A polarization modulator for the far infrared (terhahertz waves)

Abstract

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 FOR PLASMA DIAGNOSTICSPhotoelastic modulators have long been used for plasma diagnostics of plasma fusion devices. Levinton, Wroblewski and others have developed a PEM-based Stokes polarimeter for analysis of the D radiation from a beam of neutral deuterium atoms injected into a tokamak plasma. 1,2 The data is needed to determine the direction of the magnetic field vs. position in the plasma. From this the current density profile may be determined.More recently, Kawano and others have used zinc selenide PEMs in a Stokes polarimeter/interferometer instrument with 9.27 and 10.6 m CO 2 laser radiation. 3 (The interferometer and Stokes polarimeter operate in parallel.) The polarimeter is designed to measure electron density in the plasma by measuring Faraday rotation of the laser light polarization.Okajima, Nakayama and others have noted that laser radiation between 40 and 70 m would be ideal for the interferometer/Stokes polarimeter techniques. 4 To that end, they have designed and built lasers that operate at 47.6 and 57.2 m. The lasers are pumped by CO 2 lasers and use CH 3 OD molecules as the active material. 5Akiyama and others are developing an interferometer /Stokes polarimeter system using the 47.6 and 57.2 m laser beams for measuring electron density and current profiles in plasma. 6 The instrument will be used in magnetically confined fusion devices, initially the Large Helical Device (LHD) in Japan. The polarimeter will use two PEMs operating at nominal frequencies of 40 kHz and 50 kHz. The development of the first PEM at 50 kHz is the subject of this paper. *

Short wavelength far infrared laser polarimeter with silicon photoelastic modulators

“…19 The maximum transmissivities of the 57.2 and 47.7 m light through the PEM are about 70% and 60% for each PEM, respectively. Remaining multireflection components can be suppressed by tilting the incident angle of the laser light slightly based on the etalon effect.…”

“…The photoelastic material is highly resistive silicon whose absorptions of the 57.2 and 47.7 m laser light are the small. 19 Both sides of the photoelastic element are anti-…”

“…The performance was experimentally evaluated 19 and measurement of the polarization angle with the single PEM was demonstrated. 20 This paper describes performance of the dual PEM system, which is upgraded in order to improve temporal and angular resolutions.…”

Magneto-optical fingerprints of distinct graphene multilayers using the giant infrared Kerr effect

“…The first point is of particular interest, since an electrostatic gate can modulate E F , and, therefore, the polarization of infrared light, very quickly. Such a device is especially important for infrared applications, since current technologies are limited to fixed-frequency, sinusoidal modulation of infrared polarization43. In contrast, a graphene-based device is capable of modulating the polarization with arbitrary wave forms at arbitrary frequencies that are an order of magnitude larger than current technologies.…”