JP2006216923A - Mode-locked semiconductor laser equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To narrow the spectral line width in each mode of optical frequency COM as well as improving S/N in mode-locked semiconductor laser equipment, and further, to achieve mode-locked semiconductor laser equipment that is simple in structure and inexpensive. <P>SOLUTION: The mode-locked semiconductor laser equipment 10 is provided with self-excitation oscillating semiconductor laser equipment that oscillates a pulse laser beam at a cycle of τ and resonators that shut in the self-excitation oscillating semiconductor laser equipment, has a mode-locked laser beam emitter 11 for emitting a mode-locked laser beam, and has an injection laser beam emitter 12 for emitting an injection laser beam to be injected to the mode-locked laser beam. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a mode-locked semiconductor laser device that oscillates pulsed laser light.

  A mode-locked laser is a laser that can oscillate pulsed laser light. A laser usually has a gain medium for amplifying light and a resonator having a resonator length of L. The resonance frequency of the longitudinal mode of the resonator has an interval of c / 2L (c is the speed of light). The spectrum width of the laser beam emitted from the laser is very narrow. However, if the resonance frequency interval of the resonator is narrower than the spectrum width, the laser beam resonates in a plurality of modes (frequencies) in the resonator. At this time, by aligning the phase of each mode using some method (mode synchronization), the laser light is intensified at a period T = 2L / c. Thereby, a pulse laser beam having a period T is generated.

  As shown in FIG. 7, the pulse laser beam oscillated by this mode-locked laser becomes an aggregate of a large number of modes arranged in a comb shape at equal frequency intervals 1 / T on the frequency axis. Such an assembly of laser beams arranged in a comb shape on the frequency axis is called an “optical frequency comb”. The optical frequency comb can be used for optical frequency measurement as a “measurement” of the frequency of light by utilizing the property that the frequency intervals are equal. Since the frequency interval is determined by 1 / T, the difference between the frequencies of the two lights to be measured can be obtained using this. Moreover, the absolute value of the frequency of each mode can be determined by determining the absolute value of the frequency of each mode by some method. The optical frequency comb can be used as a highly accurate wavelength standard or frequency standard, or can be used as a reference frequency in wavelength division multiplexing optical communication. Further, when viewed in the time domain, the mode-locked laser is an ultrashort pulse light source that oscillates at an accurate pulse repetition frequency, and thus can be applied to ranging technology.

  As such a mode-locked semiconductor laser device, there are a monolithic mode-locked semiconductor laser described in Patent Document 1 and an external resonator-type mode-locked semiconductor laser using a modulation circuit described in Patent Document 2.

  Conventionally, as a mode-locked laser, for example, as a laser that oscillates pulsed laser light, for example, a self-excited oscillation semiconductor laser described in Patent Document 3 is known. As shown in FIG. 3, the self-pulsation type semiconductor laser has an active layer 32 and a saturable absorption layer 33 incorporated between the layers of the semiconductor chip 31, and has a simpler structure than the mode-locked semiconductor laser. Therefore, it can be manufactured at low cost. However, mode locking cannot be applied in this self-excited oscillation type semiconductor laser. The mode-locked semiconductor laser pulsates at a frequency determined by the distance between the reflecting surfaces, whereas the conventional self-excited oscillation type semiconductor laser pulsates at a frequency irrelevant to them.

  The monolithic mode-locked semiconductor laser disclosed in Patent Document 1 must be subjected to complicated microfabrication in order to form all the components on a small semiconductor chip. Since the pulse repetition frequency is determined by the chip length of the semiconductor laser, it has a drawback that it is difficult to set it to a desired value.

  The external resonator mode-locked semiconductor laser using the modulation circuit of Patent Document 2 has a problem that phase modulation is applied when the injection current is modulated. Further, the external cavity mode-locked semiconductor laser using the saturable absorption mirror has a drawback that the saturable absorption mirror is a special part and is expensive.

Although the self-excited oscillation type semiconductor laser of Patent Document 3 does not have the above-described drawbacks, since it is not mode-locked, it cannot generate an optical frequency comb and can be used for the optical frequency measurement. Can not.
JP 7-22694 A JP 2003-264335 A JP-A-6-260716

  The conventional mode-locked semiconductor laser device has a problem that the spectral line width of each mode is wide and a problem that the S / N is low.

  The present invention has been made in view of the above problems, and an object thereof is to realize a mode-locked semiconductor laser device capable of narrowing the spectral line width of each mode and improving S / N. There is to do.

  In order to solve the above problems, a mode-locked semiconductor laser device according to the present invention is a mode-locked semiconductor laser device that becomes an optical frequency comb that is an aggregate in which a large number of laser beams are arranged at equal frequency intervals when viewed on the frequency axis. A closed optical circuit comprising a self-excited oscillation type semiconductor laser device that oscillates pulsed laser light with a period τ, and at least two reflecting portions sandwiching the self-excited oscillation type semiconductor laser device, A mode-locked laser beam emitting unit that emits a mode-locked laser beam, and a mode-locked laser beam emitting unit that emits a mode-locked laser beam, and a resonator that satisfies a relationship of (2L / c) <τ (c is the speed of light). An injection laser beam emitting unit that emits an injection laser beam having a single frequency that draws one peak frequency of the mode-locked laser beam, the laser beam being injected into the laser beam; It is characterized by that.

  With the above configuration, the injection laser light emitted from the injection laser light emitting portion is injected into the mode-locked laser light. As a result of drawing one peak frequency of the mode-locked laser light by the injection laser light, the peak frequency of the mode-locked laser light matches the frequency of the injection laser light. Therefore, the mode-locked laser light obtained by the injection has an effect that the spectral line width of each mode can be narrowed and the S / N can be improved as compared with that before the injection.

  With the above configuration, the mode-locked laser beam is a self-excited oscillation type semiconductor laser device, and a closed optical circuit is configured by a resonator. A self-excited oscillation type semiconductor laser device oscillates pulse laser light that is not mode-locked. This pulse laser beam is intensified by a resonator at a mode having a frequency that is an integral multiple of c / 2L. The light in this mode is not weakened by the saturable absorber of the self-excited oscillation type semiconductor laser device, and the light of other wavelengths is weakened by the saturable absorber. As a result, mode synchronization is applied, and pulsed laser beams having specific frequencies arranged at intervals of c / 2L in the frequency domain are obtained. Therefore, in addition to the effect of the above configuration, an optical frequency comb can be generated by applying mode synchronization, and an effect is obtained that a mode-locked semiconductor laser device having a simple structure and a low cost can be realized.

  In the mode-locked semiconductor laser device according to the present invention, in the above configuration, the mode-locked laser light into which the injection laser light is injected is divided into two by a polarization analyzer, and the intensity of the divided two lights is The injection laser light emitting part or the mode-locked laser light emitting part has a difference of zero so that the frequency of one peak of the mode-locked laser light and the single frequency of the injection laser light always coincide with each other. A feedback controller for controlling one of them is provided.

  With the above configuration, feedback control is performed so that the frequency of one peak of the mode-locked laser light and the single frequency of the injection laser light coincide. The polarization analyzer includes, for example, a quarter wave plate and a polarization beam splitter. Therefore, both frequencies can be made to coincide with each other for a long time. Therefore, in addition to the effects of the above-described configuration, the spectral line width of each mode can be narrowed stably for a long time, and the S / N can be improved.

  As described above, the mode-locked semiconductor laser device according to the present invention includes a self-excited oscillation type semiconductor laser device that oscillates pulsed laser light with a period τ, and at least two reflectors sandwiching the self-excited oscillation type semiconductor laser device. And a resonator in which the optical path length 2L of one cycle of the closed circuit satisfies the relationship of (2L / c) <τ (c is the speed of light). A mode-locked laser beam emitting section that emits laser light, and a laser beam that is injected into the mode-locked laser beam and emits an injection laser beam having a single frequency that draws one peak frequency of the mode-locked laser beam And a laser beam emitting part for use.

  As a result, the mode-locked laser light obtained by the injection has an effect that the spectral line width of each mode can be narrowed and the S / N can be improved as compared with that before the injection.

  As a result, in addition to the effects of the above configuration, an optical frequency comb can be generated by applying mode synchronization, and an effect is obtained that a mode-locked semiconductor laser device having a simple structure and a low cost can be realized. .

  As shown in FIG. 1, the mode-locked semiconductor laser device 10 according to the present embodiment includes a mode-locked laser beam emitting unit 11, an injection laser beam emitting unit 12, a half mirror 13, and a mirror 14. The mode-locked laser beam emitting unit 11 emits the mode-locked laser beam A. The injection laser beam emitting unit 12 is a CW (Continuous Wave) laser, and emits the injection laser beam B injected into the mode-locked laser beam A. The frequency of the injection laser beam B is single and is assumed to be fb.

  The half mirror 13 transmits a part of the incident light and reflects a part thereof, and a pulsed laser light oscillated from a mode-locked laser light emitting side end face 21a of a self-excited oscillation type semiconductor laser element 21 described later. It is arranged on the optical path. Thereby, the mode-locked laser beam A is emitted from the mode-locked laser beam emitting section 11 and passes through the half mirror 13.

  Further, the mirror 14 and the half mirror 13 are arranged on the optical path of the injection laser beam B so that the injection laser beam B is reflected by the mirror 14 and the half mirror 13 and enters (injects) the mode-locked laser beam emitting unit 11. It is arranged so that it is also located.

  As shown in FIG. 3, a self-excited oscillation type semiconductor laser device that oscillates pulsed laser light is constituted by the self-excited oscillation type semiconductor laser element 21 and a DC power supply 22. The self-excited oscillation type semiconductor laser element 21 has an active layer 32 and a saturable absorption layer 33 incorporated between the semiconductor chips 31 and 34, and has a simpler structure than the mode-locked semiconductor laser disclosed in Patent Documents 1 and 2. Therefore, it can be manufactured at low cost. However, in this self-oscillation type semiconductor laser device, mode synchronization cannot be applied. While the mode-locked semiconductor lasers of Patent Documents 1 and 2 pulsate at a frequency determined by the distance between the reflecting surfaces, the self-excited oscillation type semiconductor laser device oscillates at a frequency irrelevant to them.

  As shown in FIG. 2, the mode-locked laser beam emitting unit 11 includes a self-excited oscillation type semiconductor laser element 21, a DC power supply 22, a collimator lens 23, and a half mirror 24. The self-excited oscillation type semiconductor laser element 21 includes a mode-locked laser light emitting side end face 21a and a mode-locked laser light reflecting side end face 21b. The distance between the mode-locked laser light reflection side end face 21b and the half mirror 24 is L.

  The DC power source 22 is for injecting a DC current into the self-excited oscillation type semiconductor laser element 21.

  The self-excited oscillation type semiconductor laser element 21 itself can oscillate a pulse laser beam having a pulse repetition frequency τ from the end face 21a on the mode-locked laser beam emission side. A reflective surface for reflecting the pulsed laser light is formed inside the mode-locked laser light reflecting side end surface 21b facing the mode-locked laser light emitting side end surface 21a.

  The half mirror 24 is provided on the optical path of the pulsed laser light oscillated from the mode-locked laser light emitting side end face 21a. The collimator lens 23 is provided on the optical path. The collimator lens 23 converts the light oscillated from the self-excited oscillation type semiconductor laser element 21 into parallel light, and reflects the light reflected by the half mirror 24 into the mode of the self-excited oscillation type semiconductor laser element 21. This is for converging on the synchronous laser light emission side end face 21a. The distance L between the mode-locked laser light reflection side end face 21b and the half mirror 24 is set to be smaller than τc / 2 (c is the speed of light).

  With such a configuration, each of the mode-locked laser light reflection side end face 21b and the half mirror 24 serves as a resonator, and includes at least two reflecting portions sandwiching the self-excited oscillation type semiconductor laser device. A closed optical circuit is configured, and an optical path length 2L of one cycle of the closed optical circuit satisfies a relationship of (2L / c) <τ. As a result, when viewed on the frequency axis, as shown in FIG. 7 described above, an optical frequency comb (comb) which is an aggregate in which a large number of laser beams are arranged in a comb shape at equal frequency intervals can be obtained.

  Further, the mode-locked semiconductor laser device 50 shown in FIG. 8 can be used instead of the mode-locked semiconductor laser device 10 shown in FIG. That is, the mode-locked semiconductor laser device 50 includes a mode-locked laser beam emitting unit 11 and an injection laser beam emitting unit 12 similar to those in FIG. A beam shaping optical system 51, a 45 ° Faraday rotator 52, and a polarization beam splitter 53 are arranged in this order from the side closer to the mode-locked laser beam emitting unit 11 on the path of the emitted light from the mode-locked laser beam emitting unit 11. Yes. A beam shaping optical system 54, an optical isolator 55, an optical attenuator 56, and a lens 57 are arranged in this order from the side closer to the injection laser light emitting unit 12 on the path of the emitted light from the injection laser light emitting unit 12. Further, the lens 57 is connected to the lens 59 via a single mode optical fiber 58, and light emitted from the lens 59 enters the polarization beam splitter 53.

  This is a configuration that operates the principle of FIG. 1 more reliably.

  In order to surely realize injection locking, the spatial mode of the mode-locked laser beam emitting unit 11 that emits a self-excited oscillation type mode-locked laser and the spatial mode of the injection laser beam emitting unit 12 that emits a single frequency laser beam. It is important to match as much as possible. In addition to realizing this with optical elements such as lenses and prisms, as a more reliable method, as shown in FIG. 9, a single mode optical fiber 58 is used to maximize the coupling of both laser beams from both directions. There is a method of making it incident with efficiency. This takes advantage of the fact that single mode fiber 58 allows propagation in only one spatial mode. A similar function can be achieved by a spatial filter using a pinhole and a lens.

  In order to avoid that the single-frequency laser beam (injection laser beam B) is injected into the self-excited oscillation type semiconductor laser beam (mode-locked laser beam A) and the operation becomes unstable, the injection laser beam emitting unit 12 An optical isolator 55 is installed immediately after emission.

  The mode-locked laser light emitting unit 11 side performs light injection with a Faraday rotator 52 and a polarization beam splitter 53 that rotate the polarization direction by 45 ° in order to inject single-frequency laser light while avoiding the influence of its own return light. Configure an isolator. In the configuration of FIG. 9, the polarization direction of the mode-locked laser light A is set to be polarized horizontally with respect to the paper surface after being emitted from the Faraday rotator 52. Since the horizontally polarized light is transmitted through the polarization beam splitter 53, reflection toward the injection laser light emitting portion 12 is very small. On the other hand, if the polarization of the single frequency laser is perpendicular to the paper surface in front of the polarizing beam splitter 53, this light is reflected by the polarizing beam splitter 53, passes through the Faraday rotator 52, and travels toward the mode-locked laser light emitting unit 11. In the Faraday rotator 52, the rotation direction of the polarization is reversed depending on the traveling direction of the beam. In the arrangement of FIG. 9, the polarization of the single frequency laser after passing through the Faraday rotator 52 and the polarization of the self-excited oscillation mode-locked semiconductor laser before entering the Faraday rotator 52 coincide with each other. It is injected into a self-oscillation type mode-locked semiconductor laser. The functions of the Faraday rotator 52 and the polarization beam splitter 53 can be realized by an optical circulator.

  In order to optimize the power of the injection laser light, an optical attenuator 56 is installed in the optical path of the single laser light as necessary.

  The shape of the beam emitted from the semiconductor laser is generally an ellipse. In order to increase the efficiency of beam utilization, such as increasing the coupling efficiency to an optical fiber, the beam shape such as an anamorphic prism pair is used to make a perfect circle. Beam shaping optical systems 51 and 54 are used. As long as the problem of return light does not occur, it may be anywhere in the optical path, but it is usually placed before and after the optical isolator immediately after emission.

  Further, for the purpose of facilitating adjustment or reducing loss due to reflection, an element (not shown) for rotating polarized light such as a half-wave plate may be appropriately inserted in the optical path. Good.

  As described above, in the configuration of FIG. 1, the injection laser beam B emitted from the injection laser beam emitting unit 12 is injected into the mode-locked laser beam A emitted from the mode-locked laser beam emitting unit. It has become.

  Here, in the configuration of FIG. 1, the frequency spectrum of the mode-locked laser light A in a state where the injection laser light B is not injected is indicated by a in FIG. In the same figure, the frequency spectrum (single frequency fb) of the injection laser beam B is also shown (indicated by b). In this state, the mode-locked laser light A has a wide spectral line width in each mode and a small S / N.

  Further, in the configuration of FIG. 1, the frequency spectrum of the mode-locked laser light A in a state where the injection laser light B is injected is indicated by c in FIG. In the figure, the frequency spectrum (single frequency fb) of the injection laser beam B is also shown (indicated by d). In this state, it can be seen that the mode-locked laser light A has a narrow spectral line width and a large S / N in each mode.

  Here, the frequency fb of the laser beam to be injected (injection laser beam B) matches with any one of a plurality of frequency peaks of the laser beam to be injected (mode-locked laser beam A). The mode-locked laser light emitting section 11 adjusts the mode-locked laser light A, or the injection laser light emitting section 12 adjusts the injection laser light B. That is, in FIG. 5 and FIG. 6, seven frequency peaks are shown, and fb is made to coincide with the second peak from the higher frequency.

  From the results of FIGS. 5 and 6, it can be said as follows. The frequency of light can be measured by using a mode-locked laser, that is, a laser having an optical frequency comb, which is an aggregate of many laser beams arranged at equal frequency intervals when viewed on the frequency axis. is there. At present, this technique is realized by using an expensive and large-sized laser such as a titanium sapphire laser, but it can be miniaturized by using the configuration shown in FIG. However, with this mode-locked semiconductor laser, if the configuration of FIG. 2 is not used, the beat signal with another CW laser cannot be detected with good S / N as shown in FIG. On the other hand, in the present invention, as shown in FIG. 1, a CW laser is injected into a mode-locked semiconductor laser, and the power of the laser light to be injected is set appropriately. Injection locking is applied to the laser, and the spectral line width of each mode of the mode-locked semiconductor laser can be narrowed.

  As described above, the beat signal with the CW laser is detected with sufficient S / N at any frequency within the spectrum range of the mode-locked semiconductor laser. The spectral line width of each mode of the beat signal is also sufficiently narrow. Therefore, it is practically sufficient for measuring the beat frequency. If this structure is used, the optical frequency measurement system of the small size, cheap, and low power consumption is realizable as the whole optical frequency measurement system. In addition to optical frequency measurement technology, for example, it can be expected to be applied to atomic clocks and to geodetic systems such as GPS (global positioning system) and resource exploration if it can be mounted on an artificial satellite.

  Further, the state where the injection locking is applied can be maintained for about several tens of seconds as it is, but if the feedback control is performed, it is possible to keep the frequency matching between the mode-locking semiconductor laser and the injection laser for a long time. For the feedback control, a known technique using polarized light can be appropriately used for detecting a frequency shift. In the present invention, a sharp error signal can be obtained under conditions where it is difficult to obtain a signal, such as injection of very weak power injection laser light into a resonator of a mode-locked semiconductor laser having a low Q factor (quality factor). .

  This feedback will be described. First, as a method for detecting a deviation between the resonance frequency of the optical resonator and the laser frequency to be resonated with the optical resonator, there is a method using the polarized light shown in FIG. The light emitted from the laser device 101 is reflected by the optical resonator 102, passes through the quarter-wave plate 61, is divided into two directions by the polarization beam splitter 62, and is received by the photodiodes 63a and 63b. The comparator 64 compares the voltages according to the light intensity and outputs them. As the optical resonator 102, a confocal optical resonator, a ring resonator, or the like can be used. The photodiodes 63a and 63b and the comparator 64 constitute a differential photodetector. The quarter wave slope 61, the polarization beam splitter 62, and the differential photodetector constitute a polarization analyzer.

  Here, the polarization of light resonating with the optical resonator 102 is linearly polarized light, the angle thereof is defined by an optical element (not shown) in the optical resonator 102, and the laser light incident from the laser device 101 is linearly polarized light. Therefore, it is assumed that the polarization direction is an angle different from the polarization direction of polarized light resonating with the optical resonator 102.

  When the laser frequency incident on the optical resonator 102 from the laser device 101 is slightly shifted from the resonant frequency, the reflected light from the optical resonator 102 becomes elliptically polarized light. In the vicinity of the resonance frequency f0, a zero cross signal as shown in FIG. 10 is obtained. The horizontal axis x is the frequency of the laser light from the laser device 101, and the vertical axis y is the output voltage of the differential photodetector. By performing feedback control so that this signal becomes zero, the frequency of the laser light from the laser device 101 can be maintained at the resonance frequency of the optical resonator 102.

  FIG. 11 shows this method applied to the present invention. Since the injected light and the reflected light are coaxial, a part of the light is extracted by the polarization beam splitter 66 and the signal is detected. Thereafter, the same configuration as 61 to 64 shown in FIG. 9 is arranged. Then, according to the output voltage of the comparator 64, the feedback controller 65 determines whether the laser light from the laser light emission unit 12 for injection or the laser light from the mode-locked laser light emission unit 11 is Control to increase or decrease the frequency. That is, when controlling the injection laser beam emitting section 12, the frequency of the laser beam is increased or decreased. Further, when the mode-locked laser beam emitting unit 11 is controlled, the frequency of the laser beam is increased or decreased by controlling the resonator length or the injection current. The resonator length can be controlled by electrically moving the position of the resonator based on the output signal of the feedback controller 65 using, for example, a piezo element. By using the output signal of the feedback controller 65, the feedback controller 65 controls one of the specific mode frequency of the mode-locked semiconductor laser and the frequency of the single frequency laser to be injected. Both frequencies can be matched to each other for a long time.

  Here, the power of the single frequency laser beam to be injected (injection laser beam B) is set to be quite weak in order to continue the mode-locked oscillation here. Since the Q value of the resonator of the mode-locked laser light emitting section 11 which is a mode-locked semiconductor laser is low, it is considered that only a signal with a very poor S / N can be obtained at first. When the semiconductor laser was turned off, no signal indicating resonance of the optical resonator could be detected. However, when the self-excited oscillation type mode-locked semiconductor laser is oscillating, the mode frequency of the mode-locked laser and the frequency of the single-frequency laser to be injected are close to each other so that the injection locking can be performed. It was found that the laser beam was amplified on the mode-locked semiconductor laser side, and the signal intensity was amplified and observed as shown in FIG.

  As described above, even when the laser on the injection side is mode-locked and the power of the laser beam to be injected must be weakened to maintain the mode-locked oscillation, the laser is weak due to the large gain of the semiconductor laser. The injection laser light is amplified, and a high S / N signal is obtained.

  Note that the mode-locked laser light emitting unit 40 in FIG. 4 may be used instead of the mode-locked laser light emitting unit 11 in FIG.

  That is, in this configuration example, the resonator of the mode-locked laser beam emitting unit 11 in FIG. 2 is replaced with an external resonator including four reflectors 41 to 44. The structure of the self-excited oscillation type semiconductor laser device is the same as that of FIG. The length 2L of the square loop having the reflectors 41 to 44 as apexes is set so as to satisfy the relationship (2L / c) <τ. In this configuration example, pulsed laser light oscillated from the self-excited oscillation type semiconductor laser element 21 circulates around the loop of the external resonator, whereby light having a frequency that is an integral multiple of c / 2L is amplified by interference, and self-excited. By the saturable absorption layer 33 in the oscillation type semiconductor laser element 21, the intensity of the light of the amplified frequency is relatively stronger than the light of other frequencies. As a result, pulse laser light that is mode-locked at a frequency interval of c / 2L is generated. This pulse laser beam can be taken out of the external resonator by using, for example, one of the reflectors 41 to 44 as a half mirror.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope shown in the claims. That is, embodiments obtained by combining technical means appropriately changed within the scope of the claims are also included in the technical scope of the present invention.

  It can also be applied to applications such as an optical frequency measurement system.

It is a figure which shows one structural example of the mode synchronous semiconductor laser apparatus which concerns on this invention. It is a figure which shows the structural example of a mode synchronous laser beam emission part. It is a figure which shows the structural example of a self-excited oscillation type semiconductor laser apparatus. It is a figure which shows the structural example of a mode synchronous laser beam emission part. It is a figure which shows the frequency spectrum of the mode locked semiconductor laser of the state which does not apply injection locking. It is a figure which shows the frequency spectrum of the mode-locking semiconductor laser of the state which applied injection locking. It is a figure which shows the frequency spectrum of a mode synchronous semiconductor laser. It is a figure which shows one structural example of the mode synchronous semiconductor laser apparatus which concerns on this invention. It is a figure which shows one structural example of the feedback control using polarization. It is a figure which shows the output of a differential photodetector. It is a figure which shows the example of 1 structure of the mode synchronous semiconductor laser apparatus based on this invention which performs feedback control using polarization | polarized-light.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Mode-locked semiconductor laser apparatus 11 Mode-locked laser beam emitting part 12 Injection laser beam emitting part 13 Half mirror 14 Mirror 21 Self-excited oscillation type semiconductor laser element (self-excited oscillation type semiconductor laser apparatus)
21a Mode-locked laser light emitting side end face 21b Mode-locked laser light reflecting side end face (resonator)
22 DC power supply (self-oscillation type semiconductor laser device)
23 Collimator lens 24 Half mirror (resonator)
DESCRIPTION OF SYMBOLS 31 Semiconductor chip 32 Active layer 33 Saturable absorption layer 34 Semiconductor chip 40 Mode-locked laser light emission part 41,42,43,44 Reflector 50 Mode-locked semiconductor laser apparatus 51 Beam shaping optical system 52 45 degree Faraday rotator 53 Polarizing beam splitter 54 Beam shaping optical system 55 Optical isolator 56 Optical attenuator 57 Lens 58 Single mode optical fiber 59 Lens 61 Quarter wave plate 62 Polarizing beam splitter 63a, 63b Photo diode 64 Comparator 65 Feedback controller 66 Polarizing beam splitter 101 Laser device 102 optical resonator

Claims (2)

In a mode-locked semiconductor laser device that becomes an optical frequency comb that is an aggregate in which a large number of laser beams are arranged at equal frequency intervals when viewed on the frequency axis,
A closed optical circuit is configured by including a self-excited oscillation type semiconductor laser device that oscillates a pulsed laser beam with a period τ and at least two reflecting portions sandwiching the self-excited oscillation type semiconductor laser device. A mode-locked laser beam emitting unit that emits a mode-locked laser beam by including a resonator in which the optical path length 2L of the cycle satisfies a relationship of (2L / c) <τ (c is the speed of light);
An injection laser beam emitting unit that emits an injection laser beam having a single frequency that is a laser beam to be injected into the mode-locked laser beam and that draws one peak frequency of the mode-locked laser beam. A mode-locked semiconductor laser device.   The mode-locked laser light into which the injection laser light is injected is divided into two by a polarization analyzer, and the difference in intensity between the two divided lights becomes zero, and the frequency of one peak of the mode-locked laser light is A feedback controller is provided for controlling one of the injection laser beam emitting unit and the mode-locked laser beam emitting unit so that the single frequency of the injection laser beam always matches. Item 2. The mode-locked semiconductor laser device according to Item 1.

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