Ultrashort Laser Pulse Phenomena, 2nd Edition

416 Femtosecond Pulse Amplification

Table 7.3

Typical parameters of pump lasers for fs pulse amplifiers.

Pulse energy Duration Repetition rate λ

Laser (mJ) (ns) (Hz) (nm)

Ar

+

(cavity dumped) 10

−3

15 3 ×10

6

514

Nd:YAG

Q-switched 300 5 10 532

Regenerative amplified 2 0. 07 10

3

532

Diode pumped 0. 05 10 800 532

Nd:YLF (Q-switched) 10 400 10

4

523. 5

Copper-vapor 2 15 5000 510, 578

Excimer 100 20 10 308

Much larger spulse energies however—ranging from millijoules to the Joule

range—can be extracted, because the active volume that can be pumped is much

larger than in dye amplifiers. As compared to solid-state or liquid materials,

another advantage of excimer gases is the smaller susceptibilities associated with

undesired nonlinear effects (such as self-focusing). Unfortunately, the relatively

narrow gain bandwidth of excimers limits the shortest pulse duration that can be

amplified and the tunability.

Essential pulse parameters, such as the achievable energy range and repeti-

tion rate, that can be reached are determined by the pump laser of the amplifier.

Table 7.3 summarizes data on lasers that have successfully been used for pump-

ing fs amplifiers. Usually these pump lasers have to be synchronized to the

high repetition rate oscillators for reproducible amplification. On a nanosecond

time scale this synchronization can be achieved electronically. With picosecond

pump pulses, satisfactory synchronism requires generally that the pump pulses

for the femtosecond oscillator and amplifier be derived from a single master

oscillator.

7.5.2. Amplifier Configurations

Usually the amplifier is expected to satisfy certain requirements for the output

radiation, which can be achieved by a suitable design and choice of the compo-

nents. Table 7.4 shows some examples. Different applications of amplified pulses

have different requirements, and subsequently various amplifier configurations

have been developed. In particular, trade-off between pulse energy and repeti-

tion rate will call for a particular choice of amplifier design and pump source.

A feature common to nearly all femtosecond amplifiers is that they are terminated

by a linear optical element to recompress the pulses.

Amplifier Design 417

Table 7.4

Design requirements of a fs pulse amplifier.

Requirements Realization

(a) Clean beam profile Homogeneously inverted gain region, no self-focusing, proper

(linear) optical design

(b) High peak power Same as above, CPA

(c) High energy amplification High pump power, amplification reaches saturation level

(d) Low background ASE suppression through spatial and/or spectral filtering, filtering

through saturable absorption

(e) Certain repetition rate Repetition rate of pump, suitable gain medium

(f) No temporal broadening GVD adjustment

(g) No spectral narrowing Gain medium with broad band-width

Multistage Amplifiers

Low repetition rate systems (<500 Hz) used for high gain amplification consist

mostly of several stages traversed in sequence by the signal pulse. A typical con-

figuration is sketched in Figure 7.12. This concept, introduced by Fork et al. [27]

for the amplification of fs pulses in a dye amplifier, has the following advantages.

(a) Each stage can be adjusted separately for maximum gain, considering the par-

ticular signal pulse energy at that stage. The splitting of the pump energy among

the various stages has to be optimized, as well as the pump focalization to match

the volume to be pumped. Typically, only a few percent of the pump pulse is

tightly focused into the first stage, resulting in a gain of several thousands. More

than 50% of the pump energy is reserved for the last stage (to pump a much

larger volume) resulting in a gain factor of about 10. (b) The unavoidable ASE

can be suppressed with filters inserted between successive stages. Ideally, these

filters are linear attenuators for the ASE, but are saturated by the signal pulse.

Pump

Filter

~5% ~25% ~70%

Filter

Stage 1

Stage 2

Stage 3

Figure 7.12 Sketch of a multistage amplifier.