FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

I
in one portion of the recombination layer but not in
another. ‘Lhis is known as filamentary lasing. Such
lasing behavior tends to produce noise. If the width
of the layer is less than 10 microns, it is too narrow
for such filamentary behavior to occur and when lasing
does begin it occura uniformly throughout the layer.
Furthermore, when the width is less than 15 microns
singlemode propagation usually occurs. Finally, the
less the width of the recombination layer the less the
required threshold current. Double-heterojunction diode
lasers with threshold currents as low as 20 ma have
been produced. However, trade-offs may be required.
For example, reducing the recombination layer width
also reduces the maximum safe photon intensity. A safe
cw optical power output that can be maintained without
danger of facet damage is approximately 1 mW for each
micron of recombination layer width. Thus, a laser with
a recombination layer 10 microns wide can produce 10 mW
of optical power safely.
A striped-geometry injection laser diode,
such as that shown in Fig. 2.47, has the desired thin
and narrow recombination region. With this geometry,
the emitted light spreads out in the vertical direction
by as much 50” and in the horizontal by 8° or more.
The dimensions of the recombination layer are of the
order of 0.3 microns thick, 10 microns wide, and Up to
500 microns long. These dimensions and light spreading
angles must be taken into account when the laser is
coupled to an optical fiber or substrate.
TRANSVERSE
(m)
u 1
Y
UDINAL (q)
FUNDAMENTAL MODE
{ (!:;)
,-I 2ndMODE
,’‘ ‘,,, (m::)
~
E LATERAL(s)
DISTANCE
Fig. 2.48 The light intensity as a function of distance
across the face of a laser for the
fundamental and second lightwave modes generated
by a laser.
METAL CONTACT
-’”’
P
(a) STRIPE CONTACT
METAL CONTACT
)
II(Zn-DIFFUSED)
/[ n
P
N
euBsTRATE
(c) DOPING-PROFILE
P
METAL ~ONTACT
PROTON
SOMSARDED
(SEMI t+SULATING)
METAL CONTACT
‘P
-4 l-+=
(b) PROTON-BOMBARDMENT
(d) STRIPE MESA
Fig. 2.49 End views of various stripe geometry diode
lasers.
Fig. 2.47
A GsAs-GcAIAs geometry CW injection laser
diode.
The distribution of optical energy across the
lasing region is shown in Fig. 2.48. The fundamental
and second harmonic of the longitudinal modes are shown.
In the longitudinal fundamental mode, the energy tends
to be concentrated more heavily towards the center, and
tapers off towards the edges in a Gaussian distribution
curve. If this lasing region is sufficiently wide, the
second harmonic mode can occur and the emitted optical
energy is concentrated in two regions.
Several techniques have been employed to fabricate
such stripe geometry. Some of these are shown
in Fig. 2.49. The upper left (a), an oxide protective
stripe is shown between the metal contact and the crystal.
The stripe is formed where the oxide layer is
omitted in the center. Electrons tend to be injected
into this region only. In this case, the current can
spread out underneath the oxide layer where it is not
confined. Another technique, shown at the lower left
(b), reduces such current spreading by increasing the
resistivity in the regions on each side of the stripe.
This can be accomplished by photon bombardment that
2-21
produces a semi-insulating layer on each side of the
stripe. A third technique, shown at the upper right
(c), uses the diffusion of a dopant, such as zinc, into
the stripe region to significantly lower the resisti-
Vity. Finally, almost complete electric current conf
inement occurs in the structure shown at the lower
right, (d). A stripe mesa (plateau or table) such as
this is formed during the process of growing the crystal.
Often such a mesa is buried by depositing additional
material over it.
For diode laser operation one major concern
has been the reduction of the spontaneous emission region
that was shown in Fig. 2.49. However, spontaneous
emission is the mechanism responsible for light emission
in LED’s. These devices are cheaper. Simpler
construction techniques may be used. The light they
produce is not coherent and is emitted over a much
wider angle (approximately 180° ) with the result that
less optical power may be coupled into a fiber. On
the other hand, the spontaneous emission portion of
the optical output power versus input direct current
curve is far less temperature dependent than the stimulated
emission region. Thus , because LED’s are less
temperature dependent than diode lasers, temperature
control and optical feedback problems are reduced.