JP2010066381A - Wavelength variable terahertz wave generating apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a generating apparatus for improving the output of terahertz waves regardless of a frequency. <P>SOLUTION: The wavelength variable terahertz wave generating apparatus includes: a first exciting light source 51 which emits first exciting light; a second exciting light source 52 which emits second exciting light whose frequency difference from the first exciting light is a terahertz band; an angle adjusting means 64 for adjusting an angle so as to turn the angle formed by the first exciting light and the second exciting light to a phase matching angle and mixing them; nonlinear optical crystal 65 on which two lines of amplified light are made incident while forming the phase matching angle, and which emits the terahertz waves by difference-frequency mixing; and spot diameter adjusting means 70, 71 and 64 for adjusting the spot diameter of the first exciting light and the second exciting light made incident on the nonlinear optical crystal. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a terahertz wave generator that generates a terahertz wave by difference frequency mixing using a nonlinear optical crystal. In particular, the present invention relates to an apparatus capable of obtaining a high output.

  In recent years, terahertz waves, which are electromagnetic waves at the boundary between radio waves and light, have attracted attention, and research and development have been conducted on spectroscopic techniques and imaging techniques using terahertz waves.

  For the generation of terahertz waves, a generation method using a difference frequency using a parametric element, a generation method using a parametric oscillator using a non-linear element, a photoconductive antenna formed on a semiconductor substrate, or a crystal having an electro-optic effect. There is a method of generating by inputting a short pulse laser beam of picosecond to femtosecond.

  Those utilizing the generation of a difference frequency using a nonlinear optical crystal are known (for example, Patent Documents 1 to 3). In this method, two pump lights having a frequency difference in the terahertz band are made incident on a nonlinear optical crystal so as to satisfy a phase matching condition, and a terahertz wave is generated by difference frequency mixing.

  When an isotropic nonlinear optical crystal such as GaP is used, phase matching cannot be performed using birefringence, and therefore non-collinear phase matching is performed so that the two excitation lights have a minute angle. Non-collinear phase matching in the case of GaP crystal is described in detail in Patent Documents 1 and 3.

  In the method using difference frequency generation, which is one of the nonlinear optical effects, two pulsed lights having different frequencies called pump laser and signal laser are incident on a semiconductor GaP, GaSe crystal or the like that is a crystal for generating terahertz electromagnetic waves. The monochromatic coherent terahertz wave corresponding to the difference in frequency can be generated. By using these terahertz light sources, spectrum measurement and imaging of a substance in the terahertz band enables identification of molecules, nondestructive inspections such as food / pharmaceutical inspections, discovery of cancer tissues, and the like.

In order to generate terahertz electromagnetic waves using two laser pulse lights, pump light, signal light, and terahertz electromagnetic waves must satisfy a phase matching condition. When a GaP crystal or the like having an isotropic refractive index is used, the two incident beams need to have a predetermined angle. When these terahertz waves are used for spectroscopic analysis, for example, it is necessary that the wavelength can be continuously changed in a wide band.
JP 2004-318028 A JP 2006-91802 A JP2007-133339

  However, in the above-described terahertz wave generator, since the output of the terahertz wave to be output is small, a detector operating at room temperature cannot be used. In addition, when GaP is used as a nonlinear optical crystal for generating terahertz, a terahertz wave in the band of 0.3 to 7.5 THz can be generated. The output gradually decreases in the frequency bands on both sides. For this reason, it is necessary to increase the number of integrations in order to perform spectrum measurement in a frequency region of 1 THz or less and 6.5 THz or more. When the number of integrations increases, there is a problem that the measurement time required for imaging, spectroscopic analysis, etc. becomes longer. Long measurement time means that accurate spectroscopic analysis cannot be performed if the field environment changes during measurement.

  Therefore, an object of the present invention is to improve the output of the output terahertz wave. Accordingly, it is possible to perform measurement such as spectroscopic analysis in a short time and obtain an accurate measurement value.

  A first invention is a terahertz wave generator for generating a wavelength-tunable terahertz wave, wherein the first pumping light source that emits the first pumping light and the second pumping light whose frequency difference between the first pumping light is in the terahertz band And an angle adjusting means for adjusting and mixing the angle so that the angle formed by the first excitation light and the second excitation light becomes the phase matching angle according to the frequency of the output terahertz wave Two excitation lights are incident at a phase matching angle, and a non-linear optical crystal that emits a terahertz wave by difference frequency mixing, and the spot diameters of the first and second excitation lights that are incident on the non-linear optical crystal, It is a terahertz wave generator characterized by comprising spot diameter adjusting means for adjusting.

  According to a second aspect of the present invention, in the terahertz wave generation device that generates a wavelength-tunable terahertz wave, the second excitation light whose frequency difference between the first excitation light and the first excitation light is in the terahertz band. A second excitation light source that emits light, a multiplexing means that combines the first excitation light and the second excitation light to generate combined light, a dividing means that divides the combined light into two, and a dividing means The angle adjusting means for adjusting and mixing the angle so that the angle formed by the two combined lights becomes the phase matching angle, and the two combined lights are made incident at the phase matching angle, and simultaneously 2 by the difference frequency mixing. A terahertz wave comprising: a nonlinear optical crystal that emits a terahertz wave in a direction; and spot diameter adjusting means that adjusts the spot diameters of the first excitation light and the second excitation light incident on the nonlinear optical crystal. Generator.

In the first and second inventions, it is desirable that the spot diameter adjusting means is changed according to the phase matching angle. The spot diameter adjusting means is preferably changed according to the phase matching angle and the length of the nonlinear optical crystal. Further, in that case, the spot diameter adjusting means uses the spot diameter Rmm, the length Lmm of the nonlinear optical crystal, the phase matching angle θ, and the proportional constant α,
It is desirable to control with.

  Further, the length of the nonlinear optical crystal in the light propagation direction is desirably set to a length that maximizes the output of the output terahertz wave with respect to a predetermined spot diameter. Since the length of the nonlinear optical crystal that maximizes the output depends on the frequency of the terahertz wave, if it is a single frequency, the maximum output power can be obtained at that frequency for a given spot diameter. Is set. In addition, when the frequency is changed and used in a certain frequency bandwidth, for example, even if the maximum output is set at the average frequency, the integrated output in the entire variable frequency band becomes the maximum. As described above, the length of the nonlinear optical crystal may be set. In addition, since the loss in the terahertz wave inside the nonlinear optical crystal increases as the frequency increases, the length of the nonlinear optical crystal may be set so that the maximum output is obtained at the maximum frequency in the variable frequency band. .

  In the first invention, the first pumping light and the second pumping light are propagated through separate optical paths, adjusted to have a phase matching angle, and made incident on the nonlinear optical crystal. On the other hand, in the second invention, the first pumping light and the second pumping light are once combined, the combined light is divided into two, and each of the divided combined lights is converted into a phase matching angle. The angle is adjusted so that the light is incident on the nonlinear optical crystal.

  In the first invention, an amplifier for amplifying the first pumping light and the second pumping light is provided before the angle adjusting unit, and the angle adjusting unit outputs the first pumping output from each amplifier. It is desirable to adjust the angle between the light and the second excitation light. A fiber amplifier can be used as the amplifier. In the second invention, it is desirable to provide an amplifier and a fiber amplifier after combining the first pumping light and the second pumping light and before dividing the combined light.

  In the first and second inventions, the first excitation light source is preferably a semiconductor laser using a DFB laser or DBR, and the second excitation light source is preferably an external cavity semiconductor laser. . Generally, when there is no mode hop, the wavelength tunable width is narrow, but when there is a mode hop, the wavelength cannot be continuously changed, but the wavelength range that can be changed even if it is discontinuous. Is wide. Therefore, in the first and second aspects of the invention, the first excitation light source is a light source having a predetermined first wavelength sweep width that can be continuously varied without mode hops, and the second excitation light source is It is sufficient to use a light source having a mode hop and capable of discretely setting the oscillation wavelength at intervals narrower than the first wavelength sweep width, and the type is not limited. DFB (distributed feedback type) semiconductor laser or DBR (distributed Bragg reflection type) semiconductor laser realizes a continuous wavelength tunable width of about 1 THz without mode hops by changing the refractive index by temperature control. Therefore, it is optimal as the first excitation light source. The external resonator type semiconductor laser is a laser that uses a diffraction grating and an external resonator to feed back resonance light to the semiconductor and oscillates, and changes the resonance wavelength depending on the incident angle to the diffraction grating. . For this reason, the resonance wavelength width is narrow, and there is a mode hop between the next resonance wavelength and the wavelength cannot be changed continuously. However, even if it is discontinuous, the range in which the wavelength can be variably set is wide, so it is optimal as the second excitation light source. In this case, although the frequency width depends on the type of the external cavity semiconductor laser, it is about 5 to 15 GHz or about 1 to 4 MHz, and the mode hop frequency interval is about 5 to 15 GHz or about 1 to 4 MHz. is there.

  In the first and second aspects of the invention, the spot diameter adjusting means is a means for adjusting the diameter of the two beams so as to have an optimum value on the incident surface of the nonlinear optical crystal, and means for changing the position of the lens. It is desirable to configure. Furthermore, a GaP crystal can be used for the nonlinear optical crystal.

  In the present invention, by changing the spot diameter of the beam in the nonlinear optical crystal that generates the terahertz wave according to the frequency to be output, depending on the frequency of the output terahertz wave, and thus the phase matching angle, the nonlinear optical crystal By combining the two excitation lights over the entire length of the light traveling direction, the output of the output terahertz wave can be maximized. As a result, the S / N ratio in spectrum measurement can be improved by increasing the output. Moreover, as a result of improving the S / N ratio, spectrum measurement can be performed in a shorter time.

  Hereinafter, specific examples of the present invention will be described with reference to the drawings. However, the present invention is not limited to the examples.

Although the present invention is not limited to the fact that the frequency of the output terahertz wave is variable, an example in which the maximum output is obtained in an apparatus capable of continuously changing the frequency of the terahertz wave in a wide band will be described. First, the principle that the frequency of the output terahertz wave can be varied in a wide band will be described. As shown in FIG. 1A, the lower limit value λ _L and the upper limit value λ _u of the first wavelength sweep width, which is the continuous wavelength variable width of the first excitation light, are expressed as c / (f ₁₀ −Δf ₁₀ ), c / (F ₁₀ + Δf ₁₀ ). Where c is the speed of light, f ₁₀ is the center frequency, and 2Δf ₁₀ is the sweep width in frequency. The center wavelength λ ₁₀ of the wavelength sweep width is c / f ₁₀ , and the wavelength sweep width 2Δλ ₁₀ satisfies the following equation.

Because it is Δf ₁₀ «f _10,

Next, the frequency and wavelength of the first pumping light within the first wavelength sweep width are f ₁ (x) and λ ₁ (x), respectively, and the frequency of the second pumping light is f ₂ (y), The wavelength is λ ₂ (y), the frequency of the obtained terahertz wave is f ₃ (x, y), and the wavelength is λ ₃ (x, y). The following relational expression holds. However, it is assumed that f ₂ (y)> f ₁ (x) with respect to possible values of x and y. Further, x is an index that determines the frequency of the first pumping light within the first wavelength sweep width, and y is an index that determines the frequency at which the second pumping light is set.

Δf ₁ (x) is a deviation frequency measured from the center frequency f ₁₀ of the first excitation light.
For the wavelength, the following equation holds.

However, f ₃ (x, y) and λ ₃₀ (x, y) are a center frequency and a center wavelength when the frequency of the first excitation light of the output terahertz wave is changed, respectively.

In addition, the frequency interval for setting the second excitation light is set to a frequency width 2Δf ₁₀ or less of the first wavelength sweep width.
That is,
And Further, the minimum value of y is 1 and the maximum value is k _max , the minimum value of the variable setting width of the second excitation light is f ₂ (1), and the maximum value is f ₂ (k _max ).

The second pumping light has mode hops, and the width in which the frequency can be continuously varied and the minimum value of the adjacent frequency interval of the frequency that can be realized are usually about 5 to 15 GHz, and the first wavelength sweeping is performed. It is sufficiently shorter than the frequency width 2Δf ₁₀ corresponding to the width and the variable setting frequency width f ₂ (k _max ) −f ₂ (1) of the second excitation light. Therefore, the frequency and wavelength of the second excitation light can be selected so as to satisfy the expression (7), including the case of the equal sign of the expression (7).

As shown in FIG. 1B, the second excitation light has discrete frequencies such as f ₂ (1), f ₂ (2),... F ₂ (k),... F ₂ (k _max ). Can be set. Of course, it is possible to oscillate at a discrete frequency between f ₂ (1), f ₂ (2),..., F ₂ (k). As shown in FIG. 1B, the frequency of the second excitation light is set at an arbitrary frequency interval.

Assume that the frequency of the second excitation light is set to f ₂ (y) and the frequency of the first excitation light is changed in the first wavelength sweep width. Then, since Δf ₁ (x) changes from −Δf ₁₀ to Δf ₁₀ , the frequency f ₃ of the output terahertz wave is obtained from f ₃₀ (y) + Δf ₁₀ as is apparent from the equation (5). changes to f ₃₀ (y) -Δf _10.

When the set frequency of the second pumping light is set to f ₂ (1) and the frequency of the first pumping light is changed, the variable range of the frequency of the output terahertz wave is A1 shown in FIG. It becomes an area. When the set frequency of the second pumping light is set to f ₂ (2) and the frequency of the first pumping light is changed, the variable range of the frequency of the output terahertz wave is shown in FIG. This is the area A2. Then, by repeating these steps and setting the set frequency of the second pumping light to f ₂ (k _max ) and changing the frequency of the first pumping light, the variable range of the frequency of the output terahertz wave is as shown in FIG. This is the region of Ak _max shown in 1 (c).

The frequency interval f ₂ (k + 1) −f ₂ (k) that can be variably set for the second pumping light can be made equal to or less than the frequency sweep width 2Δf ₁₀ corresponding to the wavelength sweep width of the first pumping light. The frequency variable widths A1, A2, ... Ak, ... Ak _max are continuous.

On the other hand, the output of the output terahertz wave, the powers I ₁ (x) and I ₂ (x) of the first excitation light and the second excitation light incident on the nonlinear optical crystal, and the cross-sectional area of the overlapping portion of these beams The following equation holds between S (x). Where x is a coordinate with the incident plane of the nonlinear optical crystal as the origin, and the light traveling direction is positive, and S (0) is the cross-sectional area of the overlap of the two excitation lights on the light incident plane. . However, the integration range is the shorter of the range in which the two excitation light spots overlap or the length of the nonlinear optical crystal in the light traveling direction.

S (x) depends on the phase matching angle θ.
When formula (8) is modified, the following formula is obtained.

Where I _THz is the output of the terahertz wave, and A (θ) is a proportionality constant that is a function of the phase matching angle θ. When A (θ) is equal, the output of the terahertz wave increases as the cross-sectional area S (0) of the overlapping spot of the first excitation light and the second excitation light on the incident surface of the nonlinear optical crystal decreases. It is understood that In addition, since there is a phase matching angle θ, the area where the two excitation lights overlap in the optical path inside the nonlinear optical crystal decreases along the light traveling direction. Since the length of the overlap becomes shorter, the output decreases. The decrease is represented by A (θ).

  As shown in FIG. 2, it is assumed that the first excitation light and the second excitation light are incident on the nonlinear optical crystal from the incident surface Q1 with the same beam cross-sectional area S and the phase matching angle θ. At this time, the length of the optical path in which the beams of the first excitation light and the second excitation light overlap, measured from the incident surface Q1 of the nonlinear optical crystal, is an optical path length β (hereinafter, this length is referred to as a coupling length). . The relationship between the coupling length β, the beam diameter R, and the phase matching angle θ is as follows.

FIG. 3 is a characteristic curve showing the relationship between the coupling length β and the phase matching angle θ with the spot diameter R as a parameter. It is understood that the coupling length β is shorter as the phase matching angle θ is larger, and the coupling length is longer as the spot diameter R is larger.

A terahertz wave is generated from the optical path having the coupling length β. When the length L of the nonlinear optical crystal is longer than the coupling length β, there exists an optical path in which the first excitation light and the second excitation light are not superimposed, and this non-coupling optical path is a loss for the terahertz wave. Will give. When GaP is used for the nonlinear optical crystal, the absorption of terahertz waves by free carriers dominant on the low frequency side is small, and the absorption of terahertz waves by dominant phonons on the high frequency side is large. Therefore, if the non-coupled optical path is long, the loss increases as the frequency range becomes higher. In other words, there is a length L _S that maximizes the output of the terahertz wave with respect to an increase in the length L of the nonlinear optical crystal, and the optimum length L _S has a high frequency of the terahertz wave. I see, it gets shorter.

Therefore, in the present invention, the optimum length L _S is obtained from the coupling length β according to the frequency of the output terahertz wave.
However, in the total coupling length β, both excitation lights are not separated in the spot cross-sectional area of the incident surface, but are not divided, but the coupling cross-section gradually decreases along the light traveling direction, Each part of the crystal contributes to loss more than it contributes to the emission of terahertz waves. For this reason, the optimum length L _S is shorter than the bond length β. Therefore, L _S ≦ β and α ≧ 1.

If the length of the nonlinear optical crystal is set to the optimum length L _S , the length cannot be changed thereafter even if the frequency of the output terahertz wave changes. When the length of the nonlinear optical crystal is fixed to the optimum length L _S , the coupling length β decreases as the spot diameter R decreases according to the equation (10), and conversely, the length of the non-coupled optical path. The length increases as the spot diameter R decreases. That is, from the viewpoint of this coupling length, the smaller the spot diameter R, the greater the loss and the lower the output of the output terahertz wave. On the other hand, since the power density of the two excitation lights increases and the output of the terahertz wave increases as the maximum cross-sectional area S (0) of the overlap of the two excitation lights decreases, the output of the terahertz wave increases. If the length is fixed at the optimum length L _S , there will be an optimum spot diameter R for obtaining the maximum output. The optimum diameter R is a function of the phase matching angle θ, that is, the frequency of the terahertz wave. When the frequency of the terahertz wave is increased, the coupling length β is shortened, so that the loss is increased. Therefore, the optimum diameter R is increased as the frequency is increased. On the other hand, as the spot diameter R increases, the coupling length increases, and when this coupling length exceeds the length of the nonlinear crystal, the effective length that can be converted into terahertz waves decreases and the output decreases. From this, the maximum output can be obtained in a wide band by controlling the spot diameter R according to the frequency of the terahertz wave so that the coupling length β is constant regardless of the frequency of the terahertz wave.

In the present invention, the spot diameter R of the first excitation light and the second excitation light is controlled from the optimum length L _S of the nonlinear optical crystal and the phase matching angle θ determined by the frequency of the terahertz wave. Thereby, the maximum output was obtained.

  FIG. 4 illustrates a configuration of the terahertz wave generation device according to the first embodiment. The terahertz wave generator 50 includes a first excitation light source 51 made of a DFB semiconductor laser and a second excitation light source 52 made of an external resonance type semiconductor laser. The first excitation light source 51 and the second excitation light source 52 are connected to fiber amplifiers 55 and 56 via half-wave plates 53 and 54, respectively. The outputs of the fiber amplifiers 55 and 56 are connected to mirrors 59 and 60 via lenses 57 and 58, respectively. The first excitation light reflected by the mirror 59 passes through the half-wave plate 62, is reflected by the mirror 63, and enters the polarization beam splitter 64. Further, the second excitation light reflected by the mirror 60 passes through the half-wave plate 61 and is input to the deflection beam splitter 64. The first excitation light and the second excitation light incident on the deflection beam sprinter 64 are incident on the GaP crystal 65. The polarizing beam splitter 64 corresponds to the angle adjusting means of the present invention, and the GaP crystal 65 corresponds to the nonlinear optical crystal of the present invention.

  The fiber amplifiers 55 and 56 are optical fiber amplifiers doped with Yb. The lenses 57 and 58 have a focal length of 500 mm. The lenses 57 and 58 can be adjusted in position in the optical axis direction by position adjusting devices 70 and 71, respectively. The focal positions of the lenses 57 and 58 are adjusted by adjusting the positions of the lenses 57 and 58 in the optical axis direction, and the first excitation light and the second excitation light output from the fiber amplifiers 55 and 56 are incident on the GaP crystal 65, respectively. The beam diameter R on the surface Q1 can be controlled. The position adjusting devices 70 and 71 constitute spot diameter adjusting means.

  The polarization beam splitter 64 is a cube-type element in which a dielectric multilayer film is formed on the slope of a triangular prism and two triangular prisms are bonded together on the slope. The polarization beam splitter 64 can be rotated and moved in the optical axis direction and in a direction perpendicular to the optical axis direction. For example, the rotation angle and the position can be controlled by placing the polarization beam splitter 64 rotatably supported on a single axis stage. The polarization beam splitter 64 controls the rotation and movement of the polarizing beam splitter 64 to adjust and mix the position overlapping the angle formed by the first excitation light and the second excitation light. Further, the diameter R of the spot of the first excitation light and the second excitation light on the incident surface Q1 of the GaP crystal 65 can also be controlled by the rotation angle and position of the polarization beam splitter 64.

  The GaP crystal 65 is polished on the (110) plane. This (110) plane is the light incident plane. The terahertz wave generating apparatus according to the first embodiment generates terahertz waves by causing two excitation lights to enter the GaP crystal 65 and causing difference frequency mixing. Since the GaP crystal 65 is an isotropic nonlinear optical crystal and does not have birefringence, the phase matching condition is satisfied by phase matching (non-collinear phase matching) performed by providing an angle between the two excitation lights. To do.

  The mirrors 59, 60, and 63 are all for controlling the light irradiation direction, and the half-wave plates 53, 54, 61, and 62 are all for adjusting the polarization direction of the light. is there. The terahertz wave generation device 50 can generate a terahertz wave having a difference frequency between the first excitation light and the second excitation light from the GaP crystal 65.

Next, a method of changing the frequency and wavelength of the output terahertz wave over a wide band using the present terahertz wave generator 50 will be described.
First, by controlling the incident angle of the excitation light to the diffraction grating of the second excitation light source 52, the frequency of the second excitation light is set to f ₂ (k). Then, by controlling the temperature of the first excitation light source 51, the frequency of the first excitation light from the f ₁₀ + Δf _10, is varied in a range of 2.DELTA.f ₁₀ to f ₁₀ -.DELTA.f _10. Then, the frequency of the output terahertz wave changes from f ₂ (k) −f ₁₀ −Δf ₁₀ to f ₂ (k) −f ₁₀ + Δf ₁₀ with a width 2Δf ₁₀ . That is, the center frequency f ₂ (k) -f _10, only Delta] f ₁₀ from a low frequency from the center frequency, from the center frequency to a higher frequency by Delta] f _10, continuously changing the frequency of the terahertz wave output be able to.

Next, the second pumping light source 52 is controlled to set the frequency of the second pumping light source to the frequency of the second pumping light at f ₂ (k + 1). Then, as in the case described above, by controlling the temperature of the first excitation light source 51, the frequency of the first excitation light from the f ₁₀ + Δf _10, is varied in a range of 2.DELTA.f ₁₀ to f ₁₀ -.DELTA.f ₁₀ . Then, the frequency of the output terahertz wave changes from f ₂ (k + 1) −f ₁₀ −Δf ₁₀ to f ₂ (k + 1) −f ₁₀ + Δf ₁₀ with a width 2Δf ₁₀ . That is, the center frequency _{f 2 (k + 1) -f} 10, only Delta] f ₁₀ from a low frequency from the center frequency, from the center frequency to a higher frequency by Delta] f _10, continuously changing the frequency of the terahertz wave output be able to. Since f ₂ (k + 1) is set to _{-f 2 (k) ≦ 2Δf 10} , a frequency region from _{f 2 (k) -f 10 -Δf} 10 to _{f 2 (k) -f 10 +} Δf 10, f The frequency region from ₂ (k + 1) −f ₁₀ −Δf ₁₀ to f ₂ (k + 1) −f ₁₀ + Δf ₁₀ is a continuous region although a part of the region overlaps. In this way, by sequentially changing the frequency f ₂ (k) of the second pumping light and similarly sweeping the frequency of the first pumping light by temperature control, the frequency of the output terahertz wave is continuous over a wide band. Frequency.

Further, by setting the frequency f ₂ (k) of the second excitation light to f ₂ (k + 1) −f ₂ (k) = 2Δf ₁₀ , the frequency of the output terahertz wave overlaps in the frequency domain. Without being continuous. This condition can be easily satisfied because the mode hop interval of the second excitation light source is about 15 GHz, which is smaller than the frequency of the output terahertz wave.

FIG. 5 shows that the frequency f ₂ (k) of the second pumping light is set to 285.11 THz, the center frequency f ₁₀ of the first pumping light is set to 283.55 THz, and the frequency sweep width 2f ₁₀ is set to 0.1. The absorption spectrum of air is measured by using the output terahertz wave as 5 THz. It can be seen that the frequency continuously changes from 1.55 THz to 2.05 THz.

  As shown in FIG. 6, the terahertz band output in FIG. 5 is swept with the first wavelength sweep width by sequentially changing the frequency of the second pump light, and as shown in FIG. In the wide band of 7 THz, the wavelength of the output terahertz wave can be continuously changed.

  When the frequency of the terahertz wave changes, the phase matching angle θ changes. When the nonlinear optical crystal is GaP, the frequency f and the phase matching angle θ have a relationship as shown in FIG. Therefore, when the frequencies of the first excitation light and the second excitation light are changed, the rotation angle and position of the polarization beam splitter 64 are controlled according to the frequency of the output terahertz wave. That is, the phase matching angle θ corresponding to the frequency f of the terahertz wave to be obtained is read from the characteristics shown in FIG. 7, and the rotation angle and position of the polarization beam splitter 64 are controlled so that the phase matching angle θ is realized. This can be automated by computer control. The polarization beam splitter 64 and the device for controlling the rotation angle and position constitute angle adjustment means.

When the variable frequency range of the terahertz wave is, for example, 1.5 to 8.0 THz, the maximum frequency is 8.0 THz, the initial diameter of the spot is set to the maximum value, and the GaP crystal is obtained so that the maximum output is obtained in that state. A length of 65 has been determined. Since the phase matching angle at the frequency of 8.0 THz is θ = 240 minutes, the coupling length β is obtained with respect to the initial diameter R of the spot by the equation (10). In consideration of the attenuation in the non-bond length, α is set to 1.2, and the length of the GaP crystal 65 is set to β / 1.2 which is the optimum length L _S.

  When the frequency f of the output terahertz wave changes in the state set in this way, the spot diameter R is determined by the equation (1) and the position adjusting means 70 and 71 are controlled accordingly. The spot diameter R is controlled by adjusting the focal positions of the lenses 57 and 58. That is, the spot diameter R is changed according to the frequency so that the coupling length β is constant regardless of the frequency. Accordingly, when the frequency is lowered, the phase matching angle θ is reduced and the coupling length β is increased. In order to keep this constant, the spot diameter R is reduced. In this way, the output of the terahertz wave can be increased over the entire variable range.

In the above description, the optimum length L _S of the GaP crystal 65 and the initial diameter R of the spot are adjusted so that the maximum output can be obtained at the maximum frequency. The initial diameter R of the spot is set to the minimum value of the variable range, and in this state, the optimum length L _S of the GaP crystal 65 is determined so that the maximum output is obtained at the minimum frequency. When the frequency of the output terahertz wave increases, the phase matching angle θ increases and the coupling length β tends to decrease. To make this coupling length β constant, the spot diameter R is increased. May be. Alternatively, the optimum length L _S of the GaP crystal 65 may be set so that the maximum output is obtained in the state where the initial diameter of the spot is set to the median value of the variable range at the center frequency of the variable range. When the terahertz frequency is increased, the spot diameter R is increased so that the coupling length β is constant, and when the terahertz frequency is decreased, the coupling length β is constant. Reduce the diameter R of the spot. As a result, the output of the terahertz wave can be increased over the entire frequency variable range.

FIGS. 8 and 9 are characteristic diagrams in which the relationship between the phase matching angle θ and the output is measured using the length of the GaP crystal 65 as a parameter in a state where the spot diameter R is constant at 0.7 mm. When the center frequency is 2 THz, it can be seen that the output decreases as the length of the GaP crystal 65 decreases from 20 mm to 2.5 mm. It can be seen from the characteristics of FIG. 8 that the optimum length L _S is 20 mm or more at a frequency of 2 THz. Further, in the characteristics of FIG. 9, it is understood that the optimum length L _S at which the output at 3 THz is maximum is 10 mm. In other words, the optimum length is 10 mm, and it is understood that the output is lowered regardless of whether the length of the GaP crystal is longer or shorter than that.

FIG. 10 is a characteristic diagram showing the relationship between the length of the GaP crystal and the output using the frequency of the serahertz wave as a parameter. In the case of 3THz, the optimum length L _s of the GaP crystal is 10 mm, in the case of 2.75THz, the optimum length L _s of the GaP crystal is 10 mm, in the case of 2.52THz the optimal GaP crystals It is understood that the optimum length L _s of the GaP crystal is longer than 20 mm when the length L _s is about 15 mm, 2 THz, and 1.5 THz.

  The second embodiment is a terahertz wave generator that can generate terahertz waves in two directions at the same time, and the frequency of the terahertz waves is variable. FIG. 11 is a diagram illustrating the structure of the terahertz wave generation device according to the second embodiment. The terahertz wave generator 1 includes a first excitation light source 11, a second excitation light source 10, a non-polarizing beam splitter 12, a fiber amplifier 13, a lens 14, and a position adjusting device 30 that changes the focal position of the lens 14. Polarizing beam splitters 15 and 16, GaP crystal 17, mirrors 18 to 20, and half-wave plates 21 to 24 are provided. The non-polarizing beam splitter 12 corresponds to the multiplexing means of the present invention, the polarizing beam splitter 15 corresponds to the dividing means of the present invention, the polarizing beam splitter 16 corresponds to the angle adjusting means of the present invention, and the GaP crystal 17 Corresponds to the nonlinear optical crystal of the present invention.

  The first excitation light source 11 is a DFB semiconductor laser as in the first embodiment. The second excitation light source 10 is an external resonant semiconductor laser as in the first embodiment. The fiber amplifier 13 is an optical fiber amplifier doped with Yb, and amplifies the combined light from the non-polarizing beam splitter 12.

  The lens 14 has a focal length of 500 mm. By adjusting the position of the lens 14 in the optical axis direction by the position adjusting device 30, the diameter R of the spot of the combined light from the fiber amplifier 13 on the incident surface Q1 of the GaP crystal 17 is controlled. This position adjusting device 30 constitutes a spot diameter adjusting means.

  The non-polarizing beam splitter 12 and the polarizing beam splitters 15 and 16 are cube-shaped elements in which a dielectric multilayer film is formed on a slope of a triangular prism, and two triangular prisms are bonded together on the slope. The polarization beam splitter 16 can be rotated and moved in the direction perpendicular to the optical axis direction. For example, the rotation angle and the position can be controlled by placing the polarization beam splitter 16 rotatably supported on a single axis stage. The light incident on the non-polarizing beam splitter 12 is combined with the reflected light and the transmitted light regardless of the polarization state. The light incident on the polarization beam splitters 15 and 16 is divided into reflected light and transmitted light, and the polarization directions of the reflected light and transmitted light are orthogonal to each other. The non-polarizing beam splitter 12 is for combining the second excitation light and the first excitation light. The polarization beam splitter 15 divides the combined light of the second excitation light and the first excitation light into two. The polarization beam splitter 16 controls the rotation and movement of the polarization beam splitter 16 so as to adjust and mix the position overlapping the angle formed by the two combined lights. The phase matching angle θ of the two excitation lights and the diameter of the spot are mixed. R is controlled.

  The GaP crystal 17 is polished on the (110) plane. This (110) plane is the light incident plane. The terahertz wave generator of Example 1 generates terahertz waves by causing two excitation lights to enter the GaP crystal 17 and causing difference frequency mixing. Since the GaP crystal 17 is an isotropic nonlinear optical crystal and does not have birefringence, the phase matching condition is satisfied by phase matching (non-collinear phase matching) performed by giving angles to the two excitation lights. To do.

  The mirrors 18 to 20 are all for controlling the light irradiation direction, and the half-wave plates 21 to 24 are for adjusting the polarization direction of the light. This terahertz wave generator is capable of simultaneously generating terahertz waves having a difference frequency between the first excitation light and the second excitation light in two directions from the GaP crystal 17.

  Hereinafter, the operation of terahertz wave generation by the terahertz wave generator will be described in more detail. The first excitation light emitted from the first excitation light source 11 passes through the half-wave plate 21 and then enters the non-polarizing beam splitter 12. On the other hand, the second excitation light emitted from the second excitation light source 10 is reflected by the mirror 18 through the half-wave plate 22 and is incident on the non-polarizing beam splitter 12. Thereby, the first excitation light and the second excitation light are combined with the polarization directions aligned, and the combined light is output.

  Next, the combined light output from the non-polarizing beam splitter 12 is incident on the fiber amplifier 13 to amplify the combined light.

  Next, the combined light is condensed by the lens 14 and the position adjusting device 30 adjusts the position of the lens 14 in the optical axis direction, thereby controlling the diameter R of the spot on the incident surface Q1 of the GaP crystal 17.

  Next, the polarization beam splitter 15 splits the combined light into two, combined light A and B. One of the combined lights A is incident on the polarization beam splitter 16 through the half-wave plate 23. The other combined light B is reflected by the mirror 19, transmitted through the half-wave plate 24, further reflected by the mirror 20, and then incident on the polarization beam splitter 16. Then, the two combined lights A and B are mixed at an angle in the GaP crystal 17 by the polarization beam splitter 16. The angle formed by the combined lights A and B and the overlapping position can be controlled by rotating and moving the polarization beam splitter 16. The angle formed by the combined lights A and B is controlled so as to satisfy the phase matching condition.

  The two combined lights A and B are incident on the (110) plane of the GaP crystal 17 in a superimposed manner. The reason for entering the (110) plane is that TO phonons are excited more efficiently than other crystal planes, and terahertz waves can be generated efficiently. The polarization directions of the combined lights A and B are set so that one is in the [001] axis direction and the other is in the [1-10] axis direction. This is because the phonon excitation efficiency derived from the Raman selection rule is excellent. Both phenomena are explained from the phonon-polariton dispersion curves of GaP crystals.

In the GaP crystal 17, phonon-polaritons are excited by difference frequency mixing, and a terahertz wave having a difference frequency between the second excitation light and the first excitation light is generated. FIG. 12 is a diagram showing the relationship between the first excitation light, the second excitation light, and the terahertz wave (phonon-polariton) wave vector in the GaP crystal 17. In FIG. 12, k _s is the wave number vector of the first excitation light in the combined light A, k _p is the wave number vector of the second excitation light in the combined light B, and k _s ′ is the first excitation light in the combined light B. , K _p ′ represents the wave vector of the second excitation light in the combined light A, and q and q ′ represent the wave vector of the generated terahertz wave. q = k _p −k _s , q ′ = k _p ′ −k _s ′. As shown in FIG. 12, the difference k _p −k _s , k _p ′ −k _s ′ between the wave number vector of the second pump light and the wave vector of the first pump light matches the wave vector q, q ′ of the terahertz wave. When the phase matching condition is satisfied. Since the two combined lights A and B are made incident, the terahertz wave (q = k _p −k _s) generated by the second excitation light in one combined light B and the first excitation light in the other combined light A, 12 (a)), terahertz waves (q ′ = k _p ′ −k _s ′) of the second excitation light in one combined light A and the first excitation light in the other combined light B, FIG. b))) are generated and emitted from the GaP crystal 17 in different directions. That is, the angle between q and k _p, the angle between q 'and k _p' equal, the terahertz wave symmetrical two directions to the direction of k _p + k _p 'is output.

  Further, since the signal light and the pump light are simultaneously condensed by the lens 14, the beam shapes of the combined lights A and B in the GaP crystal 17 are almost circular. Therefore, the combined lights A and B can be efficiently superimposed.

  As described above, terahertz waves are generated in two directions simultaneously from the terahertz wave generating apparatus according to the second embodiment. Therefore, if the spectroscopic device is constructed using the terahertz wave generation device according to the second embodiment, it is not necessary to divide the terahertz wave using optical components to generate the reference light, so that the spectroscopic device is constructed at low cost. Spectroscopic measurement can be performed with high accuracy.

In the second embodiment, similarly to the first embodiment, the wavelength of the output terahertz wave can be continuously changed in a wide wavelength band, and the output can be maximized regardless of the frequency of the terahertz wave. it can. In the above embodiment, a GaP crystal is used as the nonlinear optical crystal. However, other nonlinear optical crystals that can generate a terahertz wave by difference frequency mixing and satisfy the phase matching condition by non-collinear phase matching can be used. It may be used. For example, a LiNbO ₃ crystal can be used.

  The terahertz wave generator of the present invention can be used for spectroscopy and the like.

BRIEF DESCRIPTION OF THE DRAWINGS Explanatory drawing which showed the principle of the terahertz wave generator of Example 1. FIG. Explanatory drawing which showed the coupling length of 1st excitation light and 2nd excitation light. The characteristic view which represented the relationship between a phase matching angle and coupling length as the parameter of the diameter of the spot of excitation light. The figure which showed the structure of the terahertz wave generator of Example 1. FIG. FIG. 3 is a measurement diagram of air spectrum absorption characteristics using terahertz waves output by the apparatus of Example 1; Explanatory drawing which showed the frequency band of the terahertz wave output which shows that a wavelength is variable in a wide band. The characteristic view which showed the relationship between the phase matching angle in GaP crystal | crystallization, and the frequency of the terahertz wave output. The characteristic view which measured the relationship between the phase matching angle | corner (theta) and the output in the state which made the diameter R of the spot constant, using the length of GaP crystal as a parameter. The characteristic view which measured the relationship between the phase matching angle | corner (theta) and the output in the state which made the diameter R of the spot constant, using the length of GaP crystal as a parameter. The characteristic view which showed the relationship between the length of a GaP crystal | crystallization, and an output by using the frequency of a slahertz wave as a parameter. The figure which showed the structure of the terahertz wave generator of Example 2. FIG. The figure which showed the relationship of the wave vector.

Explanation of symbols

51, 11: First excitation light source 52, 10: Second excitation light source 12: Non-polarization beam splitter 13: Fiber amplifier 14: Lens 15, 16: Polarization beam splitter 17: GaP crystal

Claims (6)

In a terahertz wave generator for generating a wavelength-tunable terahertz wave,
A first excitation light source that emits first excitation light;
A second excitation light source that emits second excitation light having a frequency difference from the first excitation light in a terahertz band;
An angle adjusting means for adjusting and mixing the angle so that the angle formed by the first excitation light and the second excitation light becomes a phase matching angle according to the frequency of the output terahertz wave;
A nonlinear optical crystal in which the two excitation lights are incident at a phase matching angle and radiate a terahertz wave by difference frequency mixing;
A terahertz wave generator comprising: spot diameter adjusting means for adjusting spot diameters of the first excitation light and the second excitation light incident on the nonlinear optical crystal. In a terahertz wave generator for generating a wavelength-tunable terahertz wave,
A first excitation light source that emits first excitation light;
A second excitation light source that emits second excitation light having a frequency difference from the first excitation light in a terahertz band;
A multiplexing unit configured to combine the first excitation light and the second excitation light to generate combined light;
Splitting means for splitting the combined light into two;
Angle adjusting means for adjusting and mixing the angle so that the angle formed by the two combined lights divided by the dividing means becomes a phase matching angle;
A non-linear optical crystal in which two combined lights are incident at a phase matching angle and radiate a terahertz wave in two directions simultaneously by difference frequency mixing;
A terahertz wave generator comprising: spot diameter adjusting means for adjusting spot diameters of the first excitation light and the second excitation light incident on the nonlinear optical crystal.   The terahertz wave generating device according to claim 1 or 2, wherein the spot diameter adjusting means is changed according to the phase matching angle.   3. The terahertz wave generating device according to claim 1, wherein the spot diameter adjusting unit is changed according to the phase matching angle and the length of the nonlinear optical crystal. The spot diameter adjusting means uses the spot diameter Rmm, the length Lmm of the nonlinear optical crystal, the phase matching angle θ, and the proportional constant α,
The terahertz wave generator according to claim 4, wherein   6. The length of the nonlinear optical crystal in the light propagation direction is set to a length that maximizes the output of the output terahertz wave. 6. The terahertz wave generator described.