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Section 14:
Highly-Dispersive
Mirrors
Detailed descriptions of dispersion and how it signifi cantly aff ects ultrafast
laser systems can be found in Section 8: Characteristics of Laser
Grade Substrates from pages 24-28 and Section 13: Ultrafast Dispersion
from pages 40-41. The positive chirp, or dispersion, introduced by most
optical media in ultrafast systems can be compensated for using optical
components, such as highly-dispersive mirrors, that feature negative dispersion.
This gives short wavelengths a higher phase velocity than that
of long wavelengths, which balances out the positive chirp and compresses
the pulse duration (Figure 14.1).
Section 14.1:
Earlier Methods of
Dispersion Compensation
Several diff erent types of pulse compressing optics were traditionally
used in ultrafast systems before the introduction of highly-dispersive
mirrors including prisms, gratings, Gires-Tournois interferometer (GTI)
mirrors, and chirped mirrors.
Dispersive Prisms and Gratings
Several prisms and gratings with negative dispersion can be introduced
into ultrafast laser systems to balance out positive chirp and compress
pulse duration (Figure 14.2). Dispersive prisms tend to be bulky setups
that take up a lot of space. Both dispersive prisms and gratings also typically
have low throughput, high sensitivity to alignment and separation
distance, and a large amount of third-order dispersion, which increases
temporal broadening.
Gires-Tournois Interferometer Mirrors
It is necessary to understand Gires-Tournois interferometer (GTI) mirrors
to understand how highly-dispersive mirrors function. Gires-Tournois
interferometers are standing-wave resonators which use a highly-refl
ective GTI mirror to generate chromatic dispersion. The phase of light
refl ected by GTI mirrors is wavelength-dependent due to resonance in
the mirror’s coating, allowing GTI mirrors to provide angle-dependent
negative GDD for ultrafast intracavity dispersion control. However, GTI
mirrors introduce some high-order dispersion and they only provide
negative GDD over a limited wavelength range.
Ultrafast Laser
Pulse Compressing Mirrors
Optical Medium
Time
(Objective, AOM, Window, Lens, etc.)
Irradiance
Time
Irradiance
Pulse Compressing
Mirrors
Time
Irradiance
Ultrafast Laser
Pulse Compressing Prisms
Irradiance
Optical Medium
(Window, Lens, etc.)
Time
Irradiance
Time
Time
Irradiance
Figure 14.1: Pulse compressing mirrors (top) and prisms (bottom) introducing
negative dispersion to cancel out the positive dispersion experienced
by ultrafast laser pulses as they transmit through optical media
Figure 14.2: The variable layer thickness of chirped mirrors causes
negative dispersion