JP2011021930A - Waveform observation apparatus and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a waveform observation apparatus for accurately observing the waveform of a reflected pulse from a moving body which is set as an object to be inspected. <P>SOLUTION: The waveform observation apparatus 1 includes a repeated frequency generator 2 for generating a repeated frequency; a pulse generator 3 for generating a femtosecond pulse light at the repeated frequency; a pulse transmitter 4 for converting the femtosecond pulse light into a terahertz pulse wave, and transmitting it; a half mirror 5 for dividing the femtosecond pulse light into an exciting pulse light 21 and a synchronization-detecting pulse light 22; a mirror 6; a beam-scanning mechanism 7 for scanning the direction of the pulse wave; a pulse receiver 8 for receiving the reflected wave and converting it into a current pulse; a frequency control section 9 for controlling the repeated frequency generator 2; a waveform-acquiring section 15 for obtaining a received waveform; a distance measuring section 16; a waveform-measuring section 17; an output section 18; and a per-object transmission parameter calculating section 19. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a waveform observation apparatus and method for measuring a distance to a target, a relative speed, a reflection coefficient of the target, and the like using an electromagnetic wave pulse.

  In terahertz time domain spectroscopy, two femtosecond laser light sources without using a mechanical stage to time-delay scan terahertz detection probe light serving as a measurement time window with respect to terahertz pulse generation pump light There is known a spectroscopic measurement apparatus using the above (for example, Patent Document 1).

  According to this spectroscopic measurement device, the pulse period of the femtosecond laser for pump light and the pulse period of the femtosecond laser for probe light are made slightly different from each other while being highly stabilized. As a result, the time delay timing between the two pulses is automatically shifted for each pulse, so the time-expanded terahertz pulse is measured at high speed without using a mechanical stage.・ High speed measurement is possible.

International Publication No. 2006/092874

  However, according to the above-described conventional spectroscopic measurement device, it is necessary to perform frequency control of the pulse repetition frequency so that the difference between the pulse periods of the two femtosecond lasers is maintained at a constant value. There was a problem that the waveform could not be measured with high accuracy.

  In order to solve the above problems, the present invention oscillates a repetition frequency of pulse transmission, generates a pulse at the repetition frequency, transmits the generated pulse to a measurement target, receives a pulse reflected from the measurement target, In the waveform observation device for acquiring the waveform of a received pulse, the waveform observation device is configured to set a parameter of a repetition frequency of pulse transmission based on a relative relationship with a measurement target.

  According to the present invention, the pulse transmission repetition frequency parameter can be set based on the relative relationship with the measurement target, so that the reflected pulse waveform with the relative relationship changing as the measurement target is highly accurate. There is an effect that it can be observed with.

It is a block diagram which shows the structure of embodiment of the waveform observation apparatus which concerns on this invention. It is a perspective view explaining the detailed example of the pulse transmitter 4 in FIG. It is a perspective view explaining the detailed example of the pulse receiver 8 in FIG. It is a figure which shows the example of the frequency pattern in embodiment. It is a figure which shows the example of the frequency switching pattern in embodiment. It is a flowchart explaining operation | movement of the waveform observation apparatus of embodiment.

Next, embodiments of the present invention will be described in detail with reference to the drawings. The embodiment described below is an embodiment using terahertz waves (0.1 THz to 10 THz, 1 THz = 10 ¹² Hz) as the frequency of the electromagnetic wave pulse. A terahertz wave is an electromagnetic wave in a boundary region between radio waves and light, and has properties of both radio waves and light.

FIG. 1 is a block diagram showing a configuration of an embodiment of a waveform observation apparatus according to the present invention. In FIG. 1, a waveform observation apparatus 1 has a repetition frequency generator 2 that generates a repetition frequency of a pulse, and a pulse width of about 100 fs (1 fs = 10 ⁻¹⁵ s) at the repetition frequency generated by the repetition frequency generator 2. A pulse generator 3 using a femtosecond pulsed laser that generates femtosecond pulsed light, a pulse transmitter 4 that converts the femtosecond pulsed light into a terahertz pulse wave, and transmits the pulse to a measurement target, and for exciting femtosecond pulsed light The half mirror 5, which is divided into the pulse light 21 and the synchronous detection pulse light 22, the mirror 6, the beam scanning mechanism 7 which scans the direction of the transmitted / received terahertz pulse wave, and the terahertz pulse wave reflected from the measurement object are received and the current is received. A pulse receiver 8 for converting to a pulse; a frequency control unit 9 for controlling the frequency of the repetition frequency generator 2; It includes a waveform acquisition unit 15 Tokusuru, a distance measuring unit 16, a waveform measuring section 17, an output unit 18, and a target-specific transmission parameter calculating section 19.

  The repetition frequency generator 2 includes an oscillator 10 that oscillates a plurality of frequencies F1, F2,... FN, and a switcher 11 that switches and outputs the frequency oscillated by the oscillator 10.

  The frequency control unit 9 includes an oscillator control unit 12 that controls the oscillator 10, a time-division frequency pattern setting unit 13, and a switching control unit 14 that controls the switch 11.

  For example, when the pulse generator 3 uses a titanium-added sapphire laser (Ti: Al2O3), a pulse width of 12 to 100 fs, a pulse repetition frequency of 10 to 80 MHz, and an output of 500 mW to 2 W are obtained. Further, when a fiber laser is used as the pulse generator 3, a pulse width of 502 to 100 fs, a pulse repetition frequency of 10 to 50 MHz, and an output of 30 to 300 mW are obtained.

  The wavelength of the pulsed light generated by the pulse generator 3 is the wavelength of light corresponding to the band gap of a semiconductor substrate (for example, GaAs) that converts femtosecond pulsed light into terahertz waves (Eg = 1.42 eV in the case of GaAs). This is a wavelength that can excite photocarriers on an LT-GaAs substrate, which will be described later with reference to FIG.

  The waveform acquisition unit 15 synthesizes the waveform of the terahertz pulse wave received by the pulse receiver 8. Since the pulse receiver 8 sequentially outputs amplitude sample values obtained by sampling the terahertz pulse wave with a time gate described later, the waveform acquisition unit 8 combines the amplitude sample values and synthesizes the reception waveform of the terahertz pulse wave.

  The distance measurement unit 16 measures the distance (dgd) to the measurement target based on the received waveform acquired by the waveform acquisition unit 15. The waveform measurement unit 17 measures the amplitude value, waveform shape, and phase of the received waveform acquired by the waveform acquisition unit 15 and outputs the measured value to the output unit 18. The output unit 18 outputs the amplitude value, waveform shape, and phase of the received waveform measured by the waveform measurement unit 17 to a host device (not shown).

  The transmission parameter calculation unit 19 for each object calculates a parameter of the repetition frequency generated by the repetition frequency generator 2 for each object having a different distance measured by the distance measurement unit 16, and sends the calculated parameter to the frequency control unit 9. Output. The frequency control unit 9 controls the oscillator 10 and the switch 11 according to the parameters input from the target transmission parameter calculation unit 19. As a result, the repetition frequency of the pulse generated by the pulse generator 3 is controlled, and consequently, the repetition frequency of the terahertz pulse wave transmitted from the pulse transmitter 4 to the measurement target is controlled.

  FIG. 2 is a perspective view illustrating a detailed example of the pulse transmitter 4. The pulse transmitter 4 includes a laser converging lens 31 for converging the excitation pulse light 21, a silicon lens 32 for converging the generated terahertz pulse wave, an LT-GaAs substrate 33, which is a GaAs substrate grown at a low temperature, and an antenna supply. It has a pair of electrodes 34 and 35 that also serve as electric wires. The width d of the gap between the electrodes 34 and 35 is, for example, 5 μm to 10 μm, and the electrode functions as a minute dipole antenna. A bias 37 for supplying a bias voltage Vb (DC to 10 kHz, about 20 V) is connected between the electrodes 34 and 35. When the excitation pulse light 21 is irradiated to the gap between the electrodes 34 and 35, photocarriers are generated by the photoelectric effect of GaAs, and the photocarriers are accelerated by the applied voltage of the bias 37, so that an instantaneous current flows. An electromagnetic wave (ETHz (t) ∝∂J / ∂t) proportional to the time derivative of this current is generated as a terahertz pulse wave on the electrodes 34 and 35 and is radiated by the silicon lens 32. The LT-GaAs substrate 33 preferably has a carrier lifetime of several ps.

Thus, since the frequency band of the electromagnetic wave pulse transmitted by the pulse transmitter 4 is a terahertz wave (0.1 THz to 10 THz, 1 THz = 10 ¹² Hz), an optical scanning mechanism such as a oscillating prism or a polygon mirror is used. Can be used for the beam scanning mechanism 7, and the radiation direction of the terahertz pulse wave can be scanned by a relatively simple mechanism.

  FIG. 3 is a perspective view illustrating a detailed example of the pulse receiver 8. The pulse receiver 8 has basically the same structure as the pulse transmitter 4. A difference from the pulse transmitter 4 is that a current amplifier 36 is connected instead of connecting a bias 37 between the electrodes 34 and 35. In the pulse receiver 8, the electrodes 34 and 35 on the LT-GaAs substrate 33 similarly have a width of, for example, 5 to 10 μm between the electrodes, and the electrodes serve as a minute dipole antenna that operates in the terahertz wave band. Works. When photocarriers are generated by receiving the synchronous detection pulsed light 22 converged by the laser converging lens 31, when a terahertz wave converged by the silicon lens 32 is incident, light generated by the electric field of the terahertz wave in the dipole antenna A terahertz pulse wave can be received after being converted into an electric current. This means that synchronous detection is performed with synchronous detection pulse light in order to detect the terahertz waveform.

  Here, the principle of synchronous detection will be described. When the pulse width of the synchronous detection pulse light 22 is, for example, 300 fs, that is, the time gate can perform synchronous detection in a time shorter than 300 fs (resolution of about 30 fs). When the pulse width of the terahertz pulse wave is set to 1000 fs, for example, sampling is performed with the time gate being gradually delayed by changing the repetition frequency of the transmission pulse little by little. By connecting the sample points thus sampled, the waveform of the terahertz pulse wave can be observed. This technique is called terahertz time domain spectroscopy (TDS).

  Next, the operation of the waveform observation apparatus 1 will be described with reference to FIGS. First, the symbols used in the following description are defined.

d1: Minimum detection distance d2: Maximum detection distance dgd: Object detection distance (distance to the measurement object)
dmin: Detection resolution (usually determined by system requirements)
F: pulse repetition frequency c: speed of light Nitg: number of sampling points (usually set to 1000 for waveform observation)
Next, a change pattern of the repetition frequency for pulse transmission by the waveform observation apparatus 1 is defined from pattern 1 to pattern 3. The pattern 1 is a frequency change pattern for measuring the distance to the object and obtaining the object detection distance dgd. The pattern 2 is a frequency change pattern for acquiring a precise reflected waveform from the object at the distance dgd after acquiring the object detection distance dgd. The pattern 3 is a frequency pattern for detecting a moving object, and the frequency is determined based on the distance Δdm in which the measurement target that has acquired the object detection distance dgd may move within the time corresponding to the repetition period of pulse transmission. It is a pattern to change.

  When the waveform observation apparatus 1 starts operation, the pulse transmission repetition frequency is first changed by the pattern 1. In this pattern 1, whether there is a measurement object between the maximum detection distance d2 and the minimum detection distance d1, and if there is a measurement object, it is used to obtain the distance dgd to the measurement object.

  The time for the terahertz pulse wave to reciprocate from the waveform observation apparatus 1 to the measurement object existing at the distance d is 2 · d / c. Therefore, when the pulse generator 3 generates femtosecond pulsed light with a period of 2 · d / c, the next femtosecond pulsed light is transmitted at the timing when the terahertz pulse wave reflected by the measurement object enters the pulse receiver 8. It is reflected by the half mirror 5 and the mirror 6 and can enter the pulse receiver 8 as the synchronous detection pulse 22.

Therefore, in the pattern 1, the frequency control unit 9 repeats the frequency generator so that the frequency is continuously changed between the initial frequency Fs and the termination frequency Fe represented by the following expressions (1) and (2). 2 is controlled. Thereby, if a measurement target exists between the minimum detection distance d1 and the maximum detection distance d2, the object detection distance dgd is acquired.

  The distance measuring unit 16 controls the time-division frequency pattern setting unit 13 to continuously change the frequency between the initial frequency Fs and the terminal frequency Fe represented by the expressions (1) and (2). The waveform acquired by the acquisition unit 15 is monitored. Then, an object detection distance dgd that is a distance between the waveform observation apparatus 1 and the measurement target is calculated from the frequency at which the waveform is acquired. The distance measurement unit 16 outputs the calculated object detection distance dgd to the transmission parameter calculation unit 19 for each target.

When the object detection distance dgd is acquired, the waveform observation apparatus 1 then performs pulse transmission with the pattern 2 in order to observe a precise reflected waveform from the measurement object. In pattern 2, the width of detection resolution dmin × number of sampling points Nitg / 2 is set as a detection range before and after the object detection distance dgd. Therefore, the initial frequency Fs in the pattern 2 is expressed by the equation (3), and the terminal frequency Fe is expressed by the equation (4).

Further, the modulation frequency ΔF when performing frequency modulation on each repetition frequency includes the detection resolution dmin, the modulation period Tsrch, the frequency modulation initial frequency f0, the frequency modulation end frequency f1, the sampling acquisition number Nitg, and the sample accumulation number Nsum. The sampling frequency Fspl is determined. For example, the following equation (5) is determined, and the modulation frequency ΔF is determined based on the detection resolution dmin and the sampling frequency Fspl.

  The pattern 3 is a method of changing the repetition frequency in consideration of the distance Δdm that may move within the sampling time of the measurement object from which the object detection distance dgd is acquired. Refer to the equations (6) and (7). It will be described later.

  The switching control unit 14 instructs the switch 11 to select a frequency while switching a plurality of frequency patterns from the pattern 1 to the pattern 3 within a predetermined time (for example, 10 μs). As described above, since the waveform observation apparatus 1 of the present embodiment sets the parameter of the repetition frequency of pulse transmission based on the relative relationship with the measurement target such as the distance to the measurement target, the relative speed, and the reflection coefficient. Even when the measurement object moves, there is an effect that the waveform can be observed with high accuracy.

  The measurement time for each frequency pattern is 1 μs as standard, and the number of frequency multiplexing is 5 (switching time is 1 μs). Note that the switching control unit 14 gives priority to the instruction from the time division frequency pattern setting unit 13 and performs precise measurement of the object in which the object detection distance dgd is detected, according to the instruction from the transmission parameter calculation unit 19 for each target. Operate.

  FIG. 4 is a diagram illustrating an example of frequency selection control by the switching control unit 14. Reference numerals 41, 42, and 43 in the figure indicate the respective frequency patterns.

  The transmission parameter calculation unit 19 for each object, as a frequency modulation method at the object detection distance dgd, samples the distance that the measurement target can move within the sampling time of the object detection distance ddg (hereinafter referred to as “movable distance Δdm”) and the relative speed. Estimated by the product of time. The relative speed can be obtained from a change in the sampling time of the object detection distance ddg, but may be calculated from an average of a plurality of values in order to improve accuracy.

When the movable distance is Δdm, for example, the frequency controller 9 is instructed to perform frequency modulation between the initial frequency Fs and the terminal frequency Fe shown in the following equations (6) and (7). Send. This frequency pattern is called pattern 3.

  In pattern 3, since the repetition frequency of pulse transmission is corrected in consideration of the movable distance of the measurement object, when tracking the position of the mobile body once detected, the frequency modulation band is higher than in pattern 1 and pattern 2. Therefore, the object detection distance dgd can be detected in a short time. When the reflection coefficient of the terahertz pulse wave to be measured is low and the output of the pulse receiver 8 is weak as a result, the transmission time is adjusted to the switch 11 to increase the integration. For example, if the initial switching standby time is 1 μs and the pulse generation period of the pulse generator 3 is 10 ns (synchronization frequency is 10 MHz), 100 sampling is possible. At this time, for example, when a sufficient amplitude is not obtained for the received signal, the S / N can be improved by increasing the number of samplings by changing the standby time (FIG. 4). Therefore, there is an effect that a measurement object having a low reflection coefficient of the terahertz pulse wave can be accurately detected. The generated frequency pattern is output to the oscillator control unit 12 and the switching control unit 14.

  The time division frequency pattern setting unit 13 is a frequency pattern for waveform measurement based on the relationship between the switching control unit 14, the oscillator control unit 12, the distance to the measurement target of the distance measurement unit 16, and the relative speed and frequency pattern based on the time difference. A switching method of (pattern 2 and pattern 3) is determined.

  Since sampling is performed with a detection resolution corresponding to the measurement target, control is performed including the relative speed when the measurement target moves. For example, when the object to be measured is moving at a relative speed v = 10 km / h (2.7 m / s) with respect to the waveform observation apparatus 1, the time for moving the distance of the detection resolution dres is Δmv = dres / v. Become.

  Each frequency pattern (pattern 2, pattern 3) needs to be sampled at intervals of Δmv or less. For example, the frequency may be switched as shown in FIG. Pattern A (which may be either pattern 2 or pattern 3) shown in FIG. 5 has a small relative speed, a slow sampling cycle, and a characteristic that requires a long integration time. Pattern B (pattern 2 and pattern 3) (Any) may indicate that the sampling rate is fast and the integration time is short because the relative speed is large. In order to simplify the control, as shown in FIG. 5, multiplexing may be performed in accordance with a plurality of frequency patterns and the shortest necessary sampling period. As described above, the time division frequency pattern setting unit 13 transmits the time switching method of each frequency pattern to the switching control unit 14.

  In this way, the transmission parameter calculation unit 19 for each target, the time-division frequency pattern setting unit 13 and the frequency control unit 9 cooperate to determine the distance to the measurement target, the relative speed with the measurement target, and the reflection coefficient of the measurement target. Based on the calculation result, the multiplex number, modulation frequency band, and modulation time of the repetition frequency of the pulse transmission oscillated by the oscillator 10 are set. Therefore, even if the measurement target is a mobile object, high-precision waveform observation is possible. There is an effect that can be performed.

  FIG. 6 is a flowchart illustrating the operation of the waveform observation apparatus 1 according to the embodiment. First, in step (hereinafter abbreviated as “S”) 1, the oscillator control unit 12 transmits basic patterns (pattern 1 and pattern 2) to F <b> 1 to F <b> 2 of the oscillator 10. Next, in S2, the oscillator 10 starts an oscillation operation (F1, F2) at a repetition frequency of pulse generation. Next, in S <b> 3, the switch 11 switches the signal of the oscillator 10 and outputs it to the pulse generator 3.

  Next, in S4, femtosecond pulsed light is emitted from the pulse generator 3. In S5, the pulse transmitter 4 starts the operation of transmitting the terahertz pulse wave, and transmits the terahertz pulse wave from the waveform observation device 1 to the measurement target. The transmitted terahertz pulse wave is reflected by the measurement target and returns to the waveform observation apparatus 1.

  Next, in S6, the pulse receiver 8 starts receiving the terahertz pulse wave in synchronization with the femtosecond pulsed light emitted by the pulse generator 3. Next, in S <b> 7, the waveform acquisition unit 15 performs waveform measurement. Next, in S <b> 8, the distance measurement unit 16 calculates the distance to the measurement target, and generates the pattern 3 as the frequency pattern based on the target-specific transmission parameter calculation unit 19.

  Next, in S9, the setting of the oscillator control unit 12 is changed based on the output of the target transmission parameter calculation unit 19, and a new pattern is generated.

  In S10, the time division frequency pattern setting unit 13 calculates the switching timing setting method, and the switching control unit 14 generates the switching timing according to the outputs of the time division frequency pattern setting unit 13 and the target transmission parameter calculation unit 19. A new frequency pattern switching method is used to switch the next operation. If the operation is to be continued in S11, the process returns to S2, and otherwise ends.

  Next, application examples of the waveform observation apparatus according to the present invention will be described. For example, when the waveform observation apparatus of this embodiment is used for airport security applications, the detection distance can be determined, the minimum detection distance d1 is the installation height, the maximum height of the person, and the maximum detection distance d2 is the size of the security monitoring area. For example, it is 30 m. In addition, the detection resolution dmin is required when the explosive test is performed, and waveform observation and frequency conversion with a fine resolution are required. Therefore, the area where human movement is possible is about the pedestrian movement speed. Frequency modulation parameters are determined. Furthermore, a frequency pattern can be determined by integrating individual measurement objects (targets).

  In addition, when the waveform observation apparatus of this embodiment is used in a product assembly factory such as a vehicle, in one process on the factory line, voids and deterioration of the body such as painting, iron plate, plastic, crack inspection, movable parts, etc. It is conceivable to use a terahertz pulse wave for metal wear and the like, and even in such a case, the position, speed, etc. of the measurement target are often almost constant. Furthermore, inspection can be performed at high speed by selecting the frequency according to the vehicle type.

  Moreover, when the waveform observation apparatus of this embodiment is used for a vehicle surrounding sensor, the detection distance and the relative speed are variable when detecting the front. Since the use efficiency is higher when the area and the frequency modulation width are reduced as much as possible from the features of the present principle, measurement is performed by reducing the movable region Δdm according to the distance to the measurement object, the relative speed, and the acceleration. When the road surface is detected, the detection distance is almost constant. When this method is used, the measurement range and the relative velocity area are reduced, and a terahertz pulse wave from the target can be quickly acquired with a simple configuration.

DESCRIPTION OF SYMBOLS 1 Waveform observation apparatus 2 Repetitive frequency generator 3 Pulse generator 4 Pulse transmitter 5 Half mirror 6 Mirror 7 Beam scanning mechanism 8 Pulse receiver 9 Frequency control part 10 Oscillator 11 Switcher 12 Oscillator control part 13 Time division frequency pattern setting part 14 switching control unit 15 waveform acquisition unit 16 distance measurement unit 17 waveform measurement unit 18 output unit 19 subject transmission parameter calculation unit

Claims (5)

An oscillator that oscillates the repetition frequency of pulse transmission;
Frequency control means for controlling the frequency of the oscillator;
Pulse generating means for generating pulses at a repetition frequency by the oscillator;
Pulse transmitting means for transmitting the pulse generated by the pulse generating means to a measurement object;
Pulse receiving means for receiving the pulse reflected from the measurement object;
A waveform acquisition device including a waveform acquisition means for acquiring a waveform of a received pulse,
The waveform control apparatus, wherein the frequency control means sets a parameter of a frequency at which the oscillator oscillates based on a relative relationship with a measurement target.   The frequency control means, based on the calculation result of any one or more of the distance to the measurement target, the relative speed with the measurement target, and the reflection coefficient of the measurement target, the multiplex number of the frequencies oscillated by the oscillator, The waveform observation apparatus according to claim 1, wherein any one of a modulation frequency band and a modulation time is set. A transmission parameter calculation unit for each object that estimates a distance that the measurement object can move within a time corresponding to the period of the pulse transmission;
The waveform observation apparatus according to claim 1, wherein the frequency control unit corrects the frequency of the oscillator based on the movable distance.   4. The waveform observation apparatus according to claim 1, wherein the pulse transmitted by the pulse transmission unit is a terahertz wave of 0.1 THz to 10 THz. In a waveform observation method for oscillating a repetition frequency of pulse transmission, generating a pulse at the frequency, transmitting the pulse to a measurement target, receiving a pulse reflected from the measurement target, and acquiring a waveform of the received pulse,
A waveform observation method, wherein a parameter of a repetition frequency of pulse transmission is set based on a relative relationship with a measurement target.

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