Optical combiner for a ring laser gyro

• US 4,863,271 A
• Filed: 02/02/1988
• Issued: 09/05/1989
• Est. Priority Date: 02/02/1988
• Status: Expired due to Fees

• Current Assignee: Litton Systems Incorporated
• Original Assignee: Litton Systems Incorporated
• Inventors: Grant, David C., Purrazzella, Joseph J., Grobsky, Kevin D.
• Primary Examiner(s): McGraw, Vincent P.
• Application Number:
  US07/151,617
• Time in Patent Office: 581 Days
• Field of Search: 356/350
• US Class Current: 356/468
• CPC Class Codes: G01C 19/662 signal readout; dither compensators

Abstract

The apparatus of this invention is a composite combining prism for combining the beams of a ring laser gyro. It uses a composite prism whose trunk is fabricated of two substantially identical juxtaposed sub-prisms having beam splitters in the region where laser beams cross their common boundary. The beam splitter regions are very thin dielectric films, typically no thicker than a fraction of a wavelength of the laser light. Dielectric films, such as titanium dioxide, are used for the splitters so that the geometrical path lengths of interfering beams are almost exactly the same. To make the path lengths optically the same, the two sub-prisms and the substrate of the partly transmitting corner mirror of the ring laser preferably have the same index of refraction. In a first embodiment, the outwardly-extending trunk of the assembled composite prism is substantially rectangular in cross-section. It is split into two identical sub-prisms with negligible-thickness beam splitters at the common interface of the sub-prisms. It has a slanted outward end carrying two combining light heterodyne sensors. To reduce the outward extension of the prism from the ring laser gyro another embodiment has the composite prism lying flat on the substrate of the partly transmitting corner mirror. It uses a forty five degree prism surface to turn the laser light through a right angle to deliver the outwardly directed laser beams into a direction parallel to the surface of the mirror substrate.

Claims

1. In combination:

a ring laser, including at least three thin film dielectric corner mirrors, each rigidly supported by its substrate and having a narrow reflective laser band width to reflect counterpropagating laser beams around a closed path, the two beams at each said mirror defining a laser-beam-plane;

a composite combining prism, mounted upon the outside of and optically cooperating with the substantially planar outer surface of said substrate of one of said corner mirrors, said cooperating corner mirror being partly transmissive of said beams;

said composite combining prism comprising first and second substantially identical juxtaposed and touching sub-prisms at a common planar boundary, rectangular in cross-section having a thin film dielectric beam splitter, with a narrow bandwidth around the beam frequency, at said common boundary to cause said composite prism to be substantially symmetrical relative to said common boundary;

said sub-prisms being configured and positioned along an outer surface thereof with the long direction of said outer surface in contact with, and said common planar boundary perpendicular to, said substantially planar outer surface of said substrate to receive a portion of said counterpropagating laser beams,

said sub-prisms having substantially planar, substantially forty five degree faces on one end thereof intercepting said beams and turning said beams substantially ninety degrees toward said beam splitter;

said sub-prisms being configured and positioned to deliver said beams in symmetrical paths to opposite faces of said beam splitter with said beams arriving at said beam splitter at angles to combine said beams into two combined beams;

said common planar boundary composite prism being turned slightly, about an optical offset axis parallel to said common planar boundary, parallel to said laser-beam-plane, and perpendicular to said corner mirror; and

at least one heterodyne dual-light-sensor positioned to receive at least one of said combined beams from said beam splitter to produce moving interference fringes whose count rate is a measure of angular velocity of said ring laser about its sensing axis.

2. Apparatus as recited in claim 1 wherein said at least one heterodyne dual-light-sensor comprises two heterodyne dual-light-sensors, each intercepting a different one of said composite beams.

3. Apparatus as recited in claim 1 wherein said at least one heterodyne light-sensor comprises two heterodyne dual-light-sensors, each intercepting a different one of said composite beams.

4. Apparatus as recited in claim 1 and further comprising first and second auxiliary prisms attached to said first and second sub-prisms, respectively, intercepting said beams;

and

at least one auxiliary light amplitude sensor, positioned on at least one of said auxiliary prisms to sense the amplitude of at least one of said beams, the signals from said last-named sensors being useful to control and adjust the cavity length of said ring laser gyro.

5. Apparatus as recited in claim 4 in which there are two auxiliary light amplitude sensors, one positioned on each said auxiliary prism.

6. Apparatus as recited in claim 1 and further comprising absorbing means on said common planar boundary, except in the region of said beam splitter, for absorbing laser beams.

7. In combination:

a dielectric mirror on a substrate having substantially parallel faces;

first and second sub-prisms, fabricated of the same material as said substrate, each having a rectangular first face and forming a rectangular cylinder extending parallel to and attached to the opposite side of said substrate from said mirror, said sub-prisms having a forty five degree face on a first end thereof to re-direct lases beams along said cylinder, said sub-prisms being juxtaposed with one face of each in contact with a corresponding face of the other and defining an axis on said common face and centered on said forty five degree face, and a dielectric beam splitter on said axis at said common face, the outer ends of said cylinders being inclined from said common face toward said substrate to support at least one light heterodyne sensor.

8. Apparatus as recited in claim 7 and further comprising means for absorbing laser beams on said common faces except in the region of said beam splitter.