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Fourier-Transform-Based Noise-Immune Cavity-Enhanced Optical Frequency Comb Spectroscopy Amir Khodabakhsh, Alexandra C. Johansson, Lucile Rutkowski, and Aleksandra Foltynowicz* Department of Physics, Umeå University, 901 87 Umeå, Sweden
[email protected]
Abstract: We achieve absorption sensitivity of 6.4 × 10−11 cm−1 Hz−1∕2 per spectral element with near-infrared Fourier-transform-based noise-immune cavity-enhanced optical frequency comb spectroscopy (NICE-OFCS), which allows detection of CO2 at ppb concentration levels. OCIS codes: (300.1030) Absorption; (300.6360) Spectroscopy, laser; (300.6300) Spectroscopy, Fourier transforms.
1. Introduction Fourier-transform-based NICE-OFCS is a newly developed sensitive, broadband and high-resolution technique of optical frequency comb spectroscopy [1]. In NICE-OFCS the optical frequency comb (OFC) is locked to a cavity and phase-modulated at a frequency precisely equal to (a multiple of) the cavity free spectral range (FSR). Each comb line and sideband is transmitted through a separate cavity mode and thus affected by any residual frequency noise of the OFC relative to the cavity in an identical manner. The transmitted intensity contains a beat signal at the modulation frequency that is immune to frequency-to-amplitude noise conversion by the cavity, in a way similar to continuous-wave noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) [2]. 2. Experimental setup The NICE-OFCS system, shown in Fig. 1, is based on an Er:fiber femtosecond laser operating in the 1.5–1.6 μm wavelength range with a repetition rate of 250 MHz, an enhancement cavity with a finesse of ~9000 and a fastscanning Fourier-transform spectrometer (FTS) equipped with a high-bandwidth commercial detector [3]. The comb is locked to the cavity using the two-point Pound-Drever-Hall (PDH) technique [4], and phase-modulated at a frequency equal to three times the cavity FSR. The cavity is 80 cm long (FSR of 187.5 MHz) and transmits every third comb line, which yields 750 MHz comb line spacing in transmission. The modulation frequency (562.5 MHz) is generated by a direct digital synthesizer (DDS), which uses the 5th harmonic of the OFC repetition rate (i.e. 1.25 GHz) as a clock input. Thus it is passively locked to the repetition rate of the OFC, which in turn is locked to the cavity FSR, and ensures the noise immunity [3]. The NICE-OFCS signal is obtained by fast Fourier transform (FFT) of a synchronously demodulated FTS interferogram.
Fig. 1. Experimental setup: OFC – optical frequency comb; EOM – electro-optic modulator; FC – fiber collimator; λ∕2 – half-waveplate; λ∕4 – quarter-waveplate; f – mode-matching lens; PBS – polarizing beam splitter; FTS – Fourier transform spectrometer; BPF – band-pass filter; G – amplifier; LPF – low-pass filter; FFT – fast Fourier transform; Ph – phase shifter; DDS – direct digital synthesizer; PDH – Pound-Drever-Hall locking electronics; OFC Control – PI controllers for repetition rate and offset frequency of the OFC; fPDH – PDH modulation frequency; fm – NICE-OFCS modulation frequency.
3. Results The normalized NICE-OFCS absorption signal from 200 ppm of CO2 in N2 at a total pressure of 500 Torr is shown by the black curve in Fig. 2(b). The spectrum was measured in 1 s (0.5 s for the analytical and background signal, respectively) with a resolution of 750 MHz, and contains 4000 resolved spectral elements. A fit of a spectrum calculated using absorption line parameters from the HITRAN database [5] and the lineshape model described in [3] is shown by the red curve, inverted for clarity. The residual of the fit is shown in Fig. 2(c), while the wavelength dependence of the cavity finesse, measured by cavity-ringdown and used in the model, is shown in Fig. 2(a). It is
SM1O.6.pdf
CLEO:2015 © OSA 2015
worth to note that the NICE-OFCS signal does not have the dispersion-like shape of continuous-wave NICE-OHMS. Instead, the signal resembles direct cavity-enhanced absorption spectra on top of a baseline [3], which enables normalization and calibration-free determination of concentration, provided the cavity finesse is known.
Fig. 2. The experimental and theoretical NICE-OFCS CO2 absorption signals along with the Allan-Werle plot of the minimum detectable CO2 concentration. (a) Cavity finesse measured by cavity ring-down (black markers) with a 3rd order polynomial fit (red). (b) Normalized NICE-OFCS absorption signal (black, 20 averages) from 200 ppm of CO2 in N2 at 500 Torr total pressure, along with fitted spectra calculated using the HITRAN database and proper lineshape model (red, inverted for clarity). (c) Residuals of the fit. (d) The Allan-Werle plot of the minimum detectable CO2 concentration retrieved by fitting of NICE-OFCS CO2 spectra to normalized background spectra (blue markers). The dashed black line shows the linear fit to the white-noise dominated regime.
The noise equivalent absorption sensitivity, calculated as the relative noise on the baseline divided by the effective optical path length in the cavity and normalized to the square root of the number of spectral elements, is equal to 6.4 × 10−11 cm−1 Hz−1∕2 per spectral element. To estimate the CO2 concentration detection limit we use the entire molecular spectrum and the multiline fitting advantage [6]. The Allan-Werle plot [7] of the concentration obtained by fitting of the NICE-OFCS CO2 spectra to a series of background spectra (measured with the cavity filled with pure nitrogen at 500 Torr) is shown in Fig. 2(d). The dashed black line shows the τ-1/2 dependence characteristic for the white-noise-dominated regime that is fitted to the corresponding measurement points. The CO2 concentration detection limit, given by the slope of the fitted line, is 450 ppb Hz-1/2. The minimum detectable CO2 concentration is 25 ppb after 330 s integration time. 4. Conclusions The NICE-OFCS detection scheme can be implemented in commercial FTIR instruments using a single high-speed detector at the interferometer output and standard RF components for phase-sensitive detection. The high long-term stability and low-noise performance of the technique requires a perfect match of the modulation frequency to the cavity FSR. This can be passively sustained by a simple approach, in which the modulation frequency is generated by a DDS referenced to the repetition rate of the OFC and thus to the cavity FSR via the comb-cavity lock. Another step towards a wider applicability of the NICE-OFCS technique is improving the spectral resolution by resolving the comb lines in order to allow for measurements of narrower absorption lines at lower pressures, and to benefit from the high frequency accuracy of a frequency comb. Acknowledgements: We acknowledge financial support from the Swedish Research Council (621-2012-3650), Swedish Foundation for Strategic Research (ICA12-0031), and Carl Tryggers Stiftelse (CTS12:131).
5. References [1] A. Khodabakhsh, C. Abd Alrahman, and A. Foltynowicz, "Noise-immune cavity-enhanced optical frequency comb spectroscopy," Opt. Lett. 39, 5034-5037 (2014). [2] J. Ye, L. S. Ma, and J. L. Hall, "Ultrasensitive detections in atomic and molecular physics: demonstration in molecular overtone spectroscopy," J. Opt. Soc. Am. B 15, 6-15 (1998). [3] A. Khodabakhsh, A. C. Johansson, and A. Foltynowicz, "Noise-immune cavity-enhanced optical frequency comb spectroscopy: A sensitive technique for high-resolution broadband molecular detection," arXiv:1410.8800 (2014). [4] A. Foltynowicz, P. Maslowski, A. J. Fleisher, B. J. Bjork, and J. Ye, "Cavity-enhanced optical frequency comb spectroscopy in the midinfrared - application to trace detection of hydrogen peroxide," Appl. Phys. B 110, 163-175 (2013). [5] L. S. Rothman, I. E. Gordon, Y. Babikov, A. Barbe, D. C. Benner, P. F. Bernath, M. Birk, et al., "The HITRAN2012 molecular spectroscopic database," J. Quant. Spectrosc. Radiat. Transf. 130, 4-50 (2013). [6] F. Adler, P. Maslowski, A. Foltynowicz, K. C. Cossel, T. C. Briles, I. Hartl, and J. Ye, "Mid-infrared Fourier transform spectroscopy with a broadband frequency comb," Opt. Express 18, 21861-21872 (2010). [7] P. Werle, R. Mucke, and F. Slemr, "The limits of signal averaging in atmospheric trace-gas monitoring by Tunable Diode-Laser Absorption Spectroscopy (TDLAS)," Appl. Phys. B 57, 131-139 (1993).
