Page 425 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lmers 385

condition of maximum expansion) when the angle of incidence equals the Brew-
ster angle. The magnification of each prism is then equal to the index of refrac-
tion of the prism material, that is, M = 17.

7.2 Tunable Filters

The ideal filter for an ECL has a bandwidth that is less than the axial mode
spacing of the cavity and has 0-dB insertion loss at its peak. No real filter is
ideal, but a number of different types of wavelength-selective elements have
been used to tune external cavity lasers. The filters are grouped according to
whether they are actuated by mechanical means (e.g., have moving parts) or
electronically (no moving parts).

7.2. 7 Mechanically Tuned Filters

7.2.1.1 Diffraction Gratings

7.2.1.1.1 Types of Gratings
Diffraction gratings are the most common type of filter used in ECLs and
have arguably the best optical performance. A diffraction grating consists of a
large number of regularly spaced grooves on a substrate. The distance between
adjacent grooves is called the pitch. If the underlying substrate is reflective. then
we have a I;?jection gl-atiizg [Fig. 18(a)]. If the substrate is transmissive, then the
device is said to be a tl-ansmissiorz gmtiizg [Fig. 18(b)].
Diffraction gratings are also classified by the way in which they are manu-
factured. When the grooves are created by scribing with a ruling engine, the
device produced is a ruled mastel- grating. Relatively few masters are produced,
and these are rarely sold. The groove pattern of the master can be faithfully
Transferred by a contact process to a number of replica gratings, which are then
made available commercially (e.g.. by Milton Roy).
Diffraction grating groove patterns are also generated by exposing photo-
resist with the fringe pattern created bj two interfering beams of laser light,
Such gratings are called holographic and are also sold commercially (e.g., by
American Holographic).

7.2.1.1.2 Principle of Operation
When a beam of light is incident on a grating, each groove generates a dif-
fracted wavelet. For each wavelength component in the incident beam, the con-
structive interference of the diffracted components from each groove occurs at a
unique set of discrete directions called the diffraction oi-del-s of the grating.
7.2.1.1.3 The Grating Equation
The geometry of the diffraction pattern from a grating is governed by the
grating equation: