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Chirped-pulse Amplification

Acronym: CPA

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

German: Verstärkung gechirpter Pulse

Categories: light pulseslight pulses, methodsmethods


Cite the article using its DOI: https://doi.org/10.61835/3ts

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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 chirp-pulse amplification. That was recognized as an essential contribution for the development of laser devices with which one can produce 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, in order 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 also interesting in combination with coherent beam combining. There are plans to combine the outputs of a large number of such amplifiers 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.

case study fiber cpa

Case Studies

Case Study: Chirped-pulse Ytterbium-doped Fiber Amplifier System

We design an ytterbium-doped chirped-pulse fiber amplifier system. We investigate how the remaining nonlinear effects limit the possible performance parameters.


tutorial fiber amplifiers

Fiber Amplifiers
Part 8: Fiber Amplifiers for Ultrashort Pulses; Fiber Nonlinearities

We discuss fiber amplifiers for ultrashort pulses with picosecond or femtosecond durations. It is shown how and to which extent nonlinear effects can be mitigated with parabolic pulse amplification and chirped-pulse amplification.

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 power of such amplifiers: the energy stored in the amplifier can 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.

More to Learn

Tutorial on Fiber Amplifiers, Part 8: Fiber Amplifiers for Ultrashort Pulses; Fiber Nonlinearities

Case studies:

Encyclopedia articles:


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[17]F. Tavella et al., “Dispersion management for a sub-10-fs, 10 TW optical parametric chirped-pulse amplifier”, Opt. Lett. 32 (15), 2227 (2007); https://doi.org/10.1364/OL.32.002227
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[19]Y. Zaouter et al., “Transform-limited 100 μJ, 340 MW pulses from a nonlinear-fiber chirped-pulse amplifier using a mismatched grating stretcher–compressor”, Opt. Lett. 33 (13), 1527 (2008); https://doi.org/10.1364/OL.33.001527
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[21]T. Eidam et al., “Femtosecond fiber CPA system emitting 830 W average output power”, Opt. Lett. 35 (2), 94 (2010); https://doi.org/10.1364/OL.35.000094
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[24]T. Eidam et al., “Fiber chirped-pulse amplification system emitting 3.8 GW peak power”, Opt. Express 19 (1), 255 (2011); https://doi.org/10.1364/OE.19.000255
[25]J. M. Mikhailova et al., “Ultra-high-contrast few-cycle pulses for multipetawatt-class laser technology”, Opt. Lett. 36 (16), 3145 (2011); https://doi.org/10.1364/OL.36.003145
[26]Z. Wang et al., “High-contrast 1.16 PW Ti:sapphire laser system combined with a doubled chirped-pulse amplification scheme and a femtosecond optical-parametric amplifier”, Opt. Lett. 36 (16), 3194 (2011); https://doi.org/10.1364/OL.36.003194
[27]C. Jocher et al., “Sub 25 fs pulses from solid-core nonlinear compression stage at 250 W of average power”, Opt. Lett. 37 (21), 4407 (2012); https://doi.org/10.1364/OL.37.004407
[28]S. Keppler et al., “The generation of amplified spontaneous emission in high-power CPA laser systems”, Laser & Photonics Reviews 10 (2), 264 (2016); https://doi.org/10.1002/lpor.201500186
[29]R. Paschotta, “Modeling of ultrashort pulse amplification with gain saturation”, Opt. Express 25 (16), 19112 (2017); https://doi.org/10.1364/OE.25.019112
[30]W. Li, “339 J high-energy Ti:sapphire chirped-pulse amplifier for 10 PW laser facility”, Opt. Lett. 43 (22), 5681 (2018); https://doi.org/10.1364/OL.43.005681
[31]M. E. V. Pedersen et al., “175 W average power from a single-core rod fiber-based chirped-pulse-amplification system”, Opt. Lett. 47 (19), 5172 (2022); https://doi.org/10.1364/OL.471631
[32]Nobel Prizes in 2018 to Gérard Mourou (see the lecture) and Donna Strickland (lecture)

(Suggest additional literature!)

Dr. R. Paschotta

This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics AG. How about a tailored training course from this distinguished expert at your location? Contact RP Photonics to find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, training) and software could become very valuable for your business!

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