Apparatus for measuring refractive index of medium using light, displacement measuring system using the same apparatus, and direction-of-polarization rotating unit

• US 5,767,971 A
• Filed: 12/19/1995
• Issued: 06/16/1998
• Est. Priority Date: 12/20/1994
• Status: Expired due to Term

• Current Assignee: Nikon Corporation
• Original Assignee: Nikon Corporation
• Inventors: Ishida, Akira, Tsukihara, Kouichi, Kawakami, Jun, Kawai, Hitoshi
• Primary Examiner(s): Turner, Samuel A.
• Application Number:
  US08/574,704
• Time in Patent Office: 910 Days
• Field of Search: 356/351, 356/358, 356/349, 356/364, 356/361, 359/383, 359/494, 359/497, 359/501
• US Class Current: 356/493
• CPC Class Codes: G01N 21/45 using interferometric methods; using Schlieren methods

Abstract

A second harmonic generation (SHG) device is used for conversion of wavelength. The direction of polarization of light beams is suitably rotated by a direction-of-polarization rotating unit, for example by 45°, and thereafter these light beams are guided into the SHG device. This can minimize attenuation of intensity of light beam due to conversion of wavelength. Hence, using this direction-of-polarization rotating unit in the apparatus for measuring the medium, the intensity of light being incident upon the photodetector can be increased, whereby the refractive index of the medium can be measured more accurately than by the conventional apparatus.

Claims

1. An apparatus for measuring the refractive index of a medium, having a light source which can emit a light beam and a photodetector arranged in a path way of the light beam, the medium being arranged in said path way, said apparatus comprising:

at least two second harmonic generation devices each of which can convert wavelength of light passing therethrough, said second harmonic generation devices being arranged in said path way; and

a unit arranged in said path way and which can rotate a direction of polarization of light passing therethrough,

wherein said unit comprises a plurality of wave plates arranged in said path way, and said wave plates being arranged so that said unit rotates the polarization direction of light having first wavelength relative to the polarization direction of light having second wavelength.

2. An apparatus according to claim 1, wherein said unit is arranged in said path way between said second harmonic generation devices.

3. An apparatus according to claim 1, wherein said second harmonic generation devices are comprised of KTiOPO₄.

4. An apparatus according to claim 1, wherein said apparatus measures the refractive index of the medium based on a change in an intensity detected by said photodetector.

5. An apparatus according to claim 1, further comprising:

an optical separating element arranged in said path way and which separates the light beam into two components each of which has a different wavelength; and

a frequency shifter which can shift the frequency of one of the components separated by said optical separating element, wherein said apparatus measures the refractive index of said medium based on a change in an intensity detected by said photodetector.

6. An apparatus according to claim 1, further comprising:

an optical separating element arranged in said path way and which separates the light beam into two components; and

a frequency shifter which can shift the frequency of one of the components separated by said optical separating element, wherein said apparatus measures the refractive index of said medium based on a change in an intensity detected by said photodetector.

7. An apparatus according to claim 1, wherein said unit comprises:

a casing having two windows, each of which is arranged in said path way; and

a wave plate arranged in said casing, said wave plate being rotatable.

8. An apparatus according to claim 7, wherein said casing has a slot, and wherein said unit further comprises a handle attached to said wave plate, said handle being movable in said slot.

9. An apparatus according to claim 1, wherein said unit comprises:

a casing having two windows, each of which is arranged in said path way; and

a plurality of wave plates arranged in said casing, each of said wave plates being rotatable.

10. An apparatus according to claim 1, wherein said unit comprises a plurality of wave plates arranged in said path way.

11. An apparatus according to claim 1, wherein said unit comprises at least three wave plates arranged in said path way.

12. An apparatus according to claim 1, wherein said unit rotates said polarization directions of the respective lights having first and second wavelengths independently.

13. An apparatus according to claim 1, wherein said unit comprises wave plates arranged in said path way, and wherein said unit can rotate the polarization direction of light passing therethrough independently of wavelength.

14. An apparatus according to claim 1, wherein said apparatus is an apparatus for measuring a change in the refractive index of the medium.

15. An apparatus according to claim 1, further comprising:

a first beam splitter arranged in said path way, said first beam splitter dividing said light beam into two components after said light beam has passed through one of said second harmonic generation devices;

first and second mirrors, each of which is arranged to oppose said first beam splitter, said first mirror reflecting one of the components divided by said first beam splitter, the medium being arranged between said first mirror and said first beam splitter, said second mirror reflecting the other of the components divided by said first beam splitter back toward said first beam splitter, the respective components, having been reflected by said first and second mirrors and having arrived at said first beam splitter, being coupled by said first beam splitter, the coupled light beam continuing to travel on said path way;

a second beam splitter arranged in said path way between said first beam splitter and another of said second harmonic generation devices, the coupled light beam being again divided into two components by said second beam splitter, one of the components divided by said second beam splitter being detected by said photodetector; and

another photodetector arranged to detect the other of the components divided by said second beam splitter.

16. An apparatus according to claim 1, further comprising:

a beam splitter arranged in said path way, said beam splitter dividing said light beam into two components after said light beam has passed through one of said second harmonic generation devices, wherein one of the components divided by said beam splitter is detected by said photodetector; and

another photodetector arranged to detect the other of the components divided by said beam splitter.

19. A displacement measuring system comprising said apparatus of claim 1.

17. An apparatus for measuring the refractive index of a medium, having a light source which can emit a light beam and a photodetector arranged in a path way of the light beam, the medium being arranged in said path way, said apparatus comprising:

at least two second harmonic generation devices each of which can convert wavelength of light passing therethrough, said second harmonic generation devices being arranged in said path way; and

a unit arranged in said path way and which can rotate a direction of polarization of light passing therethrough, wherein said unit comprises;

a first wave plate having a retardation (2n-1)π

/2 for light having a first wavelength and a retardation (2m-1)π

for light having a second wavelength;

a second wave plate having a retardation (2p-1)π

/2 for light having a first wavelength and a retardation (2q-1)π

for light having a second wavelength;

a third wave plate having a retardation (2r-1)π

for light having a first wavelength and a retardation 2sπ

for light having a second wavelength, wherein said wave plates are arranged on a common axis, and wherein n, m, p, q, r, s are integers.

18. An apparatus for measuring the refractive index of a medium, having a light source which can emit a light beam and a photodetector arranged in a path way of the light beam, the medium being arranged in said path way, said apparatus comprising:

at least two second harmonic generation devices each of which can convert wavelength of light passing therethrough, said second harmonic generation devices being arranged in said path way; and

a unit arranged in said path way and which can rotate a direction of polarization of light passing therethrough, wherein said unit comprises;

a first wave plate having a retardation (2n-1)π

for light having a first wavelength and light having a second wavelength; and

a second wave plate having a retardation (2m-1)π

for light having a first wavelength and a retardation 2pπ

for light having a second wavelength, wherein said wave plates are arranged on a common axis, and wherein n, m, p are integers.

20. A direction-of-polarization rotating unit comprising:

first wave plate having a retardation (2n-1)π

/2 for light having a first wavelength and a retardation (2m-1)π

for light having a second wavelength;

a second wave plate having a retardation (2p-1)π

/2 for light having the first wavelength and a retardation (2q-1)π

for light having the second wavelength; and

a third wave plate having a retardation (2r-1)π

for light having the first wavelength and a retardation 2sπ

for light having the second wavelength, wherein said wave plates are arranged on a common axis, and wherein n, m, p, q, r, s are integers.

21. A direction-of-polarization rotating unit comprising:

a first wave plate having a retardation (2n-1)π

for light having a first wavelength and light having a second wavelength; and

a second wave plate having a retardation (2m-1)π

for light having the first wavelength and a retardation 2pπ

for light having the second wavelength, wherein said wave plates are arranged on a common axis, and wherein n, m, p are integers.

22. An apparatus for measuring the refractive index of a medium, having a light source which can emit a light beam and a photodetector arranged in a path way of the light beam, the medium being arranged in said path way, said apparatus comprising:

at least two second harmonic generation devices each of which can convert wavelength of light passing therethrough, said second harmonic generation devices being arranged in said path way; and

a unit arranged in said path way and which can rotate a direction of polarization of light passing therethrough, wherein said unit comprises a plurality of wave plates arranged in said path way, and said wave plates being arranged so that said unit rotates the polarization direction of light having first wavelength and does not rotate the polarization direction of light having second wavelength.