Ultrashort Laser Pulse Phenomena, 2nd Edition

450 Pulse Shaping

These losses are mainly associated with the coupling of the pulses into the fiber

and diffraction at the gratings. The overall transmission in the experiment cited

was about 7%.

8.1.3. Pulse Compression Using Bulk Materials

One drawback of fiber compressors is that only pulses of relatively small

energy can be handled. With fs input pulses the possible energies do not exceed

several tens of nJ. One possible solution is to spectrally broaden the pulse in

the fiber before amplification and subsequent compression as demonstrated by

Damm et al. [31] with pulses from a Nd:glass laser.

Previous attempts to use bulk materials for the chirping of high energy ps

pulses resulted in a relatively poor pulse quality at the compressor output owing

to the nonlinear chirp behavior. As discussed before, to obtain an almost linear

chirp across the main part of the pulse, a certain ratio of L

D

and L

NL

is necessary.

This could be achieved by utilizing pulse propagation in single-mode fibers.

The fiber lengths needed become smaller with shorter durations of the input

pulse and, for fs pulses, are of the order of several millimeters. From Eq. (8.30)

it can easily be seen that L

opt

∝ τ

p0

if the peak intensity is kept constant. Over

such propagation distances suitably focused Gaussian beams do not change their

beam diameter much in bulk materials provided self-focusing can be neglected.

The latter limits the possible propagation lengths to those shorter than the self-

focusing length. The latter in turn can be adjusted by choosing an appropriate

spot size of the focused beam, which sets an upper limit for the maximum pulse

intensity. The maximum compression factor that can be achieved under such

conditions can be estimated to be:

K

c

≈ 0. 3

-

n

0

¯n

2

P

0

λ

2



(8.38)

where P

0

is the peak power of the input pulse (cf. [32]). Figure 8.10 shows results

of a numerical evaluation of pulse compression using bulk SQ1 fused silica, and

60 fs input pulses of various energies.

Rolland and Corkum [33] demonstrated experimentally the compression of

high-power fs light pulses in bulk materials. Starting from 500 µJ, 92 fs pulses

from a dye amplifier, they obtained ≈20 fs compressed pulses at an energy of

≈100 µJ. The nonlinear sample was a 1. 2 cm piece of quartz and the pulses

were focused to a beam waist of 0. 7 mm.

At high intensities a white light continuum pulse can be generated [34] as

was discussed in Section 3.7. With fs pulses, SPM is expected to contribute