8 Tunable External-Cavity Semiconductor Lasers
-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 =
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.
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
, ...... .--:::::: ......... ::::::'" "'--:::
to Fiber
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 ~, \
"', ~",
BS I l I~
l~ AR Coatings 1- Network
~ 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
where Lca v is the total cavity length and
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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