322

IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 3, NO. 3, MAY 2013

Terahertz Comb Spectroscopy Traceable to
Microwave Frequency Standard
Yi-Da Hsieh, Yuki Iyonaga, Yoshiyuki Sakaguchi, Shuko Yokoyama, Hajime Inaba, Kaoru Minoshima,
Francis Hindle, Yoshinori Takahashi, Masashi Yoshimura, Yusuke Mori, Tsutomu Araki, and Takeshi Yasui

Abstract—The fine-structured spectrum of a terahertz (THz)frequency comb has been observed using asynchronous-optical-sampling THz time-domain spectroscopy with an extended
time window covering multiple THz pulses. Fourier transformation of 10 consecutive THz pulses enables us to obtain the
spectrum of THz comb mode having a linewidth of 25 MHz atintervals of 250 MHz. Further expansion of time window up to
100 consecutive THz pulses reduces the linewidth of THz comb
mode down to 2.5 MHz while conserving interval of 250 MHz. The
observed THz comb mode can be used as a precise and accurate
frequency marker for broadband THz spectroscopy because it isphase-locked to a microwave frequency standard by laser control.
The spectroscopy of pharmaceutical tablets and low-pressure
molecular gas is used to demonstrate the utility of this approach,indicating a spectral resolution of 250 MHz. The proposed method
enables frequency calibration of the THz spectrometer based on a
microwave frequency standard.
Index Terms—Asynchronous optical sampling, frequency comb,
gas spectroscopy, Terahertz (THz) wave, THz spectroscopy.

I. INTRODUCTIONTERAHERTZ (THz) spectroscopy is appreciated as a
powerful tool for sensing and material characterization
due to the unique characteristics inherent in THz radiation andManuscript received November 14, 2012; revised February 05, 2013;
accepted February 16, 2013. Date of publication March 15, 2013; date ofcurrent version April 29, 2013. This work was supported by Collaborative
Research Based on Industrial Demand from Japan Science and Technology
Agency, Grants-in-Aid for Scientific Research No. 23656265 from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, andRenovation Center of Instruments for Science Education and Technology inOsaka University, Japan.
Y.-D. Hsieh, Y. Iyonaga, Y. Sakaguchi, and T. Araki are with the Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka
560-8531, Japan (e-mail: sya@sml.me.es.osaka-u.ac.jp; iyonaga@sml.me.es.
osaka-u.ac.jp; sakaguti@me.es.osaka-u.ac.jp; araki@me.es.osaka-u.ac.jp).
S. Yokoyama is with the Micro Optics Company, Ltd., Nishikyo, Kyoto 6101104, Japan (e-mail: shuko.yokoyama@micro-optics.jp).
H. Inaba and K. Minoshima are with the National Metrology Instituteof Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8563, Japan (e-mail: h.inaba@aist.go.jp;
k.minoshima@aist.go.jp).
F. Hindle is with the Laboratoire de Physico-Chimie de l’Atmosphère, Université du Littoral Côte d’Opale, Dunkerque 59140, France (e-mail: francis.
hindle@univ-littoral.fr).
Y. Takahashi, M. Yoshimura, and Y. Mori are with the Graduate
School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan(e-mail: takahashi@cryst.eei.eng.osaka-u.ac.jp; yoshi@eei.eng.osaka-u.ac.jp;
mori.yusuke@eei.eng.osaka-u.ac.jp).
T. Yasui is with the Institute of Technology and Science, The University of Tokushima, Tokushima, Tokushima 770-8506, Japan (e-mail:yasui.takeshi@tokushima-u.ac.jp).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TTHZ.2013.2250333

the abundant THz spectral fingerprints unlike from the corresponding infrared signature. The most widely used techniquesin THz spectroscopy include THz time-domain spectroscopy(THz-TDS) with coherent pulsed THz radiation [1], [2], Fouriertransform far-infrared spectroscopy (FT-FIR) with incoherent
THz radiation [3], and THz frequency-domain spectroscopy
with narrow tunable continuous-wave (CW) THz radiation(CW-THz spectroscopy). The last method can be further
classified into photonic [4] and electronic [5] based systems
depending on the source and detector. Both THz-TDS andFT-FIR have the merit of providing a broad spectral coverage
but suffer from poor spectral resolution owing to mechanicaltime-delay scanning. Furthermore, since the frequency scale inTHz spectrum obtained by these methods is not assured by afrequency standard, there is a possibility that the spectrum of asample is not necessarily the same among different apparatuses.
In the case of the photonic-based CW-THz spectroscopy, highspectral resolution can be achieved but the accessible spectral
range is usually narrower than that of THz-TDS and FT-FIR.
Furthermore, the mode hopping of the CW-THz radiation often
limits the continuous tuning when a broad spectral range is required. On the other hand, the electronic-based system provides
fast sweeping of high spectral purity without mode hopping,high brightness sources, and exact frequency determination
with electronic frequency reference to CW-THz spectroscopy.
However, the accessible spectral range is still limited to a few
hundreds GHz. In this way, each of these methods has its merits
and weaknesses. If one could combine the merits of them, it
would be possible to further expand the application field of
THz science and technology based on high performance andreliable frequency metrology.
Recently, THz frequency combs have started to reveal theirpotential for THz frequency metrology [6], such as spectroscopy [7], [8], frequency measurement [9]–[12], and signal
generation [13]–[15]. A THz comb, which is a description ofpulsed THz radiation in the frequency domain, is composedof a series of CW-THz waves regularly separated by themode-locked frequency of the pulsed THz radiation. In the caseof frequency metrology a THz comb is attractive due to thenarrow-linewidth of the CW-THz waves that provide regularfrequency marks over a wide spectral band. Furthermore, theabsolute frequencies of all comb modes can be phase-locked
to a microwave frequency standard by laser control. These
characteristics enable us to use a THz comb as a precise ruler ofTHz frequency. For example, THz-comb-referenced spectrum
analyzer has been successively applied to the measurementof absolute frequency in CW-THz sources with an accuracy
to 10[9]–[12]. Furthermore, the THz combof 10
should combine the merits of both THz-TDS and CW-THz

2156-342X/$31.00 © 2013 IEEE

HSIEH et al.: THz COMB SPECTROSCOPY TRACEABLE TO MICROWAVE FREQUENCY STANDARD

Fig. 1. (a) Temporal waveform of single THz pulse and (b) corresponding THz
continuous spectrum. (c) Temporal waveform of THz pulse train and (d) corresponding THz comb spectrum.

spectrometers, so that it can show the frequency scale of thespectrum based on a frequency standard. There are somestudies that report the observation of detailed spectrum of
THz comb itself based on the multi-frequency-heterodyningtechnique [7], [8]; however, poor signal-to-noise ratio (SNR)in spectral amplitude, arising from frequency-domain measurement with a spectrum analyzer, hinders the applicationof THz comb to practical spectroscopy. In this paper, an
SNR-enhanced THz comb spectrum is obtained by the useof time-window-extended asynchronous-optical-sampling
THz-TDS (ASOPS-THz-TDS). Furthermore, the THz spectroscopy of several samples using a THz comb is demonstrated.
II. TIME-WINDOW-EXTENDED ASOPS-THZ-TDS
Here, let us consider how we can acquire the fine-structured
spectrum of a THz comb, which surpasses spectral resolution of
conventional THz spectroscopy. A traditional THz-TDS system
employing mechanical time-delay scanning usually measures
the temporal waveform of only a single THz pulse, as shown in
Fig. 1(a). Taking the Fourier transformation (FT) of the waveform gives the continuous spectrum of the broadband THz radiation without comb structure, as shown in Fig. 1(b). On the other
hand, if the time window is greatly expanded to cover morethan one pulse period, one can measure the temporal waveformof consecutive THz pulses, that is to say, a THz pulse train, as
shown in Fig. 1(c). Taking the FT of this will give the fine-structured spectrum of the THz comb because FT of the periodic THz
pulses imprints a frequency modulation on THz spectrum, as
shown in Fig. 1(d).
However, in conventional THz-TDS system it is not practical to measure such a temporal waveform of THz pulse train
because it requires scanning over a long time delay. Instead, we
focus on ASOPS-THz-TDS using two mode-locked lasers withslightly mismatched mode-locked frequencies [8], [16]–[19].
Since ASOPS-THz-TDS enables us to expand ps time scale ofTHz pulse up to the s scale based on the principle of asynchronous optical sampling [20], the resulting slow temporal
waveform can be directly measured by a standard oscilloscope
without the need for the mechanical time-delay scanning. One

323

Fig. 2. Experimental setup. Rb-FS: rubidium frequency standard; SFG-XC:
sum-frequency-generation cross-correlator; SHGs: second-harmonic-generation crystals; Ls:lenses; EDFA: erbium-doped fiber amplifier; OSC,
erbium-doped fiber oscillator; PCA1: dipole-shaped LT-GaAs photoconductive
antenna for THz generation; PCA2: dipole-shaped LT-GaAs photoconductive
antenna for THz detector; Si-Ls: silicon lenses; AMP: current preamplifier.

can, therefore, arbitrarily adjust the observed time window of
THz pulse temporal waveform by changing the time scale ofthe oscilloscope. Conventionally, the ASOPS-THz-TDS has
been used to acquire the temporal waveform of the single THz
pulse within the range of the time window equal to one pulse
period; however, it is possible to extend the time window tomore than one pulse period due to its non-mechanical nature of
the time-delay scanning.
III. EXPERIMENTAL SETUP
Our ASOPS-THz-TDS system consists of dual mode-locked
Er-fiber lasers (ASOPS TWIN 250 with P250, Menlo Systems;,
fs,mW) and a delay-stage-free THz-TDS
setup including a pair of photoconductive antenna (PCA) as
shown in Fig. 2. The individual mode-locked frequencies of thetwo lasers (Hz andHz) andthe frequency offset between themHz
were stabilized by two independent laser control systems referenced to a rubidium frequency standard (Rb-FS,
andat 1s). After wavelength conversion of the two laser beams bysecond-harmonic-generation crystals (SHGs), pulsed THz radiation was emitted by a dipole-shaped, low-temperature-grown,
GaAs PCA (LT-GaAs-PCA) triggered by pump light (PCA1;mW,), and wasthen detected by another dipole-shaped LT-GaAs-PCA triggered by probe light (PCA2;mW). The
optical path in which the THz beam propagated was purged
with dry nitrogen gas to avoid absorption by atmosphericmoisture. Portions of the output light from the two lasers
were fed into a sum-frequency-generation cross-correlator(SFG-XC). The resulting SFG signal was used to generate atime origin signal in the ASOPS-THz-TDS. After amplification
with a current preamplifier (AMP,,), the temporal waveform of theoutput signal from PCA2 was acquired with a digitizer(
MS/s,bit) by using

324IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 3, NO. 3, MAY 2013

the SFG-XC signal as a trigger signal. Then, time scale of theobserved signal was calibrated by a temporal magnification
factor of[17]. Finally,
the amplitude spectrum of THz comb was obtained by FT ofthe temporal waveform of THz pulse train.
IV. RESULTS
A. Basic Performance
To acquire the temporal waveform of 10 consecutive
THz pulses, we set the time window of the digitizer tos with a sampling rate of 2 MS/s
and 400 000 samples per acquisition. Once the SFG signal
triggered the measurement of the temporal waveform, the dataacquisition was continued until the completion of the 400 000
samples. During this data acquisition period, any SFG signal
was disregarded and was not treated as a trigger signal. As aresult, the temporal waveform with the time window size of 40
ns was measured. Since this temporal waveform was composedof 400 000 samples, the real time sampling interval is 100 fs.
Fig. 3(a) shows the acquired temporal waveform of the THz
pulse train, obtained by repeating the measurement of the temporal waveform over 10 consecutive THz pulses (acquisition
time for) 5000 times and integrating
the data (,s). Ten consecutive THz pulses are clearly observed at aninterval of 4 ns. Also, the peak-to-peak value in electric filed
has no deviation for these THz pulses, implying negligible influence of the timing jitter. Actually, 10 repetitive measurementsof the temporal waveform without signal integration indicatedthat the accumulated timing jitter was 105 fs for the tenth THz
pulse in Fig. 3(a). Small modulation observed in the interval of
the THz pulses is mainly due to electromagnetic noise propagating in free space.
Fig. 3(b) shows a magnified temporal waveform of the firstTHz pulse. The transient evolution of the pulsed THz radiationin the picosecond region was also observed clearly. A dynamic
range of time scalein this measurement is calculated
as follows:

dB

(1)

Such a wide
leads to wide dynamic range of frequency
scale that can resolve fine-structured spectrum of the broad THz
comb although the actual spectral range depends on the performance of THz emitter and detector and/or timing jitter between
the dual fiber lasers.
The red line in Fig. 3(c) shows the amplitude spectrum of
the THz comb obtained by taking the FT of the temporal waveform of the THz pulse train in Fig. 3(a). In contrast, the blueline in Fig. 3(c) shows a continuous spectrum of the THz amplitude obtained by taking the FT of the temporal waveform of
the single THz pulse
ns measured with
one-pulse-period ASOPS-THz-TDS [19]. The spectral envelopof the THz comb spectrum is in good agreement with the spectral shape of the THz continuous spectrum. This indicates that
our ASOPS-THz-TDS can capture the temporal waveform ofFig. 3. Temporal waveform of electric field in: (a) 10 consecutive THz pulses
ns and (b) the first THz pulseps .
(c) Comparison of amplitude spectrum between THz comb spectrum and THz
continuous spectrum. (d) Amplitude spectrum of THz comb mode around 0.557
THz.

THz pulse train correctly without influence of timing jitter between dual fiber lasers and/or pulse-to-pulse timing jitter.
The spectrum of THz comb is composed of a series offrequency spikes regularly separated by the mode-locked
frequency of the laser. To observe its detailed structure, we
expanded the spectral region around 0.5570 THz, as shown inHSIEH et al.: THz COMB SPECTROSCOPY TRACEABLE TO MICROWAVE FREQUENCY STANDARD

Fig. 4. (a) Temporal waveform of electric field in 100 consecutive THz pulses
ns . (b) Amplitude spectrum of THz comb mode around0.557 THz.

Fig. 3(d). The THz comb modes have a frequency spacing of
250 MHz and a linewidth of 25 MHz. The frequency spacing
was equal to the mode-locked frequency of the fiber laser,
whereas the linewidth was consistent with the reciprocal of thetemporal window in Fig. 3(a)ns . Here the dynamic
range of frequency scale
is defined as follows:(2)
In the case of Fig. 3(c) and (d), theachieved is 98 dB
from the sampling step of 25 MHz and the spectral range of 2
THz.
The time window was extended further to decrease thelinewidth of the observed THz comb mode. Fig. 4(a) shows
the acquired temporal waveform of the THz pulse train
when the observed time window was extended to 400 ns(,).
Since this time window was 100 times longer than one period of thens , it contains 100 consecutive
THz pulses. This time window corresponds to a mechanical time-delay scanning distance of 60 m if a conventional
THz-TDS scheme was used, a distance that cannot be realistically achieved with a scanning stage. Even in the case of thisincomparable time window, the accumulated timing jitter was
569 fs for the 100-th THz pulse in Fig. 4(a).
After taking the FT of the temporal waveform in Fig. 4(a),we expanded the spectral region around 0.5570 THz again, as
shown in Fig. 4(b). Due to the increase of the325

Fig. 5. (a) Amplitude spectrum of THz comb generated by DAST crystal.
(b) Spectral comparison of THz comb mode around 0.4005 THz generated by
LT-GaAs-PCA with one generated by DAST crystal.

ns a decreased linewidth of 2.5 MHz is observed for theTHz comb modes, with an identical frequency spacing as inFig. 3(d). In this case,achieved is 118 dB from the sampling step of 2.5 MHz and the spectral range of 2 THz, which isincomparable performance in THz spectroscopy.
We also tried to extend the spectral bandwidth of THz comb
by using a 4-N, N-dimetylamino-4 -N -methyl stilbazolium tosylate (DAST) crystal
m as a THz emitter
instead of PCA1. In this experiment, we changed
to be 5
Hz. The green line of Fig. 5(a) shows the amplitude spectrum of
the THz comb obtained by taking the FT of the temporal waveform of 10 consecutive THz pulses (,s). For comparison, an amplitude spectrum measured in the absence of the THz beam(noise spectrum) is shown as the black line in Fig. 5(a). The
observed spectral bandwidth was extended up to 4 THz, which
was twice as large as that in Fig. 3(c). This is because of thephase matching condition between THz pulse and 1.5 m laser
light in DAST crystal enhancing the THz spectral bandwidth[21]. Fig. 5(b) shows a spectral comparison of the THz comb
mode around 0.4005 THz between DAST-based system and
PCA1-based system. Absolute frequencies of THz comb modes
in DAST crystal are strictly consistent with those in PCA1.
This is because the absolute frequencies of all comb modes
are secured by the rubidium frequency standard within a certain level of uncertainty, which depends on the stability of the. Such the traceability to the frequency standard enables us to give the universal
frequency scale on THz spectrum among different apparatuses.

326IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 3, NO. 3, MAY 2013

B. Spectroscopy of Pharmaceutical Tablets
To evaluate the spectroscopic potential of THz comb, werecorded the absorption spectra of several pharmaceutical
tablets because they display sharp spectral fingerprints in THz
region resulting from their crystal structure. For the sake of thisdemonstration, we prepared two different sugar tablets (lactose
and -maltose powders mixed with polyethylene powder)and one reference tablet (polyethylene powder) with identicalshapes (mm andmm). Sugar
content by weight was 25% for -maltose tablet and 50% forlactose tablet, respectively.
The red and blue lines in Fig. 6(a) and (b) shows amplitude
spectra of THz comb after passing through the lactose tablet
and the -maltose one, respectively. For comparison, the amplitude spectrum of THz comb after passing through the reference tablet was also indicated as the black line in both graphs.
Spectral dips clearly appeared in both spectra. Fig. 6(c) shows
the absorption spectra of the two sugar tablets. Each absorption
spectrum was calculated from envelop of power spectra of THz
comb for the sugar tablet and the reference tablet. The two sugar
tablets exhibited characteristic spectral fingerprints of THz absorption: 0.53 THz and 1.37 THz for the lactose and 1.10 THz
for the -maltose, in good agreement with the value reported in
the literature [22]. In this way, it is possible to classify pharmaceutical tablets based on their spectral fingerprints appearing in
THz comb even though they have a similar appearance.
C. Spectroscopy of Low-Pressure Molecular Gas
The demonstration in the previous section indicated apotential of THz comb spectroscopy for pharmaceuticalapplications. However, this demonstration does not necessarily make full use of advantages offered by THz comb
spectroscopy because the spectral features of the pharmaceutical tablets are broader than the frequency interval between
MHz . From the viewpoint ofhigh-precision THz spectroscopy, another interesting application is the analysis of atmospheric molecular gases, theirrotational transitions give them particularly rich spectral fingerprints with much narrower linewidths in the THz region. To
correctly measure these densely distributed absorption lines in
THz spectroscopy requires high resolution, high accuracy, andbroad spectral coverage.
To evaluate the potential of THz comb spectroscopy for gas
analysis, the absorption spectrum of the rotational transition
at 0.557 THz of water vapor was measured. To
avoid pressure broadening of the absorption lines, water vapor
was enclosed in a low-pressure gas cell (mm,mm). To decrease the strong absorption of water
vapor, the water vapor was diluted in nitrogen gases. The partial pressures of water vapor and nitrogen were set to 0.6 kPa
and 1.3 kPa respectively. We estimated the pressure broadening
linewidth of this absorption line to be 250 MHz by the pressure
broadening coefficient [23]. The red line in Fig. 7(a) shows amplitude spectrum of THz comb mode around 0.557 THz withthe sample gas. Also, we obtained a reference spectrum withthe vacuum cell as shown in the black line in Fig. 7(a). Fromcomparison between them, decrease of spectral amplitude due toabsorption of water vapor was confirmed across only two comb

Fig. 6. Amplitude spectra of THz come after passing through (a) lactose tablet
and (b) -maltose one. (c) Comparison of absorbance spectra between lactose
and -maltose.

modes around 0.557 THz. The absorption spectrum was calculated by extracting peak of each comb mode of both spectra andnormalizing water vapor spectrum by the reference spectrum asshown in Fig. 7(b). The spectral linewidth was determined to be
265 MHz when a Lorentzian function was fitted to the spectral
shape, indicated by the red solid line in Fig. 7(b), which is consistent with the expected pressure broadening linewidth of 250
MHz. We estimated from the accumulated timing jitter of 105 fs
and the time window of 40 ns that the contribution of the timing
jitters to the spectral resolution was 66 Hz. The center frequencyof this absorption spectrum determined by the curve fitting was
0.556858 THz, also in good agreement with the value reported
in theTHz [24]. The deviation of 78
MHz from the literature values corresponds to the spectral accuracy of 10 . Here, it should be emphasized that this deviation is due to discrete distribution of comb modes at 250-MHz

HSIEH et al.: THz COMB SPECTROSCOPY TRACEABLE TO MICROWAVE FREQUENCY STANDARD

Fig. 7. (a) Amplitude spectrum and (b) absorption spectrum for the water roat partial pressure of water vapor and nitrogentational transition of
to be 0.6 and 1.3 kPa.

intervals, rather than uncertainty of absolute frequency of combmode.
To demonstrate the capacity to simultaneously probe multiple
absorption lines of low-pressure molecular gas, we performed
gas-phase spectroscopy of acetonitrile CH CN . As symmetric
top molecule CH CN has a rotational constant of 9.2 GHz[25], a manifold of rotational transitions regularly spaced byappears in THz region [26]. Fig. 8(a) shows
the absorption spectrum of CH CN at 40 Pa. Many manifoldsof the regularly spaced rotational transitions (to 51)
were clearly found within a frequency range from 0.3 to 1.0
THz. To observe the detailed structure of these absorption lines,
we expanded the spectral region around 0.58 THz, as shownin Fig. 8(b). Two adjacent manifolds (and 31) wereclearly observed. The frequency separation between them was
18.4 GHz, is consistent with thevalue of CH CN [26]. A
small spike beside the manifold around 0.57 THz
iscaused by the electric noise in the current preamplifier. Fromthese results, we can conclude that THz comb spectroscopy has
a high potential for high-precision THz spectroscopy of lowpressure molecular gas.
V. DISCUSSION
One might feel that there is little difference between the THz
comb spectroscopy and one-pulse-period ASOPS-THz-TDS[19]. Furthermore, another might think that repetitive THz
pulse waveforms are only replicas of a single THz pulse
waveform over one pulse period and thus cannot provide
any more information than a one-pulse-period THz pulse
waveform. Here we discuss the difference between the THz
comb spectroscopy and one-pulse-period ASOPS-THz-TDS
assuming that contribution of the timing jitter to the spectral
resolution is negligible. On one hand, the spectral resolution

327

Fig. 8. (a) Absorption spectrum of acetonitrile gas at 40 Pa and (b) its magnified
spectrum around 0.58 THz.

in one-pulse-period ASOPS-THz-TDS was equal to the laser
mode-locked frequency[19]. In this case, each data plot
on the spectrum was given as a mean value of the spectral
amplitude over theintervals. On the other hand, THz comb
spectroscopy provided the discrete spectra of THz comb modewith a linewidth of 25 MHz [see Fig. 3(d)]. The spectrum was
obtained by sampling the spectral amplitude with the 10%
width ofat intervals of . Therefore, the main difference
between them is that the THz spectrum is composed of theaveraging data for one-pulse-period ASOPS-THz-TDS andthe sampling data for THz comb spectroscopy. Unfortunately,
discrete distribution of THz comb mode limits the practical
spectral resolution in THz spectroscopy toeven thougheach data plot in the spectrum has the spectral resolution of
10% in . However, the fact that individual comb modes were
spectrally resolved will open the door for higher resolution THz
spectroscopy over the broad spectral range based on narrow
linewidth of the comb mode. Work is in progress to further
decrease the spectral resolution in THz spectroscopy down tolinewidth of comb mode.
VI. CONCLUSIONSA THz comb has been employed as a precise frequency ruler
for THz spectroscopy. The fine-structured spectrum of THz
comb was obtained at good SNR by applying time-window-extended ASOPS-THz-TDS to temporal waveform measurementof THz pulse train with incomparable dynamic range of the timescale. To evaluate the spectroscopic potential of THz comb, wedemonstrated the spectroscopy of pharmaceutical tablets andmolecular gases. The individual spectral fingerprints of pharmaceutical tablets clearly appeared on the amplitude spectraof THz comb, to the best of our knowledge, this is the firstexperimental demonstration of THz comb spectroscopy. The
results of low-pressure gas-phase spectroscopy indicate that the328IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 3, NO. 3, MAY 2013

established frequency scale is directly related to the THz comb
structure, having frequency interval of 250 MHz and linewidthof 25 MHz in our case. The discrete distribution of THz comb
modes limits the practical spectral resolution and accuracy inthese demonstrations to be 250 MHz and 10 , respectively.
Although the achieved resolution and accuracy may not be always sufficient for specific applications of high-precision THz
spectroscopy, an assured traceability of the frequency scale
to a microwave frequency standard is a significant advantageof this method over conventional THz spectroscopic methods
without any traceability. These spectroscopic demonstrations
make us conclude that the THz comb truly combines the
merits of both THz-TDS and CW-THz spectrometers and has
the strict traceability to a frequency standard. Furthermore,
25-MHz or 2.5-MHz linewidth of comb mode, which is muchnarrower than frequency interval between comb modes, implies
the possibility to further enhance the spectral resolution andaccuracy in THz comb spectroscopy. Therefore, THz comb
opens the door for establishment of frequency metrology in
THz spectroscopy.
ACKNOWLEDGMENT
The authors are grateful to Dr. M. Hashimoto of Osaka University and Dr. H. Hoshina of RIKEN for fruitful discussions.
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Yi-Da Hsieh received the Master’s degree in biomedical imaging and radiological sciences from the National Yang-Ming University, Taiwan, in 2008, and iscurrently working toward the Ph.D. degree from the
Graduate School of Engineering Science, Osaka University, Toyonaka, Japan.
His areas of research are THz instrumentation andgas-phase THz spectroscopy.
Mr. Hsieh is a member of the Japan Society ofApplied Physics.

HSIEH et al.: THz COMB SPECTROSCOPY TRACEABLE TO MICROWAVE FREQUENCY STANDARD

Yuki Iyonaga received the Bachelor’s degree in mechanical science and bioengineering from the Osaka
University, Toyonaka, Japan, in 2011, and is currently
working toward the Master’s degree from the Graduate School of Engineering Science, Osaka University, Toyonaka, Japan.
His areas of research are THz instrumentation andgas-phase THz spectroscopy.
Mr. Iyonaga is a member of the Japan Society ofApplied Physics.

Yoshiyuki Sakaguchi is currently a Technical staff
in Graduate School of Engineering Science, Osaka
University, Toyonaka, Japan. His area of research isprocess technology for semiconductor device.
Mr. Sakaguchi is a member of the Japan Society ofApplied Physics.

Shuko Yokoyama received the Ph.D. degree inmechanical engineering from the University ofTokushima, Tokushima, Japan, in 1996.
She is currently a CEO in Micro Optics Company, Ltd., Kyoto, Japan. Since 2006, she has beena Postdoctoral Researcher in the Department of
Mechanical Science and Bioengineering, Graduate
School of Engineering Science, Osaka University,
Toyonaka, Japan. Her areas of research are laser
stabilization and optical metrology, particularlyinterferometry and measurement related to opticaland THz frequency comb based on femtosecond laser.
Dr. Yokoyama is a member of the Japan Society of Applied Physics and the
Optical Society of Japan. She received the Best Paper Award in Measurement
Science and Technology from Institute of Physics in 1999.

Hajime Inaba received the Ph.D. degree in applied
physics from Hokkaido University, Sapporo, Japan,
in 2004.
In 1993, he joined the National Research Laboratory of Metrology (NRLM, former AIST), Tsukuba,
Japan, and he began work on continuous-wave
erbium-doped fiber lasers. His areas of research areultrafast optical science and technology and theirapplication to optical metrology, particularly generation and control of frequency combs, and frequency
metrology using frequency combs. He was a visiting
scientist at the National Measurement Institute, Australia, between 2006 and2007. He is currently a senior research scientist at the National Institute ofAdvanced Industrial Science and Technology (AIST), Tsukuba, Japan.
Dr. Inaba is a member of the Japan Society of Applied Physics. He received aPrize for Science and Technology by the Minister of Education, Culture, Sports,
Science and Technology in 2008.

Kaoru Minoshima received the Ph.D. degree inphysics from the University of Tokyo, Tokyo, Japan,
in 1993.
She is at the National Institute of Advanced
Industrial Science and Technology (AIST), Tsukuba,
Japan, and also a Guest Professor of Tokyo University of Science, Japan. In 1993, she joined the
National Research Laboratory of Metrology (NRLM,
former AIST), was briefly with the University of
Bordeaux I as an Invited Professor in 1996 anda Visiting Scientist at Massachusetts Institute of329

Technology, Cambridge, MA, USA, from 2000 to 2001. Her areas of research
are ultrafast optical science and technology and their application to opticalmetrology, particularly time-resolved imaging, generation of frequency combs,
and length metrology using frequency combs.
Dr. Minoshima is a member of the Japan Society of Applied Physics, thePhysical Society of Japan, Laser Society of Japan, and the Optical Society(OSA). She served as one of the General Chairs and Program Chairs of CLEO
conference in 2011 and 2009, respectively. She received a Prize for Scienceand Technology by the Minister of Education, Culture, Sports, Science andTechnology in 2008, and Women Researcher Award, The Japan Society ofApplied Physics in 2010.

Francis Hindle received the Ph.D. degree in electrical engineering and electronics from the Universityof Manchester, U.K., in 2000.
He is appointed lecturer at the Laboratoire dePhysicoChimie de l’Atmosphère, Université du Littoral Côte d’Opale, Dunkerque, France, in 2004. He
has significant experience with THz instrumentation
developing high-resolution spectrometers for gas
phase applications.

Yoshinori Takahashi received the B.S., M.S., andPh.D. degrees in electrical engineering from Osaka
University, Suita, Japan, in 1998, 2000, and 2007, respectively.
He is currently a Post-Doctoral Researcher in the
Division of Electrical, Electronic and Information
Engineering. His research interests include crystal
growth of nonlinear optical crystals for ultraviolet
light generation and THz wave generation, andmedicinal compound.
Dr. Takahashi is a member of the Japan Societyof Applied Physics and the Laser Society of Japan. He received the Lectures
Encouragement Award from the Japan Society of Applied Physics in 2006, and
the Progress Award from the Laser Society of Japan in 2012.

Masashi Yoshimura received the B.S., M.S., andPh.D. degrees in electrical engineering from Osaka
University, Suita, Japan, in 1994, 1996, and 1999,
respectively.
He worked as an Assistant Professor from 1999
to 2007, and is currently an Associate Professor in
the Division of Electrical, Electronic and Information
Engineering. His research interests include development of new nonlinear optical crystals for ultraviolet
light generation and THz wave generation.
Dr. Yoshimura is a member of the Optical Society(OSA), the Japan Society of Applied Physics, the Laser Society of Japan, and theJapanese Association for Crystal Growth. He received the Commendation for
Science and Technology by the Minister of Education, Culture, Sports, Scienceand Technology in the Research Category in 2007, German Innovation Award
Gottfried Wagener Prize 2012, and so on.

Yusuke Mori received the B.S., M.S., and Ph.D. degrees in electrical engineering from Osaka University, Suita, Japan, in 1989, 1991, and 1996, respectively.
He worked as an Assistant Professor from 1993
to 1999, a Lecturer from 1999 to 2000, an Associate
Professor from 2000 to 2007, and is currently a Professor in the Division of Electrical, Electronic and Information Engineering. His research interests include
crystal growth of nonlinear optical crystals for ultraviolet light generation and THz wave generation, protein, and gallium nitride. He is a Representative Director of SOSHO, Inc. since
2005.

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IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 3, NO. 3, MAY 2013

Prof. Mori is a member of the Japan Society of Applied Physics, the Laser
Society of Japan, and the Japanese Association for Crystal Growth. He received
the Commendation for Science and Technology by the Minister of Education,
Culture, Sports, Science and Technology in the Research Category in 2007 and2008, German Innovation Award Gottfried Wagener Prize 2012, and so on.

Tsutomu Araki received the Ph.D. degree in applied
physics from Osaka University, Suita, Japan, in 1977,
and one in medical sciences from the University ofTokushima, Tokushima, Japan, in 1986.
From 1978 to 1979, he worked as a Post-Doctoral
Fellow at the University of Wisconsin. He waswith the Faculty of Medicine of the University ofTokushima from 1979 to 1987, and the Faculty ofEngineering of the University of Tokushima from1987 to 1996. He is currently a Professor in the
Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, Osaka University, Toyonaka,
Japan. His research interests include development of a new biomedical andbiophysical measurement system using a fast pulse laser.
Prof. Araki is a member of the Japan Society of Mechanical Engineers, theJapan Society of Medical Electronics and Biological Engineering, and the Japan
Society of Applied Physics. He received the Sakamoto Award from the Japan
Society of Medical Electronics and Biological Engineering, the Achievement
Award from the Bioengineering Division in the Japan Society of MechanicalEngineers in 2006, the Achievement Award of Education and Research fromthe Osaka University in 2008, and the Funai Award from the Japan Society of
Mechanical Engineers in 2009.

Takeshi Yasui received the Ph.D. degree in mechanical engineering from the University of Tokushima,
Tokushima, Japan, in 1997.
From 1997 to 1999, he worked as a Post-Doctoral Research Fellow in the National ResearchLaboratory of Metrology, Japan. He was with the
Graduate School of Engineering Science, Osaka
University from 1999 to 2010, and was briefly with
the University of Bordeaux I in 2007 and 2012, andwith the University of Littoral Côte d’Opale in 2010
as an Invited Professor. He is currently a Professor
in the Institute of Technology and Science, The University of Tokushima,
Tokushima, Japan, and an Invited Professor in the Graduate School of Engineering Science, Osaka University, Toyonaka, Japan. His research interests
include THz instrumentation and metrology, laser stabilization, nonlinear
optical microscopy, and ultrafast time-resolved measurement.
Prof. Yasui is a member of the Optical Society (OSA), the Japan Society ofApplied Physics, the Optical Society of Japan, the Laser Society of Japan, theJapan Society of Medical Electronics and Biological Engineering, the Japan
Society of Mechanical Engineers, and the Institute of Electronics, Information
and Communication Engineers. He received the Award for the Most Promising
Young Scientist from the Optical Society of Japan in 1998, the Sakamoto Award
from the Japan Society of Medical Electronics and Biological Engineering in
2006, the Optics Paper Award from the Japan Society of Applied Physics, andthe Funai Award from the Japan Society of Mechanical Engineers in 2009.