Laser Diode Back Reflection

Small demonstration of how back relfected light can destroy coherence of a laser diode.
A collimated laser beam is used in a simple 'glass plate fizeau type interferometer' to create a stripe pattern. Adjusting the angle of the incident light beam to be exactly perpendicular to the glass plates surface creates a reflection that is focused back into the laser diode chip by the collimating lens.
This decreases coherence length quite dramatically so that the strip pattern vanishes.

[@shoopdawhoop]

I have an idea for an experiment but have no resources to implement it, so maybe you can do it. The idea is to split the beam with 50% mirror, and the second beam goes into the simplified Michelson interferometer which outputs a circular, or just concentric pattern at the probe area. The photoelement is placed there at the pattern center, and the voltage it creates is used to throttle the laser diode voltage through the feedback circuit connected between the source and diode - which can be built on 1 or 2 field transistors, resistors, and potentiometers to adjust the feedback. Theoretically, this scheme can lock the laser frequency (after 3-5 minutes of heating up, of course) and effectively eliminate the thermal drift, so it becomes ultra-stable and suitable for DIY holography. The longer the interferometer gap - the better precision it reaches.

[@DiffractionLimited]

This is actually what I use for an interferometer project I am working on currently. Thanks for sharing the idea.

[@shoopdawhoop]

@@DiffractionLimited May there possibly be any experiments with the DCG holograms in the future?

[@Netsroht72]

Its not always evil. Back Reflexion can also stabilise the Laser under controlled Conditions. Called External Cavity Diode Lasers (ECDL).

[@DiffractionLimited]

Umm yes the video makes quite an oversimplification. Btw if you know how to make a normal laser diode operate in external cavitiy mode to increase coherence length I would love to know. I could make it work so far.

[@Netsroht72]

Only theoretical.
Search for "W's Laser-Projects". That guy does it in practise. Works only with some kind of Laserdiodes and needs a super stable setting.

[@GrantFalkenburg]

Why does the back reflection destroy the coherence? Is it because the temporal coherence of a laser diode is low, so the back reflected light is incoherent with respect to the cavity light, and therefore gets amplified in a non-coherent way?

[@shoopdawhoop]

Every blue LED (and blue laser as well) can act like a photo element and generate electricity. So, as the light goes reflected back to the diode, a counter-current appears which increases the resistance of the diode and so its consumption drops - just as if we decreased the driving voltage. As long as the frequency of the laser diode highly depends on the power consumption, this causes the striped pattern to move. This changes the back-illumination itself and creates the feedback loop which causes laser frequency to oscillate randomly and quickly, and so do the stripes as well, resulting in a temporal incoherence. A similar trick can be used not to destroy, but to enhance the coherence - placing the glass plate close to the diode at a certain distance and a certain power level can force it to operate single-mode instead of multi-mode.

[@GrantFalkenburg]

@@shoopdawhoop makes sense!