8 Tunable External-Cavity Semiconductor Lasers
407
-85% when driven at 2.75 W. An extended-cavity configuration containing a
pair of chirp-compensating AOTFs provided 11% feedback (Fig. 34). The
reflectance of the feedback-coupling facet was 3 x 10-5, leading to a ratio of
diode cavity loss to external-cavity loss of 36 dB. The wavelength was measured
versus drive frequency in 10-kHz steps across an 80-nm range. A theoretical tun-
ing curve of the form X =
(a/fa) + b
was fit to the data. The residual of the fit was
0.036 nm rms averaged across the 80-nm tuning range.
A ring configuration was also studied (Fig. 35). The ring cavity provided
about 1% feedback but the ratio of diode cavity to external-cavity loss was
increased to 46 dB because both facets of the gain medium were AR coated
(Rface t =
5
10-4).
In this case the rms tuning error decreased to 0.018 nm. This
study demonstrates the utility of the cavity-loss ratio as a figure of merit for
optimizing tuning fidelity.
9. MODE SELECTIVITY OF GRATING CAVITIES
Of the various types of filters used to tune ECLs, diffraction gratings provide
the narrowest nonperiodic spectral bandwidth. As shown earlier, the grazing-
incidence configuration has the narrowest bandwidth. Most of this advantage
comes from the use of a steeper incidence angle (--85 ~ for the grazing-incidence
configuration versus--50 ~ for the Littrow configuration). In addition, double pass-
ing gives another factor of 2. Thus,
for identical beam diameters,
the grazing-
incidence configuration has a resolution advantage of about 2 x [tan (85 ~ / tan
(50~ -- 20 times over the Littrow configuration. However, this conclusion carries
the important stipulation that the grating must capture the full width of the beam.
Diode AOTF #1 AOTF #2 Mirror
Laser
, ...... .--:::::: ......... ::::::'" "'--:::
Output
to Fiber
[]
Matching
Networks
Sig
Gen
FIGURE 34
Extended cavity laser tuned with two chirp-compensating AOTFs. (Reproduced
with permission from Zorabedian [46]. 9 1995 IEEE.)
408 Paul Zorabedian
..'V ou pu, "'.2"..
," ," ~to Fiber ", ",.
.'" ~"" AOTF #1 ~, \
AmplifierOptical
AOTF #2
"', ~",
BS I l I~
Matching
Matching
l~ AR Coatings 1- Network
Amp
~ A
FIGURE 35 Ring
ECL tuned with two chirp-compensating AOTFs. (Reproduced with permis-
sion from Zorabedian [46]. 9 ! 995 IEEE.)
In practice, the grating resolution will ultimately be limited by the width of the
ruled area. For example, assume both configurations use a 30-mm wide grating
that is fully illuminated by the coupling optics. In this case, the grazing-incidence
geometry will have a filled depth of 30 mm x sin(85 ~ = 29.9 mm, whereas in Lit-
trow the filled depth will be 30 mm x sin(50 ~ = 23.0 mm. This reduces the spec-
tral resolution advantage of the grazing-incidence configuration to a factor of
about 2 x (30/23) -- 2.5, that is, by almost an order of magnitude.
Furthermore, the figure of merit for determining how well a cavity maintains
single-mode operation is not the filter bandwidth but rather the number of longi-
tudinal modes within the passband. Cavity parameters that are representative of a
typical grazing-incidence cavity are ~, = 670 nm, beam diameter = 1 mm, grating
angle = 85 ~ and cavity length = 7.5 to 15 cm [105]. Therefore, the number of
modes in the grating passband is between one and three. For a Littrow cavity, the
number of modes in the passband is given by
L cav
Nm~ ~ Lg
(69)
where Lca v is the total cavity length and
Lg
is the filled depth of the grating. By
eliminating as much air space as possible within the cavity, a practical limit of
about two modes can be reached. One way to minimize the cavity length for a
given resolution is to butt the grating up against the coupling lens [83]. This tends

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