APPLIED PHYSICS LETTERS 87, 061101 共2005兲

Asynchronous optical sampling terahertz time-domain spectroscopy
for ultrahigh spectral resolution and rapid data acquisition
Takeshi Yasui, Eisuke Saneyoshi, and Tsutomu Araki
Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka,
Osaka 560-8531, Japan

共Received 28 February 2005; accepted 19 June 2005; published online 1 August 2005兲
We propose a terahertz time-domain spectrometer based on asynchronous optical sampling. Thespectrometer features ultrahigh spectral resolution equivalent to a laser mode-locked frequency andrapid data acquisition. The proposed method requires no mechanical translation stages fortime-delay scanning, and hence, overcomes the inherent tradeoff between frequency resolution anddata acquisition time. Time evolution of the picosecond terahertz pulse with a temporal window of
12.1 ns is measured directly on an oscilloscope using a time-scale magnification of 764 815. A
frequency resolution of 82.6 MHz is achieved at a measurement time of 10 s. The effectiveness ofthe proposed method is confirmed by comparing it with conventional stage-scanning terahertz
time-domain spectroscopy. © 2005 American Institute of Physics. 关DOI: 10.1063/1.2008379兴
Terahertz time-domain spectroscopy 共THz-TDS兲 hasbeen attracting much attention as the basic measurement
method in THz science and technology. In conventional
THz-TDS, temporal profiles of a pulsed THz electric field
are first measured by scanning the time interval between aTHz pulse and a probe pulse using a mechanical translation
stage. Then, broadband spectra of THz amplitude and phaseare obtained through a Fourier-transform procedure. Frequency resolution and frequency range in THz-TDS are defined as reciprocals of an overall window and step incrementin the time delay, which correspond, respectively, to thestroke length and step displacement in the mechanical stage.
Thus, there is an inherent tradeoff between the frequency
resolution and the measurement time in conventional
THz-TDS. The problem would be largely solved if we could
do away with the time-consuming mechanical stage. An
effective method to realize a stage-free configuration involves the asynchronous optical sampling 共AOS兲 technique,often employed in optical communication,1,2 pump-probespectroscopy,3,4 Fourier-transform infrared spectroscopy,5and electronic THz spectroscopy6 for rapid data acquisition.
In this method, the use of two asynchronous mode-locked
lasers magnifies the time evolution of the ultrafast transient
signal arbitrarily in the time scale. The resultant signal canbe measured directly and quickly on a conventional oscilloscope. The most attractive advantage of the AOS method in
connection with THz-TDS is that it helps realize a time delay
much longer than that achievable by the conventional motordriven translation stage. When a temporal window equivalent
to the THz pulse period is obtained, the theoretical limit offrequency resolution in THz-TDS is achieved. Thus, application of the AOS concept to THz-TDS provides an additional
advantage besides rapid data acquisition: an ultrahigh spectral resolution equivalent to the laser mode-locked frequency.
In this letter, we apply the AOS technique to THz-TDS
共AOS-THz-TDS兲 to drastically improve the frequency resolution and decrease the measurement time.
The experimental setup is shown in Fig. 1. We used two
femtosecond lasers for AOS-THz-TDS: a 60-fs Kerr-lensmode-locked Ti: sapphire laser 共ML-Ti: S-1兲 and a 10-fs
Kerr-lens-mode-locked Ti: sapphire laser 共ML-Ti: S-2兲. The

mode-locked frequencies of the two lasers 共f 1 and f 2兲 are set
approximately at 82.6 MHz. The difference in mode-locked
frequency 共⌬f = f 1 − f 2兲 must be stabilized at a certain frequency to achieve the AOS method. Hence, a ring-type piezoelectric transducer 共PZT, stroke= 6 ␮m; resonancefrequency= 6 kHz兲 is attached to an output coupler in MLTi: S-1 for controlling f 1 within a ±125-Hz range. The
difference-frequency signal 共frequency= 10f 1 − 10f 2 = 10⌬f兲
between the tenth-harmonic components of the mode-locked
frequency is extracted from two signals detected with photodiodes 共PD兲 through a series of electronic heterodyned processes that employ a voltage-controlled oscillator 共VCO兲,
two double-balanced mixers 共DBM兲, three low-pass filters
共LPF兲, and an analog mixer 共M兲. Finally, f 1 is controlled
to keep ⌬f = 108 Hz via a phase-locked loop 共PLL,
reference frequency= 1.08 kHz兲 and a PZT controller. TheFIG. 1. Experimental setup. ML-Ti: S-1 and ML-Ti: S-2: mode-locked
Ti: sapphire lasers; PZT: piezoelectric transducer; PD: photodiode; DBM:
double-balanced mixer; VCO: voltage-controlled oscillator; LPF: low-passfilter; M: analog mixer; PLL: phase-locked loop. OSC: oscillator; NLC:
LiB3O5; PMT: photomultiplier; AMP1 and AMP2: current preamplifiers;
THz-EM: THz emitter; THz-DT: THz detector; Si-L: silicon lens.

0003-6951/2005/87共6兲/061101/3/$22.50
87, 061101-1© 2005 American Institute of Physics
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061101-2

Yasui, Saneyoshi, and Araki

resulting fluctuation of ⌬f is suppressed within 108± 0.1 Hz
at a gate time of 1 s. Portions of the two laser lights are fed
into a SFG 共sum frequency generation兲 cross correlator using
a nonlinear optical crystal 共LiB3O5 , thickness= 2 mm兲. The
resultant SFG cross-correlation light is detected with a photomultiplier 共PMT兲, amplified with a current preamplifier
共AMP1兲, and then used as the time origin in AOS-THz-TDS.
In THz-TDS, ML-Ti: S-1 and ML-Ti: S-2 are used as theprobe light and pump light, respectively. We use two bowtieshaped, low temperature grown GaAs photoconductive antennas 共bowtie length= 1 mm; gap= 5 ␮m兲 as a THz emitter
共THz-EM兲 and THz detector 共THz-DT兲 to improve the detection sensitivity of a weak THz signal. The THz pulse radiating from the THz emitter is detected with the THz detector via two hemispherical silicon lenses 共Si-L兲 and a THzlens 共f = 50 mm兲. After passing through another current
preamplifier
共AMP2;
sensitivity= 106 V / A , bandwidth= 200 kHz兲, the signal can be directly observed at a scan rateof ⌬f with a digital oscilloscope 共bandwidth= 500 MHz兲
triggered by the time-origin signal from the SFG crosscorrelator.
Figure 2共a兲 shows the temporal profile of a pulsed THz
electric field measured by AOS-THz-TDS, in which signal
averaging of 1000-sweep sequences was carried out by adigital oscilloscope 共required time= 10 s兲. The observed temporal profile is expanded by a temporal magnification factorof 764 815 共=f 1 / ⌬f = 82 600 000/ 108兲 according to the principle of the AOS method.4 Furthermore, the temporally magnified THz pulse is sampled as a burst of the probe pulse
with a period of 1 / f 1, which is equivalent to a step incrementof 15.8 fs in real time delay 关=⌬f / f 1 f 2 = 108/ 共82 600 000⫻ 82 600 108兲兴. The upper and lower horizontal scales inFig. 2共a兲 indicate, respectively, the time scale on the oscilloscope and the real time scale translated using the above magnification factor. The temporal wave form of the pulsed THzelectric field was measured within a temporal window of thepulse period 共=12.1 ns兲. In contrast to conventional THzTDS, AOS-THz-TDS can acquire the temporal wave form at
a fixed scan rate of ⌬f 共=108 Hz兲 regardless of the size ofthe overall temporal window. A temporal region of 100 ps
enlarged from Fig. 2共a兲 is shown in Fig. 2共b兲. The transient
evolution of the THz electric field in the picosecond region isclearly observed. In the present system, the bowtie-shaped
photoconductive antennas used as a THz emitter and detector
limit the bandwidth of the THz pulse to a considerably narrow range, resulting in the long pulse width of the THz electric field 共=6.5 ps兲. To compare with AOS-THz-TDS, we obtained the temporal profile of the THz electric field byconventional THz-TDS using a motor-driven mechanicalstage and a lock-in amplifier, as shown in Fig. 2共c兲
共measurement time= 5 min兲. The comparison indicated thatAOS-THz-TDS reproduces the temporal profile of a pulsed
THz emission accurately. The slight distortion of the observed THz pulse profile is due to the narrow bandwidth of
the AMP2 and/or insufficient temporal magnification of theAOS method. It is important to emphasize that the signal-tonoise ratio observed in AOS-THz-TDS for a measurement
time of 10 s is almost the same as that observed in conventional THz-TDS in 5 min. Furthermore, the proposed method
requires no lock-in amplifier, which is usually used in THzTDS for sensitive detection of a weak THz electric field.
Figure 3共a兲 shows the THz amplitude spectrum calculated

Appl. Phys. Lett. 87, 061101 共2005兲

FIG. 2. Temporal wave forms of pulsed THz electrical field. 共a兲 AOS-THzTDS 共full scale= 12.1 ns兲, 共b兲 AOS-THz-TDS 共full scale= 100 ps兲, and 共c兲
stage-scanning THz-TDS 共full scale= 100 ps兲. The upper horizontal scales
indicate the time scale on oscilloscope.

from the Fourier transform of the wave form in Fig. 2共a兲.
The frequency resolution and frequency span are calculatedto be 82.6 MHz and 68 THz, respectively, from an overall
window of 12.1 ns and a step increment of 15.8 fs, if fluctuation in ⌬f is neglected. However, the actual instability in⌬f fluctuates the frequency resolution and frequency span.
For example, a fluctuation of 0.1 Hz in ⌬f leads to those of
11.2 ps in the overall window and 0.015 fs in the step increment, resulting in those of 76.4 kHz in frequency resolution
and 69.3 GHz in frequency span. The spectral structure inFig. 3共a兲 is due to multiple reflection of THz pulse in ahemispherical silicon lens. The THz amplitude spectrum inFig. 3共b兲 was obtained through conventional THz-TDS, byDownloaded 02 Aug 2005 to 133.1.55.214. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

061101-3

Appl. Phys. Lett. 87, 061101 共2005兲

Yasui, Saneyoshi, and Arakiin conventional THz-TDS if a long stage scanning equivalent
to a laser cavity length 共=1.82 m兲 is adopted. However, thisresult is extremely time consuming; over 10 h under theexperimental conditions used in Fig. 2共c兲.
In conclusion, using the AOS-THz-TDS technique, wehave obtained a frequency resolution of 82.6 MHz at a dataacquisition time of 10 s. This method not only achieves thetheoretical limit of frequency resolution in THz-TDS at the
short measurement time but also opens the door to a compact, simple, and stable THz-TDS system with no mechanical parts by the combination of femtosecond mode-locked
fiber lasers and fiber-coupled photoconductive antennas. The
use of a broadband THz emitter and detector, adjustment of
the temporal magnification factor, and/or use of broadband
THz detection electronics will enable further expansion ofthe spectral bandwidth in AOS-THz-TDS.
This work was supported by the Industrial Technology
Research Grant Program in 2004 from the New Energy andIndustrial Technology Development Organization 共NEDO兲
of Japan, and the Strategic Information and Communications
R&D Promotion Programme 共SCOPE兲 from Ministry of Internal Affairs and Communications 共MIC兲 of Japan. We are
grateful to Dr. Mamoru Hashimoto at Osaka University for
productive discussions.
1FIG. 3. Spectra of THz amplitude. 共a兲 AOS-THz-TDS 共frequency
resolution= 82.6 MHz兲 and 共b兲 stage-scanning THz-TDS 共frequency
resolution= 10 GHz兲.

taking the Fourier transform of the wave form in Fig. 2共c兲. A
comparison between the two profiles reveals a remarkably
improved frequency resolution in the case of AOS-THzTDS. The frequency resolution of 82.6 MHz can be achieved

H. Ohta, S. Nogiwa, Y. Kawaguchi, and Y. Endo, Electron. Lett. 36, 737
共2000兲.
2
J. Li, J. Hansryd, P. O. Hedekvist, P. A. Andrekson, and S. N. Knudsen,
IEEE Photonics Technol. Lett. 13, 987 共2001兲.
3
P. A. Elzinga, R. J. Kneisler, F. E. Lytle, Y. Jiang, G. B. King, and N. M.
Laurendeau, Appl. Opt. 26, 4303 共1987兲.
4Y. Takagi and S. Adachi, Rev. Sci. Instrum. 70, 2218 共1999兲.
5
F. Keilmann, C. Gohle, and R. Holzwarth, Opt. Lett. 29, 1542 共2004兲.
6
D. W. van der Weide, J. Murakowski, and F. Keilmann, IEEE Trans. Microwave Theory Tech. MTT-48, 740 共2000兲.

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