Complete Mueller matrix imaging polarimeter for evaluating optical components for structured light

. We present a complete Mueller matrix (MM) imaging polarimeter based on liquid-crystal retarders and a pixelated polarization camera. The polarimeter instrument is first calibrated and optimized, and then applied for the precise characterization of optical components used for the generation of structured light, like patterned retarders and patterned polarizers.


Introduction
Structured light is typically produced with patterned polarization elements.Flat patterned polarization components can be fabricated with metamaterials [1] or with liquid-crystal (LC) geometric-phase optical elements [2].Patterned polarizers with arbitrary spatial distributions are also being fabricated, with great interest especially for the application to develop micro-polarizers for polarization cameras [3].Optoelectronic spatial light modulator (SLM) devices have also been extensively used to create spatial polarization patterns [4], having the important property of being reconfigurable.
Research related to patterned polarization elements and to polarimetric imaging features a dual sense.While patterned polarization elements are providing new tools for advanced polarization imaging [5], polarimetric imaging is also very valuable to evaluate the quality of the fabricated components [6].In this context, we present the advances in the realization of an automatic Mueller matrix imaging polarimeter designed to evaluate patterned polarization elements.
The polarimeter includes the following features compared to our previous system [7]: 1) the use of a multiwavelength LED light source to avoid interference and speckle noise, 2) the use of spectrally calibrated liquid-crystal retarders (LCR), both in the polarization state generator (PSG) and in the polarization state analyzer (PSA), 3) the application of polarimeter calibration and optimization procedure at the pixel level, and 4) the realization of a setup over a rotatable breadboard to easily adapt the polarimeter for a transmission and for a reflection configuration.

Polarimeter setup
The imaging polarimeter system (Fig. 1) consists in a multiwavelength RGB LED light source, a polarization state generator (PSG) composed of a linear polarizer and two liquid-crystal variable retarders (LCR), and a polarization state analyzer (PSA) composed of a polarization camera and two further LCRs.The PSA system is mounted onto a rotatory stage so measurements can be made in transmission or in reflection.The LED source has three bands with central wavelengths at 660 nm, 565 nm and 470 nm respectively.A diffuser (D) is used to obtain a uniform beam and a spectral filter (SF) is included to filter the spectra to about 10 nm width.The LCR devices are from ARCOptix.In the PSG, LCR1 is oriented at 45º and LCR2 vertically such that an arbitrary fully polarized state can be generated by adjusting their retardance [8].After lightsample interaction, the emerging light is analyzed with the PSA.The polarization camera (Thorlabs CS505MUP Kiralux) is a monochrome sensor with integrated micropolarizers.In this way images for horizontal, vertical, diagonal and antidiagonal linear analyzers can be measured in a single shot.An objective macro zoom lens (Computar MLH 10X) is attached to the camera, enabling the sample to be imaged with relatively large magnification at distances around 20 cm.In principle, adding one quarter-wave retarder in front of the lens camera suffices to also measure the circular components and retrieve the sample Mueller matrix (MM).Here we use LCR3 and LCR4 devices to achieve a better detection.

Patterned radial polarizer
As a first example we consider a radial polarizer from Codixx (ColorPol VIS500 BC3), made of 12 segments where the transmission axis is aligned radially, thus rotating 30º between adjacent segments.Figure 2

Patterned retarder
As a second example, Fig. 3 shows the experimental MM of a birefringent NBS 1963A resolution test (Thorlabs R2L2S1B).Now the elements in the first row and column become null, indicating a null polarizance and diattenuation, as expected for a pure retarder.

Conclusion
In summary, this work presents a complete Mueller matrix imaging polarimeter based on the use of a polarization camera and LCRs.The system is applied to analyzing optical components useful for structured light, like patterned polarizers or patterned retarders, where a precise spatial resolution is required.The good performance of this instrument is illustrated here on a radial polarizer and a birefringent resolution test.
displays the experimental MM, which shows the expected result for a diattenuator, with null values in the last row and last column.The rest of the MM elements show the variation between the different segments of the polarizer.