Today, Koji and I cleaned up the the lab space and made some space on the optical table for radius of curvature measurement of the A+ OMC curved mirrors. 

[Stephen, Thejas]

Today, a more rigorous effort was made to re-measure the position of the optics forming the Fizeau cavity and re-position the curved optic to get more contrasting fringes. Distance measurements were made using a Fluke laser displacement sensor. We obtained a contrasting fringe pattern but the phase profile measured was assymmeteric and un-satisfactory. Tomorrow an attempt will be made to place an iris infront of the curved optic to define the edge of the beam and limit it only to the curved optic surface. 

OMC test set-up

Yesterday, laser beam output from the fibre follwoing teh mode-matching lenses was picked off and beam profile was characterized using beam profiler Thorlabs BP209-VIS. 

The gaussian fit beam diameter was measured to be about wx = 939 um wy = 996 um at the location of a distance of 0.4 m from the high reflector. The mode content of this beam is about 98% TEM00. We want to use this beam within the Rayleigh range (near field) to measure radium of curvature of the curved optics. 

The Rayleigh range is about 0.74 m. 

[GariLynn, Stephen, Thejas]

Yesterday, we placed an iris (borrowed from OMC Lab) infront of the spherical transmission sphere to limit the spot size, on the other end of the cavity, to only the curved optic. This produced a crisp boundary for the interference pattern. We obtained some data at different imaging focal planes. The transmission optic here is a spherical mirror. This was replaced with a plane reference and the curved optic was moved closer to this optic. Intereference fringes were nuled for the plane mirror upon which the curved optic sits. This ensures that the curved mirror is head on to the laser beam. The spherical fringes were obscured by some diffraction artifacts. Today, we will be makign an attempt to eliminate that and try to see fringes from the whole curved optic. 

[Camille, GarriLynn, Stephen, Thejas]

Folllowing the replacement of the spherical transmission / reference mirror with a flat mirror, on Friday we were able to observe fringes that facilitated characterization of the curvature minimum. 

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By rotating the curved optic by 90 deg we couodn't reproduce consistent data. 

This is probably due to insufficient attention given to the orientation/centering of the curved mirror under the clamp. 

RoC: 2.65m ! Interesting. I'll wait for the follow-up analysis/measurements. The RoC may be dependent on the area (diameter) for the fitting. You might want to run the fitting of your own. If so, let me know. I have some Matlab code that is compatible with the CSV file exported from MetroPro data.

Today, I tried to measure the radius of curvature of the curved mirror using the input beam for the OMC test set-up. It was noticed that the half inch curved optic (ROC=2.5 m), when placed within the Rayleigh range of the beam waist, did not focus the beam. This is probably becasue the beam diameter is small for this optic's radius of curvature to produce any focussing. This can be illustrated even further using the JAMMT software by replacing a concave sperical mirror with a ocnvex lens of focal length of 1.25 m. 

Substrate: 1/2 inch optic with f= 0.25 m 

Substrate: 1/2 inch optic with f= 1.25 m

Substrate: 1/2 inch optic with f= 1.25 m

The only wasy to resolve this is by incresing the beam diameter to > 2 mm

If the mirror has the RoC, it works as a lens. And you should be able to see the effect in the beam profile.

Just what you need to do is to compare the beam profile without the mirror (or with a flat mirror) and then with the curved mirror.

Thanks for teh comment Koji. Yes, I did see this effect by comparing the beam sizes with and without the curved mirror. But the observation did not conform with the expectation that the beam should focus at a distance of 1.25 m from the curved mirror (as seen in the software images). So, I plan to use some lenses to increase the beam waist and perform the measurement.

I hope you can find useful lenses from the lens kit in the cabinet. If you need more lenses and mounts, talk to our students in WB and the 40m.

Thanks Koji, the lenses available in the cabinet in the lab actually sufficed.