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Fiber-based, hybrid terahertz spectrometerusing dual fiber combs
Takeshi Yasui,1,* Masaki Nose,1 Atsushi Ihara,1,2 Kohji Kawamoto,1Shuko Yokoyama,1,3 Hajime Inaba,2 Kaoru Minoshima,2 and Tsutomu Araki1
1Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho,Toyonaka, Osaka 560-8531, Japan
23National Metrology Institute of Japan, National Institute of Advanced Industrial
Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8563, Japan

Micro-Optics Co. Ltd., 2-20 Oe-Nakayama-cho, Nishikyoku, Kyoto, 610-1104, Japan*Corresponding author: t‑yasui@me.es.osaka‑u.ac.jp
Received December 24, 2009; revised March 31, 2010; accepted April 15, 2010;
posted April 22, 2010 (Doc. ID 122020); published May 14, 2010

We constructed a fiber-based, hybrid terahertz (THz) spectrometer having two working modes, asynchronousoptical-sampling THz time-domain spectroscopy (AOS-THz-TDS) and multiple-frequency-heterodyning THz comb
spectroscopy (MFH-THz-CS), by use of dual fiber-laser-based frequency combs. A spectral range of 2 THz and dynamic range of 100 was achieved at the single sweep measurement of 200 ms in the AOS-THz-TDS mode, whereas
the detailed structure of the THz frequency comb was clearly observed in the MFH-THz-CS mode. The spectrometerfeatures compactness, robustness, flexibility, and cost effectiveness, in addition to high spectral resolution in rapid
data acquisition, and has the potential to become a powerful tool for practical applications. © 2010 Optical Societyof America
OCIS codes: 120.6200, 300.6320, 300.6495.

Asynchronous optical sampling (AOS) using dual opticalfrequency combs emitted by two mode-locked pulsed lasers with slightly mismatched mode-locked frequencies
enables us to linearly expand the time scale of picosecond transient signals to microsecond order. The resulting slow signal can be captured rapidly on a standard
oscilloscope without the need for time-consuming, mechanical time-delay scanning. Combined use of theAOS method with Fourier transform spectroscopy permits even higher spectral resolution because the combination enables a time delay much longer than that
achievable by conventional motor-driven translation
stages. Recently, the AOS method has been effectively
coupled with terahertz (THz) time-domain spectroscopy,
namely, AOS-THz-TDS, to achieve rapid data acquisition and high spectral resolution [1,2]. Furthermore,
frequency-domain measurement based on dual THz frequency combs generated by a photoconductive process
in dual optical combs has been proposed, referred toas multiple-frequency-heterodyning THz comb spectroscopy (MFH-THz-CS) [3], which has been used for observing the detailed structure of THz combs.
Conventionally, a pair of mode-locked Ti:sapphire lasers has been widely used for dual optical combs in thosesystems [1–3]. However, use of bulky, complicated, andexpensive Ti:sapphire lasers is a major obstacle toachieve a compact, robust, flexible, and cost-effective apparatus suitable for practical applications. A promisingalternative laser source for dual optical combs is amode-locked Er-doped fiber laser working at the1550 nm telecommunication band [4], exploiting the robustness, compactness, and cost-effectiveness of thislaser source. Furthermore, direct coupling between thelaser output and a photoconductive antenna (PCA) using
an optical fiber, namely, fiber-coupled PCA, eliminates
free-space propagation of the laser light, which normally
involves many mirrors and lenses, and allows the measurement head to be placed at any position [5]. A fiber0146-9592/10/101689-03$15.00/0

based THz spectrometer using dual fiber combs andfiber-coupled PCAs is promising as a portable, alignment-free, robust, and flexible apparatus without theneed for mechanical time-delay scanning. In this Letter,
we report a fiber-based, hybrid THz spectrometer working in the AOS-THz-TDS mode and in the MFH-THzCS mode.
Figure 1 shows a schematic diagram of the experimental setup. We constructed two self-starting, stretchedpulse mode-locked, Er-doped fiber lasers (λc ¼ 1550 nm,Δτ ¼ 50 fs, P AVG ¼ 90 mW) [4] for use as laser sources
to generate and detect pulsed THz radiation (pumpand probe lasers). The individual mode-locked frequencies of the two lasers (f 1 ¼56,124,000 Hz and f 2 ¼
56,124,005 Hz) and the frequency difference between
them (Δf ¼ f 2 − f 1 ¼ 5 Hz) were stabilized by two independent PI (proportional and integral) control systems
with reference to a Rb frequency standard (accuracy ¼5 × 10−11 , instability ¼ 2 × 10−11 at 1 s). The instability ofthe mode-locked frequency was equal to that of the Rb
frequency standard, whereas the timing jitter between
the two lasers was less than 300 fs within the range ofthe time delay equal to the pulse period. Output lightFig. 1. (Color online) Experimental setup. SHG, second-harmonic-generation crystal; L, lenses; PCA1 and PCA2, dipoleshaped LTG-GaAs photoconductive antennas; Si-L, hemispherical silicon lenses; THz-L, THz lenses; AMP, current preamplifier.
© 2010 Optical Society of America

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from the pump laser was delivered by an optical fiber andwas converted by second-harmonic-generation (SHG)
to half its original wavelength by using an SHG crystal(β‐BaB2 O4 ). The resulting 775 nm SHG light was focusedon the gap of a dipole-shaped low-temperature-grown
GaAs (LTG-GaAs) PCA for THz generation (PCA1, length
7:5 μm, width 10 μm, gap 5 μm). The pulsed THz radiation from PCA1 propagated in free space through twopairs of hemispherical silicon lenses (Si-L) and THz
lenses (THz-L) and was then incident on another dipoleshaped LTG-GaAs PCA (PCA2) gated by the SHG light ofthe probe laser. A fully fiber-coupled THz spectrometer
will be achieved without the need for frequency doubling
if a PCA for a 1550 nm light is used [5]. Portions of theoutput light from the two lasers were fed into a sumfrequency-generation (SFG) cross correlator. The resulting SFG signal was used to generate a time origin signal
in the AOS-THz-TDS mode. The current signal from PCA2
was amplified with a high-gain current preamplifier(AMP, bandwidth 100 kHz and sensitivity 5 × 107 V=A).
The resulting voltage signal was measured with a fast digitizer by using the SFG signal as a trigger signal in theAOS-THz-TDS mode. In the case of the MFH-THz-CS
mode, the signal was measured with a radio frequency(RF) spectrum analyzer, in place of the digitizer, without
the need for the SFG cross correlator.
Figure 2(a) shows the temporal profile of the pulsed
THz radiation measured in the AOS-THz-TDS mode, inwhich a single sweep measurement was carried out by
the digitizer (required time 200 ms). A moderate dynamic
range (DR) was achieved at the single sweep measurement. Figure 2(b) shows a power spectrum of the pulsed
THz radiation obtained by a fast Fourier transform (FFT)of the temporal waveform in Fig. 2(a). A spectral range of2 THz and DR of 100 was achieved at the acquisition timeof 200 ms. Several absorption lines caused by atmospheric water vapor were clearly observed. For example,
the absorption line at 0:556 THz has linewidth of 10 GHz,
although the spectral resolution determined by the time
window (350 ps) in Fig. 2(a) was 2:86 GHz. This linewidth is influenced by pressure broadening. Work is inprogress to evaluate the actual spectral resolution by
the THz spectroscopy of the low-pressure water vapor.
The use of a broadband THz emitter, e.g., a DAST crystal[6], will permit further expansion of the spectral bandwidth, together with reducing the timing jitter of the dual
fiber combs.
We next demonstrated the MFH-THz-CS mode using
the same spectrometer. In this mode, an electromagnetic
THz comb (EM-THz comb, frequency spacing f 1 ) is radiated from PCA1 excited by the SHG optical comb ofthe pump light, whereas a photocarrier THz comb (PCTHz comb, frequency spacing f 2 ¼ f 1 þ Δf ) is inducedin PCA2 gated by that of the probe light [3]. When thefree-space-propagating EM-THz comb is incident on
PCA2 having the PC-THz comb, a secondary frequency
comb having the frequency spacing of Δf is observedin the RF region via the multifrequency-heterodyningphotoconductive process occurring between the EMTHz and PC-THz combs. Since the observed RF comb
is a replica of the EM-THz comb scaled down by
f 1 =Δf ¼11,224,800 in frequency, one can reconstruct
the EM-THz comb easily via direct observation of theFig. 2. (Color online) (a) Temporal waveform and (b) powerspectrum of the pulsed THz radiation measured at the single
sweep measurement in AOS-THz-TDS mode (acquisition time200 ms).

RF comb by using the RF spectrum analyzer and by calibration of the frequency scale using f 1 =Δf , without theneed for FFT calculation. Figure 3(a) shows a power
spectrum of the EM-THz comb measured in the MFHTHz-CS mode (measurement time 100 s). The spectrumof the EM-THz comb, here plotted in red, is actually composed of a series of frequency spikes regularly separatedby the mode-locked frequency. Also, the absorption lines
caused by the water vapor were confirmed. For comparison, the THz power spectrum was measured at the samemeasurement time in AOS-THz-TDS mode as shown in
Fig. 3(b), in which signal averaging of 500-sweep sequences was carried out by the digitizer. Although theactual −10 dB bandwidth of the MFH-THz-CS mode
(1:286 THz) was almost equal to that of the AOS-THzTDS mode (1:230 THz), there was a large difference in
DR between the two modes; this leads to difference ofthe spectral tail between them. The spectral bandwidthand DR in the MFH-THz-CS mode should be equal tothose in the AOS-THz-TDS mode because both of thesemodes share the same dual fiber combs and PCAs. The
only difference between the two modes is the measuring
instrument: the digitizer for the AOS-THz-TDS mode andthe spectrum analyzer for the MFH-THz-CS mode. The

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a weak signal by heterodyning with the signal of the local
oscillator, it is likely that the efficiency of signal acquisition in the spectrum analyzer is still lower than that in thedigitizer in this demonstration. However, if the zero-spanmode is used in the spectrum analyzer, the MFH-THz-CS
mode has the advantage of real-time monitoring capability of the THz amplitude at a single frequency, such as aspecified THz spectral fingerprint.
In conclusion, we have constructed a fiber-based, hybrid THz spectrometer, working in the AOS-THz-TDS
mode and the MFH-THz-CS mode, by use of dual fiber
combs. We compared the basic performance between
both modes and found that the former mode is superiorto the latter mode in terms of DR, although the lattermode offers real-time monitoring capability of the THz
amplitude at an arbitrary, single frequency, in addition
to a simple setup without the need for FFT calculation
and an SFG cross-correlator. The constructed spectrometer is expected to become a compact, alignment-free,robust, flexible, and cost-effective apparatus suitable forpractical applications.
This work was supported by Grants-in-Aid for Scientific Research 20560036, 21360039, and 21650111 from theMinistry of Education, Culture, Sports, Science, andTechnology of Japan. We also gratefully acknowledge financial support from the Renovation Center of Instruments for Science Education and Technology in Osaka
University, the Suzuki Foundation and the Mitutoyo Association for Science and Technology, Japan.

Fig. 3. (Color online) Comparison of power spectrum measured in (a) MFH-THz-CS mode (sweep time 100 s) and(b) AOS-THz-TDS mode (acquisition time 100 s).

digitizer can capture the signal components of the pulsed
THz radiation in the time domain. Conversely, since ourspectrum analyzer is based on the superheterodyne technique, it acquires only the frequency-component signalcoinciding with the frequency of its local oscillator at
any given moment, and then acquires the overall spectrum by sweeping the frequency of the local oscillator.
Although this superheterodyne technique can enhance

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