Biomedical Engineering Reference
In-Depth Information
8. Manufacturability and yield: Manufacturability of VCSELs has
been a key selling point for this technology. Because of complex
manufacturing processes and reliability issues related to COD, edge
emitters have a low yield (edge-emitter 980-nm pump chip manu-
facturers typically only get ~500 chips out of a 2″ wafer). On the
other hand, yields for VCSELs exceed 90% (corresponds to ~5000
high-power chips from a 2″ wafer). In fact, because of its planar
attributes, VCSEL manufacturing is identical to standard IC Silicon
processing.
9. Scalability: For high-power applications, a key advantage of VCSELs
is that they can be directly processed into monolithic 2-D arrays,
whereas this is not possible for edge emitters (only 1-D monolithic
arrays are possible). In addition, a complex and thermally ineffi-
cient mounting scheme is required to mount edge-emitter bars in
stacks.
10. Packaging and heat-sinking: Mounting of large high-power VCSEL
2-D arrays in a “junction-down” configuration is straightforward
(similar to microprocessor packaging), making the heat-removal
process very efficient, as the heat must traverse only a few microns
of AlGaAs material. Record thermal impedances of <0.16 K/W have
been demonstrated for 5 mm × 5 mm 2-D VCSEL arrays.
11. Cost: With the simple processing and heat-sinking technology, it
becomes much easier to package 2-D VCSEL arrays than an equiva-
lent edge-emitter bar-stack. The established silicon industry heat-
sinking technology can be used for heat removal for very high-power
arrays. This will significantly reduce the cost of the high-power
module. Currently, cost of the laser bars is the dominant cost for the
DPSS lasers.
12. High-wavelength stability and low-temperature dependence: Since
the VCSEL resonant cavity is defined by a wavelength-thick cavity
sandwiched between two DBRs, devices emit in a single longitudinal
mode and the emission wavelength is inherently stable (<0.07 nm/K),
without the need for additional wavelength stabilization schemes or
external optics, as is the case for edge emitters. Furthermore, thanks
to advances in growth and packaging technologies, the emission
wavelength is very uniform across a 5 mm × 5 mm VCSEL array,
resulting in spectral widths of 0.7-0.8 nm (FWHM). (see Figures 2.44
and 2.45.)
Wavelength stability and narrow spectral width are very signifi-
cant advantages in pumping applications, for example, where the
medium has a narrow absorption band.
Unlike edge emitters, VCSELs emit in a circularly symmetric beam
with low divergence without the need for additional optics. This has
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