Kane RACBP Development Page

Page for timeline & development of RACBP.

RACBP Spring 2024 Parts List:

Status updates: 

An initial version of the RACBP was constructed in the benchtop lab setup on Feb. 22.

Initial testing with a 532nm laser unfortunately showed strong and tightly spatially located double-bounce propagation through the system, indicating that the 30-arcmin wedge on the wedge windows purchase would be insufficient for canceling double-bounce issues. 

Consequently, plano-convex lenses of the fastest f-ratio were ordered and installed the following week as replacements to the wedge windows; initial observation reveal that this modification  appears to successfully cancel out double bounces, representing a step closer to taking controlled flux measurements through the system at this proof-of-concept stage. 

March: Modelling of Polarization Sensitivity + total Transmission: I dedicated several weeks to coding a mini-Zemax-like model of our system. The python code calculates the transmission of the system. 

Current Alignment Methodology

The following information is not critical for the purpose of modeling, but may be potentially useful for consideration of the sources of erroneous lens alignment in the system.

This past week, I’ve engaged in a more precise realignment procedure for the lenses. For each lens, I have threaded on an aperture-masked collimated beam to one side of the cube, and on an orthogonal face, I’ve threaded on a camera capable of live-feed images with 3.76 micron pixels. Using the live-feed, I’ve aligned the reflected beam to the center of the camera sensor as shown below.

Alignment state of the lens before camera-based alignment:

Alignment of the lens following camera-based alignment:

This approach has undoubtedly improved the alignment of the lenses compared to my by-eye alignment which I previously employed. 

I estimate the current alignment error of the lenses, as shown below, to be at or below 0.6 degrees in azimuth and altitude, following analysis in Adobe Illustrator, a vector graphics software that can measure angles between line segments. My hope is that Zemax modeling will tell me whether a more precise alignment is required. 

Note that this apparent slight misalignment of the lens with the cage cube is due to the fact that the lens is likely not positioned in the exact center of the 60mm cage cube.  As the image above shows, the LB5C1 lens optic mount is bolted to the LB4C kinematic prism platform, which is capable of 360 degree rotation of the lens and optic mount inside the kinematic cage cube, and so I have aligned both of these by eye as best I can such that the lens is at the center of the cage cube, but there is inherently human error in this. I assume the lens is not perfectly centered because the present alignment above is what produced a perfect alignment of the light beam on the camera sensor.

An overview of the project is described in this document, and also our modeling goals. 

May 15, 2024

We have an idea to improve our ability to redirect refracted light to a photodiode. We will use a wedge prism that has a wedge angle similar to the refraction angle of light through the glass, as governed by its index of refraction. As a result, the refracted light should travel down the length of the prism and exit normal to the opposite side of the prism, whose face is orthogonal to the long axis of the prism (not wedged). On this backside of the prism, we will graft on a condensing lens that should cleanly focus the light onto the photodiode. This results in more precise management and control over the light we will use for referencing to our photodiode.

Orders have been placed for: 

x1   Optical wedge, the 15 deg. Nom. Uncoated, N-BK7 Wedge Prism.
x1   Condenser lens, 25mm Dia., 0.25 Numerical Aperture, Uncoated, Precision  Aspheric Lens.
x1  2" clear aperture adapter, AD2-CA - Ø2" OD Adapter for Ø1" Optic with Clear Aperture, 0.28" Thick.
x1 2" to 1" center adapter, AD2 - Ø2" OD Adapter for Ø1" Optic, 0.25" Thick.

I was only able to find a 1" BK7 wedge (25.25 degrees) that fit the calculated refraction angle of BK7 / UVFS, which ranges from 26-28 degrees for BK7 and 27-29 degrees for UVFS over the range 0.2 - 1 micron. 

Alternatively, we could probably use a 30-60-90 prism, but I was unable to find a size larger than 1" made of BK7/UVFS, that was available off-the-shelf.

Since one surface is now N-BK7, I have ordered a 2" N-BK7 plano-convex lens to accompany it to ensure we don't introduce fractional polarization asymmetrically.