5X Laser Polarimeter

5. Laser Polarimeter (2021)

One aspect of working with NLOs is they require compatible polarised light, and I need to verify I am meeting that criterion. A simple way of achieving this is with a polarising cube. I bought the vertically polarised HeNe laser for use as a polarisation reference.

However polarisation is not limited to binary states and I recognised the benefit of an instrument that could reveal the angle of polarisation, particularly when assessing the necessary voltage to apply to a Q-switch pockels cell in order for it to match its associated quarter or half waveplate.

Polarimeters are expensive. The one I keep seeing on eBay is the now obsolete Thorlabs PA Series. Usually just the data interface box appears quite cheaply by itself, occasionally heads appear too but then the price shoots up, even more if they are together. But a box and head is not enough, as I discovered from the helpful description of a complete system listed at $2000:

PA430 polarimeter optical head, with FC/PC & FC/APC FO connectors to polarimeter data interface box
Keithley DAS-1702HR Data Acquisition Board (installed in the computer, with software and manual)
DAS Cable (to connect the data interface box to the DAS board in the computer)
Two [non-standard] Male/Female DB9 Cables (to connect the optical head to the data interface box
PC with software installed, configured and tested working
CD with Thorlabs Polarimeter Software and User's Manual

DriverLink software to enable the polarimeter software to interface with the Keithley DAS board
DAS Support Software CD, including registration card with registration number required for installation

video card display adapter driver

But this system measures far more than just the angle of polarisation. Understandably I abandoned the idea of the Thorlabs route and began to think if I could build my own. A simple manual setup of a PBS cube and polariser together with PD to determine the peak and relative energy should do the trick, but I didn't relish manually rotating the polariser in the beam path of a Nd:YAG, so at first I thought of motorising the polariser.

Paradigm Lasers used to sell their small self-contained $200 motorised LED-based O-Tool, but that was superseded by their $500 PolaScope http://www.paradigmlasers.com/Polascope/Polascope.htm

Roll forward to 2021 when I bought a non-functional Cambridge Research Instruments CRi PROUV UV laser stabiliser because the seller described it having a Pockels cell and UVPBS inside. I was curious to learn how it worked, and wondered if I could modify it to build my own automated polarimeter. Before now it had never occurred to me to use a Pockels cell.

CRi no longer exists, but I found Brockton Electro-Optics (BEOC) took over their laser power division
and their LS-PRO looks almost identical, also described as models LS-100: VIS 400nm-740nm), and
LS-101: UV 325nm-740nm [assumed]. http://www.brocktoneo.com/products/products.htm

Another potential use for it was to employ its Pockels cell as a Q-switch for the ruby laser, as I learned from a later auction the BBAR Inrad Pockels I bought earlier might not work up to 694nm.

The description intrigued me: 'With a 2MHz noise reduction bandwidth, this hybrid stabilizer uses a Liquid Crystal (LC) cell and Pockels cell feedback elements. This allows us to provide a high frequency noise reduction without the HIGH VOLTAGE drive requirements of typical Pockels cell stabilizers.', and the datasheet: 'Transmittance of 74 - 80%...Stabilizes CW and mode-locked laser power to 0.03%'.

The alternative use of the Pockels cell as a sensor ruled out my ruby Q-switch idea, and the LCD limited maximum power to 1W.

http://www.brocktoneo.com/products/images/LSDS0805.pdf

It wasn't all bad. If they were able to control the polarisation of a laser beam using just an LCD, surely I could also use it to measure polarisation, and although metalwork constrained the beam size to 2mm, the optics path inside looked wider.

Thorlabs offers an LCD retarder 'software...allows the user to set the retardance (in units of waves, degrees, or nm) or the voltage across the liquid crystal cell (0V to 10V, with 0.2mV resolution). For retardance sequences': https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=8831

The LCD approach reminded me of a Meredith Lasers HeNe laser LCD attenuator assembly I bought long ago at the knock down price of 6 for $25. At that time neither of us knew its original purpose but now that is no longer the case, and although still for sale, they are now (a still reasonable) $25 each. I thought again about the Meredith LCD attenuator and wondered if, rather than being an attenuator, it was always intended to be a stabiliser?

What appears from its URL to be a Russian copy of Sam's Lasers website, provides a working design for a laser stabiliser based on it:

Close up photo of the LCD assembly: http://www.laserfaq.ru/sam/attentr1.jpg

His HeNe stabiliser: http://www.laserfaq.ru/sam/seashl1.jpg

Here's the circuit: http://www.laserfaq.ru/sam/sgas1sch.pdf

THE PLAN

The plan is to see if I can employ either of these LCD assemblies to build a 'simple' polarimeter, initially consisting of the vertically polarised Rofin-Sinar hene through the LCD and PBS into an energy meter, the aim being to calibrate the vertical position of the LCD polariser.

I may be able to manually repeat the experiment using the Glan-Taylor polariser and PBS I have, although from earlier experiments I am suspicious it is misaligned, which ironically is one of the reasons for wanting a polarimeter in the first place.

Question - Will a different wavelength have a different effect? i.e. if I calibrate it with the HeNe and then send a vertically polarised CW Nd:YAG through it, will I get the same result?

If not, my idea of using the HeNe as a reference is moot.

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