Electro-Optics - Applied Optics Fundamentals and Device Applications - page 69

2. Wavelength uniformity and spectral width: Growth technology has
improved such that VCSEL 3″ wafers are produced with <2 nm stan-
dard deviation for the cavity wavelength. This allows the fabrica-
tion of VCSEL 2-D arrays with little wavelength variation between
the elements of the array (<1 nm FWHM spectral width). By contrast,
edge-emitter bar-stacks suffer from significant wavelength varia-
tions from bar to bar since there is no intrinsic mechanism to sta-
bilize the wavelength, resulting in a wide spectral width (3-5 nm
FWHM).
3. Temperature sensitivity of wavelength: The emission wavelength
in VCSELs is approximately five times less sensitive to temperature
variations than in edge emitters. The reason is that in VCSELs, the
lasing wavelength is defined by the optical thickness of the single-
longitudinal mode cavity and that the temperature dependence of
this optical thickness is minimal (the refractive index and physical
thickness of the cavity have a weak dependence on temperature).
On the other hand, the lasing wavelength in edge emitters is defined
by the peak-gain wavelength, which has a much stronger depen-
dence on temperature. As a consequence, the spectral line-width for
high-power arrays (where heating and temperature gradients can be
significant) is much narrower in VCSEL arrays than in edge-emitter
arrays (bar-stacks). Also, over a 20°C change in temperature, the
emission wavelength in a VCSEL will vary by <1.4 nm (compared to
~7 nm for edge emitters).
4. High-temperature operation (Chillerless operation for pumps):
VCSEL devices can be operated without refrigeration because they
can be operated at temperatures to 80°C, the cooling system becomes
very small, rugged, and portable with this approach.
5. Higher power per unit area: Edge emitters deliver a maximum of
about 500 W/cm 2 because of gaps between bar to bar that must be
maintained for coolant flow, while VCSELs are delivering ~1200 W/
cm 2 now and will deliver 2-4 kW/cm 2 in the near future.
6. Beam quality: VCSELs emit a circular beam. Through proper cav-
ity design, VCSELs can also emit in a single transverse mode (cir-
cular Gaussian). This simple beam structure greatly reduces the
complexity and cost of coupling/beam-shaping optics (compared to
edge emitters) and increases the coupling efficiency to the fiber or
pumped medium. This has been a key selling point for the VCSEL
technology in low-power markets.
7. Reliability: Because VCSELs are not subject to COD, their reliability
is much higher than for edge emitters. Typical FIT values (failures in
one billion device-hours) for VCSELs are <10.