Optical: systems and elements – Prism
Reexamination Certificate
2002-08-15
2004-08-24
Dunn, Drew A. (Department: 2872)
Optical: systems and elements
Prism
C359S837000, C359S290000, C385S088000, C385S092000
Reexamination Certificate
active
06781773
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to lasers and other optical instruments having stringent boresight stability requirements and used in harsh environments; and more particularly, to improving boresight stability on an optical bench that uses a beam expander near an inlet or exit aperture of the optical system.
BACKGROUND
Optical instrument technology has evolved rapidly over the past forty years. For instance, early lasers had few components and consisted of a laser rod, flashlamp and cavity reflecting mirrors. As the industry matured and lasers became more capable, laser systems became more complex, adding components to support Q-switching, amplification, novel out-coupling schemes, polarization control, lateral and angular beam alignment, power monitoring, beam divergence control, improved mechanical and thermal stability, optical parametric amplification, and frequency doubling. As complexity grew, so did the number and variety of applications, and so did the need for improved boresight stability—maintaining the optical beam angle with respect to some reference.
Light Detection and Ranging (LIDAR) units, lasers, fire control systems, missile defense systems, interferometers, and other optical instruments rely on boresight stability to function properly. Lasers and other optical instruments require thermal and mechanical stability to maintain beam quality, output power, beam divergence and mechanical boresight. Lasers and other optical instruments used in precision applications such as surveying and targeting, and used in demanding operating environments, such as military applications, have very high stability requirements.
One of the primary causes of degraded optical system boresight stability is mechanical motion. The mechanical motion arises from a number of possible sources, including thermal effects and mechanical loads within the optical system, change in index of refraction over temperature (henceforth referred to as dN/dT) effects in the components, and motion of the bench itself due to external mechanical loads. Possible sources of movement include: mounting stresses, thermally induced stresses, material dimension instabilities, vibration, acceleration loads, and pressure changes, such as result from altitude changes. For example, as internal temperatures change, each of the materials in an optical system expands or contracts at a rate different than other materials in the system, which introduces movement of the various components with respect to each other.
Differential thermal expansions and contractions cause distortions in the optical bench, chassis, and motion of other component parts in the optical system. These movements can therefore alter the alignments of optical system parts with respect to each other, and in turn, cause an adverse change in the boresight alignment of the optical system. This adverse change causes the output beam to deviate laterally and angularly from its intended path, thus degrading optical system performance. Additional external environmental factors, such as changes in altitude or aircraft g-forces, also exert mechanical forces on system components that also can adversely impact boresight alignment by causing differential movement of optical system parts. This aggregation of design and environmental factors, and their resulting adverse effects on boresight alignment, can yield an unreliable optical system, especially for precision laser applications such as surveying, targeting, missile defense, long-range free space optical communications, and the alignment of machinery and buildings.
Laser energy is Gaussian in nature and subject to divergence. In most applications, collimated light energy is used to direct a laser beam to some specific location. In may seem counter-intuitive, but to form a bright narrow spot at some distance generally requires a larger diameter beam of light. A beam expander is an afocal telescope often used as the final output element on various laser instruments like range finders, designators, laser radar equipment, free space laser communications equipment and countermeasures systems. The beam expander, whether reflective or refractive, takes collimated input beam and outputs a collimated output beam of a larger beam.
The beam expander telescope is typically mounted proximate an exit aperture and on the optical bench with other optical components. Thus, the beam expander device is subject to the same factors that detrimentally affect the optical bench and boresight alignment of other optical system components such as mechanical motion due to thermal and mechanical loads within the optical system, and external mechanical loads.
However, the state of the art implementations have yet to satisfy the commercial applications and there is considerable room for improvement. Thus, there is a need for improving the boresight stability of optical systems that use a beam expander telescope.
SUMMARY OF THE INVENTION
The invention is devised in the light of the problems of the prior art described herein, accordingly, it is a general object of the present invention to provide a novel and useful apparatus and technique that can solve the problems described herein. The improved optical boresight stability system, as disclosed herein, meets the need identified hereinabove for improving the boresight stability of an optical system that uses a beam expander apparatus near an exit aperture and that operates in a variety of environmental conditions.
A beam expander on the output end of a laser or laser instrument is commonly thought to reduce the output boresight angular error inherent in the laser and/or instrument itself by a factor equal to the magnification ratio (MR) of the beam expander. This is a significant benefit, if actually achieved, as the optical elements can be mounted at lower costs as the tilt error will be reduced by 1/MR.
In order to obtain this commonly calculated and commonly expected benefit, the beam expander must be mounted in such a way that it is isolated from the motions experienced by the other optics. Since it is the last component in a train of optics, it can be mounted somewhat separately from the other optics so that movement of the optical bench does not affect the beam expander. In one embodiment the beam expander is located near the reference feature to which boresight will be measured. The beam expander, more than any other component, should be mounted in such a separated and rugged way that it moves negligibly with respect to that reference. This can be accomplished, for example, by placing the beam expander at the mounting feet or an external wall of the structure. If the motion relative to this reference surface is negligible in magnitude with respect to the system requirements, then and only then will the overall angular tilt error of the laser beam be reduced by the magnification ratio of the beam expander. The common standard for measuring the angular error is the reflective reference. Another option is to employ a specific sighting reference, but this adds to the cost and complexity.
Except in systems with very large magnifications and very loose angular stability requirements, this theoretical benefit is not experienced because of the interaction between the motion of the beam expander itself and the motion of the optics and the optical bed upon which the optics are mounted. This movement of the beam expander device diminishes any benefit to boresight stability. The problem is more pronounced when the beam expander tilts in the same direction as the optics or opposite to the optics. In order to obtain the theoretical optical advantages of reducing angular error, the beam expander must not move with respect to the output reference surface of the instrument, and this aspect has eluded designers for many years.
Boresight stability is improved by reducing the motion of the beam expander telescope with respect to other optical system components. The motion of the beam expander telescope relative to other components is minimized by several techniques that may be combined for opti
Bae Systems Information and Electronic Systems Integration INC
Dunn Drew A.
Mainc & Asmus
Pritchett Joshua L
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