Mathias Johansson

Parallel fluorescence detection of single biomolecules in microarrays by a diffractive-optical-designed 2 x 2 fan-out element

We have developed a multifocal diffractive-optical fluorescence correlation spectroscopy system for parallel excitation and detection of single tetramethylrhodamine biomolecules in microarrays. Multifocal excitation was made possible through the use of a 2 x 2 fan-out diffractive-optical element with uniform intensity in all foci. Characterization of the 2 x 2 fan-out diffractive-optical element shows formation of almost perfect Gaussian foci of submicrometer lateral diameter, as analyzed by thermal motion of tetramethylrhodamine dye molecules in solution. Results of parallel excitation and detection in a high-density microarray of circular wells show single-biomolecule sensitivity in all four foci simultaneously.

Adaptive beam steering implemented in a ferroelectric liquid-crystal spatial-light-modulator free-space, fiber-optic switch

Active alignment of a 1 x 8 free-space optical switch was studied experimentally. Optical signals, carried on single-mode fibers, were switched by a ferroelectric liquid-crystal-on-silicon spatial light modulator. Continuous measurement of the in-coupled power to the fibers provided feedback for the switch control. The switch automatically located and locked to the output fibers. An advantage with adaptive switches of a similar kind is relaxed geometrical tolerances in the switch assembly. Further, such switches can adapt to possible geometrical changes and light wavelength drift during operation.

Robust design method for highly efficient beam-shaping diffractive optical elements using an iterative-Fourier-transform algorithm with soft operations

Abstract Beam-shaping diffractive optical elements give a desired intensity distribution in the diffraction plane over areas much larger than the diffraction limited spot size. Such elements can be designed using geometrical optics methods or iterative-Fourier-transform algorithms (IFTAs). The usefulness of geometrical optics methods is considerably limited for two reasons: first the number of cases for which a solution exists is small and second the design solution, if it exists, often does not work in practice. Then IFTAs can be used. They are applicable for any desired intensity distribution in the diffraction plane with any intensity cross-section of the incident beam. The IFTA presented in this paper uses a novel set of operations that introduce a minimum disturbance of the fields while still leading to an improved performance. This makes the method robust, insensitive to stagnation and capable of iteratively distributing an increasing portion of the light in the diffraction plane into the desired areas thus leading to a high efficiency (∼95%). Three design examples are given and one is also tested experimentally.

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.

Parallel flow measurements in microstructures by use of a multifocal 4 x 1 diffractive optical fan-out element.

We have developed a multifocal optical fluorescence correlation spectroscopy system for parallel flow analyses. Multifocal excitation was made possible through a 4 x 1 diffractive optical fan-out element, which produces uniform intensity in all four foci. Autocorrelation flow analyses inside a 20 microm x 20 microm square microchannel, with the 4 x 1 fan-out foci perpendicular to the flow direction, made it possible to monitor different flows in all four foci simultaneously. We were able to perform cross-correlation flow analyses by turning the microstructure, thereby having all four foci parallel to the direction of flow. Transport effects of the diffusion as a function of flow and distance could then also be studied.

Fan-out diffractive optical elements designed for increased fabrication tolerances to linear relief depth errors

The intensity uniformity of the spots generated by fan-out diffractive optical elements (DOEs) (or kinoforms) is often highly sensitive to any fabrication error that leads to a deviation of the surface-relief depth of the DOE from its design value. Many of the fabrication errors, such as those that are due to insufficient control of development or etch rates, increase almost linearly with the desired relief depth in every position of the DOE. We present an algorithm for designing fan-out DOEs with a significantly reduced sensitivity of the intensity uniformity to such errors. The reduced sensitivity can be obtained without reducing the efficiency of the DOE. Experimental results for fabricated DOEs show that reduced sensitivity is also obtained in practice.

Design, fabrication, and evaluation of a multichannel diffractive optic rotary joint

A multichannel diffractive optic rotary joint was designed, fabricated by electron-beam lithography, and evaluated with regard to cross talk and to output signal power variations. High cross-talk margin (>25 dB) and low output signal power variations (<2 dB) were achieved. The sensitivity to input-light-beam wavelength uncertainty was investigated. Two design examples are presented. The first design eliminates cross talk due to unwanted diffraction orders and shows that for a ten-channel joint the wavelength uncertainty of an 850-nm emitting laser must be less than 8 nm. In the second design cross talk due to the second diffraction order is permitted, which results in a tolerance level that is three times better for wavelength uncertainty.

Computer-controlled, adaptive beam steering, implemented in a FLC-SLM free-space optical switch

We have used a dynamic diffractive spatial light modulator for adaptive beam steering. With adaptive beam steering it is possible to substantially reduce the alignment accuracy needed for the assembly of a free-space optical switch and to make it adaptive to changes in the environment during operation.

Analog-to-digital conversion using diffractive optics

Summary form only given. An analog electric (current or voltage) signal to be converted is fed to a multi-electrode, wavelength tunable diode laser, the output wavelength of which is uniquely determined by the signal. A microscope lens makes the Gaussian laser output beam converge weakly. The laser beam impinges on a blazed grating, the beam diffracted from it falling upon one in an array of diffractive elements. The widths of the array elements are approximately the same as the diameter of the focused beam incident on them. The diffractive array elements are designed to fan out beams into individual spot patterns. The maximum number of spots in a pattern, n, equals the number of bits the analog signal is to be resolved into. All the fanout array elements produce spots at the same fixed locations, where n individual detectors are positioned. Through this arrangement, a particular analog electric signal amplitude is first converted into a unique wavelength, a fact which makes the laser beam strike a particular diffractive element in the array. The array element in turn produces a unique spot pattern (bit-code) on the detectors, which are read out in parallel.