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Molecular spectroscopy and theoretical chemistry

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Contact Information

Laboratory of Physical Chemistry
Department of Chemistry
A.I. Virtasen aukio 1
(P.O. BOX 55)
FI-00014 University of Helsinki
Finland

Group leader
Prof. Lauri Halonen, D.Sc.
Phone: +358(0)2941 50280
email: lauri.halonen helsinki.fi
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Experimental work in laser spectroscopy

Development of tunable continuous-wave (cw) optical parametric oscillators


Fig. 1. Photograph of the cavity of a cw OPO, which is pumped by a Ti:sapphire laser at 800 nm [3]. Blue light is generated as a result of residual nonlinear mixing in the MgO:PPLN crystal, which is shown in the middle of the picture.

The use of breath analysis in medical diagnostics is actively studied in our group. The basic idea is to detect and quantify tiny amounts of biomarker molecules found in exhaled breath using laser spectroscopy. Typically, the near-infrared region of the optical spectrum is used for such measurements. It should be possible to significantly improve the sensitivity and reliability of the biomarker detection using laser sources operating in the mid-infrared region, which occupies the strong fundamental vibrational transitions of many biomarker molecules. Due to limited number of lasers suitable for high-resolution mid-infrared spectroscopy, devices based on nonlinear optical conversion, such as optical parametric oscillators (OPOs), are commonly used for this purpose. However, the stringent requirements of stability and tuning flexibility in spectroscopic applications are difficult to meet in practice.


Fig. 2. Mid-infrared wavelength tuning range of the cw OPO pumped by a Ti:sapphire laser [3]. The mid-infrared wavelength is tuned by tuning the pump wavelength, which greatly simplifies the experimental setup compared to conventional approaches. Depending on the poling period (PP) of the MgO:PPLN crystal, the mid-infrared wavelength can be tuned either from 2.5 to 3.5 Ám (PP = 21.25 Ám) or from 3.4 to 4.5 Ám (PP = 23.25 Ám).

We have developed singly-resonant cw OPOs that combine high stability and improved tuning properties. These OPOs are capable of producing high output power (> 1 W) and narrow linewidth (1 MHz) in the mid-infrared wavelengths between 2.5 and 4.5 Ám. We have built an OPO that requires no intracavity etalon for stable single-mode operation [1, 2]. The wavelength can be adjusted simply by tuning the temperature of the nonlinear crystal or the wavelength of the pump laser. As an example, we have demonstrated a cw OPO which is pumped by a Ti:sapphire laser, and whose wavelength can be tuned in a few seconds from 2.5 to 3.5 Ám or from 3.4 to 4.5 Ám by simply changing the pump wavelength (see Fig. 2) [3]. We have also developed a novel method for simple and rapid tuning of the OPO wavelength by a rotatable diffraction grating [4].


Fig. 3. Tuning curve of a near-degenerate singly-resonant cw-pumped OPO, which was stabilized using a Bragg grating [8]. Tuning of the nonresonant wavelength from 1589 nm to 1680 nm is shown, corresponding to a tuning range of over 10 THz. The resonant wavelength of the OPO is fixed to 1584 nm by the Bragg grating. The tuning curve is shown for four different poling periods of the OPO crystal (PPLN), and as a function of the crystal temperature.

Large continuous wavelength tuning range can be obtained by operating the OPO near degeneracy, where the parametric gain bandwidth is exceptionally broad. While this method has proven extremely useful in optical frequency comb generation by synchronously pumped femtosecond OPOs [5, 6], stable operation of a cw OPO near degeneracy is challenging to obtain. We have successfully stabilized a near-degenerate cw OPO by using a Bragg grating as a frequency-selective cavity element [7]. We have shown that using this method it is possible to continuously tune the signal-idler frequency difference from 1 to 10 THz (Fig. 3) [8]. Such OPO can be used, for example, to produce single-frequency cw THz radiation by difference frequency mixing.

[1] M. Vainio, J. Peltola, S. Persijn, F. J. M. Harren and L. Halonen, "Singly resonant cw OPO with simple wavelength tuning", Opt. Express, 16, 11141-11146 (2008).
[2] M.Vainio, J. Peltola, S. Persijn, F. Harren, and L. Halonen, "Thermal effects in singly resonant continuous-wave optical parametric oscillators" Appl. Phys. B 94, 411-427 (2009).
[3] M. Siltanen, M. Vainio, and L. Halonen, "Pump-tunable continuous-wave singly resonant optical parametric oscillator from 2.5 to 4.4 Ám," Opt. Express 18, 14087-14092 (2010).
[4] M. Vainio, M. Siltanen, J. Peltola, and L. Halonen, "Continuous-wave optical parametric oscillator tuned by a diffraction grating," Opt. Express 17, 7702-7707 (2009).
[5]. N. Leindecker, A. Marandi, R.L. Byer, K. L. Vodopyanov, J. Jiang, I. Hartl, M. Fermann, and P. G. Schunemann, "Octave-spanning ultrafast OPO with 2.6-6.1 Ám instantaneous bandwidth pumped by femtosecond Tm-fiber laser," Opt. Express 20, 7046-7053 (2012).
[6] M. Vainio, M. Merimaa, L. Halonen, and K. Vodopyanov, "Degenerate 1 GHz repetition rate femtosecond optical parametric oscillator," Opt. Lett. 37, 4561-4563 (2012).
[7] M. Vainio, M. Siltanen, T. Hieta, and L. Halonen, "Continuous-wave optical parametric oscillator based on a Bragg grating," Opt. Lett. 35, 1527-1529 (2010).
[8] M. Vainio and L. Halonen, "Stable operation of a continuous-wave optical parametric oscillator near the signal-idler degeneracy," Opt. Lett. 36, 475-477 (2011).