Phase difference comparison to measure very small spacing between bodies

Abstract

An apparatus and a method for measuring very small separations between a transparent or semi-transparent first body and a second body is disclosed. A light source produces light that is split into two distinct paths. One path is directed through the first body at two locations, one where it reflects from the interface at the separation to be measured, and another where the second body does not affect the reflection. The second path is directed at a frequency shifter. The two paths are recombined and interferometric variations of intensity are detected. The difference in phase between the measurement and reference areas with the second body not present, is subtracted from the difference in phase between the measurement and reference areas with the second body present. The difference in differences yields the phase change that occurs when the second body is introduced. The separation is calculated based on this phase change.

1 . An apparatus for measuring the spacing between a first body and a second body comprising:
 at least one source of light simultaneously producing a measurement light beam and a reference light beam that are coexistent;   at least one beam splitter that separates the measurement light beam into a first measurement portion and a second measurement portion, and that separates the reference light beam into a first reference portion and a second reference portion;   a measurement optical path that directs the first measurement portion to shine upon the first and second bodies, and that directs the second measurement portion to shine upon a third body;   a reference optical path that directs the first reference portion to shine upon the first body but not the second body, and that directs the second reference portion to shine upon the third body;   a first light detector disposed to receive light that is reflected from the first, second, and third bodies via the measurement optical path, and that produces a first output signal;   a second light detector disposed to receive light that is reflected from the first and third bodies via the reference optical path, and that produces a second output signal;   a movable mount that can hold the second body in the measurement optical path during a first period, and that can hold the second body away from the measurement optical path during a second period;   a phase difference comparator coupled to the first and second light detectors, capable of comparing a phase difference between the first and second output signals during the first period with a phase difference between the first and second output signals during the second period.   
   
   
       2 . The apparatus of  claim 1  wherein said light includes light that has a frequency in the visible spectrum. 
   
   
       3 . The apparatus of  claim 1  wherein the at least one source of light includes a laser. 
   
   
       4 . The apparatus of  claim 1  wherein the measurement optical path includes an objective lens capable of focusing the first measurement portion on either the first or second body. 
   
   
       5 . The apparatus of  claim 1  wherein the reference optical path includes an objective lens capable of focusing the first reference portion on the first body. 
   
   
       6 . The apparatus of  claim 1  wherein said first and second light detectors each comprise a photodiode. 
   
   
       7 . The apparatus of  claim 1  wherein said phase difference comparator includes at least one analog-to-digital converter. 
   
   
       8 . The apparatus of  claim 1  wherein said phase difference comparator includes a microprocessor and electronic memory. 
   
   
       9 . The apparatus of  claim 1  wherein the second body is a head designed for use in an information storage device. 
   
   
       10 . The apparatus of  claim 1  wherein the first body is a rotating disk that is at least partially transparent. 
   
   
       11 . A method for measuring the spacing between a first body and a second body comprising the acts of:
 producing at least two simultaneously coexisting light beams, including a measurement light beam and a reference light beam;   separating the measurement light beam into a first measurement portion and a second measurement portion;   separating the reference light beam into a first reference portion and a second reference portion;   directing the first measurement portion to shine upon the first and second bodies along at least a portion of a measurement optical path;   directing the second measurement portion to shine upon a third body;   directing the first reference portion to shine upon the first body but not the second body;   directing the second reference portion to shine upon the third body;   detecting light from the measurement light beam that is reflected from the first, second, and third bodies, and producing a corresponding first output signal;   detecting light from the reference light beam that is reflected from the first and third bodies, and producing a corresponding second output signal;   holding the second body in the measurement optical path during a first period;   holding the second body away from the measurement optical path during a second period;   comparing a phase difference between the first and second output signals during the first period with a phase difference between the first and second output signals during the second period; and   determining the spacing based on the phase difference comparison.   
   
   
       12 . The method of  claim 11  wherein the act of producing at least two light beams includes producing light that has a frequency in the visible spectrum. 
   
   
       13 . The method of  claim 11  wherein the act of producing at least two light beams includes producing laser light. 
   
   
       14 . The method of  claim 11  wherein the second body is a head designed for use in an information storage device and wherein the act of determining includes determining a flying height of the head based on the phase difference comparison. 
   
   
       15 . The method of  claim 11  further comprising rotating the first body. 
   
   
       16 . An apparatus for measuring the spacing between a first body and a second body comprising:
 at least one source of electromagnetic radiation simultaneously producing at least two coexisting electromagnetic radiation beams, including a measurement beam and a reference beam;   at least one beam splitter that separates the measurement beam into a first measurement portion and a second measurement portion, and that separates the reference beam into a first reference portion and a second reference portion;   a measurement radiation path that directs the first measurement portion to impinge upon the first and second bodies, and that directs the second measurement portion to impinge upon a third body;   a reference radiation path that directs the first reference portion to impinge upon the first body but not the second body, and that directs the second reference portion to impinge upon the third body;   a first detector disposed to receive radiation that is reflected from the first, second, and third bodies via the measurement radiation path, and that produces a first output signal;   a second detector disposed to receive radiation that is reflected from the first and third bodies via the reference radiation path, and that produces a second output signal;   a movable mount that can hold the second body in the measurement radiation path during a first period, and that can hold the second body away from the measurement radiation path during a second period;   a phase difference comparator coupled to the first and second detectors, capable of comparing a phase difference between the first and second output signals during the first period with a phase difference between the first and second output signals during the second period.   
   
   
       17 . The apparatus of  claim 16  wherein the second body is a head designed for use in an information storage device. 
   
   
       18 . The apparatus of  claim 16  wherein the first body is a rotating disk that is at least partially transparent to the electromagnetic radiation. 
   
   
       19 . A method for measuring the spacing between a first body and a second body comprising the acts of:
 simultaneously producing at least two coexisting beams of electromagnetic radiation, including a measurement beam and a reference beam;   separating the measurement beam into a first measurement portion and a second measurement portion;   separating the reference beam into a first reference portion and a second reference portion;   directing the first measurement portion to impinge upon the first and second bodies along at least a portion of a measurement radiation path;   directing the second measurement portion to impinge upon a third body;   directing the first reference portion to impinge upon the first body but not the second body;   directing the second reference portion to impinge upon the third body;   detecting electromagnetic radiation from the measurement beam that is reflected from the first, second, and third bodies, and producing a corresponding first output signal;   detecting electromagnetic radiation from the reference beam that is reflected from the first and third bodies, and producing a corresponding second output signal;   holding the second body in the measurement radiation path during a first period;   holding the second body away from the measurement radiation path during a second period;   comparing a phase difference between the first and second output signals during the first period with a phase difference between the first and second output signals during the second period; and   determining the spacing based on the phase difference comparison.   
   
   
       20 . The method of  claim 19  wherein the second body is a head designed for use in an information storage device and wherein the act of determining includes determining a flying height of the head based on the phase difference comparison. 
   
   
       21 . The method of  claim 19  further comprising rotating the first body.