Method and apparatus for simultaneously interferometrically measuring optical characteristics in a noncontact manner

• US 6,172,752 B1
• Filed: 08/05/1997
• Issued: 01/09/2001
• Est. Priority Date: 08/04/1996
• Status: Expired due to Fees

• Current Assignee: Matsushita Electric Industrial Company Limited (Panasonic Holdings Corporation)
• Original Assignee: Matsushita Electric Industrial Company Limited (Panasonic Holdings Corporation)
• Inventors: Maruyama, Hideki, Haruna, Masamitsu
• Primary Examiner(s): Turner, Samuel A.
• Application Number:
  US08/906,392
• Time in Patent Office: 1,253 Days
• Field of Search: 356/345, 356/349, 356/355, 356/357, 356/359, 356/360, 356/361, 356/351
• US Class Current: 356/503
• CPC Class Codes: G01B 11/06 for measuring thickness ; e.g. of sheet material thickness measurement by thermal means G01B21/085
• CPC Class Codes: G01N 21/45 using interferometric methods; using Schlieren methods

Abstract

Light from a light source is converged by a converging lens and is irradiated to an object to be measured through a converging lens, the object to be measured or the converging lens and the reference light mirror are displaced so as to maximize intensities of interference light at the reference light mirror and the front and rear surfaces of the object to be measured, and displaced distances of the object to be measured or the converging lens and the reference light mirror at a position where an intensity of interference light becomes maximum at the rear surface and a position where an intensity of interference light becomes maximum are obtained in order to simultaneously measure a refractive index and a thickness of the object to be measured. With this arrangement, simultaneous measurement of a phase index and a thickness of an object to be measured, simultaneous measurement of a birefringence and a thickness of an object to be measured, and a phase index and a group index of an object to be measured can be carried out.

Claims

1. A measuring method for measuring an optical characteristic of an optically transmissive object having a front surface and a rear surface, using an optical system including a light source for emitting light, a converging lens, a reference light mirror, a light receiving element, drive means for displacing one of (i) the object to be measured and said reference light mirror and (ii) said converging lens and said reference light mirror, said method comprising the steps of:

simultaneously converging said light from the light source through said converging lens, and onto the object, and irradiating said light from the light source onto the reference light mirror;

synthesizing a reflected light beam from the front surface of the object and a reflected light beam from the reference light mirror through interference therebetween into a first synthesized interference light beam, and directing the first synthesized interference light beam into the light receiving element, while synthesizing a reflected light beam from the rear surface of the object to be measured, and a reflected light beam from the reference light mirror through interference therebetween into a second synthesized interference light beam, and directing the second synthesized interference light beam into the light receiving element;

displacing one of (i) the object and the reference light mirror and (ii) the converging lens and the reference light mirror so as to maximize an intensity of each of the first and second interference light beams; and

obtaining distances of displacement of said one of (i) the object and the reference light mirror and (ii) the converging lens and the reference light mirror, from a position where an intensity of each of the first and second interference light beams becomes maximum in order to simultaneously measure a refractive index and a thickness of the object to be measured.

5. A method as set forth in claim 1 further comprising applying arithmetic formulae in consideration with a wavelength dispersion of the refractive index of the object to be measured to permit the simultaneous measurement of the refractive index and the thickness and deriving simultaneously the phase index and the thickness of the object.

7. A method as set forth in claim 1 or 4, wherein the light is one for emitting low coherence light.

8. A method as set forth in claim 7, wherein the light source emitting the low coherence light is one of a linearly polarized light source and an unpolarized light source.

9. A method as set forth in claim 7, wherein the light source for emitting the low coherent light is one of a linearly polarized light source, a non-polarized light source and a randomly polarized light source and has a coherence length Δ

lc (=((ln(2))×

(2/π

)×

(λ

c²/Δ

λ

))/2)) which is less than 30 μ

m.

10. A method as set forth in claim 9, wherein the light source for emitting the low coherent light is a white light source whose output is subjected to spectroscopy for a specific wavelength range by a monochrometer.

17. A method as set forth in claim 1, 2, 3 or 4, wherein said interferometric system includes a means for branching and synthesizing light from the light source.

18. A method as set forth in any one of claims 1, 2, 3 or 4, wherein each of the drive means for displacing said one of the object and the converging lens and said drive means for displacing the reference light mirror comprises a slight motion stage.

19. A method as set forth in claim 1, 2, 3 or 4 wherein the reference light mirror in the interferometric system is secured to a vibrator for vibrating the reference light mirror to phase modulate the reference light.

20. A method as set forth in claim 19, wherein phase-modulation of the reference light is carried out by applying vibration having (i) an amplitude which is less than λ

c/2 where λ

c is the oscillating center wavelength of the light source, and (ii) a frequency higher than 100 Hz.

21. A method as set forth in claim 1, 2, 3 or 4, wherein the light receiving element is a photodiode for heterodyne detection.

22. A method as set forth in claim 1, 2, 3 or 4, wherein a detection signal subjected to the heterodyne detection is converted into a digital signal by a detecting circuit.

23. A method as set forth in claim 1, 2, 3 or 4, wherein the object is a medium which does not completely absorb light from the light source.

24. A method as set forth in claim 1, 2, 3 or 4 wherein said object is a biological tissue.

2. A measuring method for measuring an optical characteristic of an optically transmissive object having a front surface and a rear surface, using an optical system including a light source for emitting light, a reference light mirror, a light receiving element, drive means for displacing one of the object to be measured and the reference light mirror, said method comprising the steps of:

simultaneously irradiating light from the light source onto the object and onto the reference light mirror;

synthesizing a reflected light beam from the front surface of the object and a reflected light beam from the reference light mirror through interference therebetween into a first synthesized interference light beam and directing the first synthesized interference light beam to the light receiving element;

synthesizing a reflected ordinary light beam from the rear surface of the object and a reflected light beam from the reference light mirror through interference therebetween into a second synthesized interference light beam and directing the second synthesized interference light beam to the light receiving element;

synthesizing a reflected extraordinary light beam from the rear surface of the object and a reflected light beam from the reference light mirror through interference therebetween into a third synthesized interference light beam and directing the third synthesized interference light to the light receiving element;

displacing one of (i) the object and (ii) the reference light mirror so as to maximize an intensity of each of the first, second and third interference light beams; and

obtaining a distance of displacement of said one of (i) the object and (ii) the reference light mirror, from a position where each of the first, second and third interference light beams becomes maximum in order to simultaneously measure a birefringence and a thickness of the object to be measured.

11. A method as set forth in claim 2 or 3, wherein the light source is one for emitting low coherence light.

12. A method as set forth in claim 11, wherein the light source emitting the low coherence light is a unpolarized light source.

13. A method as set forth in claim 12, wherein the light source for emitting the low coherent light is a super luminescent diode.

14. A method as set forth in claim 11, wherein the light source for emitting the low coherent light is one of a non-polarized light source and a randomly polarized light source and has a coherence length Δ

lc (=((ln(2))×

(2/π

)×

(λ

c²/Δ

λ

))/2) which is less than 30 μ

m.

15. A method as set forth in claim 14, wherein the light source for emitting the low coherent light is a super luminescent diode.

16. A method as set forth in claim 14, wherein said light source for emitting the low coefficient light is a white light source whose output is subjected to spectroscopy for specific wavelength range by a monochrometer.

3. A measuring method for measuring an optical characteristic of an optically transmissive object having a front surface and a rear surface, using an optical system including a light source for emitting light, a converging lens, a reference light mirror, a light receiving element, a drive means for displacing one of (i) the object and the reference light mirror and (ii) the converging lens and the reference light mirror, said method comprising the steps of:

simultaneously converging said light from the light source through the converging lens and onto the object, and irradiating said light from the light source onto the reference light mirror;

synthesizing a reflected light beam from the front surface of the object and a reflected light beam from the reference light mirror through interference therebetween into a first synthesized interference light beam and directing the first synthesized interference light beam to the light receiving element;

synthesizing a reflected ordinary light beam from the rear surface of the object and a reflected light beam from the reference light mirror through interference therebetween into a second synthesized interference light beam and directing the second synthesized interference light beam to the light receiving element;

synthesizing a reflected extraordinary light beam from the rear surface of the object and a reflected light beam from the reference light mirror through interference therebetween into a third synthesized interference light beam and directing the third synthesized interference light beam to the light receiving element,

displacing one of (i) the object and the reference light mirror and (ii) the converging lens and the reference light mirror so as to maximize an intensity of each of the first, second and third interference light beams; and

obtaining distances of displacement of said one of (i) the object and the reference light mirror and (ii) the converging lens and the reference light mirror, from a position where each of the first, second and third interference light beams becomes maximum in order to simultaneously measure a birefringence and a thickness of the object to be measured.

6. A method as set forth in claim 3 further comprising applying formulae in consideration with a wavelength dispersion of the refractive index of the object to be measured to permit the simultaneous measurement of the birefringence and the thickness, and deriving simultaneously the phase index and the thickness of the object to be measured.

4. A measuring method for measuring an optical characteristic of an optically transmissive object having a front surface and a rear surface, using an optical system including a light source for emitting light, a converging lens, a reference light mirror, a light receiving element, drive means for displacing one of (i) the object to be measured and the reference light mirror and (ii) the converging lens and the reference light mirror, said method comprising the steps of:

simultaneously converging said light from the light source through said converging lens and onto the object and irradiating said light from the light source onto the reference light mirror;

synthesizing a reflected light beam from the front surface of the object and a reflected light beam from the reference light mirror through interference therebetween into a first synthesized interference light beam, and directing the first synthesized interference light beam into the light receiving element, while synthesizing a reflected light beam from the rear surface of the object and a reflected light beam from the reference light mirror through interference therebetween into a second synthesized interference light beam, and directing the second synthesized interfering light into the light receiving element;

displacing one of (i) the object and the reference light mirror and (ii) the converging lens and the reference light mirror so as to maximize an intensity of each of the first and second interference light beams; and

obtaining distances of displacement of said one of (i) the object and (ii) the converging lens and the reference light mirror, from a position where an intensity of each of the interference light beams becomes maximum in order to simultaneously measure a phase refractive index and a group index of the object to be measured.

25. An apparatus for measuring optical characteristics including refractive index and thickness of a light transmissive object, said apparatus comprising:

a light source for emitting light;

means for splitting said light from the light source into first light and second light;

a reference light mirror for receiving and reflecting one of said first light and said second light;

a converging lens;

means for passing the other one of said first light and said second light through said converging lens onto said object;

means for displacing one of (i) the object and the reference light mirror and (ii) the converging lens and the reference light mirror;

a light receiving element for synthesizing reflected light from the object and reference light from the reference light mirror for interference so as to deliver an interference light signal; and

means for applying arithmetic formulae in consideration with a wavelength dispersion of the refractive index of the object for simultaneous measurement of the refractive index and the thickness, so as to simultaneously derive the phase index and the thickness of the object to be measured.

26. An apparatus for measuring birefringence and thickness of an object to be measured, said apparatus comprising:

a light source for emitting light;

means for splitting light from the light source into first light and second light;

a reference light mirror for receiving and reflecting one of said first light and said second light;

a converging lens;

means for passing the other one of said first light and said second light through said converging lens onto said object;

means for displacing one of (i) the object and the reference light mirror and (ii) the converging lens and the reference light mirror;

a light receiving element for synthesizing reflected light from the object and reference light from the reference light mirror for interference so as to deliver an interference light signal; and

means for applying arithmetic formulae in consideration with a wavelength dispersion of the object for simultaneous measurement of the birefringence and the thickness, so as to simultaneously derive the phase index and the thickness of the object to be measured.