JP2013068524A - Terahertz wave generation detecting device, and femtosecond laser generating device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a terahertz wave generation detecting device capable of making a femtosecond laser optical pulse which has high power and narrow pulse width and is modulated, incident to a terahertz generating portion, by a simple structure; and a femtosecond laser generating device.SOLUTION: An optical pulse from a fiber laser oscillator 108 is branched into two beams by a beam splitter 109, and modulation is applied by an intensity modulator 110 to the branched one optical pulse in s seed light stage. Then, the power of the modulated optical pulse is amplified to an output value by a fiber amplifier 115, and the pulse width of the modulated optical pulse is narrowed to the output value by a fiber compressor 116, thereby emitting a modulated femtosecond laser optical pulse having high power and narrow pulse width to a terahertz wave generating portion 102.

Description

  The present invention relates to a terahertz wave generation detection device and a femtosecond laser generation device, and more specifically, a terahertz wave is generated by a laser and incident on a measurement object, and is emitted from the measurement object by reflection or transmission. The present invention relates to a terahertz wave generation detection device and a femtosecond laser generation device that detect terahertz waves.

  Conventionally, in terahertz time domain spectroscopy (THz-TDS) disclosed in Patent Document 1, a delay line that forms part of a path through which pump light propagates is displaced by an optical path length scanning unit, and the optical path length scanning unit In the rapid scan method in which the signal is continuously scanned at a predetermined repetition period, the displacement of the delay line is accurately measured by a laser interferometer or the like. Thereby, the delay line can be scanned at high speed. In the technique disclosed in Patent Document 1, the measurement data is integrated to increase the sensitivity. However, since the measurement time per scan is shorter than the conventional step scan THz-TDS, the intensity fluctuation of the laser, etc. Accurate measurement is possible because it is less affected by

  Patent Document 2 discloses a technique for pulsing continuous light with a Mach-Zehnder interferometer. In this case, since the repetition period and intensity of the optical pulse can be easily modulated by the Mach-Zehnder interferometer, a mechanical chopper becomes unnecessary. Further, in Patent Document 2, two ultrashort optical pulse lasers are used, and two optical pulse trains having different repetition frequencies are created from the two ultrashort optical pulse lasers, so that the arrival time of an optical pulse at an arbitrary position can be determined. It is changing. This eliminates the need for a delay line.

  In Patent Document 3, a terahertz time domain spectroscopic device that can support a plurality of measurement forms such as a transmission type and a reflection type, in a propagation path of pulse excitation light between a femtosecond laser oscillation unit and a terahertz wave generation element. In addition, in order to adjust the repetition time of the pulsed excitation light in accordance with the relaxation time of the sample, an acousto-optic modulator (hereinafter also simply referred to as “AOM (Electro Optical Modulator)”) or an electro-optic modulator as frequency adjusting means (Hereinafter also simply referred to as “EOM (Acousto Optical Modulator)”). With this configuration, it is possible to modulate the repetition frequency of the laser light pulse for generating the terahertz wave using AOM or EOM, thereby reducing the repetition frequency of the pulse excitation light, which is higher than the original laser repetition frequency. Slow response can be observed.

  In terahertz time domain spectroscopy using a fiber laser with a wavelength of 1.55 μm band disclosed in Patent Document 4, a nonlinear crystal capable of generating a second harmonic is divided into two before dividing into pump light and probe light. A short pulse laser beam to be a wave is incident. As a result, a fundamental wave that is a short pulse laser beam that is not wavelength-converted and a second harmonic that is a wavelength-converted short pulse laser beam are emitted from the nonlinear crystal, and the short pulse laser beam that is not wavelength-converted. Is used as the pump light, and the wavelength-converted short pulse laser light is used as the probe light.

Japanese Patent No. 4654996 JP 2009-80007 A Japanese Patent No. 4248665 JP 2008-10737 A

  As described above, the invention disclosed in Patent Document 1 is directed to a technique for performing sampling at a predetermined timing when performing one scan of the optical path length scanning unit while changing the optical path length by the optical path length scanning unit. Yes, it is not aimed at technology for modulating pump light. Therefore, when the terahertz wave is generated using the photoconductive antenna, the terahertz wave generated by modulating the bias voltage can be modulated. However, when a terahertz wave is generated using a nonlinear crystal capable of generating a broadband terahertz wave, the terahertz wave cannot be modulated. Therefore, a highly sensitive lock-in detection for detecting a weak signal is required. Cannot be used.

  Since lock-in cannot be detected as described above, it is necessary to increase the number of integrations in order to improve sensitivity. However, in order to increase the number of integrations within a short measurement time, the optical path length scanning unit must be swung at high speed, and the delay line position information corresponding to the high-speed movement of the optical path length scanning unit. In order to obtain the accuracy, a high-precision rangefinder such as a laser interferometer is required. Therefore, the control becomes complicated. Moreover, the number of spatial optical systems that require high-precision adjustment, such as the assembly of the laser interferometer, increases, and the setup becomes complicated.

  Further, in the technique of pulsing the continuous light by the Mach-Zehnder interferometer in Patent Document 2, the passive mode-locked laser is not used, and the pulses are electrically aligned by the electric signal applied to the Mach-Zehnder interferometer. There may be variations in the spacing. Therefore, the accuracy of the delay line step is deteriorated. For this reason, the accuracy of broadband terahertz wave detection in which step accuracy is important deteriorates, and it is difficult to increase the bandwidth of terahertz waves.

  Further, in the above embodiment, the Mach-Zehnder interferometer functions as an optical switch to form a pulse train from continuous light. However, in the Mach-Zehnder interferometer, it is difficult to reduce the pulse width of the generated pulse train, and the narrow pulse width It is difficult to generate a femtosecond laser (for example, 50 fs). Also from this point, it is understood that it is difficult to generate a broadband terahertz wave with the technique disclosed in Patent Document 2.

  On the other hand, in the form using two pulse trains having different repetition frequencies in Patent Document 2, even if a mode-locked laser is used as a laser that emits the two pulse trains, the laser that emits the two pulse trains is a separate laser. Therefore, if each laser does not emit a pulse train having a stable repetition frequency, the terahertz wave (that is, the pulse train having one repetition frequency) and the probe light (the pulse train having the other repetition frequency) overlap at the time of each sampling. Timing deviation is not constant. Thus, if the timing deviation is not constant, it becomes difficult to detect a fast response, that is, a high-frequency component. However, complicated control is required to stabilize the repetition frequency of the two lasers with high accuracy. Moreover, it can be said that the use of two lasers is problematic both in terms of cost and size.

  As described above, in order to generate a broadband terahertz wave, it is necessary to use an optical pulse having a wide spectrum width and a narrow pulse width as a laser beam for generating the terahertz wave.

However, as in the technique disclosed in Patent Document 3, when a femtosecond laser having a condition to be incident on the terahertz wave generating element is passed through the AOM or EOM, the pulse width of the incident optical pulse is widened due to the effect of wavelength dispersion. End up. That is, when a femtosecond laser having a condition to be incident on the terahertz wave generating element is emitted from the femtosecond laser generator and the femtosecond laser having the condition is passed through the AOM or EOM, The pulse width widens. Therefore, even if a femtosecond laser beam pulse having a narrow pulse width is emitted from the femtosecond laser generator aiming at widening the terahertz wave, the invention disclosed in Patent Document 3 uses the femtosecond laser beam pulse. In order to lower the repetition frequency of the femtosecond laser light pulse, the light is incident on the AOM or EOM, so that the pulse width increases. In particular, as the spectral band of the femtosecond laser light pulse incident on the AOM or EOM increases, the pulse width also increases. In addition, when a femtosecond laser pulse having a condition to be incident on the terahertz wave generating element is modulated by AOM or EOM, it is unavoidable that the insertion loss into the device and the light intensity become half or less due to the modulation. Therefore, it is difficult for a femtosecond laser having high power and a narrow pulse width to enter the terahertz wave generating element.
As described above, in Patent Document 3, the repetition frequency can be lowered, but at the same time, the pulse width of the passing light pulse is widened, and the light intensity is also lowered. Therefore, in the invention disclosed in Patent Document 3, it is difficult to generate a broadband terahertz wave.

  Moreover, in patent document 3, in order to make pulse excitation light into intermittent light of a predetermined time interval, the optical chopper is provided in the said propagation path. In such a mechanical optical chopper, it is impossible to perform modulation at high speed, and as a result, high speed measurement is impossible.

  In the technique disclosed in Patent Document 4, laser light is incident on a nonlinear crystal before being split into pump light and probe light, and the fundamental wave and the second harmonic emitted from the nonlinear crystal are pump light. Since it is used as probe light, all of the laser light can be used for generation and detection of terahertz waves. Therefore, since the invention according to Patent Document 4 can improve energy efficiency, it is a very powerful technique, but there remains a problem to be improved. In general, it is very difficult to generate high-intensity ultrashort pulses with a fiber laser, but even if such a high-intensity ultrashort pulse laser is generated, it is inefficient to divide the generated energy. Is. In addition, since the pulse width of the laser light transmitted through the crystal without wavelength conversion is wider than that before the crystal is incident, it is not convenient from the viewpoint of the generation of broadband terahertz waves. Moreover, since a mechanical chopper is used, high-speed measurement is impossible.

  Thus, conventionally, when modulating a light pulse incident on a terahertz wave generation unit, a laser light pulse having a power and a pulse width (for example, femtosecond) that should be incident on the terahertz wave generation unit from the laser generator. Laser light pulse), and the laser light pulse is modulated by a predetermined modulator and is incident on the terahertz wave generation unit. However, when the modulator is a chopper, high-speed modulation cannot be applied due to mechanical limitations. Therefore, high-speed measurement cannot be performed. When the modulator is an AOM or EOM, the pulse width is widened due to the effect of wavelength dispersion. In particular, when an optical pulse having a narrow pulse width is incident with the aim of generating a broadband terahertz wave, the pulse width further increases. Accordingly, in this case as well, it is difficult to form a femtosecond laser light pulse having a narrow pulse width (for example, 50 fs). Furthermore, when half of the light intensity is discarded due to modulation and insertion loss is taken into account, the intensity of the generated terahertz wave becomes weak, and as a result, high-sensitivity measurement cannot be performed at high speed.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide femtoseconds that have a simple structure, a high power, a narrow pulse width, and an electrical modulation at high speed. An object of the present invention is to provide a terahertz wave generation detection device and a femtosecond laser generation device capable of entering a laser light pulse into a terahertz wave generation unit.

  In order to achieve such an object, a first aspect of the present invention is a terahertz wave generation detection device, a laser oscillator that oscillates an optical pulse, a branching unit that divides the optical pulse into two, Modulating means for applying a predetermined modulation to one of the two optical pulses branched, an amplifying means for amplifying the power of the one modulated optical pulse, and amplified by the amplifying means, A compression means for narrowing a pulse width of one modulated optical pulse; a terahertz wave generating means for generating a terahertz wave by the one modulated optical pulse emitted from the compression means; and The other optical pulse branched into two and the terahertz wave emitted from the object to be measured on which the terahertz wave generated from the terahertz wave generating means is incident, and Terahertz wave detecting means for detecting the emitted terahertz wave, and delay means for delaying one of the two optical pulses branched to the two and the other optical pulse by a predetermined delay time. Features.

  According to a second aspect of the present invention, there is provided a femtosecond laser generating apparatus that emits a femtosecond laser light pulse having a predetermined power and a predetermined pulse width and that has been subjected to predetermined modulation. A laser oscillator that oscillates an optical pulse, a modulation unit that applies the predetermined modulation to the optical pulse, an amplification unit that amplifies the power of the modulated optical pulse to the predetermined power, and The pulse width of the modulated optical pulse amplified by the amplifying means is reduced to the predetermined pulse width, and is emitted as a femtosecond laser light pulse having the predetermined power and the predetermined pulse width. And compression means.

  According to the present invention, a terahertz wave is generated from a femtosecond laser light pulse having high power (for example, 500 mW), a narrow pulse width (for example, 50 fs), and modulated at high speed with a simple configuration. It can be incident on the part.

1 is a schematic configuration diagram of a terahertz wave generation detection device according to an embodiment of the present invention. 1 is a schematic configuration diagram of a terahertz wave generation detection device according to an embodiment of the present invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to the embodiments. In the drawings described below, components having the same function are denoted by the same reference numerals, and repeated description thereof may be omitted.

(First embodiment)
FIG. 1 is a schematic configuration diagram of a terahertz wave generation detection device according to the present embodiment.
In FIG. 1, a terahertz wave generation detection device 100 is a femtosecond laser light pulse that is ready for incidence on a terahertz wave generation unit (hereinafter, sometimes referred to as “terahertz wave generation light pulse” for simplicity). , And a femtosecond laser generator 101 that emits a light pulse of a femtosecond laser to be incident on the terahertz wave detection unit (hereinafter also referred to as “terahertz wave detection light pulse” for simplicity). Further, the terahertz wave generation detection device 100 uses a terahertz wave generation light pulse emitted from the femtosecond laser generation device 101 to generate a terahertz wave optical pulse (hereinafter sometimes referred to as “terahertz wave pulse” for simplicity). A terahertz wave generation unit 102 that generates, a terahertz wave detection unit 103 that detects a terahertz wave pulse emitted from the object 105 by reflection or transmission, and a propagation module 104 that propagates the terahertz wave pulse are provided. The propagation module 104 has a plurality of off-axis parabolic mirrors, collimates the terahertz wave incident from the terahertz wave generation unit 102, collects it, and makes it incident on the object 105 to be measured. The terahertz waves incident from the laser beam are collimated, condensed, and incident on the terahertz wave detection unit 103.

  The terahertz wave detection unit 103 receives a current generated when the terahertz wave detection unit 103 receives the terahertz wave detection light pulse and the terahertz wave emitted from the device under test 105 and passing through the propagation module 104. An input current amplifier 106 is electrically connected, and a lock-in amplifier 107 is electrically connected to the current amplifier 106.

  The terahertz wave generating optical pulse is an optical pulse of a femtosecond laser that should be incident on the terahertz wave generating unit 102, and is a power set for generating a desired terahertz wave in the terahertz wave generating unit 102. And a modulated optical pulse having a pulse width. In this embodiment, the optical pulse for generating a terahertz wave has, as an example, a high power for generating a high-intensity terahertz wave (in this embodiment, 500 mW as an example), and for generating a broadband terahertz wave A description will be given as a modulated optical pulse having a narrow pulse width (in this embodiment, 50 fs as an example).

  The femtosecond laser generator 101 includes a fiber laser oscillator 108, an inline beam splitter 109, an intensity modulator 110, a delay line scanner 111, an optical path length adjuster 112, optical fiber couplers 113a and 113b, a fiber An amplifier 115, a fiber compressor 116, a fiber amplifier 117, and a fiber compressor 118 are provided. In this embodiment, each component of the femtosecond laser generator 101 is an optical fiber device, and each component is connected by an optical fiber to eliminate the space propagation portion. Therefore, since the optical fiber that becomes the propagation part of the light pulse can be wound compactly, the apparatus can be downsized and stabilized. In the present embodiment, it is desirable that the optical fiber connecting the above-described components is a polarization maintaining fiber (PMF). Therefore, it is possible to stabilize the intensity of the generated optical pulse, the pulse waveform, and the polarization direction against environmental changes and fiber bending.

  In FIG. 1, a fiber laser oscillator 108 is a passive mode-locked fiber laser, and has a power lower than the power of a terahertz wave generating optical pulse that is an optical pulse that is finally output from the femtosecond laser generating apparatus 101. An optical pulse having a pulse width wider than the pulse width of the terahertz wave generating optical pulse is oscillated. That is, the fiber laser oscillator 108 oscillates seed light pulses of a terahertz wave generating optical pulse and a terahertz wave detecting optical pulse. In the present embodiment, the fiber laser oscillator 108 outputs a femtosecond optical pulse train having a pulse width of 500 fs, an average intensity of 40 mW, and a repetition frequency of 100 MHz.

  Thus, in this embodiment, the repetition frequency of the laser oscillated from the fiber laser oscillator 108 is set higher than that of a general passive mode-locked fiber laser. If the average intensity is the same, the pulse peak power is reduced by increasing the repetition frequency, and the influence of nonlinear effects in the optical fiber is reduced. Therefore, by setting the repetition frequency high as described above, it is possible to achieve both shortening of the terahertz wave generating optical pulse and high average intensity.

  The exit end of the fiber laser oscillator 108 and the entrance end of the inline beam splitter 109 are connected by PMF. The beam splitter 109 has one incident end and two exit ends, branches the seed light pulse incident from the entrance end into two, and outputs the branched seed light pulse to the two exits. Emits from the end. That is, the beam splitter 109 branches the seed light pulse incident from the fiber laser oscillator 108 into two, emits a seed light pulse for generating a terahertz wave from one emission end, and emits a terahertz wave from the other emission end. A seed light pulse for detection is emitted.

  One emission end of the beam splitter 109 and the intensity modulator 110 are connected by PMF. The intensity modulator 110 applies predetermined modulation to an optical pulse that becomes a terahertz wave generating optical pulse when emitted from the femtosecond laser generator 101. That is, the intensity modulator 110 modulates an optical pulse (seed light pulse) that is lower than the power of the terahertz wave generating optical pulse and wider than the pulse width of the pulse. Before the intensity modulator 110 adjusts the power and pulse width to be possessed as a terahertz wave generating optical pulse, the optical pulse to be the terahertz wave derivation pulse is modulated in advance. Therefore, the terahertz wave generating optical pulse can be a modulated optical pulse.

  In this embodiment, as a configuration for branching into an optical pulse for generating terahertz waves and an optical pulse for detecting terahertz waves, an inline beam splitter 109 is used instead of a bulk beam splitter as in the prior art. The apparatus can be reduced in size, and the apparatus can be operated stably.

  In this embodiment, an AOM is used as the intensity modulator 110, but an EOM may be used. In the present embodiment, the intensity modulator 110, which is an AOM, is configured to apply 90 kMz modulation to an incident seed light pulse train for generating a terahertz wave. This modulation frequency is a value set since the upper limit of the band of the lock-in amplifier 107 is 100 kHz, and higher-speed modulation may be applied depending on the device.

  In this way, by using AOM or EOM as the intensity modulator 110, it is possible to apply higher-speed modulation and to detect terahertz waves at high speed. The intensity modulator 110 is electrically connected to the lock-in amplifier 107, and the modulation signal of the intensity modulator 110 is output to the lock-in amplifier 107. Therefore, high sensitivity detection using the lock-in amplifier 107 can be performed. In the present embodiment, high-speed modulation can be performed by AOM or EOM. Therefore, by making the modulation faster, high sensitivity detection using a band pass filter instead of the lock-in amplifier can be performed.

  In this embodiment, it is important to perform modulation at the stage of the seed light pulse, and in particular, the power and pulse width of the optical pulse for detecting the terahertz wave are set values (power higher than the seed light pulse, and the seed light pulse). It is important to apply predetermined modulation to the optical pulse for detecting the terahertz wave at a stage before the pulse width is made narrower than the optical pulse. Therefore, it is not essential that the intensity modulator 110 is AOM or EOM. As the intensity modulator 110, any one that can apply predetermined modulation to an input optical pulse train, such as an optical switch, is used. It may be used. Further, if the modulation frequency may be slow, an optical chopper may be used as the intensity modulator 110.

The optical fiber coupler 113a has two entrance ends and one exit end, and one of the two entrance ends and the exit end of the intensity modulator 110 are connected by a PMF. The other and the excitation laser 114a are connected by PMF, and light incident from each of the two incident ends is emitted from the emission end. The pump laser 114a oscillates pump light for giving a fiber amplifier 115 described later an amplification function. Therefore, from one incident end of the optical fiber coupler 113a, a seed light pulse for generating a terahertz wave, which is emitted from the intensity modulator 110 and subjected to predetermined modulation, is incident, and the modulated seed light pulse is transmitted. It exits from the exit end. On the other hand, the excitation light emitted from the excitation laser 114a is incident from the other incident end of the optical fiber coupler 113a, and the excitation light is emitted from the emission end.
In this embodiment, the pump light for giving the amplification function to the fiber amplifier 115 is incident on the fiber amplifier 115 from the seed light pulse incident side of the fiber amplifier 115. However, the pump light is incident on the fiber amplifier 115. The seed light pulse may be incident on the fiber amplifier 115 from the emission side. In this case, an optical fiber coupler 113a connected to the pump laser 114a is provided between the fiber amplifier 115 and the fiber compressor 116, and the pump light emitted from the pump laser 114a is passed through the optical fiber coupler 113a to the fiber amplifier. What is necessary is just to make it inject into this fiber amplifier 115 from the output side of 115 seed light pulses.

  The exit end of the optical fiber coupler 113a and the entrance end of the fiber amplifier 115 are connected by PMF. The fiber amplifier 115 is an erbium-doped fiber whose dispersion characteristic has a normal dispersion value. In this way, by using a normal dispersion erbium-doped fiber as an amplification fiber, it is possible to reduce nonlinear effects that adversely affect measurement performance such as pulse splitting. The fiber amplifier 115 can amplify the seed light pulse train for generating the terahertz wave by injecting the excitation light from the excitation laser 114a which is an excitation laser diode through the optical fiber coupler 113a. As the fiber amplifier 115, a fiber amplifier to which ytterbium, thulium, neodymium or the like is added in addition to erbium can be used, and an additive may be selected in accordance with a necessary wavelength. In this embodiment, the fiber amplifier 115 is configured to amplify the power of the incident seed light pulse for generating the terahertz wave to 500 mW. That is, the power of the seed light pulse for generating the terahertz wave is amplified by the fiber amplifier 115 to the power that should be possessed as the optical pulse for generating the terahertz wave that is the optical pulse that is finally output.

When the fiber amplifier 115 is not a polarization maintaining fiber, a polarization controller may be installed in front of the fiber amplifier 115 (between the optical fiber coupler 113a and the fiber amplifier 115) to adjust the polarization.
In order to facilitate this polarization adjustment, the fiber amplifier 115 may have a multi-stage configuration, and a polarization controller may be installed for each. Further, an isolator may be inserted after each fiber amplifier (on the output side of the fiber amplifier) to prevent return light. Each polarization controller observes the output power of the TAP monitor installed after each fiber amplifier and optimizes it so that the intensity becomes maximum. By repeating amplification and polarization adjustment in multiple stages, polarization disturbance due to nonlinear polarization rotation occurring in the fiber can be suppressed as compared with the case of intensity amplification with a single-stage fiber amplifier.

  The output end of the fiber amplifier 115 and the fiber compressor 116 are connected via a PMF. The fiber compressor 116 is a large mode area fiber such as a large-diameter constant polarization photonic crystal fiber, and is configured to compress the pulse width of the incident seed light pulse for generating the terahertz wave to 50 fs. Yes.

  The optical pulse output from the fiber amplifier 115 (seed light pulse for generating a terahertz wave) has a positive monotonous chirp at the same time as the spectrum spreads due to the nonlinear effect of the fiber amplifier 115 and the influence of normal dispersion. . In this embodiment, since a large mode area fiber such as a large-diameter photonic crystal fiber is used as the fiber compressor 116, this chirp can be compensated. Although this photonic crystal fiber has a mode field diameter of 20 microns or more, it can propagate a single mode of an optical pulse. Therefore, even in the propagation of an optical pulse having a high peak intensity by amplification, an excessive nonlinear effect can be suppressed, and a single peak high peak intensity short pulse can be generated. In this embodiment, the pulse width of the seed light pulse for generating the terahertz wave is 50 fs due to the optical spectrum broadening generated in the fiber amplifier 115 and the chirp compensation and soliton compression generated in the photonic crystal fiber as the fiber compressor 116. Compressed to: That is, the fiber compressor 116 narrows the pulse width of the seed light pulse for generating the terahertz wave to the pulse width that should be possessed as the optical pulse for generating the terahertz wave that is the optical pulse that is finally output.

  The photonic crystal fiber has a shape with fine pores and may be affected by dust or moisture, so the characteristics may deteriorate. Therefore, end sealing the fiber end face or fixing the lens. When doing so, it is advisable to reduce the influence of dust and moisture by sealing the fiber holder. In addition, when using a fiber that has a large bending loss and affects the output due to the fluctuation of the fiber, such as a photonic crystal fiber, create a resin guide that can fix the fiber with the minimum allowable bending radius. Then, the apparatus can be miniaturized while maintaining stability.

  The other exit end of the beam splitter 109 and the delay line scanner 111 are connected by a PMF. The delay line scanner 111 is configured to delay the seed light pulse for detecting the terahertz wave by a predetermined delay time, and is electrically connected to a control device (not shown) such as a PC (personal computer). An inline delay line scanner with a fiber pigtail. The delay line scanner 111 has a mirror that can be driven by a drive unit, and the control device drives the drive unit so as to give a predetermined delay to the seed light pulse for terahertz wave detection. The mirror can be moved. That is, when sampling the terahertz wave signal, the time difference between the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse is adjusted by driving the delay line scanner 111 according to an instruction from the control device.

  The delay line scanner 111 and the incident end of the inline type optical path length adjuster 112 are connected by a PMF. The optical path length adjuster 112 has a mirror whose position can be manually displaced. By displacing the position of the mirror, the optical path length of the seed light pulse for detecting the terahertz wave can be adjusted. This optical path length adjuster 112 is used to adjust the optical path length of the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse. Once adjusted, it is necessary to adjust the time difference for each measurement thereafter. Absent. Fiber length or terahertz wave propagation in advance so that the time difference between the optical pulse train for terahertz wave generation (some sections propagate as terahertz waves) and the optical pulse train for terahertz wave detection falls within the adjuster range of this optical path length adjuster 112 It is necessary to adjust the length.

  In this embodiment, a predetermined delay time is given to the seed light pulse for detecting the terahertz wave. However, a predetermined delay time may be given to the seed light pulse for generating the terahertz wave. In this case, the delay line scanner 111 may be arranged at any position between the beam splitter 109 and the optical fiber coupler 113a.

The optical fiber coupler 113b has two entrance ends and one exit end, and one of the two entrance ends and the exit end of the optical path length adjuster 112 are connected by PMF, and the two entrance ends. The other of the two and the excitation laser 114b are connected by a PMF, and light incident from each of the two incident ends is emitted from the emission end. The pump laser 114b oscillates pump light for giving a fiber amplifier 117 described later an amplification function. Accordingly, the terahertz wave detection seed light pulse emitted from the optical path length adjuster 112 is incident from one incident end of the optical fiber coupler 113b, and the seed light pulse is emitted from the emission end. On the other hand, the excitation light emitted from the excitation laser 114b is incident from the other incident end of the optical fiber coupler 113b, and the excitation light is emitted from the emission end.
In this embodiment, the pumping light for giving the fiber amplifier 117 an amplifying function is incident on the fiber amplifier 117 from the incident side of the seed light pulse of the fiber amplifier 117. However, the pumping light is incident on the fiber amplifier 117. The seed light pulse may be incident on the fiber amplifier 117 from the emission side. In this case, an optical fiber coupler 113b connected to the pumping laser 114b is provided between the fiber amplifier 117 and the fiber compressor 118, and the pumping light emitted from the pumping laser 114b is passed through the optical fiber coupler 113b to the fiber amplifier. What is necessary is just to make it inject into this fiber amplifier 117 from the output side of 117 seed light pulses.

  The exit end of the optical fiber coupler 113b and the entrance end of the fiber amplifier 117 are connected by PMF. The fiber amplifier 117 has a structure in which a fiber amplifier having an anomalous dispersion characteristic and a fiber amplifier having a normal dispersion characteristic are provided in two stages. That is, a first fiber amplifier having anomalous dispersion characteristics is provided on the incident side of the seed light pulse for detecting the terahertz wave to the fiber amplifier 117, and normal dispersion is provided on the rear side in the traveling direction of the seed light pulse. A second fiber amplifier having characteristics is provided. By using a fiber amplifier having anomalous dispersion characteristics in the first half of amplification in the fiber amplifier 117, when the power (light intensity) of the incident optical pulse is relatively weak, pulse compression and pulse amplification are performed simultaneously. Can do. After power (light intensity) is amplified to a certain level, optical amplification is performed using a fiber amplifier having normal dispersion characteristics. This is because, by inducing a nonlinear effect in normal dispersion, the spectral width can be broadened by self-phase modulation without causing pulse splitting, and a positive monotonous chirp can be added to an optical pulse. With such a configuration, in the present embodiment, the fiber amplifier 117 is configured so that the pumping light from the pumping laser 114b, which is a pumping laser diode, is injected through the optical fiber coupler 113b. The power of the pulse can be amplified up to 500 mW.

  The exit end of the fiber amplifier 117 and the entrance end of the fiber compressor 118 are connected by PMF. The fiber compressor 118 is a single mode optical fiber, and the single mode optical fiber can pulse-compress the seed light pulse for generating the terahertz wave having the positive chirp emitted from the fiber amplifier 117. In the present embodiment, the fiber compressor 118 is configured to reduce the pulse width of the seed light pulse for generating the terahertz wave to 50 fs.

  With such a configuration, the femtosecond laser generation apparatus 101 emits a terahertz wave generating optical pulse, which is a light pulse having a high power (light intensity) and a narrow pulse width, to the terahertz wave generation unit 102.

The exit end of the fiber compressor 116 (the exit end of the photonic crystal fiber) is connected to the entrance end of the terahertz wave generation unit 102. The terahertz wave generation unit 102 includes a collimating lens connected to an output end of a large mode area fiber that is a fiber compressor 116, and a condensing lens provided to condense light collimated by the collimating lens. And a non-linear crystal for generating a terahertz wave provided so that light condensed by the condenser lens is incident thereon. The photonic crystal fiber emitting end, collimating lens, condensing lens, and DAST crystal are modularized. With such a configuration, the terahertz wave generating optical pulse emitted from the photonic crystal fiber is collimated and condensed by the collimating lens and the condensing lens, and is incident on the DAST crystal which is a nonlinear crystal for generating the terahertz wave. Generates terahertz waves.
In addition to the nonlinear crystal, a terahertz wave can be generated using a photoconductive antenna. In that case, it is desirable to perform wavelength conversion using PPLN.

The output end of the fiber compressor 118 (the output end of the single mode optical fiber) is connected to the terahertz wave detection unit 103. The terahertz wave detection unit 103 is connected to an output end of a single mode optical fiber that is a fiber compressor 118, and can generate a second harmonic, and a non-linear crystal (in this embodiment, PPLN (Periodically Polluted Lithium Niobate)) An optical filter for extracting the second harmonic from the light generated from the nonlinear crystal, a lens for allowing the second harmonic extracted by the optical filter to enter the photoconductive antenna, and the optical filter. And a photoconductive antenna provided so that the second harmonic is incident thereon. A silicon hemisphere or super hemisphere lens is installed on the back surface of the photoconductive antenna (the surface opposite to the fiber compressor 118). In the present embodiment, one module is formed from the exit end of the single mode optical fiber to the silicon lens, and the size and the robustness are realized as compared with the space propagation. With such a configuration, when the terahertz wave detection optical pulse emitted from the fiber compressor 118 enters the PPLN, a second harmonic is generated from the PPLN, and only the second harmonic is extracted by the optical filter, The extracted second harmonic is condensed on the photoconductive antenna by the lens. At this time, when the terahertz wave emitted from the DUT 105 is incident from the silicon lens side of the photoconductive antenna, the terahertz wave is detected.
In the present embodiment, a terahertz wave is detected using a photoconductive antenna, but EO detection may be performed using a nonlinear crystal instead. In EO detection using a nonlinear crystal, a terahertz wave can be detected without generating the second harmonic.

Next, the operation of the terahertz wave generation detection device 100 according to the present embodiment will be described.
First, the fiber laser oscillator 108 is lower than the power (light intensity) (500 mW) set as a terahertz wave generating optical pulse that is an incident light pulse to the terahertz wave generating unit 102, and the terahertz wave generating optical pulse. A seed light pulse (average power 40 mW, pulse width 500 fs, repetition frequency 100 MHz) having a pulse width wider than the pulse width (50 fs) set as is oscillated. The seed light pulse is branched into two by the beam splitter 109, and one of the branched light beams enters the intensity modulator 110 as a seed light pulse for generating a terahertz wave (average power 20 mW, pulse width 500 fs, repetition frequency 100 MHz). On the other hand, the other is incident on the delay line scanner 111 as a seed light pulse for terahertz wave detection (average power 20 mW, pulse width 500 fs, repetition frequency 100 MHz).

  The intensity modulator 110 modulates 90 kHz with respect to the incident seed light pulse for generating the terahertz wave (average power 20 mW, pulse width 500 fs, repetition frequency 100 MHz), and transmits the modulated signal to the lock-in amplifier 107. . The modulated seed light pulse for generating the terahertz wave (average power 20 mW, pulse width 500 fs) is incident on the fiber amplifier 115 via the optical fiber coupler 113a. The optical fiber amplifier 115 amplifies the power of the modulated seed light pulse for generating the terahertz wave to an output value of 500 mW by the pumping light input from the pumping laser 114a through the optical fiber coupler 113a. The light is emitted to the fiber compressor 116. The fiber compressor 116 is modulated by the incident light, and narrows the pulse width of the amplified seed light pulse (power 500 mW, pulse width 500 fs) for generating the terahertz wave to 50 fs. Accordingly, the femtosecond laser generator 101 emits a terahertz wave generating optical pulse having a power of 500 mW and a pulse width of 50 fs.

  The terahertz wave generating optical pulse emitted from the femtosecond laser generator 101 is incident on the terahertz wave generating unit 102, and the terahertz wave generating unit 102 generates a terahertz wave. The generated terahertz wave enters the object 105 to be measured via the propagation module 104. The terahertz wave incident on the device under test 105 is reflected by the device under test 105, and the reflected terahertz wave enters the terahertz wave detection unit 103 via the propagation module 104.

  On the other hand, in order to observe the terahertz wave waveform, the delay line scanner 111 through which the seed light pulse for terahertz wave detection (average power 20 mW, pulse width 500 fs, repetition frequency 100 MHz) passes is driven by the control device, and the object 105 to be measured The photoconductive antenna output current of the terahertz wave detecting unit 103 is detected while changing the time difference between the reflected terahertz wave emitted from the terahertz wave and the terahertz wave detecting optical pulse. The scan width and scan rate of the scan by the delay line scanner 111 may be adjusted according to the object 105 to be measured.

  The terahertz wave detection seed light pulse (average power 20 mW, pulse width 500 fs, repetition frequency 100 MHz) to which a predetermined delay time is applied by the delay line scanner 111 is incident on the fiber amplifier 117 via the optical fiber coupler 113b. . The optical fiber amplifier 117 uses the pumping light input from the pumping laser 114b through the optical fiber coupler 113b to increase the power of the seed light pulse for detecting the terahertz wave to which the predetermined delay time is given to an output value of 500 mW. Amplified and output to the fiber compressor 118. The fiber compressor 118 is given a predetermined delay time, and narrows the pulse width of the amplified seed light pulse for detecting the terahertz wave (power 200 mW, pulse width 500 fs, repetition frequency 100 MHz) to 50 fs. As a result, the femtosecond laser generator 101 emits a terahertz wave detection optical pulse having a power of 200 mW and a pulse width of 50 fs.

  The terahertz wave detection unit 103 is a photoconductive antenna based on the terahertz wave detection light pulse emitted from the femtosecond laser generator 101 and the terahertz wave emitted from the propagation module 104 and reflected by the measured object 105. Is output to the lock-in amplifier 107 via the current amplifier 106. The lock-in amplifier 107 measures the electric field time waveform of the terahertz wave from the modulation signal output from the intensity modulator 110 and the current output from the terahertz wave detection unit 103.

  Conventionally, for example, when an AOM (or EOM) is used as an intensity modulator for high-speed modulation, an AOM is provided between the femtosecond laser generator and the terahertz wave generator. The pulse width spread due to the effect. In addition, even if a high-intensity laser is prepared as the ultrashort pulse laser, the AOM performs ON / OFF modulation with an intensity modulator, so that half of the laser light intensity is discarded. In addition, insertion loss of these devices also occurs. Therefore, it is difficult to efficiently use all energy. However, in view of increasing the intensity of the terahertz wave, it is necessary to increase the intensity (high power) of the femtosecond laser light pulse to be incident on the terahertz wave generation unit. Even if the power is reduced, the pulse width is inevitably widened due to the effect of chromatic dispersion of AOM as described above, and it is difficult to generate a femtosecond laser light pulse with a narrow pulse width while performing modulation. there were. Thus, conventionally, it has been difficult to emit femtosecond laser light pulses having high power and a narrow pulse width while performing modulation.

  On the other hand, in the present embodiment, before the power and pulse width necessary for the optical pulse for generating the terahertz wave to be incident on the terahertz wave generating unit 102 are set, the optical pulse for generating the terahertz wave (seed light pulse) is predetermined. After the modulation, the optical pulse for generating the terahertz wave is set to a desired power and a desired pulse width. That is, modulation is performed at the seed light stage of the optical pulse for generating the terahertz wave, and then the power is increased to a required value by an amplifier, and the pulse width is reduced to a required value by a pulse compressor.

  Therefore, when modulating the seed light pulse for generating the terahertz wave by the intensity modulator 110 such as AOM or EOM, even if the pulse width is expanded due to the effect of wavelength dispersion of the intensity modulator 110, Since the pulse width of the seed light pulse for generating the terahertz wave is narrowed by the fiber compressor 116, the optical pulse of a fine femtosecond laser with a narrow pulse width is emitted as a result, although it has been modulated. Can do. Therefore, a femtosecond laser beam having a narrow pulse width can be incident on the terahertz wave generation unit 102, and the generated terahertz wave can be widened. That is, in the present embodiment, the intensity modulator 110 is provided in the propagation path of the optical pulse for generating the terahertz wave, and the fiber compressor 116 is further provided downstream of the intensity modulator 110 with respect to the traveling direction of the optical pulse. By providing, it is possible to avoid the influence of chromatic dispersion, which is fatal when generating a broadband terahertz wave.

  In addition, it is necessary to increase the intensity of the optical pulse for generation in order to generate a high-intensity terahertz wave. After modulating the seed light pulse for generating the terahertz wave, the power is amplified to the value that should be the final output. Therefore, even if a low-power optical pulse is incident on the intensity modulator 110, a terahertz wave generating optical pulse having a desired high power can be obtained at the final output. Furthermore, by performing modulation before amplification, it is possible to increase the intensity by efficiently using the energy of the excitation laser as compared with the case where the final laser output is modulated.

  As described above, in the present embodiment, the femtosecond laser generation apparatus 101 transmits the femtosecond laser light pulse having a high power and a narrow pulse width to the terahertz wave generation unit 102 while performing electrical high-speed modulation. Terahertz waves can be generated using femtosecond laser light pulses that can be emitted, are high-speed modulated, have high power, and have a narrow pulse width. Therefore, it is possible to generate a high-intensity and broadband terahertz wave.

  In this embodiment, since the seed light pulse is modulated before the final condition of the terahertz wave generation pulse is adjusted, detection by a lock-in amplifier can be performed. In addition, a simple configuration in which the intensity modulator 110 is provided so as to perform modulation before final pulse shaping can achieve a broad band and high intensity of the terahertz wave.

(Second Embodiment)
In the present invention, the optical pulse for generating the terahertz wave is subjected to predetermined modulation at the seed light stage, and the modulated optical pulse is subjected to the pulse width of the optical pulse to be finally output (in the terahertz wave generating unit). The power (light intensity) of the optical pulse that is shaped to have a pulse width set to be incident and finally output (the light intensity that is set to be incident on the terahertz wave generation unit) It is important to amplify so that For this purpose, an intensity modulator is provided in the propagation path of the optical pulse for generating the terahertz wave in order to apply a predetermined modulation. The present invention is characterized in that an amplifier for amplifying the power of the optical pulse to the output value and a pulse compressor for narrowing the pulse width of the modulated optical pulse to the pulse width of the output value are provided. Therefore, as long as such a configuration is adopted, the configuration is not limited to the configuration in which each component from the laser oscillator to the terahertz wave generation unit and the terahertz wave detection unit is connected by an optical fiber as in the first embodiment. A space propagation system may be used, or a form in which a space propagation system and an optical fiber connection are mixed may be used.

FIG. 2 is a schematic configuration diagram of a terahertz wave generation and detection device that spatially propagates between each component from the laser oscillator to the terahertz wave generation unit and the terahertz wave detection unit according to the present embodiment.
In FIG. 2, a terahertz wave generation detection apparatus 200 includes a femtosecond laser generation apparatus 201 that emits a terahertz wave generation optical pulse and a terahertz wave detection optical pulse, and a terahertz wave generation output from the femtosecond laser generation apparatus 201. A terahertz wave generation unit 202 that generates a terahertz wave pulse by an optical pulse, a terahertz wave detection unit 203 that detects a terahertz wave pulse emitted from the object 205 by reflection or transmission, and propagation for propagating the terahertz wave pulse Module 204. The propagation module 204 has the same configuration as the propagation module 204 described above.

  The femtosecond laser generator 201 includes a laser oscillator 206, a beam splitter 207, an intensity modulator 208, amplifiers 209 and 212, pulse compressors 210 and 213, and a delay line scanner 211.

  The laser oscillator 206 is a passive mode-locked laser, and is configured to oscillate a femtosecond optical pulse train having a pulse width of 500 fs, an average intensity of 40 mW, and a repetition frequency of 100 MHz. That is, the laser oscillator 206 oscillates a seed light pulse of a terahertz wave generating optical pulse and a terahertz wave detecting optical pulse. A beam splitter 207 is provided in the subsequent stage of the light pulse traveling direction of the laser oscillator 206. The beam splitter 207 is a seed light pulse (pulse width 500 fs, average intensity (power) 40 mW) emitted from the laser oscillator 206. , The repetition frequency is 100 MHz). One of the branched light pulses is a seed light pulse for generating a terahertz wave, and the other light pulse is a seed light pulse for detecting a terahertz wave. An intensity modulator 208 is provided on the transmission side of the beam splitter 207, and one seed light pulse branched by the beam splitter 207 enters the intensity modulator 208 as a seed light pulse for generating a terahertz wave. A delay line scanner 211 is provided on the reflection side of the beam splitter 207, and the other seed light pulse branched by the beam splitter 207 is incident on the delay line scanner 211 as a seed light pulse for terahertz wave detection. To do.

  The intensity modulator 208 is a member that can apply a predetermined modulation to an optical pulse, such as an AOM, an EOM, an optical chopper, or an optical switch. Therefore, the intensity modulator 208 performs a predetermined modulation on the seed light pulse for generating the terahertz wave emitted from the beam splitter 207 and emits it.

  An amplifier 209 is provided at the subsequent stage of the light modulator traveling direction of the intensity modulator 208. The amplifier 209 is configured to amplify the power of a modulated seed light pulse (average power 20 mW, pulse width 500 fs) emitted from the intensity modulator 208 to an output value of 500 mW. Yes. Therefore, the amplifier 209 emits a modulated seed light pulse (power 500 mW, pulse width 500 fs) for generating a terahertz wave.

  A pulse compressor 210 is provided downstream of the amplifier 209 in the traveling direction of the optical pulse, and a terahertz wave generation unit 202 is disposed downstream of the traveling direction of the optical pulse of the pulse compressor 210. The pulse compressor 210 is configured to reduce the pulse width of the modulated seed light pulse (power 500 mW, pulse width 500 fs) emitted from the amplifier 209 to an output value of 50 fs. . Therefore, the pulse compressor 210 emits a modulated terahertz wave generating optical pulse (power 500 mW, pulse width 50 fs) to the terahertz wave generating unit 202.

  The delay line scanner 211 is configured to give a predetermined delay time to the terahertz wave detection seed light pulse emitted from the beam splitter 207. The delay line scanner 211 is electrically connected to a control device (not shown) such as a PC, and gives a predetermined delay time to the seed light pulse for detecting the terahertz wave by the control of the control device. Operate.

  An amplifier 212 and a pulse compressor 213 are provided in this order in the subsequent stage of the optical pulse traveling direction of the delay line scanner 211. The amplifier 212 amplifies the power of the terahertz wave detection seed light pulse (average power 20 mW, pulse width 500 fs) emitted from the delay line scanner 211 and provided with a predetermined delay time to an output value of 500 mW. It is configured. The pulse compressor 213 narrows the pulse width of the seed light pulse (power 500 mW, pulse width 500 fs) emitted from the amplifier 212 and provided with a predetermined delay time to an output value of 50 fs. Is configured to do. Therefore, the optical pulse emitted from the pulse compressor 213 becomes a terahertz wave detection optical pulse (power 200 mW, pulse width 50 fs) to which a predetermined delay time is applied.

  As described above, also in this embodiment, predetermined modulation is performed at the seed light stage, and after modulation, the power of the optical pulse is amplified to a necessary value, and the pulse width of the optical pulse is reduced to a necessary value. Yes. Therefore, the femtosecond laser generator 201 can emit a modulated femtosecond laser light pulse having high power and a narrow pulse width. Therefore, the terahertz wave generation detection device 200 has high intensity and can generate a broadband terahertz wave.

100, 200 Terahertz wave generation detection device 201, 201 Femtosecond laser generation device 102, 202 Terahertz wave generation unit 103, 203 Terahertz wave detection unit 109, 207 Beam splitter 110, 208 Intensity modulator 115, 117 Fiber amplifier 116, 118 Fiber Compressor 111, 211 Delay line scanner 209, 212 Amplifier 210, 213 Pulse compressor

Claims (8)

A laser oscillator that oscillates an optical pulse;
Branching means for branching the optical pulse into two;
Modulation means for applying a predetermined modulation to one of the two optical pulses branched;
Amplifying means for amplifying the power of one of the modulated optical pulses;
Compression means for narrowing a pulse width of one of the modulated optical pulses amplified by the amplification means;
Terahertz wave generating means for generating a terahertz wave by one of the modulated optical pulses emitted from the compression means;
The other optical pulse branched into the two and the terahertz wave emitted from the object to be measured on which the terahertz wave generated from the terahertz wave generating unit is incident are incident and the emitted Terahertz wave detecting means for detecting terahertz waves;
A terahertz wave generation and detection apparatus comprising: delay means for delaying one of the two optical pulses branched into the two and the other optical pulse by a predetermined delay time. The laser oscillator is a laser oscillator that oscillates an optical pulse having a first power and a first pulse width,
The amplifying means is configured to amplify the power of one of the modulated optical pulses to a second power higher than the first power,
The compression means is configured to narrow the pulse width of one of the modulated optical pulses amplified by the amplification means to a second pulse width that is smaller than the first pulse width,
The terahertz wave generation means is incident with one of the modulated optical pulses having the second power and the second pulse width emitted from the compression means. The terahertz wave generation detection device according to claim 1. Second amplifying means provided to amplify the power of the other optical pulse branched into the two;
The second compression means provided so as to narrow the pulse width of the other optical pulse amplified by the second amplification means, The method according to claim 1, further comprising: Terahertz wave generation detection device.   The laser oscillator and the branching unit, the branching unit and the modulation unit, the modulation unit and the amplification unit, the amplification unit and the compression unit, the compression unit and the terahertz wave generation unit, The terahertz wave generation and detection apparatus according to claim 1, wherein each of the terahertz waves is connected via an optical fiber.   5. The terahertz wave generation detection device according to claim 1, wherein the branching unit is an in-line type beam splitter.   6. The terahertz wave generation and detection apparatus according to claim 1, wherein the modulation unit is one of an acousto-optic modulator and an electro-optic modulator. A femtosecond laser generator that emits a femtosecond laser light pulse having a predetermined power and a predetermined pulse width, the femtosecond laser light pulse having a predetermined modulation applied thereto,
A laser oscillator that oscillates an optical pulse;
Modulation means for applying the predetermined modulation to the optical pulse;
Amplifying means for amplifying the power of the modulated optical pulse to the predetermined power;
The pulse width of the modulated optical pulse amplified by the amplification means is reduced to the predetermined pulse width and emitted as a femtosecond laser light pulse having the predetermined power and the predetermined pulse width. A femtosecond laser generator, comprising: A branching unit that is provided between the laser oscillator and the modulating unit and splits the optical pulse into two;
8. The femtosecond laser generation apparatus according to claim 7, wherein the modulation unit applies the predetermined modulation to the one of the two optical pulses branched.

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