JP5844694B2 - Terahertz wave generator - Google Patents

Description

  The present invention relates to a terahertz wave generation device, and more particularly to a terahertz wave generation device used for a spectroscopic analysis device, a fluoroscopic device, and the like.

  The conventional terahertz wave generator in the 1.5-7.0 THz frequency range using the Er-doped glass fiber mode-locked laser 10, the lens 12, and the nonlinear optical crystal DAST14 is an ultrashort pulse light with a pulse width of about 70-100fs. Is applied to the nonlinear optical crystal DAST14, and a terahertz wave can be generated by the optical rectification effect, that is, the difference frequency generation between two different wavelengths within the wavelength band of the optical pulse (see FIG. 1).

  An example of a terahertz wave generator using a non-linear optical crystal DAST having a thickness of 0.36 mm has actually been reported (see Non-Patent Document 1 and Non-Patent Document 2). In this example, a relatively thin non-linear optical crystal is used. Since DAST is used, there is a problem that it is difficult to obtain a high-intensity terahertz wave.

  In this method, a terahertz light source can be constructed at low cost because a semiconductor laser-excited Er-doped glass fiber mode-locked laser is used instead of the commonly used solid-laser-excited Ti sapphire laser. Is possible.

"Terahertz spectroscopy system TR-1000", [online], Otsuka Electronics Co., Ltd., [searched on June 11, 2012], Internet <URL: http://www.photal.co.jp/product/tr10_0.html > "Yomimono [3] Physical property evaluation by terahertz spectroscopy-Methods other than time domain spectroscopy and terahertz light sources", [online], Otsuka Electronics Co., Ltd., [June 11, 2012 search], Internet < URL: http://www.photal.co.jp/book/thz_03_01.html> Jun Takayanagi, So Kanamori, Koji Suizu, Masatsugu Yamashita, Toshihiko Ouchi, Shintaro Kasai, Hideyuki Ohtake, Hiromasa Uchida, Norihiko Nishizawa, and Kodo Kawase, `` Generation and detection of broadband coherent terahertz radiation using 17-fs ultrashort pulse fiber laser '', Opt. Express 16, 2008, p.12859 S. Ishibashi and K. Naganuma, `` Diode-pumped Cr4 +: YAG single crystal fiber laser '', in Advanced Solid State Lasers, OSA Technical Digest Series (Optical Society of America, 2000), paper MD4. Z. Zhang, T. Nakagawa, K. Torizuka, T. Sugaya, and K. Kobayashi, `` Gold-reflector-based semiconductor saturable absorber mirror for femtosecond mode-locked Cr4 +: YAG lasers '', Applied Physics B: Lasers and Optics, Volume 70, Supplement 1, 2004, S59-S62 Yuzo Ishida and Kazunori Naganuma, "Characteristics of femtosecond pulses near 1.5 μm in a self-mode-locked Cr4 +: YAG laser", Optics Letters, vol. 19, no. 23, 1994, p. 2003

  In order to increase the generation intensity of the terahertz wave, it is desirable to use a thick nonlinear optical crystal DAST. Specifically, it is desirable to use a nonlinear optical crystal DAST having a thickness of 0.5 mm or more.

  However, even when the nonlinear optical crystal DAST having a thickness of 0.5 mm or more is used, as shown in FIG. 1, the center wavelength of the ultrashort pulse light generated by the Er-doped glass fiber mode-locked laser 10 is 1.55 μm. Therefore, the phase matching of terahertz wave generation cannot be sufficiently obtained in the vicinity of 3 THz, 4-5 THz, and 6-7 THz (see Non-Patent Document 3). Therefore, there is a problem that the terahertz wave output power in these frequency ranges does not reach the output obtained by perfect phase matching. In order to solve this problem, it is very important to avoid an increase in device price.

The present invention is a terahertz wave generator composed of a pulsed light generator and a nonlinear crystal DAST irradiated with pulsed light generated from the pulsed light generator, wherein the pulsed light generator is in order from the output side, It consists of an output mirror, a Cr ⁴⁺ : YAG single crystal fiber waveguide, a first spherical mirror, a quartz glass rod for dispersion compensation, a second spherical mirror, and an optical path switching mirror. In order to propagate light in either the first optical path or the second optical path, switching is possible by an electric signal.

  One embodiment of the present invention is characterized in that a semiconductor laser as an excitation light source is further provided in front of the first spherical mirror.

  In one embodiment of the present invention, in the pulsed light generator, the first semiconductor saturable absorber mirror is disposed in the first optical path, and the first semiconductor saturable absorber mirror exceeds 1.45 to 1.65 μm. A saturable absorption wavelength range, a high reflectance in the range of 1.45 to 1.65 μm, a dispersion compensating mirror and a second semiconductor saturable absorber mirror are disposed in the second optical path, The semiconductor saturable absorber mirror No. 2 has a saturable absorption wavelength range exceeding 1.30 to 1.50 μm, a high reflectance in the range of 1.30 to 1.50 μm, and a single value in the range of 1.47 to 1.57 μm. And an optical pulse having a central wavelength (average wavelength) in the range of 1.35 to 1.45 μm.

  The present invention relates to a terahertz wave generator that generates terahertz waves by irradiating a nonlinear optical crystal DAST with an ultrashort pulse laser beam.

In the terahertz wave generator according to the present invention, a Cr ⁴⁺ : YAG single crystal fiber is used as an amplifying medium, and a pulse center wavelength (average wavelength) is between 1.47-1.57 μm and a pulse center wavelength ( An ultrashort pulse laser that can variably output a total of two wavelengths of light, one with an average wavelength between 1.35-1.45 μm. With such a configuration, a terahertz wave having an intensity that has no room for improvement in phase matching over a frequency range of 1.5 to 7.0 THz through a nonlinear optical crystal DAST having a thickness of 0.5 mm or more that enables generation of a high-power terahertz wave. Can be output.

  A terahertz wave is generated by the optical rectification effect, that is, the difference frequency generation between two different wavelengths within the wavelength band of the optical pulse. In the terahertz wave generator according to the present invention, when generating a high-power terahertz wave at a frequency of 1.5 to 2.5 THz, a nonlinear optical signal is used for an ultrashort pulse light having a single wavelength between 1.47-1.57 μm in the center wavelength (average wavelength). When irradiating a crystal DAST and generating a high-power terahertz wave at a frequency of 2.5-7.0 THz, irradiates the nonlinear optical crystal DAST with one short-wavelength light with a center wavelength (average wavelength) of 1.35-1.45 μm. By doing so, phase matching can be sufficiently obtained over 1.5 to 7.0 THz (see Non-Patent Document 3).

Further, since the terahertz wave generating apparatus according to the present invention is composed of a Cr ⁴⁺ : YAG single crystal fiber that can directly excite a semiconductor laser, it can be constructed at low cost.

It is a figure for demonstrating the terahertz wave generator in the 1.5-7.0 THz frequency range comprised from the conventional Er dope glass fiber mode-locked laser and the nonlinear optical crystal DAST. It is a figure which shows the structure of the pulsed light generator concerning one Example of this invention. FIG. 2 is a diagram for explaining a terahertz wave generator in a frequency range of 1.5 to 7.0 THz composed of a Cr ⁴⁺ : YAG single crystal fiber mode-locked laser with a variable center wavelength and a nonlinear optical crystal DAST according to the present invention. .

  The present invention is an apparatus for generating terahertz waves by irradiating a nonlinear optical crystal DAST with pulsed light. Here, the present invention is an apparatus for generating pulsed light for irradiating DAST. One pulsed light having a pulse center wavelength (average wavelength) of 1.47-1.57 μm and the center wavelength (average wavelength) of the pulse. Is used, and a light source capable of switching and generating these two types of pulsed light is used.

Specifically, an apparatus for generating pulsed light includes a Cr ⁴⁺ : YAG single crystal (more specifically, a Cr ⁴⁺ : YAG single crystal fiber) and a dispersion compensation medium (specifically, a laser resonator). Is a device in which a quartz glass rod and a dispersion compensating mirror) and a saturable absorber (specifically, a semiconductor saturable absorber mirror) are arranged. The pulsed light generator further includes an optical path switching mirror, and the center wavelength (average wavelength) can be selected by the optical path switching mirror. The pulsed light generator is a Cr ⁴⁺ : YAG pulse laser (more specifically, a Cr ⁴⁺ : YAG single crystal fiber pulse laser).

A Cr ⁴⁺ : YAG single crystal fiber pulsed laser used as a pulsed laser is a mode-locked laser and has a configuration that is directly pumped by a semiconductor laser in the same manner as an Er-doped glass fiber mode-locked laser according to the prior art. Cr ⁴⁺ : YAG single crystal fiber mode-locked lasers are inexpensive, and the price is comparable to Er-doped glass fiber mode-locked lasers.

The oscillation pulse width of the Cr ⁴⁺ : YAG single crystal fiber mode-locked laser can be 50 fs or less, and has a sufficient frequency band for generating terahertz waves up to a frequency of 7.0 THz.

In order to obtain perfect phase matching in the 1.5-7.0 THz frequency range, the Cr ⁴⁺ : YAG single crystal fiber mode-locked laser has a center wavelength (average wavelength) of the output ultrashort pulse of, for example, 1.55 μm and 1.40. It is designed to be variable with a binary value of μm.

The Cr ⁴⁺ : YAG single crystal fiber mode-locked laser is an external resonator type composed of a single crystal fiber waveguide and a spatial optical system, and an optical path switching mirror is installed in the optical path of the spatial optical system. A saturable absorber mirror for generating an ultrashort pulse with a center wavelength (average wavelength) of 1.55 μm is disposed in one optical path from the switching mirror. A dispersion compensating mirror and a saturable absorber mirror for generating an extremely short pulse having a center wavelength (average wavelength) of 1.40 μm are disposed in the other optical path from the switching mirror. The switching mirror is switched by an electrical signal as necessary.

  When a high output is generated at a terahertz wave frequency of 1.5 to 2.5 THz, the nonlinear optical crystal DAST is irradiated with an extremely short pulse having a center wavelength (average wavelength) of 1.55 μm. When a high output is generated at a terahertz wave frequency of 2.5-7.0 THz, the nonlinear optical crystal DAST is irradiated with an extremely short pulse having a center wavelength (average wavelength) of 1.40 μm. The validity of these settings regarding the center wavelength can be confirmed from the phase matching data (see Non-Patent Document 3).

  In the terahertz wave generator according to the present invention having the structure as described above, perfect phase matching is achieved in a frequency range from 1.5 to 7.0 THz, including around 3.0 THz, around 4.0 to 5.0 THz, and around 6.0 to 7.0 THz. Terahertz waves can be generated. Therefore, according to the present invention, a thickness of 0.5 mm, which was a problem in a terahertz wave generator having a configuration in which the nonlinear optical crystal DAST is irradiated with an output pulse from an ultrashort pulse laser with a center wavelength fixed to 1.55 μm according to the prior art, When the above nonlinear optical crystal DAST is used, the problem that the output power of the terahertz wave that should be generated by perfect phase matching cannot be obtained can be solved.

The terahertz wave generator according to the present embodiment includes a pulsed light generator and a nonlinear crystal DAST irradiated with pulsed light generated from the pulsed light generator. The pulsed light generator is a mode-locked laser that is directly pumped by a semiconductor laser using a Cr ⁴⁺ : YAG single crystal fiber as an amplification medium. The mode-locked laser output pulse is applied to the nonlinear crystal DAST to generate a terahertz wave with a frequency of 1.5-7.0 THz.

  The features (1) to (4) of this embodiment are listed below.

  (1) The resonator has two optical paths separated by a switching mirror. In one optical path, a semiconductor saturable absorber mirror is disposed so as to generate an extremely short pulse having a central wavelength (average wavelength) of 1.55 μm. In the other optical path, a dispersion compensating mirror and a semiconductor saturable absorber mirror are arranged so as to generate an ultrashort pulse having a center wavelength (average wavelength) of 1.40 μm.

  (2) The switching mirror has an automatic switching function using an electrical signal.

(3) In this embodiment, the pulsed light generator is a Cr ⁴⁺ : YAG single crystal fiber mode-locked laser, and uses a Cr ⁴⁺ : YAG single crystal fiber as an amplification medium. Therefore, a configuration in which the semiconductor laser can be directly excited is possible, and the cost is as low as when the Er-doped glass fiber mode-locked laser according to the prior art is used.

  (4) When the terahertz wave generator according to this embodiment is used to generate a high output at a terahertz wave frequency of 1.5 to 2.5 THz, an ultrashort pulse light having a center wavelength (average wavelength) of 1.55 μm is used as a nonlinear crystal. When irradiating the DAST and generating a high output at a frequency of 2.5-7.0 THz, the nonlinear crystal DAST is irradiated with an ultrashort pulse light having a center wavelength (average wavelength) of 1.40 μm. With the terahertz wave generating apparatus according to the present embodiment, phase matching can be sufficiently obtained over 1.5 to 7.0 THz, and a terahertz wave having an intensity with no room for improvement in phase matching can be output.

FIG. 2 shows a configuration of a pulsed light generator according to the present embodiment, that is, a Cr ⁴⁺ : YAG single crystal fiber mode-locked laser having a structure that is directly excited by the semiconductor laser 36.

As shown in FIG. 2, the pulsed light generator according to this embodiment includes, in order from the output side, an output mirror 20, a Cr ⁴⁺ : YAG single crystal fiber waveguide 22, a first spherical mirror 24, and dispersion compensation. And a second spherical mirror 28 and a structure that reaches the optical path switching mirror 30 (refer to Non-Patent Document 4 regarding the Cr ⁴⁺ : YAG single crystal fiber waveguide).

  The reflectivity of the output mirror 20 is 99% at the oscillation wavelength.

  The reflectance of the first spherical mirror 24 is 99.9% or more at the oscillation wavelength, and the transmittance of the first spherical mirror 24 is 95% or more at the excitation wavelength.

  The quartz glass rod 26 is arranged so that the oscillation light enters or exits at a Brewster angle at both ends.

  The reflectance of the second spherical mirror 28 is 99.9% or more at the oscillation wavelength.

  The optical path switching mirror 30 has a reflectance of 99.9% or more at a wavelength of 1.30-1.50 μm. When pulse light having a center wavelength (average wavelength) of 1.55 μm is generated, the light is transmitted as it is without being reflected and guided to the first optical path. When pulse light having a center wavelength (average wavelength) of 1.40 μm is generated, the light is reflected and guided to the second optical path. Switching of the optical path switching mirror 30 is automatically performed by an electric signal.

  A first semiconductor saturable absorber mirror 32 is disposed on the transmission side, that is, the first optical path. Regarding the operating wavelength range of the semiconductor saturable absorber mirror, the semiconductor saturable absorber mirror has a wide saturable absorption wavelength range. The wide saturable absorption wavelength region of the semiconductor saturable absorber mirror greatly exceeds the wavelength region of the optical pulse generated and used in the present invention. The value of the saturable absorption changes little by little depending on the wavelength.

  The first semiconductor saturable absorber mirror 32 has a high reflectivity of 98% or more at a wavelength of 1.45-1.65 μm, a saturable absorption wavelength region in the range exceeding the wavelength of 1.45-1.65 μm, and its value is 1.55 μm. The reflectance change is 1% at (see cited document 5). Oscillation light in the resonator is folded back by the first semiconductor saturable absorber mirror 32.

  Two dispersion compensating mirrors 34 and a second semiconductor saturable absorber mirror 36 are disposed on the reflection side, that is, the second optical path. The dispersion compensation mirror 34 and the second semiconductor saturable absorber mirror 36 are arranged so that each of the two dispersion compensation mirrors 34 is reflected twice each way. The light reflected by the optical path switching mirror 30 is reflected a total of four times by the two dispersion compensating mirrors 34 and then folded by the second semiconductor saturable absorber mirror 36. The second semiconductor saturable absorber mirror 36 has a high reflectance of 98% or more at a wavelength of 1.30-1.50 μm, and has a saturable absorption wavelength range exceeding 1.30-1.50 μm, and its value is 1.40 μm. The reflectance change is 1%.

From the outside of the first spherical mirror 24, pumping light output from the semiconductor laser 38 (wavelength 0.9-1.1 μm, output 10 W or more) is output to the end of the Cr ⁴⁺ : YAG single crystal fiber waveguide 22 by a lens (not shown). Condensate. In this example, a single crystal fiber having a diameter of about 120 μm was used. Since the Cr ⁴⁺ : YAG single crystal fiber waveguide 22 has a SiO ₂ cladding on the side surface, even if the pumping light is a diffusive high-power semiconductor laser beam, it can be confined in the fiber (see Non-Patent Document 4). reference).

When generating ultrashort pulse light, it is important to correct the group delay dispersion in the optical path in the resonator. When a Cr ⁴⁺ : YAG single crystal fiber waveguide having a length of 40 mm is used, a group delay dispersion of +400 fs ^{2 at} a wavelength of 1.55 μm and +1900 fs ^{2 at} a wavelength of 1.40 μm occurs. The amount of group delay dispersion required for generating an extremely short pulse with a pulse width of about 50 fs is about −2300 fs ² (see Non-Patent Document 5). Therefore, generally -2700Fs ^2, is generally dispersion compensation -4200Fs ² at a wavelength of 1.40μm is determined at a wavelength of 1.55μm for the dispersion compensation medium (silica glass rod 26, the dispersion compensation mirror 34, etc.).

Setting the length of the quartz glass rod 26 to 48.4mm, -2700fs ² at a wavelength of 1.55 .mu.m, group delay dispersion of -1130Fs ² is obtained at a wavelength of 1.40 .mu.m. By arranging ^two dispersion compensation mirrors 34 that obtain a dispersion compensation of approximately -380 fs ² at a wavelength of 1.40 μm in the second optical path that generates a pulse with a center wavelength (average wavelength) of 1.40 μm, and reflecting it eight times in total. In both the first optical path and the second optical path, group delay dispersion necessary for generating an ultrashort pulse can be obtained.

The output light of the pulsed light generator (specifically, a Cr ⁴⁺ : YAG single crystal fiber mode-locked laser with a variable center wavelength) 40 according to the present embodiment described above is applied to a nonlinear crystal DAST 14 having a thickness of 1.0 mm. A terahertz wave was generated by condensing with the lens 12 (see FIG. 3).

  When high output is required at terahertz wave frequency 1.5-2.5THz, the switching mirror is controlled so that light propagates through the transmission side (first optical path), and the center wavelength (average wavelength) is 1.55μm, Ultrashort pulse light having a pulse width of about 50 fs is generated and irradiated to the nonlinear crystal DAST14. When generating a high output at a frequency of 2.5-7 THz, the switching mirror is controlled so that the light propagates on the reflection side (second optical path), the center wavelength (average wavelength) is 1.40 μm, and the pulse width is 50 fs. An extremely short pulse light of a certain degree is generated and irradiated to the nonlinear crystal DAST14.

  With the terahertz wave generator as described above, the phase matching of the optical rectification effect can be sufficiently obtained over 1.5-7 THz, and a terahertz wave having an intensity that cannot be improved can be output.

10 Er-doped glass fiber mode-locked laser 12 Lens 14 Nonlinear optical crystal DAST
20 output mirror 22 Cr ⁴⁺ : YAG single crystal fiber waveguide 24 first spherical mirror 26 quartz glass rod 28 second spherical mirror 30 optical path switching mirror 32 first semiconductor saturable absorber mirror 34 dispersion compensation mirror 36 second Semiconductor saturable absorber mirror 38 Semiconductor laser 40 Center wavelength tunable Cr ⁴⁺ : YAG single crystal fiber mode-locked laser

Claims (2)

A pulsed light generator;
A terahertz wave generator composed of a nonlinear crystal DAST irradiated with pulsed light generated from a pulsed light generator,
The pulsed light generator is in order from the output side,
An output mirror;
Cr ⁴⁺ : YAG single crystal fiber waveguide,
A first spherical mirror;
A quartz glass rod for dispersion compensation;
A second spherical mirror;
It consists of an optical path switching mirror,
The optical path switching mirror, in order to propagate the light to either the first optical path or the second optical path, Ri switchable der by an electric signal,
A first semiconductor saturable absorber mirror is disposed in the first optical path, and the first semiconductor saturable absorber mirror has a saturable absorption wavelength region in a range exceeding 1.45 to 1.65 μm, and is 1.45. High reflectivity in the range of ~ 1.65μm,
A dispersion compensation mirror and a second semiconductor saturable absorber mirror are disposed in the second optical path, and the second semiconductor saturable absorber mirror has a saturable absorption wavelength range in a range exceeding 1.30 to 1.50 μm. And has a high reflectance in the range of 1.30 to 1.50 μm,
The pulsed light generator generates an optical pulse having a central wavelength (average wavelength) of one value in the range of 1.47 to 1.57 μm and one value in the range of 1.35 to 1.45 μm.
The terahertz wave generator is characterized by being adjusted as described above .   The terahertz wave generating device according to claim 1, further comprising a semiconductor laser as an excitation light source in front of the first spherical mirror.

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