Page 29 - Tunable Lasers Handbook

12 F. J. Duarte

where the output is coupled via the reflection losses of the grating, at a cost of
higher amplified spontaneous emission (ASE) levels [2,18,24]. In addition to the
information given in Table 1, this class of oscillator design has also been applied
to optical parametric oscihtors [32] (see Chapter 6).
Class I1 oscillators incorporating multiple-prism beam expanders are, in
general, more efficient than pure grazing-incidence designs but they are also
more complex. In Fig. 2, MPL oscillators using multiple-prism beam expanders
deployed in (+,+,+,-) and (+,-,+,-) configurations are illustrated. In a (+,+,+,-)
configuration, the first three prisms are deployed in an additive configuration
with the fourth prism deployed in a compensating mode to neutralize the cumu-
lative dispersion of the first three prisms. In a (+,-,+,-) configuration, two pairs
of compensating prisms are utilized [ 1,2]. These configurations are used to yield
zero dispersion at a wavelength of design thus reducing beam deviations due to
(an/&") factors and leaving the tuning characteristics of the oscillator dependent
on the grating. Extensive details on multiple-prism design have been given by
Duarte [l] and relevant mathematical formulas are given in a later section on
intracavity dispersion. The main design constraint is to provide the necessary
beam expansion to achieve total illumination of the grating at a maximum trans-
mission efficiency and a minimum intracavity length.
The intrinsic intracavity dispersion of a grazing-incidence grating design is
higher than the dispersion achieved by an MPL grating configuration. A configu-
ration that provides higher intracavity dispersion than MPL designs and higher
conversion efficiency than pure grazing-incidence cavities is the HMPGI grating
cavity mentioned earlier [20,24] (Fig. 2c). In HMPGI oscillators the grating is
deployed in a near grazing-incidence configuration that is far more efficient than
a pure grazing-incidence configuration [24] (see Section 9). Further, because the
required intracavity beam expansion is far less than that typical of MPL oscilla-
tors, efficient and compact multiple-prism expanders can be readily designed to
provide the necessary intracavity preexpansion. Today, HMPGI oscillators are
widely used in research and commercial tunable laser systems.
Improved oscillator designs use a polarizer output coupler rather than a tra-
ditional mirror as the output coupler [23,33] (see Fig. 2). The output-coupler
polarizer is made of a Glan-Thompson polarizer with an antireflection-coated
inner surface and an outer surface that is coated for partial reflection. Dispersive
oscillators incorporating multiple-prism grating assemblies yield strongly p-
polarized narrow-linewidth emission [1,2,20]. In this context, the function of the
output-coupler polarizer is to suppress single-pass unpolarized ASE in high-gain
lasers. Thus, the use of a polarizer output coupler in dispersive dye laser oscilla-
tors has yielded extremely low levels of ASE in the 10-7 to 10-9 range [22,23].
The Glan-Thompson polarizer output coupler is illustrated in Fig. 3.