Page 27 - Tunable Lasers Handbook

10 F. J. Duarte
2. DISPERSIVE OSClLlATOR CONFIGURATIONS

Dispersive oscillators can be divided into two major classes [l]: Class I
oscillators use a narrow and intrinsic TEM,, intracavity beam, and Class I1
oscillators use intracavity beam expansion. Examples of Class I oscillators are
grating-mirror resonators, which incorporate intracavity etalons, and pure graz-
ing-incidence grating cavities (Fig. 1). Class I1 oscillators employ intracavity
beam expansion to magnify the original narrow TEM,, beam waist in order to
illuminate the grating completely (Fig. 2). Intracavity beam expansion can be
accomplished using multiple-prism beam expanders and two-dimensional
transmission or reflection telescopes, such as Galilean and Cassegrain tele-
scopes, respectively [1,2]. In Fig. 2, two alternative Class I1 oscillators are
illustrated: multiple-prism Littrow (MPL) grating oscillators (Figs. 2a,b) and
hybrid multiple-prism grazing-incidence (HMPGI) grating oscillators (Fig. 2c).
Table 1 lists reported performance characteristics for Class I and I1 dispersive
oscillators for gain media in the gaseous, liquid, and solid states.
Class I oscillators using intracavity etalons can yield excellent narrow-
linewidth performance [SI. The main concerns are the use of intracavity etalons
with coatings that may be susceptible to damage by high intracavity energy
fluxes. Also, broadband tuning can demand a fine degree of control on the vari-
ous intracavity elements. The pure grazing-incidence cavity offers very narrow-
linewidth emission, compactness, and excellent broadband synchronous tuning
capabilities. The main disadvantage of grazing-incidence cavities deployed in a
closed-cavity configuration (as shown in Fig. lb), is their relatively lower effi-
ciency. Higher efficiencies can be obtained in an open-cavity configuration,

Grating M
FIGURE 1 Class I oscillators. (a) Grating-mirror resonator incorporating intracavity etalons. (b)
Grazing-incidence cavity.