Page 454 - Tunable Lasers Handbook

414 Paul Zorabedian

POLARIZATION
CONTROLLER
FlGU RE 3 8 Delayed self-heterodyne interferometer for linewidth measurement.

Typical values for a GaInAsP buried crescent device with Lint = 300 pm at 1300
nm are [yZL,(ho) + a]Lint = 0.7 and yLint = 0.12 mA-1.

1 2.2 External-Cavity Reflectance
The following calculations pertain to the basic extended-cavity configura-
tion. They can readily be modified for the double-ended ECL or ring ECL con-
figurations. It is relatively straightforward to determine the transmission losses
of the various intracavity elements separately. However, the mode-conversion
loss due to the coupling lens is not easy to determine directly. Therefore. the
external-cavity reflectance usually must be deduced from changes in the thresh-
old current caused by modifications to the feedback. To determine the external
feedback strength, place the laser diode in the external cavity, tune to the vicinity
of the solitary lasing wavelength A,, and measure the external-cavity threshold
current versus wavelength over one period of its ripple pattern. Compute the
average external-cavity threshold current

which is related to the wavelength-averaged external-cavity reflectance 1% by

At this point, assuming yLht is independently known from test devices, the external-
cavity reflectance can now be determined from