JP4666619B2 - Terahertz wave imaging device - Google Patents

Description

  The present invention relates to an imaging apparatus using terahertz waves.

  Conventionally, an imaging apparatus using a terahertz wave (an electromagnetic wave having a frequency ranging from about 0.01 THz to 100 THz) is known (see the following non-patent document). An imaging apparatus is used as an inspection apparatus that uses a substance permeability of terahertz waves to determine, for example, a drug (sample) in an envelope without opening it.

The imaging device is an optical system that forms an image of terahertz light from a sample on an electro-optic crystal, and the distribution of birefringence that appears on the electro-optic crystal according to the electric field strength of the terahertz light guided to this optical system. And an CCD camera that captures an image obtained by the electro-optic crystal and the analyzer.
Physics in Medicine and Biology, 47, 3749 (2002)

  Since the optical system of the imaging apparatus described above is configured by arranging two convex lenses side by side, there is a problem described below.

  FIG. 3 is a diagram for explaining the convergence state of the terahertz wave with respect to the electro-optic crystal of the conventional imaging apparatus.

  As shown in the figure, in the case of the optical system of the conventional imaging apparatus, the terahertz wave TW is incident perpendicularly to the incident end face of the electro-optic crystal 111 at the center of the electro-optic crystal 111, but the peripheral edge of the electro-optic crystal 111. Part is incident obliquely on the incident end face of the electro-optic crystal 111 and a propagation loss of the terahertz wave TW occurs. Therefore, there is a difference in the efficiency with which birefringence is induced between the central portion and the peripheral portion of the electro-optic crystal 111, and there is also a difference in the efficiency with which the polarization state of the probe light PB changes. As a result, when the imaging region is expanded, the above-described imaging apparatus causes vignetting in the peripheral portion of the region to be imaged (imaging region) with respect to the sample, and the light amount difference between the central portion and the peripheral portion of the imaging region increases. There is a problem that image quality deteriorates.

  The present invention has been made in view of such circumstances, and is to provide a terahertz wave imaging apparatus capable of obtaining a high-quality image.

In order to solve the above-mentioned problems, the invention described in claim 1 is directed to a first optical system that forms an image of terahertz light from a sample on an electro-optic crystal, and an electric field intensity of the terahertz light guided to the first optical system. Accordingly, a light intensity converting means for converting the distribution state of birefringence appearing on the electro-optic crystal into light intensity, and a second optical system for forming an image obtained by the light intensity converting means on the imaging means. The first optical system and the second optical system are both telecentric optical systems.

  According to the present invention, a high-quality image can be obtained.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a conceptual diagram showing a configuration of a terahertz wave imaging apparatus according to an embodiment of the present invention, and FIG. 2 is a diagram for explaining a convergence state of a terahertz wave with respect to an electro-optic crystal.

This terahertz wave imaging apparatus includes beam splitters 2 and 10, a delay stage 3, beam expanders 4 and 8, a polarizer 9, a terahertz generator 5, and a first telecentric optical system (first optical system). ) 7, an electro-optic crystal 11, an analyzer (light intensity converting means) 12, a second telecentric optical system (second optical system) 13, a CCD (Charge Coupled Device) camera (imaging means) 14, and It has.

  The beam splitter 2 splits the pulsed light P1 from the laser light source 1 into pump light P3 and probe light P2. As the laser light source 1, for example, a femtosecond pulse laser is used.

  The beam splitter 10 transmits the terahertz wave P5 and reflects the probe light P2 divided by the beam splitter 2.

  The delay stage 3 performs time delay of the pump light P3 divided by the beam splitter 2. The delay stage 3 is composed of a movable movable mirror 3a and a fixed planar reflecting mirror 3b. The movable mirror 3a moves as indicated by the arrow to change the optical path length of the pump light P3.

  The terahertz generator 5 generates a terahertz wave by irradiation with the pump light P3. The terahertz generator 5 includes a so-called large-diameter photoconductive antenna in which two electrodes having an antenna shape are provided on a semiconductor substrate made of semi-insulating GaAs or the like, and a bias voltage is applied between these electrodes, or ZnTe Nonlinear optical crystals such as are used.

  The first telecentric optical system 7 is composed of a polyethylene lens, a diaphragm or the like (not shown). The sample 6 is disposed at the front focal position of the first telecentric optical system 7, and the electro-optic crystal 11 is disposed at the rear focal position. As the electro-optic crystal 11, a crystal such as ZnTe is used.

  The second telecentric optical system 13 is composed of a polyethylene lens, a diaphragm or the like (not shown). The electro-optic crystal 11 is disposed at the front focal position of the second telecentric optical system 13, and the CCD camera 14 is disposed at the rear focal position.

  The beam expander 4 has a concave lens 4A and a convex lens 4B, and widens the beam diameter of the pump light P3.

  The beam expander 8 has a concave lens 8A and a convex lens 8B, and widens the beam diameter of the probe light P2.

  The polarizer 9 is disposed between the beam expander 8 and the beam splitter 10. The polarizer 9 makes the probe light P2 linearly polarized light.

  The analyzer 12 is disposed between the electro-optic crystal 11 and the second telecentric optical system 13. The analyzer 12 converts the change in the polarization state of the probe light P2 into light intensity. The analyzer 12 is arranged orthogonally with respect to the polarizer 9 and is arranged so that only the component whose polarization has changed is imaged in the light receiving region of the CCD camera 14.

  The CCD camera 14 images the change in polarization state as the intensity of light.

  The pulsed light P1 emitted from the laser light source 1 is split into two pulsed lights by the beam splitter 2 through the plane reflecting mirror M1. The pulsed light P1 is linearly polarized pulsed light having a center wavelength of about 780 to 800 nm in the near infrared region, a repetition period of about several kHz to about 100 MHz, and a pulse width of about 10 to 150 fs.

  One pulsed light split by the beam splitter 2 becomes pump light P3 for exciting the terahertz generator 5 to generate a terahertz wave. The pump light P3 is guided to the terahertz generator 5 through the plane reflecting mirror M2, the delay stage 3, the beam expander 4, and the plane reflecting mirror M3. When the terahertz generator 5 is irradiated with pulsed light of about 100 femtoseconds, a terahertz wave P4 having a frequency in the terahertz region is emitted from the terahertz generator 5.

The terahertz wave P4 is an electromagnetic wave in a frequency range from about 0.01 × 10 ¹² to 100 × 10 ¹² hertz. The terahertz wave P4 generated by the terahertz generator 5 becomes parallel light and is irradiated on the sample 6.

  The terahertz wave P5 that has passed through the sample 6 forms an image of the sample 6 on the electro-optic crystal 11 via the first telecentric optical system 7 and the beam splitter 10.

  At this time, as shown in FIG. 2, the terahertz wave P <b> 5 indicated by the solid line is perpendicularly incident on the incident end face of the electro-optic crystal 11 not only at the central portion but also at the peripheral portion of the electro-optic crystal 11.

  The other pulsed light split by the beam splitter 2 becomes probe light P2, and enters the beam expander 8 via the plane reflecting mirrors M4, M5, and M6. The beam expander 8 widens the beam diameter of the probe light P2, and the polarizer 9 turns the probe light P2 into linearly polarized light. The probe light P2 is reflected by the beam splitter 10 and guided to the same propagation path as the terahertz wave P5, and irradiates the electro-optic crystal 11.

  At this time, as shown in FIG. 2, the probe light P <b> 2 indicated by the dotted line is incident perpendicularly to the incident end face of the electro-optic crystal 11 not only in the central portion but also in the peripheral portion.

  Therefore, the same degree of birefringence is induced in both the central portion and the peripheral portion of the electro-optic crystal 11 (Pockels effect).

  When the probe light P2 enters the electro-optic crystal 11, a phase change occurs due to birefringence induced by the electric field of the terahertz wave P5. The probe light P2 passes through the electro-optic crystal 11 and changes its polarization state to become elliptically polarized light.

  The change in the polarization state of the probe light P2 is converted into a light intensity level by the analyzer 12. Only the component whose polarization state has changed is imaged on the light receiving area of the CCD camera 14 by the second telecentric optical system 13.

  At this time, at least the size of the field of view of the second telecentric optical system 13 and the size of the beam diameter of the probe light P2 need to be larger than the size of the sample image formed by the first telecentric optical system 7, respectively. . It is preferable that the size of the sample image formed by the first telecentric optical system 7, the size of the field of view of the second telecentric optical system 13, and the size of the beam diameter of the probe light P2 are equal.

According to this embodiment, since the second telecentric optical system 13 is adopted as the second optical system, there is no vignetting at the peripheral portion of the photographing region, and the light quality difference between the central portion and the peripheral portion of the photographing region is small. Thus, an image with a wide field of view can be obtained. In addition, an image in the terahertz wave region can be visualized using a camera that is not sensitive to terahertz waves. Furthermore, since the first telecentric optical system 7 is employed as the first optical system, a higher quality image can be obtained.

  In the above embodiment, the CCD camera 14 is used. However, a CMOS (Complementary Metal Oxide Semiconductor) camera may be used instead of the CCD camera 14.

  In the above embodiment, the first optical system and the second optical system are telecentric optical systems, but only the second optical system may be a telecentric optical system.

  Furthermore, although the case where the transmission optical system was used as the first optical system has been described in the above embodiment, a reflection optical system may be used instead of the transmission optical system.

FIG. 1 is a conceptual diagram showing a configuration of a terahertz wave imaging apparatus according to an embodiment of the present invention. FIG. 2 is a diagram for explaining the convergence state of the terahertz wave with respect to the electro-optic crystal. FIG. 3 is a diagram for explaining the convergence state of the terahertz wave with respect to the electro-optic crystal of the conventional imaging apparatus.

Explanation of symbols

6: sample, 7: first telecentric optical system (first optical system), 11: electro-optic crystal, 12: analyzer (light intensity converting means), 13: second telecentric optical system (second optical system) System), 14: CCD camera (imaging means).

Claims (1)

A first optical system for imaging terahertz light from a sample on an electro-optic crystal;
A light intensity converting means for converting the distribution state of birefringence appearing on the electro-optic crystal into light intensity according to the electric field intensity of the terahertz light guided to the first optical system;
A second optical system for forming an image obtained by the light intensity conversion unit on the imaging unit,
The first optical system and the second optical system are both telecentric optical systems.

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