Page 424 - Tunable Lasers Handbook

384 Paul Zorabedian

7.1.1.5 Camera Lenses
There are at least three published reports on the use of camera lenses as col-
limators in ECLs. Heckscher and Rossi [57] reported the use of a TV camera
lens for intracavity collimation of a Littrow grating cavity, but gave no indication
of the feedback strength obtained. Sommers [58] evaluated several camera
lenses from f10.99 (25-mm focal length) to f12.0 (50-mm focal length). The
lenses gave only about 1% feedback when used with a grating, and it was con-
cluded that spherical aberration was responsible for the poor performance since
the lenses were not used in their intended geometry. Fleming and Mooradian
successfully employed camera lenses in an ECL [38]. They used 50-mm focal
length,fll.4 seven-element lenses. All air-glass surfaces were AR coated.

7.1.1.6 Ball Lenses
Glass spheres can be used to couple the gain medium to waveguide or fiber-
pigtailed external filters. However, the spherical aberrations are too great to be
useful for collimation in bulk optic cavities.
7.1.1.7 Lensed Fiber
Lensed optical fiber [59] can be used to couple the gain medium to fiber-
pigtailed external cavities. However. this method requires the fiber to be in very
close proximity to the facet, which gives rise to the danger of facet damage.
There is also a very high sensitivity of the coupling loss to lateral misalignment.
7.7.2 Optics for Beam Expansion and Shaping
7.1.2.1 Cylindrical Lenses
A cylindrical lens can be used in an ECL [60] to form a line illumination on
a diffraction grating. This implements a degenerate resonator in one dimension
and provides a high degree of angular misalignment tolerance while maintaining
high spectral selectivity. Critical to the success of this technique is the fact that
the cylinder axis can be inclined with respect to the optical axis at a large angle
to match the grating angle of incidence without introducing a large amount of
spherical aberration. This is because the cylinder lens has no power in this plane
and appears to be a tilted plate.
7.1.2.2 Prisms
The use of prism beam expanders allows the use of a compact, high-resolution
grating-tuned extended-cavity laser [61]. A particularly useful geometry is when the
apex angle 8, is cut so that

8, = 90" - tan-' (11) . (44)

where ri is the index of refraction of the prism material. For this choice of apex
angle, the output beam is normal to the exit face of the prism (which is the