Page 155 - High Power Laser Handbook

124 Diode Lasers Semiconductor Laser Diodes 125

is not intended to be comprehensive; rather it represents the current
state of the industry.
The housing consists of a steel frame that is brazed to a copper
(Cu) base for low R . Electrical leads are sealed by low-MP glass. If
th
low CTE is needed (e.g., ~7 ppm/ºC to mount a TEC), Kovar hous-
ings with Cu/W bases and alumina electrical feedthroughs can be
used at a substantial cost penalty. The COS and other internal compo-
nents (e.g., fiber mount) inside the package are bonded with low-
er-MP solders in the range of 120ºC < MP < 260ºC to avoid reflowing
the AuSn-solder joint below the laser diode. A lid that is CTE-matched
to the housing frame is then attached to the seal ring, using resistive
sealing or laser welding. Solder sealing is not a viable option for
short-wavelength (< 980 nm) diodes due to oxygen added intention-
ally to prevent catastrophic optical damage (COD) failures.
A chisel lens or FAC collimates the light for efficient coupling into
the fiber-tail assembly. Antireflection (AR) coatings are needed to
increase coupling efficiency, as well as to prevent back reflections into
the diode, which would degrade linearity and short-term power
stability. Worse yet, laser diodes will fail catastrophically from tran-
sient high-peak-power pulses generated from the fiber laser; as such,
manufacturers now offer optical isolation inside the pump (i.e., highly
transmissive < 975 nm [laser diode] and highly reflective > 1050 nm
[fiber laser]). Dichroic coatings are capable of creating greater than
42
30-dB isolation without any efficiency penalty. State-of-the-art
pumps routinely achieve 95 percent average coupling efficiency
(AR-coated output) into 0.22 NA fiber and 92.5 percent into 0.15 NA
fiber by employing FACs that improve coupling at low NA, as compared
with chisel lens (due to reduced spherical aberrations of the former).
The fiber lens or FAC may be attached directly with low-MP lead-
solder glass (~300ºC) or ultraviolet epoxy, whereas AuSn soldering
requires metallized fiber. With either lens design, the working distance
remains less than 10 mm to avoid overfilling the fiber core in the lateral
dimension. Each technique requires tailoring the cure or stress relief to
stabilize the lens relative to the laser for eventual deployment, as well
as to preserve low NA with varying case temperatures (0 to 75ºC).
The fiber pigtail is secured to the package frame centered within
the snout and generally maintains a hermetic seal under static or
dynamic force. The most common bonding techniques include epoxy-,
glass-, and solder-sealing of ferrules or directly to the fiber (metal-
lized, to bond with metal solder). Additional strain relief allows the
fiber to be coiled and assembled without weakening the fiber, as well
as to protect against accidental tugging on the pigtail that might either
break the fiber at greater than 5 newtons (N) or degrade coupling effi-
ciency if the force is transmitted internally to the fiber attachment.
Moisture is well known to cause a variety of failure mechanisms
43
in components and metallurgy. For diodes, the most worrisome
form of corrosion occurs at the laser facet that promotes COD, even