Page 138 - High Power Laser Handbook

108 Diode Lasers Semiconductor Laser Diodes 109

index is achieved. The magnitude of the index step depends on how
deeply the ridge is etched and is typically less than 10 .
–2
A larger lateral index step and confinement can be achieved in
buried-heterostructure lasers. This structure is formed by a deep
8
etch through the active layer and subsequent regrowth with wider-
band-gap, lower-index layers to provide lateral mode confinement.
Current blocking layers are also formed during regrowth (Fig. 5.6b).
Although the regrowth process is well developed for InP/InGaAsP-
based material systems, it is much more difficult in GaAs/AlGaAs
material systems, due to oxidation of the aluminum-containing lay-
ers during the various process steps.

5.6 Efficiency of Laser Diodes
One of the major attributes of semiconductor diode lasers is the
electrical-to-optical conversion efficiency, or wall-plug efficiency,
which is the ratio of optical power P over electrical power (or the
product of electrical current I and voltage V). High power conversion
efficiency (PCE) is especially important for high-power lasers, because
excessive heat can result in degradation of device performance. To
dissipate excess heat, effective cooling is needed, which, in turn,
requires more electric power and space, as well as additional packaging
costs. Power loss in a semiconductor laser is divided among waste
voltage, waste current, and optical loss. Waste voltage is a combina-
tion of the excess of the laser’s turn-on voltage V compared with the
to
voltage corresponding to the emitted photon energy V and the volt-
λ
age dropped over the series resistance R , which comprises semicon-
s
ductor layers, metal layers, and wire bonds. Waste current is divided
between the threshold current needed to reach the required gain and
the leakage current that is commonly described by the internal quan-
tum efficiency parameter η . Optical loss is due to distributed internal
i
loss α —usually free carrier absorption—and external loss, such as
int
light lost out the rear facet. A simple expression for PCE is
 I 
η i 1 − I th  

PCE = (5.1)
 α   V − V I IR 
1 + int  1  + to λ + s 
 α m   V λ V λ 

where mirror loss α is
m
1  1 
α = 2 L ln    R R r   (5.2)

m
f
hc
V = eλ (5.3)
λ