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8 Tunable External-Cavity Semiconductor Lasers 41 5
If yLm, is not independently known, then an alternative method must be used.
This method involves inserting a filter of known amplitude transmittance .x and
negligible wavefront distortion into the external cavity so that its round-trip
amplitude reflectance is reduced by a factor 12. Then experimentally determine
the new wavelength-averaged threshold current Text* for the modified external
cavity. The value is related to the modified external feedback by
From the various threshold currents, the known additional filter loss, and the
bare-fact Fresnel reflectance. the external-cavity reflectance is then given by
The preceding formulas are only valid at the original oscillation wavelengffi
of the solitary diode laser prior to coating. To obtain a value of external-cavity
reflectance at other wavelengths, one can factor in any known spectral \,ariation
in the losses of the individual intracavity optical components and the filter.
12.3 Facet Reflectance
There are basically three methods for measuring the reflectance of AR-
coated facets. These ape summarized as follows.
The first method is a simple approach. but it can be used only for devices
with one coated facet and only when the beams from both facets are unob-
structed. After coating. the laser is operated above its new. higher threshold cur-
rent and the L-I curves from both facets are measured. The unknown reflectance
of the coated facet is related to the assumed-known Fresnel reflectance of the
uncoatedfacet (RL, = 0.31) by [117,118]
where q, and q, are the slope efficiencies for the uncoated and coated facets,
respectively (Fig. 39). Obviously, this method only determines the facet reflectance
at the emission wavelength of the solitary diode laser.
The second method [ 1191 can be applied to gain media with AR coatings on
one or both facets, and it does not require an unobstructed beam from either
facet, It relates a change in the round-trip amplification factor to a change in the
