JPH0310531A - Analog signal demodulation system - Google Patents

Abstract

PURPOSE:To reduce the band characteristic required to an optical receiver circuit after a photodetector by applying AM modulation to a received optical signal with a carrier frequency before photoelectric conversion is performed. CONSTITUTION:An optical signal L1 received from a transmission line 1 is inputted at first to an external modulator 2. The external modulator 2 modulates an inputted optical signal L1 with a prescribed carrier frequency(f) supplied from a transmitter 6 and the result is outputted as an optical signal L2. The optical signal L2 outputted from the external modulator 2 is inputted to a light receiving device 4 and demodulated and outputted as a base band signal S1. Thus, the band characteristic required to the light receiving device 4 is low enough corresponding to the optical signal L2.

Description

[Detailed description of the invention] Industrial applications The present invention relates to an analog signal demodulation system.

More specifically, the present invention relates to a novel configuration of a system that demodulates multiplexed optical analog signals by AM modulation upon reception.

Conventional technology Conventionally, to receive an AM modulated optical analog signal, the transmitted optical signal was input to an optical receiver, converted to an AM modulated electrical signal, and then AM demodulated to obtain a baseband signal. . Furthermore, AM multiplexed signals are similarly converted into electrical signals and then processed.

Here, two-channel analog image transmission will be described as an example.

The band of a normal image signal is 4.2 MHz, but when actually transmitting it, it is generally carried on a carrier around 100 MHz. Here, 1 channel is 100M)
Place it on the lz carrier and set the other channel to 106MHz.
If the transmission signal is placed on a carrier, the spectrum of the carrier component and the spectrum of the 4.2 MHz signal component on one side of the spectrum are observed from the transmission signal.

FIG. 2 is a diagram showing the power spectrum of such a signal. When receiving an image signal having such a power spectrum, the optical receiver has a power spectrum of 100 to 110.
Needless to say, an optical receiving circuit configuration having effective sensitivity in a high band up to 2 MHz is required.

Problems to be Solved by the Invention However, an AM modulated or AM multiplexed optical signal has a frequency component in a wide band approximately 100 times larger than that of the baseband signal. Since an optical receiver that receives this signal is required to have an extremely wide reception band, it is difficult to ensure reception sensitivity, and as a result, the circuit configuration becomes complicated.

Therefore, the present invention solves the problems of the above-mentioned prior art, and A.
The object of the present invention is to provide an analog signal demodulation system with a novel configuration that can receive M-modulated and/or AM multiplexed optical signals with a simpler circuit.

Means for Solving the Problems According to the present invention, in a system that receives and demodulates an analog optical signal transmitted by AM modulation and/or AM multiplexing, the received optical signal is transmitted at the same frequency as its carrier wave. Equipped with an external light intensity modulator that modulates the frequency,
A signal demodulation system is provided, characterized in that normal demodulation processing is performed on the output light of the external optical intensity modulator.

The analog signal demodulation system according to the present invention reduces the band characteristics required of the optical receiver below the light receiving element by AM modulating the received optical signal at the carrier frequency before photoelectrically converting it. This is its main feature.

This will be explained using two-channel analog image transmission as an example.

That is, similar to the example described in the prior art section, 4.2
M) tz band image signal, one channel is 100M/
Place it on a Hz carrier and set the other channel to 106MHz.
z carrier and further superimposed to receive an optical signal including a band from 100 to 110.2 MHz as shown in FIG.

Here, the received signal is further 100MHz as an optical signal.
When remodulated at the frequency z, the resulting optical signal is between 0 and 1
It becomes an optical signal including a band up to 0.2M&. Figure 3 shows
FIG. 3 is a diagram showing a power spectrum of a signal obtained by further modulating the received optical signal at a frequency of 100 MHz. As is clear from the power spectrum shown in FIG. 3, this optical signal can be received and demodulated as a two-channel signal if there is an optical receiver having a reception band of 10.2 MHz.

Also, if the transmitted signal is only a signal carried on a 106 MHz carrier, and if the frequency to be remodulated is 106 M7, then the signal carried on a 106 M)lz carrier will be 4.2
It can be received/demodulated with an optical receiver in the MHz band.

In this way, in the system according to the present invention, by re-modulating the optical signal on the receiving side, it becomes possible to demodulate the received signal in the baseband signal band. Therefore, the bandwidth required for the optical receiver is also reduced, and it becomes possible to configure the demodulation system with simple hardware.

The demodulation system according to the present invention will be described in more detail below with reference to the drawings, but the disclosure below is only one embodiment of the present invention and does not limit the technical scope of the present invention in any way. .

Embodiment FIG. 1 is a block diagram showing the basic configuration of an analog signal demodulation system according to the present invention.

As shown in FIG. 1, in the demodulation system according to the present invention,
The optical transmission line 1 is first coupled to an external modulator 2, and the output of the external modulator 2 is coupled to an optical receiver 4. Furthermore, a transmitter 6 is connected to the external modulator 2 .

In the demodulation system configured as described above, an optical signal received from the transmission line 1 is first input manually to the external modulator 2. The external modulator 2 modulates the manually input optical signal with a predetermined carrier frequency f supplied from the transmitter 6, and outputs it as an optical signal 2. The optical signal L2 output from the external modulator 2 is manually demodulated by the optical receiver 4 and output as a baseband signal S1.

That is, the optical signal L1 input to this system is an AM modulated signal having a power spectrum as shown in FIG. 2 is a low band signal having a power spectrum as shown in FIG. Therefore, it is sufficient that the optical receiver 4 has a low band characteristic corresponding to the optical signal L2. Furthermore, the signal S1 output from the optical receiver 4 is the baseband signal itself.

Manufacturing Example AM The multiplexed optical signal L1 can be, for example, modulated light of a semiconductor laser beam. In addition, the demodulation process itself is
Since a known optical receiver can be used, the following
A specific example of the configuration of an external modulator that can be used in the system of the present invention will be described.

FIG. 4 is a diagram showing a configuration example of an external modulator that can be advantageously used in an analog signal demodulation system according to the present invention.

As shown in FIG. 4, this external modulator has a polarizer 10, a quarter-wave plate 12, a bulk waveguide 14, and an analyzer 16 arranged in this order for the AM-modulated human-powered optical signal L1. It is configured as follows. The bulk waveguide 14 is
It is made of lithium niobate, and is connected to an oscillator 6.

In the external modulator configured as described above, a human-powered optical signal is first converted into a circularly polarized wave by a polarizer 10 and a quarter-wave plate 12, and then transferred to a bulk waveguide made of lithium niobate. Guided by 14. The bulk optical waveguide 14 includes
A voltage modulated according to the modulation frequency is applied by the oscillator 6 to intensity-modulate the optical signal propagating inside. The optical signal modulated in this manner is outputted as a baseband optical signal L2 via the analyzer 16. Note that such an external modulator configuration is suitable for AM with a carrier frequency of IMHz or less.
Suitable for optical signals.

FIG. 5 is a diagram showing another configuration example of an external modulator that can also be applied to the demodulation system according to the present invention.

As shown in FIG. 5, this external modulator includes a leading waveguide 26 formed on a lithium niobate substrate 21, electrodes 29a, 29b, and These electrodes 29a and 29b
Electrodes 28a and 28b formed between and on both sides
, 28c, and an oscillator 6 further connected to electrodes 29a and 28b. Here, the optical waveguide 26 has an optical branching section 20 formed on the incident side, where two
After branching into the main waveguide to form a pair of phase modulation sections 22, it is again branched to the optical multiplexing section 24 near the output side to form a single waveguide.

In the external modulator configured as described above, the incident optical signal L1 is first branched at the optical branching section 20, and each branched light is phase modulated at the frequency generated by the oscillator 60 at the phase modulation section 22. Thereafter, it is converted into an intensity-modulated optical signal L2 in the optical multiplexer 24. Therefore,
If the modulation frequency generated by the oscillator 6 is the same as the AM modulation carrier frequency of the human-powered optical signal L1, the output optical signal L2 becomes a baseband signal. Note that this external modulator effectively operates up to IGHz or higher.

FIG. 6 is a diagram showing still another configuration example of an external modulator that can also be used in the demodulation system according to the present invention.

As shown in FIG. 6, this external modulator includes a pair of parallel optical waveguides 34a and 34b formed on a substrate 32,
Electrode 36a1 formed on each optical waveguide 34a, 34b
36b, and an oscillator 6 that applies a voltage between the electrodes.

In this external modulator, the human-powered optical signal L1 is transmitted through the optical waveguide 34.
b. Here, by applying a voltage to the electrodes 36a, 36b, the equipolarized refractive index of the optical waveguides 34a, 34b is changed, and the optical signal L2 modulated at the frequency generated by the oscillator 6 is output. . Therefore, by setting the oscillation frequency of the oscillator 6 to the carrier frequency of the received optical signal, the output optical signal L2 can be made into a baseband signal. Note that it is specified in 13R that this external modulator also operates effectively up to IGHz or higher.

As described in detail of the invention, in the analog signal demodulation system according to the present invention, it is possible to receive/demodulate an AM-modulated analog signal containing a high frequency component using a low frequency receiver of about the baseband frequency. Therefore, the receiver circuit configuration becomes simple and high sensitivity can be easily achieved.

Further, by changing the frequency signal applied to the external light intensity modulator, it is also possible to select an AM multiplexed baseband signal. Therefore, it can be advantageously used, for example, as a receiving system for multi-channel analog image transmission.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the basic configuration of an analog signal demodulation system according to the present invention, FIG. 2 is a diagram showing the power spectrum of a transmitted optical signal by AM modulation and AM multiplexing, and FIG. The figure is a diagram showing the power spectrum of the optical signal manually input to the optical receiver in the demodulation system according to the present invention. FIG. 3 is a diagram illustrating an example of a configuration of a possible external modulator. [Main reference numbers] 1... Optical transmission line, 2... External modulator, 4... Optical receiver, 6... Transmitter, 10... Polarizer, 1
2...1/4 wavelength plate, 14...Bulk type waveguide,
16... Analyzer, 20... Optical branching section, 21... Lithium niobate substrate, 22... Phase modulation section, 24... Optical multiplexing section, 26...
・Optical waveguide, 28a, 28b, 28c, ・29a, 2
9b... Electrode, 32... Substrate, 34a, 34b --- Optical waveguide, 36a, 36
b -・-electrode 2, external modulator 4 optical receiver

Claims (4)

[Claims] (1) A system that receives and demodulates analog optical signals transmitted through AM modulation and/or AM multiplexing, which includes an external optical intensity modulator that modulates the received optical signal at the same frequency as its carrier wave frequency. , a signal demodulation system characterized in that normal demodulation processing is performed on the output light of the external optical intensity modulator. (2) The analog signal demodulation system according to claim 1, wherein the external light intensity modulator includes a polarizer, a quarter-wave plate, an electro-optical element, and an analyzer. (3) The analog signal demodulation system according to claim 1, wherein the external optical intensity modulator includes an optical branching section, a phase modulation section, and an optical multiplexing section. (4) The analog signal demodulation system according to claim 1, wherein the external optical intensity modulator includes a pair of mutually parallel optical waveguides formed on an electro-optical element.