Mid-infrared wavelength- and freque

Mid-infrared wavelength- and frequency- modulation spectroscopy with a pump-modulated singly-resonant optical parametric oscillator

I.D. Lindsay, P. Groß, C.J. Lee, B. Adhimoolam and K.-J. Boller
Laser Physics and Nonlinear Optics Group, Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands i.d.lindsay@tnw.utwente.nl

Abstract: We describe the implementation of the wavelength- and frequency-modulation spectroscopy techniques using a singly-resonant optical parametric oscillator (OPO) pumped by a fiber-amplified diode laser. Frequency modulation of the diode laser was transferred to the OPO’s mid-infrared idler output, avoiding the need for external modulation devices. This approach thus provides a means of implementing these important techniques with powerful, widely tunable, mid-infrared sources while retaining the simple, flexible modulation properties of diode lasers.
©2006 Optical Society of America
OCIS codes: (190.4970) Parametric oscillators and amplifiers; (140.3510) Lasers, fiber; (140.3600) Lasers, tunable; (300.6380) Spectroscopy, modulation.


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1. Introduction

Frequency modulation techniques are widely used in laser spectroscopy to achieve a high signal-to-noise ratio and, therefore, sensitive spectroscopic detection. These techniques exploit the fact that, on passage through a medium having frequency-dependent absorption, frequency-modulation of a laser results in a transmitted power variation at the modulation frequency and its harmonics. Phase-sensitive detection at frequencies beyond the range of technical noise sources is thus possible. In practice, these techniques are typically classified into two approaches [1,2]. Wavelength-modulation spectroscopy (WMS) conventionally describes the case where the modulation frequency is much less than the width of the spectral feature of interest and the modulation index is high. In practice this typically corresponds to modulation frequencies from a few kilohertz to a few megahertz. In the case usually termed frequency-modulation spectroscopy (FMS) the modulation frequency is comparable to, or greater than, the spectral width of the target feature and the modulation index is sufficiently low that only the first two sidebands of the modulated laser spectrum have significant amplitude. In this case, modulation frequencies are typically in excess of 100 MHz.
Both techniques have been most widely applied in diode-laser spectroscopy due to the
ease with which diode lasers can be frequency modulated via their injection current. In contrast to external electro-optic modulators required by other laser types, current modulation of diode lasers can be achieved over broad bandwidths, extending to several gigahertz, with minimal RF power requirements and simple control of the modulation index over a wide range. This flexibility allows the same laser system to be easily reconfigured for different FM techniques [2]. Many variations on these techniques have been demonstrated with diode lasers including two-tone FMS [2], high modulation index WMS [3] and photo-acoustic WMS [4]. To access fundamental molecular vibrational bands in the mid-infrared (mid-IR) FM spectroscopic techniques have been demonstrated with lead salt diode lasers [1], and, more recently, quantum cascade lasers [5], which offer similar advantages to diode lasers in terms of ease of modulation. Mid-IR WMS with milliwatt-level powers has also been demonstrated by difference frequency generation (DFG) between amplified near-IR diode lasers, whose modulation then transfers to the mid-IR output [6].
Advances in nonlinear optical materials and pump lasers have made continuous-wave singly-resonant optical parametric oscillators (OPOs) attractive sources for mid-IR spectroscopy. These devices can produce watt-level output powers, far exceeding those of other mid-IR sources, while having tuning ranges of hundreds of wavenumbers [7]. Combining these attributes with the benefits of WMS and FMS techniques would, therefore, be highly attractive. Hybrid wavelength-amplitude modulation has been used with a pulsed OPO for systematic background cancellation in photoacoustic spectroscopy [8]. However, the intrinsically low modulation frequency (30Hz) would preclude many of the signal to noise advantages WMS usually offers. WMS and FMS in the conventional sense, as described above, appear never to have been demonstrated with an OPO source.
Recently, we have shown that pump-tuned singly-resonant OPOs represent particularly attractive spectroscopic sources [9]. In this case, the resonant signal wave remains fixed in frequency and tuning of the pump is transferred directly to the mid-IR idler output. This approach allows rapid tuning over hundreds of wavenumbers [10], wide-range continuous tuning [9], and mid-IR tuning with narrow linewidth [11]. In this paper, we extend this approach to include transfer to the idler of pump laser modulation and use this to demonstrate mid-IR WMS and FMS detection. Use of a diode laser-based pump source allows this to be achieved while retaining simplicity of modulation and avoiding the requirement for an external modulator in the mid-IR. We believe this to be the first reported demonstration of the WMS and FMS detection techniques with an OPO.

2. Experimental arrangement

The optical configuration used for both WMS and FMS investigations is shown schematically in Fig. 1. The pump source consisted of a commercial multi-section DBR diode laser seeding