Under a DARPA program known as Steered Agile Beams (STAB), novel liquid-crystal devices are being developed by Rockwell Scientific Company (RSC) to enable compact, high-speed laser beam steering subsystems for applications in free-space laser communications over distances ranging from 1 to 100 km. For such distances, the subsystem must have precise control of the laser beam to account for atmospheric effects or any slight movements in the transmitting or receiving stations. This allows for high data rate communications between link partners without interruptions. Critical for this task is a device that can steer light continuously at a high rate of speed (approximately 1 kHz). A key element in the multistage architecture of this project is a liquid-crystal optical phased array (OPA) that requires a large number of high-precision synchronized analog outputs.
The OPA basically acts as a tunable array of prisms that adjusts the beam's direction through the physics of diffraction. It consists of a thin piece of glass that has 512 striped electrodes on its surface, each 10 microns wide and 5 mm long, resulting in an optical aperture of approximately 5 x 5 mm (see Figure 1). Another piece of glass coated with a transparent conductor is glued peripherally on top of the electrodes to create a cell type structure, which is filled with a special class of high-speed liquid crystals before being sealed.
With the OPA in an un-driven state (liquid crystal in homogenous orientation), a laser beam would travel through the liquid crystal and not be affected, and the light would then reflect off the patterned electrodes, creating a diffraction pattern corresponding to electrodes structure. However, by sending analog voltages to each electrode, the system can create a ramp (linear in phase) of diffraction gratings that act like birefringent prisms to deflect the beam as much as ±1.5 degrees. This gives continuous steering through these rewritable phase ramps, which steer the light to any angle in its field of regard. Coupled with wide steering devices, entirely electro-optic beam steering over a 40-degree field of regard has been demonstrated, with many improvements coming.
The OPA driving all of the electrodes in a precise, synchronized fashion is the PD2-AO-96 PCI board from United Electronic Industries (UEI). The board provides 96 independent analog outputs with an aggregate output rate as high as 1.3 MHz. Five cards are used for a total of 480 channels (see Figure 2). This configuration does not provide a drive signal for all 512 electrodes, but is sufficient for demonstrating proof of concept.
For each electrode, the analog-output level changes at either 1 kHz or 20 kHz. The amplitude on each electrode is different, and the LCDs require a net AC voltage; they are either all positive or all negative. Thus, a squarewave is used, rather than a sinewave, to get the highest possible rms voltage from the drivers, creating the optimal control waveform for each channel.
Synchronization of all 480 channels is vital. Based on the physical layout of the device, neighboring pixels are interleaved, which results in different D/A boards driving adjacent electrodes. This also puts a heavy constraint on the timing of all the D/A converters. If any pixels are out of phase, the result is a floating ground, which redirects the electric field generated by the electrodes. This field deforms the liquid crystal, making an optically inert area on the device. The card specs a channel-to-channel jitter of 5 μsec, which is well within the acceptable range of this application.
All five PCI cards are in the same backplane. A master/slave triggering and clocking scheme keeps the cards synchronized so that one command can update all the D/As simultaneously. Intercard cabling and firmware changes are necessary for each board's onboard DSP.
To drive this device in an optimal fashion new software has also been developed with LabVIEW - running on a Windows® 2000 machine. LabVIEW provides ease of access to DAC devices and interfaces as well as debugging capabilities, which saves time when implementing new drivers and RSC's proprietary drive algorithms.
The end system demonstrates an update of the LCD every 20 msec, which is faster than similar devices and motor/gimbal-based schemes. This electro-optic solution is completely random access, meaning the beam can jump from one point to another. With a mirror system, on the other hand, the beam must move across continually from one point to a new destination.
Advanced drive schemes have also been developed to manipulate the far field spot, allowing for additional advantages over mechanical systems, such as multi-target designation/tracking and spot deformation for assistance with agile targets. Another is "make-before-break-steering," which allows the link partner to be continuously illuminated while changes are being made to portions of the OPA's liquid crystal orientation.