JPH0748692B2 - Light beam tracking device - Google Patents

Description

The present invention relates to a light beam tracking device for tracking a light beam of received light in an optical space communication device.

[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 of the optical antenna 1 is rotated. Control the direction of light transmission and reception.

The tracking detector 9 is, as shown in FIG. 7, four photodetectors each made of, for example, an avalanche photodiode.
A known four-quadrant light receiving element having 9a to 9d is provided, and the signal voltages e ₁ to e ₄ output from the photodetectors 9a to 9d are output to the signal processing circuit 11. In response to this, the signal processing circuit 11 receives 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 ′ are extracted, normalized error voltages Vx and Vy represented by the following equations are calculated based on the low frequency components e ₁ ′ to e ₄ ′, 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 receives the received light based on the normalized error voltages Vx and Vy as is known. The received light is controlled so as to be located on the central axis by changing the inclination angle of the optical deflector 2 with respect to.

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 for the intensity modulated signal on the electric signal, and for example 100M
It is converted into a digital reception signal which is an information signal having a transmission rate of b / s and is output to the reception signal processing device 14.

On the other hand, the transmission signal generator 15 uses a modulator 16 to transmit a digital transmission signal, which is an information signal having a transmission rate of 100 Mb / s, for example.
Output to. 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 a transmission signal light in response 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 100 is collected.
Thus, 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 100
Of the tracking detector 9 when the arrival directions of the
Signal voltages e ₁ to e ₄ output from 9a to 9e, respectively
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),
The center of the light spot 110a due to 100a and the center of the tracking detector 9 do not coincide with each other, and a difference occurs in the signal voltages e ₁ to e ₄ output from the respective photodetectors 9a to 9e. Therefore, the arrival angle of the received light beam can be detected based on this difference.

Signal voltage output from each photodetector 9a to 9d
As shown in FIG. 7, e ₁ to e ₄ are respectively input to the signal processing circuit 11, and then are input through the preamplifiers 21a to 21d and the LPFs 22a to 22d having a cutoff frequency of, for example, about several kHz, to the arithmetic circuit. Entered in 23. Arithmetic circuit 23
Calculates the normalized error voltages Vx, V calculated by the above formulas (1) and (2) based on the signal voltages.
Output y to the optical deflector controller 12. In response to this, the optical deflector control device 12 determines, based on the normalized error voltages Vx, Vy, the horizontal direction (hereinafter, x) of the tracking detector 9 in FIG.
Called direction. ) And the elevation angle corresponding to the vertical direction (hereinafter referred to as y direction) of the detector 9 are 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, each of the tracking detectors 9 is detected. Photo detector
Since the DC component due to the noise light at the output terminals of 9a to 9d becomes extremely larger than the DC component due to the signal light, the signal voltage-to-noise voltage ratio (hereinafter, referred to as SN ratio) of the normalized error voltages Vx, Vy. It is significantly reduced, and the light beam of the signal light cannot be tracked. That is, the above-described optical space communication device cannot track the light beam when receiving the strong background light, and therefore cannot communicate. Further, it is necessary to reacquire and track the light beam when the background light has passed.

As a method for solving the above problems, a beacon method is known, and in this method, when receiving background light such as sunlight, the partner station replaces the information signal with, for example, several digits.
An optical beacon signal obtained by modulating an optical signal with a sine wave signal of 10 kHz is transmitted, and in response to the signal output from each of the photodetectors 9a to 9d of the tracking detector 9 in the local station, The frequency component of the sine wave signal is extracted using a bandpass filter (hereinafter referred to as BPF), and the light beam is tracked based on the frequency component.

However, in order to perform bidirectional communication in this beacon method, two optical systems are required to transmit an optical signal modulated with a sine wave signal for tracking in the partner station and an information signal light. There was a problem that it was necessary. For example, even when the background light such as sunlight is received and the bidirectional communication is abandoned and only the light beam is tracked, the optical signal is modulated by the sine wave signal instead of the information signal, so that the information signal is transmitted at the own station. There is a problem that the signal processing circuit for receiving the signal must be replaced with a signal processing circuit for receiving the sine wave signal.

An object of the present invention is to solve the above problems, to receive a relatively strong background light such as sunlight when receiving a signal light including an information signal, and the power of the signal light is smaller than that of a conventional example. Even in the case, it is to provide a light beam tracking device capable of stably tracking the light beam of the signal light.

[Means for Solving the Problems] A light beam tracking device according to the present invention receives an optical signal modulated by a digital signal which is an information signal of a predetermined transmission bit rate, and photoelectrically converts the optical signal into an electric signal. Receiving means for outputting the high-pass filter so as to remove a direct current component from the electric signal output from the receiving means, and outputting the high-pass filter. Non-linear means for extracting a timing component having the same frequency as the transmission bit rate and outputting an electrical signal containing the timing component by performing a predetermined non-linear operation on the output electrical signal; Second filtering means having a center frequency substantially the same as the frequency and a predetermined pass band width, and band-pass filtering the electric signal output from the non-linear means to output the electric signal; By detecting the electric signal output from the filtering means, the timing component is frequency-converted into a direct-current component, and the frequency component other than the timing component is frequency-converted into a lower frequency, and the detection means is output. Of the electric signal output from the detection means, a third filtering means for low-pass filtering the timing component, which is a direct current component, and outputting the same, and a timing component output from the third filtering means. Tracking means for tracking the light beam of the optical signal received by the receiving means based on the above.

At the output of the reception means configured light beam tracking device as [action] or more, a predetermined transmission bit rate (hereinafter referred to. The transmission bit rate and f _o [bps]) digital signal is information signals the power, for example, as shown in FIG. 3 (a), whereas consisting continuous spectrum components from the DC component to the frequency f _o, the noise power due to the background light is relatively large DC component and said receiving means And a white shot noise component which is proportional to the noise power of the background light and is smaller than the DC component and is a random noise component irrelevant to the frequency. Furthermore, in the output of the receiving means, there is actually a thermal noise component which is a continuous spectrum component from the direct current component to the high frequency regardless of the frequency generated by the circuit of the receiving means.

In the prior art apparatus of FIG. 7, LPFs 22a, 22b, 22c and 22d substantially eliminate the high frequency noise including the white shot noise component and the thermal noise component with respect to the spectral components in this state, As the tracking signal component (DC component), the DC component of the digital signal and the noise component due to the background light are added. If the power of the noise component due to the background light is larger than the power of the DC component of the digital signal, the digital signal is hidden by the background light and the background light is tracked instead of the digital signal. The beam cannot be tracked at all.

On the other hand, according to the present invention, it is possible to remove a relatively large direct current component of the noise component due to the background light by the first filtering means, and then by the non-linear means, the frequency of the same frequency as the transmission bit rate. A timing component can be extracted and an electrical signal containing the timing component can be output, and the electrical signal is passed through the second filtering means to remove most of the white shot noise component and the thermal noise component. After that, the timing component is frequency-converted into a direct-current component by detecting with the detecting means, and only the timing component can be extracted by further low-pass filtering with the third filtering means. The light beam can be tracked based on the extracted timing component. That is, in the present invention, a relatively large DC component of background light can be removed, and thereafter, the timing component extracted by the non-linear means is detected and frequency-converted into a DC component, thereby substantially eliminating the background light noise. It is possible to obtain the timing component that has been removed.

By the processing by the non-linear means, noise is generated from the white shot noise due to the background light and the correlation between the spectral component of the digital signal and the white shot noise due to the background light. This noise is generated by the third filtering means. The signal power to noise power ratio can be improved.

Therefore, when receiving a signal light including a digital signal,
Even when a relatively strong background light such as sunlight is received and the power of the signal light is smaller than that of the conventional example, as described later in detail with reference to FIG. If the above optical signal power can be received, the optical beam of the signal light can be stably tracked.

As described above, since the timing component extracted from the received optical signal is the same as the transmission bit rate of the information signal included in the optical signal and exists in the relatively high frequency region, the DC component due to the background light is generated. Can be easily separated from each other, and the tracking of the light beam of the optical signal is not affected by the DC component of the background light. For this reason,
Even when the background light is received, stable tracking of the light beam can be performed.

Further, since the timing component is extracted from the received signal light, as long as the partner station transmits the digital signal, the own station can always extract the timing component. Therefore, it is possible to avoid the problem of requiring two optical systems, an optical system for tracking a light beam by the beacon method and an optical system for transmitting digital signal light. It is also possible to avoid the problem that the signal processing circuit for receiving the information signal must be replaced with a signal processing circuit for receiving the sine wave signal at the station.

[Embodiment] FIG. 1 is a block diagram of a signal processing circuit 11a 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 circuit 24a of FIG.

In the signal processing circuit 11a of this embodiment, the conventional LPF 22a
2 to 22d in place of the signal arithmetic processing circuits 24a to 24d shown in FIG. In addition,
At the other station, the optical signal is transmitted at a transmission bit rate f _o [bp
[s] information signal, which is a non-return-to-zero (hereinafter referred to as NRZ) digital signal.

In the signal arithmetic processing circuit 24a of FIG. 2, the preamplifier 21a
The signal output from is input to the equalization circuit 32 via a high-pass filter (hereinafter referred to as HPF) 31 for removing a DC component. The equalization circuit 32 removes frequency components exceeding the frequency f _o [Hz] included in the input signal to limit the noise band included in the signal, and the nonlinear element 33 in the next stage.
In order to easily extract the timing component of the information signal in FIG. 3, after performing waveform shaping of the signal by a known method such as the full cosine roll off method or the Gaussian equalization method, the signal of FIG. The signal having the spectrum is output to the non-linear element 22. The non-linear element 22 is composed of, for example, one or a plurality of diodes, and performs known non-linear operation on the input signal by, for example, full-wave rectification or square characteristic, and as a result, as shown in FIG. 3 (B). like,
The timing component TS of the frequency f _o [Hz] is extracted, and the signal including the timing component TS is output to the BPF 34. The BPF 34 is a bandpass filter having a center frequency substantially equal to the frequency f _o [Hz] of the timing component TS and a predetermined passband width, and the input signal is in the vicinity of the timing component TS on the frequency axis. After passing only the noise located at, the signal is output to the detection circuit 35. As shown in FIG. 3 (C), the detection circuit 35 frequency-converts the timing component TS into a DC component and frequency-converts a noise component near the timing component TS into a low frequency near DC, It is output to the arithmetic circuit 23 via the LPF 36 for removing the noise component.

The signal arithmetic processing circuits 24b to 24d are the signal arithmetic processing circuits 24
It is constructed and operates in the same way as a.

Each timing component TS extracted by the signal arithmetic processing circuits 24a to 24d is input to the arithmetic circuit 23,
Reference numeral 23 is a unit for calculating the normalized error voltages Vx and Vy based on the respective timing components TS in the same manner as in the conventional example, and the optical polarizer control device 1
Output to 2. The optical deflector controller 12 controls the optical deflector 2 so as to track the light beam of the received light based on the input normalized error voltages Vx and Vy, as is known.

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

The noise equivalent angle NEA is used as a parameter for evaluating the tracking characteristics 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 the detection error of the arrival angle of the equivalent light beam. For example, JD Barry and GS Mekhale "Free installation on satellite" Beam pointing error as an important design parameter for spatial optical communication systems ", Optical Engineering, Vol. 24, No. 6, December 1985, is given by Here, the tracking detector 9 of FIG. 1 has sensitivity over the entire light receiving surface, and the four photodetectors 9a to 9a
It is assumed that 9d is completely electrically separated and the light spot formed on the light receiving surface of the tracking detector 9 is an Airy image.

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

FIG. 4 shows the noise equivalent angle NE of the light beam tracking system and the beacon system of the apparatus of this embodiment when there is background light from sunlight.
It is a figure which shows A. Note that the conditions shown in Table 1 were used in the calculation of these noise equivalent angles NEA, and in the beacon system of the conventional example, as described above, after detecting the optical signal, the sine wave signal was synchronized. It shall be detected. The horizontal axis in FIG. 4 represents 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, as shown in FIG. 4, when the bit rate of the information signal is 400 Mbps, the tracking optical power of -52 dBm is required. Also, when the bit rate is 100 Mbps, optical power for tracking of -53 dBm is required, and the bit rate is 10 Mb.
For ps, a tracking optical power of -54 dBm is required. On the other hand, the beacon system requires a tracking optical power of -61 dBm.

For example, in order to perform information transmission at a transmission bit rate of 400 Mbps with a bit error rate of 10 ^-9 when the background light interferes with sunlight, optical power of -42 dBm is required on the receiving side. If this condition is satisfied, 10% of the received optical power, or −52
If the optical power of dBm is allocated for tracking, the light beam can be tracked while performing bidirectional communication using a part of the communication signal light even when the background light is interfered by sunlight.

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 pickups for optical disks such as compact disks, laser radars using signal light, and various measuring devices using light beams.

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.
Control of the device 10 based on the output of the signal processing circuit 11 by using the Cassegrain type optical antenna including the function of 12 in the device 10 and not including the optical deflector 2 but including, for example, a sub-reflecting mirror. The angle of the sub-reflecting mirror may be changed to change the beam direction of the optical antenna.

[Effects of the Invention] As described in detail above, according to the present invention, the relatively large DC component of the noise component due to the background light can be removed by the first filtering means, and then by the non-linear means, It is possible to extract a timing component having the same frequency as the transmission bit rate and output an electric signal including the timing component. The electric signal is passed through the second filtering means to generate the white shot noise component. After removing most of the thermal noise component, the timing component is frequency-converted into a DC component by detection by the detection means, and the timing is obtained by further low-pass filtering by the third filtering means. Only the component can be extracted, and the light beam can be tracked based on the extracted timing component. That is, in the present invention, a relatively large DC component of background light can be removed, and thereafter, the timing component extracted by the non-linear means is detected and frequency-converted into a DC component, thereby substantially eliminating the background light noise. It is possible to obtain the timing component that has been removed.

Therefore, when receiving a signal light including a digital signal,
Receiving relatively strong background light such as sunlight, and even if the power of the signal light is small compared to the conventional example, if the optical signal power of a predetermined threshold or more can be received, The light beam can be stably tracked.

Further, since the timing component is extracted from the received signal light, as long as the partner station transmits the digital signal, the own station can always extract the timing component. Therefore, it is possible to avoid the problem of requiring two optical systems, an optical system for tracking a light beam by the beacon method and an optical system for transmitting digital signal light. It is also possible to avoid the problem that the signal processing circuit for receiving the information signal must be replaced with a signal processing circuit for receiving the sine wave signal at the station.

[Brief description of drawings]

FIG. 1 is a block diagram of a signal processing circuit 11a of an optical space communication device according to an embodiment of the present invention, FIG. 2 is a block diagram of a signal arithmetic processing circuit 24a of FIG. 1, and FIG. 2 shows the output power spectrum of the equalizing circuit 32, FIG. 3 (B) shows the output power spectrum of the nonlinear element 33 of FIG. 2, and FIG. 3 (C) shows the LPF 36 of FIG. FIG. 4 is a diagram showing an output power spectrum, FIG. 4 is a diagram showing noise equivalent angles of a light beam tracking system and a beacon system of the apparatus of the present embodiment when there is background light from sunlight, and FIG. 5 is an optical space of a conventional example. FIG. 6A is a block diagram of a communication device, FIG. 6A is a diagram showing a conventional light beam tracking method, FIG. 6B is a plan view of the optical tracking detector of FIG. 6A, and FIG. 6 is a block diagram of the signal processing circuit 11 of FIG. 5. FIG. 1 ... Optical antenna, 2 ... Optical deflector, 9 ... Tracking detector, 11a ... Signal processing circuit, 12 ... Optical deflector control circuit, 2
1a to 21d ...... Preamplifier, 23 ...... Operation circuit, 24a to 24d ...... Signal processing circuit, 31 ...... High-pass filter (HPF), 32 …… Equalization circuit, 33 …… Nonlinear element, 34 ……
Band pass filter (BPF), 35 ...... Detection circuit, 36 ...... Low pass filter (LPF).

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kenichi Araki, 5 Seiraya, Seika-cho, Seika-cho, Kyoto Prefecture Prefectural ATR Optical & Radio Communications Laboratory, Inc. (72) Inventor, Koji Yasukawa Soraku, Kyoto Prefecture Gunma Seika-cho Osamu Osamu Osamu Osamu 5 Hiratani, Ltd. Optical Communication Research Laboratory, ATR Co., Ltd. (56) Reference JP-A-54-149406 (JP, A) JP-A-60-35282 (JP, A)

Claims (1)

[Claims] 1. Receiving means for receiving an optical signal modulated by a digital signal which is an information signal of a predetermined transmission bit rate, photoelectrically converting the optical signal into an electric signal and outputting the electric signal, and output from the receiving means. First filtering means for high-pass filtering so as to remove a DC component from the electric signal and outputting the electric signal, and a predetermined non-linear operation for the electric signal output from the first filtering means. Thus, a non-linear means for extracting a timing component having the same frequency as the transmission bit rate and outputting an electric signal containing the timing component, a center frequency substantially the same as the frequency of the timing component, and a predetermined pass band. A second filtering means having a width for band-pass filtering and outputting the electric signal output from the non-linear means; and detecting an electric signal output from the second filtering means. Yo The frequency component of the timing component is converted to a direct current component, and the frequency component other than the timing component is frequency-converted to a lower frequency and output, and a DC component of the electric signal output from the detection device. Third filtering means for low-pass filtering the timing component and outputting the signal, and light of the optical signal received by the receiving means based on the timing component output from the third filtering means. An optical beam tracking device comprising: a tracking means for tracking a beam.