JP4090707B2 - Optical receiver - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical receiver that demodulates and receives an optical subcarrier modulated signal, and more particularly to an optical receiver that performs a multiplication operation by modulating an optical signal with an electric signal.
[0002]
[Prior art]
For performing conventional optical subcarrier communication, as described in, for example, Japanese Patent Laid-Open No. 2000-138644, a modulated optical signal subcarrier-multiplexed from a central control station to each antenna station is used as a photodiode. Then, the signal is demodulated after being filtered by a band-pass filter circuit that passes the subcarrier frequency as a center.
[0003]
FIG. 7 is a diagram illustrating a configuration of a conventional example of a communication system that performs optical subcarrier communication. In FIG. 7, the central control station 10 and a plurality of antenna stations 20 are connected via an optical fiber 30, and an optical branching / integrating unit 31 is provided in the middle of the optical fiber 30. The input optical signal is branched and distributed to each antenna station 20. The centralized control station 10 includes an input terminal 11 through which each data signal is input, a modulator 12, a frequency converter 13, two pilot carrier generators 14 and 15, a synthesizer 16, a laser driver 17, and a laser. The antenna station 20 includes a photodiode 21 to which an optical signal is input from an optical fiber 30, a data signal separator 22, two pilot carrier separators 23 and 24, a frequency converter 25, An antenna 26 is included.
[0004]
In the central control station 10, the signal modulated by the modulator 12 with the data input from each input terminal 11 is frequency-converted by the frequency converter 13 and input to the synthesizer 16. The synthesizer 16 receives pilot carrier signals having different frequencies from the pilot carrier generators 14 and 15, and the synthesizer 16 synthesizes and outputs the pilot carrier signal and the frequency-converted signal. The laser driver 17 drives the laser element 18 in response to the output signal, and outputs a modulated optical signal that has been subcarrier multiplexed to the optical fiber 30.
[0005]
The antenna station 20 performs photoelectric conversion on the input optical signal by the photodiode 21, and then separates and passes the data signal frequency and the subcarrier frequency by the data signal separator 22 and the pilot carrier separators 23 and 24. The frequency converter 25 converts the frequency and sends the data to the antenna 26.
[0006]
[Problems to be solved by the invention]
However, according to the above conventional technique, for example, when a signal having a remarkably poor signal-to-noise ratio (SN ratio) is received, such as a supervisory control signal in which a subcarrier is superimposed on a high-speed optical signal that is a main signal only slightly. In order to avoid the saturation of the amplifier due to noise and obtain a predetermined gain, it is necessary to improve the S / N ratio by using a band pass filter circuit having a very narrow band centered on the subcarrier frequency.
[0007]
However, in general, when the band of the band-pass filter circuit is narrowed, the demand for temperature characteristics and aging of the center frequency becomes severe, and when the center frequency is low, the circuit size increases. there were.
[0008]
The present invention has been made in view of the above problems, and an object of the present invention is to obtain a receiver capable of accurately demodulating a signal having a poor signal-to-noise ratio.
[0009]
[Means for Solving the Problems]
In order to solve the above-described problems and achieve the object, the optical receiver according to the present invention modulates an input optical signal in the optical receiver that demodulates and receives an optical subcarrier-modulated signal. Oscillation means for driving the modulation means, photoelectric conversion means for photoelectrically converting the modulated optical signal into an electrical signal, and controlling the oscillation frequency or phase of the oscillation means by the converted electrical signal And a control means.
[0010]
According to the present invention, the optical modulation means is disposed in front of the photoelectric conversion means, the input optical signal is modulated by the output from the oscillation means, and the multiplication operation conventionally performed in the electric circuit is performed in the state of the optical signal. In addition, by controlling the oscillation frequency and phase of the oscillation means using the electrical signal output from the photoelectric conversion means, and synchronizing the output of the oscillation means with the subcarrier signal, the photoelectric conversion means The baseband signal is obtained directly at the output of.
[0011]
In the optical receiver according to the next invention, in an optical receiver for demodulating and receiving an optical subcarrier modulated signal, the first and second optical modulation means for modulating the input optical signal, Oscillating means for driving the first and second optical modulation means to output signals of mutually orthogonal phases to the first and second optical modulation means, and the modulated optical signal as an electrical signal, respectively. First and second photoelectric conversion means for electrical conversion, multiplication means for multiplying the converted electrical signal, and control means for controlling the oscillation frequency or phase of the oscillation means by the multiplied electrical signal It is characterized by providing.
[0012]
According to the present invention, the oscillating means provides the first and second optical modulation means with outputs of phases orthogonal to each other to modulate the optical signal, and photoelectrically converts the two modulated optical signals. By obtaining the electrical signal and controlling the oscillation frequency and phase of the oscillating means using the multiplied electrical signal and synchronizing the output of the oscillating means with the subcarrier signal, the output of the photoelectric conversion means is directly Get a band signal.
[0013]
In the optical receiver according to the next invention, in the above invention, the optical receiver further includes multiplexing means for multiplexing the optical signals transmitted by the plurality of optical fibers, and the combined optical signal is converted into the optical modulation means. It is characterized by being output to.
[0014]
According to the present invention, when a plurality of optical signals are inputted, they are combined and then modulated and multiplied, and the oscillation frequency of the oscillating means using the electrical signal output from the photoelectric conversion means And the phase is controlled so that the output of the oscillation means is phase-synchronized with the subcarrier signal.
[0015]
The optical receiver according to the next invention is characterized in that, in the above invention, the optical modulator comprises at least one of an optical modulator, an optical switch, and an optical amplifier for modulating a gain with an electric signal. .
[0016]
According to the present invention, the optical modulation means is not limited to an optical modulator, and any device such as an optical switch, an optical amplifier that modulates a gain with an electric signal, or an optical amplifier that combines an optical modulator may be used. Allows modulation of optical signals.
[0017]
In the optical receiver according to the next invention, in the above invention, the optical modulation means comprises a differential output type optical modulation means, outputs at least two modulated optical signals, and performs the photoelectric conversion. The means comprises differential input type photoelectric conversion means, and converts each optical signal into an electrical signal.
[0018]
According to the present invention, a differential output type optical modulation means modulates an input optical signal with an electrical signal and outputs two optical signals, and the differential input type photoelectric conversion means modulates the optical signal. Each optical signal is converted into an electrical signal, and two signals equivalent to those obtained by multiplying two types of oscillation outputs with inverted phases are obtained.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of an optical receiver according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.
[0020]
Embodiment 1 FIG.
FIG. 1 is a block diagram showing the configuration of the optical receiver according to the first embodiment of the present invention. In FIG. 1, an optical receiver 40 includes a plurality of input terminals 41 through which optical signals are input via a plurality of optical fibers (not shown), an optical multiplexer / demultiplexer 42, an optical modulator 43, and a photoelectric conversion element 44. An electric amplifier 45, an oscillator 46 whose oscillation frequency can be variably set by an input current / voltage, a loop circuit 47 for controlling the oscillator 46, a low-pass filter (hereinafter referred to as "LPF") 48, a base And an output terminal 49 for outputting a band signal (data signal).
[0021]
The optical multiplexer / demultiplexer 42 is composed of, for example, an optical coupler, a planar optical waveguide, or a wavelength multiple demultiplexer, and multiplexes and outputs optical signals input from the input terminals 41. The optical multiplexer / demultiplexer 42 can be omitted when there is a single optical fiber to which an optical signal is input. The optical modulator 43 includes, for example, an LN (Lithium Niobate) modulator, an EA (electro-absorption type) modulator, an acousto-optic switch, a mechanical switch, a semiconductor optical amplifier, a fiber amplifier capable of modulating the output of an excitation laser, or an optical modulator. A built-in fiber amplifier or the like can be used, and driving is performed so as to modulate the optical signal from the optical multiplexer / demultiplexer 42 with the signal of the oscillation frequency input from the oscillator 46.
[0022]
The photoelectric conversion element 44 is made of, for example, a photodiode, and the photoelectric conversion element 44 and the electric amplifier 45 constitute a photoelectric converter. As shown in FIG. 2, the loop circuit 47 includes an input terminal 47a for inputting an electric signal, a capacitor 47b for cutting off a DC component of the signal, a multiplier 47c, a loop filter 47d, an amplifier 47e, and an adder 47f. A dither oscillator 47g, an output terminal 47h that outputs a control signal to the oscillator 46, an LPF 47i, and an output terminal 47j that outputs a data signal to the LPF 48.
[0023]
In such a configuration, when an optical signal is input from each input terminal 41, the optical multiplexer / demultiplexer 42 combines these optical signals and outputs them to the optical modulator 43. The optical modulator 43 performs a multiplication operation by modulating the input optical signal with an electrical signal having an oscillation frequency from the oscillator 46. The modulated optical signal is converted into an electric signal by the photoelectric conversion element 44, then amplified by the electric amplifier 45 and output to the loop circuit 47.
[0024]
The electric signal output from the electric amplifier 45 includes frequency difference and phase difference information between the subcarrier signal and the oscillation frequency signal from the dither oscillator 47g. Therefore, the loop circuit 47 uses the multiplier 47c to multiply the output of the electric amplifier 45 by the output from the dither oscillator 47g, thereby synchronizing the oscillation frequency signal of the oscillator 46 with the subcarrier signal from the optical multiplexer / demultiplexer 42. I am letting. That is, this is to perform synchronous detection of the baseband signal using the optical modulator 43, and the baseband signal can be obtained directly from the outputs of the photoelectric conversion element 44 and the electric amplifier 45.
[0025]
The output of the multiplier 47c becomes an output of a low-frequency component by the loop filter 47d having the LPF function, and after being amplified by the amplifier 47e, is input to the adder 47f and the LPF 47i.
[0026]
Incidentally, since the output from the electric amplifier 45 includes a direct current component which is an average value of the input optical signal, the loop circuit has an appropriate low-pass characteristic like a normal PLL (phase locked loop). It is not possible to obtain a phase-synchronized operation simply by making it. Therefore, in the first embodiment, the adder 47f adds the output from the dither oscillator 47g to the oscillator control signal that is the output of the amplifier 47e, superimposes it, performs minute phase modulation, and inputs the signal to the input terminal 47a. Synchronous detection of band signals. By this operation, the phase detection output characteristic performed by the optical modulator 43 is equivalently differentiated and the DC component is removed.
[0027]
Therefore, when the subcarrier signal is an ASK (Amplitude Shift Keying) signal or an FSK (Frequency Shift Keying) signal, an appropriate determination, for example, a threshold value is provided after the output of the loop circuit 47 passes through the LPF 48, and the amplifier 47e. It is possible to demodulate the data signal by making a determination in comparison with the output of.
[0028]
According to the first embodiment, the optical modulator is arranged in front of the photoelectric conversion element, the optical signal is modulated by the output from the oscillator, and the multiplication operation performed in the electric circuit in the conventional example is performed on the optical signal. The output of the photoelectric converter by controlling the oscillation frequency and phase of the oscillator using the electric signal output from the photoelectric converter and the amplifier, and synchronizing the output of the oscillator to the subcarrier signal. By directly obtaining the baseband signal, a predetermined S / N ratio can be obtained, and a signal with a poor S / N ratio can be demodulated with high accuracy.
[0029]
Further, according to the first embodiment, since a baseband signal can be obtained directly from the photoelectric converter, LPF may be used for band limitation to obtain a predetermined S / N ratio. Therefore, it is not necessary to use a large band-pass filter, and space saving and cost reduction are achieved.
[0030]
Embodiment 2. FIG.
FIG. 3 is a block diagram showing a configuration of the optical receiver according to the second embodiment of the present invention. In the following drawings, the same components as those shown in FIG. 1 are denoted by the same reference numerals.
[0031]
In FIG. 3, in the second embodiment, a differential output optical modulator 50 is used as an optical modulator, and two types of optical signals having different phases are sent to the photoelectric conversion elements 44a and 44b, respectively. The optical signal is converted into an electrical signal by the photoelectric conversion elements 44a and 44b. Other configurations are the same as those of the optical receiver shown in FIG.
[0032]
The differential output optical modulator 50 includes, for example, an optical multiplexer / demultiplexer 50a, modulators 50b and 50c, and a phase inverter 50d as shown in the block diagram of FIG. In FIG. 4, modulators 50b and 50c are single-phase output modulators such as semiconductor optical amplifiers and EA modulators, and modulate the optical signal from the optical multiplexer / demultiplexer 50a with the oscillation frequency signal input from the oscillator 46. is doing. The phase inverter 50d inverts the phase of the transmission frequency signal from the oscillator 46 and outputs it to the modulator 50c. The photoelectric conversion elements 44a and 44b are connected to the electrical amplifier 45 in reverse phase.
[0033]
Next, the operation of the differential output optical modulator will be described with reference to the block diagram of FIG. In FIG. 5, the optical signal S <b> 1 from the optical multiplexer / demultiplexer 42 is input to the differential output optical modulator 50 via the input terminal 51. Here, the optical signal S1 is modulated by the output from the oscillator 46 and subjected to a multiplication operation, and becomes two optical signals S2 and S3 equivalent to those obtained by multiplying the two types of oscillator outputs whose phases are inverted. These optical signals S2 and S3 are input to the photoelectric conversion elements 44a and 44b via the output terminals 52 and 53, respectively, where they are converted into electric signals. The output polarity of the photoelectric conversion elements 44a and 44b and the electric amplifier 45 in which the photoelectric conversion elements 44a and 44b are connected in opposite phases are reversed when an optical signal is input to the photoelectric conversion elements 44a and 44b. Since it operates as a differential input photoelectric converter, when the two optical signals S2 and S3 from the differential output optical modulator 50 are simultaneously input to the photoelectric conversion elements 44a and 44b, the DC component corresponding to the average optical power is Only the multiplication result of the subcarrier signal and the output of the oscillator 46 can be taken out from the electric amplifier 45 as an electric signal.
[0034]
Therefore, if this electrical signal is used, the output signal of the oscillator can be easily synchronized with the subcarrier signal by using an appropriate loop circuit. This is because synchronous detection is performed using an optical modulator. A baseband signal can be obtained directly from the output of the photoelectric converter comprising the photoelectric conversion elements 44a and 44b and the electric amplifier 45.
[0035]
As described above, since the DC component corresponding to the average optical power is canceled out, it is possible to use a very low-pass characteristic circuit such as a PLL as the loop circuit 47, for example. In addition, the electric amplifier 45 can be easily provided with a narrow-band low-frequency selection type amplification characteristic by, for example, arranging a capacitor in parallel in the feedback circuit, thereby having a poor S / N ratio. A high gain can be easily obtained for a signal.
[0036]
Therefore, when the subcarrier signal is, for example, an ASK signal or an FSK signal, after the output of the loop circuit 47 passes through the LPF 48, a threshold value is provided and compared with the output of the amplifier 47e, for example, as in the first embodiment. By performing the determination, the data signal can be demodulated.
[0037]
According to the second embodiment, using a differential output optical modulator as an optical modulator, two optical signals equivalent to those obtained by multiplying two types of oscillator outputs whose phases are inverted are obtained, and these optical signals are obtained. Are simultaneously input to the photoelectric converter to cancel the DC component corresponding to the average optical power, so that only the multiplication result of the subcarrier signal and the oscillator output can be extracted from the photoelectric converter. Thereby, in this embodiment, a high gain can be easily obtained even for a signal with a poor S / N ratio, and a signal with a poor S / N ratio can be accurately demodulated.
[0038]
As the differential output optical modulator, in addition to the one shown in the second embodiment, for example, the combined portion of the two arms of the Mach-Zehnder modulator is a coupler type, an acousto-optic switch, an optical path switching mechanical A switch or the like can be used.
[0039]
Embodiment 3 FIG.
FIG. 6 is a block diagram showing the configuration of the optical receiver according to the third embodiment of the present invention. In FIG. 6, an optical receiver 40 includes a plurality of input terminals 41 to which an optical signal is input, an optical multiplexer / demultiplexer 42, differential output optical modulators 50e and 50f, photoelectric conversion elements 44a to 44d, Amplifiers 45a and 45b, an oscillator 46 whose oscillation frequency can be variably set by an input current / voltage, a loop circuit 47 for controlling the oscillator 46, LPFs 48a and 48b, an output terminal 49 for outputting a baseband signal, and an LPF 48a , 48b and a phase shifter 56 for shifting the phase of the output signal from the oscillator 46 by 90 degrees.
[0040]
The optical multiplexer / demultiplexer 42 combines the optical signals input from the input terminals 41 and outputs the combined optical signals to the two differential output optical modulators 50e and 50f. The phase shifter 56 can be easily configured, for example, by using the oscillator 46 as an oscillator having an oscillation frequency 2N (N is an arbitrary integer) times the subcarrier frequency, and combining this oscillator with a shift register type frequency divider. . 6 includes the differential output optical modulators 50e and 50f having a function as a multiplier described in the second embodiment and a differential input photoelectric converter (photoelectric conversion elements 44a and 44b and an electric amplifier). 45a is a tracking loop known as a Costas loop except that the photoelectric conversion elements 44c and 44d and the electric amplifier 45b) are used. The multiplier 55 multiplies the signals from the LPFs 48a and 48b. The oscillator 46 is controlled by a signal equivalently multiplied in phase.
[0041]
The optical signal from the optical multiplexer / demultiplexer 42 is input to the differential output optical modulators 50e and 50f, where the optical signal is modulated by the output from the oscillator 46, subjected to multiplication operation, and phase-inverted 2 Two optical signals (that is, four outputs of the differential output optical modulators 50e and 50f) equivalent to those obtained by multiplying the types of oscillator outputs are obtained. These optical signals are respectively input to the photoelectric conversion elements 44a to 44d, where they are converted into electric signals, respectively. As in the second embodiment, the electric signals are converted into electric signals in which the DC component corresponding to the average optical power is canceled. Output from the amplifiers 45a and 45b, respectively. These electric signals are output as low frequency components by the LPFs 48 a and 48 b and multiplied by the multiplier 55. The loop circuit 47 controls the oscillator 46 using this electrical signal, and synchronizes the output of the oscillator 46 with the subcarrier signal.
[0042]
As described above, also in the third embodiment, since the DC component corresponding to the average optical power is canceled out, it is possible to use a loop circuit 47 having a very low-pass characteristic such as a PLL. is there. In addition, the electric amplifier 45 can be easily provided with a narrow band low-frequency selection type amplification characteristic by arranging a capacitor in parallel in the feedback circuit. In contrast, a high gain can be easily obtained.
[0043]
Therefore, when the subcarrier signal is, for example, a BPSK (Binary Phase Shift Keying) signal, a demodulated data signal can be extracted from the output terminal 49.
[0044]
According to the third embodiment, four differential optical signals and a Costas loop are used to obtain four optical signals equivalent to those obtained by multiplying the output of the oscillator whose phase is inverted, and these are photoelectrically converted. Since the DC component corresponding to the average optical power is canceled by inputting to the converter, only the multiplication result of the subcarrier signal and the oscillator output can be extracted from the photoelectric converter. Thus, in this embodiment, as in the second embodiment, a high gain can be easily obtained even for a signal with a poor S / N ratio, and a signal with a poor S / N ratio can be accurately demodulated.
[0045]
In the third embodiment, the case where a differential output type optical modulator is used has been described. However, the present invention is not limited to this, and for example, the single-phase output optical modulator shown in the first embodiment is used. It is also possible to use it.
[0046]
【The invention's effect】
As described above, according to the present invention, the input optical signal is modulated with the oscillation output, the multiplication operation is performed in the state of the optical signal, and the oscillation frequency and phase are further changed using the electrical signal that has been photoelectrically converted. By controlling the phase of the oscillation output and the subcarrier signal, the baseband signal can be obtained directly from the photoelectrically converted output, so that a predetermined S / N ratio can be obtained and the signal-to-noise ratio can be obtained. It is possible to accurately demodulate a bad signal.
[0047]
According to the next invention, by using two optical modulators and a Costas loop, the direct current component corresponding to the average optical power is canceled, so that only the multiplication result of the subcarrier signal and the oscillator output is extracted from the photoelectric converter. Therefore, it is possible to easily obtain a high gain even for a signal with a poor S / N ratio, and it is possible to accurately demodulate a signal with a poor S / N ratio.
[0048]
According to the next invention, the optical signals input from a plurality of optical fibers are combined and modulated to perform a multiplication operation, and further, the oscillation frequency and phase are controlled using an electrical signal obtained by converting the optical signal, Since the oscillation output is phase-synchronized with the subcarrier signal, it is possible to accurately demodulate a plurality of optical signals.
[0049]
According to the next invention, the optical signal is modulated using an optical modulator, an optical switch, an optical amplifier that modulates the gain with an electric signal, an optical amplifier that combines the optical modulator, and the like. Even if optical modulation is performed using the above-described apparatus having a modulation function, a predetermined S / N ratio can be obtained, and the optical signal can be demodulated with high accuracy.
[0050]
According to the next invention, two signals equivalent to those obtained by multiplying two types of oscillation outputs whose phases are inverted by the differential output type optical modulation means and the differential input type photoelectric conversion means are obtained. Since the optical signal is simultaneously input to the photoelectric converter and the DC component corresponding to the average optical power is canceled, a high gain can be easily obtained even for a signal with a poor S / N ratio, and a signal with a poor S / N ratio can be obtained. The effect is that demodulation can be performed with high accuracy.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of an optical receiver according to a first embodiment of the present invention.
FIG. 2 is a circuit diagram showing a configuration of the loop circuit shown in FIG. 1;
FIG. 3 is a block diagram showing a configuration of an optical receiver according to a second embodiment of the present invention.
4 is a block diagram showing a configuration of the differential output optical modulator shown in FIG. 3. FIG.
5 is a block diagram for explaining the operation of the differential output optical modulator shown in FIG. 3; FIG.
FIG. 6 is a block diagram showing a configuration of an optical receiver according to a third embodiment of the present invention.
FIG. 7 is a diagram illustrating a configuration of a conventional example of a communication system that performs optical subcarrier communication.
[Explanation of symbols]
40 optical receiver, 41, 47a, 51 input terminal, 42, 50a optical multiplexer / demultiplexer, 43 optical modulator, 44, 44a, 44b, 44c, 44d photoelectric conversion element, 45, 45a, 45b electric amplifier, 46 oscillator 47 loop circuit, 47b capacitor, 47c, 55 multiplier, 47d loop filter, 47e amplifier, 47f adder, 47g dither oscillator, 47h, 47j, 49, 52, 53 output terminal, 47i, 48, 48a, 48b LPF, 50, 50e, 50f differential output optical modulator, 50b, 50c modulator, 50d phase inverter, 56 phase shifter.

Claims (3)

In an optical receiver that demodulates and receives an optical subcarrier modulated signal,
First and second optical modulation means for performing a multiplication operation by modulating an input optical signal with an electrical signal of an oscillation frequency output from the oscillation means;
And outputting the electrical signal of the phase orthogonal to each other in the first and second light modulating means, said oscillating means for driving said first and second light modulation means,
First and second photoelectric conversion means for photoelectrically converting the modulated optical signals into electrical signals, respectively;
Multiplying means for multiplying the converted electrical signal;
Control means for controlling the oscillation frequency or phase of the oscillation means by the multiplied electrical signal ;
Combining means for combining optical signals transmitted by a plurality of optical fibers, and outputting the combined optical signals to the optical modulation means;
Optical receiver characterized by comprising a. Optical modulator as before Symbol light modulating means, an optical switch, an optical amplifier for modulating the gain in electric signals, according to claim 1, characterized in that it comprises at least one of an optical amplifier that combines optical modulator Optical receiver. The optical modulation means comprises differential output type optical modulation means, modulates the input optical signal with an electrical signal and outputs at least two optical signals, and the photoelectric conversion means comprises a differential input type optical modulation means. It consists photoelectric conversion unit, an optical receiver according to claim 1 or 2, characterized in that for converting the optical signals said modulated electrical signal.

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