Optical Parametric Chirped-pulse Amplification
Acronym: OPCPA
Definition: parametric amplification of chirped ultrashort pulses
Categories: light pulses, methods
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Author: Dr. Rüdiger Paschotta
The concept of chirped-pulse amplification was originally developed for the amplification of ultrashort pulses with laser amplifiers, but it was soon realized that it is also very suitable for optical parametric amplifiers (OPAs). At high pulse energies, these also profit from a strong reduction in the peak intensities by amplifying temporally stretched (chirped) pulses. Stretching to chirped signal pulse durations of the order of 1 ns makes it possible to apply much higher pump energies and therefore to obtain much higher amplified pulse energies. Furthermore, one no longer needs ultrashort pump pulses, but can rather resort to powerful and comparatively simple Q-switched lasers as pump sources in the nanosecond regime. (Note that a parametric amplifier, in contrast to a laser amplifier, requires pump pulses with durations similar to those of the signal pulses, because there is no energy storage in the gain medium.)
Advantages of the OPCPA Concept
Compared with classical chirped-pulse amplification based on laser gain media, OPCPA has a number of important advantages:
- The parametric gain within a single pass through a nonlinear crystal can be many tens of decibels, so that OPCPA systems require fewer amplification stages (often just one), usually do not involve complicated multipass geometries, and can thus be built with much simpler and more compact setups.
- Parametric amplification is possible in a wide range of wavelengths. (Note, however, that an ultrabroad gain bandwidth is achieved only under certain phase-matching conditions.)
- With optimized phase-matching conditions, the gain bandwidth can be very large, allowing very short (few-femtosecond) high-energy pulses to be generated.
- Thermal effects in the amplifier crystal, such as thermal lensing, are much weaker than in a laser amplifier, since there is only a small amount of heating due to weak parasitic absorption. This together with the very high quantum efficiency allows for scaling to very high energy and peak power levels, and also to a high beam quality of the amplified pulses.
- Sometimes the generated idler wave can also be used.
- As the parametric gain occurs only within the duration of the pump pulse, one avoids the problems of power losses by amplified spontaneous emission in high-gain laser amplifiers and can easily generate high-energy pulses with very high intensity contrast, i.e. with a very low level of power before the actual pulse.
On the other hand, disadvantages of the OPCPA concept (compared with classical CPA with laser amplifiers) are
- the requirement to match the pump and signal pulse durations, and to synchronize seed and pump laser
- the requirement for a high pump beam quality
- the limited aperture of most available nonlinear crystals
- the complicated details of phase-matching issues
Terawatt and Petawatt Peak Powers
Some large laser facilities, which originally started with more traditional chirped-pulse amplification, have adopted the OPCPA technique for achieving extremely high peak powers [6, 7, 14–16]. Such systems employ at least two amplification stages, with a preamplifier typically based on a borate crystal (BBO or LBO), whereas KDP is used for the final amplifier stage because of the possibility to fabricate KDP crystals with very large useful apertures. A titanium–sapphire laser can serve as the seed source, and high-energy frequency-doubled Q-switched lasers generate the pump pulses. In some cases, a laser amplifier (with moderate gain) is used for the last amplifier stage, but all-parametric systems are also under investigation. The latter already reach peak powers of hundreds of terawatts [17], and it is expected that multi-petawatt peak powers will be reached soon.
Few-cycle Pulse Amplification
The largest amplification bandwidth can be reached with certain noncollinear phase-matching schemes, based on, e.g., a BBO amplifier crystal pumped with few-picosecond pulses from a frequency-doubled mode-locked titanium–sapphire laser. The term noncollinear optical parametric amplifier (NOPA) has been coined. Compared with the above-mentioned high-energy systems, NOPAs typically operate with a relatively short interaction length, much shorter pump pulses and correspondingly lower amplified pulse energies, but reach compressed pulse durations in the few-cycle regime down to ≈ 4–5 femtoseconds. For simplicity, the seed pulse can be taken from a supercontinuum derived from the pump laser itself, avoiding the need to synchronize a separate seed laser with the pump laser.
A range of interesting concepts are currently being explored in this domain. For example, there is the idea of wideband phase matching (“achromatic phase matching”) by angularly dispersing the signal beam such that each frequency component of the signal is properly phase-matched. Similar effects are achieved with tilting of the pulse fronts in the amplifier crystal (pulse front matched geometry); even in a collinear geometry, this allows for a very large phase-matching bandwidth [2, 5]. Wavelength tuning, which makes such systems very interesting for various scientific applications, is sometimes possible in a relatively wide range. Another important issue is the precise optimization of stretcher/compressor setups [9].
Compact Systems
The use of highly nonlinear quasi-phase-matched crystals allows for very high gains with moderate pump pulse energies. Although such systems typically generate pulses with durations of hundreds of femtoseconds and energies of microjoules or up to a few millijoules, these performance values are sufficient for a wide range of applications, and such systems can be made very compact and efficient.
Suppliers
The RP Photonics Buyer's Guide contains 8 suppliers for optical parametric chirped-pulse amplifiers. Among them:
Fastlite
Our Starzz systems are custom-made mid-IR OPCPA dedicated to High Harmonic Generation (HHG) and attosecond science. With record CEP stability, high average power or high pulse energy, this new generation of ultrafast light sources combines Yb-doped industrial grade pump lasers with Fastlite landmark technologies to set new standards in ultrafast laser science.
EKSPLA
Featuring a success story in design and manufacturing of ultrafast high intensity OPCPA system of ELI, EKSPLA introduces a wide range of ultrafast and nanosecond high intensity lasers and amplifiers. Our broad knowledge in high energy laser physics, nonlinear materials and more that 27 years of experience in laser design enables us to offer unique solutions for high pulse energy systems.
Our high intensity laser systems feature OPCPA based technology, flash lamp pump for ultra-high pulse energy, and diode pumping for high average power. Innovative solutions for pulse shaping, precise synchronization between different laser sources enables users to fit these systems to numerous experiments of modern fundamental science.
TOPTICA Photonics
TOPTICA’s FemtoFiber dichro midIR generates radiation at 3 μm – 15 μm. Based on difference frequency generation of two optically synchronized laser pulses at tunable wavelengths of 1 – 2 μm a highly stable broadband emission of approximately 400 cm-1 is generated. Here, the output at 1560 nm of an erbium-doped ultrafast fiber laser is superimposed with the long or short wavelength part of a supercontinuum.
The CEO-free mid-IR laser pulses are applied to attosecond spectroscopy where the extreme UV pulses consist of only a few optical cycles. The conversion of mid-IR radiation to extreme UV is accomplished by high harmonic generation. First, the mid-IR pulses are subject to optical parametric chirped amplification (OPCPA). Then, the intense laser fields are launched into an atomic beam or a gas-filled hollow core fibre to generate extreme UV attosecond laser pulses via high harmonics.
Class 5 Photonics
Class 5 Photonics offer powerful and high-performance femtosecond lasers based on optical parametric chirped pulse amplification (OPCPA):
WHITE DWARF OPCPA powered by Coherent up to 5 W:
- < 9 fs at 800 nm @ 1 MHz
- most compact few-cycle laser system
- stable and reliable
- CEP stability optional
- dual output for time-resolved measurements
- pumped by Coherent Monaco industrial femtosecond laser
WHITE DWARF HE OPCPA up to 30 W:
- high-performance OPCPA system
- pumped by Yb:Fiber or Yb:YAG laser up to 300 W and 3 mJ
- output power up to 30 W
- pulse durations down to 9 fs
- add-ons EUV to MID-IR
- pump-probe configuration available
SUPERNOVA OPCPA up to 100 W – our flagship product:
- highest average power OPCPA for demanding applications
- pumped by kW-class Yb:YAG Innoslab amplifiers or thin-disk
Light Conversion
Light Conversion has developed high-energy OPCPA systems, which are pumped with picosecond Nd:YAG lasers. Their attractive features:
- multi-TW power pulses produced at up to 1 kHz repetition rate
- pulse duration down to <9 fs
- pre-pulse contrast exceeding 1012
- sub-220-mrad CEP noise and <1% energy fluctuations maintained throughout the full day of operation
- safe and simple spectral-temporal shaping of output pulses possible
- integrated control and diagnostics system
- less than 1 hour warm-up time
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See also: chirped-pulse amplification, optical parametric amplifiers, optical amplifiers, chromatic dispersion, nonlinearities, ultrashort pulses, pulse compression
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