Polarization interferometer apparatus using the polarization dependent phase lag in a birefringent retarder

• US 5,841,536 A
• Filed: 08/01/1997
• Issued: 11/24/1998
• Est. Priority Date: 08/01/1997
• Status: Expired due to Fees

• Current Assignee: National Security Agency The United States Government As Represented By The Director
• Original Assignee: The United States of America As Represented By The Director National Security Agency
• Inventors: Dimmick, Timothy E.
• Primary Examiner(s): Turner, Samuel A.
• Application Number:
  US08/904,535
• Time in Patent Office: 480 Days
• Field of Search: 356/346, 356/351
• US Class Current: 356/491
• CPC Class Codes: G01J 2009/0261 polarised
• CPC Class Codes: G01J 9/0246 Measuring optical wavelength

Abstract

An inexpensive and accurate wavemeter for measuring the wavelength of monochromatic light is described. The device uses the wavelength dependent phase lag between principal polarization states of a length of birefringent material (retarder) as the basis for measuring the optical wavelength. The retarder is sandwiched between a polarizer and a polarizing beamsplitter and is oriented such that its principal axes are non-orthogonal to the axis of the polarizer and the principal axes of the beamsplitter. As a result of the disparity in propagation velocities between the principal polarization states of the retarder, the ratio of the optical power exiting the two ports of the polarizing beamsplitter is wavelength dependent. If the input wavelength is known to be within a specified range, the measurement of the power ratio uniquely determines the input wavelength. The device offers the advantage of trading wavelength coverage for increased resolution simply through the choice of the retarder length. Implementations of the device employing both bulk-optic components and fiber-optic components are described.

Claims

1. A polarization interferometer apparatus using the polarization dependent phase lag in a birefringent retarder, comprising:

a controllable birefringent element for accepting light and varying retardance so as to change the polarization of the accepted light in response to control inputs and transmitting that light of changed polarization;

a first bulk optic polarizing beamsplitter for dividing the light received from said controllable birefringent element into two orthogonally polarized components, having a principle axis, orthogonal to the direction of travel of the received light, a first output port for providing one of the orthogonally polarized components of the received light, and a second output port for providing the other component of the orthogonally polarized components of the received light;

an undesired polarization output photodetector arranged to detect the light provided by the first output port of said first bulk optic polarizing beamsplitter and to produce an undesired polarization output photodetector photocurrent;

power minimizing means to accept the undesired polarization output photodetector photocurrent from said undesired polarization output photodetector and provide control inputs to said controllable birefringent element such that the undesired polarization output photodetector photocurrent from said undesired polarization output photodetector is minimized;

a bulk optic birefringent retarder, having a fast, a slow, and an other principle axes, arranged to receive light of linear polarization from the second output port of said first bulk optic polarizing beamsplitter and to transmit components of light polarized along the fast and slow axes, with the fast and slow axes oriented at an angle non-orthogonal and non-parallel to the direction of linear polarization and the other principle axis oriented in the direction of propagation of the received light;

a second bulk optic polarizing beamsplitter for dividing received light into two orthogonally polarized components, arranged to receive the transmitted components of light polarized along the fast and slow axes of said bulk optic birefringent retarder, having a principle axis, oriented to accept the light exiting said bulk optic birefringent retarder such that the principle axis is non-orthogonal and non-parallel to both the fast and slow axes of said bulk optic birefringent retarder, a first output port for providing one of the orthogonally polarized components of the received light, and a second output port for providing the other component of the orthogonally polarized components of the received light;

a first photodetector arranged to detect the light provided by the first output port of said second bulk optic polarizing beamsplitter and having means for producing a first photocurrent that corresponds to the power of the detected light on its photocurrent output;

a second photodetector arranged to detect the light provided by the second output port of said second bulk optic polarizing beamsplitter and having means for producing a second photocurrent that corresponds to the power of the detected light on its photocurrent output; and

means for comparing the first and second photocurrents produced by said first and second photodetectors and providing an output signal that corresponds to the ratio of the photocurrent produced by said first photodetector to the photocurrent produced by said second photodetector which in turn corresponds to the wavelength of the light received by said controllable birefringent element.

2. A polarization interferometer apparatus using the polarization dependent phase lag in a birefringent retarder, comprising:

a fixed birefringent retarder, having a fast, a slow, and an other principle axes, arranged to receive monochromatic light of linear polarization and to transmit components of light polarized along the fast and slow axes, with the fast and slow axes oriented at an angle non-orthogonal and non-parallel to the direction of linear polarization and the other principle axis oriented in the direction of propagation of the received monochromatic light;

a polarizing beamsplitter for dividing received light into two orthogonally polarized components, arranged to receive the transmitted components of light polarized along the fast and slow axes of said birefringent retarder, having a principle axis, oriented to accept the light exiting said fixed birefringent retarder such that the principle axis is non-orthogonal and non-parallel to both the fast and slow axes of said fixed birefringent retarder, a first output port for providing one of the orthogonally polarized components of the received light, and a second output port for providing the other component of the orthogonally polarized components of the received light;

a first photodetector arranged to detect the light provided by the first output port of said polarizing beamsplitter and having means for producing a first photocurrent that corresponds to the power of the detected light on its photocurrent output;

a second photodetector arranged to detect the light provided by the second output port of said polarizing beamsplitter and having means for producing a second photocurrent that corresponds to the power of the detected light on its photocurrent output; and

means for comparing the first and second photocurrents produced by said first and second photodetectors and providing an output signal that corresponds to the ratio of the photocurrent produced by said first photodetector to the photocurrent produced by said second photodetector, with a single observation, wherein the ratio of photocurrents produced by said first and second photodetectors corresponds to the wavelength of, and is insensitive to the power of, the monochromatic light received by said fixed birefringent retarder.

3. A polarization interferometer apparatus using the polarization dependent phase lag in a birefringent retarder according to claim 2, further comprising an input polarizer oriented so as to accept light of unknown polarization and transmit only light of a known linear polarization to be received by said fixed birefringent retarder.

4. A polarization interferometer apparatus using the polarization dependent phase lag in a birefringent retarder according to claim 2, wherein:

said fixed birefringent retarder is comprised of a bulk optic fabrication of birefringent material, having a fast, a slow, and an other principle axes, arranged to receive light of linear polarization and to transmit components of light polarized along the fast and slow axes, with the fast and slow axes oriented at an angle non-orthogonal and non-parallel to the direction of linear polarization and the other principle axis oriented in the direction of propagation of the received light; and

said polarizing beamsplitter is comprised of a bulk optic beamsplitter for dividing the received light into two orthogonally polarized components, arranged to receive the transmitted components of light polarized along the fast and slow axes of said bulk optic fabrication of birefringent material, having a principle axis, oriented to accept the light exiting said bulk optic fabrication of birefringent material such that the principle axis is non-orthogonal and non-parallel to both the fast and slow axes of said bulk optic fabrication of birefringent material, a first output port for providing one of the orthogonally polarized components of the received light, and a second output port for providing the other component of the orthogonally polarized components of the received light.

5. A polarization interferometer apparatus using the polarization dependent phase lag in a birefringent retarder according to claim 3, wherein:

said input polarizer is comprised of a length of polarizing fiber of sufficient length to ensure adequate polarization of the accepted monochromatic light that transmits light polarized along a preferred axis with low loss and light polarized orthogonally to the preferred axis with high loss;

said fixed birefringent retarder is comprised of a length of birefringent fiber; and

said polarizing beamsplitter is comprised of an all-fiber device.