Electro-Optics - Applied Optics Fundamentals and Device Applications - page 61

2.11.2 Vertical Cavity Surface Emitting Lasers for Illumination
VCSELs were historically limited to low-power applications; but through
several Defense Advanced Research Projects Agency (DARPA) spon-
sored programs [88], the output power of VCSELs dramatically improved.
Princeton Optronics (PO) Corporation developed the world's highest power
single devices and 2-D arrays [89]. They have successfully demonstrated sin-
gle devices with >3 W continuous wave (CW) output power and large 2-D
arrays with >230 W CW output power. These arrays comprise thousands of
small, low-power, single-mode devices. The output of this array can then
be focused into a very small, low-diverging spot using a microlens array
focusing-lens system. VCSELs have several major advantages in illumination
applications [89]:
• A circular output beam.
• Much lower wavelength dependence to temperature (5× less than
for edge emitters). Since the VCSEL resonant cavity is defined by a
wavelength-thick cavity sandwiched between two distributed Bragg
reflectors (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.
• A much higher reliability than edge emitters since VCSELs are not
subject to catastrophic optical damage (COD) failures.
• High-temperature operation. VCSELs and VCSEL arrays can be
operated at temperatures up to 80°C ambient without chillers.
• High power from the arrays (demonstrated CW power density of
1200 W/cm 2 from the arrays).
• Emission wavelength is very uniform across a 5 mm × 5 mm VCSEL
array, resulting in spectral widths of ~0.7 to 0.8-nm full width at half
maximum (FWHM).
• VCSELs can be easily processed into large 2-D arrays (do not need
stacking) to scale up the power. The 2-D array configuration pro-
vides for more efficient heat-sinking and better pump power density,
as the devices can be very closely packed.
• Low thermal resistance for CW operation. A thermal resistance of
0.15°C/W from the junction temperature to the microcooler coolant
makes it lower by more than a factor of 2 compared with edge-emit-
ter arrays.
• VCSEL arrays are designed for coupling match to COTS microlens
arrays.
• PO makes single-mode devices at wavelengths of 976 and 1064 nm,
as well as custom wavelengths between 808 and 1064 nm.