Optimizing integrated optical chips for label-free (bio-)chemical sensing.

Label-free sensing is an important method for many (bio-)chemical applications in fields such as biotechnology, medicine, pharma, ecology and food quality control. The broad range of applications includes liquid refractive index sensing, molecule detection, and the detection of particles or cells. Integrated optics based on the use of waveguide modes offers a great potential and flexibility to tailor the sensor properties to these applications. In this paper, the results of a numerical study are presented, showing that this flexibility is founded on the many degrees of freedom that can be used for the integrated optical chip design, in contrast to other technologies such as those based on surface plasmon resonance, for which the materials' properties limit the range of choices. The applications that are explicitly considered and discussed include (1) bulk refractometry, (2) thin-layer sensing, for example biosensors monitoring molecular adsorption processes occurring within some 10 nm of the chip's surface, (3) thick-layer sensing with processes involving molecules or ions to be monitored within a sensing matrix extending to some 100 nm from the chip's surface, for example hydrogel-based layers and chemo-optically sensitive membranes, and (4) particle sensing with particles or, for example, biological cells to be monitored within probe volumes extending to some 1,000 nm from the chip's surface. The peculiarities for the different types of applications will be discussed, and suitable modeling methods presented. Finally, the application-specific design guidelines supplied will enable the optimization of various types of integrated optical sensors, including interferometers and grating-based sensors.

Compact integrated optical sensor system.

A compact integrated optical sensor system for a large variety of different (bio-) chemical applications using replicated sensor chips is described. Features of the refractometric system to be emphasized for practical applications include a high-resolution window that can be positioned within a wide measuring range, an in situ chip testing and characterization procedure, and on-chip referencing. As an application example, experimental results on refractometric measurements as well as on the suppression of non-specific binding are given.

Single-pad scheme for integrated optical fluorescence sensing.

A new scheme for integrated optical fluorescence sensing is presented. The principle is based on a planar waveguide containing multiple sensing units, each consisting of a single-pad grating coupler structure. Single-pad means that all the following functions are incorporated in one single pad: laser light input, excitation of the labeled analyte molecules, efficient collection of the emitted fluorescent light into the waveguide, background suppression, and coupling the guided wave out to the detector. The results demonstrate a high efficiency of the fluorescence light excitation and collection, as well as a good suppression of the volume background.

Wavelength-interrogated optical sensor for biochemical applications.

A novel miniature integrated-optical sensor for versatile multichannel applications is described. Wavelength-modulation techniques using laser diodes (vertical-cavity surface-emitting lasers) are used to interrogate multiple-waveguide sensing regions on a single integrated-optical chip for accurate measurement of effective refractive-index changes at a high data rate. With the experimental miniature sensor system, a resolution of DN(PP) = 10(-7) (short term, peak to peak) of the effective refractive index was demonstrated. In terms of surface-mass coverage, this resolution corresponds to D?(PP) = 130 fg/mm(2).

Novel integrated optical sensor based on a grating coupler triplet.

A novel approach for accomplishing robust integrated optical biosensors is presented. The principle is based on a symmetric grating coupler structure with the inherent feature of compensating disturbances originating from different kinds of mechanical instabilities. The complete sensor system has no moving parts and provides the high sensitivities typical for integrated optical sensors based on grating couplers. The feasibility of this approach is demonstrated by determining the refractive index of liquids as well as by measuring the specific binding of biomolecules (anti-chicken IgG) to immobilized chicken IgG on the sensor chip surface.

Compact Integrated Optical Immunosensor using Replicated Chirped Grating Coupler Sensor Chips.

A novel compact multichannel integrated optical sensor module isdescribed. Its performance is demonstrated by measurement of the mass adsorption of an analyte molecule to the transducer surface by refractometry in an immunosensor experiment. The signal transduction is achieved by means of chirped grating couplers, which allow simple yet highly functional sensor modules to be built. The experiments were performed with high-sensitivity replicated polycarbonate TiO(2) waveguide sensor chips for detecting the binding of rabbit immunoglobulin to immobilized protein A. Aresolution of ?3 pg/mm(2) surface mass coverage was obtained in a dual-channel module with size 10 cm x 10 cm x10 cm.

Analysis and optimization of fabrication of continuous-relief diffractive optical elements.

The fabrication of continuous-relief diffractive optical elements by direct laser beam writing in photoresist is analyzed. The main limitation and tolerances are identified, and their influence on optical performance is quantified. Fabricated structures show rounded profile steps resulting from the convolution of the desired profile with the writing beam. This leads to a reduction in diffraction efficiency. Optimization techniques are presented to minimize this effect. Scaling the profile depth by a factor of mu > 1 increases the first-order diffraction efficiency for blazed elements. This method is also applied to suppress the zeroth diffraction order in computer-generated holograms. A nonlinear compensation of the exposure data for the Gaussian beam convolution results in an 18% increase of the diffraction efficiency for a blazed grating with a 10-mum period to a value of 79%.

Arrays of anamorphic phase-matched Fresnel elements for diode-to-fiber coupling.

A method for designing microlens arrays that inherently takes into account application requirements and fabrication constraints is presented. Elements with numerical apertures of up to 0.5 have been designed and fabricated by laser beam writing in photoresist and replication in plastic material. In a laser-diode-to-fiber array coupling experiment, an overall optical throughput of 60% was achieved. By means of anamorphic microlens arrays, correction of the laser-diode longitudinal astigmatism and circularization of the image-plane irradiance distribution are demonstrated.

Refractive and diffractive properties of planar micro-optical elements.

The refractive and the diffractive properties of planar micro-optical elements are investigated. The transition between purely refractive and purely diffractive planar microlenses is numerically simulated for the example of differently designed phase-matched Fresnel elements. Results obtained from numerical simulations and experiments show that the refractive and diffractive types exhibit a distinctly different behavior in the presence of small fabrication errors or wavelength deviations. Based on these results, design rules for various applications, including low- and high-numerical-aperture lenses and hybrid refractive-diffractive elements, are derived. For a high-numerical-aperture (ƒ /# = 1.0) lens the experimental characterization of the irradiance distribution in the image space is presented and shown to agree well with theoretical predictions.

Transverse plantar incision for heel spur surgery. Four-year follow-up survey of 35 patients.

A transverse plantar incisional approach was used on 35 patients who underwent heel spur surgery from 1982 through 1990. Ten bilateral procedures were performed on a total of 45 feet. A medial approach was used for five of the bilateral surgeries. The average age of the patient was 47 years, and the average follow-up period was 49 months. Of the 34 plantar approach cases in which complete data were obtained, 94% showed either good or excellent results. The authors illustrate a technique using a transverse plantar incision as an alternative to the medial approach for heel spur surgery.