Page 446 - Tunable Lasers Handbook

406 Paul Zorabedian

AO-tuned ECLs must have excellent suppression of diode cavity Fabry-Perot
modes in order to achieve good tuning fidelity.
An AOTF imparts a frequency chirp 4vopr = ? fa,,,,, to the filtered light on
each pass through the filter. The sign of the chirp depends on the polarization
state of the input light and the direction of passage through the filter. The sign of
the chirp is invariant with respect to simultaneous 90" rotation of the input polar-
ization and reversal of the propagation direction. With a single intracavity AOTF,
the normal modes of the laser are chirping modes [ 1001 with normal frequencies
given by

where q = 1 for a ring laser and q = 2 for an extended-cavity laser. To obtain a
set of stationary longitudinal modes, AOTFs must be arranged in chirp-compen-
sation pairs inside the laser. There have been several reports of laser tuning using
AOTFs both singly and in pairs.
Tuning of a dye laser with a single collinear AOTF was demonstrated by a
Stanford group almost 25 years ago [loll. Without chirp compensation, a filter
at h = 780 nm with a bandwidth of 0.7 nm (FWHM) resulted in a laser linewidth
of - 0.14 nm.
Tuning of a 0.85-ym semiconductor laser with an AOTF was reported by
Coquin and Cheung [102]. They also showed that the filter chirp could be com-
pensated with an intracavity A0 modulator driven at the same frequency as the
filter. Shortly thereafter, Coquin and coworkers [ 1031 reported a 1.3-ym
extended-cavity laser tuned with a chirp-compensating pair of AOTFs. Tuning
over a total range of 83 nm was demonstrated. However, oscillation was restricted
to the wavelengths of the residual laser diode Fabry-Perot modes, resulting in
nanometer-size tuning gaps. It was pointed out that this restriction was not funda-
mental and that with a combination of reduced AR coating reflectance and nar-
rower fiber bandwidth, quasi-continuous tuning would be possible.
A 1.3-pm semiconductor laser in a fiber ring cavity tuned with an AOTF
was reported by Oshiba and coworkers [104]. The cavity was identical to the
grating-tuned fiber ring laser described previously [ 871 except that the grating
was replaced with an AOTF reported to have a bandwidth of <5 nm and a peak
efficiency of 80%. The tuning range was about 80 nm. A minimum linewidth of
15 kHz was reported. The method of linewidth measurement was not described,
but it is unlikely that 15 kHz could represent the true spectral width of the
chirped-mode laser.
The elimination of nanometer-size gaps in the tuning range of acousto-
optically tuned extended and ring cavity lasers has recently been demonstrated
[46]. AOTFs specifically designed for tuning of 1.2- to 1.6-pm ECLs were fabri-
cated. The devices had bandwidths of -1.0 nm (FWHM) and peak efficiencies of