Page 422 - Tunable Lasers Handbook

382 Paul Zorabedian

focal lengths. In the geometric optics approximation, the degenerate resonator has
the property that rays emitted from a point on either mirror return to that point
after one round-trip (recycle). independent of mirror tilt or lateral displacement of
the source point from the optical axis of the cavity. The degenerate-resonator con-
cept can be usefully applied to ECL design (Fig. 17). The degenerate extended-
cavity laser contains two lenses. The first lens. adjacent to the laser diode, creates a
central collimation region in the cavity where the tunable filter can be inserted.
The second lens refocuses the intracavity beam onto the external mirror. The com-
bination of the second lens and the external mirror forms a cat’s-eye retroreflector
[51]. The retroreflecting property of the cat’s eye is highly insensitive to mirror tilt.
Provided the distance between the lenses is fii +f7, the feedback from the cavity is
also insensitive to lateral displacement of the acgve area with respect to the optic
axis. If the distance between the two lenses is not equal tofi +fi but the distance
between the second lens and the external mirror is equal tof,, the feedback is still
insensitive to mirror tilt and the cavity is called .‘quasi-degenerate.’‘ The degener-
ate-resonator concept can be independently applied in one dimension by using a
cylindrical lens for refocusing onto the external mirror.

6.5 Chromatic Aberration
Chromatic aberration refers to the variation of the focal length of a lens with
wavelength. Because ECLs operate over wide wavelength ranges and the posi-
tion of the collimating lens is critical to laser performance, chromatic aberration
can require the collimating lens working distance to be adjusted as the wave-
length is varied. Multielement lenses such as microscope lenses can be corrected
for chromatic aberration. Single-element lenses such as graded-index rod lenses
and ball lenses cannot be corrected.

6.6 Birefringence

Some lenses, especially plastic lenses, may be birefringent due to stresses
built up during the manufacturing process. Birefringence will change the polar-
ization state of the intracavity light, reducing the external feedback.

fl fl +f2 f2
FIGURE 1 7 Generic laser based on a degenerate extended resonator.