Fabry-Perot Laser Diodes
Fabry-Perot is the simplest laser
diode there is. It is made from a Fabry-Perot optical cavity produced in III-V
semiconductor with mirrors. Optical gain is sourced by current injected into
The laser diode act like a LED is
the supplied current is below a certain threshold.
FP laser diodes are multimode
diodes. Emitted light contains many wavelengths. Large pulse stretching occurs.
FP resonant frequency occurs
Where: L = Physical
length of the diode (m)
m = Number of
n = Refractive index
of the material
Longitudinal mode spacing is given by:
Single Mode Laser Diodes
DFB (distributed feedback) and
DBR (distributed Bragg reflector) lasers are single mode lasers that emit light
of one wavelength. The have minimum chromatic dispersion effects in single mode
fibers. This can be achieved with wavelength selective optical feedback. Thus
mirrors in LDs are replaced with grating material.
In distributed feedback lasers
(DFB) grating is placed parallel to the length of the active region. DFB LD
optical power: 10 – 50 mW
spectral line width: 10 – 100 MHZ
sidemode suppression ration: SMSR > 50 dB
good performance can be achieved
Frequency chirp that occurs in
DFB LDs increases the spectral line width during modulation. The following
change in supplied current results in change in
electron density which changes the refractive index.
Instantaneous optical frequency then changes.
Frequency changes in both front and back edges
Typical frequency chirp for a DFB
laser is 250 MHz/mA.
In Bragg reflector LDs laser
grating is placed at both ends of the active region. The addition of individual
reflected waves in phase result in Bragg reflection. The following condition
must be true for Bragg reflection:
Where: m = Bragg