Björn Löfving

Fabrication and simulation of diffractive optical elements with superimposed antireflection subwavelength gratings

With the aim of reducing surface reflections and increasing the diffraction efficiency we investigated the superposition of subwavelength phase gratings onto blazed phase gratings. With direct-write electron-beam lithography bare blazed gratings and blazed gratings carrying subwavelength gratings were fabricated and their optical performances compared. For TE polarization the subwavelength-carrying gratings showed a maximum diffraction efficiency of 90.6%, whereas the corresponding maximum value for the bare grating was 86.3%. The experiment was simulated with rigorous diffraction theory.

Study of an ultrafast analog-to-digital conversion scheme based on diffractive optics

A potentially ultrafast optical analog-to-digital (A/D) converter scheme is proposed and was partly studied experimentally. In the A/D converter scheme the input signal controls the wavelength of a diode laser, whose output beam is incident on a grating. The beam from the grating hits a diffractive optical element in an array. The wavelength determines which element is illuminated. Each element fans out a unique spot-pattern bit code to be read out in parallel by individual detectors. In the experiment all patterns but one from 64 array elements were read out correctly.

Beam steering with two ferroelectric liquid-crystal spatial light modulators

Ferroelectric liquid-crystal spatial light modulators (FLC SLMs) are binary modulating devices that can be used for flexible beam steering in the microsecond range. Because they are binary, at least 60% of the light power is lost to unwanted diffraction orders, however. We suggest the use of a second FLC SLM, synchronously operating in amplitude-modulating mode, to block the unwanted orders. The proposed beam-steering method is demonstrated in a simple scanner experiment. The efficiency of the suggested beam steerer is independent of the number of steering directions.

Beam steering experiment with two cascaded ferroelectric liquid-crystal spatial light modulators

The design, construction, and evaluation of a laser beam steerer that uses two binary ferroelectric liquid-crystal (FLC) spatial light modulators (SLMs) operated in conjunction are presented. The system is characterized by having few components and is in principle lossless. Experimentally, a throughput of approximately 20% was achieved. The simple system design was achieved because of the high tilt angle FLC material used in the SLMs, which were specifically designed and manufactured for this study. By coherently imaging the first SLM onto the second SLM, pixel by pixel, we obtained an effective four-level phase structure with a phase step of 90 degrees. An appropriate alignment procedure is presented. The beam steering performance of the system is reported and analyzed.

Optical switching with two FLC SLMs

A free-space optical switch concept that uses two binary phase modulating FLC SLMs is studied. The first SLM steers the input beams to output ports on the second SLM which redeflects the incident beams. The second SLM acts as an angle selective filter which reduces the crosstalk. Single mode optical fibers are assumed both at the input and at the output. The switch design is analyzed, part of the switch was studied experimentally and the full switch was computer simulated with regard to crosstalk and throughput. With the ports arranged in a 2-dimensional array in the switch, it is shown that the switch can handle a few tens of channels at a few percent crosstalk.

Self-adjusting dynamic binary phase holograms

An optical system constructed around a dynamic diffractive optic element, a ferroelectric liquid-crystal spatial light modulator in binary phase-only modulation mode, is investigated. The spatial light modulator is successively adjusted according to the direct binary search technique to diffract an incoming laser light beam into a predecided intensity distribution by use of feed back from the diffracted light. It was found that the feedback signal was noisy and that vibrations and limited bistability in the spatial light modulators pixels were the main noise sources. The final diffraction efficiency depends on the degree of noise in the feedback signal, but even under fairly noisy conditions the iterations were found to converge properly.

Measurement of the spatial phase modulation of a ferroelectric liquid-crystal modulator

Ferroelectric liquid crystal is used as a dynamic light-wave phase modulator. A device that performs phase-only modulation is investigated, especially with regard to the boundary effect caused by the inevitable gap between controlling electrodes. The dependence of cell retardation and polarizer alignment is discussed. Two phase-retrieving algorithms are used to determine the complex lightwave front after traversing the device. In operation it was found that the electrode gap region was partly controlled.