Boulder Nonlinear Systems (BNS) is developing capabilities to greatly improve the speed of nematic SLMs using the OverdrivePlus technique.
OverdrivePlus is an addressing technique for nematic SLM systems that have the ability to load images at a rate faster than the normal response time of the LC modulator. As a nematic SLM forms a new phase hologram, the hologram is optically realized in less time (>3× faster than its normal rate) using multiple transition images. This accelerated SLM response is achieved using a combination of two different concepts. The first is the “Overdrive” concept, which uses transient voltage changes that exceed the target steadystate voltage needed for the desired phase change. As an example, if a pixel is at 0.5V and the target voltage for the next value is 1.0V, the SLM will apply a transient voltage at or near the maximum value (e.g., 5V), rolling off slightly, until the desired phase is reached, at which point the voltage will revert to the steady-state target value of 1.0V. This “overdriving” of the voltage results in a faster transition between target phases. The same concept applies in reverse for transitioning from higher voltages to lower voltages.
The second concept is phase-wrapping, which takes advantage of the cyclic nature of light to provide the desired phase shift. In other words, phase-wrapping capitalizes on the fact that phase shifts of, for example, 0.5π and 2.5π are equivalent. BNS has demonstrated this capability using modulators that can provide more than a single wave (2π) of phase shift at the desired wavelength. When the software receives the command to change the phase at a given pixel, it first reads what the actual, previously driven, phase of the pixel is and the finds the nearest absolute phase value that will result in the desired modulo-2π phase. For example, if a pixel is at 0.9π and the software receives the command to shift that pixel to 0.1π, the pixel will phase-wrap to the closer, but equivalent, phase of 2.1π for a faster transition.
To provide a representative demonstration of the new OverdrivePlus capabilities, a high-voltage 256-pixel SLM was used at 1064 nm to generate 8-pixel full wave phase ramps, as shown above. The SLM was switched between “left” and “right” steering ramps while monitoring the signal from a photodetector at one of the beam positions. The full rise time of the photodetector signal was used to indicate switching speed. By incorporating a mixture of both large and small phase transitions during the transition from left to right, this test is a fair and repeatable representation of the type of improvements that can be expected from OverdrivePlus. In this particular test, the transition time decreased from 8.5 ms without OverdrivePlus to approximately 1 ms with OverdrivePlus activated: an 8.5-fold improvement in switching speed.
OverdrivePlus capabilities that BNS is developing:
Software implementations which include:
- “Checkbox” functionality incorporated into Blink and Blink SDK routines
- Use of OpenCL for fast computation, e.g., on a GPU
- Pre-calculation of OverdrivePlus transitions to reduce real-time calculation burdens on resource-limited system
Firmware implementations which feature:
- FPGA pipeline processing to eliminate software overhead and latency
- On-board transition image generation to greatly reduce computer-to-driver transfer rates
SLM characterization and calibration procedures for 256×256,
512×512 and higher resolution backplanes
Optimization algorithms to determine:
- When to implement phase wrapping based on both speed and accuracy
- The minimum number of transition images required for a given phase transition to increase overall system speed