Page 343 - High Power Laser Handbook

312 So l i d - S t at e La s e r s Ultrafast Solid-State Lasers 313

designed system, the entire optical system’s net dispersion, including
the stretcher, the amplifier components, and the compressor, is
designed to be as nearly zero as possible. Typically, the pulse com-
pressor consists of a pair of diffraction gratings (Fig. 12.4) or an
equivalent configuration. In some past work, prisms, or a combina-
tion of prisms and chirped mirrors, have been used for the compres-
26
sion process. The use of prisms, rather than gratings, has also been
employed to avoid some of these limitations. However, prisms do not
avoid spatial dispersion effects; furthermore, prisms typically need to
use specially designed mirrors to compensate for residual higher-or-
27
der dispersion.
With downchirped pulse amplification (DPA), the pulse is
stretched using negative dispersion. The pulse injected into the ampli-
fier is thus negatively chirped—in other words, the blue colors come
first in the pulse, and the red colors come later. This pulse stretching
can be accomplished using a grating or prism pair, which is the same
type of negative-dispersion element that is normally employed for
recompressing the pulse. Other possible optical elements that might
be included are specially designed mirrors, which compensate for dis-
persion or which correct for high-order dispersion errors introduced
by other optical elements, or pulse shapers, which use adaptive-optics
devices to adjust pulse dispersion in either a predetermined or a pro-
grammable manner. The use of grisms (grating-prism combinations)
has also been successfully made (as is discussed later in this section).
Compression of the optical pulses after amplification is accom-
plished using positive dispersion. Perhaps the most advantageous way
of doing this is by using material dispersion, or propagating the pulse
through a block of glass or other transparent material. Other devices,
such as the positive-dispersion grating arrangement used for pulse
stretchers in CPA systems, could also be used. However, the use of a
simple, transparent optical element has a number of significant advan-
tages over past pulse compressor designs. First, a transparent material
can be virtually lossless, thus avoiding the 30 to 50 percent loss in aver-
age power typical of a grating pulse compressor. Furthermore, it also
helps avoid thermal distortion effects. Second, a simple block of glass
is alignment-insensitive, making alignment of the pulse compressor, as
well as accurate dispersion compensation, much simpler to obtain.
Unlike conventional CPA, the fully compressed femtosecond-
duration pulse will emerge from a material, such as a block of glass or
similar, that compresses the pulse. Thus, the possibility exists for
nonlinear distortion of the pulse due to Kerr self-phase modulation,
17
or the B integral [Eq. (12.9)]. However, this problem is not funda-
mental and unavoidable. After amplification, the pulse beam will
typically be expanded to a larger physical cross section. By expand-
ing the beam, the peak power inside the compressor can be kept low
enough to avoid nonlinear distortions. The necessity for expanding
the beam is not a major disadvantage over conventional CPA, because