Supervisor: Dr. Aleksandar Rakic
Aim of the thesis Matlab simulation of laser power fluctuations
This project investigates a novel technique of using a laser
diode as both light source and detector in order to measure
medium-length distances with sub-centimetre accuracy.
Light in a laser cavity
Lasers can be considered as cavities, with reflective edges,
that cause light to oscillate inside them. The wavelength of
this light must satisfy the following,
where L is the length of the laser cavity, is the wavelength of
the light and m is an integer.
How a compound cavity works:
r1 r2s r2ext
Light in a compound cavity
Light can also be reflected off external objects and feed back
into the laser cavity. This results in changes in the laser’s
output wavelength and power.
Creating more resonant modes Oscilloscope
A laser’s wavelength is also proportional to its operating
current. So ramping the laser’s operating current induces a
number of resonant frequencies that satisfy the first equation,
given that = c / . By subtracting two adjacent frequencies
then solving for m = 1, 0, we get the external cavity length: Electronic Workbench circuit response
Lext = c / 2 0
Measuring the output
Instead of measuring frequency we measure the corresponding
power fluctuations -- enhanced by differentiation -- that occur as
the laser light hops between resonant frequency modes. This is
achieved by using an internal photodiode, which is attached to
the rear of the laser cavity.
A Vertical-Cavity Surface-Emitting Laser (VCSEL) showing the
lazing cavity and the internal photodiode:
• Maximum error of 11 mm for target distances of 0.25 to 2m.
• Range dependent on strength of external feedback.
• VCSEL response similar to conventional laser diode.
• Develop control system and improve existing circuitry.
• Implement as a 3D system.
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