JP2006128828A - Optical/electric clock signal extraction device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical/electric clock signal extraction device extracting optical and electric clock signals directly and simultaneously from optical data signal having a deteriorated waveform in which clock extraction sensitivity from the optical data signal is enhanced while reducing required power of the optical data signal, and extraction of an optical clock signal at a bit rate different from that of the optical data signal, wavelength of the optical clock signal and wavelength of the optical data signal can be set freely. <P>SOLUTION: The optical/electric clock signal extraction device constituting a closed loop positive feedback circuit receives optical data signal and generating an optical clock signal of constant amplitude having a phase synchronized with that of an optical data signal, and an electric clock signal simultaneously comprises an optical clock signal source being driven by the electric clock signal, and an electric clock extraction circuit receiving an optical clock signal from the optical clock signal source and the optical data signal through an optical directional coupler and outputting an electric clock signal. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention can simultaneously generate an optical clock signal and an electric clock signal that are phase-synchronized with an optical data signal and have a constant amplitude from an optical data signal that has caused waveform degradation such as amplitude / timing noise and pulse broadening. With regard to the clock extractor, in particular, an optical / electrical clock signal extractor having a clock extractor function required for signal regeneration and demultiplexing used in a repeater or receiver of an optical fiber communication system having a bit rate of 40 to 160 Gbit / s. Relates to the device.

In an optical fiber communication system having a bit rate of 40 to 160 Gbit / s, a technique for extracting high-speed optical and electrical clock signals is necessary to reproduce data signals in a repeater and to perform demultiplexing in a receiver. .
Therefore, the conventional clock signal extraction device uses a phase-locked loop circuit having an optical / electric hybrid configuration. The phase-locked loop circuit is composed of a phase comparator and a high-frequency voltage-controlled oscillator (VCO), but the phase comparator is composed of an optical method, and the voltage-controlled oscillator VCO is composed of an electronic circuit. Is done.
For example, in the “ultra-high-speed clock extraction circuit” of Patent Document 1, an error signal is detected by a phase comparator using an optical modulator to control the voltage-controlled oscillator VCO, and a local oscillation pulse is output from the electrical output of the voltage-controlled oscillator VCO. The laser light source is driven.
In the “ultra-high-speed clock extraction circuit” of Patent Document 2, an optical modulator is driven by an electric output of a voltage controlled oscillator VCO to obtain an optical clock signal.

Further, in the “optical clock signal extraction circuit” of Patent Document 3, an optical clock signal is obtained by driving an electroabsorption optical modulator with an electrical output of a voltage controlled oscillator VCO.
Patent Document 4 “Optical Clock Signal Extraction Device” uses an apparatus that directly extracts an optical clock signal using an optoelectronic oscillator and a mutual gain modulation effect or a mutual phase modulation effect in a semiconductor optical amplifier. Yes.
FIG. 10 is a diagram for explaining the optical clock signal extraction device disclosed in Patent Document 4. In FIG.
The optical clock extraction device is an optical clock extraction device that directly generates an optical clock signal that is phase-synchronized with an input optical data signal and has a constant amplitude, and generates an optical clock signal by driving a high-frequency electrical signal output from a bandpass filter 8. An optical clock signal source 1, an optical directional coupler 2, a variable optical delay line 3, and an optical / optical modulation circuit 4 for intensity-modulating the optical clock signal output from the optical clock signal source 1 with the optical data signal. And an optical circulator 5, a photodetector 6, and an amplifier 7.
An optical data signal that is an input signal includes, for example, a signal with an uneven amplitude or pulse width of an optical pulse.

  The optical clock extraction device passes a part of the output from the optical clock signal source 1 through the variable optical delay line 3 and the optical / optical modulation circuit 4 to which the deteriorated optical data signal is input, and then the optical circulator 5 The high-frequency electrical signal received by the photodetector 6 and output from the photodetector 6 is positively fed back as a driving electrical signal of the optical clock signal source 1 through the amplifier 7 and the bandpass filter 8 to thereby generate an optoelectronic oscillator. The optical circulator 5 generates an optical clock signal that is phase-synchronized with the input optical data signal and has a constant amplitude. Although the optical circulator 5 is described for convenience of signal processing, the optical circulator 5 and the light / light modulation circuit 4 are constituted by semiconductor optical amplifiers.

Even when no optical data signal is input to the optical / optical modulation circuit 4, the optical clock signal source 1 performs self-excited oscillation, and the oscillation frequency is determined by the center frequency of the bandpass filter 8 and the delay time of the positive feedback loop. In order to extract the optical clock signal from the deteriorated optical data signal, it is necessary to set the self-excited oscillation frequency of the optical clock signal source 1 to a value close to the bit rate of the optical data signal in advance. The self-oscillation frequency of the optical clock signal source 1 is adjusted by changing the delay time of the positive feedback loop by the variable optical delay line 3.
JP 2003-143073 A Japanese Patent Application Laid-Open No. 09-230291 Japanese Patent Laid-Open No. 09-102767 Japanese Patent Application No. 2004-214661

Since the conventional phase-locked loop circuit having an optical / electric hybrid structure as shown in Patent Documents 1 to 3 uses a voltage controlled oscillator VCO configured by an electronic circuit, it is extracted as an output of the voltage controlled oscillator VCO. The clock signal is only an electric signal. Since signal regeneration in a repeater and demultiplexing in a receiver require an optical clock signal, a pulse laser light source or an optical modulator is driven by an electrical clock signal output from a voltage controlled oscillator VCO. It is necessary to generate a clock signal. For this reason, an apparatus becomes large sized and complicated, and causes an increase in power consumption.
The “optical clock signal extraction device” of Patent Document 4 provides a device that directly extracts an optical clock signal, and increases the size, complexity, and power consumption of the conventional phase-locked loop circuit device having an optical / electric hybrid configuration. The problem can be solved.

However, since the mutual gain modulation effect or the mutual phase modulation effect in the semiconductor optical amplifier is used, the sensitivity of extracting the clock component contained in the optical data signal is low, and it is necessary to increase the power of the optical data signal. Further, although the clock component extraction sensitivity is low, an optical clock signal having the same bit rate as the optical data signal can be extracted, but an optical clock signal having a bit rate different from that of the optical data signal, for example, 160 Gbit / s optical data. It is difficult to extract a 40 Gbit / s optical clock signal from the signal.
Furthermore, in the “optical clock signal extraction device” of Patent Document 4, in order to suppress crosstalk between the extracted optical clock signal and the optical data signal, the optical clock signal must be set to a wavelength different from that of the optical data signal. There are constraints on the wavelength.

  In order to solve the above problems, the present invention provides an optical / electrical clock signal extraction device capable of directly extracting optical and electrical clock signals simultaneously from an optical data signal whose waveform has deteriorated, and also extracts a clock from the optical data signal. Optical / electricity that improves sensitivity, reduces the power of the required optical data signal, extracts the optical clock signal with a bit rate different from that of the optical data signal, and can freely set the wavelength of the optical clock signal and the wavelength of the optical data signal An object of the present invention is to provide a clock signal extraction device.

In order to achieve the above object, in the optical / electrical clock extraction apparatus of the present invention, an optoelectronic oscillator (Optoelectronic Oscillator) capable of directly generating optical and electric clocks simultaneously instead of a conventional voltage controlled oscillator VCO configured by an electronic circuit. Is used. In particular,
(1) The optical / electrical clock extraction device constitutes a closed-loop positive feedback circuit, receives an optical data signal as input, and simultaneously generates an optical clock signal and an electric clock signal that are phase-synchronized with the optical data signal and have a constant amplitude. And an optical clock signal source driven by an electrical clock signal, an optical clock signal output from the optical clock signal source, and the optical data signal completely by an optical directional coupler. And an electrical clock extraction circuit for separately inputting and outputting an electrical clock signal.
(2) In the optical / electrical clock signal extraction device according to (1) above, after passing a part of the optical clock signal output from the optical clock signal source through an optical delay line, the optical data signal and Combined and input to the electrical clock extraction circuit, the electrical clock signal output from the electrical clock extraction circuit is fed back as a drive electrical signal of the optical clock signal source, phase-synchronized with the optical data signal, and constant amplitude The optical clock signal and the electrical clock signal are generated simultaneously.

(3) The optical clock signal source described in the above (1) or (2) is characterized by comprising a single frequency laser light source and an optical intensity modulator for modulating the output light by the electric clock signal.
(4) The optical clock signal source described in the above (1) or (2) uses an active mode-locked semiconductor laser instead of the single frequency laser light source described in the above (3) and the optical intensity modulator. And
(5) The electrical clock extraction circuit according to any one of (1) to (4) is configured by a photodetector, an amplifier, and a bandpass filter tuned to the electrical clock signal. And

  In the optical clock extracting apparatus according to the present invention, since an optical clock signal and an optical clock oscillator OEO capable of directly generating an electrical clock signal at the same time are used in place of the voltage controlled oscillator VCO based on a conventional electronic circuit, the clock signal is converted from electricity to light. No device is required, the configuration is simplified, and power consumption can be reduced. Further, by using an optical clock signal source and a photodetector capable of high-speed operation, it is possible to operate at higher speed than a voltage-controlled oscillator VCO using an electronic circuit. Further, since the extraction sensitivity of the clock component is improved, the power of the required optical data signal can be reduced, and an optical clock signal having a bit rate different from that of the optical data signal can be extracted. Furthermore, since the wavelength of the optical clock signal and the wavelength of the optical data signal can be arbitrarily set, the applicable range can be expanded.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram for explaining the outline of an optical / electrical clock signal extraction apparatus according to the present invention. The optical / electrical clock signal extracting device is an optical / electrical clock extracting device that directly generates an optical clock signal having a constant amplitude and an electric clock signal simultaneously in phase with an input optical data signal. An optical directional coupler 2, a variable optical delay line 3, an electrical clock extraction circuit 9, and a power divider 10. The optical clock signal source 1 has a function of generating an optical clock signal by inputting a high frequency electrical modulation signal. The electrical clock extraction circuit 9 has a function of receiving an optical clock signal output from the optical clock signal source 1 and an optical data signal and outputting an electrical clock signal.
The optical data signal that is an input signal includes, for example, a signal in which the amplitude, position, and pulse width of the optical pulse are not uniform. In the optical / electrical clock extraction signal device, the output of the optical clock signal source 1 is bifurcated by an optical directional coupler 2, one branch output is taken out as an optical clock signal, and the other branch output is a variable optical delay. After passing through the line 3, the optical data signal deteriorated by the optical directional coupler 2 is merged and input to the electric clock extraction circuit 9.

Thus, since the optical data signal and the optical clock signal are completely separated by the optical directional coupler 2 provided between the optical clock signal source 1 and the variable optical delay line 3, no crosstalk occurs. Thereby, the wavelength of the optical data signal and the wavelength of the optical clock signal can be arbitrarily set.
The high frequency electric clock signal output from the electric clock extraction circuit 9 is divided into two by the power divider 10, and one is output as an electric clock signal. The other output of the power divider 10 is positively fed back as a drive electric signal of the optical clock signal source 1 to constitute an optoelectronic oscillator, and generates an optical clock signal having a constant amplitude and phase-synchronized with the input optical data signal.

Compared with the optical / electrical clock signal extraction device of the present invention, whether the optoelectronic transmitter generates a phase-synchronized optical clock signal by inputting a deteriorated optical data signal, or does it operate at the self-excited oscillation frequency of the circuit? It depends on.
In other words, the optoelectronic transmitter and the optical / electrical clock signal extraction device have the same circuit configuration. However, an optical / electrical clock signal extracting device inputs a data signal, and an optoelectronic transmitter operates independently without inputting a data signal.
1 includes an optical clock signal source, an optical directional coupler 2, a variable optical delay line 3, an optical directional coupler 2, an electric clock extraction circuit 9, and a power divider 10.

FIG. 2 is a diagram for explaining an optical clock signal source using a single frequency laser light source and an optical intensity modulator.
The optical clock signal source 1 comprises a single frequency laser light source 11 and a light intensity modulator 12 that modulates the output light with an electric signal.
As the optical intensity modulator 12, a Mach-Zehnder type optical intensity modulator or an electroabsorption optical modulator is used, and an optical clock signal having a constant amplitude, timing, and pulse width is output.

FIG. 3 is a diagram for explaining an optical clock signal source using the active mode-locked semiconductor laser.
The optical clock signal source 1 can use an active mode-locked semiconductor laser 13 in place of the single frequency laser light source 11 and the light intensity modulator 12.
The active mode-locked semiconductor laser 13 needs to be directly modulated and operable at the same frequency as the bit rate of the optical data signal. The method using the active mode-locked semiconductor laser 13 is functionally the same as the method using the single frequency laser light source 11 and the light intensity modulator 12, but can generate an optical clock signal with a smaller pulse width. Is possible.
The electric clock extraction circuit 9 in FIG. 2 has a function of extracting a clock component from an optical clock signal or an optical data signal and outputting it as a high frequency electric signal.

  FIG. 4 is a diagram for explaining the electric clock extraction circuit 9. The electrical clock extraction circuit 9 includes a photodetector 6, an amplifier 7, and a band pass filter 8. The optical signal input to the photodetector 6 is converted into an electric signal, and the clock frequency component is extracted by the band pass filter 8. Since the optical data signal is directly input to the photodetector 6, the extraction sensitivity to the clock component is improved, and the purity of the extracted optical clock signal and the signal-to-noise ratio are significantly improved. Further, by improving the extraction sensitivity, not only optical clock signal extraction with the same bit rate as the optical data signal but also optical clock signal extraction with a bit rate different from the optical data signal, for example, optical data with a bit rate of 160 Gbit / s. An optical clock signal with a bit rate of 40 Gbit / s can be extracted from the signal.

  Even when no optical data signal is input, the optoelectronic oscillator performs self-oscillation, and the oscillation frequency is determined by the center frequency of the bandpass filter 8 and the delay time of the positive feedback loop. In order to extract the optical clock signal from the degraded optical data signal, it is necessary to set the self-excited oscillation frequency of the optoelectronic oscillator to a value close to the bit rate of the optical data signal in advance. The self-oscillation frequency of the optoelectronic oscillator is adjusted by changing the delay time of the positive feedback loop by the variable optical delay line 3.

  When an optical data signal is input to an optoelectronic oscillator operating in self-excited oscillation, a clock frequency component included in the optical data signal is extracted by the electric clock extraction circuit 9 and positively fed back to the optical clock signal source 1. As a result, the optoelectronic oscillator shifts to an injection locking operation, and generates an optical clock signal that is phase-locked with the input optical data signal and has a constant amplitude. Further, the optoelectronic oscillator generates an electric clock signal having a constant amplitude and phase-synchronized with the input optical data signal via the power divider 10.

FIG. 5 is a diagram for explaining a 39.81312 Gbit / s clock extraction experiment using the optical / electrical clock signal extraction device of the present invention. The optical / electrical clock extraction device used in the experiment is mainly composed of an optical clock signal source shown in FIG. 2 and an electric clock extraction circuit shown in FIG. As shown in FIG. 5, the optical clock extraction apparatus according to the present invention includes an optical data signal generation unit that generates an optical data signal having a bit rate of 40 Gbit / s or 160 Gbit / s, and an optoelectronic oscillator that extracts the optical clock signal. Consists of
The optical data signal generator includes a synthesizer 14 that generates a signal having a predetermined frequency, for example, ITU-T standard 9.95328 GHz, and an active mode-locked optical fiber ring that generates an optical clock signal based on the signal from the synthesizer 14. A laser 15, a code generator 16 that generates a 1 and 0 pulse pattern based on a signal from the synthesizer 14, an optical modulator 12 that modulates the optical clock signal with a 1 and 0 pulse pattern signal, and a modulated signal It comprises an OTDM multiplexer 17 that converts an optical data signal into a higher bit rate optical data signal.

In the optical data signal generation unit, an optical pulse having a pulse width of about 3 ps generated from the active mode-locked optical fiber ring laser 15 having a repetition frequency of 9.95328 GHz is input to the optical intensity modulator 12 and output from the code generator 16. After performing data modulation with 2 ³¹ -1 pseudo-random bit sequence, optical time division multiplexing (OTDM: Optical Time Division Multiplexing) 4 times or 16 times is performed by the OTDM multiplexer 17, and 39.81312 Gbit / s, Alternatively, a data signal of 159.25246 Gbit / s is generated. The center wavelength of the optical data signal is 1550 nm.

  On the other hand, the optoelectronic oscillator includes a single frequency laser light source 11, a light intensity modulator 12, an optical directional coupler 2, a variable optical delay line 3, a photodetector 6, an amplifier 7, and a band pass filter 8. The light having a wavelength of 1540 nm output from the single frequency laser light source 11 is converted into an optical clock signal by passing through the light intensity modulator 12, and a part thereof is extracted to the outside by the optical directional coupler 2. The remaining optical clock signal passes through the variable optical delay line 3 and is then converted into an electrical signal by the photodetector 6, and is a two-stage amplifier 7 and a band whose center frequency is 39.813 GHz and 3 dB pass bandwidth is 41 MHz. Positive feedback is provided to the light intensity modulator 12 via the pass filter 8. In the figure showing the outline of the optical / electrical clock signal extraction device in FIG. 1, the power divider 10 is used to extract the electric clock signal. However, in FIG. 5, the external termination of the optical intensity modulator 12 is used. The electric clock signal is taken out from the output terminal.

  When no optical data signal is input to the optical directional coupler 2, the optoelectronic oscillator self-oscillates and generates an optical clock signal having a frequency of around 39.813 GHz. The repetition frequency of the self-excited oscillation depends on the delay time of the loop and the DC bias voltage of the light intensity modulator 7, and is adjusted to about 39.8112 GHz by adjusting the variable optical delay line 3. When an optical data signal is input via the optical directional coupler 2 in this state, light having a frequency of 39.8112 GHz synchronized with the optical data signal and an electric clock signal are generated simultaneously.

  FIG. 6 is a diagram showing the spectrum of the extracted electrical clock signal. The horizontal axis represents the frequency (MHz) based on the offset frequency fc, and the vertical axis represents the normalized RF power (dB). The bit rate of the optical data signal is 39.83112 Gbit / s, and the resolution bandwidth is 10 kHz. Even in a free-running state in which no optical data signal is input, high spectral purity is obtained. However, when optical data signal is input and injection locking occurs, the intensity of the sideband is reduced and the spectral purity is further improved. ing.

  FIG. 7 is a diagram showing the optical spectrum of the extracted optical clock signal. The horizontal axis represents the wavelength (nm) based on the offset wavelength λc, and the vertical axis represents the normalized power (dB). The bit rate of the optical data signal is 39.83112 Gbit / s, and the resolution bandwidth is 10 pm. It can be seen that the clock frequency components included in the optical data signal are extracted from the many sidebands appearing in the optical clock signal.

  FIG. 8 shows an optical sampling waveform. FIG. 8 (upper diagram) shows an optical data signal with a bit rate of 39.8112 Gbit / s, and FIG. 8 (lower diagram) shows an optical sampling waveform of the extracted optical clock signal. The horizontal axis represents time (ps), and the vertical axis represents light intensity (mW). The sampling interval is 2.5 fs, and the time resolution is about 800 fs. The data component is removed, and an optical clock signal with a repetition frequency of 39.81312 GHz is extracted. The pulse width of the optical clock signal is 9.8 ps.

FIG. 9 shows an optical sampling waveform. FIG. 9 (upper diagram) shows an optical data signal with a bit rate of 159.25246 Gbit / s, and FIG. 9 (lower diagram) shows an optical sampling waveform of the extracted optical clock signal. The horizontal axis represents time (ps), and the vertical axis represents light intensity (mW). An optical clock signal with a repetition frequency of 39.81312 GHz corresponding to ¼ of the bit rate of the optical data signal is extracted. The pulse width of the optical clock signal is 11.3 ps. Extraction of the frequency-divided clock signal is an effective technique for realizing demultiplexing in the receiver.
The present invention produces the desired effect by being configured as described above. The components constituting the optical clock extracting apparatus of the present invention can be changed as long as the desired functions are exhibited.

  The present invention enables the simultaneous and direct generation of an optical clock signal and an electrical clock signal, which was impossible with a conventional oscillator VCO. Since a conversion device from an electrical clock signal to an optical clock signal is not required, the system can be simplified and power consumption can be greatly reduced. As a result, application to a repeater or a receiver in an optical fiber communication system having a bit rate of 40 Gbit / s or more becomes possible.

It is a figure explaining the outline of the optical / electrical clock extraction apparatus concerning this invention. It is a figure explaining the optical clock signal source using a single frequency laser light source and a light intensity modulator. It is a figure explaining the optical clock signal source using an active mode synchronous semiconductor laser. It is a figure explaining an electric clock extraction circuit. It is a figure explaining 39.81312 Gbit / s optical clock extraction experiment using the optical / electrical clock signal extraction device of the present invention. It is a figure showing the spectrum of an electrical clock signal. It is a figure showing the spectrum of an optical clock signal. It is a figure showing the waveform of a 39.81312 Gbit / s optical data signal (upper) and several 39.8112 GHz optical clock signal (lower). It is a figure showing the waveform of a 159.25246 Gbit / s optical data signal (top) and 39.81212 GHz optical clock signal (bottom). It is a figure explaining the optical clock signal extraction device concerning invention of patent documents 4.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical clock signal source 2 Optical directional coupler 3 Variable optical delay line 4 Optical / optical modulation circuit 5 Optical circulator 6 Optical detector 7 Amplifier 8 Band pass filter 9 Electric clock extraction circuit 10 Power divider 11 Single frequency laser light source 12 Optical intensity modulator 13 Active mode-locked semiconductor laser 14 Synthesizer 15 Active mode-locked optical fiber ring laser 16 Code generator 17 OTDM multiplexer

Claims (5)

  An optical / electrical clock signal extraction device that constitutes a closed-loop positive feedback circuit, inputs an optical data signal, and simultaneously generates an optical clock signal that is phase-synchronized with the optical data signal and has a constant amplitude, and an electrical clock signal. The optical clock signal source driven by the electrical clock signal, the optical clock signal output from the optical clock signal source and the optical data signal are completely separated by the optical directional coupler and respectively input, and the electrical clock signal An optical / electrical clock extracting apparatus comprising: an electric clock extracting circuit that outputs   A part of the optical clock signal output from the optical clock signal source is passed through an optical delay line, and then merged with the optical data signal and input to the electrical clock extraction circuit, from the electrical clock extraction circuit The output electric clock signal is fed back as a driving electric signal of an optical clock signal source, and an optical clock signal having a constant amplitude and a constant amplitude is generated simultaneously with the optical data signal. The optical / electrical clock signal extracting device according to claim 1.   3. The optical / electrical clock signal extraction device according to claim 1, wherein the optical clock signal source comprises a single frequency laser light source and an optical intensity modulator for modulating the output light by the electric clock signal. .   4. The optical / electrical clock signal extraction device according to claim 3, wherein an active mode-locked semiconductor laser is used in place of the single frequency laser light source and the light intensity modulator. 5. The optical / electrical device according to claim 1, wherein the electrical clock extraction circuit includes a photodetector, an amplifier, and a band-pass filter tuned to the electrical clock signal. 6. Clock signal extraction device.