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 ω_p into two output waves of lower frequency (ω_s, ω_i) and ω_s+ ω_i= ω_p. The two output waves are signal (ω_s) and idler (ω_i), with the signal being the output wave with the greater frequency. A special case is the degenerate OPO, i.e; ω_s= ω_i= ω_p/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 ω_p. 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 ω_p= ω_s+ ω_i. 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

• Q-switched Pump: Most of the OPOs are pumped with a Q-switched laser. The laser emits pulses in the order of nanoseconds that help to overcome the threshold for oscillation.  At this excitation level, stimulated emission drives the laser's output, and the system is said to be "lasing." Generally, output pulses from oscillators with Q-switched pumps are shorter and have a wider linewidth. Usually, a single Nd: YAG laser with an active Q-switch is used to pump these systems. The generated short pulses have frequencies in the near- to mid-infrared region and have energy in the microjoule to millijoule range.
• Continuous Wave Pump: OPOs are also pumped with continuous wave lasers. With a highly nonlinear crystal gain medium, like LiNbO₃, this pump performs well. Continuous wave pumps are the best pumps for applications requiring single frequency.
• Mode-Locked Pump: In the research field, mode-locked lasers are used to pump OPOs to produce ultrashort pulses of light. The frequency within the resonator of these oscillators and the pulse repetition rate of the pump match. High energy output pulses are produced while the pump requires a lot less power than 1 watt.

Types of feedback resonators

Depending on the number of resonating waves, feedback resonators are distinguished: 

• Singly Resonant Oscillator: In a singly resonant oscillator, a single frequency is amplified and it is the frequency of signal wave ωs. As a result of the parametric process, an idler wave with frequency ω_i is generated. But the reflectivity mirror is fixed in such a way that signal wave ωs will make a round trip and then is amplified accordingly. It requires high pump power at a threshold, which exceeds several watts. Most OPOs are singly resonant. These OPOs have a bandwidth limited by phase matching. 

Figure 3: Singly-resonant oscillator

• Doubly Resonant Oscillator: In this both signal and idler waves are resonated. Both of the waves make round trips inside the cavity and hence oscillate. The pump power at the threshold is reduced by one to three orders of magnitude. Tuning is complicated for these OPOs. The signal and idler wavelengths jump when the crystal temperature or pump wavelength changes, and tuning is highly irregular. This is because simultaneous resonance for both the idler and the signal waves, but not just a phase-matching condition, determines the operation wavelengths. There is another case where signal and pump waves are resonated. These OPOs are called Pump enhanced singly resonant optical parametric oscillators (PESOPO). In this, the frequency depends linearly on the cavity length, and the cavity length is limited by pump resonance width.

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.

Frequency Tuning 

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.

Features

• Wavelength Versatility: Ultraviolet → Mid IR/THz
• Temporal Versatility: CW → Femtosecond
• Operation: >Room temperature
• High Power/Pulse Energies: 30 W, 200 mJ
• High Efficiency: 50-90%
• Compact, Solid-State design

Applications

• Laser spectroscopy and many other scientific applications: Optical parametric oscillators can emit light in the mid-infrared or far-infrared range. This makes them useful for spectroscopy, a field in which lasers are limited. The molecules attain a characteristic vibrational spectrum in spectroscopy and this property is especially particularly common within the mid-infrared region where a large number of molecules undergo strong vibrational transitions. Therefore, mid-IR spectroscopy provides an important method to study molecular structure and properties.OPOs can also detect and monitor gases by measuring their absorption spectrum in the infrared. 
• Trace Gas Detection: A gas that is present in small concentrations is known as a trace gas. Despite comprising less than 1% of the atmosphere, they can have a very negative impact on the ecosystem. Greenhouse gases are one of these trace gases. Because of their limited scanning range, laser spectroscopy instruments are only able to examine a single type of gas. However, OPOs and spectroscopic methods show to be far more versatile in the detection of trace gases. For the purpose of detecting trace gases, researchers can tune either a Q-switched or continuous-wave optical parametric oscillator through a broad range of the mid-infrared.
• Biomedical Imaging: Optical microscopy is an imaging method that can be used to examine living tissue at a resolution superior to ultrasound or MRI but this microscopy technique is limited in detecting the chemical composition of tissue structures. To best determine the biochemistry of tissues, researchers are currently studying a nonlinear Raman scattering method that makes use of an optical parametric oscillator. This method employs strong vibrational signals for high-sensitivity imaging using a mode-locked pump. Anti-Stokes Raman scattering microscopy has so far been quite successful at capturing images of mouse tissues. The images are real-time, extremely high resolution, and non-intrusive. OPO-based biomedical imaging may be extremely helpful for tissue pathology.

Some of the other applications:

• Military application
• Digital projection displays
• Light Detection and Ranging (LiDAR)
• Precision frequency metrology