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