Page 153 - High Power Laser Handbook

122 Diode Lasers Semiconductor Laser Diodes 123

materials to create an effective CTE matched closely to GaAs. How-
ever, interest in these ceramics is tempered by longer cavity diodes
(i.e., lower thermal resistance) and challenged by high material and
fabrication costs, which continue to delay serious reception of these
advances by the industry.
Gold wire bonding to gold-metallized submounts (or lead frames
inside the package) has proven to be an extremely mature and reli-
able process. Ball bonding is common for high-power diodes; whereas
wedge bonding can reduce the wire height and length needed for
high-speed applications. Standard process parameters are force, tem-
perature, ultrasonic power, and time. Small diameter (1–1.5 milli-inch
[mil]) wires may be dedicated for the chip exclusively because larger
wires require greater bond force, which may introduce damage to the
underlying active region. When bonding to the submount or leads,
therefore, larger diameter wire (e.g., greater than 2 mil) or even rib-
bon bonding is desirable to reduce the number of bonds while main-
taining acceptable PCE at higher drive currents.

5.12 Fiber-Coupled Package Design and Processes

A package (or housing) can add greater functionality to the COS (e.g.,
by adding monitor photodiode, thermistor, or thermal electric cooler
[TEC]). However, these components are not typically employed for
industrial applications, and TECs usually dissipate too much heat to
be practical at ~10 W output power levels. The housing also enables
fiber coupling, which is the focus of this section.
As mentioned previously, the industry has adopted designs and pro-
cesses borrowed from the telecom and submarine industry, with strict
assembly protocols and reliability standards. Best practices have been
promulgated by customers who seek greater quality and reliability assur-
41
ance to minimize the total cost of ownership. Fortunately, there is par-
ticularly favorable synergy with 980-nm single-mode telecom pumps,
and they share most of the same material components, assembly pro-
cesses, and equipment. As shown by Fig. 5.20, a multimode fiber-coupled
laser package resembles a single-mode pump, with the exception of the
diode and corresponding larger fiber-core diameter.
The laser diode is bonded p-side down to a submount that is sub-
sequently bonded to the base of the housing. A chisel lens or fast-axis
collimating (FAC) lens, or fiber rod, is mounted near the laser facet
for efficient coupling, and the fiber is bonded to the snout and strain
3
relief that exits the wall. The outside package occupies ~15 × 13 × 8 mm
volume with a ~15-mm strain relief to meet fiber-integrity require-
ments. At this time, industrial laser diode manufacturers have not
standardized form factors, as the telecom industry does.
The basic components and assembly methods used for fiber-
coupling single-emitter diodes are summarized in Table 5.3. This list