Highly accurate calibration of polarimeters

#The present invention relates to optical measurement devices, and more particularly to a method of high accuracy calibration of complete polarimeters.

#Commercially available polarimeters work either with only a detector and a rotatable wave plate or they are based on a multi-detector arrangement with four (or more) detectors. Complete polarimeters are polarimeters that measure all four Stokes parameters. On multi-detector polarimeters, the incident light power is broken down into at least four separate power components. At least three of the four power components pass polarising elements and thus become polarisation-dependent. One or several birefringent elements, which are arranged in front of the polarising elements, ensure that the maxima of the four detector currents occur at different input polarisations.

#A 4×4 calibration matrix B for the polarimeter gives a clear connection between the four detector currents I0 . . . I3 and the four Stokes parameters S0 . . . S3.

#FIG. 1 is a block diagram view for the absolute calibration of a polarimeter with a deterministic polarisation controller according to the present invention.

#FIG. 2 is a block diagram view for the calibration of a polarimeter with a non-deterministic polarisation controller and a comparison polarimeter according to the present invention.

#FIG. 3 is a block diagram view for the relative calibration of a polarimeter with a non-deterministic polarisation controller but without a comparison polarimeter according to the present invention.

#FIG. 1 shows a first arrangement for the calibration of a polarimeter. The polarisation of a laser 1 is adjustable through a deterministic polarisation controller 3 having a rotatable polariser 4, a rotatable λ/4 wave plate 5 and a rotatable λ/2 wave plate 6. A manual polarisation controller 2 arranged in front has the task of adjusting the polarisation in front of the polariser 4 so that minimum loss occurs.

#The polariser 4 guarantees a degree of polarisation of 100% (DOP=1). The adjustment angle of the λ/4 wave plate 5 influences the ellipticity, and the adjustment angle of the λ/2 wave plate 6 influences the azimuth of the polarisation at the output of the deterministic polarisation controller 3. Since the polarisation parameters are deterministically adjustable, i.e., the Stokes parameters are known, an absolute calibration is thus possible. The calibration refers to the co-ordinate system of the polarisation controller 3. The polarisations with DOP=1 thus generated reach the input of a polarimeter to be calibrated 7 featuring an evaluation and display unit 8.

#Using an arrangement according to FIG. 2, a polarimeter 7 may be calibrated with a comparison polarimeter 11. The laser light is 100% polarised in the polariser 4. The manual polarisation controller 2 arranged in front ensures minimum loss through the polariser 4. With the aid of a second manual polarisation controller 9 all possible states of polarisation may be generated while retaining the degree of polarisation DOP=1. A coupler 10 splits the optical power into two components: one is made available to the polarimeter to be calibrated 7 and one is made available to the comparison polarimeter 11. An absolute calibration is only achieved with respect to the reference level of the comparison polarimeter 11. If the coupler 10 is a fibre-optic device, an absolute calibration is achievable if the fibres involved in the set-up are not subjected to any movement. If the fibre is moved after all, any change to the input polarisation both in the polarimeter to be calibrated 7 and in the comparison polarimeter 11 effects the same changes in the display, not, however, the display of the same absolute value.