Absolute distance measurement of optically roughobjects using asynchronous-optical-sampling
terahertz impulse ranging
Takeshi Yasui,1,2,* Yasuhiro Kabetani,1 Yoshiyuki Ohgi,1Shuko Yokoyama,1,3 and Tsutomu Araki1
1

Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama,
Toyonaka, Osaka 560-8531, Japan

2

Institute of Technology and Science, The University of Tokushima, 2-1 Minami-Josanjima,
Tokushima 770-8506, Japan
3

Micro-Optics Co. Ltd., 2-20 Oe-Nakayamacho, Nishikyoku, Kyoto 610-1104, Japan*Corresponding author: yasui@me.tokushima‑u.ac.jp
Received 23 June 2010; revised 21 August 2010; accepted 23 August 2010;
posted 24 August 2010 (Doc. ID 130579); published 22 September 2010

We report on a real-time terahertz (THz) impulse ranging (IPR) system based on a combination of timeof-flight measurement of pulsed THz radiation and the asynchronous-optical-sampling (ASOPS) technique. The insensitivity of THz radiation to optical scattering enables the detection of various objects
having optically rough surfaces. The temporal magnification capability unique to ASOPS achieves precise distance measurements of a stationary target at an accuracy of −551 μm and a resolution of 113 μm.
Furthermore, ASOPS THz IPR is effectively applied to real-time distance measurements of a moving
target at a scan rate of 10 Hz. Finally, we demonstrate the application of ASOPS THz IPR to a shapemeasurement of an optically rough surface and a thickness measurement of a paint film, showing
the promise of further expanding the application scope of ASOPS THz IPR. The reported method will
become a powerful tool for nondestructive inspection of large-scale structures. © 2010 Optical Society ofAmerica
OCIS codes: 110.6795, 120.0280, 280.3400, 280.5600.

1. Introduction

Recent progress made in the construction of largescale structures on the scale from a few to several
tens of meters, such as buildings, bridges, airplanes,
ships, and parabolic antennae, increases the demandfor precise distance and/or shape measurement techniques. One promising method to achieve noncontact
distance measurement is to use an optical distancemeter based on intensity-modulated continuouswave (CW) light [1,2], ultrashort pulsed light [3,4],or an optical frequency comb [5,6]. For example, absolute distance measurement at a distance of 240 m
0003-6935/10/285262-09$15.00/0© 2010 Optical Society of America
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APPLIED OPTICS / Vol. 49, No. 28 / 1 October 2010is achieved with a resolution of 50 μm using theoptical beat component between longitudinal modesof an optical frequency comb [5]. However, since mostlarge-scale structures have optically rough surfaces,such as metal, plastic, rubber, or paint, optical scattering at their surfaces often hinders the use of optical distance meters. To avoid optical scatteringand achieve specular reflection, a corner reflector
has to be fixed on the target; however, this is an obstacle to attaining in situ distance measurement inopen fields and shape measurement. One alternative
approach for in situ measurement of optically roughobjects is millimeter-wave or microwave radar, because the wavelengths used are much larger than
the typical spatial dimensions of the surface roughness, making this technique insensitive to opticalscattering. However, the precision of these radar devices is relatively low. For example, in conventional
microwave and millimeter-wave radars [7–9], themeasurement of the short range less than 10 m is extremely difficult. In short, the conventional distancemeasurement methods based on optical waves andelectric waves have their own merits and demerits.
Combining the merits of both methods would allow
ideal distance measurement of optically rough objects to be achieved.
One potential electromagnetic wave combining thecharacteristics of both methods is terahertz (THz)radiation (0:1–10 THz frequency; 30–3000 μm wavelength) because it lies at the boundary between
optical waves and electric waves and, hence, possesses characteristics of both [10]. Recently, pulsed
THz radiation (THz pulses) has emerged as a newmode for sensing and imaging. Because the wavelength of THz radiation is still larger than the typicalspatial dimensions of the surface roughness, insensitivity to optical scattering enables measurement ofobjects having optically rough surfaces. Furthermore, time-of-flight measurement of a reflected THz
pulse, a technique known as THz impulse ranging(IPR) [11], possesses the possibility of realizing precise distance measurement because the pulse duration is extremely short, typically subpicoseconds topicoseconds. Therefore, THz IPR is highly promisingfor use in the precise distance measurement of optically rough objects, although pioneering work of THz
IPR reports an application for a scale-model simulator of microwave radar [11].
However, when THz IPR is applied to these measurements, we are confronted with one technicaldifficulty: the requirement for mechanical stage
scanning to produce a time delay due to the fact that
detection of the THz pulses is based on pump–probe
schemes in which a single mode-locked pulsed laser
provides both the pump and the probe pulses. When
we measure a target located at an unknown distance,scanning for the time delay has to be extended to thetime period of the THz pulse train to ensure temporal
overlapping between the THz pulse and the probe
pulse. For example, to fully cover the THz pulse period of 12:2 ns used in the system presented here requires a mechanical stage with a scanning range of
1:83 m. If a conventional stepper motor-driven translation stage is used for this scanning, the measurement time will reach several tens of minutes,
which will be a serious obstacle to practical adoptionof THz IPR because the method is only applied to stationary objects. If THz IPR can be achieved in realtime, the number of applications will be greatly increased, for example, moving targets and imaging
measurement.
One effective method to realize rapid data acquisition of the temporal waveform is to use a rotary optical delay line equipped with fast rotating mirrors. A
scan rate of up to 400 Hz and a time delay of up to
1 ns are demonstrated with this approach [12,13].
However, it is still difficult to cover the overall time

period of the THz pulse train. Furthermore, there areseveral significant disadvantages, such as mechanical and acoustic vibrations of the fast rotating motor,
spot size variations, and positional changes of thelaser beam during scanning.
One promising method to achieve scanning with arapid scan rate and long time delay at the same time
is asynchronous optical sampling (ASOPS) using twomode-locked pulsed lasers with slightly mismatchedmode-locked frequencies [14,15]. The ASOPS method
enables us to linearly expand the time scale of ultrafast transient signals. The resulting slow signal tocover the overall time period can be captured directly
on a standard oscilloscope without the need formechanical time-delay scanning. Also, the ASOPS
method can avoid the above-mentioned significant
disadvantages inherent in the rotary optical delayline. Furthermore, this method always ensures temporal overlapping of the THz pulse and the probe
pulse regardless of the difference of their optical path
lengths because the period of the THz pulse isslightly detuned from that of the probe pulse. Therefore, it is no longer necessary to match the optical
path lengths. Recently, the ASOPS method has beeneffectively applied to terahertz time-domain spectroscopy (TDS) to achieve rapid data acquisition andhigh spectral resolution at the same time [16–19].
However, other applications of the ASOPS method
are still lacking in the THz region. Applying the
ASOPS method to THz IPR will open the door for distance measurement of an optically rough object at an
unknown distance in real time.
In the work described in this paper, we constructeda real-time THz IPR system based on the ASOPS
method, which we call ASOPS THz IPR. We performed absolute displacement measurement of a stationary target and a moving one located at a distancewithin the range of several meters, which is the sameorder of magnitude as for large-scale structures. We
further demonstrated a shape measurement of a
stepped diffusing surface and a thickness measurement of a paint film to assess the effectiveness of thissystem as a nondestructive inspection tool for largescale structures.
2. Principle of Operation

In the impulse ranging method based on time-offlight measurement, the target distance is obtained
from the time delay of a reflected pulse relative to atime-origin signal, which is independent of the target
distance. In our ASOPS THz IPR system, a THz echo
signal and a sum-frequency-generation (SFG) crosscorrelation signal are used as the reflected pulse andthe time-origin signal, respectively. Figure 1(a)shows a timing chart of the SFG signal and the THz
echo signal, where ϕ is the time delay of the THz echo
signal and T is the period of the SFG and THz pulse
trains. Here, we define a spatial period of the THz
pulse train Lhsp as follows:
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to T and T þ ΔT. The change of T by ΔT results in achange of Δϕ in the time delay ϕ. The target distanced before and after the change is given byd¼

cðmT þ ϕÞ c½mðT þ ΔTÞ þ ϕ þ Δϕ
¼
:
2ng
2ng

ð4Þ

Assuming that the target distance d is constant before and after the change of ΔT, ΔT and Δϕ mustsatisfy the following relationship:
Δϕ ¼ −mΔT:ð5Þ

Therefore, we can determine the order m by measuring ΔT and Δϕ.
Fig. 1. (Color online) Principle of distance measurement. (a) Timing chart for SFG signal and THz echo signal when target distanced is shorter and longer than half of the spatial period Lhsp of a THz
pulse train. (b) Timing chart for SFG signal and THz echo signal
when the pulse period is set to T and T þ ΔT.

Lhsp ¼

cT
;
2ng

ð1Þ

where ng is the group refractive index of a medium
for propagation of the THz pulse and c is the velocity
of light in a vacuum. The ng value is equal to 1 when
the THz pulse propagates in air. When the target distance d is shorter than Lhsp , d is simply determined
as follows [see the middle of Fig. 1(a)]:
d¼

cϕ
:
2ng

ð2Þ

However, Lhsp of the THz pulse train is usually too
short for ranging of a distant target. For example,
when a THz pulse train with T ¼ 12:22 ns, corresponding to a repetition frequency of 81:85 MHz, isused, as demonstrated in Section 4, Lhsp is only
1:833 m. If d exceeds Lhsp, we have to apply a modified equationd¼

cðmT þ ϕÞ
;2ng

We modify a transmission setup used in ASOPS THz
TDS [16] into a reflection setup for ASOPS THz IPR.
The experimental setup is shown in Fig. 2. We use
two Kerr-lens mode-locked Ti:sapphire lasers forgeneration and detection of the THz pulse (pumpand probe lasers). The individual mode-locked frequencies f 1 ð¼ 81:84746 MHzÞ and f 2 ð¼ f 1 þ 10 Hz ¼
81:84747 MHzÞ of the two lasers, and the frequency
difference between them Δf ð¼ f 2 − f 1 ¼ 10 HzÞ arestabilized by two independent phase-locked loop
control systems (PLL1 and PLL2). In the PLL electronics, the 150th harmonic component of themode-locked frequency is filtered, amplified, andused to stabilize the mode-locked frequency using
a piezoelectric transducer attached to an output coupler of the laser cavity. By using a rubidium (Rb) frequency standard (Rb FS; Stanford Research Systems
FS725 with 10 MHz frequency, 5 × 10−11 accuracy,and 2 × 10−11 stability at 1 s) as a reference signal
source for laser stabilization, the achieved instabilityof the mode-locked frequency is 2 × 10−11 at a gate
time of 1 s. Precise control of f 1 and f 2 leads toð3Þ

where m is the order of the measured THz pulse. For
example, when m ¼ 2, the timing chart of the THz
echo signal is as illustrated in the lower part in
Fig. 1(a). Because an oscilloscope or digitizer canmeasure only the time delay of the THz pulse relative to the SFG signal nearest in time, i.e., ϕ, we haveto determine m and T for absolute distance measurement of a target at a distance over Lhsp. The period T
can be obtained by measuring the laser output with aphotodetector and a frequency counter. To determine
m, T is changed by ΔT using a laser control system.
Figure 1(b) shows the timing chart for the SFG signal
and the THz echo signal when the pulse period is set
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3. Experimental Setup

APPLIED OPTICS / Vol. 49, No. 28 / 1 October 2010

Fig. 2. (Color online) Experimental setup: pump and probe lasers, mode-locked Ti:sapphire lasers; PD, photodiodes; PLL1
and PLL2, phase-locked loop control systems; Rb FS, rubidium frequency standard; BS, beam splitter; THz PCA emitter, planar,large-area GaAs photoconductive antenna with an interdigitatedelectrode structure for THz generation; THz PCA detector, bowtieshaped, low-temperature-grown GaAs photoconductive antenna
for THz detection; OAPM-1 and OAPM-2, off-axis parabolic mirrors; M, plane mirror.

stabilization of Δf . The resulting timing jitter between the two lasers is suppressed to less than
400 fs, as discussed in Subsection 4.A.
Portions of the two laser beams are fed into an SFG
cross correlator equipped with a nonlinear opticalcrystal by beam splitters, and the resulting SFG signal is used as the time origin for the ASOPS THz IPR
measurement, which is independent of the target
distance. The remaining parts of the two laser beams
are incident on photoconductive antennas (PCAs) forTHz generation and detection, namely, a THz PCAemitter and THz PCA detector, respectively. A planar, large-area GaAs PCA with an interdigitatedelectrode structure (Gigaoptics GmbH, Tera-SED
3) is used as the THz PCA emitter, whereas a bowtie-shaped, low-temperature-grown GaAs PCA (Hamamatsu Photonics; 1 mm bowtie length and a 5
μm gap) is used as the THz PCA detector. The
THz pulse radiating from the THz PCA emitter is collimated by an off-axis parabolic mirror (OAPM-1)
and then directed to a target object by a plane mirror(M). The THz pulse reflected at the surface of the object is collected and focused onto the THz PCA detector by another off-axis parabolic mirror (OAPM-2). To
simulate the impulse ranging applications in open
fields, the measurement is performed with humidity
in normal atmosphere. The current signal from theTHz PCA detector was amplified with a high-gaincurrent preamplifier (AMP; 100 kHz bandwidth and5 × 107 V=A sensitivity). The resulting voltage signal
is measured at a scan rate of Δf ð¼ 10 HzÞ with a fast
digitizer (National Instruments, PCI-5122) by using
the SFG signal as a trigger signal in the ASOPS THz
IPR measurement.
4. Results
A.

Distance Measurement of a Stationary Target

We first evaluated the basic performance of the
ASOPS THz IPR system with a stationary target.
A metal plate with a machined surface was placed
at a distance of about 1 m. Figure 3 shows the temporal waveforms of the THz echo signal returned
from the target; a measurement time of 10 s was
required for signal integration of 100 temporal waveforms. The observed temporal profile was expanded
by a temporal magnification factor of f 1 =Δf ð¼
81; 847; 460=10 ¼ 8; 184; 746Þ according to the principle of the ASOPS method [16]. The upper and lower
horizontal coordinates in Fig. 3 indicate, respectively,
the actual time scale measured by the digitizer andthe real time scale calibrated using the above magnification factor. Despite the optically rough surface,a temporal waveform of the pulsed THz radiation
was measured at a good dynamic range due to theinsensitivity of the THz radiation to optical scattering. Although the sample used has a machined surface with 10-point average roughness (RZJIS ) of
1:03 μm and calculated average roughness (Ra ) of
0:16 μm, the observed magnitude of the electric field
was the same level as that when a metal mirror

Fig. 3. Temporal waveforms of the THz echo signal returned fromthe machined surface of a metal plate. The upper and lower horizontal coordinates indicate the actual time scale measured by thedigitizer and the real time scale calibrated using a temporal magnification factor of f 1 =Δf , respectively.

(RZJIS ¼ 0:06 μm and Ra ¼ 0:01 μm) was set as a sample (result not shown), indicating that the machined
surface functions as a mirror for the THz pulse. The
scattering effect of the THz wave on different surface
roughness was investigated in detail elsewhere [20].
When the target position ϕ ¼ 0 ps was defined as a
distance origin, the absolute distance d of the sample
was determined to be 1:0522 m from the observed ϕof 7:0147 ns using Eq. (2).
Here, let us consider the advantage when the
ASOPS technique is applied for the impulse ranging
method. In the conventional methods, the distanceresolution and precision are largely limited by the response time of the receiver rather than the pulse
duration of the source because the pulse duration
usually exceeds the response time. For example, a response time of 100 ps limits the distance resolution to
30 mm even if a 1 ps pulse is used. To measure thetemporal waveform of the pulse accurately without
the limitation of the response time, we have to use
the pump–probe scheme with mechanical time-delayscanning, resulting in a long measurement time. In
this way, there is an inherent trade-off between thedata acquisition speed and the distance precision in
the conventional impulse ranging methods. On theother hand, the temporal magnification capability
achieved by the ASOPS method resolves this tradeoff issue by reducing the response time requirementof the receiver. For example, when the temporal magnification factor is set to 107 , a 1 ps pulse signal isextended to a 10 μs pulse by the ASOPS method and,hence, can be accurately measured by a standard receiver having a response time of the order of nanoseconds or microseconds. In this case, a peak position
precision of 0:1 μs in the actual time scale is equivalent to that of 10 fs in the real time scale, considering
the temporal magnification factor of 107. Therefore,
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the ASOPS method can achieve precise ranging inreal time.
We next measured the displacement of a target located at a distance of about 1 m. The target was intermittently moved at intervals of 5 mm within arange of 200 mm by a translation stage. The black circle in Fig. 4 shows the relationship between the stage
displacement and the measured displacement with
the ASOPS THz IPR. A good linear relationshipwas confirmed between them. Discrepancy between
them at each stage position is shown as the red triangle in Fig. 4. Because the displacement precision of
the translation stage (<1 μm) is much smaller thanthese discrepancies, the red point in Fig. 4 reflects
the accuracy of the displacement measurement and,hence, the accuracy of distance measurement in
ASOPS THz IPR. The mean accuracy in this demonstration was −551 μm. We consider that this negative
accuracy is caused by insufficient parallelism between the optical path of the THz beam and the moving direction of the translation stage.
In the distance measurement based on the ASOPS
THz IPR, timing jitter between the pump and theprobe lasers is a dominant factor in limiting the resolution of the distance measurement because thetiming jitter causes the temporal magnification factor f 1 =Δf to fluctuate. Furthermore, the amount oftiming jitter is dependent on the time delay ϕ between the THz pulse and the SFG signal [18]. To
evaluate the effect of the timing jitter on the distancemeasurement, we measured the timing jitter of theTHz pulse with respect to various ϕ values, as shownin Fig. 5. Here, the timing jitter was defined as thestandard deviation of the temporal position of the positive signal peak in 100 repeated single-sweep measurements of the THz echo signal. The maximum
timing jitter was 383 fs at ϕ ¼ 12 ns. The axis of the
distance resolution estimated from the timing jitteris indicated at the right vertical coordinate in Fig. 5.
Therefore, distance measurement could be achieved

Fig. 4. (Color online) Relationship between stage displacement
and measured displacement (black circle) and discrepancy between them (red triangle).
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APPLIED OPTICS / Vol. 49, No. 28 / 1 October 2010

Fig. 5. Relationship between time delay ϕ and timing jitter. Distance resolution calculated by the timing jitter is given at the rightvertical coordinate.

within the distance resolution of 113 μm by the present ASOPS THz IPR system.
We next determined the absolute distance of a metal plate located at a distance greater than half of thespatial period Lhsp of 1:833 m. The target was a metal
plate placed at an unknown distance greater than
1:833 m. When the initial T was set to 12:21785 ns,
the temporal waveform of the THz echo pulse was observed as the upper blue curve in Fig. 6. At this target position, the time positions of the peaks in theFig. 6. (Color online) Absolute distance measurement of the target located at a distance greater than Lhsp . The temporal waveform
after shift of T (lower green curve) is offset for clarity.

SFG signal and the THz echo signal temporally overlapped, indicating that ϕ ¼ 0 ps. Then, T was shiftedby −1:49 psð¼ ΔTÞ. The resulting THz echo signal isshown as the lower green curve in Fig. 6. This shift
caused a change in ϕ of 2:98 psð¼ ΔϕÞ. Therefore, theorder m of the measured THz pulse was calculated tobe 2 based on Eq. (5). To avoid ambiguities when determining the m value, the precision to determine
the temporal position of the THz pulse peak should
be much better than Δϕ. The actual precision in thepresent system was limited to within 0:383 ps by thetiming jitter between the two lasers (see Fig. 5).
Therefore, the absolute distance was correctly determined in this demonstration that Δϕ ¼ 2:98 ps. To
determine the absolute distance without the ambiguities of the m value, the ΔT value should be set byconsidering the precision of the peak determination.
Finally, we determined the target distance to be
3:6629 m based on Eq. (3). This value is in good
agreement with the actual distance measured witha scale.
The main limiting factor for long distance measurement is THz absorption caused by the atmospheric water vapor because it attenuates anddistorts the temporal waveform of the pulsed THz radiation. Here, we estimate the maximum measurable distance in the present system. When DR is
defined as the ratio of the maximum magnitude of
amplitude in the presence of the THz beam to standard deviation of amplitude in the absence of theTHz beam [21], the present system achieved a DRof 377 at the acquisition time of 10 s (see Fig. 3).
On the other hand, attenuation of the THz wave in
the atmosphere depends on the weather and THz frequency. For example, in the case of sunny weather,
the attenuation is 100 dB=km at the frequency of
0:5 THz [22]. When the sample has a flat, smooth surface for the THz wave and is adjusted to be at a good
incident angle, most of the THz pulse will be retuned
to the THz PCA detector as specular reflection, as
shown in Fig. 3. Under these conditions, when the
maximum measurable distance is defined as a distance that the DR is equal to 2, it is estimated to
be 228 m at the acquisition time of 10 s. However,if the sample has a tilted, uneven, or bent surface,
the collection efficiency of the reflected THz pulse
with the THz PCA detector will largely decrease
due to the specular reflection. In this case, mechanical scanning of the THz beam or phased array antenna will be required. Furthermore, diffraction of theTHz beam during the long propagation will decrease
the collection efficiency.
B.

forms of the THz echo signal acquired in a singlesweep measurement; the lower horizontal scale
indicates the time delay ϕ between the THz echo signal and the SFG signal. In this case, the absolute distance d of the sample was calculated from Eq. (2) [seethe middle of Fig. 1(a)] and is indicated on the upperhorizontal coordinate in Fig. 7. When a target position indicating ϕ ¼ 0 ps was defined as the distanceorigin, the initial distance of the moving target wasdetermined to be 1:035 m. The temporal change of
the THz echo signal is shown as a movie in Media 1,
indicating that the moving target is continuously approaching the distance origin.
In this demonstration, an acquisition time of
100 ms was required to scan the time window of
12:22 ns, although Fig. 7 indicates only a part of thewhole time window to be measured. If a steppermotor-driven translation stage with a moving speedof 1 mm=s is used for this time-delay scanning, themeasurement time will reach 1833 s. Therefore, themeasurement time achieved by the ASOPS method
is 4 orders of magnitude shorter than that achievedby the conventional mechanical-time-delay method.
We next compare the DR of the pulsed THz electric field between the ASOPS and the conventional

Distance Measurement of a Moving Target

To evaluate the real-time measurement capability of
the ASOPS THz IPR, we measured the distance of amoving target in real time. The target was a metal
plate placed at a distance of about 1 m. Then, the target was continuously moved by 200 mm at a speed of
25 mm=s using a translation stage. Figure 7 illustrates the results of six consecutive temporal wave-

Fig. 7. (Color online) Six consecutive measurements of the THz
echo signal when a target was continuously moved by 200 mm at a
speed of 25 mm=s using a translation stage (Media 1). The lower
horizontal coordinate indicates the time delay ϕ between the THz
echo signal and the SFG signal. The target distance d determinedfrom ϕ is given at the upper horizontal scale.
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5267

mechanical-time-delay methods. The DR of the THz
temporal waveforms in Fig. 7 was 32. One may feelthat the DR of these THz signals is very low compared with the conventional mechanical-time-delay
method. However, there is a large difference in measurement time between the two methods. Because
the DR of the electric field is proportional to thesquare root of the measurement time in the ASPOS
method [17], the DR in the present system (¼ 32 at ameasurement time of 100 ms) is comparable to thatin the conventional mechanical-time-delay method(typically, a DR ¼ 1000 at a measurement time of
100 s). Furthermore, it should be emphasized that
this DR is maintained at any temporal position in
the time window of 12:22 ns, which is technically difficult in the conventional method due to the slow acquisition time.
Here, let us consider the distance resolution andthe maximum detectable speed when the target isunidirectionally moved at a constant speed along the
propagation direction of the THz beam. In this case,the distance resolution is limited by the relationshipbetween the sample speed and the scan rate of themeasurement because the target distance changes
between one scan and the next scan. Figure 8 shows
the relationship between the sample speed and the
distance resolution with respect to three different
scan rates of 10 Hz (red solid line), 100 Hz (bluedotted line), and 1000 Hz (green broken line). The
distance resolution increases in proportion to the increase of the sample speed or the decrease of the scanrate. In the case of the demonstration in Fig. 7 andMedia 1, the distance resolution was 2:5 mm. On theother hand, when the distance resolution exceeds
half of the spatial period of the THz pulse train
Lhsp ð¼ 1:833 mÞ by increasing the sample speed,
the target distance can no longer be determined.
Therefore, the sample speed gives the maximum detectable speed of the moving target. The maximum
detectable speed at a scan rate of 10 Hz was

18:33 m=sð¼ 65:99 km=hÞ. The higher scan rate of
the measurement is important in measuring the distance of a faster-moving target at high resolution.
Although the scan rate in the present system waslimited to 10 Hz by the electrical bandwidth of thecurrent preamplifier (¼ 100 kHz) and the actual
spectral bandwidth of the pulsed THz radiation(¼ 1 THz), it should be possible to further increase
Δf by using a faster THz detector and/or higherrepetition-rate mode-locked lasers [17,18].

Fig. 8. (Color online) Relationship between the sample speed andthe distance resolution with respect to three different scan rates of
10 Hz (red solid line), 100 Hz (blue dotted line), and 1000 Hz (green
broken line).

Fig. 9. (Color online) Shape measurement of a stepped metal object: (a) schematic drawing of the object and scanned THz beamand (b) temporal waveform of the THz echo signal measured at fivedifferent steps on the object (Media 2).

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C.

Shape Measurement of a Stepped Object

The real-time measurement capability of the ASOPS
THz IPR system enables us to expand its scope toimaging applications. Next, we demonstrate the surface shape measurement of a stepped metal object. A
schematic drawing of the stepped object is shown in
Fig. 9(a). The THz beam was unidirectionallyscanned across the steps by moving the object witha translation stage. Figure 9(b) shows the temporal
waveforms of the THz echo signal measured at fivedifferent steps on the object. Also, the temporal behavior of the THz echo signal is shown as a movie
in Media 2. The change of the THz echo signals
clearly reflected the surface shape of the object under
test. The transverse resolution in this demonstration
would be equal to a diameter of the THz beam(¼ 20 mm) if the diffraction of the THz beam during
the propagation is negligible. The reason we used an
unfocused THz beam is that it is difficult to apply an
imaging system with a fixed focal length for a sample
located at an unknown distance. If an automatic focusing system of the THz beam is used for such asample, the transverse resolution will be improved
to less than a few millimeters.
In this demonstration, it is important to note that
the ASOPS THz IPR can easily achieve remote measurement of the object shape without knowledge ofthe object distance because it is unnecessary tomatch the optical path lengths between the THz

pulse and the probe pulse. Therefore, the ASOPS
THz IPR will be a powerful tool for real-time
profilometry of industrial products moved on a conveyor belt.
D.

Thickness Measurement of Paint Film

From the viewpoint of maintenance inspection oflarge-scale structures, there is also considerable
need to detect delamination of paint films on metal
substrates, which is an important problem in practice due to the fact that metal substrates exhibit corrosion resulting from moisture or salt penetration
through areas where paint has peeled off. Although
tap testing is normally used for such an inspection,
maintenance workers are exposed to the risk of falling from large-scale structures during this work. Unfortunately, since paint films are usually opaque andcause strong scattering and/or absorption of visible
and infrared light, it is difficult to apply optical probe
methods for this purpose. In contrast, since THz radiation highly penetrates dry, nonmetallic, nonpolarmaterials, it is suitable for remote nondestructive inspection of paint films based on THz tomography[23]. A device based on this technique, called a THz
paintmeter, is reported [24–26]; however, conventional THz paintmeters can be applied only to targets
located at a known distance due to the limited time
window in the mechanical time-delay scanning [24,25]
or electro-optical time-to-space conversion [26].
Therefore, long-distance THz paintmeters are another interesting application of the ASOPS THz IPR.
We demonstrated thickness measurement of asingle-layer paint film using the ASOPS THz IPR
system. As a sample, we used an oil-based, alkyd
paint film (white color) on an aluminum substrate located at a distance of about 1 m. The group refractive
index in the THz region ng , geometric thickness D,and optical thickness ng D of the paint film were
determined in advance to be 2.59, 220 μm, and570 μm, respectively, using a conventional THz paintmeter equipped with mechanical time-delay scanning [24], and a contact-type thickness meter(eddy-current type, precision 3% of the actual thickness). Figure 10(a) shows an original signal of theTHz echo pulse for the single-layer paint film;
however, it is a little difficult to distinguish the firstfrom the second THz echo signals accurately, whichcome from the air/paint (paint surface) and paint/
substrate (paint back surface) interfaces, respectively. We next calculated an impulse response signalof the THz echo pulse as shown in Fig. 10(b), based on
numerical Fourier deconvolution [23]. As a result, we
confirmed that two THz echo signals were separatedby 3:8 ps. The time separation Δt between the two
echoes is given by
Δt ¼

2ng D
:
c

ð6Þ

Therefore, the geometric thickness D of the paintfilm was determined to be 205 μm from Δt of 3:8 ps
and ng of 2.59. There was a discrepancy of 7% be-

Fig. 10. (a) Original signal and (b) impulse response signal ofTHz echo pulse returned from a single-layer paint film on an Al
plate.

tween this result and the result obtained with thecontact-type thickness meter. Considering that themeasurement spot in the ASOPS THz IPR is notcompletely consistent with those measured with theconventional THz paintmeter and the contact-type
thickness meter, and that the paint sample preparedby us indicates an uneven thickness distribution of
about 10% [24,25], this discrepancy was within theestimated uncertainty. The minimum detectable
thickness of the paint film is limited to 51:5 μm by thepulse duration of 0:89 ps (see Fig. 3) and the ng valueof 2.59 of the paint film [24]. If the ng value ofthe paint film is unknown, one can determine only
the optical thickness ng D from Eq. (6). It should bepossible to apply the ASOPS THz IPR to detection
and two-dimensional mapping of areas where painthas peeled off, as demonstrated with the previous
THz paintmeter [24].
5. Conclusions

We constructed an ASOPS THz IPR system combining the advantages of both optical distance meters
1 October 2010 / Vol. 49, No. 28 / APPLIED OPTICS

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and electric-wave radars. The insensitivity of THz
radiation to optical scattering enables detection ofvarious objects having optically rough surfaces.
Combination of time-of-flight measurement using
an 0:89 ps THz pulse with temporal magnification
capability achieved by the ASOPS method was used
for distance measurement of a stationary target at an
accuracy of −551 μm and a resolution of 113 μm.
Although the achieved ranging performance could
not go beyond that in the optical distance meter[5], the proposed method has the big advantage ofthe in situ distance measurement of optically roughobjects over the optical distance meters due to insensitivity to optical scattering. We proposed a uniquemethod to determine the absolute distance of a target
located at a distance in excess of half of the spatial
period of the THz pulse train (¼ 1:833 m). Furthermore, the ASOPS THz IPR system was effectively applied to real-time distance measurement of a moving
target at a scan rate of 10 Hz. Finally, we demonstrated the application of the ASOPS THz IPR system to shape measurement of an optically roughsurface and thickness measurement of a paint film,
showing the potential to further expand the application scope of the THz IPR method. The ASOPS THz
IPR has the potential to become a powerful tool for
nondestructive inspection of large-scale structures
if the system can be made more compact, robust, flexible, and cost effective by use of a fiber-laser-based
ASOPS system [16] and fiber-coupled PCAs [27].
This work was supported by Grants-in-Aid for
Scientific Research, 20560036 and 21650111, from
the Ministry of Education, Culture, Sports, Science,
and Technology of Japan. We also gratefully acknowledge financial support from Renovation Centerof Instruments for Science Education and Technology of Osaka University, Japan.
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