Gradient-index Rod Lenses Research Articles

Birefringence in gradient-index rod lenses: a direct measurement method and interferometric polarization effects

Gradient-index (GRIN) media can contain stress birefringence resulting from the variation in material composition. Anisotropy in a GRIN rod lens affects ray propagation and can degrade image quality. A technique, believed to be new, for measuring birefringence in GRIN rod lenses has been developed. The change in optical path difference (OPD) for orthogonal polarizations is measured directly. With this method, effects on OPD from standard imaging aberrations are excluded. Birefringence measurements for various GRIN rod samples are presented. The data are compared with results obtained previously by use of a Twyman-Green measurement method. Also, the polarization effects on tilt fringes observed with the direct measurement method and the Twyman-Green method are presented and modeled theoretically. Tilt fringes for large birefringence test cases are also modeled.

Gradient-index rods as flux concentrators with applications to laser fiber optic surgery

Gradient-index rod lenses have been developed and ana- lyzed as imaging optical elements, but their capability as flux concentra- tors has not been explored. Both analytic methods and computer ray- trace simulations are used for a comprehensive evaluation of the concentration and efficiency of these rods. They appear to be well suited as concentrators for the distal end of laser fiber optic surgical units, toward improving surgical efficacy and reducing expensive laser power. © 2001 Society of Photo-Optical Instrumentation Engineers. (DOI: 10.1117/1.1347998) Subject terms: concentrators; biomedical optics; nonimaging optics; gradient in- dex; optical design; geometric optics.

Birefringence measurements in gradient-index rod lenses

Stress birefringence can be found in gradient-index (GRIN) materials because they contain a variation in composition. GRIN glass fabricated by ion exchange may contain stress from two different processes. These include a size difference between the exchange and the diffusing ions and a variation in the thermal-expansion coefficient across the gradient region. The optical properties of the stressed material are polarization dependent, and therefore image quality is directly affected. We examine birefringence in GRIN rod lenses that have lengths more than ten times greater than their diameters. The effects are more easily observed in long rod lenses because of the large optical path lengths.

General formula for coupling-loss characterization of single-mode fiber collimators by use of gradient-index rod lenses: errata

General formula for coupling-loss characterization of single-mode fiber collimators by use of gradient-index rod lenses: errata

Interferometric chromatic dispersion measurements of long, gradient-index rod lenses in the visible and the near infrared

A new procedure is presented for testing long, radial-gradient-index rod lenses in a phase-stepping Twyman-Green interferometer. In addition, a method for converting the measured optical phase data to the rod's index of refraction profile is reviewed. This procedure was used to measure refractive-index profile dispersions over the wavelength range of 441 to 1064 nm. These measurements provide the first data for refractive-index profiles of rod lenses in the infrared and show the existence of positive dispersion, negative dispersion, and near-infrared dispersion reversal in some samples. The dispersion results suggest the need for modifications to the current glass model.

General formula for coupling-loss characterization of single-mode fiber collimators by use of gradient-index rod lenses

A general formula for determining the coupling loss between two single-mode fiber collimators with the simultaneous existence of separation, lateral offset and angular tilt misalignments, and spot-size mismatch is theoretically derived by use of the Gaussian field approximation. Based on this general formula, the formulas for coupling losses that are due to the misalignment of insert separation, lateral offset, and angular tilt are given. The formula for the coupling loss that is due to Gaussian spot-size mismatch of two single-mode collimators is also given. Good agreement between these formulas and experimental results is demonstrated with gradient-index rod lens-based fiber collimators operating in the 1300-nm band.

Reduction of coupling loss in many-to-many collimating system for optomechanical matrix switch

In a many-to-many collimating system using gradient-index- rod lenses (GRINs) for an optomechanical matrix switch, there are multiple cross-connections having different optical path lengths so that excess coupling losses arise. The total coupling loss is greatly reduced by properly choosing the GRIN lengths. Two schemes can do so. In the equal ?Z scheme, all GRINs have the same excess length ?Z0 than the quarter pitch, which is optimized at 13, 25, and 49 µm for the three focusing parameters of 0.237, 0.326, and 0.417 mm-1, respectively. The total coupling loss is further reduced in the unequal ?Z scheme, in which the ?Zµ of each GRIN is independently determined and optimized by numerical computation. The results are preferred rather than the zero ?Z scheme where just quarter-pitch GRINs are used.

An Artificial Compound Eye Using a Microlens Array and Its Application to Scale-Invariant Processing

An artificial compound eye corresponding to apposition eyes of insects has been demonstrated using a two-dimensional array of gradient-index rod lenses known as a Selfoc lens plate (a SLP). A single erect Moire image is reconstructed from a number of inverted micro-images by an experimental optical system composed of the SLP, a pinhole array and a plano-concave lens. Characteristics of the compound eye is briefly discussed from the viewpoints of space-bandwidth product, viewing angle, and focal depth. Capability of a scale-invariant image sensor that has been proposed as an application of the compound-eye-type imager was also proved experimentally.

Gradient-index rod lens with a high acceptance angle for color use by Na^+ for Li^+ exchange

The relationship between the characteristics of gradient-index rod lenses obtained by ion exchange and their mother glass compositions is studied. It is observed that Li(2)O content, R(2)O (Li(2)O + Na(2)O) content, and the Li(2)O/Na(2)O ratio are important factors in attaining a high acceptance angle and a small field curvature. Thus the gradient-index lens array is fabricated by the use of a newly developed glass composition and its optical performance; the modulation transfer function, luminous intensity, and chromatic aberration are precisely evaluated. The array is found to have a sufficiently high performance to be applicable for color use.

A twin-beam fibre optic laser light scattering system

A simple remote sensing single-particle detection system is described which uses light scattering to determine the size and concentration of scatterers. They report the initial results obtained from the system. A dual wavelength conformation consisting of an inner trigger beam and outer analysis beam is used to eliminate edge effects of the Gaussian profile of the laser beam. This is achieved without the use of complicated optics, by employing an optical fibre and a chromatically dispersive gradient index (GRIN) rod lens to give the correct optical configuration in the scattering volume.

Optical receiver design for high optical return loss

A new fiber pigtailed optical receiver design using a single gradient-index rod lens with a beveled exit face achieved a broadband optical return loss of > 65 dB, which was limited by the diffuse reflection from the photodetector front surface. By contrast the optical return loss for a receiver with an unbeveled lens exit face and an on-axis optical path was limited to a smaller value by the specular reflection from the lens exit face.

Characteristics of a grating-external-cavity semiconductor laser containing intracavity prism beam expanders

A grating tuned external cavity semiconductor laser incorporating a gradient-index rod lens and a pair of intracavity silicon prism beam expanders was constructed using a configuration aimed at simultaneously achieving the following objectives: the ability to operate at any external cavity longitudinal mode without tuning gaps caused by residual resonances of the laser diode cavity, stable feedback coupling between the diode and external cavities, narrow optical linewidth, and a high degree of external cavity longitudinal sidemode suppression.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Inline optical filter using lifted-off GaAs/AlGaAs multilayer

A new inline optical filter is reported. A GaAs/AlGaAs multilayer interference filter was lifted off its GaAs substrate by selective etching, and was sandwiched between two gradient-index rod lenses. Near-theoretical performance was obtained, with over 20 dB rejection in the stop band and less than 0.5 dB insertion loss at transmission peaks. This type of filter could be extremely useful in wavelength division multiplexed systems.

Ion beam assisted deposition of a thin film coating on a gradient-index lens array

A new coating method which employs ion bombardment has been developed for a gradient-index (GRIN) rod lens array using silicone rubber and fiber reinforced plastic plates in its construction. The thin film coatings deposited using this method passed the durability tests on the basis of MIL-M-13508C. The adhesion of the thin film coating was strong and durable enough to allow for use on GRIN rod lens arrays in photocopiers. The effect of ion bombardment has been investigated with thin film analysis data by Fourier transform infrared microspectroscopy and secondary ion mass spectrometry.

Gradient-index rod lens made by a double ion-exchange process

Gradient-index rod lenses with a parabolic-index profile have been fabricated by a double Na-Ag ion-exchange process, and their optical characteristics have been evaluated. The numerical aperture and minimum focused spot diameter of the 2-mm diam rod lens were 0.58 and 2.5 microm for lambda = 0.63 microm, respectively. Because of the high diffusing rate for Ag ions, this technique offers the possibility of making large-sized (larger than 10 mm-diam) rod lenses for photographic uses.

Holographic endoscopy with gradient-index optical imaging systems and optical fibers

Single or assembled gradient-index (GRIN) rod lenses as endoscopic optical devices combined with optical fibers or articulating optical arms for flexible illuminating object beam and reference beam guidance provide the possibility of constructing an easy-to-handle holographic endoscopic arrangement for practical use in industry and medicine. Using the same GRIN optical device for both endoscopic imaging as well as illumination simultaneously, an enlargement of the limiting aperture for speckle noise reduction can be gained within the given outer diameter of the instrument compared with endoscopic systems with separate fiber bundle illumination. Reflections, which destroy the image information, can be suppressed by holographic subtraction.

Fifth-order analysis of GRIN lenses

A new fifth-order algebraic ray-tracing formula valid in any radial gradient is proposed. This leads to algebraic fifth-order aberration formulas for gradient-index (GRIN) rods or Wood lenses. Such formulas can be used for profiling GRIN rod lens samples.

Gradient-index optical systems in holographic endoscopy

Small limiting apertures of endoscopes cause gross speckles when using coherent light sources in holographic endoscopy. Thus, for a high optical signal-to-noise ratio, imaging systems with high numerical apertures and low f/Nos. are required, which under given geometrical constraints can be provided in particular by gradient-index rod lenses. For additional speckle reduction the limiting aperture has to be enlarged within the given outer diameter of the instrument. This can be achieved by using the same gradient-index rod lens for both imaging and illumination, so that no separate illuminating fiber bundle is necessary. Holographic recording problems due to reflections of the illuminating object beam from the end faces can be solved by appropriate positioning and shaping of the rod as well as by holographic subtraction.

Reduction/enlargement gradient-index lens array

A novel concept of a gradient-index lens array for reduction and enlargement is described. The array is composed of radial gradient-index rod lenses arrayed in a fanlike manner. Design and radiometric equations are presented. Some image aberrations are discussed in a qualitative fashion. The imaging performance and radiometric properties of a sample lens array are given.

Single GRIN-Lens Directional Couplers

Two types of small, rugged, gradient-index (GRIN) rod-lens directional couplers have been made. One is a three-port directional coupler and the other is an asymmetric four-port coupler. Each coupler can have various splitting ratios from 1:1 to 20:1. Excess insertion loss is less than 1.0 dB, and directivity is greater than 30 dB for the three-port couplers, while these same figures are 1.2 dB and 40 dB for the asymmetric four-port couplers. The couplers are stable between −40 and +80°C.