Heterodyne polarimeter with a background subtraction system

A method for measuring the optical rotation of a polarized light beam by an optically active substance using a polarimeter with an efficient background subtraction system, comprising the steps of:

• a) generating by a laser a first coherent light beam of a first frequency;
• b) linearly polarizing said first light beam by passing it through a first polarizer, so that its electric field vector becomes parallel to the main plane of the polarimeter, defined by the polarizer's transmission axis and the beam's direction;
• c) directing said first linearly polarized light beam through an interferometer, whereby:
• (i) said first linearly polarized light beam of (a) is split, so that one part of it is reflected at an angle with respect to the original direction, creating a second linearly polarized beam, and the other part is transmitted in the original direction, creating a third linearly polarized beam;
• (ii) the electric field vector of said reflected second linearly polarized beam is rotated by mirroring it around an axis set at 45 degrees with respect to the polarimeter's main plane, so that its polarization becomes orthogonal to the polarization of said first linearly polarized beam;
• (iii) the optical frequency of said second beam is shifted by an amount ranging, typically, from several hundred hertz to several kilohertz, so that said reflected beam now has a second frequency different from said first frequency;
• (iv) said second beam is reflected so that it returns exactly along its original path;
• (v) said transmitted third linearly polarized light beam is passed through a sample comprising an optically active substance and its electric field vector is rotated by said optically active substance;
• (vi) the relative phase of the orthogonal components of said third linearly polarized light beam is changed, either from 0 to π/2 radians or from π/2 to π radians, said change can be continuous following, for example, a sine waveform at a low frequency of several hertz;
• when the relative retardance angle is either 0 or π radians, said third linearly polarized light beam becomes a references beam, and when the relative retardance angle is π/2 radians, said third linearly polarized light beam becomes a measurement beam, the transition between said reference and measurement beams can be continuous, following, for example, a sine waveform at a low frequency of several hertz;
• (vii) said reference beam is reflected, so that it returns along the path of said third light beam and the relative phase of its orthogonal components is, again, changed by the same angle of either 0 or π radians, so that when it passes through the sample in the reverse direction, its electric field vector is rotated by said optically active substance in the sample, in such a way that said linearly polarized reference beam accumulates no net optical rotation with respect to said first linearly polarized light beam;
• (viii) said measurement beam is also reflected and returns along the path of said third light beam and the relative phase of its orthogonal components is, again, changed by the same angle of π/2 radians, so that when it passes through the sample, its electric field vector is rotated by said optically active substance in the sample, and its optical rotation, imparted by said optically active substance, is doubled with respect to said third linearly polarized light beam;
• (ix) said linearly polarized reference and measurement light beams, transmitted by said sample, are combined and interfere, in their turn, with said second, frequency-shifted, linearly polarized light beam to generate a fourth light beam;
• d) passing said fourth linearly polarized light beam through a second polarizer, whose transmission axis is perpendicular to the transmission axis of said first polarizer;
• e) directing the transmitted component of said fourth light beam to a photodetector to generate a heterodyne signal;
• when said fourth light beam is the result of interfering said reference beam with said second beam, said heterodyne signal is a reference signal proportional only to the amount of polarization noise in the polarimeter;
• when said fourth light beam is the result of interfering said measurement beam with said second beam, said heterodyne signal is a measurement signal proportional also to the optical rotation imparted by said optically active substance, the transition between said reference and measurement signals can be continuous following, for example, a sine waveform whose minima are said reference signal and maxima are said measurement signal;
• and
• f) storing and processing said reference and measurement signals and generating comparison signals using said reference signals to subtract any background signals from said measurement signals, said comparison signals indicating the polarization rotation angle imparted to said linearly polarized measurement light beam by said optically active substance, said polarization rotation angle can be used to infer the concentration of said optically active substance.