Page 208 - High Power Laser Handbook

176 So l i d - S t at e La s e r s Intr oduction to h igh-Power Solid-State Lasers 177

considerations underlying the trade between oscillators and amplifi-
ers for high-power lasers.

7.4.1 Stable Resonators
Stable resonators are geometrically stable in the sense that they con-
fine a cone of rays upon reflection between two curved mirrors. This
allows a near-planar wavefront to build up during laser oscillation,
providing a simple, robust means of generating good beam quality.
Stable resonators are typically configured to support only a single,
TEM (Gaussian) mode via selective gain competition against higher-
00
order modes. The TEM mode experiences higher net round-trip
00
gain through improved geometric overlap with the pumped gain
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volume or lower clipping losses from intracavity apertures.
Stable resonator ray confinement naturally leads to tightly
focused spots within the cavity, with spot dimensions determined by
diffraction and typically on the order of ∼(λL) 1/2 = 1 mm for 1-µm
wavelengths and cavity lengths L ∼ 1 m. With such small beam sizes,
the resulting high intensity allows easy saturation and efficient extrac-
tion of the gain material. Their simplicity and robustness allow stable
resonators to form the cornerstone of most low- to moderate-power
SSLs. However, they are poorly suited for generating good beam
quality from high-power SSLs with large gain apertures, because the
fundamental stable mode cannot be easily scaled to diameters beyond
the order of a few millimeters without impractically long cavity
lengths or alignment sensitivities. Nevertheless, for applications
where multimode output is acceptable, the high circulating power
achievable in a high-Q stable resonator enables efficient extraction of
low-gain materials, such as Yb:YAG, or low-gain extraction geome-
tries, such as thin disks.

7.4.2 Unstable Resonators
When the output power from SSLs grows to the point at which ther-
mal or damage limits become prohibitive for millimeter-class spots,
another extraction geometry must be adopted. Unstable resonators
are often employed for high-power SSLs, because they allow very
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large mode areas with excellent BQ. Instead of supporting cavity
modes whose size is determined by diffraction, unstable resonator
modes are not geometrically confined. Laser oscillation initially
builds up within a Fresnel core of diameter ~(λL) 1/2 , in which dif-
fractive beam spreading dominates the cavity mirror curvatures
(Fig. 7.8). The mirror curvatures are chosen to magnify the beam by
a factor of M upon each round trip, so that beam sizes are constrained
only by the limiting aperture of the primary mirror or the intracavity
gain element. The final beam is outcoupled either by spreading past
the clear aperture of the secondary mirror or by using a larger sec-
ondary mirror with spatially varying reflectivity that tapers to zero