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A polarization modulator for the far infrared (terahertz waves) - art. no. 66820X
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.
The remarkable electronic properties of graphene strongly depend on the thickness and geometry of graphene stacks. This wide range of electronic tunability is of fundamental interest and has many applications in newly proposed devices. Using the mid-infrared, magneto-optical Kerr effect, we detect and identify over 18 interband cyclotron resonances (CR) that are associated with ABA and ABC stacked multilayers as well as monolayers that coexist in graphene that is epitaxially grown on 4H-SiC. Moreover, the magnetic field and photon energy dependence of these features enable us to explore the band structure, electron-hole band asymmetries, and mechanisms that activate a CR response in the Kerr effect for various multilayers that coexist in a single sample. Surprisingly, we find that the magnitude of monolayer Kerr effect CRs is not temperature dependent. This unexpected result reveals new questions about the underlying physics that makes such an effect possible.
Measurement of an internal magnetic field distribution in magnetically confined fusion devices is indispensable for both understanding the plasma physics and controlling plasmas. A polari-interferometer based on the Faraday effect has been used for such a purpose. This paper describes performance evaluations of a part of the polarimeter with photoelastic modulators (PEMs) of the short-wavelength far-infrared (FIR) laser polari-interferometer. The wavelengths of the light source are 57.2 and 47.7 μ m, which are suitable for a high-density operation and large fusion devices. The PEM for the FIR region is newly developed with high-resistive silicon in which absorption of the FIR laser is small. The polarization angle is successfully measured and an angle resolution of 0.01° with a time resolution of 1 ms is achieved. A drift of the baseline of about 0.1° for 1000 s is observed and is found to be caused by changes in the room temperature.
A short wavelength far infrared laser whose wavelength lambda is about 50 microm is preferable for a polarimeter and an interferometer for high density operations in the Large Helical Device (LHD) and on future large fusion devices such as ITER. This is because the beam bending effect (proportional to lambda(2)) in a plasma, which causes fringe jump errors, is small and the Faraday and the Cotton-Mouton effects are moderate. We have developed a polarimeter with highly resistive silicon photoelastic modulators (PEMs) for the CH(3)OD laser (lambda=57.2 and 47.7 microm). We performed bench tests of the polarimeter with a dual PEM and demonstrated the feasibility for the polarimeter. Good linearity between actual and evaluated polarization angles is achieved with an angular resolution of 0.05 degrees and a temporal resolution of 1 ms. The baseline drift of the polarization angle is about 0.1 degrees for 1000 s.
The multi-layer optical films realize various functions such as wide-band anti-reflection coating, high reflection coating, low-pass filter, high-pass filter, band-pass filter, polarizing beam splitter, and non- polarizing beam splitter, etc. Although the multi-layer optical coatings are commonly used in light-wave regions (visible, near-infrared, and mid-infrared regions), it has not yet fully realized in the tera-hertz electromagnetic wave region (customarily defined as f = 0.3 - 10 THz, λ = 1000 - 30 μ m). Single-layer coatings used at terahertz frequencies have realized by some methods, however, there has been almost no report on the fabrication method for the multi-layer coatings. Because each layer in the multi-layer structure must be as thick as several to several tens of micrometers, which is far thicker than coating used in optical regions. A method for manufacturing multi-layer optical coatings for optics used at tera-hertz frequencies has therefore been developed. The method forms a multi-layer structure, consisting of silicon and silicon-oxide layers, through plasma-enhanced chemical vapor deposition (CVD) using silane (SiH 4) and oxygen (O 2) as source gases. This method has lots of advantages over other methods. The details of the method and also the optical properties of a single layer anti-reflection coating and a four-layer wide-band anti-reflection coating on a germanium substrate are reported.
The infrared CO2 laser polarimetry for electron density measurement based on the tangential Faraday rotation has been progressed in JT-60U tokamak. The reliability of polarimetry in a pellet-injected plasma was confirmed. The Faraday rotation was measured with good angle resolution of ∼0.01° under the sufficiently fast temporal resolution of 4 ms. Electron density evaluated from the tangential Faraday rotation measured by polarimetry shows good agreement with that measured by interferometry for an identical probing laser beam. The long-time continuous measurement was successful with good stability and accuracy. The basic feasibility of the infrared CO2 laser polarimetry for electron density measurement in long-time and steady-state operation in large fusion devices was demonstrated. © 2001 American Institute of Physics.
Powerful lasers in the far-infrared wavelength range (47.6 and 57.2 μ m ) have been developed to measure the plasma density in the Large Helical Device at National Institute for Fusion Science and future plasma devices such as the International Thermonuclear Experimental Reactor. The intensification of these lasers has been done by cooling the laser tube wall, adding He as the buffer gas, and using a chemical-vapor-deposited diamond output window. The output powers for the 57.2 and 47.6 μ m lasers have been found to be 1.6 and 0.8 W, respectively. © 2004 American Institute of Physics.
For high density operation of the large helical device and for a future large plasma machine such as ITER, a powerful 57 μm CH 3 OD laser pumped by a continuous wave 9R(8) CO 2 laser has been developed. The 57 μm (5.2 THz) laser light has been successfully detected by using a GaAs Schottky barrier diode detector with a corner reflector. For optical windows of the plasma vessel and the far-infrared laser crystal quartz etalons have been designed under a concept of a two-wavelength etalon for 119 and 57 μm lights. © 2001 American Institute of Physics.
Although the q profile plays a key role in theories of instabilities and plasma equilibrium, it has been quite difficult to measure until the recent development of the motional Stark effect (MSE) diagnostic. A multichannel motional Stark effect polarimeter system has recently been installed on the Tokamak Fusion Test Reactor (TFTR). The diagnostic can measure the magnetic field pitch angle [γ p =tan-1(B p /B T )] at ten radial locations. The doppler shifted D α radiation from a TFTR heating beam is viewed near tangential to the toroidal magnetic field via a re‐entrant front surface reflecting mirror. The field of view covers from inboard of the magnetic axis to near the outboard edge of the plasma with a radial spatial resolution of 3–5 cm. A high throughput f/2 optics system results in an uncertainty for γ p of ∼0.1°–0.2° with a time resolution of ∼5–10 ms. Initial pitch angle profiles from TFTR have been obtained. The MSE data is consistent with the estimated magnetic axis position from external magnetic measurements and the q=1 radius is in good agreement with the inversion radius from the electron cyclotron emission temperature measurements.
A method for reducing the reflections from silicon optics at
terahertz frequencies has been investigated. In this study, we used thin
films of parylene as an anti-reflection (AR) layer for silicon optics
and show low-loss behavior well above 1 THz. Transmittance spectra are
acquired on double-sided-parylene-coated, high-resistivity,
single-crystal silicon etalons between 0.45 THz and 2.8 THz. Modeling
the optical behavior of the three-layer system allowed for the
determination of the refractive index and absorption coefficient of
parylene at these frequencies. Our data indicate a refractive index, n,
of 1.62 for parylene C and parylene D, and a reasonably modest
absorption coefficient make these materials a suitable AR coating for
silicon at terahertz frequencies. Coatings sufficiently thick for AR
performance reduced the average transmittance of the three-layer system
by <10% compared to a lossless AR coating with an ideal refractive
index