Page 179 - High Power Laser Handbook

148 Diode Lasers High-Power Diode Laser Arrays 149

the overall result is that the composite beam that emerges from the
2
beam shaper has a smaller value for M . In the y direction, the beam
x
size is increased, but the divergence remains approximately constant
(assuming that mirrors A and B are sufficiently parallel); hence, the
2
2
emerging beam has its M value increased. The factor by which M
y
y
is increased depends on the number of times the beam is cut in the
lateral (x) direction.
The disadvantages of this design are the different path lengths of the
individual beams and the losses due to multiple reflections (28x for a
uniform beam) on the HR coating, which is not 100 percent reflective.
Step Mirror FhG-ILT
7
The second design for a beam-shaping device also consists of reflect-
ing surfaces. A first-step mirror (Fig. 6.14) divides the line emission
from a diode laser bar into individual line segments, while a second-
step mirror stacks the line segments in the direction of the better beam
quality (similar to the double-mirror Southampton beam shaper). Each
beam has the same path length and hits the mirror surfaces only twice.
A typical step size is 1 mm to match the beam size after fast-axis col-
limation; thus, a 10-mm bar can be cut into 10 segments. This reduces
the beam parameter product from 500 mm-mrad for a 30 percent fill
factor bar to 50 mm-mrad in the lateral direction and increases the
value to 20 mm-mrad in the vertical direction. To couple into a fiber
with minimized losses, the sum of the beam parameter products must
be the same or less than the product of the diameter times the fiber’s
NA. With a value of 70 mm-mrad for the diode beam and a typical NA
of 0.2, the smallest fiber diameter that can be used with this approach
is about 200 mm. Using a diode bar with fewer emitters and increased

2
M ≈ N ⋅ M f 0 2
f
M s 0 2
2
M ≈ N
s
M 2
N = s 0
2
M f 0 2

Propagation
Fast axis
Slow axis
Figure 6.14 A step-mirror beam shaper rotating single line elements and
formulas to calculate the number of steps.