JPH0622350B2 - Optical signal detection method and device - Google Patents

Description

The present invention relates to an optical space communication method and device, and more particularly to an optical signal detection method and device for detecting an optical signal, and the optical signal detection method and device. The present invention relates to a used light beam tracking method and apparatus.

[Prior Art] FIG. 5 is a block diagram of a conventional space optical communication apparatus.

In FIG. 5, after the signal light transmitted from the partner station is focused by the optical antenna 1, the optical deflector 2, the optical coupler 3, and background noise light such as sunlight (hereinafter referred to as background light). ) Is incident on the beam splitter 5 via the optical filter 4 which is a band pass filter having a predetermined pass band in order to limit the band. The beam splitter 5 outputs a signal light having a signal power of, for example, about 1% of the incident received light to the capture detector 6, and outputs a signal light having a signal power of about 99% to the beam splitter 7.
The beam splitter 7 outputs a signal light having a signal power of, for example, about 90% of the received light incident on the beam splitter 7 to the optical signal detector 8 and tracks a signal light having a signal power of about 10%. Output to the detector 9.

The capture detector 6 includes an image sensor, and detects the deflection direction and deflection angle of the incident received light from the central axis for receiving light of the photodetector 8 and the tracking detector 9 using the image sensor. Then, the information on the deflection direction and the deflection angle is output to the optical antenna direction control device 10. Based on these pieces of information, the optical antenna direction control device 10 rotates the optical antenna 1 in the predetermined two axial directions so that the received signal light is approximately in the vicinity of the central axis, and the optical antenna 1 is rotated.
Control the direction of light transmission and reception.

As shown in FIG. 7, the tracking detector 9 includes a known four-quadrant light receiving element having four photodetectors 9a to 9d, each of which is, for example, an avalanche photodiode.
The signal voltages e ₁ to e ₄ output from the photodetectors 9a to 9d are output to the signal processing circuit 11. Correspondingly, the signal processing circuit 11 receives the low-frequency components e ₁ ′ from the input signal voltages e ₁ to e ₄ by the low-pass filters (hereinafter referred to as LPF) 22a to 22d of FIG. To e ₄ ′,
Based on the low frequency components e ₁ ′ to e ₄ ′, the normalized error voltages Vx and Vy represented by the following equations are calculated and output to the optical deflector controller 12.

Vx = (e ₁ ′ + e ₄ ′ −e ₂ ′ −e ₃ ′) / (e ₁ ′ +
_{e 2 '+ e 3' +} e 4 ') ...... (1) Vy = (e 1' + e 2 '-e 3' -e 4 ') / (e 1' +
e ₂ ′ + e ₃ ′ + e ₄ ′) (2) The optical deflector controller 12 uses the normalized error voltages Vx and Vy.
On the basis of the above, the tilt angle of the optical deflector 2 with respect to the received light is changed as is known, and the received light is controlled to be positioned on the central axis.

The optical signal detector 8 detects the incident signal light, converts it into an electric signal, and outputs the electric signal to the demodulator 13. In response to this, the demodulator 13 performs processing such as demodulation and waveform shaping on the electric modulation signal on the electric signal and converts it into a digital reception signal which is an information signal having a transmission rate of 100 Mb / s, for example. And outputs it to the reception signal processing device 14.

On the other hand, the transmission signal generator 15 outputs a digital transmission signal, which is an information signal having a transmission rate of 100 Mb / s, to the modulator 16. In response to this, the modulator 16 outputs a drive current, whose intensity is modulated based on the input digital transmission signal, to the semiconductor laser 18 via the laser drive device 17. The laser 18 generates transmission signal light corresponding to the input drive current and outputs it to the optical coupler 3. The optical coupler 3 has an incident transmission signal light in an axial direction that is the same as the optical axis of the reception signal light output from the optical deflector 2 or that is deviated by a certain angle but in the opposite direction to the reception signal light. , The optical deflector 2 and the optical antenna 1 to transmit to the partner station.

FIG. 6 (A) is a principle diagram showing a method for detecting the angle of arrival of a light beam using the tracking detector 9, and FIG. 6 (B) is a front view of the tracking detector 9.

In FIG. 6A, the received light beam 100 is collected by the optical antenna 1, and the collected light beam 1
00, the light spot 110 is imaged on the tracking detector 9. Each respective light detectors 9a to 9d of the tracking detector 9 in response thereto, detects the optical signal incident to the signal voltage e ₁ not detected outputs the e _4.

Here, if the optical axis of the optical antenna 1 and the light beam 10
When the arrival directions of 0 coincide, the signal voltage e ₁ output from each of the photodetectors 9a to 9e of the tracking detector 9
To e ₄ are the same. On the other hand, if the optical axis of the optical antenna 1 does not coincide with the arrival direction of the light beam 100, as shown by the dotted line in FIGS. 6 (A) and 6 (B), a light spot 110a of the light beam 100a is generated. The center and the center of the tracking detector 9 do not coincide with each other, which causes a difference in the signal voltages e ₁ to e ₄ output from the photodetectors 9a to 9e, respectively. Therefore, the arrival angle of the received light beam can be detected based on this difference.

The signal voltages e ₁ to e ₄ output from the photodetectors 9a to 9d, respectively, are as shown in FIG.
After being input to the signal processing circuit 11, the arithmetic circuit 2 is passed through preamplifiers 21a to 21d and LPFs 22a to 22d having a cutoff frequency of, for example, about kHz.
Input to 3. The arithmetic circuit 23 performs the arithmetic operations of the equations (1) and (2) based on the signal voltages, and calculates the normalized error voltages Vx and Vy calculated by the optical deflector controller 1.
Output to 2. In response to this, the optical deflector controller 12
Is an azimuth angle corresponding to the horizontal direction (hereinafter, referred to as x direction) of the tracking detector 9 in FIG. 7 and the vertical direction (hereinafter, referred to as the following) based on the normalized error voltages Vx and Vy. The elevation angle corresponding to the y direction) is estimated, and the optical deflector 2 is controlled so that the received light is located on the central axis.

[Problems to be Solved by the Invention] However, in the above-described conventional optical space communication device, when strong background light such as sunlight is received together with the received signal light, the tracking detector 9 detects Since the DC component due to the noise light at the output terminals of the photodetectors 9a to 9d becomes significantly larger than the DC component due to the signal light, the signal voltage-to-noise voltage ratio (hereinafter referred to as the SN ratio) of the normalized error voltages Vx and Vy. That is, the optical beam of the signal light cannot be tracked.

As a method for solving the above problems, a beacon method is known, and in this method, when receiving background light such as sunlight, a partner station uses a sine wave signal of, for example, several tens of kHz instead of an information signal. An optical beacon signal obtained by modulating the optical signal is transmitted, and the frequency component of the sine wave signal is output from the signal output from each of the photodetectors 9a to 9d of the tracking detector 9 in the local station. Is extracted using a bandpass filter (hereinafter referred to as BPF), and the light beam is tracked based on the frequency component.

However, in this beacon method, the optical signal is modulated with a sine wave signal instead of the information signal in the other station,
There is a problem in that an information signal cannot be transmitted from the partner station to the own station, and bidirectional communication cannot be performed.

An object of the present invention is to solve the above problems and, when receiving a signal light including an information signal, provide a signal including the above information signal in a good condition even when receiving strong background light such as sunlight. Optical signal detecting method and device capable of detecting with signal power to noise power ratio, and optical beam tracking capable of detecting including the information signal and stably tracking the optical beam of the signal light A method and apparatus are provided.

[Means for Solving the Problems] The first invention is to convert the received signal light containing only an information signal and noise or an information signal into an electric signal, and nonlinearly detect the converted electric signal, and It is characterized in that the noise after the nonlinear detection is removed based on the relationship between the DC component and the AC component of the noise after the nonlinear detection to extract the information signal after the nonlinear detection.

In the first invention, the converted electric signal is converted into 2
After multiplicative detection, the noise after the square-law detection is removed based on the relationship that the power of the AC component of the noise after the square-law detection is twice the power of the DC component of the noise after the square-law detection. Then, the information signal after the square-law detection is extracted.

A second invention is a photo-detecting means for converting a received signal light containing only an information signal and noise or an information signal into an electric signal,
Detection means for performing non-linear detection of the output of the photo-detecting means, and noise after the non-linear detection is removed based on the relationship between the DC component and the AC component of the noise in the output of the detection means to obtain the information signal after the non-linear detection. It is characterized in that extraction means for extracting is provided.

In the second invention, the detection means is a square-law detector, and the extraction means has a power of an AC component of noise at an output of the detection means of a power of a DC component of noise at an output of the detection means. It is characterized in that the information signal after the square-law detection is removed and the information signal after the square-law detection is extracted based on the relationship of being doubled.

A third aspect of the present invention converts a received signal light including an information signal and noise or only an information signal into an electric signal, performs a non-linear detection of the converted electric signal, and outputs a DC component of noise after the non-linear detection. Extract the information signal after the non-linear detection by removing the noise after the non-linear detection based on the relationship with the AC component, the light of the received signal light based on the extracted information signal after the non-linear detection The feature is that the beam is tracked.

In the third invention, the converted electric signal is converted into
Based on the relationship that the power of the AC component of the noise after the square-law detection is twice the power of the DC component of the noise after the square-law detection, the noise after the square-law detection is performed. Is removed, and the information signal after the square-law detection is extracted.

A fourth invention is a photo-detecting means for converting a received signal light containing an information signal and noise or only the information signal into an electric signal,
The detection means for performing non-linear detection of the output of the photo detection means, and the non-linear detection by removing the noise after the non-linear detection based on the relationship between the DC component and the AC component of the noise after the non-linear detection in the output of the detection means Extraction means for extracting the subsequent information signal, and tracking means for tracking the light beam of the received signal light based on the information signal after the non-linear detection output from the extraction means. .

In the fourth aspect of the invention, the photodetector includes a plurality of photodetectors. Further, the detection means is a square-law detector, and the extraction means is such that the power of the AC component of the noise at the output of the detection means is twice the power of the DC component of the noise at the output of the detection means. It is characterized in that the noise after the square-law detection is removed based on the relationship and the information signal after the square-law detection is extracted. further,
The photodetection means is a four-quadrant photodetector having four photodetectors, the detection means square-detects the output of each photodetector of the four-quadrant photodetector, and the extraction means is The information signal at each output is extracted by removing the noise at each output for each output of the detection means,
The tracking means generates two tracking signals corresponding to deviations of the light beam in two directions from the center of the four-quadrant photodetector on the basis of each information signal output from the extraction means, and the two tracking signals are generated. It is characterized in that the light beam is tracked based on a tracking signal. Furthermore, the extracting means includes a low-pass filter that extracts a component having a predetermined frequency or less from the output of the detecting means, a high-pass filter that removes a DC component from the output of the detecting means, and the high-pass filter. An arithmetic circuit that performs a root mean square arithmetic process on the output of the filter, an attenuator that attenuates the output power of the arithmetic circuit by half, and the output of the attenuator from the output of the low-pass filter. It has a subtractor for

[Operation] According to the first invention, after the received signal light containing only the information signal and the noise or the information signal is converted into an electric signal and the converted electric signal is subjected to the non-linear detection, the non-linear detection is performed. The noise after the non-linear detection is removed based on the relationship between the DC component and the AC component of the noise in (1) to extract the information signal after the non-linear detection. Therefore, it is possible to remove the noise component from the received signal light and extract the information signal after the non-linear detection with a good signal power to noise power ratio.

In the first invention, for example, after the square-law detection of the converted electric signal, the power of the AC component of the noise after the square-law detection is 2 times the power of the DC component of the noise after the square-law detection.
By removing the noise after the square-law detection and extracting the information signal after the square-law detection based on the relationship of being doubled, the noise component is removed from the received signal light, and a good signal power pair is obtained. The information signal after the square-law detection can be extracted by the noise power ratio.

According to the second aspect of the invention, the photodetector converts the received signal light containing the information signal and noise or only the information signal into an electric signal, and the detector detects the output of the photodetector by nonlinear detection. . Next, the extraction means removes the noise after the nonlinear detection based on the relationship between the DC component and the AC component of the noise in the output of the detection means, and extracts the information signal after the nonlinear detection. This makes it possible to remove the noise component from the received signal light and extract the information signal after the nonlinear detection with a good signal power to noise power ratio.

In the second aspect of the invention, for example, the detection means is a square-law detector, and the extraction means is such that the power of the AC component of noise at the output of the detection means is the power of the DC component of noise at the output of the detection means. By removing the noise after the square-law detection and extracting the information signal after the square-law detection based on the relationship of being doubled, the noise component is removed from the received signal light to obtain a good signal power pair. The information signal after the square-law detection can be extracted by the noise power ratio.

According to the third aspect of the present invention, the received signal light including only the information signal and the noise or the information signal is converted into an electric signal, the converted electric signal is subjected to the nonlinear detection, and then the noise after the nonlinear detection is removed. Extract the information signal after the non-linear detection by removing the noise after the non-linear detection based on the relationship between the DC component and the AC component, the optical beam of the received signal light based on the extracted information signal To track. Therefore, it is possible to remove the noise component from the received signal light, extract the information signal after the non-linear detection with a good signal power to noise power ratio, and track the light beam based on the extracted information signal.

In the third invention, for example, after square-law detection of the converted electric signal, the power of the AC component of the noise after the square-law detection is twice the power of the DC component of the noise after the square-law detection. By removing the noise after the square-law detection and extracting the information signal after the square-law detection on the basis of the existence relationship,
It is possible to remove noise components from the received signal light, extract the information signal after the square-law detection with a good signal power to noise power ratio, and stably track the light beam based on the extracted information signal. .

According to the fourth aspect of the invention, the photo-detecting means converts the received signal light containing the information signal and noise or only the information signal into an electric signal, and the detecting means non-linearly detects the output of the photo-detecting means. . Then, the extraction means removes the noise after the non-linear detection based on the relationship between the DC component and the AC component of the noise in the output of the detection means to extract the information signal after the non-linear detection, and then the tracking means The optical beam of the received signal light is tracked based on the information signal after the non-linear detection output from the extraction means. Therefore,
Remove the noise component from the received signal light to extract the information signal after the non-linear detection with a good signal power to noise power ratio,
The light beam can be stably tracked based on the extracted information signal.

In the fourth aspect of the invention, the same operation is performed when the photodetection means includes a plurality of photodetectors.

In the fourth aspect of the invention, the detection means is a square-law detector, and the extraction means has a power of an AC component of noise in an output of the detection means and a power of a DC component of noise in an output of the detection means. The noise component is removed from the received signal light by removing the noise after the non-linear detection and extracting the information signal after the non-linear detection based on the relationship that it is two times It is possible to extract the information signal after the non-linear detection with the power ratio and stably track the light beam based on the extracted information signal.

Further, for example, the photodetection means is a four-quadrant photodetector having four photodetectors, and the detection means square-detects the output of each photodetector of the four-quadrant photodetector, and extracts the Means removes noise at each output from each output of the furniture detection means to extract an information signal at each output, and the tracking means outputs the information signal based on each information signal output from the extraction means. The received signal light is generated by generating two tracking signals corresponding to deviations of the light beam from the center of the quadrant photodetector in two directions and tracking the light beam based on the two tracking signals. It is possible to remove the noise component from the signal, extract the information signals with a good signal power to noise power ratio, and stably track the light beam based on the extracted information signals.

Still further, for example, a low-pass filter that extracts a component of a predetermined frequency or less from the output of the detection means, a high-pass filter that removes a DC component from the output of the detection means, and an output of the high-pass filter. The output power of the arithmetic circuit that performs the arithmetic processing of the root mean square with respect to
The extracting means for removing the noise component and extracting the information signal after the square-law detection by including an attenuator for attenuating to 2 and a subtractor for subtracting the output of the attenuator from the output of the low-pass filter. Can be configured.

[Embodiment] FIG. 1 is a block diagram of a circuit 11a for signal processing of an optical space communication apparatus according to an embodiment of the present invention, and FIG. 2 is a block diagram of the signal arithmetic processing circuits 24a to 24d of FIG. Is.

In the signal processing circuit 11a of the present embodiment, the conventional LP
It is characterized by including the signal arithmetic processing circuits 24a to 24d of FIG. 2 in place of the F22a to 22d.
At the other station, the optical signal is a non-information
It shall be modulated with a return-to-zero (hereinafter referred to as NRZ) digital signal.

In the signal arithmetic processing circuit 24a of FIG. 2, the signal voltage output from the preamplifier 21a has a cutoff frequency sufficiently lower than the bit rate of the information signal modulated into the optical signal, and removes the DC component. A high-pass filter (hereinafter, referred to as HPF) 31 for inputting to a square-law detector 32. The square-law detector 32 is composed of an operational amplifier or a diode, square-detects the input signal voltage, and then cuts off the detected signal voltage z at a frequency sufficiently lower than the bit rate of the information signal. The signal is output to the first input terminal of the subtractor 34 and the HPF 35 via the LPF 33 having a frequency.

The HPF 35 has a cutoff frequency similar to that of the HPF 31, removes the DC component from the input signal voltage,
It outputs to the root mean square calculation circuit 36. Root mean square calculation circuit 3
Reference numeral 6 denotes a square-law detector configured similarly to the square-law detector 32, and L having a cutoff frequency similar to that of the LPF 33.
An attenuator 37 composed of a PF and having an attenuation amount of 3 dB after an input signal voltage is subjected to a mean square calculation process.
Through the second input terminal of the subtractor 34. The subtractor 34 subtracts the signal voltage input to the second input terminal from the signal voltage input to the first input terminal, and outputs the subtracted signal voltage to the arithmetic circuit 23.

In the signal arithmetic processing circuit 24a configured as described above, the signal voltage which is the detection voltage of the information signal which is modulated into the optical signal at the partner station and detected by the tracking detector 9 and the random noise voltage are input to the HPF 31. It Here, the signal voltage is the NRZ signal, and the random noise voltage is the shot noise superimposed on the optical signal, the shot noise and DC component due to the background light such as sunlight, the thermal noise generated in the preamplifiers 21a to 21d, and the optical noise. It includes noise voltage due to dark current generated in detectors 9a-9d.

Here, when the signal voltage input to the square-law detector 32 is s and the noise voltage is n, the output signal voltage z of the square-law detector 32 is expressed by the following equation.

z = (s + n) ² (3) When the autocorrelation function of the output voltage z is Rz (τ), Rz (τ) = E [(s ₁ + n ₁ ) ² (s ₂ + n ₂ ) ² ] = E [s ₁ ² s ₂ ² ] + E [s ₁ ² n ₂ ² ] +4 E [s ₁ s ₂ n ₁ n ₂ ] + E [s ₂ ² n ₁ ² ] + E [n ₁ ²
n ₂ ² ] ＝ E [s ₁ ² s ₂ ² ] + 2E [s ₁ ² n ₁ ² ] + 4E [s ₁ s ₂ n ₁ n ₂ ] + E ² [n ₁ ² ] + 2E ² [n ₁ n ₂ ] = E [s ₁ ⁴ ] + 2E [s ₁ ² ] E [n ₁ ² ] + 4E [s ₁ s ₂ ] + E [n ₁ n ₂ ] + E ² [n ₁ ² ] + 2E ² [n ₁ n ₂ ] (4) where E [·] represents an average operation, and the signal voltage s
And the subscripts 1 and 2 of the noise voltage n indicate time t and time t + τ, respectively. The first to fifth terms on the right side of the above equation (4) are respectively denoted by A to E as in the following equations.

A = E [s ₁ ⁴ ] ... (5a) B = 2E [s ₁ ² ] E [n ₁ ² ] ... (5b) C = 4E [s ₁ s ₂ ] E [n ₁ n ₂ ] ... (5c) D = E ² [n ₁ ² ] (5d) E = 2E ² [n ₁ n ₂ ] (5e) Among the above items A to E, the item represented by only the signal voltage is A. , Term A is used for tracking, while terms B through E above are noise components. Here, the signal amplitude of the signal voltage s is r, and the power density on both sides of the noise voltage n is N.
₀ , the transmission bit rate of the information signal is B, and the power density spectra of the signal voltage s and the noise voltage n are Ss (f) and Sn (f), respectively, and the components A to E are Fourier transformed. , The power density spectrum of each of the A to E terms is expressed by the following equation.

F [A] = r ⁴ δ (f) (6a) F [B] = 2F [2r ² N ₀ B] = 4N ₀ r ² Bδ (f) (6b) F [C] = 4F [E [ s ₁ s ₂ ]] * F [E [n ₁ n ₂ ]] = 4Ss (f) * Sn (f) (6c) F [D] = (2N ₀ B) ² δ (f) = 4N ₀ ² B ² δ (f) (6d) F [E] = 2F [E [n ₁ n ₂ ]] * F [E [n ₁ n ₂ ]] = 2Sn (f) * Sn (f) (6e) Here, F [•] represents a Fourier transform operation, δ (•) represents a delta function, and * represents a convolution operation.
FIG. 3 shows the power density spectra of the components of the respective terms A to E, the total power at the output of the square detector 32, and the LP.
It is a figure which shows the total electric power in the output of F33. here,
_BL is the pass bandwidth of the LPF 33.

As shown in FIG. 3, the powers of the noise DC components B and D after the square-law detection are respectively noise continuous spectrum components C and E.
It is 1/2 of the electric power of. Therefore, based on this relationship, in the signal arithmetic processing circuits 24a to 24d of FIG.
The noise DC components B and D are canceled by a component having a power half that of the noise continuous spectrum components C and E, whereby the noise DC components B and D and the noise continuous spectrum components C and E are removed. , Only the signal voltage s ² is extracted.

That is, as shown in FIG. 2, the noise DC components B and D after the square-law detection are extracted using the LPF 33 and the subtractor 34 is used.
Of the noise continuous spectrum components C and E after square-law detection using the HPF 35, and then the extracted components are converted into signal power by the root mean square circuit 36 to obtain 3 dB. The second of the subtractor 34 is passed through the attenuator 37.
Input to the input terminal of. Further, in the subtractor 34,
The signal voltage component A and the noise DC components B and D input to the first input terminal are input to the second input terminal.
The signal voltage component A alone is extracted by subtracting the component having the power of 1/2 of the noise continuous spectrum components C and E.
As described above, since the noise component can be removed and the signal voltage s ² can be extracted, the signal power to noise power ratio at the output of the subtractor 34 can be significantly improved compared to the conventional example. It should be noted that in the present apparatus, the signal voltage s ² can be extracted even when there is no background light, and the light beam of the signal light received based on this signal voltage s ² can be tracked.

Further, the signal arithmetic processing circuits 24b to 24d are configured similarly to the circuit 24a as shown in FIG. 2 and have the same operation and effect.

Then, similarly to the conventional example, the signal voltage component A output from the subtractor 34 is output to the arithmetic circuit 23 as a signal voltage, and the normalized error voltages Vx and Vy are output based on the signal voltage.
Is calculated and then the optical deflector control device 12 tracks the light beam of the received optical signal based on the voltages Vx and Vy, whereby strong background light noise such as sunlight is received. However, compared to the conventional example, there is an advantage that the light beam of the optical signal can be tracked with good tracking accuracy.

In the above embodiment, the HPF 31 is provided between the preamplifiers 21a to 21d and the square wave detector 32, but not limited to this, the photodetectors 9a to 9d and the preamplifier 21 are provided.
It may be provided between a and 21d.

An example of evaluation of the tracking characteristic of the optical space communication device using the signal arithmetic processing circuits 24a to 24e configured as above will be described below.

The noise equivalent angle NEA is used as a parameter for evaluating the tracking characteristic of the optical space communication device. This noise equivalent angle NEA
Is obtained by replacing the noise in the output of the signal processing circuit 11a with an equivalent detection error of the arrival angle of the light beam,
For example, JD Barry and GS Meker, "Beam Directive Error as an Important Design Parameter for Sanitary Free Space Optical Communication Systems," Optical Engineering, Vo1.24, No. 6,
According to December 1985, it is given by the following formula. Here, the tracking detector 9 has sensitivity over the entire light receiving surface,
It is assumed that the four photodetectors 9a to 9d are completely electrically separated and the light spot imaged on the light receiving surface of the tracking detector 9 is an Airy image.

NEA = {3π (λ / D)} / {16 (SNR) v ...
(7) where λ is the wavelength of the optical signal, D is the diameter of the optical antenna 1, and (SNR) v is the signal voltage to noise voltage ratio at the output ends of the signal arithmetic processing circuits 24a to 24d.

The signal voltage-to-noise voltage ratio (S
NR) va is given by the following equation.

^{(SNR) va = r 2 /} (8N 0 r 2 B L + 8B B L N 0 2) 1/2 ... (8) Furthermore, the signal processing circuit 24a to the signal voltage to noise voltage ratio at the output of 24d ( SNR) v is given by the following equation from the above equation (8).

(SNR) v = 4r ² / [4 (8N ₀ r ² _BL + 8B _BL N ₀ ² )] ^1/2 = r ² / (2N ₀ r ² _BL + 2B _BL N ₀ ² ) ^{1 / 2} = 2 (SNR) va (9) FIG. 4 (A) is a diagram showing the noise equivalent angle NEA of the light beam tracking system of the device of the present embodiment and the beacon system of the conventional example when there is no background light. FIG. 4 (B) is a diagram showing the noise equivalent angle NEA of the light beam tracking system and the beacon system of the apparatus of the present embodiment when there is background light from sunlight. In the calculation of these noise equivalent angles NEA, the first
The conditions shown in the table are used, and in the beacon system of the conventional example, as described above, after detecting the optical signal, the sine wave signal is synchronously detected. Also, FIG. 4 (A)
The horizontal axes in and (B) are the optical signal power [dBm] at the input end of the tracking detector 9.

For example, according to Araki et al. “Interaction between two stations of optical ISL tracking / orientation system”, IEICE, Space Navigation Electronics Research Group, SANE87-23, September 1987, as a condition for stable tracking. For example, a noise equivalent angle NEA of about 0.3 μrad or less is required. Therefore, in order to satisfy this requirement, when there is no background light, as shown in FIG. 4 (A), when the transmission bit rate Br of the information signal is 2 Mbps, about -63 dBm at the input end of the tracking detector 9. Optical power for tracking is required. Below, when the transmission bit rate Br is 10 Mbps, -61 dBm, and the transmission bit rate Br is 100 Mbps.
s is -59 dBm, transmission bit rate Br is 400 M
For bps, a tracking optical power of about -57 dBm is required, and further, a beacon system requires a tracking optical power of -66 dBm.

On the other hand, when there is background light from sunlight as shown in Table 1, in order to satisfy the above requirement, as shown in FIG. 4 (B), the transmission bit rate Br of the information signal is 2 Mb.
At ps, optical power for tracking of about -59 dBm is required at the input end of the tracking detector 9, and hereinafter, when the transmission bit rate Br is 10 Mbps, -58 dBm, when the transmission bit rate Br is 100 Mbps, -56 dBm, and transmission bit rate Br. At 400 Mbps, optical power for tracking of about -54 dBm is required, and optical power for tracking of -63 dBm is required for the beacon system.

For example, in the absence of the background light, the required optical signal power for obtaining a bit error rate of 10 ⁻⁹ when the transmission bit rate Br of the information signal is 400 Mbps is −48 d, for example.
It is about Bm. Of this, about 10% optical power-58d
If Bm is assigned for tracking, the noise equivalent angle NEA
Is 0.36 μrad, which substantially satisfies the above-mentioned requirements for tracking. Next, the transmission bit rate Br is 4
When the background light is received when the optical power is 00 MHz and the optical power is −58 dBm for tracking, the noise equivalent angle NEA becomes about 2 μrad, and the requirement for tracking cannot be satisfied, and stable tracking is performed. I can't. Here, if the transmission bit rate Br is reduced to 10 Mbps, the noise equivalent angle NEA becomes about 0.3 μrad, which satisfies the requirement for tracking. Therefore, as shown in FIG. 4 (B), although the tracking characteristic is deteriorated as compared with the beacon method, even when the background light is interfered by sunlight,
It is possible to perform stable tracking while performing bidirectional communication.

Further, in the case where the transmission power of the other station can be increased when the background light is interfered by the sunlight, in order to perform transmission at a transmission bit rate of 400 Mbps with a bit error rate of 10 ⁻⁹ , the receiving side needs −42 dBm. Optical power is required, and if this optical power is obtained at the receiving side, when the optical power of −52 dBm of 10% is allocated to the tracking as described above, the noise equivalent angle NEA becomes about 0.1 μrad. Therefore, since the requirement for tracking is satisfied, 40
Even at a transmission bit rate of 0 Mbps, stable tracking can be performed while performing bidirectional communication.

Although the square wave detector 32 is used in the above embodiments, the square wave detector 32 is not limited to this, and has square and half wave detection characteristics, even and even power full wave and half wave detection characteristics, absolute value detection characteristics, and the like. A non-linear detector that detects an input signal and outputs only a positive signal voltage may be used. In this case, the configurations of the arithmetic circuit 36 and the attenuator 37 are different from those of this embodiment.

In the above embodiment, the tracking detector 9 includes four photodetectors 9a to 9d, but the present invention is not limited to this, and may include a plurality of photodetectors.

The optical space communication device configured as described above can be widely applied to communication between satellite stations, between ground fixed stations, and between mobile stations such as automobiles or between ground fixed stations and satellite stations. . Further, the light beam tracking device and method included in the above communication device can be widely applied to a pickup such as an optical disc such as a compact disc, and a laser radar using signal light.

Although the optical antenna direction control device 10 and the optical deflector control device 12 are separately provided in FIG. 5, the invention is not limited to this.
Based on the output of the signal processing circuit 11 by using the Cassegrain type optical antenna which includes the function of the device 12 in the device 10 and does not include the optical deflector 2 but includes, for example, a sub-reflecting mirror. The angle of the sub-reflecting mirror may be changed by control so as to change the beam direction of the optical antenna.

[Effects of the Invention] As described in detail above, according to the present invention, the received signal light including only the information signal and the noise or the information signal is converted into an electric signal, and the converted electric signal is subjected to nonlinear detection. , Since the noise after the nonlinear detection is removed based on the relationship between the DC component and the AC component of the noise of the nonlinear detection to extract the information signal after the nonlinear detection, a strong background light such as sunlight Even when receiving, the information signal after the non-linear detection can be detected from the signal light with a better signal power to noise power ratio than the conventional example.

Furthermore, by tracking the light beam of the received signal light based on the information signal after the non-linear detection from which the noise after the non-linear detection is removed, a strong background light such as sunlight as described above can be obtained. Even when receiving, there is an advantage that the light beam can be stably tracked.

[Brief description of drawings]

FIG. 1 is a block diagram of a signal processing circuit 11a of an optical space communication apparatus which is an embodiment of the present invention, and FIG. 2 is a signal arithmetic processing circuit 24a to 24d of FIG.
FIG. 3 is a diagram showing the power density spectrum at the output end of the square-law detector 32 in FIG. 2, the total power at the output end of the square-law detector 32, and the total power at the output end of the LPF 33. FIG. 4 (A) is a diagram showing noise equivalent angles of the light beam tracking system of the apparatus of the present embodiment and the beacon system of the conventional example in the absence of background light. FIG. 4 (B) is the case of background light due to sunlight. FIG. 5 is a diagram showing the noise equivalent angles of the light beam tracking system and the beacon system of the device of the present embodiment, FIG. 5 is a block diagram of the optical space communication device of the conventional example, and FIG. 6 (A) is the light beam tracking of the conventional example. FIG. 6 (B) is a plan view of the optical tracking detector of FIG. 6 (A), and FIG. 7 is a block diagram of the signal processing circuit 11 of FIG. 1 ... Optical antenna, 2 ... Optical deflector, 9 ... Tracking detector, 11a ... Signal processing circuit, 12 ... Optical deflector control circuit, 21a to 21d ... Preamplifier, 23 ... Arithmetic circuit, 24a To 24d ... Signal arithmetic processing circuit, 31, 35 ... High-pass filter (HPF), 32 ... Square-law detector, 33 ... Low-pass filter (LPF), 34 ... Subtractor, 36 ... Root mean square calculation circuit, 37 ... Attenuator.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor, Koji Yasukawa, 5 Seiraya, Seika-cho, Soraku-gun, Kyoto Pref. 5 Hiratani, A-R Co., Ltd. Optical and Radio Communications Research Laboratories, Inc. (56) Reference JP 62- 34433 (JP, A) JP 61-212127 (JP, A) JP 60-157345 (JP, A) JP 55-150628 (JP, A)

Claims (11)

[Claims] 1. A received signal light containing an information signal and noise or only an information signal is converted into an electric signal, the converted electric signal is subjected to non-linear detection, and then a DC component of noise after the non-linear detection and an alternating current A method for detecting an optical signal, characterized in that noise after the non-linear detection is removed based on a relation with a component to extract the information signal after the non-linear detection. 2. After square-law detection of the converted electric signal, the power of the AC component of the noise after the square-law detection is twice the power of the DC component of the noise after the square-law detection. 2. The optical signal detection method according to claim 1, wherein the noise after the square-law detection is removed based on the relationship, and the information signal after the square-law detection is extracted. 3. A light detecting means for converting received signal light containing an information signal and noise or only an information signal into an electric signal, a detecting means for nonlinearly detecting an output of the light detecting means, and an output of the detecting means. 2. An optical signal detecting device comprising: an extracting means for removing the noise after the non-linear detection and extracting the information signal after the non-linear detection based on the relationship between the DC component and the AC component of the noise in FIG. 4. The detection means is a square-law detector, and the extraction means is such that the power of the AC component of noise at the output of the detection means is twice the power of the DC component of noise at the output of the detection means. 4. The optical signal detection device according to claim 3, wherein the noise after the square-law detection is removed based on the existence relationship to extract the information signal after the square-law detection. 5. A received signal light containing an information signal and noise or only an information signal is converted into an electric signal, the converted electric signal is subjected to a non-linear detection, and then a DC component of noise after the non-linear detection and an AC Extract the information signal after the non-linear detection by removing the noise after the non-linear detection based on the relationship with the component, the optical beam of the received signal light based on the extracted information signal after the non-linear detection A method for tracking a light beam, which is characterized by tracking a beam. 6. After square-law detection of the converted electric signal, the power of the AC component of noise after the square-law detection is 2
The information signal after the square-law detection is extracted and the information signal after the square-law detection is extracted based on the relation that the power of the DC component of the noise after the multi-law detection is twice. The light beam tracking method according to the fifth item. 7. A photodetector for converting received signal light containing an information signal and noise or only an information signal into an electric signal, a detector for nonlinearly detecting the output of the photodetector, and an output of the detector. In the extraction means for removing the noise after the non-linear detection based on the relationship between the DC component and the AC component of the noise after the non-linear detection in to extract the information signal after the non-linear detection, An optical beam tracking device comprising a tracking means for tracking the received light beam of the signal light based on the information signal after the nonlinear detection. 8. A light beam tracking device according to claim 7, wherein said light detecting means comprises a plurality of light detectors. 9. The detecting means is a square-law detector, and the extracting means is such that the power of the AC component of noise at the output of the detecting means is twice the power of the DC component of noise at the output of the detecting means. 8. The light beam tracking device according to claim 7, wherein the noise after the square-law detection is removed and the information signal after the square-law detection is extracted on the basis of the existence relationship. 10. The photodetection means is a four-quadrant photodetector having four photodetectors, and the detection means square-detects the output of each photodetector of the four-quadrant photodetector, The extracting means removes noise at each output from each output of the detecting means to extract an information signal at each output, and the tracking means based on each information signal output from the extracting means. 2. Two tracking signals corresponding to deviations of the light beam in two directions from the center of the four-quadrant photodetector are generated, and the light beam is tracked based on the two tracking signals. Item 9. The light beam tracking device according to item 9. 11. A low-pass filter for extracting a component having a frequency equal to or lower than a predetermined frequency from the output of the detecting means, a high-pass filter for removing a DC component from the output of the detecting means, and the high-pass filter. An arithmetic circuit that performs a root mean square arithmetic processing on the output of the low-pass filter, an attenuator that attenuates the output power of the arithmetic circuit to 1/2, and an output of the low-pass filter from the output of the attenuator. The light beam tracking device according to claim 9 or 10, further comprising a subtractor for subtracting.