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					                                                                              Garth Niethe
                                                                              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,
                             L=m/2
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:
          L                  Lext




         r1       r2s                        r2ext
                                                                     System set-up
                                                Target
       Laser Cavity
              L
                                                                             Signal
                                                                                                        Driver
                                                                            Generator
         r1       r2




                                                                                                                                Target
        Compound
          Cavity
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.
                                                                                                   d
Creating more resonant modes                                                                                     Oscilloscope
A laser’s wavelength is also proportional to its operating
                                                                                                   dt
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:
                                                                     Conclusions
                                                                     • 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.

                                                                     Future projects
                                                                     • Develop control system and improve existing circuitry.

                                                                     • Implement as a 3D system.




                           QR CSEE Innovation Expo 2000 posters proudly sponsored by Able Systems

				
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posted:2/9/2012
language:English
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