High-sensitivity infrared polarimeter

• US 4,818,881 A
• Filed: 05/04/1987
• Issued: 04/04/1989
• Est. Priority Date: 05/04/1987
• Status: Expired due to Fees

• Current Assignee: The United States Of America As Represented By The Secretary Of The Army
• Original Assignee: The United States Of America As Represented By The Secretary Of The Army
• Inventors: Grisham, John A., Tanton, George A.
• Primary Examiner(s): Howell, Janice A.
• Assistant Examiner(s): NOT, DEFINED
• Application Number:
  US07/051,872
• Time in Patent Office: 701 Days
• Field of Search: 250/338.1, 250/370, 356/367, 356/368
• US Class Current: 250/338.1
• CPC Class Codes: G01J 4/04 Polarimeters using electric detection means G01J4/02 takes precedence

Abstract

A sensitive infrared polarimeter that measures the amount of degree of rotation of the plane of polarization of plane polarized radiation after the radiation has passed through a wafer of cadmium sulfide. The wafer is placed in a relatively small magnetic field that is varied. Increased sensitivity is realized by using a chopping wheel to chop the laser beam, providing a reference frequency and by detecting the signals with a synchronous detector such as a lock-in amplifier to obtain high signal to noise ratios of the detected signal. The greater sensitivity of the high-sensitivity infrared polarimeter allows electronic carrier concentrations as low as 10¹⁵ cm^-3 to be measured in cadmium sulfide with magnetic fields as low as 0.1 Tesla.

Claims

1. A high sensitivity infrared polarimeter comprising:

a source of CO₂ laser radiation for generating a beam of radiant energy of wavelength of 10.6 microns along a beam path;

an attenuator, a polarizer, a magnetic field generating means, an analyzer, said magnetic field generating means comprising an electromagnetic having two coaxial coils for developing a magnetic field, said coils having coaxially aligned apertures therethrough, said apertures being positioned in said beam path such that any impinging radiation passes through the apertures in going from said polarizer to said analyzer, said two coaxial coils further having a space therebetween for individual placement within said space of a plurality of cadmium sulfide wafers; and

a mercury-cadmium-telluride detector disposed in series alignment along the beam path so that impinging radiant energy is coupled sequentially from said source of radiation through said attenuator, polarizer, magnetic field generating means, and analyzer to terminate on said detector;

beam directing means disposed in said beam path between said source of laser radiation and said attenuator for guiding the direction of said beam along said path; and

interrupting means disposed on said beam path adjacent said directing means for selectively opening and closing said beam path to laser radiation traversing said path;

driving means coupled to said magnetic field generating means for changing the electrical potential driving said magnetic field generating means, said driving means providing, selectively, a positive voltage, a negative voltage, or no voltage to said magnetic field generating means for controlling the degree and direction of an electromagnetic field within said apertures and the space between said coils.

2. A high sensitivity infrared polarimeter as set forth in claim 1 and further comprising recording means coupled to said detector for recording signals detected by said detector;

said signals being indicative of the amount of Faraday rotation of the plane of polarization of plane polarized radiation passing through said cadmium sulfide wafers along said beam path.