Definition: An amplifier that amplifies ultrashort optical pulses.
Ultrafast amplifiers are optical amplifiers used to amplify ultrashort pulses. Some ultrafast amplifiers are used to amplify high repetition rate pulse trains to get very high average power while the pulse energy is still at moderate levels, in other cases lower repetition rate pulses get more gain and get very high pulses energy and relatively large peak power. When these intense pulses are focused on some targets, very high light intensities are obtained, sometimes even greater than 1016 W/cm2.
As an example, consider the output of a mode-locked laser with a pulse repetition rate of 100 MHz, a length of 100 fs, and an average power of 0.1 W. So the pulse energy is 0.1W/100MHz=1nJ, and the peak power is less than 10kW (related to the pulse shape). A high power amplifier, acting on the entire pulse, can increase its average power to 10W, thus increasing the pulse energy to 100nJ. Alternatively, a pulse pickup can be used before the amplifier to reduce the pulse repetition rate to 1 kHz. If the high-power amplifier still increases the average power to 10W, then the pulse energy is 10mJ at this time, and the peak power can reach 100GW.
Special requirements for ultrafast amplifiers:
In addition to the usual technical details of optical amplifiers, ultrafast devices face additional problems:
Especially for high energy systems, the gain of the amplifier must be very large. In the ions discussed above, a gain of up to 70dB is required. Since single-pass amplifiers are limited in gain, multi-channel operation is usually employed. Very high gains can be achieved with positive feedback amplifiers. In addition, multi-stage amplifiers (amplifier chains) are often employed, where the first stage provides high gain and the last stage is optimized for high pulse energy and efficient energy extraction.
High gain also generally means more sensitivity to back-reflected light (with the exception of positive feedback amplifiers) and a greater tendency to produce amplified spontaneous emission (ASE). To a certain extent, ASE can be suppressed by placing an optical switch (acousto-optical modulator) between the two stages of amplifiers. These switches only open for very short time intervals around the peak of the amplified pulse. However, this time interval is still long compared to the pulse length, so suppressing the ASE background noise near the pulse is unlikely. Optical parametric amplifiers perform better in this regard because they only provide gain when the pump pulse is passed through. Backpropagating light is not amplified.
Ultrashort pulses have significant bandwidth, which can be reduced by the gain-narrowing effect in the amplifier, thus resulting in longer amplified pulse lengths. When the pulse length is less than tens of femtoseconds, an ultra-wideband amplifier is required. Gain narrowing is especially important in high gain systems.
Especially for systems with high pulse energies, various nonlinear effects can distort the temporal and spatial shape of the pulse, and even damage the amplifier due to self-focusing effects. An effective way to suppress this effect is to use a chirped pulse amplifier (CPA), where the pulse is first dispersion broadened to a length of, for example, 1 ns, then amplified, and finally dispersion compressed. Another less common alternative is to use a sub-pulse amplifier. Another important method is to increase the mode area of the amplifier to reduce the light intensity.
For single-pass amplifiers, efficient energy extraction is only possible if the pulse length is long enough to allow the pulse flux to reach saturation flux levels without causing strong nonlinear effects.
The different requirements for ultrafast amplifiers are reflected in differences in pulse energy, pulse length, repetition rate, average wavelength, etc. Accordingly, different devices need to be adopted. Below are some typical performance metrics obtained for different types of systems:
The ytterbium-doped fiber amplifier can amplify the pulse train of 10ps at 100MHz to an average power of 10W. (A system with this capability is sometimes referred to as an ultrafast fiber laser, even though it is actually a master oscillator power amplifier device.) Peak powers of 10 kW are relatively easy to achieve using fiber amplifiers with large mode areas. But with femtosecond pulses, such a system would have very strong nonlinear effects. Starting with femtosecond pulses, followed by chirped pulse amplification, energies of a few microjoules can easily be obtained, or in extreme cases greater than 1 mJ. An alternative approach is to amplify a parabolic pulse in a fiber with normal dispersion, followed by dispersion compression of the pulse.
A multi-pass bulk amplifier, such as a Ti:Sapphire-based amplifier, can provide a large mode area, resulting in output energies on the order of 1 J, with relatively low pulse repetition rates, such as 10 Hz. Pulse stretching by a few nanoseconds is necessary to suppress nonlinear effects. Later compressed to say 20fs, the peak power can reach tens of terawatts (TW); the most advanced large systems can achieve peak power greater than 1PW, which is on the order of picowatts. Smaller systems, for example, can generate 1 mJ pulses at 10 kHz. The gain of a multipass amplifier is usually on the order of 10dB.
A high gain of tens of dB can be obtained in a positive feedback amplifier. For example, a 1 nJ pulse can be amplified to 1 mJ using a Ti:Sapphire positive feedback amplifier. In addition, a chirped pulse amplifier is required to suppress nonlinear effects.
Using a positive feedback amplifier based on an ytterbium-doped thin-disk laser head, pulses less than 1 ps in length can be amplified to several hundred microjoules without the need for CPA.
Fiber parametric amplifiers pumped with nanosecond pulses generated by Q-switched lasers can amplify the stretched pulse energy to several millijoules. High gain of several decibels can be achieved in single-channel operation. For special phase matching structures, the gain bandwidth is very large, so a very short pulse can be obtained after dispersion compression.
The performance specifications of commercial ultrafast amplifier systems are often well below the best performance obtained in scientific experiments. In many cases, the main reason is that the devices and techniques employed in the experiments often cannot be applied to commercial devices due to their lack of stability and robustness. For example, complex optical fiber systems contain multiple transition processes between optical fibers and free-space optics. All-fiber amplifier systems can be constructed, but these systems do not achieve the performance of systems employing bulk optics. There are other cases where optics operate near their damage thresholds; however, for commercial devices, higher safety assurances are required. Another problem is that some special materials are required, which are very difficult to obtain.
Application:
Ultrafast amplifiers have many applications:
Many devices are used for basic research. They can provide strong pulses for strong nonlinear processes, such as high-order harmonic generation, or to accelerate particles to very high energies.
Large ultrafast amplifiers are used in research for laser-induced fusion (inertial confinement fusion, fast ignition).
Picosecond or femtosecond pulses with energies in millijoules are beneficial in precision machining. For example, very short pulses allow very fine and accurate cutting of thin metal sheets.
Ultrafast amplifier systems are difficult to implement in industry due to their complexity and high price, and sometimes because of their lack of robustness. In this case, more technologically advanced developments are needed to improve the situation.