JP5435443B2 - Synchronous optical signal generator and synchronous optical signal generation method - Google Patents

Description

The present invention relates to a synchronous optical signal generator and a synchronous optical signal generation method .

  Conventionally, optical communication networks that perform high-speed information communication are known. For example, in an optical communication network of 100 [Gbit / s] or more, a repeater or a receiver has a retiming function that aligns optical pulses whose timings are shifted after optical fiber transmission at the original timing. This retiming function requires an optical signal synchronized with the optical signal to be communicated.

  As a device for generating the synchronous optical signal, there has been a system using OPLL (Optical Phase-Lock Loop) or PLL (Phase-Lock Loop). For example, an apparatus that generates a beat optical signal synchronized with a reference optical signal by OPLL is considered (for example, see Patent Document 1). With reference to FIG. 17, a synchronous optical signal generator for outputting a synchronous optical signal by conventional OPLL will be described. FIG. 17 shows the configuration of a conventional synchronous optical signal generator 7.

  As shown in FIG. 17, the synchronous optical signal generator 8 includes an optical coupler 81 as a multiplexing unit, an Si-APD (Avalanche Photo Diode) 82 as an optical phase comparison unit, a loop filter 83, and an OVCO (Optical). Voltage Controlled Oscillator (Voltage Controlled Light Signal Source) 84). An input optical signal to be synchronized is set as a reference optical signal. In the synchronous optical signal generator 8, a beat optical signal synchronized with the reference optical signal is output from the OVCO 84.

  The optical element (optical path) is connected by a normal optical fiber that does not maintain polarization unless otherwise specified, and the electrical element (electrical path) is connected between the optical element and the optical element. Connected by. In the drawing, the optical path is indicated by a bold line, and the electrical path is indicated by a thin line. The same shall apply hereinafter.

  The OVCO 84 includes LDs (Laser Diodes) 841 and 842 and an optical coupler 843. In the OVCO 84, the output lights of the two LDs 841 and 842 having different output light wavelengths are multiplexed by the optical coupler 843. Here, the polarizations of the optical outputs of the LDs 841 and 842 are the same. As a result, the two optical outputs interfere in the optical coupler 843, and the optical coupler 843 outputs a beat optical signal that vibrates at a frequency corresponding to the wavelength difference. The restriction on the wavelength difference is determined by the band of the optical coupler 843. The LDs 841 and 842 can change the wavelength of the output light depending on the drive current and the control temperature. Therefore, the OVCO 84 can control the wavelength of the output light of the LDs 841 and 842 in accordance with the reference light signal, change the frequency of the beat light signal to be output, and output the beat light signal synchronized with the reference light signal.

  The Si-APD 82 outputs an electric signal (TPA signal) corresponding to the phase difference between the two input signal light intensities as a phase error signal using two-photon absorption as a nonlinear effect. The loop filter 83 averages and shapes the phase error signal and outputs it as a control signal for the OVCO 84.

  The phase error signal will be described with reference to FIG. FIG. 18A shows the waveform and time difference τ between the beat optical signal and the reference optical signal. FIG. 18B shows a TPA (Two Photon Absorption) signal (TPA current signal) I (τ) with respect to the time difference τ.

When receiving light, the Si-APD 82 outputs a TPA signal I of a reference light signal and a beat light signal by two-photon absorption. Assuming that the instantaneous light power received by the Si-APD 82 is P, a relationship of I∝P ² is established. As shown in FIG. 18A, a case is considered in which a time difference τ is generated between the reference optical signal and the beat optical signal in the synchronous optical signal generator 8. The TPA signal I (τ) output from the Si-APD 82 has a relationship as shown in FIG. 18B with respect to the time difference τ. That is, the TPA signal I (τ) becomes maximum when τ = 0, and oscillates around a predetermined offset current.

  Then, the synchronous optical signal generator 8 operates so that the time difference τ (phase difference) between the TPA signal I (τ) output from the Si-APD 82 and the beat optical signal becomes zero. In this way, the phase comparison between ultrahigh-speed optical signals in optical communication can be performed by the Si-APD 82 or the like.

  A synchronous optical signal generator that outputs a synchronous beat optical signal with a small line width by OPLL is also considered (see, for example, Patent Document 2). Here, with reference to FIG. 19, the synchronous optical signal generator 9 which outputs a synchronous beat optical signal using an optical signal with a narrow line width will be described. FIG. 19 shows the configuration of the synchronous optical signal generator 9.

  As shown in FIG. 19, the synchronous optical signal generator 9 includes an optical coupler 11 as a multiplexing unit, an Si-APD 121, a TIA (TransImpedance Amplifier) 19, a loop filter 20, a VCO (Voltage Controlled Oscillator: voltage). Control electric signal generation source) 31, double frequency generators 311, 312, RF (Radio Frequency) amplifier 32, DFB (Distributed FeedBack) -LD331, LN (Lithium Niobete: lithium niobate) A phase modulator 332, a circulator 341, an AWG (Arrayed Waveguide Grating) 342, an FRM (Faraday Rotator Mirror) 351, 352, an EDFA (Erbium Doped Fiber Amplifier) 361, And an optical coupler 37.

  The synchronized optical signal generator 9 generates a synchronized beat optical signal of 160 [GHz], for example. The optical coupler 11 combines the reference optical signal and the beat optical signal output from the optical coupler 37 and outputs the combined optical signal. The Si-APD 121 receives the combined optical signal input from the optical coupler 11 and outputs the TPA signal as a phase error signal. The TIA 19 converts and amplifies the phase error signal input from the Si-APD 121 into a voltage signal and outputs the voltage signal. The loop filter 20 shapes and outputs the phase error signal input from the TIA 19.

  The VCO 31 generates and outputs an electrical signal whose frequency is changed according to the phase error signal input from the loop filter 20. The double frequency generator 311 generates and outputs an electric signal having a frequency twice that of the electric signal input from the VCO 31. The double frequency generator 312 generates and outputs an electric signal having a frequency twice that of the electric signal input from the double frequency generator 311. The RF amplifier 32 amplifies and outputs the electric signal input from the double frequency generator 312. If the frequency of the electrical signal output from the VCO 31 is about 10 [GHz], the frequency of the electrical signal output from the RF amplifier 32 is about 40 [GHz].

  The DFB-LD 331 outputs a single frequency laser beam. The LN phase modulator 332 phase-modulates the laser light input from the DFB-LD 331, and an optical comb signal having narrow spectral lines with a frequency (about 40 [GHz]) interval of the electric signal input from the RF amplifier 32. Is generated and output. The optical comb signal output from the LN phase modulator 332 is input to the AWG 342 via the circulator 341. The AWG 342 selects and outputs an optical signal with spectral lines having wavelengths λ 1 and λ 2 from the optical comb signal input from the LN phase modulator 332. The frequency interval between the wavelengths λ1 and λ2 is set to 160 [GHz].

  The optical signals of wavelengths λ1 and λ2 output from the AWG 342 have the same polarization, are reflected by the FRMs 351 and 352, and output through the AWG 342 again. Further, the FRMs 351 and 352 output an optical signal having a polarization orthogonal to the polarization of the input optical signal by reflection. For this reason, the optical signal output from the AWG 342 becomes a beat optical signal with wavelengths λ1 and λ2. The polarization purity of the beat optical signal can be increased by the AWG 342, and two optical spectral lines to be selected pass through the AWG 342 twice, so that the optical SNR (Signal to Noise Ratio: S / N ratio) is increased (frequency purity). Becomes higher).

  The beat optical signals having the wavelengths λ1 and λ2 (frequency is 160 [GHz]) output from the AWG 342 are input to the EDFA 361 via the circulator 341. The EDFA 361 amplifies and outputs the input beat optical signals having wavelengths λ1 and λ2. The optical coupler 37 demultiplexes the beat optical signals having the wavelengths λ 1 and λ 2 input from the EDFA 361, outputs one to the outside, and feeds back the other to the optical coupler 11. In this way, in the synchronous optical signal generator 8, the line width synchronized with the reference optical signal by selecting and combining the optical spectral lines of the wavelengths λ1 and λ2 from the optical comb signal generated based on the electrical signal having a narrow line width. A narrow optical beat signal is generated. For this reason, the loop band can be reduced and the phase noise and timing jitter can be reduced.

  In addition, a synchronous electric signal generator that outputs an electric signal synchronized with a reference light signal by OPLL is also considered (for example, see Non-Patent Documents 1 and 2). Here, with reference to FIG. 20, the synchronous electric signal generator 10 which outputs a synchronous electric signal is demonstrated. FIG. 20 shows a configuration of the synchronous electric signal generator 10.

  As shown in FIG. 20, the synchronous electric signal generator 10 includes an optical coupler 101, an EDFA 102, a Si-APD 103, a loop filter 104, a VCO 105, an RF amplifier 106, a mode-locked laser 107, and a PC (Polarization). Controller: polarization controller) 108.

  The mode-locked laser 107 outputs a clock optical signal corresponding to the input electric signal. First, the reference optical signal and the clock optical signal are combined and output by the optical coupler 101. The optical signal output from the optical coupler 101 is amplified by the EDFA 102 and input to the Si-APD 103.

  The Si-APD 103 outputs, as a phase error signal, a TPA signal of an optical signal obtained by combining the reference optical signal input from the EDFA 102 and the clock optical signal by two-photon absorption. The phase error signal output from the Si-APD 103 is shaped by the loop filter 104 and output to the VCO 105. The VCO 105 generates an electrical signal having a frequency corresponding to the phase error signal input from the loop filter 104, outputs the electrical signal to the outside, and outputs it to the RF amplifier 106. The electric signal output from the VCO 105 is amplified by the RF amplifier 106 and input to the mode-locked laser 107. The polarization of the clock signal generated by the mode-locked laser 107 is adjusted by the PC 108 and input to the optical coupler 101. In this way, an electric signal synchronized with the reference light signal is obtained.

  With reference to FIG. 21, the polarization dependence of the TPA signal of the synchronous electrical signal generator 10 will be described. FIG. 21A shows the relationship of the TPA signal with respect to the time difference τ. FIG. 21B shows the relationship of the TPA signal with respect to time t.

  FIGS. 21A and 21B show TPA signals output from the Si-APD 103 when the clock signal polarization is changed by the PC 108 in the synchronous electric signal generator 10. 21A and 21B, a TPA signal when a linearly polarized clock signal is output by adjustment of the PC 108 is indicated by a solid line, and a TPA signal when a circularly polarized clock signal is output is indicated by a dotted line.

  As shown in FIG. 21A, the offset value between the waveform of the TPA signal in which the linearly polarized clock signal is input to the optical coupler 101 and the waveform of the TPA signal in which the circularly polarized clock signal is input to the optical coupler 101 has an offset value. Different. For this reason, as shown in FIG. 21B, when the offset is adjusted, the waveform of the TPA signal in which the linearly polarized clock signal is input to the optical coupler 101 and the TPA signal in which the circularly polarized clock signal is input to the optical coupler 101 are obtained. There is a timing deviation (timing offset) of about one-tenth of the period with respect to time t.

  Further, a technique for reducing the polarization dependence of the phase error signal in the Si-APD as described above is considered. Specifically, one of the two optical signals incident on the Si-APD is circularly polarized (for example, see Non-Patent Document 3).

  With reference to FIG. 22, the polarization dependence of the TPA signal output from the Si-APD when two optical signals are combined and input to the Si-APD will be described. FIG. 22A shows a TPA signal when two optical signals, at least one of which is linearly polarized, are combined with respect to the time difference τ. FIG. 22B shows a TPA signal when at least one of the optical signals having a circular polarization is multiplexed with respect to the time difference τ.

  In FIG. 22A, a phase error signal when two optical signals of linearly polarized light in the same direction are combined is indicated by a solid line, and a phase error when two optical signals of linearly polarized light orthogonal to each other are combined. A signal is indicated by a dotted line, and a phase error signal when a linearly polarized light signal and a circularly polarized light signal are combined is indicated by a broken line. As shown in FIG. 22A, when the polarization is different, both the offset value and the amplitude of the phase error signal are different.

  In FIG. 22B, the TPA signal when the circularly polarized light signal and the linearly polarized light signal are combined is indicated by a solid line, and the TPA signal when the two circularly polarized light signals having the same rotation direction are combined is broken. A TPA signal when two circularly polarized optical signals in the reverse rotation direction are combined is indicated by a dotted line. As shown in FIG. 22B, the amplitude of the TPA signal is the same even if the polarization is different compared to FIG.

The TPA signal I (τ) when one of them is combined with circular polarization is described by the following equations (1) to (4).

However,
I (τ): photocurrent (TPA signal) generated in Si-APD by two-photon absorption,
E (t): electric field as a function of time,
η: constant for two-photon absorption efficiency,
S _{1 to} S ₃ : Normalized Stokes vector of one incident light,
S ′ _{1 to} S ′ ₃ : Normalized Stokes vector of the other incident light,
g (t): envelope of one incident light,
g ′ (t): envelope of the other incident light,
B1: Offset value of I (τ),
C1: amplitude of I (τ),
It is.

FIG. 23A shows a waveform of the envelope g (t) with respect to time t. FIG. 23B shows the waveform of the TPA signal I (τ) with respect to the time difference τ. Envelopes g (t) and g ′ (t−τ) shown in equations (1) to (4) have waveforms as shown in FIG. At this time, the TPA signal I (τ) shown in Expression (2) has a waveform as shown in FIG. 23B, and has an offset value B1 and an amplitude C1 expressed by Expressions (3) and (4). If one of the two input optical signals to the Si-APD is circularly polarized, the amplitude C1 does not change and the offset value B1 changes.
WO03 / 104886 pamphlet International Publication No. 2008/007615 Pamphlet Reza Salem, TEMurphy, “Broad-Band Optical Clock Recovery System Using Two-Photon Absorption,” Photonics Technology Letters, vol. 16, no. 9, 2141-2143, 2004 Reza Salem, GETudury, TUHorton, GMCarter and TEMurphy, “Polarization-Insensitive Optical Clock Recovery at 80 Gbi / s Using a Silicon Photociode,” Photonics technology letters, vol. 17, no. 9, pp. 1968-1970, 2005. Reza Salem and Thomas E. Murphy, “Polarization-insensitive cross correlation using two-photon absorption in a silicon photodiode,” Optics letters, vol. 29, no. 13, pp. 1524-1526, 2004

  However, in the conventional synchronous optical signal generator using OPLL, even if one input optical signal is circularly polarized in order to eliminate the polarization dependence of the TPA signal I (τ) in Si-APD, the TPA signal I Although the amplitude C1 of (τ) can be made independent of polarization, the offset value B1 cannot be made independent of polarization. For this reason, a timing offset of the synchronous beat signal is generated by changing the polarization of the reference optical signal or the beat optical signal to be fed back (hereinafter, these two are referred to as input optical signals).

  An object of the present invention is to generate a synchronous beat optical signal at a synchronous timing independent of the polarization of the input optical signal.

In order to solve the above problems, a synchronous optical signal generator according to the present invention provides:
A local oscillation unit that generates a modulation signal of a predetermined frequency;
A combining unit for combining the reference light signal and the temporally modulated circularly polarized beat light signal;
A phase comparison unit that generates a phase comparison signal by comparing phases of the combined reference light signal and beat light signal by a non-linear effect;
An electric mixer that multiplies the phase comparison signal and the modulation signal and outputs a differential value signal of the phase comparison signal as a phase error signal;
A shaping unit for shaping the phase error signal;
Based on the shaped phase error signal, a beat optical signal generator that generates and outputs a beat optical signal that sets the phase error signal to 0,
A temporal modulation unit that temporally modulates the beat optical signal generated by the beat optical signal generation unit based on the modulation signal;
A circularly polarized light adjusting unit that converts the temporally modulated beat optical signal into circularly polarized light and outputs the circularly polarized light signal to the multiplexing unit.

  Preferably, the circular polarization adjustment unit is a polarization controller.

Preferably, the multiplexing unit is a half mirror.
The circularly polarized light adjusting unit is a quarter wavelength plate,
An optical path from the quarter-wave plate to the phase comparison unit is configured by a spatial optical system,
A polarization holding unit that holds the polarization of the optical signal passing through the optical path from the beat optical signal generation unit to the quarter-wave plate as linearly polarized light;

  Preferably, the temporal modulation unit includes a phase modulation unit that phase-modulates an optical signal having one wavelength constituting the beat optical signal.

Preferably, the temporal modulation unit is
An arrayed waveguide diffraction grating for extracting optical signals of the first and second wavelengths constituting the beat optical signal;
A Faraday rotator mirror that reflects the optical signal of the second wavelength and the optical signal of the first wavelength that is phase-modulated,
The phase modulation unit phase modulates the optical signal of the first wavelength output from the arrayed waveguide diffraction grating,
The arrayed waveguide diffraction grating multiplexes the optical signal of the second wavelength and the optical signal of the first wavelength modulated by the phase reflected by the Faraday rotator mirror, and outputs it as a beat optical signal.

Preferably, the beat optical signal generator is
An optical comb generator for generating an optical comb signal at a frequency interval according to the phase error signal;
A two-mode selection unit for selecting and extracting optical signals of the first and second wavelength line spectra from the generated optical comb signal;
A two-mode multiplexing unit that combines the extracted optical signals of the first and second wavelengths to generate and output a beat optical signal.

  Preferably, the two-mode selection unit and the two-mode multiplexing unit are configured by a Fabry-Perot etalon.

  Preferably, the temporal modulation unit temporally modulates the optical signal of the first wavelength selected by the two-mode selection unit, and the temporally modulated optical signal of the first wavelength and the second wavelength Are combined and output.

  Preferably, the two-mode selection unit and the two-mode multiplexing unit include the temporal modulation unit, and generate and output a beat optical signal temporally modulated by the temporal modulation unit.

  Preferably, the temporal modulation unit includes a phase modulation unit that phase-modulates the beat optical signal.

  Preferably, a phase shifter that shifts the phase of the modulation signal from the local oscillation unit and outputs the shifted signal to the electric mixer or the temporal modulation unit is provided.

The synchronous optical signal generation method according to the present invention includes:
Combining the reference light signal and the temporally modulated circularly polarized beat light signal;
A step of generating a phase comparison signal by comparing phases of the combined reference light signal and beat light signal by a non-linear effect;
Multiplying the phase comparison signal by a modulation signal of a predetermined frequency and outputting a differential value signal of the phase comparison signal as a phase error signal;
Shaping the phase error signal;
Generating and outputting a beat optical signal that sets the differential value signal to 0 based on the differential value signal of the shaped phase error signal;
Temporally modulating the generated beat optical signal based on the modulation signal;
And converting the temporally modulated beat optical signal into circularly polarized light and outputting it as a temporally modulated circularly polarized beat optical signal used for the multiplexing.

  According to the present invention, the differential value of the phase comparison signal can be obtained as a phase error signal by temporally modulating the beat optical signal to be fed back and demodulating it into circularly polarized light, which depends on the polarization of the input optical signal. A synchronous beat optical signal can be generated at a synchronous timing that is not.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to the illustrated examples.

  First, with reference to FIG. 1, the apparatus structure of the synchronous optical signal generator 1 of this Embodiment is demonstrated. FIG. 1 shows a configuration of a synchronous optical signal generator 1 according to the present embodiment.

  As shown in FIG. 1, the synchronous optical signal generator 1 includes an optical coupler 11 as a multiplexing unit, a phase comparison unit 12, an electric mixer 13, a loop filter 20, and an OVCO 30 as a beat optical signal generation unit. The local oscillation unit 15, the temporal modulation unit 16, and the circular polarization adjustment unit 17 are configured. The OVCO 30 includes a VCO 31, an RF amplifier 32, an optical comb generator 33, a two-mode selector 34, and a two-mode multiplexer 35. The multiplexing unit is not limited to the optical coupler 11 and may be another multiplexing unit such as a half mirror.

  The optical coupler 11 receives a reference light signal to be synchronized input from an external device (not shown) and a beat light signal output from the circular polarization adjustment unit 17, and combines and outputs the two lights. The phase comparison unit 12 receives the combined optical signal output from the optical coupler 11 and outputs a phase comparison signal indicating the phase difference between the reference optical signal and the beat optical signal from the optical signal. The phase comparison unit 12 is a phase comparator that outputs a TPA signal (TPA current) as a phase comparison signal by two-photon absorption that is a nonlinear effect. Hereinafter, as an example, the phase comparison unit 12 will be described as being Si-APD, but other phase comparison units based on two-photon absorption such as Si-PD may be used. The output signal of the phase comparison unit 12 is an RF signal.

  The electric mixer 13 receives the phase comparison signal output from the phase comparison unit 12 and the modulation signal output from the local oscillation unit 15, and multiplies the two electric signals to demodulate the phase comparison signal to generate a phase error. Output as a signal. The output signal of the electric mixer 13 is an IF (Intermediate Frequency) signal.

  The loop filter 20 shapes and outputs the phase error signal output from the electric mixer 13. The OVCO 30 outputs beat optical signals with wavelengths λ1 and λ2 synchronized with the reference optical signal in accordance with the shaped phase error signal input from the loop filter 20.

  The VCO 31 receives the shaped phase error signal output from the loop filter 20 and generates and outputs an electrical signal having a frequency corresponding to the phase error signal. The RF amplifier 32 receives the electrical signal output from the VCO 31 and amplifies and outputs the electrical signal. The optical comb generator 33 receives the electrical signal output from the RF amplifier 32 and outputs an optical comb signal at a frequency interval (repetition frequency) of the electrical signal. The optical comb signal is an optical signal having a plurality of spectral lines at a frequency interval of the electric signal of the VCO 31. The line width of the difference frequency between each spectral line is narrow. In this way, the OVCO 30 outputs a beat optical signal that makes the phase error signal input from the loop filter 20 zero.

  The two-mode selector 34 receives the optical comb signal output from the optical comb generator 33, and selects the optical signals of the spectral lines (predetermined two modes) of the predetermined wavelengths λ1 and λ2 among the optical comb signals. Select and output. The two-mode multiplexing unit 35 receives the optical signals of the wavelengths λ1 and λ2 output from the two-mode selection unit 34, combines the optical signals of the wavelengths λ1 and λ2, and outputs the resultant optical beat optical signal to the outside. Also output to the temporal modulation section 16. It is assumed that the polarizations of the optical signals having the wavelengths λ1 and λ2 combined by the two-mode multiplexing unit 35 match.

  The local oscillation unit 15 outputs an electric signal having a predetermined frequency as a modulation signal. The output signal of the local oscillating unit 15 is an LO signal. The temporal modulation unit 16 receives the beat optical signal output from the two-mode multiplexing unit 35 and the local frequency signal output from the local oscillation unit 15, and temporally converts the beat optical signal to the beat optical signal based on the local frequency signal. Output with modulation. The circular polarization adjustment unit 17 receives the time-modulated beat light signal output from the time modulation unit 16, and adjusts the beat light signal to be circularly polarized when input to the phase comparison unit 12. 11 is output. The beat optical signal is fed back to the phase comparison unit 12. Thus, the synchronous optical signal generator 1 is locked when the phase error signal becomes 0, and outputs a beat optical signal synchronized with the reference optical signal.

  Next, the operation of the synchronous optical signal generator 1 will be described with reference to FIGS. As described in the prior art, when one of the two optical signals input to the two-photon absorption phase comparison unit 12 (Si-APD) is circularly polarized, the phase comparison signal output from the phase comparison unit 12 The amplitude of (TPA current) is constant regardless of the polarization of the input optical signal, and the offset value depends on the polarization of the input optical signal. Therefore, if the TPA current is differentiated with respect to time, the offset value is canceled, and the differentiated value signal becomes the same signal regardless of the polarization of the input optical signal. In the synchronous optical signal generator 1, the beat optical signal is temporally modulated by the temporal modulator 16, is adjusted to circularly polarized light by the circular polarization adjuster 17, and is input to the phase comparator 12 together with the reference signal. 13, the TPA signal of the phase comparison unit 12 is demodulated, so that a temporal differential value signal of the TPA signal is obtained as a phase error signal.

Here, it will be described that a temporal differential value signal of the TPA signal can be obtained by temporal modulation of the beat optical signal and demodulation of the TPA signal. First, let T (τ) be the TPA signal of the phase comparison unit 12 obtained with respect to the time difference τ between the reference light signal and the beat light signal. At a predetermined time difference τ _{0, the} temporal modulation sin ωt by the local oscillating unit 15 is amplitude D and the temporal modulation unit 16
Is added to the beat light signal, the TPA signal X (τ) is described by X (τ ₀ + Dsinωt).

Here, when the TPA signal X (τ) is Taylor-expanded, it is expressed by the following equation (5). Here, in the limit where D is small, it means that the second term on the right side of Equation (5) is equal to the differential value of X (τ).

  Further, when the local oscillation unit 15 is driven by Dsinωt and input to the electric mixer 13, and the TPA signal of Expression (5) is input to the electric mixer 13, the output of the electric mixer 13 is the right side of Expression (5). The second term is multiplied by sinωt. This is because an input DC component is ignored in a commonly used electric mixer.

A value obtained by multiplying the second term on the right side of the equation (5) by sinωt is expressed by the following equation (6). Further, when the DC component is extracted from the equation (6), the value of the following equation (7) is obtained.

  In the limit where the time amplitude D is small, Equation (7) matches the differential value of the TPA signal. The value of equation (7) is the output signal of the electric mixer 13. In a normal operation, the time amplitude D takes a finite value, and the value is suppressed as small as possible. Therefore, the output signal of the electric mixer 13 substantially matches the differential value of the TPA signal. However, the magnitude of the time amplitude D in the normal operation needs to be a sufficiently small value as compared with the cycle of the beat optical signal and a value larger than the timing jitter.

  Next, an example of temporal modulation will be described with reference to FIG. FIG. 2 shows a waveform of a signal subjected to temporal modulation. In FIG. 2, “period” on the horizontal axis is one cycle of the beat optical signal, and the same applies to the following figures. The temporal modulation period is indicated by Δt.

In the temporal modulation unit 16, for example, as shown in FIG. 2, temporal modulation is performed so that the modulation amount oscillates from a predetermined time t to t + 4Δt. Further, the frequency of the temporally modulated signal needs to be smaller than the OPLL signal frequency. That is, the following expression (8) is satisfied.
(Temporal modulation period Δt) >> (one period of the OPLL output signal) (8)

Further, the frequency of the temporally modulated signal needs to be larger than the OPLL loop band. That is, the following expression (9) is satisfied.
(Temporal modulation period Δt) << (reciprocal of loop band) (9)
In the present embodiment, for example,
(Reciprocal of loop band = 10 [μs]) >> (temporal modulation period = 100 [ns]) >> (period of OPLL output signal (beat optical signal) = 6.25 [ps])
It is set to satisfy the relationship.

  Next, with reference to FIG. 3 and FIG. 4, the state of temporal modulation / demodulation will be described. FIG. 3A shows an example of the TPA signal with respect to the time difference τ and points A, B, and C for temporal modulation. FIG. 3B shows a differential value signal of the TPA signal shown in FIG. FIG. 4A shows the waveform of the TPA signal after temporal modulation at points A, B, and C in FIG. FIG. 4B shows waveforms of the LO signal of the local oscillating unit 15 and the demodulated signals at points A, B, and C in FIG.

  The output signal of the phase comparison unit 12 is a TPA signal having a different offset value with respect to the time difference τ, as shown by a solid line, a broken line, and a dotted line in FIG. Become. In such a case, the temporal modulation unit 16 applies temporal modulation at the frequency of the modulation signal generated by the local oscillation unit 15. Here, TPA signals having predetermined values with time differences τ <0, τ = 0, and τ> 0 are assumed to be points A, B, and C in order.

  Then, the output signal (TPA signal) of the phase comparison unit 12 at points A, B, and C has a waveform as shown in FIG. Furthermore, the LO signal (modulation signal) output from the local oscillation unit 15 and the output signal (demodulation signal) of the electric mixer 13 at points A, B, and C have waveforms as shown in FIG. . As shown in FIG. 4B, the DC component (average value) of the demodulated signal has a positive value at point A, a zero value at point B, and a negative value at point C. That is, it can be seen that the demodulated signal is a differential value of the TPA signal of FIG. Note that the differential value signal of the TPA signal in FIG. 3A has the waveform shown in FIG.

  Next, an appropriate frequency range of the modulation signal of the local oscillation unit 15 will be described with reference to FIG. FIG. 5 shows the frequency distribution of the output signal of the electric mixer 13.

The output signal of the electric mixer 13 has a frequency distribution as shown in FIG. Here, the frequency of the modulation signal of the local oscillation unit 15 is assumed to be f _LO . Since the TPA signal output from the phase comparison unit 12 is temporally modulated by the modulation signal of the local oscillation unit 15, it has a signal component at the frequency f _LO . Further, due to insufficient isolation of the electric mixer 13, it appears in the output signal of the electric mixer 13. Since the demodulated signal output from the electric mixer 13 has a multiplication component of the TPA signal and the modulation signal of the local oscillation unit 15, it has a DC component and a 2 × f _LO component.

From the output signal of the electric mixer 13, only the demodulated signal DC component is extracted and shaped by the loop filter 20, and is input to the OVCO 30 (VCO 31). Therefore, it is preferable to use a low-pass filter that efficiently attenuates the f _LO component and the 2 × f _LO component from the output signal of the electric mixer 13 as the loop filter 20. For this reason, the frequency f _LO of the modulation signal of the local oscillation unit 15 is preferably larger than the loop band of OPLL. Further, it is more preferable that the frequency f _LO is larger than 10 times the loop band of OPLL.

  Next, with reference to FIGS. 6 to 10, first to fourth temporal modulation methods will be sequentially described as temporal modulation methods of the temporal modulation unit 16. Conventionally, there has been a configuration in which an optical signal is phase-modulated using a phase modulator (for example, see Patent Document 2). However, there has been no technique for applying temporal modulation to an optical signal (particularly a beat optical signal).

  First, the first temporal modulation method will be described with reference to FIGS. FIG. 6 shows an apparatus configuration using the phase modulation unit 16A according to the first temporal modulation method.

  The first temporal modulation method is a method of performing temporal modulation by phase-modulating one (wavelength λ1) of CW (Continuous Wave) light of wavelengths λ1 and λ2 constituting the beat optical signal. is there. As shown in FIG. 2, the temporal modulation can be realized by phase modulation. In the first temporal modulation method, the temporal modulation unit 16 includes a phase modulation unit 16A and an optical coupler 160, as shown in FIG. For example, the modulation signal output from the local oscillation unit 15 is amplified by the RF amplifier 151 and output to the phase modulation unit 16A.

  The phase modulation unit 16A phase-modulates the wavelength λ1 component of the beat optical signal with the voltage of the modulation signal input from the local oscillation unit 15. The wavelength λ1 component of the beat optical signal phase-modulated by the phase modulating unit 16A and the wavelength λ2 component of the beat optical signal are combined by the optical coupler 160, and the circularly polarized light adjusting unit 17 is used as the beat optical signal after temporal modulation. Is output.

Here, the change in the phase of the beat optical signal by the first temporal modulation method will be described. First, the electric field of the wavelength λ1 component of the beat optical signal is E ₁ (t), and the electric field of the wavelength λ2 component is E ₂ (t). In addition, the electric field E ₁ (t) = sin (ω ₁ t + φ) and the electric field E ₂ (t) = sinω ₂ t are set. Where ω _{1 is} an angular velocity corresponding to the wavelength λ1, ω _{2 is} an angular velocity corresponding to the wavelength λ2, and φ is a phase modulation amount by the phase modulation unit 16A. In the present embodiment, for example, a beat optical signal of about 193 [THZ] (wavelength 1550 [nm]) is assumed, but the present invention is not limited to this.

Then, the power P (t) of the beat optical signal after temporal modulation is described by the following equation (10).

In Expression (10), the component of (1 + cos ((ω ₂ −ω ₁ ) t−φ)) / 2 is the envelope component (for example, 160 [GHz]) of the beat optical signal, and sin ² (((ω ₁ The component of + ω ₂ ) t + φ) / 2) (for example, 193 [THz]) is the component of the carrier wave. From equation (10), it can be seen that when the phase modulation amount φ of the phase modulation section 16A is changed from 0 to 2π, the phase of the beat optical signal also changes from 0 to 2π. That is, the beat optical signal can be freely temporally modulated by arbitrary phase modulation of the wavelength λ1 component.

  With reference to FIG. 7, the voltage of the modulation signal input to the phase modulation unit 16A will be described. FIG. 7 shows an intensity time waveform of the beat optical signal with respect to the voltage applied to the phase modulation unit 16A.

In FIG. 7, _Vπ is an applied voltage necessary for the phase modulation unit 16A to shift the phase of the carrier wave constituting the beat optical signal by π. When a normal LN phase modulator is used, _Vπ is about 3 to 5 [V].

但し、Ｔ_{period-of-BeatSignal}：入力ビート光信号の一周期である。ビート光信号の周波数が１６０［ＧＨｚ］である場合に、Ｔ_{period-of-BeatSignal}は、６．２５［ｐｓ］となる。一般的には、Ｔ_{period-of-BeatSignal}＝１／ｆ_beatである。ただし、ｆ_beat：ビート光信号の周波数である。 A modulation amount of time T _m of a light passing through the phase modulation unit, the voltage V required for the temporal modulation, according to the following equation (11).

Where T _{period-of-BeatSignal} is one period of the input beat optical signal. When the frequency of the beat optical signal is 160 [GHz], T _{period-of-BeatSignal} is 6.25 [ps]. Generally, T _{period-of-BeatSignal} = 1 / f _beat . Where f _beat is the frequency of the beat optical signal.

For example, as shown in FIG. 7, in order to temporally modulate the frequency of the beat optical signal of 160 [GHz] by 3.125 [ps] which is a half period, V = _Vπ Requires applied voltage. Therefore, the voltage value of the modulation signal of the local oscillation unit 15 input to the phase modulation unit 16A in order to temporally modulate the beat optical signal is relatively small.

  In the first temporal modulation method, the upper limit of the modulation frequency is determined by the band of the phase modulation unit 16A. For this reason, for example, a modulation frequency up to about 50 [GHz] is possible. The amount of temporal modulation can be up to several times the period of the beat optical signal. For example, if the beat optical signal is 160 [GHz], 6.25 [ps] can be easily modulated.

  Here, with reference to FIG. 8, a temporal modulation unit 161 as an example of a temporal modulation unit that performs the first temporal modulation method will be described. FIG. 8 shows the configuration of the temporal modulation unit 161.

  As illustrated in FIG. 8, the temporal modulation unit 161 includes a circulator 1611, an AWG 1612, an LN phase modulator 1613, an FRM 1614, a VOA (Variable Optical Attenuator) 1615, and an FRM 1616.

  The circulator 1611 outputs the optical comb signal (beat optical signal) input from the OVCO 30 to the AWG 1612 and outputs the beat optical signal input from the AWG 1612 (to the circular polarization adjustment unit 17). The AWG 1612 selects an optical signal having a wavelength of λ1 or λ2 from the optical comb signal (beat optical signal) input from the circulator 1611, outputs the optical signal having the wavelength λ1 to the LN phase modulator 1613, and outputs the optical signal having the wavelength λ2. The optical signal is output to the VOA 1615.

  The LN phase modulator 1613 receives the modulation signal from the local oscillation unit 15 and the optical signal having the wavelength λ1 from the AWG 1612. The LN phase modulator 1613 modulates and outputs the phase of the optical signal having the wavelength λ1 in accordance with the voltage of the input modulation signal. The optical signal having the wavelength λ1 output from the LN phase modulator 1613 is reflected by the FRM 1614 and is input again to the AWG 1612 via the LN phase modulator 1613. The VOA 1615 receives the optical signal having the wavelength λ 2 from the AWG 1612, and outputs the optical signal having the wavelength λ 2 with the same attenuation as the LN phase modulator 1613. The optical signal having the wavelength λ2 output from the VOA 1615 is reflected by the FRM 1616 and input again to the AWG 1612 through the VOA 1615.

  The optical signals of wavelengths λ1 and λ2 input through the LN phase modulator 1613 and the VOA 1615 are combined by the AWG 1612 and output to the circular polarization adjustment unit 17 through the circulator 1611 as a beat optical signal.

  The optical signals of wavelengths λ1 and λ2 output from the AWG 1612 have the same polarization, are reflected by the FRMs 16141 and 1615 via the LN phase modulator 1613 and the VOA 161, respectively, and output through the AWG 1612 again. Further, the FRMs 16141 and 1615 output an optical signal having a polarization orthogonal to the polarization of the input optical signal by reflection. For this reason, the optical signal output from the AWG 1612 is a beat optical signal having wavelengths λ1 and λ2. The polarization purity of the beat optical signal is increased by the AWG 1612, and the two optical spectrum lines to be selected pass through the AWG 1612 twice, so that the optical SNR is increased.

  Next, the second temporal modulation method will be described with reference to FIGS. 9 and 10. FIG. 9 shows an apparatus configuration according to the second temporal modulation method.

  The second temporal modulation method is a method of phase-modulating the input optical signal (beat optical signal) itself. As shown in FIG. 9, in the second temporal modulation method, as shown in FIG. 6, the temporal modulation unit 16 includes a phase modulation unit 16B. For example, the modulation signal output from the local oscillation unit 15 is amplified by the RF amplifier 152 and output to the phase modulation unit 16B. The phase modulation unit 16B performs phase modulation on the input beat optical signal according to the voltage of the modulation signal input from the RF amplifier 152 and outputs the result.

  The second temporal modulation method has the simplest configuration. Moreover, not only a beat optical signal but arbitrary optical signals can be temporally modulated. Further, high-speed modulation is possible. For example, the maximum value of the modulation frequency can be set to 50 [GHz].

  The voltage required for the second temporal modulation method will be described with reference to FIG. FIG. 10 shows the intensity time waveform of the beat optical signal with respect to the voltage applied to the phase modulation unit 16B.

10, the V _[pi, an applied voltage required for phase modulation section 16B is shifted the phase of the beat light signal is the carrier [pi, for example, about 3 to 5 [V].

但し、Ｔ_period：入力光信号の搬送波の一周期である。 A modulation amount of time T _m of a light passing through the phase modulation unit, the voltage V required for the temporal modulation, according to the following equation (12).

Where T _period is one period of the carrier wave of the input optical signal.

For example, if the wavelength of the input optical signal is 1533.33 [nm] (193.0 [THz]), T _m = 5.18 [fs]. At this time, 160 for delaying the optical beat signal of a half cycle 3.125 [ps] temporally [GHz], from equation (12), the voltage of as 1206 · V _[pi shown in FIG. 10 Application is required.

  Next, a third temporal modulation method will be described. The third temporal modulation method is a method of performing temporal modulation by vibrating the optical path length of the space in the spatial optical system. For example, consider a configuration in which an input optical signal is reflected by a first fixed mirror, the reflected light is reflected by first and second movable mirrors, and the reflected light is reflected by a second fixed mirror and output. . Actuators such as piezo elements are attached to the first and second movable mirrors. The actuator is vibrated by the modulation signal output from the local oscillation unit and amplified by the RF amplifier. According to the third temporal modulation method, the delay amount can be increased. On the other hand, since the actuator vibrates mechanically, it is difficult to perform temporal modulation using a modulation signal in the megahertz order.

  Next, a fourth temporal modulation method will be described. The fourth temporal modulation method is a method of performing temporal modulation by modulating the repetition frequency of the mode-locked laser. The mode-locked laser is driven by the modulation signal output from the local oscillating unit to output laser light. As described above, the timing of the output pulse is modulated by increasing or decreasing the repetition frequency. The timing modulation degree is a value obtained by multiplying the frequency deviation by time. According to the fourth temporal modulation method, large timing modulation can be performed. On the other hand, a modulation frequency larger than the loop band is required, and it is difficult to temporally modulate with a megahertz order modulation signal.

  As described above, according to the present embodiment, the optical coupler 11 combines the reference optical signal and the temporally modulated circularly-polarized beat optical signal, and the phase comparison unit 12 combines the combined reference optical signal and beat. A phase comparison signal (TPA signal) of the optical signal is generated, and the electric mixer 13 multiplies (demodulates) the phase comparison signal and the modulation signal of the local oscillation unit 15 to obtain a phase error from the differential value signal of the phase comparison signal. It outputs as a signal and the loop filter 20 shapes. Then, based on the shaped phase error signal, the OVCO 30 generates and outputs a beat optical signal in which the phase error signal becomes 0, and the temporal modulation unit 16 locally oscillates the beat optical signal generated by the OVCO 30. The time-modulated signal is modulated based on the modulation signal of the unit 15, and the circularly polarized light adjusting unit 17 converts the time-modulated beat optical signal into circularly polarized light and outputs it to the optical coupler 11. In this way, in OPLL, the beat optical signal is temporally modulated to be circularly polarized, and the phase comparison signal is demodulated to obtain a differential value signal of the phase comparison signal as a phase error signal. Therefore, a synchronized beat optical signal can be generated at a synchronization timing that does not depend on the polarization of the input optical signal (reference optical signal, feedback beat optical signal) in OPLL.

  The OVCO 30 includes a VCO 31, an optical comb generator 33, a two-mode selector 34, and a two-mode multiplexer 35. For this reason, the line width of the synchronous beat optical signal output from the OVCO 30 can be reduced, the phase noise can be reduced, the loop band can be reduced, and the loop length can be made appropriate.

  Further, when the temporal modulation unit 16 performs the first temporal modulation method, the phase of the beat optical signal can be freely changed from 0 to 2π by changing the phase from 0 to 2π by the phase modulation unit 16B. Can change. In addition, it is possible to easily perform temporal modulation of the beat optical signal using a relatively low voltage modulation signal from the local oscillation unit 15.

  The temporal modulation unit 161 that performs the first temporal modulation method includes an AWG 1612 and FRMs 1614 and 1616, and optical signals having wavelengths λ1 and λ2 pass through the AWG 1612 twice. For this reason, the optical SNR of the beat optical signal to be returned can be increased.

  Further, when the temporal modulation unit 16 performs the second temporal modulation method, the temporal modulation of the beat optical signal can be performed using the modulation signal from the local oscillation unit 15, and the temporal modulation is performed. The apparatus configuration of the modulation unit 16 can be simplified.

  With reference to FIG. 11, Example 1 as a specific example of the above embodiment will be described. In FIG. 11, the structure of the synchronous optical signal generator 2 of a present Example is shown. However, the same reference numerals are given to the same components as those described in the above embodiment, and the description thereof is omitted.

  The synchronized optical signal generator 2 according to the present embodiment is a device that applies time modulation to the beat optical signal by the first temporal modulation method and outputs a synchronized beat optical signal synchronized with the reference optical signal. As shown in FIG. 11, the synchronous optical signal generator 2 includes an optical coupler 11, an Si-APD 121, a TIA 19, an electric mixer 13, a loop filter 20, an OVCO 301, a local oscillator 15, and temporal modulation. The unit 161, the phase shifter 18, and the PC 171 are configured. The OVCO 301 includes a VCO 31, double frequency generators 311 and 312, an RF amplifier 32, a DFB-LD 331, an LN phase modulator 332, a circulator 341, an AWG 342, an FRM 351 and 352, an EDFA 361, and an optical coupler. 37.

  The local oscillating unit 15 generates and outputs a local electrical signal (modulated signal) having a predetermined frequency. The optical coupler 11 combines the reference optical signal and the circularly polarized beat optical signal input from the PC 171 and outputs the combined optical signal. The Si-APD 121 receives the combined optical signal output from the optical coupler 11 and outputs a phase comparison signal (TPA signal) between the reference optical signal and the beat optical signal. The phase comparison signal is converted into a voltage signal by the TIA 19 and amplified, multiplied by the modulation signal input from the local oscillation unit 15 by the electric mixer 13, and demodulated and output. This demodulated signal is a differential value signal of the phase comparison signal and is used as a phase error signal.

  The phase error signal is shaped by the loop filter 20 and input to the VCO 31 of the OVCO 301. The VCO 31 generates and outputs an electrical signal whose frequency is changed according to the phase error signal input from the loop filter 20. The electric signal output from the VCO 31 is quadrupled in frequency by the double frequency generators 311 and 312 and amplified by the RF amplifier 32. The DFB-LD 331 generates and outputs CW light. The LN phase modulator 332 converts the CW light output from the DFB-LD 331 into an optical comb signal having a frequency interval of the electrical signal output from the RF amplifier 32 and outputs the optical comb signal.

  The optical comb signal output from the LN phase modulator 332 is input to the AWG 342 via the circulator 341, and the optical signals of the spectral lines having the wavelengths λ1 and λ2 are selected by the AWG 342, reflected by the FRMs 351 and 352, and multiplexed. Then, beat optical signals with wavelengths λ1 and λ2 are output. The beat optical signal output from the AWG 342 is amplified by the EDFA 361 via the circulator 341 and output to the outside, and also output to the temporal modulation unit 161. Thus, in the OVCO 301, the optical signals having the wavelengths λ1 and λ2 pass through the AWG 342 twice. For this reason, the optical SNR of the beat optical signal can be increased.

  The phase shifter 18 receives the modulation signal output from the local oscillation unit 15, shifts the modulation signal by a set phase amount, and outputs the shifted signal to the LN phase modulator 1613 of the temporal modulation unit 161. This phase shift amount is adjusted so that the phase error signal amplitude becomes maximum, depending on the path of the synchronous optical signal generator 2 and the like.

  The beat optical signal temporally modulated by the temporal modulation unit 161 is output from the circulator 1611 and input to the PC 171. The PC 171 adjusts the polarization so that the beat optical signal input from the circulator 1611 becomes circularly polarized at the input of the Si-APD 121, and outputs it to the optical coupler 11.

Here, the function of the phase shifter 18 will be described. In FIG. 3A, a time path example when time modulation is applied at points A, B, and C will be considered. In the synchronous optical signal generator 2, the LO signal input to the electric mixer 13 can be described as cos (ωt + φ) depending on the length of the electric path. Further, the signals of points A, B, and C can be described in order as cos (ωt + π), cos (2ωt + π), and cosωt. The electric mixer 13 performs multiplication. For this reason, the signals of points A, B, and C are sequentially described by the following equations (13) to (15).

  At points A and C, the DC component (low frequency component) extracted by the loop filter 20 is the first term of each of the equations (13) and (15). According to Equation (14), at point B, the DC component is always zero regardless of φ. At points A and C, the DC component increases or decreases depending on the value of φ, and the sign is reversed. In order to realize the synchronous operation, φ is adjusted by the phase shifter 18 so that the phase error signal amplitude becomes the maximum value while the output frequency of the OPLL is almost the same as the bit rate of the reference light signal. . However, if the lengths of the conductors from the local oscillation unit 15 to the LN phase modulator 1613 and from the local oscillation unit 15 to the electric mixer 13 are the same with an error of 1/5 or less of the wavelength of the modulation wavelength, the phase is Even without the shifter 18, a phase error signal with a certain amplitude can be obtained.

  As described above, according to the present embodiment, a synchronous beat optical signal that does not depend on the polarization of the input optical signal can be generated as in the above-described embodiment. Further, since the first temporal modulation method is performed, the temporal modulation of the beat optical signal can be easily performed. Further, the optical signal having the wavelengths λ 1 and λ 2 passes through the AWG 1612 twice by the temporal modulation unit 161. For this reason, the optical SNR of the beat optical signal to be returned can be increased. Furthermore, the apparatus configuration can be easily realized using ready-made optical components.

  Further, a phase shifter 18 that shifts the phase of the modulation signal of the local oscillation unit 15 by an amount that maximizes the phase error signal amplitude is provided. For this reason, the amplitude of the phase error signal can be maximized. Thereby, the SNR of the phase error signal can be increased and the timing jitter can be reduced.

  With reference to FIG. 12, Example 2 as a specific example of the above embodiment will be described. FIG. 12 shows the configuration of the synchronous optical signal generator 3 of this embodiment. However, the same components as those described in the above embodiments and examples are denoted by the same reference numerals, and the description thereof is omitted.

  The synchronized optical signal generator 3 of this embodiment is a device that applies temporal modulation to the beat optical signal by the first temporal modulation method and outputs a synchronized beat optical signal synchronized with the reference optical signal. As shown in FIG. 12, the synchronous optical signal generator 3 includes a half mirror 111, a lens 112, a Si-APD 121, a TIA 19, an electric mixer 13, a loop filter 20, an OVCO 302, and a polarization holding unit. PMF (Polarization Maintaining Fibers) 45 and 46, a local oscillation unit 15, a phase shifter 18, a temporal modulation unit 162, and a quarter wavelength plate 172.

  The OVCO 302 includes a VCO 31, double frequency generators 311 and 312, an RF amplifier 32, a DFB-LD 331, PMFs 41 to 44 as polarization holding units, an LN phase modulator 332, and a polarization holding unit. PBS (Polarized Beam Splitter) 343, AWG 342, FRM 351, 352, PM (Polarization Maintaining) -EDFA 362 as a polarization maintaining unit, and optical coupler 37. The The temporal modulation unit 162 includes a PBS 1617 as a polarization maintaining unit, an AWG 1612, an LN phase modulator 1613, an FRM 1614, a VOA 1615, and an FRM 1616. The PBSs 343 and 1617 may be PBC (Polarized Beam Combiner).

  The half mirror 111, the lens 112, the Si-APD 121, and the quarter wavelength plate 172 constitute a spatial optical system F. In the spatial optical system F, the optical elements are configured to pass through a space rather than an optical fiber, and polarization does not change in the space. The half mirror 111 transmits the reference light signal, reflects the circularly polarized beat light signal emitted from the quarter-wave plate 172, and emits it in the same direction (lens 112) as the reference light signal. The lens 112 collects the optical signal emitted from the half mirror 111 on the surface of the Si-APD 121. The Si-APD 121 generates and outputs a phase comparison signal of the incident optical signal.

  In the OVCO 302, the DFB-LD 331 and the LN phase modulator 332 are connected via the PMF 41, the LN phase modulator 332 and the PBS 332 are connected via the PMF 42, and the PBS 332 and the PM-EDFA 362 are connected. The PMF 43 is connected, and the PM-EDFA 362 and the optical coupler 37 are connected via the PMF 44. The polarization of the optical signal passing through the PMFs 41 to 44 is maintained. The linearly polarized CW light output from the DFB-LD 331 is input to the LN phase modulator 332 via the PMF 41, converted into a linearly polarized optical comb signal by the LN phase modulator 332, and output. The PBS 343 reflects the linearly polarized optical comb signal input from the LN phase modulator 332 via the PMF 42 and outputs the reflected optical comb signal to the AWG 342, and also outputs the beat optical signals of the linearly polarized wavelengths λ 1 and λ 2 input from the AWG 342 to the PMF 43. To the PM-EDFA 362. The PM-EDFA 362 maintains and amplifies the polarization of the linearly polarized beat optical signal input from the PBS 343 and outputs the amplified signal.

  The linearly-polarized beat optical signal output from the PM-EDFA 362 is demultiplexed by the optical coupler 37 via the PMF 44, output to the outside, and output to the PBS 1617 of the temporal modulation unit 162 via the PMF 45. The PBS 1617 reflects the linearly-polarized beat light signal input via the PMF 45 and outputs it to the AWG 1612. The PBS 1617 converts the linearly-polarized beat light signal input from the AWG 1612 into a time-modulated beat light signal. To the quarter-wave plate 172. The quarter wave plate 172 has a function of converting linearly polarized light into circularly polarized light. Therefore, the quarter wavelength plate 172 converts the linearly polarized beat light signal input via the PMF 46 into circularly polarized light and outputs the circularly polarized light to the half mirror 111.

  As described above, according to the present embodiment, a synchronous beat optical signal that does not depend on the polarization of the input optical signal can be generated as in the above-described embodiment. Further, since the first temporal modulation method is performed, the temporal modulation of the beat optical signal can be easily performed. Furthermore, since the quarter-wave plate 172 is used for the circular polarization adjustment unit 17, the adjustment of the polarization as in the PC 171 can be made unnecessary.

  As in the first embodiment, the optical signal having the wavelengths λ 1 and λ 2 passes through the AWG 1612 twice by the temporal modulation unit 162. For this reason, the optical SNR of the beat optical signal to be returned can be increased. Further, the phase shifter 18 can maximize the amplitude of the phase error signal, increase the SNR of the phase error signal, and reduce the timing jitter.

  With reference to FIG. 13, Example 3 as a specific example of the above embodiment will be described. FIG. 13 shows the configuration of the synchronous optical signal generator 4 of this embodiment. However, the same components as those described in the above embodiments and examples are denoted by the same reference numerals, and the description thereof is omitted.

  The synchronized optical signal generator 4 of this embodiment is a device that applies temporal modulation to the beat optical signal by the first temporal modulation method and outputs a synchronized beat optical signal synchronized with the reference optical signal. As shown in FIG. 13, the synchronous optical signal generator 4 includes an optical coupler 11, an Si-APD 121, a TIA 19, an electric mixer 13, a loop filter 20, an OVCO 303, a local oscillator 15, and a phase shifter 181. And a temporal modulation unit 163 and a PC 171.

  The OVCO 303 includes a VCO 31, an RF amplifier 32, a DFB-LD 331, an LN phase modulator 332, an AWG 342, optical couplers 344 and 345, a PC 353, and an optical coupler 354. The temporal modulation unit 163 includes an LN phase modulator 1613, a PC 1618, an optical coupler 1619, and an EDFA 1620.

  In the synchronous optical signal generation device 4, the phase shifter 181 shifts the modulation signal input from the local oscillation unit 15 by a phase amount so that the phase error signal amplitude is maximized, and outputs the modulated signal. The electric mixer 13 receives the time-adjusted phase comparison signal from the TIA 19 and the modulation signal output from the local oscillation unit 15 and phase-shifted by the phase shifter 181, and receives these two signals. To generate a differential value signal of the phase comparison signal and output it as a phase error signal.

  In the OVCO 303, the optical comb signal output from the LN phase modulator 332 is input to the AWG 342. The AWG 342 outputs an optical signal having a line spectrum of wavelengths λ1 and λ2 from the optical comb signal input from the LN phase modulator 332. The optical signal having the wavelength λ1 output from the AWG 342 is demultiplexed by the optical coupler 344, output to the optical coupler 354, and output to the LN phase modulator 1613 of the temporal modulation unit 163. The optical signal of wavelength λ 2 output from the AWG 342 is demultiplexed by the optical coupler 345, output to the optical coupler 354 via the PC 353, and output to the PC 1618 of the temporal modulation unit 163. The PC 353 adjusts the polarization of the optical signal having the wavelength λ 2 input from the AWG 342 to the polarization of the optical signal having the wavelength λ 1 input to the optical coupler 354, and outputs it. The optical signal of wavelength λ1 output from the optical coupler 344 and the optical signal of wavelength λ2 output from the PC 353 are combined by the optical coupler 354 and output to the outside as a beat optical signal.

  In the temporal modulation unit 163, the LN phase modulator 1613 outputs an optical signal having the wavelength λ 1 input from the optical coupler 343 in accordance with the voltage of the modulation signal output from the local oscillation unit 15 and amplified by the RF amplifier 151. The phase is modulated and output to the optical coupler 1619. The PC 1618 adjusts the polarization of the optical signal having the wavelength λ2 input from the optical coupler 344 to the polarization of the optical signal having the wavelength λ2 input to the optical coupler 1619, and outputs the adjusted signal. The optical signal of wavelength λ1 output from the LN phase modulator 1613 and the optical signal of wavelength λ2 output from the PC 1618 are combined by an optical coupler 1619, amplified by an EDFA 1620, and temporally modulated beat optical signal. Is output to the PC 171.

  As described above, according to the present embodiment, a synchronous beat optical signal that does not depend on the polarization of the input optical signal can be generated as in the above-described embodiment. Further, since the first temporal modulation method is performed, the temporal modulation of the beat optical signal can be easily performed. In addition, the OVCO 303 and the temporal modulation unit 163 share the wavelength selection portion (two-mode selection unit 34) of the wavelengths λ1 and λ2, and the apparatus configuration can be simplified. In particular, as compared with the synchronous optical signal generators 2 and 3, only one AWG is required, and the manufacturing cost can be reduced.

  Similarly to the first embodiment, the phase shifter 181 can maximize the amplitude of the phase error signal, increase the SNR of the phase error signal, and reduce the timing jitter.

  Referring to FIG. 14, Example 4 as a specific example of the above embodiment will be described. FIG. 14 shows the configuration of the synchronous optical signal generator 5 of this embodiment. However, the same components as those described in the above embodiments and examples are denoted by the same reference numerals, and the description thereof is omitted.

The synchronized optical signal generator 5 of this embodiment is a device that applies temporal modulation to the beat optical signal by the second temporal modulation method and outputs a synchronized beat optical signal synchronized with the reference optical signal. As shown in FIG. 14, the synchronous optical signal generator 5 includes an optical coupler 11, an Si-APD 121, a TIA 19, an electric mixer 13, a loop filter 20, an OVCO 301, a local oscillator 15, and a phase shifter 18. And an LN phase modulator 164 as a temporal modulation unit and a PC 171.

  The LN phase modulator 164 phase-modulates the beat optical signal input from the optical coupler 37 of the OVCO 301 in accordance with the voltage of the modulation signal input from the phase shifter 18, and sends it to the PC 17 as a time-modulated beat optical signal. Output.

  As described above, according to the present embodiment, a synchronous beat optical signal that does not depend on the polarization of the input optical signal can be generated as in the above-described embodiment. In addition, since the second temporal modulation method is performed, the beat optical signal can be temporally modulated and the apparatus configuration can be simplified. In particular, as compared with the synchronous optical signal generators 2 and 3, only one AWG is required, and the manufacturing cost can be reduced.

  Similarly to the first embodiment, the phase shifter 18 can maximize the amplitude of the phase error signal, increase the SNR of the phase error signal, and reduce the timing jitter.

  With reference to FIG. 15, Example 5 as a specific example of the above embodiment will be described. FIG. 15 shows the configuration of the synchronous optical signal generator 6 of this embodiment. However, the same components as those described in the above embodiments and examples are denoted by the same reference numerals, and the description thereof is omitted.

  The synchronous optical signal generator 6 of this embodiment is a device that applies temporal modulation to the beat optical signal by the first temporal modulation method and outputs a synchronous beat optical signal synchronized with the reference optical signal. As shown in FIG. 15, the synchronous optical signal generator 6 includes an optical coupler 11, an Si-APD 121, a TIA 19, an electric mixer 13, a loop filter 20, an OVCO 304, a local oscillator 15, and a phase shifter 18. And a PC 171.

  The OVCO 304 includes a VCO 31, double frequency generators 311 and 312, an RF amplifier 32, a DFB-LD 331, an LN phase modulator 332, a temporal modulation unit 161, an EDFA 361, and an optical coupler 37. Configured. The optical comb signal output from the LN phase modulator 332 is output to the AWG 1612 via the circulator 1611 of the temporal modulation unit 161. In addition, the time-modulated beat optical signal output from the LN phase modulator 332 output from the AWG 1612 is amplified by the EDFA 361, demultiplexed by the optical coupler 37, and output to the outside and the PC 171.

  That is, the beat optical signal output to the outside from the synchronous optical signal generator 6 is a time-modulated beat optical signal. For this reason, when an application using an external beat optical signal or the like permits a time-modulated beat optical signal, the synchronous optical signal generator 6 can be applied.

  As described above, according to the present embodiment, a synchronous beat optical signal that does not depend on the polarization of the input optical signal can be generated as in the above-described embodiment. Further, since the first temporal modulation method is performed, the temporal modulation of the beat optical signal can be easily performed. Furthermore, since the OVCO 304 includes the temporal modulation unit 161, the apparatus configuration can be simplified. In particular, as compared with the synchronous optical signal generators 2 and 3, only one AWG is required, and the manufacturing cost can be reduced.

  As in the first embodiment, the optical signal having the wavelengths λ 1 and λ 2 passes through the AWG 1612 twice by the temporal modulation unit 161. For this reason, the optical SNR of the beat optical signal to be returned can be increased. Further, the phase shifter 18 can maximize the amplitude of the phase error signal, increase the SNR of the phase error signal, and reduce the timing jitter.

  With reference to FIG. 16, Example 6 will be described as a specific example of the above embodiment. FIG. 16 shows the configuration of the synchronous optical signal generator 7 of this embodiment. However, the same components as those described in the above embodiments and examples are denoted by the same reference numerals, and the description thereof is omitted.

  The synchronized optical signal generator 7 of this embodiment is a device that applies temporal modulation to the beat optical signal by the first temporal modulation method and outputs a synchronized beat optical signal synchronized with the reference optical signal. As shown in FIG. 16, the synchronous optical signal generator 7 includes an optical coupler 11, an Si-APD 121, a TIA 19, an electric mixer 13, a loop filter 20, an OVCO 305, a local oscillator 15, and a phase shifter 18. And a temporal modulation unit 161 and a PC 171.

  The OVCO 305 includes a VCO 31, double frequency generators 311, 312, an RF amplifier 32, a DFB-LD 331, an LN phase modulator 332, a Fabry-Perot etalon 346, an EDFA 361, and an optical coupler 37. Composed.

  The Fabry-Perot etalon is an optical element in which the inter-interference distance is fixed in the Fabry-Perot interference system, and a specific wavelength of the input light is enhanced and output. The interval between specific wavelengths transmitted by the Fabry-Perot etalon is set as an FSR (Free Spectral Range). In addition, the Fabry-Perot etalon requires temperature adjustment in order to maintain the wavelength accuracy of the output light.

  In the Fabry-Perot etalon 346, for example, the FSR is set to 160 [GHz]. The Fabry-Perot etalon 346 receives the optical comb signal output from the LN phase modulator 332 and transmits an optical signal having a wavelength (λ1, λ2) corresponding to the FSR from the optical comb signal (for example, 160 [GHz]). ) Output to the EDFA 361 as a beat light signal.

  As described above, according to the present embodiment, a synchronous beat optical signal that does not depend on the polarization of the input optical signal can be generated as in the above-described embodiment. Further, since the first temporal modulation method is performed, the temporal modulation of the beat optical signal can be easily performed. Furthermore, since the OVCO 304 is configured using the Fabry-Perot etalon 346 without using AWG, the apparatus configuration can be simplified. In particular, as compared with the synchronous optical signal generators 2 and 3, only one AWG is required, and the manufacturing cost can be reduced.

  As in the first embodiment, the optical signal having the wavelengths λ 1 and λ 2 passes through the AWG 1612 twice by the temporal modulation unit 161. For this reason, the optical SNR of the beat optical signal to be returned can be increased. Further, the phase shifter 18 can maximize the amplitude of the phase error signal, increase the SNR of the phase error signal, and reduce the timing jitter.

  Note that the descriptions in the above-described embodiments and modifications are examples of the synchronous optical signal generation device, the temporal modulation device, the synchronous optical signal generation method, and the temporal modulation method according to the present invention, and are not limited thereto. Absent.

  For example, at least two of the above-described embodiments, each example, and some or all of the configurations described below may be appropriately combined.

  In the above embodiment and each example, the OVCO is configured to generate a beat optical signal having a narrow line width by the optical comb generator, the two-mode selector, and the two-mode multiplexer, but the present invention is not limited thereto. Is not to be done. For example, like the conventional synchronous optical signal generator 7, it is good also as a structure using other OVCOs, such as OVCO using LD.

  Moreover, in the said embodiment and each Example, although it was set as the structure which uses Si-APD121 using two-photon absorption as a nonlinear effect as the phase comparison part 12, it is not limited to this, Other nonlinear effects It is good also as a thing using. For example, a phase comparator that uses four-wave mixing (FWM) in an optical fiber may be used. The phase comparison unit using the FWM extracts FWM light newly generated by the FWM using an optical filter, converts it into a current with a photodiode, and outputs it as a phase comparison signal. The phase comparison unit using FWM uses, for example, a nonlinear medium such as HNLF (Highly Nonlinear Fiber), SOA (Semiconductor Optical Amplifier), or the like.

  Further, as the phase comparison unit, a phase comparison unit that uses cross phase modulation (XPM) in an optical fiber may be used. The phase comparison unit using XPM shifts the wavelength of the optical signal by XPM, extracts the XPM light having the shifted wavelength by an optical filter, converts it to a current by a photodiode, and outputs it as a phase comparison signal.

  Further, a phase comparison unit using second harmonic generation (SHG) in the nonlinear optical crystal may be used. A phase comparison unit using SHG extracts SHG light with an optical filter, converts it into a current with a photodiode, and outputs it as a phase comparison signal.

  Further, a phase comparator using SHG in pseudo phase matching (PPLN: Periodically Poled Lithium Niobate) may be used. A phase comparison unit using SHG in the PPLN extracts SHG light with an optical filter, converts it into a current with a photodiode, and outputs it as a phase comparison signal. In addition, as the phase comparison unit, a unit using a PMT (Photo Multiplier Tube), a unit using the Kerr effect, or the like may be used.

  The detailed configuration and detailed operation of each component of the synchronous optical signal generator described in the above embodiments and examples can be changed as appropriate without departing from the spirit of the present invention. Of course.

It is a figure which shows the structure of the synchronous optical signal generator of embodiment which concerns on this invention. It is a figure which shows the waveform of the signal by which time modulation was made. (A) is a figure which shows an example of the TPA signal with respect to time difference (tau), and the points A, B, and C which time-modulate. (B) is a figure which shows the differential value signal of the TPA signal of (a). (A) is a figure which shows the waveform of the TPA signal after the time modulation | alteration in the points A, B, and C of Fig.3 (a). (B) is a figure which shows the waveform of the LO signal of the local oscillation part 15, and the demodulated signal in point A, B, C of Fig.3 (a). It is a figure which shows the frequency distribution of the output signal of an electric mixer. It is a figure which shows the apparatus structure using the phase modulation part by the 1st temporal modulation method. It is a figure which shows the intensity | strength time waveform of the beat optical signal with respect to the voltage added to the phase modulation part. It is a figure which shows the structure of a temporal modulation part. It is a figure which shows the apparatus structure by the 2nd time modulation method. It is a figure which shows the intensity | strength time waveform of the beat optical signal with respect to the voltage applied to the phase modulator. It is a figure which shows the structure of the synchronous optical signal generator of Example 1 which concerns on this invention. It is a figure which shows the structure of the synchronous optical signal generator of Example 2 which concerns on this invention. It is a figure which shows the structure of the synchronous optical signal generator of Example 3 which concerns on this invention. It is a figure which shows the structure of the synchronous optical signal generator of Example 4 which concerns on this invention. It is a figure which shows the structure of the synchronous optical signal generator of Example 5 which concerns on this invention. It is a figure which shows the structure of the synchronous optical signal generator of Example 6 which concerns on this invention. It is a figure which shows the structure of the conventional 1st synchronous optical signal generator. (A) is a figure which shows the waveform and time difference (tau) of a beat light signal and a reference light signal. (B) is a diagram showing a phase error signal I (τ) with respect to the time difference τ. It is a figure which shows the structure of the 2nd conventional synchronous optical signal generator. It is a figure which shows the structure of the conventional synchronous electric signal generator. (A) is a figure which shows the relationship of the TPA signal with respect to time difference (tau). (B) is a figure which shows the relationship of the TPA signal with respect to time t. (A) is a figure which shows a TPA signal at the time of combining two optical signals with at least one linearly polarized light with respect to the time difference τ. (B) is a figure which shows a TPA signal at the time of combining the optical signal with at least one circularly polarized light with respect to the time difference (tau). (A) is a figure which shows the waveform of envelope g (t) with respect to time t. (B) is a diagram showing a waveform of the TPA signal I (τ) with respect to the time difference τ.

Explanation of symbols

1, 2, 3, 4, 5, 6, 7, 8, 9 Synchronous optical signal generator 10 Synchronous electric signal generator 11 Optical coupler F Spatial optical system 111 Half mirror 112 Lens 12 Phase comparator 121 Si-APD
DESCRIPTION OF SYMBOLS 15 Local oscillation part 151,152 RF amplifier 16 Temporal modulation part 16A, 16B Phase modulation part 160 Optical coupler 161,162,163 Temporal modulation part 1611 Circulator 1612 AWG
1613 LN phase modulators 1614 and 1616 FRM
1615 VOA
1617 PBS
1618 PC
1619 Optical coupler 1620 EDFA
164 LN phase modulator 17 Circular polarization adjustment unit 171 PC
172 1/4 wavelength plate 18 phase shifter 19 TIA
20 Loop filter 30, 301, 302, 303, 304, 305 OVCO
31 VCO
311, 312 Double frequency generator 32 RF amplifier 33 Optical comb generator 331 DFB-LD
332 LN phase modulator 34 2 mode selection unit 341 circulator 342 AWG
343 PBS
344, 345 Optical coupler 346 Fabry-Perot etalon 35 2-mode multiplexing unit 351, 352 FRM
353 PC
354 Optical Coupler 361 EDFA
362 PM-EDFA
37 Optical coupler 41, 52, 53, 44, 45, 46 PMF
81 Optical coupler 82 Si-APD
83 Loop filter 84 OVCO
841,842 LD
843 Optical coupler 101 Optical coupler 102 EDFA
103 Si-APD
104 Loop filter 105 VCO
106 RF amplifier 107 Mode-locked laser 108 PC

Claims (12)

A local oscillation unit that generates a modulation signal of a predetermined frequency;
A combining unit for combining the reference light signal and the temporally modulated circularly polarized beat light signal;
A phase comparison unit that generates a phase comparison signal by comparing phases of the combined reference light signal and beat light signal by a non-linear effect;
An electric mixer that multiplies the phase comparison signal and the modulation signal and outputs a differential value signal of the phase comparison signal as a phase error signal;
A shaping unit for shaping the phase error signal;
Based on the shaped phase error signal, a beat optical signal generator that generates and outputs a beat optical signal that sets the phase error signal to 0,
A temporal modulation unit that temporally modulates the beat optical signal generated by the beat optical signal generation unit based on the modulation signal;
A synchronous optical signal generator comprising: a circularly polarized light adjusting unit that converts the temporally modulated beat optical signal into circularly polarized light and outputs the circularly polarized light to the combining unit.   The synchronous optical signal generator according to claim 1, wherein the circular polarization adjustment unit is a polarization controller. The multiplexing unit is a half mirror,
The circularly polarized light adjusting unit is a quarter wavelength plate,
An optical path from the quarter-wave plate to the phase comparison unit is configured by a spatial optical system,
The synchronous optical signal generation device according to claim 1, further comprising a polarization holding unit that holds a polarization of an optical signal passing through an optical path from the beat optical signal generation unit to the quarter-wave plate as linearly polarized light.   4. The synchronous optical signal generation device according to claim 1, wherein the temporal modulation unit includes a phase modulation unit that performs phase modulation on an optical signal having one wavelength constituting the beat optical signal. 5. The temporal modulation unit is
An arrayed waveguide diffraction grating for extracting optical signals of the first and second wavelengths constituting the beat optical signal;
A Faraday rotator mirror that reflects the optical signal of the second wavelength and the optical signal of the first wavelength that is phase-modulated,
The phase modulation unit phase modulates the optical signal of the first wavelength output from the arrayed waveguide diffraction grating,
The arrayed waveguide diffraction grating combines the optical signal of the second wavelength reflected by the Faraday rotator mirror and the optical signal of the first wavelength that has been phase-modulated and outputs it as a beat optical signal. Item 5. The synchronous optical signal generator according to Item 4. The beat optical signal generator is
An optical comb generator for generating an optical comb signal at a frequency interval according to the phase error signal;
A two-mode selection unit for selecting and extracting optical signals of the first and second wavelength line spectra from the generated optical comb signal;
6. A two-mode multiplexing unit that multiplexes the extracted optical signals of the first and second wavelengths to generate and output a beat optical signal. 6. Synchronous optical signal generator.   The synchronous optical signal generation device according to claim 6, wherein the two-mode selection unit and the two-mode multiplexing unit are configured by a Fabry-Perot etalon.   The temporal modulation unit temporally modulates the optical signal of the first wavelength selected by the two-mode selection unit, and the temporally modulated optical signal of the first wavelength and the optical signal of the second wavelength The synchronous optical signal generator according to claim 6, which outputs a combined signal.   The synchronized light according to claim 6, wherein the two-mode selection unit and the two-mode multiplexing unit include the temporal modulation unit, and generate and output a beat optical signal temporally modulated by the temporal modulation unit. Signal generator.   The synchronized optical signal generation device according to claim 1, wherein the temporal modulation unit includes a phase modulation unit that performs phase modulation on the beat optical signal.   11. The synchronous optical signal generator according to claim 1, further comprising: a phase shifter that shifts a phase of a modulation signal from the local oscillation unit and outputs the shifted signal to the electric mixer or the temporal modulation unit. Combining the reference light signal and the temporally modulated circularly polarized beat light signal;
A step of generating a phase comparison signal by comparing phases of the combined reference light signal and beat light signal by a non-linear effect;
Multiplying the phase comparison signal by a modulation signal of a predetermined frequency and outputting a differential value signal of the phase comparison signal as a phase error signal;
Shaping the phase error signal;
Generating and outputting a beat optical signal that sets the differential value signal to 0 based on the differential value signal of the shaped phase error signal;
Temporally modulating the generated beat optical signal based on the modulation signal;
And generating a time-modulated beat optical signal as circularly polarized light and outputting it as a time-modulated circularly-polarized beat optical signal used for the multiplexing.

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