JP2007242794A - Passive mode-locked semiconductor laser device, and optical clock signal extractor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable to extract a high-speed optical clock signal exceeding the upper limit speed of an electronic device, not depending on the polarization direction of an input optical signal. <P>SOLUTION: This is an MLLD device 200 which has a multielectrode semiconductor laser element 70 and a polarization rotator 32 in an optical resonator. The resonator length L of the optical resonator is so set that an optical pulse train occurring at frequent time intervals of a fraction of a natural number of the time required for light to go around once inside the optical resonator may be output outside the optical resonator. The multielectrode semiconductor laser element consists of a gain region 20 and a saturable absorber region 10, which are arranged in series. Into the gain region, a current is injected from a constant current source 36 via an n-side common electrode 26 and a p-side electrode 22 of the gain region. Meanwhile, the saturable absorber region is applied with inverse bias voltage from a constant voltage source 38 via the n-side common electrode and a p-side electrode 12 of the saturable absorber region. Optical guides 24 and 14 are interposed between a first clad layer 18 and a second clad layer 16 which are common to both regions. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a passive mode-locked semiconductor laser device, and an optical clock signal extraction device and an optical clock signal extraction method using the passive mode-locked semiconductor laser device.

  In optical communication networks, transmission distance and capacity have been increased. As transmission becomes longer, optical loss in the optical transmission line, S / N ratio decrease due to the use of multiple optical amplifiers, and waveform distortion due to optical fiber group velocity dispersion and nonlinear optical effects in the optical fiber occur. As a result, the quality of the optical signal deteriorates. Generation of frequency waveform distortion and time waveform distortion becomes a more serious problem as the transmission capacity increases.

  Therefore, a repeater is provided in the middle of the optical transmission line at intervals of several tens to several hundred kilometers, and so-called optical signal regeneration is performed by which the frequency waveform and time waveform of the optical signal are restored to the original shape by this repeater. ing. One of the main roles of this repeater is clock signal extraction. The clock signal extraction is to generate a pulse output (or sine wave output) signal having a frequency corresponding to the bit rate from an optical signal composed of an optical pulse whose time waveform is distorted, that is, an optical signal with a deteriorated quality. Sometimes called timing extraction.

  The clock signal may be extracted as an electrical signal or an optical signal. However, only when it is necessary to clearly indicate in which form the clock signal is extracted, Sometimes written separately from the optical clock signal. Further, the frequency corresponding to the bit rate of the optical signal indicates the frequency of f (GHz) when the bit rate of the optical signal is f (Gbit / s). Hereinafter, the frequency corresponding to the bit rate of the optical signal may be referred to as a bit rate frequency.

  One of the methods conventionally known as clock signal extraction methods is to photoelectrically convert an optical signal with degraded quality to a photodiode or the like, filter the output electrical signal from the photodiode with a narrowband electrical filter, In this method, an electric clock signal is extracted by extracting only frequency components corresponding to the bit rate of the input optical signal. Hereinafter, an optical signal from which a clock signal is extracted including an optical signal whose quality has deteriorated is referred to as an input optical signal.

  In the following description, an optical signal (or input optical signal) is an RZ (Return) signal that is a binary digital signal that is a transmission signal obtained by optically modulating a sequence of optical pulses regularly arranged at regular intervals on the time axis. to Zero) signal. On the other hand, the expression “optical pulse train” is used to indicate the total of optical pulses arranged at regular intervals at regular intervals on the time axis.

  In general, since the photoelectric conversion characteristics of a photodiode have a small polarization dependence, a clock signal can be stably extracted by using the photodiode even if there is temporal fluctuation in the polarization plane of the input optical signal. be able to.

  In addition, when it is necessary to extract an optical clock signal according to the system configuration, an optical pulse light source such as a mode-locked laser is driven using the electrical clock signal extracted by the above-described method. An optical clock signal can be generated (see, for example, Patent Documents 1 and 2).

  On the other hand, as a technique for increasing the transmission capacity of an optical communication network, a multiplex transmission technique such as optical time division multiplexing (Optical Time Division Multiplexing) has been studied. Since the bit rate of the multiplexed signal is a multiple of the number of channels per multiplexed channel, the bit rate is very high.

  When the bit rate of the multiplexed signal exceeds 40 Gbit / s, it is difficult for the electronic device to extract the clock signal. This is because a photodiode that operates also for an optical signal having a bit rate of 40 Gbit / s or more and an electric narrowband filter that operates also for an electrical signal of 40 GHz or more have not been developed.

  Therefore, in order to extract the clock signal from the high-speed optical signal, it is desirable to employ a method of directly extracting the optical clock signal without going through photoelectric conversion. Hereinafter, a method of extracting an optical clock signal without going through photoelectric conversion may be referred to as an all-optical clock signal extraction method.

  Conventionally, as an all-optical clock signal extraction method, a method using a mode-locked laser (for example, see Non-Patent Document 1) or a method using a self-excited light pulse generation laser such as a self-pulsation laser (for example, Non-Patent Document 2) See). In any of these methods, an input optical signal is input to a mode-locked laser or self-pulsation laser that generates optical pulses at a repetition frequency close to the bit rate of the input optical signal, and the output of the mode-locked laser or self-pulsation laser is output. A technique is adopted in which the optical clock signal is extracted by synchronizing the optical pulse with the bit rate of the input optical signal.

  The advantage of these methods is that, as described above, a high-speed clock signal at a level that cannot be realized by using an electronic device can be extracted in the form of an optical clock signal. For example, an example in which an optical clock signal is successfully extracted from an input optical signal of 160 Gbit / s has been reported (for example, see Non-Patent Document 3).

In addition, when an external resonator type mode-locked semiconductor laser is used as the mode-locked laser used for extracting the all-optical clock signal, the resonator length of the external resonator is set to realize passive mode locking by the input optical signal. It is easy to adjust, and it is also easy to change the center wavelength of the wavelength filter inserted inside the resonator. That is, it is possible to configure an all-optical clock signal extraction device in which the frequency of the clock signal and the wavelength of the optical clock signal are variable.
JP 09-008741 A JP 11-340954 A T. Ono, T. Shimizu, Y. Yano, and H. Yokoyama, "Optical clock extraction from 10-Gbit / s data pulses by using monolithic mode-locked laser diodes,"OFC'95 Technical Digest, ThL4. M. Jinno and T. Matsumoto, "All-optical timing extraction using a 1.5 μm self pulsating multielectrode DFB LD," Electron. Lett., Vol. 24, No. 23 pp. 1426-1427, 1988. S. Arahira, S. Sasaki, K. Tachibana, Y. Ogawa, "All optical 160-Gb / s clock extraction with a mode-locked laser diode module," IEEE Photon. Technol. Lett. Vol. 16, No. 6 , pp. 1558-1560, 2004.

  However, the above-described conventional all-optical clock signal extraction method has the following problems. That is, the operation for extracting the optical clock signal depends on the polarization direction of the input optical signal. In order to extract an optical clock signal using a mode-locked semiconductor laser or a self-pulsation laser, it is necessary to make the direction of the polarization plane of the input optical signal coincide with the direction of the polarization plane of the oscillation light of these lasers. For this reason, if the direction of the polarization plane of the input optical signal varies, the optical clock signal cannot be extracted stably.

  In general, an input optical signal propagates over a long distance through a single-mode optical fiber transmission line where preservation of the polarization direction is not ensured, and therefore the polarization direction is not constant and varies with time. For this reason, when such an input optical signal whose polarization direction is not constant is input to these lasers, the operation becomes unstable, and it becomes difficult to stably extract the optical clock signal.

  Accordingly, a first object of the present invention is to provide a passive mode-locked semiconductor laser device suitable for use in this optical clock signal extraction device.

  A second object of the present invention is to provide an optical clock signal extraction device that is independent of the polarization direction of an input optical signal and that can extract a high-speed optical clock signal that exceeds the upper limit speed of an electronic device, and an optical clock signal extraction It is to provide a method.

  In order to achieve the above first object, according to the gist of the present invention, an external resonator type passive mode-locked semiconductor laser device having the following configuration is provided. A passively mode-locked semiconductor laser device includes a multi-electrode semiconductor laser element and a polarization rotation element in an optical resonator, and has a time interval that is a fraction of a natural number of the time required for light to circulate in the optical resonator. The resonator length of the optical resonator and the magnitude of the optical gain in the gain region are set so that an optical pulse train having a repetition period is output to the outside of the optical resonator. As will be described later, the resonator length of this optical resonator approximates the reciprocal of the bit rate frequency of the input optical signal so that the time required for the light to make one round in the optical resonator is such that frequency pull-in is caused. Adjusted to be equal to each other.

  Here, the multi-electrode semiconductor laser device is configured such that a gain region in which an inversion distribution is formed and a saturable absorption region having a function of modulating light intensity are arranged in series. The polarization rotation element has a function of rotating the plane of polarization by 90 ° when linearly polarized light makes a round trip of the polarization rotation element. The magnitude of the optical gain in the gain region is a time interval that is one-tenth of the natural number of the time required for light to make one round in the optical resonator of the external resonator type passive mode-locked semiconductor laser device of the present invention. An optical pulse train having a repetition period is set so as to be output to the outside of the optical resonator.

More specifically, the meaning that an optical pulse train having a repetition period with a time interval that is a fraction of the natural number of times required for one round of light to travel inside the optical resonator is output to the outside of the optical resonator is described below. It is as follows. If the time required for one round of light to travel through the optical resonator is T _rt and the repetition period of the optical pulse train is T _ML , this means that T _ML = T _rt / M. Here, M is a natural number, that is, an integer of 1 or more (M = 1, 2, 3,...). Further, the resonator frequency f _rt is equal to 1 / T _rt, and the repetition frequency f _ML of the optical pulse train generated as a result of the passive mode synchronization operation is equal to 1 / T _ML .

  The active layer provided with the above-described saturable absorption region is preferably formed by a quantum well structure having a bulk semiconductor crystal layer or a quantum well into which an extension strain is introduced.

  The polarization rotator described above can use a quarter wave plate or a Faraday rotator.

  In order to achieve the second object described above, according to the gist of the present invention, there are provided first and second optical clock signal extraction devices having the following configurations. That is, the first optical clock signal extraction device of the present invention is a device that includes the passive mode-locked semiconductor laser device of the present invention, an input unit and an output unit, and generates an optical pulse train as an optical clock signal. The difference between the repetition frequency of the optical pulse train generated by the passive mode-locked semiconductor laser device and the frequency corresponding to the bit rate of the input optical signal is the difference between the input optical signal input to the optical clock signal extraction device and the passive mode-locked semiconductor. It is assumed that the input optical signal has a bit rate small enough to cause the frequency pull-in necessary for the passive mode synchronization operation of the laser device.

  Thereafter, the difference between the bit rate of the input optical signal input to the optical clock signal extraction device and the repetition frequency of the optical pulse train generated by the passive mode-locked semiconductor laser device is small enough to cause frequency pulling as described above. In some cases, the bit rate frequency of the input optical signal approximates the repetition frequency of the optical pulse train. That is, as will be described later, when the frequency of the optical clock signal reproduced by the optical clock signal extraction device and the self-oscillation frequency of the passive mode-locked semiconductor laser device constituting the optical clock signal extraction device are in the above relationship, It is expressed that both frequencies are approximately equal.

  The input unit inputs an input optical signal to the passive mode-locked semiconductor laser device. On the other hand, the output unit outputs an optical pulse train generated by the passive mode-locking semiconductor laser device from the input optical signal as an optical clock signal.

  The input unit includes a first optical isolator for preventing the return light from the optical clock signal extraction device from being input to the input side optical transmission line for transmitting the input optical signal, and for inputting the input optical signal to the passive mode-locked semiconductor laser device. The first coupling optical system is preferably included.

  The output unit is configured to prevent the return light from the output side optical transmission line that transmits the optical clock signal from being input to the passive mode-locked semiconductor laser device, and to input the optical clock signal to the output side optical transmission line. It is preferable that the second coupling optical system is provided.

  The output unit preferably includes a polarization separator that outputs the optical clock signal by separating the optical clock signal into two components whose polarization directions are orthogonal to each other, and further, the light extracted by the passive mode-locked semiconductor laser device. It is preferred to provide a wavelength filter for filtering the clock signal.

  A second optical clock signal extraction device of the present invention is an optical clock signal extraction device comprising the passive mode-locked semiconductor laser device of the present invention and an input / output unit. Here, the bit rate frequency of the input optical signal input to this optical pulse signal extraction device and the repetition frequency of the optical pulse train generated by the passive mode-locking semiconductor laser device of the present invention are characterized by being approximately equal. is there.

  The input / output unit inputs an input optical signal to the passive mode-locked semiconductor laser device and outputs an optical pulse train generated from the input optical signal by the passive mode-locked semiconductor laser device as an optical clock signal.

  The input / output unit preferably includes an optical circulator and a coupling optical system. The optical circulator includes a first port for inputting an input optical signal, a second port for outputting the input optical signal and inputting an optical clock signal extracted by a passive mode-locking semiconductor laser device, and an optical clock signal. Has a third port to output. The coupling optical system outputs the input optical signal output from the second port to the passive mode-locked semiconductor laser device, and outputs the optical clock signal extracted by the passive mode-locked semiconductor laser device to the second port.

  The input / output unit preferably includes a polarization separator that separates and outputs the optical clock signal output from the passive mode-locked semiconductor laser device into two components whose polarization directions are orthogonal to each other. It is preferable to provide a wavelength filter for filtering the optical clock signal.

  In order to achieve the second object described above, according to the gist of the present invention, the following optical clock signal extraction method is provided. That is, the optical clock signal extraction method of the present invention extracts an optical pulse train output as a passive mode-locked semiconductor laser device in passive mode synchronization as an optical clock signal. Here, in order to perform passive mode synchronization of the passive mode-locked semiconductor laser device, the condition that the repetition frequency of the optical pulse train generated by the passive mode-locked semiconductor laser device is approximately equal to the bit rate frequency of the input optical signal is satisfied. It is characterized in that an input optical signal is input to a passive mode-locked semiconductor laser device.

  The external resonator type passively mode-locked semiconductor laser device of the present invention comprises a multi-electrode semiconductor laser element and a polarization rotation element in an optical resonator, and the time required for the light to make one round in the optical resonator. The resonator length is set so that an optical pulse train having a repetition period with a natural fraction of a time interval is output outside the optical resonator. Outputting an optical pulse train having a repetition period with a time interval of one-tenth of the natural number of times required for one round of light to travel through the optical resonator to the outside of the optical resonator can be expressed as follows. That is, although details will be described later, this means that the passive mode-locked semiconductor laser device is operated in even-order harmonic mode synchronization.

  Even if the polarization state of the external input light fluctuates, the phase of the optical clock signal generated as an optical pulse train is achieved by operating the even-order harmonic mode-locked operation of the external resonator type passively mode-locked semiconductor laser device of the present invention. There will be no fluctuations. That is, by using the passive mode-locked semiconductor laser device of the present invention, it becomes possible to extract the optical clock signal with its phase stabilized independently of the polarization direction of the input optical signal.

  If the external cavity type passively mode-locked semiconductor laser device of the present invention is used, an optical clock signal is generated from the input optical signal by the mode-locking operation of the passively mode-locked semiconductor laser device, so that no electric circuit is used. Can extract the optical clock signal. The mode-locking operation of the mode-locking semiconductor laser device is much faster than the operation speed of the electric circuit. Therefore, by using the passive mode-locked semiconductor laser device of the present invention, it is possible to extract a high-speed optical clock signal exceeding the upper limit speed of the electronic device.

  A multi-electrode semiconductor laser element is configured by arranging a gain region in which an inversion distribution is formed and a saturable absorption region having a function of modulating light intensity in series, which are components of the saturable absorption region. By forming a certain active layer with a bulk semiconductor crystal layer or a quantum well structure having a quantum well into which an extensional strain is introduced, the above-described even-order harmonic mode-locking operation can be more reliably realized. It becomes possible.

  The first optical clock signal extraction device of the present invention comprises the passive mode-locked semiconductor laser device of the present invention, an input unit and an output unit. When the bit rate of the input optical signal input from the input unit satisfies the following conditions, the passive mode-locked semiconductor laser device of the present invention can be operated to extract the optical clock signal. That is, the bit rate frequency of the input optical signal is set to a frequency that approximates the repetition frequency of the optical pulse train generated by the passive mode-locked semiconductor laser device of the present invention.

  By configuring the input unit with the first optical isolator, it becomes possible to prevent the return light from the optical clock signal extraction device from being input to the input side optical transmission line. When the return light from the optical clock signal extraction device is input to the input side optical transmission path, the input optical signal travels backward to the input side optical transmission path and is returned to the device that output this input optical signal. As a result, there is a possibility that problems such as malfunctions occur in a device that outputs this input optical signal. That is, by providing the input unit with the first optical isolator, it is possible to prevent such an inconvenience from occurring.

  In general, an input optical signal propagates through an optical transmission line such as an optical fiber and is input to an optical clock signal extraction device. In addition, a multi-electrode semiconductor laser element configured by arranging a gain region and a saturable absorption region in series to extract an optical clock signal is formed as a waveguide type device. Therefore, by providing the first coupling optical system for inputting the input optical signal that has propagated through the optical transmission line into the optical waveguide of the multi-electrode semiconductor laser element, the input optical signal that has propagated through the optical transmission line can be efficiently obtained. It becomes possible to input to the optical waveguide of the multi-electrode semiconductor laser element. That is, it is desirable that the first coupling optical system is installed in the input unit.

  On the other hand, by providing the output unit with the second optical isolator, it is possible to prevent the return light from the output side optical transmission line that transmits the optical clock signal from being input to the passive mode-locked semiconductor laser device. When the return light from the output-side optical transmission line is input to the passive mode-locked semiconductor laser device, the gain of the passive mode-locked semiconductor laser device varies and the passive mode-locking operation becomes unstable. This may lead to a situation where the optical clock signal cannot be extracted stably.

  Similarly to the input unit, it is desirable that a second coupling optical system for inputting an optical clock signal to the output side optical transmission path is also installed in the output unit. By installing the second coupling optical system, the extracted optical clock signal can be efficiently input to the output side optical transmission line.

  Furthermore, by providing the output unit with a polarization separator that separates and outputs the optical clock signal into two components whose polarization directions are orthogonal to each other, the polarization direction of the optical clock signal can be selected and used. . When an optical clock signal is used, if the device that processes the optical clock signal is dependent on the polarization direction, there is an advantage that the polarization direction of the optical clock signal can be selected conveniently for this device.

  Also, by providing the output unit with a wavelength filter that filters the optical clock signal extracted by the passive mode-locked semiconductor laser device, the wavelength component of the input optical signal is removed and only the wavelength component of the optical clock signal is output. Can do. As a result, the input optical signal component input to the passive mode-locked semiconductor laser device can be removed from the output optical clock signal, and can be extracted and used as an optical clock signal with a small noise component. .

  The second optical clock signal extraction device of the present invention comprises the passive mode-locked semiconductor laser device of the present invention and an input / output unit. The difference from the first optical clock signal extraction device is that the input side of the input optical signal and the output side of the optical clock signal are provided separately or are provided together on one side. In other respects, the first optical clock signal extraction device and the second optical clock signal extraction device have the same configuration.

  In the first optical clock signal extraction device, the input unit and the output unit are provided on both sides of the passive mode-locked semiconductor laser device. On the other hand, in the second optical clock signal extraction device, the input side of the input optical signal and the output side of the optical clock signal are provided together on one side of the passive mode-locked semiconductor laser device.

  By providing the input side of the input optical signal and the output side of the optical clock signal together on one side of the passive mode-locked semiconductor laser device, light must be output from the other end of the passive mode-locked semiconductor laser device. There is no. For this reason, if the intensity of the input optical signal is the same, it is advantageous to realize the passive mode synchronization operation as compared with the case where the input side of the input optical signal and the output side of the optical clock signal are provided separately. . The reason for this is as follows.

  When the input side of the input optical signal and the output side of the optical clock signal are provided separately, a part of the input optical signal is output from the side opposite to the input side. The input optical signal intensity in the active region of the multi-electrode semiconductor laser element constituting the semiconductor laser device is reduced. On the other hand, when the input side of the input optical signal and the output side of the optical clock signal are provided together on one side of the passive mode-locked semiconductor laser, all of the input optical signal is reflected on the other side. Therefore, the input optical signal intensity in the active layer of the multi-electrode semiconductor laser device constituting the passive mode-locked semiconductor laser device is maintained. Therefore, there is no loss due to partial output of the input optical signal from the side opposite to the input side, and the input optical signal can be effectively used for the passive mode synchronization operation.

  However, when the optical clock signal extraction device is incorporated into an optical communication system, it may be advantageous to provide an input optical signal input side and an optical clock signal output side separately for mounting. In some cases, the intensity of the input optical signal can be secured sufficiently. Accordingly, which one of the first and second optical clock signal extraction devices is to be used may be appropriately selected according to the form of use.

  The input / output unit of the second optical clock signal extraction device can be configured by using an optical circulator. In addition, the input / output unit of the second optical clock signal extraction device also includes a coupling optical system, a polarization separator, and a wavelength filter, similar to the input unit or output unit of the first optical clock signal extraction device, so that The same effects as those of the first optical clock signal extraction device can be obtained.

  As described above, according to the optical clock signal extraction method of the present invention, by satisfying the condition that the bit rate frequency of the input optical signal approximates the repetition frequency of the optical pulse train generated by the passive mode-locked semiconductor laser device. It is possible to extract an optical clock signal.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Each figure shows one configuration example according to the present invention, and only schematically shows the arrangement relationship of each component to the extent that the present invention can be understood. It is not limited to. In the following description, specific materials and conditions may be used. However, these materials and conditions are only one of preferred examples, and are not limited to these. In addition, overlapping description of the same components in each drawing may be omitted.

<Passive mode-locked semiconductor laser device>
A configuration of an external resonator type passive mode-locked semiconductor laser device 200 of the present invention will be described with reference to FIG. FIG. 1 is a schematic configuration diagram of a passive mode-locked semiconductor laser device 200 of the present invention. Hereinafter, the passive mode-locked semiconductor laser may be simply abbreviated as MLLD (Mode-Locked Laser Diode). An external resonator type passively mode-locked semiconductor laser device may also be simply abbreviated as a mode-locked semiconductor laser.

  FIG. 1 is a schematic configuration diagram of the MLLD device 200 as viewed from the direction of the side surface perpendicular to the direction of the optical waveguide (the optical waveguide 14 in the saturable absorption region 10 and the optical waveguide 24 in the gain region 20).

  The external resonator type MLLD device 200 of the present invention includes a multielectrode semiconductor laser element 70 and a polarization rotation element 32 in an optical resonator, and is a natural number of times required for light to make one round of the optical resonator. The resonator length of the optical resonator and the magnitude of the optical gain in the gain region are set so that the optical pulse train having a repetition period of 1 time interval is output to the outside of the optical resonator. This optical resonator is composed of an end face 28 on the side where external input light of the multi-electrode semiconductor laser element 70 is input, and a half mirror 34. That is, the resonator length of this optical resonator is the distance L between the end face 28 and the semi-transparent mirror 34.

  The reason why the MLLD device 200 is of the external resonator type is that one end constituting the optical resonator is other than the end face of the multi-electrode semiconductor laser element 70, that is, the half mirror 34. Incidentally, the internal resonator type multi-electrode semiconductor laser element means a case where both ends constituting the optical resonator are both end faces of the multi-electrode semiconductor laser element. In the multi-electrode semiconductor laser device 70 shown in FIG. 1, the end surfaces refer to the end surface 28 and the end surface 30.

  Since the end face 30 of the multielectrode semiconductor laser element 70 is an interface inside the external resonator, it is necessary to prevent reflection from occurring at this interface. Therefore, the end face 30 is subjected to a non-reflective coating process. On the other hand, the end face 28 as one end constituting the optical resonator may remain a cleaved face, but it is preferable to form an end face protective film.

  The external resonator type mode-locked semiconductor laser can easily change its resonator length by shifting the position of the semi-transparent mirror 34 constituting one end of the optical resonator. On the other hand, in the multi-electrode semiconductor laser element of the internal resonator type, since both ends constituting the optical resonator are both end faces of the multi-electrode semiconductor laser element, this interval cannot be geometrically changed. The adjustment of the resonator length of the internal resonator type multi-electrode semiconductor laser device is performed by providing an optical path length adjusting portion in a part of the optical waveguide provided in the multi-electrode semiconductor laser device, and the effective refraction of the optical waveguide of this optical path length adjusting portion. This is realized by changing the resonator length effectively by changing the rate by a method such as current injection. In any case, with a simple method of adjusting the position of the semi-transparent mirror, such as an external resonator type mode-locked semiconductor laser, it is possible to adjust the resonator length of the internal resonator type multi-electrode semiconductor laser device. Can not.

  The multi-electrode semiconductor laser device 70 includes a gain region 20 in which an inversion distribution is formed and a saturable absorption region 10 having a function of modulating light intensity, and the gain region 20 and the saturable absorption region 10 are connected in series. This is a multi-electrode semiconductor laser device arranged. The gain region 20 and the saturable absorption region 10 are monolithically formed as shown in FIG. A current is injected into the gain region 20 by the constant current source 36 via the n-side common electrode 26 and the p-side electrode 22 in the gain region. A reverse bias voltage is applied to the saturable absorption region 10 by the constant voltage source 38 via the n-side common electrode 26 and the p-side electrode 12 of the saturable absorption region 10.

  An optical waveguide 24 (hereinafter sometimes referred to as “optical waveguide 24 in the gain region 20”) existing in the gain region 20 and an optical waveguide 14 (hereinafter referred to as “optical waveguide in the saturable absorption region 10”). The waveguide 14 ”is sometimes sandwiched between the common first cladding layer 18 and the second cladding layer 16. Here, the first cladding layer 18 is an n-type cladding layer, and the second cladding layer 16 is a p-type cladding layer. The crystal materials constituting the optical waveguide 14 and the optical waveguide 24 are determined by the wavelength of the external input light input to the multi-electrode semiconductor laser element 70. For example, if the wavelength of this external input light is in the 1.5 μm band, a quantum well structure made of InP semiconductor bulk crystal material or InP semiconductor crystal material is used.

  The cross-sectional shape (not shown) in the direction perpendicular to the waveguide direction of the optical waveguide 14 and the optical waveguide 24 is w in the direction parallel to the pn junction direction and perpendicular to the pn junction direction. It is a rectangular optical waveguide having a thickness in the direction d. Here, in general, w> d.

  The state of polarization of external input light is defined as follows. That is, when the direction of the polarization plane of the light guided through the optical waveguide 14 and the optical waveguide 24 (vibration plane including the electric field vector of the light) is parallel to the width direction of these optical waveguides, TE (Transverse Electric wave) polarized light, and TM (Transverse Magnetic Wave) polarized light when the direction is parallel to the thickness direction. Hereinafter, the propagation mode propagating with TE polarization is referred to as TE mode, and the propagation mode propagating with TM polarization is sometimes referred to as TM mode. The polarization direction means the vibration direction of the electric field vector of light.

  Here, by forming the optical waveguide 24 in the gain region 20 with a quantum well structure into which a bulk crystal or an extension strain is introduced, an optical clock signal extraction operation can be satisfactorily realized as will be described later.

  The polarization rotator 32 has a function of rotating the plane of polarization by 90 ° when linearly polarized light makes a round trip through the polarization rotator, and a quarter wave plate or a Faraday rotator can be used. is there.

  The passive mode-locked semiconductor laser device 200 shown in FIG. 1 has an optical resonator as a Fabry-Perot type resonator structure, but may be configured as a ring-type optical resonator. It is not excluded to configure the optical resonator of the laser device as a ring type. Even when a ring-type optical resonator structure is adopted, if the multi-electrode semiconductor laser element 70 and the polarization rotation element 32 are provided in the ring-type optical resonator, the configuration is the same as that of a ring-type optical resonator. The effect is obtained.

<First optical clock signal extraction device>
With reference to FIG. 2, the configuration of the first optical clock signal extraction device of the present invention will be described. FIG. 2 is a schematic configuration diagram of the first optical clock signal extraction device. The first optical clock signal extraction device is an optical pulse signal extraction device including the MLLD device 200 of the present invention, an input unit 80, and an output unit 90. The input unit 80 causes the input optical signal 79 to be input to the MLLD device 200. On the other hand, the output unit 90 outputs an optical pulse train generated by the passive mode-locked semiconductor laser device from the input optical signal as the optical clock signal 87.

  The input unit includes a first optical isolator 82 for preventing the return light from the optical clock signal extraction device from being input to an input side optical transmission line that transmits the input optical signal, and an input optical signal 79 for inputting the MLLD device 200. A first coupling optical system 84.

  The input optical signal 79 that has propagated through the input side optical transmission line (not shown) is input to the MLLD device 200 via the first optical isolator 82 and the first coupling optical system 84. The first optical isolator 82 transmits light in the direction from the input side optical transmission line to the MLLD device 200, but blocks light traveling in the direction from the MLLD device 200 in the opposite direction to the input side optical transmission line. There is a nature to do. Accordingly, in the first optical isolator 82, a part of the input optical signal 79 input to the MLLD device 200 is reflected by the end face 28 on the side where the external input light of the multi-electrode semiconductor laser element 70 is input, and is input again. It plays the role which prevents returning to the side optical transmission line.

  The first coupling optical system 84 is configured to input the input optical signal 79 output from the first optical isolator 82 to the optical waveguide 14 in the saturable absorption region 10, and the incident end of the optical waveguide 14 (end surface of the optical waveguide 14). It plays a role of converging the input optical signal 79 at a position where it intersects 28).

  The input unit 80 is not limited to the example shown in FIG. 2, and can be appropriately configured as an optical system that plays the same role as the roles played by the first optical isolator 82 and the first coupling optical system 84.

  The output unit 90 has a second optical isolator 92 for preventing the return light from the output-side optical transmission line that transmits the optical clock signal to the MLLD device 200 and the optical clock signal 87 to be input to the output-side optical transmission line. Second coupling optical systems 96b and 96c. The output unit 90 also splits the optical clock signal 87 into two components whose polarization directions are orthogonal to each other, and outputs a polarization separator 42, and a wavelength filter 94b that filters the optical clock signal 87 extracted by the MLLD device 200. And with 94c.

  The optical clock signal 87 output from the MLLD device 200 is separated into two optical clock signals 91b and 91c whose polarization planes are orthogonal to each other by the polarization separator 42 via the second optical isolator 92 provided in the output unit 90. . That is, the optical clock signal 87 that has passed through the second optical isolator 92 is input from the input port a of the polarization separator 42, is polarized and separated, and the optical clock signals 91b and 91c are output from the output ports b and c, respectively. .

  The polarization directions of the optical clock signal 91b and the optical clock signal 91c are orthogonal to each other. For example, setting the polarization separator 42 so that the optical clock signal 91b is TE-polarized and the optical clock signal 91c is TM-polarized is that the incident surface of the optical clock signal 87 on the reflective surface of the polarization separator 42 This can be done easily by adjusting the surface direction.

  The optical clock signal 91b output from the output port b is transmitted through the wavelength filter 94b to remove the wavelength component of the input optical signal, and is collected by the second coupling optical system 96b and output as the optical clock signal 91b ′. It is input to the input end of the side optical transmission line (not shown). On the other hand, the optical clock signal 91c output from the output port c is transmitted through the wavelength filter 94c to remove the wavelength component of the input optical signal, and is collected by the second coupling optical system 96c, and the optical clock signal 91c ′ Is input to the input end of the output side optical transmission line (not shown).

  The output unit 90 is not limited to the example shown in FIG. 2, and the arrangement order of the wavelength filters 94b and 94c and the second coupling optical systems 96b and 96c can be reversed. It is also possible to use one wavelength filter instead of two. In this case, a wavelength filter may be inserted between the second optical isolator 92 and the input port a of the polarization separator. Further, the wavelength filters 94b and 94c may be installed in close contact with the output ports b and c of the polarization separator 42, respectively.

<Second optical clock signal extraction device>
With reference to FIG. 3, the configuration of the second optical clock signal extraction device of the present invention will be described. FIG. 3 is a schematic configuration diagram of the second optical clock signal extraction device. The second optical clock signal extraction device is an optical clock signal extraction device including the MLLD device 200 and the input / output unit 100 of the present invention. Although described in detail later, the input / output unit 100 inputs the input optical signal 79 to the MLLD device 200 and outputs an optical pulse train generated by the MLLD device 200 from the input optical signal 79 as the optical clock signal 57.

  In the second optical clock signal extraction device, there is no need to output light at all on the opposite side of the input / output unit 100, so that the first optical clock signal extraction device is provided to constitute one end of the optical resonator. Instead of the semi-transparent mirror 34, a reflecting mirror 54 is provided.

  The input / output unit 100 includes an optical circulator 40 and a coupling optical system 110. The optical circulator 40 includes a first port a for inputting the input optical signal 79, a second port b for outputting the input optical signal 79 and inputting the optical clock signal 57 extracted by the MLLD device 200, A third port c for outputting the clock signal 111 is provided. Further, the coupling optical system 110 inputs the input optical signal 109 output from the second port b into the MLLD device 200 as the input optical signal 55 via the coupling optical system 110, and the light extracted by the MLLD device 200. The clock signal 57 is input to the second port b.

  Also, the input / output unit 100 separates the optical clock signal 111 output from the MLLD device 200 and output from the third port via the optical circulator 40 into two components whose polarization directions are orthogonal to each other, and outputs the polarization component. The container 42 is provided.

  An input optical signal 79 that has propagated through an input side optical transmission path (not shown) is input from the first port a of the optical circulator 40 and output from the second port b as the input optical signal 109. The coupling optical system 110 receives the input light signal 109 at the input / output end of the optical waveguide 14 (the position where the end face 28 of the optical waveguide 14 intersects) in order to input the input optical signal 109 to the optical waveguide 14 in the saturable absorption region 10. It serves to collect the signal 109.

  The optical clock signal 57 output from the input / output end of the optical waveguide 14 of the MLLD device 200 is input to the second port b of the optical circulator 40 via the coupling optical system 110 and is output from the third port c. 111 is input to the input port a of the polarization separator 42. The optical clock signal 111 input from the input port a of the polarization separator 42 is separated by the polarization separator 42 into two optical clock signals 113b and 113c whose polarization planes are orthogonal to each other. That is, the optical clock signal 111 is input from the input port a of the polarization separator 42, subjected to polarization separation, and output as optical clock signals 113b and 113c from the output ports b and c, respectively.

  The optical clock signal 113b output from the output port b is transmitted through the wavelength filter 94b to remove the wavelength component of the input optical signal, and is collected by the second coupling optical system 96b and output as the optical clock signal 113b ′. It is input to the input end of the side optical transmission line (not shown). On the other hand, the optical clock signal 113c output from the output port c is transmitted through the wavelength filter 94c to remove the wavelength component of the input optical signal, and is collected by the second coupling optical system 96c to be optical clock signal 113c ′. Is input to the input end of the output side optical transmission line (not shown). The polarization directions of the optical clock signal 113b and the optical clock signal 113c are orthogonal to each other. For example, setting the polarization separator 42 so that the optical clock signal 113b is TE-polarized and the optical clock signal 113c is TM-polarized is that the incident surface of the optical clock signal 111 with respect to the reflective surface of the polarization separator 42 This can be done easily by adjusting the surface direction.

  The portion constituted by the polarization separator 42, the wavelength filters 94b and 94c, and the second coupling optical systems 96b and 96c has the same configuration as the corresponding portion of the output unit 90 of the first optical clock signal extraction device.

<Operation principle of MLLD device>
With reference to FIGS. 4A to 4C, the operation principle of the first and second optical clock signal extraction devices will be described. 4A shows the time waveform of the input optical signal, and FIGS. 4B and 4C show the time waveform of the optical clock signal. The horizontal axis is the time axis and is scaled in units of the reciprocal of the bit rate frequency f _bit-rate (bit / s) of the input optical signal. That is, one scale on the horizontal axis is time slot per _bit 1 / f _bit-rate (s) = T _br (s). Hereinafter, f _bit-rate (bit / s) may be simply expressed as f _br (bit / s) or simply as f _br without a unit within a range where no misunderstanding occurs. The time slot T _br (s) may be simply expressed as T _br without the unit.

  4 (A) to 4 (C) are perspective views of the input optical signal and the optical clock signal with the TE polarization component and the TM polarization component in the direction orthogonal to the horizontal axis (time axis). It is shown in the form. The TE polarization component is shown perpendicular to the horizontal axis and the TM component is shown obliquely to the horizontal axis.

  Since the input optical signal 79 of the first and second optical clock signal extraction devices is an optical signal including both TE and TM polarized components, the time waveform of the input optical signal 79 shown in FIG. Includes both TM polarized components. Since the input optical signal 79 is an RZ-encoded binary digital optical pulse signal, there is a time slot in which there is no optical pulse reflecting the content of the signal. That is, assuming that “0” is assigned to a time slot in which no optical pulse exists and “1” is assigned to a time slot in which an optical pulse exists, the time waveform shown in FIG. 4 (A) is (1, 1, 1, 0). , 1, 0, 0, 1).

  In the case of the first optical clock signal extraction device, if the polarization separator 42 is set so that the optical clock signal 91b is in the TE mode and the optical clock signal 91c is in the TM mode, FIGS. ) Show time waveforms of the optical clock signal 91b and the optical clock signal 91c, respectively. Further, in the case of the second optical clock signal extraction device, if the polarization separator 42 is set so that the optical clock signal 113b is in the TE mode and the optical clock signal 113c is in the TM mode, FIG. (C) shows the time waveforms of the optical clock signal 113b and the optical clock signal 113c, respectively.

FIGS. 4B and 4C show an optical pulse train having a repetition frequency of f _br (time slot T _br ). That is, since FIGS. 4B and 4C are the time waveforms of the optical clock signals 91b and 113b and the optical clock signals 91c and 113c extracted from the input optical signal 79, respectively, the first and second optical clock signals It is a figure explaining that an extraction apparatus is an apparatus which produces | generates an optical pulse train as an optical clock signal.

  An optical clock signal extraction device using an external resonator type MLLD configured by inserting a quarter wavelength plate into an external resonator is disclosed in a patent document (Japanese Patent Laid-Open No. 6-088981). is there. However, the optical clock signal extraction device disclosed in this patent document is fundamentally different from the first and second optical clock signal extraction devices of the present invention as described later. Since the first and second optical clock signal extraction devices of the present invention have this difference, the optical clock signal is extracted in a stable state without depending on the polarization direction of the input optical signal. The effect that it is possible to obtain is obtained.

  First, in order to contribute to an understanding of the operating principles of the passive mode-locked semiconductor laser device of the present invention and the first and second optical clock signal extraction devices in the following, with reference to FIGS. The prior art disclosed in the Kaihei 6-088981 will be described.

  FIG. 5A is a diagram for explaining how the propagation mode (TE mode or TM mode) changes when light goes around the optical resonator. The optical resonator includes a multi-electrode semiconductor laser element 70 and a quarter-wave plate 32, and both end surfaces constituting the optical resonator are a saturable absorption region side end surface 28 and a semi-transparent mirror 34.

  FIG. 5B is a diagram for explaining the polarization direction of light. In the following description, as shown in FIG.5 (B), the oscillation direction of the electric field vector of light, that is, the case where the polarization direction is a direction perpendicular to the paper surface is referred to as TE polarization, and the polarization direction is included in the paper surface and The case where the direction is perpendicular to the optical axis direction of the optical resonator is defined as TM polarized light.

  FIG. 5C is a diagram for explaining how to set the direction of the optical axis of the quarter-wave plate 32. In FIG. 5C, the directions of the two optical axes of the quarter-wave plate 32 are indicated by broken line segments with arrows at both ends. If the direction of one of these optical axes is set to make an angle of 45 ° with the vibration direction of the electric field vector of TE-polarized light and TM-polarized light, when this quarter-wave plate is reciprocated once, the electric field of light The vector vibration direction is rotated 90 °. In the following description, it is assumed that the quarter-wave plate (polarization plane rotating element) 32 is installed in the optical resonator in this way.

In describing the operation of the conventional optical clock signal extraction device, first, the operation in a state where no input optical signal is input to the optical resonator will be described. A time required for one optical pulse to make one round of the optical resonator is _defined as T _rt . When one optical pulse first passes through one arbitrary point P in the optical resonator in the TE mode to the right, it is input to the 1/4 wavelength plate 32 in the TE mode, converted into circularly polarized light, and output. . The light pulse converted to circularly polarized light is reflected by the semi-transparent mirror 34 (or the reflecting mirror 54) and input again from the right end face of the quarter-wave plate 32 in the state of circularly polarized light. At this time, a linearly polarized light pulse is output from the left end face of the quarter-wave plate 32. The direction of oscillation of the electric field vector of this linearly polarized light pulse is rotated by 90 ° when converted to the 1/4 wavelength plate 32 from the left end face in the TE mode, and is converted to the TM mode.

  The same effect as that of the quarter-wave plate 32 described above can also be obtained by using a Faraday rotator. By adjusting the thickness of the optical crystal composing the Faraday rotator and the strength of the magnetic field applied to this optical crystal, the oscillation direction of the linearly polarized electric field vector is rotated by 90 ° when the Faraday rotator is reciprocated once. Can be set.

As described above, when an arbitrary point P in the optical resonator is first passed in the TE mode from the left to the right on the paper surface of FIG. When passing in the left direction, it is TM mode. That is, since the propagation mode of the optical pulse that circulates in the optical resonator every time T _rt elapses is converted from the TE mode to the TM mode, or from the TM mode to the TE mode, the TE is transmitted at intervals of the time T _rt. The mode and TM mode are output from the optical resonator alternately.

  Therefore, the oscillation threshold value of the external resonator type passively mode-locked semiconductor laser device is given by the condition that the gain and the loss are balanced when the optical pulse makes two rounds of the optical resonator. That is, since a gain equal to the absolute value of this loss is a threshold gain, it is one condition of laser oscillation (hereinafter, referred to as a gain above this threshold gain formed in the gain region 20 of the multi-electrode semiconductor laser device 70). It may also be called “amplitude condition”. Another condition for laser oscillation (hereinafter referred to as “phase”) is that the phase change when the optical pulse rotates through the optical resonator twice while rotating the polarization direction by 90 ° for each rotation is an integer multiple of 2π. It may be called "condition".)

  These amplitude conditions and phase conditions are the same regardless of whether the propagation mode when the optical pulse first passes through the point P is the TE mode or the TM mode. Therefore, the external resonator type passively mode-locked semiconductor laser device oscillates in both the TE mode and the TM mode in the propagation mode, with the same wavelength and with the same intensity.

With reference to FIG. 6, the time change of the polarization state of the optical pulse output from the external resonator type passively mode-locked semiconductor laser device operating as described above will be described. FIG. 6 is a diagram showing a time waveform of an output light pulse of the external resonator type passively mode-locked semiconductor laser device. The horizontal axis in FIG. 6 is the time axis, and is scaled in units of time T _rt required for the optical pulse to make one round in the optical resonator. FIG. 6 shows the intensities of the TE-polarized component and the TM-polarized component of the optical pulse train in the form of a perspective view in the direction orthogonal to the horizontal axis (time axis). The TE polarization component is shown perpendicular to the horizontal axis and the TM component is shown obliquely to the horizontal axis.

FIG. 6 shows a state in which the TE mode and the TM mode are alternately output from the external resonator type passive mode-locked semiconductor laser device at the time T _rt as described above. Therefore, the TE mode optical pulse is output at a cycle of 2T _rt , and the TM mode optical pulse is output at a cycle of 2T _rt . That is, the self-excited oscillation frequency for the TE mode and TM mode optical pulses of self-excited oscillation, which is an oscillation operation when no input optical signal is input to the optical resonator, is f _rt / 2. Here, since f _rt is a frequency, it is the reciprocal of the time interval T _rt of optical pulse expression, that is, f _rt = 1 / T _rt . The TE mode optical pulse and the TM mode optical pulse are output from the external resonator type passive mode-locked semiconductor laser device with a relative time difference of time T _rt .

  The time change of the polarization state of the TE mode optical pulse and the TM mode optical pulse is the same also in the optical resonator of the external resonator type passive mode-locked semiconductor laser device. That is, when a single point Q is set inside the optical resonator of the external resonator type passively mode-locked semiconductor laser device, and at this point Q, it is observed whether the propagation mode of the optical pulse is the TE mode or the TM mode. In addition, the state is the same as that shown in FIG.

Next, the case where an optical clock signal is extracted by inputting an input optical signal to the optical resonator of the external resonator type passively mode-locked semiconductor laser device will be described. Here, the frequency of the extracted optical clock signal is a value approximating the self-excited oscillation frequency of the external resonator type passively mode-locked semiconductor laser device, and the optical clock signal extraction operation is f _rt / _{2≈f bit-rate} It is executed under the condition of satisfying. 2T _rt = T _bit-rate and T _bit-rate = 1 / f _bit-rate . That is, twice the time _Trt required for one optical pulse to make one round of the optical resonator is equal to T _bit-rate which is the reciprocal of the bit rate frequency f _bit-rate of the input optical signal.

  7 (A1) to (C2), when the polarization direction of the input optical signal changes, the conventional external resonator type disclosed in the above-mentioned patent document (Japanese Patent Laid-Open No. 6-088981) The effect received by the optical clock signal output from the passive mode-locked semiconductor laser device will be described. This external resonator type passively mode-locked semiconductor laser device is a laser similar to the mode-locked semiconductor laser disclosed in Non-Patent Document 1, Non-Patent Document 3, and Japanese Patent Application Laid-Open No. 11-326974. That is, this external resonator type passively mode-locked semiconductor laser device has the same input as the mode-locked semiconductor laser disclosed in Non-Patent Document 1, Non-Patent Document 3, and Patent Document (Japanese Patent Laid-Open No. 6-088981). When the polarization direction of the optical signal matches the polarization direction of the laser oscillation light, it is possible to extract the clock signal stably.

FIGS. 7A1 to 7C2 are diagrams for explaining the polarization dependency of the conventional passive mode-locked semiconductor laser device. In FIG. 7 (A1) to (C2), the horizontal axis is the time axis and is scaled in units of time _Trt . FIGS. 7A1 to 7C2 show the intensity of the TE-polarized component and TM-polarized component of the optical pulse train in the form of a perspective view in the direction orthogonal to the horizontal axis (time axis). . The TE polarization component is shown perpendicular to the horizontal axis and the TM component is shown obliquely to the horizontal axis.

  7A1 shows the time waveform of the TE mode input optical signal, and FIG. 7A2 shows the time waveform of the optical clock signal extracted from the input optical signal having the time waveform shown in FIG. 7A1. 7B1 shows a time waveform of the TM mode input optical signal, and FIG. 7B2 shows a time waveform of the optical clock signal extracted from the input optical signal having the time waveform shown in FIG. 7B1. Fig. 7 (C1) shows the time waveform of the input optical signal including both the TE mode component and TM mode component, and Fig. 7 (C2) shows the light extracted from the input optical signal with the time waveform shown in Fig. 7 (C1). The time waveform of a clock signal is shown.

  In the following description, it is assumed that the polarization direction of the mode-locked oscillation light pulse of the mode-locked semiconductor laser device is the TE mode. Of course, even if the polarization direction of this mode-locked oscillation light pulse is the TM mode, the following explanation is based on replacing the TE mode with the TM mode and replacing the TM mode with the TE mode. The same holds true.

A case where an optical clock signal is extracted from an input optical signal in TE mode will be described with reference to FIGS. 7A1 and 7A2. When an input optical signal is input to the mode-locked semiconductor laser, the optical pulses that make up the input optical signal circulate around the optical resonator of the mode-locked semiconductor laser and optically change the optical absorption coefficient of the saturable absorption region multiple times. Modulate. Assuming that an optical pulse is input to the saturable absorption region at time 0, the optical pulse passes through the saturable absorption region in the TE mode at time 0, 2T _rt , 4T _rt , 6T _rt ,. Further, at time T _rt , 3T _rt , 5T _rt , 7T _rt ,..., This optical pulse passes through the saturable absorption region in the TM mode.

  The optical clock signal extraction operation is executed so that the polarization direction of the input optical signal matches the polarization direction of the oscillation light of the mode-locked light pulse of the mode-locked semiconductor laser. That is, the polarization direction of the extracted optical clock signal (one optical pulse constituting the optical clock signal) is output in the TE mode at time 0.

A case where an optical clock signal is extracted from an input optical signal in TM mode will be described with reference to FIGS. 7B1 and 7B2. In this case, if an optical pulse is input to the saturable absorption region at time 0, at time 0, 2T _rt , 4T _rt , 6T _rt , ..., this optical pulse passes through the saturable absorption region in TM mode. pass. Further, at times T _rt , 3T _rt , 5T _rt , 7T _rt ,..., This optical pulse passes through the saturable absorption region in the TE mode. Accordingly, the polarization direction of the extracted optical clock signal (one optical pulse constituting the optical clock signal) is output in the TM mode at time 0. That is, as shown in FIG. 7 (B2), the TE mode input optical signal is input at the moment when the TE mode optical pulse is output among the optical pulses constituting the extracted optical clock signal. Compared to the case, the output timing of (1/2) T _rt is shifted.

For the case of extracting an optical clock signal from an input optical signal in which a TE mode component and a TM mode component are mixed using a conventional passive mode-locked semiconductor laser device, refer to FIGS. 7 (C1) and (C2). explain. In this case, if an optical pulse is input to the saturable absorption region at time 0, since the TE mode component and TM mode component are mixed in this optical pulse, the time 0, 2T _rt , 4T _rt , 6T _At the time T _rt , ... and the times T _rt , 3T _rt , 5T _rt , 7T _rt , ..., the TE mode component and the TM mode component simultaneously pass through the saturable absorption region.

For this reason, among the optical pulses that make up the extracted optical clock signal, the moment when the TE mode optical pulse is output is output by τ compared to when the TE mode input optical signal is input. The timing is different. The optical clock signal extraction operation is executed so that the polarization direction of the input optical signal matches the polarization direction of the oscillation light of the mode-locked light pulse of the mode-locked semiconductor laser (here, the TE mode). Therefore, if the input optical signal does not include the TM mode component and only the TE mode component, the timing deviation amount τ is the smallest and zero. On the other hand, if the input optical signal does not include the TE mode component and only the TM mode component, the polarization direction of the input optical signal and the polarization direction of the oscillation light of the mode-locked optical pulse (here, the TE mode) are orthogonal However, it is the farthest polarization state. In this case, as shown in FIGS. 7 (B1) and (B2), the timing deviation amount τ is the largest (1/2) T _rt .

That is, this timing deviation amount τ approaches 0 when the TE mode component of the input optical signal is large, and approaches (1/2) T _rt as the TM mode component increases, and the TE mode component and the TM mode component are mixed. When the optical clock signal is extracted from the input optical signal, the timing deviation amount τ is a value in the range of 0 to (1/2) T _rt depending on the mixing ratio of the TE mode component and the TM mode component Take.

  As described above, when the optical clock signal is extracted using the conventional technique, the timing shift amount τ of the extracted optical clock signal varies according to the polarization direction of the input optical signal. The variation in the timing deviation amount τ causes trouble in the operation of the signal processing circuit or the like that uses the extracted optical clock signal. Therefore, it is necessary to compensate by some method so that τ = 0.

  Compensation for setting the timing deviation amount τ to 0 can be performed by using a variable optical delay device or the like. However, in order to optimally compensate by this variable optical delay device or the like (compensation so that τ = 0), Therefore, a means for detecting the mixing ratio of the TE mode component and the TM mode component of the input optical signal (hereinafter also referred to as “TE / TM mixing ratio detector”) is required. Furthermore, a control device for controlling the TE / TM mixing ratio detector, variable optical delay device, and the like is also required. In particular, when the bit rate of the input optical signal is high, high-speed operation is guaranteed to drive the variable optical delay device, the TE / TM mixing ratio detector, and the control device for controlling them. Requires electronic devices and optical devices.

  In order to extract an optical clock signal using the prior art, there is a problem that a complicated apparatus configuration is required as described above, and high cost and high power consumption are required. In contrast, according to the first and second optical clock signal extraction devices using the passively mode-locked semiconductor laser device of the present invention, as described below, after solving these problems, the input optical signal A high-speed optical clock signal that is independent of the polarization direction and exceeds the upper limit speed of the electronic device can be extracted. That is, the passive mode-locked semiconductor laser device of the present invention solves the above-mentioned problems by expressing even-order harmonic mode-locked operation, and extracts a high-speed optical clock signal independent of the polarization direction of the input optical signal. Can be done.

The even-order harmonic mode synchronization operation means that the repetition frequency f _ML of the optical pulse train generated as a result of the passive mode synchronization operation and the resonator circulation frequency f _rt satisfy the following equation (1).
f _ML = (2M) x (f _rt / 2) (1)
Here, M is an integer (natural number) of 1 or more, and 2M is called the order of harmonic mode synchronization.

Since Equation (1) can be written as f _ML = (2M) × (f _rt / 2) = M · f _rt , the time required for one round of light in the optical resonator is expressed as T _rt (= 1 / f _rt) and then, when the repetition period of the optical pulse train and _{T ML (= 1 / f ML} ), which means that the T _ML = T _rt / M. In other words, equation (1) indicates that an optical pulse train having a repetition period _TML with a time interval of 1 / (M) is a natural number of time T _rt required for one round of light to travel through the optical resonator. The condition that it is output outside the container is given.

  Harmonic mode-locked operation is known to be achieved by increasing the current injected into the gain region of a passively mode-locked laser (eg, S. Sanders, A. Yariv, J. Paslaski, JE Ungar, and HA Zarem "Passive mode locking of a two-section multiple quantum well laser at harmonics of the cavity round-trip frequency," Appl. Phys. Lett., vol. 58, No. 7 pp. 681-683, 1991. This document is hereinafter referred to as Non-Patent Document A).

  That is, in order to operate the passive mode-locked semiconductor laser device 200 of the present invention in the harmonic mode-locked operation, the current injected into the gain region 20 of the multi-electrode semiconductor laser device 70 is increased to increase the optical gain of the gain region 20. You just need to make it large enough. Therefore, in the passive mode-locked semiconductor laser device 200 of the present invention, it is necessary to set the magnitude of the optical gain of the gain region 20 of the multi-electrode semiconductor laser element 70. In addition, the resonator of this optical resonator is output so that an optical pulse train having a repetition period with a time interval of one-tenth of the natural number of times required for one round of light to travel inside the optical resonator is output to the outside of this optical resonator It is necessary to set the magnitude of the optical gain in the long and gain regions. The resonator length of this optical resonator is approximately equal to the reciprocal of the bit rate frequency of the input optical signal so that the time required for the light to make one round in the optical resonator is such that frequency pulling is caused. It is necessary to adjust so that it becomes.

  In setting the magnitude of the optical gain of the gain region, for example, while observing the optical pulse train output from the passive mode-locked semiconductor laser device 200 without inputting the input optical signal to the passive mode-locked semiconductor laser device 200, The current injected into the gain region 20 of the multi-electrode semiconductor laser device 70 is increased until the harmonic mode synchronization operation is realized. When the optical clock signal extraction device is used as a passive mode-locked semiconductor laser device, the multi-electrode semiconductor laser device 70 is driven with a current exceeding the injection current value being injected into the gain region 20. And use it.

  With reference to FIGS. 8A and 8B, the polarization state of the output light pulse when the passive mode-locked semiconductor laser device of the present invention is operated in an even-order harmonic mode-locked operation will be described. 8 (A) and 8 (B) are diagrams for explaining the polarization state of the output light pulse output from the passive mode-locked semiconductor laser device during the harmonic mode-locked operation, and FIG. Fig. 8 (B) shows the time waveform indicating the polarization state of the output light pulse during the third harmonic mode synchronization operation, and Fig. 8 (B) shows the time waveform indicating the polarization state of the output light pulse during the third harmonic mode synchronization operation. Show. The second harmonic mode synchronization corresponds to the case where 2M = 2 in Equation (1), that is, M = 1. The third-order harmonic mode synchronization is an odd-order harmonic mode synchronization operation described below.

The odd-order harmonic mode synchronization operation means that the repetition frequency f _ML of the optical pulse train generated as a result of the passive mode synchronization operation and the resonator circulation frequency f _rt satisfy the following equation (2).
f _ML = (2M-1) x (f _rt / 2) (2)
Here, M is an integer equal to or greater than 1, and (2M-1) is referred to as the order of harmonic mode synchronization. The third-order harmonic mode synchronization operation shown in FIG. 8B is a case where 2M−1 = 3, that is, M = 2.

As shown in FIG. 8 (A), the optical pulse train output by the second-order harmonic mode synchronization operation is such that both the TE mode optical pulse and the TM mode optical pulse are arranged on the time axis with a period T _rt. Yes. As described above, there is a time difference of time T _rt between the TE mode optical pulse and the TM mode optical pulse. As a result, as shown in FIG. 8A, the TE mode optical pulse and the TM mode optical pulse are arranged at the same timing. That is, as described with reference to FIGS. 7 (C1) and (C2), when the optical clock signal is extracted by the conventional optical clock signal extraction device, the TE mode optical pulse and the TM mode optical pulse Are alternately arranged on the time axis, whereas the optical pulse train output by the second-order harmonic mode synchronization operation is the same as the TE mode optical pulse on the time axis, as shown in FIG. The TM mode light pulses are arranged at the same position.

  In general, as shown in FIG. 8 (A), the optical pulse train output by even-order harmonic mode synchronization operation given by M ≧ 2 includes a TE mode optical pulse and a TM mode optical pulse on the time axis. Are arranged in the same position. That is, when the optical clock signal is extracted by the even-order harmonic mode synchronization operation, the TE mode optical pulse and the TM mode optical pulse constituting the optical clock signal are simultaneously generated and arranged at the same position on the time axis. .

On the other hand, the optical pulse train output by the third-order harmonic mode synchronization operation shown in FIG. 8 (B) has a time axis with a period (2/3) T _rt of both the TE mode optical pulse and the TM mode optical pulse. They are lined up. A relative time difference of time T _rt occurs between the TE mode optical pulse and the TM mode optical pulse. As a result, TE mode optical pulses and TM mode optical pulses are alternately arranged on the time axis at a time interval of (1/3) T _rt . In general, in an optical pulse train output by an odd-order harmonic mode synchronization operation given by M ≧ 2, a TE mode optical pulse and a TM mode optical pulse are alternately arranged on the time axis at regular time intervals. . That is, when an odd-order harmonic mode synchronization operation is performed, a TE mode optical pulse and a TM mode optical pulse are alternately generated at regular time intervals.

  Incidentally, the case where the optical clock signal is extracted by the conventional optical clock signal extraction device corresponds to the primary odd-order mode synchronization operation given by M = 1 in the equation (2). The primary odd-order mode synchronization operation is sometimes called basic mode synchronization.

  When the passive mode-locked semiconductor laser device is subjected to an odd-numbered harmonic mode-locking operation to extract an optical clock signal, the TE mode optical pulse and the TM mode optical pulse are not aligned at the same position on the time axis. For this reason, as described with reference to FIGS. 7A1 to 7C2, a timing shift occurs according to the polarization direction of the input optical signal. This is a practical problem as described above.

  On the other hand, when the passive mode-locked semiconductor laser device performs even-order harmonic mode-locked operation to extract the optical clock signal, the TE mode optical pulse and the TM mode optical pulse are the same on the time axis. Line up in the position.

  Referring to FIGS. 9 (A1) and (C2), with the passive mode-locked semiconductor laser device of the present invention, when the optical clock signal is extracted by the even-order harmonic mode-locked operation, the TE mode component An optical clock signal extracted from an input optical signal in which TM mode components are mixed will be described. FIGS. 9 (A1) and (C2) are diagrams for explaining the polarization dependence of the passive mode-locked semiconductor laser device of the present invention, and the same drawings as FIGS. 7 (A1) to (C2) are shown in FIG. It shows a case where an optical clock signal is extracted using a passive mode-locked semiconductor laser device.

  As shown in FIGS. 9 (A1) and (C2), regardless of how the polarization direction of the input optical signal fluctuates, the passive mode-locked semiconductor laser device always generates TE mode optical pulses and TM mode optical pulses. Are output at the same time. For this reason, even if the polarization direction of the input optical signal fluctuates, the optical clock signal can be extracted with the phase of the optical clock signal unchanged. That is, by using the passive mode-locked semiconductor laser device of the present invention, the optical clock signal can be extracted independently of the polarization direction of the input optical signal.

  Further, according to the optical clock signal extraction device of the present invention, since the external resonator type passive mode-locked semiconductor laser device of the present invention is used, it is variable in the optical resonator of the passive mode-locked semiconductor laser device. By inserting an element (not shown) that can change the optical path length, such as an optical delay device, the resonator length of this optical resonator can be effectively changed, and the repetition frequency of the optical clock signal can be changed. Is possible. Furthermore, it is possible to change the wavelength of the extracted optical clock signal by inserting a wavelength filter (not shown) having a variable center transmission wavelength into the optical resonator.

<Saturable absorption region>
The optical waveguide 14 in the saturable absorption region 10 of the multi-electrode semiconductor laser device 70 constituting the passive mode-locked semiconductor laser device of the present invention is a quantum well structure having a quantum well in which a bulk crystal or an extension strain is introduced (hereinafter, (It may also be referred to as “stretched strain quantum well structure”), and the effect that the above-described optical clock signal can be extracted in a state in which the phase remains unchanged will be further ensured. That is, by explaining that the optical waveguide 14 has a bulk crystal or an extended strain quantum well structure, it is possible to further easily realize even-order higher-order mode-locking operation for the multielectrode semiconductor laser element 70. To do.

  As described above, in order to realize an even-order higher-order mode-locking operation in the external-cavity-type passive mode-locked semiconductor laser device 200, the multi-electrode semiconductor laser element 70 constituting the passive-mode-locked semiconductor laser device 200 is used. The current injected into the gain region 20 may be increased. However, it is generally known that there are a plurality of harmonic mode-locking orders including odd and even orders that can exist stably with a constant optical gain in the gain region ( For example, see FIG. 4 and FIG. 5 of Non-Patent Document A described above).

  Therefore, even if the current injected into the gain region 20 is constant, the external resonator type passive mode-locked semiconductor laser device 200 can realize an odd-order or even-order harmonic mode-locking operation. Unless special measures are taken, there is no general factor that can actively limit the order of harmonic mode-locked operation, and it is affected by external disturbances such as input optical signals and temperature changes of the multi-electrode semiconductor laser device 70. The mode-locking operation of the passive mode-locking semiconductor laser device can vary. That is, the odd-order harmonic mode synchronization operation changes to the even-order harmonic mode synchronization operation, and vice versa, the even-order harmonic mode synchronization operation changes to the odd-order harmonic mode synchronization operation. .

  Therefore, in order to realize even-order harmonic mode synchronization operation stably even under the influence of the external disturbance described above, it is necessary to take some new means.

Before describing this new means, the structure and operation of the saturable absorption region 10 will be described with reference to FIGS. 10 (A1) to (B3). 10 (A1) to (B3) are diagrams for explaining the operation of the saturable absorption region, and FIGS. 10 (A1) to (A3) are optical pulses in which the absorption saturation in the saturable absorption region is TE mode. FIGS. 10 (B1) to (B3) show the case where the absorption saturation in the saturable absorption region occurs with an optical pulse in which the TE mode and the TM mode are mixed. In each of FIGS. 10 (A1) to 10 (B3), the horizontal axis is the time axis and is scaled in units of time _Trt .

10 (A1) and (B1) show the optical pulse train passing through the saturable absorption region, and FIGS. 10 (A3) and (B3) show the TE polarization component and TM of the optical pulse train output from the passive mode semiconductor laser device. The intensity of the polarization component is shown in the form of a perspective view for each direction perpendicular to the horizontal axis (time axis). The TE polarization component is shown perpendicular to the horizontal axis and the TM component is shown obliquely to the horizontal axis. FIGS. 10A2 and 10B2 are diagrams showing changes in the light absorption coefficient of the saturable absorption region, and are scaled in units of time _Trt with the horizontal axis as the time axis. Further, the vertical axis shows the magnitude of the light absorption coefficient on an arbitrary scale, which means that the light absorption coefficient decreases as the distance from the time axis decreases. That is, it means that the transparency of the saturable absorption region increases as the distance from the time axis decreases.

Here, a description will be given assuming that a primary (odd order) basic mode synchronization operation has occurred. FIG. 10 (A1) shows the time waveform of the mode-synchronized optical pulse passing through the saturable absorption region for each polarization direction (TE or TM mode). The mode-synchronized optical pulse that circulates in the external resonator of the passive mode-locked semiconductor laser device passes through the saturable absorption region with the time interval T _rt alternating between the TE mode and the TM mode.

If the saturation due to the absorption saturation of the saturable absorption region occurs only in the TE mode, the transparency due to the absorption saturation of the saturable absorption region occurs when the light pulse passes through the saturable absorption region in the TE mode. Only happens. Accordingly, as shown in FIG. 10 (A2), the saturable absorption region becomes transparent at the time interval 2T _rt . Therefore, as shown in FIG. 10 (A3), both TE mode and TM mode optical pulses are generated with a period of 2T _rt , and they are alternately generated with a relative time difference of T _rt .

On the other hand, if the saturation due to the absorption saturation of the saturable absorption region occurs in both the TE mode and the TM mode, the transparency due to the absorption saturation of the saturable absorption region is caused by changing the saturable absorption region to the TE mode. Or it occurs even if the light pulse of any polarization of TM mode passes. Therefore, in this case, as shown in FIG. 10 (B2), the saturable absorption region becomes transparent at the time interval _Trt . Although transparency due to absorption saturation of the saturable absorption region occurs at the time interval T _rt , for example, the TE mode light pulse is generated at the time interval 2T _rt as shown in FIG. 10 (B1) It is an unnatural and contradictory phenomenon.

This contradiction is due to the interpretation that the generated mode-locked optical pulse is an optical pulse generated by the primary (odd-order) basic mode-locking operation as shown in FIG. 10 (B1). . In this case, the generated mode-synchronized optical pulse train is inconsistent by interpreting that both the TE mode and TM mode optical pulses are generated at the time interval T _rt as shown in FIG. 10 (B3). It will be resolved. This state is a state in which the passive mode-locked semiconductor laser is operating in the second harmonic mode-locked manner.

  More generally, even in the higher-order odd-order mode-locked operation as shown in FIG. 8B, when the saturation due to the absorption saturation in the saturable absorption region occurs in both the TE mode and the TM mode. The same contradiction occurs as described above. It is more consistent to interpret the generated mode-synchronized optical pulse that both the TE mode and TM mode optical pulses are generated at the same time in the same period. That is, this state is a state in which the passive mode-locked semiconductor laser is operating in a higher-order harmonic mode-locked operation.

  As described above, when transparency due to absorption saturation in the saturable absorption region occurs in both the TE mode and the TM mode, the external resonator type passive mode-locked semiconductor laser device inevitably has an even order. Performs harmonic mode synchronization. Therefore, when transparency due to absorption saturation in the saturable absorption region occurs in both the TE mode and the TM mode, if the optical clock signal is extracted by this passive mode-locked semiconductor laser device, the extracted optical clock signal is The phase can be extracted with no change.

  Therefore, if the transparency due to absorption saturation in the saturable absorption region can occur in both the TE mode and the TM mode, the passive mode-locked semiconductor laser device can realize even-order higher-order mode-locking operation even more. Can be easier. In the passive mode-locked semiconductor laser device 200 of the present invention, in order to make the saturation of the saturable absorption region 10 transparent due to the absorption saturation, the optical waveguide 14 in the saturable absorption region 10 is made of a bulk crystal. Alternatively, it can be realized by an extension strain quantum well structure. It is already known that if the optical waveguide 14 in the saturable absorption region 10 has a bulk crystal or an extended strain quantum well structure, the optical waveguide 14 can be made transparent by absorption saturation without depending on the polarization direction. (For example, F. Devaux, S. Chelles, A. Ougazzaden, A. Mircea, F. Huet, and M. Carre, “10 Gbit / s operation of polarisation insensitive, strained InGaAsP / InGaAsP MQW electroabsorption modulator,” Electron. Lett , vol. 29, No. 13, pp. 1201-1203, 1993).

  In order to realize even-order harmonic mode synchronization operation even under the influence of the external disturbance described above, the transparency due to the absorption saturation of the saturable absorption region is achieved in both the TE mode and the TM mode. A new means for making this happen is that the active layer, which is a component of the saturable absorption region, has a bulk semiconductor crystal layer or an extended strain quantum well structure. In the present invention, the active layer that is a component of the saturable absorption region refers to the optical waveguide 14 in the saturable absorption region 10. By this new means, even-order harmonic mode synchronization operation can be realized more reliably. Therefore, the saturable absorption region having the above-described configuration contributes to further stabilization of the optical clock signal extraction operation.

It is a schematic block diagram of a passive mode-locking semiconductor laser device. FIG. 2 is a schematic configuration diagram of a first optical clock signal extraction device. FIG. 3 is a schematic configuration diagram of a second optical clock signal extraction device. It is a figure which shows the time waveform of an input optical signal and an optical clock signal. It is a figure where it uses for description of passive mode synchronous operation | movement. It is a figure which shows the time waveform of the output optical pulse of a passive mode synchronous semiconductor laser apparatus. It is a figure where it uses for description of the polarization dependence of the conventional passive mode-locking semiconductor laser apparatus. It is a figure with which it uses for description of the polarization state of the output optical pulse output from the passive mode-locking semiconductor laser apparatus at the time of harmonic mode locking operation. It is a figure where it uses for description of the polarization dependence of the passive mode-locking semiconductor laser apparatus of this invention. It is a figure where it uses for description of operation | movement of a saturable absorption area | region.

Explanation of symbols

10: Saturable absorption region
12: p-side electrode in saturable absorption region
14: Optical waveguide in saturable absorption region
16: p-type cladding layer
18: n-type cladding layer
20: Gain region
22: p-side electrode in gain region
24: Optical waveguide in the gain region
26: n-side common electrode
28, 30: End face
32: Polarization rotation element
34: Semi-transparent mirror
36: Constant current source
38: Constant voltage source
40: Optical circulator
42: Polarization separator
54: Reflector
70: Multi-electrode semiconductor laser device
80: Input section
82: First optical isolator
84: First coupling optics
90: Output section
92: Second optical isolator
94b, 94c: Wavelength filter
96b, 96c: Second coupling optics
100: Input / output section
110: Coupled optics
200: Passive mode-locked semiconductor laser device

Claims (15)

An external cavity type passively mode-locked semiconductor laser device,
A multi-electrode semiconductor laser device in which a gain region in which an inversion distribution is formed and a saturable absorption region having a function of modulating light intensity are arranged in series;
The linearly polarized light is provided in the optical resonator with the polarization rotating element that rotates the polarization plane of the linearly polarized light by 90 ° when reciprocating once through the polarization rotating element;
The resonator length of the optical resonator is such that an optical pulse train having a repetition period of a time interval that is one-tenth of the natural number of times required for one round of light to travel through the optical resonator is output to the outside of the optical resonator. And a passive mode-locked semiconductor laser device, wherein the magnitude of the optical gain in the gain region is set.   2. The passive mode-locked semiconductor laser device according to claim 1, wherein the active layer provided in the saturable absorption region is formed of a bulk semiconductor crystal layer.   2. The passive mode-locked semiconductor laser device according to claim 1, wherein the active layer provided in the saturable absorption region is formed with a quantum well structure having a quantum well into which an extension strain is introduced.   4. The passively mode-locked semiconductor laser device according to claim 1, wherein the polarization rotation element is a ¼ wavelength plate.   4. The passively mode-locked semiconductor laser device according to claim 1, wherein the polarization rotator is a Faraday rotator. The passively mode-locked semiconductor laser device according to any one of claims 1 to 5,
An input unit for inputting an input optical signal to the passive mode-locked semiconductor laser device;
An output unit for outputting an optical pulse train generated by the passive mode-locked semiconductor laser device from the input optical signal as an optical clock signal;
The difference between the repetition frequency of the optical pulse train generated by the passive mode-locked semiconductor laser device and the frequency corresponding to the bit rate of the input optical signal is determined by the passive mode-locked semiconductor laser device. An optical clock signal extraction apparatus characterized in that an input optical signal having a bit rate small enough to cause a frequency pull-in phenomenon necessary for operation is generated.   7. The optical clock signal extraction device according to claim 6, wherein the output unit includes a polarization separator that separates and outputs the optical clock signal into two components whose polarization directions are orthogonal to each other.   A first optical isolator for preventing the return light from the optical clock signal extraction device from being input to an input-side optical transmission line through which the input optical signal is transmitted, and the input mode optical signal to the passive mode-locked semiconductor laser. 8. The optical clock signal extraction device according to claim 6, further comprising a first coupling optical system for inputting to the device.   A second optical isolator for preventing the return light from the output-side optical transmission line for transmitting the optical clock signal to the passive mode-locked semiconductor laser device, and the output-side optical transmission of the optical clock signal; 9. The optical clock signal extraction device according to claim 6, further comprising a second coupling optical system for inputting to the path.   10. The optical clock signal extraction according to claim 6, wherein the output unit includes a wavelength filter that filters the optical clock signal extracted by the passive mode-locking semiconductor laser device. apparatus. The passively mode-locked semiconductor laser device according to any one of claims 1 to 5,
An input / output unit for inputting an input optical signal and outputting the optical pulse train generated by the passive mode-locking semiconductor laser device from the input optical signal as an optical clock signal;
The difference between the repetition frequency of the optical pulse train generated by the passive mode-locked semiconductor laser device and the frequency corresponding to the bit rate of the input optical signal is the passive mode-locking operation of the passive mode-locked semiconductor laser device. An optical clock signal extracting apparatus characterized in that the input optical signal has a bit rate small enough to cause a frequency pull-in phenomenon necessary for the optical signal.   12. The optical clock signal extraction device according to claim 11, wherein the input / output unit includes a polarization separator that separates and outputs the optical clock signal into two components whose polarization directions are orthogonal to each other. The input / output unit is
A first port for inputting the input optical signal; a second port for outputting the input optical signal and inputting the optical clock signal extracted by the passive mode-locking semiconductor laser device; and the optical clock signal. An optical circulator having a third port to output;
Coupled optics for outputting the input optical signal output from the second port to the passive mode-locked semiconductor laser device and inputting the optical clock signal extracted by the passive mode-locked semiconductor laser device to the second port 13. The optical clock signal extraction device according to claim 11 or 12, further comprising: a system.   14. The optical clock signal extraction device according to claim 13, wherein the input / output unit further includes a wavelength filter that filters the optical clock signal extracted by the passive mode-locked semiconductor laser device. In the passively mode-locked semiconductor laser device according to any one of claims 1 to 5,
The difference between the repetition frequency of the optical pulse train generated by the passive mode-locked semiconductor laser device and the frequency corresponding to the bit rate of the input optical signal is a frequency pulling phenomenon necessary for the passive mode-locking operation of the passive mode-locked semiconductor laser device. By inputting an input optical signal with a bit rate that is small enough to generate passive mode-locking operation,
An optical clock signal extraction method, wherein an optical pulse train output from the passive mode-locked semiconductor laser device is extracted as an optical clock signal.

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