Abstract
We have carried out dual-comb spectroscopy and observed in a simultaneous acquisition a 140-THz-wide spectrum from 1.0 to 1.9 µm using two fiber-based frequency combs phase-locked to each other. This ultrabroad-wavelength bandwidth is realized by setting the difference between the repetition rates of the two combs to 7.6
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This article was made open access on 16 September 2015
Optical frequency combs have been expected to work as broadband light sources for advanced spectroscopy in addition to their use as optical frequency rulers for precise measurements. An optical frequency comb is a collection of lasers with identical frequency intervals, and molecular responses can be recorded in the amplitude and phase of each comb mode. To benefit from the advantage that these combs have over thermal light sources, individual comb modes must be separated in the spectrum. Several methods have been demonstrated for accomplishing this, including the combination of an optical grating and a Fabry–Perot cavity,1,2) a virtually imaged phased array (VIPA),3) and Fourier transform spectroscopy (FTS).4,5) Dual-comb spectroscopy is one such method, in which atomic or molecular absorption information is stored in each mode of the first comb (signal comb) and retrieved for the individual comb mode by the second comb (local comb) through multi-heterodyne detection.6,7) Frequency accuracy and acquisition time superior to those of conventional FTS and a spectral resolution equal to the repetition rate of the signal comb have been demonstrated using dual-comb spectrometers based on mode-locked fiber lasers.8–13)
In the dual-comb spectroscopy, optical pulses from signal and local combs with respective repetition rates of frep,S and frep,L = frep,S −
Previously, a spectral bandwidth of 43 THz was observed in a simultaneous acquisition by setting
Figure 1 shows the configuration of our dual-comb spectrometer. The signal and local combs employ an erbium-based mode-locked fiber laser as the comb oscillator. It has an electrooptic modulator (EOM) for fast servo control14,15) and a delay line for the tuning of frep in the laser cavity. The repetition rates of the combs are about 48 MHz. The output of the individual combs is divided into three branches, amplified by erbium-doped fiber amplifiers, and then spectrally broadened with highly nonlinear fibers. The waves from the branches are used to detect the carrier-envelope offset beat (fceo), to detect the beat note between a CW reference laser and the nearest comb mode (fbeat), and to record the dual-comb spectrum. fceo is phase-locked at a reference frequency. One of the individual comb modes is phase-locked to a 1.54 µm CW laser, which is stabilized to an ultrastable cavity. The error signal is fed back to the EOM, a piezoelectric transducer, and a Peltier element in the comb oscillator with different time constants. By virtue of the fast control of the cavity length with the EOMs, the relative linewidth between the combs falls to less than the sub-Hz level. This is confirmed by observing the beat note between the two combs with an RF spectrum analyzer. The narrow relative linewidth allows us in principle to set
Fig. 1. Configuration of dual-comb spectrometer. EOM: electrooptical modulator; FFT: fast Fourier transform. A 50-cm-long single-pass cell is filled with 2.7 kPa CH4, and a 15-cm-long 26-pass White cell is filled with 2.6 kPa C2H2.
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Standard image High-resolution imageThe optical pulses from the signal comb are transmitted through a 50-cm-long absorption cell filled with 2.7 kPa CH4 and a 15-cm-long 26-pass White cell of 2.6 kPa C2H2. The transmitted signal wave overlaps with the local wave at a polarization beam splitter. These waves are divided into two beams and detected with two InGaAs detectors for balanced detection. The detected signal is an interferogram of the local and signal waves, which is guided to a 14-bit digitizer through an anti-aliasing filter with a pass band from 0.5 to 21.4 MHz, and is sampled at the repetition rate of the local comb.
To enhance the sensitivity by the accumulation of interferograms, we employ a coherent averaging technique.17) Thus, we are able to accumulate interferograms coherently for a few seconds, which is determined by the inverse of the sub-Hz-level relative linewidth of the combs. In addition, the following two real-time compensations are employed to realize a long-term accumulation. First, we correct the carrier phase drift of the interferograms caused by the fluctuations in optical pass length on a computer.18) Second, fbeat,L and fceo,L are actively controlled to compensate the frequency drift of the CW laser (∼9
Figure 2(a) shows an observed spectrum containing the 2
Fig. 2. Observed dual-comb spectrum. (a) Entire spectrum from 1.0 to 1.9 µm. The black structures represent the variation of the comb spectrum. (b)–(g) Expanded spectra of (a). (b) H2O 2
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Standard image High-resolution imageFigure 3 shows a normalized absorption spectrum of C2H2 in the 1.52 µm band and a calculated spectrum derived from the line parameters of HITRAN201219) together with the discrepancies between them. The absorption spectrum is recorded with a sample pressure of 60 Pa,
Fig. 3. Normalized (red) and calculated (blue) spectra and residuals between them (green) for the
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Standard image High-resolution imageFigure 4 shows the absorption spectrum around the R(11) line of the
Fig. 4. Observed (black circles) and calculated (red curve) spectra and the residual between them (red circles) for the R(11) line of the
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Standard image High-resolution imageTable I. Frequency list of the
Line | |||
---|---|---|---|
P(12) | 195 660 692.5 | 2.0 | −0.3 |
P(11) | 195 739 648.8 | 0.8 | −0.7 |
R(11) | 197 343 962.5 | 0.7 | −0.1 |
R(12) | 197 404 396.2 | 1.6 | 0.6 |
Here, we discuss the SNR of broadband dual-comb spectroscopy. We estimate the noise level of our dual-comb spectrometer on the basis of the discussion in Ref. 21. The relative intensity noise (RIN) of the spectrally broadened comb is dominant in our measurement across 1.0–1.9 µm (Fig. 2). The measured RIN is about −116
Our spectrometer is able to accumulate interferograms for a long period of time to obtain a required SNR. The inset of Fig. 2(g) shows a spectrum of the same lines recorded using 220,000 times averaging over 8 h. When we accumulate interferograms for 8 h, the SNR improved proportionally to the accumulation time, and significant spectral fringes of about 3 GHz appear, as shown in the inset of Fig. 2(g). We consider that these fringes originate from the spectrum of the comb oscillators and remain even after the spectral broadening in the highly nonlinear fiber. They become residual fringes on the base line of the normalized spectrum and cause a center-frequency shift. Therefore, comb oscillators with smooth spectra are required for the center-frequency determinations in such high-SNR spectroscopy.
In conclusion, we have developed an ultra-broadband dual-comb spectrometer, which simultaneously possesses a broad bandwidth, a high spectral resolution, and a high frequency accuracy. These excellent characteristics are superior to those of the conventional FTS and will stimulate various advanced applications such as the identification of the chirality of chiral molecules. In addition, a broader spectral coverage will offer new spectroscopic approaches to applications such as the multicomponent analysis and investigation of the relaxation associated with several vibrational states.22) Combs with a sub-Hz-level linewidth simply realize ultra-broadband dual-comb spectrometers, which will become ubiquitous tools in the near future.
Acknowledgments
This work is supported by MEXT/JSPS KAKENHI Grant Numbers 23244084, 23560048, and 22540415, Collaborative Research Based on Industrial Demand from the Japan Science and Technology Agency, and the Photon Frontier Network Program of the Ministry of Education, Culture, Sports, Science and Technology, Japan.