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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 14 — Jul. 2, 2012
  • pp: 15071–15078
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Enhancement of spectral resolution and accuracy in asynchronous-optical-sampling terahertz time-domain spectroscopy for low-pressure gas-phase analysis

Takeshi Yasui, Kohji Kawamoto, Yi-Da Hsieh, Yoshiyuki Sakaguchi, Mukesh Jewariya, Hajime Inaba, Kaoru Minoshima, Francis Hindle, and Tsutomu Araki  »View Author Affiliations


Optics Express, Vol. 20, Issue 14, pp. 15071-15078 (2012)
http://dx.doi.org/10.1364/OE.20.015071


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Abstract

The spectral resolution and accuracy of asynchronous-optical-sampling terahertz time-domain spectroscopy (ASOPS-THz-TDS) were evaluated by examining low-pressure gas-phase samples. Use of dual 56-MHz, erbium (Er)-doped, mode-locked femtosecond fiber lasers enhanced the spectral resolution to as low as 50.5 MHz and the spectral accuracy to as low as 6.2 × 10−6. The results indicate that ASOPS-THz-TDS has the potential to achieve high spectral resolution, high spectral accuracy, and wide spectral coverage at the same time. ASOPS-THz-TDS will open a new door to gas-phase spectroscopy of multiple chemical species in the field of atmospheric gas analysis.

© 2012 OSA

1. Introduction

One potential method to simultaneously achieve high spectral resolution, high spectral accuracy, and broad spectral coverage in THz spectroscopy is asynchronous-optical-sampling THz-TDS (ASOPS-THz-TDS) using two mode-locked lasers with slightly mismatched mode-locked frequencies (mode-locked frequency = f1 and f2, offset frequency = ∆f = f2 - f1) for generation and detection of the pulsed THz radiation [6

6. C. Janke, M. Först, M. Nagel, H. Kurz, and A. Bartels, “Asynchronous optical sampling for high-speed characterization of integrated resonant terahertz sensors,” Opt. Lett. 30(11), 1405–1407 (2005). [CrossRef] [PubMed]

, 7

7. T. Yasui, E. Saneyoshi, and T. Araki, “Asynchronous optical sampling terahertz time-domain spectroscopy for ultrahigh spectral resolution and rapid data acquisition,” Appl. Phys. Lett. 87(6), 061101 (2005). [CrossRef]

]. Since ASOPS-THz-TDS helps to realize a time delay much longer than that achievable with a motor-driven translation stage in conventional THz-TDS, it has the potential to achieve high spectral resolution with broad spectral coverage. For example, when the measurement time window of the pulsed THz radiation is extended to one pulse period, the spectral resolution can be enhanced to the mode-locked frequency f1. In this case, the frequency scale of the spectrum is determined on the basis of the measurement time scale and a temporal magnification factor of f1/∆f [7

7. T. Yasui, E. Saneyoshi, and T. Araki, “Asynchronous optical sampling terahertz time-domain spectroscopy for ultrahigh spectral resolution and rapid data acquisition,” Appl. Phys. Lett. 87(6), 061101 (2005). [CrossRef]

]. However, the actual stability of f1 and ∆f may cause fluctuations in the temporal magnification factor, and hence influence the spectral resolution and accuracy. Although f1 and/or ∆f can be stabilized by referencing to a reference signal synthesized from a microwave frequency standard, the residual fluctuation will ultimately limit the performance of this technique. Therefore, the actual spectral resolution and accuracy achieved by ASOPS-THz-TDS should be evaluated using a standard material, for example, the absorption lines of a molecular gas with a narrow linewidth. Spectral resolution and accuracy in an ASOPS-THz-TDS system equipped with ∆f-stabilized, dual 1-GHz Ti:Sapphire lasers (f1 = 1,000,000,000 Hz, f2 = 1,000,002,000 Hz, and ∆f = 2 kHz) were evaluated using molecular gasses at atmospheric pressure, resulting in the resolution of 1 GHz and accuracy of 5.7 × 105 [8

8. G. Klatt, R. Gebs, H. Schäfer, M. Nagel, C. Janke, A. Bartels, and T. Dekorsy, “High-resolution terahertz spectrometer,” IEEE J. Sel. Top. Quantum Electron. 17(1), 159–168 (2011). [CrossRef]

]. Further enhanced performance will be required to increase the ability to identify spectral fingerprints and expand the selectivity of target gas molecules, for example, analysis of multiple chemical species in gas-phase spectroscopy under low pressure [9

9. R. A. Cheville and D. Grischkowsky, “Far-infrared terahertz time-domain spectroscopy of flames,” Opt. Lett. 20(15), 1646–1648 (1995). [CrossRef] [PubMed]

, 10

10. D. Bigourd, A. Cuisset, F. Hindle, S. Matton, E. Fertein, R. Bocquet, and G. Mouret, “Detection and quantification of multiple molecular species in mainstream cigarette smoke by continuous-wave terahertz spectroscopy,” Opt. Lett. 31(15), 2356–2358 (2006). [CrossRef] [PubMed]

]. In the work reported in this paper, we enhanced the spectral resolution and accuracy in ASOPS-THz-TDS by use of dual 56-MHz, f1-f2-∆f-stabilized, erbium (Er)-doped, mode-locked fiber lasers [11

11. T. Yasui, M. Nose, A. Ihara, K. Kawamoto, S. Yokoyama, H. Inaba, K. Minoshima, and T. Araki, “Fiber-based, hybrid terahertz spectrometer using dual fiber combs,” Opt. Lett. 35(10), 1689–1691 (2010). [CrossRef] [PubMed]

].

2. Experimental setup

Figure 1
Fig. 1 Experimental setup. FS1 and FS2, frequency synthesizers; Rb-FS, rubidium frequency standard; DBMs, double-balanced mixers; LPFs, low-pass filters; PD, photodetectors; PIs, proportional and integral control systems; SFG-XC, sum-frequency-generation cross-correlator; SHGs, second-harmonic-generation crystals; Ls, lenses; PCA1, photoconductive antenna for THz generation; PCA2, photoconductive antenna for THz detector; Si-Ls, silicon lenses; AMP, current preamplifier.
shows a schematic diagram of the experimental setup in our fiber-laser-based ASOPS-THz-TDS system. A pair of self-starting, stretched-pulse, mode-locked, Er-doped fiber lasers (λc = 1550 nm, ∆t = 50 fs, PAVG = 110 mW, f1f2 ≈56.1 MHz) [12

12. 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, “Long-term measurement of optical frequencies using a simple, robust and low-noise fiber based frequency comb,” Opt. Express 14(12), 5223–5231 (2006). [CrossRef] [PubMed]

] was used to generate and detect the pulsed THz radiation. The 20th-harmonic components of the mode-locked frequency (20f1 and 20f2) were extracted from two signals detected with two fast photodetectors (PDs, bandwidth = 1.2 GHz) through electronic heterodyned processes that employ two frequency synthesizers (FS1, output freq. = fFS1 = 1,112,480,000 Hz; FS2, output freq. = fFS2 = 1,112,480,100 Hz) phase-locked to a rubidium frequency standard (Rb-FS, accuracy = 5 × 10−11 and instability = 2 × 10−11 at 1 s), two double-balanced mixers (DBMs), and two low-pass filters (LPFs). The resulting beat signals (freq. = 20f1fFS1 and 20f2fFS2) were used to generate the final error signals (freq. = 20f1fFS1 - fRb and 20f2fFS2 - fRb) by mixing with output signals from Rb-FS (output freq. = fRb = 10 MHz). Finally, based on these error signal, the cavity length in fiber lasers was adjusted with piezoelectric actuators (quick, fine, and narrow control) and Peltier heaters (slow, coarse, and wide control) through proportional and integral control systems (PIs) so that 20f1fFS1 - fRb = 0 and 20f2fFS2 - fRb = 0, or 20f1 = fFS1 + fRb and 20f2 = fFS2 + fRb. In this way, the individual mode-locked frequencies of the two lasers (f1 = 56,124,000 Hz and f2 = 56,124,005 Hz), and thus the frequency offset between them (∆f = f2f1 = 5 Hz), were stabilized by two independent laser control systems. After wavelength conversion of the two laser beams with second-harmonic-generation crystals (SHGs), pulsed THz radiation was emitted by a dipole-shaped, low-temperature-grown, GaAs photoconductive antenna triggered by pump light (PCA1), and was then detected by another dipole-shaped LT-GaAs-PCA triggered by probe light (PCA2). 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 a time origin signal in the ASOPS-THz-TDS. The temporal waveform of the pulsed THz radiation was measured with a digitizer (sampling frequency = 500 kHz) by using the SFG-XC signal as a trigger signal after amplification with a current preamplifier (AMP, bandwidth = 100 kHz, gain = 5 × 107 V/A). The time window of the measured temporal waveform was set equal to one pulse period in order to achieve a spectral resolution equal to the mode-locked frequency. To avoid pressure broadening of absorption lines, molecular gasses were enclosed in a low-pressure gas cell (length = 380 mm, diameter = 17 mm) with a pair of Brewster windows made of polypropylene plates. The optical path in which the THz beam propagated, except for the gas cell, was purged with dry nitrogen gas to avoid absorption by atmospheric moisture. Finally, the absorption spectrum was obtained by using a zero-filling factor of 10, fast Fourier transformation, and normalization with a reference spectrum obtained under identical conditions.

3. Results

3.1 Evaluation of spectral resolution using low-pressure water vapor

To evaluate the spectral resolution, we measured the absorption spectrum of the rotational transition 110 ←101 at 0.557 THz in water vapor. To decrease the strong absorption of water vapor, we diluted the water vapor with a foreign gas (dry argon gas). Figure 2(a)
Fig. 2 (a) Water absorption spectrum of rotational transition 110-101 at 0.557 THz (total pressure = 1 kPa) and (b) pressure broadening of 0.557 THz water line as a function of total pressure.
shows the absorption spectrum of the 0.557 THz water line at a total pressure of 1 kPa. In usual, signal integration of numerous scans is required in ASOPS-THz-TDS due to the fact that a lock-in amplifier cannot be used. In the present system, signal averaging of 5,000-sweep sequences under the low scan rate ( = ∆f = 5Hz) resulted in the required time of 1,000 s. If the scan rate is increased by use of a fast current preamplifier or free-space electro-optic sampling equipped with a fast photodetector, the data acquisition time will be decreased. The spectral linewidth was determined to be 50.5 MHz when a Lorentzian function was fitted to the spectral shape, indicated by the solid line in Fig. 2(a), which was reasonably consistent with the mode-locked frequency of 56.1 MHz. The reason for the negative absorption coefficient is mainly due to the system instability of THz spectral amplitude in the acquisition time.

Next, we investigated pressure broadening of the same water line when the total pressure was increased. Figure 2(b) shows the half-width at half-maximum (HWHM) of the observed absorption line as a function of the total pressure, which was varied between 1 kPa and 50 kPa. The linear relationship without saturation indicated that the slope gives the pressure broadening parameter of this mixed gas sample without any influence of the instrumentation linewidth or the Doppler linewidth. We determined the broadening parameter to be 31.7 MHz/kPa ( = 3.17 MHz/mbar) with a fitting error R of 0.99955, by performing a linear approximation to y = αx, indicated by the solid line in Fig. 2(b), where y is the HWHM, α is the pressure broadening parameter, and x is the total pressure. Considering the obtained parameter, the self-broadening coefficient of water ( = 13.8 MHz/mbar), the collision-broadening coefficient of argon ( = 1.63 MHz/mbar), and the total pressure, the mixture ratio of water vapor to the argon gas was estimated to 10.4% in this experiment. From these experiments, we can conclude that our ASOPS-THz-TDS system has the potential to perform a spectral resolution comparable to the mode-locked frequency in low-pressure gas-phase spectroscopy.

3.2 Evaluation of spectral accuracy using low-pressure acetonitrile gas

To evaluate the spectral accuracy and demonstrate the capacity to simultaneously probe multiple absorption lines, we performed gas-phase spectroscopy of acetonitrile (CH3CN). As CH3CN is a symmetric top molecule, the frequencies of absorption lines caused by its rotational transition are given by
ν=2B(J+1)2DJKK2(J+1)
(1)
where B is the rotational constant, J and K are rotational quantum numbers, and DJK is the centrifugal distortion constant. Since CH3CN has a B value of 9.2 GHz [13

13. M. Kessler, H. Ring, R. Trambarulo, and W. Gordy, “Microwave spectra and molecular structures of methyl cyanide and methyl isocyanide,” Phys. Rev. 79(1), 54–56 (1950). [CrossRef]

], a manifold of rotational transitions regularly spaced by 2B ( = 18.4 GHz) is expected from the first term in Eq. (1). Furthermore, from the second term in Eq. (1), each absorption line composing the manifold has hyperfine structure determined by DJK. Conventionally, the manifold of the rotational transitions and the hyperfine structure have been separately measured by using broadband THz-TDS [2

2. D. M. Mittleman, R. H. Jacobsen, R. Neelamani, R. G. Baraniuk, and M. C. Nuss, “Gas sensing using terahertz time-domain spectroscopy,” Appl. Phys. B 67(3), 379–390 (1998). [CrossRef]

] and a high-resolution CW-THz spectrometer [4

4. S. Matsuura, M. Tani, H. Abe, K. Sakai, H. Ozeki, and S. Saito, “High-resolution terahertz spectroscopy by a compact radiation source based on photomixing with diode lasers in a photoconductive antenna,” J. Mol. Spectrosc. 187(1), 97–101 (1998). [CrossRef] [PubMed]

]. Here, on the other hand, we evaluated the possibility that the ASOPS-THz-TDS system allows us to observe both features solely due to wide dynamic range of the frequency scale.

Figure 3(a)
Fig. 3 (a) Amplitude spectrum of pulsed THz radiation after passing though a gas cell filled with CH3CN at 40 Pa, (b) absorption spectrum of CH3CN between 0.6 and 0.7 THz, and (c) expanded absorption spectrum around 0.65 THz.
shows an amplitude spectrum of pulsed THz radiation after passing though a gas cell filled with CH3CN at 40 Pa, in which the manifold of the regularly spaced rotational transitions was clearly found within a frequency range from 0.3 to 0.9 THz in Fig. 3(a). To observe the detailed structure of the manifold, we calculated the absorption spectrum around 0.65 THz, as shown in Fig. 3(b). The six observed absorption lines had a mutual frequency spacing of 18.4 GHz, in good agreement with the value reported in the literature [2

2. D. M. Mittleman, R. H. Jacobsen, R. Neelamani, R. G. Baraniuk, and M. C. Nuss, “Gas sensing using terahertz time-domain spectroscopy,” Appl. Phys. B 67(3), 379–390 (1998). [CrossRef]

]. We expanded the range to observe hyperfine structure of the J = 35 to J = 34 transition around 0.64 THz and assigned lines K = 3 to 10, as shown in Fig. 3(c). To determine the center frequencies of these absorption lines, we performed multi-peak fitting analysis using a Lorentzian function, indicated by the black solid line in Fig. 3(c). Table 1

Table 1. Comparison of CH3CN absorption line positions as reported in the literature [14] and obtained using ASOPS-THz-TDS in this work.

table-icon
View This Table
summarizes the results for the absorption lines, showing a comparison between experimental values and values reported in JPL database [14

14. H. M. Pickett, R. L. Poynter, E. A. Cohen, M. L. Delitsky, J. C. Pearson, and H. S. P. Muller, “Submillimeter, millimeter, and microwave spectral line catalog,” J. Quant. Spectrosc. Radiat. Transf. 60(5), 883–890 (1998). [CrossRef]

], and the frequency discrepancy between them. The mean and standard deviation of the discrepancy for the eight assigned absorption lines were 4 MHz and 3 MHz, respectively. When spectral accuracy is defined as the ratio of the frequency discrepancy to the value reported in the literature, the mean spectral accuracy was 6.2 × 10−6 in this experiment.

Accuracy of the absorption strength is also important to make the reliable quantitative analysis in gas-phase spectroscopy under low pressure. To evaluate the quality of the obtained spectrum, one should compare the obtained spectrum with that calculated from spectral database. However, HITRAN database is not possible as this database presently only has CH3CN at wavenumbers from 890 - 946 cm−1 [15

15. L. S. Rothman, I. Gordon, A. Barbe, D. Benner, P. Bernath, M. Birk, V. Boudon, L. Brown, A. Campargue, J.-P. Champion, K. Chance, L. Coudert, V. Dana, V. Devi, S. Fally, J.-M. Flaud, R. Gamache, A. Goldman, D. Jacquemart, I. Kleiner, N. Lacome, W. Lafferty, J.-Y. Mandin, S. Massie, S. Mikhailenko, C. Miller, N. Moazzen-Ahmadi, O. Naumenko, A. Nikitin, J. Orphal, V. Perevalov, A. Perrin, A. Predoi-Cross, C. P. Rinsland, M. Rotger, M. Šimečková, M. A. H. Smith, K. Sung, S. A. Tashkun, J. Tennyson, R. A. Toth, A. C. Vandaele, and J. Vander Auwera, “The HITRAN 2008 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 110(9-10), 533–572 (2009). [CrossRef]

]. Although JPL database gives integrated intensities of each absorption line in this molecule [14

14. H. M. Pickett, R. L. Poynter, E. A. Cohen, M. L. Delitsky, J. C. Pearson, and H. S. P. Muller, “Submillimeter, millimeter, and microwave spectral line catalog,” J. Quant. Spectrosc. Radiat. Transf. 60(5), 883–890 (1998). [CrossRef]

], it is impossible to make the calculation of the complete spectrum, as there are no values for the broadening coefficients. Here, we made a simple comparison of our spectrum with the integrated intensities in JPL database, indicated as red lines in Fig. 3(c). The relative variation between neighboring lines is reasonably well reproduced and is limited by the signal-to-noise ratio of the measurements and the accuracy of the line strength calculations included in the database.

4. Discussions

5. Conclusion

Acknowledgments

This work was supported by Collaborative Research Based on Industrial Demand from the Japan Science and Technology Agency, and Grants-in-Aid for Scientific Research 21360039 and 23656265 from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. We also gratefully acknowledge financial support from the Renovation Center of Instruments for Science Education and Technology at Osaka University. We are grateful to Dr. Hiromichi Hoshina of RIKEN for fruitful discussions.

References and links

1.

M. Exter, Ch. Fattinger, and D. Grischkowsky, “Terahertz time-domain spectroscopy of water vapor,” Opt. Lett. 14(20), 1128–1130 (1989). [CrossRef] [PubMed]

2.

D. M. Mittleman, R. H. Jacobsen, R. Neelamani, R. G. Baraniuk, and M. C. Nuss, “Gas sensing using terahertz time-domain spectroscopy,” Appl. Phys. B 67(3), 379–390 (1998). [CrossRef]

3.

A. S. Pine, R. D. Suenram, E. R. Brown, and K. A. McIntosh, “A terahertz photomixing spectrometer: application to SO2 self broadening,” J. Mol. Spectrosc. 175(1), 37–47 (1996). [CrossRef]

4.

S. Matsuura, M. Tani, H. Abe, K. Sakai, H. Ozeki, and S. Saito, “High-resolution terahertz spectroscopy by a compact radiation source based on photomixing with diode lasers in a photoconductive antenna,” J. Mol. Spectrosc. 187(1), 97–101 (1998). [CrossRef] [PubMed]

5.

G. Mouret, F. Hindle, A. Cuisset, C. Yang, R. Bocquet, M. Lours, and D. Rovera, “THz photomixing synthesizer based on a fiber frequency comb,” Opt. Express 17(24), 22031–22040 (2009). [CrossRef] [PubMed]

6.

C. Janke, M. Först, M. Nagel, H. Kurz, and A. Bartels, “Asynchronous optical sampling for high-speed characterization of integrated resonant terahertz sensors,” Opt. Lett. 30(11), 1405–1407 (2005). [CrossRef] [PubMed]

7.

T. Yasui, E. Saneyoshi, and T. Araki, “Asynchronous optical sampling terahertz time-domain spectroscopy for ultrahigh spectral resolution and rapid data acquisition,” Appl. Phys. Lett. 87(6), 061101 (2005). [CrossRef]

8.

G. Klatt, R. Gebs, H. Schäfer, M. Nagel, C. Janke, A. Bartels, and T. Dekorsy, “High-resolution terahertz spectrometer,” IEEE J. Sel. Top. Quantum Electron. 17(1), 159–168 (2011). [CrossRef]

9.

R. A. Cheville and D. Grischkowsky, “Far-infrared terahertz time-domain spectroscopy of flames,” Opt. Lett. 20(15), 1646–1648 (1995). [CrossRef] [PubMed]

10.

D. Bigourd, A. Cuisset, F. Hindle, S. Matton, E. Fertein, R. Bocquet, and G. Mouret, “Detection and quantification of multiple molecular species in mainstream cigarette smoke by continuous-wave terahertz spectroscopy,” Opt. Lett. 31(15), 2356–2358 (2006). [CrossRef] [PubMed]

11.

T. Yasui, M. Nose, A. Ihara, K. Kawamoto, S. Yokoyama, H. Inaba, K. Minoshima, and T. Araki, “Fiber-based, hybrid terahertz spectrometer using dual fiber combs,” Opt. Lett. 35(10), 1689–1691 (2010). [CrossRef] [PubMed]

12.

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, “Long-term measurement of optical frequencies using a simple, robust and low-noise fiber based frequency comb,” Opt. Express 14(12), 5223–5231 (2006). [CrossRef] [PubMed]

13.

M. Kessler, H. Ring, R. Trambarulo, and W. Gordy, “Microwave spectra and molecular structures of methyl cyanide and methyl isocyanide,” Phys. Rev. 79(1), 54–56 (1950). [CrossRef]

14.

H. M. Pickett, R. L. Poynter, E. A. Cohen, M. L. Delitsky, J. C. Pearson, and H. S. P. Muller, “Submillimeter, millimeter, and microwave spectral line catalog,” J. Quant. Spectrosc. Radiat. Transf. 60(5), 883–890 (1998). [CrossRef]

15.

L. S. Rothman, I. Gordon, A. Barbe, D. Benner, P. Bernath, M. Birk, V. Boudon, L. Brown, A. Campargue, J.-P. Champion, K. Chance, L. Coudert, V. Dana, V. Devi, S. Fally, J.-M. Flaud, R. Gamache, A. Goldman, D. Jacquemart, I. Kleiner, N. Lacome, W. Lafferty, J.-Y. Mandin, S. Massie, S. Mikhailenko, C. Miller, N. Moazzen-Ahmadi, O. Naumenko, A. Nikitin, J. Orphal, V. Perevalov, A. Perrin, A. Predoi-Cross, C. P. Rinsland, M. Rotger, M. Šimečková, M. A. H. Smith, K. Sung, S. A. Tashkun, J. Tennyson, R. A. Toth, A. C. Vandaele, and J. Vander Auwera, “The HITRAN 2008 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 110(9-10), 533–572 (2009). [CrossRef]

16.

A. Bartels, R. Cerna, C. Kistner, A. Thoma, F. Hudert, C. Janke, and T. Dekorsy, “Ultrafast time-domain spectroscopy based on high-speed asynchronous optical sampling,” Rev. Sci. Instrum. 78(3), 035107 (2007). [CrossRef] [PubMed]

OCIS Codes
(120.6200) Instrumentation, measurement, and metrology : Spectrometers and spectroscopic instrumentation
(300.6320) Spectroscopy : Spectroscopy, high-resolution
(300.6495) Spectroscopy : Spectroscopy, teraherz

ToC Category:
Spectroscopy

History
Original Manuscript: May 1, 2012
Revised Manuscript: June 9, 2012
Manuscript Accepted: June 10, 2012
Published: June 20, 2012

Citation
Takeshi Yasui, Kohji Kawamoto, Yi-Da Hsieh, Yoshiyuki Sakaguchi, Mukesh Jewariya, Hajime Inaba, Kaoru Minoshima, Francis Hindle, and Tsutomu Araki, "Enhancement of spectral resolution and accuracy in asynchronous-optical-sampling terahertz time-domain spectroscopy for low-pressure gas-phase analysis," Opt. Express 20, 15071-15078 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-14-15071


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References

  1. M. Exter, Ch. Fattinger, and D. Grischkowsky, “Terahertz time-domain spectroscopy of water vapor,” Opt. Lett.14(20), 1128–1130 (1989). [CrossRef] [PubMed]
  2. D. M. Mittleman, R. H. Jacobsen, R. Neelamani, R. G. Baraniuk, and M. C. Nuss, “Gas sensing using terahertz time-domain spectroscopy,” Appl. Phys. B67(3), 379–390 (1998). [CrossRef]
  3. A. S. Pine, R. D. Suenram, E. R. Brown, and K. A. McIntosh, “A terahertz photomixing spectrometer: application to SO2 self broadening,” J. Mol. Spectrosc.175(1), 37–47 (1996). [CrossRef]
  4. S. Matsuura, M. Tani, H. Abe, K. Sakai, H. Ozeki, and S. Saito, “High-resolution terahertz spectroscopy by a compact radiation source based on photomixing with diode lasers in a photoconductive antenna,” J. Mol. Spectrosc.187(1), 97–101 (1998). [CrossRef] [PubMed]
  5. G. Mouret, F. Hindle, A. Cuisset, C. Yang, R. Bocquet, M. Lours, and D. Rovera, “THz photomixing synthesizer based on a fiber frequency comb,” Opt. Express17(24), 22031–22040 (2009). [CrossRef] [PubMed]
  6. C. Janke, M. Först, M. Nagel, H. Kurz, and A. Bartels, “Asynchronous optical sampling for high-speed characterization of integrated resonant terahertz sensors,” Opt. Lett.30(11), 1405–1407 (2005). [CrossRef] [PubMed]
  7. T. Yasui, E. Saneyoshi, and T. Araki, “Asynchronous optical sampling terahertz time-domain spectroscopy for ultrahigh spectral resolution and rapid data acquisition,” Appl. Phys. Lett.87(6), 061101 (2005). [CrossRef]
  8. G. Klatt, R. Gebs, H. Schäfer, M. Nagel, C. Janke, A. Bartels, and T. Dekorsy, “High-resolution terahertz spectrometer,” IEEE J. Sel. Top. Quantum Electron.17(1), 159–168 (2011). [CrossRef]
  9. R. A. Cheville and D. Grischkowsky, “Far-infrared terahertz time-domain spectroscopy of flames,” Opt. Lett.20(15), 1646–1648 (1995). [CrossRef] [PubMed]
  10. D. Bigourd, A. Cuisset, F. Hindle, S. Matton, E. Fertein, R. Bocquet, and G. Mouret, “Detection and quantification of multiple molecular species in mainstream cigarette smoke by continuous-wave terahertz spectroscopy,” Opt. Lett.31(15), 2356–2358 (2006). [CrossRef] [PubMed]
  11. T. Yasui, M. Nose, A. Ihara, K. Kawamoto, S. Yokoyama, H. Inaba, K. Minoshima, and T. Araki, “Fiber-based, hybrid terahertz spectrometer using dual fiber combs,” Opt. Lett.35(10), 1689–1691 (2010). [CrossRef] [PubMed]
  12. 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, “Long-term measurement of optical frequencies using a simple, robust and low-noise fiber based frequency comb,” Opt. Express14(12), 5223–5231 (2006). [CrossRef] [PubMed]
  13. M. Kessler, H. Ring, R. Trambarulo, and W. Gordy, “Microwave spectra and molecular structures of methyl cyanide and methyl isocyanide,” Phys. Rev.79(1), 54–56 (1950). [CrossRef]
  14. H. M. Pickett, R. L. Poynter, E. A. Cohen, M. L. Delitsky, J. C. Pearson, and H. S. P. Muller, “Submillimeter, millimeter, and microwave spectral line catalog,” J. Quant. Spectrosc. Radiat. Transf.60(5), 883–890 (1998). [CrossRef]
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