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The photoelastic modulator (PEM) is a polarization modulator that operates at the resonant frequency of its optical element. The PEM is made of isotropic optical materials, in contrast to birefringent materials used in electro-optic modulators. These two characteristics, operation at resonance and the use of isotropic optical materials, give the PEM unique optical features, such as high modulation purity and efficiency, broad spectral range, high power handling capability, large acceptance angle, large useful aperture and high retardation stability. These features make the PEM an effective polarization modulator in a variety of applications. Sometimes it is the only choice for high sensitivity applications. In an effort to characterize the PEMs more thoroughly, we are carrying out a series of tests on the basic properties of the PEM. Residual birefringence is an important property that affects the quality of a PEM. In the second paper in a series, we focus on the measurement of residual birefringence in the optical element of a PEM and maintaining the residual birefringence at a low level in the final PEM product.

Photoelastic modulator (PEM) based polarimeters have been used for plasma diagnostics of magnetically confined fusion devices for over 15 years. With the invention of a new laser operating at 47.7 and 57.2 microns, using this radiation for plasma diagnostics has become possible, providing that PEMs can be made for these wavelengths of radiation. Recently, a PEM has been made which meets these requirements. The device uses a silicon optical element with a single-layer polymer anti-reflective coating. Design decisions during the development and performance characteristics of the new PEM will be discussed. Topics include the choice of silicon as an optical element material, antireflective coating design and material choice, optical transmission, maximum retardation, useful aperture and modulation frequency.

Photoelastic modulator (PEM) based polarimeters have been used for plasma diagnostics of magnetically confined fusion devices for over 15 years. With the invention of a new laser operating at 47.7 and 57.2 microns, using this radiation for plasma diagnostics has become possible, providing that PEMs can be made for these wavelengths of radiation. Recently, a PEM has been made which meets these requirements. The device uses a silicon optical element with a single-layer polymer anti-reflective coating. Design decisions during the development and performance characteristics of the new PEM will be discussed. Topics include the choice of silicon as an optical element material, antireflective coating design and material choice, optical transmission, maximum retardation, useful aperture and modulation frequency.

The two-modulator generalized ellipsometer (2-MGE) is a spectroscopic polarization-sensitive optical instrument that is sensitive to both standard ellipsometric parameters from isotropic samples as well as cross polarization terms arising from anisotropic samples. In reflection mode, the 2-MGE has been used to measure the complex dielectric functions of several uniaxial crystals, including TiO2, ZnO, and BiI3. The 2-MGE can also be used in the transmission mode, in which the complete Mueller matrix of a sample can be determined (using 4 zone measurements).

We report in this paper an instrument for measuring the Stokes parameters of a light beam. This Stokes polarimeter employs two low birefringence photoelastic modulators (PEMs) operating at different resonant frequencies. A computer program calculates and displays the intensity parameter and the normalized Stokes parameters of the light beam measured. Common laboratory lasers are measured as examples.

Measurements of circular dichroism (CD) in the UV and vacuum UV have used photoelastic modulators (PEMs) for high sensitivity (to about 10-6). While a simple technique for wavelength calibration of the PEMs has been used with good results, several features of these calibration curves have not been understood. The authors have calibrated a calcium fluoride PEM and a lithium fluoride PEM using the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory as a light source.

Birefringence in refractive components such as lenses has become an increasingly serious problem in semiconductor lithography as exposure wavelength decreases. Most measurements of birefringence are made with visible light but the light used for photolithography is in the UV and deep UV spectral regions. The method uses photoelastic modulators with optical elements made from fused silica and calcium fluoride.

Circular birefringence is also called optical rotation. An experimental set-up using the photoelastic modulator (PEM) is developed in our lab for measuring small optical rotation in chiral solutions. Sugar solutions at known concentrations are used as standards to test the feasibility of the method and the sensitivity of the instrument. The sensitivity of this current instrument is determined to be 0.001°.

The author reports in this paper a sensitive method for measuring low-level linear birefringence in optical materials. A photoelastic modulator is employed as the polarization modulation device in the set-up. The sensitivity of this method is evaluated to be at ~0.003nm (~0.002° at 632.8nm) by measuring the mechanically induced linear birefringence in a fused silica optical element. The capability of the method is demonstrated in the residual linear birefringence below 0.1nm in several high quality optical elements.

Infrared reflection-absorption spectroscopy (IRRAS) is an important IR technique for studying and monitoring chemical species adsorbed on a metal surface. This article describes, on the “how-to” level, double-modulation IRRAS instruments and discusses different demodulation approaches for obtaining the IRRAS signal.

Measurements of low levels of strain birefringence in fused silica glass have been made using a system based on a photoelastic modulator. Measurements of sample net retardation have been made with a resolution of 0.1 nanometers. Measured values of a strain birefringence constant for fused silica are in good agreement with established data.

A method for measurement of low-level strain birefringence in optical elements and materials will be described. This method provides for the simultaneous measurement of magnitude and direction of the net retardation without the necessity of sample rotation. Good agreement was obtained between measured retardation and independent measurements of a polymer waveplate. Measurements were also made of uncalibrated samples with retardation magnitudes down to 1.5 nanometers.

A system for measurement of waveplate retardation using a photoelastic modulator will be described. The system is intended for incoming quality inspection of quarter-wave plates at 632.8 nm and 900 nm. Measurement of several polymer waveplates were in good agreement with the waveplate manufacturer's calibration data.

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 life’s unique chiral signature.

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.

We report in this paper an instrument for measuring the Stokes parameters of a light beam. This Stokes polarimeter employs two low birefringence photoelastic modulators (PEMs) operating at different resonant frequencies. A computer program calculates and displays the intensity parameter and the normalized Stokes parameters of the light beam measured. Common laboratory lasers are measured as examples.

In this paper, the author reports a photoelastic modulator (PEM) based linear birefringence measurement system (BMS) using a near infrared (NIR) laser (He-Ne @ 1532 nm). This instrument determines both the magnitude and angle (of fast axis) of a birefringent sample without rotating the sample. It records birefringence images by scanning a sample that is mounted on a computer-controlled X-Y translation stage. The accuracy, repeatability and other key performance tests for the NIR-BMS are provided in this report. Selected samples, including silicon optical component, silicon wafer and waveplates commonly used at visible wavelengths, are studied using the NIR-BMS.

This paper describes an instrument for measuring linear retardance in transparent optical materials using two photoelastic modulators. The instrument contains a He-Ne laser (632.8 nm), a polarizer, two low birefringence photoelastic modulators at different frequencies, an analyzer and a silicon-photodiode detector. A sample is placed between the two modulators. The detector signals corresponding to linear retardance in a sample are analyzed using lock-in amplifiers. A computer program calculates and displays both the retardation magnitude and angle of fast axis. The instrument reported here is essentially a polarimeter specifically designed for measuring low level linear retardance in high quality optical components. It provides a retardation sensitivity of better than 0.005 nm (~0.003o with a He-Ne laser at 632.8 nm).

This paper describes a method for assessing the accuracy of a new linear birefringence measurement system based on PEM technology. We used a Soleil-Babinet compensator in the experiment and observed that its retardation varies significantly across the optical aperture. To use this compensator as a meaningful retardation standard, we fixed the beam position relative to the Soleil-Babinet compensator before and after its calibration. Our results show that the birefringence measurement system is capable of providing accurate measurements for linear retardation below 125 nm with a relative uncertainty below 1%. Experimental results support that this birefringence measurement system is self-calibrating for measuring linear retardation.

In this paper, the author reports a photoelastic modulator (PEM) based linear birefringence measurement system (BMS) using a near infrared (NIR) laser (He-Ne @ 1532 nm). This instrument determines both the magnitude and angle (of fast axis) of a birefringent sample without rotating the sample. It records birefringence images by scanning a sample that is mounted on a computer-controlled X-Y translation stage. The accuracy, repeatability and other key performance tests for the NIR-BMS are provided in this report. Selected samples, including silicon optical component, silicon wafer and waveplates commonly used at visible wavelengths, are studied using the NIR-BMS.

We recently developed a linear birefringence measurement instrument, known as the Exicor system, using photoelastic modulator (PEM) technology. We have reported the precision and short-term stability of this instrument. In this paper, the author further evaluates the accuracy, long-term stability, and instrumental performance under low light intensity levels of the Exicor system.

This paper describes a sensitive method for measuring both the magnitude and the angle of the fast axis of low level linear birefringence in optical materials. Several different approaches have been investigated and one of them has been chosen as the basis for a linear birefringence measurement instrument. The instrument employs a low birefringence photoelastic modulator (PEM) for modulating the polarization states of a He-Ne laser beam. After the modulated laser beam passes through the sample, two detecting channels analyze the polarization change caused by the sample. An algorithm has been developed to calculate the magnitude and angle of the linear birefringence in a sample. A computer program implements the algorithm and displays the calculated values. Using this instrument, selected samples with different levels of linear birefringence have been studied. The resulting instrument achieves high sensitivity for the final measurements. The magnitude of linear birefringence is better than ±0.005 nm (~±0.003o with a He-Ne laser at 632.8 nm), and the angle of the fast axis of the sample is < 2o (for retardance > 0.5 nm).

Residual linear birefringence is an important property for quality control of optical components used in optical lithographic instruments. This paper shows that it is especially critical to control the residual linear birefringence in the substrate of photomasks at a very low level. A birefringence measurement system, known as Exicor®, was used for measuring both the magnitude and angular orientation of residual linear retardance in photomask substrates. Different patterns and levels of residual linear birefringence in these samples were identified. The effect of residual linear birefringence in photomask substrates, in determining wafer imaging quality, is discussed.

This article examines birefringence present in calcium fluoride

Fused silica and calcium fluoride are the standard lens materials for the 193 nm (ArF excimer laser) generation step and scan systems. In this paper, the author reports measurements of induced birefringence in both fused silica and calcium fluoride samples exposed to ArF excimer laser irradiation. A new birefringence measurement instrument, known as the ExicorTM system, was used for mapping birefringent images. Different patterns of induced birefringence in fused silica samples were observed for samples irradiated with 'unpolarized' and polarized excimer lasers. Birefringence induced in fused silica with linearly polarized excimer laser irradiation contradicts the traditional compaction model. The measurement result of a CaF2 element confirms that 193 nm excimer laser irradiation induces no observable birefringence in CaF2.

This article studies the residual birefringence in calcium fluoride and fused silica optical component samples. The effect of birefringence on optical lithography processes is also discussed. Several sample measurements are shown.

In this paper we introduce an instrument developed recently for measuring of low level birefringence. Known as the Exicor™ system, this instrument has two detecting channels for measuring both the magnitude and orientation of linear birefringence in transparent optical materials. The Exicor system, employing a low birefringent photoelastic modulator (PEM), provides high level sensitivity of ~0.005nm and good time resolution < 2s per data point. We present applications of the Exicor system to a variety of optical samples with industrial importance, including PEM optical elements, compact disc blanks, photomask blanks and other optical components.