JPH0392732A - Optical-beam tracking apparatus - Google Patents

Abstract

PURPOSE:To obtain a DC signal which is proportional to the intensity of incident light for each photodetector, to simplify the circuit constitution and to obtain a highly accurate error signal by regenerating only the same frequency component as a reference signal by a synchronous detection based on the received signal of each photodetector. CONSTITUTION:Received light which is guided into a four-division photodetector 2 forms an optical spot 1 on a detecting surface. An optical spot 23 of emitted light 22 from a laser generating device 19 is superimposed on the spot 1. The modulated signals are outputted from photodetetors 2a-2d by the interference of the spots and inputted into an adding circuit 12 and narrow BPFs 15a-15d through amplifiers 3a-3d. The added output signal is mixed with a reference signal from an oscillator 17 in a heterodyne detecting circuit 15. The output signal is divided into two signals. The signals are inputted into a clock regenerating circuit 18 and an identifying and regenerating circuit 14. Thus the clock signal is taken out and regenerated, and the data signal is regenerated.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a tracking device that generates a tracking error signal for tracking a light beam in, for example, an inter-satellite optical communication system.

[Conventional technology] In inter-satellite optical communication systems, optical beams are normally tracked using direct current control when detecting communication signals. If the light beams are received at the same time, noise light will be generated and the DC component will become extremely large, making it impossible to obtain a highly accurate tracking error signal, which will cause the problem of not only being unable to track the light beam but also cutting off the communication line. A point occurs. 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 method called

There is a tracking method described below that can maintain a communication line while performing two-way communication when there is background noise light due to sunlight.

Figure 2 shows, for example, Kashiki et al.
This figure shows a block diagram of the signal processing circuit of a conventional optical beam tracking device, which was announced at the Spring National Conference of the Institute of Electronics, Information and Communication Engineers in 1988 as a proposal for an L-beam tracking system. 2 is a 4-split photodetector for detecting the incident digital signal light, 2a, 2b,
2c and 2d are photodetectors that replace the 4-split photodetector 2; 3 is an amplifier that amplifies the output signal of the 4-split photodetector 2; and 4 is for removing the DC component in the output signal of the amplifier 3. 1st
5 is a square-law detection circuit for square-law detection of the output signal of the circuit 4, 6 is a low-pass filter, and 7 is a 2-square-law detection circuit.
a second DC signal removal circuit that removes the DC signal in the output signal of the multiplicative detection circuit 5, 8 a root-mean-square calculation circuit that averages the square of the output signal of the circuit 7, 9 a two-attenuator, 0 is a differential amplifier that takes and amplifies the output signal difference between the low-pass filter 6 and the % attenuator 9, and 11 is a tracking error signal calculation circuit that calculates a tracking error signal from the output signal of the differential amplifier 10. .

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 4-split photodetector 2, and an optical sensor 1-1 is formed on the detector surface. do. Since each of the photodetectors 2a, 2b2c, and 2d making up the four-split photodetector 2 is electrically insulated, it is possible to output an electrical signal according to the intensity of incident light. Each of the above photodetectors 2a, 2b, 2c, 2
The circuit connected to the photodetector 2a is the same as that connected to the photodetector 2a, so the following explanation will be based on the photodetector 2a.
The circuit structure from a to the tracking error signal calculation circuit 11 will be described. The communication signal is a rectangular NRZ code O N/O F
Assume F signal. The electrical signal from the photodetector 2a is
After being amplified by the converter 3, the signal is input to the first DC component removal circuit 4, where 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 for the noise component, assuming random noise, the DC power component and the continuous spectral component are obtained. Although it is necessary to use the DC component obtained by squaring the communication signal to track the light beam, the DC component of unnecessary noise superimposed on this is removed as shown below.

The signal strength after passing through the first DC component removal circuit 4 is S
, the noise intensity 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 signal of the square-law detection circuit 5 is
It is expressed as equation 1.

R(τ) 1 E C(S1+n+)'
(Sz+nz) 21 H (S l2 Sz'
) + 2E (51' n I t )
+4E(S+Sz Lnz) +IE2(nI2)
+2I! 2(r++nz) ・
...(11 Here, E[.] represents the average calculation.The above (
1) In formula 1. 2. 4th term is DC command,
Section 3.5 corresponds to the continuous spectrum distribution. Further, the first term represents signal discrimination, and the second to fifth terms represent noise discrimination.

Here, focusing on the noise predetermined power, the power of the DC predetermined signal, ie, the second term and the fourth term, becomes the power of the continuous spectrum component, ie, the third term and the fifth term A.

When the output signal of the square-law detection circuit 5 is divided into two and one of the output signals is inputted to the low-pass filter 6, terms 1 to 5 2.4 shown in equation (1) are obtained as the output signal. A signal containing On the other hand, when the other output signal of the square law detection circuit 5 is input to the second DC component removal circuit 7, a continuous spectrum component including the third term and the fifth term shown in equation (1) is obtained. The output signal of the second DC component removal circuit 7 is sent to an A attenuator 9 via a root mean square calculation circuit 8.
(1) as the output signal of the former queen A annihilator 9
A signal that is half the DC value in term 3.5 of the equation is obtained. The output signal of the low-pass J overfilter 6 and the 'A'$i attenuator 9
When inputting the output signal to the differential amplifier 10, the (1st
) The first term of the equation, that is, the DC value proportional to the signal amplitude is obtained.

Similar signals are obtained for the output signals from each of the photodetectors 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 the DC signal from the communication signal, and perform autocorrelation calculations on the square law detection output. Performing subtraction using a differential amplifier caused the following problems.

That is, first, square law detection is performed, but as the input signal becomes smaller, the detection efficiency of the output signal of the square law detection decreases and the uncertainty increases. For example, in FIG. 2, the intensity of light incident on the photodetector 20 is different from that of the other photodetectors 2a and 2b.
, 2d, the communication signal and background noise may be the same or may be larger. Even if such a signal is subjected to square law detection, accurate signal components cannot be obtained.

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

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

[Means to solve the problem]

The light beam tracking device according to the present invention includes a plurality of divided photodetectors 2a22b, 2c, 2 for detecting incident digital signal light.
d, and each of the photodetectors 2a. 2b2c. 2d, and each of the photodetectors 2a.
, 2b, 2c, and 2d.
5a, 15b 15C, 15d, and each of the band pass filters 15a, 15b. Each detection circuit 1.6a detects a predetermined frequency band component extracted by 15c, 15d using a reference signal. 16b. 16c, 16d, and each low-pass filter 6a, 6b, which passes the low frequency component in the output signal of each of the detection circuits 16a, 16b, 16c, L6d.
6c, 6d, and each of the above-mentioned low-pass filters 5a, 6b, 6
Tracking error signal calculation circuit I that calculates a tracking error signal for tracking the light beam based on the output signals of c and 5d.
II.

[Effect]

Plurally divided photodetectors 2a, 2b. 2c and 2d detect incident digital signal light. The laser generator 19 irradiates each photodetector 2a, 2b, 2c, and 2d with a laser beam. Each bandpass filter 15a15b, 15c, 15d extracts a predetermined frequency band signal from the modulated signal output from each photodetector 2a, 2b, 2c, 2d. Each detection circuit L6a
, 16b, 1.6c. 16d detects the predetermined frequency band signal using a reference signal. Each low pass filter 9 filter 6a, 6b, 6c, 6d is each detection circuit 16a16b
, 16c. The low frequency components in the output signal of 16d are passed. A tracking error signal calculation circuit 111 calculates a tracking error signal based on the output signals of each of the low-pass filters 6a, 6t+, 6c, and 6d.

[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, the same reference numerals are given to the components corresponding to the structural precepts shown in Fig. 2.
The explanation will be omitted. In FIG. 1, 3a, 3b. 3
12 are amplifiers 3a, 3b,
13 is an adder circuit which adds the output signals of 3c3d; 13 is an adder circuit; 14 is a heterolog detection circuit which performs heterolog detection on the output signal of 2 based on the reference signal from the reference oscillator I7 and reproduces the baseband signal; An identification and regeneration circuit 18 regenerates a data signal from the output signal of the detection circuit I3, and a clock regeneration circuit 18 regenerates a clock signal from the output signal of the heterolog detection circuit 13. 15a, 15b, 1
5c, 15d are narrow band pass filters 16a which extract the power of each predetermined frequency band from the output signals of the amplifiers 3a, 3b, 3c, 3d (modulation signals output from each photodetector 2a, 2b, 2c, 2d); , 16b, 16c, and 16d are detection circuits for detecting each of the predetermined frequency band components based on the reference signal from the reference oscillator 17, and 6a, 6b, and 6c6d are detection circuits for detecting the respective predetermined frequency band components based on the reference signal from the reference oscillator 17. Low-pass filter that passes low frequency components, 111
is a tracking error signal calculation circuit that calculates a tracking error signal for tracking a light beam based on the output signals of the low-pass filters 6a, 6b, 6c, and 6d. 19 is a laser generator that generates laser light to irradiate the photodetectors 2a, 2b, 2c, and 2d; 20 is a laser drive port that drives and controls the laser generator I9; and 21 is a laser beam that is generated from the laser generator 19. Frequency 11i! This is a frequency control circuit that performs I control. 22 is the emitted light from the laser generator 19, and 23 is the spot of the emitted light 22 on the 4-split light detection 1ifZ.

Next, the operation will be explained. The received light guided to the four-split photodetector 2 forms a light beam 1 on the detector surface. An optical subsystem 23 of the output light 22 from the laser generator 19 is superimposed on this optical subsystem 1. Here, I
111 If the oscillation frequency fL of the laser generator 19 is set to a value close to the frequency f1 of the received light, the optical subsoto 1
The frequency f,4=lf due to the interference between the
A modulated signal having a carrier frequency of +-fc is output from each photodetector 2a, 2b, 2c, and 2d. Each detector'la. 2b, 2c, and 2d output electric signals corresponding to the intensity of the phallus light, and input them to amplifiers 3a, 3b, 3c, and 3d connected to the respective ones.

The output of each amplifier 3a, 3b, 3c, 3d is divided into two parts, one of which is input to the adder circuit 12, and the other is connected to each narrow band pass filter 15a, 15b with a center frequency of fM.
1.5c. 15d.

In the adder circuit 12, the amplifiers 3a. 3b. The output signals of 3c3d are amplified and added and output. Said addition circuit I2
The output signal is mixed with the reference signal of the oscillation frequency f8 from the heterolog detection circuit 17 to perform heterolog detection, thereby outputting a communication signal of the Haas band of the received signal. This output signal is divided into two parts, one of which is input to the clock regeneration circuit 18 and the other input to the identification and regeneration circuit 14. Although a detailed explanation will be omitted here, the cross-resonance reproducing circuit 18 is implemented as a known circuit including a phase loop, and extracts and regenerates a cross-reference signal from a communication signal. An identification reproducing circuit 1 based on the clock signal as a reference.
4, the data signal is regenerated.

Next, the process of obtaining the tracking error signal will be explained. The circuits connected to each of the amplifiers 3b, 3c, and 3d6 are the same as those connected to the amplifier 3a, so the following explanation will be from the amplifier 3a to the tracking error signal calculation circuit 11.
The operation leading up to 1 will be explained. The electrical signal from the amplifier 3a is centered] 3. Frequency f. 4, the frequency f. Components near are extracted. This signal is input to the detection circuit 16a. On the other hand, the signal from the reference oscillator l7 is also input to the detection circuit Q1i.

Here, the electrical signal of the narrow band pass filter 5a is
, cos ((ωs+ωm)t+ψ,}+E,,
COS (ω,,1+ψ,,), and the reference oscillator 17
If the output signal of is cos (ω t + ψ 8), then the output E of the first detection circuit 16a. becomes like No. (2) 2.

EO- or [cos (ω, 14ψ, ψM)”cos
((2ωi+ωm)t+ψ1l+ψi})
os((ω7-ωs)t+ψ.-ψ,}"-1 cos((ω,+ωM)1+ψl,4 ψM)
) −{2) Here, E. is the amplitude of the signal component, ω. 4
−2π′8, ω. is the narrow band pass filter 5a as i
Modulation signal angular frequency after JIl, ψ. is the phase of the signal component, En is the amplitude of the noise component, ω7 is the angular frequency of the noise component,
ψ. is the phase of the noise component, ψ8 is the phase of the reference signal, and 14? Ru.

The output E0 is passed through the low-pass filter 6a and ω
■If only the DC component that is 10 is extracted, only the first term of equation (2) is demodulated, and the magnitude of the output signal E
p c is BDC−ke cos (ψ7−ψM) −(3
It becomes 1. Here, if the phase of the preliminary reference oscillator is adjusted so that ψ1=ψ8, the magnitude of the output signal will be E. is the maximum value E. /2 can be obtained. The third term shown in equation (2) may fall within the passband of the low-pass filter 6a, but the angular frequency ω of the noise component
The probability that 7 and frequency ω9 match is small 《and the phase ψ of the noise component. Since EDC is random, the signal after passing through the low-pass i111 filter 6a in the third term of Equation (2) is sufficiently smaller than the signal power output EDC. Therefore S/
Due to the N ratio, a DC signal proportional to the amount of light incident on the photodetector 2a can be obtained from the low-pass filter 6a. Similar signals are obtained with respect to the outputs from each of the photodetectors 2b, 2c, and 2d. Tracking error signal calculation circuit for the above 4 signals! ．． 1. 1 and performs calculations, a tracking error signal with a good S/N ratio from which background light noise has been removed can be obtained.

This tracking error signal is given to an antenna drive device (not shown) and is used for controlling the antenna position.

From the tracking error signal calculation circuit 111 to the laser generator 1
The feedback circuit leading to 9 is a laser generator 19
The difference between the oscillation frequency of the signal and the frequency of the received signal lr+. This is to keep fcl always at a constant value rH, and the frequency control circuit 21 controls the oscillation frequency of the laser generator 19 so that the sum of the output signals of the low-pass filters 6a, 6b, 6c, and 6d is maximized. do.

Although the above embodiment has been described for use in inter-satellite optical communication, it goes without saying that the present invention may also be used in a two-way spatial optical transmission device used in ground-to-ground communication.

As described above, according to the above embodiment, by using the signal from the reference oscillator as a reference and reproducing only the same frequency component as the reference signal from the received signal of each photodetector by optical heterogeneous detection by synchronous detection, Since it is possible to obtain a DC signal proportional to the incident light intensity of each photodetector, the influence of background noise light has been removed, the circuit structure is simplified, and
This has the effect of obtaining a highly accurate tracking error signal.

〔Effect of the invention〕

As described above, according to the present invention, there is a plurality of divided photodetectors that detect incident digital signal light, a laser generator that irradiates laser light to each of the photodetectors, and a laser beam output from each of the photodetectors. Each bandpass filter extracts a predetermined frequency band component from the modulated signal, each detection circuit detects the predetermined frequency band component extracted by each of the bandpass filters based on a reference signal, and the output of each of the above detection circuits. Each low-pass filter that passes the low-frequency component in the signal, and a tracking error signal calculation circuit that calculates a tracking error signal for tracking the light beam based on the output signal of each of the above-mentioned low-pass filters. For example, the intermediate frequency component of heterodyne detection in optical coherent communication can be extracted by the bandpass filter 17, and its noise can be removed, and the detection circuit can detect the output signal of each photodetector. Even if the level of the output signal is smaller than the level of the background noise, the DC signal proportional to the intensity of the incident light of each photodetector from which the influence of the background noise light has been removed is passed through the low-pass filter. As a result, a highly accurate tracking error signal with a high S/N ratio can be obtained, and therefore the reliability of the optical beam tracking process is improved. The effect of doing so can be obtained.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the structure of a light beam tracking device according to an embodiment of the present invention, and FIG. 2 is a block diagram showing the structure of a conventional light beam tracking device. 2a, 2b, 2c, 2d--・Photodetector, 5a, 6b,
6c. 6d...Low pass filter, 15a, 15b.
15c. 15d---Narrow band pass filter (band pass filter), 16a, 1 6 b. 1 6 c,
I6d...detection circuit, I9...laser generator, 111...tracking error signal calculation circuit.

Claims (1)

[Claims] a multi-segmented photodetector that detects incident digital signal light;
A laser generator that irradiates laser light onto each of the photodetectors, a bandpass filter that extracts a predetermined frequency band component from the modulated signal output from each of the photodetectors, and extraction by each of the bandpass filters. each detection circuit that detects the predetermined frequency band component based on the reference signal, each low-pass filter that passes the low frequency component in the output signal of each of the above-mentioned detection circuits, and the output of each of the above-mentioned low-pass filters. A light beam tracking device comprising: a tracking error signal calculation circuit that calculates a tracking error signal for tracking a light beam based on the signal.