WO2005083502A1

WO2005083502A1 - Optical clock extracting device and method

Info

Publication number: WO2005083502A1
Application number: PCT/JP2005/003292
Authority: WO
Inventors: Yoichi Hashimoto
Original assignee: Nec Corporation
Priority date: 2004-02-27
Filing date: 2005-02-28
Publication date: 2005-09-09
Also published as: JP4807514B2; JPWO2005083502A1

Abstract

An optical clock extracting device for generating an optical pulse having an oscillation frequency synchronizing with the reference frequency of an incident signal light. The optical clock extracting device comprises a mode-locked semiconductor laser which has a saturable absorption region producing a phase modulation effect based on a control voltage applied to the saturable absorption region and a gain region and in which the oscillation frequency to generate an optical pulse can be varied, an optical mixer section for optically synchronously detecting the synchronization error between the reference frequency of the signal light and the oscillation frequency of the output optical pulse of the mode-locked semiconductor laser and thereby generating a phase error signal, and a phase-locked loop for negatively feed-backing the phase error signal to the saturable absorption region.

Description

Specification

Optical clock extraction device and method

Technical field

The present invention relates to a device that generates clock light used for optical communication, optical measurement, optical information signal processing, and the like. In particular, the present invention relates to an apparatus and a method for generating an optical clock reference signal with low phase noise, and a relative timing that is independent of the polarization of the optical signal and has small jitter when generating an optical timing pulse from the optical signal. The present invention relates to an optical clock extracting apparatus and method for generating or extracting an optical clock reference signal with low intensity noise (RIN) with high stability. Background art

[0002] Mode-locked semiconductor lasers (MLLDs) are relatively easy and stable at 10-160GHz! Because it can generate a clock pulse train with a pulse width on the order of picoseconds at an extremely high repetition frequency, industrially important clock pulses such as optical communication light sources, optical signal processing light sources, and measurement light sources can be used. Light source.

[0003] When an MLLD is used as such a clock pulse light source, a method for establishing synchronization with an external reference signal source such as an optical signal or an electric signal and a method for stabilizing the clock light without depending on the external reference signal source are used. Techniques for shading and techniques for reducing phase noise in synchronized clock signals are important.

[0004] In MLLD, a phenomenon is seen in which the pulse repetition frequency fluctuates randomly. This is called jitter. Jitter is considered to occur because the length of the optical resonator fluctuates at random by determining the repetition frequency of the output optical pulse in the MLLD. In more detail, the noise power generated in the region of the MLLD that has an optical amplification effect randomly fluctuates the carrier density and refractive index in the MLLD, and as a result, the effective optical cavity length changes randomly. it is conceivable that.

[0005] Until now, a method for establishing synchronization between the clock light output from the MLLD and an external reference signal source, particularly an optical signal, and reducing the phase noise when the clock light is regenerated. There are two methods: (1) an optical method that directly injects signal light into the MLLD and synchronizes it, that is, a method that uses optical injection locking, and (2) a method that converts an optical signal into an electrical signal once Techniques for taking time are well known. The former method is an all-optical method because the processing is performed with the optical signal as it is, and the latter method is an electric method because it is converted into an electric signal.

[0006] A configuration using a light injection locking technique of directly injecting signal light into an MLLD to achieve synchronization is disclosed in, for example, Japanese Patent Application Laid-Open No. 2-183236. FIG. 1A shows an example of a configuration based on light injection locking.

[0007] In optical injection locking, an optical signal is injected into an MLLD 91 that passively generates an optical pulse, thereby forcibly modulating the saturable absorption region and gain region of the MLLD with the optical signal. This is a method of achieving synchronization in the output pulse light. In the illustrated example, a band-pass filter 92 is arranged at the output of the M LLD 91. In this case, the phase noise of the clock extraction light reproduced from the MLLD generally does not become better than the phase noise of the incident optical signal, and also strongly depends on the optical signal as an external signal source. Furthermore, since the MLLD itself has polarization dependence, in optical injection locking, phase noise strongly depends on the polarization state of the signal light incident on the MLLD.

[0008] FIG. 1B shows an example of a configuration of a technique for once converting an optical signal into an electric signal and then achieving synchronization. In this method, a high-speed optical receiver 93 such as a high-speed PIN diode is used to convert an optical signal into an electric signal by the high-speed optical receiver 93, and then an electric phase-locked loop ( (PLL) for synchronization.

[0009] The PLL includes a phase comparator 94 such as an electric mixer that can handle up to high frequencies, a loop filter 95, and a voltage controlled oscillator (VCO) 96 that generates a frequency signal as an electric signal corresponding to up to high frequencies. ing. The phase comparator 94 detects the frequency difference by comparing the phase of the electric signal obtained by converting the input optical signal by the high-speed optical receiver 93 with the signal from the VC096, and detects the signal corresponding to the phase difference. Is output. The signal corresponding to the phase difference is smoothed by the loop filter 95 and supplied to the VC096 as a control signal. The electric high-frequency signal output from the VC096 is fed back to the phase comparator 94 and is applied to the MLLD 98 via the high-frequency amplifier 97 so that the MLLD 98 can be directly modulated. In this configuration, the output frequency of the VC096 matches the frequency of the optical signal after being converted to an electrical signal, which allows the optical signal input to the high-speed receiver 93 and the MLLD9 Synchronization with the output clock light from 8 is established. Further, in this configuration, the phase noise can be reduced by designing a loop filter constituting the PLL. The high-speed light receiver 93, the phase comparator 94, the loop filter 95, the VC096, and the high-frequency amplifier 97 constitute an electric control unit 99, that is, an electric high-frequency circuit.

[0010] In an optical clock extraction device of the PLL system using a high-frequency circuit such as an electric oscillator or a VCO, technology for reducing phase noise, for example, JB Georges et al., "Stable picosecond pulse generation at 46Griz by modelocking or a semiconductor laser operating in an optoelectronic phaselocked loop, Electronics Letters, Vol. 30, No. 1, pp. 69-71, JP-A-8-340154, H. Ono et al, "Low jitter pulse train generation using a regeneratively mode-locked laser diode, "10th International Workshop on Femtosecond Technology, TP-5, pp. 154, Chiba, Japan, July 16-17, 2003.

Patent Document 1: JP-A-2-183236

Patent Document 2: JP-A-8-340154

Non-patent literature 1: J. B. ueorges et al., Staole picosecond pulse generation at 46GHz by modelocking of a semiconductor laser operating in an optoelectronic phaselocked loop, Electronics Letters, Vol. 30, No. 1, pp. 69-71

Non-Patent Document 2: H. Ono et al., "Low jitter pulse train generation using a

regeneratively mode-locked laser diode, "10th International Workshop on

Femtosecond Technology, TP— 5, pp. 154, Chiba, Japan, July 16-17, 2003 Disclosure of the Invention

Problems to be solved by the invention

[0011] In the method of establishing synchronization between signal light and output clock light by optical injection locking, synchronization can be achieved with a simple configuration. However, the synchronization state fluctuates depending on the polarization state of the optical signal. Also, there is a problem that the phase noise of the obtained clock light depends on the phase noise of the injected optical signal. The reason is that optical injection locking uses the effect of optically modulating the saturable absorption region of the MLLD, so that the polarization state of the MLLD oscillating light and the polarization state of the incident light are the same. Otherwise the modulation effect will be constant This is because a stable clock light cannot be obtained. Further, the effect of stabilizing the frequency of the output clock light does not include the light injection locking itself.

[0012] In the method of once converting an optical signal into an electric signal, there is no synchronization fluctuation due to the polarization state of the optical signal, and the frequency of the output clock light is stabilized by the PLL, but the MLLD is directly electrically modulated. Therefore, since a VCO based on an electric circuit is used as a high-frequency reference signal source, there is a problem that it cannot follow an optical signal having a clock frequency of, for example, 40 GHz or more. In addition, the conventional optical clock extraction device based on the electrical PLL method has a problem that it is difficult to integrate the optical clock extraction device itself into a quartz waveguide, a polymer optical waveguide (PLC), or the like. The reason is that the higher the frequency of the clock reference signal, the higher the price, power consumption, and heat generation of the high-frequency reference signal source constituting the PLL, and the higher the clock reference signal frequency becomes, for example, 40 GHz or higher, the higher the efficiency. This is because it becomes difficult to provide high-frequency wiring.

An object of the present invention is to be able to reproduce clock light without depending on the polarization of signal light as a reference signal source, without requiring an electrical high-frequency circuit, and with low phase noise! An object is to provide an optical clock extraction device.

Another object of the present invention is to be able to reproduce clock light without depending on the polarization of signal light as a reference signal source, without requiring an electrical high-frequency circuit, and with low phase noise. It is to provide a simple optical clock extraction method.

Means for solving the problem

An object of the present invention is to provide an optical clock extracting device for obtaining an optical pulse having an oscillation frequency synchronized with a reference frequency of an incident signal light, wherein the saturable absorption region having a phase modulation effect is provided. A mode-locked semiconductor laser (MLLD) that has a gain region and can change the oscillation frequency that generates optical pulses by the phase modulation effect based on the control voltage applied to the saturable absorption region; Means for obtaining a phase error signal by optically and synchronously detecting the synchronization deviation between the reference frequency and the oscillation frequency of the output light pulse of the MLLD. Feedback means, and is achieved by an optical clock extraction device that synchronizes the output light pulse of the MLLD with the signal light. [0016] Such an optical clock extracting apparatus of the present invention typically has an optical clock that directly injects at least a part of an optical pulse output from one output end face of the MLLD from the other end face of the MLLD. Self-feedback loop. In the present invention, by providing such an optical self-feedback loop, the MLLD operates as an optical VCO, and the phase noise in the optical VCO output is reduced.

[0017] Another object of the present invention is to provide a saturable absorption region and a gain region having a phase modulation effect, and an oscillation frequency for generating an optical pulse by a phase modulation effect by a control voltage applied to the saturable absorption region. This is an optical clock extraction method that synchronizes the oscillation frequency of the output light pulse of the MLLD force with the reference frequency of the incident signal light, and the reference frequency of the signal light and the oscillation frequency of the output light pulse. Obtaining a phase error signal by optically synchronously detecting the synchronization shift of the optical signal, and forming a phase locked loop by negatively feeding back the phase error signal to a region having a phase modulation effect, thereby forming an optical noise and an incident signal light. Synchronizing the optical clock.

According to the present invention, a phase-locked loop that does not require a high-frequency electric circuit such as an electric VCO can be constructed. As a result, miniaturization, high-speed operation, low power consumption, high integration, An optical clock extraction device with improved operability is provided.

According to the present invention, clock extraction can be performed without depending on the speed or polarization state of signal light. In addition, other signal processing such as an optical 3R repeater and an optical DEMUX can be simultaneously performed by the optical mixer as the phase comparator. In the present invention, for example, a power using an optical gate switch that can operate at high speed is used as the optical mixer. As this optical gate switch, an optical fiber type optical gate constructed with a polarization independent SOA or a polarization independent configuration is used. This is because the use of the switch enables phase comparison independent of the polarization state of the signal light. At this time, if the characteristics of the loop filter are selected to properly design the synchronization holding range of the negative feedback of the phase-locked loop, the synchronization holding range can be made larger than the fluctuation due to the polarization dependence of the optical mixer. Then, more stable clock extraction can be realized.

Since the optical mixer is a high-speed optical gate switch, once synchronization is established, the optical mixer can perform signal processing at the optimal timing between the signal light and the clock light. Therefore, in addition to generating an error signal, the optical mixer is required to perform optical 3R regeneration processing and optical DEMUX processing. Can be used simultaneously for processing.

Further, according to the present invention, an optical clock can be extracted by PLL control in multiple stages.

FIG. 1A is a block diagram showing a configuration of a conventional optical clock extraction device using light injection locking, which extracts a clock light whose optical signal power is also synchronized therewith.

FIG. 1B is a block diagram showing a configuration of a conventional optical clock extraction device using a PLL, which extracts a clock light that is also synchronized with the optical signal power.

FIG. 2 is a diagram illustrating an optical clock extraction method based on the present invention for reducing phase noise in pulsed light using an optical VCO by an MLLD having a self-feedback loop.

FIG. 3 is a block diagram illustrating an optical clock extraction device according to a first embodiment of the present invention.

FIG. 4 is a graph showing an RF spectrum in an optical PLL operation for reducing phase noise in an optical VCO using an MLLD having a self-feedback loop, and an RF spectrum when the PLL operation is performed without self-feedback. .

[Figure 5] Sampling oscilloscope waveforms in an optical PLL operation with reduced phase noise in an optical VCO using an MLLD with a self-feedback loop, and sampling oscilloscope waveforms when the PLL is operated without self-feedback FIG.

FIG. 6 is a graph showing an RF spectrum of a side band generated by a self-feedback loop.

FIG. 7 is a block diagram illustrating an optical clock extraction device according to a second embodiment in which two MLLDs having different wavelengths are provided in a loop for suppressing sideband spectra.

FIG. 8 is a block diagram illustrating a clock extraction device according to a third embodiment.

FIG. 9A is a block diagram showing a configuration for suppressing a sideband spectrum using wavelength conversion by CW (continuous wave) light.

FIG. 9B is a block diagram showing a configuration for suppressing a sideband spectrum by a supercontinuum using an optical fiber.

[Figure 9C] Sideband spectrum is suppressed by placing an optical filter in the loop FIG. 3 is a block diagram illustrating a configuration for controlling the power consumption.

FIG. 10 is a block diagram showing a configuration in which a phase comparison function is provided to an optical gate unit of an optical 3R repeater in a fourth embodiment.

FIG. 11 is a block diagram showing a configuration in which an electro-absorption modulator is used for a phase comparison unit in Embodiment 5.

FIG. 12 is a block diagram showing a configuration in a case where PLL control is performed in multiple stages in Embodiment 6.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 2 is a diagram illustrating an optical clock extraction method according to the present invention. In the optical clock extraction method of the present invention, phase noise at the optical VCO frequency is reduced by using an MLLD having a self-feedback loop configuration.

According to the present invention, a saturable absorption region and a gain region having a phase modulation effect are provided, and the oscillation frequency at which an optical pulse is generated is changed by the phase modulation effect by a control voltage applied to the saturable absorption region. Use an MLLD (mode-locked semiconductor laser diode). Then, when synchronizing the oscillation frequency of the output optical pulse with the MLLD power with the reference frequency of the incident signal light, optically synchronous detection of the synchronization deviation between the reference frequency of the signal light and the oscillation frequency of the output optical pulse is performed. In addition to obtaining the phase error signal of these signals, the phase error signal is negatively fed back to the saturable absorption region of the MLLD to form a phase locked loop, and the optical pulse and the incident signal light are synchronized to form a clock. Perform the extraction. Here, the oscillation frequency of the output light pulse corresponds to the repetition frequency of the light pulse train.

[0025] Since the refractive index of light in a semiconductor generally depends on the carrier density, the effective resonator length of the MLLD can be controlled via the carrier density. This makes it possible to control the repetition frequency of the MLLD, and the MLLD can be used as a voltage-Z current controlled oscillator (VCO) that outputs an optical signal. However, in order to use an MLLD as an optical VCO, it is important to reduce the phase noise in the output clock light of the MLLD power, that is, to reduce the jitter. Therefore, in the present invention, since an MLLD is used as an optical VCO without the need for an electrical high-frequency oscillator, the method of reducing the phase noise of the clock light from the MLLD, that is, reducing the jitter, as shown in FIG. Of the clock light from the MLLD100 The optical loop system is constructed by taking out the part and re-injecting the light into the same MLLDIOO, and adopts a configuration in which the clock light self-feeds back. In the figure, the optical signal portion is indicated by a thick line, and the electric signal portion is indicated by a thin line.

[0026] By adjusting the optical path length so that the timing of the generation of the clock light pulse from the MLLDIOO and the timing of the clock light pulse circulating the optical loop and returning to the MLL D100 become the same, the saturable MLLD is formed. The absorption saturation phenomenon that occurs in the absorption region, that is, the gate operation becomes steep, and the phase noise caused by the randomness of absorption saturation and the randomness of noise light in the gain region due to gain saturation is reduced. As a result, the phase noise of the output clock light can be reduced, and the frequency accuracy of the clock light pulse train can be improved without applying a method of forcibly modulating by applying an electric high-frequency circuit. You.

Further, in the present invention, in order to establish synchronization between the input signal light and the reproduced clock light, the external signal light and the reproduced clock light from the optical VCO using the MLLDIOO are connected to the semiconductor optical amplifier ( The optical signal is input to the optical mixer 200 using a nonlinear effect such as SOA), and the phase of the external signal light is compared with the reference signal of the optical VCO. The optical beat signal generated by the optical mixer 200 is a signal corresponding to the phase difference between the external signal light and the reproduced clock light, and this signal is converted into an electric signal by the optical receiver 310 which has a strong force such as a PIN diode, and is converted into a loop signal. It is smoothed by the filter 320 and applied as a control voltage to the saturable region of the MLLDIOO. As described above, in the present embodiment, a clock light synchronized with the external signal light can be obtained by employing the configuration of the phase locked loop.

In the present embodiment, of the basic constituent elements of the PLL, a part that requires high-speed processing, such as a phase comparison unit (mixer) and a voltage-controlled oscillator (VCO), is optically configured. A phase difference detection unit, an error amplifier, a loop filter, and a part for performing a sufficient process with a low-speed response on the order of MHz are electrically configured by an electric control unit 300 including a photodetector 310 and a loop filter 320. I have. As described above, in order to suppress the frequency fluctuation caused by the amplified spontaneous emission light (ASE) of the MLLD, a method of optically self-feedback looping the MLLD is introduced, and the frequency accuracy is remarkably improved. .

As described above, according to the present embodiment, an optical mixer using a polarization-independent SOA or the like is provided. By using this, an optical clock extraction device that does not depend on the polarization state of the external signal light can be realized.

. Also, by combining an optical mixer and a phase comparator with an optical switch utilizing a non-linear effect such as an SOA or an optical fiber, high-speed operation can be measured. Furthermore, since the optical mixer can have the function of optical identification signal processing at the same time, the operation of optical phase comparison and the operation of optical signal processing such as optical 3R regeneration and optical DEMUX can be performed simultaneously. As a result, their operations can be further optimized.

In the present embodiment, the electrical control unit is only a part that smoothes the signal corresponding to the phase difference with the loop filter, and has a relatively low-speed PIN diode photodetector and a band on the order of MHz. And a loop filter using an operational amplifier. The electrical control unit does not need to be a high frequency circuit, and this part can be configured as an integrated circuit (IC). Therefore, miniaturization, high integration, and low power consumption of the electric control unit can be achieved. As described above, the optical clock extraction device has a configuration in which components such as the MLLD, SOA, isolator, PIN, and IC circuit can be easily mounted on the PLC, so that the size can be reduced.

[0031] Further, in this optical clock extracting device, the light intensity of the clock light pulse circling the optical loop is adjusted by an amplifier, an attenuator, and the like arranged in a configuration in which feedback is performed by the optical loop system. Also, the timing can be adjusted by changing the optical loop length. This makes it possible to variably control the fundamental frequency of the clock light pulse train that also generates MLLD power, and also to control amplifiers and attenuators as PLL control circuits, thus realizing a multi-stage PLL control circuit. it can.

Hereinafter, the present invention will be described in more detail based on examples.

Example 1

Fig. 3 shows an optical clock extraction device that has an optical VCO using an MLLD with a self-feedback loop configuration and that reduces phase noise at the oscillation frequency of the optical VCO. The illustrated optical clock extraction device includes an optical mixer and a loop filter together with an optical VCO, and an optical PLL is configured by the optical mixer, the optical VCO, and the loop filter.

[0034] The MLLD 1 used as an optical VCO has at least a gain region 2, a saturable absorption region 3, and a power structure. It is made. The gain region 2 and the saturable absorption region 3 are electrode-separated. A lens 4 and an isolator 5 are arranged close to each other on both end faces of the MLLD1. Then, in order to form an optical self-feedback loop for directly injecting a part of the optical pulse output from one output end face of the MLLD1 to the MLLD1 from the other end face, an optical waveguide 6 that externally connects both end faces is used. In the optical waveguide 6, an optical delay unit 7, an optical amplifier 8, and an optical attenuator 9 are inserted. The optical delay unit 7 is adjusted so that the timing at which the MLLD 1 generates an optical noise and the timing at which the optical north circulates the optical waveguide 6 and enters the other end face of the MLLD 1 are simultaneous. . With this configuration, the absorption saturation phenomenon occurring in the saturable absorption region 3 constituting the MLLD 1, that is, the optical gate operation is made steep, and the phase noise caused by the randomness of the absorption saturation in the saturable absorption region 3 when an optical pulse is generated. , The phase noise in the output clock light is reduced, and the frequency accuracy of the recovered clock light can be increased.

[0035] In order to establish synchronization between the external signal light and the output clock light from the MLLD1, that is, the recovered clock light, an optical mixer (MIX) 10, an optical attenuator (ATT) 9, and a noise detector 11 , An error amplifier 12, a loop filter 13, and a voltage adder 14. In the optical mixer 10, a part of the recovered clock light extracted from the MLLD 1 and the external signal light enter the optical mixer 10, and the output light of the optical mixer 10 passes through the optical attenuator 9 to one of the balance detectors 11. And the balance detector 11 detects a frequency difference between the oscillation frequency of the MLLD1 that is the optical VCO and the reference frequency of the external signal light. A signal corresponding to the phase difference is amplified by the error amplifier 12, smoothed by the loop filter 13, and applied as a control voltage to the saturable region 3 of the MLLD 1, which is an optical VCO, by the voltage adder 14. As a result, the recovered clock light synchronized with the external signal light is obtained from the MLLD1.

[0036] In the optical mixer 10, modulation according to the phase difference between the reference frequency of the external signal light and the oscillation frequency of the clock light from the MLLD 1 can be added to both the external signal light and the clock light. For example, if mutual gain modulation using SOA is used, intensity modulation according to the phase difference between both the external signal light and the clock light can be added. Either one of these receiving the intensity modulation is output to a balance detector 11 having a low-speed frequency characteristic on the order of several MHz. When converted into an electric signal by one of the light receiving units, the high-speed external signal light and clock light are output as DC voltage components, and the intensity modulation component is output as an error signal component. it can.

When the clock light or the external signal light is incident on the other light receiver of the balance detector 11 so that the DC voltage component is canceled, the DC voltage component is canceled and only the error signal can be extracted. . This error signal is amplified by an error amplifier 12, and a high-frequency component contained in the error signal is completely removed by a loop filter 13 and smoothed, and a bias source is applied to a saturable absorption region 3, which is a phase modulation region of the MLLD 1. Applied as a modulation component. At this time, if the MLLD 1 passively generates pulsed light by applying a reverse noise to the saturable absorption region 3 by the voltage adder 14, stable optical PLL operation can be performed.

[0038] FIG. 4 shows an RF VCO using an MLLD with a self-feedback loop configuration and an RF spectrum in an optical PLL operation when reducing phase noise at an oscillation frequency, and no self-feedback. The RF spectrum when the PLL is operated is shown. In Fig. 4, "self-feed knock + optical PLL" shows the spectrum when operating the optical PLL using self-feedback, and "only photoelectric PLL" shows the spectrum when self-feedback is not used. I have. When the jitter component is reduced by using the self-feedback, a sharp spectrum peak is obtained. When the jitter component is large without using the self-feedback, the spectrum peak spreads broadly. As described above, it can be confirmed that the signal-to-noise ratio (SNR) in the frequency spectrum when the self-feedback is used is improved and the peak becomes sharp. Figure 4 also shows the RF spectrum when electrical synchronization has been established and the RF spectrum when synchronization has not been established!

FIG. 5 shows a waveform observed by a sampling oscilloscope of the recovered clock light from the optical VCO using the MLLD having the above-described self-feedback loop configuration. The upper waveform 81 is a sampling waveform in the asynchronous state, that is, when the optical PLL operates, and the state is low.The middle waveform 82 is a sampling waveform when the PLL operation is performed without self feedback, and the lower waveform 81 is the lower waveform. Waveform 83 activates both the optical PLL and self-feedback. 7 shows a sampling waveform when the optical PLL is operated so as to reduce the phase noise.

In general, the greater the jitter of the sampling waveform, the smaller the fluctuation of the sampling waveform 83 when self-feedback is performed, as the jitter is seen. This means that the effect of improving the SNR of the frequency spectrum has been observed in the form of jitter reduction.

Example 2: Sideband Spectrum Suppression and Two-Wavelength Synchronized Clock Light Generation

In the configuration shown in the first embodiment, if the effect of self-feedback performed by self-injection on the MLLD, which is an optical VCO, becomes too large, a sideband spectrum depending on the self-feedback loop length may be generated. FIG. 6 shows an example of the RF spectrum of such a sideband, and shows a sideband generated by a self-feedback loop configuration. Sidebands occur because the wavelength corresponding to the frequency of the MLLD is the same as the wavelength corresponding to the frequency determined by the length of the self-feedback loop, so that a composite resonator is formed. In order to remove the side band, it is important to make the wavelength of the optical pulse output from the MLLD 1 different from the wavelength of the optical pulse circulating in the optical waveguide 6.

FIG. 7 shows a configuration of an optical clock extracting apparatus according to the second embodiment, in which two MLLDs having different wavelengths are inserted in the same loop to suppress sideband spectrum. Is shown. In the second embodiment, as shown in FIG. 7, two MLLDs 1 and MLLDs 100 having different wavelengths are provided in an optical loop, MLLD 1 is operated as an optical PLL, and a part of the output optical pulse is transmitted to the other MLLD 100. Light injection. Since the MLLD 1 outputs an optical pulse with the same polarization, the MLLD 100 on the injection side stably performs light injection locking. In this case, as in the above case, the optical delay unit 7 is used so that the timing at which the clock light north is generated from the MLLD 1 and the timing at which the clock light pulse reaches the saturable absorption region of each MLLD around the optical waveguide are synchronized. By adjusting, the absorption saturation phenomenon (gate operation) becomes sharp, and phase noise caused by the randomness of absorption saturation is reduced.

At this time, if the optical filter 15 and the optical filter 150 that transmit only the optical pulse generated from each MLLD are inserted into the optical waveguide 6, the wavelength of the optical pulse output from the MLLD 1 is reduced. Since the wavelength of the optical pulse that goes around the optical waveguide 6 and is again injected into the MLLD 1 is different, a synchronized optical pulse around which no sideband spectrum is generated can be created.

In this configuration, MLLDs 1 and 100 output clock lights of different wavelengths. Such a configuration that can simultaneously obtain two different wavelengths of clock light is an optimal configuration for a clock light source used for optical signal processing that requires synchronized clock light of two wavelengths, such as an optical 3R repeater. is there.

[Example 3: Configuration in which wavelength conversion unit is placed in self-feedback loop to suppress sideband spectrum]

In order to prevent only the optical pulse of a specific wavelength from resonating strongly, the method of Example 2 was used as a method for making the optical pulse output from the MLLD 1 different from the wavelength of the optical pulse circulating in the optical waveguide 6. In addition, a method of inserting the wavelength converter 16 into the waveguide 6 is also conceivable. The optical clock extraction device shown in FIG. 8 is the same as the one shown in FIG. 3, an optical filter 15 is provided near the output end face of the MLLD1, and a wavelength converter 16 is inserted into the optical waveguide 6. It is different in that it is.

FIG. 9A shows a CW (continuous wave) light source 17 operating at a wavelength different from the oscillation wavelength of the MLLD 1 as the wavelength conversion unit 16 for suppressing the side mode spectrum, a reproduced clock light from the MLLD 1 and a CW light source 17. FIG. 2 shows a configuration having a wavelength conversion 18 to which continuous light is input. The wavelength converter 18 includes an EA (electroabsorption) modulator operating at high speed, an optical gate switch using an SOA, and the like, and is inserted into the optical waveguide 6.

FIG. 9B shows a wavelength converter 16 using wavelength conversion by super continuum using an optical fiber. In this configuration, the spectrum of the clock light from the MLLD1 is expanded by the nonlinear effect of the optical amplifier 8 and the supercontinuum element (SC) 19 such as an optical fiber and a photonic crystal, and part of the spectrum is filtered by an optical filter. Wavelength conversion is performed by clipping at 15. In the figure, the SC spectrum is the output spectrum from the supercontinuum element 19, and the seed corresponds to the spectral position of the clock light before magnification. At this time, if the bandwidth of the optical filter 15 is widened and the optical pulse output from the wavelength conversion unit 16 is compressed so as to shorten the pulse width, the absorption of the MLLD is reduced. Since the sum phenomenon (gate operation) can be made steeper, phase noise caused by the randomness of absorption saturation can be further reduced.

FIG. 9C shows the simplest configuration, in which a narrow band optical filter 15 cuts out a part of the optical spectrum from the clock light from the MLLD 1 so that the optical pulse of the MLLD 1 and the self-feedback loop can be obtained. A configuration in which the center wavelength and the band wavelength range of the circulating light pulse are different is called. With such a configuration, the composite resonance effect can be suppressed, and the same effect as when the wavelength is changed can be obtained.

Example 4: Configuration in which the optical gate section of the optical 3R repeater has a phase comparison function

In the present invention, the optical mixer includes, in addition to the SOA, for example, a polarization-separated symmetric Mach-Zehnder interferometer (hereinafter, referred to as PD-SMZ), a symmetric Mach-Zehnder interferometer (hereinafter, referred to as SMZ), and a nonlinear optical loop. High-speed optical switches such as mirrors (NOLM), terahertz optical asymmetric demultiplexers (TOAD), and transmissive cross-phase modulation (T XPM) can be used. At the same time, optical gate switches such as PD—SMZ, SMZ, NOLM, TOAD, and T XPM can simultaneously perform optical signal processing such as optical 3R regeneration and optical DEMUX.

FIG. 10 shows a configuration in which the optical gate section of the optical 3R repeater has a phase comparison function. An example in which the optical clock extraction based on the present invention is applied to an optical 3R repeater using the PD-SMZ101 will be described with reference to FIG.

The optical clock extracting unit has the same configuration as that described in the second embodiment, and generates two clock lights having different wavelengths and small phase noise. The PD-SMZ 101 includes a polarization controller 20, a calcite (polarizer) 21, an SOA (semiconductor optical amplifier) 22, an optical phase controller 23, and an optical filter 15. The clock light output from the MLLD 1 passes through the polarization controller 20, undergoes a time delay in the first calcite 21, is separated into two polarization states, and enters the SOA 22. At this time, if the optical pulse of the signal light is adjusted to enter the SOA 22 so that it falls within the delay time, the clock optical pulse that has received the time delay among the two separated clock optical pulses that have entered the SOA 22 Only the phase is modulated. Further, the optical pulse whose phase has been modulated and the optical pulse which has not been phase-modulated are passed through the second calcite 21 to cancel the time delay caused by the first calcite 21. In this way two By generating interference between clock lights, a clock light that has been gated by a logic light can be obtained.

. The output light of the second calcite 21 reaches the optical filter 15 via the optical phase controller 23 and the polarization controller 20.

The positive logic and the negative logic at this time are controlled by the polarization plane of the polarization controller 20. In order to operate with the positive logic, the interference condition of the PD-SMZ 101 may be adjusted so that only clock light that has been optically gated passes through the polarization controller 20 only when phase modulation occurs.

In this case, if the synchronization of the signal light and the clock light is not synchronized, no signal light pulse will enter within the delay time, so that sufficient phase modulation will not be performed, which will deviate from the interference condition, and will be optically gated. The clock light undergoes strong intensity modulation. When the optical PLL is operated using this intensity modulated signal as an error signal and a clock extraction operation is performed, the conditions under which the signal light and the clock light are synchronized are the same as the optimal conditions for the optical gate operation. This makes it possible to simultaneously execute clock extraction by the optical PLL and optimal operation of optical signal processing. In the second and subsequent stages, the optical gated optical signal and the clock light having different wavelengths shown in the second embodiment are made incident on another PD-SMZ 102. Thus, the signal processing of the optical 3R repeater can be completed.

FIG. 10 illustrates an example in which the signal light that has been optically gated is used as an error signal. As the force error signal, the signal light directly output from the SOA or the clock separated into two is used. The same processing can be executed with or without using light.

[Example 5: Optical PLL circuit using EA modulator as phase comparison]

FIG. 11 shows a configuration in which an electro-absorption modulator (EA modulator) 103 is used for a phase comparison unit (optical mixer unit). In place of the EA modulator 103, a saturable absorption modulator can be used.

A reverse bias voltage is applied to EA modulator 103 via bias T circuit 140. If the high-speed frequency signal can be extracted from the other terminal of the bias T circuit 140, the clock light output from the MLLD1 will be input to the EA modulator 103 when the clock light An electric signal as a component can be extracted from the terminal of the bias T circuit 140. In this state, when external signal light enters the EA modulator 103, An error signal is superimposed on the electric signal output from the noise T circuit 140. When the frequency of this error signal is amplified by the error amplifier 113 composed of a low-speed operational amplifier or the like, since the high-frequency component appears as a DC component signal, only the error signal can be separated and extracted. The DC component can be canceled by receiving the signal light power clock light by the low-speed light receiver 111 or the like and balancing the signal light by the adder 112 that also has power such as an operational amplifier. The error signal extracted in this way is passed through the loop filter 13 to be smoothed, and is applied as a bias source to the saturable absorption region 3 of the MLLD 1 via the bias T circuit 140 to form a phase locked loop. As a result, the signal light and the clock light can be synchronized.

Example 6

In the first to fifth embodiments described above, the method of feeding back the error signal to the saturable absorption region having the phase modulation effect in order to feed back the error signal to the MLLD 1 has been mainly described. As a method of constructing a phase locked loop, there is a method other than feeding back the error signal to the saturable absorption region. For example, (1) the loop length of self-feedback is changed according to the error signal, (2) the light intensity injected into the MLLD in self-feedback is modulated according to the error signal, (3) the gain region 2 of the MLLD Modulation of the applied current according to the error signal, (4) PLL control by applying voltage to the cavity length adjustment area 27 (see Fig. 12) newly provided in the MLLD, etc. Thus, the oscillation frequency of the MLLD1 can be changed by the multi-stage PLL control.

[0058] In the self-feedback of the MLLD1, the loop length is set by the time delay unit 7 or the like so that the timing at which the MLLD1 oscillates an optical pulse and the timing at which the optical pulse circulating around the optical waveguide is injected into the MLLD again. When the frequency is changed, the oscillation frequency of the MLLD1 is reduced to a frequency that is an integer multiple of the frequency determined by the loop length in the range of several hundred MHz. This is because the light pulse intensity in the resonator that absorbs and saturates the saturable absorption region 3 plays an important role in the self-excited light pulse oscillation of the MLLD 1, but the injection of the light pulse causes the injection. The generated optical pulse also plays a major role in the self-excited optical pulse oscillation of the MLLD. Therefore, the frequency modulation required for PLL control can be realized by changing the loop length of the self-feedback. [0059] The oscillation frequency of MLLD1 can be controlled by the effective resonator length. An effective resonator length can be obtained by changing the refractive index of light according to the carrier density in the semiconductor constituting the MLLD 1. In other words, by changing the optical pulse intensity injected by the optical amplifier 8 and the optical attenuator 9 in the loop and the injection current in the gain region 2, the carrier density in the semiconductor can be controlled to change the frequency. Is possible.

When an MLLD is newly constructed in which a waveguide having a grating such as a DFB (distributed feedback) structure or a DBR (distributed Bragg reflector) or a cavity length control region 27 of a passive waveguide is added. Also, by controlling the voltage in the resonator length control region 27, the refractive index inside the MLD can be changed, and the oscillation frequency of the MLLD can be controlled.

[0061] According to the experiment of the present inventor, when comparing the frequency variable bandwidth for each control method, it is found that saturable absorption region voltage control <light intensity control <loop length control ≤gain region current control <resonator length adjustment region Voltage control

Met.

As shown in FIG. 12, by designing loop filters 13, 25, 26, 28, and 29 that can effectively control such a frequency variable range, by performing PLL control in multiple stages for MLLD 1, It is possible to realize a wide frequency synchronization range and fine synchronization establishment.

Claims

The scope of the claims

[1] An optical clock extraction device for obtaining an optical pulse having an oscillation frequency synchronized with a reference frequency of an incident signal light,

A mode-locked semiconductor laser having a saturable absorption region having a phase modulation effect and a gain region, and capable of changing an oscillation frequency for generating an optical pulse by a phase modulation effect based on a control voltage applied to the saturable absorption region. When,

Means for obtaining a phase error signal by optically synchronously detecting a synchronization shift between a reference frequency of the signal light and an oscillation frequency of an output light pulse of the mode-locked semiconductor laser;

Means for negatively feeding back the phase error signal to the saturable absorption region so as to form a phase locked loop;

Has,

An optical clock extraction device for synchronizing an output light pulse of the mode-locked semiconductor laser with the signal light.

[2] An optical clock extraction device for obtaining an optical pulse having an oscillation frequency synchronized with a reference frequency of an incident signal light,

An optical self-feedback loop for directly injecting at least a part of an optical pulse output from one output end face of the mode-locked semiconductor laser from the other end face of the mode-locked semiconductor laser;

Means for obtaining a phase error signal by optically synchronously detecting a synchronization shift between the oscillation frequency of the output light pulse output from the mode-locked semiconductor laser and the reference frequency of the signal light;

Has,

An optical clock for synchronizing the output light pulse of the mode-locked semiconductor laser with the signal light. Extraction equipment.

[3] A mode-locked semiconductor laser including a saturable absorption region and a gain region separated from each other, and capable of changing an oscillation frequency for generating an optical pulse by a control voltage; and a repetition frequency of the mode-locked semiconductor laser. An optical mixer unit that optically detects a phase error between the incident signal light and the optical pulse and optically generates a synchronization shift signal when synchronizing the incident signal light with a frequency close to the reference signal.

A light receiving unit that converts the synchronization error signal into a phase error signal that is a voltage signal; and a loop filter for negatively feeding back the phase error signal.

Based on the phase error signal that is negatively fed back through the loop filter, the synchronization deviation of the repetition frequency and phase of the output light pulse from the mode-locked semiconductor laser with respect to the repetition frequency and phase of the incident signal light is minimized. A phase-locked loop for controlling the voltage applied to the electrodes of the mode-locked semiconductor laser,

An optical clock extracting device having:

[4] A saturable absorption region and a gain region separated from each other by electrodes, and an oscillation frequency for generating an optical pulse can be changed by a control voltage, and a first wavelength for oscillating an optical pulse of a first wavelength is provided. A mode-locked semiconductor laser,

A second mode-locked semiconductor laser including a saturable absorption region and a gain region separated from each other, and oscillating an optical pulse having a second wavelength different from that of the first mode-synchronized semiconductor laser;

At least one output end face force of the first mode-locked semiconductor laser at least a part of the light pulse to be output is directly injected from one end face of the second mode-locked semiconductor laser; and An optical self-feedback loop for directly injecting an optical pulse output from the other end face of the synchronous semiconductor laser from the other end face of the first mode-locked semiconductor laser;

An optical delay unit provided in the self-feedback loop,

When synchronizing the incident signal light with a frequency close to the repetition frequency of the first mode-locked semiconductor laser as a reference signal, optically detects the phase error between the signal light and the light pulse, and optically detects the synchronization shift signal. Optical mixer part generated in A light receiving unit that converts the synchronization error signal into a phase error signal that is a voltage signal; and a loop filter for negatively feeding back the phase error signal.

Based on the phase error signal that is negatively fed back through the loop filter, the synchronization deviation of the repetition frequency and phase of the output optical pulse from the first mode-locked semiconductor laser with respect to the repetition frequency and phase of the incident signal light is minimized. So that

An optical clock extraction device comprising: a phase-locked loop that controls a voltage applied to an electrode of the mode-locked semiconductor laser.

[5] A mode-locked semiconductor that has a saturable absorption region and a gain region separated from each other by electrodes and can change the oscillation frequency for generating an optical pulse by a control voltage, and oscillates an optical pulse of a first wavelength. A laser,

A light source that oscillates continuous light having a second wavelength different from the first wavelength,

A wavelength converter that transfers the light pulse of the first wavelength to continuous light of the second wavelength and converts the light pulse into light of a second wavelength;

An optical self-feedback loop for directly injecting the second wavelength pulsed light converted by the wavelength converter from the other end face of the first mode-locked semiconductor laser; and Optical delay device,

When synchronizing the incident signal light with a frequency close to the repetition frequency of the mode-locked semiconductor laser as a reference signal, the phase error between the incident signal light and the light pulse of the first wavelength is optically detected, and the synchronization error signal is detected. An optical mixer for optically generating

An optical clock extracting device having:

[6] An optical pulse from the mode-locked semiconductor laser is incident, and the input is caused by a nonlinear effect. An optical fiber that broadens the spectrum band of the emitted light pulse, and a filter that cuts out a part of the spectrum band of the light pulse output from the optical fiber and generates a light pulse having a wavelength different from that of the mode-locked semiconductor laser. 4. The optical clock extracting device according to claim 1, wherein the optical clock extracting device has a wavelength conversion capable of compressing a time width of the optical pulse.

7. The mode-locked semiconductor laser according to claim 1, further comprising an optical filter that cuts off a part of a spectrum band of the optical pulse of the mode-locked semiconductor laser, and having a wavelength variation that generates an optical pulse having a wavelength different from that of the mode-locked semiconductor laser. 4. The optical clock extraction device according to any one of 3.

[8] The optical clock extraction method according to any one of claims 3 to 5, wherein the optical mixer section has an optical signal processing function.

[9] The optical clock extracting method according to claim 8, wherein the optical signal processing function is an optical 3R reproducing function.

[10] A mode in which a saturable absorption region having a phase modulation effect and a gain region are provided, and the oscillation frequency for generating an optical pulse can be changed by the phase modulation effect based on the control voltage applied to the saturable absorption region. A synchronous semiconductor laser,

An optical absorption type modulator used for optical signal processing and capable of absorbing an optical signal to generate a photocurrent;

When synchronizing the oscillation frequency of the output light pulse from which the mode-locked semiconductor laser power is also output with the reference frequency of the incoming signal light, the output light pulse and the signal light enter the light absorption modulator. Means for optically detecting the out-of-synchronization as a photocurrent to obtain a phase error signal;

Means for negatively feeding back the phase error signal to the saturable absorption region so as to form a phase locked loop, thereby synchronizing the output light pulse with the signal light;

An optical clock extraction device, comprising:

11. The optical clock according to claim 10, wherein the light absorption type modulator has an optical 3R reproducing function. Extraction method.

[12] A saturable absorption region, a gain region, and a resonator adjustment region separated from each other by electrodes are provided, and by controlling a voltage or a current applied to the electrodes in each of the regions, an oscillation frequency for generating an optical pulse is reduced. A mode-locked semiconductor laser that can be varied;

A pulse delayer provided in the self-feedback loop,

An optical amplifier provided in the self-feedback loop;

An optical attenuator provided in the self-feedback loop,

When synchronizing the oscillation frequency of the output light pulse from which the mode-locked semiconductor laser power is also output with the reference frequency of the incident signal light,

Means for optically synchronously detecting a synchronization shift between the oscillation frequency of the output light pulse output from the mode-locked semiconductor laser and the reference frequency of the incident signal light to obtain a phase error signal;

The phase error signal is negatively fed back to at least one of the gain region, the resonator adjustment region, the pulse delay device, the optical amplifier, and the optical attenuator so that phase locked loop control is performed. Means for synchronizing the output light pulse with the signal light.

13. The optical clock extraction device according to claim 12, comprising a plurality of phase locked loops.

[14] A mode-locking system including a saturable absorption region having a phase modulation effect and a gain region, wherein the oscillation frequency for generating an optical pulse can be changed by the phase modulation effect by a control voltage applied to the saturable absorption region. An optical clock extraction method for synchronizing an oscillation frequency of an output light pulse from a semiconductor laser with a reference frequency of an incident signal light, comprising: a synchronization shift between the reference frequency of the signal light and the oscillation frequency of the output light pulse. Obtaining a phase error signal by optically synchronous detection of

Negatively feeding back the phase error signal to the region having the phase modulation effect to form a phase synchronization loop, and synchronizing the light pulse with an incident signal light; An optical clock extraction method, comprising:

[15] A mode lock that includes a saturable absorption region having a phase modulation effect and a gain region, and is capable of changing an oscillation frequency for generating an optical pulse by a phase modulation effect by a control voltage applied to the saturable absorption region. An optical clock extraction method for synchronizing an oscillation frequency of an output light pulse from a semiconductor laser with a reference frequency of an incident signal light, comprising: Partially injecting light directly from the other end face of the mode-locked semiconductor laser, and synchronizing the oscillation frequency of the output light pulse of the mode-locked semiconductor laser with the reference frequency of the signal light. Obtaining a phase error signal by optically synchronously detecting a synchronization shift between a reference frequency of the signal light and an oscillation frequency of the output light pulse; Negatively feeding back the error signal to the saturable absorption region to form a phase locked loop, and synchronizing the output light pulse with the signal light;

An optical clock extraction method comprising:

16. The optical clock extraction method according to claim 14, wherein an optical signal processing is performed simultaneously with the optical synchronous detection of the synchronization shift.