JPS5910098B2 - Space transmission optical communication device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical communication device, and particularly to a spatial transmission optical communication device that utilizes space as a transmission path for multi-channel transmission of television signals.

Various applications of optical communication have been attempted due to the development of light emitting devices.

Television multiplex transmission is also an example of its application. However, when performing multiplex transmission, distortion occurring between signals has been a problem because the light emitting element has nonlinearity. In televisions in particular, the resulting distortion appears on the screen demodulated by the receiver, severely degrading the quality. Therefore, when performing television multiplex transmission, it is necessary to prevent distortion, but since distortion increases each time the number of transmission channels increases, the number of signals that can be multiplexed is naturally limited. The present invention has been made in view of the above-mentioned circumstances, and its object is to provide a space transmission optical communication device that enables distortion-free television multiplex transmission.

An embodiment of the present invention will be described below with reference to the drawings.
Generally, a light emitting diode or a semiconductor laser (hereinafter abbreviated as LD) is used as a light emitting element of an optical communication device for television multiplex transmission.

Here, a case using an LD will be explained. Figure 1 shows an optical communication device that uses space as a transmission path.
This is an example in which a transmission signal is divided into two systems and transmitted using one optical system, and is composed of a light transmitter 1 and a light receiver 2.

In the light transmitter 1, a television multiplex signal is inputted from a VHF signal input terminal 3. This signal is converted into a VHF signal having an appropriate signal strength by the VHF amplifier 4 and sent to the distributor 5. Further, the signals are sent to band-pass filters (hereinafter abbreviated as BPF) 6 and 7, each having a different pass band, and are divided into two frequency groups. The output signals of BPFs 6 and 7 are sent to LD drive circuits 8 and 9, respectively, and drive LDIO and 11 with the respective output signals. LDIO, 11
use ones with different emission center wavelengths. L
Although the emission center wavelengths of D are slightly different, the bandwidth thereof is extremely narrow, so that multiplexing of optical signals is possible. L.D.
The optical signals of IO and 11 are emitted from near the focal point of lens 14 of light transmitter 1 through optical fibers 12 and 13 connected to each other. Since the diameters of the optical fibers 12 and 13 are extremely small, their ends can be placed near the focal point of the lens 14 at the same time. A single optical signal from the terminals of the optical fibers 12 and 13 is radiated into space by the lens 14 in a shape close to parallel light, and is emitted by the lens 15 of the optical receiver 2.
reach.

The optical signal focused by the lens 15 is transmitted to a light receiving element such as a photodiode (hereinafter referred to as
(abbreviated as PD) 16 converts the signal into an electrical signal again.
Although two light-emitting elements are used, only one light-receiving element is sufficient. Therefore, the multiplexed optical signal enters PDl6,
The output signal is a multiplexed signal having the same frequency arrangement as that applied to the HF input signal terminal 3 of the light transmitter 1. P
The output terminal of Dl6 is connected to the HF amplifier 17, and a signal of arbitrary magnitude can be obtained. The output is output from the HF signal output terminal 18. An embodiment of the present invention has been described above regarding the case where two LDs and one PD are used.

When applied to Japanese television broadcasting, this applies to channels 1 to 3 (90 to 108M).
This corresponds to two television broadcast bands: the so-called low band (Hz) and the so-called high band from the 4th channel to the 12th channel (170 to 222 MHz). In this way, the frequency band is divided and different LDs are used.
When obtaining optical output by using LD, it is possible to deeply modulate each channel because the number of channels is smaller than when all channels are transmitted using one LD.

Therefore, the signal-to-noise ratio (hereinafter S
/N) can be reduced. This greatly contributes to distortion reduction since it is known that the smaller the current flowing through the LD, the less distortion occurs. Furthermore, since the number of channels per LD is reduced, distortion between channels occurring within the same LD is also reduced.

From this, it can be seen that by increasing the number of LDs, that is, by increasing the number of divisions of the multiplexed signal, it is possible to further eliminate the influence of distortion. FIG. 2 shows a method for attaching the optical fibers 12 and 13 near the focal point.

The end surfaces 20 and 21 of the optical fibers 12 and 13 mounted on the optical fiber mount 19 are the optical fiber mount 1
It is attached so that it coincides with the end face of 9. Since the optical fibers 12, 13 are extremely thin, it is possible to arrange the end faces of a plurality of optical fibers within the size of the focal point of the lens.

FIG. 3 is a sectional view taken along line AA' in FIG. 2.

This is an example in which optical fibers 12 and 13 are arranged symmetrically with respect to the optical axis. Grooves 22 and 23 are provided for fixing the optical fibers 12 and 13, making it possible to accurately locate the center of the optical fibers. The optical fibers 12 and 13 are
It is fixed onto the optical fiber mount 19 with adhesive or a suitable metal band. At this time, the optical fiber can be fixed more firmly by passing it through a stainless steel pipe and fixing it. Furthermore, if a mechanism for moving the optical fiber mount 19 on the optical axis is provided, it is possible to finely adjust the amount of light transmitted to a light receiver located far away. Further, when using the method of guiding the optical output from the LDIO, 11 to a focal point using an optical fiber as described above, it is possible to configure the LDIO, 11 section and the lens 14 section separately. Therefore, LDlO,ll can be easily cooled.
Furthermore, in an extreme case, the LDIO, 11 section and the lens 14 section can be housed in separate cases. That is, since the space transmission optical communication device is often used outdoors, if it is made into a separate case, only the LDlO,ll section can be installed indoors, making it possible to protect the LDlO,ll. Although the above description has been made using an LD as a light emitting element and a lens as an optical system, a light emitting diode or a parabolic mirror may also be used.

Moreover, if the number of light emitting elements is two or more, the more the number increases,
Needless to say, an optical communication device with improved S/N and distortion characteristics can be obtained. As described above, according to the present invention, since the television signal per channel can be deeply modulated, the S/N can be improved compared to the case where only one LD is used.

Furthermore, the distortion between each channel after being divided decreases as the number of channels to be acquired decreases. Therefore, S/N
It is possible to perform television multiplex transmission with good performance and low distortion.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of a space transmission optical communication device according to an embodiment of the present invention, FIG. 2 is a perspective view showing a method of fixing the optical fiber near the focal point in the device, and FIG. It is a sectional view taken along the line A-A' in the figure. 1... Light transmitter, 2... Light receiver, 3...
...Input terminal, 4...Amplifier, 5...
...Distributor, 6,7...Band pass filter,
8, 9... Semiconductor laser drive circuit, 10, 11
... Semiconductor laser, 12, 13... Optical fiber, 14, 15... Luns, 16...
...Photodiode, 17...Amplifier, 18
...Output terminal, 19...Optical fiber mount, 20, 21...Optical fiber end surface, 22
, 23...V groove.

Claims (1)

[Claims] 1 A light transmitter that drives light emitting elements with different emission wavelengths using a plurality of divided frequency multiplexed signals and radiates the output light from each light emitting element into space from near the focal point of the optical system, and a light transmitter that emits light from the light transmitter. 1. A space transmission optical communication device comprising a light receiver that focuses a signal through an optical system and converts it into an electrical signal by a light receiving element placed near the focal point.