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I have 2 months to do a project on characterization of a laser parameter. We have typical optics lab equipment (DFB Lasers, oscilloscopes, random number generator, etc.). I was told to choose a laser parameter to characterize experimentally, but after hours of reading research papers, they all seem like they will take much longer to understand and test experimentally. Any ideas for which parameter is relatively simple to characterize experimentally with my short time window?

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What sort of detectors/sensors do you have available? – Carl Witthoft Feb 6 '14 at 15:27
@user40077 assuming you mean the lifetime of a photon inside the cavity, a quick google search brings up a plethora of analytic notes and discussions of test methodologies. – Carl Witthoft Feb 6 '14 at 18:05
While I think this is an interesting skrt of question to have on the site, it would be good if you could have a look at the guidelines in Good list, bad list, and maybe incorporate some of them. – Emilio Pisanty Feb 6 '14 at 22:59
up vote 3 down vote accepted

One of the simplest measurements you can do is to characterize the spatial mode coming out of the laser. This can be done with a simple photodiode and a scanning slit (optical chopper or razor blade mounted on a movable stage). I wrote a brief note on making such a measurement here. This isn't the most interesting of measurements if you aren't going to apply it to anything though.

Do you have any curved mirrors? Do you have a phase modulator? If so, why don't you try to measure the frequency noise of the laser or characterize its modal content. To do so you need to make a Fabry-Perot cavity with the curved mirrors. Be sure to mount them on a fixed spacer with sturdy, thick mirror mounts so that you don't get killed by mechanical vibrations.

Measuring the modal content is relatively easy. Just sweep the laser frequency back and forth over the tranmission peaks of the cavity using a function generator. The relative height of these peaks to the 00 peak at the FSR tells you about the relative modal content between the laser and your cavity. For this measurement you don't need the phase modulator.

Measuring the frequency noise is a little more complicated. It requires using a simple servo (such as a PID) to feedback the Pound-Drever-Hall signal to the laser frequency and recording the feedback signal with some sort of spectrum analyzer or ADC. You could also do it with tilt locking if you don't have a phase modulator, but this is probably beyond a two month project.

I would recommend starting with the beam size measurements I described at the beginning with the goal of making it to the second measurement. Playing around with an optical cavity is fun. Put a camera at the output and you will be amazed by the agreement between the mathematics you write down on paper and the shapes you see on your camera. Have fun!

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You might try measuring the temporal coherence of the laser. (This came to mind 'cause it's closely related to the PhD work I would have done if I didn't quit with an ABD :-( ).

Basically, you split the beam with, e.g., a 50-50 splitter, and recombine one way or another to get an intereference pattern. Then use an "optical trombone" (the meaning should clear :-) ) to change the path length of one leg and observe the degradation in the interference peak-valley. I'd recommend generating an analytical estimate of the coherence length so you don't build a system with path length differences limited to 1% of the expected coherence length.

Whether this is sufficiently complicated to rate 2 month's effort isn't obvious.

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Temperature sensitivity, beam divergence, spectral purity, power, beam shape, beam profile.

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This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. – centralcharge Feb 6 '14 at 13:46
It's a list of 6 parameters that the questioner may wish to characterise... – Nic Feb 6 '14 at 13:54
But it doesn't provide any justification for the difficulty and time required to get any useful data. – Carl Witthoft Feb 6 '14 at 14:20
@Carl if that's the case, I believe that would make it a bad answer (for which the appropriate response is a downvote), not a non-answer. – David Z Feb 6 '14 at 16:45
I appreciate the responses nonetheless. I just met with my professor and it looks like I will be using the transient response of the laser to characterize the photon lifetime. If anyone has any ideas as to how I can experimentally go about doing this, it would save me a lot of time. – Leo Feb 6 '14 at 17:52

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