Page 393 - Tunable Lasers Handbook
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8 Tunable External-Cavity Semiconductor lasers 353
Ith I
FIGURE 2 Schematic light output versus current curve.
internal loss given by al,,. The cleaved-facet ends of the active region constitute
a mirror loss amil. given by
where Lint is the physical length of the internal cavity bounded by the facets with
power reflectances Rfl and Rf2. The Fresnel reflectance of a bare facet is
where 17 = 3.5 is the semiconductor index of refraction.
2.3.3 Confinement Factor
Only a fraction r of the optical field lies within the active region and sees its
gain. The factor l- is called the confinement factor.
2.3.4 Threshold Condition
The threshold condition requires the optical field to be periodic with respect
to one round-trip of the diode cavity. This leads to magnitude and phase condi-
tions on the optical field. The magnitude part of the threshold condition requires
the gain g,, to be equal to the total round-trip loss:
