JP2001077760A - Optical space communication device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve code error prevention characteristics of a received digital signal by a pilot signal included in a main signal and to enable stable identification of a digital signal. SOLUTION: A light beam from a partner device enters a lens 30 from the left side, passes through a lens 31, and has a movable mirror 32.
, And is reflected downward to reach the polarization beam splitter 33.
Since this light beam is polarized perpendicular to the plane of the paper, it is reflected rightward on the bonding surface of the polarizing beam splitter 33 and split by the light beam splitting mirror 36 in two directions. The light beam reflected by the light beam splitting mirror 36 is received by the light receiving element 40 through the lens 39 and converted into an electric signal, and then amplified by the amplifier 45 to an appropriate level. After being removed, only the main signal component is reproduced into a digital signal by the identification circuit 47 and output from the reception signal output terminal 48.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical space communication apparatus for transmitting an optical signal modulated by a transmission signal in the form of a beam and propagating through the atmosphere to perform communication between two distant points.

[0002]

2. Description of the Related Art In general, communication using optical signals is capable of transmitting a large amount of information at a high speed. In particular, optical space communication using a transmission path as a free space is possible compared with wired communication such as optical fiber. There is an advantage that a communication path can be easily established with high portability. 2. Description of the Related Art In a conventional communication device, in order to improve the reliability of optical space communication, automatic tracking (auto tracking) for correcting an angle of an emission direction of a light beam is performed so that an optical signal does not deviate from the device.

FIG. 6 shows a configuration of a conventional optical space communication apparatus having a tracking function. When a digital signal is transmitted using this optical space communication apparatus, when a transmission signal is input from a transmission signal input terminal 1 during transmission, an amplifier 2
The pilot signal from the oscillator 3 is further multiplexed by the multiplexer 4 and then output to the light emitting element 5.
The light emitting element 5 intensity-modulates the oscillating light according to the input signal to convert the oscillating light into an optical signal. The light beam from the light emitting element 5 reaches the polarizing beam splitter 7 via the lens 6, and since this light beam is polarized in parallel with the paper surface, it passes through the polarizing beam splitter 7 as it is, and is reflected to the left by the movable mirror 8, The light is emitted through the lenses 9 and 10 in the direction of the other device.

At the time of reception, a light beam from the partner device enters the lens 10 from the left, passes through the lens 9, is reflected downward by the movable mirror 8, and reaches the polarization beam splitter 7. Since this light beam is polarized perpendicular to the plane of the paper, it is reflected rightward on the bonding surface of the polarizing beam splitter 7, and
The light is split in two directions by the light beam splitting mirror 11. The light beam reflected by the light beam splitting mirror 11 is received by the light receiving element 13 through the lens 12 and converted into an electric signal, then amplified to an appropriate level by the amplifier 14, and further reproduced by the identification circuit 15 into a digital signal. And the received signal output terminal 1
6 is output.

On the other hand, the light beam transmitted through the light beam splitting mirror 11 is condensed by a lens 17 and received as a spot image S by a position detecting element 18. The position detecting element 18 calculates the position of the spot image S and outputs it to the tracking control circuit 19 as a position signal. Based on the position signal, the tracking control circuit 19 calculates the angle formed by the light beam from the partner device with the optical path of the device, and calculates the angle of the actuator 20a,
A drive signal of 20b is created. Actuator 20a,
20b adjusts the angle of the movable mirror 8 to adjust the spot image S
Is received at the center of the position detecting element 18.

Accordingly, the position of the light emitting element 5 is also adjusted, so that the optical paths of the outgoing beam and the incident beam coincide, and the light beam is accurately emitted toward the partner device. When the optical path of the received light shifts due to the device tilting during communication and the position of the spot image S of the position detecting element 18 shifts from the center, the movable mirror 8 immediately moves, and the spot image S
Is received at the center of the position detecting element 18 so that the incident optical path of the light beam is sequentially corrected so that the incoming light beam does not deviate from the device.

At this time, the position detection signal 18 is attracted in the direction of strong background light by the influence of the background light around the partner device, thereby causing an error. In order to prevent this, only the AC pilot signal of a specific frequency which is generated from the oscillator 3 and multiplexed with the transmission signal by the multiplexer 4 and sent to the partner device is selected and detected.

FIG. 7 shows a position detecting element 18 divided into four parts.
Is shown, and the four divided photodetectors 18a to 18a to
The position of the spot image S is obtained by comparing the respective outputs of 18d. Background light other than the spot image S is incident on the position detecting elements 18a to 18d. However, components other than the frequency component of the pilot signal are not detected by being removed by an electric circuit such as a filter or a frequency selective amplifier at a subsequent stage.

The tracking function described above does not operate unless the light beam from the partner device reaches a receivable level and a spot image S is not received on a part of the position detecting element 18. For this reason, in the initial adjustment at the time of installation of the apparatus, the operator fixes the movable mirror 8 at the initial position near the midpoint and manually adjusts the direction of the apparatus while observing the other apparatus with the collimating scope 21. ing.

[0010]

However, in the above-mentioned prior art, even if the position detecting element 18 has a slight angle shift, the light receiving area is required to prevent the spot image S from deviating from the light receiving surface. A wide element is required. However, if the light receiving area is large, the response speed becomes slow, so that the frequency of the pilot signal cannot be increased.

When transmitting a digital signal as a main signal for transmission / reception, direct intensity modulation (IMD) for transmitting the digital signal as an optical signal obtained by directly converting the digital signal into a signal of the intensity of light.
D) method is the most efficient, but general digital signals have frequency components widely distributed from low frequency to high frequency as shown in FIG. 8, and when a pilot signal of frequency fps is combined with this, the frequency components overlap. When this is viewed in the time domain, a digital signal as shown in FIG. 9 has a signal waveform undulating with an envelope of the frequency fps.

When this signal is received and reproduced by the discrimination circuit 15 into the original digital signal, if the discrimination level indicated by the dotted line between the H level and the L level is set to Lo, the portion A has a margin for the H level. Becomes small, and the margin for the L level becomes small in the portion B. Therefore, when the attenuation of the transmission path increases due to bad weather or the like, and the received power decreases to lower the SN ratio of the received signal, there is a problem that the probability of causing a code error increases.

An object of the present invention is to solve the above-mentioned problems,
It is an object of the present invention to provide an optical space communication apparatus capable of improving a code error prevention characteristic of a received digital signal by a pilot signal included in a main signal and enabling stable digital signal identification.

[0014]

SUMMARY OF THE INVENTION An optical space communication apparatus according to the present invention for achieving the above object is an optical communication apparatus which is installed between two distant points so as to communicate by transmitting an optical signal into a free space. A spatial communication device, wherein in the transmission unit, means for generating a pilot signal for angle detection for automatic tracking,
Means for multiplexing the pilot signal with a main signal which is a communication signal, and means for converting the multiplexed signal into an optical signal. And a means for selectively removing a frequency component of the pilot signal from a main signal including the pilot signal converted into the electric signal.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to the embodiments shown in FIGS. FIG. 1 shows a configuration diagram of an optical space communication device having a tracking function according to the first embodiment,
A transmission / reception lens 30 is disposed at a position facing the partner device, and a lens 31 and a movable mirror 32 are arranged behind the lens 30. In the incident direction of the movable mirror 32, a light emitting element 35 including a polarization beam splitter 33, a lens 34, a semiconductor laser light source, and the like is arranged.
In the reflection direction of the polarization beam splitter 33, a light beam splitting mirror 36, a lens 37, and a position detecting element 38 are arranged.
9, light receiving elements 40 are arranged.

The output of a multiplexer 41 is connected to the light emitting element 35, the output of a transmission signal input terminal 42 is connected to the multiplexer 41 via an amplifier 43, and a pilot signal oscillator for position detection. Forty-four outputs are connected. The output of the light receiving element 40 is sequentially connected to an amplifier 45, a band-elimination filter 46 for selectively removing a pilot signal component fps, a discrimination circuit 47, and a reception signal output terminal 48.

The output of the position detecting element 38 is connected to a tracking control circuit 49, and the output of the tracking control circuit 49 for changing the angle of the movable mirror 32 is sent to the movable mirror 32 via two actuators 50a and 50b. It is connected. A collimating scope 51 for the examiner to visually confirm is provided substantially parallel to the optical axis of the lens 30.

The band elimination filter 46 inserted between the amplifier 45 and the identification circuit 47 has a transmission characteristic as shown in FIG. 2, and cuts only the frequency fps of the pilot signal. Examples of the band elimination filter 46 include an inductor and a capacitor as shown in FIG. 3, and a ceramic filter as shown in FIG.

Although the band elimination filter 46 is inserted after the amplifier 45, the order may be reversed so that the band elimination filter 46 is inserted between the light receiving element 40 and the amplifier 45.

When a digital signal is transmitted using this optical space communication apparatus, when a transmission signal is input from a transmission signal input terminal 42 at the time of transmission, it is amplified by an amplifier 43 and further combined with a pilot signal from an oscillator 44. Corrugator 41
And then output to the light emitting element 35. The light emitting element 35 modulates the intensity of the oscillating light according to the input signal to convert it into an optical signal. The light beam from the light emitting element 35 reaches the polarizing beam splitter 33 via the lens 34, and since this light beam is polarized in parallel with the paper surface, it passes through the polarizing beam splitter 33 as it is, and is reflected to the left by the movable mirror 32. ,
The light is emitted through the lenses 31 and 30 in the direction of the other device.

At the time of reception, the light beam from the partner device enters the lens 30 from the left, passes through the lens 31, is reflected downward by the movable mirror 32, and is reflected by the polarization beam splitter 3
Reaches 3. Since this light beam is polarized perpendicular to the plane of the paper, the light beam is reflected rightward on the bonding surface of the polarizing beam splitter 33 and split in two directions by the light beam splitting mirror 36. The light beam reflected by the light beam splitting mirror 36 is received by the light receiving element 40 through the lens 39, converted into an electric signal, and then amplified to an appropriate level by the amplifier 45. The signal is removed, reproduced into a digital signal by the identification circuit 47, and output from the reception signal output terminal 48.

On the other hand, the light beam transmitted through the light beam splitting mirror 36 is condensed by a lens 37 and received by a position detecting element 38 as a spot image S. The position detecting element 38 obtains the position of the spot image S and outputs it to the tracking control circuit 49 as a position signal. The tracking control circuit 49 calculates, based on the position signal, the angle formed by the light beam from the partner device with the optical path of its own device,
a, 50b are generated. According to the drive signal, the actuators 50a and 50b adjust the angle of the movable mirror 32 and perform control so that the spot image S is received at the center of the position detection element 38.

As described above, by removing the pilot signal components before the identification circuit 47, the signal waveform entering the identification circuit 47 has a constant envelope as shown in FIG. In the identification circuit 47, the identification levels of H and L are set to Lo.
Then, there is no place where the margin of the level is small as shown in FIG. 9 of the conventional example, and the code error characteristic is greatly improved.

Here, the band elimination filter 46 shown in FIG.
Has a certain width, and if the digital signal as the main signal has a frequency component in this region, the band elimination characteristic is lost. However, since the frequency spectrum of the actual digital signal is a set of discontinuous line spectra, if the width of the attenuation band of the band elimination filter 46 is set sufficiently small, the probability that the spectrum of the digital signal overlaps the attenuation band is small. Even if the spectra are overlapped, even if the spectra overlap, only a small part of the entire spectral components is lost, and the waveform of the digital signal is slightly distorted, so that there is almost no problem in practical use.

[0025]

As described above, the optical space communication apparatus according to the present invention has a receiving section for receiving a digital signal.
By selectively removing the pilot signal for position detection from the main signal, it is possible to reduce the occurrence of code errors and perform stable reception even when the reception state is bad such as in bad weather.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a free-space optical communication apparatus according to an embodiment.

FIG. 2 is a graph showing characteristics of a band elimination filter.

FIG. 3 is a configuration diagram of a band elimination filter.

FIG. 4 is a configuration diagram of a band elimination filter.

FIG. 5 is a graph of an envelope waveform of a digital signal.

FIG. 6 is a configuration diagram of a conventional example.

FIG. 7 is a front view of a position detection element.

FIG. 8 is a graph showing frequency components of a digital signal and a time axis waveform.

FIG. 9 is a graph of an envelope waveform of a digital signal.

[Explanation of symbols]

 Reference Signs List 30 transmitting / receiving lens 32 movable mirror 33 deflection beam splitter 35 light emitting element 36 light splitting mirror 38 position detecting element 40 light receiving element 41 multiplexer 44 pilot signal oscillator 46 band elimination filter 47 identification circuit 49 tracking control circuit 50a, 50b actuator

Claims (1)

[Claims] 1. An optical space communication device which is installed between two distant points so as to communicate by transmitting an optical signal into a free space, wherein a transmitting unit is provided for detecting an angle for automatic tracking. Means for generating a pilot signal, means for multiplexing the pilot signal with a main signal that is a communication signal, and means for converting the multiplexed signal into an optical signal, A light receiving means for receiving a signal light and converting it into an electric signal; and a means for selectively removing a frequency component of the pilot signal from a main signal including the pilot signal converted into the electric signal. Spatial communication device.