Electro-Optics - Applied Optics Fundamentals and Device Applications - page 72

13. Feedback insensitivity: In VCSELs, the as-grown output coupler
reflectivity is very high (typically >99.5%) compared to edge emitters
(typically <5%). This makes VCSELs extremely insensitive to optical
feedback effects, thus eliminating the need for expensive isolators or
filters in some applications.
14. Low thermal impedance and ease of packaging: PO developed
advanced packaging technologies, enabling efficient and reliable
die-attach of large 2-D VCSEL arrays on high-thermal-conductivity
submounts. The resulting submodule layout allows for straightfor-
ward packaging on a heat exchanger.
For high-power devices packaged on microcoolers, PO demonstrated mod-
ules with thermal impedances as low as 0.15 K/W (between the chiller and
the chip active layer).
2.12.4 High-Power CW and QCW VCSEL Arrays
PO designs and manufactures advanced high-power CW and QCW diode
lasers for the industrial, medical, and defense markets. Unlike edge emit-
ters, the light emits perpendicular to wafer surface for VCSELs. It is therefore
a straightforward to process 2-D arrays of small VCSEL devices driven in
parallel to obtain higher output powers. The advantage of 2-D arrays is the
simple silicon IC chip-like configuration. Many of the silicon IC packaging
and cooling technologies can be applied to VCSEL arrays.
PO took the VCSEL technology to very high power levels by develop-
ing very large (5 mm × 5 mm) 2-D VCSEL arrays packaged on high-thermal-
conductivity submounts. (see Figure 2.46). These arrays are composed
of thousands of low-power single devices driven in parallel. Using this
approach, record CW output powers in excess of 230 W from a 0.22 cm 2
emission area (>1 kW/cm 2 ) have been demonstrated, without sacrificing
wall-plug efficiency.
In addition to CW VCSEL arrays, PO has developed very high power den-
sity VCSEL arrays for quasi-CW (QCW) operation. QCW powers in excess of
925 W have been demonstrated from very small arrays (5 × 5 mm chip size),
resulting in record power densities >4.2 kW/cm 2 (see Figure 2.47). These
small arrays can easily be connected in series to form larger arrays with high
output powers. These arrays are ideal for applications requiring very com-
pact high-power laser sources.
Because VCSELs can operate reliably at temperatures up to 80°C, they do
not necessarily require refrigeration. Additionally, since the wavelength
change with temperature is small, the cooling system design can be consid-
erably simplified. The cooling system thus becomes very small, rugged, and
portable with this approach. We have been operating the VCSELs and VCSEL