Laser Diodes
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 semiconductor.
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
when:
Where: L = Physical
length of the diode (m)
m = Number of
wavelengths
n = Refractive index
of the material
Longitudinal mode spacing is given by:
Therefore:
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
characteristics:
·
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
occurs:
1.
change in supplied current results in change in
electron density which changes the refractive index.
2.
Instantaneous optical frequency then changes.
3.
Frequency changes in both front and back edges
optical signals.
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
diffraction order
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