Photoelastic Modulator
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.
The photoelastic modulator (PEM) is a resonant polarization modulator. It
operates at the resonant frequency of a desired mechanical vibration mode of its
optical element. The PEM is made of isotropic optical materials, in contrast to
the 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 properties, such as high modulation
purity and efficiency, broad spectral range, high power handling capability,
large acceptance angle, large useful aperture and good retardation stability.
These properties make the PEM an effective polarization modulator in a variety
of high sensitivity applications. In this first paper in a series, we focus on
studying two basic optical properties of the PEM: useful aperture and acceptance
angle.
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.
Polarimeters
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.
Exicor
Instrumentation:
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).
Application:
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
