JP2017059683A - Laser oscillator and spectrometer provided with laser oscillator, optical coherence tomography device, asynchronous optical sampling device, long-distance absolute distance measurement device, and cw laser high-speed, high-resolution spectrometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser oscillator of which size can be reduced and an optical measurement device equipped with the laser oscillator.SOLUTION: The laser oscillator is provided with a circulating optical path 9 including a first mirror 2, a second mirror 3 disposed so as to face the first mirror 2, and a gain medium 4 capable of producing a Kerr-lens effect as a nonlinear crystal disposed between the first mirror 2 and the second mirror 3. Light 11 that is induced/emitted by an excitation light source 7 circulates in a first direction in the sequence of the first mirror 2, the gain medium 4, and the second mirror 3, and in a second direction in the sequence of the second mirror 3, the gain medium 4, and the first mirror 2. The optical path length in the gain medium 4 differs between the first direction and the second direction. The laser oscillator emits a first pulse laser in which the light 11 is circulated through the circulating optical path 9 in the first direction and is mode-locked, and a second pulse laser in which the light 11 is circulated through the circulating optical path 9 in the second direction and is mode-locked, the repetition frequency of the second pulse laser being different from that of the first pulse laser.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a laser oscillator and a spectroscopic device including the laser oscillator, an optical coherence tomography device, an asynchronous optical sampling device, a long-range absolute distance measuring device, and a CW laser high-speed high-resolution spectroscopic device.

  In order to measure a molecular vibration spectrum, Fourier spectroscopy (also referred to as FT-IR) using a Michelson interferometer is widely used. In Fourier spectroscopy, it is necessary to mechanically move the mirror during measurement. Therefore, in Fourier spectroscopy, the moving speed of the mirror becomes a bottleneck in improving the measuring speed, and it is difficult to improve the measuring speed.

  Thus, attention has been focused on dual-comb spectroscopy that can acquire a spectral spectrum in a shorter time with higher resolution than Fourier spectroscopy (see Non-Patent Document 1).

  In dual-comb spectroscopy, two ultrashort pulse lasers with different repetition frequencies are overlapped and irradiated onto the specimen, the light transmitted through the specimen is detected, and the detected transmitted light is Fourier transformed to obtain a spectral spectrum. obtain. As described above, in the dual-comb spectroscopy, it is possible to measure in a shorter time because the mirror is not mechanically moved and the light transmitted through the test object is detected and Fourier-transformed.

A. Schliesseret al., Optics Express 13, 9029 (2005) S.-J. Lee et al., Japanese Journal of Applied Physics. 40, L878-L880 (2001) A. Bartels et al., Review of Scientific Instruments 78, 035107 (2007) I. Coddington et al., Nature Photonics 3, 351-356 (2009) F. R. Giorgettaet al., Nature Photonics 4, 853-857 (2010)

  However, in a measurement method using lasers with different frequencies such as dual comb spectroscopy, it is necessary to prepare two laser oscillators and a repetition frequency stabilization mechanism, and it is difficult to downsize the apparatus.

  Therefore, the present invention provides a laser oscillator that can reduce the size of the apparatus, a spectroscopic device including the laser oscillator, an optical coherence tomography device, an asynchronous optical sampling device, a long-range absolute distance measuring device, and a CW laser high-speed high-resolution spectroscopic device. For the purpose.

  The laser oscillator according to the present invention includes a first mirror, a second mirror disposed opposite to the first mirror, and a nonlinear crystal disposed between the first mirror and the second mirror, and is guided by an excitation light source. A circulating optical path in which the emitted light circulates in the first direction in the order of the first mirror, the nonlinear crystal, and the second mirror and in the second direction in the order of the second mirror, the nonlinear crystal, and the first mirror. A first pulsed laser in which the optical path length in the nonlinear crystal is different between the first direction and the second direction, and the light is mode-locked around the circulating optical path in the first direction, and The light circulates in the second optical path in the second direction and is mode-locked, and a second pulse laser having a repetition frequency different from that of the first pulse laser is emitted.

  A spectroscopic device according to the present invention includes the laser oscillator according to any one of claims 1 to 8, a photopolymerization unit that superimposes the first pulse laser and the second pulse laser emitted from the laser oscillator, An interference wave detection unit that detects an interference wave obtained by superimposing the first pulse laser and the second pulse laser by the photopolymerization unit; and a Fourier transform unit that Fourier-transforms the detected interference wave. To do.

  An optical coherence tomography apparatus according to the present invention includes the laser oscillator according to any one of claims 1 to 8.

  The asynchronous optical sampling apparatus of this invention is equipped with the laser oscillator of any one of Claims 1-8, It is characterized by the above-mentioned.

  The long distance absolute distance measuring device of the present invention is provided with the laser oscillator according to any one of claims 1 to 8.

  A CW laser high-speed and high-resolution spectroscopic device according to the present invention comprises the laser oscillator according to any one of claims 1 to 8.

  Since the optical path length in the nonlinear crystal is different between the first direction and the second direction, the laser oscillator of the present invention can oscillate pulse lasers with different repetition frequencies, and requires lasers with two different frequencies such as a dual comb spectrometer. By using it for a simple device, the device can be miniaturized.

  Since the spectroscopic device of the present invention includes the laser oscillator according to any one of claims 1 to 8, it can be miniaturized.

  Since the optical coherence tomography apparatus of the present invention includes the laser oscillator according to any one of claims 1 to 8, it can be miniaturized.

  Since the asynchronous optical sampling apparatus of the present invention includes the laser oscillator according to any one of claims 1 to 8, it can be miniaturized.

  Since the long distance absolute distance measuring apparatus of the present invention includes the laser oscillator according to any one of claims 1 to 8, it can be miniaturized.

  Since the CW laser high-speed and high-resolution spectroscopic apparatus of the present invention includes the laser oscillator according to any one of claims 1 to 8, it can be miniaturized.

It is the schematic which shows the whole structure of the laser oscillator of embodiment. It is the schematic which shows the whole structure of the laser oscillator of a modification. It is the schematic which shows the whole structure of the spectrometer of embodiment. It is a spectrum of two pulse lasers emitted from the laser oscillator of the embodiment. FIG. 5A is a graph showing temporal changes in the repetition frequency of two pulse lasers emitted from the laser oscillator of the embodiment, and FIG. 5B is a graph showing the difference in repetition frequency between the two pulse lasers emitted from the laser oscillator of the embodiment. It is a graph which shows a time-dependent change. FIG. 6A is a graph showing the change over time of the repetition frequency of two pulse lasers emitted from the laser oscillator according to the embodiment with the box covered, and FIG. 6B is emitted from the laser oscillator according to the embodiment with the box covered. It is a graph which shows a time-dependent change of the difference of the repetition frequency in two pulsed lasers. FIG. 7A is a schematic diagram showing the moving direction of the gain medium, and FIG. 7B is a graph showing the relationship between the distance from the initial position of the gain medium and the difference between the repetition frequencies of the two pulse lasers of the laser oscillator of the embodiment. is there. FIG. 8A is a schematic diagram showing the moving direction of the second mirror, and FIG. 8B shows the relationship between the distance from the initial position of the second mirror and the difference between the repetition frequencies of the two pulse lasers of the laser oscillator of the embodiment. It is a graph. FIG. 9A is a schematic diagram showing the moving direction of the lens, and FIG. 9B is a graph showing the relationship between the distance from the initial position of the lens and the difference between the repetition frequencies of the two pulse lasers of the laser oscillator of the embodiment. It is a graph showing the spectrum of the spectrum measured with the spectroscopic device of the embodiment and the spectrum of the pulse laser of the laser oscillator of the embodiment.

(1) Configuration of Laser Oscillator According to Embodiment of Present Invention As shown in FIG. 1, a laser oscillator 1 includes a first mirror 2, a second mirror 3 disposed opposite to the first mirror 2, and a first mirror. The gain medium 4 that is disposed between the second mirror 3 and the second mirror 3 and can generate a Kerr lens effect (optical Kerr effect) as a nonlinear crystal, and the first mirror 2 and the second mirror 3 that are adjacent to each other are disposed opposite to each other. 3 mirror 5, and output coupler 6 disposed so as to face second mirror 3 and adjacent first mirror 2.

  The first mirror 2 and the second mirror 3 are arranged so that the focal point of the first mirror 2 and the focal point of the second mirror 3 overlap each other.

In the present embodiment, the first mirror 2 and the second mirror 3 are concave mirrors having a reflectivity of 99.9% and a radius of curvature of 30 mm, and the gain medium 4 is a titanium sapphire crystal (Ti: Al ₂ O) in which sapphire is doped with titanium. ₃ ), the third mirror 5 is a convex mirror having a reflectivity of 99.8% and a radius of curvature of 1000 mm, and the output coupler 6 is a plane mirror having a reflectivity of 99.0% and can transmit 1% of light.

  Furthermore, in the case of this embodiment, the 1st mirror 2, the 2nd mirror 3, and the 3rd mirror 5 are chirp mirrors. Thus, by making each mirror a chirp mirror, the group velocity dispersion of the light 11 that circulates around the circular optical path 9 is compensated. A normal mirror can be used instead of the chirp mirror. In this case, the group velocity dispersion is compensated by arranging a prism pair on the light path.

The shape and reflectivity of each mirror are not particularly limited as long as the circulating optical path 9 described below can be formed. As the gain medium 4, Yb: YAG, Yb: KYW, Yb: KY (WO 4) 2, Yb: YVO 4, Yb: Sc 2 O 3, Nd: YAG, Cr: YAG, Cr: forsterite, Cr : LiSGaF, Cr: LiSAF, Cr: LiCAF, Cr: ZnS, Cr: ZnSe, Pr: YLF, Yb: Y ₂ O ₃ , Yb: Lu ₂ O ₃ and the like can be used.

  The laser oscillator 1 further includes an excitation light source 7 and a lens 8. The excitation light source 7 is disposed so as to face the gain medium 4 with the first mirror 2 interposed therebetween, and irradiates the gain medium 4 with the excitation light 10. When the excitation light 10 is applied to the gain medium 4, the gain medium 4 is excited and light 11 is stimulated and emitted from the gain medium.

The excitation light source 7 is appropriately selected according to the type of the gain medium 4. In the present embodiment in which the gain medium 4 is a titanium sapphire crystal, the second harmonic (wavelength 532 nm) of an Nd: YVO ₄ laser is used as the excitation light source 7. As the excitation light source 7, a laser diode such as a Nd: YAG laser, Nd: YLF laser, Nd: YVO4 laser, AlInGaN or the like, a second harmonic of a Yb fiber laser, a light excitation semiconductor laser, or the like may be used.

  The lens 8 is disposed between the excitation light source 7 and the first mirror 2, collects the excitation light 10, and irradiates the gain medium 4 with the condensed excitation light 10. The lens 8 is disposed so that the focal point is located in the gain medium 4. In the present embodiment, the lens 8 is a convex lens having a focal length of 40 mm.

  The first mirror 2, the second mirror 3, the third mirror 5, and the output coupler 6 are configured such that the light 11 stimulated and emitted from the gain medium 4 is the gain medium 4, the second mirror 3, the third mirror 5, and the output coupler. 6, the first mirror 2 and the gain medium 4 are arranged with their orientation and position adjusted so that they can circulate in the first direction (the direction of the arrow B shown in FIG. 1) in the order of the ring-shaped circular optical path 9 Is forming.

  The light 11 stimulated and emitted from the gain medium 4 is in the order of the gain medium 4, the first mirror 2, the output coupler 6, the third mirror 5, the second mirror 3, and the gain medium 4, which are reverse to the first direction. The circulating optical path 9 can also circulate in the second direction (the direction of arrow A shown in FIG. 1). In this way, the stimulated light 11 circulates around the circulating optical path 9 in two directions, the first direction and the second direction.

  In the case of this embodiment, the optical path length of the circulating optical path 9 is set to about 30 cm so that the repetition frequency of the pulse laser oscillated by the laser oscillator 1 is about 1 GHz. The optical path length of the circulating optical path 9 can be appropriately changed according to the repetition frequency to be obtained.

  In order to produce a Kerr lens effect in the gain medium 4, the light 11 that circulates in the circular optical path 9 is mode-locked. The light 11, which is circulated in the first optical path 9 in the first direction and mode-locked, is emitted from the output coupler 6 as a first pulse laser (Output B shown in FIG. 1), and circulated in the second optical path 9 in the second direction. The locked light 11 is emitted from the output coupler 6 as a second pulse laser (Output A shown in FIG. 1). Thus, the laser oscillator 1 emits two pulse lasers.

  In the case of this embodiment, the gain medium 4 is formed in a prismatic shape having a parallelogram shape with a bottom surface of 2.3 mm × 5.0 mm and a height of 3.0 mm. The gain medium 4 is arranged so that the light 11 that circulates in the first direction along the circular optical path 9 and the light 11 that circulates in the second direction on the circular optical path 9 can enter at an angle close to the Brewster angle.

  Further, the gain medium 4 is arranged so that the center of the gain medium 4 is at a position different from the focal point of the first mirror 3 and the focal point of the second mirror 4. In the case of this embodiment, since the curvature radii of the first mirror 3 and the second mirror 4 are the same, the focal point of the first mirror 3 and the focal point of the second mirror 4 are the centers between the first mirror 2 and the second mirror 3. Located in. That is, the gain medium 4 is arranged so that the center of the gain medium 4 is at a position different from the center between the first mirror 2 and the second mirror 3.

  In this specification, the center between the first mirror 2 and the second mirror 3 refers to the midpoint of the light path between the first mirror 2 and the second mirror 3, and the center of the gain medium 4 refers to the inside of the gain medium 4. Says the midpoint of the light path. Arrangement at different positions means that the center of the gain medium 4 is shifted in the first mirror 2 direction or the second mirror 3 direction from the center between the first mirror 2 and the second mirror 3.

  In the present embodiment, since the center of the gain medium 4 is at a position different from the center between the first mirror 2 and the second mirror 3, the light 11 that strikes the gain medium 4 circulates in the first direction. And the light 11 that circulates in the second direction. Since the way in which the light 11 strikes the gain medium 4 is different, the optical path length in the gain medium 4 is different between the case where the light 11 passes through the gain medium 4 in the first direction and the case where the light 11 passes through the second direction. Different.

  Since the optical path length in the gain medium 4 differs depending on the circulation direction, the optical path length of the circulation optical path 9 when the light 11 circulates in the first direction and the optical path length of the circulation optical path 9 when the light 11 circulates in the second direction. Will also be different. As a result, the laser oscillator 1 emits a first pulse laser and a second pulse laser having a repetition frequency different from that of the first pulse laser.

  In the case of the present embodiment, the gain medium 4 is disposed so that the center is located closer to the first mirror 2 side than the centers of the first mirror 2 and the second mirror 3. Therefore, the light 11 that circulates around the circulating optical path 9 in the first direction (the direction of arrow B in FIG. 1) and the light 11 that circulates around the circulating optical path 9 in the second direction (the direction of arrow A in FIG. 1). The optical path length in the gain medium 4 is different. In the case of this embodiment, the optical path length of the circular optical path 9 is shorter in the second direction, and the second pulse laser (Output A in FIG. 1) has a higher repetition frequency than the first pulse laser (Output B in FIG. 1). high.

  The fact that the center of the gain medium 4 is at a position different from the center between the first mirror 2 and the second mirror 3 is that the photograph of the laser oscillator 1 is taken from directly above, and the distance is measured from the obtained photographed image. This can be confirmed.

  Further, the laser oscillator 1 includes a frequency difference adjusting unit 13 that adjusts a difference in repetition frequency between the first pulse laser and the second pulse laser. In the case of this embodiment, the frequency difference adjustment unit 13 holds the gain medium 4 as a nonlinear crystal, and moves the frequency difference adjustment unit 13 with an actuator or the like (not shown) provided in the frequency difference adjustment unit 13. Thus, the gain medium 4 held is manufactured so as to be movable. The frequency difference adjustment unit 13 is not particularly limited as long as the gain medium 4 can be moved. For example, the frequency difference adjustment unit 13 may be manufactured as a pedestal on which the gain medium 4 is placed.

  The frequency difference adjustment unit 13 moves the gain medium 4 between the first mirror 2 and the second mirror 3 by moving the gain medium 4 in the first direction or the second direction along the circular optical path 9 between the first mirror 2 and the second mirror 3. Of the light 11 (the focal point of the first mirror 3 and the focal point of the second mirror 4) and the center of the gain medium 4 are adjusted, and the light 11 that circulates in the first direction is adjusted by adjusting how the light 11 strikes the gain medium 4 By changing the optical path length in the gain medium 4 of the light 11 circulating in the second direction, the difference between the repetition frequency of the first pulse laser and the repetition frequency of the second pulse laser can be adjusted. By providing such a frequency difference adjusting unit 13, the difference in repetition frequency between the eleventh pulse laser and the second pulse laser can be easily adjusted.

  Moreover, it is preferable that the frequency difference adjustment part 13 adjusts so that the difference of the repetition frequency of a 1st pulse laser and a 2nd pulse laser may be larger than 0 kHz and 10 kHz or less. When the difference in repetition frequency between the first pulse laser and the second pulse laser is greater than 0 kHz and less than or equal to 10 kHz, the laser oscillator 1 can be measured more reliably when used in a dual comb spectrometer.

(2) Operation and Effect In the above configuration, the laser oscillator 1 of the present embodiment includes the first mirror 2, the second mirror 3 disposed opposite to the first mirror 2, the first mirror 2, and the second mirror 3. The gain medium 4 arranged between them and capable of producing the Kerr lens effect as a nonlinear crystal, the third mirror 5, and the output coupler 6 capable of emitting a laser are included.

  In the laser oscillator 1, the light 11 irradiated with the excitation light 10 from the excitation light source 7 and stimulatedly emitted by the excitation light source 7 is converted into the first mirror 2, the gain medium 4, the second mirror 3, and the third mirror. 5, output coupler 6, first direction of first mirror 2 and second mirror 3, gain medium 4, first mirror 2, output coupler 6, third mirror 5, second mirror 3 It was comprised so that the surrounding optical path 9 which can be circulated in two directions was provided.

  Further, the laser oscillator 1 is arranged at a position where the center of the gain medium 4 is different from the center between the first mirror 2 and the second mirror 3 (different from the focal point of the first mirror 2 and the focal point of the second mirror 3). A first pulse laser in which the optical path length in the medium 4 is different between the first direction and the second direction, the light 11 circulates around the circulating optical path 9 in the first direction, and the light 11 circulates in the second direction. A second pulse laser that is mode-locked around the optical path 9 and has a repetition frequency different from that of the first pulse laser is emitted from the output coupler 6.

  Therefore, in the laser oscillator 1, since the optical path length in the gain medium 4 is different between the first direction and the second direction, the optical path length of the circulating optical path 9 is mode-locked when the light 11 circulates in the first direction. Unlike the case where the light 11 circulates in two directions and is mode-locked, two pulse lasers having different repetition frequencies can be oscillated.

  In addition, since the first pulse laser and the second pulse laser are excited by the same gain medium 4 and are mode-locked, the laser oscillator 1 can oscillate two pulse lasers having the same spectrum but different repetition frequencies. And can be used in a dual comb spectrometer. When the laser oscillator 1 is used in an apparatus that requires two different repetition frequency lasers, such as a dual comb spectrometer, the laser oscillator and the repetition frequency stabilization mechanism can be omitted, and the apparatus can be downsized.

  Further, the laser oscillator 1 adjusts the deviation between the center between the first mirror 2 and the second mirror 3 and the center of the gain medium 4 by moving the position of the gain medium 4, and the repetition frequency of the first pulse laser and the first frequency are changed. By providing a frequency difference adjustment unit that adjusts the difference between the repetition frequencies of the two-pulse lasers, the difference between the repetition frequencies of the first pulse laser and the second pulse laser can be easily adjusted.

(3) Other Embodiments The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the gist of the present invention. For example, the laser oscillator shown in FIG. 21, the nonlinear crystal 24 is further arranged in the laser oscillator 21 in addition to the gain medium 34, and the gain medium 34 is irradiated with the excitation light 30 and the light 31 stimulated and emitted from the gain medium 34 is obtained by the gain medium 34. It may be configured to be mode-locked and oscillate two pulse lasers having different repetition frequencies by disposing the center of the nonlinear crystal 24 by shifting the focus of the first mirror 22 and the focus of the second mirror 23.

  As described above, by providing the gain medium 34 and the nonlinear crystal 24, the laser oscillator 21 causes the nonlinear crystal 24 to generate a difference between the repetition frequencies of the two pulse lasers. Good conditions for mode locking can be maintained, and a stable mode locking state can be realized.

  Furthermore, since the laser oscillator 21 includes the gain medium 34 and the nonlinear crystal 24, the nonlinear crystal 24 that produces a nonlinear effect larger than that of the gain medium 4 of the above embodiment can be used. can do.

  The laser oscillator 21 includes a fourth mirror 32, a fifth mirror 35 disposed opposite to the fourth mirror 32, a gain medium 34 disposed between the fourth mirror 32 and the fifth mirror 35, and the fourth mirror. The third mirror 25 arranged to face the fifth mirror 35, the output coupler 26 arranged to face the fifth mirror 35 and the fourth mirror 32, and the third mirror 25 to the neighboring output. The second mirror 23 arranged to face the coupler 25, the first mirror 22 arranged to face the output coupler 26 and the third mirror 25 and the second mirror 23, and the first mirror 22 arranged to face the second mirror 23. And a nonlinear crystal 24 disposed between the first mirror 22 and the second mirror 23. The 1st mirror 22 and the 2nd mirror 323 are arrange | positioned so that a focus may overlap.

  In this modification, the gain medium 34, the third mirror 25, and the output coupler 26 have the same configuration as the gain medium 4, the third mirror 5, and the output coupler 26 of the above embodiment, and the first mirror 22, The configuration is the same as that of the second mirror 22, the fourth mirror 32, the fifth mirror 35, and the first mirror 22 of the above embodiment. In the case of this modification, the nonlinear crystal 24 is made of quartz, and various types of glass or ZnS can be used in addition to quartz.

  The laser oscillator 21 further includes an excitation light source 27 and a lens 28. The excitation light source 27 is disposed so as to face the gain medium 34 with the first mirror 32 interposed therebetween, and irradiates the gain medium 34 with the excitation light 30. The lens 28 is the same lens as the lens 8, condenses the excitation light 30 and irradiates the gain medium 34.

  Excitation light 30 emitted from the excitation light source 27 is collected by the lens 28, passes through the fourth mirror 32, and is applied to the gain medium 34. When the excitation light 30 is irradiated, the gain medium 34 is excited by the excitation light source and stimulates and emits the light 31.

  The laser oscillator 21 includes a fifth mirror 35, a third mirror 25, a first mirror 22, a nonlinear crystal 24, a second mirror 23, an output coupler 26, a fourth mirror 32, and the light 31 stimulated and emitted from the gain medium 34. The positions and orientations of these mirrors and the like are adjusted so that the gain medium 34 can circulate in the first direction in the order (the direction of arrow B in FIG. 2), and the circular optical path 29 is formed. In the circulating optical path 29, the fourth mirror 32, the output coupler 26, the second mirror 23, the nonlinear crystal 24, the first mirror 22, the third mirror 25, and the fifth mirror 35 are arranged so that the light 31 is in the direction opposite to the first direction. Further, the gain medium 34 can also circulate in the second direction in the order (the direction of arrow A in FIG. 2). Other configurations are the same as those of the laser oscillator 1.

  The gain medium 34 mode-locks the light 31 that circulates around the circulating optical path 29. In the case of this modification, the center of the gain medium 34 is arranged so as to overlap the center between the fourth mirror 32 and the fifth mirror 35, but the center of the gain medium 34 is between the fourth mirror 32 and the fifth mirror 35. You may arrange | position so that it may become a position different from the center of. As the fourth mirror 32 and the fifth mirror 35, the same lens may be used, or lenses having different curvature radii may be used.

  In such a laser oscillator 21, the nonlinear crystal 24 is arranged such that the center of the nonlinear crystal 24 is deviated from the focus of the first mirror 22 and the focus of the second mirror 23. In the case of this modification, since the curvature radii of the first mirror 22 and the second mirror 23 are the same, the center of the nonlinear crystal 24 is the center between the first mirror 22 and the second mirror 23 (definition of the center of the nonlinear crystal 24). Is the same as that of the central embodiment of the gain medium 4, and the definition of the center between the first mirror 22 and the second mirror 23 is the same as the center between the first mirror 2 and the second mirror 3. Are arranged as follows.

  Therefore, in the laser oscillator 21, the way the light strikes the nonlinear crystal 24 is different between the first direction and the second direction, and the optical path length in the nonlinear crystal 24 is different between the first direction and the second direction. The optical path length of 29 is different between the case where the light 31 circulates in the first direction and the case where the light 31 circulates in the second direction, and two pulse lasers having different repetition frequencies can be emitted.

  In the case of this modification, the nonlinear crystal 24 is formed in the same shape as the gain medium 34, but the shape thereof is not particularly limited, and may be, for example, a plate shape.

  Further, the nonlinear crystal 24 is arranged so that both the light 31 that circulates in the first optical path 29 in the first direction and the light 31 that circulates in the second optical path 29 in the second direction can enter at an angle close to the Brewster angle.

  The laser oscillator 21 may further include a plurality of nonlinear crystals.

  Furthermore, the laser oscillator 21 of the present modification may include a frequency difference adjustment unit that holds the nonlinear crystal 24 and moves the position of the nonlinear crystal 24, and the frequency difference adjustment unit may adjust the frequency difference repeatedly. .

  Moreover, although the said embodiment demonstrated the case where the curvature radius of the 1st mirror 2 and the 2nd mirror 3 was equal, and the said modification demonstrated the case where the curvature radius of the 1st mirror 22 and the 2nd mirror 23 was equal, The invention is not limited to this, and the first mirror 2 (22) and the second mirror 3 (23) may have different radii of curvature.

  Also in this case, the first mirror 2 (22) and the second mirror 3 (23) are arranged so that the focal point of the first mirror 2 (22) and the focal point of the second mirror 3 (23) overlap, and the gain medium 4 The (nonlinear crystal 24) is arranged such that the center of the gain medium 4 (nonlinear crystal 24) is different from the focal point of the first mirror 2 (22) and the focal point of the second mirror 3 (23).

  As a result, the laser oscillator 1 (21) is different in how the light 11 (31) strikes the gain medium 4 (nonlinear crystal 24) in the first direction and the second direction, and the gain medium 4 (nonlinear crystal 24). Since the optical path lengths are different, two pulse lasers having different repetition frequencies can be emitted.

  Furthermore, in the above-described embodiment, the frequency difference adjusting unit 13 holds the gain medium 4 and moves the position of the gain medium 4, so that the focal point of the first mirror 2 and the focal point of the second mirror 3 (first mirror 2 and The center of the second mirror 3) and the center of the gain medium 4 are adjusted to adjust the difference in repetition frequency between the first pulse laser and the second pulse laser, but the present invention is not limited to this. Absent.

  For example, the frequency difference adjusting unit 13 holds the first mirror 2 or the second mirror 3 and positions the first mirror 2 or the second mirror 3 along the circulating optical path 9 between the first mirror 2 and the second mirror 3. To adjust the difference between the center of the gain medium 4 and the focal point of the first mirror 2 and the focal point of the second mirror 3 to adjust the difference in repetition frequency.

  Further, the frequency difference adjustment unit 13 holds the lens 8 and moves the lens 8 along the excitation light 10 so that the focal point of the lens 8 is set at a position different from the center of the gain medium 4. May be adjusted. In this case, since the focal position of the lens 8 is shifted from the center of the gain medium 4, the overlap of the light 11 stimulated and emitted by the gain medium 4 and the excitation light 10 in the gain medium 4 is in the first direction and the first direction. It becomes different in two directions. As a result, the mode 11 of the light 11 is different in the first direction and the second direction, and the way the light hits the gain medium 4 is different in the first direction and the second direction. As a result, the optical path length in the gain medium 4 differs between the first direction and the second direction. Therefore, the frequency difference adjustment unit 13 can adjust the difference in the repetition frequency by moving the lens 8.

  Further, the frequency difference adjusting unit 13 may adjust the frequency difference repeatedly by moving at least two of the first mirror 2, the second mirror 3, the gain medium 4, and the lens 8.

  In this case, the frequency difference adjusting unit 13 is configured by a driving mechanism such as an actuator that holds the first mirror 2, the second mirror 3, the gain medium 4, and the lens 8 and can move them separately.

(4) Use of Laser Oscillator The first pulse laser and the second pulse laser emitted from the laser oscillator 1 of the present embodiment are ultrashort pulse lasers of about 1 GHz. Ultrashort pulse lasers generally have a discrete spectral structure, so-called optical frequency comb, in which longitudinal modes are arranged at repetition frequency intervals. The laser oscillator 1 can oscillate two pulse lasers having the same spectrum but different repetition frequencies. Therefore, the laser oscillator 1 can be used as a laser light source of a dual comb spectrometer.

  As shown in FIG. 3, the dual comb spectroscopic device 40 as a spectroscopic device using the laser oscillator 1 of the present embodiment includes a laser oscillator 1 and a first pulse laser emitted from the laser oscillator 1 (Output B shown in FIG. 3). ) And the second pulse laser (Output A shown in FIG. 3) are overlapped and interfered with each other, an interference wave detection unit 43 for detecting the interference wave of the first pulse laser and the second pulse laser, and detection A Fourier transform unit 44 for Fourier transforming the interference wave is provided.

  The photopolymerization unit 41 includes a beam splitter 41a and a mirror 41b. The beam splitter 41a is disposed on the light beam of the second pulse laser. The mirror 41b is disposed on the light beam of the first pulse laser, reflects the first pulse laser, and enters the beam splitter 41a. The directions of the beam splitter 41a and the mirror 41b are adjusted so that the first pulse laser and the second pulse laser interfere with each other.

  In the present embodiment, a plate-type beam splitter (manufactured by THORLABS, model number: BSW11) is used as the beam splitter 41a, and a plane mirror is used as the mirror 41b. The configuration of the photopolymerization unit 41 is not particularly limited as long as the first pulse laser and the second pulse laser can interfere with each other.

  The interference wave detection unit 43 is disposed on the interference light beam of the first pulse laser and the second pulse laser emitted from the photopolymerization unit 41, detects the interference wave, and converts it into an electrical signal. In the present embodiment, a Si high-speed PIN (RF) amplification photodetector (FPD310-FV manufactured by MenloSystems) is used as the interference wave detection unit 43, but it is not particularly limited as long as the interference wave can be detected.

  The Fourier transform unit 44 receives the electrical signal of the interference wave transmitted from the interference wave detection unit 43, and Fourier transforms the interference wave converted to the electrical signal. The Fourier transform unit 44 includes a low-pass filter 44a and a digitizer 44b. The low pass filter 44a removes high frequency components such as noise included in the electric signal of the interference wave. The digitizer 44b converts the electrical signal of the interference wave that has passed through the low-pass filter 44a into a digital signal.

  The Fourier transform unit 44 includes a computation unit (not shown), processes the digital signal with the computation unit, and Fourier transforms the interference wave.

  In the present embodiment, BLP-550 + manufactured by Mini-Circuits is used as the low-pass filter 44a, ATS9440 manufactured by Alazar Tech is used as the digitizer 44b, and a PC (personal computer) is used as the calculation unit. The electrical signal of the interference wave is digitized by the digitizer 44b and stored as data in a storage device of the PC. Thereafter, the interference wave data stored in the storage device of the PC is processed by the PC, and the interference wave is Fourier transformed. The spectrum data of the interference wave obtained by the Fourier transform is stored in the storage device of the PC.

  The Fourier transform unit 44 is not particularly limited as long as it can Fourier transform the interference wave, and may perform Fourier transform by calculation using a PC or the like as in the present embodiment, or a dedicated hardware such as a Fourier transform processing circuit. The Fourier transform may be performed using wear.

  The Fourier transform unit 44 may include a display unit (in the case of the present embodiment, a PC monitor (not shown)) that displays spectral spectrum data, and may display the spectral spectrum on the display unit. Furthermore, the Fourier transform unit 44 may include a transmission unit that transmits spectral spectrum data to another device.

  In the dual-comb spectrometer 40, a test object to be measured is disposed between the output coupler 6 of the laser oscillator 1 and the mirror 41b, and the test object is irradiated with the first pulse laser. The frequency characteristics of the first pulse laser change in accordance with the light absorption characteristics of the test object in the process of passing through the test object.

  As a result, the frequency characteristic of the interference wave between the first pulse laser and the second pulse laser that has passed through the test object also changes according to the light absorption characteristic of the test object. By obtaining the spectrum, the light absorption characteristics of the test object can be analyzed.

  Even if the test object is disposed between the output coupler 6 and the beam splitter 41a and the test object is irradiated with the second pulse laser, the test object is detected by the beam splitter 41a and the interference wave detector 43. Even if it is arranged in between and the interference wave is irradiated to the test object, a spectrum corresponding to the light absorption characteristic of the test object can be obtained, and the light absorption characteristic of the test object can be analyzed.

  A conventional dual-comb spectrometer has had to prepare two laser oscillators and a repetition frequency stabilization mechanism. However, by using the laser oscillator 1, one laser oscillator and the repetition frequency stabilization mechanism can be eliminated. The device can be simplified and miniaturized.

(5) Other Uses of Laser Oscillator The laser oscillator 1 of the present embodiment has the characteristics as described above. Therefore, it is necessary to use two pulse lasers having different repetition frequencies, for example, Non-Patent Document 2. Optical coherence tomography device described, asynchronous optical sampling device described in Non-Patent Document 3, long-range absolute distance measuring device described in Non-Patent Document 4, CW described in Non-Patent Document 5 It can also be used for a laser high-speed and high-resolution spectroscope. The laser oscillator 1 can be simplified and miniaturized by using it in an apparatus that needs to use two pulse lasers having different repetition frequencies.

(6) Verification Test (6-1) Pulse Laser Spectrum The spectrum of the first pulse laser and the second pulse laser oscillated from the laser oscillator 1 of the present embodiment is spectrograph (product name: LSM, manufactured by Ocean Photonics). -Mini). The result is shown in FIG.

  The horizontal axis of FIG. 4 represents the wavelength of each pulse laser, and the vertical axis represents the intensity (arbitrary unit) of each wavelength of each pulse laser. Output A and Output B in FIG. 4 correspond to Output A and Output B in FIG. 1, respectively. The solid line in FIG. 4 is the spectrum of the second pulse laser, and the broken line is the spectrum of the first pulse laser.

  As shown in FIG. 4, the spectrum of the first pulse laser and the spectrum of the second laser are almost the same, and it can be seen that the laser oscillator 1 can oscillate a laser having a similar spectrum.

(6-2) Repetition frequency of pulse laser The repetition frequency of the first pulse laser and the second pulse laser oscillated from the laser oscillator 1 was measured using a frequency counter (product name: FCA3003, manufactured by Tektronix). The result is shown in FIG.

  In FIG. 5A, the horizontal axis represents time, the vertical axis represents the repetition frequency, and represents the change over time of the repetition frequency of each pulse laser. In FIG. 5B, the horizontal axis represents time, the vertical axis represents the difference in repetition frequency, and the change with time in the difference in repetition frequency between the second pulse laser and the first pulse laser is represented. Output A and Output B in FIG. 5 are the same as those in FIG.

  As shown in FIG. 5A, it can be seen that the repetition frequency of the first pulse laser is different from the repetition frequency of the second pulse laser. Further, as shown in FIG. 5B, the difference between the repetition frequencies of the first pulse laser and the second pulse laser is stable at about 455 Hz ± 3 Hz. Thus, it can be seen that the laser oscillator 1 can oscillate two pulse lasers having different repetition frequencies.

  Subsequently, the laser oscillator 1 was covered with a rectangular parallelepiped box made of aluminum having a thickness of about 5 mm, and the frequency was measured in the same manner. The result is shown in FIG. In FIG. 6, the repetition frequency monotonously decreases with respect to time for both the first pulse laser and the second pulse laser because the room temperature changed during the measurement.

  The horizontal axis in FIG. 6 is time, and the vertical axis is the repetition frequency. In FIG. 6A, the horizontal axis represents time, the vertical axis represents the repetition frequency, and represents the change over time of the repetition frequency of each pulse laser. In FIG. 6B, the horizontal axis represents time, the vertical axis represents the difference in repetition frequency, and the time-dependent change in the difference in repetition frequency between the second pulse laser and the first pulse laser is represented.

  As shown to FIG. 6A, it turns out that the repetition frequency of a 1st pulse laser and a 2nd pulse laser differs. Moreover, as shown to FIG. 6B, it turns out that the difference of the repetition frequency of a 1st pulse laser and a 2nd pulse laser is very stable with about 216.55 +/- 0.15Hz. Thus, it can be seen that the difference in the number of repetitions can be further stabilized by covering the laser oscillator 1 with a box. Therefore, by covering the laser oscillator 1 with a box, the difference in repetition frequency can be stabilized, and the laser oscillator 1 can be measured with higher accuracy by using, for example, a dual-comb spectrometer.

(6-3) Difference in Repetition Frequency of Two Pulse Lasers The position of the gain medium 4 was moved by the frequency difference adjusting unit 13, and the difference between the repetition frequencies of the two pulse lasers of the laser oscillator 1 was measured each time. The method for measuring the repetition frequency is the same as described above. The positions of the second mirror 3 and the lens 8 were similarly moved, and the difference between the repetition frequencies of the two pulse lasers was measured each time.

First, as shown in FIG. 7A, the same components as those in FIG. 1 are given the same reference numerals, and the gain medium 4 is moved in the direction of the second mirror 3 (the direction of the arrow X _crystal shown in FIG. 7A). The position of the center 4a was shifted from the center between the first mirror 2 and the second mirror 3, and the relationship between the position of the gain medium 4 and the difference between the repetition frequencies of the two pulse lasers was examined. FIG. 7B shows the result.

  The horizontal axis of FIG. 7B represents the distance from the initial position of the gain medium 4, and the vertical axis represents the difference between the repetition frequencies of the two pulse lasers. In this test, the state where the center 4a of the gain medium 4 and the center between the first mirror 2 and the second mirror 3 are shifted is set as the initial position. For this reason, even when placed at the initial position, the difference in repetition frequency is about 210 Hz.

  As shown in FIG. 7B, it can be seen that the difference in the repetition frequency of the pulse laser changes by moving the gain medium 4. As described above, the laser oscillator 1 moves the gain medium 4 in the first direction or the second direction along the circular optical path 9 between the first mirror 2 and the second mirror 3 by the frequency difference adjusting unit 13, thereby It can be seen that the difference in repetition frequency between the pulse laser and the second pulse laser can be adjusted.

Next, as shown in FIG. 8A, the same components as those in FIG. 1 are denoted by the same reference numerals, and the second mirror 3 is moved in the direction opposite to the gain medium 4 by the frequency difference adjusting unit 13 (the arrow X _mirror shown in FIG. The position of the center 3a between the first mirror 2 and the second mirror 3 is shifted from the center of the gain medium 4, and the relationship between the position of the second mirror 3 and the repetition frequency difference between the two pulse lasers is examined. It was. FIG. 8B shows the result.

  The horizontal axis of FIG. 8B represents the distance from the initial position of the 2nd mirror 3, and a vertical axis | shaft represents the difference of the repetition frequency of two pulse lasers. In this test, the state where the center of the gain medium 4 and the center 3a between the first mirror 2 and the second mirror 3 were shifted was set as the initial position. For this reason, the difference in repetition frequency is about 120 Hz even when placed at the initial position in the figure.

  As shown in FIG. 8B, it can be seen that when the position of the second mirror 3 changes, the difference between the repetition frequencies of the two pulse lasers changes. As described above, the laser oscillator 1 moves the second mirror 3 along the circular optical path 9 between the first mirror 2 and the second mirror 3 by the frequency difference adjusting unit 13, so that the first pulse laser and the second pulse laser are moved. It can be seen that the difference in the repetition frequency can be adjusted.

Next, as shown in FIG. 9A in which the same components as those in FIG. 1 are denoted by the same reference numerals, the lens 8 is moved from the initial position toward the second mirror by the frequency difference adjusting unit 13, and then the lens 8 is moved to the first. The lens 8 is moved in the opposite direction (the direction of the arrow X _lens shown in FIG. 9A), the position of the focal point 8a of the lens 8 is shifted from the center of the gain medium 4, and the position of the lens 8 and the repetition frequency of the two pulse lasers. The relationship between the differences was investigated. FIG. 9B shows the result.

  The horizontal axis of FIG. 9B represents the distance from the initial position of the lens 8, and the vertical axis represents the difference between the repetition frequencies of the two pulse lasers.

  As shown in FIG. 9B, it can be seen that when the position of the lens 8 changes, the difference between the repetition frequencies of the two pulse lasers changes. Thus, it can be seen that the laser oscillator 1 can adjust the difference between the repetition frequencies of the first pulse laser and the second pulse laser by moving the lens 8 along the excitation light 10 by the frequency difference adjusting unit 13.

(6-4) Dual Comb Spectrometer Using the above-described dual comb spectrometer 40, a spectrum was measured in a state where no test object was installed. The spectrum of the pulse laser of the laser oscillator 1 used in the dual comb spectrometer 40 was measured by the same method as described above. The result is shown in FIG.

  The horizontal axis in FIG. 10 represents the wavelength, and the vertical axis represents the intensity (arbitrary unit) of each wavelength component. FT shown in FIG. 10 is a spectrum measured by the dual comb spectrometer 40, and Spectrometer is the square root of the product of the spectrum of the first pulse laser of the laser oscillator 1 and the spectrum of the second laser. As shown in FIG. 10, both spectra are almost the same.

  When the spectrum is measured by the dual comb spectrometer 40 with no test object installed, the spectrum is considered to be the same as the spectrum of the pulse laser emitted from the laser oscillator 1.

  Therefore, it was confirmed that the spectroscopic measurement can be correctly performed by the dual comb spectroscope 40.

DESCRIPTION OF SYMBOLS 1 Laser oscillator 2 1st mirror 3 2nd mirror 4 Gain medium 5 3rd mirror 6 Output coupler 7 Excitation light source 8 Lens 9 Circulating optical path 13 Frequency difference adjustment part 40 Dual comb spectrometer 41 Photopolymerization part 43 Interference wave detection part 44 Fourier transform

Claims (15)

A first mirror; a second mirror disposed opposite to the first mirror; and a nonlinear crystal disposed between the first mirror and the second mirror. A first mirror, the nonlinear crystal, a first optical path in the order of the second mirror, and the second mirror, the nonlinear crystal, a circular optical path that circulates in the second direction in the order of the first mirror,
The optical path length in the nonlinear crystal is different between the first direction and the second direction,
A first pulse laser in which the light is mode-locked by circling the circulating optical path in the first direction;
A laser oscillator, wherein the light circulates in the second optical path and is mode-locked, and emits a second pulse laser having a repetition frequency different from that of the first pulse laser. The optical path length in the nonlinear crystal is different between the first direction and the second direction,
2. The laser according to claim 1, wherein the nonlinear crystal is arranged so that a center of the nonlinear crystal is different from a focal point of the first mirror and a focal point of the second mirror. Oscillator. The center of the nonlinear crystal and the focal point of the first mirror and the focal point of the second mirror are at different positions,
The laser oscillator according to claim 2, wherein the nonlinear crystal is arranged so that a center of the nonlinear crystal is different from a center between the first mirror and the second mirror. . A frequency difference adjustment unit that adjusts a difference between a repetition frequency of the first pulse laser and a repetition frequency of the second pulse laser by moving at least one position of the first mirror, the second mirror, and the nonlinear crystal. The laser oscillator according to any one of claims 1 to 3, wherein the laser oscillator is provided. The laser oscillator according to claim 1, wherein the circulating optical path includes a gain medium that is excited by the excitation light source and stimulates and emits the light, and the nonlinear crystal. A lens that collects the excitation light of the excitation light source and irradiates the nonlinear crystal;
The optical path length in the nonlinear crystal is different between the first direction and the second direction,
The lens is arranged so that the focal point of the lens is different from at least one of the center of the nonlinear crystal and the focal point of the first mirror and the focal point of the second mirror and the center of the nonlinear crystal. The laser oscillator according to claim 1, wherein A frequency difference adjusting unit that adjusts a difference between a repetition frequency of the first pulse laser and a repetition frequency of the second pulse laser by moving the position of the lens;
The laser oscillator according to claim 6. The laser oscillator according to any one of claims 1 to 7, wherein a difference in repetition frequency between the first pulse laser and the second pulse laser is greater than 0 kHz and equal to or less than 10 kHz. The laser oscillator according to any one of claims 1 to 8,
Photopolymerization means for superimposing the first pulse laser and the second pulse laser emitted from the laser oscillator;
Interference wave detecting means for detecting an interference wave obtained by superimposing the first pulse laser and the second pulse laser by the photopolymerization means;
And a Fourier transform unit for Fourier transforming the detected interference wave. The test object is irradiated with the interference wave,
The spectroscopic apparatus according to claim 9, wherein the interference wave detection unit detects the interference wave transmitted through the test object. The test object is irradiated with the first pulse laser or the second pulse laser,
The said photopolymerization means superimposes the said 1st pulse laser or the said 2nd pulse laser which permeate | transmitted the said test object, and the said 2nd pulse laser or the said 1st pulse laser. The spectroscopic device described.   An optical coherence tomography apparatus comprising the laser oscillator according to claim 1.   An asynchronous optical sampling device comprising the laser oscillator according to claim 1.   A long-distance absolute distance measuring device comprising the laser oscillator according to claim 1.   A CW laser high-speed, high-resolution spectroscopic apparatus comprising the laser oscillator according to claim 1.

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