JP2013044727A - Electromagnetic wave imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable simultaneous imaging of multiple parts of a measured object.SOLUTION: An electromagnetic wave for detection irradiated on a measured object enters an electro-optic crystal. The electro-optic crystal is irradiated with a probe wave whose wavefront is at an angle with the wavefront of the electromagnetic wave for detection. The probe wave that has passed through the electro-optic crystal is detected by an imaging device. The beam cross-section of the electromagnetic wave for detection or the probe wave is divided into a plurality of first unit areas of different optical path length on a first virtual plane that is orthogonal to the wavefront of the electromagnetic wave for detection or the wavefront of the probe wave. Phase shifts between the wavefront of the electromagnetic wave for detection and the wavefront of the probe wave are compensated at a plurality of points in the direction of intersection of a surface of the electro-optic crystal with the first virtual plane. The beam cross-section of the probe wave is divided into a plurality of second unit areas on a second virtual plane that is orthogonal to the first virtual plane and parallel to the travel direction of the probe wave. Further, each of the second unit areas is divided into a plurality of areas with different wavefront delay time of the probe wave.

Description

  The present technology relates to an electromagnetic wave imaging apparatus using two electromagnetic waves. For example, a technique for performing nondestructive inspection of an object using terahertz electromagnetic waves is known.

  Various methods for observing an object using electromagnetic waves are known. For example, there are a method of obtaining visible data using visible light and a method of observing an internal structure that cannot be viewed using X-rays.

  A terahertz (THz) wave is an electromagnetic wave that exists in a region having a frequency of 100 GHz to 10 THz and a wavelength of 30 μm to 3 mm, and is transmitted through plastic, cloth, paper, semiconductor, and the like, and has an intrinsic absorption spectrum. For this reason, it is possible to perform physical property analysis, inspection, fluoroscopic imaging, and the like using terahertz waves, and applied technologies are being developed. For the terahertz wave region, terms for light such as an optical axis, a polarizer, and an analyzer may be used.

  Since non-destructive component analysis imaging is possible, it is expected as a component analysis type internal fluoroscopy unit replacing a conventional internal fluoroscopy unit (X-ray, ultrasonic wave, etc.). As a recent countermeasure against terrorism and crime, it cannot be detected by conventional X-ray inspections such as explosives (plastic bombs, flammable liquids, etc.) in airport baggage inspection, or prohibited drugs (narcotics, stimulants, etc.) in postal seals. The use of terahertz spectroscopic imaging using characteristic THz absorption has been proposed for a certain measurement object.

  A method has been proposed in which a pulsed terahertz wave and probe pulse light are incident on an electro-optic (EO) crystal non-coaxially, an image of the probe pulse light is captured by a digital camera, and a terahertz time waveform is measured with a single shot. Yes.

  There has also been proposed a method in which a pulsed terahertz wave and probe pulse light are incident on an electro-optic (EO) crystal coaxially. In this method, the plane (pulse plane) connecting the peak positions of the intensity of the probe pulse light is inclined with respect to the wavefront of the terahertz wave. By tilting the pulse surface of the probe pulse light with respect to the wavefront of the terahertz wave, the same effect as in the non-coaxial case can be obtained.

WO2006 / 085403 JP 2008-96210 A

  In a method that uses a non-coaxial optical system to focus the detection electromagnetic wave in a line and apply it to the object to be measured, and use the imaging electromagnetic wave in the non-coaxial direction, the information in the direction perpendicular to the linear region is obtained. Scanning is necessary to obtain. It is efficient if imaging can be performed at a plurality of locations simultaneously in a direction orthogonal to the linear region.

According to one aspect of the invention,
An electro-optic crystal;
A first optical system that irradiates an object to be measured with a pulsed electromagnetic wave for detection and transmits or reflects the electromagnetic wave for detection incident on the electro-optic crystal;
A second optical system that irradiates the electro-optic crystal with the probe wave by inclining the wave front of the pulsed probe wave with respect to the wave front of the electromagnetic wave for detection;
An imaging device that detects the probe wave transmitted through the electro-optic crystal;
One of the first optical system and the second optical system is configured to detect the detection electromagnetic wave or the probe wave in a first virtual plane orthogonal to the wavefront of the detection electromagnetic wave and the wavefront of the probe wave. A compensation optical member that divides a beam cross-section into a plurality of first unit regions and makes an optical path length of a beam passing through the first unit region different for each of the first unit regions, the electro-optic crystal A compensation optical member that compensates for a phase shift between the wavefront of the electromagnetic wave for detection and the wavefront of the probe wave at a plurality of locations in the direction of the intersection of the surface of the first virtual plane and the first virtual plane,
The second optical system has a beam section of the probe wave in a plurality of second unit regions in a second virtual plane orthogonal to the first virtual plane and parallel to the traveling direction of the probe wave. Further comprising a time delay device for dividing each of the second unit regions into a plurality of regions having different delay times of the wavefronts of the probe waves,
The time delay device includes a step mirror in which two kinds of reflection surfaces having different heights are arranged in a first direction, and a counter mirror facing the step mirror, and the probe wave is reflected on the reflection surface of the step mirror. The probe wave is divided into a plurality of regions having different wavefront delay times by reciprocating between the step mirror and the counter mirror a plurality of times while shifting the reflection position in the first direction. An electromagnetic imaging device is provided.

  A plurality of locations can be measured without scanning the incident position of the detection electromagnetic wave within the object to be measured.

FIG. 1 is a schematic diagram of an electromagnetic wave imaging apparatus according to the first embodiment. FIG. 2A is a schematic diagram illustrating the positional relationship of the compensation optical element, the electro-optic crystal, the analyzer, and the probe wave path of the electromagnetic wave imaging apparatus according to the first embodiment, and FIG. 2B is defined on the imaging surface of the imaging apparatus. It is a front view which shows the cell made. FIG. 3 is a perspective view of the compensation optical element and the electro-optic crystal of the electromagnetic wave imaging apparatus according to the first embodiment. 4A is a front view of the step mirror used in Example 1, and FIGS. 4B and 4C are cross-sectional views taken along one-dot chain lines 4B-4B and 4C-4C in FIG. 4A, respectively. FIG. 5 is a perspective view of one light beam of the step mirror, the counter mirror, and the probe wave. 6A and 6B are perspective views of a step mirror, a counter mirror, and one light beam of a probe wave. 7A and 7B are diagrams showing the positional relationship between the step mirror and the beam section of the probe wave, and the distribution of delay time in the beam section. 7C and 7D are diagrams showing the positional relationship between the step mirror and the beam section of the probe wave, and the distribution of delay time in the beam section. FIG. 8 is a graph showing the delay time distribution of the beam cross section of the probe wave. FIG. 9 is a diagram showing the positional relationship between the wavefronts of the probe waves. 10A is a front view showing division of cells in the imaging plane of the imaging apparatus, FIG. 10B is a diagram in which one cell is divided into regions having different delay times, and FIG. 10C is an electromagnetic wave for detection. It is the graph which connected the detected waveform. FIG. 11 is a cross-sectional view of a step mirror used in the electromagnetic wave imaging apparatus according to the second embodiment. FIG. 12 is a schematic diagram of an electromagnetic wave imaging apparatus according to the third embodiment. FIG. 13A is a schematic diagram of an adaptive optical element used in the electromagnetic wave imaging apparatus according to the fourth embodiment, and FIG. 13B is a schematic diagram illustrating another configuration example of the adaptive optical element. It is the schematic for demonstrating the principle of operation of the electromagnetic wave imaging device which injects a terahertz wave and a probe wave into an electro-optic crystal non-coaxially.

  Before describing the embodiment, the principle of operation of a terahertz wave imaging apparatus that causes a terahertz wave and a probe wave to enter the electro-optic crystal non-coaxially will be described.

  FIG. 14 is a principle diagram of a terahertz imaging apparatus in which a terahertz wave and a probe wave are incident on an electro-optic crystal non-coaxially. The pulsed terahertz wave 101 traveling in the z direction is condensed into a linear shape parallel to the x axis by the cylindrical lens 108 and is incident on the object 110 to be measured. The pulsed terahertz wave 101 carries information on a linear region along the x-axis direction of the DUT 110. The pulsed terahertz wave may be reflected by the measurement object instead of being transmitted through the measurement object. The terahertz wave that has passed through the linear region of the measurement object 110 having a depth is returned to a parallel light beam having a spread in the xy plane by the cylindrical lens 109. Information on the linear region in the x-axis direction is expanded in the y direction.

  A pulsed terahertz wave that has passed through the DUT 110 is incident on the electro-optic crystal 103. The electro-optic crystal 103 causes a birefringence change due to the electro-optic effect caused by the electric field of the pulsed terahertz wave 101 transmitted through the target object. The wavefront (equal phase surface) indicated by the solid line precedes the wavefront (equal phase surface) indicated by the broken line in time. The pulsed probe wave 102 is incident on the electro-optic crystal 103 in the yz plane direction and in a non-coaxial arrangement with the pulsed terahertz wave 101. The probe wave 102 is affected by the birefringence change caused by the pulsed terahertz wave 101 in the electro-optic crystal 103.

  Since they are non-coaxial, a time difference occurs at the timing at which the wavefronts of the pulsed terahertz wave 101 and the probe wave 102 overlap, and a time change appears in the y-axis direction. That is, the temporal change information of the linear region in the x-axis direction is developed in the y-axis direction. On the optical axis of the probe wave 102, the polarizer 104 and the analyzer 105 are arranged in a crossed Nicols relationship with the electro-optic crystal 103 interposed therebetween. A probe wave 102 affected by a change in birefringence induced by the pulsed terahertz wave 101 is taken out and measured by a CCD camera 106 including an imaging lens.

  The wavefront of the probe wave 102 and the wavefront of the pulsed terahertz wave 101 intersect at an intersection angle θ. When viewed from the wavefront of the probe wave 102, in FIG. 14, when it matches the wavefront of the pulsed terahertz wave on the upper side, it matches the wavefront of the broken line of the pulsed terahertz wave on the lower side. That is, time has passed from the upper side to the lower side.

  The time-varying information developed in the traveling direction (z-axis direction) of the pulsed terahertz wave 1 is converted into a direction orthogonal to the one-dimensional image in the x-axis direction (time of the probe wave 102) by time-space conversion in the electro-optic crystal 103. It is developed in the beam width direction. By detecting this with a two-dimensional imaging device, information in the time axis (depth) direction can be obtained. Similar to the interference spectroscopy, a spectrum can be obtained by Fourier transforming the time-varying information.

  One axis of the two-dimensional imaging image obtained by the two-dimensional imaging device is used for measuring the time-series waveform of the terahertz pulse. The other one axis is used for one-dimensional imaging (one-dimensional imaging in a two-dimensional plane) of a terahertz pulse transmitted in the state of line focusing (line beam). With respect to the y-axis direction in the real space, only one point in the DUT 110 is measured. In order to obtain information about the y direction, it is necessary to scan the linear condensing region in the y direction within the object 110 to be measured. The inventor considered to simultaneously measure in-plane information by decomposing the wavefront.

[Example 1]
FIG. 1 shows a schematic diagram of an electromagnetic wave imaging apparatus according to the first embodiment. A femtosecond laser is emitted from the light source 20. As the light source 20, for example, a femtosecond laser oscillator manufactured by Spectra Physics Co., Ltd. can be used. This laser oscillator emits a pulse laser having a pulse energy of 1 mJ, a pulse width of 100 fs, a center wavelength of 800 nm, and a repetition frequency of 1 kHz.

  The output of the light source 20 is branched by the beam splitter 21. One of the branched pulse lasers is adjusted in the optical path by the mirror 22 or the like as necessary to become the probe wave 2. The other pulse laser is incident on the terahertz generator using the ZnTe electro-optic crystal 23, and a pulsed terahertz wave (detection electromagnetic wave) 1 is generated. The detection electromagnetic wave 1 includes a terahertz wave in a wide wavelength range. The time width of the pulsed electromagnetic wave 1 for detection is 1 to 2 psec.

  Cylindrical lens rows 13 and 14 are arranged on the optical path of the collimated electromagnetic wave 1 for detection. The cylindrical lens array 13 condenses the detection electromagnetic wave 1 in a plurality of parallel linear regions. The device under test 8 is arranged so that the condensed area is located inside the device under test 8. The detection electromagnetic wave 1 transmitted through the object to be measured 8 is returned to a plurality of collimated beam bundles by the cylindrical lens array 14. Each of the collimated beam bundles spreads in the width direction, but is an electromagnetic wave that has passed through the linear region of the object 8 to be measured, and the light beam at any position in the width direction can be transmitted to the same linear region of the object 8 to be measured. It carries information. The detection electromagnetic wave 1 that has passed through the cylindrical lens array 14 enters the electro-optic crystal 3 perpendicularly. The wavefront (phase plane) of each collimated beam bundle of the detection electromagnetic wave 1 is parallel to the surface of the electro-optic crystal 3.

  The probe wave 2 is incident on the electro-optic crystal 3 at an incident angle θ through the polarizer 4, the time delay device 9, and the compensation optical element 10. That is, the detection electromagnetic wave 1 and the imaging probe wave 2 are incident on the electro-optic crystal 3 non-coaxially. The adaptive optics element 10 divides the probe wave 2 into a plurality of unit beam bundles having optical path length differences (phase differences). Each unit beam bundle of the probe wave 2 intersects a corresponding collimated beam bundle of the detection electromagnetic wave 1 on the electro-optic crystal 3. Accordingly, the number of collimated beam bundles generated by the cylindrical lens arrays 13 and 14 is equal to the number of unit beam bundles generated by the adaptive optics 10.

  For the compensation optical element 10, a step-like transparent block is used. Each of the step surfaces (step surfaces) of the step-like transparent block is perpendicular to the traveling direction of the probe wave 2 (parallel to the wave surface). The tread surface of the step-like transparent block and the unit light flux of the probe wave 2 correspond one-to-one. The wavefront of the unit beam of the probe wave 2 and the wavefront of the collimated beam of the detection electromagnetic wave 1 intersect at an angle θ.

  The electro-optic crystal 3 causes a birefringence change due to the electro-optic effect due to the electric field of the detection electromagnetic wave 1 that is transmitted through the workpiece 8 and influenced by the workpiece 8. The linearly polarized probe wave 2 changes its polarization state due to a change in birefringence of the electro-optic crystal 3 (generally, it becomes elliptically polarized light). The probe wave 2 whose polarization state has been changed by the electro-optic crystal 3 passes through the analyzer 5 and enters an imaging device 6 such as a digital camera. The polarizer 4 and the analyzer 5 are arranged in a crossed Nicols relationship. For this reason, only the change component of the polarization state of the probe wave 2 passes through the analyzer 5. The outside air serving as the optical path of the detection electromagnetic wave 1 is a nitrogen atmosphere or a vacuum as necessary.

  The electro-optic crystal 3 is formed of, for example, a 30 mm × 30 mm × 2 mm ZnTe crystal. The compensation optical element 10 is made of, for example, glass (BK7) having a wavelength of 800 nm and a refractive index of 1.51. Other transparent materials such as other types of glass and organic resins transparent at a wavelength of 800 nm may be used. For example, a CCD camera including an imaging lens, having a sensitivity at a wavelength of 800 nm, a pixel number of 512 × 512, and a pixel pitch of 20 μm is used as the imaging device 6. The magnification of the imaging lens is, for example, 0.3 times.

  Xyz orthogonal where the traveling direction of the probe wave 2 is the reverse direction of the z-axis, the width direction (the direction in which the unit beam bundles of the probe wave 2 are arranged) is the y-axis direction, and the vertical direction (height) direction is the x-axis direction. Define a coordinate system. The traveling direction of the detection electromagnetic wave 1 and the traveling direction of the probe wave 2 are parallel to the yz plane. That is, a plane perpendicular to both the wavefront of the detection electromagnetic wave 1 and the wavefront of the probe wave 2 is a yz plane.

  In the first embodiment, the compensation optical element 10 is arranged on the upstream side of the electro-optic crystal 3 on the optical path of the probe wave 2. The compensation optical element 10 has a refractive index higher than that of the outside air having a refractive index of 1.0. In FIG. 1, the adaptive optical element 10 has the largest dimension in the z direction along the optical path on the negative side of the y axis, and the dimension in the z direction decreases stepwise toward the positive direction of the y axis. A tread surface parallel to the xy plane extends in the x-axis direction. The beam cross section of the probe wave 2 is divided into a plurality of regions (y division unit regions) 12 having different optical path lengths in the beam width direction (y direction). Within one y-division unit region 12, the optical path length is uniform. Specifically, the plurality of y-divided unit regions 12 are defined so that the optical path length increases stepwise in the negative direction of the y-axis. One y-divided unit region 12 corresponds to one unit beam bundle of the probe wave 2.

  FIG. 2A shows a cross-sectional view of the compensation optical element 10, the electro-optic crystal 3, and the analyzer 5. The adaptive optical element 10 has a surface 10A parallel to the xz plane and a surface (bottom surface) 10B parallel to the xy plane, and a plurality of treads 10C parallel to the xy plane on the opposite side of the bottom surface 10B. The number of steps of the tread surface 10C is, for example, five. A kick surface 10D parallel to the xz plane is connected between the tread surfaces 10C. Therefore, in the beam cross section of the probe wave 2, five y-divided unit regions 12 having different optical path lengths with respect to the beam width direction (y-axis direction) are formed.

  The higher the tread surface 10C (the negative side of the y-axis) of the compensating optical element 10, the longer the optical path length and the more delayed the wavefront. As a result, wavefronts WF1 to WF5 of five stages are formed in which the wavefront of the unit beam bundle relatively positioned on the positive side of the y-axis advances from the wavefront of the unit beam bundle positioned on the negative side. The surface of the electro-optic crystal 3 is inclined at an angle θ from a plane perpendicular to the traveling direction of the probe wave 2. The step in the z direction (the height of the kick-up surface 10D) of each step of the compensation optical element 10 is designed so that the wavefronts WF1 to WF5 reach the electro-optic crystal 3 at the same time. As a result, the wavefront of the detection electromagnetic wave 1 and the wavefront of the probe wave 2 coincide with each other (at five points) in the intersecting direction between the surface of the electro-optic crystal 3 and the yz plane (the phase shift is compensated). .

  Next, the operation of the electromagnetic wave imaging apparatus according to the first embodiment will be described assuming that the time delay device 9 is not disposed.

  FIG. 2A shows a state in which the lower ends (ends on the y-axis negative side) of the wavefronts WF1 to WF5 have reached the electro-optic crystal 3 at the same time. As the wavefront advances, the upper portions of the wavefronts WF 1 to WF 5 gradually reach the electro-optic crystal 3. That is, in each y-division unit area 12, time information is developed in the y-axis direction as described with reference to FIG. Information on the position of the real space in the y-axis direction can be measured for the number of steps, here five steps.

  FIG. 2B schematically shows the imaging surface of the imaging device 6. The imaging surface is divided into five rectangular regions that are long in the x direction corresponding to the y-divided unit region 12. Each rectangular area is referred to as y-divided cells CLy1 to CLy5. 90 pixels are arranged in the width direction (y-axis direction) of each of the y-divided cells CLy1 to CLy5. A pixel column arranged in the y-axis direction of each of the y-divided cells CLy1 to CLy5 corresponds to the time axis of the time waveform of the electromagnetic wave 1 for detection. In each of the y-divided cells CLy1 to CLy5, time elapses toward the positive direction of the y-axis.

  FIG. 3 is a perspective view of the detection electromagnetic wave 1, the probe wave 2, the electro-optic crystal 3, the imaging device 6, and the compensation optical element 10. The compensation optical element 10 that forms an optical path length difference with respect to the probe wave 2 having a wavelength of 800 nm is formed of glass (BK7). In performing spectroscopic imaging, in order to obtain a time waveform with a resolution of 0.15 psec and a time width of 13.5 psec, and a spectrum with a resolution of 0.07 THz and a band of 3.33 THz, The incident angle θ is 42.45 °. The refractive index of the compensation optical element 10 is 1.51, the dimension in the y-axis direction is 22.13 mm, the dimension in the x-axis direction is 30 mm, the distance in the z-axis direction from the bottom surface to the highest tread surface is 39.71 mm, the tread surface The dimension in the y-axis direction is 4.43 mm, and the step in the optical path direction (the height of the kick-up surface) is 7.94 mm. By arranging such a step-shaped compensation optical element 10 in the optical path of the probe wave 2 having a wavelength of 800 nm, the width direction of the object to be measured 8 (perpendicular to both the traveling direction of the electromagnetic wave 1 for detection and the x-axis direction). At the same time, the time waveform of the detection electromagnetic wave 1 can be measured.

  Returning to FIG. 1, the configuration of the time delay device 9 will be described. The beam section of the probe wave 2 that has passed through the polarizer 4 is shaped into a shape that is long in the x-axis direction by the beam shaper 31. The probe wave 2 transmitted through the beam shaper 31 reciprocates a plurality of times between the step mirror 34 and the counter mirror 35 arranged to face each other, and then enters the beam shaper 32. For each of the beam shapers 31 and 32, an afocal optical system including two cylindrical lenses is used. The beam shaper 32 restores the original shape by stretching the cross section of the probe wave 2 in the y-axis direction. The probe wave 2 that has passed through the beam shaper 32 enters the adaptive optical element 10.

  FIG. 4A shows a front view of the step mirror 34. 4B and 4C are cross-sectional views taken along one-dot chain lines 4B-4B and 4C-4C in FIG. 4A, respectively. Relatively high reflective surfaces 34H and relatively low reflective surfaces 34L are alternately arranged in the x-axis direction. The difference in height between the high reflective surface 34H and the low reflective surface 34L is H. The shapes and dimensions of the high reflective surface 34H and the low reflective surface 34L are all the same. In Example 1, three high reflective surfaces 34H and three low reflective surfaces 34L are arranged. The reflective surface of the counter mirror 35 is a plane.

  FIG. 5 shows a schematic perspective view of the path of the step mirror 34, the counter mirror 35, and the probe wave 2. The normal lines of the high reflection surface 34H and the low reflection surface 34L of the step mirror 34 are obtained from the direction opposite to the traveling direction of the incident probe wave 2 (the positive direction of the z axis), the positive direction of the x axis, and the y axis. Slightly inclined in the positive direction. The flat reflecting surface of the counter mirror 35 is parallel to the high reflecting surface 34H and the low reflecting surface 34L of the step mirror 34.

  When attention is paid to a light beam having the probe wave 2, the reflection position P on the surface of the step mirror 34 moves in the positive direction of the y axis as the reflection is repeated between the step mirror 34 and the counter mirror 35. Also move in the positive direction. In the first embodiment, the light is reflected by the step mirror 34 and the counter mirror 35 six times and then enters the beam shaper 32 (FIG. 1). FIG. 5 shows an example in which the reflection positions P on the reflection surface of the step mirror 34 are all located within the high reflection surface 34H.

  FIG. 6A shows a light beam in which the first to fifth reflection positions P on the reflection surface of the step mirror 34 are located on the high reflection surface 34H and the sixth reflection position P is located on the low reflection surface 34L. The optical path length of this light beam is 2 × H longer than the optical path length of the light beam shown in FIG. The time during which the probe wave 2 propagates through the optical path length 2 × H is td. In FIG. 6B, the first to second reflection positions P on the reflection surface of the step mirror 34 are located on the high reflection surface 34H, and the third to sixth reflection positions P are located on the low reflection surface 34L. Is shown. The optical path length of this light beam is longer by 8 × H than the optical path length of the light beam shown in FIG. The number of times of reflection by the high reflection surface 34H and the low reflection surface 34L depends on the position in the x direction within the beam cross section 38 of the probe wave 2.

  The light beam reflected by the low reflection surface 34L travels a longer path than the light beam reflected by the high reflection surface 34H. This causes a time delay. The longer the light beam reflected by the lower reflective surface 34L, the longer the delay time.

  The delay time distribution in the beam cross section 38 will be described with reference to FIGS. 7A to 7D.

  In FIG. 7A, the first incidence area 38A of the probe wave 2 is indicated by a solid line, and the second to sixth incidence areas 38B to 38F are indicated by a broken line. The incident regions 38A to 38F have a shape that is long in the x direction. The first reflection area 38A extends from the lowermost lower reflective surface 34L in FIG. 7A to the uppermost lower reflective surface 34L. Thereby, the inside of the beam section 38 is divided into a region A0 reflected by the high reflecting surface 34H and a region A1 reflected by the low reflecting surface 34L.

  The light beam reflected by the high reflective surface 34H in all six times is used as a reference for the delay time. At the time of one reflection, the delay time of the light beam passing through the region A0 is 0, and the delay time of the light beam passing through the region A1 is td.

  The second to sixth incidence areas 38B to 38F move in the positive y-axis direction and shift in the positive x-axis direction as the number of reflections increases. The amount of shift in the x-axis direction per number of reflections is 1/7 of the dimension in the x-axis direction of each of the high reflection surface 34H and the low reflection surface 34L.

  FIG. 7B shows the second incident region 38B of the probe wave 2 and the delay time distribution in the beam cross section 38 after being reflected twice. A high reflection area between an area A0 having two reflections on the high reflection surface 34H and a delay time of 0 and an area A2 having two reflections on the low reflection surface 34L and a delay time of 2td A region A1 having a delay time of td is defined by the number of reflections at 34H and the low reflection region 34L once. The dimension of the region A1 in the x-axis direction is equal to the distance by which the incident region of the probe wave 2 is shifted in the x-axis direction per one reflection.

  FIG. 7C shows the third incident region 38C of the probe wave 2 and the delay time distribution in the beam section 38 after being reflected three times. Regions A0, A1, A2, and A3 having delay times of 0, td, 2td, and 3td are defined.

  FIG. 7D shows the sixth incident region 38F of the probe wave 2 and the delay time distribution in the beam section 38 after being reflected six times. Regions A0, A1, A2, A3, A4, A5, and A6 of delay times 0, td, 2td, 3td, 4td, 5td, and 6td are defined.

  As shown in FIG. 8, the inside of the beam cross section 38 is divided in the x direction, and four x division unit regions 39 are defined. Each of the x-divided unit areas 39 includes seven areas A0 to A6 whose delay times are 0, td, 2td, 3td, 4td, 5td, and 6td. That is, the step size of the delay time of the wavefront of the probe wave 2 in the seven regions A0 to A6 is td. Six regions A1 to A6 in which the delay times are td, 2td, 3td, 4td, 5td, and 6td are defined at the end on the positive side of the x-axis in the beam cross section 38. Since this area does not include the area A0 where the delay time is 0, the x-divided unit area 39 is not configured.

  When attention is paid to two x-divided unit regions 39 adjacent to each other, the regions A0 to A6 are arranged in a line-symmetric relationship with respect to the boundary line between them. That is, the area A0 is arranged on both sides of the boundary line of the x-divided unit area 39, and the areas A0 to A6 are sequentially arranged in the direction away from the boundary line, or the area A6 is arranged on both sides of the boundary line. The areas A6 to A0 are arranged in order in a direction away from the area.

  FIG. 9 shows the positional relationship of the wavefront of the probe wave 2. 9 shows a wavefront in one y-divided unit region 12 among the plurality of y-divided unit regions 12 shown in FIG. 2A. In each of the x-divided unit regions 39, the wavefront of the region A0 is the most advanced, and the wavefront of the region A6 is the most delayed. For this reason, the time when the wavefront reaches the electro-optic crystal 3 is the earliest in the region A0 and the latest in the region A6. The wavefront delay time between adjacent regions is equal to td.

  FIG. 10A shows a front view of the imaging surface of the imaging device 6 (FIG. 1). As illustrated in FIG. 2B, the imaging surface is divided into y-divided cells CLy1 to CLy5 in the y direction. The imaging surface is divided into four x-divided cells CLx1 to CLx4 corresponding to the x-divided unit region 39 (FIG. 8). A region where the y-divided cell CLyj and the x-divided cell CLxi intersect each other constitutes one cell CL (i, j). Here, i is an integer from 1 to 4, and j is an integer from 1 to 5.

  As shown in FIG. 10B, one cell CL (i, j) is divided into seven regions A0 to A6 corresponding to regions A0 to A6 (FIG. 8) having different delay times. In each of the regions A0 to A6, as described with reference to FIG. 2B, the y axis corresponds to the time axis of the time waveform of the electromagnetic wave 1 for detection. Specifically, time elapses in the positive direction of the y-axis.

  As shown in FIG. 9, since the wavefront of the probe wave 2 is delayed from the region A0 toward the region A6, the time waveform of the detection electromagnetic wave 1 detected in the region A1 is detected in the region A0. This is after the time waveform of the electromagnetic wave 1 for detection. By adjusting the delay time td between the areas A0 to A6, the time corresponding to the positive end of the y axis in the area A0 and the time corresponding to the negative end of the y axis in the area A1 Can be matched. Specifically, one end of one wavefront of the wavefronts WF1 to WF5 in the y-division unit region 12 of the probe wave 2 shown in FIG. 2A is incident on the electro-optic crystal 3, and the other end is placed on the electro-optic crystal 3. What is necessary is just to make the time until the incident and the step size td of the delay time by the time delay device 9 equal.

  At this time, the time waveform of the detection electromagnetic wave 1 detected in the region A0 and the time waveform of the detection electromagnetic wave 1 detected in the region A1 are connected on the time axis, whereby the detection electromagnetic wave 1 A continuous time waveform can be obtained. The delay time td can be adjusted by the height difference H between the high reflective surface 34H and the low reflective surface 34L shown in FIG. 4B.

  As shown in FIG. 10C, the time width of the detection electromagnetic wave 1 that can be detected in one area A0 is extended seven times by connecting the time waveforms of the detection electromagnetic waves 1 detected in the areas A0 to A6. Can do.

  In the first embodiment, the number of y division unit regions 12 (FIG. 2A) is five and the number of x division unit regions 39 (FIG. 8) is four. However, the number of divisions may be other numbers. The number of y-divided unit regions 12 is determined by the number of steps of the adaptive optical element 10 (FIG. 2A). The number of x-divided unit regions 39 is determined by the number of the high reflection surface 34H and the low reflection surface 34L in the beam cross section 38 (FIG. 7A) of the probe wave 2. For example, when it is desired to set the number of x-divided unit regions 39 to 14, the number of the high reflection surface 34H and the low reflection surface 34L of the step mirror 34 is 8 respectively, and the dimension of the beam section 38 in the x direction is set. The total of the high reflection surface 34H and the low reflection surface 34L may be 14 planes.

  In the first embodiment, the x division unit area 39 is divided into seven areas A0 to A6 having different delay times, but the number of sections may be other than seven. For example, when the probe wave 2 is reciprocated seven times between the step mirror 34 and the counter mirror 35 (FIG. 1), the number of regions having different delay times can be made eight.

  In the first embodiment, three or more regions A0 to A6 having different delay times can be provided by using the step mirror 34 having two types of reflection surfaces 34H and 34L having different heights. Three or more regions having different delay times can also be provided by using three or more stages of reflecting mirrors and reflecting the probe wave 2 only once. However, it becomes difficult to manufacture the step mirror as the number of steps increases. The step mirror 34 used in Example 1 is easier to manufacture than a multi-stage mirror having three or more stages.

[Example 2]
FIG. 11 is a sectional view of the step mirror 34 used in the electromagnetic wave imaging apparatus according to the second embodiment. The configuration other than the step mirror 34 is the same as the configuration of the electromagnetic wave imaging apparatus according to the first embodiment.

  Reflective films 41 such as gold (Au), silver (Ag), and dielectric multilayer films are formed on the optically polished end faces of the plurality of glass plates 40. The reflective film 41 becomes a high reflective surface 34H and a low reflective surface 34L. The glass plate 40 that defines the high reflective surface 34H is fixed to the base body 36 so that the height of the reflective film 41 is uniform. The glass plates 40 that define the low reflective surface 34 </ b> L are aligned with each other so that the heights of the reflective films 41 are aligned, and are attached to the mother body 36 via a step adjustment mechanism 37. For the step adjustment mechanism 37, for example, a piezoelectric element is used.

  The step adjustment mechanism 37 can displace the glass plate 40 that defines the low reflective surface 34L in the height direction of the reflective surface. For this reason, the height difference H between the high reflective surface 34H and the low reflective surface 34L can be changed. Thereby, the delay time td between the plurality of areas A0 to A6 having different delay times in the x-divided unit area 39 (FIG. 8) can be changed.

  By changing the delay time td, the measurement conditions of the detection electromagnetic wave 1 can be changed.

[Example 3]
FIG. 12 shows a schematic diagram of an electromagnetic wave imaging apparatus according to the third embodiment. Hereinafter, differences from the first embodiment shown in FIG. 1 will be described, and description of the same configuration will be omitted.

  In the first embodiment, the compensation optical element 10 is disposed on the optical path of the probe wave 2. However, in the third embodiment, the compensation optical element 10 is disposed on the optical path of the detection electromagnetic wave 1. For this reason, the beam cross section of the detection electromagnetic wave 1 is divided into a plurality of y-divided unit regions 12. The probe wave 2 is perpendicularly incident on the electro-optic crystal 3 and the detection electromagnetic wave 1 is obliquely incident. The wavefront of the probe wave 2 is parallel to the surface of the electro-optic crystal 3.

  The wavefront WFt of the detection electromagnetic wave 1 in the y-divided unit region 12 is simultaneously incident on the electro-optic crystal 3 in the same manner as the wavefronts WF1 to WF5 of the probe wave 2 shown in FIG. 2A. That is, the phase shift between the wavefront of the detection electromagnetic wave 1 and the wavefront of the probe wave 2 is compensated at a plurality of locations in the direction of the intersection of the surface of the electro-optic crystal 3 and the yz plane. Therefore, in Example 3, the same measurement as in Example 1 can be performed.

[Example 4]
FIG. 13A shows a schematic diagram of the adaptive optical member 10 used in the electromagnetic wave imaging apparatus according to the fourth embodiment. In the first embodiment, a step-like transparent block is used as the compensation optical member 10, but in the fourth embodiment, a beam splitter 10J and a step-like mirror 10I are used. Other configurations are the same as those of the electromagnetic wave imaging apparatus according to the first embodiment.

  The step-like mirror 10I has a plurality of step-like reflecting surfaces with different heights. For example, a half mirror is used as the beam splitter 10J. A half mirror for a probe wave with a wavelength of 800 nm can be composed of, for example, a thin film of reflective metal such as Al, Ag, or Au formed on a transparent material plate such as glass, or a partially reflecting mirror (stripes, etc.). . The step-like mirror 10I is manufactured by forming a reflective metal mirror on the surface of a structural material that forms a step-like surface. The material of the structural material is not particularly limited as long as it is physically supported and can form a mirror surface.

  A part of the probe wave 2 is reflected by the beam splitter 10J and travels toward the stepped mirror 10I. The probe wave 2 is reflected by each reflecting surface of the stepped mirror 10I and travels toward the beam splitter 10J. The probe wave 2 transmitted through the beam splitter 10J enters the electro-optic crystal 3 (FIG. 1). An optical path length difference corresponding to twice the step of the stepped mirror 10I is generated in the probe wave 2 reflected by the reflecting surfaces adjacent to each other.

  The beam splitter 10J and the stepped mirror 10I shown in FIG. 13A may be arranged at the position of the adaptive optics member 10 according to the third embodiment shown in FIG.

  FIG. 13B shows another configuration example of the adaptive optical member 10. In the adaptive optical member 10 shown in FIG. 13B, stepped mirrors 10M and 10N are used at an oblique incidence. The raised surface is arranged in parallel to the incident wave or reflected wave so that the raised surface of the step does not kick the beam. The case where the incident angle to the reflecting surface is 45 ° and the change in the traveling direction of the electromagnetic wave is 90 ° will be described as an example. In the case of the configuration shown in FIG. 13B, the stepped mirror 10M has a kicked surface parallel to the incident wave, and the stepped mirror 10N has a kicked surface parallel to the reflected wave. Between the plurality of reflected light beams reflected by the plurality of reflecting surfaces of the stepped mirror 10M, gaps are generated corresponding to the depth (height) of the kick-up surface. The gap disappears in the plurality of reflected light beams reflected by the reflecting surfaces of the stepped mirror 10N. A desired optical path length difference (phase difference) is formed between the plurality of light beams reflected by the plurality of reflecting surfaces of the stepped mirrors 10M and 10N.

  Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

1 Electromagnetic wave for detection (terahertz wave)
2 Probe wave 3 Electro-optic crystal 4 Polarizer 5 Analyzer 6 Imaging device 8 Measured object 9 Time delay device 10 Compensation optical element 10A Surface 10B Bottom surface 10C Tread surface 10D Kick-up surface 10I Step-like mirror 10J Beam splitter 10M, 10N Step shape Mirror 12 y-divided unit region 13, 14 Cylindrical lens array 20 Light source 21 Beam splitter 22 Mirror 23 Electro-optic crystal 31, 32 Beam shaper 34 Step mirror 34H High reflection surface 34L Low reflection surface 35 Opposing mirror 36 Base body 37 Step adjustment mechanism 38 Probe wave cross section 39 x division unit area 40 glass plate 41 reflective film 101 terahertz wave 102 probe wave 103 electro-optic crystal 104 polarizer 105 analyzer 106 CCD camera 108, 109 cylindrical lens 110 object to be measured CLx1 to CL x4 x-divided cells CLy1 to CLy5 y-divided cells WF1 to WF5 Wavefront WFt of probe beam unit beam bundle Wavefront in y-divided unit region of electromagnetic wave for detection

Claims (4)

An electro-optic crystal;
A first optical system that irradiates an object to be measured with a pulsed electromagnetic wave for detection and transmits or reflects the electromagnetic wave for detection incident on the electro-optic crystal;
A second optical system that irradiates the electro-optic crystal with the probe wave by inclining the wave front of the pulsed probe wave with respect to the wave front of the electromagnetic wave for detection;
An imaging device that detects the probe wave transmitted through the electro-optic crystal;
One of the first optical system and the second optical system is configured to detect the detection electromagnetic wave or the probe wave in a first virtual plane orthogonal to the wavefront of the detection electromagnetic wave and the wavefront of the probe wave. A compensation optical member that divides a beam cross-section into a plurality of first unit regions and makes an optical path length of a beam passing through the first unit region different for each of the first unit regions, the electro-optic crystal A compensation optical member that compensates for a phase shift between the wavefront of the electromagnetic wave for detection and the wavefront of the probe wave at a plurality of locations in the direction of the intersection of the surface of the first virtual plane and the first virtual plane,
The second optical system has a beam section of the probe wave in a plurality of second unit regions in a second virtual plane orthogonal to the first virtual plane and parallel to the traveling direction of the probe wave. Further comprising a time delay device for dividing each of the second unit regions into a plurality of regions having different delay times of the wavefronts of the probe waves,
The time delay device includes a step mirror in which two kinds of reflection surfaces having different heights are arranged in a first direction, and a counter mirror facing the step mirror, and the probe wave is reflected on the reflection surface of the step mirror. The probe wave is divided into a plurality of regions having different wavefront delay times by reciprocating between the step mirror and the counter mirror a plurality of times while shifting the reflection position in the first direction. Electromagnetic imaging device. The second optical system includes:
A first beam shaper that shapes a beam cross section of the probe wave into a shape that is long in the first direction, and causes the shaped probe wave to enter the step mirror;
The electromagnetic wave imaging apparatus according to claim 1, further comprising: a second beam shaper that expands a beam cross section of the probe wave reciprocating between the step mirror and the counter mirror in a direction orthogonal to the first direction. .   The electromagnetic wave imaging apparatus according to claim 1, wherein the step mirror includes a step adjustment mechanism that adjusts a difference in height between two types of reflection surfaces having different heights.   In the first imaginary plane, a time until one end of the wavefront in the first unit region reaches the electro-optic crystal and the other end reaches the electro-optic crystal is determined by the time delay device. The electromagnetic wave imaging apparatus according to claim 1, wherein the electromagnetic wave imaging apparatus is equal to a step size of a delay time of a wavefront of the probe wave.

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