JP5376366B2 - Electromagnetic wave generating apparatus and electromagnetic wave generating method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electromagnetic wave generating device which has a simple and inexpensive structure and generates pulse train laser light, a terahertz wave, or a millimeter wave, and to provide an electromagnetic wave generating method. <P>SOLUTION: The electromagnetic wave generating device includes a spatially distributed pulse train converting device 20 and a condenser lens 26. The spatially distributed pulse train converting device 20 has first and second mirrors 22, 24, and the first mirror 22 has first and second reflecting surfaces 22a, 22b having a step portion on a boundary line 23 where a part of a surface 22a side body and a part of a surface 22b side body come into contact with each other. The spatially distributed pulse train converting device 20 converts pulsed laser light of incident femtosecond laser pulses or picosecond laser pulses into spatially distributed pulse train laser light 3 composed of a plurality of spatially distributed pulse laser lights with time differences proportional to level difference intervals through reflection by the reflecting surfaces 22a, 22b across the boundary line 23. The condenser lens 26 converges the spatially distributed pulse train laser light 3 to generate pulse train laser light 4 on same axis composed of a plurality of pulse laser lights which are on the same axis at temporally equal intervals. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to an electromagnetic wave generating apparatus and an electromagnetic wave generating method for generating pulse train laser light, terahertz waves or millimeter waves.

An electromagnetic wave is one in which a periodic change in the electromagnetic field propagates as a wave, and anything in which a periodic change in the electromagnetic field propagates as a wave is included in the electromagnetic wave.
In particular, in the present application, a pulse laser beam, a pulse train laser beam, a terahertz wave, and a millimeter wave electromagnetic wave having a wavelength of 10 ⁻⁷ m (0.1 μm) to 10 ⁻² m (10 mm) are targeted.
In addition, what emits laser light including femtosecond laser light of near infrared (wavelength 800 nm) and femtosecond laser of visible light is included in the electromagnetic wave in the present application at all wavelengths.

The frequency of the terahertz wave (THz wave) is 0.3 to 10 THz, and the wavelength is 1 mm to 30 μm when converted into the wavelength from the relational expression ν (frequency) = c (speed of light) / λ (wavelength).
A terahertz wave is an electromagnetic wave that strikes an intermediate region between radio waves and light waves, and has transparency to various substances such as paper and plastic like radio waves, and at the same time has an appropriate spatial resolution like light. Furthermore, since the substance has an intrinsic absorption spectrum, it can be expected to be applied to detect dangerous substances such as explosives by identifying the type of the substance hidden in a parcel or the like by spectroscopic measurement, for example. To achieve this, a terahertz wave generator that is tuned at various frequencies is indispensable.

  For example, Patent Documents 1 and 2 and Non-Patent Documents 1 to 3 disclose means for generating a terahertz wave suitable for spectroscopic measurement by irradiating a non-linear optical element with femtosecond laser light.

Patent Document 1 and Non-Patent Document 1 disclose generation of a femtosecond laser pulse train using a spatial phase modulator and single frequency generation means.
By modulating the phase spectrum of the broadband optical pulse, a pulse train at a plurality of variable wavelengths can be generated. When a non-linear optical element is irradiated with an equidistant femtosecond pulse train generated using this principle, a terahertz wave pulse forms a continuous wave waveform. Therefore, a frequency component having a period of the terahertz wave pulse interval is strongly generated. This means has an advantage that the time interval and the number of pulses of the femtosecond pulse train can be freely changed.

Non-Patent Document 2 discloses generation of a femtosecond pulse train using a birefringence of a birefringent crystal and a single frequency generating means.
When femtosecond laser light is transmitted through a birefringent crystal, a time delay occurs between femtosecond laser pulses of different polarization directions due to the birefringence. Using this principle, it is possible to generate a pulse train that is equally spaced in time and generate a single-frequency terahertz wave. However, this means requires birefringent crystals having different crystal lengths in order to change the time interval of the femtosecond pulse train.

Non-Patent Document 3 discloses a single frequency generating means using difference frequency mixing of spatially dispersed beams.
By irradiating the diffraction grating with femtosecond laser light, a spatially dispersed elliptical beam (spatial dispersion beam) is generated and divided into two. A terahertz wave is generated by irradiating a nonlinear optical element by superimposing two beams while spatially shifting one beam. At this time, since the wavelength difference between the two lights is substantially constant, a single frequency terahertz wave is generated by the difference frequency mixing. Also, frequency tuning of the terahertz wave is possible by changing a value for spatially shifting one of the spatially dispersed beams.

  Patent Document 2 discloses a broadband generation means using a single femtosecond laser pulse. This is a means for generating a broadband terahertz wave pulse using a broadband femtosecond pulse laser.

JP 2001-174664 A Japanese Patent Laid-Open No. 2002-2223017

J. et al. Ahn, A.A. V. Efimov, R.M. D. Averitt, A.A. J. et al. Taylor, “Terahertz waveform synthesis via optical rectification of shaped ultralaser pulses” Opt. Express 11, (2003.10.6) Froberg, N.M. M.M. Bin Bin Hu, Xi-Chang Zhang, Austin, D.B. H, “Terahertz radiation from a photoducting antenna array” IEEE J. Quantum Electron 2291-2301 (1992.10) Ken-ichiro Maki, Chiko Otani, “Terahertz beam steering and frequency tuning the use of the spatial dispersal of ultrafast lasers. Express 16, 10158-10169 (2008.7.7)

  The femtosecond pulse train generation and single-frequency terahertz wave generation methods (Patent Document 1 and Non-Patent Document 1) using the spatial phase modulator described above are about 2 million yen in commercially available spatial light modulators. There is a problem that the response speed is slow (about several tens of Hz) because it depends on the display switching speed of the liquid crystal pixel to be phase-modulated. In addition, since two diffraction gratings and a spatial light modulator (SLM) are used, there is a problem that the laser light intensity is lowered and the output of the terahertz wave is lowered.

  In the means using the above-described birefringent crystal (Non-Patent Document 2), the frequency of the terahertz wave generated by the length of the crystal is uniquely determined. Therefore, in order to tune to a desired frequency, a birefringent crystal having a predetermined length must be prepared. Therefore, frequency tuning to an arbitrary frequency is virtually impossible. Furthermore, there is a problem that dispersion compensation is necessary to prevent a decrease in the signal intensity of the terahertz wave because the pulse width is expanded by passing through the crystal and the peak value of the laser beam is reduced.

  The above-described means using the difference frequency mixing of the spatially dispersed beam (Non-patent Document 3) has a problem that the peak value of the femtosecond pulse is lowered due to the formation of the spatially dispersed beam. In addition, since the terahertz wave generation uses a second-order nonlinear optical effect, the output is proportional to the square of the laser light intensity. Therefore, there is a drawback that the output of the terahertz wave is reduced.

  The above-described broadband generation means (Patent Document 2) using a single femtosecond laser pulse can obtain information on various frequency components by performing Fourier transform on the time waveform of the measured short pulse terahertz wave. However, since all frequency components are included in one terahertz pulse, it cannot be used for high-resolution spectroscopic measurement in a specific frequency region.

  The present invention has been made to improve at least some of these problems. An object of an embodiment of the present invention is to provide an electromagnetic wave generating apparatus and an electromagnetic wave generating method capable of generating pulse train laser light, terahertz waves, or millimeter waves, for example, with a simple and inexpensive structure.

The electromagnetic wave generator of one embodiment of the present invention has the following characteristic points.
(1) It has the 1st and 2nd mirror arrange | positioned facing each other, and this 1st mirror has 1st and 2nd which has a level | step-difference part straddling the boundary line which mutually closes in part A pulse laser beam of a femtosecond laser pulse or a picosecond laser pulse that has a reflecting surface and is incident between the first and second mirrors is reflected on the first and second reflecting surfaces across the boundary line. A spatial distribution pulse train conversion device for converting into a spatial distribution pulse train laser beam composed of a plurality of pulse laser beams spatially distributed with a time difference proportional to the step interval between the first and second reflecting surfaces by reflection;
And a condensing lens that collects the spatially-distributed pulse train laser light and generates coaxial pulse train laser light composed of a plurality of pulse laser lights that are equidistant in time on the same axis.

(2) Moreover, as an embodiment of the present invention, an electromagnetic wave generator of the present invention includes a pulse laser device that outputs a pulse laser beam of the femtosecond laser pulse or the picosecond laser pulse.
(3) The electromagnetic waves are pulse train laser light, terahertz waves, or millimeter waves.
(4) Further, a photoconductive antenna or a nonlinear optical crystal that generates a terahertz wave or a millimeter wave having a narrow frequency band from the coaxial pulse train laser light is provided.
(5) In addition, a pulse interval adjusting device that continuously moves one or both of the first and second reflecting surfaces of the first mirror in a direction orthogonal to the reflecting surface to change the step interval on the boundary line. With
Thereby, the time interval of the coaxial pulse train laser beam is continuously changed.

(6) In one embodiment of the present invention, on the first and second reflection surfaces, a boundary line direction is a y-axis, a direction perpendicular to the boundary direction is a z-axis, and a direction orthogonal to the reflection surface is an x-axis. In a three-dimensional space
The reflecting surface of the second mirror is parallel to the first and second reflecting surfaces in the xz plane and the xy plane,
The pulse laser beam is incident between the first and second mirrors at least with respect to the x-axis.
(7) In another embodiment of the present invention, on the first and second reflecting surfaces, a boundary line direction is a y-axis, a direction orthogonal to the z-axis is a direction orthogonal to the reflecting surface, In a three-dimensional space with x-axis,
The reflective surface of the second mirror is parallel to the first and second reflective surfaces in the xz plane and non-parallel to the first and second reflective surfaces in the xy plane.
(8) Furthermore, in another embodiment of the present invention, the second mirror has third and fourth reflecting surfaces having step portions straddling a boundary line in close contact with each other.

Moreover, the electromagnetic wave generation method of one embodiment of the present invention has the following characteristics.
(9) First and second mirrors arranged opposite to each other are prepared, and the first and second mirrors have step portions straddling a boundary line in close contact with each other on the first mirror. Provide a reflective surface,
A pulse laser beam of a femtosecond laser pulse or a picosecond laser pulse is incident between the first and second mirrors, and the pulse laser beam is reflected by the first and second reflecting surfaces across the boundary line. Is converted into a spatially distributed pulse train laser beam consisting of a plurality of pulsed laser beams spatially distributed with a time difference proportional to the step interval between the first and second reflecting surfaces,
The spatially-distributed pulse train laser beam is condensed to generate a coaxial pulse train laser beam composed of a plurality of pulse laser beams that are equally spaced in time on the same axis.

(10) As an embodiment of the present invention, in the electromagnetic wave generation method of the present invention, the coaxial pulse train laser beam is incident on a photoconductive antenna or a nonlinear optical crystal to generate a terahertz wave or a millimeter wave having a narrow frequency band.
(11) Further, the coaxial pulse train may be configured such that one or both of the first and second reflecting surfaces of the first mirror are continuously moved in a direction orthogonal to the reflecting surface to change the step interval at the boundary line. The time interval of the laser beam is continuously changed.

According to the present invention, the problems of the prior art can be improved.
The following are the effects of the embodiment of the present invention.
In an embodiment of the present invention, a single pulse laser beam (femtosecond laser beam or picosecond laser beam) is used to generate a coaxial pulse train laser beam that is equidistant in time and further variably adjusted using the same. One frequency terahertz wave or millimeter wave is generated.

  According to the apparatus and method of the present invention described above, a single pulse laser beam is converted into a spatial distribution pulse train laser beam composed of a plurality of pulse laser beams by the spatial distribution pulse train converter, and the spatial distribution pulse train is converted by the condenser lens. It is possible to generate a coaxial pulse train laser beam composed of a plurality of pulse laser beams that are equally spaced in time on the same axis on the same axis.

  In addition, according to the embodiment of the present invention, a photoconductive antenna or a nonlinear optical crystal is provided, and the coaxial pulse train laser light is incident on the photoconductive antenna or the nonlinear optical crystal, thereby generating a terahertz wave or a millimeter wave having a narrow frequency band. Can be generated.

  Furthermore, a pulse interval adjusting device is provided, and one or both of the first mirror first and second reflecting surfaces are continuously moved in a direction perpendicular to the reflecting surface to change the step interval at the boundary line, By continuously changing the time interval of the coaxial pulse train laser light, it is possible to change the tuning frequency of the generated terahertz wave or millimeter wave having a narrow frequency band.

  Further, by configuring the pulse interval adjusting device with, for example, a piezo element and a power supply device that applies a voltage to the piezo element, the tuning frequency can be changed at a high response speed (for example, 1 to 3 kHz).

The above-described apparatus and method according to the embodiments of the present invention have the following effects compared to the prior art.
(1) Compared with the generation of a broadband terahertz wave using a single pulse laser beam (femtosecond pulse), spectral information of a desired frequency can be obtained from a broadband frequency component, so there is no need to separate the frequency component. .
(2) In the conventional means for generating a pulse train laser beam (femtosecond pulse train) using a birefringent optical crystal, it is necessary to prepare a birefringent optical crystal having a predetermined length in order to tune to a desired frequency. There are problems that it is practically impossible to tune to an arbitrary frequency and that dispersion compensation is necessary to prevent a decrease in output of the terahertz wave due to the pulse width expansion. However, in the present invention, if the step difference between the first and second reflecting surfaces is changed, the frequency can be easily tuned and only the reflection between the mirrors is used, so there is no need for dispersion compensation.
(3) In the case of the difference frequency mixing of the spatially dispersed beam, there is a disadvantage that the output of the terahertz wave is reduced due to the reduction of the laser light intensity and the expansion of the pulse width, but the present invention only uses reflection between mirrors. The problem is solved.
(4) Compared with a spatial phase modulator, the step mirror can be controlled with a linear actuator, and the frequency can be tuned at a response speed of several kHz, which is several hundred times faster, and the number of mirrors and piezoelectric elements is several. It is about 100,000 yen and can be (1/10). Furthermore, in the spatial phase modulator, the two diffraction gratings and the spatial light modulator have a problem that the laser light intensity is reduced and the output of the terahertz wave is also reduced. The problem is solved because it is used.

It is a block diagram of the level | step difference mirror by this invention. It is action | operation explanatory drawing of the level | step difference mirror by this invention. It is explanatory drawing of the coaxial pulse-train laser beam production | generation by the lens condensing in this invention. It is a wave form diagram of the terahertz wave generated by the present invention. It is a schematic diagram of a nonlinear optical element that generates a terahertz wave having a narrow frequency band from coaxial pulse train laser light. 1 is an overall configuration diagram of an electromagnetic wave generator according to an embodiment of the present invention. It is a supplementary explanatory drawing of FIG. It is a figure which shows the positional relationship when a level | step difference mirror and a counter mirror are parallel. It is a figure which shows the positional relationship when a level | step difference mirror and an opposing mirror are not parallel. It is a figure which shows 1st Example of this invention. It is a figure which shows 2nd Example of this invention. It is a figure which shows 3rd Example of this invention.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. In addition, the same code | symbol is attached | subjected to the common part in each figure, and the overlapping description is abbreviate | omitted.

  First, the principle of the present invention will be described with reference to FIGS. In the following description, the pulsed laser light is femtosecond laser light, but the present invention is not limited to this, and may be picosecond laser light.

  FIG. 1 is a configuration diagram of a step mirror according to the present invention. In this figure, (A) is a front view and a side view of the step mirror 22, and (B) is a schematic diagram of reciprocal reflection between the counter mirror 24 and the step mirror 22. Here, the step mirror is a reflection mirror having first and second reflection surfaces 22a and 22b having a step portion across a boundary line 23 which is in close contact with each other, and the counter mirror 24 counters this. It is a reflection mirror.

FIG. 2 is an operation explanatory view of the step mirror according to the present invention. In this figure,
(A) is explanatory drawing of reflection with the level | step difference mirror of a laser beam, (B)-(D) is a spatial distribution figure of the time delay in a laser beam.
FIG. 3 is an explanatory diagram of the generation of coaxial pulse train laser light by lens focusing in the present invention.
FIG. 4 is a waveform diagram of a terahertz wave generated by the present invention. In this figure, (A) shows a waveform of a terahertz wave generated from a single femtosecond pulse, and (B) shows a waveform of a terahertz wave generated from a femtosecond pulse train. Note that the arrows pointing to the right in FIGS. 4A and 4B represent the time axis.

In the present invention, for example, a single pulse laser beam (femtosecond laser beam or picosecond laser beam) is used to generate a coaxial pulse train laser beam in which a plurality of pulse laser beams are arranged at equal intervals in time. It is used to generate a single frequency terahertz wave or millimeter wave.
Here, both terahertz waves and millimeter waves are electromagnetic waves. The generated electromagnetic wave can be frequency-tuned as will be described later.

As shown in FIG. 1A, the reflecting surfaces 22a and 22b (first and second reflecting surfaces) of the step mirror 22 are shifted by several hundred μm so that there is no gap. That is, the step mirror 22 has two reflection surfaces 22a and 22b that are parallel to each other, and has a step interval ΔL that can be variably adjusted with a boundary line 23 between the reflection surfaces as a boundary.
When the laser beam is reflected by the boundary portion 23 of the step mirror 22, as shown in the right figure of FIG. 1A, the laser beam 2 reflected by the upper first reflecting surface 22a and the lower second reflecting surface 22b. A difference in the optical distance is generated between the laser beams 2 reflected by the step 2 by the step distance ΔL of the step mirror 22. Accordingly, two pulse trains are generated from the single pulse laser beam 2.

Next, as shown in FIG. 1B, the laser beam 2 is reflected back and forth between the counter mirror 24 and the step mirror 22.
At this time, as shown in FIG. 2A, the reflection position of the laser beam 2 with respect to the boundary line 23 of the step mirror 22 is shifted in the order of (1), (2), and (3). In FIGS. 2B to 2D, reference numerals 21a, 21b, and 21c denote positions of lasers that hit the boundary line 23 of the step mirror 22 at the reflection positions (1), (2), and (3), respectively. .

Laser light 2 reflected by the lower second reflecting surface 22b in the first reflection (1) at the step mirror 22 with reference to the laser light 2 reflected by the upper first reflecting surface 22a in FIG. In the meantime, a time delay is caused by the optical distance of the step interval ΔL of the step mirror 22 (FIG. 2B).
Next, in the second reflection (2), the laser beam reflected by the upper first reflecting surface 22a in (1) and (2) is used as a reference and reflected by the lower second reflecting surface 22b in (1). The laser beam reflected by the upper first reflecting surface 22a in (2) is delayed by an optical distance of the step interval ΔL and reflected by the lower second reflecting surface 22b in (1) and (2). The laser light is delayed in time by an optical distance of 2ΔL (FIG. 2C).
Finally, in the third reflection (3), the laser beam reflected by the upper first reflection surface 22a in (1), (2) and (3) is used as a reference, and the upper reflection in (1) and (2). The laser light reflected by the first reflecting surface 22a and reflected by the lower second reflecting surface 22b in (3) is delayed in time by the optical distance of the step interval ΔL. In (1), the upper first reflecting surface 22a is reflected. The laser light reflected and reflected by the lower second reflecting surface 22b in (2) and (3) is delayed by an optical distance of 2ΔL, and the lower side in (1), (2) and (3) The laser beam reflected by the second reflecting surface 22b is delayed by an optical distance of 3ΔL (FIG. 2D).
In this way, as shown in FIG. 2D, the laser light 2 is converted into a pulse train 3 (referred to as “spatial distribution pulse train laser light”) composed of a plurality of pulse laser light at time intervals corresponding to the step interval ΔL. Converted.

Then, the spatially distributed pulse train laser light 3 spatially distributed with different time delays is condensed by the condenser lens 26, so that the pulse laser light 4 ("" Called "coaxial pulse train laser light").
In addition, by condensing with the condensing lens 26, the pulse train equally spaced in time is not produced | generated. By condensing with the condensing lens 26, the laser light distributed temporally and spatially is converted into the laser light that forms a line only in terms of time.

Next, the principle of generating a single frequency terahertz wave will be described.
First, the time waveform of the terahertz wave generated when one pulse laser beam 3 (femtosecond laser beam) is applied to a dipole antenna, which is one of the photoconductive antennas, is as shown in the right diagram of FIG. Become. This is because the carrier is excited between the electrodes by the femtosecond pulse irradiation, and the electromagnetic wave is radiated corresponding to the time change of the carrier. Details will be described in the description of the apparatus configuration described later.

Next, FIG. 4B shows a time waveform when the terahertz wave 5b is generated by the coaxial pulse train laser beam 4 (femtosecond pulse train) having a time interval of about several ps. The terahertz wave pulse forms a continuous wave waveform. Therefore, a frequency component having a cycle of the THz pulse interval is strongly generated.
Furthermore, the pulse interval can be arbitrarily changed by adjusting the step interval of the step mirror 22. That is, the frequency of the terahertz wave can be tuned to a desired frequency by a simple method of moving the first reflecting surface 22a or the second reflecting surface 22b.
Similarly, millimeter waves can be generated instead of terahertz waves.

In the above example, when a kind of photoconductive antenna dipole antenna is used as the electromagnetic wave generating element, the frequency variable range is 0.6 THz to 1 THz.
However, it is known that the frequency of a THz wave when a femtosecond pulse train generated by a spatial phase modulator is generated by irradiating a ZnTe crystal which is a kind of nonlinear optical element is variable from 1 THz to 3 THz. is there.
Even if femtosecond laser light having the same pulse width of about 100 fs is used as the light source, for example, if a ZnTe crystal which is a kind of nonlinear optical element is used as the generating element, electromagnetic waves having a frequency from the millimeter wave band to the terahertz wave band are used. Can be generated. Therefore, it is possible to generate a millimeter wave, and it is also possible to generate an electromagnetic wave having a frequency from the millimeter wave band to the terahertz wave band.

  FIG. 5 is a schematic diagram of a nonlinear optical element 30 that generates a terahertz wave 5 having a narrow frequency band from the coaxial pulse train laser beam 4. This figure shows a dipole photoconductive antenna element which is one of nonlinear optical elements.

  In the photoconductive antenna element 30, an electrode 32 is attached on a low-temperature grown gallium arsenide substrate 31 that responds at high speed so as to form a minute gap 32a at the center. When a voltage is applied between the gaps 32a by the power source 33 and the laser pulse 4 having a photon energy larger than the band gap of gallium arsenide is irradiated to the gaps 32a, carriers are excited. In response to this time change, the terahertz wave 5 is emitted.

  FIG. 6 is an overall configuration diagram of an electromagnetic wave generator according to the present invention, and FIG. 7 is a supplementary explanatory diagram of FIG. In FIG. 6, the electromagnetic wave generator of the present invention includes a spatial distribution pulse train converter 20 and a condenser lens 26.

  As shown in FIG. 1A, the spatial distribution pulse train converter 20 includes first and second mirrors 22 and 24 arranged to face each other, and the first mirror 22 has one of them. It has the 1st and 2nd reflective surfaces 22a and 22b which have a level | step-difference part across the boundary line 23 which closely_contact | connects. The first and second reflecting surfaces 22a and 22b are preferably parallel to each other. The first mirror 22 is a step mirror, and the second mirror 24 is a counter mirror without a step in this example.

  In addition, the spatial distribution pulse train converter 20 is configured so that the femtosecond incident between the first and second mirrors 22 and 24 as shown in FIG. 1B and FIG. 2A to FIG. The pulse laser beam 2 of the laser pulse or the picosecond laser pulse is reflected by the first and second reflecting surfaces 22a and 22b across the boundary line 23 with a time difference proportional to the step interval between the first and second reflecting surfaces. The laser beam is converted into a spatially distributed pulse train laser beam 3 composed of a plurality of spatially distributed pulse laser beams.

In FIG. 6, the spatial distribution pulse train conversion device 20 further includes a pulse interval adjustment device 28.
The pulse interval adjusting device 28 has a linear actuator capable of continuously moving one or both of the first and second reflecting surfaces 22a and 22b constituting the step mirror 22 in a direction perpendicular to the reflecting surface.
For example, a linear actuator capable of variably adjusting the step interval ΔL is provided on at least one of the first and second reflecting surfaces 22a and 22b, and the step interval ΔL can be continuously moved. The linear actuator can be composed of, for example, a piezoelectric element and a power supply device that applies a voltage to the piezoelectric element.

  As shown in FIG. 3, the condensing lens 26 condenses the spatially distributed pulse train laser light 3 to generate a coaxial pulse train laser light 4 composed of a plurality of pulse laser lights that are coaxially spaced at regular intervals.

In FIG. 6, the electromagnetic wave generator of the present invention further includes a pulse laser device 10.
The pulse laser device 10 outputs a pulse laser beam 1 of a femtosecond laser pulse or a picosecond laser pulse. The pulse width of the femtosecond laser light is (1 to several hundreds) × 10 ⁻¹⁵ seconds (fs), or the pulse width of the picosecond laser light is (1 to several hundreds) 10 ⁻¹² seconds (ps).

  The spatial distribution pulse train conversion device 20 described above generates a coaxial pulse train laser beam 4 (see FIG. 7D) having an equal pulse interval from a single pulse laser beam 1 (see FIG. 7C). Here, the coaxial pulse train laser beam 4 means a plurality of pulse laser beams 4 that are equally spaced in time as shown in FIG. However, it is not essential to be equally spaced in time.

  In this example, the electromagnetic wave generator of the present invention further includes reflection mirrors 12a and 12b, cylindrical lenses 14a and 14b, a reflection mirror 16, and reflection mirrors 18a and 18b.

  The reflection mirrors 12a and 12b have a function of reflecting the pulse laser beam 1 and guiding it to the two cylindrical lenses 14a and 14b.

The two cylindrical lenses 14a and 14b have different focal lengths f ₁ and f ₂ , and the beam cross-sectional shape of the pulsed laser light 1 is circular (see FIG. 7A) to elliptical (FIG. 7B). See). This is converted so that the focal length ratio (f ₂ / f ₁ ) of the cylindrical lens is equal to the ratio of the major axis to the minor axis of the laser beam shape. A pulsed laser beam having an elliptical cross section is referred to as an elliptical pulsed laser beam 2.

The reflection mirror 16 has a function of reflecting the elliptical pulse laser beam 2 emitted from the cylindrical lenses 14 a and 14 b and guiding it between the step mirror 22 and the counter mirror 24.
In this example, the step mirror 22 and the counter mirror 24 are arranged in parallel to each other, and the reflection mirror 16 reciprocally reflects the elliptical pulse laser beam 2 between the step mirror 22 and the counter mirror 24 and reflects it at the step mirror 22. Each position is positioned so that the reflection position is shifted in a direction intersecting the boundary line 23.

  Reflecting the pulse laser beam 2 incident between the first and second mirrors 22 and 24 on the first and second reflecting surfaces 22a and 22b across the boundary line 23 by the spatial distribution pulse train converter 20 described above. Thus, it is possible to convert into a spatially distributed pulse train laser beam 3 composed of a plurality of pulse laser beams spatially distributed with a time difference proportional to the step interval between the first and second reflecting surfaces.

  That is, the elliptical pulse laser beam 2 reflected a plurality of times between the step mirror 22 and the counter mirror 24 is spaced at equal pulse intervals in time, as shown in FIGS. 2 (B) to 2 (D). The spatially distributed pulse train laser beam 3 is distributed.

  The reflection mirrors 18 a and 18 b have a function of reflecting the spatially distributed pulse train laser light 3 emitted from between the step mirror 22 and the counter mirror 24 and guiding it to the condenser lens 26.

  The condensing lens 26 condenses the spatially distributed spatially-distributed pulse train laser light 3 and generates pulse train laser light (coaxial pulse train laser light 4) that is equally spaced in time.

In FIG. 6, the electromagnetic wave generator of the present invention further includes an electromagnetic wave conversion device 30.
The electromagnetic wave conversion device 30 generates a terahertz wave 5 or a millimeter wave having a narrow frequency band from the coaxial pulse train laser beam 4. For the electromagnetic wave conversion device 30, a photoconductive antenna, a nonlinear optical crystal, or the like can be used.

6 and 7 described above show the configuration of a device that generates a femtosecond pulse train (pulse train laser beam 4) and generates a terahertz wave 5 using the same.
From the circular femtosecond laser beam 1 having a beam diameter of 2 mm, the shape of the laser beam 2 is made elliptic by using two cylindrical lenses 14a and 14b having different focal lengths. Then, the laser beam 2 is reflected back and forth between the step mirror 22 and the counter mirror 24 as shown in FIG. 1, and converted into a femtosecond pulse train (spatial distribution pulse train laser beam 3). Thereafter, the coaxial pulse train laser beam 4 is generated from the spatially distributed spatially-distributed pulse train laser beam 3 by the condensing lens 26 and is collected in the gap 32a of the dipole photoconductive antenna 30 shown in FIG. Then, current modulation is applied by a femtosecond pulse train (coaxial pulse train laser beam 4) equally spaced in time, and a terahertz wave 5 is emitted from the back surface of the low-temperature grown gallium arsenide substrate 31.

FIG. 8 and FIG. 9 are embodiment diagrams showing the positional relationship between the first mirror and the second mirror constituting the spatial distribution pulse train converter 20.
8A and 8B show the case where the first mirror and the second mirror are parallel, and FIGS. 9A and 9B show the case where the first mirror and the second mirror are not parallel. And (A) has a stepped portion across a boundary line where the second mirror 24B is in close contact with each other when the second mirror 24A has a single reflecting surface. This is a case where the third and fourth reflecting surfaces 24a and 24b are provided.

  That is, in these drawings, the second mirror may be either a plane mirror or a step mirror. Hereinafter, the case where the second mirror is a plane mirror is referred to as an “opposing step mirror”, and the case where the second mirror is a step mirror is referred to as an “opposing step mirror”.

  8 and 9, there are four positional relationships between the first mirror and the second mirror as shown in FIGS. 8A and 8B and FIGS. 9A and 9B.

In FIG. 8A, the reflecting surface of the opposing plane mirror 24A facing the step mirror 22 is parallel on both the xy plane and the xz plane (provided that the xyz axes are orthogonal to each other, the same applies hereinafter). This is the case. That is, on the first and second reflection surfaces, the boundary line direction is the y-axis, the direction orthogonal to this is the z-axis, and the direction orthogonal to the first and second reflection surfaces 22a and 22b is the x-axis. In the dimensional space, the reflecting surface of the second mirror (opposing plane mirror 24A) is configured in parallel to the first and second reflecting surfaces 22a and 22b in the xz plane and the xy plane.
The positional relationship in FIG. 8A corresponds to FIG. 6 described above.

FIG. 8B shows a case where the reflecting surface of the opposing step mirror 24B facing the step mirror 22 is in a parallel relationship in both the xy plane and the xz plane. That is, on the first and second reflection surfaces, the boundary line direction is the y-axis, the direction orthogonal to this is the z-axis, and the direction orthogonal to the first and second reflection surfaces 22a and 22b is the x-axis. In the dimensional space, the reflection surface of the second mirror (opposing step mirror 24B) is configured to be parallel to the first and second reflection surfaces 22a and 22b in the xz plane and the xy plane.
The positional relationship in FIG. 8A corresponds to the case where the counter mirror 24 in FIG. 6 described above is replaced with the counter step mirror 24B. Other configurations are the same as those in FIG.

In the example of FIGS. 8A and 8B described above, the pulsed laser beam 2 is incident between the first and second mirrors 22 and 24 at an inclination with respect to at least the x-axis. This inclination angle is set according to the distance by which the reflection position is shifted in the direction intersecting the boundary line 23 for each reflection.
With the configuration described above, the elliptical pulse laser beam 2 is incident with being slightly inclined with respect to the x-axis, whereby the elliptical pulse laser beam 2 is reflected back and forth between the step mirror 22 and the counter mirror 24, and the step mirror 22 For each reflection at, the reflection position can be shifted in a direction intersecting the boundary line 23.
In the arrangement shown in FIG. 8B, the reflection position of the opposed step mirror 24B can be shifted in the direction intersecting the boundary line between the third and fourth reflecting surfaces 24a and 24b.

  FIG. 9A shows a case where the reflecting surface of the opposing flat mirror 24A facing the step mirror 22 is non-parallel in the xy plane and parallel in the xz plane. That is, on the first and second reflecting surfaces, the second mirror in a three-dimensional space in which the boundary line direction is the y-axis, the direction perpendicular to the z-axis is the z-axis, and the direction perpendicular to the reflecting surface is the x-axis. The reflective surface of the (opposing flat mirror 24A) is parallel to the first and second reflective surfaces 22a and 22b in the xz plane and non-parallel to the first and second reflective surfaces 22a and 22b in the xy plane. It is configured.

  FIG. 9B shows a case where the reflecting surface of the opposing step mirror 24B facing the step mirror 22 is non-parallel on the xy plane and parallel on the xz plane. That is, on the first and second reflecting surfaces, the second mirror in a three-dimensional space in which the boundary line direction is the y-axis, the direction perpendicular to the z-axis is the z-axis, and the direction perpendicular to the reflecting surface is the x-axis. The reflective surface of the (opposing step mirror 24B) is parallel to the first and second reflective surfaces 22a and 22b in the xz plane, and is not parallel to the first and second reflective surfaces 22a and 22b in the xy plane. It is configured.

In the example of FIGS. 9A and 9B described above, the reflection surface of the opposed flat mirror 24A or the opposed step mirror 24B is set so as to reflect the pulse laser beam 2 to a different position of the step mirror 22.
With the configuration described above, the pulse laser beam 2 is incident between the step mirror 22 and the counter mirror 24, so that the pulse laser beam 2 is reflected back and forth between the step mirror 22 and the counter flat mirror 24A or the counter step mirror 24B. The reflection position can be shifted in a direction intersecting the boundary line 23 for each reflection at the step mirror 22.
In the arrangement shown in FIG. 8B, the reflection position of the opposed step mirror 24B can be shifted in the direction intersecting the boundary line between the third and fourth reflecting surfaces 24a and 24b.

In FIGS. 8 and 9, the reflecting surfaces of the two mirrors facing each other need to be parallel in the xz plane. Otherwise, the coordinate interval of position x between 21a (FIG. 2 (B)) and 21b (FIG. 2 (C)) and the position between 21b (FIG. 2 (C)) and 21c (FIG. 2 (D)). The coordinate intervals of x are not equal. If they are not equal, there will be variations in the laser light intensity of each femtosecond pulse, and the output intensity of the generated terahertz wave will also vary.
Examples of the present invention will be described below.

FIG. 10 is a diagram showing a first embodiment of the present invention.
In this figure, FIG. 10A shows a time waveform measured when the dipole photoconductive antenna 30 is irradiated with a single pulse laser beam 1 (femtosecond pulse) output from the pulse laser device 10. FIG.
From this figure, it can be seen that one electromagnetic pulse having a pulse width of about 0.4 ps is generated.

A curve A (single pulse) in FIG. 10C is a diagram illustrating a spectrum of a terahertz wave calculated using Fourier transform based on the time waveform in FIG.
From this figure, it can be seen that the line width of the electromagnetic wave pulse in FIG. 10A is about 650 GHz, and a terahertz wave is generated in a wide band.

Next, FIG. 10 (B) uses the electromagnetic wave generator of the present invention shown in FIG. 6 to generate four femtosecond pulses (pulse train laser beam 4) from one femtosecond pulse, which is a dipole type light. It is a figure which shows the time waveform measured when making the conductive antenna 30 irradiate.
From this figure, four terahertz wave pulses 5 can be confirmed, and it can be seen that the time intervals are substantially equal intervals of about 1.3 ps. Further, in the electromagnetic wave spectrum of the curve B (pulse train) in FIG. 10C, the center frequency component is 0.8 THz corresponding to the period, and the line width is about 130 GHz.

  It can be seen that curve B (pulse train) in FIG. 10C clearly has a narrower frequency band than curve A (single pulse). From these results, it can be seen that a femtosecond pulse train (pulse train laser beam) is generated by the apparatus of FIG. 6 using the principle described with reference to FIGS. 1 to 4, and a narrow-band terahertz wave 5 is generated. Thereby, a spectrum with a higher frequency resolution can be obtained in the spectroscopic measurement.

  Next, in order to show that frequency tuning is possible, the step interval between the first and second reflecting surfaces when a terahertz wave having a center frequency of 1 THz is generated is 0 (reference), 0, 30 μm, 70 μm The experiment was conducted with the interval of 210 μm.

FIG. 11 is a diagram illustrating a second embodiment of the present invention, and is a diagram illustrating a time waveform of a terahertz wave when one reflecting surface (22a or 22b) is shifted.
In this figure, (A), (B), and (C) show the time waveforms of terahertz waves when the step intervals are 0, 70 μm, and 210 μm, respectively. It can be seen that the pulse interval becomes wider in the order of (A), (B), and (C).

FIG. 12 is a diagram showing a third embodiment of the present invention, and is a diagram showing a change in the terahertz wave spectrum when one of the reflection surfaces (22a or 22b) is shifted.
In this figure, power peaks occur at 0.6 THz, 0.8 THz, 0.9 THz, and 1 THz. Further, if the step interval on the reflecting surface is increased, the center frequency of the terahertz wave is shifted to the low frequency side. Thus, an experimental result consistent with the principle was obtained, and it was shown that the frequency was variable from 0.6 THz to 1 THz.

Hereinafter, the features of the present invention will be described in comparison with the prior art.
A feature of the present invention is that a single pulse laser beam (femtosecond laser beam or picosecond laser beam) is generated from a coaxial pulse train laser beam in which a plurality of pulse laser beams are arranged at equal intervals in time. It can be used to generate a single frequency terahertz wave or millimeter wave.

  Conventionally, in single frequency terahertz wave generation using a femtosecond laser, there are means by difference frequency mixing of spatially dispersed beams and terahertz wave generation means by a femtosecond pulse train using a birefringent crystal.

  In the former case (difference frequency mixing of spatially dispersed beams), the peak value of the pulse is lowered because the laser light is dispersed. The generation of terahertz waves by excitation of the nonlinear optical element is a second-order nonlinear optical effect, and the output of the terahertz waves is proportional to the square of the laser light intensity, so that the output of the terahertz waves is reduced.

  In the latter case (a femtosecond pulse train using a birefringent crystal), a femtosecond pulse train is generated by a time delay proportional to the length of the birefringent crystal. In order to arbitrarily change the frequency of the terahertz wave, it must be replaced with a crystal having a different length. Further, when transmitted through the crystal, the pulse width expands due to group velocity dispersion. In order to prevent a decrease in the output of the terahertz wave due to a decrease in the peak value of the pulse, it is necessary to perform dispersion compensation that gives negative dispersion and narrows the pulse width. In addition, there is a method using a spatial light modulator in terms of generating a femtosecond pulse train. Although the spatial light modulator can freely shape the waveform, there are problems such as high price (about 2 million yen) and slow response speed (tens of Hz).

  On the other hand, since the present invention uses only the reflection of the mirror, there is almost no influence on the expansion of the pulse width due to the group velocity dispersion. Further, since the femtosecond pulse interval can be easily controlled simply by changing the step interval of the step mirror, tuning of the generated terahertz wave is easy. For the control, a piezo element or the like is conceivable, frequency tuning is possible at several kHz, and the price is about several hundred thousand yen.

The range of frequencies that can be tuned according to the present invention is limited by the type of nonlinear optical element that is laser-excited. Moreover, since the number of pulses can be increased in proportion to the number of reflections, it is technically easy to narrow the line width of the terahertz wave.
The case of a femtosecond laser with a commercially available output power of 1 W is taken as an example. In terahertz wave generation by a single pulse, the laser power is 10 mW and the output of the terahertz wave is saturated. Accordingly, 100 pulses can be generated with 10 mW per pulse, and a line width of 10 GHz can be achieved at a frequency of 1 THz.

The light source may be a picosecond laser instead of a femtosecond laser. The laser mentioned above refers to a laser having a pulse width between 1 fs and 33 ps.
Here, 33 ps is the longest pulse width for generating an electromagnetic wave having a narrow line width spectrum by the method of the present invention at a frequency of 30 GHz.
In addition, in FIG. 1 and FIG. 2, the D-shaped reflection mirror is used for forming the step mirror, but it is not essential that the shape is D-shaped. Any mirror that can reflect at the edge portion of the boundary line 23 may be used.
Furthermore, the terahertz wave generation unit may have a device having nonlinearity such as a nonlinear optical crystal as well as a photoconductive antenna.

  In addition, this invention is not limited to embodiment mentioned above, Of course, it can change variously, unless it deviates from the summary of this invention.

1 Pulse laser light (femtosecond laser light or picosecond laser light),
2 elliptical pulse laser light, 3 spatially distributed pulse train laser light,
4 Coaxial pulse train laser beam,
10 pulse laser device, 12a, 12b reflecting mirror,
14a, 14b cylindrical lens,
16 reflection mirror, 18a, 18b reflection mirror,
20 Spatial distribution pulse train converter,
22 first mirror (step mirror),
22a 1st reflective surface, 22b 2nd reflective surface, 23 boundary line,
24 second mirror (opposite mirror),
24A facing flat mirror, 24B facing step mirror,
24a 3rd reflective surface, 24b 4th reflective surface,
26 condenser lens,
28 Pulse interval adjustment device (piezo element and power supply device),
30 Electromagnetic wave conversion device (nonlinear optical element, photoconductive antenna or nonlinear optical crystal),
31 low temperature growth gallium arsenide substrate, 32 electrodes,
32a gap, 33 power supply

Claims (11)

The first and second mirrors are disposed opposite to each other, and the first mirror includes first and second reflecting surfaces having stepped portions across a boundary line in close contact with each other. A pulse laser beam of a femtosecond laser pulse or a picosecond laser pulse incident between the first and second mirrors is reflected by the first and second reflecting surfaces across the boundary line. A spatial distribution pulse train conversion device for converting into a spatial distribution pulse train laser beam composed of a plurality of pulse laser beams spatially distributed with a time difference proportional to the step interval between the first and second reflecting surfaces;
An electromagnetic wave generator comprising: a condensing lens that condenses the spatially-distributed pulse train laser light and generates a coaxial pulse train laser light composed of a plurality of pulse laser lights that are equidistant in time on the same axis.   The electromagnetic wave generator according to claim 1, further comprising a pulse laser device that outputs a pulse laser beam of the femtosecond laser pulse or the picosecond laser pulse.   The electromagnetic wave generator according to claim 1, wherein the electromagnetic wave is a pulse train laser beam, a terahertz wave, or a millimeter wave.   The electromagnetic wave generator according to claim 1, further comprising a photoconductive antenna or a nonlinear optical crystal that generates a terahertz wave or a millimeter wave having a narrow frequency band from the coaxial pulse train laser beam. A pulse interval adjusting device that continuously moves one or both of the first and second reflecting surfaces of the first mirror in a direction perpendicular to the reflecting surface and changes a step interval at the boundary line;
The electromagnetic wave generator according to claim 1, wherein the time interval of the coaxial pulse train laser beam is thereby changed continuously. On the first and second reflecting surfaces, in a three-dimensional space in which the boundary line direction is the y axis, the direction orthogonal to the z axis is the z axis, and the direction orthogonal to the reflecting surface is the x axis,
The reflecting surface of the second mirror is parallel to the first and second reflecting surfaces in the xz plane and the xy plane,
2. The electromagnetic wave generator according to claim 1, wherein the pulse laser beam is incident between the first and second mirrors at least with respect to the x-axis. On the first and second reflecting surfaces, in a three-dimensional space in which the boundary line direction is the y axis, the direction orthogonal to the z axis is the z axis, and the direction orthogonal to the reflecting surface is the x axis,
The reflective surface of the second mirror is parallel to the first and second reflective surfaces in the xz plane and non-parallel to the first and second reflective surfaces in the xy plane. The electromagnetic wave generator according to claim 1.   2. The electromagnetic wave generator according to claim 1, wherein the second mirror has third and fourth reflecting surfaces having stepped portions across a boundary line that is in close contact with each other. First and second mirrors arranged opposite to each other are prepared, and first and second reflecting surfaces having a stepped portion across a boundary line in close contact with each other are provided on the first mirror. Provided,
A pulse laser beam of a femtosecond laser pulse or a picosecond laser pulse is incident between the first and second mirrors, and the pulse laser beam is reflected by the first and second reflecting surfaces across the boundary line. Is converted into a spatially distributed pulse train laser beam consisting of a plurality of pulsed laser beams spatially distributed with a time difference proportional to the step interval between the first and second reflecting surfaces,
An electromagnetic wave generating method comprising: condensing the spatially-distributed pulse train laser light to generate a coaxial pulse train laser light composed of a plurality of pulse laser lights at equal intervals on the same axis on the same axis.   10. The electromagnetic wave generation method according to claim 9, wherein the coaxial pulse train laser beam is incident on a photoconductive antenna or a nonlinear optical crystal to generate a terahertz wave or a millimeter wave having a narrow frequency band. One or both of the first and second reflecting surfaces of the first mirror are continuously moved in a direction orthogonal to the reflecting surface to change the step interval at the boundary line, and the time interval of the coaxial pulse train laser light The electromagnetic wave generation method according to claim 9, wherein the electromagnetic wave is continuously changed.

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