Fiber-based, hybrid terahertz spectrometerusing dual fiber combs

Takeshi Yasui,1,* Masaki Nose,1 Atsushi Ihara,1,2 Kohji Kawamoto,1

Shuko Yokoyama,1,3 Hajime Inaba,2 Kaoru Minoshima,2 and Tsutomu Araki11Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho,

Toyonaka, Osaka 560-8531, Japan2National Metrology Institute of Japan, National Institute of Advanced Industrial

Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8563, Japan3Micro-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, asynchronous-optical-sampling THz time-domain spectroscopy (AOS-THz-TDS) and multiple-frequency-heterodyning THz combspectroscopy (MFH-THz-CS), by use of dual fiber-laser-based frequency combs. A spectral range of 2 THz and dy-namic range of 100 was achieved at the single sweep measurement of 200 ms in the AOS-THz-TDS mode, whereasthe detailed structure of the THz frequency combwas clearly observed in the MFH-THz-CS mode. The spectrometerfeatures compactness, robustness, flexibility, and cost effectiveness, in addition to high spectral resolution in rapiddata acquisition, and has the potential to become a powerful tool for practical applications. © 2010 Optical Societyof AmericaOCIS codes: 120.6200, 300.6320, 300.6495.

Asynchronous optical sampling (AOS) using dual opticalfrequency combs emitted by two mode-locked pulsed la-sers with slightly mismatched mode-locked frequenciesenables us to linearly expand the time scale of pico-second transient signals to microsecond order. The re-sulting slow signal can be captured rapidly on a standardoscilloscope without the need for time-consuming, me-chanical time-delay scanning. Combined use of theAOS method with Fourier transform spectroscopy per-mits even higher spectral resolution because the combi-nation enables a time delay much longer than thatachievable by conventional motor-driven translationstages. Recently, the AOS method has been effectivelycoupled with terahertz (THz) time-domain spectroscopy,namely, AOS-THz-TDS, to achieve rapid data acquisit-ion and high spectral resolution [1,2]. Furthermore,frequency-domain measurement based on dual THz fre-quency combs generated by a photoconductive processin dual optical combs has been proposed, referred toas multiple-frequency-heterodyning THz comb spectro-scopy (MFH-THz-CS) [3], which has been used for obser-ving the detailed structure of THz combs.Conventionally, a pair of mode-locked Ti:sapphire la-

sers 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 ap-paratus 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 ro-bustness, compactness, and cost-effectiveness of thislaser source. Furthermore, direct coupling between thelaser output and a photoconductive antenna (PCA) usingan optical fiber, namely, fiber-coupled PCA, eliminatesfree-space propagation of the laser light, which normallyinvolves many mirrors and lenses, and allows the mea-surement head to be placed at any position [5]. A fiber-

based THz spectrometer using dual fiber combs andfiber-coupled PCAs is promising as a portable, align-ment-free, robust, and flexible apparatus without theneed for mechanical time-delay scanning. In this Letter,we report a fiber-based, hybrid THz spectrometer work-ing in the AOS-THz-TDS mode and in the MFH-THz-CS mode.

Figure 1 shows a schematic diagram of the experimen-tal setup. We constructed two self-starting, stretched-pulse mode-locked, Er-doped fiber lasers (λc ¼ 1550 nm,Δτ ¼ 50 fs, PAVG ¼ 90 mW) [4] for use as laser sourcesto generate and detect pulsed THz radiation (pumpand probe lasers). The individual mode-locked frequen-cies of the two lasers (f 1 ¼56,124,000 Hz and f 2 ¼56,124,005 Hz) and the frequency difference betweenthem (Δf ¼ f 2 − f 1 ¼ 5 Hz) were stabilized by two inde-pendent PI (proportional and integral) control systemswith 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 Rbfrequency standard, whereas the timing jitter betweenthe two lasers was less than 300 fs within the range ofthe time delay equal to the pulse period. Output light

Fig. 1. (Color online) Experimental setup. SHG, second-har-monic-generation crystal; L, lenses; PCA1 and PCA2, dipole-shaped LTG-GaAs photoconductive antennas; Si-L, hemispheri-cal silicon lenses; THz-L, THz lenses; AMP, current preamplifier.

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0146-9592/10/101689-03$15.00/0 © 2010 Optical Society of America

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‐BaB2O4). The resulting 775 nm SHG light was focusedon the gap of a dipole-shaped low-temperature-grownGaAs (LTG-GaAs) PCA for THz generation (PCA1, length7:5 μm, width 10 μm, gap 5 μm). The pulsed THz radia-tion from PCA1 propagated in free space through twopairs of hemispherical silicon lenses (Si-L) and THzlenses (THz-L) and was then incident on another dipole-shaped LTG-GaAs PCA (PCA2) gated by the SHG light ofthe probe laser. A fully fiber-coupled THz spectrometerwill be achieved without the need for frequency doublingif a PCA for a 1550 nm light is used [5]. Portions of theoutput light from the two lasers were fed into a sum-frequency-generation (SFG) cross correlator. The result-ing SFG signal was used to generate a time origin signalin the AOS-THz-TDS mode. The current signal from PCA2was 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 di-gitizer by using the SFG signal as a trigger signal in theAOS-THz-TDS mode. In the case of the MFH-THz-CSmode, the signal was measured with a radio frequency(RF) spectrum analyzer, in place of the digitizer, withoutthe 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 bythe digitizer (required time 200 ms). A moderate dynamicrange (DR) was achieved at the single sweep measure-ment. Figure 2(b) shows a power spectrum of the pulsedTHz 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 atmo-spheric 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 timewindow (350 ps) in Fig. 2(a) was 2:86 GHz. This line-width is influenced by pressure broadening. Work is inprogress to evaluate the actual spectral resolution bythe 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 band-width, together with reducing the timing jitter of the dualfiber combs.We next demonstrated the MFH-THz-CS mode using

the same spectrometer. In this mode, an electromagneticTHz comb (EM-THz comb, frequency spacing f 1) is ra-diated from PCA1 excited by the SHG optical comb ofthe pump light, whereas a photocarrier THz comb (PC-THz comb, frequency spacing f 2 ¼ f 1 þΔf ) is inducedin PCA2 gated by that of the probe light [3]. When thefree-space-propagating EM-THz comb is incident onPCA2 having the PC-THz comb, a secondary frequencycomb having the frequency spacing of Δf is observedin the RF region via the multifrequency-heterodyningphotoconductive process occurring between the EM-THz and PC-THz combs. Since the observed RF combis a replica of the EM-THz comb scaled down byf 1=Δf ¼11,224,800 in frequency, one can reconstructthe EM-THz comb easily via direct observation of the

RF comb by using the RF spectrum analyzer and by cali-bration of the frequency scale using f 1=Δf , without theneed for FFT calculation. Figure 3(a) shows a powerspectrum of the EM-THz comb measured in the MFH-THz-CS mode (measurement time 100 s). The spectrumof the EM-THz comb, here plotted in red, is actually com-posed of a series of frequency spikes regularly separatedby the mode-locked frequency. Also, the absorption linescaused by the water vapor were confirmed. For compar-ison, the THz power spectrum was measured at the samemeasurement time in AOS-THz-TDS mode as shown inFig. 3(b), in which signal averaging of 500-sweep se-quences 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-THz-TDS mode (1:230 THz), there was a large difference inDR 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. Theonly difference between the two modes is the measuringinstrument: the digitizer for the AOS-THz-TDS mode andthe spectrum analyzer for the MFH-THz-CS mode. The

Fig. 2. (Color online) (a) Temporal waveform and (b) powerspectrum of the pulsed THz radiation measured at the singlesweep measurement in AOS-THz-TDS mode (acquisition time200 ms).

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digitizer can capture the signal components of the pulsedTHz radiation in the time domain. Conversely, since ourspectrum analyzer is based on the superheterodyne tech-nique, it acquires only the frequency-component signalcoinciding with the frequency of its local oscillator atany given moment, and then acquires the overall spec-trum by sweeping the frequency of the local oscillator.Although this superheterodyne technique can enhance

a weak signal by heterodyning with the signal of the localoscillator, it is likely that the efficiency of signal acquisi-tion 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-CSmode has the advantage of real-time monitoring capabil-ity of the THz amplitude at a single frequency, such as aspecified THz spectral fingerprint.

In conclusion, we have constructed a fiber-based, hy-brid THz spectrometer, working in the AOS-THz-TDSmode and the MFH-THz-CS mode, by use of dual fibercombs. We compared the basic performance betweenboth 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 THzamplitude at an arbitrary, single frequency, in additionto a simple setup without the need for FFT calculationand an SFG cross-correlator. The constructed spectro-meter 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 Scienti-fic Research 20560036, 21360039, and 21650111 from theMinistry of Education, Culture, Sports, Science, andTechnology of Japan. We also gratefully acknowledge fi-nancial support from the Renovation Center of Instru-ments for Science Education and Technology in OsakaUniversity, the Suzuki Foundation and the Mitutoyo As-sociation for Science and Technology, Japan.

References

1. T. Yasui, E. Saneyoshi, and T. Araki, Appl. Phys. Lett. 87,061101 (2005).

2. A. Bartels, A. Thoma, C. Janke, T. Dekorsy, A. Dreyhaupt,S. Winnerl, and M. Helm, Opt. Express 14, 430–437 (2006).

3. T. Yasui, Y. Kabetani, E. Saneyoshi, S. Yokoyama, andT. Araki, Appl. Phys. Lett. 88, 241104 (2006).

4. H. Inaba, Y. Daimon, F.-L. Hong, A. Onae, K. Minoshima,T. R. Schibli, H. Matsumoto, M. Hirano, T. Okuno, M. Onishi,and M. Nakazawa, Opt. Express 14, 5223 (2006).

5. B. Sartorius, H. Roehle, H. Künzel, J. Böttcher, M. Schlak,D. Stanze, H. Venghaus, and M. Schell, Opt. Express 16,9565 (2008).

6. J. Takayanagi, S. Kanamori, K. Suizu, M. Yamashita, T.Ouchi, S. Kasai, H. Ohtake, H. Uchida, N. Nishizawa, andK. Kawase, Opt. Express 16, 12859 (2008).

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

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