Baoliang Wang

Mueller matrix polarimetry with four photoelastic modulators: theory and calibration

A spectroscopic Mueller matrix polarimeter with four photoelastic modulators (PEMs) and no moving parts is introduced. In the 4-PEM polarimeter, all the elements of the Mueller matrix are simultaneously determined from the analysis of the frequencies of the time-dependent intensity of the light beam.

The Exicor DUV birefringence measurement system and its application to measuring lithography-grade CaF2 lens blanks

Optical lithography continues its transition to shorter wavelengths to support the semiconductor industry’s production of faster microchips to meet evolving market demands. The next step for optical lithography is likely to use the F2 excimer laser at 157.63 nm (157 nm,according to the industry’ s naming convention).At 157 nm, among the limited number of fluoride crystals with acceptable optical properties calcium fluoride is the only practical lens material for step and scan systems due to its readiness for mass production. Since the discovery of intrinsic birefringence in CaF2 at deep ultraviolet (DUV)wavelengths,the optical lithography industry has developed a critical interest in measuring birefringence at 157 nm. In response to this need, we have developed a DUV birefringence measurement system. In this article,we describe the working principle, system construction, technical performance and selected applications for measuring lithography grade calcium fluoride lens blanks at DUV wavelengths.

Evaluation of a dual PEM Stokes polarimeter using different signal processing methods

In our laboratory we built a Stokes polarimeter using two photoelastic modulators (PEMs). We applied two different signal processing methods to this dual PEM Stokes polarimeter. In one method, we used lock-in amplifiers to measure the PEM modulated signals. In the other method, we used Fourier analysis of a digitized waveform that contains the modulated signals. The dual PEM Stokes polarimeter can measure all normalized Stokes parameters. In this paper we present and compare results obtained using both signal processing methods. We discuss how each method benefits selected applications of the dual PEM Stokes polarimeter.

A polarimeter using two photoelastic modulators

We describe in this paper a special polarimeter for measuring linear birefringence in transparent optical samples. The polarimeter employs two photoelastic modulators that operate at different frequencies (50 KHz and 60 KHz). The resulting electronic signal at the detector is processed using two methods: demodulation by lock-in amplifiers or using Fourier analysis of a digitized waveform. We compare the results obtained by using the two signal processing methods in this special polarimeter and discuss how each method benefits selected applications.

A spectroscopic polarimeter for detecting chiral signatures in astrobiological samples

We have developed a polarimeter for accurately measuring both the circular and linear polarization components of a light beam from 400 nm to 800 nm. This polarimeter is designed to work at low light levels that are typical in astronomical applications. It is optimized to detect the circular polarization signal that is orders of magnitude weaker than the linear polarization signal. Two photoelastic modulators (PEMs) are the key polarization components employed in this polarimeter to afford the high sensitivity required for the application. Using this instrument, we have quantified the circular polarization signal produced by astrobiologically relevant microorganisms and compared the results to macroscopic vegetation (such as leaves) and abiotic minerals. Our aim is to understand whether circular polarization offers a viable technique for remote detection of chiral signatures and hence will be useful as an element of telescopic searches for life elsewhere in the Universe. We see unambiguous circular polarization from photosynthetic microbes. The circular polarization of reflected light is related to the circular dichroism of photosynthetic molecules. Therefore, circular polarization spectroscopy offers the prospect of remotely sensing lifes unique chiral signature.

Measuring stress birefringence in industrial-sized photovoltaic Si ingots

During the production of photovoltaic (PV) Si solar panels, a high level of residual stress in Si crystals often remains undetected far into the fabrication process. Available metrology is primarily limited to testing wafers, cells and panels, which is late in the manufacturing process. We describe in this poster an instrument, known as the Exicor® PV-Si birefringence measurement system, which can measure stress birefringence in Si ingots before they are sawed into wafers. This instrument measures stress-induced retardation in Si ingots at either 1550 nm or 1310 nm using photoelastic modulator (PEM) technology. It maps residual stress birefringence in squared Si ingots of typical industrial dimensions (for example, 156mm × 156mm × 500mm). It can also provide a line of stress birefringence data along the length of an as-grown Si ingot that has a circular cross-section.

A photoelastic modulator-based birefringence imaging microscope for measuring biological specimens

The photoelastic modulator (PEM) has been applied to a variety of polarimetric measurements. However, nearly all such applications use point-measurements where each point (spot) on the sample is measured one at a time. The main challenge for employing the PEM in a camera-based imaging instrument is that the PEM modulates too fast for typical cameras. The PEM modulates at tens of KHz. To capture the specific polarization information that is carried on the modulation frequency of the PEM, the camera needs to be at least ten times faster. However, the typical frame rates of common cameras are only in the tens or hundreds frames per second. In this paper, we report a PEM-camera birefringence imaging microscope. We use the so-called stroboscopic illumination method to overcome the incompatibility of the high frequency of the PEM to the relatively slow frame rate of a camera. We trigger the LED light source using a field-programmable gate array (FPGA) in synchrony with the modulation of the PEM. We show the measurement results of several standard birefringent samples as a part of the instrument calibration. Furthermore, we show results observed in two birefringent biological specimens, a human skin tissue that contains collagen and a slice of mouse brain that contains bundles of myelinated axonal fibers. Novel applications of this PEM-based birefringence imaging microscope to both research communities and industrial applications are being tested.

A near infrared stokes polarimeter for fiber applications

We describe in this paper a near infrared (NIR) Stokes polarimeter. This instrument measures the Stokes parameters of a light beam that exits an optical fiber. The key components of this polarimeter include two photoelastic modulators (PEMs) that operate at different resonant frequencies, a calcite analyzer and a Ge-photodiode detector. The two PEMs in the instrument are oriented at 0° and 45°, respectively, and the analyzer is oriented at 22.5°. The electronic signal from the detector is processed using two different demodulation methods. The normalized Stokes parameters are calculated and displayed on the Poincare sphere. We have shown the high accuracy, sensitivity and stability of this instrument in measuring normalized Stokes parameters.

P‐55: A High Accuracy Instrument for Measuring Both In‐Plane and Out‐Of‐Plane Birefringence in Birefringence Films

We report in this paper a new instrument, known as the Exicor® birefringence measurement system, that measures both in-plane and out-of-plane (vertical) birefringence in birefringence films. This instrument can be used as a quality control (QC) tool for birefringence film suppliers to test the accuracy and uniformity of their products. It can also be used as a research tool to study birefringence in birefringence films and their effects on LCD performance.