Page 407 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lasers 367
where nl and tl are the index and thickness, respectively. of the lnner layer and
n, and t, are the index and thickness, respectively, of the outer layer. This prin-
ciple can be extended to three layers by incorporating a third quarter-wave layer
with an intermediate index of refraction tz3 = no (iznJn0)~ between the two layers
specified above. Other index and thickness combinations for two- and three-
layer antireflection coatings are also possible.
Antireflection coatings with three dielectric layers in a low-high-low sequence
of refractive indices have been used to relax the tolerances and broaden the low-
reflectance banda idth [29,30].

2.10.2.3 Antireflection Coating Materials
The most widely used material for antireflection coatings on AlGaAs and
InGaAsP facets is nonstoichiometric SiOl, which can be deposited by thermal
[25] or electron-beam evaporation [31]. The composition and the film index
can be adjusted by varying the oxygen pressure in the deposition chamber.
Sputtered Si3N, films have also been used on 0.85-ym AlGaAs and 1.3 and
1.55 ym InGaAsP laser diodes. resulting in facet reflectances in the 0.01 to
0.03% range [32].

2.7 0.3 Passivation Layers
Commercial telecommunication and CD laser diodes are often shipped with
h/2 hcet passivation layers. It is possible to etch off the passivation layer prior to
coating or to deposit the antireflection coating over the passivation layer, but it is
preferable to start with unpassivated devices if possible.
2.7 0.4 Angled Facets
4n alternative to antireflection coatings is to use an optical amplifier with an
angled gain stripe in an external or ring cavity. The waveguide is slanted from
the cleavage plane so that the internal Fresnel reflection from the facet is not
coupled back into the waveguide and lost. The effective reflectance of the lonest
order TE mode decreases exponentially with the slant angle [33]. However. the
reflectance of the higher modes increases with the slant angle. Therefore. caution
must be exercised if the stripe is wide enough to support higher order modes.
For a 2-ym stripe, the calculated facet reflectance is on the order of 10-3 for a
slant angle of 10" [31].
Even though the internal facet reflections do not couple back to the wave-
guide. there are still reflections at the semiconductor-air interface that represent
loss in coupling to the external cavity. The coupling loss can be redwed by
applying antireflection coatings to the angled facets [35].

2. 70.5 Buried Facets
Another means of facet reflectance reduction is the use of gain media with
buried facets [36]. In these devices the waveguide stops several microns inside
the chip, with semi-insulating material between the end of the guide and the