JPS6148284A - Optical picture transmitter - Google Patents

Abstract

PURPOSE:To eliminate interference between channels due to nonlinear distortion of a laser diode by restoring a signal into a frequency multiplex signal by an N:M photo synthesizing and branching device and transmitting simultaneously the signal to M-set of receivers being the same number or over the channel number. CONSTITUTION:Each output signal separated at each channel by band amplifiers 4-1-4-n is inputted to laser diodes 7-1-7-n through separate laser diode drive circuits 6-1-6-n and converted into optical signals. The optical output signal of the diodes 7-1-7-n is inputted to the optical synthesizing/branching device 12 by optical paths 11-1-11-n and after they are synthesized once at the device 12, they are branched again uniformly into M lines of optical fibers 8-1-8-m being the same number or over the channel number N and led to photodetectors 9-1-9-m, which convert the inputted optical signals into electric signals and give them to TV receivers 10-1-10-m.

Description

Detailed Description of the Invention [Technical Field of the Invention] The present invention relates to an optical image transmission/device that transmits multiple channels of frequency-multiplexed video signals in the VHF band or UHF band using a laser diode and an optical fiber. It is something.

[Prior Art] Conventionally, as this type of device, an example of which is shown in FIG. 2 has been known. In Figure 2, (1) is a receiving antenna that receives broadcast waves that are frequency-multiplexed video signals of multiple channels, and (2) is the receiving antenna (1).
preamplifiers, (3-1) to (3-n) that amplify the output of
is a bandpass filter that separates the output of the preamplifier (2) for each channel, and (4-1,) to (4-ri,) are the outputs of the bandpass filters (3-1) to (3-n). The band amplifier (5) amplifies the band amplifier (4-1) to (4
-n) is a signal synthesizer that combines the levels of each output signal; (6) is a laser diode drive circuit that superimposes the output of the signal synthesizer (5) on a bias current and supplies it to the laser diode (7); , (8) is an optical fiber, (9) is an optical receiver that photoelectrically converts and amplifies the optical signal transmitted by the optical fiber (8), and (10) is a TV receiver.

Next, the operation of the conventional optical image transmission device will be explained based on the above configuration. Broadcast waves as video signals frequency-multiplexed in the VHF band or UHF band are received by an antenna (1
), is amplified by the preamplifier (2), is once frequency-separated for each channel by bandpass filters (3-1) to (3-n), and is then passed through bandpass amplifiers (4-s) to
(4-n), the signals are amplified and level-adjusted so that the output levels of each channel are equal, and then combined again in a signal synthesizer (5).

The output signal of the signal synthesizer (5) is applied to the laser diode drive circuit (6), where it is set together with a DC bias current to satisfy a predetermined modulation condition, and then output to the laser diode (7). is converted into an optical signal. The optical signal output from the laser diode (7) is transmitted over a predetermined distance through an optical fiber (8) and then input to an optical receiver (9), where it is converted into an electrical signal and transmitted to a TV receiver. (10) is output.

Here, the received C/N (carrier-to-noise ratio) when transmitting a frequency-multiplexed video signal in the V) IF band using the conventional optical image transmission apparatus having the configuration shown in FIG. 2 is calculated. now,
The elementary quantity of electricity is e, the multiplication factor of APD is M, the light receiving sensitivity is S, the excess noise figure is
) is the relative noise intensity of RIN, the demodulation bandwidth of the optical receiver (8) is B, and the root mean square value of the input equivalent noise current is 7 q 2
Then, the root mean square value T82 of the received signal current of the optical receiver (9) and the root mean square value Tn2 of the total noise current are respectively expressed by the following equations.

Completion s2 = % e (S Mm Pr
) 2 =・(1) 〒n2 = (T
q2 +2e (S Pr+Id)M +RIN
@(S M Pr ) 2't B −(2)(
Using equations 1) and (2), the reception CAM is expressed as follows.

= 1101o [(%) * (S Mm
Pr ) 2” (1/ (iq2+2e(S
Pr+ Id)M”+ RIN ψ (SMPr
) 2) B) ]...(3) In equation (3), the APD multiplication factor Mopt that maximizes G/N is determined as shown in the following equation.

From equations (3) and (4), an example of numerical calculation of the average received light level (Pr)-C:/N characteristic when transmitting 7 VHF broadcast waves is shown in Figure 3D.
=0.48A/W, x=0.3, Id=0.1nA
, m = 0.1 /ch, B = 4.2 MHz, 7 stones F = 4 PA / "n, = L, the relative noise intensity (RIN) of the laser autonode is used as a parameter.

From FIG. 3, it can be seen that the received C/N is greatly influenced by the relative noise intensity (RIN) of the laser diode (7). Here, if the required CAM is 42 dB, the relative noise intensity (RIN) of the laser diode (7) is -133
It is necessary to keep it below dB/Hz.

On the other hand, in order to prevent beat disturbance due to cross-modulation between channels, it is necessary to use a laser diode with small nonlinear distortion in the operating region.

- In the conventional optical image transmission device configured as described above, for example, the VHF band or U) IF band, such as TV broadcast waves,
When transmitting video signals of multiple channels that are frequency-multiplexed in the band, the relative noise intensity (
In order to avoid interference (cross-modulation) between channels, a laser diode with low non-linear distortion is required, and in some cases it is necessary to select the laser diode.

[Summary of the invention]

The present invention has been made to solve the above-mentioned drawbacks.
Frequency multiplexed video signals of multiple channels are separated for each channel, converted into optical signals by N separate laser diodes, and then the respective optical outputs are combined and the number of channels is the same as the number of channels N or more than M signals. The laser beam is converted back into a frequency multiplexed signal by an N-to-M optical combiner/brancher that equally splits the signals into optical fibers, and simultaneously transmits the signal to M receivers with the same number of channels or more. It is an object of the present invention to provide an optical image transmission device whose transmission characteristics are not easily influenced by the relative noise intensity (RIN) and nonlinear distortion of the diode itself.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described based on the overall circuit block diagram shown in FIG. 1, in which the same or corresponding parts as those of the conventional device shown in FIG. 2 described above are given the same reference numerals. In the figure, the optical image transmission device according to the present embodiment includes a reception antenna (1) that receives broadcast waves, a preamplifier (2) that amplifies the broadcast waves received by the reception antenna (1), The preamplifier (
Bandpass filters (3-1) to (2) operate as signal separators that frequency separate the output of each channel (N pieces).
3-n), and a bandpass amplifier (4-n) that amplifies each output signal separated by the bandpass filters (3-1) to (3-n).
s ) to (4-n) and the band amplifiers (4-t ) to (
A laser diode drive circuit (Ef-s
) to (B-n) and the laser diode drive circuit (
Laser diodes (?-1) to (8-1) to (+3-TI) which use the outputs of 8-1) to (+3-TI) as bias inputs and convert them into optical signals, respectively.
7-n) and the laser diodes (7-1) to (?-n
), and the optical waveguides (11-z) to (11-n
), and this combined optical signal is branched into the same number of channels (N) or more as optical signals corresponding to the original frequency-multiplexed video signal. A fist branching device (12) and optical fibers (8-1) to 8-1 provided corresponding to the number of branches branched by the optical synthesis/branching device (12) and transmitting the branched optical signals.
(8-m) and the optical fibers (8-s) to (8-n)
optical receivers (9-1) to (8-m) that photoelectrically convert and amplify the optical signals transmitted by the optical receivers (9-1), respectively;
) to (9-m), and a TV receiver that displays images based on frequency-multiplexed video signals output as electrical signals. -Next, the operation of the image transmission apparatus according to this embodiment based on the above configuration will be explained. In addition, a receiving antenna (1), a preamplifier (2), a bandpass filter (3-
1) to (3-n) and band amplifiers (4-+) to (4,
-n) is completely the same as that of the conventional device shown in FIG. 2, and will therefore be omitted. First, the band amplifier (4-i)
Each output signal separated for each channel by (4-n) is sent to a separate laser diode drive circuit (8
= 1) to (8-n) to each laser diode (7
-1) to (7-n) and are converted into optical signals. −
The optical output signals of the laser diodes (7-1) to (?-n) are transmitted through optical waveguides (11-t) to (11-n), respectively.
n) is input to the optical combiner/brancher (12), where it is once combined and then equally distributed over M optical fibers (8-1) to (8-m), which are equal to or greater than the number of channels N. The signal is branched again and guided to each optical receiver (L1) to (9-m). Each of the above optical receivers (9-1') to (9-m)
converts the input optical signal into an electrical signal and connects each TV receiver (10
-1) Output to (10-m). In other words, frequency-multiplexed video signals of multiple channels are transmitted through separate N laser diodes (?-1) for each channel.
(?-n) into an optical signal, it is returned to the original frequency multiplexed signal by an N to M optical combiner/brancher (12) and connected to M optical fibers with the same number of channels or more. 8-1) to (8-n+), the same signal is simultaneously transmitted to M optical receivers (9-,) to (θ1).

As described above, in the embodiment shown in FIG. 1, separate laser diodes (7-1
) to (7-n), unlike the conventional example shown in Fig. 2, the laser diode (
7-s) to (7-n), the problem of intermodulation (cross modulation) between channels caused by distortion is solved. - Also, as shown below, each laser diode C7-1) to (7-n) required to satisfy the required CAM is compared to the conventional example shown in FIG.
The relative noise intensity (RIM) requirement for is relaxed.

Hereinafter, calculations regarding the RIN improvement effect described above will be performed. Now, the number of channels is N, and each laser diode (7-1')
The modulation degree and relative noise intensity of ~(7-n) are respectively mi
' , RINi ' (i=1.2...,N)
Also, the number of branches of the photosynthetic divider (12) is Na,
N laser diodes (7-1) to (7-n) input to each of the Na optical receivers (9-1) to (9-m)
The optical input from Pi′−(i=1.2,...,N
) far. where - 1 ml' of the above.

Assume that RINi' and Pi' satisfy the following equation.

mi' = Nm (i=1.2...,N)
...(5) RINi'=RIN (i=1.2.・
..., N) ... (6) Pi' = Pr/Ha
(i=1.2.-, N) - (7) However, m, RIN
, Pr are the modulation degree per channel in the C/N calculation formula (3) in the conventional example shown in FIG. 2, the relative noise intensity of the laser diode (7), and the optical receiver (9-1).
) to (9-m) respectively.

Here, each of the above laser diodes (7-1) to (7-r
+) are unrelated to each other, the received C of each channel when the total average received light level Pr is
AM is expressed by the following formula.

Crm5/N' rn+s = 101og [(34) ・(SMm i'Pi'
)2・(1/1, - (iq2+2e(S Pr+Id)M +(S M
Pi') 2ΦR1 old')t3)]...
・From formulas (8), (5), (6), -(7), and (8), C
rm5/N' rms = 1101o [(34) ” (M, S Mm
Pr) 2・(1/(Tq2+2e(S Pr+
Id)M”+(S M Pr ) 2・M¥RIN)B
)] ...(9) Here, Ma≧N
-(10) Comparing the denominators of equations (3) and (8), it is found that the embodiment of this invention has the same RI as the conventional example.
Relative noise intensity RIN of each laser diode viewed from the receiver side when using eight laser diodes with M
is equivalently given by the following equation.

That is, compared to the conventional example shown in FIG. 2, the relative noise intensity (RIN) of laser diodes (7-1) to (7-n) is
) is equivalently improved by 1101oΣ(dB).

Note that especially when Ma=N is satisfied, equation (9) becomes as shown in the following equation.

erIIls/N' TIIS = 101og [(q) e (S Mm Pr)
2m(1/(dq2+2e(S Pr+I)
d. It can be seen that the required value of the relative noise intensity (RIN) of the laser diodes (7-1) to (?-n) necessary to obtain the characteristics is relaxed by 1010 gN101O.

In Fig. 1, the configurations of the reception antenna (1), preamplifier (2), bandpass filters (3-1) to (3-n), and bandpass amplifiers (4-1) to (4-n) are as follows. This is exactly the same as the conventional example shown in Figure 2, and the number of laser diode optical transmitters per receiver is on the order of N/Ha (≦1), so if the cost of the optical combiner/brancher can be reduced, Compared to the conventional example shown in FIG. 2, the cost per recipient can be reduced to the same level or lower.

In the above embodiment, the case of transmitting TV broadcast waves has been described, but it goes without saying that the present invention can also be applied to the case of transmitting other frequency-multiplexed video signals.

〔Effect of the invention〕

As described above, according to the present invention, a plurality of frequency-multiplexed image signals are separated for each channel, each converted into an optical signal by a separate diode, and then the respective optical outputs are combined and the number of channels is equal to the number of channels. This is a configuration in which the signal is returned to a frequency multiplexed signal using an optical combiner/brancher that evenly splits the signal into a number of optical fibers equal to or more than the number of channels, and simultaneously transmits the signal to the same number of optical receivers as or more than the number of channels. The relative noise intensity (RIM) of the laser diode can be solved.
Since the required value of is relaxed, there are effects such as no need to select laser diodes. Further, due to the interference effect of the photosynthesis/branching device, it is also possible to suppress the influence of reflected light from the optical fiber connector portion, etc.

[Brief explanation of drawings]

FIG. 1 is an overall circuit block diagram of an optical image transmission device according to an embodiment of the present invention, FIG. 2 is an overall circuit block diagram of a conventional optical image transmission device, and FIG. 3 is an optical image transmission device that uses VHF broadcast waves. Average received light level (Pr)-C/ when transmitting 7 waves
The N characteristic diagram is shown. (1)...Receiving antenna, (2)...Preamplifier,
(3-1) to (3-n)...Band filter, (4-s
)~(4-n)...bandwidth amplifier, (5)...signal synthesizer, (fli), (8-1)~(ft-n)...laser diode drive circuit, (7), (7-1) to (7-n)... Laser diode, (8), (8-1) to (8-+s)... Optical fiber, (9), (9-1) to (9 -m)...optical receiver,
(10), (10-1) ~(10-m)...TV receiver, (11-1) ~(11-n)...optical waveguide,
(12)...Photosynthesis-brancher. In the drawings, the same or equivalent parts are designated by the same reference numerals.

Claims (4)

[Claims] (1) In an optical image transmission device that transmits multiple channels of frequency-multiplexed video signals in the VHF band or UHF band using laser diodes and optical fibers, a signal separator that separates the frequency of input electrical signals for each channel. , laser diodes of the same number as the number of channels N for converting each output electrical signal of the signal separator into an optical signal, and combining each output light of the laser diodes to form a signal having the same number or more than the number of channels. 1. An optical image transmission device comprising: an N to M optical combiner and splitter that equally re-branches into M optical fibers. (2) The optical image transmission device according to claim 1, wherein the laser diode is constructed using a laser diode that oscillates in longitudinal multimode. (3) The optical image transmission device according to claim 1 or 2, wherein the laser diode is constructed using a large number of laser diodes integrated on the same substrate. (4) The light combining/branching device is constructed using a light combining/branching device having an optical waveguide structure formed on the same substrate as a laser diode, or any one of claims 1 to 3. The optical image transmission device according to claim 1.