Page 410 - Tunable Lasers Handbook

370 Paul Zorabedian

4. FIRST-ORDER PROPERTIES

This section defines the main parameters and reviews the major perfor-
mance features of external-cavity diode lasers.

4.1 External-Cavity Axial Modes
In the case of strong external feedback, the solitary resonances are strongly
suppressed by the facet 4R coating(s). In this case the axial modes of the system
are the Fabry-Perot modes of the external cavity, with modal frequencies vq and

and

where fXt is the total external-cavity path length and IZ,~ and lint are, respec-
tively, the effective index and physical length of the gain medium waveguide.
For an extended cavity or double-ended external cavity, q is an integer; for a ring
cavity q may only be an even integer. The frequency spacing between modes for
an extended cavity or double-ended external cavity is given by

The mode spacing for a ring external cavity is given by

In terms of wavelength the mode spacing is not constant but can be approxi-
mated by

for linear configurations and