Demodulator and a polarization diversity receiver for coherent optical communication provided with the demodulator
First Claim
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1. A demodulator in an optical receiver for demodulating a modulated optical signal, comprising:
- an OR/NOR circuit having an input port supplied with an intermediate-frequency signal obtained from the modulated optical signal and a first and a second output port, adapted such that, when the input level at said input port is at a low level, output levels of said first and second output ports are respectively brought to a low level and a high level and, when the input level at said input port is at a high level, output levels of said first and second output ports are respectively brought to a high level and a low level;
a delay circuit for delaying the output signal from the second output port of said OR/NOR circuit by a predetermined delay time τ and
outputting the delayed signal; and
an exclusive OR/NOR circuit having a first input port supplied with the output signal from the first output port of said OR/NOR circuit, a second input port supplied with the output signal from said delay circuit, and a first and a second output port from which demodulated signals are output, adapted such that, when input levels at said first and second input ports are in phase, output levels at said first and second output ports are respectively brought to a low level and a high level, and when input levels at said first and second input ports are in antiphase, output levels at said first and second output ports are respectively brought to a high level and a low level.the demodulator being for use in a direct modulation phase shift keying system, in which an injection current supplied to a semiconductor laser, which outputs light with a frequency corresponding to the injection current, is varied for a predetermined modulation time t, which is shorter than one time slot T of a binary coded input signal, such that the integrated value of the frequency varied according to the injected current becomes a phase amount of π
or -π
,said predetermined delay time τ
being set to a time corresponding to one time slot T of the modulated signal.
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Abstract
A demodulator comprising an OR/NOR circuit, an exclusive OR/NOR circuit, and a delay circuit is disclosed as that capable of being suitably formed into a monolithic IC. Also, a polarization diversity receiver for coherent optical communication using the above demodulator as its constituent is disclosed as that having a smaller number of variable gain amplifiers and accordingly having the controlling circuit made simpler in structure.
34 Citations
20 Claims
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1. A demodulator in an optical receiver for demodulating a modulated optical signal, comprising:
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an OR/NOR circuit having an input port supplied with an intermediate-frequency signal obtained from the modulated optical signal and a first and a second output port, adapted such that, when the input level at said input port is at a low level, output levels of said first and second output ports are respectively brought to a low level and a high level and, when the input level at said input port is at a high level, output levels of said first and second output ports are respectively brought to a high level and a low level; a delay circuit for delaying the output signal from the second output port of said OR/NOR circuit by a predetermined delay time τ and
outputting the delayed signal; andan exclusive OR/NOR circuit having a first input port supplied with the output signal from the first output port of said OR/NOR circuit, a second input port supplied with the output signal from said delay circuit, and a first and a second output port from which demodulated signals are output, adapted such that, when input levels at said first and second input ports are in phase, output levels at said first and second output ports are respectively brought to a low level and a high level, and when input levels at said first and second input ports are in antiphase, output levels at said first and second output ports are respectively brought to a high level and a low level. the demodulator being for use in a direct modulation phase shift keying system, in which an injection current supplied to a semiconductor laser, which outputs light with a frequency corresponding to the injection current, is varied for a predetermined modulation time t, which is shorter than one time slot T of a binary coded input signal, such that the integrated value of the frequency varied according to the injected current becomes a phase amount of π
or -π
,said predetermined delay time τ
being set to a time corresponding to one time slot T of the modulated signal.
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2. A demodulator in an optical receiver for demodulating a modulated optical signal, comprising:
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an OR/NOR circuit having an input port supplied with an intermediate-frequency signal obtained from the modulated optical signal and a first and a second output port, adapted such that, when the input level at said input port is at a low level, output levels of said first and second output ports are respectively brought to a low level and a high level and, when the input level at said input port is at a high level, output levels of said first and second output ports are respectively brought to a high level and a low level; a delay circuit for delaying the output signal from the second output port of said OR/NOR circuit by a predetermined delay time τ and
outputting the delayed signal; andan exclusive OR/NOR circuit having a first input port supplied with the output signal from the first output port of said OR/NOR circuit, a second input port supplied with the output signal from said delay circuit, and a first and a second output port from which demodulated signals are output, adapted such that, when input levels at said first and second input ports are in phase, output levels at said first and second output ports are respectively brought to a low level and a high level, and when input levels at said first and second input ports are in antiphase, output levels at said first and second output ports are respectively brought to a high level and a low level, wherein said demodulator for use in a continuous phase frequency shift keying system, and wherein said predetermined delay time τ
is set to τ
=T/2m, where m represents the modulation index expressed as m=Δ
F/B, where Δ
F represents the frequency shift and B represents the bit rate of the modulated signal, and T represents one time slot of the modulated signal.
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3. A demodulator in an optical receiver for demodulating a modulated optical signal, comprising:
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an OR/NOR circuit having an input port supplied with an intermediate-frequency signal obtained from the modulated optical signal and a first and a second output port, adapted such that, when the input level at said input port is at a low level, output levels of said first and second output ports are respectively brought to a low level and a high level and, when the input level at said input port is at a high level, output levels of said first and second output ports are respectively brought to a high level and a low level; a delay circuit for delaying the output signal from the second output port of said OR/NOR circuit by a predetermined delay time τ and
outputting the delayed signal; andan exclusive OR/NOR circuit having a first input port supplied with the output signal from the first output port of said OR/NOR circuit, a second input port supplied with the output signal from said delay circuit, and a first and a second output port from which demodulated signals are output, adapted such that, when input levels at said first and second input ports are in phase, output levels at said first and second output ports are respectively brought to a low level and a high level, and when input levels at said first and second input ports are in antiphase, output levels at said first and second output ports are respectively brought to a high level and a low level, wherein said demodulator for use in a direct modulation phase shift keying system, in which an injection current supplied to a semiconductor laser, which outputs light with a frequency corresponding to the injection current, is varied for a predetermined modulation time t, which is shorter than one time slot T of a binary coded input signal, such that the integrated value of the frequency varied according to the injected current becomes a phase amount of π
or -π
,wherein said predetermined delay time τ
is set to a time corresponding to one time slot T of the modulated signal, andwherein said predetermined modulation time t is set to t=T/2 m, where m represents a modulation index expressed as m=Δ
F/B, where Δ
F represents the frequency shift and B represents the bit rate of the modulated signal, and T represents one time slot of the modulated signal. - View Dependent Claims (4)
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5. A polarization diversity receiver for coherent optical communication comprising:
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an optical local oscillator outputting local light; an optical-electrical converter for performing optical-electrical conversion of received signal light and said local light for each of polarization components, of which planes of polarization are orthogonal to each other, thereby outputting two intermediate-frequency signals having the frequency corresponding to the difference between the frequency of said signal light and the frequency of said local light, for each of said polarization components; a first and a second demodulator respectively supplied with said intermediate-frequency signals; and a combining circuit for combining demodulated signals from said first and second demodulators;
wherein each of said first and second demodulators comprises;an OR/NOR circuit having an input port supplied with the intermediate-frequency signal and a first and a second output port and adapted such that, when the input level at said input port is at a low level, output levels at said first and second output ports are respectively brought to a low level and a high level and, when the input level at said input port is at a high level, output levels at said first and second output ports are respectively brought to a high level and a low level; a delay circuit for delaying the output signal from the second output port of said OR/NOR circuit by a predetermined delay time τ and
outputting the delayed signal; andan exclusive OR/NOR circuit having a first input port supplied with the output signal from the first output port of said OR/NOR circuit, a second input port supplied with the output signal from said delay circuit, and a first and a second output port outputting demodulated signals, and adapted such that, when the input levels at said first and second input ports are in phase, output levels at said first and second output ports are respectively brought to a low level and a high level and, when the input levels at said first and second input ports are in antiphase, output levels at said first and second output ports are respectively brought to a high level and a low level. - View Dependent Claims (6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20)
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Specification