Optical phase regeneration method and device

Abstract

In a signal regeneration device in which recovery of a signal quality which has been degraded during transmission in optical communication and extension of a transmission distance are achieved, the most representative method of quantizing an optical phase is a phase sensitive amplifier (PSA) and a technique that utilizes an optical parametric process through use of a highly nonlinear optical medium, but there is a demand for a technique of quantizing an optical phase which is not accompanied with an optical parametric gain, has small-sized elements, is easily integrated, and does not require high power pump light. By a technique of a hybrid optical phase squeezer (HOPS), when a phase of input light is quantized to M levels (M>2), phase conjugate light of the input light and (M1)th phase harmonic light of the input light are subjected to power modulation to be coherently added, so that quantization of the optical phase is performed through use of a simple four-wave mixing (FWM) that is not accompanied with the optical parametric gain and a general optical amplifier by using a general nonlinear optical medium such as silicon, and accordingly, a GER of equal to or higher than 30 dB can be obtained, even if a nonlinear optical element having a low nonlinearity is used.

Claims

1. An optical phase regeneration method of regenerating a phase of binary phase shift keying signal light, the method comprising: multiplexing the binary phase shift keying signal light whose frequency is .sub.s and pump light whose frequency is .sub.p at a predetermined mixing ratio, introducing the multiplexed signal light into a nonlinear optical element, and taking out the signal light and phase conjugate light whose frequency is .sub.i.sup.(1) by four-wave mixing; setting each light power of the signal light and the phase conjugate light to a predetermined same level in an optical amplifier; and performing power modulation of the signal light and the phase conjugate light which are set to be the same level, by using a half of a difference frequency between the signal light and the phase conjugate light as a modulation frequency, and generating interference signal light at the frequency .sub.p of the pump light as phase-regenerated signal light in a light power modulator.

2. The optical phase regeneration method according to claim 1, further comprising: introducing the generated interference signal light into a phase-preserving amplitude regenerator and performing amplitude regeneration.

3. An optical phase regeneration device that generates a phase of binary phase shift keying signal light, the optical phase regeneration device comprising: a multiplexer configured to multiplex the binary phase shift keying signal light whose frequency is .sub.s and pump light whose frequency is .sub.p at a predetermined mixing ratio; a nonlinear optical element configured to receive the multiplexed signal light to output the signal light and phase conjugate light whose frequency is .sub.i.sup.(1) by four-wave mixing; an optical amplifier configured to set each light power of the signal light and the phase conjugate light to a predetermined same level; and a light power modulator configured to perform power modulation of the signal light and the phase conjugate light which are set to be the same level by using a half of a difference frequency between the signal light and the phase conjugate light as a modulation frequency to generate interference signal light at the frequency .sub.p of the pump light as phase-regenerated signal light.

4. The optical phase regeneration device according to claim 3 further comprising: a phase-preserving amplitude regenerator, wherein the generated interference signal light is introduced into the amplitude regenerator and subjected to amplitude regeneration.

Description

BRIEF DESCRIPTIONS OF THE DRAWINGS

(1) FIG. 1(a) is a diagram illustrating a phase relation between input light and output light in optical phase quantization;

(2) FIG. 1(b) is a diagram illustrating a relation between a phase of input light and power of output light in the optical phase quantization;

(3) FIG. 2 is a diagram illustrating wavelength arrangement at the time of performing phase regeneration of a BPSK signal through use of a DP-PSA;

(4) FIG. 3 is a constellation diagram illustrating a state in which phase noise of the BPSK signal is suppressed by a phase squeezing effect;

(5) FIG. 4 is a diagram illustrating wavelength arrangement at the time of performing phase regeneration of a QPSK signal through use of the DP-PSA;

(6) FIG. 5(a) is a diagram illustrating a relation of light waves generated in four-wave mixing through use of one pump light beam;

(7) FIG. 5(b) is a diagram illustrating a relation of light waves generated in four-wave mixing through use of two pump light beam;

(8) FIG. 6 is a diagram illustrating a principle of a two-level HOPS;

(9) FIG. 7 is a diagram illustrating a temporal variation of interference signal power at the time of free running of a VCO;

(10) FIG. 8 is a constellation diagram illustrating a phase noise suppression effect of the BPSK signal obtained by the two-level HOPS;

(11) FIG. 9 is a diagram illustrating a principle of a four-level HOPS;

(12) FIG. 10 is a diagram illustrating a principle of an M-level HOPS (dual-pump system);

(13) FIG. 11 illustrates an embodiment in which a BPSK signal is subjected to phase regeneration through use of the two-level HOPS;

(14) FIG. 12 is a diagram illustrating an embodiment in which a BPSK signal is subjected to amplitude and signal regeneration through use of the two-level HOPS together with a constellation diagram;

(15) FIG. 13 illustrates an embodiment in which a QPSK signal is subjected to phase regeneration through use of the four-level HOPS;

(16) FIG. 14 is a diagram illustrating an embodiment in which a QPSK signal is subjected to amplitude and signal regeneration through use of the four-level HOPS together with a constellation diagram;

(17) FIG. 15 is a diagram illustrating an embodiment in which an M-level PSK signal is subjected to phase regeneration through use of the M-level HOPS (dual-pump system);

(18) FIG. 16 is a diagram illustrating an embodiment in which an M-level PSK signal is subjected to amplitude and signal regeneration through use of the M-level HOPS (dual-pump system) together with a constellation diagram; and

(19) FIG. 17 is a diagram illustrating an embodiment in which a QPSK signal is separated into two BPSK signals by utilizing the HOPS together with a constellation diagram.

DETAILED DESCRIPTION

First Embodiment

(20) FIG. 11 illustrates an embodiment in which a BPSK signal is subjected to phase regeneration through use of a two-level HOPS.

(21) Signal light (frequency: .sub.s) is multiplexed with pump light (frequency: .sub.p), and the multiplexed signal light is amplified to a suitable level and then is introduced into a nonlinear optical element to generate phase conjugate light.

(22) At this time, an optical amplifier generates amplified spontaneous emission light (ASE), and thus, the ASE in a frequency domain in which the phase conjugate light is generated is removed in advance through use of a filter or the like.

(23) When the pump light power is sufficiently high, the optical amplifier and the filter may be omitted.

(24) The output of the nonlinear optical element is guided to a spectrum forming device, a component except for the signal light and the phase conjugate light is removed, and further, the component of signal light () is attenuated such that each power of both light beams becomes equal.

(25) These operations can be performed by using a wavelength-dependent optical element such as a notch filter and a band-pass filter.

(26) This pair of the signal light and the phase conjugate light is guided to a power modulator (may be guided to a phase modulator), and interference signal light (frequency: .sub.p) is output through use of the filter from the output of the power modulator.

(27) A part of the output light is guided to a photodetector by a splitter in order to monitor the power thereof.

(28) The power modulator is driven by a VCO, and the oscillation frequency thereof is feedback-controlled by a phase-locked loop (PLL) such that an output voltage of the photodetector is constant.

Second Embodiment

(29) FIG. 12 illustrates an embodiment in which an amplitude noise and a phase noise of a BPSK signal are suppressed and signal regeneration is performed through use of the two-level HOPS.

(30) The BPSK signal is subjected to the phase regeneration through use of the method illustrated in FIG. 11 and then is introduced into a phase-preserving amplitude regenerator to perform amplitude regeneration.

(31) An example of the phase-preserving amplitude regenerator is an injection-locked semiconductor laser.

Third Embodiment

(32) FIG. 13 illustrates an embodiment in which a QPSK signal is subjected to phase regeneration through use of a four-level HOPS.

(33) Signal light (frequency: .sub.s) is multiplexed with pump light (frequency: .sub.p), and the multiplexed signal light is amplified to a suitable level and then is introduced into a nonlinear optical element to generate phase conjugate light and third phase harmonic light.

(34) At this time, an ASE in a frequency domain in which these light waves are generated is removed in advance through use of a filter or the like.

(35) When the pump light power is sufficiently high, an optical amplifier and the filter may be omitted.

(36) When it is difficult to obtain the third phase harmonic light with a sufficient S/N ratio, the nonlinear optical element may be cascaded in two stages.

(37) At this time, it is possible to obtain a favorable S/N ratio when an output of a nonlinear optical element in the first stage is spectrally shaped by the filter and amplified and then is introduced to a nonlinear optical element in the second stage.

(38) The output of the nonlinear optical element is guided to a spectrum forming device, the components except for the phase conjugate light and the third phase harmonic light are removed, and further, the spectrum shaping is performed such that the two signal light beams have a preferable power ratio.

(39) These operations can be performed by using a wavelength-dependent optical element such as a notch filter and a band-pass filter.

(40) The two power-adjusted signal light beams are guided to a power modulator, and interference signal light (frequency: .sub.s) is output through use of the filter from the output of the power modulator.

(41) A part of the output light is guided to a photodetector by a splitter in order to monitor the power thereof.

(42) The power modulator is driven by a VCO, and the oscillation frequency thereof is feedback-controlled by a PLL such that an output voltage of the photodetector becomes constant.

(43) An object of the power modulator is to shift each frequency of the phase conjugate light and the third phase harmonic light by 2(.sub.p.sub.s), and another device such as a phase modulator may be used as long as such an object can be achieved.

Fourth Embodiment

(44) FIG. 14 illustrates an embodiment in which an amplitude noise and a phase noise of a QPSK signal are suppressed and regeneration is performed through use of a four-level HOPS.

(45) The QPSK signal is subjected to phase regeneration through use of the method illustrated in FIG. 13 and then is introduced into a phase-preserving amplitude regenerator to perform amplitude regeneration.

(46) An example of the phase-preserving amplitude regenerator is an injection-locked semiconductor laser.

Fifth Embodiment

(47) FIG. 15 illustrates an embodiment in which an M-level PSK signal (M2) is subjected to phase regeneration through use of an M-level HOPS (dual-pump system).

(48) Laser light is introduced into an optical comb generator to generate optical combs, and two modes thereof are extracted as pump light through use of a filter.

(49) Signal light is multiplexed with these two pump light beams, and the multiplexed signal light is introduced into a nonlinear optical element to generate phase conjugate light ((e.sup.i*)) and (M1)th phase harmonic light (e.sup.(M1)i*).

(50) At this time, an ASE in a frequency domain in which these light waves are generated is removed in advance through use of a filter or the like.

(51) When it is difficult to obtain the (M1)th phase harmonic light with a sufficient S/N ratio, the nonlinear optical element may be cascaded in two stages.

(52) At this time, it is possible to obtain a favorable S/N ratio when an output of a nonlinear optical element in the first stage is spectrally shaped by the filter and amplified and then is introduced to a nonlinear optical element in the second stage.

(53) The output of the nonlinear optical element is guided to a spectrum shaper, the components except for the phase conjugate light (frequency: .sub.i.sup.(1)) and the (M1)th phase harmonic light (frequency: .sub.i.sup.(M1)) are removed, and further, the spectrum shaping is performed such that the two signal light beams have a preferable power ratio.

(54) These operations can be performed by using a wavelength-dependent optical element such as a notch filter and a band-pass filter.

(55) The two power-adjusted light waves are guided to a power modulator and are modulated at a frequency which is the half of a difference frequency between the two light waves.

(56) Interference signal light (frequency: .sub.s=[.sub.i.sup.(1)+.sub.i.sup.(M1)]/2) which is generated at an intermediate frequency of the two light waves is output through use of the filter from the output of the power modulator.

(57) A part of the output light is guided to a photodetector by a splitter in order to monitor the power thereof.

(58) The power modulator is driven by a VCO, and the oscillation frequency thereof is feedback-controlled by a PLL such that an output voltage of the photodetector becomes constant.

Sixth Embodiment

(59) FIG. 16 illustrates an embodiment in which an amplitude noise and a phase noise of an M-level PSK signal is suppressed and regeneration is performed through use of an M-level HOPS (dual-pump system).

(60) The M-level PSK signal is subjected to phase regeneration through use of the method illustrated in FIG. 15 and then is introduced into a phase-preserving amplitude regenerator to perform amplitude regeneration.

(61) An example of the phase-preserving amplitude regenerator is an injection-locked semiconductor laser.

Seventh Embodiment

(62) FIG. 17 illustrates an embodiment in which a QPSK signal is separated into two BPSK signals (composed of data of an I-component and a Q-component of the input QPSK signal, respectively) by using a two-level HOPS and a four-level HOPS in combination.

(63) Signal light is multiplexed with pump light through use of a multiplexer (with two inputs and two outputs) such as a 3-dB coupler. Each of two outputs of the multiplexer is guided to the two-level HOPS (first and second arms).

(64) A part of the output of the multiplexer is divided and guided to the four-level HOPS (a third arm).

(65) All power modulators used in these three HOPSs are driven by the output of the same VCO.

(66) In the first and the second arms, an operation of converting the QPSK signal into the BPSK signals each of which contains only one of components of the QPSK signal is performed through use of a quadrature phase squeezing effect.

(67) In the third arm, an operation of setting a frequency of the VCO to be equal to a difference frequency between the signal light and the pump light is performed.

(68) Phase conjugate light and input signal light are extracted by a filter from the output of the nonlinear optical element and are coherently added in the first and the second arms.

(69) Meanwhile, phase conjugate light and third phase harmonic light are extracted by a filter from the output of the nonlinear optical element and are coherently added in the third arm.

(70) The output of the third arm is the phase-regenerated QPSK signal, and an oscillation frequency of the VCO is constantly kept to be equal to the difference frequency between the signal light and the pump light by controlling the frequency of the VCO such that the power of the QPSK signal is constant.

(71) Accordingly, the first and the second arms function as the two-level HOPS, and the input QPSK signal thereof is influenced by the phase squeezing effect.

(72) Here, it is possible to selectively suppress one component between the two quadrature phase components forming the QPSK signal when a relative phase between the signal light and the pump light is suitably adjusted.

(73) Accordingly, when the relative phase between the signal light and the pump light is adjusted such that the I-component and the Q-component are left, respectively, in the first and the second arms, the two BPSK signals which are separated into the I-component and the Q-component are output.

(74) The present invention is a technique that can be utilized to regenerate signal light which has been degraded in quality during transmission in optical communication and to increase a transmission distance.

(75) In addition, the technique can be utilized for format conversion of multilevel phase shift keying signal light, thereby facilitating flexible operation of all-optical network aiming at low power consumption.

(76) While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure.