Page 173 - High Power Laser Handbook

142 Diode Lasers High-Power Diode Laser Arrays 143

they suffer from poor beam quality. Although the beam quality in the
2
fast axis (assuming no bar smile) is diffraction limited (M < 1.2), the
beam quality in the slow axis is poor. For example, an industry stan-
dard of an 808-nm, 19-emitter bar with a 150-mm emitter width on a
500-mm pitch and a divergence angle of 6 degrees (90 percent power)
2
has an M of about 800. The degradation of beam quality is attributed
to three factors: First is the large emitter width, which is needed to
deliver the high power per emitter. Second is the emitter count in the
diode laser bar. And third is the fill factor (30 percent, in this example).
The emitter widths can be decreased but not by a large amount,
because in high-power laser bars, the goal is to maximize power per
emitter. As a result, the only two variables that can be optimized to
improve beam quality are the emitter count and the fill factor. A lower
emitter count and a lower fill factor laser bar improve the beam quality.
The lower fill factor assumes that the nonemitting areas between
emitters are filled after slow-axis collimation in order to recover beam
quality. Another variable that is often used to improve beam quality
by reducing the slow-axis divergence is the cavity length—a longer
cavity length can reduce slow-axis divergence, while at the same time
increasing power per emitter.
Low fill-factor bars, with emitter counts in the range of 5 to 10 and
fill factors of about 10 percent with powers approaching 10 W per
emitter in CW mode, are emerging as the preferred architecture for
high-brightness applications. The low fill-factor bars aim to capture the
beam quality of a single emitter, while delivering the power of a laser
bar. For example, an 808-nm, 10 percent fill-factor bar with an emitter
width of 100 mm and 10 emitters with a slow-axis divergence of
2
6 degrees (90 percent power) has a slow-axis M of about 800. However,
after slow-axis collimation (i.e., filling of the nonemitting areas), the M
2
value drops to 80, whereas the standard bar after slow-axis collimation
2
has an M equal to 240. For the same power output, the brightness of a
low fill-factor bar is ~3 times higher than the standard bar.
Other techniques for improving beam quality and brightness are
described further in Sec. 6.6.

6.5.3 Wavelength Locking
High-power diode lasers are multimode lasers; therefore, their spec-
tral brightness is low. Although the centroid wavelength can be tuned
fairly accurately at any given temperature, the FWHM (full width half
maximum) is approximately 3 nm, and the FW 1/e (full width at 1/e
2
2
of the maximum) is approximately 5 nm. Furthermore, the wavelength–
temperature coefficient for these lasers is around 0.3 nm/°C. For
some applications, this broad bandwidth and sensitivity to tempera-
ture create operational challenges. For example, pumping of standard
ytterbium (Yb) fiber lasers in the 980-nm pump region requires a nar-
row bandwidth, due to the narrow absorption band. In some specific