Ultrafast Amplifiers

Definition: amplifiers for ultrashort optical pulses

Ultrafast amplifiers are optical amplifiers which are applied to ultrashort pulses. Some of these devices are used for amplifying high repetition rate pulse trains, leading to a high average power whereas the pulse energy remains moderate. In other cases, a much higher gain is applied to pulses at lower repetition rates, leading to high pulse energies and correspondingly huge peak powers. Enormously high optical intensities, sometimes above 10¹⁶ W/cm², are achieved by focusing such intense pulses tightly on some target.

As an example, consider the output of some mode-locked laser, consisting of 100-fs pulses with a repetition rate of 100 MHz and an average power of 0.1 W. The pulse energy is then 0.1 W / 100 MHz = 1 nJ, and the peak power is somewhat below 10 kW (depending on the pulse shape). A high-power amplifier, applied to the full pulse repetition rate, may raise the average power to 10 W, thus increasing the pulse energy to 100 nJ. In addition, a pulse picker before the amplifier may be used to reduce the pulse repetition rate to 1 kHz. If the high-power amplifier can still raise the average power to 10 W, the pulse energy is now 10 mJ, and the peak power nearly 100 GW.

Special Requirements for Ultrafast Amplifiers

Beyond the general technical issues of optical amplifiers, ultrafast devices meet a number of additional challenges:

• Particularly for high-energy systems, the amplifier gain must be huge. In the example discussed above, a gain as high as 70 dB is needed. As single-pass amplifiers are limited in terms of gain, multipass arrangements are frequently used. Very high gain is possible with regenerative amplifiers. Also, it is common to use multiple amplifier stages (→ amplifier chains), where the first stages provide a high gain while the last ones are optimized for high pulse energies and efficient energy extraction.
• A high gain also implies a high sensitivity against any back-reflected light (except for regenerative amplifiers), and a tendency for emitting amplified spontaneous emission (ASE). To some extent, ASE in laser amplifiers can be suppressed with optical switches (such as acousto-optic modulators) between the amplifier stages. These switches are opened only for short time intervals around the peaks of the amplified pulses. However, these time intervals are usually still long compared with the pulse duration, so that it is not possible to suppress some ASE background around the pulses. Optical parametric amplifiers are better in this respect, as they provide gain only during the passage of the pump pulse. Also, there is no amplification for light propagating in the backward direction.
• Ultrashort pulses have some significant optical bandwidth, which can be reduced by the effect of gain narrowing in an amplifier, so that the amplified pulses become longer. For pulse durations below a few tens of femtoseconds, ultrabroadband amplifiers are required. Note that gain narrowing is particularly important in high-gain systems.
• Particularly for systems with high pulse energies, various optical nonlinearities can distort the temporal and spectral pulse shape, or may even lead to damage of the amplifier via self-focusing. An effective technique for mitigating such effects is chirped-pulse amplification (CPA), where the pulses are first dispersively stretched to e.g. 1 ns duration, then amplified, and finally dispersively recompressed. A novel and not yet common alternative is divided-pulse amplification. Another important measure is to increase the mode areas in the amplifiers so as to reduce the optical intensities.
• For a single-pass amplifier, efficient energy extraction is possible only if the pulse duration is long enough to reach a pulse fluence of the order of the saturation fluence without causing strong nonlinear effects.

Examples of Possible Performance Figures

The requirements on ultrafast amplifiers in terms of pulse energy, pulse duration, repetition rate, mean wavelength, etc., are very diverse. Correspondingly, very different types of devices are used. The following list gives some typical examples of the performance reached by different types of systems:

• An ytterbium-doped fiber amplifier may amplify a 100-MHz train of 10-ps pulses to an average power of 10 W. (A system with such output performance is sometimes called an ultrafast fiber laser, although it is actually a master oscillator power amplifier device.) The peak power of roughly 10 kW can be relatively easily handled with a large mode area amplifier fiber. For femtosecond pulses, however, such a system would exhibit severe nonlinear effects. Starting with femtosecond pulses and applying chirped-pulse amplification, several microjoules can easily be reached, or in extreme cases > 1 mJ. An alternative is the amplification of parabolic pulses in a fiber with normal dispersion, and subsequent dispersive compression.
• A multipass bulk amplifier, based e.g. on Ti:sapphire and realized with a large mode area, may provide an output energy of the order of 1 J at a low repetition rate of e.g. 10 Hz. Strong pulse stretching to a duration of e.g. a few nanoseconds is necessary in order to limit nonlinear effects. After compression to e.g. 20 fs, the peak power is tens of terawatts (TW); the most advanced large systems already reach peak powers above 1 PW, i.e. in the petawatt domain. Smaller systems generate e.g. 1-mJ pulses at 10 kHz. The gain of a multipass amplifier is usually of the order of 10 dB.
• A much higher gain of several tens of decibels is possible with a regenerative amplifier. Made with Ti:sapphire, e.g., such a device may amplify 1-nJ pulses to 1 mJ. Again, chirped-pulse amplification has to be applied in order to limit nonlinear effects.
• Using a regenerative amplifier based on an Yb-doped thin-disk laser head, pulses with durations somewhat below 1 ps can be amplified to hundreds of microjoules without applying CPA.
• An optical parametric amplifier, pumped with nanosecond pulses from a Q-switched laser, can amplify stretched pulses to energies of several millijoules. A high gain of tens of decibels can be achieved in a single pass. For special phase-matching configurations, the gain bandwidth can be very large, so that very short pulse durations are possible after dispersive compression.

The performance figures of commercial ultrafast amplifier systems often greatly lag behind the best performance values achieved in scientific experiments. In many cases, a main reason is that experimental systems rely on techniques or components which cannot be used in commercial devices due to a lack of stability and robustness. For example, complex fiber-based systems have been demonstrated which contain multiple transitions between fibers and free-space optics. All-fiber amplifier systems can be made, but these do not reach the performance levels of systems containing bulk-optical elements. In other cases, optical components are operated close to their damage threshold; for commercial devices, a larger safety margin is required. Another problem can be the use of very special parts, which are difficult to procure.

Applications

Ultrafast amplifiers find diverse applications:

• Many devices are used in fundamental research. They can provide intense pulses for highly nonlinear processes such as high harmonic generation, or for the acceleration of particles to high energies.
• Large ultrafast amplifiers are under consideration for the use in laser-induced nuclear fusion (inertial confinement fusion, fast ignition).
• Millijoule pulse energies in picosecond or femtosecond pulses are interesting for micromachining. Short pulse durations make it possible to achieve very precise cutting of thin metal sheets, for example.

Industrial applications are often made difficult by the complexity and cost of ultrafast amplifier systems, sometimes also by a lack of robustness. Further technological progress may improve the situation.

Bibliography

See also: amplifiers, multipass amplifiers, regenerative amplifiers, chirped-pulse amplification, divided-pulse amplification, ultrafast lasers, Spotlight article 2008-06-20

Categories: amplifiers, pulses