Terahertz wave generator and terahertz wave generation method

Abstract

To provide a terahertz wave generator and a terahertz wave generation method that can generate terahertz waves with high efficiency. [Solution] The terahertz wave generator 1 comprises an oscillator 2 that oscillates ultrashort pulse light L, a fiber amplifier 3 that broadens the spectrum of the ultrashort pulse light L oscillated by the oscillator 2, a soliton shift fiber 6 that modulates the wavelength of the ultrashort pulse light L whose spectrum has been broadened by the fiber amplifier 3 using soliton self-frequency shift, and an organic crystal 9 that generates terahertz waves when irradiated with the ultrashort pulse light L modulated by the soliton shift fiber 6.

Description

Technical Field

[0001] One aspect of the present disclosure relates to a terahertz wave generation device.

Background Art

[0002] A pulse light generation device including an oscillation unit that oscillates pulsed light and a modulation unit that modulates the wavelength of the pulsed light oscillated by the oscillation unit using soliton self-frequency shift is known. In such a pulse light generation device, by increasing the intensity of the pulsed light before modulation by the modulation unit, it is intended to split the pulsed light into a plurality of pulsed lights with different wavelengths (output multi-colored solitons) by this modulation (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] A technique is known in which terahertz waves are generated by irradiating an organic crystal with pulsed light output from a pulsed light generation device as described above, and the generated terahertz waves are detected. Here, in the above pulsed light generation device, when the intensity of the pulsed light before modulation by the modulation unit is increased, the pulsed light may split due to this modulation and a plurality of pulsed lights may be formed (hereinafter, also referred to as "multi-solitonization"). In this case, pulse splitting may also occur in the terahertz waves generated in the subsequent stage, and there is a risk of inhibiting highly efficient terahertz wave generation.

[0005] One aspect of the present disclosure has been made in view of the above circumstances, and an object thereof is to provide a terahertz wave generation device and a terahertz wave generation method capable of generating terahertz waves with high efficiency.

Means for Solving the Problems

[0006] The terahertz wave generator of the present disclosure is a terahertz wave generator comprising: [1] an oscillator that emits pulsed light; a first amplifier that broadens the spectrum of the pulsed light emitted by the oscillator; a modulation unit that modulates the wavelength of the pulsed light whose spectrum has been broadened by the first amplifier using soliton self-frequency shift; and a terahertz wave generator that generates terahertz waves when the pulsed light modulated by the modulation unit is irradiated.

[0007] As a result of diligent research, the inventors have found that multi-solitonization can be suppressed by broadening the bandwidth of the pulse light spectrum before modulation using soliton self-frequency shift. Therefore, in the terahertz wave generator of this disclosure, the spectrum of the pulse light is broadened, and the wavelength of the broadened pulse light is modulated using soliton self-frequency shift. This makes it possible to suppress multi-solitonization even when, for example, the intensity of the pulse light before modulation is increased. When pulse light with suppressed multi-solitonization is irradiated onto the terahertz wave generator, terahertz waves can be generated without causing pulse splitting. This makes it possible to generate terahertz waves with high efficiency.

[0008] The terahertz wave generator of the present disclosure may also be [2] "the terahertz wave generator according to [1], wherein the terahertz wave generating unit comprises an organic crystal." Such a configuration makes it possible to generate terahertz waves with higher efficiency.

[0009] The terahertz wave generator of the present disclosure may also be the "terahertz wave generator according to [2], wherein the terahertz wave generating unit is configured to include at least one of DAST, DASC, and DSTMS as the organic crystal." Such a configuration makes it possible to generate terahertz waves with higher efficiency.

[0010] The terahertz wave generator of the present disclosure may also be [4] "the terahertz wave generator according to any one of [1] to [3], wherein the terahertz wave generating unit generates terahertz waves having a frequency in the range of 0.01 to 30 THz." By using such a frequency range, the absorption characteristic of terahertz waves increases, and appropriate spectroscopy can be performed.

[0011] The terahertz wave generator of the present disclosure may also be [5] "a terahertz wave generator according to any one of [1] to [4], further comprising a second amplification unit configured to include a thulium-doped fiber amplifier for amplifying the pulsed light modulated by the modulation unit, wherein the terahertz wave generator generates terahertz waves when pulsed light modulated by the modulation unit and amplified by the second amplification unit is irradiated onto the terahertz wave generator." In this way, terahertz waves can be generated with higher efficiency by irradiating the terahertz wave generator with pulsed light amplified by the second amplification unit.

[0012] The terahertz wave generator of the present disclosure may also be [6] "a terahertz wave generator according to any one of [1] to [5], further comprising a terahertz wave detection unit for detecting the terahertz waves output from the terahertz wave generator." In this way, by providing a terahertz wave detection unit in the terahertz wave generator, the terahertz waves generated with high efficiency can be easily and quickly detected.

[0013] The terahertz wave generator of the present disclosure may also be [7] "a terahertz wave generator according to any one of [1] to [6], further comprising: an optical branching unit that branches the pulse light modulated by the modulation unit into pump light irradiated onto the terahertz wave generator and probe light; an optical path delay unit that delays the probe light in time by changing the optical path length of the probe light; and a terahertz wave detection crystal into which the pump light irradiated onto the sample and the probe light that has passed through the optical path delay unit are incident." With such a configuration, the complex refractive index of the sample can be obtained using terahertz time-domain spectroscopy, and the state of the sample can be appropriately detected. Furthermore, since residual light components that did not contribute to terahertz wave generation are used as probe light for terahertz wave detection, terahertz waves can be detected efficiently while making use of light without waste.

[0014] The terahertz wave generator of the present disclosure may also be [8] "a terahertz wave generator according to any one of [1] to [5], further comprising a terahertz wave measuring unit for measuring the terahertz waves output from the terahertz wave generating unit, wherein the terahertz wave measuring unit includes a photomultiplier tube that is sensitive to the optical band including terahertz waves." With such a configuration, the interference time waveform and spectrum of efficiently generated terahertz waves can be measured quickly and efficiently.

[0015] The terahertz wave generator of the present disclosure may also be [9] "a terahertz wave generator according to any one of [1] to [5], further comprising a terahertz wave measuring unit for measuring the terahertz waves output from the terahertz wave generating unit, wherein the terahertz wave measuring unit includes an image intensifier for electronically converting the terahertz waves to acquire an image." With such a configuration, the interference time waveform and spectral image of the efficiently generated terahertz waves can be measured at high speed and efficiently.

[0016] The terahertz wave generation method of the present disclosure may also be

[10] "a terahertz wave generation method comprising oscillating pulsed light, broadening the spectrum of the pulsed light, modulating the wavelength of the broadened pulsed light using soliton self-frequency shift, and generating terahertz waves by irradiating an organic crystal with the modulated pulsed light." [Effects of the Invention]

[0017] According to one aspect of this disclosure, it is possible to provide a terahertz wave generator and a terahertz wave generation method that can generate terahertz waves with high efficiency. [Brief explanation of the drawing]

[0018] [Figure 1] This is a block diagram showing a terahertz wave generator according to an embodiment. [Figure 2] Figure 2(a) is a graph showing the time waveform of the ultrashort pulse light output from the oscillator in Figure 1. Figure 2(b) is a graph showing the spectrum of the ultrashort pulse light output from the oscillator in Figure 1. Figure 2(c) is a graph showing the time waveform of the ultrashort pulse light output from the fiber amplifier in Figure 1. Figure 2(d) is a graph showing the spectrum of the ultrashort pulse light output from the fiber amplifier in Figure 1. [Figure 3] Figure 3 is a graph showing a specific example of the spectrum of ultrashort pulse light output from the fiber amplifier in Figure 1. [Figure 4] Figure 4(a) is a graph showing the time waveform of the ultrashort pulse light output from the acousto-optic modulator shown in Figure 1. Figure 4(b) is a graph showing the spectrum of the ultrashort pulse light output from the acousto-optic modulator shown in Figure 1. [Figure 5]Fig. 5(a) is a graph showing the temporal waveform of the ultrashort pulsed light output from the soliton shift fiber of Fig. 1. Fig. 5(b) is a graph showing the spectrum of the ultrashort pulsed light output from the soliton shift fiber of Fig. 1. Fig. 5(c) is a graph showing the temporal waveform of the ultrashort pulsed light output from the filter. Fig. 5(d) is a graph showing the spectrum of the ultrashort pulsed light output from the filter. [Figure 6] Fig. 6(a) is a graph showing the relationship between the intensity of the ultrashort pulsed light input to the soliton shift fiber, the wavelength of the soliton, and the number of solitons. Fig. 6(b) is a graph showing the relationship between the spectral width of the ultrashort pulsed light input to the soliton shift fiber, the wavelength of the soliton, and the number of solitons. [Figure 7] Fig. 7 is a diagram for explaining terahertz wave generation. [Figure 8] Fig. 8 is a diagram for explaining terahertz wave detection. [Figure 9] Fig. 9 is a flowchart showing an example of a terahertz wave generation and detection method. [Figure 10] Fig. 10 is a diagram for explaining terahertz wave detection according to a modified example. [Figure 11] Fig. 11 is a diagram for explaining terahertz wave detection according to a modified example. [Figure 12] Fig. 12 is a diagram showing an example of the configuration of a terahertz measurement unit.

Embodiments for Carrying Out the Invention

[0019] Hereinafter, embodiments will be described in detail with reference to the drawings. In each figure, the same or corresponding parts are denoted by the same reference numerals, and duplicate explanations are omitted.

[0020] As shown in Figure 1, the terahertz wave generator 1 according to this embodiment generates long-wavelength ultrashort pulse light (pulsed light) L using soliton self-frequency shift (Raman soliton shift). The terahertz wave generator 1 comprises an oscillator 2 (oscillator), a fiber amplifier 3 (first amplifier), an acousto-optic modulator 4, a compressor 5, a soliton-shift fiber 6 (modulation unit), a stretcher fiber 7, a fiber amplifier 8 (second amplifier), an organic crystal 9 (terahertz wave generation unit), and a detection unit 10 (terahertz wave detection unit). The terahertz wave generator 1 functions as a tunable laser device, and its wavelength range may be 1500 nm to 4000 nm, its average power 1 mW or more, its repetition frequency 1 MHz or more, and its pulse width 10 ps or less (for example, 100 fs or less).

[0021] Oscillator 2 constitutes an oscillator that oscillates ultrashort pulse light L. As shown in Figure 2(a), oscillator 2 generates an ultrashort pulse train with a predetermined period. Here, oscillator 2 oscillates ultrashort pulse light L having a spectrum with a first spectral width H1 and a first intensity K1, as shown in Figure 2(b). For example, oscillator 2 oscillates ultrashort pulse light L with a wavelength band of 1550 nm or less. Oscillator 2 is not particularly limited, and various oscillators can be used.

[0022] The fiber amplifier 3 constitutes an amplification section that broadens the spectrum of the ultrashort pulse light L oscillated by the oscillator 2. The fiber amplifier 3 broadens the spectrum of the ultrashort pulse light L and increases the output power of the ultrashort pulse light L by simillariton amplification. The fiber amplifier 3 is positioned between the oscillator 2 and the soliton-shift fiber 6 in the optical path of the ultrashort pulse light L.

[0023] The fiber amplifier 3 includes a fiber amplifier. The fiber amplifier in the fiber amplifier 3 is a normally dispersed fiber, specifically a double-clad fiber co-doped with erbium and ytterbium. That is, the fiber amplifier 3 performs amplification while introducing a nonlinear effect using a normally dispersed double-clad fiber without stretching, thereby obtaining ultrashort pulse light L as broadband amplifier light. A normally dispersed fiber is a fiber with a negative dispersion parameter D (ps / nm / km). The dopants used in the fiber amplifier 3 are not particularly limited, and various dopants may be employed.

[0024] As shown in Figures 2(c) and 2(d), the fiber amplifier 3 broadens the spectral width of the ultrashort pulse light L to a second spectral width H2, which is wider than the first spectral width H1. The fiber amplifier 3 also increases the intensity of the ultrashort pulse light L to a second intensity K2, which is higher than the first intensity K1. Specifically, as shown in Figure 3, the fiber amplifier 3 sets the spectral width of the ultrashort pulse light L to 100 nm or more. In Figure 3, the horizontal axis represents the wavelength of the ultrashort pulse light L, and the vertical axis represents the relative value of the intensity of the ultrashort pulse light L relative to a predetermined intensity.

[0025] The acousto-optic modulator 4 constitutes an optical intensity control unit that controls the intensity of the ultrashort pulse light L pulse by pulse. The acousto-optic modulator 4 is a device that modulates the ultrashort pulse light L using the power of sound (sound waves), and is called an AOM (Acousto Optic Modulator). In this embodiment, the acousto-optic modulator 4 is placed between the fiber amplifier 3 and the soliton shift fiber 6 in the optical path of the ultrashort pulse light L. The acousto-optic modulator 4 may be placed at any position between the oscillator 2 and the soliton shift fiber 6. As shown in Figures 4(a) and 4(b), the acousto-optic modulator 4 controls the intensity of the ultrashort pulse light L so that it changes with each pulse. For example, as shown in Figure 4(a), when intensity modulations M1 and M2 are applied, ultrashort pulse lights LM1 and LM2 corresponding to the intensities applied by M1 and M2 are generated, as shown in Figure 4(b). The intensity modulation range and accuracy of the ultrashort pulse light L (LM1, LM2) depend on the performance of the acousto-optic modulator 4. The intensity of each pulse in the pulse train of ultrashort pulse light L can be arbitrarily modulated by the acousto-optic modulator 4.

[0026] The compressor 5 constitutes a pulse compression unit that compresses the pulse duration of the ultrashort pulse light L. In this embodiment, the compressor 5 is positioned between the acousto-optic modulator 4 and the soliton shift fiber 6 in the optical path of the ultrashort pulse light L. The compressor 5 may be positioned at any location between the fiber amplifier 3 and the soliton shift fiber 6. Even if the ultrashort pulse light L is stretched (for example by several picoseconds) by the fiber amplifier 3, the compressor 5 compresses the pulse duration of the ultrashort pulse light L and outputs an ultrashort pulse light L with a duration of less than or equal to a certain extent (less than 1 picosecond). The compressor 5 is not particularly limited, and various compressors can be used.

[0027] The soliton shift fiber 6 constitutes a modulation section that modulates the wavelength of the ultrashort pulse light L, whose spectrum has been broadened and output increased by the fiber amplifier 3, using soliton self-frequency shifting. The soliton shift fiber 6 is positioned downstream of the fiber amplifier 3 in the optical path of the ultrashort pulse light L. As shown in Figures 5(a) and 5(b), the soliton shift fiber 6 lengthens the wavelength of the ultrashort pulse light L and generates soliton S1. For example, the soliton shift fiber 6 can be a single-mode anomalous dispersion fiber that exhibits anomalous dispersion in the wavelength band of the ultrashort pulse light L generated by the fiber amplifier 3. In addition, by controlling the acousto-optic modulator 4, it is also possible to generate solitons with wavelengths different from soliton S1. When the wavelength of soliton S is modulated with intensities M1 and M2, as shown in Figure 5(c), the wavelength of soliton S shifts to wavelengths corresponding to the intensities M1 and M2, as shown in Figure 5(d) (solitons S1 and S2). The shift wavelength range and accuracy of soliton S depend on the performance of the acousto-optic modulator 4. The shift wavelength of each soliton S in the soliton train generated from the pulse train of ultrashort pulse light L can be arbitrarily changed by applying intensity modulation to the pulse train with the acousto-optic modulator 4. In the illustrated example, the ultrashort pulse light L modulated by soliton self-frequency shift includes a non-soliton component S0 (a component that did not become soliton S1 or S2).

[0028] A filter (not shown) filters the ultrashort pulse light L whose wavelength has been modulated by the soliton shift fiber 6. The filter is located downstream of the soliton shift fiber 6 in the optical path of the ultrashort pulse light L. In the example shown, the filter cuts out the non-soliton component S0 of the ultrashort pulse light L, as shown in Figures 5(b) and 5(d). The filter preferably has an OD value of 3 or higher. The filter is not particularly limited, and various filters can be used.

[0029] The reason for broadening the spectrum by the fiber amplifier 3 in front of the soliton shift fiber 6 will be explained. Figure 6(a) is a graph showing the relationship between the intensity of the ultrashort pulse light L input to the soliton shift fiber 6, the wavelength of the solitons, and the number of solitons. Figure 6(b) is a graph showing the relationship between the spectral width of the ultrashort pulse light L input to the soliton shift fiber 6, the wavelength of the solitons, and the number of solitons. As shown in Figure 6(a), increasing the intensity of the ultrashort pulse light L input to the soliton shift fiber 6 lengthens the wavelength of the solitons generated by soliton self-frequency shift. In this case, the wavelength tuning range of the ultrashort pulse light L is thought to widen. On the other hand, if the intensity of the ultrashort pulse light L input to the soliton shift fiber 6 is too high, a phenomenon called multi-solitonization may occur, resulting in multiple solitons. From a practical standpoint, for example, it is desirable to suppress multi-solitonization. Therefore, the inventors conducted further intensive studies and, as shown in Figure 6(b), found that multi-solitonization can be suppressed by broadening the spectral width of the ultrashort pulse light L input to the soliton shift fiber 6, that is, by broadening the spectrum of the ultrashort pulse light L before modulation using soliton self-frequency shift. Thus, by broadening the spectrum of the ultrashort pulse light L with the fiber amplifier 3 and modulating the wavelength of the broadened ultrashort pulse light L using soliton self-frequency shift, it is possible to increase the intensity of the ultrashort pulse light L input to the soliton shift fiber 6, efficiently lengthen the wavelength of the solitons, broaden the wavelength tuning range, and suppress multi-solitonization.

[0030] Returning to Figure 1, the stretcher fiber 7 is a stretcher that widens the time width of ultrashort pulse light L. The wavelength band of the ultrashort pulse light L widened by the stretcher fiber 7 is, for example, 1800 nm to 2000 nm. The stretcher fiber 7 is composed of a first fiber that widens the time width of ultrashort pulse light L with a first characteristic and a second fiber that widens the time width of ultrashort pulse light L with a second characteristic different from that of the first fiber. The first and second fibers are configured to widen the time width of ultrashort pulse light L by creating a difference in the optical path length for each wavelength due to the difference in refractive index of each wavelength when passing through the ultrashort pulse light L. The first fiber widens the time width of ultrashort pulse light L, which is output from the soliton shift fiber 6 and includes a wavelength band of, for example, 1800 nm to 2000 nm, with the first characteristic and outputs it to the second fiber. The first fiber may be, for example, a normal dispersion fiber. The second fiber is connected to the first fiber and widens the time width of the ultrashort pulse light L input from the first fiber according to the second characteristic, and outputs it to the fiber amplifier 8. The second fiber may be, for example, a normal dispersion fiber or an anomalous dispersion fiber.

[0031] The fiber amplifier 8 amplifies (increases power output of) ultrashort pulse light L, which is modulated by the soliton shift fiber 6 and widened in time by the stretcher fiber 7. The fiber amplifier 8 includes a fiber amplifier. The fiber amplifier of the fiber amplifier 8 is an anomalous dispersion fiber, for example, a thulium-doped fiber. The laser medium added to the fiber of the fiber amplifier 8 is not particularly limited and may be a rare earth element such as ytterbium, erbium, neodymium, or Bi. The wavelength range of the ultrashort pulse light L amplified by the fiber amplifier 8 is, for example, 1800 nm to 2000 nm. A fiber amplifier 8 capable of reliably amplifying the ultrashort pulse light L in a broadband wavelength range may be configured, for example, by comprising a first fiber amplifier (not shown) having a high gain G1 (not shown) on the first wavelength side, which is the short wavelength side of the ultrashort pulse light L, and a second fiber amplifier (not shown) having a high gain G2 (not shown) on the second wavelength side, which is the long wavelength side, with a filter that attenuates the amplified light of noise generated by ASE and soliton self-frequency shift being combined between these amplifiers.

[0032] The organic crystal 9 constitutes the terahertz wave generation unit and generates terahertz waves when irradiated with ultrashort pulse light L, which is modulated by the soliton shift fiber 6 and amplified by the fiber amplifier 8. The organic crystal 9 may be composed of at least one of the following: DAST (4-N,Ndimethylamino-4′-N′-methylstilbazolium tosylate), DASC (4-dimethylamino-N-methyl-4-stilbazolium p-chlorobenzenesulfonate), and DSTMS (4-N,N-dimethylamino-4′-N′-methyl-stilbazolium 2,4,6trimethylbenzenesulfonate). In this embodiment, the organic crystal 9 will be described as being composed of DAST. The organic crystal 9 may generate terahertz waves with a frequency in the range of 0.01 to 30 THz, or it may generate terahertz waves in the range of 0.1 to 10 THz. Such frequency ranges have many absorptions characteristic of the terahertz wave band and are suitable for spectroscopy. Furthermore, the beam energy of the ultrashort pulse light L irradiated onto the organic crystal 9 may be determined taking into consideration the damage density of the organic crystal 9.

[0033] Figure 7 illustrates the generation of terahertz waves. Note that some components are omitted in Figure 7. As shown in Figure 7, an ultrashort pulse light L with a wavelength of, for example, 1550 nm or less is emitted from the oscillator 2, the spectrum of the ultrashort pulse light L is broadened in the fiber amplifier 3, the wavelength of the broadened ultrashort pulse light L is modulated in the soliton shift fiber 6 using soliton self-frequency shifting, the time width of the ultrashort pulse light L is widened in the stretcher fiber 7, and the ultrashort pulse light L is amplified in the fiber amplifier 8. When the ultrashort pulse light L output from the fiber amplifier 8 is irradiated onto the organic crystal 9, a terahertz wave with a wavelength of, for example, 300 μm or less is generated.

[0034] As shown in Figures 1 and 8, the terahertz waves generated by the organic crystal 9 are detected by the detection unit 10. The detection unit 10 functions as a terahertz wave detection unit that detects the terahertz waves output from the organic crystal 9. The detection unit 10 may also detect terahertz waves that have passed through the sample S. The detection unit 10 may be, for example, a photomultiplier tube sensitive to the light band including terahertz waves, an image intensifier that converts terahertz waves into electrons to acquire an image, a pyrodetector, a Goray cell, a Schottky barrier diode, a Fermi level barrier diode, etc.

[0035] Figure 9 is a flowchart illustrating an example of a terahertz wave generation and detection method. As shown in Figure 9, an ultrashort pulse light L is first emitted from oscillator 2 (step S1). Subsequently, the spectrum of the ultrashort pulse light L is broadened by fiber amplifier 3 (step S2).

[0036] Next, the ultrashort pulse light L is modulated in the soliton shift fiber 6 using soliton self-frequency shifting (step S3), the time width of the ultrashort pulse light L is widened in the stretcher fiber 7, and the ultrashort pulse light L is amplified in the fiber amplifier 8 (step S4).

[0037] Then, the ultrashort pulse light L output from the fiber amplifier 8 is irradiated onto the organic crystal 9 to generate terahertz waves (step S5), and these terahertz waves are detected by the detection unit 10 (step S6).

[0038] Next, the effects and benefits of the terahertz wave generator 1 according to this embodiment will be described.

[0039] The terahertz wave generator 1 comprises an oscillator 2 that oscillates ultrashort pulse light L, a fiber amplifier 3 that broadens the spectrum of the ultrashort pulse light L oscillated by the oscillator 2, a soliton shift fiber 6 that modulates the wavelength of the ultrashort pulse light L whose spectrum has been broadened by the fiber amplifier 3 using soliton self-frequency shift, and an organic crystal 9 that generates terahertz waves when irradiated with the ultrashort pulse light L modulated by the soliton shift fiber 6.

[0040] As a result of diligent research, the inventors have found that multi-solitonization can be suppressed by broadening the bandwidth of the spectrum of ultrashort pulse light before modulation using soliton self-frequency shift. Therefore, in the terahertz wave generator 1, the spectrum of ultrashort pulse light L is broadened, and the wavelength of the broadened ultrashort pulse light L is modulated using soliton self-frequency shift. This makes it possible to suppress multi-solitonization even when, for example, the intensity of the ultrashort pulse light L before modulation is increased. When ultrashort pulse light L with suppressed multi-solitonization is irradiated onto the organic crystal 9, terahertz waves can be generated without causing pulse splitting. This makes it possible to generate terahertz waves with high efficiency.

[0041] As described above, in the terahertz wave generator 1 according to this embodiment, an organic crystal 9 is used as the terahertz wave generation unit. With this configuration, it is possible to generate terahertz waves with higher efficiency.

[0042] The terahertz wave generating unit may be composed of at least one of DAST, DASC, and DSTMS as the organic crystal 9. Such a configuration makes it possible to generate terahertz waves with higher efficiency.

[0043] The organic crystal 9 may generate terahertz waves with a frequency in the range of 0.01 to 30 THz. By using this frequency range, the absorption characteristic of terahertz waves increases, allowing for proper spectroscopy.

[0044] The terahertz wave generator 1 further comprises a fiber amplifier 8 which includes a thulium-doped fiber amplifier that amplifies ultrashort pulse light L modulated by a soliton-shift fiber 6. The organic crystal 9 may generate terahertz waves when irradiated with ultrashort pulse light L modulated by the soliton-shift fiber 6 and amplified by the fiber amplifier 8. By irradiating the organic crystal 9 with ultrashort pulse light L amplified by the fiber amplifier 8 in this way, terahertz waves can be generated with higher efficiency.

[0045] The terahertz wave generator 1 may further include a detection unit 10 for detecting terahertz waves output from the organic crystal 9. By providing the detection unit 10 in the terahertz wave generator 1 in this way, the terahertz waves generated with high efficiency can be easily and quickly detected.

[0046] The embodiments described above are not limited to the above-described embodiments.

[0047] Figure 10 illustrates a modified version of terahertz wave detection. The terahertz wave generator shown in Figure 10 obtains the complex refractive index of a sample S using terahertz waves by terahertz time-domain spectroscopy. In addition to the oscillator 2, fiber amplifier 3, acousto-optic modulator (not shown), compressor (not shown), soliton shift fiber (not shown), stretcher fiber 7, organic crystal 9, etc., the terahertz wave generator shown in Figure 10 includes an optical branching unit 21, an optical path delay unit 22, a terahertz wave detection crystal 25, a polarization adjustment unit 23, and a photodetector unit 24.

[0048] The optical splitter 21 splits the ultrashort pulse light L modulated by the soliton shift fiber into pump light irradiated onto the organic crystal 9 and probe light. The optical splitter 21 may be composed of, for example, a beam splitter. Terahertz waves are generated when the pump light irradiates the organic crystal 9. The terahertz waves may be irradiated onto a sample S that is designed to be insertable and removable. By using residual light components (probe light) with a wavelength band of, for example, 1550 nm, which did not contribute to terahertz wave generation, for terahertz wave detection, light can be utilized without waste. In addition, while 2000 nm photodetectors are expensive, 1550 nm photodetectors are relatively inexpensive, thus reducing costs.

[0049] The optical path delay unit 22 is configured to delay the probe light in time by changing the optical path length of the probe light. The optical path delay unit 22 may include, for example, a mechanical stage.

[0050] The terahertz wave detection crystal 25 is a crystal for terahertz wave detection, into which pump light irradiated onto the sample S (or pump light that reaches the sample S without being irradiated) and probe light that has passed through the optical path delay unit 22 are incident.

[0051] Here, since terahertz waves can only exist for an extremely short period of time, waveform observation is not easy. Therefore, a method is employed in which terahertz time-domain spectroscopy is used, in which terahertz waves are repeatedly generated, and the optical path delay unit 22 is placed on the probe light side to change the optical path length, thereby delaying the probe light in time and observing the waveform while gradually shifting the timing of detection.

[0052] Terahertz time-domain spectroscopy is performed, for example, by the following procedure. First, a pulsed laser with a wavelength of approximately 1.5 μm is incident on the terahertz wave detection crystal 25 and then incident on the photodetector 24 via the polarization adjustment unit 23. At this time, the angle of the polarization adjustment unit 23 (e.g., polarizer) is adjusted so that the light intensity after transmission is minimized.

[0053] First, with the sample S absent from the optical path, the generated terahertz wave and the probe light are spatially and temporally superimposed and incident on the terahertz wave detection crystal 25. Then, by adjusting the optical path delay unit 22, the time delay of the probe light and the signal output from the photodetector unit 24 are measured, and the terahertz waveform is acquired.

[0054] Next, with the sample S present in the optical path, the optical path delay unit 22 is adjusted to measure the time delay of the probe light and the signal output from the photodetector unit 24, thereby acquiring the terahertz waveform transmitted through the sample S.

[0055] Finally, the terahertz waveforms for both the presence and absence of sample S in the optical path are Fourier transformed, and the complex refractive index of sample S is derived from the difference in amplitude and phase. This allows the state of sample S to be identified.

[0056] Figure 11 illustrates a terahertz wave detection method according to another modified example. The terahertz wave generator shown in Figure 11 includes the oscillator 2, fiber amplifier 3, acousto-optic modulator (not shown), compressor (not shown), soliton shift fiber (not shown), stretcher fiber 7, fiber amplifier 8, organic crystal 9, etc., as well as a terahertz measurement unit 30. The terahertz measurement unit 30 is configured to measure the terahertz waves output from the organic crystal 9.

[0057] Figure 12 shows an example of the configuration of the terahertz measurement unit 30. As shown in Figure 12, the terahertz measurement unit 30 includes an interference optical system 31, a photomultiplier tube 32, an interference intensity measurement unit 33, an electric field amplitude calculation unit 34, and an analysis unit 35.

[0058] The interference optical system 31 includes a beam splitter 31a, a mirror 31b, and a mirror 31c, and has the configuration of a Michelson interferometer. The beam splitter 31a splits the light output from the oscillator 2 into a first-branched beam and a second-branched beam, outputs one of the first-branched beams to mirror 31b, and outputs the other of the second-branched beams to mirror 31c. The beam splitter 31a also receives the first-branched beam reflected by mirror 31b and the second-branched beam reflected by mirror 31c, combines these input first-branched and second-branched beams, and outputs them to the photomultiplier tube 32. The beam splitter 31a may be made of, for example, silicon or an ITO mirror. The sample S is placed on the optical path of the first-branched beam between the beam splitter 31a and the mirror 31b. The sample S may also be placed on the optical path of the second-branched beam. Both or either of the mirrors 31b and 31c can be moved in a direction perpendicular to the reflective surface, thereby making the difference in optical path length between the first branched light and the second branched light variable.

[0059] The photomultiplier tube 32 is sensitive to the light band including terahertz waves and outputs an electrical signal with a value corresponding to the incident light intensity. The interference intensity measurement unit 33 measures the intensity of the interference light caused by the first branched light and the second branched light incident on the photomultiplier tube 32 based on the electrical signal output from the photomultiplier tube 32.

[0060] The electric field amplitude calculation unit 34 calculates the electric field amplitude E of the interference light for each value of the time difference Δt corresponding to the optical path length difference Δd, based on the relationship between the electric field amplitude of the light incident on the photomultiplier tube 32 and the value of the electrical signal output from the photomultiplier tube 32. The optical path length difference Δd corresponds to twice the difference in distance from the beam splitter to each of the two mirrors. There is a relationship between the optical path length difference Δd and the time difference Δt: Δt = Δd / c, where c is the speed of light in a vacuum.

[0061] The analysis unit 35 performs a Fourier transform based on the dependence of the electric field amplitude E of the interference light, obtained by the electric field amplitude calculation unit 34, on the time difference Δt, thereby analyzing the sample S.

[0062] With this configuration, the interference time waveform and spectrum of efficiently generated terahertz waves can be measured quickly and efficiently.

[0063] The terahertz measurement unit may also include an image intensifier that electronically converts terahertz waves to acquire an image. With such a configuration, the interference time waveform and two-dimensional spectral image of efficiently generated terahertz waves can be measured quickly and efficiently. [Explanation of Symbols]

[0064] 1...Terahertz wave generator, 2...Oscillator (oscillating unit), 3...Fiber amplifier (first amplification unit), 6...Soliton shift fiber (modulation unit), 8...Fiber amplifier (second amplification unit), 9...Organic crystal (terahertz wave generation unit), 10...Detection unit (terahertz wave detection unit), 21...Optical branching unit, 22...Optical path delay unit, 25...Terahertz wave detection crystal, 32...Photomultiplier tube, L...Ultrashort pulse light (pulsed light), S...Sample.

Claims

1. An oscillator that emits pulsed light, A first amplification unit that broadens the spectrum of the pulsed light oscillated by the oscillation unit, A modulation unit modulates the wavelength of the pulsed light whose spectrum has been broadened by the first amplification unit using soliton self-frequency shift, A terahertz wave generating device comprising: a terahertz wave generating unit that generates terahertz waves when the pulsed light modulated by the modulation unit is irradiated onto it; and a terahertz wave generating device.

2. The terahertz wave generating device according to claim 1, wherein the terahertz wave generating unit is composed of an organic crystal.

3. The terahertz wave generating device according to claim 2, wherein the terahertz wave generating unit is configured to include at least one of DAST, DASC, and DSTMS as the organic crystal.

4. The terahertz wave generating device according to claim 1, wherein the terahertz wave generating unit generates terahertz waves having a frequency in the range of 0.01 to 30 THz.

5. The system further comprises a second amplification unit which includes a thulium-doped fiber amplifier for amplifying the pulsed light modulated by the modulation unit, The terahertz wave generating device according to claim 1, wherein the terahertz wave generating unit generates the terahertz wave by irradiating the pulsed light that has been modulated by the modulation unit and amplified by the second amplification unit.

6. The terahertz wave generator according to claim 1, further comprising a terahertz wave detection unit for detecting the terahertz waves output from the terahertz wave generator.

7. The pulsed light modulated by the modulation unit is split into a pump light irradiated onto the terahertz wave generating unit and a probe light by the optical branching unit, An optical path delay unit that delays the probe light by changing the optical path length of the probe light, The terahertz wave generator according to claim 1, further comprising: a terahertz wave detection crystal into which the pump light irradiated onto a sample and the probe light that has passed through the optical path delay unit are incident.

8. The system further comprises a terahertz wave measuring unit for measuring the terahertz waves output from the terahertz wave generating unit, The terahertz wave generator according to claim 1, wherein the terahertz wave measuring unit is configured to include a photomultiplier tube that is sensitive to the light band including terahertz waves.

9. The system further comprises a terahertz wave measuring unit for measuring the terahertz waves output from the terahertz wave generating unit, The terahertz wave generator according to claim 1, wherein the terahertz wave measurement unit is configured to include an image intensifier that electronically converts terahertz waves to acquire an image.

10. Oscillating pulsed light, To broaden the spectrum of the pulsed light, The wavelength of the broadbanded pulsed light is modulated using soliton self-frequency shift, A method for generating terahertz waves, comprising generating terahertz waves by irradiating an organic crystal with the modulated pulsed light.

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