Page 412 - Tunable Lasers Handbook

372 Paul Zorabedian

When the gain medium is pumped sufficiently hard, oscillation will occur at
the external-cavity mode that sees the lowest net cavity loss. Ideally, the loss
ripple due to the facet reflections of the gain medium is weak and the filter
bandwidth is narrow compared to the period of the ripple. In this case the oscil-
lation will occur at the external-cavity mode that is closest to the loss minimum
of the wavelength-selective filter. Usually the wavelengths of the external-cavity
modes are fixed, that is, they do not shift as the filter is tuned. In this case, as the
minimum-loss wavelength of the filter is varied, the laser oscillation repeatedly
hops to the next external-cavity mode. The output wavelength tracks the filter
peak wavelength in a quasi-continuous linear fashion (Fig. lla). This type of
tuning behavior is known as stepise tuning. It is also sometimes called pseudo-
continuous or quasi-continuous tuning. (Cautionary note: Some authors refer to
stepwise tuning among external cavity modes as “continuous tuning” because
the spacing is much finer than that of the solitary diode-cavity modes. However.
in this chapter, this terminology is reserved for lasers that tune without any
mode hops whatsoever. This type of tuning is discussed in a later section.)
In practice, the loss modulation caused by the diode-cavity etalon will cause
some amount of tuning nonlinearity. Measurable tuning nonlinearity can occur
even with surprisingly low facet reflectances (Le.. <l%). If the diode-etalon loss
ripple is strong and the filter bandwidth is comparable to, or wider than, the soli-
tary cavity mode spacing, then the tuning relation tends toward a staircase (Fig.
1 lb) with wavelength jumps approximately equal to Avmt. Such discontinuities
in the h,,, vs h,, characteristic are called tuning gaps. The suppression of soli-
tary cavity etalon effects is a major reason why the double-ended or ring exter-
nal-cavity configurations are sometimes used.

4.3 Spectral Narrowing
Many solitary Fabry-Perot diode lasers, especially long-wavelength infrared
lasers, run on a multiplicity of axial modes spread over several nanometers.

1 Solitary Cavity
T Mode Spacing
Mode Spacing

filter A filter
FIGURE 1 1 Schematic tuning curves: (a) Limit of ne& facet reflectances and narrov. filter
bandwidth. (b) Limit of strong facet reflectances and broad filter bandwidth.