Molecular Spectroscopy in the mid-IR,
Optical Parametric Oscillators,
and
Difference Frequency Generators

EU-STREP project
Versatile Infrared Laser source for Low-cost Analysis of Gas Emissions (VILLAGE)

2006-2009

(collaboration of Thales SA (F), Univ. Southampton (UK), Univ. Duesseldorf, Univ. Valladolid (E), Norsk Elektro-Optikk (N))

Specific features of molecular gases spectra in mid-infrared (MIR) range attract a growing interest for a number of scientific and commercial applications: fundamental spectroscopy, trace gas detection, remote sensing. Yet, many promising results have remained confined to laboratories for lack of suitable MIR laser sources.

The objective of the VILLAGE project is the development of a cost-effective widely tunable narrowband MIR laser source, emitting from 5 µm to 15 µm. This source will combine a 2 µm Thulium (Tm)-doped fibre laser and a high spectral purity optical parametric oscillator (OPO) using as nonlinear frequency converting medium a semiconductor crystal, Orientation-Patterned Gallium Arsenide (OP-GaAs).

 

Our first spectroscopic device based on OP-GaAs is a difference frequency generator (Vasilyev et al, Opt. Lett. 33, 1413 (2008)). The wave from an diode laser + Er:glass fiber amplifier and the wave from a Tm:glass fiber laser were mixed in periodically patterned GaAs crystal to produce radiation in the range 7.6 - 8 µm.

 

 

DFG setup.png

Figure: Setup for the generation of widely tunable cw mid-IR radiation. A 9 W 1.5 µm laser (diode laser + Er-fiber amplifier) and a 1 W 2 µm laser (Tm-fiber laser) were mixed in OP-GaAs crystal. The DFG output wavelength is tunable in the 7.6 – 8.2 µm range by tuning the diode laser.

 

 

With appropriately high input powers, the output power was near 1 mW. With the source we demonstrated spectroscopy on methane recording hundreds of lines.

Computer control of the DFG output wavelength via the diode laser wavelength was implemented.

There are good prospects to increase the output power to the 5 mW level in an upcoming demonstration, using higher laser power and an AR coated crystal.

 

 

 

Optical Parametric Oscillators (OPOs)

An optical parametric oscillator consists of a nonlinear medium inside an optical resonator. The pump laser beam wpump generates photons wsignal and widler as it interacts with the nonlinear medium (spontaneous parametric process). Under suitable (phase-matched) conditions signal and idler fields undergo amplification. By use of the optical resonator narrowband oscillations for the certain signal and idler wavelengths can be achieved. Tuning of the OPO output can be realized by pump laser tuning or by variation of the nonlinear medium parameters (temperature, grating period)

 

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Continuous-wave narrowband OPO for the 2.4 - 3.9 µm spectral region

(Collaboration with the University of Bonn)

In this work we have used periodically poled lithium-niobate crystal (PPLN) as a nonlinear medium. The pump source was continuous wave single-frequency 2.5 Watt Nd:YAG laser @ 1064nm. Photorefractive damage of the nonlinear crystal was avoided by heating the PPLN in an oven up to 150-200 C. The optical cavity was resonant for signal wavelength as well as for pump wavelength. The resonant enhancement of the intracavity pump intensity has lowered OPO threshold and improved pump conversion efficiency. The OPO output wavelength was tunable between 2.4 and 3.9 µm. The coarse tuning was performed by variation of the crystal’s poling period and the fine tuning was achieved by the crystal’s temperature adjustment. A galvanometer-mounted etalon (E) was inserted into the signal cavity to suppress spontaneous mode hops.

 

 

 

Figure: Schematic and photograph oft he standing-wave OPO system. Note that there are two nested standing-wave cavities, for the pump and the signal waves, respectively. 

 

The developed OPO has been used for trace gas detection by both photoacoustic detection and cavity-leak-out detection, achieving excellent sensitivity.

Figure: Time series of the measured absorption (dots) while a stepwise diluted ethane concentration (solid line) flows through the CALOS (cavity leak out spectroscopy) cell (total flow: 2.4 l/h, pressure: 100 hPa); left y-axis: measured absorption; right y-axis: corresponding concentration, calculated from FTIR; inset: data points during grade 5 nitrogen flow, taken to calculate a noise-equivalent absorption (A.Popp et al. Appl. Phys. B 75, 751-4 (2002)).

 

References:

Poster (DIN-A4): "Cw-OPO based photoacoustic and cavity-leak-out spectroscopy" F. Müller, A. Popp, F. Kühnemann, S. Schiller, G.v. Basum, H. Dahnke, P. Hering, M. Mürtz

For published papers, see our Publications page