Page 416 - Tunable Lasers Handbook

376 Paul Zorabedian

elements such as an optical isolator, the coupling between the two facets obeys
reciprocity, i.e., cll(h) = czl(h). Multiple reflections between the facets and the
external mirrors defining the ring can be neglected because there are no (inten-
tional) standing waves in the cavity. However. sometimes spurious etalons exist
between the residual reflections of the facets and the intracavity optics.

5.2 Threshold Current

The gain coefficient per unit length is given by

where y is a constant independent of h, I is the pump current, and I$) is the
transparency current. Note that the previously-defined confinement factor r has
been lumped in with the constant y. The threshold magnitude condition states
that the round-trip gain equals the total round-trip loss. This leads to the follow-
ing general expression for the threshold current:

where amir is represents the mirror loss for the appropriate cavity configuration.
These are given below.

5.3 Mirror Losses
The mirror loss in an extended-cavity configuration is given by

TABLE 4 External Feedback Model Parameters

Amplitude reflectances of facets
r,1, rf:
V Optical frequency
re,tl(v), reXI2(v) Amplitude reflectances of extended cavity sections (lumping. coupling, filter,
and mirror losses)
C Speed of light
Round-mp time of extended cavity section with length Lektl
Ltl = C/%Xtl
Round-trip time of extended cavity section with length Lexrl
=at2 = C/%\I?