Page 419 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lasers 379

extended-cavity or dauble-ended configurations is that the collimating lens must
transform the Gaussian beam waist at the gain medium facet into another waist
at the surface of the external mirror (Fig. 15). In ring external cavities, the intra-
cavity optics must transform the output bemi from each facet into a beam uaist
that matches the output beam from the opposite facet. Obtaining efficient cou-
pling puts requirements on the numerical aperture, wavefront distortion. and
attenuation of the collimating lens.

6.2.7 Numerical Aperture
Perpendicular to the junction plane the output beam from the gain medium
has a divergence angle of up to 0, = 40" FWHM. For the Gaussian intensity pro-
file the l/$ point is 1,7 times the FWHM value. The collimating lens should
have a sufficiently large numerical aperture (NA) to capture the b- Lam out to
these points. that is,
NA 2 sin [YO,]
o.55
17
=
6.2.2 Wavefront Distortion
Wavefiront distortion reduces the overlap integral between the waists of the
output and return beams (Fig. 16) [47]. In an extended cavity. the effect of \\we-
front distortion is multiplied by 3 since the beam transits the collimating lens
twice. A peak-to-peak wavefront distortion of ?~/4 results in a 2-dB reduction m
coupling efficiency. A maximum peak-to-peak wavefront distortion of A/4 over
the usable aperture of the collimating lens should be specified.

6.2.3 Attenuation
One-way coupling efficiencies for good large-aperture coupling lenses are
typically in the range of 50 to 70%. Therefore. the maximum round-trip feedback
efficiency is limited to the range of 25 to 50% by the coupling lens, assuming no
4
other intracavity losses. In general, an overall feedback efficiency of >lo% is
desirable from an extended cavity. Therefore, the total of the additional round-trip

Optics
\ Coupling
t t

Beam Beam
Waist Waist
FIGURE 1 5 Intracavitq beam transformation by coupling optics.