The use of phase only spatial light modulators for holographic optical trapping results in the appearance of ghost orders,
creating unwanted traps with uncontrolled intensity and causing variations in the intensity of the desired traps. By
introducing dummy areas in the diffraction plane during the hologram optimization, the intensity in the ghost orders can
be significantly reduced. By directing a variable fraction of the light to the dummy area, the optical power in the traps
can be controlled independently and kept constant also while moving traps to different arrangements. We present and
evaluate an algorithm for hologram generation which utilizes dummy areas and allows arbitrary spot positioning in three
dimensions. The method enables the use of holographic optical trapping for applications requiring precise control of the
intensity in traps, such as optical force measurement.
We have implemented several algorithms for hologram generation, aimed for holographic optical tweezers
applications, using the parallel computing architecture CUDA. We compare required computation time for different
implementations of the Gerchberg-Saxton algorithm and provide guidelines for choosing the best suited version with
respect to the application. We also show that additional calculations, compensating for limitations in the used spatial
light modulator and optical system, can be included in the hologram generating software with little or no loss in computational speed.
The positioning accuracy when a phase-only one dimensional spatial light modulator (SLM) is used for beam
steering is limited by the number of pixels and their quantized phase modulation. Optimizing the setting of the
SLM pixels individually can lead to the inaccuracy being a significant fraction of the diffraction limited spot size.
This anomalous behaviour was simulated numerically, and experiments showed the same phenomena with very
good agreement. However, by including an extra degree of freedom in the optimization of the SLM setting, we
show that the accuracy can be improved by a factor proportional to the number of pixels in the SLM.
The analog switching mode (sometimes referred to as V-shaped switching mode) of the ferroelectric liquid crystal cell is a recently developed type of liquid crystal cell in which the molecular director can be arbitrarily positioned with high speed on the surface of a cone depending on the steering voltage over the cell. This changes the orientation of the slow and fast axes as well as the amount of the birefringence. We show that theoretically, it is possible to use a V-shaped switched ferroelectric liquid crystal cell to achieve near lossless analog phase modulation between zero and π radians for a special ellipticity of the polarized input light. We also fabricated a cell which slightly deviates from the ideal (tilt cone half-angle 38° instead of 45°) for which near-lossless transmission was obtained, manifested as a < 4% modulation of the amplitude, and a continuous phase modulation between 0 and ~0.8π radians; the values agree very well with numerical simulations.
A novel retrocommunication link utilizing reflective multiple quantum well (MQW) optical modulators and nonmechanical beam steering and tracking is demonstrated. Large aperture reflective MQW modulators using AlGaAs/GaAs are optimized and manufactured. The modulators exhibit a contrast ratio larger than 4:1 and a modulation bandwidth of 10 MHz. Nonmechanical beam steering and tracking are studied using nematic liquid crystal (NLC) spatial light modulators (SLMs). The communication link is comprised of a retromodulating array with four MQW modulators and a transceiver using a NLC SLM for beam steering and tracking. Transfer of audio, real-time image data and pseudorandom bit sequences over 100-m range while tracking the moving retromodulator is shown. The link is capable of transferring data at approximately 8 Mbps.
Retrocommunication is a new technique for asymmetric free-space optical communication that has attracted interest during recent years. Novel technologies such as multiple quantum well (MQW) optical modulators and non-mechanical laser beam steering and tracking have been studied for implementation in a retrocommunication link. Large and small aperture reflective AlGaAs/GaAs MQW modulators were optimised and fabricated. The modulators exhibit high contrast ratios (from 5 to 100) and high modulation rates (up to 16 Mbit/s). A retroreceiver consisting of four large aperture MQW modulators, associated optics and drive electronics was fabricated. Nematic liquid crystal spatial light modulators have been evaluated, characterised for beam steering and tracking and implemented in a transceiver. Small area MQW modulators, used in focal plane configurations, were studied for static communication links. Results from a novel retrocommunication link utilising a retroreceiver and non-mechanical laser beam steering and tracking will be presented. Bit rates of 8 Mbit/s were observed during non-mechanical tracking of a moving retroreceiver over 100 m range. The demonstrator system was capable of transferring audio-, real-time images or bit streams. The demonstrated principles show promising features for future low weight free-space communication links. Performance calculations including requirements for a retrocommunication link using MQW modulators and non-mechanical beam steering are discussed.
Ferroelectric Liquid Crystal (FLC) Spatial Light Modulators (SLMs) are attractive because of their high switching speed. However, conventional FLC SLMs are only capable of binary phase modulation. This is inconvenient for beam steering since as much as 60% of the incident power is lost to unwanted diffraction orders. To overcome this problem two cascaded FLC SLMs were used in this work. By coherently imaging a 180° binary-phase FLC SLM onto a 90° FLC SLM, with high precision, an effective four-level phase modulator was realized experimentally. Beam steering was demonstrated in the angular range ±10.9 mrad. The angular inaccuracy of the steered beam was found to be about 0.1 mrad, which equals about 25% of the beam diameter. The beam steering device has also been used for tracking experiments.