In this project, several of the important characteristics of diode lasers will be
investigated. We will examine the power, spectral characteristic, beam profile, frequency
and current characteristics of diode lasers as well as their phase and amplitude noise
Laser geometry of the output beam characteristics of a diode laser are determined by its wave guide characteristics and vary somewhat depending on the waveguide construction and the mode (transverse) in which the beam is lasing. The radiation field
from these faces is dependent on the operating mode of the laser, and for fundamental
mode operation the diode has a major lobe as shown in Figure 1. When the laser is operated in a higher order mode, then a double lobe pattern is observed as illustrated in Figure 1 for θy. Higher mode patterns are less efficiently coupled into single-mode fibers and are avoided in single-mode communications. The half power points
are approximately given by
which for λ=0.8 μm, x =0.9 μm, y =4 μm produces θx~19°, θy~4.4°. Values
represent typical single-mode laser beam characteristics. A second important characteristic of diode lasers which affects coupling is astigmatism. Astigmatism is the property that the apparent focus of the beams are not collocated. Figure 1 shows the foci of the beams at different places relative to the cleaved facet.
Laser characteristics when a current is passed through the diode, electron/hole pairs recombine in the active region of a forward biased diode laser and emit radiation. As the intensity of light increases, threshold for stimulated emission is reached and the laser begins to oscillate. At this point, the light output from the laser increases dramatically as shown in Figure 2.
Below threshold, the diode acts as a light emitting diode (LED) with a very broad spectral
output (~150 nm spread) whereas as one increases the current well above threshold,
single-mode lasing is obtained...