Page 200 - High Power Laser Handbook

168 So l i d - S t at e La s e r s Intr oduction to h igh-Power Solid-State Lasers 169

lasers, the large emission cross section enables high-gain extraction
geometries with reduced sensitivity to optical losses and relatively
low saturation intensity I = hν/στ = 2.8 kW/cm for efficient extrac-
2
sat
tion. The corresponding low saturation fluence makes Nd:YAG
attractive for moderate energy pulse lasers, where efficiency and
damage resistance are of paramount importance. However, the high
cross section makes Nd:YAG generally ill suited for high pulse ener-
gies (> ~10 J) due to the onset of parasitics and ASE.
Yb:YAG
With the recent advent of diode pumping, Yb:YAG has emerged as an
12
attractive alternative to Nd:YAG in numerous HAP SSL architectures.
Yb:YAG’s predominant spectroscopic feature is its simple energy level
structure, with essentially only two energy levels (Fig. 7.4). These levels
are Stark-split into thermally populated manifolds, allowing energeti-
cally close pump and lasing transitions at 940 nm and 1030 nm, respec-
tively. The corresponding ~9 percent quantum defect is two to three
times smaller than for Nd:YAG, so that Yb:YAG is intrinsically high effi-
ciency, generating relatively little waste heat per emitted photon.
Yb:YAG is a quasi-three-level laser, with about 5 percent Boltzmann
population in the terminal laser level at room temperature. Hence, bulk
Yb:YAG SSLs typically exhibit rather high lasing thresholds, because
the material must first be pumped to transparency before exhibiting net
gain. Nevertheless, when operated high above threshold, Yb:YAG lasers
can be extremely efficient (c.f., Chap. 10).
–20
Yb:YAG’s low emission cross section σ = 2.2 × 10 cm leads to a
2
low gain for most CW devices, requiring careful management of opti-
cal losses and typically multiple lasing passes to fully extract the
material. Whereas Yb:YAG’s long ∼1-ms upper-state lifetime would

−1
Energy levels (cm )
2.5 11000
Yb absorption 2
F 5/2
Yb emission
Cross section (10 −20 cm 2 ) 1.5 1 10000 1030 nm
2

2000
0.5

0 940 nm
1000
900 950 1000 1050 1100
2
Wavelength (nm) F 7/2
0
Figure 7.4 Yb:YAG energy levels, absorption, and emission cross sections.