Optical Parametric Oscillators
An optical parametric oscillator (OPO) [1, 2] is a light source similar to a laser, also using a kind of laser resonator, but based on optical gain from parametric amplification (usually in a nonlinear crystal) rather than from stimulated emission of radiation. Like a laser, such a device exhibits a threshold for the pump power, below which there is negligible output power (only some parametric fluorescence).
A main attraction of OPOs is that the signal and idler wavelengths, which are determined by a phase-matching condition, can be varied in wide ranges. Thus it is possible to access wavelengths (e.g. in the mid-infrared, far-infrared or terahertz spectral region) which are difficult or impossible to obtain from any laser, and wide wavelength tunability (often by affecting the phase-matching condition) is also often possible. This makes OPOs very valuable, for example, as light sources for laser spectroscopy.
A limitation is that any OPO requires a pump source with high optical intensity and relatively high spatial coherence. Therefore, a laser is essentially always required for pumping an OPO, and as the direct use of a laser diode is in most cases not possible, the system becomes relatively complex, consisting e.g. of laser diodes, a diode-pumped solid-state laser, and the actual OPO.
Comparison with Lasers
Although parametric oscillators are in many respects similar to lasers, there are also a couple of important differences:
- Whereas many lasers can be operated with spatially incoherent pump sources, a parametric oscillator requires relatively high spatial coherence of its pump. In most cases, a diode-pumped solid-state laser is used.
- While the emission wavelength of most lasers can be tuned only in a narrow range, many parametric oscillators offer the potential for wavelength tuning with extremely wide tuning ranges. These may span regions in the visible, near or mid-infrared part of the electromagnetic spectrum. Particularly in the mid-infrared region, OPOs are very commonly used, because there is little competition from mid-IR lasers.
- The parametric amplification process requires phase matching to be efficient. The phase-matching details also determine the oscillation wavelength. Wavelength tuning is in most cases achieved by influencing the phase-matching conditions, e.g. by changing the crystal temperature, the angular orientation of the crystal (for critical phase matching), or the poling period (for quasi-phase matching in periodically poled crystals). Within the phase-matching bandwidth, tuning is also possible with an intracavity optical filter. The tuning range can be limited either by restrictions of phase matching (see below), or by the transparency region of the nonlinear material or by the spectral region with high reflectivity of the resonator mirrors.
- The parametric amplification occurs only in the direction of the pump beam (as another consequence of phase matching), which means that a unidirectional operation in a ring resonator is automatically obtained. (In fact, ring resonators are often used, due to various advantages, see below.)
- No heat is deposited in the nonlinear crystal, unless there is some parasitic absorption at the pump, signal or idler wavelength. As OPOs are mostly operated with all wavelengths involved lying well within the transparency region, there is normally not much heating. Only at fairly high power levels may a disturbance of the phase-matching conditions occur. Thermal lensing is significant only at very high power levels.
- An idler wave is generated, which carries away the difference between the generated signal power and the absorbed pump power. (Only in the rarely used case of degenerate parametric oscillation, is there no idler wave.) More precisely, the photon energy of the idler wave is the difference in the photon energies of the pump and signal. The idler wave plays an essential role in the nonlinear conversion process; when an OPO is operated in a spectral region with strong idler absorption in the crystal, the threshold pump power can be much higher, and the efficiency lower.
- No energy is stored in the nonlinear crystal. Therefore, the gain is present only as long as the pump wave is there, and pump fluctuations directly affect the signal power. The dynamics are therefore different to laser dynamics.
- Other than the fluorescence of a laser gain medium, parametric fluorescence occurs only in the direction of the pump beam. More precisely, it is observed in those modes which experience parametric gain.
Singly Resonant Versus Doubly Resonant OPOs
Most OPOs are singly resonant, i.e., they have a resonator which is resonant at either the signal or the idler wavelength, but not for both. (For the non-resonant wave, dichroic resonator mirrors or some polarizing optics lead to high resonator losses, so that there is very little optical feedback.) However, there are also doubly resonant OPOs, where both signal and idler are resonant. The latter makes sense only with a single-frequency pump laser.
The advantage of doubly resonant OPOs is that the threshold pump power can be much lower. This is interesting particularly for continuous-wave operation. However, the tuning behavior is complicated: when the crystal temperature or pump wavelength is changed, the signal and idler wavelengths undergo jumps, and the tuning is generally non-monotonous. This is because the operation wavelengths are determined primarily by the requirement for simultaneous resonance for signal and idler (mode clusters), and not only by a phase-matching condition.
Another possibility is resonant enhancement of the pump wave, which is sometimes applied when the pump laser is a single-frequency device. In a triply resonant OPO, pump, signal and idler waves are resonant at the same time. Such a device is delicate to operate, however. A simpler option is to make an intracavity pumped OPO, where the nonlinear crystal is placed within the resonator of the pump laser, exploiting the high intracavity power.
Linear and Ring Resonators
OPOs can be built either with linear (standing-wave) or ring resonators. The choice of resonator type can have various implications:
- A linear resonator is often easier to build and align. This is particularly so in cases where a compact setup for a short resonator round-trip time is required. However, there are highly compact monolithic ring resonators where alignment is also no problem.
- A ring resonator may require larger angles of incidence on curved resonator mirrors, which can cause problems with astigmatism.
- The parametric gain occurs only in the direction of the pump light. Therefore, the backward path in a linear resonator does not provide additional amplification, but only parasitic power losses. The obtainable threshold power and power conversion efficiency are therefore often higher with a ring resonator.
- A linear resonator can cause substantial back-reflection of pump light into the laser source, which in some cases would destabilize that laser even when a Faraday isolator is used.
Pumping of OPOs
There are basically three different options for pumping optical parametric oscillators:
- For continuous-wave operation, an OPO can be pumped with a (possibly frequency-doubled) continuous-wave laser (see e.g. ). The threshold pump power for singly-resonant OPOs (see above) is relatively high – usually at least a few watts, sometimes below 1 W. Some doubly-resonant OPOs can be continuously pumped with only a few tens of milliwatts.
- Most OPOs are pumped with nanosecond pulses from a Q-switched laser. In this mode, it is easy to overcome the threshold even of a singly-resonant OPO. The output pulses are often slightly shorter than the pump pulses, since parametric oscillation sets in with some delay. The output linewidth is often relatively large, and the pulse-to-pulse fluctuations are significant, since a pulsed OPO often has insufficient time during a pulse to settle to the steady state, and is thus relatively strongly influenced by noise .
- For generating ultrashort pulses, an OPO can be synchronously pumped with a mode-locked laser. For synchronous pumping, the length of the OPO resonator is usually adjusted such that the resonator round-trip frequency matches the pulse repetition rate of the pump. (In rare cases, the resonator frequency is a multiple or some fraction of the pump repetition frequency.) During many resonator round trips, the pulses generated reach a steady state, and the noise can be relatively weak. The pulse duration is usually comparable to the pump pulse duration, but may under certain conditions (with significant group velocity mismatch) be significantly shorter than that . Due to the low duty cycle of typical mode-locked lasers, the required average pump power can be well below 1 W.
In most cases, the pump light for an OPO comes either directly from some near-infrared laser or from a frequency doubler, generating e.g. green light. In less common cases, OPOs are pumped with ultraviolet or mid-infrared light.
Types of OPOs
The following list shows that there is a wide variety of OPOs:
- Continuous-wave OPOs are usually based on highly nonlinear crystal materials such as periodically poled LiNbO3  or KTP, pumped e.g. with a 1-μm ytterbium-doped laser, or with a frequency doubled solid-state laser. Single-frequency operation concerning the OPO output is possible, in the case of a singly resonant OPO even if the pump source is not single-frequency.
- Other continuous-wave OPOs, particularly those for very high output power, are intracavity pumped. The nonlinear crystal is then placed within the laser resonator of typically a neodymium-based high-power laser.
- The most typical OPOs are singly-resonant and pumped with an actively Q-switched Nd:YAG laser. They emit nanosecond pulses with microjoule or millijoule pulse energies in the near- or mid-infrared region. For operation at relatively long wavelengths, one sometimes uses tandem OPOs, where a first OPO does a wavelength conversion from the 1-μm to the 2-μm region, and its output is used for pumping a mid-IR OPO (e.g. based on ZGP).
- Typical synchronously pumped OPOs have a picosecond or femtosecond mode-locked laser as a pump source, such as a 1-μm neodymium-doped laser or a titanium–sapphire laser. Their average pump power is between a few hundred milliwatts and a few watts, the pulse repetition rate is between 100 MHz and 1 GHz, and the power conversion efficiency is of the order of 30–50%.
- The high parametric gain achievable with intense ultrashort pulses allows the construction of fiber-feedback OPOs, where the OPO resonator contains a single-mode fiber [25, 29]. Such devices have some practical benefits, such as a remarkable insensitivity to resonator length changes.
- Sync-pumped OPOs with extremely high pulse repetitions rates of higher than 80 GHz have been demonstrated . Here, the challenge is that the average pump threshold power scales linearly with the pulse repetition rate, whereas high average powers are most difficult to obtain from passively mode-locked lasers when high repetition rates are also required. Therefore, a MOPA pump source is required for extremely high repetition rates.
- Less common are fiber OPOs, based on the χ(3) nonlinearity of an optical fiber rather than the χ(2) of a crystal. Earlier fiber OPOs often had a pump wavelength near the signal and idler wavelengths and were interesting mainly for telecom applications. Using photonic crystal fibers with special chromatic dispersion properties, one can obtain output in very wide wavelength regions [32, 33].
Applications of OPOs
The potential application areas of OPOs are very diverse. Some examples are:
- Laser spectroscopy and many other scientific applications can profit from the ability of OPOs to cover very wide spectral regions, and to deliver outputs with narrow linewidth and high power.
- A common military application is the generation of broadband high-power light in the 3–5-μm region for blinding heat-seeking missiles when they attack airplanes (infrared countermeasures).
- An OPO can be part of a high-power RGB source as used for, e.g., digital projection displays.
Problems for Commercial Realization
Despite their amazing capabilities, as demonstrated in years of interesting research, optical parametric oscillators have so far found only limited use in commercial products. Some of the reasons for this are briefly discussed in the following:
- Containing at least some pump laser and an OPO, and possibly a temperature-stabilized crystal oven, parametric oscillator systems are more complex than pure laser systems and correspondingly expensive.
- The requirement for phase matching makes the operation of nonlinear conversion stages more delicate than e.g. laser gain media, which are usually more forgiving e.g. in terms of crystal temperature. OPOs requiring a temperature-stabilized oven for the crystal are certainly less attractive for many applications due to the complexity, turn-on time, heat dissipation, etc.
- Some nonlinear crystal materials are hygroscopic, others are subject to gray tracking (i.e., increasing parasitic losses), and some are difficult to obtain with robust anti-reflection coatings (e.g. due to non-isotropic thermal expansion).
- Finally, a detailed understanding of nonlinear optics and particularly the physics of parametric amplification is not very widespread in the laser industry.
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See also: optical parametric amplifiers, parametric amplification, nonlinear crystal materials, nonlinear frequency conversion, tunable lasers, synchronous pumping, mid-infrared laser sources, The Photonics Spotlight 2006-07-30, The Photonics Spotlight 2006-09-03, The Photonics Spotlight 2006-09-21, The Photonics Spotlight 2007-08-23
and other articles in the categories nonlinear optics, photonic devices, non-laser light sources
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