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(Acronym: OPO)
Definition: coherent light sources based on parametric amplification within an optical resonator
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 in a nonlinear crystal rather than from stimulated emission. Like a laser, such a device exhibits a threshold for the pump power, below which there is negligible output power (only some parametric fluorescence).
Figure 1: Schematic of an optical parametric oscillator.
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 is also often possible. 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.
Figure 2: Setup of a typical optical parametric oscillator with a ring resonator.
Comparison with Lasers
Although parametric oscillators are in many respects similar to lasers, there are also a couple of important differences:
- While lasers can often be operated with rather 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 middle infrared part of the electromagnetic spectrum. Particularly in the mid infrared, 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). 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.)
- 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 involved wavelengths lying well within the transparency region, there is normally not much heating. Only at rather high power levels, a disturbance of the phase-matching conditions may occur. Thermal lensing is usually not significant.
- An idler wave is generated, which carries away the difference between the generated signal power and the absorbed pump power. (Only in the relatively exotic case of degenerate parametric oscillation, there is no idler wave.) More precisely, the photon energy of the idler wave is the difference of the photon energies of 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 quite 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 either at the signal or the idler wavelength, but not for both signal and idler. (For the non-resonant wave, dichroic resonator mirrors or some polarizing optics lead to rather 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 very 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 of simultaneous resonance for signal and idler (→ mode clusters), and not only by a phase-matching condition.
Another matter 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 rather delicate to operate, though. A somewhat simpler option is to realize intracavity pumped OPO, where the nonlinear crystal is placed within the resonator of the pump laser, exploiting the high intracavity power.
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. [14]). 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 a bit 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 [19].
- For generation 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 so 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 generated pulses normally 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 [20]. 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 great variety of OPOs:
- Continuous-wave OPOs are usually based on highly nonlinear crystal materials such as periodically poled LiNbO3 [14] or KTP, pumped e.g. with a 1-μm neodymium-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 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 [23]. 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 [25]. 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.
- Somewhat exotic are fiber OPOs, based on the χ(3) nonlinearity of an optical fiber rather than the χ(2) of a crystal. Fiber OPOs often have a pump wavelength near the signal and idler wavelength and can be interesting for telecom applications.
Application of OPOs
The potential application areas of OPOs are very diverse. Some examples are:
- 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-μm to 5-μm region for blinding heat-seeking missiles when they attack airplanes.
- An OPO can be part of a high-power RGB source as used e.g. for digital projection displays.
Problems for Commercial Realization
Despite their amazing capabilities, as demonstrated in years of interesting research, optical parametric oscillators have so far not found widespread use in commercial products. Some of the reasons for that 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.
- The requirement of phase matching makes the operation of nonlinear conversion stages somewhat 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 involved 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 many 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.
Bibliography
| [1] | R. H. Kingston, "Parametric amplification and oscillation at optical frequencies" (an early theoretical investigation), Proc. IRE 50, 472 (1962) |
| [2] | J. A. Giordmaine and R. C. Miller, "Tunable coherent parametric oscillation in LiNbO3 at optical frequencies", Phys. Rev. Lett. 14 (24), 973 (1965) (first experimental demonstration of an optical parametric oscillator) |
| [3] | S. T. Yang et al., "Power and spectral characteristics of continuous-wave parametric oscillators: the doubly to singly resonant transition", J. Opt. Soc. Am. B 10 (9), 1684 (1993) |
| [4] | R. Byer and A. Piskarskas, Eds., Feature issue on optical parametric oscillation and amplification, J. Opt. Soc. Am. B 9, 1656–1791 (1993) and vol. 10, pp. 2148–2243 (1993) |
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| [6] | J. D. Kafka et al., "Synchronously pumped optical parametric oscillators with LiB3O5", J. Opt. Soc. Am. B 12 (11), 2147 (1995) |
| [7] | Special feature on optical parametric oscillators, J. Opt. Soc. Am. B 12 (11), 1995 |
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| [13] | M. A. Arbore and M. M. Fejer, "Singly resonant optical parametric oscillation in periodically poled lithium niobate waveguides", Opt. Lett. 22 (3), 151 (1997) |
| [14] | R. G. Batchko et al., "Continuous-wave 532-nm-pumped singly resonant optical parametric oscillator based on periodically poled lithium niobate", Opt. Lett. 23 (3), 168 (1998) |
| [15] | S. Guha, "Focusing dependence of the efficiency of a singly resonant optical parametric oscillator", Appl. Phys. B 66 (6), 663 (1998) |
| [16] | M. E. Klein et al., "Singly resonant continuous-wave optical parametric oscillator pumped by a diode laser", Opt. Lett. 24 (16), 1142 (1999) |
| [17] | M. H. Dunn and M. Ebrahimzadeh, "Parametric generation of tunable light from continuous-wave to femtosecond pulses", Science 286, 1513 (1999) |
| [18] | A. V. Smith et al., "Numerical models of broad-bandwidth nanosecond optical parametric oscillators", J. Opt. Soc. Am. B 16 (4), 609 (1999) |
| [19] | G. Arisholm, "Quantum noise initiation and macroscopic fluctuations in optical parametric oscillators", J. Opt. Soc. Am. B 16 (1), 117 (1999) |
| [20] | L. Lefort et al., "Generation of femtosecond pulses from order-of-magnitude pulse compression in a synchronously pumped optical parametric oscillator based on periodically poled lithium niobate", Opt. Lett. 24 (1), 28 (1999) |
| [21] | T. Südmeyer et al., "Femtosecond fiber-feedback OPO", Opt. Lett. 26 (5), 304 (2001) |
| [22] | D. C. Hanna et al., "Synchronously pumped optical parametric oscillator with diffraction-grating tuning", J. Phys. D 34, 2440 (2001) |
| [23] | T. Südmeyer et al., "Novel ultrafast parametric systems: high repetition rate single-pass OPG and fiber-feedback OPO", J. Phys. D: Appl. Phys. 34 (16), 2433 (2001) |
| [24] | U. Strößner et al., "Singly-frequency continuous-wave optical parametric oscillator system with an ultrawide tuning range of 550 to 2830 nm", J. Opt. Soc. Am. B 19 (6), 1419 (2002) |
| [25] | S. Lecomte, R. Paschotta et al., "Synchronously pumped optical parametric oscillator with a repetition rate of 81.8 GHz", Photon. Technol. Lett. 17, 483 (2005) |
See also: optical parametric amplifiers, parametric amplification, nonlinear crystal materials, nonlinear frequency conversion, tunable lasers, synchronous pumping, Spotlight article 2006-07-30, Spotlight article 2006-09-03, Spotlight article 2007-08-23
Categories: nonlinear optics, photonic devices
This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) could become very valuable for your business!
