The overall goal of this project is to characterize a violet diode
laser (nominal wavelength = 404 nm) by careful measurements of such
properties as: the laser wavelength and wavelength changes with
changing temperature, the output power and its stability over time,
the degree of polarization, and the shape and size, quality, and
divergence of the elliptical beam.

Diode lasers are made from a semiconductor material such a gallium
arsenide (GaAs) or gallium nitride (GaN). Since the light is emitted
from a tiny cavity within the material it is highly divergent and must
be focussed with some sort of lens. Besides a lens, laser diode
assemblies often include a photodiode for monitoring and controlling
the output power and a Peltier (thermoelectric) cooler for controlling
the diode temperature. Temperature control is important because
wavelength changes with temperature in a complicated discontinuous way
(relatively small steady changes interrupted by larger jumps called
"mode hops").

Red diode lasers are the most common type and these are widely used in
CD players, laser printers, and CDROM drives. These applications and
others will benefit from recent advances in laser technology, which
have made possible lasers in the blue, violet and even ultra-violet
(UV). These shorter wavelength diode lasers will allow for increased
storage capacity of DVDs and higher resolution laser printers.
Although shorter wavelengths have been achieved in the past through
frequency doubling, being able to directly create these wavelengths is
much simplier and more cost effective.

Our laser is a Model PPMT manufactured by Power Technology
Incorporated that was donated to the Laser Teaching Center. It was
manufactured about five years ago and originally cost around $5,000.

The wavelength of the laser was measured by analyzing the diffraction
pattern from a grating that had been previously calibrated using the
known 632.8 nm wavelength of a red HeNe laser. The grating was found
to have a groove spacing "d" of 1.35 (?) microns, and the measured
laser wavelength was 407 nm, about 1% higher than the nominal value.

Wavelength changes with temperature where studied by adding a 500 mm
focal length lens and a linear-diode-array detector to the setup. The
array consists of 1000 elements spaced 25.4 microns (1/1000 inch)
apart. The light intensity hitting each of these elements could be
observed on an oscilloscope. By taking photographs of the display the
centroid (mathematical center) of a spectral line could be determined
to a precision of better than 0.01 nm. Observations were made in
second order, where the dispersion (beam spot movement per unit
wavelength change) is 800 microns/nm [??].

Wavelength changes were recorded as the temperature was changed in
steps of about 0.4 C from 20 C (the lowest attainable temperature) to
30 C (the maximum safe value). One mode hop was observed over this
range, at about 21 C. Over the remainder of the temperature range
wavelength increased with temperature at a rate of 0.018 nm/C. [is
this the final value?]

Laser power was measured as 3.5 mW in the central spot, which was
surrounded by a diffuse halo. [Do we know what diode current this was
for?] The laser was found to be highly polarized, but an accurate
result for the degree of polarization will require a better type of
polarizer for the test.

-------------------------------------------------------------------

Lindsey Garay, August 2005