chirped-pulse amplification (original) (raw)

Acronym: CPA

Definition: a technique for amplifying pulses to very high optical intensities while avoiding excessive nonlinear pulse distortions or optical damage

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What is Chirped-pulse Amplification?

In amplifiers for ultrashort optical pulses, the optical peak intensities that occur can become very high, so that detrimental nonlinear pulse distortion or even destruction of the gain medium or of some other optical element may occur. This can be effectively prevented by employing the method of chirped-pulse amplification (CPA), which was originally developed in the context of radar technology, but later applied to optical amplifiers [4]:

Before passing through the amplifier medium, the pulses are chirped and temporally stretched to a much longer duration by means of a strongly dispersive element (the pulse stretcher, e.g. a grating pair or a long fiber).
Now, the pulses are sent through an optical amplifier. The long pulse duration reduces the peak power to a level where the above-mentioned detrimental effects in the amplifier gain medium are avoided.
After the gain medium, a dispersive compressor is used, i.e., an element with opposite dispersion (typically a grating pair), which removes the chirp and temporally compresses the pulses to a duration similar to the input pulse duration. As the peak power becomes very high at the compressor, the beam diameter on the compressor grating has to be large. For the most powerful devices, a beam diameter of the order of 1 m is required.

From Gigawatts to Terawatts and Petawatts

pulse evolution in CPA system

Figure 1: Evolution of the temporal pulse shape in a chirped-pulse amplifier.

The method of chirped-pulse amplification has allowed the construction of table-top amplifiers which can generate pulses with millijoule energies and femtosecond durations, leading to peak powers of several terawatts. (1 TW = 1012 W, corresponding to the electric output of 1000 large nuclear power stations).

For the highest peak powers in ultrashort pulses, amplifier systems consisting of several regenerative and/or multipass amplifiers are used, which are mostly based on titanium–sapphire crystals. Such amplifiers can be used e.g. for high harmonic generation in gas jets. Large-scale facilities even reach peak powers in the petawatt range (1 PW = 1000 TW = 1015 W).

It is also possible to use optical parametric amplifiers, leading to the concept of optical parametric chirped-pulse amplification (see below).

Nobel Prize in Physics 2018

In October 2018, the Nobel Prize in physics has been awarded with one half to Arthur Ashkin for work on optical tweezers and the other half jointly to Gérard Mourou and Donna Strickland [32]. The latter two have pioneered the method of chirped-pulse amplification. This was recognized as an essential contribution to developing lasers capable of producing enormously high optical intensities.

Stretcher and Compressor

Several aspects of dispersive stretchers and compressors can be of crucial importance:

Particularly in fiber-based systems (see below), it is important to obtain very long stretched pulses, with durations e.g. of the order of 1 ns, to minimize nonlinear effects in the fiber amplifier. This requires a large difference of group delay within the optical spectrum of the input pulses. Strong stretching and compression is possible e.g. with pairs of diffraction gratings, or with fiber Bragg gratings and volume Bragg gratings. A simple Treacy compressor [1] can provide anomalous dispersion, whereas a Martínez-type setup containing a telescope with two lenses between the gratings [3] can provide normal dispersion.
At least for high-energy systems, the compressor needs to tolerate high optical peak powers without introducing nonlinear distortions. Grating compressors (with bulk-optical diffraction gratings) are good in this respect, since they can be made for operation with large mode areas, while the use of fiber Bragg gratings limits the pulse energy to well below 1 μJ. Volume Bragg gratings allow for relatively high energies, but are difficult to fabricate with large thickness and length.
The optical losses, which are relevant particularly in the compressor, can be substantial. For example, when ordinary diffraction gratings are used for the compressor, the four reflections on gratings can easily cause a loss of ≈ 50%. In order not to lose half of the output power, special transmission gratings, fabricated with electron beam lithography, have been developed with losses of only ≈ 3% or even less per reflection (at least for one polarization direction). Another approach for reducing the compressor losses is downchirped-pulse amplification, where (e.g. for the 1-μm spectral region) the stretcher has anomalous dispersion, so that the compressor can be a simple glass block with normal dispersion.
The quality of recompression depends on a good match between the dispersive properties of stretcher and compressor, and may also be spoiled by residual nonlinear effects and possibly by chromatic dispersion in the amplifier. Grating compressors introduce substantial amounts of higher-order dispersion, which is difficult to match with dispersive fibers, but tailored nonlinearly chirped fiber Bragg gratings can be a solution.
The total amount of chromatic dispersion required in the stretcher and the compressor also depends on the initial pulse bandwidth, which should thus not be too small. On the other hand, very large bandwidths introduce problems with matching higher-order dispersion, and gain narrowing in the amplifier is another issue. Therefore, CPA systems work best for pulse durations between roughly 20 fs and a few hundred femtoseconds.

Depending on the performance required and on other requirements, different types of stretchers and compressors can present the best solution. The highest performance is achieved if at least the compressor is made with bulk diffraction gratings.

Fiber Versus Bulk Amplifiers

The concept of chirped-pulse amplification is also applied to fiber amplifiers. Due to the inherently high nonlinearity of long fibers, CPA has to be applied already for relatively low pulse energies, and even with strong temporal stretching of the pulses, the achievable pulse energies remain limited to roughly 10 mJ. (Stretching the pulses to more than a few nanoseconds is not practical, and that pulse duration combined with a few megawatts of peak power, which is limited by catastrophic self-focusing, leads to the order of 10 mJ of energy.) However, high average powers of tens of watts or even more than 100 W can be generated [12, 15]. Fiber-based CPA systems are therefore most suitable for high pulse repetition rates combined with high average powers. The fibers used for such systems should be optimized in various respects; they should have features such as a high gain per unit length, polarization-maintaining properties (strong birefringence) and core-less end caps.

All-fiber solutions are possible, but very limited in terms of pulse energy. Therefore, at least the compressor is often made with bulk-optical components. In the future, it may become possible to replace bulky diffraction gratings with volume Bragg gratings while still achieving high pulse energies.

Fiber-based CPA systems are also interesting in combination with coherent beam combining. There are plans to combine the outputs of a large number of such amplifier systems for reaching much higher pulse energies.

See also the article on fiber lasers versus bulk lasers, touching upon various aspects which also apply to CPA systems.

Optical Parametric Chirped-pulse Amplification

The CPA concept is also applied to optical parametric amplifiers, and is then called optical parametric chirped-pulse amplification ({OPCPA=optical parametric chirped-pulse amplification} [8]). The article on optical parametric chirped-pulse amplification contains more details.

A Simple Variant

A simple variant of CPA can be realized with a fiber amplifier where the chirp of the pulses is automatically generated in the fiber, rather than with a pulse stretcher before the amplifier. This can occur in fibers with normal chromatic dispersion, where parabolic pulses are formed. Apart from not requiring a stretcher, an advantage of this method is that the chirp obtained is very close to linear. See the article on parabolic pulses for more details.

CPA with Semiconductor Amplifiers

The CPA concept can also be utilized for semiconductor optical amplifiers (SOAs) [14]. In that case, the duration of the stretched pulses can be well beyond the carrier lifetime of the amplifier. As a consequence, the achievable energy is no longer limited by the low saturation energy of such amplifiers: The energy stored in the amplifier can be replenished during amplification of the stretched pulse. This means that the main purpose of CPA is in that case not avoiding effects of Kerr nonlinearity and optical damage, but increasing the extractable energy. However, this energy is still low compared with that from amplifiers based on ion-doped gain media.

CPA in the Picosecond Regime

As mentioned above, CPA may not be practical for pulses with relatively long durations (several picoseconds or longer), since very large amounts of chromatic dispersion would be required in the stretcher and compressor. In this regime, the technique of divided-pulse amplification may be an interesting alternative.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is chirped-pulse amplification (CPA)?

Chirped-pulse amplification is a technique for amplifying ultrashort optical pulses to very high energy levels. It works by stretching a pulse in time, amplifying it at a lower peak power, and then compressing it back to its original duration.

Why is CPA necessary for amplifying ultrashort pulses?

Without CPA, the intense peak power of an ultrashort pulse would cause detrimental nonlinear effects or even damage the amplifier medium. Stretching the pulse reduces its peak power to a safe level during amplification.

What are the essential components of a CPA system?

A CPA system consists of three main stages: a pulse stretcher (e.g., a grating pair or fiber) that introduces a chirp, an optical amplifier for energy gain, and a pulse compressor that removes the chirp and shortens the pulse.

Who received the Nobel Prize for developing CPA?

Gérard Mourou and Donna Strickland were jointly awarded a share of the 2018 Nobel Prize in Physics for their pioneering work on chirped-pulse amplification.

What limits the pulse energy in fiber-based CPA systems?

In fiber amplifiers using CPA, the pulse energy is typically limited to around 10 mJ. This is because even with pulse stretching, the high intensity in the small fiber core can lead to effects like catastrophic self-focusing.

What is the difference between CPA and OPCPA?

OPCPA stands for Optical Parametric Chirped-Pulse Amplification. It is a variant of CPA that uses an optical parametric amplifier for the amplification stage instead of a traditional gain medium like a doped crystal or fiber.

Can CPA be done without a pulse stretcher?

Yes, a simplified version is possible, particularly in fiber amplifiers. If pulses propagate in a fiber with normal dispersion, they can evolve into parabolic pulses with a nearly linear chirp, which can then be compressed after amplification, eliminating the need for a separate stretcher.

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Bibliography

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[32]	Nobel Prizes in 2018 to Gérard Mourou (see the lecture) and Donna Strickland (lecture)

(Suggest additional literature!)

Picture of Dr. Rüdiger Paschotta

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