Page 462 - Tunable Lasers Handbook

422 Paul Zorabedian

Fabry-Perot etalons can serve as frequency references in any wavelength
region, but their resonance frequencies are susceptible to the same disturbances
that cause residual FM noise in the laser. Absorption lines provide better stability.
Atomic absorption lines originating from the ground state are available for stabi-
lizing visible and near-infrared lasers. Due to the lack of atomic absorption lines
originating from the ground state in the 1.3- and 1.5-ym wavelength regions.
molecular transitions have been used for stabilization of long-wavelength com-
munications lasers. Molecular spectra are generally more complex and weaker
than atomic spectra. These complications have motivated some workers to use
nonlinear crystals to generate the second harmonic of 1.3- and 1.5-pm laser out-
put in order to make use of atomic lines at shorter wavelengths. An alternative for
stabilizing 1.3- and 1.5-ym lasers is to use excited state transitions of noble gases
such as neon, argon, krypton, or xenon inside a discharge lamp. Excited state
absorption increases the population of the upper excited state of the transition.
The upper state has a higher ionization probability because it requires less energy
to be ionized. Thus, under resonant irradiation the lamp requires less discharge
current to maintain the steady state so that its impedance is increased. This is
called the optogalvanic effect [ 1321. A large number of optogalvanic transitions
have been surveyed [ 1331. Table 8 lists a number of reports of solitary diode laser
and ECL frequency stabilization.

16. ADVANCED MODELING TOPICS

16.1 Variation of Threshold Gain with Length

The oscillation threshold condition for an extended cavity laser is given by

where rj, is the reflectance of the outer facet and reeft(v) is the frequency-dependent
effective reflectance of the external cavity defined by Eq. (32). In the following,
the assumption is made that the oscillation frequency v is an independent variable
that is identical to the peak feedback wavelength of the external-cavity filter. The
threshold gain as a function of frequency is then determined by simultaneous solu-
tion of the magnitude and phase parts of the threshold condition [23]:

and