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 modulators (PEMs) are polarization modulation devices used in a wide range of experiments to probe the interaction between polarized light and matter. Experimental setups using PEMs rely on common detector types (photodiodes, photomultipliers, etc.) but care must be taken with their use. Specifically,
1. Care must be exercised to ensure proper impedance matching between the detector and the signal analyzing electronics, for example by using a trans-impedance preamplifier.
2. For silicon detectors, the responsivity at PEM frequencies (e.g. 100 kHz) decreases markedly for light wavelengths above about 900 nm
3. For circular dichroism experiments in the UV and visible, care must be taken in selecting a photomultiplier tube to minimize signal "artifacts" due to birefringence in optical components such as the sample cell and the PEM optical element.
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. These experiments showed calibration graphs that are linear but do not pass through the graph origin. A second “multiple pass” experiment with laser light of a single wavelength, performed on the calcium fluoride PEM, demonstrates the linearity of the PEM electronics. This implies that the calibration behavior results from intrinsic physical properties of the PEM optical element material. An algorithm for generating calibration curves for calcium fluoride and lithium fluoride PEMs has been developed. The calibration curves for circular dichroism measurement for the two PEMs investigated in this study are given as examples.
In this paper we introduce an instrument developed recently for measuring 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 approximately 0.005 nm and good time resolution of < 2s per data point. We present applications of the Exicor system to a variety of otpical samples with industrial importance, including PEM optical elements, compact disc blanks, photomask blanks and other optical components.
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. Measurements of the relative variation of stress-optic constants have been made for fused silica and calcium fluoride, the two primary transmissive optical materials used by this industry.
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.
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.
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.
When photoelastic modulators (PEMs) are used with lasers as light sources, modulated interference effects may appear as spurious signals at the fundamental and harmonic frequencies of the PEM. They are correlated with the modulator reference signal and are at precisely the same frequencies as the polarization modulation effects being studied. This modulated interference does not appear to be a problem with any light source other than lasers. The modulated interference effects arise because of interference between light reflected at the surfaces of the modulator optical element and the primary beam and relative motion of these two surfaces synchronized with the modulator oscillations. Interference in modulator optical elements, which is similar to multiple beam interference in thin films, is examined. Criteria for estimating the strength of the modulated interference are presented and a simple test for the presence of modulated interference in an optical system that includes a PEM is described. Several strategies are presented for reducing or eliminating these troublesome effects including (1) careful positioning of the PEM, (2) use of antireflective (AR) coatings, and (3) techniques that physically separate the primary beam from multiple reflected beams. Data are given for suppression of the modulated interference by using AR coatings and for one method of beam separation.
When photoelastic modulators (PEMs) are used with lasers as light sources, modulated interference effects may occur. Interference which occurs due to the reflection of light at the surfaces of the PEM optical element is modulated because the surfaces of the optical element are in relative motion synchronized with the oscillations of the modulator. Since the modulated interference frequencies are exactly the same as the polarization modulation frequencies being studied, they can be very troublesome. This paper will describe these effects quantitatively, give a simple test for their existence in an optical system and describe some techniques for their suppression or elimination.