Page 457 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lasers 41 7

This method, like the previous one, is strictly accurate only at the original oscil-
lation wavelength of the solitary laser diode. At other wavelengths the gain is
lower. and lhis method tends to underestimate the reflectance. Furthermore.
when both facets have been coated. it is often difficult to measure m, and the
method only gives the product of the two coating reflectivities.
The third method determines the facet reflectance as a function of wave-
length but requires operating the gain medium inside an ECL [115]. The ECL
containing the gain medium under test is operated over wavelength and the
threshold current is recorded as a function of wavelength. Let Zmal(h) and Zmln(x)
be, respectively, the local maxima and minima. The facet reflectance is given by

where the methods for determining y Lm, and rest were described in the previous
two subsections. The use of this method for the measurement of the reflectances
of single-layer facet coatings has given good agreement with theory (Fig. 40).

? 3. MULTIMODE SUPPRESSION

There are a number of ways to deal with the problem of multi-longitudinal-
mode oscillation. The first line of defense is in the design of the cavity;

t

Wavelength, nm
FIGURE 40 Reflectance versus wavelength measured with a diode in an extended-cavity laser
and calculated for an ideal single-layer coating. (Reproduced with permission from Stokes [ 1151.
0 1993 IEEE.)