IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 17, NO. 1, JANUARY/FEBRUARY 2011

191

Terahertz Frequency Metrology Basedon Frequency Comb
Takeshi Yasui, Shuko Yokoyama, Hajime Inaba, Kaoru Minoshima,
Tadao Nagatsuma, Senior Member, IEEE, and Tsutomu Araki(Invited Paper)

Abstract—Two techniques for terahertz (THz) frequency metrology based on frequency comb, namely, a THz-comb-referenced
spectrum analyzer and a continuously tunable, single-frequency
continuous-wave (CW)-THz generator, are reviewed. Since the frequency comb enables to coherently link the frequency among microwave, optical, and THz regions, it is possible to establish theTHz frequency metrology traceable to time of the SI base units.
Using a THz-comb-referenced spectrum analyzer based on a stable THz comb generated in a photoconductive antenna for THz
detection, the absolute frequency of CW test sources in the subTHz and THz regions was determined at a precision of 10−1 1 .
Furthermore, a continuously tunable, single-frequency CW-THz
generator was demonstrated around 120 GHz by photomixing ofan accurately tunable CW laser and a tightly fixed CW laser inthe optical frequency region, phase locked to two independent optical combs. The combination of the CW-THz generator with theTHz-comb-referenced spectrum analyzer will open the door forestablishment of frequency metrology in the THz region.
Index Terms—Frequency comb, frequency measurement, frequency metrology, frequency synthesizers, terahertz (THz) wave.

I. INTRODUCTIONERAHERTZ (THz) electromagnetic (EM) wave, lying at
the boundary between optical and electrical waves, has
emerged as a new innovative mode for sensing, spectroscopy,
communication, and other applications [1]. Along with recent
progress in THz technology, the requirements of THz metrology have increased in various applications [2]. In particular,

T

Manuscript received January 30, 2010; revised March 8, 2010 and March
19, 2010; accepted March 24, 2010. Date of publication June 21, 2010; date ofcurrent version February 4, 2011. This work was supported in part by the Ministry of Education, Culture, Sports, Science, and Technology of Japan throughGrants-in-Aid for Scientific Research under Grant 18650121, Grant 18686008,
Grant 20560036, Grant 21360039, and Grant 21650111, in part by the New Energy and Industrial Technology Development Organization of Japan through theIndustrial Technology Research Grant Program under Grant 02A51005d, and inpart by the Ministry of Internal Affairs and Communications of Japan throughStrategic Information and Communications R&D Promotion Programme underGrant 042107005.
T. Yasui, T. Nagatsuma, and T. Araki are with the Graduate Schoolof Engineering Science, Osaka University, Toyonaka, Osaka 560-8531,
Japan (e-mail: t-yasui@me.es.osaka-u.ac.jp; nagatuma@ee.es.osaka-u.ac.jp;
araki@me.es.osaka-u.ac.jp).
S. Yokoyama is with the Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan, and also with Micro Optics Company
Ltd., Kyoto 621-0252, Japan (e-mail: yokoyama@sml.me.es.osaka-u.ac.jp).
H. Inaba and K. Minoshima are with the National Metrology Institute of Japan,
National Institute of Advanced Industrial Science and Technology, Tsukuba
305-8563, Japan (e-mail: h.inaba@aist.go.jp; k.minoshima@aist.go.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/JSTQE.2010.2047099

frequency and power are important quantities to be measured in
the metrology because they are fundamental physical quantities
of EM waves. The THz frequency metrology can be establishedby realizing precise frequency measurement equipments (THzspectrum analyzers), signal sources of standard frequency (THzclocks), and accurate, tunable, single-frequency signal generators (THz synthesizers). The establishment of the THz frequency
metrology will enable to expand the scope of THz applications based on its high reliability because frequency accuracy
in the THz region is relatively lower than that in the optical andelectrical regions, namely, “THz gap of frequency metrology.”
For example, when the THz wave is used as a carrier wave
for broad-band wireless communications, its transmission frequency should be highly accurate and stable in order to securenecessary and sufficient bandwidth for broad-band communication without interferences with other applications, such as
astronomy or sensing. The THz clocks will be used to generate
the accurate and stable transmission frequency. Also, the THz
frequency metrology will play an important role in frequency
calibration of various types of commercial THz instruments,
such as sources, detectors, and systems. The absolute frequencyof the THz source is accurately determined by the THz spectrum
analyzers, whereas frequency scale of the THz spectrometeris precisely calibrated by the THz synthesizers. The precisely
calibrated THz spectrometer increases identification power inspectroscopic applications based on THz spectral fingerprints.
For example, high-resolution spectroscopy in gas sensing andastronomy requires high accuracy for determination of the absolute frequency because many absorption lines of moleculargasses exist in the THz frequency range [3].
A spectrum analyzer is a fundamental frequency measurement instrument widely used for RF, microwave, and millimeter waves. However, it is still difficult to use in the THz region
although steady efforts are bring made to extend its frequency
range. The electrical heterodyne method with a superconductorinsulator-superconductor mixer [4] or a hot-electron-bolometer
mixer [5] enables frequency measurement of continuous wave
(CW) in the sub-THz and THz regions. Conversely, optical interferometric method can be used as an optical spectrum analyzer
in the THz region. However, these methods often require cryogenic cooling of the mixer or detector to suppress thermal noise,
which is a major obstacle to practical use. Recently, new types ofspectrum analyzers based on harmonic mixing technique with
optical frequency comb (OFC) have been proposed and developed, which can measure the absolute frequency and spectral
shape of CW-THz wave without the need for the cryogenic

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cooling. One such device is a THz-comb-referenced spectrum
analyzer [6], [7] using a frequency comb of photocarriers (PC),namely PC-THz comb, generated in a photoconductive antenna(PCA) for THz detection [8]. By precisely stabilizing a modelocked frequency of a femtosecond Ti:Sapphire laser and hence
the PC-THz comb with a rubidium (Rb) atomic clock, the absolute frequency of CW-THz wave in the sub-THz and THz
regions could be measured to a precision of 10−11 [6]. Furthermore, a fiber-based, THz-comb-referenced spectrum analyzer has been constructed, which has the advantages of being
a portable, alignment-free, robust, and flexible apparatus suitable for practical use [7]. Other types of spectrum analyzers are
based on a metal-semiconductor point contact diode [9] and anelectrooptic sampling technique [10], using a free-running femtosecond Ti:Sapphire laser. The latter method has been achievedto frequency measurements of a CW CO2 laser at 28 THz [10].
On the other hand, accurate, stable, and/or tunable singlefrequency signal generation is an essential technique to achieve
the THz clock and synthesizer. Frequency-stabilized CW-THz
sources have been achieved by active control of a THz quantum cascade laser (THz-QCL) [11], [12]; however, it is difficult
to tune the frequency in a broad range due to the restricted
tunable range of the THz-QCL, typically a few hundreds GHz
depending on its band structure [13]. One promising alternative method for widely tunable CW-THz sources is photomixing of two CW near-IR lasers of adjacent wavelengths with aphotomixer, such as a unitraveling-carrier photodiode (UTCPD) [14] or a PCA [15]. However, if the two CW lasers areoperated in the free-running mode, it is difficult to generate theaccurate and stable frequency in the CW-THz wave. One attractive frequency reference for simultaneous control of two CW
lasers is an OFC [16]. A stable millimeter-wave synthesizer has
been realized by photomixing two CW lasers locked to an optical
comb generator based on an electrooptic modulator [17]. Recently, accurate, stable, phase-locked CW-THz wave has been
discretely generated at four different frequencies (0.30, 0.56,
0.84, and 1.1 THz) by photomixing of two CW lasers locked
to a single optical comb based on a mode-locked Ti:Sapphirelaser [18]. Furthermore, the output frequency was tuned continuously by scanning the frequency interval of the optical comb
while locking the CW lasers to the comb; however, the rangeof continuous tuning was limited to several kilohertz. In thecase where the two CW lasers share the same optical comb,
when scanning the frequency interval of the comb, the opticalfrequencies of the two CW lasers change simultaneously. This
common-mode change cancels most of the optical frequencychange in the two CW lasers. As a result, the continuous tuning
range of the CW-THz wave is much smaller than that of the
optical frequency in the CW lasers. To increase the continuous
tuning range of the CW-THz wave while maintaining high frequency accuracy and stability, a CW-THz generator has beenachieved by photomixing of an accurately tunable CW laser anda tightly fixed CW laser in the optical frequency region, phaselocked to two independent optical combs [19].
In this paper, we review two techniques for THz frequency
metrology based on frequency comb. After describing the coherent link of frequency among microwave, optical, and THz
regions achieved by frequency comb techniques in Section II,

Fig. 1.

Optical comb and THz comb.

a THz-comb-referenced spectrum analyzer based on the PCTHz comb and its application for measurement of the CW-THz
test sources are presented in Section III [6], [7]. In Section
IV, a continuously tunable, single-frequency, CW-THz generator using combination of two independent optical combs withthe photomixing technique and its potential for application toa THz clock and synthesizer are demonstrated [19]. Section V
then provides conclusions.
II. COHERENT LINK OF FREQUENCY USING FREQUENCY COMB
Fig. 1 shows behavior of optical comb and THz comb intime and frequency domains. A femtosecond mode-locked (fsML) laser generates a sequence of pulses that are essentiallycopies of the same pulse separated by an interval equal to theinverse of the mode-locked frequency of frep . The highly stable fs-ML pulse train is synthesized by a series of frequency
spikes regularly separated by the mode-locked frequency frep
in the optical frequency domain. This structure is referred to asa frequency comb. Since the frequency-comb structure can beused as a precision frequency ruler in the optical domain, thefs-ML-laser-based OFC has received a lot of interest as a powerful metrological tool capable of covering the optical region by
virtue of precise laser stabilization [16]. Recently, the conceptof the frequency comb has been extended to the THz region by
combination of the fs-ML pulse trains with the photoconductive
process [8]. When a PCA for THz generation is triggered by
the fs-ML optical pulse train, a free-space-propagating, modelocked THz pulse train is radiated from the PCA. This THz pulse
train is composed of a regular comb of sharp lines of EM waves
in the THz frequency domain, namely, EM-THz comb. On theother hand, when the fs-ML optical pulse train is incident to a
PCA for THz detection, a sequence of photoconductive gatingis induced in the PCA. Such the photoconductive, mode-locked
THz pulse train constructs frequency comb structure of the PC in
the THz region, namely, PC-THz comb. In this way, the optical
comb is downconverted to the THz region without any change
in its frequency spacing. The resulting THz comb is a harmonicfrequency comb without any frequency offset, composed of afundamental component and a series of harmonic components ofa mode-locked frequency of frep . This is the biggest differencecompared to an OFC having a carrier-envelope offset frequency,
which has to be stabilized, and it enables us to achieve simple,

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Fig. 2.

193

Coherent link of frequency using frequency comb.

practical THz frequency metrology including stabilization of a
THz comb. The THz combs provide attractive features for frequency metrology, namely, simple, broad-band selectivity, highspectral purity, offset free, and absolute frequency calibration.
Therefore, if the mode-locked frequency of frep is well stabilized by the laser control, the THz comb can be used as a precise
frequency ruler in the THz region.
Fig. 2 shows the concept of THz frequency metrology basedon the frequency comb. Atomic clock in the microwave region
has been widely used as a standard of time and frequency. However, it has been difficult to transfer the frequency accuracy of
the atomic clock to the optical region due to the large gap of frequencies between the microwave and optical regions. Althoughsuch the gap has been bridged by the phase-coherent frequency
measurement using the frequency chain [20], it is quite bulky
and complicated apparatus. Furthermore, the frequency uncertainty is accumulated while passing through many intermediateoscillators in the chain. Recently, optical comb has emerged as a
powerful tool to link the frequency between the microwave andoptical regions directly without losing its accuracy by achieving
the precise laser stabilization with the atomic clock [16]. Furthermore, the optical comb has been extended to the THz region
using the PCA, as shown in Fig. 1 [8]. Here, most important
point is that connection of frequency among microwave, optical, and THz regions is based on the coherent process, such as
laser control and photoconductive process. This coherent linkof frequency enables to maintain the same frequency accuracy
in three different regions of EM wave. Therefore, based on theTHz comb or optical comb, one can construct the THz frequency
metrology, directly connected to a standard of frequency. In thisway, THz frequency metrology traceable to time of the SI baseunits can be established.
III. THZ-COMB-REFERENCED SPECTRUM ANALYZER
A. Principle of Operation
In the field of microwave technology, harmonic mixing technique based on electrooptic sampling has been often used todownconvert microwave spectrum to RF spectrum, using the
comb spectrum of the mode-locked laser with a nonlinear detection technique [21], [22]. Futhermore, this technique has been
extended to the THz region [10]. We here modified this technique to downconvert THz spectrum to RF spectrum using photoconductive detection. Our THz spectrum analyzer is based on

Fig. 3. (a) Principle of THz-comb-referenced spectrum analyzer and (b) corresponding spectral behavior in THz and RF regions.

a heterodyne technique, as shown in Fig. 3(a). In this heterodyne
method, the PC-THz comb is used as an RF signal with multiple frequencies covering from the sub-THz to THz regions,whereas a measured CW-THz wave works as a local oscillator. Therefore, the moderate power is required for the measured
CW-THz wave [6], [7] compared with cryogenic-cooling heterodyne methods. However, combined use of the PCA and the
PC-THz comb enables heterodyne mixing covering from the
sub-THz to THz regions at room temperature.
Consider a PCA detector when an fs-ML laser light (probe
pulse with repetition frequency frep ) is incident on the antenna
gap of a photoconductive film. Fig. 3(b) illustrates the corresponding spectral behaviors in THz and RF regions. The probe
pulse train emitted from the fs-ML laser constructs an OFC
in the frequency domain, whose mode spacing is equal to amode-locked frequency (see Fig. 1). When the PCA is triggeredby such pulse train, the PC-THz comb is induced in the PCA.
Next, consider what happens when a measured CW-THz wave
(frequency = fTHz ) is incident on a PCA detector triggered by
the probe pulse train. The photoconductive mixing process in
the PCA generates a group of beat signals between the CWTHz wave and the PC-THz comb in the RF region. Focus ona beat signal at the lowest frequency (= fb ), namely, fb beatsignal. Since this fb beat signal is generated by mixing the
CW-THz wave (frequency = fTHz ) with the mth mode of the
PC-THz comb (frequency = mfrep ) nearest in frequency to
the CW-THz wave, fTHz is given as follows
fTHz = mfrep ± fb .

(1)

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Since frep and fb can be measured by RF frequency instruments, fTHz can be determined if the value of m and the signof fb are measured. To this end, the mode-locked frequency ischanged from frep to frep + δfrep by adjustment of the laser
cavity length with the laser control system. This results in achange of the beat frequency from fb to fb + δfb . Since |δfb | isequal to |mδfrep |, the value of m is determined asm=|δfb |.
|δfrep |(2)

The sign of δfb /δfrep is opposite to that of fb . Finally, theabsolute frequency of the measured CW-THz wave can be determined by measuring frep , fb , δfrep , and δfb becauseδfbfTHz = mfrep + fb
<0
(3a)
δfrep
and


fTHz = mfrep − fb


δfb
>0 .
δfrep

Fig. 4. Experimental setup of THz-comb-referenced spectrum analyzer.
L: lens; PCA: bowtie-shaped, low-temperature-grown InGaAs/InAlAs PCA;
THz-L: THz lenses.

(3b)

B. Experimental Setup
Fig. 4 shows a schematic diagram of the experimental setup.
The THz-comb-referenced spectrum analyzer was composedof a home-built, fs-ML erbium (Er)-doped fiber laser (center
wavelength = 1550 nm, pulse duration = 40 fs, and frep =56 122 206 Hz) [23], a PCA for THz detection, and RF frequency instruments. The mode-locked frequency frep was stabilized using a laser control system referenced to an Rb atomic
clock (Stanford Research Systems FS725 with frequency = 10
MHz, accuracy = 5 × 10−11 , stability = 2 × 10−11 at 1 s). The
output of the fiber laser was delivered to a bowtie-shaped, lowtemperature-grown InGaAs/InAlAs PCA for a 1550-nm laser
light (Menlo Systems GmbH, BT10) by an optical fiber. This results in generation of the PC-THz comb in the PCA equivalent to
the Rb atomic clock. The CW-THz wave from a test source propagated in free space through a pair of THz lens (Pax Company,
Tsurupica), and was then incident on the PCA. Photoconductive
heterodyne mixing between the CW-THz wave and the PC-THz
comb in the PCA generates beat signals in the RF region. The
fb beat signal is amplified by a high-gain current preamplifier(bandwidth = 10 MHz and sensitivity = 105 V/A) and wasmeasured with an RF spectrum analyzer (Agilent E4402B witha frequency range of 100 Hz to 3 GHz) and an RF frequency
counter (Agilent 53132 A with a frequency range to 225 MHz)to determine its spectral shape and center frequency. A portionof the laser light is detected with a photodetector, and its modelocked frequency (=frep ) is measured using the RF frequency
counter. The RF spectrum analyzer and RF frequency counterare synchronized to the Rb atomic clock.
C. Results
1) Test Source Based on Active Frequency Multiplier Chain:
The first test source (output power = 4 dBm = 2.5 mW,
frequency range = 75–110 GHz, and linewidth < 0.6 Hz)
is achieved by combination of an active frequency multiplierchain (AFMC) (Millitech AMC-10-R0000 with multiplication

Fig. 5. Spectra of the fb beat signal of the AFMC source measured by the RF
spectrum analyzer (RBW = 1 Hz and sweep time = 690 ms).

factor = 6) and a microwave frequency synthesizer (Agilent
E8257D with frequency = 12.5–18.33 GHz), referred to as an
AFMC source. Since the frequency synthesizer is synchronized
to the Rb atomic clock, output frequency of this AFMC sourceis accurate and stable. We first evaluated the spectral linewidthof the fb beat signal when the output frequency of the AFMC
source is set to be 100 GHz. Fig. 5 shows a linear-scale spectrumof this signal, measured by the RF spectrum analyzer [resolution bandwidth (RBW) = 1 Hz and sweep time = 690 ms].
The resulting linewidth of the beat signal was 1.8 Hz. This
result indicates that each mode of the PC-THz comb has sufficiently narrow linewidth to perform the frequency measurementwith high precision. On the other hand, the signal-to-noise ratio (SNR) of the fb beat signal reached 40 dB. From this SNR
and the output power of the test source (namely, +4 dBm =2.5 mW), the detection limit of the THz power is estimatedto −36 dBm, or 250 nW. Comparison of SNR among the fb
beat signals obtained by three different PCAs and electrooptic
sampling is given in detail elsewhere [7].
To determine the absolute frequency of the test source, itis necessary to measure the deviation of fb before and after
changing frep . The initial frep and fb values were measured to
be 56 122 206.03 and 356 156 Hz as indicated by the blue color
in Fig. 6 (RBW = 1 kHz and sweep time = 902 ms). Then,the frequency frep was changed by δfrep (=+25 Hz), using
the laser control system. This resulted in a change of the beatYASUI et al.: TERAHERTZ FREQUENCY METROLOGY BASED ON FREQUENCY COMB

Fig. 6. Spectra of the fb beat signal when the laser mode-locked frequency(fre p ) is set at 561 222 06 (blue curve) Hz and 561 222 31 Hz (red curve)(RBW = 1 kHz and sweep time = 902 ms).

195

Fig. 7. Precision of frequency measurement in available frequency range of
the AFMC source.

frequency by δfb of 44 549 Hz (see the red color in Fig. 6).
Since |δfb | is equal to |mδfrep |, m is determined asm=|δfb ||+44 549|== 1781.96 ≈ 1782.
|δfrep |
|+25|

(4)

Since the sign of δfb /δfrep (positive in this case) is opposite
to that of fb , the value of fTHz was determined as follows:
fTHz = mfrep − fb
= 1 782 × 56 122 206.03 − 356 156= 100 009 414 989.46 Hz.

(5)Since the actual set frequency of the AFMC source was 100009 414 988.9 Hz from the output frequency of the microwave
synthesizer, the error between the set and measured frequencies
was only 0.56 Hz.
To evaluate the precision of frequency measurement in available frequency range of the AFMC source, we determined theabsolute frequency of the source while tuning its output frequency from 75 to 110 GHz at 5-GHz intervals. When precisionof frequency measurement is defined as the ratio of the error
to fTHz , the precision for eight different measurement frequencies was shown in Fig. 7. A mean precision of 2.4 × 10−11 was
obtained for this source, which is limited by the performanceof the Rb atomic clock. The frequency precision will be further
improved if we use a frequency standard having higher accuracy, such as cesium atomic clock, optical clock [24], or opticallattice clock [25].
2) Test Source Based on Photomixing of Two Free-RunningCW Lasers With UTC-PD: The second test source was produced by photomixing of two CW near-IR lasers at adjacent
wavelengths with a UTC-PD [14]. The two CW lasers were
external cavity wavelength-tunable laser diodes with an emission wavelength of 1550 nm and operated in free-running mode
(Koshin Kogaku Company, LS-601 A-15S1; spectral linewidth≤100 kHz, frequency fluctuation <100 MHz/h). The opticalfrequency difference between them was set to be approximately
120 GHz. The outputs of the lasers were combined using a fiber
coupler, amplified with an Er-doped fiber amplifier, and thenFig. 8. Spectrum of the fb beat signal of the photomixing source (RBW =1 kHz and sweep time = 13 ms).

photomixed by a F-band UTC-PD [Nippon Telegraph and Telephone (NTT) Electronics, available frequency = 90–140 GHz]
equipped with a horn antenna. The output power was set to be
100 μW at a frequency of 120 GHz. We used the THz spectrum analyzer for monitoring of the photomixing source. The
resulting image of the beat spectrum is shown in Fig. 8 (RBW =1 kHz, sweep time = 13 ms). Also, a movie of the beat spectrum
is given elsewhere [7]. In contrast to the stable AFMC source,
the beat frequency of the photomixing source exhibits large fluctuations within a spectral window of 1 MHz. This is because
the two CW lasers used for the photomixing are operated infree-running mode without any frequency control.
3) Test Source Based on EM-THz Comb: It is interesting
to investigate the applicability of this THz spectrum analyzer
to frequencies above 1 THz because frequency range of thetwo test sources above were limited to the sub-THz region.
Unfortunately, we have no available CW test sources in thisfrequency region. One possible alternative test source is the
EM-THz comb radiating from a PCA emitter triggered by a
fs-ML laser (see Fig. 1). Because the EM-THz comb is a groupof many CW-THz waves having a frequency spacing equal to
the laser mode-locked frequency, it can be used as a test sourcewith multiple frequencies ranging from sub-THz to a few THz.

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Fig. 9. Spectrum of the EM-THz comb (RBW = 1 kHz and sweep time =1000 s).

The THz spectrum analyzer was used for capturing the spectrumof the EM-THz comb. Principle and experimental setup of thisdetection scheme are given in detail elsewhere [7], [8]. The
resulting spectrum is shown in Fig. 9 (RBW = 1 kHz andsweep time = 1000 s). The EM-THz comb spectrum was clearlyobserved over 1 THz. Although it is not so simple to compare
the sensitivity between the time-gated sampling measurementin Fig. 9 and the heterodyne measurements in Figs. 5 and 8, theTHz spectrum analyzer has sensitivity in the frequencies above
1 THz. The spectral bandwidth of the THz spectrum analyzer
should be limited by that of the PCA including characteristics of
its photoconductive film and antenna shape, and pulse durationof the fs-ML laser. The spectral sensitivity of the PCA as a
THz detector has been previously investigated using THz timedomain spectroscopy, for which a bandwidth of 170 THz wasachieved [26]. We consider that the THz spectrum analyzer canbe further used for frequency measurement of CW-THz sources
oscillating at higher THz frequency, such as THz-QCL.
IV. CONTINUOUSLY TUNABLE, SINGLE-FREQUENCY CW-THZ
GENERATOR REFERENCING TWO INDEPENDENTOPTICAL COMBS
A. Principle of Operation
Let us compare the continuous tuning range of photomixingbased CW-THz wave between the previous method, using a single optical comb [18], and our proposed method, using two independent optical combs [19]. We consider the previous method,
as shown in Fig. 10(a). Two CW lasers (CWL1 and CWL2) are,respectively, phase locked to two different modes (n1 and n2 )of a single OFC(frequency interval of comb modes = frep ) andare then optically heterodyned by a photomixer to generate CWTHz wave. The frequency of the CW-THz wave (fTHz ) is determined by the optical frequency separation of the two CW lasers.
Therefore, it is possible to generate fTHz discretely in increments of frep within the spectral bandwidth of the photomixer
by selection of the targeted modes n1 and n2 . Furthermore, fTHz
can be tuned continuously by scanning frep while locking thetwo CW lasers to the comb. In this case, the range of continuous
tuning (ΔfTHz ) is determined by the product of the scanning

Fig. 10. Continuous tuning of the CW-THz wave based on photomixing of (a)two CW lasers (CWL1 and CWL2) referenced to a single OFC and (b) two CW
lasers (CWL1 and CWL2) referenced to two independent optical combs (OFC1and OFC2).

range of frep (Δfrep ) and the number of comb lines between
the two CW lasers (Δn = n2 –n1 ). For example, Δn = 20 000
when fTHz = 1 THz and frep = 50 MHz, and Δn = 2000,
when fTHz = 0.1 THz and frep = 50 MHz. Therefore, whenΔfrep = 500 kHz, which is 1% of frep , ΔfTHz = 10 GHz, and
1 GHz, for fTHz = 1 THz and 0.1 THz, respectively.
In our method, consider that the two CW lasers (CWL1and CWL2) are, respectively, phase locked to two independent optical combs (OFC1 and OFC2), as shown in Fig. 10(b).
We assume that the carrier-envelope offset frequencies (fceo1
and fceo2 ) of the two combs are fixed by respective laser
control. If the repetition frequency of OFC2 (=frep2 ) is tunable (tunable range = Δfrep2 ), whereas that of OFC1 (=frep1 )is fixed, the continuously tuning range of the CW-THz wave
(ΔfTHz ) is determined by the product of Δfrep2 and the comb
mode (n2 ) to which the CW laser used for tuning (CWL2)
is locked. The value of n2 reaches 3 880 000 when the optical frequency of the CWL2 is 194 THz (corresponding wavelength = 1550 nm) and frep2 is 50 MHz, which is two ordersof magnitude higher than the value of Δn for photomixing with
fTHz = 1 THz. Furthermore, when fTHz is set to 0.12 THz,
as demonstrated in the following experiments, the value ofn2 is three orders of magnitude higher than the value of Δn.
In this method, with Δfrep = 500 kHz, the frequency tuningYASUI et al.: TERAHERTZ FREQUENCY METROLOGY BASED ON FREQUENCY COMB

Fig. 11. Experimental setup of continuously tunable, single-frequency CWTHz generator. OFC1, OFC2, and OFC3: OFCs; CWL1 and CWL2: CW near-IR
lasers; λ/2: half-wave plate; λ/4: quarter-wave plate; UTC-PD for photomixing;
L: lenses; PCA: bowtie-shaped, low-temperature-grown GaAs PCA; THz-L:
THz lenses; SHG crystal; UTC-NMIJ.

range of the THz wave, ΔfTHz , could reach 2 THz. Therefore,
our method enables a large increase in the continuously tuning
range ΔfTHz of the CW-THz wave compared with the previous
method.
B. Experimental Setup
The experimental setup of the proposed method is shown in
Fig. 11. We prepared two CW lasers operating at a wavelength of
1542 nm for photomixing. One was a distributed feedback fiber
laser (CWL1; Koheras A/S, Inc., AdjustiK E15-PM), and theother was an external cavity laser diode (CWL2; Optical Comb,
Inc., LT-5001). The optical frequency difference between themwas set to approximately 120 GHz by using an optical wavemeter (Advantest Corporation, Q8326). CWL1 and CWL2 were,respectively, phase locked to two independent optical combs
of home-built, fs-ML Er-doped fiber lasers operating at a center wavelength of 1550 nm (OFC1 with frep1 = 48.683 MHz,and OFC2 with frep2 = 51.150 MHz) [23]. Details of the phaselocking process are given elsewhere [27]. The frequencies frep1 ,frep2 , and fceo2 were phase locked to microwave references synthesized from a hydrogen maser linked to coordinated universal time operated by the National Metrology Institute of Japan(UTC-NMIJ). Although fceo1 is also an important parameter for
the final characteristics of this CW-THz generator as discussed
later, it was not stabilized in the presented system. However, weconsider that principle and usefulness of this CW-THz generator
can be confirmed even though fceo1 is not stabilized. CWL1,
which was phase locked to OFC1, was used as a fixed CW
light source in our method, whereas CWL2, which was phaselocked to OFC2, was used as a tunable CW light source. In
the present system, the optical frequency of n2 in OFC2 could
be continuously tuned over 1.7 THz by making full use of the197

scanning range of frep2 (Δfrep2 = 450 kHz). However, continuously tuning range of CWL2 without the mode hopping was
limited to 110 GHz due to the stroke of a piezoelectric actuator
used to tilt a diffraction grating for wavelength tuning. As aresult, the optical frequency of the tunable light source could becontinuously tuned over 110 GHz without losing phase locking
of CWL2 to OFC2 by varying the repetition frequency over 30
kHz with a variable optical delay line module in OFC2 [27]. If
we use another CW laser having broader tunability without themode hopping for CWL2, it should be possible to extend tuning
range of the tunable light source to 1.7 THz. Work is in progress
to extend the tuning range over 1 THz.
The outputs of CWL1 and CWL2 were combined with afiber coupler and then photomixed by an F-band UTC-PD (NTTElectronics, available frequency = 90–140 GHz) equipped witha horn antenna. The polarization of CWL1 was well alignedwith that of the CWL2 by adjusting a half-wave plate (λ/2) anda quarter-wave plate (λ/4). The photocurrent of the UTC-PD
was set to 6.9 mA by controlling the output powers of CWL1
and CWL2. This resulted in CW-THz wave with an average
power of 250 μW at a frequency of 120 GHz.
To evaluate the spectral characteristics of this CW-THz generator, we used a fiber-based, THz-comb-referenced spectrum
analyzer [7]. The free-space-propagating CW-THz wave passing through a pair of THz lenses (Pax Company, Tsurupica)
was made incident on a bow-tie-shaped, low-temperature-grown
GaAs PCA in the THz spectrum analyzer. The PCA was triggered by the second-harmonic-generation (SHG) light (center wavelength = 775 nm) from another repetition-frequencylocked fiber-laser comb (OFC3 with frep3 = 49.996 MHz). This
resulted in the generation of a photoconductive self-beat signalof OFC3 in the PCA, namely, a PC-THz comb [8]. The beat current signal from the PCA was amplified by a high-gain current
preamplifier (bandwidth = 10 MHz and sensitivity = 105 V/A)and was measured with an RF spectrum analyzer. To check thevalidity of the absolute frequency measured by the THz spectrum analyzer, we also determined the absolute frequency of the
CW-THz generator by measuring the optical frequency of twolight sources with the optical wavemeter.
C. Results
1) Generation of Frequency-Locked CW-THz Wave: To
evaluate the potential performance of this system as a THz clock,we generated frequency-locked CW-THz wave by photomixing
two fixed light sources. Here, the combination of CWL2 andOFC2 in Fig. 11 was also used as a fixed light source. The blue
curve in Fig. 12 illustrates the linear-scale spectrum of the fb
beat signal, measured by the RF spectrum analyzer (RBW =10 kHz, sweep time = 189 ms, number of integrated signals =100). Also, a movie of the beat spectrum is given elsewhere [19].
Compared with the frequency spectrum of the CW-THz wave
generated by photomixing of two free-running CW lasers (seeFig. 8), the frequency fluctuation of the CW-THz wave is effectively suppressed in the presented CW-THz generator due
to precise stabilization control of two CW lasers based on two
optical combs. The linewidth of the CW-THz wave was 1.093 ±198

IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 17, NO. 1, JANUARY/FEBRUARY 2011

Fig. 12. Spectra of fb beat signal when fre p 3 is set at 499 961 05 Hz (bluecurve) Hz and 499 944 95 Hz (red curve) (RBW = 10 kHz, sweep time =189 ms, number of integrated signals = 100).

0.008 MHz when a Lorentzian function was fitted to the spectral
shape. CWL1 and CWL2 had linewidths of 0.890 and 1.07 MHz
when phase locked to OFC1 and OFC2, respectively. Since
the convolution of them gave a linewidth of 0.97 MHz, slightbroadening of the linewidth occurred. We consider that the freerunning fceo1 was the source of this broadening. The linewidthof the CW lasers used in the present system was limited by thefrequency response of piezoelectric transducers used to control
their optical frequencies in addition to the spectral linewidth
in the free-running mode. Fast feedback control of CW lasers
further decreases the linewidth of the CW-THz wave.
To obtain the absolute frequency of the CW-THz wave(=fTHz ), we measured the deviation of fb before and after
changing frep3 . The initial frep3 was measured to be 49 996
105 Hz with an RF frequency counter. The value of fb was
calculated to be 5.264 MHz by the curve fitting, in which uncertainty of ±2 kHz was achieved using the regression analysis
based on Levenberg–Marquardt algorithm. To determine m andthe sign of fb , frep3 was changed by δfrep3 (=−1.610 kHz).
This resulted in a change of the beat frequency by δfb (=+3.924
MHz), as shown in the red curve in Fig. 12. Since |δfb | is equal
to |mδfrep3 |, m is determined from
+3.924 × 106 |δfb |== 2.437 × 103 . (6)m=|δfrep3 ||−1.610 × 103 |Finally, because the sign of δfb /δfrep3 (negative in this case)is opposite the sign of fb , fTHz was determined as follows:
fTHz = mfrep3 + fb
= 2.437 × 103 × 4.999 610 5 × 107 + 5.264 × 106= 121.85 GHz.

(7)

We next measured the optical frequencies of CWL1 andCWL2 using an optical wavemeter. Since the resulting frequencies were 194.405 46 and 194.526 55 THz, respectively,
fTHz was 121.09 GHz. Therefore, there was a discrepancy of
760 MHz, or 0.63%, between the THz spectrum analyzer measurement and the optical wavemeter measurement. Since theoptical frequency resolution of the wavemeter (=10 MHz) weremuch smaller than this discrepancy, the error caused by the
wavemeter measurement is likely to be negligible. Since theTHz spectrum analyzer had the sufficiently high precision of
frequency measurement as the demonstrated in the previous section, we estimated the two major components for the uncertaintyof the CW-THz generator induced in the THz spectrum analyzer
measurement as follows. One component originated in the peakanalysis used to determine fb by the curve-fitting analysis of thespectral shape. The actual uncertainty in this peak analysis wasestimated to be 2 kHz for the fb value of 5.264 MHz, which is
caused by the distorted spectral shape and broad linewidth of thebeat signal. This causes an error of ±2 when determining the mvalue based on (6). Therefore, the uncertainty in the peak analysis would lead to an uncertainty of 100 MHz, or 0.083%, in thevalue of fTHz obtained based on (7). The other component was
fluctuation of fceo1 because it was not stabilized. To evaluate thefluctuation of fceo1 , we measured the frequency deviation of the
fb beat signal during a 4-min period, which was equal to the measurement time required for the experiment, as shown in Fig. 12,
using the THz spectrum analyzer. The resulting frequency deviation was 25.5 kHz. This fb deviation leads to a deviation of
800 MHz, or 0.65%, in the value of fTHz obtained based on (6)and (7). Taking the root-sum-square of these two major components, the total uncertainty was estimated to be 810 MHz, or
0.65%. Therefore, the discrepancy between the THz spectrum
analyzer measurement and the optical wavemeter measurement
was within the estimated uncertainty of the CW-THz generator.
If fceo1 was further stabilized in the same manner as fceo2 , therelative standard uncertainty in the CW-THz generator would besubstantially decreased. Further improvement is expected if thelinewidth of the CW-THz wave can be narrowed by fast feedback control of the CW lasers. The resulting CW-THz generator
can be used as an F-band THz clock with accuracy and stability
equal to those of the reference frequency standard used.
2) Generation of Frequency-Tuned CW-THz Wave: The current CW-THz generator can fully cover the available spectral
bandwidth of the F-band UTC-PD photomixer because CWL2
achieves continuous tuning of 110 GHz without losing phaselocking to the COMB2 [27]. We here show the result of continuous tuning of the CW-THz wave around 121.85 GHz byscanning frep2 at 0.14 Hz intervals. The resulting consecutive
spectra of the CW-THz wave are shown in Fig. 13 (RBW =10 kHz, sweep time = 189 ms, number of integrated signals =100), in which the horizontal coordinate is scaled by the THz
spectrum analyzer. Furthermore, a movie of the beat spectrum
is given elsewhere when the output frequency is continuously
tuned over 16 MHz [19]. The reason why the amplitude of theTHz spectrum decreases as the frequency increases is frequency
dependence of the gain in the current preamplifier used in theTHz spectrum analyzer rather than decrease of the output powerfrom the UTC-PD.
We fitted their spectral shapes to Lorentzian functions anddetermined the center frequencies of the CW-THz wave (fTHz )in nine consecutive spectra in Fig. 13. The relationship between the change of frep2 (Δfrep2 ) and that of fTHz (ΔfTHz ) isshown by the red color in Fig. 14. A relationship was found in
the form ΔfTHz = n2 Δfrep2 , where n2 = 3 757 241, with a

YASUI et al.: TERAHERTZ FREQUENCY METROLOGY BASED ON FREQUENCY COMB

Fig. 13. Continuous tuning of the CW-THz wave around 121.85 GHz byscanning fre p 2 at 0.14-Hz intervals (RBW = 10 kHz, sweep time = 189 ms,
number of integrated signals = 100).

199

beginning and end of the experiment of Fig. 13, it leads toan error in n2 of 238 679, or 6.3%. Therefore, the discrepancy between the two n2 values is within the uncertainty of the
CW-THz generator. This can be further improved if the rangeof the frequency scanning is increased, together with stabilizing fceo1 . Smooth tuning of the CW-THz wave over 16 MHz
was achieved elsewhere [19], although the signal power became
smaller at higher frequencies due to the frequency response of
the current preamplifier used in the THz spectrum analyzer. It
should be emphasized that the tuning range of this demonstration is not limited by the proposed CW-THz generator but by theelectrical bandwidth of the current preamplifier. Therefore, we
believe that it should be possible to extend the tuning range to
110 GHz in the current system because we have already demonstrated continuous tuning of 110 GHz in the optical domain,using the same setup [27]. Although the THz-QCL [13] and the
AFMC source (see Fig. 7) have greater or similar tuning range at
the sub-THz and THz regions, the proposed CW-THz generator
has the potential to exceed them in wide tunability. If we use abroad-band photomixer, such as PCA [15], and extend tuning
range of CWL2 over a few THz, it will be possible to extend
tuning range of this CW-THz generator to 1.7 THz at arbitrary
center frequencies.
V. CONCLUSION

Fig. 14. Relationship between change of fre p 2 or fre p (Δfre p 2 or Δfre p )and that of fT H z (ΔfT H z ) for the presented method, using a dual optical comb,
and the previous method, using a single optical comb.

regression coefficient of 0.999 92. Although Δfrep2 waschanged by only 1.12 Hz in Fig. 14, the tuning rate n2 is maintained in full range of Δfrep2 (=450 kHz). In this case, ΔfTHz
reaches 1.7 THz. For comparison, we also indicated a theoretical
relationship between Δfrep and ΔfTHz (ΔfTHz = ΔnΔfrep )in the previous method, using a single optical comb (frequency
interval of comb modes = frep = 51.150 MHz) by the blue
color when fTHz was set to around 121.85 GHz. The resulting
Δn was 2382. It is important to note that the tuning rate n2 of
the present method was three orders of magnitude larger than
the tuning rate Δn of the previous method. This resulted in alarge expansion of the continuous tuning range of the CW-THz
wave.
From the optical wavemeter measurement, we also determined n2 to be 3 803 061. Although there is a discrepancy of
45 820, or 1.2%, in these n2 values, we attributed the main causeof this discrepancy to the fluctuation of fceo1 , as discussed in theprevious section. The acquisition time for all spectra in Fig. 13
was about 25 min. The frequency drift of the measured fb beatsignal indicated that fceo1 changed by 250 kHz in 25 min. Since
this deviation of fceo1 shifts ΔfTHz by 250 kHz between the

We reviewed two techniques of THz frequency metrology
based on the frequency comb. Most important point in this review is that the coherent link of frequency established by thefrequency comb enables to realize the THz frequency metrology
traceable to time of the SI base units. The THz-comb-referenced
spectrum analyzer achieved the absolute frequency measurement of the CW test sources at the precision of 10−11 , which
is limited by the performance of the Rb atomic clock used. We
also confirmed that spectral bandwidth of this THz spectrum
analyzer was extended over 1 THz. Next, continuously tunable,
single-frequency CW-THz generator was demonstrated around
120 GHz by photomixing of two stabilized CW lasers referenced to two independent optical combs. The continuous tuning
rate n2 in the presented method was enhanced by three orders ofmagnitude larger than that Δn of the previous method, using thesingle comb. There was a discrepancy of the absolute frequencyof the CW-THz generator between the THz spectrum analyzer
measurement and the optical wavemeter measurement due to thefree-running fceo1 . However, if the fceo1 is further stabilized, itshould be emphasized that the absolute THz frequency can be
determined at the precision of the frequency standard used, suchas the hydrogen maser or Rb atomic clock, by measuring fceo1(fixed), fceo2 (fixed), frep1 (fixed), frep2 (tunable), n1 (fixed),
n2 (fixed), and beat frequency (fixed) between CWL1 (CWL2)
and OFC1 (OFC2). This is the big advantage over other tunable
CW-THz sources [28]. Therefore, this CW-THz generator has
the high potential for application to a THz clock and synthesizer.
On the other hand, from the standpoint of practical use, thesefiber-based techniques have the advantage of being a portable,alignment-free, robust, and flexible apparatus. The combination of the CW-THz generator with the THz-comb-referenced

200IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 17, NO. 1, JANUARY/FEBRUARY 2011

spectrum analyzer will pave the way for establishment of frequency metrology in the THz region.
ACKNOWLEDGMENT
The authors would like to thank R. Nakamura, A. Ihara,
M. Nose, K. Kawamoto, and Y. Fujimoto of the Graduate Schoolof Engineering Science, Osaka University, and H. Takahashi and
Y. Iwamoto of the Faculty of Science and Technology, Tokyo
University of Science for their experimental support for the partof the work. T. Yasui is grateful to Drs. I. Hosako and N. Sekine
from the National Institute of Information and Communications
Technology for fruitful discussions.
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Takeshi Yasui received the Ph.D. degree in mechanical engineering from the University of Tokushima,
Tokushima, Japan, in 1997.
From 1997 to 1999, he was a Postdoctoral Research Fellow at the National Research Laboratory ofMetrology, Tsukuba, Japan. In 2007, he was with the
University of Bordeaux I, as an Invited Professor. He
is currently an Assistant Professor at the Departmentof Mechanical Science and Bioengineering, Graduate School of Engineering Science, Osaka University,
Toyonaka, Japan. His research interests include THz
instrumentation, laser stabilization, second-harmonic-generation imaging of biological tissues, and ultrafast time-resolved measurement.
Dr. Yasui is a member of the Optical Society of America, the Japan Society of Applied Physics, the Optical Society of Japan, the Laser Society ofJapan, the Japan Society of Medical Electronics and Biological Engineering,and the Japan Society of Mechanical Engineers. He was the recipient of the Most
Promising Young Scientist Award from the Optical Society of Japan in 1998, theSakamoto Award from the Japan Society of Medical Electronics and BiologicalEngineering in 2006, the Optics Paper Award from the Japan Society of Applied
Physics, and the Funai Award from the Japan Society of Mechanical Engineers in2009.

YASUI et al.: TERAHERTZ FREQUENCY METROLOGY BASED ON FREQUENCY COMB

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

Hajime Inaba received the Ph.D. degree in applied
physics from Hokkaido University, Sapporo, Japan,
in 2004.
He is currently a Senior Research Scientist at
the National Metrology Institute of Japan, National
Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan. In 1993, he joined
the National Research Laboratory of Metrology (formerly AIST), Tsukuba, Japan, where he was involvedin continuous-wave erbium-doped fiber lasers. From2006 and 2007, he was a Visiting Scientist at the National Measurement Institute, Australia. His research interests include ultrafast
optical science and technology and their application to optical metrology, particularly generation and control of frequency combs, and frequency metrology
using frequency combs.
Dr. Inaba is a member of the Japan Society of Applied Physics. He was therecipient of the Prize 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.
In 1993, she joined the National Research Laboratory of Metrology (formerly National Institute ofAdvanced Industrial Science and Technology, AIST),
where she is currently a Group Leader at the National
Metrology Institute of Japan, AIST, Tsukuba, Japan.
She is also a Guest Professor at Tokyo University of
Science, Tokyo, Japan. In 1996, she was with the University of Bordeaux I, as an Invited Professor. During
2000–2001, she was a Visiting Scientist at Massachusetts Institute of Technology, Cambridge. Her research interests include ultrafast optical science andtechnology and their application to optical metrology, 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 ofAmerica. She served as a Program Cochair of the Lasers and Electro-Optics
Conference in 2009. She was the recipient of a Prize for Science and Technology by the Minister of Education, Culture, Sports, Science and Technology in
2008.

201

Tadao Nagatsuma (M’93–SM’02) received the B.S.,
M.S., and Ph.D. degrees in electronic engineeringfrom Kyushu University, Fukuoka, Japan, in 1981,
1983, and 1986, respectively.
During his Ph.D. studies, he was involved inmillimeter-wave and submillimeter-wave oscillators
based on flux-flow phenomenon in superconducting
devices. In 1986, he joined the Electrical Communications Laboratories, Nippon Telegraph and Telephone Corporation (NTT), Atsugi, Kanagawa, Japan,
where he was engaged in research on the design andtesting of ultrahigh-speed semiconductor electronic/photonic devices and integrated circuits. From 1999 to 2002, he was a Distinguished Technical Member
with NTT Telecommunications Energy Laboratories. From 2003 to 2007, he
was a Group Leader with NTT Microsystem Integration Laboratories. He is currently a Professor at the Division of Advanced Electronics and Optical Science,
Department of Systems Innovation, Graduate School of Engineering Science,
Osaka University, Toyonaka, Japan. His research interests include millimeterwave and terahertz photonics and their application to sensors and wirelesscommunications.
Prof. Nagatsuma is a member of the Institute of Electronics, Information
and Communication Engineers (IEICE), Japan, the Technical Committee on
Microwave Photonics of the IEEE Microwave Theory and Techniques Society, and the Microwave Photonics Steering Committee. He was the recipientof the 1989 Young Engineers Award presented by the IEICE, the 1992 IEEE
Andrew R. Chi Best Paper Award, the 1997 Okochi Memorial Award, the 1998
Japan Microwave Prize, the 2000 Minister’s Award of the Science and Technology Agency, the 2002 Asia–Pacific Microwave Conference Prize, the 2004
Yokosuka Research Park Award, the 2006 Asia–Pacific Microwave-Photonics
Conference Award, the 2006 European Microwave Conference Prize, the 2007
Achievement Award presented by the IEICE, and the 2008 Maejima Award
presented by the Post and Telecom Association of Japan.

Tsutomu Araki received the Ph.D. degree in applied
physics from Osaka University, Toyonaka, Japan, in
1977, and the Ph.D. degree in medical sciences fromthe University of Tokushima, Tokushima, Japan, in
1986.
From 1978 to 1979, he was a Postdoctoral Fellow at the University of Wisconsin. From 1979 to
1987, he was with the Faculty of Medicine, University of Tokushima, where he was with the Faculty ofEngineering from 1987 to 1996. He is currently a Professor at the Department of Mechanical Science andBioengineering, Graduate School of Engineering Science, Osaka University.
His research interests include development of a new biomedical and biophysical measurement system using a fast pulse laser.
Prof. Araki is a member of the Society for Applied Spectroscopy, the Optical Society of America, the Japan Society of Mechanical Engineers, and theJapan Society of Applied Physics. He was the recipient of the Sakamoto Award
from the Japan Society of Medical Electronics and Biological Engineering, theAchievement Award from the Bioengineering Division in the Japan Society of
Mechanical Engineers in 2006, the Achievement Award of Education and Research from the Osaka University in 2008, and the Funai Award from the Japan
Society of Mechanical Engineers in 2009.