This invention relates generally to optical coatings for controlling the reflection and transmission of particular optical wavelengths at an optical surface. This invention relates particularly to a structure and method of fabrication of an ultraviolet resistive, antireflective coating for a component of an optical system. Still more particularly, this invention relates to an ultraviolet resistive, antireflective coating for an intra-cavity element of a multioscillator ring laser gyroscope.
A ring laser gyroscope employs the Sagnac effect to measure rotation. Counterpropagating light beams in a closed path have transit times that differ in direct proportion to the rotation rate about an axis perpendicular to the plane of the path. In a ring laser gyroscope the closed path is defined by mirrors that direct the light beams around the path. The path is generally either square or triangular in shape, although any closed polygonal path could, in principle, be used. The closed path is typically in a cavity formed in a frame or body that is formed of a glass ceramic material.
The cavity is evacuated and then filled with a mixture of helium and neon, which is the gain medium for the laser. An electrical discharge excites the gain medium, which produces light amplification and a coherent light source. The mirrors must be precisely aligned to direct the light beams around the closed path in a manner that promotes proper operation of the laser. The mirror surface must be free of impurities to provide a laser beam intensity that will result in a usable signal.
Once laser oscillation occurs in the system at resonant frequencies, the difference in the length of the pathways traversed by the counterpropagating laser beams result in a difference or beat frequency which is sensed by a photodetector and amplified by an amplifier. The beat frequency is a result of optically heterodyning the counter propagating beams.
It is well-known that a ring laser gyroscope that employs two counterpropagating waves is subject to the phenomenon of lock-in at low rotation rates. At rates below a threshold value, a two-mode ring laser gyroscope will erroneously indicate that the rotation rate is zero. When the frequency difference between the waves is too small, the frequencies appear to be equal, which gives an erroneous indication of a zero rotation rate.
One type of ring laser gyroscope that eliminates lock-in is the Zeeman ring laser gyroscope, which is sometimes referred to as the Zero Lock rotation sensor. Zero Lock is a trademark of Litton Systems, Inc. The Zero Lock rotation sensor has two pairs of (counterpropagating) circularly polarized waves propagating in the resonant cavity simultaneously. One pair of counterpropagating waves consists of right circularly polarized light waves propagating in the clockwise and counterclockwise directions. The other pair consists of left circularly polarized waves which are also propagating in the clockwise and counterclockwise directions within the same resonant cavity. Such a four mode ring laser gyroscope configuration is described in detail in U.S. Pat. No. 3,741,657, issued Jun. 26, 1973 to Enduring and in U.S. Pat. No. 4,213,705, issued Jul. 22, 1980 to Sanders. Operation of a four mode laser gyroscope is briefly described below.
Disposed in the path of the propagating waves within the cavity are reciprocally anisotropic and non reciprocally anisotropic dispersive elements. A reciprocally anisotropic dispersive element, such as an optical rotator made of crystalline quartz, provides different propagation time delays (or different optical indices) to right and left circularly polarized waves. This difference in optical index due to sense of polarization results in an optical path length difference between oppositely polarized waves resonating within the same cavity. The non reciprocally anisotropic dispersive element, for example a Faraday cell, presents different refractive indices for light waves traveling in opposite directions. The waves traveling in the counterclockwise and clockwise directions thus have different propagation time delays in the Faraday cell. This difference in propagation time delays produces different path lengths for light waves traveling in opposite directions. Therefore, the combination of the two types of anisotropy can be used to adjust the frequency separation between resonant modes such that all four modes resonate at different frequencies.
Separation between the resonant mode frequencies is accomplished so that the resonant frequencies of the two waves traveling in one direction are spaced between the resonant frequencies of the two waves traveling in the opposite direction. The two highest frequency modes have the same circular polarization but opposite directions of propagation. Likewise, the two lowest frequency modes have the same circular polarization, opposite from the sense of polarization of the other pair and are also counter rotating in the cavity. Each pair of like-polarized modes operates in a separate two mode laser gyro. As the ring laser system is rotated about an axis perpendicular to the plane of the propagating waves, the frequency separation between the two higher frequency modes will either decrease or increase while the frequency separation between the two lower frequency modes will either increase or decrease. The output beat signal resulting from combining the two lower frequency modes is subtracted from the output beat signal resulting from combining the two higher frequency modes. This produces a substantially linear representation of the rotation rate of the laser system. The direction of rotation is determined by monitoring one of the pair of modes.
The intracavity element presents difficulties in using a Zero Lock rotation sensor for rotation sensing. The cavity is basically evacuated and has a refractive index of about 1.00. The intracavity element has a refractive index that is greater than that of the optical medium in the cavity. Therefore, reflections occur when the waves impinge upon the intracavity element. These reflections cause undesirable mode coupling and resonator losses, which reduce the accuracy of the output from the Zero Lock rotation sensor.
Antireflective coatings have been used on intracavity elements to reduce the amount of light reflected by the intracavity elements. These coatings have had to meet stringent requirements for thickness. The prior art procedure for fabricating such coatings involves depositing the first layer to a thickness greater than required, measuring the thickness of the first coating, and then milling down to the desired thickness. Difficulty in satisfying the requirements for thickness of the coatings causes errors and increases the cost and time for constructing a suitable intracavity element.
There are several prior art antireflective coatings. For example U.S. Pat. No. 4,966,437, issued Oct. 30, 1990 to Rahn and U.S. Pat. No. 4,907,846, issued Mar. 13, 1990 to Tustison et al. describe previous antireflective mirror coatings. The disclosures of U.S. Pat. Nos. 4,966,437 and 4,907,846 are hereby incorporated by reference into the present disclosure.