Page 418 - Tunable Lasers Handbook

378 Paul Zorabedian

instabilities [24] and the fidelity with which the oscillation wavelength tracks the
peak feedback wavelength [46]. The figure of merit for external feedback
strength is the cavity loss ratio, that is, the ratio of the mirror loss of the solitary
cavity to the loss of the external cavity. Table 5 defines the cavity loss ratio for
the three different external cavity classes. The cavity loss ratio should be at least
20 dB for any external cavity design.
There are two figures of merit for wavelength selectivity. The first is the
solital? cavih mode selectivity,

which is the ratio of the filter FWHM bandwidth to the solitary cavity axial
mode spacing. Provided the cavity loss ratio is >20 dB. good tuning fidelity in
the tracking between the oscillation wavelength and the peak feedback wave-
length will be obtained with Nint less than about 0.3. For Nln, = 1, a cavity loss
ratio of at least 30 dB is needed to obtain reasonably linear tuning.
The other figure of merit for wavelength selectivity is the atel-nul-cavio
mode selectivity,

which is the ratio of the filter FWHM bandwidth to the external cavity mode
spacing. To ensure single mode operation, it is necessary to have Ney, I 1.

6.2 Component Throughput
The most critical component for determining the feedback strength is the
intracavity collimating lens. A general requirement for strong feedback in

TABLE 5 Figures of Merit for External
Feedback Strength
~ ~ ~
Cavity configuration Cavity loss ratio

R
Extended

Double-ended 1Olog

Ring