Page 411 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lasers 71

for a ring configuration, where h is assumed to be the midpoint of the tuning
range. It is often the case that LeYr>>neKLmr, and in practice the term nzffLlnt is
often neglected when calculating the external cavity modes. Assuming Lext = 5-20
cm. we find that Avext = 1-3 GHz. Thus the axial mode spectrum for an external
cavity laser is about 100 times more dense than for a solitary diode laser.

4.2 Wavelength Selection
The principle of wavelength selection in an ECL can be explained with a
sketch of the round-trip gain and loss terms (Fig. 10). The gain profile and the fil-
ter pass band are familiar from other types of tunable lasers such as dye lasers.
However, two other features are atypical of other lasers. First, there is a sizable
coupling loss (-5 to 15 dB round trip, between the guided wave in the active
region and the free-space beam in the external cavity. Second, interference
between the reflections from the gain-medium facets creates an additional intra-
cavity loss that is modulated at the period of the gain-medium mode spacing Avlnt.

A

Semiconductor
.- K gain
a
0
External
cavity modes
IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII ,A

FIGURE 1 0 Round-trip gain and loss terms for external-cavity diode lasers.