Page 443 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lasers 403

INTERFERENCE FILTER
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OUTP

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TUNING
FIGURE 3 3 Interference filter tuned extended-cavity laser with degenerate external resonator.
(Reproduced with permission from Zorabedian and Trutna [14].)

coupler mii-ror in an invar structure with a cavity length of -20 cm. The lasers
ran in a single longitudinal mode with sidemode suppression of >40 dB. When
shielded in Plexiglas enclosures and temperature controlled to f0.5@C, the
instantaneous beat linewidth between the two lasers was 4 kHz; the total free-
running relative frequency excursion over 5 min was only 600 kHz. Although
the etalon had a free spectral range of 45 nm, tuning over more than one
external-cavity free spectral range was not mentioned.
Liquid crystal filled Fabry-Perot filters have been used to tune ECLs. One such
laser tvas constructed as follows [90]: The etalon comprised a sandwich of a liquid
crystal layer between two glass plates on which dielectric mirrors (R = 98.5%) and
indium tin oxide transparent electrode layers had been deposited. A 12+m gap
(which gave a 64-nm free spectral range) was maintained by a deposited alignment
layer. The bandwidth and peak transmittance depended on the spot size because of
plate waviness and were found to be, respectively, 0.35 nm and 50% for a 100-ym
test beam. The etalon was tuned by varying the amplitude of a 20-kHz voltage
applied to 1.he electrodes. The gain element was an AR-coated InGa4sP multiple-
quantum-well laser diode with a 300-ym-long gain section and a 70-ym-long
phase-control section. The extended cavity contained a coupling lens, the etalon,
and an external mirror. The lens focused the intracavity beam on the mirror 3 cm
from the diode. The etalon was placed near the rear focus to help iniprove the
etalon perfmnance. Nevertheless, the external feedback was estimated to be less
than 1%. Because of the weak feedback, coordinated adjustment of the etalon volt-
age and phase-control current was necessary to select arbitrary external-cavity
modes. By varying the plate voltage from 0 to 20 V and phase-control current by
less than 1 mA, tuning from 1522 to 1563 nm was obtained, with a peak power of 5
dBm at 100-mA pump current.
A fiber Fabry-Perot etalon has also been used to tune a fiber ring ECL [91].
The filter lhad a 0.3-nm bandwidth and a 30-nm free spectral range. The gain