A regenerative amplifier is a device which is used for strong amplification of light pulses, usually with ultrashort pulse durations in the picosecond or femtosecond domain (→ ultrafast amplifiers). Multiple passes through the laser gain medium (nearly always a solid-state medium) are achieved by placing the gain medium in an optical resonator, together with an optical switch, which is usually realized with an electro-optic modulator and a polarizer. The number of round trips in the resonator can be controlled with the optical switch, and it can be very large, so that a very high overall amplification factor (gain) is achieved.
Note that although a pulse makes multiple passes in a regenerative amplifier, the term multipass amplifier is often reserved for a device where a fixed number of passes is obtained – not with an optical switch, but just according to the geometric beam path. It would be more natural, though, to consider a regenerative amplifier as one type of multipass amplifier.
Operating Principle of a Regenerative Amplifier
The principle of operation can be understood as follows:
- First, the gain medium is pumped for some time, so that it accumulates some energy.
- Then, the input light pulse is injected into the resonator through a port which is opened for a short time (shorter than the round-trip time) with an electro-optic (or sometimes acousto-optic) switch.
- After that, the pulse can undergo many (typically tens or hundreds) of resonator round trips, being amplified to a high energy level. Typically, one will let the pulse energy grow until a substantial part of the energy is extracted from the gain medium.
- Finally, the pulse is released from the resonator. That can be achieved either with a second electro-optic switch, or with the same one as used for coupling in.
This operation principle makes it possible to achieve very high gain and thus pulse energies in the millijoule range with amplifiers of moderate size, or even higher energies with larger devices. Typical pulse repetition rates are of the order of 1 kHz, although repetition rates of hundreds of kHz are sometimes possible. The highest pulse energies are achieved at lower repetition rates.
Note that the seed laser, normally a mode-locked laser, may have a much higher pulse repetition rate of e.g. 80 MHz; only a small number of the generated seed pulses is used by the amplifier. A separate pulse picker is in principle not required, since the regenerative amplifier itself can “pick” an input pulse when required, but one may want to improve the suppression of additional pulses.
A common gain medium for regenerative amplifiers is Ti:sapphire, which has a broad gain bandwidth and high thermal conductivity. Other gain media, e.g. doped with ytterbium or neodymium, are suitable for diode pumping and have a good energy storage capability due to their long upper-state lifetimes, but the generated pulses are longer (typically hundreds of femtoseconds or more) due to the smaller gain bandwidth.
For high repetition rates (e.g. tens of kilohertz or higher), the amplifier gain medium is usually pumped continuously. For lower repetition rates (particularly for pulse periods well above the upper-state lifetime), pulsed pumping (e.g. with a Q-switched laser) is preferable. A common configuration is to use a frequency-doubled YAG laser to pump a regenerative Ti:sapphire amplifier.
Applications of regenerative amplifiers include laser material processing (e.g. cutting of metals with millijoule pulse energies) and scientific experiments e.g. in high-intensity physics, such as high harmonic generation.
Fast and Accurate Switching
Pulse injection and ejection require the use of a Pockels cell and a Pockels cell driver with a short switching time (well below the resonator round-trip time) and precision. These requirements are substantially more challenging than those for simple Q switching of a laser, for example.
Limited Repetition Rates
The possible output pulse repetition rate may be limited by different factors:
- Frequently, one requires an output pulse energy as high as possible, and needs to have a long pumping time between the application cycles – often of the order of the inverse upper-state lifetime of the gain medium. In extreme cases (with intense pulsed pumping), the repetition rate needs to be further decreased because cooling of the gain medium would otherwise be difficult.
- If low pulse energies are sufficient, one can realize far higher pulse repetition rates. A possible limit then arises from the time for the required number of resonator round trips. However, the round-trip time can be limited to a couple of nanoseconds, so that hundreds of round trips can be done within a microsecond, allowing for a repetition rate of the order of 1 MHz, or more with fewer round trips.
The duration of the amplified pulses may be increased due to the limited gain bandwidth of the amplifier medium, which reduces the pulse bandwidth. This so-called gain narrowing effect may be reduced by inserting a properly designed spectral filter in the amplifier, which makes the net gain spectrum wider and more flat, but reduces the power conversion efficiency.
Chromatic Dispersion and Nonlinearities
At least in femtosecond devices, the chromatic dispersion introduced by the Pockels cell (in addition to that from the gain medium and other components) has to be carefully compensated. Nonlinear effects may also occur; these can be effectively suppressed with the methods of chirped-pulse amplification or divided-pulse amplification. If this is not done, the obtainable pulse energy may be limited by nonlinear effects or even optical damage phenomena.
Influences of Losses
The power conversion efficiency of a regenerative amplifier can be severely reduced by the effect of intracavity losses (particularly in the electro-optic switch). Note that the sensitivity to such losses is particularly high in cases with low round-trip gain, because this increases the number of resonator round trips required to achieve a certain overall amplification factor.
Finite Lower-state Lifetime
Regenerative amplifiers can also have a reduced gain and power efficiency due to a finite lower-state lifetime, leading to a significant population of the lower laser level during amplification of a pulse, and thus to reabsorption on the laser transition. That problem occurs primarily in amplifiers for very short pulses, but not with chirped-pulse amplification, where the pulse duration within the amplifier is relatively large.
For some applications, it is important to keep the energy of any satellite pulses very low, i.e. to achieve a high pulse contrast. Such satellite pulses can occur in various ways, e.g. via imperfect switching of the Pockels cell or parasitic reflections in the amplifier. An additional pulse picker may be used to suppress pre- and post-pulses.
For high pulse repetition rates, it can happen that the pulse amplification process exhibits strong fluctuations. Bifurcations with period doubling can also occur, where the pulse energy first alternates between two values, later four or even eight values. For further increased repetition rates, this can lead into deterministic chaos [8, 9, 12]. Fig 2 shows a numerical example case.
Alternatives to Regenerative Amplifiers
A possible alternative to a regenerative amplifier is a multipass amplifier, where multiple passes (each time with a slightly different propagation direction) are arranged with a set of mirrors. That approach does not require a fast modulator, but becomes complicated (also in terms of alignment) if the number of passes through the gain medium is high.
As a rule of thumb, a multipass amplifier may be more appropriate for very high pulse energies if a limited gain is required (because the input pulse energy is already relatively high), whereas a regenerative amplifier is more suitable for very high gains.
Another technique for reaching high pulse energies, although not nearly as high as with regenerative amplifiers, is cavity dumping of a mode-locked laser. That technique is particularly suitable for very high pulse repetition rates of several megahertz.
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See also: multipass amplifiers, ultrafast amplifiers, chirped-pulse amplification, ultrashort pulses, titanium–sapphire lasers, cavity dumping, electro-optic modulators, Pockels cells, Pockels cell drivers, lower-state lifetime, The Photonics Spotlight 2008-06-20
and other articles in the categories optical amplifiers, light pulses
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