Optical Parametric Oscillator

Why OPOs?

In the past, several detection methods were used for trace gas measurements in biomedical applications, ranging from mass spectrometry, gas chromatography, chemiluminescence, electronic nose, to different laser-based spectroscopic methods. As many molecular gases have strong absorption in the mid-IR wavelength region, it is advantageous to use laser absorption spectroscopy; it is an excellent method for highly sensitive and selective detection of VOCs. For trace gas sensing, OPOs have to be developed in the mid-infrared wavelength region between 2.5 and 25 µm.

History

In 1965, five years after the demonstration of the first laser, Joseph Giordmaine and Bob Miller developed the Optical Parametric Oscillator (OPO) at Bell lab's. While lasers found almost directly widespread use in the field of gas sensing, this only happened for OPOs thirty years later. Its initial slow development was caused by the fact that OPOs were far from being a workhorse. Lack of proper non-linear crystals with a sufficiently high damage threshold, the unavailability of good pump lasers, and the need for skilled operators restricted their usefulness in applications. The advent of new crystals like Periodically Poled Lithium Niobate (PPLN) and a new generation of (fiber) pump lasers have changed this situation. Nowadays, OPOs are commercially available from many suppliers covering the visible, near- and mid-infrared and (recently) part of the ultraviolet wavelength region, using the pulsed and continuous wave (cw) regime.

Physical basics of OPOs

In an optical parametric process the pump photon (ωp) is split in two parts, forming two new photons with different energies (Fig. 2). The generated photon with the highest energy is termed signal (ωs), and the other photon is termed idler (ωi). The energy conservation has to be considered for the generated pair of photons, but is not sufficient to determine which frequencies will be amplified. In addition to that, destructive interference between the photons needs prevention. This gives rise to a second restriction, the phase matching condition. In bulk crystals birefringence is used to offset the dispersion and it satisfies the phase matching condition for specific wavelength combinations. Altering the propagation direction in such crystals (angle tuning) changes the selected frequency combination. Changing the crystal temperature may also change the refractive indices nidler, nsignal and npump resulting in temperature tuning.