A high-resolution polarimeter formed from inexpensive optical parts

Abstract

We describe a high resolution laser polarimeter built from commodity optical components. the optical rotation angle is determined by measuring the phase difference between two harmonically modulated polarised laser beams-an 'object beam' that passes through the sample under test and a 'reference beam' that bypasses the sample. The complete polarimeter may be assembled from low cost off-the-shelf parts for less than £300 (UK Sterling). Data acquisition and analysis are carried out on a microcontroller running an efficient algorithm based on the sliding Discrete Fourier Transform. Despite its low cost, the polarimeter is a fully automatic, research-grade instrument with an accuracy of ±0.0013° and a precision of ±0.0028°-comparable to far costlier commercial instruments. The polarimeter's ease of use, compact size, fast measurement times and high angular resolution make it a capable and versatile tool for analytical science, while its low cost means it is ideally suited for use in resource-constrained environments and process monitoring. the polarimeter is released here as open hardware, with technical diagrams, a full parts list, and source code for its firmware included as Supplementary information. The tremendous advances in electronic hardware over recent decades have led to substantial improvements in the speed, accuracy, precision and functionality of scientific instrumentation. The electromechanical automation of intricate physical and chemical procedures, the development of brighter and more stable light-sources, the proliferation of low-noise sensors, improvements in amplifier technology, and the availability of better signal processing and data analysis techniques (to name but a few) have all contributed positively to analytical performance. Taking the subject of this manuscript-polarimetry-as an example, virtually all polarimeters in the early 1960s were manually operated instruments with hand-rotated analysers and telescopic eye-pieces that relied on human judgement to determine the angle of rotation. Today, they are motorised instruments with integrated photodetectors that automatically measure optical rotation angles at the push of a button. However, while functionality has improved tremendously over the past fifty years, equipment costs have changed very little: in 1965, a manually

The Open Polarimeter: A High‐Resolution Instrument Made from Inexpensive Optomechanical Parts

“…Opol's low build cost is made possible by the use of a phasebased measurement technique (instead of an angular one), which removes the need for any precision-engineered parts. [5] Stripped to its essentials (Figure 1a, top), the polarimeter consists of two vertically polarised laser beams L1 and L2 that pass through the centre of a polariser P before striking two photodetectors D1 and D2. The polariser is made to rotate at a constant speed and the two detectors are read by separate analogue input pins of a microcontroller, with the polariser rotation causing the measured signals to vary in a sinusoidal fashion.…”

“…The optical layout used here is similar to the one described in our previous paper (see Figure S1), [5] except to reduce costs we have eliminated one plane mirror, one optical density filter, Figure 1. Optical lay-out and measurement principle.…”

The Open Polarimeter: A High-Resolution Instrument Made from Inexpensive Optomechanical Parts

“…Opol's low build cost is made possible by the use of a phase-based measurement technique (instead of an angular one), which removes the need for any precision-engineered parts. 5 Stripped to its essentials (Fig. 1a, top), the polarimeter consists of two vertically polarised laser beams L1 and L2 that pass through the centre of a polariser P before striking two photodetectors D1 and D2.…”

“…The measurement of the phase difference is therefore carried out twice, first without ( ' ) and then with ( ) ) an optically active sample present in the cell. The change Δ = ) − -in the measured phase difference is equal to twice the optical rotation angle $%& , 5,7 i.e.…”

“…The optical layout used here is similar to the one described in our previous paper (see Fig. S1), 5 except to reduce costs we have eliminated one plane mirror, one optical density filter, and one 50:50 beam-splitter. In the new design, the two glass slides BS1 and BS2 reflect approximately four percent of the incident beam intensity into the reference and object beam, respectively, while the transmitted light is dumped into a beam-stop.…”