Page 437 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lasers 397

UPSHIFT 1 DOWNSHIFT 1
0 =o +o 0 =o -w
d o a d o a

UPSHIFT 2 DOWNSHIFT 2

FIGURE 25 Sign of AQTF frequency chirp for various combinations of input polarization and
propagation direction.

7.2.2.2.6 Acousto-Optic Tuning Speed
The wavelength switching time is given by

where ic0 is the Gaussian beam parameter of the input beam in the filter. The
acceptance angle can be satisfied with input beams focused down to a few hundred
microns in diameter. Because un - 700 misec in TeO,, - T~,~, < I ps is achievable.

7,3 Optical Isolators
ECLs are sensitive to spurious optical feedback reverse-coupled through the
output mirror. For very short cavities, the feedback tolerance is as high as -20
dB [79]. However, sensitivity increases with cavity length. Isolation of at least
30 dB is typically used for external-cavity lengths of 1 to 10 cm. Up the 60 dB
isolation is sometimes used. High isolation from backreflections is especially
important when the output of the laser is being obsenied with a highly reflective
instrument such as a scanning Fabry-Perot interferometer. Miniature Faraday
optical isolators that provide about 30 to 10 dB of isolation per stage are com-
mercially available [EO].