Object imaging system using changing frequency interferometry method

Abstract

The drift of a laser frequency can be used to improve the imaging of an object where the laser experiences frequency drift for various reasons. The system uses an interferometry method using a laser of nominally fixed—but unknown and changing—frequency and phase angle, for example, attributable to laser drift. The interference pattern is periodically sampled at a frequency considerably higher than the phase shift of the object. The wavelength is reconstructed from the sampled patterns using a correlation algorithm. The phase angle is determined using an n-bucket algorithm. After all of the complex information has been determined at all of the multiple wavelengths, the surface of the object is calculated using conventional interferometry techniques. Accordingly, laser drift—which is typically considered a negative attribute—is used positively to improve imaging techniques.

1 . A method for generating an image of an object comprising the steps of: performing at least the following image information gathering steps a first number of times: 
 Illuminating an object at an illumination frequency to create interference patterns;    sampling said interference patterns at a sampling frequency that is a multiple of the phase shift of said object; and    reconstructing said wavelength and said phase angle of said illumination frequency as a function of the sampled interference patterns; and    changing said illumination frequency;    after performing said image information gathering steps, creating an image of said object as a function of said collected and reconstructed complex information.    
   
   
       2 . The method recited in  claim 1 , wherein said sampled interference patterns are collected at different phase angles.  
   
   
       3 . The method as defined in  claim 2  wherein said sampling frequency is at least about an order of magnitude greater than said phase shift of the object.  
   
   
       4 . The method as defined in  claim 3  wherein said sampling frequency is about eighteen times said phase shift of said object.  
   
   
       5 . The method as defined in  claim 1  wherein said reconstructing step includes correlating said sampled interference patterns.  
   
   
       6 . A method as defined in  claim 1  wherein said reconstructing step includes using an n-bucket algorithm to determine the phase angle of the wavelength.  
   
   
       7 . A method as defined in  claim 6  wherein the n-bucket algorithm is a four-bucket algorithm.  
   
   
       8 . A method as defined in  claim 1  wherein said creating step includes using a synthetic aperture radar technique.  
   
   
       9 . A method for generating an image of an object comprising the steps of: performing at least the following image information gathering steps a first number of times: 
 Illuminating an object at an illumination frequency to create interference patterns;    sampling said interference patterns at a sampling frequency that is a multiple of the phase shift of said object,    reconstructing said wavelength and said phase angle of said illumination    frequency as a function of the sampled interference patterns; and    changing the known range of said illumination frequency;    after performing said image information gathering steps, creating an image of said object as a function of said collected and reconstructed complex information.    
   
   
       10 . The method as defined in  claim 9  wherein said sampling frequency is at least about an order of magnitude greater than said phase shift of said object.  
   
   
       11 . The method as defined in  claim 10  wherein said sampling frequency is at least about eighteen times greater than said phase shift of said object.  
   
   
       12 . The method as defined in  claim 9  wherein said reconstructing step includes correlating said sampled interference patterns.  
   
   
       13 . The method as defined in  claim 9  wherein said reconstructing step includes using an n-bucket algorithm.  
   
   
       14 . The method as defined in  claim 13  wherein said n-bucket algorithm is a four-bucket algorithm.  
   
   
       15 . An improved object imaging system including: 
 a source of coherent radiation for illuminating an object and creating an interference pattern, wherein said coherent radiation source is capable of illuminating said object at multiple frequencies;.    a frame sampling system including a detector for sampling said interference patterns at a target sampling frequency;    a wavelength reconstruction module generating a illumination frequency and a phase angle for said object as a function of said sampled interference patterns;    a frequency changing system capable of changing the frequency of said coherent radiation source; and    an image reconstruction module configured to generate an image of said object as a function of said collected and reconstructed complex information wherein said target frequency is a multiple of a phase shift of said object.    
   
   
       16 . The system as recited in  claim 15 , wherein said frequency changing system is passive.  
   
   
       17 . The system as recited in  claim 15 , wherein said image reconstruction module includes a correlation module for correlating interference patterns.  
   
   
       18 . An improved object imaging system using changing frequency interferometry, said system comprising: 
 multiple-frequency illumination means for illuminating an object and creating interference patterns, at a plurality of illumination frequencies;    interference patterns sampling means for sampling at a frequency that is a multiple of the phase shift of the object;    wavelength reconstruction means for determining said wavelength and phase angle of said illumination frequency as a function of said sampled interference patterns;    object image means being for generating an image of said object as a function of said collected and reconstructed complex information.    
   
   
       19 . The system as recited in  claim 18 , wherein said interference pattern sampling means collects patterns at different phase angles.  
   
   
       20 . The system as recited in  claim 18  wherein said wavelength reconstruction means uses an n-bucket algorithm.