Page 444 - Tunable Lasers Handbook

404 Paul Zorabedian

medium was a 400-ym-long, 1.5-ym semiconductor amplifier with a reflectance
of -10-4 for the AR coating on each facet. The 18-m fiber loop contained two in-
line optical isolators and a polarization controller. Output was obtained with a
90:lO directional coupler (90% feedback. 10% output). The cavity had an esti-
mated loss of 12 dB. The wavelength was tunable between 1505 to 1535 nm by
applying 0 to 15 V to the filter. Single-mode operation with 30-dB sidemode
suppression was obtained at 0.9-nm intervals over this range in coincidence with
the residual Fabry-Perot modes of the optical amplifier. The tuning range was
extended to 1495 to 1544 nm by insertion of an additional Fabry-Perot filter
with a broader free spectral range, but the addition of the second filter prevented
single-mode operation.

8.4 Etalon-Grating Corn binations

Gratings have been used in tandem with Fabry-Perot etalons to tune ECLs.
There are two basic ways in which the relative spectral selectivity can be parti-
tioned between the grating and the etalon. In the first approach. the grating is illu-
minated with a broad beam and provides most of the spectral selectivity. A fairly
low-finesse etalon provides a resolution “boost” to the grating and improves the
stability of single-mode operation [92]. In the second approach, a high-finesse
etalon provides a comb of sharp transmission peaks while the grating, illuminated
with a small spot. provides sufficient resolution to reject all but one interference
order [93].

8.5 Birefringent Filter Tuning

Birefringent tuning lends itself to electronic tuning without the use of any
moving parts by using the electro-optic effect or the birefringence of liquid crys-
tals, A disadvantage of electro-optic birefringent tuning is that the large voltage
required tends to limit the tuning to significantly less than the full semiconductor
gain bandwidth.
Jopson and co-workers [94] described a 1.55-pm traveling-wave semicon-
ductor amplifier in an optical fiber ring laser that used fiber birefringence to pro-
vide Lyot-filter-like wavelength control.
A 1.55-ym extemal-cavity laser comprising a InGaAsP/InP gain medium
coupled by a short piece of lensed fiber to an integrated optic, birefringence-
tuned, narrow-band TE-TM polarization converter/filter was built by Heissman
and coworkers at AT&T Bell Laboratories [95]. The polarization converter and
polarizer were integrated into a titanium-diffused waveguide on a 4-cm-long, x-
cut, ?-propagating lithium niobate wafer. Metallization overlaying an SiO, buffer
layer was patterned into transverse interleaved electrodes for electro-optically
tuning the wavelength of peak TE-TM conversion [72]. Metal directly overlaying
the waveguide without a buffer layer provided a strong differential attenuation