Page 421 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lasers 38 1

I. The relay optics must transform the Gaussian beam waist ac the
waveguide output in the facet plane into another waist at the surface
of the end mirror.
2. The end reflector must be aligned so as to retroreilect the incident
beam back to the active spot on the diode facet. The alignment sensi-
tivities can be deduced using fairly simple arguments from Gaussian
beam theory. Note that the sensitivities may be different in the tan-
gential and sagittal planes of the cavity.

6.3. 7 Codlimating lens Axial Position Tolerance
This refers to the sensitivity to axial positioning of the intracavity elements
with respect to the diode facet. It can be shown that the axial tolerance 6-irol of
the collimating lens is approximately given by the Rayleigh range zR [49] of the
beamwaist at the diode facet:

where wrd is the spot size at the feedback-coupling facet of the gain diode.
6.3.2 Mirror Angular Alignment Tolerance
The angular alignment tolerance 6Or0, of external mirror is approximately
given by the divergence angle O,, of the waist spot incident on it:

where MI,. is the spot size at the retroreflector.
6.3.3 Gain Medium Transverse Position Tolerance
A small lateral displacement 6-x between the gain medium and the collimat-
ing lens is equivalent to angular misalignment of the external reflector by an
angle 6slf, wherefis the focal length of the lens. Thus, the lateral lens tolerance
is given by

6.4 The Degenerate Resonator
A degenerate Fabry-Perot resonator [50] comprises two end mirrors and two
intracavity lenses; each lens is spaced from the adjacent mirror by its respective
focal length, and the two lenses are separated from each other by the sum of their