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High Frequency Faraday Modulator

IP.com Disclosure Number: IPCOM000081033D
Original Publication Date: 1974-Mar-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 2 page(s) / 25K

Publishing Venue

IBM

Related People

Chambers, DW: AUTHOR [+2]

Abstract

In conventional high-speed ellipsometry a single Faraday modulator is used to rotate the plane of polarization of a beam of plane polarized light to a datum, and to vary the angle of rotation about the datum. In the present device this function is performed by two Faraday modulators, the first modulator coil is driven by DC and the second modulator coil is driven by AC and tuned to resonance by a series capacitor. This avoids the high voltage required by the conventional approach, and also allows the DC current to be measured without having to filter out the AC component.

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High Frequency Faraday Modulator

In conventional high-speed ellipsometry a single Faraday modulator is used to rotate the plane of polarization of a beam of plane polarized light to a datum, and to vary the angle of rotation about the datum. In the present device this function is performed by two Faraday modulators, the first modulator coil is driven by DC and the second modulator coil is driven by AC and tuned to resonance by a series capacitor. This avoids the high voltage required by the conventional approach, and also allows the DC current to be measured without having to filter out the AC component.

In the diagram, a beam of plane polarized light passes through a datum Faraday cell and a modulator Faraday cell. Coil L1 of the datum cell is driven by a DC current, to rotate the plane of polarization by a required angle to a datum. Coil L2 of the modulator cell is driven by an AC current, to continuously oscillate the angle of the plane of polarization about the datum. The inductance of coil L2 is tuned to resonance by series capacitor C at the frequency of the AC supply.

For a coil inductance of 6 mH, at a frequency of 1 KHz, the conventional approach requires 225 volts to deliver 6A drive current. The present arrangement achieves the same current with a 20V AC drive, due to the low resistance at series resonance.

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