clock reproduction circuit 13 reproduces a clock signal based on the output of the addition circuit 12. Besides, the variable phase shifters 16a - 16d vary the phase of the clock signal from the clock reproduction circuit 13. Then, analog multiplication circuits 15a - 15d respectively multiply the outputs of the respective photodetectors 2a - 2d by the outputs of the respective variable phase shifters 16a - 16d and give the multiplied result to the tracking error signal arithmetic operation circuit 11 through low-pass filters 6a - 6d. Thus, the background light noise is eliminated and simultaneously the accurate tracking error signal having the high S/N is obtained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an optical system that detects a deviation in the direction of arrival of a light beam by obtaining a tracking error signal for tracking a light beam in, for example, an inter-satellite optical communication system. This relates to a beam tracking device.

[Conventional technology]

In inter-satellite optical communication systems, optical beam tracking is normally performed using the DC component during communication signal detection, but when strong background noise light such as the sun is received at the same time as the signal light, the DC component generated by the noise light is Since it becomes extremely large, it becomes impossible to obtain a highly accurate tracking error signal, which makes it impossible to track the light beam and also causes the communication line to be cut off. As a countermeasure against background noise caused by sunlight, a beacon method is used in which one satellite modulates a light beam with a sine wave instead of an information signal, and the other satellite extracts only the modulated signal from the received light. There is a light beam tracking method called .

In addition, there is a light beam tracking method described below that is capable of maintaining a communication line while performing two-way communication when there is background noise light due to sunlight.

Figure 2 shows, for example, the conventional optical system that was announced at the 1988 IEICE Spring National Conference by Mr. Kashiki and two others as "Proposal of an optical ISL (optical inter-satellite communication) beam tracking system that is resistant to background noise light." The block diagram of the signal processing circuit of a beam tracking device is shown. In the figure, l is the light spot, 2 is 4
Split photodetector (four-split photodetector), 2a, 2b, 2c
, 2d are each light receiving element constituting the 4-split photodetector 2,
3 are light receiving elements 2a, 2b. Amplifier 4 amplifies the outputs of amplifiers 2c and 2d, and 4 a first amplifier that removes the DC component of the output of amplifier 3.
5 is a square-law detection circuit for square-law detection of the output of the first DC-component removal circuit 4; 6 is a low-pass filter; and 7 is a square-law detection circuit for further removing the DC component of the output of the square-law detection circuit 5. 8 is a root-mean-square calculation circuit that averages the square of the output of the DC-component removal circuit 7; 9 is a 1/2 fIi that attenuates the output of the root-mean-square calculation circuit 8 by half; 10 is a differential amplifier, 11 is a tracking error signal calculation circuit (
tracking error signal calculation means). The differential amplifier 10 takes the difference between the output of the low-pass filter 6 and the output of the 1/2 attenuator 9 and amplifies it.

The tracking error signal calculation circuit 11 includes each light receiving element 2a, 2b,
The shift in the direction of arrival of the light beam is detected by performing calculations based on signals proportional to the intensity of the incident digital signal light incident on the light beams 2c and 2d.

Next, the operation will be explained. Although not shown in FIG. 2, the communication signal light and background noise light received by the receiving optical system are guided to the four-split photodetector 2, and form a light spot 1 on the detector surface. Since each of the light receiving elements 2a, 2b, 2c, and 2d constituting the 4-split photodetector 2 is electrically insulated, it is possible to output an electric signal according to the intensity of the incident signal light. Each of the light receiving elements 2a. 2b, 2c
. Communication signal is rectangular NRZ code ON
Assume /OFF signal. The electrical signal from the light receiving element 2a is amplified by the amplifier 3 and then sent to the first DC component removal circuit 4.
The DC component is removed. The communication signal from which the DC component has been removed and the signal containing noise are input to a square law detector 5. Here, when performing square law detection, only the DC component is obtained for the communication signal, and assuming random noise, the DC component and the continuous spectrum component are obtained for the noise component. Although it is necessary to use the DC component obtained by squaring the communication signal for tracking the light beam, the DC component of unnecessary noise superimposed on this is removed as follows.i The first DC component removal The signal component after passing through circuit 4 is S
, where the noise component is n, and the signal component and noise component at times t and t+τ are distinguished by subscript 1 and subscript 2, respectively.
The autocorrelation R(τ) of the output of the square law detection circuit 5 is expressed as in the first equation.

R (r)=E ((S++n+)”CSz+nz
)” )”E (S+”S.”) +2E (Sl”n
l”)+4E (S.S.nu n.) 10E” (n+”) +2E” (n+ nz
)・・・・・・・・・・・・ +11 Here, E[.] represents the average calculation. In the first equation,
The 1.2.4 terms correspond to the DC component, and the 3rd and 5th terms correspond to the continuous spectrum components. Also, the first term is the signal component, the second to fifth terms
The terms represent noise components.

Here, focusing on the noise component, the power of the DC component, ie, the second term and the fourth term, is 1/2 of the power of the continuous spectrum component, ie, the third term and the fifth term.

When the output of the square-law detection circuit 5 is divided into two parts and one is inputted to the low-pass filter 6, a signal containing terms 1.2.4 shown in the first equation is obtained as the output. In addition, the above 2
The other output of the multiplicative detection circuit 5 is connected to a second DC component removal circuit 7.
By inputting , a continuous spectral component including the third and fifth terms shown in the first equation is obtained. The output of the second DC component removal circuit 7 is converted into a root mean square calculation circuit 8 and a 1/2vA attenuator 9.
When inputted to , a signal of half the DC value of term 3.5 of the first equation is obtained as the output of the 1/2M attenuator 9. When the output of the low-pass filter 6 and the output of the 1/2 attenuator 9 are input to the differential amplifier 10, the first term of the first equation, that is, the DC value proportional to the signal component is obtained.

Similar signals are obtained with respect to the outputs from each of the light receiving elements 2b, 2C, and 2d. When the above four signals are input to the tracking error signal calculation circuit 11 and calculated, a tracking error signal from which background light noise has been removed is obtained.

[Problem to be solved by the invention]

Conventional optical beam tracking devices use square law detection when regenerating DC components from communication signals, perform autocorrelation calculations on the output of the square law detection, and perform differential subtraction to remove noise components. The use of amplifiers caused the following problems.

First, square law detection is used, but as the input signal becomes smaller, the detection efficiency decreases and the uncertainty increases in the output of square law detection. For example, in FIG. 2, the intensity of the light incident on the light receiving element 2c is lower than the intensity of the light incident on the other light receiving elements 2a, 2b, and 2d, so the communication signal and the background noise are the same or the background noise is higher than the communication signal. It may become larger. Even if such a signal is subjected to square law detection, accurate signal components cannot be obtained.

Second, autocorrelation calculation is performed, but if there is no correlation between the signal and noise and the noise is random, the autocorrelation output will be large for only the signal component, but if the noise is not random, the autocorrelation output will be large. Since the output is not a continuous spectrum but has a bright line spectrum component, the output of the root mean square calculation circuit 8 has the root mean square output of the bright line spectrum, and the output of the differential amplifier 10 It is not possible to obtain a signal with noise components removed. Finally, the differential amplifier 10 performs calculations to remove noise components, but there is a possibility that an offset may occur in the output due to amplifier drift, etc., making it impossible to obtain a highly accurate tracking error signal.

This invention has been made to solve the above-mentioned problems, and provides an optical beam tracking device that can remove background optical noise and obtain a sensitive tracking error signal with a high S/N ratio. The purpose is to

[Means for Solving the Problems] A light beam tracking device according to the present invention includes each light receiving element 2a,
Adding means (adding circuit 12) for adding the outputs of 2b, 2c, and 2d, clock reproducing means (clock reproducing circuit 13) for reproducing a clock signal from the output of this adding means, and A plurality of variable phase means for varying the phase (variable phase shifter 16a.16b.1
6c, 16d) and each light receiving element 2a, 2b, 2c, 2d
and the output of each variable phase means, and the results are sent to the tracking error signal calculation means (tracking error signal calculation circuit 11) via low-pass filters 6a, 6b, 6c, and 6d.
) (analog multiplication circuit 1 5 a.)
1 5 b, 1 5 c, 1 5 d).

[Effect]

The addition means (addition circuit 12) includes each light receiving element 2a2b, 2c.
．． The clock reproducing means (clock reproducing circuit 13) regenerates a clock signal from the output of the adding means. 1,000 stages of variable phase (variable phase shifters 16a, 16b,
16c, 16d) vary the phase of the clock signal from the clock reproducing means (clock reproducing circuit 13). Multiplication means (analog multiplication circuits 15a, 15b, 15c, 15d
) is the output of each light receiving element 2a, 2b, 2c, 2d and each variable phase means (variable phase shifter 16a, 16b, 1
6c, 16d) respectively, and the results are applied to the low-pass filters 6a, 6b, 6c, 6d.
The signal is supplied to the tracking error signal calculation means (tracking error signal calculation circuit 11) via the tracking error signal calculation circuit 11.

[Embodiments of the invention]

FIG. 1 is a block diagram showing the configuration of a light beam tracking device according to an embodiment of the present invention. In FIG. 1, components corresponding to those shown in FIG. 2 are given the same reference numerals,
The explanation will be omitted. In Figure 1, 3a, 3b, 3
c. 3d is a light-receiving element 2a constituting the 4-split photodetector 2;
An amplifier that amplifies each output of the light receiving elements 2b, 2c, and 2d; 12 is an adder circuit (adding means) that adds the outputs of the light receiving elements 2a, 2b, 2c, and 2d; l3 reproduces a clock signal from the output of the adder circuit l2. Clock regeneration circuit (kurofuku regeneration means), 14 is an identification and regeneration circuit that inputs the addition result of the adder circuit 12, identifies the incident digital signal light based on the clock signal from the clock regeneration circuit 13, and obtains a reproduced data signal, 16a.1 6 b. 1 6 c, 1 6
d is a variable phase shifter (variable phase means) that varies the phase of each clock signal from the clock regeneration circuit 13;

15a. 15b. 15c, 15d are amplifiers 3a, 3b,
3c, 3d (outputs of light receiving elements 2a, 2b.2c.2d) and variable phase shifters 16a. 16b. 16c. Analog multiplication circuits (multiplying means) that multiply the outputs of 16d and 6a, 6b, 6c, and 6d are analog multiplication circuits 15a.
, 15b, 15c, and 15d. The tracking error signal calculation circuit l1 in this embodiment includes an analog multiplication circuit 15 that passes through low-pass filters 6a, 6b, 6c, and 6d.
a. 15b. The multiplication results of 15c and 15d are input to obtain a tracking error signal indicating the shift in the direction of arrival of the light beam.

Next, the operation of this embodiment will be explained. The incident digital signal light guided to the 4-split photodetector 2 is transmitted to its light receiving element 2a.
．． A light spot 1 is formed on surfaces 2b, 2c, and 2d. Each of the light receiving elements 'la, 2b, 2C, and 2d outputs an electric signal corresponding to the intensity of the incident light, and inputs the electrical signal to the amplifiers 3a, 3b, 3c, and 3d connected thereto. The output of each amplifier is divided into two parts, one of which is input to the adder circuit 12,
The other one is an analog multiplication circuit 15a, 15 connected to each
b. tsc. Enter in 15d. The adder circuit 12 amplifies and adds the output signals of the amplifiers 3a, 3b, 3c, and 3d, and outputs the resultant signals. The output of the adder circuit 12 is divided into two parts, one of which is input to a clock regeneration circuit 13 and the other input to an identification regeneration circuit 14. Although a detailed explanation will be omitted here, the clock reproduction circuit 13 is formed of a known circuit including a phase-locked loop, and extracts and reproduces a clock signal from a communication signal. The data signal is regenerated by the identification reproducing circuit 14 using the clock signal as a reference.

Next, we will explain the process of obtaining the tracking error signal. Since the circuits connected to each of the amplifiers 3b, 3c, and 3d are the same as the circuits connected to the amplifier 3a, the following description will focus on the operation from the amplifier 3a to the tracking error calculation circuit 11. The electrical signal from the amplifier 3a is input to the analog multiplication circuit 15a, while the clock signal from the clock regeneration circuit 13 is input to the variable phase shifter 16.
After passing through a, the signal is input to the analog multiplication circuit 15a. Here, the electric signal of the amplifier 3a is converted to E. co
s( (Wc +Wa)t+'7+'s) +
El1cos(wa t + <i)a), and the output signal of the variable phase shifter 16a is cos(Wct + 4P
C) Then, the output E0 of the analog multiplication circuit 15a becomes as shown in the second equation. E, = (Es cos (( W, +W
# )t voicePea ) +Ea cos(W.t
+ y, ) ) cos(Wct +'l'c
)E. cos ((2 Wc +W*)t+%'
a +ψ− } ]+ cos ( (We +W
c) t+? a + ¥'c ) )"・(2) Here, E. is the amplitude of the signal component, Wc is the clock angular frequency, and W
, is the modulation signal angular frequency, ψ1 is the phase of the signal component, E. is the amplitude of the noise component, and W7 is the angular frequency of the noise component. is the phase of the noise component, and ψ0 is the phase of the reproduced clock signal.

The output E0 is passed through the low-pass filter 6a and W
．． If we take out only the DC component where = 0, the first of the second equation
Only the component of the term is demodulated, and the magnitude of the output signal EDC
Hato becomes. Here, if the phase amount ψ of the variable phase shifter 16a is changed in advance and adjusted so that ψゎ=ψ1, the magnitude of the output signal EflC will be the maximum value E. /2 can be obtained. The third term shown in the second equation is the pass band of the low-pass filter 6a - (cos (W
@ t ” 'f'm '%' c) ” However, if the angular frequency W1 of the noise component and the clock angular frequency Wc match, the probability is small and the phase ψ of the noise component is Since it is random, the signal after passing through the low-pass filter 6a in the third term of the second equation is sufficiently smaller than the signal component output ]Eoc. Therefore S
A DC signal proportional to the amount of light incident on the light receiving element 2a with a good /N ratio can be obtained. Each of the light receiving elements 2b, 2C,
A similar signal is obtained for the output from 2d. When the four signals based on the signals from each of the light receiving elements 2a, 2b, 2c, and 2d are input to the tracking error signal calculation circuit 1l and calculated, a tracking error signal with a good S/N ratio from which background light noise is removed can be obtained. It will be done.

In the above embodiment, the case where the present invention is used for inter-satellite optical signals has been described, but it goes without saying that the present invention may also be used for a facing-type spatial optical transmission device used in terrestrial communication. Further, in the above embodiment, the output from the amplifier 3 is sent to the analog multiplication circuit 15.
However, a bandpass filter having the clock frequency as its center frequency may be inserted between the amplifier 3 and the analog multiplication circuit 15.

〔Effect of the invention〕

As described above, according to the present invention, there is an adding means for adding the outputs of each light receiving element, a clock reproducing means for reproducing a clock signal from the output of the adding means, and a variable phase of the clock signal from the clock reproducing means. and multiplication means for multiplying the output of each light-receiving element by the output of each variable phase means and applying the result to the tracking error signal calculation means via a low-pass filter. Therefore, the clock signal regenerated from the received signal is used as the reference signal, and the influence of background noise light caused by regenerating only the same frequency component as the clock signal from the received signal of each light receiving element by synchronous detection is removed. A DC signal proportional to the intensity of light incident on the element can be obtained, and even if the output of each light-receiving element is small, or the signal output is smaller than the noise, the S/N ratio is high and only the signal component can be detected. Therefore, it is possible to obtain a highly accurate tracking error signal, and it is possible to obtain the effect that the deviation in the direction of arrival of the light beam can be detected with high accuracy.

[Brief explanation of drawings]

FIG. 1 is a block diagram of the configuration of an optical beam tracking device according to an embodiment of the present invention, and FIG. 2 is a block diagram of a conventional optical beam tracking device 2a, 2.
b, 2 c, 2 d. −． Light receiving element, 5a
, 6b. 6c, 6d...Low pass filter, 1
l... Tracking error signal calculation circuit (tracking error signal calculation means), 12... Addition circuit (addition means), 1
3...Clock regeneration circuit (clock regeneration means)
, 1.5a, 15b, 15c, 15d... Analog multiplication circuit (multiplication means), 16a, 16b, 16c.
16d...Variable phase shifter (variable phase means). Agent Masuo Oiwa (2 idiots)

Claims (1)

[Claims] A plurality of divided light detection means that has a plurality of light receiving elements corresponding to a plurality of quadrants and detects an incident signal light, and a plurality of divided light detection means that has a plurality of light receiving elements corresponding to a plurality of quadrants, and a plurality of divided light detection means that has a plurality of light receiving elements corresponding to a plurality of quadrants. A light beam tracking device is provided with a tracking error signal calculation means for performing calculations based on the signals obtained and detecting deviations in the direction of arrival of the light beam. A clock reproducing means for reproducing a clock signal from the output, a plurality of variable phase means for varying the phase of the clock signal from the clock reproducing means, and multiplying the output of each of the light receiving elements by the output of each of the variable phase means. and multiplication means for applying this result to the tracking error signal calculation means via a low-pass filter.