Fiber-Optic Sensors - Applied Optics Fundamentals and Device Applications - page 180

where
G i = g i α is the optical gain of the i th mode
g i is the Lorentzian distribution =

( )

+ b i

2

1 1

α is the optical gain of mode zero
b is the mode selectivity of the i th mode

For a laser structure with a gain spectrum of several hundred Angstroms,
the mode selectivity is in the range of 10 −4 so that it takes a relatively long
time for the different modes to settle to their steady amplitude. Assuming
α occurs at the peak of the optical gain curve and that spontaneous emis-
sion is negligible, then at t = ∞ only one mode can exist because all other
modes will decay to zero. The above equation cannot be used to describe the
time-dependent spectrum if the laser is modulated continuously at high fre-
quency. It has been observed that the lasing spectrum under high-frequency
(microwave) modulation will remain single mode if a critical modulation
depth is not exceeded [27].

4.17.3 Experimental Observations

Following are some important observations concerning mode stability in
semiconductor laser diodes.

• Carrier density fluctuations that occur under certain conditions of
high-frequency modulation result in increased linewidths of the
individual lasing modes [28].
• The spectrum of direct modulated lasers depends on the amount of
mode selectivity in the laser.
• Lasers can be constructed with a frequency selective element. This
frequency selective element is used in the DFB laser. The DFB laser
can sustain single-mode oscillation even under turn on transients
and high-frequency modulation [29].
• Lasers having a very short cavity length sustain single-mode oscilla-
tion under turn on as well as under high-frequency modulation [30].
• Short cavity and composite cavity lasers settle to a single mode faster
than a long cavity laser of similar construction [31].

4.17.4 Guided Index and DFB Laser Operation

Many critical applications such as optical disc reading and recording or
coherent communications require a stable single-mode laser. One method
presently used to achieve single-mode operation involves the use of a con-
trolled perturbation in the plane of the p-n junction [32] to act as an internal
waveguide to control the mode of operation. This structure is known as