Page 417 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lasers 377

The mirror loss in a double-ended external-cavity configuration is given by

The mirror loss in a ring-extemal-cavity configuration is given by

5.4 Strong-Feedback Regime
Because the gain of the semiconductor medium is strongly coupled IO its
index of refraction, threshold gain ripple caused by the diode-cavity etalon effect
gives rise to a number of undesirable phenomena such as bistability, tuning non-
linearities, and in some cases axial mode instabilities [24]. To avoid these prob-
lems, it is very desirable to operate a tunable external cavity laser in the strong-
feedback regime, in which Y:,~>>T-+. For a bare cleaved facet in air, rf = 0.31.
Due to mode coupling losses, it is not possible to obtain external feedback much
greater than rixr = 0.40. Therefore, operation in the strong feedback regime
requires solme method of facet reflectance reduction. The most common approach
is to use a dielectric AIR coating. Strong feedback also requires proper design of
the external cavity to ensure low-loss coupling of the cavity and waveguide
modes. Having previously discussed facet reflectance reduction, we now discuss
external cavity optical design.

6. EXTERNALCAVITY DESIGN

The first subsection presents general cavity design principles that are
broadly applicable regardless of the implementation. The succeeding subsections
give specifications for various intracavity optical components and their position-
ing in the cavity.

6.1 General Design Principles
In any extemal-cavity design, one should try to maximize the external feed-
back strength and wavelength selectivity of the cavity. Brief explanations of the
importance of these two conditions and definitions of their respective figures of
merit are as follows. Strong external feedback is needed to obtain low output
power ripple with respect to wavelength and to avoid bistability [35]. It also
improves the ability to obtain single-mode oscillation without mode-hopping