JP2011117957A - Terahertz wave imaging device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a terahertz imaging device capable of moving a terahertz source generating a terahertz wave having a variable frequency to the periphery of a measuring object, and measuring the terahertz wave. <P>SOLUTION: In this terahertz imaging device 100, a sensor head part 120 is separated from a body part 110 and connected through a composite cable 130, and only the sensor head part 120 can be moved and used. The body part 110 combines each laser light having each different wavelength emitted from two wavelength variable lasers 111, 112 by an optical coupler 115, and transmits the combined wave to the sensor head part 120 via an optical waveguide 131. The sensor head part 120 allows the transmitted combined wave to enter the terahertz source 121, and generates a terahertz wave having a frequency equivalent to a beat component of the combined wave. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a terahertz imaging apparatus for measuring characteristics of a measured object from a transmitted wave or a reflected wave when the measured object is irradiated with a terahertz wave, and in particular, a terahertz wave corresponding to a difference frequency component from a plurality of laser light sources. The present invention relates to a terahertz imaging apparatus that measures the characteristics of an object to be measured.

  In recent years, the possibility of a new analysis technique using terahertz waves has attracted attention. The terahertz wave is an electromagnetic wave having a wavelength positioned in a boundary region between the microwave and the light wave, and is a radio wave having a wavelength of about 300 μm to 3 mm. Reagents, biopolymers, and the like have a characteristic of showing absorption spectra specific to terahertz waves. In addition, terahertz waves are difficult for conventional infrared spectroscopy because they transmit non-metal and non-polar materials such as paper, plastic, vinyl, ceramic, wood, bone, teeth, fat, powder, and dried food. It is expected that identification and non-destructive inspection of materials will be realized using terahertz waves.

  In general, the generation and detection technology of terahertz waves is based on the characteristics of the generated terahertz waves: (1) a technology targeting terahertz having an impulsive property on the time axis (a method using a terahertz wave controlled in the time domain) and (2) A technique for a terahertz having a specific frequency (method using a terahertz wave controlled in a frequency domain) can be classified. The technique (1) is often applied to spectroscopy and the like utilizing its broadband and high resolution, and the technique (2) is often applied to imaging and the like utilizing the response characteristics of the inherent frequency components.

  For each of the techniques (1) and (2), a terahertz wave generation method and a detection method have been studied so far (for example, Patent Document 1). In particular, in the technique (2), generally, two laser beams separated by a frequency corresponding to a desired terahertz (THz) are superimposed, and this is superimposed on a nonlinear crystal (GaP, DAST, KTP, etc.) called a photoconductive antenna. This technique is known as a difference frequency mixing technique that generates a desired terahertz beat component as a terahertz wave by irradiation.

  A configuration example of a conventional imaging apparatus using terahertz waves is shown in FIG. A Cr: Forsterite laser 901 is used for the two laser light sources, and a light wave generated from the laser light source is guided to the optical mixer 902 by an optical system component such as a mirror, and a terahertz source 903 formed based on GaP. Is irradiated. The terahertz source 903 converts the beat component of the light wave in which two different wavelengths overlap into a terahertz wave and emits it. The terahertz wave emitted from the terahertz source 903 is applied to the sample 904, and the terahertz wave that has passed through the sample 904 is detected by the detector 905.

JP 2006-313803 A

  In a conventional imaging apparatus using terahertz waves as illustrated in FIG. 15, an expensive laser light source having extremely high stability is used to generate a terahertz wave having a stable frequency. Optical parts are positioned with high accuracy and guided to the terahertz source. That is, the laser light source to the terahertz source are integrally configured, and it is necessary to strictly manage the positional accuracy of the optical system components and the like. In addition, there is a problem that the apparatus is enlarged. For this reason, places where measurement is possible are limited, and it is a heavy burden to position and arrange the laser light source to the terahertz source integrally with respect to the object.

  Further, such a laser light source has a problem that it is difficult to vary the output wavelength, and the frequency of the terahertz wave that can be generated is limited. Due to such problems, application of terahertz imaging using terahertz waves has been greatly restricted.

  The present invention has been made to solve the above problem, and provides a terahertz imaging apparatus capable of measuring by moving a terahertz source that generates a terahertz wave having a variable frequency to the vicinity of the object to be measured. With the goal.

  According to a first aspect of the terahertz wave imaging apparatus of the present invention, there are provided two or more wavelength tunable lasers having different oscillation wavelengths, and an optical coupler that synthesizes laser beams output from the wavelength tunable laser and outputs a synthesized wave. An optical waveguide for propagating a synthetic wave output from the optical coupler; and a terahertz source for outputting a terahertz wave having a frequency corresponding to a beat component of the synthetic wave when the synthetic wave is irradiated from the optical waveguide. And generating a terahertz wave having a predetermined frequency by irradiating the terahertz source with high accuracy through the optical waveguide with the synthesized wave obtained by synthesizing the laser beams output from the wavelength tunable laser.

  In another aspect of the terahertz wave imaging apparatus of the present invention, the two or more wavelength tunable lasers and the optical coupler are provided in a main body part, the terahertz source is provided in a sensor head part, and the sensor head part includes the sensor head part, It is configured to be movable independently from the main body connected by an optical waveguide.

  According to the present invention, it is possible to perform measurement by moving the sensor head portion including the terahertz source to the vicinity of the object to be measured. Further, by using a plurality of wavelength tunable lasers as light sources, it is possible to change the difference in output wavelength of each light source in time series, and the frequency of the terahertz wave to be generated can be made variable.

  Another aspect of the terahertz wave imaging apparatus of the present invention further includes an LD control circuit that adjusts the wavelength of the output laser light by controlling the temperature of the wavelength tunable laser, and the LD control circuit includes the two or more wavelength tunable circuits. The frequency of the terahertz wave output from the terahertz source is adjusted in time series by adjusting the wavelength of the laser beam by controlling the temperature of at least one of the lasers.

  In another aspect of the terahertz wave imaging apparatus of the present invention, the LD control circuit includes a TEC (Thermo Electric Cooler) that controls the temperature of the wavelength tunable laser, and a part of the laser light output from the wavelength tunable laser. A photodiode (PD) that receives light, an etalon filter of the same number as the wavelength tunable laser that allows only a portion of the synthesized wave to enter and passes a laser beam of a predetermined frequency, and a laser beam of the predetermined frequency from the etalon filter TEC for controlling the TEC so that the output intensity and wavelength of the laser beam are stably set to predetermined values by inputting another measurement result from another photodiode that receives light and the measurement result from the photodiode and the other photodiode. And a driving circuit.

  In another aspect of the terahertz wave imaging apparatus of the present invention, the terahertz source has an impurity level corresponding to an output wavelength of laser light output from the wavelength tunable laser, and has a short carrier with respect to the output terahertz wave. A substrate made of a semiconductor material having a long life and high mobility is provided.

In another aspect of the terahertz wave imaging apparatus of the present invention, the wavelength tunable laser outputs a laser beam having a wavelength of 1.5 μm, and the terahertz source includes a GaN substrate.
By applying GaN to the terahertz source material, a highly reliable terahertz source can be realized even in a high withstand voltage environment.

  In another aspect of the terahertz wave imaging apparatus of the present invention, the wavelength tunable laser outputs a laser beam having a wavelength of 1.3 μm or 1.5 μm, and the terahertz source is from InGaAsP / InP or InGaAs / AlGaAs. It comprises the board | substrate which becomes.

  In another aspect of the terahertz wave imaging apparatus of the present invention, the wavelength tunable laser outputs a laser beam having a wavelength of 650 nm to 1.5 um, and the terahertz source is a II-VI group semiconductor or a III-V group semiconductor. It comprises the board | substrate which consists of.

  In another aspect of the terahertz wave imaging apparatus of the present invention, the wavelength tunable laser outputs laser light having a wavelength of 650 nm to 1.5 μm, and the terahertz source is a metal-doped II-VI group semiconductor or metal And a substrate made of a III-V group semiconductor doped with.

  In another aspect of the terahertz wave imaging apparatus according to the present invention, the wavelength tunable laser outputs a laser beam having a wavelength of 0.98 um, and the terahertz source includes a substrate made of GaAs.

  Another aspect of the terahertz wave imaging apparatus of the present invention is characterized in that the optical waveguide has a polarization plane adjusting means for rotating or holding the polarization plane of the propagating laser beam.

  Another aspect of the terahertz wave imaging apparatus of the present invention further includes a detection unit including an antenna having sensitivity in a predetermined frequency band of the terahertz wave, and a broadband detection circuit that detects a signal received by the antenna, The detection unit receives a reflection signal or a transmission signal when the object to be measured is irradiated with the terahertz wave output from the terahertz source.

  Another aspect of the terahertz wave imaging apparatus of the present invention further includes a detection unit having a terahertz camera having sensitivity in a predetermined frequency band of the terahertz wave, and irradiates the object to be measured with the terahertz wave output from the terahertz source The reflected signal or transmitted signal is received by the detection unit.

  According to another aspect of the terahertz wave imaging apparatus of the present invention, a wavelength detection unit that receives a part of the terahertz wave output from the terahertz source and converts it to a voltage value, and inputs the voltage value from the wavelength detection unit. Voltage calibration means for estimating the wavelength of the terahertz wave from the voltage value using a calibration table stored in advance and outputting the estimated wavelength to the LD control circuit, the LD control circuit from the voltage calibration means When the wavelength of the terahertz wave is input, the output of the wavelength tunable laser is controlled so that the wavelength of the terahertz wave matches a predetermined target wavelength.

  In another aspect of the terahertz wave imaging apparatus of the present invention, the terahertz source includes two transmission lines arranged in parallel, and a protrusion formed at substantially the center of each of the transmission lines. The tip of the part is formed in a concave curved shape.

  Another aspect of the terahertz wave imaging apparatus of the present invention includes two or more light source units each including the two or more wavelength tunable lasers and the optical coupler, and each of the two or more light source units and the optical waveguide. A combined unit that inputs and combines the respective combined waves output from the optical coupler connected to each other, and another optical waveguide that irradiates the terahertz source with the mixed wave combined by the combined unit It is characterized by providing.

  In another aspect of the terahertz wave imaging apparatus of the present invention, the two or more wavelength tunable lasers emit laser light having the same oscillation wavelength in each of the two or more light source units.

  Another aspect of the terahertz wave imaging apparatus of the present invention is characterized in that the two or more wavelength tunable lasers emit laser beams having the same frequency difference and different oscillation wavelengths in each of the two or more light source units.

  Another aspect of the terahertz wave imaging apparatus of the present invention is characterized in that the multiplexing unit is formed using a PLC (Planar LIghtwave Circuit).

  In another aspect of the terahertz wave imaging apparatus of the present invention, the PLC includes a phase adjustment unit that aligns the phases of the combined waves input from the two or more optical waveguides.

  In another aspect of the terahertz wave imaging apparatus according to the present invention, the PLC includes a dispersion compensation unit that performs dispersion compensation on the combined wave input from the two or more light source units.

  Another aspect of the terahertz wave imaging apparatus of the present invention includes another phase adjustment unit that aligns the phases of the combined waves input from the two or more light source units and outputs each to the two or more optical waveguides. It is characterized by.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the terahertz imaging device which can measure by moving the terahertz source which generates the terahertz wave with a variable frequency to the vicinity of a to-be-measured object. In the present invention, the main body portion including the laser light source and the sensor head portion including the terahertz source are separated from each other, whereby the terahertz wave can be generated by moving the sensor head portion to the vicinity of the object to be measured. Further, by using two or more wavelength tunable lasers as light sources, a terahertz wave having a variable frequency can be generated.

1 is a block diagram illustrating an overall configuration of a terahertz imaging apparatus according to a first embodiment of the present invention. It is a block diagram of the main-body part which shows the detailed structure of LD control circuit. It is a block diagram which shows the structure of a wavelength variable light source. It is a wavelength discrimination curve of a wavelength variable light source. It is a flowchart which shows a process when the wavelength change command and the optical output power change command are received in the optical output off state. It is a flowchart which shows a process when the optical output power change command is received in the optical output on state. It is a perspective view of a terahertz source and a detection part. It is a graph which shows an example which adjusts the frequency of a terahertz wave in time series. It is a graph which shows another example which adjusts the frequency of a terahertz wave in time series. It is a graph which shows typically the frequency response of a to-be-measured object. It is a block diagram which shows the structure of the sensor head part which detects the reflected wave of a terahertz wave. It is a block diagram which shows the whole structure of the terahertz imaging device which concerns on 2nd Embodiment of this invention. It is a block diagram which shows the whole structure of the terahertz imaging device which concerns on 3rd Embodiment of this invention. It is a top view of the antenna with which the terahertz source of the terahertz imaging device which concerns on 4th Embodiment of this invention was equipped. It is a block diagram which shows the structural example of the conventional detection apparatus using a terahertz wave. It is a block diagram which shows the whole structure of the terahertz imaging device which concerns on 5th Embodiment of this invention. It is a perspective view which shows an example of PLC provided with the dispersion compensation part. It is a block diagram which shows the whole structure of the terahertz imaging device which concerns on 6th Embodiment of this invention.

  A configuration of a terahertz wave imaging apparatus according to a preferred embodiment of the present invention will be described in detail below with reference to the drawings. In addition, about each structural part which has the same function, the same code | symbol is attached | subjected and shown for simplification of illustration and description.

(First embodiment)
A preferred first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing the overall configuration of the terahertz imaging apparatus of the present embodiment. The terahertz imaging apparatus 100 according to the present embodiment includes a main body 110, a sensor head 120, and a composite cable 130 that connects the two. In this embodiment, the sensor head part 120 is separated from the main body part 110 and connected by the composite cable 130, and only the sensor head part 120 can be moved and used.

  The main body 110 controls the two wavelength tunable lasers 111 and 112, semiconductor optical amplifiers (SOA) 113 and 114 that amplify the laser beams from the wavelength tunable lasers 111 and 112, an optical coupler 115, and the wavelength tunable lasers 111 and 112. An LD control circuit 116, an analog circuit 117, and a data processing circuit 118 are provided.

  The two wavelength tunable lasers 111 and 112 emit laser beams having different wavelengths, and the two laser beams are amplified by the SOAs 113 and 114, respectively, and then combined by the optical coupler 115. The synthesized wave synthesized by the optical coupler 115 is propagated to the sensor head 120 via the composite cable 130. In the present embodiment, two tunable lasers 111 and 112 are used, but the present invention is not limited to this, and three or more tunable lasers may be provided.

  The composite cable 130 is processed by the optical waveguide 131 that propagates the combined wave synthesized by the optical coupler 115 to the sensor head unit 120, the power cable 132 that supplies power necessary for the sensor head unit 120, and the sensor head unit 120. A signal line 133 for transmitting the received signal to the data processing circuit 118 of the main body 110. Here, an optical fiber is used as the optical waveguide 131.

  The sensor head unit 120 outputs a terahertz wave 121 having a frequency corresponding to a beat component of the synthesized wave when irradiated with a synthesized wave propagated from the optical coupler 115 of the main body unit 110 via the optical waveguide 131. A lens 122 for suitably irradiating the device under test 10 with the terahertz wave output from the terahertz source 121, a detection unit 123 for detecting the terahertz wave transmitted through the device under test 10, and the sample 10 through the device under test 10. The lens 124 for making the terahertz wave suitably incident on the detection unit 123 is provided. Here, hemispherical lenses are used for the lenses 123 and 124.

となる。 The terahertz imaging apparatus 100 of the present embodiment uses two DFB lasers with different output wavelengths for the wavelength tunable lasers 111 and 112, and synthesizes the laser light emitted from each of them to irradiate the terahertz source 121. A terahertz wave is generated from the terahertz source 121. When the output frequencies of the wavelength tunable lasers 111 and 112 are ω1 and ω2, respectively, the frequency ωT of the terahertz wave generated by the terahertz source 121 is

It becomes.

  As described above, in order to generate a terahertz wave having a desired frequency ωT, the frequencies ω1 and ω2 of the two DFB lasers used for the wavelength tunable lasers 111 and 112 may be suitably selected. The terahertz imaging apparatus 100 of the present embodiment is configured such that the frequency of the terahertz wave is variable, and the two DFB lasers used for the wavelength tunable lasers 111 and 112 can be adjusted with the temperature dependence of the output wavelength. The range is selected.

  As the wavelength tunable lasers 111 and 112, for example, 1.5 μm band optical communication DFB lasers can be used. Alternatively, a 0.98 μm band optical fiber amplifier single mode pump laser or a 650 to 1.5 μm band commercially available infrared laser may be used. When a DFB laser for optical communication in the 1.5 μm band is used, it is possible to acquire a clear image of the object to be measured 10 and to perform good identification.

  As an example, when the frequency of the terahertz wave to be generated is 1 to 2 THz, a DFB laser having a wavelength of 1.52955 μm (when the temperature is 25 ° C.) is used for the wavelength tunable laser 111, and a wavelength of 1.54135 μm ( DFB laser (when the temperature is 25 ° C.) can be used. At this time, by fixing the wavelength of the wavelength tunable laser 111 and adjusting the wavelength of the wavelength tunable laser 112 in a range of approximately 1.5374 to 1.54532 μm, a terahertz wave having a predetermined frequency can be generated.

  The LD control circuit 116 provided in the main body 110 has a function of controlling the temperature of the wavelength tunable lasers 111 and 112, and outputs the output wavelength by controlling the temperature of at least one of the wavelength tunable lasers 111 and 112. Is controlling. The analog circuit 117 supplies necessary power to the sensor head unit 120 via the power cable 132 in the composite cable 130. Further, the data processing circuit 118 inputs data detected by the detection unit 123 of the sensor head unit 120 via the signal line 133 in the composite cable 130, and performs an imaging process or the like of the device under test 10.

  A detailed configuration of the LD control circuit 116 will be described with reference to FIG. FIG. 2 is a block diagram of the main body 110 showing a detailed configuration of the LD control circuit 116. The LD control circuit 116 includes heating and cooling elements (TEC: Thermo Electric Coolers) 141 and 142 that control the temperatures of the wavelength tunable lasers 111 and 112, and TEC drive circuits 143 and 144 that control the TECs 141 and 142, respectively. .

  Also, light receivers (PD) 145 and 146 for monitoring the output power of the laser light from the wavelength variable lasers 111 and 112, PDs 147 and 148 for monitoring the output wavelength of the laser light from the wavelength variable lasers 111 and 112, and inputs Etalon filters 149 and 150 that pass only light of a predetermined frequency from the light are provided. The etalon filters 149 and 150 are provided with TECs 151 and 152, respectively. In order to monitor the laser beam with the PDs 145 to 148, an optical branching element that branches a part of the laser beam is appropriately used.

  The PDs 145 and 146 receive a part of the laser beams output from the wavelength tunable lasers 111 and 112, respectively, branched by an optical branching element, and measure respective output powers (intensity levels). The measurement results are transmitted to the TEC drive circuits 143 and 144, respectively. Further, a part of the combined wave output from the optical coupler 115 is branched by an optical branching element, and is incident on the PDs 147 and 148 via the etalon filters 149 and 150. The PDs 147 and 148 measure the intensity of laser light having a predetermined frequency incident via the etalon filters 149 and 150, and the measurement results are transmitted to the TEC drive circuits 143 and 144, respectively.

  The etalon filters 149 and 150 pass only light of a predetermined frequency band, and are adjusted so as to match the frequency bands of the laser beams output from the wavelength variable lasers 111 and 112, respectively. The pass frequency bands of the etalon filters 149 and 150 can be adjusted by temperature control using the TECs 151 and 152 provided respectively. Accordingly, the PDs 147 and 148 can measure the intensity of laser light having a predetermined frequency output from the wavelength tunable lasers 111 and 112, and can monitor the output wavelengths (output frequencies) of the wavelength tunable lasers 111 and 112. it can.

  The TEC drive circuits 143 and 144 receive the respective measurement results from the PDs 145 to 148, and the respective frequencies of the laser beams output from the wavelength tunable lasers 111 and 112 stably match the desired frequencies. The TECs 141 and 142 are controlled so that the output powers match. In the present embodiment, SOAs 113 and 114, TECs 141 and 142, and TEC drive circuits 143 and 144 are integrated with the wavelength tunable lasers 111 and 112 to form a module, so that a laser light source unit that has been extremely expensive in the past can be obtained. It is possible to reduce the size and cost. Further, in the modularization, the thermal isolation between the TECs 141 and 142 is ensured.

  An example of a wavelength tunable light source used in the wavelength tunable lasers 111 and 112 and the LD control circuit 116 according to this embodiment will be described in detail with reference to the drawings. Hereinafter, the wavelength tunable laser 111 will be described, but the wavelength tunable laser 112 may have the same configuration. FIG. 3 is a block diagram showing a configuration of the wavelength tunable light source 20 when a temperature-adjustable semiconductor laser (DFB laser) is used for the wavelength tunable laser 111. A semiconductor laser (LD) 21 (corresponding to the wavelength tunable laser 111) and an SOA 22 (corresponding to the SOA 113) are mounted on a TEC 23 to which a TEC temperature detector 24 is attached. Two optical branch elements 25 and 26 are arranged in the laser light output path. The branched light from the optical branching element 25 is incident on the first PD 28 (corresponding to PD 147) via a wavelength filter 27 (corresponding to etalon filter 149) such as an etalon filter. The branched light from the optical branching element 26 is directly incident on the second PD 29 (corresponding to the PD 145). The above components are all housed in the same module as shown by the broken line in FIG.

  The output analog value of the LD drive current monitor circuit 31 is acquired by the CPU 30 as an LD current monitor value via an A / D converter (ADC) 32. Based on the difference information between the LD current monitor value and the predetermined LD current target value, the CPU 30 updates the LD current control instruction value and outputs it to the LD current drive circuit 34 via the D / A converter (DAC) 33. . In the LD current drive circuit 34, automatic current control (ACC: Automatic Current Control) of the LD 21 is realized based on the LD current control instruction value.

  The output analog value of the second PD monitor circuit 35 is taken into the CPU 30 as a current monitor value corresponding to the received light power of the second PD 29 via the ADC 36. Based on the difference information between the current target value corresponding to the optical output power target value and the current monitor value of the second PD 29, the SOA current control instruction value is updated and output to the SOA current drive circuit 38 via the DAC 37. In the SOA current drive circuit 38, automatic light output control (APC: Automatic Power Control) of the SOA 22 is realized based on the SOA current control instruction value. Since the SOA current monitor circuit 39 is mounted, a method of using the automatic current control (ACC) of the SOA 22 together or a method of selecting either APC or ACC may be used.

  The output analog value of the TEC temperature monitor circuit 41 is taken into the CPU 30 via the ADC 42 as a TEC temperature monitor value. The TEC temperature control instruction value is updated based on the difference information between the TEC temperature target value and the TEC temperature monitor value, and is output to the TEC drive circuit 44 (corresponding to the TEC drive circuit 143) via the DAC 43. In the TEC drive circuit 44, automatic temperature control (ATC: Automatic Temperature Control) of the TEC 23 is realized based on the TEC temperature control instruction value.

  The output analog values of the first PD monitor circuit 45 and the second PD monitor circuit 35 are taken into the CPU 30 as the first PD current monitor value and the second PD current monitor value via the ADCs 46 and 36, respectively, and the PD current ratio monitor value (first PD Current monitor value / second PD current monitor value) is derived. Based on the difference information between the PD current ratio target value corresponding to the wavelength target value and the PD current ratio monitor value, the wavelength control instruction value is updated and output to the TEC drive circuit 44 via the DAC 43. The TEC drive circuit 44 implements automatic wavelength control (AWC) based on the wavelength control instruction value. There is a periodic relationship as shown in FIG. 4 between the wavelength and the PD current ratio (first PD current monitor value / second PD current monitor value). This curve is generally called a wavelength discrimination curve, and converges to a PD current ratio target value determined in advance for each wavelength.

  Both the ATC and the AWC update the control instruction value of the TEC drive circuit 44. However, the ATC and the AWC may be used together, or either one may be selected.

  The stable states that can be shifted after the initialization of the wavelength variable light source 20 are the light output off state and the light output on state. The light output off state means a state in which light output to the outside as a wavelength variable light source can be ignored. If this is guaranteed, both the ATC of the TEC 23 and the ACC of the LD 21 may be operated, or only the ATC of the TEC 23 may be operated. It is not always necessary to operate both the ATC of the TEC 23 and the ACC of the LD 21. The optical output on state means a state in which a desired wavelength designated by the host device and a desired optical output power can be output.

  When the wavelength change command and the optical output power change command are received in the optical output off state, target values for various controls are determined according to the flowchart of the target value determination process shown in FIG. That is, the AWC target value (first PD current / second PD current) corresponding to the combination of the target wavelength and target optical output power at that time, the determination of the APC target value (second PD current), and the ACC target corresponding to the target wavelength The value (LD current) and ATC target value (TEC temperature) are determined. Although four target values are determined, there is no problem even if the order is changed.

  Here, even when an optical output power change command is received, four target values are changed, but only two target values related to the target optical output power may be updated. That is, it is only necessary to determine the AWC target value (first PD current / second PD current) and the APC target value (second PD current) corresponding to the combination of the target wavelength and the target optical output power at that time.

  When an optical output on command is received in the optical output off state, the optical output is turned on according to the following procedure, and target values for various controls corresponding to the target wavelength and target optical output power at that time are determined. If it is already executed in the optical output off state, it is not always necessary. Subsequently, the ATC and ACC control processes are performed so that the difference between the ATC target value and the TEC temperature monitor value and the difference between the ACC target value and the LD current monitor value converge below the allowable values. At this time, if the ATC and ACC are already in operation with the light output off, the automatic control is continued by changing the respective target values.

  After confirming that the target values have been converged in the ATC and ACC control processes, the APC and AWC control processes are started. Each of the difference between the ATC target value and the TEC temperature monitor value, the ACC target value and the LD current monitor value, the difference between the APC target value and the second PD current monitor value, and the difference between the AWC target value and the PD current ratio monitor value ATC, ACC, APC, and AWC are controlled so as to converge below the allowable value. If all the convergence conditions can be satisfied, the light output is turned on.

  Here, the APC and the AWC are operated simultaneously, but the AWC may be operated after the APC converges. Further, although the ATC and the AWC are operated at the same time, the ATC may be stopped when the AWC is operated.

  On the other hand, when a wavelength change command and an optical output power change command are received while the optical output is on, the optical output is turned on according to the same procedure as when the optical output on command is received when the optical output is off. Transition. However, the point that the APC and AWC control processes are stopped in this order before the target value determination process is executed is different from the case where the light output on command is received in the light output off state. This is a measure for suppressing emission of signal light other than the designated wavelength that cannot be shared. If the optical output level cannot be ignored even in this case, the ACC control process is also stopped before the AWC is stopped.

  Here, if the fluctuation amount of the output wavelength in the process of changing the optical output power is an operationally acceptable level, when the optical output power change command is received in the optical output on state, the above APC and AWC control processing is performed. The target value determination process may be executed without stopping. That is, the control process follows the follow chart shown in FIG.

  When the optical output off command is received in the optical output on state, the APC and AWC control processes are stopped in this order, and the optical output is turned off. This is a measure for preventing signal light having a wavelength other than the desired wavelength from being output to the outside during the transition to the optical output off state. If the optical output level cannot be ignored simply by stopping the APC, the ACC of the LD 21 is also stopped before stopping the AWC.

  As described above, the target values of various control processes, that is, the AWC target value, the APC target value, the ACC target value, and the ATC target value are determined by the combination of the target wavelength and the target optical output power. The influence of the heat generation amount and scattered light generated inside can be suppressed, and the setting accuracy and stability of the output wavelength and optical output power required for the wavelength variable light source can be ensured. The target value determination process described above does not change its effect even if it is adjusted as follows according to the control method employed.

When the APC control using the second PD light receiving power itself as the target value is adopted instead of the APC targeting the second PD current value corresponding to the light receiving power of the second PD 29, the second PD current value is converted into the second PD light receiving power. This can be done by updating the coefficient α (λ, Pow). This conversion coefficient α (λ, Pow) is uniquely determined by the combination of the target wavelength λ and the target optical output power Pow at that time, as shown in Expression (1). The conversion coefficient α (λ, Pow) is tabulated and stored in the memory 47, and can be dealt with by a selection method or a complementary method depending on the combination of the target wavelength and the target optical output power.
Second PD light receiving power = α (λ, Pow) × second PD current (1)

  Further, the PD current value (first PD current, second PD current) and the TEC temperature corresponding to each PD light receiving power may vary depending on the operating environment temperature. In this case, the coefficients A (T) and B (T) for converting the ADC value to the PD current, and the coefficients C (T) and D (T) for correcting the TEC target temperature are set as the environmental temperature T (case temperature) at that time. : Refer to FIG. 3) By adopting the correction method, the environmental temperature dependency can be suppressed. As a result, the wavelength variable light source can be stably operated with high accuracy.

The conversion coefficients A (T), B (T), C (T), and D (T) are uniquely determined by the environmental temperature T at that time, as shown in the following equations (2) and (3). Is. The conversion coefficients A (T), B (T), C (T), and D (T) are tabulated and stored in the memory 47, and can be handled by a method of selection or a method of complementing depending on the environmental temperature T. When the relationship between the ADC value and the physical value is non-linear, the conversion itself may be handled by a table reference method.
Second PD current = A (T) × ADC value + B (T) (2)
TEC temperature = C (T) × ADC value + D (T) (3)

  Further, in the case of a control method that employs the expression (1) as a process for converting the second PD current to the second PD light receiving power, the target wavelength dependence of the coefficient β (λ) for converting the second PD current to the second PD light receiving power is determined. And a mechanism for compensating the dependency of the coefficient γ (Pow) for correcting the second PD light receiving power on the target light output power. As a result, the variable wavelength light source can be stably operated with high accuracy.

As shown in the following equation (4), this conversion coefficient is uniquely determined by the target wavelength and the target optical output power at that time. The conversion coefficient β (λ) and the correction coefficient γ (Pow) are tabulated and stored in the memory 47, and can be handled by a selection method or a complementary method depending on the combination of the target wavelength and the target light output power. The conversion coefficient β (λ) and the correction coefficient γ (Pow) in Expression (4) are obtained by dividing the conversion coefficient α (λ, Pow) in Expression (1) by paying attention to the independence of λ and Pow. When this method can be used, since the capacity of the conversion coefficients and correction coefficients tabulated can be compressed, it is more efficient than the method of equation (1).
Second PD light receiving power = β (λ) × γ (Pow) × second PD current (4)

  As described above, a highly accurate and stable operation as a wavelength variable light source can be ensured by appropriately adjusting the target value determination process in accordance with the control method of the wavelength variable light source. In short, it is only necessary to adjust the target value of various controls, the conversion coefficient and the correction coefficient of the monitor value in accordance with the combination of the target wavelength and the target optical output power at that time.

  Furthermore, if the influence of the environmental temperature cannot be ignored, the target value for each control, the conversion value and the correction coefficient for the monitor value can be set according to the environmental temperature in addition to the target wavelength and target optical output power at that time. By adjusting, it is possible to realize a more accurate and stable wavelength variable light source.

  In the example of the wavelength tunable light source described above, the semiconductor laser has been described by exemplifying a temperature adjustment type semiconductor laser (DFB laser). However, the present invention is not limited to this, and may be replaced with a current control type semiconductor laser. However, the current control type semiconductor laser (DBR laser) and the temperature adjustment type semiconductor laser (DFB laser) are different in the following points. In the case of a temperature adjustment type semiconductor laser (DFB laser), AWC is realized by updating the control instruction value of the TEC drive circuit. In the case of a current control type semiconductor laser (DBR laser), the LD drive circuit The control instruction value is updated. Therefore, in the case of a current-controlled semiconductor laser (DBR laser), both the ACC and AWC of the LD update the control instruction value of the LD drive circuit, but a method using both ACC and AWC may be used. Either one may be selected.

  Further, in the case of the temperature control type semiconductor laser (DFB laser), the ACC of the LD is operated independently, but in the case of the current control type semiconductor laser (DBR laser), the TEC ATC is operated independently. It will be.

  In the above description, the PDs 145 to 148 are used to separately monitor the output power and the frequency of the laser light output from the wavelength tunable lasers 111 and 112. However, instead of the PDs 145 to 148, output is performed. An OPM that can simultaneously monitor power and frequency may be used. The OPM can be provided on the output side of the optical coupler 115, for example. By using the OPM, it is possible to accurately separate the two laser beams output from the wavelength tunable lasers 111 and 112 into their respective wavelengths and monitor the respective output powers.

  Next, the terahertz source 121 and the detection unit 123 of the sensor head unit 120 will be described in detail with reference to FIG. As shown in FIG. 1, the sensor head unit 120 includes a terahertz source 121 and a detection unit 123, and 7 (a) and 7 (b) show respective perspective views.

  The terahertz source 121 is configured by arranging an antenna 162 on one surface of a substrate 161 made of a predetermined semiconductor material. The antenna 162 includes transmission lines 163 and 164 that are arranged in parallel, and protrusions 165 and 166 that are formed at substantially the centers of the transmission lines 163 and 164, respectively. The protrusions 165 and 166 are provided with a small gap between them to form a minute dipole antenna. A DC bias is applied from the DC power supply 167 between the transmission lines 163 and 164.

  In the terahertz source 121 configured as described above, when a synthetic wave emitted from the optical waveguide 131 is irradiated onto the gap portion of the antenna 162, a beat signal corresponding to the terahertz composed of the irradiated synthetic wave is converted into a terahertz wave. Generated and emitted from the other surface of the substrate 161. The emitted terahertz wave beam is magnified by the hemispherical lens 122 and applied to the object to be measured 10, and the terahertz wave transmitted through the object to be measured 10 is condensed by the hemispherical lens 124 and incident on the detection unit 123. The detection unit 123 detects a signal corresponding to the incident terahertz wave.

  As a nonlinear material used for the substrate 161 of the terahertz source 121, it has an impurity level (absorption band) corresponding to the output wavelength of the wavelength tunable lasers 111 and 112, and has a short carrier lifetime and high mobility with respect to the output terahertz wave. A semiconductor material is preferably used. Conventionally, for example, an organic crystal called DAST or low-temperature grown GaAs has been often used for a light source of 1.5 μm band, for example. However, there has been a problem in the reliability of the terahertz source due to deterioration when a DC bias is applied at a high voltage in order to generate a high-output terahertz wave, deterioration due to light wave irradiation on the crystal, and the like.

  Therefore, in this embodiment, when laser light having a wavelength of 1.5 μm is emitted from the wavelength tunable lasers 111 and 112, a GaN substrate is used as the substrate 161 of the terahertz source 121. The inventors have found that GaN is a material containing many defects in the crystal because of the characteristics of the crystal growth process, and is suitable as a material for generating a terahertz wave similarly to low-temperature grown GaAs. . GaN can realize a terahertz source having high reliability even in a high voltage environment because of its pressure resistance and reliability. In addition, due to the high-frequency characteristics of GaN, high-power terahertz waves can be generated by integrating amplifying elements using GaN in the terahertz source 121.

  When laser light having a wavelength of 1.3 μm or 1.5 μm is emitted from the wavelength tunable lasers 111 and 112, a substrate made of InGaAsP / InP or InGaAs / AlGaAs is used as the substrate 161 of the terahertz source 121. it can.

  When laser light having a wavelength of 650 nm to 1.5 μm is emitted from the wavelength tunable lasers 111 and 112, a substrate made of an II-VI group semiconductor or a III-V group semiconductor or a metal is used as the substrate 161 of the terahertz source 121. A substrate made of II-VI semiconductor doped with or III-V semiconductor doped with metal can be used. When the wavelength of the laser light emitted from the wavelength tunable lasers 111 and 112 is widened to the 650 nm to 1.5 μm band, the terahertz wave depends on which material of the II-VI group semiconductor or the III-V group semiconductor is combined. The output level will be different. In particular, in the 1.5 μm band, a II-VI group semiconductor or a III-V group semiconductor may be doped with a metal such as Er or Be.

  Furthermore, when laser light having a wavelength of 0.98 μm is emitted from the wavelength tunable lasers 111 and 112, a substrate made of GaAs may be used as the substrate 161 of the terahertz source 121.

  The detection unit 123 includes a terahertz wave reception unit 170 and a broadband detection circuit 180, as shown in FIG. The terahertz wave receiving unit 170 has the same configuration as the terahertz source 121, and an antenna 172 is disposed on one surface of a substrate 171 made of a predetermined semiconductor material. Similarly to the antenna 162, the antenna 172 includes transmission lines 173 and 174 arranged in parallel, and protrusions 175 and 176 formed at substantially the center of each of the transmission lines 173 and 174. The protrusions 175 and 176 are provided with a small gap between them to form a minute dipole antenna.

  In the antenna 172, unlike the antenna 162, no DC bias is applied between the transmission lines 173 and 174, and an ammeter 181 is connected instead. The current measured by the ammeter 181 is converted into a digital signal by the ADC 182 and transmitted to the data processing circuit 118 via the signal line 133. The data processing circuit 118 converts a digital signal input via the signal line 133 into an intensity signal and the like, and performs an imaging process and the like of the device under test 10 based on the converted signal. In the present embodiment, the broadband detection circuit 180 includes an ammeter 181 and an ADC 182.

  By configuring the detection unit 123 using the terahertz wave reception unit 170 and the broadband detection circuit 180 as described above, the detection unit 123 can be easily downsized. As another embodiment of the detection unit 123, a terahertz camera having sensitivity in a predetermined frequency band of the terahertz wave can be used instead of the terahertz wave reception unit 170 and the broadband detection circuit 180 described above. Even with such a configuration, the detection unit can be downsized.

  A method for measuring the DUT 10 using the terahertz imaging apparatus 100 of the present embodiment will be described below. The terahertz imaging apparatus 100 includes two wavelength tunable lasers 111 and 112 in the main body 110, and the temperature of each TEC 141 and 142 can be controlled to adjust the wavelength of each emitted laser beam. ing. Thereby, by performing the temperature control of the TECs 141 and 142 in time series, the frequency of the terahertz wave output from the terahertz source 121 can be adjusted in time series.

  An example of adjusting the frequency of the terahertz wave in time series will be described with reference to FIGS. In FIG. 8, the output frequency (indicated by symbol S1) of one wavelength tunable laser 111 is fixed to a predetermined value, and the output frequency (indicated by symbol S2) of the other tunable laser 112 is continuously time-sequentially. The example which changed is shown. Since the frequency of the terahertz wave is a difference between the frequencies of the wavelength tunable lasers 111 and 112, it continuously changes in time series as indicated by reference numeral S3.

  As shown in FIG. 8, by irradiating the device under test 10 with a terahertz wave whose frequency changes in time series, a frequency response (imaging) unique to the device under test 10 can be obtained. An example of the frequency response is schematically shown in FIG. In the figure, the frequency responses of the substances A, B, and C to be detected are schematically shown. A frequency response is obtained in the data processing circuit 118 by irradiating the DUT 10 with a terahertz wave whose frequency continuously changes in time series as shown in FIG. By comparing the obtained frequency response with the frequency response of the substance to be detected shown in FIG. 10, it is determined that the DUT 10 is any of the substances A, B, and C to be detected or is not the substance to be detected. Can do.

  In FIG. 9, the output frequency of one wavelength tunable laser 111 (indicated by symbol S4) is fixed to a predetermined value, and the output frequency of the other wavelength tunable laser 112 (indicated by symbol S4) is predetermined in time series. An example in which the frequency is changed stepwise is shown. In this case, the frequency of the terahertz wave also changes in a stepwise manner to a specific frequency in time series as indicated by reference numeral S3. By irradiating the DUT 10 with such a terahertz wave, the frequency response of the DUT 10 with respect to a specific frequency can be obtained. When the substance to be detected shows a specific response at a specific frequency, it is preferable to use a terahertz wave as shown in FIG.

  The sensor head unit 120 of the above embodiment is configured to detect the terahertz wave transmitted through the device under test 10 by the detection unit 123, but instead of this, the terahertz wave reflected by the device under test 10 is detected by the detection unit 123. It is also possible to configure so as to detect with An embodiment of the sensor head unit 120 configured to detect the terahertz wave reflected by the device under test 10 is shown in the block diagram of FIG.

  In the above embodiment, the terahertz wave receiving unit 170 and the hemispherical lens 124 of the detection unit 123 are both provided in the sensor head unit 120. However, the terahertz wave receiving unit 170 is provided on the main body unit 110 side, and the sensor head unit 120 is provided. The tera health source 121 and the hemispherical lens 124 may be provided. In this case, a predetermined terahertz wave waveguide is routed in the composite cable 130 instead of the signal line 133, and the terahertz light transmitted or reflected by the device under test 10 is collected by the hemispherical lens 124. Then, the condensed terahertz wave is incident on a predetermined terahertz wave waveguide, and is emitted to the terahertz wave receiving unit 170 provided on the main body 110 side via the terahertz wave waveguide.

  Furthermore, in the sensor head unit 120 of the present embodiment, the terahertz wave generated by the terahertz source 121 is irradiated to the object to be measured 10 in a wide range, and the terahertz wave transmitted through the object to be measured 10 is incident on the detection unit 123. Although hemispherical lenses are used for the lenses 123 and 124, respectively, the present invention is not limited to this. For example, a concave lens and a convex lens may be used.

  According to the present invention, it is easy to reduce the size of the sensor head portion provided with the terahertz source, and the measurement can be performed by moving the sensor head portion in the vicinity of the object to be measured. In addition, by integrating the LD control circuit that allows the wavelength of the laser light source to be integrated with the wavelength variable laser, it is possible to reduce the size and cost of the laser light source unit that has been extremely expensive. In addition, by adopting GaN as the material of the terahertz source that generates the terahertz wave, a highly reliable terahertz source can be realized even under high driving voltage conditions. Furthermore, if an amplifying element using GaN is also integrated, it contributes to higher output of terahertz waves.

(Second Embodiment)
A second embodiment of the present invention will be described with reference to FIG. FIG. 12 is a block diagram showing the overall configuration of the terahertz imaging apparatus 200 of the present embodiment. The generation efficiency of the terahertz wave in the terahertz source 121 is affected by what state the synthetic wave propagated to the sensor head unit 120 via the optical waveguide 131 is when the terahertz source 121 is irradiated. . That is, when the terahertz source 121 is irradiated with the synthesized wave, when the polarization of the two laser beams satisfies a predetermined condition and the output powers of the two laser beams are substantially equal, the terahertz wave is generated from the terahertz source 121. It is generated efficiently.

  Regarding the polarization of the two laser beams, the polarization planes when the laser beams are emitted from the wavelength tunable lasers 111 and 112, respectively, or the polarization planes are rotated (circular polarization) Terahertz waves are efficiently generated when Therefore, in the present embodiment, the optical waveguide 231 includes a polarization plane adjusting unit 201 that rotates or maintains the polarization plane of the laser light propagating through the optical waveguide 131.

  In the terahertz imaging apparatus 200 of the present embodiment shown in FIG. 12, a depolarizer is arranged in the middle of the optical waveguide 131 as the polarization plane adjusting means 201 in order to rotate the polarization planes of the two laser beams propagating through the optical waveguide 131. ing. The depolarizer can rotate the polarization planes of the two laser beams. Further, instead of disposing a depolarizer, the optical fiber used for the optical waveguide 131 may be connected with the fusion surface with the optical coupler 115 shifted by a predetermined width. In this case, the fusion plane between the optical fiber used for the optical waveguide 131 and the optical coupler 115 becomes the polarization plane adjusting means.

  On the other hand, a panda fiber can be used for the optical waveguide 131 as polarization plane adjusting means for maintaining the polarization planes of the two laser beams propagating through the optical waveguide 131.

  As described above, by providing the function of rotating or maintaining the polarization planes of the two laser beams propagating through the optical waveguide 131, the level fluctuation of the beat component of the combined wave of the two laser beams is suppressed, and the efficiency is improved. High terahertz wave generation can be realized.

  In the present embodiment, the output monitoring unit 202 is further provided in front of the terahertz source 121 in order to control the output powers of the two laser beams output from the optical waveguide 131 to be equal. The output monitoring means 202 measures the output power of each of the two laser beams and calculates the difference between them (hereinafter referred to as output error). For example, OPM can be used to measure the output power of each of the two laser beams.

  The output error calculated by the output monitoring unit 202 is transmitted to the LD control circuit 116 via the control signal line 203 added to the composite cable 130. The LD control circuit 116 controls the drive currents of the wavelength variable lasers 111 and 112 or the drive currents of the SOAs 113 and 114 in accordance with the output error input from the output monitoring unit 202, so that the two laser beams irradiated to the terahertz source 121 The output power is adjusted to be equal. Thereby, a terahertz wave is efficiently generated by the terahertz source 121.

(Third embodiment)
A third embodiment of the present invention will be described with reference to FIG. FIG. 13 is a block diagram showing an overall configuration of the terahertz imaging apparatus 300 of the present embodiment. The terahertz wave generated by the terahertz source 121 has a frequency corresponding to the difference between the frequencies of the two laser beams. However, if there is a difference in output power between the two when the terahertz source 121 is irradiated with a composite wave composed of two laser beams, the frequency of the generated terahertz wave may change. Therefore, in this embodiment, the frequency of the terahertz wave generated by the terahertz source 121 is directly measured.

  In the present embodiment, out of the terahertz wave generated by the terahertz source 121, the wavelength detector 301 receives leaked light that is not irradiated in the direction of the object to be measured 10. The wavelength detection unit 301 converts the received voltage value into a voltage value corresponding to the wavelength or frequency of the terahertz wave, and transmits the voltage value to the LD control circuit 116 of the main body unit 110 via the control signal line 303 added to the composite cable 130. To transmit.

  However, the wavelength detection unit 301 may not be accurately converted to a voltage value corresponding to the wavelength of the received terahertz wave due to variations in conversion efficiency to voltage values. In addition, there is a possibility that the converted voltage value may fluctuate due to the influence of the control signal line 303 or the like before being transmitted to the main body 110 via the control signal line 303. Thus, when the voltage value transmitted to the main body 110 does not accurately correspond to the wavelength of the terahertz wave, the frequency of the terahertz wave cannot be controlled to a desired frequency.

  Therefore, in the terahertz imaging apparatus 300 according to the present embodiment, the main body 110 is provided with the voltage calibration unit 302, where the transmitted voltage is removed so as to remove the above-described variation in conversion efficiency, the influence of the control signal line 303, and the like. The value is calibrated.

  A method for calibrating the voltage value transmitted via the control signal line 303 will be described below. First, at the time of factory shipment, the frequency of the terahertz wave generated by the terahertz source 121 is directly measured using a predetermined highly accurate wavelength detector. At the same time, the terahertz wave received by the wavelength detector 301 is converted into a voltage value, and the voltage value transmitted to the main body 110 via the control signal line 303 is acquired. Thus, a calibration table is created in which the wavelength of the terahertz wave accurately measured by the wavelength detector is associated with the voltage value transmitted to the main body 110. The voltage calibration means 302 stores the calibration table prepared in advance and uses it to estimate the wavelength of the terahertz wave.

  A method for controlling the wavelength of the terahertz wave using the calibration table in the voltage calibration unit 302 will be described below. When a voltage value is transmitted from the wavelength detection unit 301 via the control signal line 303, the voltage calibration unit 302 converts the voltage value into a terahertz wave wavelength using a calibration table stored in advance. Then, the converted wavelength is output to the LD control circuit 116. The LD control circuit 116 controls the output power and the like of the wavelength variable lasers 111 and 112 so that the wavelength input from the voltage calibration unit 302 matches the wavelength of the desired terahertz wave.

  As described above, the present embodiment is configured to receive the terahertz wave generated by the terahertz source 121 and to control the wavelength variable lasers 111 and 112 so that the wavelength thereof matches a desired wavelength. Thus, the frequency response of the non-object can be measured with high accuracy.

(Fourth embodiment)
A fourth embodiment of the present invention will be described with reference to FIG. FIG. 14 is a plan view of the antenna 462 provided in the terahertz source 121 of the present embodiment. In the terahertz imaging apparatus 200 of the second embodiment, a panda fiber is used for the optical waveguide 131 in order to maintain the polarization planes of the two laser beams propagating through the optical waveguide 131. However, there is a possibility that the polarization planes of the two laser beams are shifted between the time when the two laser beams are emitted from the optical waveguide 131 and before the terahertz source 121 is irradiated. If the plane of polarization is deviated when the two laser beams reach the terahertz source 121, the output of the generated terahertz wave is reduced.

  Therefore, in the present embodiment, the antenna 462 of the terahertz source 121 has a shape as shown in FIG. That is, the projecting portions 465 and 466 of the antenna 462 are formed so that the tip portion has a concave curved shape. As described above, when the tip portions of the protrusions 465 and 466 are formed in a concave curved shape, an effect is obtained that the polarization planes of the two laser beams irradiated between them coincide with each other.

  In any of the above embodiments, it is easy to reduce the size of the sensor head portion provided with the terahertz source, and measurement can be performed by moving the sensor head portion in the vicinity of the object to be measured. In addition, by integrating the LD control circuit that allows the wavelength of the laser light source to be integrated with the wavelength variable laser, it is possible to reduce the size and cost of the laser light source unit that has been extremely expensive. In addition, by adopting GaN as the material of the terahertz source that generates the terahertz wave, a highly reliable terahertz source can be realized even under high driving voltage conditions. Furthermore, if an amplifying element using GaN is also integrated, it contributes to higher output of terahertz waves.

(Fifth embodiment)
A fifth embodiment of the present invention will be described with reference to FIG. FIG. 16 is a block diagram showing an overall configuration of the terahertz imaging apparatus 500 of the present embodiment. In the terahertz imaging apparatus 500 of the present embodiment, a plurality of light source units 501 are provided in the main body unit 510 in order to obtain a terahertz wave with high power from the terahertz source 121. In FIG. 16, four light source units 501 a to 501 d are provided as an example. Note that the number of the light source units 501 provided in the main body unit 510 is not limited to four, and can be increased or decreased as necessary.

  Each light source unit 501 (each of 501a to 501d) includes wavelength tunable lasers 111 and 112, SOAs 113 and 114 provided in each of them, an optical coupler 115, and an LD control circuit, as in the main body 110 of the first embodiment. 116. The main body 510 includes the above-described four light source units 501a to 501d, an analog circuit 117, and a data processing circuit 118.

  Optical light guides 531a to 531d are connected to the light source portions 501a to 501d, respectively, and the synthesized wave generated by the optical coupler 115 of each light source portion 501a to 501d is detected by the optical waveguides 531a to 531d connected to the light source portions 501a to 501d. It is transmitted to the head unit 120 side. The combined waves output from the optical couplers 115 of the light source units 501a to 501d are input to the respective optical waveguides 531a to 531d with the same phase. A PLC (Planar Lightwave Circuit) 535 is provided on the sensor head unit 120 side, and the optical waveguides 531 a to 531 d are connected to the PLC 535. In the PLC 535, a combining unit 535a that combines the combined waves output from the respective optical waveguides 531a to 531d is formed, and the combined light is output to the terahertz source 121 via the optical waveguide 531e. .

  In the present embodiment, the PLC 535 is used to multiplex a plurality of synthesized waves output from the optical waveguides 531a to 531d. However, by using the PLC 535, the phases of the synthesized waves are aligned and then the merging unit 535a. Can be combined. That is, by adjusting the length of the optical waveguide from each connection point between the optical waveguides 531a to 531d and the PLC 535 to the multiplexing unit 535a (phase adjustment unit), It is possible to make the phases of the synthesized wave uniform. It is possible to output a terahertz wave having a higher power to the terahertz source 121 by aligning the phases of the synthesized waves and combining them by the combining unit 535a. As another method for aligning the phases of the synthesized waves, the lengths of the optical waveguides 531a to 531e can be adjusted.

  As described above, a plurality of light sources 501 are provided in the main body 510, a plurality of synthesized waves are output to the PLC 535 provided on the sensor head unit 120 side, and a plurality of synthesized waves are combined by the PLC 535, and then the optical waveguide With the configuration in which the terahertz source 121 outputs the signal via the 531e, the terahertz source 120 can be irradiated with a high-power synthetic wave. As a result, a high-power terahertz wave can be output from the terahertz source 121.

  In the first embodiment, laser beams having different frequencies ω1 and ω2 are output from the wavelength tunable lasers 111 and 112, respectively, but also in the present embodiment, the frequencies from the wavelength tunable lasers 111 and 112 of the light source units 501a to 501d, respectively. Laser light of ω1 and ω2 can be output. This makes it possible to increase the power of the combined wave composed of laser light with the frequencies ω1 and ω2, and to output a high-power terahertz wave by applying a high-power combined wave to the terahertz source 121.

Alternatively, it is also possible to output laser light having different frequencies from the wavelength variable lasers 111 and 112 in each of the light source units 501a to 501d. As an example, laser light having frequencies ω1 and ω2, ω3 and ω4, ω5 and ω6, ω7, and ω8 can be output from the wavelength variable lasers 111 and 112 of the light source units 501a to 501d, respectively. At this time, each frequency is set so as to satisfy the following equation from Equation (5).
| Ω1-ω2 | = | ω3-ω4 | = | ω5-ω6 | = | ω7-ω8 | (6)

  In this way, by setting the frequencies of the laser beams output from the wavelength variable lasers 111 and 112 of the light source units 501a to 501d to be all different, the mixed wave combined by the PLC 535 is compared at each frequency. Will have low power. On the other hand, when the laser light of the same frequency ω1 and ω2 is output by the respective wavelength tunable lasers 111 and 112 of the light source units 501a to 501d, it has high power at the frequencies ω1 and ω2, which is the optical waveguide. It will be output to the terahertz source 121 via 531e.

  When light having high power at a specific frequency is guided through the optical waveguide (optical fiber) 531e, the light is output to the terahertz source 121 due to nonlinear effects due to the nonlinear characteristics of the optical fiber. As a result, for example, when the optical waveguide 531e becomes long, it may be impossible to output a terahertz wave having a sufficiently high power. On the other hand, if the frequencies of the laser beams output from the wavelength variable lasers 111 and 112 of the light source units 501a to 501d are all different, the peak power of the mixed wave guided to the optical waveguide 531e can be lowered. And non-linear effects can be sufficiently reduced. As a result, a terahertz wave having high power can be output from the terahertz source 121.

  In the present embodiment, the composite wave generated by the main body 510 is transmitted to the PLC 535 via the optical waveguides 531a to 531d made of optical fibers. For this reason, when the optical waveguides 531a to 531d become longer, the pulse width of the optical pulse (synthetic wave) may increase due to the influence of dispersion in the optical fiber, and the peak intensity may decrease. Thus, by performing dispersion compensation (distortion compensation) on the synthesized wave input to the PLC 535, the spread of the optical pulse can be suppressed. In the present embodiment, the PLC 535 can be provided with a dispersion compensation unit that performs dispersion compensation.

  An example of a PLC including a dispersion compensation unit is shown in FIG. FIG. 17 is a perspective view illustrating an example of a PLC 535 ′ including the dispersion compensation unit 535 b. The dispersion compensation unit 535b is formed of a nonlinear device capable of dispersion compensation. After the composite wave combined by the multiplexing unit 535a is input and subjected to dispersion compensation, the sensor head passes through the optical waveguide 531e. The data is output to the unit 120. In this manner, by providing the dispersion compensation unit 535b and performing dispersion compensation of the synthesized wave output to the terahertz source 121, it becomes possible to generate a higher-power and stable terahertz wave. Instead of providing the dispersion compensation unit 535b in the PLC 535, a dispersion compensation fiber (DCF) may be connected to the optical waveguides 531a to 531d or the optical waveguide 531e.

(Sixth embodiment)
A sixth embodiment of the present invention will be described with reference to FIG. FIG. 18 is a block diagram showing the overall configuration of the terahertz imaging apparatus 600 of this embodiment. In the fifth embodiment, in order to align the phases of the plurality of synthesized waves output from the optical waveguides 531a to 531d, the length of the optical waveguide from the connection point with the optical waveguides 531a to 531d on the PLC 535 to the multiplexing unit 535a. Explained the adjustment. In the terahertz imaging apparatus 600 according to the present embodiment, the terahertz imaging apparatus 500 according to the fifth embodiment is configured so that the phases of the synthesized waves output from the light source units 501a to 501d can be easily aligned with higher accuracy. Furthermore, PLC636 is added.

  One end of the PLC 636 is connected to the light source units 501a to 501d, and the other end is connected to the optical waveguides 531a to 531d. The light source units 501a to 501d are connected to the optical waveguides 531a to 531d via the waveguides 636a to 636d on the PLC 636, respectively. Each of the light source units 501a to 501d is configured to be output to the PLC 636 in a state where the phases of the combined waves output from the respective optical couplers 115 are substantially aligned. In this embodiment, the phase of each combined wave is changed to the PLC 636. With even higher accuracy. That is, the waveguides 636a to 636d that connect the light source units 501a to 501d and the optical waveguides 531a to 531d are formed on the PLC 636, and the lengths of the waveguides 636a to 636d are adjusted to generate a combined wave having the same phase. The light is output to the optical waveguides 531a to 531d.

  As described above, in the terahertz imaging apparatus 600 of the present embodiment, the phases of the synthesized waves output from the light source units 501a to 501d can be aligned with high accuracy, and the synthesized waves with the phases aligned with high accuracy can be obtained. It can be transmitted to the sensor head portion 120 side via the waveguides 636a to 636d. Further, on the sensor head unit 120 side, the length of the waveguide formed in the PLC 535 is adjusted so that the phase error generated while passing through the waveguides 636a to 636d is corrected and the phases are aligned again with high accuracy. . As a result, it is possible to output a synthesized wave having the same phase with high accuracy to the terahertz source 121, and to output a terahertz wave having high power from the terahertz source 121.

  In each of the above embodiments, the example in which two wavelength tunable lasers are provided has been described. However, the number of wavelength tunable lasers is not limited to two, and three or more wavelength tunable lasers may be provided. The description in the present embodiment shows an example of the terahertz wave imaging apparatus according to the present invention, and the present invention is not limited to this. The detailed configuration and detailed operation of the terahertz wave imaging apparatus according to the present embodiment can be appropriately changed without departing from the spirit of the present invention.

10 DUT 100, 200, 300, 500, 600 Terahertz imaging device 110, 510 Main unit 111, 112 Tunable laser 113, 114 SOA
115 Optical Coupler 116 LD Control Circuit 117 Analog Circuit 118 Data Processing Circuit 120 Sensor Head Unit 121 Terahertz Source 122, 124 Lens 123 Detection Unit 130 Composite Cable 131, 531 Optical Waveguide 132 Power Cable 133 Signal Lines 141, 142, 151, 152 TEC
143, 144 TEC drive circuit 145, 146, 147, 148 PD
149, 150 Etalon filter 161, 171 Substrate 162, 172, 462 Antenna 163, 164, 173, 174 Transmission line 165, 166, 175, 176, 465, 466 Protrusion 167 DC power supply 170 Terahertz wave receiver 180 Broadband detection circuit 181 Ammeter 182 ADC
201 Polarization plane adjustment unit 202 Output monitoring unit 203 Control signal line 301 Wavelength detection unit 302 Voltage calibration unit 303 Control signal line 501 Light source unit 535, 636 PLC
535a multiplexing unit 535b dispersion compensation unit

Claims (22)

Two or more tunable lasers each having a different oscillation wavelength;
An optical coupler for synthesizing laser beams output from the wavelength tunable laser and outputting a combined wave;
An optical waveguide for propagating a composite wave output from the optical coupler;
A terahertz source that outputs a terahertz wave having a frequency corresponding to a beat component of the synthetic wave when the synthetic wave is irradiated from the optical waveguide;
With
A terahertz wave imaging apparatus, wherein the terahertz wave is generated by irradiating the terahertz source through the optical waveguide with the synthesized wave obtained by synthesizing laser beams output from the wavelength tunable laser. The main body unit includes the two or more wavelength tunable lasers and the optical coupler,
The terahertz source is provided in a sensor head part,
The terahertz wave imaging apparatus according to claim 1, wherein the sensor head unit is configured to be movable independently of the main body unit connected by the optical waveguide. An LD control circuit for adjusting the wavelength of the output laser beam by controlling the temperature of the wavelength tunable laser;
The LD control circuit controls the temperature of at least one of the two or more wavelength tunable lasers to adjust the wavelength of the laser light, so that the frequency of the terahertz wave output from the terahertz source is changed in time series. The terahertz wave imaging apparatus according to claim 1, wherein the terahertz wave imaging apparatus is adjusted. The LD control circuit
TEC (Thermo Electric Cooler) for controlling the temperature of the wavelength tunable laser;
A photodiode (PD) that receives a portion of the laser light output from the wavelength tunable laser;
The same number of etalon filters as the wavelength tunable laser that allows a part of the combined wave to enter and pass only laser light of a predetermined frequency;
Another photodiode that receives the laser light of the predetermined frequency from the etalon filter;
A TEC driving circuit that inputs measurement results from the photodiode and the other photodiode and controls the TEC so that the output intensity and wavelength of the laser light are stably set to predetermined values. The terahertz wave imaging apparatus according to claim 3. The terahertz source is
A substrate comprising a semiconductor material having an impurity level corresponding to an output wavelength of laser light output from the wavelength tunable laser and having a short carrier lifetime and high mobility with respect to the output terahertz wave. The terahertz wave imaging apparatus according to any one of claims 1 to 4. The wavelength tunable laser outputs a laser beam having a wavelength of 1.5 μm,
The terahertz wave imaging apparatus according to any one of claims 1 to 5, wherein the terahertz source includes a GaN substrate. The wavelength tunable laser outputs a laser beam having a wavelength of 1.3 um band or 1.5 um band,
The terahertz wave imaging apparatus according to any one of claims 1 to 5, wherein the terahertz source includes a substrate made of InGaAsP / InP or InGaAs / AlGaAs. The wavelength tunable laser outputs laser light having a wavelength of 650 nm to 1.5 μm,
The terahertz wave imaging apparatus according to any one of claims 1 to 5, wherein the terahertz source includes a substrate made of a II-VI group semiconductor or a III-V group semiconductor. The wavelength tunable laser outputs laser light having a wavelength of 650 nm to 1.5 μm,
6. The terahertz wave imaging according to claim 1, wherein the terahertz source includes a substrate made of a metal-doped II-VI group semiconductor or a metal-doped III-V group semiconductor. apparatus. The wavelength tunable laser outputs a laser beam having a wavelength of 0.98 um,
The terahertz wave imaging apparatus according to claim 1, wherein the terahertz source includes a substrate made of GaAs. 11. The terahertz wave imaging apparatus according to claim 1, wherein the optical waveguide includes a polarization plane adjusting unit that rotates or holds a polarization plane of a propagating laser beam. A detector having an antenna having sensitivity in a predetermined frequency band of the terahertz wave, and a broadband detection circuit for detecting a signal received by the antenna;
The terahertz wave according to any one of claims 1 to 11, wherein the detection unit receives a reflected signal or a transmitted signal when the object to be measured is irradiated with the terahertz wave output from the terahertz source. Imaging device. A detection unit having a terahertz camera having sensitivity in a predetermined frequency band of the terahertz wave;
The terahertz wave according to any one of claims 1 to 11, wherein the detection unit receives a reflected signal or a transmitted signal when the object to be measured is irradiated with the terahertz wave output from the terahertz source. Imaging device. A wavelength detector that receives a portion of the terahertz wave output from the terahertz source and converts it into a voltage value;
Voltage calibration means for inputting the voltage value from the wavelength detection unit, estimating the wavelength of the terahertz wave from the voltage value using a calibration table stored in advance, and outputting the wavelength to the LD control circuit;
The LD control circuit, when receiving the wavelength of the terahertz wave from the voltage calibration unit, controls the output of the wavelength tunable laser so that the wavelength of the terahertz wave matches a predetermined target wavelength. Item 14. The terahertz wave imaging apparatus according to any one of Items 3 to 13. The terahertz source has two transmission lines arranged in parallel, and a protrusion formed at substantially the center of each of the transmission lines,
The terahertz wave imaging apparatus according to claim 12, wherein a tip of the protrusion is formed in a concave curved shape. Including two or more light source units each including the two or more wavelength tunable lasers and the optical coupler;
Further, a combining unit that inputs and combines each of the two or more light source units that are connected by the optical waveguide and that is output from the optical coupler;
The terahertz wave imaging apparatus according to any one of claims 1 to 15, further comprising: another optical waveguide that irradiates the terahertz source with the mixed wave combined by the combining unit. The terahertz wave imaging apparatus according to claim 16, wherein the two or more wavelength tunable lasers emit laser light having the same oscillation wavelength in each of the two or more light source units. 17. The terahertz wave imaging apparatus according to claim 16, wherein the two or more wavelength tunable lasers emit laser beams having the same frequency difference and different oscillation wavelengths in each of the two or more light source units. The terahertz wave imaging apparatus according to claim 16, wherein the multiplexing unit is formed using a PLC (Planar LIghtwave Circuit). The terahertz wave imaging apparatus according to claim 19, wherein the PLC includes a phase adjustment unit that aligns phases of synthesized waves input from the two or more optical waveguides. 21. The terahertz wave imaging apparatus according to claim 19, wherein the PLC includes a dispersion compensation unit that performs dispersion compensation on a combined wave input from the two or more light source units. The phase adjustment part which aligns the phase of the synthetic wave input from the said 2 or more light source part, and each outputs to the said 2 or more optical waveguide is provided, The any one of Claim 16 thru | or 21 characterized by the above-mentioned. The terahertz wave imaging apparatus described in the item.

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