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Chirped-pulse Amplification

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Ask RP Photonics for advice on building a chirped-pulse amplifier system, or for checking whether a simpler approach may suffice in your case. See also how the RP Fiber Power software can be used to simulate the performance of a chirped-pulse fiber laser/amplifier system.

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: methods, light pulses

How to cite the article; suggest additional literature

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 [3]:

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 stretcher, e.g. a grating pair or a long fiber). This reduces the peak power to a level where the above-mentioned detrimental effects in the 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.

pulse evolution in CPA system

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

From Gigawatts to Terawatts and Petawatts

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 (see below).

Stretcher and Compressor

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

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 > 100 W can be generated [11, 14]. 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.

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 [7]). 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.

… and an Exotic Application

The CPA concept can also be utilized for semiconductor optical amplifiers (SOAs) [13]. 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.

Final Remarks

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.

Bibliography

[1]E. B. Treacy, “Optical pulse compression with diffraction gratings”, IEEE J. Quantum Electron. QE-5, 454 (1969)
[2]O. E. Martínez et al., “Negative group-velocity dispersion using refraction”, J. Opt. Soc. Am. A 1 (10), 1003 (1984)
[3]D. Strickland and G. Mourou, “Compression of amplified chirped optical pulses”, Opt. Commun. 56, 219 (1985) (first paper on CPA)
[4]M. Pessot et al., “1000 times expansion/compression of optical pulses for chirped pulse amplification”, Opt. Commun. 62, 419 (1987)
[5]G. Vaillancourt et al., “Operation of a 1-kHz pulse-pumped Ti:sapphire regenerative amplifier”, Opt. Lett. 15 (6), 317 (1990)
[6]S. Sauteret et al., “Generation of 20-TW pulses of picosecond duration using chirped-pulse amplification in a Nd:glass power chain”, Opt. Lett. 16 (4), 238 (1991)
[7]A. Dubietis et al., “Powerful femtosecond pulse generation by chirped and stretched pulse parametric amplification in BBO crystal”, Opt. Commun. 88, 433 (1992)
[8]C. Horvath et al., “Compact directly diode-pumped femtosecond Nd:glass chirped-pulse-amplification laser system”, Opt. Lett. 22 (23), 1790 (1997)
[9]M. D. Perry et al., “Petawatt laser pulses”, Opt. Lett. 24 (3), 160 (1999)
[10]A. Galvanauskas, “Mode-scalable fiber-based chirped pulse amplification systems”, IEEE J. Sel. Top. Quantum Electron. 7 (4), 504 (2001)
[11]J. Limpert et al., “High average power femtosecond fiber CPA system”, Opt. Lett. 28 (20), 1984 (2003)
[12]F. Röser et al., “131 W 220 fs fiber laser system”, Opt. Lett. 30 (20), 2754 (2005)
[13]K. Kim et al., “eXtreme chirped pulse amplification-beyond the fundamental energy storage limit of semiconductor opt...”, IEEE J. Sel. Top. Quantum Electron. (2), ()
[14]J. Limpert et al., “High-power ultrafast fiber laser systems”, IEEE J. Sel. Top. Quantum Electron. 12 (2), (233)
[15]K.-H. Liao, “Large-aperture chirped volume Bragg grating based fiber CPA system”, Opt. Express 15 (8), 4876 (2007)
[16]F. Tavella et al., “Dispersion management for a sub-10-fs, 10 TW optical parametric chirped-pulse amplifier”, Opt. Lett. 32 (15), 2227 (2007)
[17]F. Röser et al., “Millijoule pulse energy high repetition rate femtosecond fiber chirped-pulse amplification system”, Opt. Lett. 32 (24), 3495 (2007)
[18]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)
[19]A. Amani Eilanlou et al., “Direct amplification of terawatt sub-10-fs pulses in a CPA system of Ti:sapphire laser”, Opt. Express 16 (17), 13431 (2008)
[20]T. Eidam et al., “Femtosecond fiber CPA system emitting 830 W average output power”, Opt. Lett. 35 (2), 94 (2010)
[21]L. G. Li et al., “Distortionless large-ratio stretcher for ultra-short pulses using photonic crystal fiber”, Opt. Express 18 (12), 12341 (2010)
[22]M. Y. Shverdin et al., “Chirped-pulse amplification with narrowband pulses”, Opt. Lett. 35 (14), 2478 (2010)
[23]T. Eidam et al., “Fiber chirped-pulse amplification system emitting 3.8 GW peak power”, Opt. Express 19 (1), 255 (2011)
[24]J. M. Mikhailova et al., “Ultra-high-contrast few-cycle pulses for multipetawatt-class laser technology”, Opt. Lett. 36 (16), 3145 (2011)
[25]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)
[26]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)
[27]R. Paschotta, case study on a chirped-pulse fiber amplifier
[28]R. Paschotta, tutorial on "Fiber Amplifiers", part 8 on ultrafast amplifiers

(Suggest additional literature!)

See also: amplifiers, dispersion, nonlinearities, nonlinear pulse distortion, laser-induced damage, ultrashort pulses, pulse compression, parabolic pulses, regenerative amplifiers, divided-pulse amplification


Dr. R. Paschotta

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) and software could become very valuable for your business!

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