An optical parametric oscillator (OPO) is a coherent source of light like a laser but uses the process of optical amplification in a nonlinear crystal rather than from stimulated emission. It is possible to tune these lasers over a very broad range of wavelengths since no energy levels are involved in the amplification process. In OPOs, the pump is another laser used to pump a non-linear crystal within a resonant cavity. The non-linear interaction in the crystal leads to the conversion of the pump laser into two waves at new wavelengths. Giordmaine and Miller demonstrated the first successful operation of an OPO in 1965.
A parametric oscillator that oscillates at optical frequencies is known as an optical parametric oscillator (OPO). It converts an input laser wave with frequency ωp into two output waves of lower frequency (ωs, ωi) and ωs+ ωi= ωp. The two output waves are signal (ωs) and idler (ωi), with the signal being the output wave with the greater frequency. A special case is the degenerate OPO, i.e; ωs= ωi= ωp/2, which can result in a half-harmonic generation. Figure 1 shows the geometry and energy level of the optical parametric oscillator.
Figure 1: Geometry and Energy Level Diagram of optical parametric oscillator
OPOs mainly are of two types: continuous-wave and pulsed. The pulsed OPOs are easier to build since the high intensity lasts only for a small fraction of a second, which damages the nonlinear optical material and the mirrors less than a continuous high-intensity device. An important property of the OPO is the coherence and the spectral width of the generated radiation. When the pump power is significantly above the threshold, the two output waves are at a good approximation, coherent states. The linewidth of the resonated wave is very narrow and if a pump wave of narrow linewidth is employed, the non-resonated generated wave also exhibits narrow linewidth. Narrow-linewidth OPOs are widely used in spectroscopy.
Operation of OPO
The pump source, the gain medium, and the feedback resonator are the three important components of an optical parametric oscillator. Figure 2 shows the schematic of an optical parametric oscillator.
Intense coherent light from a laser beam propagates through an optically nonlinear crystal with the frequency ωp. The crystal is placed inside an optical resonator. Due to the nonlinearity in the crystal, parametric generation takes place and a pump photon is converted into a signal and idler photon fulfilling photon-wise energy conservation ωp= ωs+ ωi. The signal wave is fed back into the crystal via the resonator, where it is amplified again.
Figure 2: Schematic of Optical Parametric Oscillator
Optical parametric oscillation starts, when the increased signal intensity surpasses the threshold which means the amplified signal wave compensates for the round-trip losses in the resonator. Reaching the threshold pump intensity, a significant portion of the pump intensity is converted into signal and idler intensity. This power transfer from the pump to the generated waves reduces the pump intensity inside the nonlinear crystal and thus the signal gain. This effect is called gain saturation.
Pumping of OPOs
Types of feedback resonators
Depending on the number of resonating waves, feedback resonators are distinguished:
Figure 3: Singly-resonant oscillator
Figure 4: Doubly-resonant oscillator
Figure 3 and Figure 4 show the schematic representation of the singly-resonant oscillator and doubly-resonant oscillator respectively.
OPOs can be made using either ring resonators or linear (standing-wave) resonators. A linear resonator is easy to build and align whereas a ring resonator requires a larger angle of incidence on curved resonator mirrors that might cause astigmatism.
The ability to tune the wavelength of laser oscillation is one of the main advantages of the parametric oscillator. For a given pump frequency, the signal and idler frequency will get amplified and are determined by the phase matching condition. Any parameter that can change the indices can be used to tune the frequency of oscillation since the phase-matching condition depends on the refractive index of the medium at the three frequencies. Thus, by changing the temperature, applying an external electric field that changes the indices by electro-optic effect, or by changing the orientation of the crystal if one of the waves is an extraordinary wave, the frequency of oscillation can be tuned.
Some of the other applications:
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