JP2013068525A - Terahertz wave generation detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a terahertz wave generation detector capable of reducing fluctuation of relative time difference between an optical pulse for generating terahertz waves and an optical pulse for detecting terahertz waves due to environmental variation, and reducing deformation of time waveform and/or spectrum waveform of the optical pulses.SOLUTION: An optical pulse for generating terahertz waves from a first path, and an optical path for detecting terahertz waves from a second path of orthogonal polarization are multiplexed with a beam combiner 107, and made incident on birefringence axes of a fiber transmission part 108, respectively. The second path is provided with a pulse time interval adjustment part 105 which avoids that the optical pulse for generating terahertz waves and the optical pulse for detecting terahertz waves temporally overlap each other at the fiber transmission part 108. A pulse time interval correction part 110 which makes the optical pulse for generating terahertz waves and the optical pulse for generating terahertz waves which do not temporally overlap each other overlap temporally is provided between a polarization beam splitter 109 and a THz wave generation part 111.

Description

  The present invention relates to a terahertz wave generation detection device, and more specifically, a terahertz wave generated by a laser to be incident on a measurement object and detected from the measurement object by reflection or transmission. The present invention relates to a wave generation detection device.

  Conventionally, in the terahertz spectroscopy / imaging apparatus proposed in Patent Document 1, an optical pulse generated from an ultrashort pulse laser 1 is converted into a terahertz wave generating optical pulse and a terahertz wave by a beam splitter 2 as shown in FIG. The terahertz wave generating probe head 6 branches to a detection light optical pulse, and the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse are separately used as transmission paths. And it is made to transmit to the probe head 9 for terahertz wave detection. That is, the terahertz wave generating optical pulse that is one of the optical pulses branched by the beam splitter 2 is incident on the optical fiber 5 through the condensing optical system unit 3 and propagates through the optical fiber 5 to propagate the terahertz. The light enters the wave generating probe head 6. On the other hand, the terahertz wave detection optical pulse, which is the other optical pulse branched by the beam splitter 2, enters the optical fiber 8 through the condensing optical system unit 7, propagates in the optical fiber 8, and propagates through the optical fiber 8. The light enters the wave detection probe head 9.

  The terahertz wave generating probe head 6 generates a terahertz wave by the terahertz wave generating optical pulse incident from the optical fiber 5 and irradiates the object to be measured 10 with the terahertz wave. The terahertz wave detection probe head 9 receives the terahertz wave emitted from the device under test 10 and the terahertz wave generating optical pulse emitted from the optical fiber 8 and detects the terahertz wave.

  On the other hand, Non-Patent Document 1 is directed to pulse capture, and an optical pulse is propagated to each birefringence axis (high-speed axis and low-speed axis) of one polarization maintaining optical fiber (PMF). Non-patent document 1 describes the interaction between pulses that sometimes occurs. As described above, the technique of Non-Patent Document 1 relates to pulse capture, and a signal pulse and a control pulse whose polarization directions are orthogonal to each other are temporally overlapped and coupled to the PMF.

JP 2008-122278 A

Eiji Shiraki, Norihiko Nishizawa, “Wideband amplified using orthologously polarized pulse trapping in birefringent fibres”, OPTIC EXPR. 18, no. 7, March 29, 2010

  As described above, in Patent Document 1, when the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse are supplied to the terahertz wave generating probe head 6 and the terahertz wave detecting probe head 9, respectively, they are independent of each other. The transmission line (optical fibers 5 and 8) is used for transmission. Accordingly, the optical fiber 5 for transmitting the optical pulse for generating the terahertz wave and the optical fiber 8 for transmitting the optical pulse for detecting the terahertz wave are stretched independently of each other by temperature change. Accordingly, the relative time between the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse is shifted with respect to the environmental change, particularly the temperature change, and the time axis accuracy becomes unstable.

  Further, in Non-Patent Document 1, since two optical pulses whose polarization directions are orthogonal to each other, which are temporally overlapped by capturing the optical pulse, are propagated on the fast axis and the slow axis of the PMF, cross-phase modulation ( Cross Phase Modulation (XPM) occurs, resulting in a spectrum change (wavelength shift) and energy transfer between pulses (change in optical pulse intensity). Therefore, since the two optical pulses whose polarization directions propagating through the PMF are perpendicular to each other overlap each other, the two optical pulses propagating through the PMF have a pulse time waveform and / or a spectrum waveform caused by XPM. It will be deformed.

  The present invention has been made in view of such a problem, and an object of the present invention is to reduce the fluctuation of the relative time difference between the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse due to environmental changes. It is another object of the present invention to provide a terahertz wave generation and detection device capable of reducing deformation of a time waveform and / or a spectrum waveform of an optical pulse.

  In order to achieve such an object, one aspect of the present invention is a terahertz wave generation and detection device, which includes a laser oscillation unit that oscillates an optical pulse and two emission ends, and oscillates from the laser oscillation unit. Branching means for branching the optical pulse into two, two incident ends, the first linearly polarized light incident from one of the two incident ends, and the first incident from the other First combining optically connecting one of the two emitting ends of the branching unit and one of the incident ends of the combining unit; A first light beam that propagates an optical pulse emitted from one of the two exit ends of the branching unit and enters the optical pulse that is the first linearly polarized light at one incident end of the combining unit. And the other of the two exit ends of the branching means and the path A second path optically connected to the other of the incident ends of the generating means, the optical pulse emitted from the other of the two outgoing ends of the branching means being propagated, and the other incident end of the combining means And a delay unit provided in at least one of the first path and the second path, and the delay unit is provided. Delay means for delaying an optical pulse passing through a predetermined path by a predetermined delay time; and adjustment means provided in at least one of the first path and the second path, wherein the combining means When the light pulse that is the first linearly polarized light and the light pulse that is the second linearly polarized light are combined, the combined light pulse that is the first linearly polarized light and the combined second linearly polarized light Does not overlap in time with the light pulse An adjusting means for delaying a light pulse passing through a path provided with the adjusting means by a fixed delay time; and a polarization maintaining fiber having birefringence, one end of which is connected to the output end of the combining means. The optical pulse that is the first linearly polarized light is incident on one of the fast axis and the slow axis of the polarization maintaining fiber, and the optical pulse that is the second linearly polarized light is incident on the other of the fast axis and the slow axis. Thus, by being connected to the synthesizing means, there are provided a polarization-maintaining fiber through which both the optical pulse that is the first linearly polarized light and the optical pulse that is the second linearly polarized light propagate, and two emission ends. And one of the light pulse that is the first linearly polarized light and the light pulse that is the second linearly polarized light that is connected to the other end of the polarization maintaining fiber and is incident from the other end of the polarization maintaining fiber. The other of the light pulse that is the first linearly polarized light and the light pulse that is the second linearly polarized light that is emitted from one of the two emission ends and incident from the other end of the polarization maintaining fiber is the two emission. A polarization branching unit that emits light from the other of the ends, a terahertz wave generating unit that generates a terahertz wave by an optical pulse emitted from one of the two output ends of the polarization branching unit, and two output ends of the polarization branching unit The optical pulse emitted from the other 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 the detection of the emitted terahertz wave is performed. A terahertz wave detecting means for performing, a third path for optically connecting one of the two outgoing ends of the polarization branching means and the terahertz wave generating means, and the polarization component Provided in at least one of the fourth path, the third path, and the fourth path for optically connecting the other of the two emission ends of the means and the terahertz wave detecting means, and And a correction unit configured to temporally overlap the light pulse that is the first linearly polarized light and the light pulse that is the second linearly polarized light.

  According to the present invention, the fluctuation of the relative time difference between the optical pulse for generating terahertz waves and the optical pulse for detecting terahertz waves due to environmental changes (for example, temperature changes) is reduced, and the time waveform of the optical pulses and / or Alternatively, the deformation of the spectrum waveform can be reduced.

It is a schematic block diagram of the conventional terahertz spectroscopy and imaging apparatus. 1 is a schematic configuration diagram of a terahertz wave generation detection device according to an embodiment of the present invention. It is a figure for showing the mode of the optical pulse for terahertz wave generation and the optical pulse for terahertz wave detection combined with the shared fiber transmission part concerning one embodiment of the present invention. 1 is a schematic configuration diagram of a terahertz wave generation 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. 2 is a schematic configuration diagram of the terahertz wave generation detection device according to the present embodiment.
In FIG. 2, a terahertz wave generation detection device 100 includes a fiber laser 101, an inline beam splitter 102, an intensity modulator 103, an inline delay line scanner 104, a pulse time interval adjustment unit 105, and an intensity modulation. , A polarization beam combiner 107, a fiber transmission unit 108 which is a polarization maintaining optical fiber (PMF) having birefringence, an in-line type polarization beam splitter 109, a pulse time interval correction unit 110, a terahertz wave (THz) Wave) generator 111, terahertz wave (THz wave) detector 112, and propagation module 113. The propagation module 113 includes a plurality of off-axis parabolic mirrors, collimates the terahertz wave incident from the terahertz wave generation unit 111, collects it, and makes it incident on the object to be measured 114. The terahertz waves incident from the laser beam are collimated, condensed, and incident on the terahertz wave detection unit 112.

  The fiber laser 101 is a passively mode-locked laser, which is a femtosecond laser light pulse that is linearly polarized light in the first polarization direction, and the conditions to be incident on the terahertz wave generation unit 111 and the terahertz wave detection unit 112 are prepared Oscillates femtosecond laser light pulse. The exit end of the fiber laser 101 and the entrance end of the beam splitter 102 are connected by a polarization maintaining fiber (PMF). The beam splitter 102 has one entrance end and two exit ends, branches the femtosecond laser light pulse incident from the entrance end into two, and splits the split light pulse into the 2 The light exits from one exit end. That is, the beam splitter 102 divides the femtosecond laser light pulse incident from the fiber laser 101 into two, and the terahertz wave generating optical pulse having the first polarization direction (conditions to be incident on the terahertz wave generating unit) (Emitted femtosecond laser light pulse), and emits a terahertz wave detection light pulse having the first polarization direction (femtosecond laser light pulse to be incident on the terahertz wave detection unit) from the other emission end. To do. As described above, the beam splitter 102 transmits the femtosecond laser light pulse propagation path, the first path for transmitting the terahertz wave generating optical pulse, and the first path for transmitting the terahertz wave detecting optical pulse. Branches to the second route.

  One emission end of the beam splitter 102 and the incident end of the intensity modulator 103 are connected by PMF. The intensity modulator 103 applies predetermined modulation to the terahertz wave generating optical pulse incident from the beam splitter 102 so that the terahertz wave generating optical pulse propagates in a soliton in a fiber transmission unit 108 to be described later. That is, the intensity modulator 103 is an intensity modulator for adjusting the primary soliton condition in the fiber transmission unit 108. As described above, in this embodiment, the optical pulse for generating the terahertz wave is propagated through the fiber transmission unit 108 as an optical soliton (an optical pulse that is propagated with no waveform deformation or with reduced waveform deformation).

  The soliton propagation is a balance between the effect of pulse broadening due to wavelength dispersion in the optical fiber and the effect of pulse compression due to the nonlinear optical effect, so that the pulse does not change the time shape in the optical fiber, or the time shape. It is a phenomenon that propagates with reduced deformation. Therefore, the intensity modulator 103 causes the peak power (light quantity) so that the soliton order is “1” with respect to the terahertz wave generating optical pulse incident from the beam splitter 102 in order to cause such soliton propagation. ). That is, the intensity modulator 103 receives the incident terahertz wave generating optical pulse so that the terahertz wave generating optical pulse propagating through the fiber transmission unit 108 has an intensity and / or a pulse time width satisfying the soliton condition. Apply modulation to Thus, the terahertz wave generating optical pulse can propagate through the fiber transmission unit 108 in the soliton.

  The output end of the intensity modulator 103 is connected to one incident end of the polarization beam combiner 107 via a PMF. In the present embodiment, the optical pulse propagation path that optically connects one output end of the beam splitter 102 and one incident end of the polarization beam combiner 107 by the PMF is the first path (for generating terahertz waves). And an intensity modulator 103 is inserted in the first path. Since the optical fiber included in the first path is PMF, the terahertz wave generating optical pulse emitted from the first path, that is, the terahertz wave generating incident on one incident end of the polarization beam combiner 107 is used. The light pulse is linearly polarized light having a first polarization direction.

  On the other hand, the other exit end of the beam splitter 102 and the entrance end of the delay line scanner 104 are connected by a PMF. In this embodiment, the propagation path of the optical pulse that optically connects the other exit end of the beam splitter 102 and the other entrance end of the polarization beam combiner 107 by the PMF is the second path (for terahertz wave detection). A delay line scanner 104, a pulse time interval adjusting unit 105, and an intensity modulator 106, which will be described later, are inserted in the second path.

  The delay line scanner 104 is configured to delay the optical pulse for detecting the terahertz wave by a predetermined delay time, and is a fiber pigtail electrically connected to a control device (not shown) such as a PC (personal computer). This is an inline-type delay line scanner. The delay line scanner 104 has a mirror that can be driven by a driving unit, and the control device drives the driving unit to cause the mirror to give a predetermined delay to the terahertz wave detection optical pulse. Can be moved. Therefore, the delay line scanner 104 can apply each of the delay times corresponding to the positions of the fluctuating mirrors to the terahertz wave detection optical pulse. 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 104 according to an instruction from the control device.

  In the present embodiment, a predetermined delay time is given to the terahertz wave detecting optical pulse, but a predetermined delay time may be given to the terahertz wave generating optical pulse. In this case, the delay line scanner 104 may be disposed at any position between the beam splitter 102 and the polarization beam combiner 107 on the first path. Alternatively, the delay line scanner 104 may be provided in both the first path and the second path.

  The exit end of the delay line scanner 104 and the entrance end of the pulse time interval adjusting unit 105 are connected by a PMF. The pulse time interval adjusting unit 105 transmits the first path and enters the polarization beam combiner 107 and the terahertz wave generating optical pulse, and transmits the second transmission path and enters the polarization beam combiner 107. The terahertz wave detection optical pulse is configured to be given a certain delay time so that the terahertz wave detection optical pulse does not overlap in time at the incident end of the fiber transmission unit 108. That is, the pulse time interval adjusting unit 105 performs a relative time difference between these two optical pulses so that the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse do not overlap in time in the fiber transmission unit 108. Adjust.

  In the present embodiment, the pulse time interval adjustment unit 105 has the same structure as the delay line scanner 104, and has a mirror that can be driven by the drive unit. The mirror can be displaced by control from a control device (not shown), and the mirror is positioned so that the optical path length of the second path is longer than the optical path length of the first path. Accordingly, the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse emitted from the beam splitter 102 to the first path and the second path at the same time arrive at the polarization beam combiner 107 with a time difference. . That is, the terahertz wave detecting optical pulse emitted from the beam splitter 102 to the second path at the same time as the terahertz wave generating optical pulse emitted from the beam splitter 102 to the first path is adjusted in the pulse time interval. The light beam enters the polarization beam combiner 107 with a delay corresponding to the optical path length given by the unit 105. Therefore, at the time of incidence on the fiber transmission unit 108, an intentional time difference is provided between the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse, and the two optical pulses are not overlapped in time. It can propagate through the transmission unit 108.

  In the present embodiment, the same configuration as the delay line scanner 104 is used as the pulse time interval adjustment unit 105, but the present invention is not limited to this. For example, a predetermined optical path length in the second path through which the optical pulse for terahertz wave detection propagates such as PMF additionally provided so that the optical path length of the first path is longer than the optical path length of the second path Any configuration may be adopted as long as the above can be added.

  In the present embodiment, the delay line scanner 104 gives a predetermined delay time to each pulse of the optical pulse for terahertz wave detection while varying the delay time in order to measure the waveform of the terahertz wave by time resolution. Is functioning. In contrast, the pulse time interval adjusting unit 105 does not overlap the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse in time along the optical axis of the fiber transmitting unit 108 in the fiber transmitting unit 108. In order to achieve this, the optical pulse for detecting the terahertz wave (or the optical pulse for generating the terahertz wave) functions to give a certain delay time.

  The emission end of the pulse time interval adjustment unit 105 and the intensity modulator 106 are connected by a PMF. The intensity modulator 106 performs a predetermined modulation on the terahertz wave detection optical pulse incident from the pulse time interval adjustment unit 105 so that the terahertz wave generation optical pulse propagates in a soliton in a fiber transmission unit 108 described later. multiply. That is, the intensity modulator 106 performs the same operation as the intensity modulator 103. Thus, the terahertz wave detection optical pulse can propagate through the fiber transmission unit 108 in a soliton.

  In the present embodiment, the fiber transmission unit 108 is used as a fiber in which the optical fiber for transmitting the optical pulse for generating the terahertz wave and the optical fiber for transmitting the optical pulse for detecting the terahertz wave are used in common. It is necessary to make the polarization directions of the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse orthogonal to each other at the time of incidence on the polarization beam combiner 107 connected to the incident end. Therefore, in the present embodiment, the PMF included in the second path is twisted, and the terahertz wave detection optical pulse having the first polarization direction incident from the incident end of the second path is converted into the second path. At the emission end, the polarization direction is controlled so as to obtain a terahertz wave detecting optical pulse having a second polarization direction orthogonal to the first polarization direction. That is, the polarization direction of the optical pulse for generating a terahertz wave that was the first polarization direction at the time of incidence on the second path is the exit end of the second path (that is, the other incident end of the polarization beam combiner 107). The PMF of at least a part of the second path is twisted so as to be in the second polarization direction.

  In the present embodiment, the polarization direction of the optical pulse for generating the terahertz wave when reaching the polarization beam combiner 107 is rotated by 90 degrees by twisting the second path, but either one of the second paths In addition, for example, a member that rotates the polarization direction of the incident light pulse by 90 degrees, such as a half-wave plate, may be inserted. That is, as a result, any configuration may be adopted as long as the terahertz wave detection light pulse having the second polarization direction is emitted from the second path to the polarization beam combiner 107.

  In the present invention, it is not essential to rotate the polarization direction of the terahertz wave detection optical pulse by 90 degrees in the second path, and the terahertz wave generation optical pulse and the terahertz wave are incident on the polarization beam combiner 107. It is essential that the polarization directions of the detection light pulses are orthogonal to each other. Therefore, the polarization direction of the terahertz wave detection light pulse is maintained at the first polarization direction by twisting the first path or providing a member that rotates the polarization direction by 90 degrees on the first path. The polarization direction of the terahertz wave generating optical pulse may be rotated by 90 degrees.

  The polarization beam combiner 107 has two entrance ends and one exit end, and synthesizes and emits two linearly polarized lights orthogonal to each other. In the present embodiment, the terahertz wave generating optical pulse having the first polarization direction is incident from one incident end, and the terahertz wave detecting optical pulse having the second polarization direction is incident from the other incident end. The beam combiner 107 combines the first polarized terahertz wave generating optical pulse incident from the two incident ends and the second polarized terahertz wave detecting optical pulse, and emits the resultant from the outgoing end.

  The exit end of the polarization beam combiner 107 and the entrance end of the fiber transmission unit 108 are connected. The fiber transmission unit 108 is a PMF having birefringence, and has a fast axis and a slow axis. The polarization beam combiner 107 is configured so that the terahertz wave generating light pulse having the first polarization direction is incident on the high-speed axis of the fiber transmission unit 108 and the terahertz wave detecting light pulse having the second polarization direction is incident on the low-speed axis. The exit end and the entrance end of the fiber transmission unit 108 are connected. Therefore, in the fiber transmission unit 108, the terahertz wave generating optical pulse propagates so that the high-speed axis and the first polarization direction coincide with each other, and the terahertz wave detection so that the low-speed axis and the second polarization direction coincide with each other. Propagation light pulse propagates. In other words, in the present embodiment, the fiber transmission unit 108 is a fiber transmission unit that shares a terahertz wave generating optical pulse transmission optical fiber and a terahertz wave detecting optical pulse transmission optical fiber. Therefore, not only can the number of parts be reduced compared to the case of constructing the optical system for the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse, respectively, but also relative to environmental changes such as temperature changes. The pulse time difference can be stabilized (even if the fiber length changes due to a temperature change, the relative time difference does not change if the fiber length is shared).

  In the present embodiment, the pulse time interval adjusting unit 105 gives a fixed delay time to the terahertz wave detection optical pulse. Therefore, as shown in FIG. 3, there is a time difference between the terahertz wave generating optical pulse 201 incident on the high speed axis and the terahertz wave detecting optical pulse 202 incident on the low speed axis. These two light pulses do not overlap in time within the fiber transmission unit 108. Therefore, in the fiber transmission unit 108, it is possible to independently propagate two orthogonal optical pulses 201 and 202 by reducing deformation of the time waveform and / or spectrum caused by XPM.

  In this embodiment, the terahertz wave generating optical pulse having the first polarization direction is incident on the high-speed axis, and the terahertz wave detecting optical pulse having the second polarization direction is incident on the low-speed axis. The terahertz wave generating optical pulse having the first polarization direction may be incident on the slow axis, and the terahertz wave detecting optical pulse having the second polarization direction may be incident on the fast axis.

  The exit end of the fiber transmission unit 108 and the entrance end of the polarization beam splitter 109 are connected. The polarization beam splitter 109 has one incident end and two exit ends, and the optical pulse for generating a terahertz wave in the first polarization direction emitted from the fiber transmission unit 108 is output from one of the two exit ends. The terahertz wave detecting optical pulse having the second polarization direction emitted from the fiber transmission unit 108 is emitted from the other of the two emission ends.

  One exit end of the polarization beam splitter 109 and the entrance end of the pulse time interval correction unit 110 are connected by a PMF. The pulse time interval correction unit 110 detects the terahertz wave generating optical pulse in the first polarization direction and the terahertz wave detection in the second polarization direction, which are transmitted without overlapping the time in the fiber transmission unit 108. The temporal deviation between the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse is corrected so that the optical pulse overlaps in time.

  In the present embodiment, as described above, since the predetermined delay time is given to the optical pulse for terahertz wave detection by the pulse time interval adjusting unit 105, the terahertz wave is incident upon the incidence of the fiber transmission unit 108. A certain time difference is given between the generation light pulse 201 and the terahertz wave detection light pulse 202. Further, since the terahertz wave generating optical pulse 201 is incident on the high-speed axis and propagates through the fiber transmission unit 108, the terahertz wave generating optical pulse 201 propagates through the fiber transmission unit 108 earlier. Therefore, in addition to the time difference given by the pulse time interval adjusting unit 105 between the terahertz wave generating optical pulse 201 and the terahertz wave detecting optical pulse 202 at the output end of the fiber transmission unit 108, the fiber A time difference due to the birefringence of the transmission unit 108 is given. Therefore, the pulse time interval correction unit 110 is configured to compensate for the time difference given by the pulse time interval adjustment unit 105 and the time difference due to the birefringence of the fiber transmission unit 108.

  In the present embodiment, the pulse time interval correction unit 110 has the same structure as the pulse time interval adjustment unit 105 and the delay line scanner 104, and includes a mirror that can be driven by the drive unit. The mirror can be displaced by control from a control device (not shown), and is incident on the high-speed axis generated by the delay time given by the pulse time interval adjustment unit 105 and the birefringence of the fiber transmission unit 108. The mirror is positioned so that the terahertz wave generating optical pulse is delayed by the total time of the time difference between the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse incident on the low-speed axis.

  The emission end of the pulse time interval correction unit 110 and the incident end of the terahertz wave generation unit 111 are connected by a PMF. In the present embodiment, the propagation path of the optical pulse that optically connects one output end of the polarization beam splitter 109 and the terahertz wave generation unit 111 by the PMF is the third path, and the third path is included in the third path. A pulse time interval correction unit 110 is inserted.

  The terahertz wave generation unit 111 collects the collimated lens connected to the emission end of the PMF that connects the pulse time interval correction unit 110 and the terahertz wave generation unit 111 and the light collimated by the collimator lens. And a non-linear crystal for generating a terahertz wave provided so that the light condensed by the condensing lens is incident thereon. The PMF exit end, collimating lens, condenser lens, and DAST crystal are modularized. With such a configuration, the first polarization direction terahertz wave generating optical pulse emitted from the pulse time interval correction unit 110 is a non-linear crystal for generating a terahertz wave that is collimated and condensed by the collimating lens and the condensing lens. It is incident on the DAST crystal, and the DAST crystal generates a terahertz wave.

The other exit end of the polarization beam splitter 109 and the terahertz wave detection unit 112 are connected by a PMF. In the present embodiment, the path through which the optical pulse that optically connects the other emission end of the polarization beam splitter 109 and the terahertz wave detection unit 112 by the PMF is the fourth path. In the present embodiment, in the fourth path, the polarization direction of the terahertz wave detection optical pulse having the second polarization direction is set to the first polarization direction at the emission end of the fourth path. For example, the PMF included in the fourth path is twisted, and the terahertz wave detection light pulse having the second polarization direction incident from the incident end of the fourth path is output from the output end of the fourth path. In, the polarization direction is controlled so as to be a terahertz wave detecting optical pulse having the first polarization direction. That is, the fourth polarization direction is such that the polarization direction of the terahertz wave generating optical pulse that was the second polarization direction at the time of incidence on the fourth path is the first polarization direction at the exit end of the fourth path. Twist the PMF of at least part of the path.
In the present embodiment, a member that rotates the polarization direction by 90 degrees may be provided in the fourth path.

The terahertz wave detection unit 112 is connected to the emission end of the PMF that connects the other emission end of the polarization beam splitter 109 and the terahertz wave detection unit 112, and can generate a second harmonic wave (in this embodiment, PPLN (Periodically Polluted Lithium Niobate), an optical filter for extracting second harmonic from light generated from the nonlinear crystal, and a lens for making the second harmonic extracted by the optical filter enter a photoconductive antenna And a photoconductive antenna provided so that the second harmonic extracted by the optical filter 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 polarization beam splitter 109). In the present embodiment, one module is formed from the emission end of the PMF to the silicon lens, and the size and the robustness are realized as compared with the case of spatial propagation. With such a configuration, when a terahertz wave detection light pulse that is emitted from the polarization beam splitter 190 and whose polarization direction is converted from the second polarization direction to the first polarization direction is incident on the PPLN, the second harmonic is generated from the PPLN. A wave is generated, and only the second harmonic is extracted by the optical filter, and 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.

  In the present embodiment, 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.

Next, the operation of the terahertz wave generation detection device 100 according to the present embodiment will be described.
The fiber laser 101 oscillates a femtosecond laser light pulse having a first polarization direction. The femtosecond laser light pulse in the first polarization direction is branched into two by the beam splitter 102, and one of the branched light pulses is passed through the first path as a terahertz wave generation light pulse in the first polarization direction. The other optical pulse is coupled to the second path as a terahertz wave detecting optical pulse in the first polarization direction.

  The optical pulse for generating a terahertz wave having the first polarization direction coupled to the first path is incident on the intensity modulator 103. The intensity modulator 103 modulates the incident terahertz wave generating optical pulse in the first polarization direction so that the terahertz wave generating optical pulse propagates soliton in the fiber transmission unit 108. To do. Modulation for propagating the emitted soliton is applied, and the terahertz wave generating optical pulse having the first polarization direction is incident on the polarization beam combiner 107.

  On the other hand, the terahertz wave detecting optical pulse having the first polarization direction coupled to the second path is incident on the delay line scanner 104. The delay line scanner 104 is driven by the control of the control device to observe the terahertz wave waveform, and changes the time difference between the reflected terahertz wave emitted from the object to be measured 114 and the optical pulse for detecting the terahertz wave. The photoconductive antenna output current of the wave detector 112 is detected. The scan width and scan rate of the scan performed by the delay line scanner 104 may be adjusted according to the object 114 to be measured.

  The terahertz wave detection optical pulse given a predetermined delay time by the delay line scanner 104 is incident on the pulse time interval adjusting unit 105, and the pulse time interval adjusting unit 105 performs terahertz at the fire transmission unit 108. A fixed delay time is further provided so as not to overlap with the wave generating optical pulse in time. The terahertz wave detection optical pulse to which the constant delay time is applied is incident on the intensity modulator 106. The intensity modulator 106 modulates the incident terahertz wave detection light pulse so that the terahertz wave detection light pulse propagates soliton in the fiber transmission unit 108 and emits the modulated light pulse. The emitted terahertz wave detection light pulse modulated for soliton propagation is incident on the polarization beam combiner 107. The second path is twisted so as to be rotated 90 degrees from the polarization direction at the incident end of the second path at the exit end of the second path. The polarization direction of the terahertz wave detection light pulse is the second polarization direction.

  The polarization beam combiner 107 combines the incident light pulse for generating the terahertz wave in the first polarization direction and the light pulse for detecting the terahertz wave in the second polarization direction, and generates the terahertz wave in the first polarization direction. The optical pulse is incident on the high-speed axis of the fiber transmission unit 108 and the terahertz wave detection optical pulse having the second polarization direction is incident on the low-speed axis of the fiber transmission unit 108. At this time, since the terahertz wave detection optical pulse is given a fixed delay time by the pulse time interval adjustment unit 105, at the incident end of the fiber transmission unit 108, the terahertz wave generation optical pulse and the terahertz wave detection are performed. It does not overlap with the light pulse for use in time. In this way, both the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse propagate through one optical fiber (fiber transmission unit 108) without overlapping in time. Since both the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse are modulated so as to propagate solitons by the intensity modulators 103 and 106, the terahertz wave generating optical pulse and the terahertz wave detecting Both optical pulses propagate as optical solitons in the fiber transmission unit 108.

  The terahertz wave generating optical pulse having the first polarization direction and the terahertz wave detecting optical pulse having the second polarization direction transmitted through the fiber transmission unit 108 are incident on the polarization beam splitter 109. The polarization beam splitter 109 couples the terahertz wave generating optical pulse in the first polarization direction to the third path, and couples the terahertz wave detection optical pulse in the second polarization direction to the fourth path.

  The optical pulse for generating terahertz waves in the first polarization direction coupled to the third path is incident on the pulse time interval correction unit 110. The pulse time interval correction unit 110 includes a fixed delay time given to the terahertz wave detection optical pulse by the pulse time interval adjustment unit 105, and a terahertz wave generation optical pulse and terahertz wave due to birefringence of the fiber transmission unit 108. The terahertz wave generation optical pulse is delayed by a time corresponding to the sum of the time differences from the detection optical pulse so that the terahertz wave generation optical pulse and the terahertz wave detection optical pulse that do not overlap in time overlap in time. To. The terahertz wave generating optical pulse having the first polarization direction thus corrected is incident on the terahertz wave generating unit 111, and the terahertz wave generating unit 111 generates a terahertz wave. The generated terahertz wave is incident on the measurement object 114 via the propagation module 113. The terahertz wave incident on the device under test 114 is reflected by the device under test 114, and the reflected terahertz wave enters the terahertz wave detector 112 via the propagation module 113.

  On the other hand, the terahertz wave detecting optical pulse having the second polarization direction coupled to the fourth path is incident on the terahertz wave detecting unit 112. The fourth path is twisted so as to be rotated 90 degrees from the polarization direction at the incident end of the fourth path at the exit end of the fourth path, so that the fourth path is incident on the terahertz wave detecting unit 112. The polarization direction of the terahertz wave detection light pulse is the first polarization direction.

  The terahertz wave detection unit 112 is generated at the photoconductive antenna by the terahertz wave detection light pulse in the first polarization direction and the terahertz wave reflected from the measurement object 114 emitted from the propagation module 113. The electric current time waveform of the terahertz wave is measured by measuring the current.

  As described above, in this embodiment, the optical pulse for generating the terahertz wave, which is the first linearly polarized light, is incident on one of the fast axis and the slow axis of the birefringent polarization maintaining optical fiber (PMF), and the first is applied to the other. The terahertz wave detecting optical pulse, which is the second linearly polarized light orthogonal to the linearly polarized light, is incident, so that an optical fiber for transmitting the terahertz wave generating optical pulse and an optical fiber for transmitting the terahertz wave detecting optical pulse are provided. It is shared by one optical fiber. Therefore, even if a temperature change occurs and the optical fiber expands in response to the temperature change, both the optical fiber for transmitting the terahertz wave generating optical pulse and the optical fiber for transmitting the terahertz wave detecting optical pulse are both Since they are the same fiber transmission unit 108, both the terahertz wave generation optical pulse and the terahertz wave detection optical pulse passing through the fiber transmission unit 108 experience the same fiber extension. Therefore, the relative time difference between the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse with respect to the environmental change can be stabilized.

  Further, when the optical fiber for transmitting the optical pulse for generating the terahertz wave and the optical fiber for transmitting the optical pulse for detecting the terahertz wave are shared by one PMF, if the light enters the same axis of the PMF, the terahertz wave is generated from the PMF. When taking out the optical pulse for terahertz and the optical pulse for detecting terahertz wave, a device for temporally demultiplexing is required, and the system becomes complicated. However, in this embodiment, since the terahertz wave generating optical pulse is incident on one of the orthogonal birefringence axes (fast axis and slow axis) of the PMF and the terahertz wave detecting optical pulse is incident on the other, it is passive. The device can be configured with various components (polarization beam combiner, polarization beam splitter). Therefore, the apparatus can be simplified.

  In the present embodiment, as described above, either one of the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse is incident on each of the orthogonal birefringence axes of the fiber transmission unit 108, and the terahertz wave is incident. A terahertz wave generating light pulse and a terahertz wave detecting light are provided in the fiber transmission unit 108 by intentionally delaying one of the wave generating light pulse and the terahertz wave detecting light pulse by giving a certain delay time. The pulse is not overlapped in time. Therefore, when propagating the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse in the shared fiber transmission unit 108, generation of XPM due to the action of these two optical pulses can be suppressed. Deformation of the time waveform and / or spectrum waveform of the optical pulse propagating through the fiber transmission unit 108 can be prevented or reduced.

  In the present embodiment, both the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse are transmitted as optical solitons through the fiber transmission unit 108. Therefore, the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse can be propagated through the fiber transmission unit 108 without causing pulse distortion over a relatively long distance (for example,> 10 m). Therefore, the fiber transmission unit 108 can be a fiber cable that connects the controller and the optical head. Therefore, the terahertz wave generation unit 111 and the terahertz wave detection unit 112 can be separated from the optical system of the fiber transmission unit 108 in the preceding stage of the optical pulse traveling direction. That is, when applied to a form using an optical head, the optical head includes at least a polarization beam splitter 109, a pulse time interval correction unit 110, a terahertz wave generation unit 111, a terahertz wave detection unit 112, and a propagation module 113. What is necessary is just to comprise. Therefore, the components of the optical head can be reduced, reduced in size, and reduced in weight.

  Furthermore, since the femtosecond laser light pulse is delivered from the fiber laser 101 to the vicinity of the terahertz wave generation unit 111 and the terahertz wave detection unit 112 using an optical fiber, compared with a terahertz wave generation detection device using a spatial optical system. Thus, it is possible to provide a robust configuration with respect to misalignment caused by environmental changes such as vibration.

  One of the important things in the present invention is to reduce the influence of XPM in the fiber transmission unit shared for transmitting the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse. In addition, in the fiber transmission unit, it is necessary to prevent the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse from overlapping in time. In order to realize this, it is important to give a certain delay time to one of the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse. In the above embodiment, since a constant delay time is given to the terahertz wave detection optical pulse, the pulse time interval adjustment unit 105 is provided in the second path. However, the terahertz wave generation optical pulse has a constant value. When providing a delay time, the pulse time interval adjustment unit 105 may be provided in the first path. Alternatively, the pulse time interval adjustment unit 105 may be provided on both the first route and the second route. In this case, the amount of delay time provided by the pulse time interval adjusting unit provided in the first path may be different from the amount of delay time provided by the pulse time interval adjusting unit provided in the second path.

  Similarly, as described above, it is necessary to eliminate the intentionally provided time difference in the fiber transmission unit 108, and for this purpose, the pulse time interval correction unit 110 is provided. Therefore, if the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse that are not temporally overlapped in the fiber transmission unit 108 can be temporally overlapped, the pulse time interval is set. Needless to say, the correction unit 110 may be provided in the fourth path. Alternatively, the pulse time interval correction unit 110 may be provided on both the third route and the fourth route. In this case, the correction amount of the pulse time interval correction unit provided in the third path is such that the terahertz wave generation optical pulse transmitted through the fiber transmission unit 108 and the terahertz wave detection optical pulse overlap in time. The correction amount of the pulse time interval correction unit provided in the fourth path may be adjusted.

(Second Embodiment)
FIG. 4 is a schematic configuration diagram of the terahertz wave generation detection device according to the present embodiment.
In FIG. 4, the terahertz wave generation detection device 400 includes an optical unit 401 incorporated in a controller, an optical head 402, and a fiber transmission unit 108 that connects the optical unit 401 and the optical head 402. The optical unit 401 includes a fiber laser oscillator 403, a beam splitter 102, an ON / OFF modulator 404, an intensity modulator 103, a delay line scanner 104, a pulse time interval adjustment unit 105, an intensity modulator 106, And a polarization beam combiner 107. The optical head 402 includes a fiber amplifier 406, a fiber compressor 407, a polarization beam splitter 109, a pulse time interval correction unit 110, a terahertz wave generation unit 111, a terahertz wave detection unit 112, and a propagation module 113. Have

  In the second embodiment, the terahertz wave generating optical pulse is a femtosecond laser light pulse to be incident on the terahertz wave generating unit 111, and the terahertz wave generating unit 111 generates a desired terahertz wave. Therefore, description will be made on the assumption that the optical pulse having the power and the pulse width is set. The terahertz wave detection optical pulse is also a femtosecond laser light pulse to be incident on the terahertz wave detection unit 112, and the power and pulse set for the terahertz wave detection in the terahertz wave detection unit 112. It will be described as an optical pulse having a width.

  In the present embodiment, the fiber laser oscillator 403 oscillates a seed light pulse of a terahertz wave generating optical pulse and a terahertz wave detecting optical pulse, and has been modulated at the time of emission from a fiber compressor 407 described later. The terahertz wave generating optical pulse and the terahertz wave detecting optical pulse are used.

  The fiber laser oscillator 403 is a passive mode-locked fiber laser, and has a power lower than the power of the terahertz wave generating optical pulse that is the optical pulse that is finally output from the fiber compressor 407, and the terahertz wave generating light An optical pulse having a pulse width wider than the pulse width of the pulse and having the first polarization direction is oscillated. That is, the fiber laser oscillator 403 oscillates seed light pulses of a terahertz wave generating optical pulse and a terahertz wave detecting optical pulse. In this embodiment, the fiber laser oscillator 403 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.

  An ON / OFF modulator 404 is inserted in the first path, and one output end of the beam splitter 102 and an incident end of the ON / OFF modulator 404 are connected by a PMF, and the ON / OFF modulator 404 is connected. And the intensity modulator 103 are connected by a PMF. The ON / OFF modulator 404 performs predetermined modulation on an optical pulse that becomes a terahertz wave generating optical pulse when emitted from the fiber compressor 407 (for example, modulation for lock-in detection for improving terahertz wave detection sensitivity). It is for applying. That is, the ON / OFF modulator 404 modulates an optical pulse (seed light pulse) that is lower than the power of the optical pulse for generating the terahertz wave and wider than the pulse width of the pulse. In the embodiment, before the ON / OFF modulator 404 adjusts the power and pulse width to be possessed as the optical pulse for generating the terahertz wave, the seed light pulse to be the pulse for deriving the terahertz wave is modulated in advance. Therefore, the terahertz wave generating optical pulse can be a modulated optical pulse.

  In this embodiment, an AOM is used as the ON / OFF modulator 404, but an EOM may be used. In the present embodiment, the ON / OFF modulator 404, which is an AOM, is configured to modulate 90 kHz on the incident seed light pulse train for generating a terahertz wave. For example, when a lock-in amplifier is used, the modulation frequency may be set based on the upper limit of the band of the lock-in amplifier, and higher-speed modulation may be applied depending on the device.

  As described above, in the present embodiment, the fiber laser oscillator 403 oscillates the seed light pulse of the terahertz wave generating optical pulse and the terahertz wave detecting optical pulse. Therefore, in the first path and the second path, The seed light pulse will propagate. That is, when the seed light pulse emitted from the fiber laser oscillator 403 is incident, the beam splitter 102 emits the seed light pulse of the terahertz wave generating light pulse from one emission end, and the terahertz wave from the other emission end. A seed light pulse of the generation light pulse is emitted. Therefore, the seed light pulse of the terahertz wave generating light pulse emitted from one emission end of the beam splitter 102 propagates through the first path and enters one incident end of the polarization beam combiner 107, and from the other emission end. The emitted seed light pulse of the terahertz wave detection light pulse propagates through the second path and enters the other incident end of the polarization beam combiner 107.

  In FIG. 4, reference numeral 405 denotes a shared fiber for transmitting both the seed light pulse for generating the terahertz wave and the seed light pulse for detecting the terahertz wave. The shared fiber 405 includes a fiber transmission unit 108 having an incident end connected to the output end of the polarization beam combiner 107, a fiber amplifier 406 having an incident end connected to the output end of the fiber transmission unit 108, and a fiber amplifier having an incident end. And a fiber compressor 407 connected to the output end of 406. The exit end of the fiber compressor 407 is connected to the entrance end of the polarization beam splitter 109. Therefore, both the seed light pulse for generating the terahertz wave and the seed light pulse for detecting the terahertz wave incident from the polarization beam combiner 107 pass through the fiber transmission unit 108, the fiber amplifier 406, and the fiber compressor 407. Is incident on the polarization beam splitter 109.

  The fiber amplifier 406 is an erbium-doped fiber having birefringence and a dispersion characteristic having 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 406 can amplify a seed light pulse for generating a terahertz wave and a seed light pulse for detecting a terahertz wave by injecting pump light from a pump laser (not shown). As the fiber amplifier 406, a fiber amplifier to which ytterbium, thulium, neodymium, or the like is added in addition to erbium can be used. In the present embodiment, the fiber amplifier 406 is configured to amplify the power of incident seed light pulses for generating terahertz waves and seed light pulses for detecting terahertz waves to 500 mW. That is, the power of the seed light pulse for generating the terahertz wave and the seed light pulse for detecting the terahertz wave is converted by the fiber amplifier 406 into the light pulse for generating the terahertz wave and the light for detecting the terahertz wave, which are finally output optical pulses. It amplifies to the power it should have as a pulse.

  The fiber compressor 407 is a large-diameter constant-polarization photonic crystal fiber having birefringence, and the pulse width of the incident seed light pulse for generating the terahertz wave and the seed light pulse for detecting the terahertz wave is 50 fs. It is configured to compress.

  The optical pulse output from the fiber amplifier 406 (the seed light pulse for generating the terahertz wave and the seed light pulse for detecting the terahertz wave) spreads as the spectrum increases due to the nonlinear effect of the fiber amplifier 406 and the influence of normal dispersion. Has a monotonous chirp. In this embodiment, since a large-diameter photonic crystal fiber is used as the fiber compressor 406, 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 the present embodiment, a seed light pulse for generating a terahertz wave and a seed light for detecting a terahertz wave are obtained by optical spectrum broadening that occurs in the fiber amplifier 406 and soliton compression that occurs in a photonic crystal fiber as the fiber compressor 407. The pulse width of the pulse is compressed to 50 fs or less. That is, the fiber compressor 407 detects the pulse width of the terahertz wave generation seed light pulse and the terahertz wave detection seed light pulse, and the terahertz wave generation light pulse and terahertz wave detection, which are finally output optical pulses. Narrow down to the pulse width that should be possessed as an optical pulse.

  In this way, the seed light pulse for generating the terahertz wave and the seed light pulse for detecting the terahertz wave incident on the shared fiber 405 are subjected to predetermined amplification and pulse compression by the fiber amplifier 406 and the fiber compressor 407. The fire compressor 407 emits modulated terahertz wave generating optical pulses (pulse width: 50 fs, power: 500 mW) and terahertz wave detecting optical pulses (pulse width: 50 fs, power: 500 mW).

  The present embodiment is characterized in that two types of optical pulses such as a seed light pulse for generating a terahertz wave and a seed light pulse for detecting a terahertz wave are propagated through the shared optical fiber 405 that is one path. One. In the fiber transmission unit 108 which is one of the components of the shared optical fiber 405, a seed light pulse for generating a terahertz wave is incident on the high speed axis, and a seed light pulse for detecting the terahertz wave is incident on the low speed axis. Thus, two independent linearly polarized lights can propagate. In the present embodiment, the terahertz wave generation seed light pulse emitted from the high-speed axis of the fiber transmission unit 108 is incident on the high-speed axis of the fiber amplifier 406, and the terahertz wave detected from the low-speed axis of the fiber transmission unit 108 is detected on the low-speed axis. The fiber transmission unit 108 and the fiber amplifier 406 are connected so that the seed light pulses for use are incident. Similarly, a terahertz wave generation seed light pulse emitted from the high speed axis of the fiber amplifier 406 is incident on the high speed axis of the fiber compressor 407, and a terahertz wave detection seed emitted from the low speed axis of the fiber amplifier 406 is incident on the low speed axis. The fiber amplifier 406 and the fiber compressor 407 are connected so that the light pulse is incident. Therefore, both the fiber amplifier 406 and the fiber compressor 407 can propagate two independent linearly polarized lights.

  As described above, in this embodiment, the fiber amplifier 406 and the fiber compressor 407 are added as shared fibers to the first embodiment. Accordingly, the pulse time interval correction unit 110 adds the time difference caused by the birefringence of the fiber amplifier 406 and the fiber compressor 407 in addition to the time difference given by the pulse time interval adjustment unit 105 and the time difference caused by the birefringence of the fiber transmission unit 108. It is configured to compensate for the time difference due to the birefringence.

  As described above, in this embodiment, the optical pulse for generating the terahertz wave is subjected to predetermined modulation at the seed light stage, and then the power is increased to a necessary value and the pulse width is narrowed to the necessary value. The femtosecond laser light pulse that has been modulated, has high power (for example, 500 mW), and has a narrow pulse width (for example, 50 fs) can be incident on the terahertz wave generation unit 111. Therefore, it is possible to generate a high-intensity and broadband terahertz wave.

  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 this case, it is necessary to incorporate a chirp compensator in the subsequent stage of the femtosecond laser generator or AOM so that the pulse width is minimized at the terahertz wave generator. Further, since the transmittance of the modulator is not 100%, a high-power optical amplifier that compensates for the loss in the modulator is required. However, when the power is high, nonlinear optical effects that cause time distortion and spectral deformation are generated. 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 111 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, even when the ON / OFF modulator 404, which is an AOM or EOM, modulates the seed light pulse for generating the terahertz wave, even if the pulse width is widened due to the wavelength dispersion effect of the ON / OFF modulator 404. After the modulation, the pulse width of the seed light pulse for generating the terahertz wave is narrowed by the fiber compressor 407. As a result, the optical pulse of a clean femtosecond laser having a narrow pulse width despite being modulated. Can be emitted. Therefore, a femtosecond laser beam having a narrow pulse width can be incident on the terahertz wave generation unit 111, and the generated terahertz wave can be widened. In other words, in the present embodiment, an ON / OFF modulator 404 is provided in the propagation path of the optical pulse for generating the terahertz wave, and further, the fiber of the ON / OFF modulator 404 is disposed downstream of the traveling direction of the optical pulse. By providing the compressor 407, 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 ON / OFF modulator 404, 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.

  In the present embodiment, the terahertz wave generation seed light pulse is incident on one of two orthogonal birefringence axes, and the terahertz wave detection seed light pulse is incident on the other fiber 405. An amplifier 406 and a fiber compressor 407 are provided. Therefore, the same amplification and pulse compression are applied to both the seed light pulse for generating terahertz waves and the seed light pulse for detecting terahertz waves, which are two independent light pulses, by one fiber amplifier and one fiber compressor. be able to.

  By the way, since a relatively large controller such as a driver is generally required to generate femtosecond laser light pulses, in order to realize a portable terahertz wave generation and detection device mounted on a robot arm or the like. In many cases, the optical head is separated from the controller for weight reduction and compactness. On the other hand, when a pulse width of about 50 fs is generated by an optical fiber, since the fiber length between the modules (oscillator, amplifier, compressor) is generally as short as several tens of centimeters to several meters, femtosecond laser light is used. In the case where the pulse is delivered by an optical fiber to the terahertz generator and the terahertz wave detector, it is difficult to place a part of the module on the controller side. Therefore, inevitably, the femtosecond laser light generation unit needs to be inserted into the optical head. In this case, it is difficult to reduce the weight and size of the optical head.

  In this embodiment, by utilizing “soliton propagation” which is one of the nonlinear optical effects in the optical fiber, relatively large modules such as the fiber laser oscillator 403 and the delay line scanner 104 are separated from the optical head 402. It is configured to be stored on the controller side. By using the above-described soliton propagation, pulse distortion can be propagated in an optical fiber over a relatively long distance (> 10 m) or with reduced pulse distortion, so that the optical head 402 and the optical unit 401 of the controller can be transmitted. A fiber cable can be realized. Therefore, the number of modules (components) that should be stored in the optical head 402 can be reduced.

100, 400 Terahertz wave generation detection device 101 Fiber laser 102 Beam splitter 103, 106 Intensity modulator 104 Delay line scanner 105 Pulse time interval adjustment unit 107 Polarization beam combiner 108 Fiber transmission unit 109 Polarization beam splitter 110 Pulse time interval correction unit 111 Terahertz Wave generator 112 Terahertz wave detector 403 Fiber laser oscillator 404 ON / OFF modulator 405 Shared fiber 406 Fiber amplifier 407 Fiber compressor

Claims (7)

Laser oscillation means for oscillating an optical pulse;
A branching unit that has two emission ends and branches the optical pulse oscillated from the laser oscillation unit into two;
The first linearly polarized light incident from one of the two incident ends and the second linearly polarized light incident from the other and orthogonal to the first linearly polarized light are combined. And combining means for emitting,
A first path that optically connects one of the two exit ends of the branching means and one of the entrance ends of the combining means, the optical pulse emitted from one of the two exit ends of the branching means , And a light path that is the first linearly polarized light is incident on one incident end of the combining unit;
A second path for optically connecting the other of the two exit ends of the branching unit and the other of the entrance ends of the combining unit, the optical pulse emitted from the other of the two exit ends of the branching unit And a second path for inputting the light pulse that is the second linearly polarized light to the other incident end of the combining means;
Delay means provided in at least one of the first path and the second path, the delay means for delaying an optical pulse passing through the path provided with the delay means by a predetermined delay time;
Adjusting means provided in at least one of the first path and the second path, wherein the light pulse that is the first linearly polarized light and the light pulse that is the second linearly polarized light in the combining means The adjusting means is provided so that the combined light pulse that is the first linearly polarized light and the light pulse that is the combined second linearly polarized light do not overlap in time. Adjusting means for delaying a light pulse passing through a given path by a certain delay time;
A polarization maintaining fiber having birefringence, one end of which is connected to the output end of the combining means, and the light pulse which is the first linearly polarized light is incident on one of the fast axis and the slow axis of the polarization maintaining fiber. The optical pulse that is the second linearly polarized light is connected to the synthesizing means so that the optical pulse that is the second linearly polarized light is incident on the other of the high-speed axis and the low-speed axis. A polarization-maintaining fiber through which both light pulses that are linearly polarized light propagate,
The light pulse that is the first linearly polarized light and the light that is the second linearly polarized light that has two emission ends, is connected to the other end of the polarization maintaining fiber, and is incident from the other end of the polarization maintaining fiber One of the pulses is emitted from one of the two exit ends, and the other of the optical pulse that is the first linearly polarized light and the optical pulse that is the second linearly polarized light incident from the other end of the polarization maintaining fiber. Polarization branching means for emitting from the other of the two exit ends;
Terahertz wave generating means for generating a terahertz wave by a light pulse emitted from one of the two exit ends of the polarization branching means;
The optical pulse emitted from the other of the two emission ends of the polarization branching unit 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 configured to be incident. Terahertz wave detection means for detecting the emitted terahertz wave;
A third path for optically connecting one of the two exit ends of the polarization splitting means and the terahertz wave generating means;
A fourth path for optically connecting the other of the two exit ends of the polarization splitting means and the terahertz wave detecting means;
The light pulse as the first linearly polarized light and the light pulse as the second linearly polarized light that are provided in at least one of the third path and the fourth path and do not overlap in time overlap in time. A terahertz generation detecting device, comprising: a correction unit configured to do so.   2. The terahertz wave generation and detection apparatus according to claim 1, wherein light pulses having the same polarization direction are emitted from the third path and the fourth path.   A fiber compressor that is provided between the polarization maintaining fiber and the polarization branching unit and has birefringence and narrows the pulse width of an optical pulse incident on its fast axis and slow axis, and the polarization maintaining A light pulse emitted from the high-speed axis of the fiber is incident on the high-speed axis of the fiber compressor, and a light pulse emitted from the low-speed axis of the polarization maintaining fiber is incident on the low-speed axis of the fiber compressor. The terahertz wave generation detection device according to claim 1, further comprising a fiber compressor.   A fiber amplifier that is provided between the polarization maintaining fiber and the polarization branching unit and has birefringence and amplifies the power of an optical pulse incident on its own fast axis and slow axis, A fiber amplifier provided so that an optical pulse emitted from a high-speed axis is incident on the high-speed axis of the fiber amplifier and an optical pulse emitted from the low-speed axis of the polarization maintaining fiber is incident on the low-speed axis of the fiber amplifier; The terahertz wave generation detection device according to any one of claims 1 to 3, wherein the terahertz wave generation detection device is provided. The terahertz wave generation detection device is configured such that an optical pulse that is the first linearly polarized light emitted from the first path is incident on the terahertz wave generation means,
5. The terahertz wave according to claim 1, further comprising a modulation unit that is provided in the first path and applies predetermined modulation to an optical pulse that passes through the first path. 6. Occurrence detection device.   6. The terahertz wave generation detection device according to claim 1, wherein both the light pulse incident on the high-speed axis and the light pulse incident on the low-speed axis of the polarization maintaining fiber propagate solitons. An optical pulse that is provided in the first path and propagates through the first path so that an optical pulse that is the first linearly polarized light emitted from the first path propagates through the soliton in the polarization-maintaining fiber. Modulation means for modulating the
An optical pulse that is provided in the second path and propagates through the second path so that an optical pulse that is the second linearly polarized light emitted from the second path propagates through the soliton in the polarization-maintaining fiber. The terahertz wave generation detection device according to claim 6, further comprising: a modulation unit that modulates the signal.

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