Figure 1
From: A high-resolution polarimeter formed from inexpensive optical parts
Schematic of optical setup. The beam (L) from a laser diode is passed through a 1-mm aperture (A1), an optional neutral density filter (ND), and a second 1-mm aperture (A2). A 50:50 plate-type beam-splitter (BS1) divides the laser beam into an object beam (O) and a reference beam (R). The reference beam is directed by a plane mirror (M1) through a fixed thin-film polariser (P1) onto a rotatable thin-film polariser (P3), which is mounted on a hollow-shaft motor. The object beam is directed by a plane mirror (M2) through a fixed thin-film polariser (P2); it then passes through a 5-cm optical cell, before striking the same point on the rotating polariser. The object and reference beams pass through the centre of P3 and are then directed by a third mirror (M3) onto a pair of amplified photodiodes (AP1 and AP2). P3 is rotated at a fixed angular frequency f, causing the intensity of light striking the photodiodes to be modulated at a frequency 2 f. The laser intensity is continuously monitored by means of a second beam-splitter (BS2), which directs a weak monitor beam (M) onto a light-to-frequency converter (LTFC). The digital signal from the LTFC and the analogue signals from the two amplified photodiodes are measured using a microcontroller (µC). The microcontroller analyses the input signals and generates a digital output signal equal to the phase-difference between the object and reference channels.
