Beam-control systems for high-average-power lasers began in the late s and early s. Early systems propagated the beams across laboratories using heavy-water-cooled copper optics and open-beam trains with commercial fans to provide fresh air. They have evolved in the intervening plus years to include highly sophisticated gimbaled-control systems with extremely high-reflectance.
In the first embodiment, an aircraft 30 is fitted with a laser system module 34 whose focused laser beam 38 may be directed by its crew controlled beam director 42 to treetop or ground foliage 40 L beyond an advancing wildfire in order to set backfires 40 R which cause said wildfire to subside when reaching the depleted fuel.
Consider, in general, how the components of a pointing and tracking system fit together. Figure 4 is a cartoon of the gimbaled portion of a beamcontrol system to illustrate basic servo-control functions.
A tracking telescope and optical sensor are mounted on the elevation over azimuth gimbal. Their purpose is to generate an electronic image of the target and send it to the. This tracker is a special-purpose computer, which then processes the target image, identifies the desired aim point, and measures the angle between it and the optical boresight of the telescope.
Its output is a command to the gimbals to rotate until the optical axis of the tracking telescope is following the target and pointing at the aim point. If the gimbals are mounted on a moving or vibrating platform, these disturbances introduce additional tracking errors.
Unlike target motion, this base motion disturbance can be directly measured using gyros and accelerometers, which are attached to the telescope. The cartoon in Fig. However, the way it is shown, mounted on the telescope it would be sensitive to any vibration modes in the structure. One way to avoid this coupling could be to use an inertial reference unit IRU that includes a stable platform independent of the telescope structure. The HEL, shown simply in this cartoon as a box, provides the weapon beam to a pointing telescope, which is then mechanically boresighted optical axis made parallel to the tracking telescope.
As shown in the cartoon, the HEL is mounted on the telescope, most systems have the HEL mounted off the telescope and coupled into the telescope by a beam path that enters the telescope along the rotation axes. Finally, the range to the target must be measured so that a parallax correction can be applied to the pointing telescope. This slightly tilts its optical axis to intersect the tracking telescopes optical axis at the range of the target and thus place the HEL beam on the aim point.
The team was led by Dr. Whitney Co. An aerial view of the laser facility is shown as Fig. The laser device and the pointing and tracking system were in the building on the left of the figure. The very basic pointing and tracking system are shown in Fig. The test series investigated tracking, pointing, focusing, thermal blooming, and atmospheric distortions with a slewing beam to a moving target.
The row of large room fans seen in Fig. The gimbaled flat pointed the beam at a down-range target board that was mounted on an instrumented railroad car, known as the Everglazer, which could be moved while being tracked. This target board is shown in Fig.
This large and distributed laser test system was not something that could be mounted in a vehicle as a weapon, but the test series investigated an large number of critical laser system problems that would all be studied for years afterward. These included pointing, tracking, jitter control, higher order aberrations, adaptive optics, and beam diagnostics.
This test introduced many of the later concepts of beam control, although the packaging of the system needed many changes to support an effective weapon. It was a three-gimbal mount outer and inner azimuth plus elevation. The HEL beam entered the beam director from the bottom along the azimuth axis of rotation and was routed up and around one of the side arms with folding mirrors.
It entered the telescope along the elevation axis of rotation and used a steering mirror to turn the beam and point it at the secondary mirror. This beam director has been used for many purposes since and is still in service. It is shown in Fig. This system was the first demonstration of a coarse and fine gimbal to reduce line-of-sight jitter. It also used a gyro on the inner gimbals to inertially stabilize the pointing telescope.
Both beam directors were built by Hughes Aircraft in the mids and were similar in design. Optical Engineering. They both had four gimbals: coarse azimuth and elevation gimbals for large angular coverage plus fine azimuth and elevation gimbals that were inertially stabilized using gyros.
They also both had tracking telescopes mounted to the inner gimbals but with their own line-of-sight optics. Figure 9 shows the ALL with both the forward and aft fairing installed. The forward fairing was later removed and replaced with a much smaller fillet-like fairing.
There were several experiments on the ALL that measured the aero-optical effects of the airflow around the turret and how it affected the propagation of the laser beam. The APT introduced several beam-control concepts. Note the beam angle sensor in the lower section of the APT. Additionally, there is a translation sensor just past the aerodynamic window close to the laser device. These sensors measured tilt and translation between the laser device and the base of the APT and used beam steering mirror 1 and 2 Fig.
Another alignment system controlled the beam from the base of the APT to the output beam expander. This system. This system sent a cylindrical beam around the outside of the HEL beam and measured the tilt through the beam path from the base to the output beam expander.
The inertial reference was obtained by a gyro on the back of the primary mirror that used the inner gimbal and beam steering mirror 3, in the beam expander, to keep the outgoing beam inertially stable.
This system proved to be a problem area. The beam-expanding telescope had several vibration bending modes and the autoalignment reference annulus flexed so much that the autoalignment system did not adequately measure or control the jitter on the output beam. A fix for this was to replace the autoalignment annulus mirror with a flat annulus mirror mounted around the secondary mirror. This meant the alignment system no longer measured the motion of the primary mirror, but it permitted operation of the system through the final flight tests.
There were also test flights that removed the output window from the APT to see if the airflow around the turret would permit propagating a beam without using the window. The result was that an acoustic mode existed in the open port, which significantly increased the jitter of the output beam.
The output window was flown for the remaining flight tests. Figure 11 is a reversed photograph of the NPT. At that point, there was a desire for higherperformance systems using larger pointing telescopes but needing lower jitter. This conflicting set of requirements larger diameter telescopes while having higher structural. The inner elevation and azimuth gimbals were eliminated, and a cm telescope was hard-mounted directly on the elevation gimbal. Instead of using the inner gimbal as the inertial reference, a separate low mass IRU was used.
This IRU was a small assembly consisting of a gyro and a flexible mount to as an inertially stable platform, which provided the line-of-sight reference for the alignment system. Using an optical reference mirror attached to the IRU, its measurement of the base motion disturbance was relayed to the SLBDs optical alignment sensors, which then commanded small fast steering mirrors to stabilize the HEL beam.
The large telescope was allowed to experience the base motion disturbance even though the HEL beam passing through it was stabilized. It remained operational until Figure 13 shows the routing of HEL beam through the gimbals and into the beam-expanding telescope.
Note that the beam was aligned along the center of the azimuth and elevation bearings. When the gimbal is rotated, the beam also rotates relative to the beam train, but stays aligned with the outgoing beam path.
The beam path entering the beam expander is rather complex; after entering the beam expander tube, the beam reflects off of a relay mirror to the tertiary and then to the secondary. Beam expansion occurs between the secondary and primary mirrors. The laser. Dynamically variable spot size laser system A Dynamically Variable Spot Size DVSS laser system for bonding metal components includes an elongated housing containing a light entry aperture coupled to a laser beam transmission cable and a light exit aperture.
A plurality of lenses contained within the housing focus a laser beam from the light entry aperture through the light exit aperture. Beam-control systems for high-average-power lasers began in the late s and early s. Early systems propagated the beams across laboratories using heavy-water-cooled copper optics and open-beam trains with commercial fans to provide fresh air.
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