JP3079249B2 - Optical signal transmission method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical signal transmission method for effectively utilizing the power of a transmission laser in spatial laser communication and improving the accuracy of optical signal detection and tracking.

[0002]

2. Description of the Related Art In laser communication in space, it may be necessary to transmit a laser beam having a narrow divergence angle depending on the intensity and transmission distance of a transmission laser and the signal detection performance on the receiving side. In addition, when the structure on which the optical transmitter / receiver is mounted is a moving structure such as an aircraft or an artificial satellite, a change in the attitude of the structure causes a change in the transmission direction of the laser beam or a change in the viewing direction of the light receiver. Occurs.
When these fluctuation angles are larger than the beam width, it is necessary to control the direction of the laser and the direction of the field of view of the photodetector in order to continuously transmit and receive the laser beam. One method is to receive light propagating from the other side of communication, detect fluctuations in the direction of incidence of the light on the light receiving telescope, and based on this, the direction of the laser to transmit and the field of view of the receiver. Tracking is performed by changing the direction.
In this case, tracking is performed by separately incorporating a tracking laser beam having a different wavelength from the communication laser beam, or when a part of the laser beam for optical communication is branched and used for tracking after receiving the laser beam for optical communication. There is. The latter method is advantageous in that the configuration of the transmission / reception optical system is simple and lightweight. At present, the light modulation method in spatial laser communication is a method in which an intensity-modulated optical signal is directly detected in reception, and the intensity modulation method is an on / off method that modulates a digital signal into light intensity according to the presence or absence of light. Methods such as a keying method and a pulse position modulation method for modulating a digital signal according to the position of an optical pulse are employed.

[0003]

However, in the above-mentioned conventional technique, when the distance between the transmitting and receiving points is long, for example, in the case of laser communication between artificial satellites, the spread or attenuation due to propagation causes There has been a problem that the amount of light received on the receiving side becomes weak, and errors in tracking and errors in demodulating optical signals increase due to the influence of background light peculiar to spatial laser communication. The problem to be solved is to improve tracking and signal detection accuracy in spatial laser communication. Therefore, an object of the present invention is to provide an optical signal transmission method with improved light detection accuracy in spatial optical communication.

[0004]

In order to solve the above problems SUMMARY OF THE INVENTION The optical signal transmission method according to claim 1, the transmitting side, depending on the level of the binary digital signal to be transmitted, the polarization direction of the linearly polarized light Transmits laser light that has been polarization-modulated into two types of polarized light that are orthogonal to each other, and on the receiving side, makes the received laser light incident on a polarizer, and converts the two types of polarized light according to the polarization direction of linearly polarized light. Separated, one polarized light was received as an optical signal, and the other polarized light was used as tracking detection light.

[0005]

Next, an embodiment of an optical signal transmission method according to the present invention will be described with reference to the accompanying drawings. FIG.
(A) shows a schematic configuration of a transmitting side device of a spatial optical communication device to which an optical signal transmission method according to the present invention is applied, and is configured by combining a laser oscillator 1, an electro-optic modulator 2, and a transmission telescope 3. Is done.

A laser beam 4 emitted from a laser oscillator 1 is polarized into a linearly polarized light 5,
Is used to generate a light 6 that is polarization-modulated into two types of polarizations in which the directions of linear polarization are orthogonal to each other, thereby performing modulation of a transmission signal. The polarization modulation shown in FIG. 1A shows a case of an NRZ code signal as an example. After the polarization modulation, the transmission laser beam 7 is emitted from the transmission telescope 3 toward the reception side.

FIG. 1 (b) shows a schematic configuration of a receiving side device of a spatial optical communication apparatus to which an optical signal transmitting method according to the present invention is applied. A receiving telescope 8, a polarizing beam splitter 9, an optical signal receiving device. Device 10, two-dimensional optical sensor 11 for tracking
Are configured in combination.

On the receiving side, the receiving laser beam 12 is received by the receiving telescope 8 and is incident on the polarizing beam splitter 9. The transmitted light 13 of the polarization beam splitter is a light having a polarization direction parallel to the plane of incidence including the incident light and the reflected light in the plane with respect to the polarization beam splitter 9, and the reflected light 14 of the polarization beam splitter is reflected on the plane of incidence. It becomes light in the vertical direction. The optical signal receiver 10 can detect a signal that is polarization-modulated on the transmission side. The degree of modulation of the signal is 100% when one of the two polarization directions of the polarization-modulated light 6 generated on the transmission side coincides with the direction of the plane of incidence on the polarization beam splitter 9. The degree of modulation is obtained.

The reflected light 14 of the polarization beam splitter enters the tracking two-dimensional optical sensor 11 and is used for tracking . The present invention is to position it receiving side polarization child, performs a reception of an optical signal, is used for transmission and reception of optical signals countryside
The light in the one-way polarization direction is used for tracking.

[0010] When there is a shift in the rotation direction about the reception laser beam 12 between the transmission side and the reception side, the modulation degree in receiving the optical signal decreases. In this case,
As shown in FIG. 2, a half-wave plate 15 is inserted between the receiving telescope 8 and the polarization beam splitter 9, and the polarization direction of the light incident on the polarization beam splitter 9 is changed with respect to the plane of incidence of the polarization beam splitter 9. By adjusting the optical axis direction of the half-wave plate 15 so as to be parallel or vertical,
A 100% modulation degree can be obtained in receiving an optical signal.

When there is a change in the attitude in the rotation direction about the reception laser beam 12, as shown in FIG. 3, a quarter-wave plate 16 is installed on the transmission side and the reception side to detect an optical signal. Can be prevented from lowering.
The transmitting quarter-wave plate 16 is adjusted so that the optical axis is at an angle of 45 degrees with respect to the incident polarization direction, so that the transmitted light becomes right-handed or left-handed circularly polarized light 17. To do. On the receiving side, the quarter-wave plate 16 is aligned at an angle of 45 degrees with respect to the incident surface of the polarizing beam splitter 9 to convert the received circularly polarized light 17 into linearly polarized light. As a result, it is possible to prevent a decrease in the degree of modulation due to a change in attitude in the rotation direction about the reception laser beam 12.

Although the electro-optic modulator 2 has been described as a device for changing the polarization direction, a half-wave plate can be used instead of the electro-optic modulator 2 because it is only necessary to generate two orthogonal linearly polarized lights. The polarization modulation of light can be applied even by using a device that rotates the light according to a transmission signal, and the modulation can be applied depending on conditions such as the modulation speed and the laser beam diameter.

[0013]

As described above, when the optical signal transmission method according to the present invention is used, the amount of light received on the receiving side is compared with the method in which a single intensity-modulated laser beam is shared for tracking and data reception. Is doubled, the error at the time of light detection for tracking is reduced, and the loss due to branching is also eliminated in optical signal reception, so that the error rate of signal detection can be reduced. This method is rich in applicability because it can be applied to the case where the attitude is shifted or fluctuated between the transmitting side and the receiving side.

[Brief description of the drawings]

FIGS. 1 (a) and 1 (b) are schematic structural diagrams of a transmitting side and a receiving side of a spatial optical communication apparatus to which an optical signal transmission method according to the present invention is applied.

FIG. 2 is an explanatory diagram of inserting a half-wave plate on the receiving side when there is a shift in the rotation direction about the transmission laser optical axis between the transmitting side and the receiving side.

FIG. 3 is an explanatory diagram using a quarter-wave plate when there is a posture change in the rotation direction about the transmission laser optical axis on the transmission side and the reception side.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 laser oscillator 2 electro-optic modulator 3 transmission telescope 4 laser light 5 linearly polarized light 6 polarization-modulated light 7 transmission laser beam 8 reception telescope 9 polarization beam splitter 10 optical signal receiver 11 tracking two-dimensional optical sensor 12 reception Laser beam 13 Transmitted light from polarizing beam splitter 14 Reflected light from polarizing beam splitter 15 Half-wave plate 16 Quarter-wave plate 17 Circularly polarized light turning right or left

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) H04B 10/00-10/28 G02B 27/00 H04B ^7/ 14-7/22

Claims (1)

(57) [Claims] 1. A transmitting side transmits a laser beam that has been polarized and modulated into two types of polarizations in which the polarization directions of linearly polarized light are orthogonal to each other in accordance with the level of a binary digital signal to be transmitted. The received laser light is incident on a polarizer, and the two kinds of polarized light are separated according to the polarization direction of the linearly polarized light. One of the polarized lights is received as an optical signal, and the other polarized light is detected as tracking light for tracking. An optical signal transmission method characterized by using as an optical signal.