Ultrasonic sub-aperture polishing of an optical element

Abstract

Aspects of an ultrasonic polishing system include an ultrasonic actuator and a polishing arm. The ultrasonic actuator is configured to generate ultrasonic vibrations and the polishing arm is coupled to the ultrasonic actuator. The polishing arm includes a horn and a polishing ball. The horn has a proximal end and a distal end. The proximal end is coupled to receive the ultrasonic vibrations and the horn is configured to propagate the ultrasonic vibrations from the proximal end to a distal end. The polishing ball is attached to the distal end of the horn, where the polishing ball vibrates in response to the ultrasonic vibrations for polishing a surface of an optical element. The polishing ball is configured to provide a polishing area on the surface of the optical element that is smaller than an aperture of the optical element.

What is claimed is: 
     
         1 . An ultrasonic polishing system for polishing an optical element, the system comprising:
 an ultrasonic actuator configured to generate ultrasonic vibrations; and   a polishing arm coupled to the ultrasonic actuator, wherein the polishing arm includes:
 a horn having a proximal end coupled to receive the ultrasonic vibrations, wherein the horn is configured to propagate the ultrasonic vibrations from the proximal end to a distal end of the horn; and 
 a polishing ball attached to the distal end of the horn, wherein the polishing ball is configured to vibrate in response to the ultrasonic vibrations for polishing a surface of the optical element, and wherein the polishing ball is configured to provide a polishing area on the surface of the optical element that is smaller than an aperture of the optical element. 
   
     
     
         2 . The ultrasonic polishing system of  claim 1 , wherein the aperture of the optical element is 3 millimeters or smaller. 
     
     
         3 . The ultrasonic polishing system of  claim 1 , wherein the polishing area has a diameter of 10 micrometers or smaller. 
     
     
         4 . The ultrasonic polishing system of  claim 1 , wherein the polishing ball has a spherical shape. 
     
     
         5 . The ultrasonic polishing system of  claim 1 , wherein the polishing ball comprises sapphire. 
     
     
         6 . The ultrasonic polishing system of  claim 1 , wherein the polishing arm has a natural frequency that matches a frequency of the ultrasonic vibrations. 
     
     
         7 . The ultrasonic polishing system of  claim 1 , wherein a frequency of the ultrasonic vibrations is greater than 20 kHz. 
     
     
         8 . The ultrasonic polishing system of  claim 1 , wherein a frequency of the ultrasonic vibrations is between 20 kHz and 40 kHz. 
     
     
         9 . The ultrasonic polishing system of  claim 1 , further comprising:
 a computer numerical control (CNC) positioner coupled to the polishing arm to vary a position of the polishing ball relative to the surface of the optical element.   
     
     
         10 . The ultrasonic polishing system of  claim 9 , further comprising:
 a computing device that includes:
 at least one processor; and 
 at least one memory coupled to the at least one processor, the at least one memory having instructions stored therein, which when executed by the at least one processor, direct the computing device to: 
 generate one or more control signals to direct the CNC positioner to vary the position of the polishing ball relative to the surface of the optical element. 
   
     
     
         11 . The ultrasonic polishing system of  claim 10 , wherein the instructions to generate the one or more control signals to direct the CNC positioner to vary the position of the polishing ball comprise instructions to direct the polishing ball along a polishing path on the surface of the optical element. 
     
     
         12 . The ultrasonic polishing system of  claim 11 , wherein the instructions to direct the polishing ball along the polishing path comprise instructions to vary at least one of: (a) a load applied to the polishing arm, or (b) a velocity of the polishing ball along the polishing path, to adjust an amount of material removed from the surface of the optical element at one or more positions along the polishing path. 
     
     
         13 . The ultrasonic polishing system of  claim 11 , further comprising:
 an interferometer disposed to obtain one or more surface measurements of the optical element, wherein the at least one memory further comprises instructions to direct the computing device to generate a surface error map of the optical element based on the surface measurements, and wherein the instructions to vary the load or velocity are responsive to the surface error map.   
     
     
         14 . The ultrasonic polishing system of  claim 1 , wherein the ultrasonic actuator comprises a magnetostrictive actuator and wherein the polishing ball is configured to vibrate along an elliptical stroke path on the surface of the optical element in response to the ultrasonic vibrations generated by the magnetostrictive actuator. 
     
     
         15 . The ultrasonic polishing system of  claim 1 , wherein the ultrasonic actuator comprises a piezoelectric actuator and wherein the polishing ball is configured to vibrate along a linear stroke path on the surface of the optical element in response to the ultrasonic vibrations generated by the piezoelectric actuator. 
     
     
         16 . A method of ultrasonic sub-aperture polishing of an optical element, the method comprising:
 enabling an ultrasonic actuator to generate ultrasonic vibrations; and   generating one or more control signals to direct a computer numerical control (CNC) positioner to vary a position of a polishing arm relative to a surface of the optical element, wherein the polishing arm includes:
 a horn having a proximal end coupled to receive the ultrasonic vibrations generated by the ultrasonic actuator, wherein the horn is configured to propagate the ultrasonic vibrations from the proximal end to a distal end of the horn; and 
 a polishing ball attached to the distal end of the horn, wherein the polishing ball is configured to vibrate in response to the ultrasonic vibrations for polishing the surface of the optical element, and wherein the polishing ball is configured to provide a polishing area on the surface of the optical element that is smaller than an aperture of the optical element. 
   
     
     
         17 . The method of  claim 16 , wherein varying the position of the polishing ball comprises directing the polishing ball along a polishing path on the surface of the optical element, the method further comprising:
 generating one or more additional control signals to direct the CNC positioner to vary at least one of: (a) a load applied to the polishing arm, or (b) a velocity of the polishing ball along the polishing path, to adjust an amount of material removed from the surface of the optical element at one or more positions along the polishing path.   
     
     
         18 . The method of  claim 17 , further comprising:
 receiving one or more surface measurements of the optical element; and   generating a surface error map of the optical element based on the surface measurements, wherein varying the load or velocity is responsive to the surface error map.   
     
     
         19 . An optical element polished by a method comprising:
 enabling an ultrasonic actuator to generate ultrasonic vibrations; and   generating one or more control signals to direct a computer numerical control (CNC) positioner to vary a position of a polishing arm relative to a surface of the optical element, wherein the polishing arm includes:
 a horn having a proximal end coupled to receive the ultrasonic vibrations generated by the ultrasonic actuator, wherein the horn is configured to propagate the ultrasonic vibrations from the proximal end to a distal end of the horn; and 
 a polishing ball attached to the distal end of the horn, wherein the polishing ball is configured to vibrate in response to the ultrasonic vibrations for polishing the surface of the optical element, and wherein the polishing ball is configured to provide a polishing area on the surface of the optical element that is smaller than an aperture of the optical element. 
   
     
     
         20 . The optical element polishing by the method of  claim 19 , the method further comprising:
 receiving one or more surface measurements of the optical element;   generating a surface error map of the optical element based on the surface measurements;   generating one or more additional control signals in response to the surface error map to direct the CNC positioner to vary at least one of: (a) a load applied to the polishing arm, or (b) a velocity of the polishing ball along a polishing path, to adjust an amount of material removed from the surface of the optical element at one or more positions of the polishing arm along the polishing path.