Multi-heterodyne spectroscopic techniques using FabryPérot quantum cascade lasers for trace gas detection Jonas Westberg1, Lukasz Sterczewski1,2, Eric Zhang1,Andreas Hangauer1, and Gerard Wysocki1 Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, USA Department of Electrical Engineering, Wroclaw University of Technology, Wroclaw, Poland Email:
[email protected]
Abstract: We have studied various spectroscopic detection techniques for applications in multiheterodyne spectroscopy performed in a dual comb configuration. Self-coherence properties of a multi-mode QCL are also investigated. OCIS codes: (140.5965) Semiconductor lasers, quantum cascade; (300.6310) Spectroscopy, heterodyne; (120.2230) FabryPerot; (300.6380) Spectroscopy, modulation.
Since the invention of the quantum cascade laser (QCL) more than two decades ago, this semiconductor light source has been implemented in a wide variety of laser-based spectroscopic systems for trace gas detection in the midinfrared region. These systems have mainly incorporated single mode DFB-QCLs due to their ease-of-operation, but while these lasers are inherently selective, they are often limited in terms of wavelength tunability. This limitation has been the main motivation behind the work on QCL frequency combs [1]. This technology has great potential to enable fully electronically controlled spectrometers, capable of targeting broadly absorbing chemical species in the mid-infrared spectral region. -3
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Fig. 1. Schematic overview of the optical setup for direct absorption and wavelength modulated multi-heterodyne spectroscopy.
The dual comb spectroscopy technique, which was originally proposed by Schiller in 2002 [2], provides an elegant method to down-convert the optical modes of a frequency comb source to the radio frequency (RF) domain by incorporating a second comb source as a local oscillator. This technique was originally intended for near-infrared mode-locked frequency combs, but can be applied to any coherent multi-mode light source. Since FP-QCLs exhibit comb-like characteristics, these lasers provide a promising, cost-effective and robust alternative in the mid-infrared region. For this reason, the applicability of standard detection techniques, commonly used for single mode DFB lasers, has been explored using a pair of multi-mode QCLs in the dual comb configuration shown in Fig. 1. The most straight-forward approach to acquire absorption information is the direct absorption technique, where the attenuation of the optical modes that interact with molecular transitions are down-converted to the corresponding RF beat notes. Continuous monitoring of the beat note amplitudes as the laser is scanned in frequency over the transition, gives direct access to the absorption information. The result of a scan of one of the optical modes over the
Fig. 2. Direct absorption measurement of N2O at ~8.5 µm.
Fig. 3. Wavelength modulation measurement of N2O at ~8.5 µm.
R27f transition of N2O at ~8.5 µm is shown in Fig. 2. The residual of the Voigt fit indicates a noise-equivalent absorption (NEA) of ~3⨯10-3/√Hz per mode. Amplitude noise measurements of a single beat note reveal a significant contribution from 1/f-noise at lower detection frequencies [3], which can be substantially suppressed by shifting the detection to higher frequencies with a suitable modulation scheme. Taking this into account, wavelength modulation at 10 kHz followed by 2f lock-in detection was implemented. The result of the same transition with a wavelength-modulated multi-heterodyne spectrometer is shown in Fig. 3, where residuals in the Voigt fit indicate a NEA of ~4.5⨯10-4/√Hz per mode. PD BS
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Fig. 4. Schematic overview of the optical setup for self-heterodyne measurements. The optical delay is controlled by a high precision translation stage (Zaber Technologies) and the RF spectrum is measured by a Tektronix RSA6106A spectrum analyzer.
The performance of multi-heterodyne spectrometers in the dual comb configuration is often limited by the degree of coherence [4] and stability between the two combs. In order to investigate the degree of self-coherence of a single FP-QCL comb a series of time-delayed self-heterodyne measurements has been performed. The measurements were performed in a Mach-Zehnder interferometer configuration with a variable optical time-delay in one of the arms to create an optical time-delay for the interfering waves (a schematic of this arrangement is displayed in Fig. 4).
Fig. 5. RF beat note spectra acquired as a function of optical delay.
Figure 5 shows measurements of the RF beat note spectrum around the AOM frequency of 40 MHz with a total delay of ~1060 ps (~4.5 m). A clear beat note broadening pattern is noticeable every round-trip time of 24.5 ps. The total linewidth of the beat note does not exceed 10 kHz, which indicates the ultimate limit of the optical resolution that could be achieved with this source. Currently we are exploring single-laser self-heterodyne methods that could approach that limit. In conclusion, multi-heterodyne spectroscopy with FP-QCLs offers a cost-effective and robust spectroscopic method that gives access to the fundamental rotational-vibrational transitions of large number of important molecules simultaneously. Our implementations of multi-heterodyne spectroscopy demonstrate excellent compatibility with existing modulation techniques, which enable high resolution, high-sensitivity and broadband spectroscopy of chemical species. Acknowledgments: The authors gratefully acknowledge financial support by the DARPA SCOUT program (grant# W31P4Q161001) and by the NSF ERC MIRTHE award EEC-0540832.
References [1] A. Hugi, et al., “Mid-infrared frequency comb based on a quantum cascade laser,” Nature 492(7428), 229-233 (2012). [2] S. Schiller, “Spectrometry with frequency combs,” Opt. Lett., vol. 27, pp. 766–768 (2002). [3] A. Hangauer et al., "Noise properties in multi-heterodyne spectrometers based on quantum- and interband-cascade lasers," in CLEO (2015). [4] D. Burghoff et al., “Evaluating the coherence and time-domain profile of quantum cascade laser frequency combs”, Opt. Exp. 23(2) (2015).
