TOTALLY REFLECTING LASER REFRACTOMETER

• US 3,770,352 A
• Filed: 05/08/1972
• Issued: 11/06/1973
• Est. Priority Date: 05/08/1972
• Status: Expired due to Term

• Current Assignee: Loral Aerospace Corp. (MHR Holdings LLC (New York))
• Original Assignee: Matthew B. White
• Inventors: White, Matthew B.
• Primary Examiner(s): Schonberg, David
• Assistant Examiner(s): Clark, Conrad
• Application Number:
  US05/250,877
• Time in Patent Office: 547 Days
• Field of Search: 356/134,135,136,118 331/94.5A
• US Class Current: 356/133
• CPC Class Codes: G01N 21/43 by measuring critical angle
• CPC Class Codes: H01S 3/121 using intracavity mechanical devices

Abstract

A laser having isotropic polarization characteristics, is operated with a confocal mirror cavity and is adjusted in terms of mirror reflectivity, spacing, and laser active medium to operate at a particular wavelength. A total internal reflection prism is contained inside the laser optical cavity and is rotated to produce an optical path angle slightly greater than the critical angle. For this condition the prism will constrain the laser to oscillate in such a way that adjacent modes have orthogonal polarizations. In the case of two mode operation one mode will be polarized parallel to the plane of incidence at the prism reflecting surface and the other will be perpendicular to the first. The phase shifts produced upon total reflection are different for light polarized parallel and perpendicular to the plane of incidence. Futhermore the difference in the phase shifts for these two polarization directions is a function of the refractive indices of the prism and its facing material as well as the angle of incidence. Since the difference in phase shift is equivalent to a difference in optical cavity length for the two orthogonally polarized modes the frequency of one of the modes is slightly displaced with respect to the other. Measurement of the beat frequency difference between the orthogonal modes will therefore yield information on the refractive index of the facing material provided that prism index and angle of incidence are known. In one embodiment the prism is provided with a calibrating sample of known refractive index placed in close contact with its reflecting face. The prism and laser cavity are then adjusted to produce a stable mode difference beat frequency. Very small changes in refractive index of the material in contact with the prism face can be correlated with changes in beat frequency thereby giving a very sensitive measure of refractive index. If the comparison sample is a fluid confined inside a chamber sealed to the prism face, extremely slight differences in composition of the fluid can be detected. Since the output is read immediately as a frequency, the system is useful in monitoring flowing fluids.

Claims

1. A refractometer comprising:

a. a laser, b. means associated with said laser to produce dual-polarization operation, said means comprising a prism rotated to produce total internal reflection of the light signal in said laser, c. means for converting a portion of the output of said laser to an electrical signal having a frequency equal to the difference in frequency between dual polarization laser modes, and d. means for maintaining in close optical contact with the reflecting face of said prism, a sample of material to be evaluated, whereby said electrical signal is a function of the ratio of the refractive index of said sample to the refractive index of said prism.

2. means for locating a sample to be tested in intimate contact with said face of said prism, e. means for extracting a portion of the optical energy in said cavity, and f. means for converting the optical signals resulting from said anisotropy into an electrical signal having a frequency related to the refractive index of said sample.

3. The refractometer of claim 1 wherein said means for maintaining is adapted to handle a fluid sample.

4. The refractometer of claim 3 wherein said means for maintaining is adapted to handle a fluid sample that is in motion.

5. The refractometer of claim 4 wherein said sample is being processed and said electrical signal is used in a process control system.

6. A refractometer comprising:

a. an optical resonant cavity having a well defined optical axis, b. a laser active medium contoured to avoid favoring any polarization state of optical energy therein, and located inside said resonant cavity, c. means for pumping said laser active medium to a level sufficient to establish laser action along said optical axis, d. means for producing dual-polarization laser action by introducing polarization anisotropy inside said cavity comprising;

7. The refractometer of claim 6 wherein said means for locating is adapted to accommodate a solid sample having at least one flat polished face thereon to conform with said face of said prism.

8. The refractometer of claim 6 wherein said means for locating is adapted to maintain a sample fluid in contact with said face of said prism by means of an enclosure mounted on said prism.

9. The refractometer of claim 8 wherein said means for locating is further adapted to contain a fluid in motion in a fluid process and said means for converting of clause (f) provides a continuous readout related to the refractive index of said fluid.

10. The refractometer of claim 9 wherein said fluid emanates from a process control operation regulated by a servomechanism, said refractometer providing control information to said servomechanism to stabilize the refractive index of said fluid in said process.