Precision length standard for coherent laser radar

Abstract

This disclosure presents a version of a coherent laser radar system that is immune to environmental effects on Mach-Zehnder-configured interferometers and reference length standards comprising such systems. The present disclosure also describes methods and materials which alleviate interferometer drift due to vapor absorption using a polyimide coated polarization maintaining fiber to reduce the change in the length of a reference arm.

Description

CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application claims priority benefit of U.S. provisional patent application Ser. No. 60 / 662,367 filed Mar. 17, 2005.BACKGROUND OF THE INVENTION [0002] 1. Field of Invention [0003] The present invention generally relates to Mach-Zehnder-configured interferometers and reference length standards in counter chirp FM laser radars. More specifically, methods and materials are described which alleviate interferometer drift due to vapor absorption observed for conventional fiber jacketing materials [0004] 2. Description of Related Art [0005] There are a number of optical systems that can measure a distance to a target. Such systems typically utilize an open beam propagated through free space between the laser source and the target. However, when the target location is such that limited free space is available for beam propagation, such systems are of limited use. Thus, well known systems may be able to perform distance measurements, but the...

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Benefits of technology

[0021] Polyimide has been used successfully for coating space grade optical fiber and it comes with the advantage of a 125° C. temperature rating. However, it was not an obvious choice for use in a laser metrology device because of its known disadvantages. Polyimide has exhibited some disadvantages that have made it a less than desirable first choice for coating optical fibers. For example, polyimide tends to make the fiber seem stiffer than an acrylate coated fiber. Second coatings have been added, but the known additional coatings come with their own problems that have limited the desirability of polyimide for mainstream applications. For example, such second coatings have exhibited outgassing and contamination problems. Of even more significance are the difficulties associated with stripping polyimide. Polyimide coating can be stripped chemically, with hot sulfuric acid, or mechanically, using a hot tweezers. Users find hot sulfuric acid to be dangerous and non-portable. Mechanical stripping methods are not recommended because they have the capacity to introduce surface flaws to the glass fiber, compromising long term reliability. As noted elsewhere, the realization that it offered superior vapor absorption and outgassing propertie

Problems solved by technology

Its disadvantages are that acrylates tend to have low temperature ratings (usually around 85° C although some are available that are rated as high as 200° C.) and it is a well known outgasser.

Investigation and analysis showed that length variations of the acrylate-coated fiber was at fault, and it was determi

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