Page 396 - Tunable Lasers Handbook

356 Paul Zorabedian

A change in the real part of the index of refraction is related to frequency
chirp by

A change in the imaginary part of the index of refraction is related to a change in
the optical gain by

2.5 Spatial Properties of Output
2.5. I Transverse Modes

The beam emanating from the facet of a properly designed laser diode is a
Gaussian beam. Some lasers with excessively wide active regions may emit
higher order transverse modes, especially at currents well above threshold. The
onset of a higher order mode is often accompanied by a telltale kink in the L-I
curve. It is very undesirable to use a laser diode that emits in a higher order
transverse mode as a gain medium in an ECL because this may degrade the cou-
pling efficiency and the wavelength resolution of the cavity.
2.5.2 Divergence
The near-field radiation emitted from a diode facet is a few-micron spot
somewhat elongated parallel to the p-iz junction. Ideally this spot is a Gaussian
beam waist at the facet surface with planar wavefronts in both the parallel and
perpendicular directions. The far field is a highly divergent beam characterized
by full width at half-maximum (FWHM) angles for the directions parallel and
perpendicular to the junction (Fig. 3).
2.5.3 Astigmatism
In some laser diodes the facet spot has a planar wavefront perpendicular to
the junction but it has convex curvature in the direction parallel to the junction.
Thus the parallel rays appear to diverge from a point inside the laser (Fig. 4).
This condition is known as astigmatism. and it depends on the waveguiding
structure used in the laser diode (discussed later). Even a few microns of astig-
matism is undesirable, and astigmatic laser diodes should be considered unsuit-
able for use as external cavity gain media.
2.5.4 Polarization
Laser diodes have modes that are polarized parallel to junction (TE) and
perpendicular to the junction (TM). TE modes are usually more strongly guided