Optical Parametric Amplifiers

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Crystal materials lacking inversion symmetry can exhibit a so-called <$\chi^{(2)}$> nonlinearity. Apart from frequency doubling and sum and difference frequency generation, this allows for parametric amplification. Here, the signal beam propagates through the crystal together with a pump beam of shorter wavelength. Photons of the pump wave are then converted into (lower-energy) signal photons and the same number of so-called idler photons; the photon energy of the idler wave is the difference between the photon energies of pump and signal wave. As the pump energy is fully converted into energy of signal and idler beams, the crystal material is not heated in this process.

In the usual non-degenerate case, signal and idler waves constitute physically separate beams. However, there are degenerate parametric amplifiers where signal and idler wave are identical, i.e. have the same frequency and same polarization. The signal frequency then has to be exactly half the pump frequency, and the phase relationship between signal and pump determines the direction of energy flow, i.e., whether there is amplification or deamplification of the signal. This phase-sensitive amplification does not occur in a non-degenerate amplifier; here, a signal with arbitrary phase can be amplified, and the phase of the generated idler will automatically adjust accordingly. (More details are given in the article on parametric amplification.)

Phase Matching

Parametric processes are sensitive to the optical phases of the waves involved. Efficient conversion usually requires phase matching to be arranged for the wavelength range of interest. The gain bandwidth is largely determined by the phase-matching bandwidth, which depends on the chromatic dispersion properties and can in certain situations be extremely large.

Parametric amplifiers are particularly attractive for the generation of wavelengths which are very hard to access directly with lasers and laser amplifiers due to the lack of suitable gain media, or when properties such as a large tuning range or a high gain in a short length are required. Tuning in a very wide wavelength range is often achieved simply by rotating the critically phase-matched nonlinear crystal.

Pulsed Operation

In most cases, parametric amplifiers are used for pulses, which may be nanosecond pulses from Q-switched lasers or ultrashort pulses from mode-locked lasers. With the high pump intensities achievable with pulses, the parametric gain can be very high – sometimes more than 80 dB within a few millimeters of crystal material. This high gain, together with the potential to operate at very different wavelengths and the minimal heating (only due to parasitic absorption, which is usually very weak), makes parametric amplifiers very attractive for a range of applications, including optical parametric chirped-pulse amplification of femtosecond pulses. Due to the high gain, the effect of gain guiding must be considered in numerical models. Purely analytical models usually neglect this effect.

Seed Sources

An optical parametric amplifier is often used with the purpose of generating intense radiation at a certain output wavelength, or some range of wavelengths. One then also requires a suitable seed laser source at the same wavelength – or alternatively at the corresponding idler wavelength.

Different kinds of seed lasers can be used:

• One often uses a laser diode; such devices are available for a wide range of wavelengths. Special kinds of diode lasers, for example external-cavity diode lasers, are used for satisfying special requirements such as single-frequency operation. Unfortunately, broad wavelength tunability is then normally not possible.
• For pulsed operation, one may use an optical parametric generator (OPG) for generating the seed, if the OPG alone is not sufficient to reach the required power level.
• Particularly for operation with ultrashort pulses, it has become quite common to use a supercontinuum source. It is not tuned to the required wavelength, but rather the OPA just utilizes a narrow part of the broad supercontinuum and ignores the rest. Because of the high achievable nonlinear gain of the OPA, the low power efficiency of the seed pulse generation does not matter too much.

Gain Saturation

As any other kind of optical amplifier, a parametric amplifier exhibits gain saturation (i.e., a reduction of gain) when amplifying a signal of substantial strength. The physical mechanism behind that is that an intense signal leads to pump depletion (by conversion of pump photons to signal and idler photons), and that in turn reduces the gain during further propagation.

In contrast to a laser amplifier, gain saturation in a parametric amplifier is essentially instantaneous. Also note that gain saturation will be strongest in those parts of the beam cross-section where the signal intensity is highest. Therefore, the shape of the transverse pump intensity profile and thus the gain profile can be changed, and that can have complicated effect on the amplification process.

High-power Operation

Generally, parametric amplifiers are suitable for fairly high output powers, but there are some partly non-trivial issues to observe.

A very beneficial property of optical parametric amplifiers is that fundamentally there is not dissipative process in the nonlinear crystal, so that thermal effects could in principle be entirely absent. However, there are some parasitic absorption losses, the strength of which depends very much on the crystal material. Unfortunately, even a moderate amount of local heating can be detrimental as it disturbs the phase matching. Therefore, significant thermal effects can in practice seriously limit the output power capability of a parametric amplifier. This holds for, e.g., amplifiers based on KTP and KTA, whereas LBO shows only weak absorption.

Another issue, which is less well known, is gain guiding. In an amplifier with high gain, the effect of gain guiding can strongly influence the output profile of the amplified wave. This creates a problem for devices with large mode areas [9]: the signal beam becomes smaller and smaller, even if the input signal beam is large. In the last section of the amplifier, where significant power is transferred from the pump beam to the signal beam, the signal beam is then too small to utilize the pump energy fully. This problem does not occur for devices with small mode area, as diffraction counteracts the effect of gain guiding, but high-power devices require larger mode areas.

The conclusion is that gain guiding spoils the apparent power scalability of OPAs [9], even in the absence of parasitic losses: with the apparent power scaling procedure of increasing the mode area in proportion to the power level, one would run into the explained problem with gain guiding, so that one would have to sacrifice the output beam quality and/or the conversion efficiency. Interestingly, the compromise between these performance figures can be improved by using a material with lower nonlinearity, operating with correspondingly smaller mode area.

Long Output Wavelengths

Optical parametric amplifiers are often used to amplify light with relatively long wavelengths. The accessible wavelength range is usually limited by the transparency range of the used nonlinear crystals.

One requires special optical elements working at the relatively long wavelengths; see the article on infrared optics.

Comparison with Laser Amplifiers

The following list summarizes the most important differences of parametric amplifiers as compared with laser amplifiers:

• Parametric amplifiers are not limited to narrow wavelength ranges as defined by laser transitions of laser gain media.
• The gain bandwidth is determined by the dispersive properties and the length of the amplifier crystal, rather than by details of a laser transition.
• With pulsed pumping, the gain per unit length can be much higher than for laser gain.
• A parametric amplifier has no energy storage, i.e., it amplifies only while being pumped. This allows for, e.g., very high intensity contrast between an amplified pulse and any satellite pulses.
• The absence of heat generation (at least provided that parasitic absorption losses are small) makes OPAs suitable for high-power operation.
• In the (rarely used) case of degenerate parametric amplification, the amplification is phase-sensitive, and quantum excess noise can be avoided.

Fiber-optic Parametric Amplifiers

There are also fiber-optic parametric amplifiers, which are based on the <$\chi^{(3)}$> nonlinearity of a fiber, rather than the <$\chi^{(2)}$> nonlinearity of a crystal, and often contain a fiber loop. In that case, there can be up to four waves interacting with each other (→ four-wave mixing). In many cases, there is a degeneracy where two photons from a single pump wave are converted into a pair of a signal and idler photon, with signal and idler frequencies to both sides of the pump frequency. However, it is also possible to use a non-degenerate interaction with two separate pump waves. In any case, the gain bandwidth is determined by dispersive properties of the fiber medium, but there can also be a significant influence of the Kerr nonlinearity of the fiber.

More to Learn

Suppliers

The RP Photonics Buyer's Guide contains 23 suppliers for optical parametric amplifiers. Among them:

Thorlabs

Amidst our extensive selection of femtosecond lasers and related products for control and characterization, Thorlabs manufactures a compact, tunable repetition rate Ytterbia OPA with integrated microjoule-class pump laser. Complimenting our ultrafast laser systems is a suite of nonlinear crystals, chirped mirrors, low GDD optics, and related products for pulse measurement, pre-compensation, and dispersion measurement.

Fluence

Find harmony in four perfectly synchronised and precisely tunable wavelength outputs produced by the Fluence Harmony Optical Parametric Amplifier (OPA).

Fluence Harmony is a device that precisely converts ultrafast pulses of one band (e.g. 1030-nm pulses coming from Fluence Jasper) into a set of pulsed laser beams tunable in an extremely broad spectrum of wavelengths, ranging from 210 nm up to 2600 nm. All automated, all software-controlled.

A growing number of ultrafast spectroscopy techniques has prompted the need for a robust and reliable, self-diagnostic device like Harmony. The optical parametric amplifier is fully compatible with all femtosecond lasers and provides automated tuning across the basic tuning range.

Covesion

Researchers have used MgO:PPLN crystals to build a compact and dependable, tunable, CW seeded synchronization-free OPA with a robust, commercial pico-second pump laser.

Utilizing our knowledge and experience in poling techniques, Covesion can offer a variety of stock and custom free space bulk crystal solutions for amplifying low power near-IR CW seed lasers for applications such as spectroscopy and environmental monitoring.

Our custom design capabilities include:

• multiple grating, chirped or fan-out designs
• tailored AR coatings
• custom grating periods and apertures
• compatibility with both CW and pulsed lasers

EKSPLA

For researchers demanding a wide tuning range, high conversion efficiency and narrow line-width, EKSPLA optical parametric oscillators/amplifiers are an excellent choice. All models feature hands-free wavelength tuning, a protection system for valuable optical components as well as a wide range of accessories and extension units.

EKSPLA's long-term experience and close cooperation with scientific institutions made it possible to create a range of models, offering probably the widest tuning range: from 193 nm to 16000 nm. Versions offering near transform-limited linewidth as well as operating at kilohertz repetition rates are available.

Class 5 Photonics

Class 5 Photonics delivers ultrafast, high-power laser technology at outstanding performance to advance demanding applications from bio-imaging to ultrafast material science and attosecond science. Our robust optical parametric chirped pulse amplifiers (OPCPA) provide high-power, tunable femtosecond pulses at user-friendly operation.

Features of the White Dwarf OPCPA 5 W:

• compact and user-friendly
• CEP stability available
• pumped by Coherent Monaco industrial femtosecond laser

White Dwarf HE OPCPA 30 W:

• high-performance, ultrafast OPCPA
• pump-probe configuration
• pumped by Yb-based laser up to 300 W and 3 mJ

Supernova OPCPA 100 W:

• our award-winning flagship product
• highest average power OPCPA for demanding applications
• pumped by kW-class Yb:YAG Innoslab amplifiers or thin-disk lasers

Light Conversion

The highlight of our tunable wavelength sources is the ORPHEUS series of femtosecond optical parametric amplifiers (OPAs) for pumping with Yb-doped lasers. The ORPHEUS series enables operation at high repetition rates while maintaining the best properties of the TOPAS series OPAs, such as a wide tuning range from deep-UV to mid-IR and high output stability. When combined with the PHAROS or CARBIDE femtosecond lasers, it becomes an invaluable source for ultrafast spectroscopy, nonlinear microscopy, and a variety of other applications. The list of tunable wavelength sources has been extended by the addition of I-OPA, a compact industrial-grade OPA, and CRONUS-3P, an OPA-based ultrafast source with GDD control for advanced nonlinear microscopy.

APE

AVUS is the very latest Optical Parametric Amplifier (OPA) providing widely tunable high-energy pulses. It is ideal for use with 1-μm femtosecond lasers and opens doors for up to 50 W pump power. The user-friendly and maintenance-free unit is air-cooled and constructed with a monolithic case design for long-term thermal stability, even at maximum pump power. Versions with pulse durations <200 fs or alternatively <70 fs are available.

Stuttgart Instruments

The Stuttgart Instruments Alpha is an ultrafast and fully wavelength-tunable frequency conversion system in an ultra-compact and completely passively stable system based on revolutionary parametric oscillator design which guarantees outstanding stability, reproducibility and shot-noise limited performance.

The revolutionary design of Stuttgart Instruments Alpha, characterized by outstanding low noise and passive long-term stability, is based on the fiber-feedback optical parametric oscillator (FFOPO) technology and results in outstanding performance and high flexibility at the same time.

The Alpha covers a gap-free rapid tunable spectral range from 700 nm to 20 µm wavelengths, while maintaining high output power up to the Watt-level with femto- or picosecond pulses at several MHz pulse repetition rates. It provides multiple simultaneously tunable outputs with a selectable bandwidth from a few to 100 cm-1. Shot-noise limited performance above 300 kHz, passive spectral stability (< 0.02% rms) and wavelength-independent stable beam pointing (< 30 µrad) enable excellent sensitivity. In addition, each Alpha is equipped with a user-friendly ethernet and Wi-Fi interface and a matching graphical user interface (GUI) as well as easy to access API interfaces for e.g. LabView, Python, C++.

Typically, the Alpha is pumped by an ultra-low-noise Primus pump laser, which provides more than 8 W average output power at 1040 nm wavelength and 450 fs pulse duration at 42 MHz repetition rate. In addition, the Alpha can be operated with other pump lasers around 1 µm wavelength and enough power.

Due to our modular platform, the Alpha can be adapted and optimized for various applications and is particularly suited for spectroscopic applications requiring a robust and reliable tunable radiation with low noise.

Active Fiber Systems

AFS offers customized OPCPA/OPA add-ons to supplement our high-power beamlines.

RPMC Lasers

Serving North America, RPMC Lasers offers a single-box optical parametric amplifier that converts ultrafast pulses from Ytterbium-based lasers into broadly tunable pulses with wavelengths ranging from 210 nm to > 10 µm. With intuitive, user-friendly PC software, an integrated mini spectrometer enables automatic tuning from 315 to 2600 nm. Designed to operate from a single pulse up to 300kHz rep. rate with pump pulse energy from 30 µJ to 200 µJ, Harmony is compatible with all Ytterbium-based femtosecond lasers, but it works best with the Jasper series: the Jasper X0 & Jasper Flex lasers featuring exceptional beam-pointing stability. Standard and custom options available. Let RPMC help you find the right laser today!

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