Page 466 - Tunable Lasers Handbook

426 Paul Zorabedian

FIGURE 45 Facet reflectance 2t onset of axial mode instability versus external feedback and
linewidth broadening factors. (Reproduced UI. ith permission from Zorabedian [24]. 0 1994 IEEE.)

facet, that is, -31%. Second, the linewidth broadening factor is usually not a
parameter that is directly under the control of the cavity designer. For 1300- and
1550-nm Fabry-Perot diode lasers, (Y. has a typical value of 4 to 7 near the gain
peak and increases to >10 at the band edge [6], which implies that stability is
more difficult to achieve at the long-wavelength edge of the tuning range. This
has been experimentally confirmed.
Note that this analysis has its limitations. Although it is intuitively obvious
that the resolution of the intracavity filter should have a strong influence on axial
mode stability, this parameter is not taken into account. Therefore. the analysis
should be interpreted in the following way: If unstable as well as stable modes
lie within the filter bandwidth. the laser is likely to go into a multimode state.
Narrowing the filter bandwidth increases the ability to reject the unstable modes
and achieve steady-state single-mode operation.

16.3 Tuning Nonlinearity

Oscillation occurs at the cavity mode with the lowest threshold gain. Due to
residual facet feedback, this mode may differ appreciably from the peak wave-
length of the filter. For a given set of cavity parameters (filter bandwidth, facet
reflectance, and external feedback) the lowest threshold mode can be found
numerically for e\7ery position of the filter peak [145]. A plot of oscillation
wavelength versus filter peak wavelength shows the tuning fidelity of the laser. A
filter bandwidth that is narrow compared to the solitary mode spacing and a
large ratio of external feedback to facet feedback results in a nearly linear tuning
curve (Fig. 46). A broader filter bandwidth and a smaller feedback ratio gives a
more nonlinear tuning cune that tends toward a staircase (Fig. 47). Overall tun-