Munir M. Ahmad

Fiber-device-fiber gain from a sol-gel erbium-doped waveguide amplifier

An erbium-doped silica-on-silicon planar waveguide optical amplifier is described. The active core is a topographic guide formed from aluminophosphosilicate glass doped with erbium and ytterbium. The buffer is formed from silica deposited by thermal oxidation and the cladding from borophosphosilicate glass obtained by plasma-enhanced chemical vapor deposition. The use of low process temperatures allows relatively heavy doping and careful control of the core etching allows low background insertion losses to be obtained. Spontaneous emission and gain measurements are given and 5.4-dB fiber-device-fiber gain is demonstrated using a 5-cm-long chip pumped using a 980-nm laser diode at 175-mW pump power.

Chemical sensing using nonoptical microelectromechanical systems

Micromachining is a technology that miniaturizes mechanical sensors and actuators through the use of tools and materials commonly used in the integrated circuit industry. The physics of the devices formed in this way are sometimes similar and sometimes different to achieve the same functionality of current macroscopic devices, but in sizes that are hundreds or thousands of times smaller. This article will discuss a few chemical detectors being developed as examples of the application of microelectromechanical systems (MEMS) for nonoptical laboratory instruments. In this article, we will report on the development and demonstration of two approaches (quadrupole and magnetic Lorentz) to fabricate a mass filter as part of a mass spectrometer. This article is not meant as a review of the field of MEMS chemical sensors, but rather a tutorial on how miniaturization is achievable through the use of MEMS fabrication techniques for mass spectrometers.

Design of a microengineered electrostatic quadrupole lens

The design of a miniature quadrupole lens based on metallized glass electrodes mounted in two anisotropically etched silicon mounts that are separated by precision spacers is described. The analysis of conventional quadrupole lenses is first reviewed. The fabrication process is then described, and the analysis techniques are applied to the new geometry. The potential distribution near the axis is first calculated using a finite difference method. Expansion into solutions of Laplaces equation in cylindrical coordinates is then used to determine the potential distortion introduced by the microengineered mount. Design rules are given for a mount and an electrode radius that achieve minimal distortion.

Sol-Gel Fabrication of Rare-Earth Doped Photonic Components

The use of sol-gel to fabricate silica-on-silicon waveguides, and particularly erbium-doped waveguide amplifiers, is reviewed. In particular, efforts to use sol-gel to improve molecular homogeneity in heavily Er-doped silica-based films is discussed. A variety of material studies carried out to investigate the gain limitations found in these materials is then presented. These include x-ray diffraction, ellipsometry and Rutherford backscattering. Excess heat treatment is used to force crystallisation of the films, and analysis of the resulting structure is used to infer properties of the glass before the additional heating. The use of erbium alkoxide precursors is shown to alter the erbium environment in the final glass, in comparison to the use of inorganic erbium salts.

Performance improvements for a miniature quadrupole with a micromachined mass filter

Conventional quadrupole mass filters are usually constructed from metallic rods of length 50—225 mm and diameters 5—15 mm. In this study the conventional arrangement has been replaced with a micromachined mass filter made from silicon with Au metallised specially drawn glass fibres of length 30 mm and diameter 0.5 mm. This assembly was mounted on a vacuum flange with a VG ANAVAC ion source and a Faraday plate collector. Conventional electronics were adapted to run at 6 MHz and mass spectra in the range 0—50 amu were obtained. The results indicate a linear mass scale with 5—10% valley separation between O 2 /N 2 peaks and a best resolution at 10% peak height of 2.7 amu at mass 40. It is believed that the recent improvements in performance are due to optimisation of the electronics coupled with refinements in the micromachining technique employed. ( 1999 Elsevier Science Ltd. All rights reserved.

Improved polymer–glass adhesion through micro-mechanical interlocking

Mechanical interlocking provides a simple and effective means of improving adhesion between dissimilar materials in micro-electro-mechanical systems (MEMS). Following successful implementation in hybrid Si-polymer systems (Larsson, Syms and Wojcik 2005 J. Micromech. Microeng. 15 2074–82), it was established that maximum interface strengthening does not necessarily rely on the presence of overhang between interlocking lobes. Instead, careful design of the lobe profile is advised in order to balance the opposing actions of physical restraint and lobe pull-out and to obtain optimal interface strength. When an interlocked interface is immersed in aggressive liquid media, however, the situation is clearer: chemical bonds are degraded or completely destroyed and lobe overhang provides the only source of physical restraint. Generating overhanging features in Si substrates is possible through reactive ion etching (RIE), but in the case of glass, the situation is more problematic. A straightforward, robust process is now described that extends mechanical interlocking to generic MEMS substrates, avoiding the need for RIE. By using inexpensive and established processes such as electroplating and wet etching, interlocking features with an overhanging profile are generated in glass substrates. Peel tests on cured strips of SU-8 confirm an increase in average peel strength by a factor of 3.5, compared with strips peeled from smooth substrates. The method can readily be applied to a number of substrates, including Si, providing a low-cost route towards attaining mechanical interlocking.

Strip-Loaded High-Confinement Waveguides for Photonic Applications

Sol-gel is a promising deposition technique for the fabrication of silica-on-silicon photonic components. Silica-titania compositions provide large index differences, and thus strong mode confinement, and are suitable hosts for Er-doped optical amplifiers. We have developed channel waveguides based on phosphosilicate strip loads over high index titania-doped guiding layers. Here we demonstrate that low propagation loss (0.3 dB/cm) can be obtained in such guides at optical communication wavelengths. Alumina co-doped guiding layers are shown to inhibit OH retention without causing titania segregation, and reduced humidity during spinning is shown to be necessary to achieve low propagation losses.

MEMS Helmholtz coils for magnetic resonance imaging

Miniature coils for magnetic resonance imaging and spectroscopy are demonstrated. A rectangular Helmholtz arrangement based on a pair of substrates separated by spherical alignment spacers is investigated. An optimal geometry is developed using simple theory and verified by numerical analysis. Prototypes are fabricated using silicon substrates shaped by anisotropic etching to form a trough to hold an internal sample and pits for use as alignment features, and carrying Cu/Au conductors fabricated by electroplating. Q values of 11 are obtained, and devices containing an internal proton source are developed for use as fiducial markers in 1 H magnetic resonance imaging at 1.5 T. Magnetic resonance spectroscopy is also performed on the internal source, with a SNR of ≈85.

Catheter-based flexible microcoil RF detectors for internal magnetic resonance imaging

Flexible catheter probes for magnetic resonance imaging (MRI) of the bile duct are demonstrated. The probes consist of a cytology brush modified to accept a resonant RF detector based on a spiral microcoil and hybrid integrated capacitors, and are designed for insertion into the duct via a non-magnetic endoscope during endoscopic retrograde cholangiopancreatography (ERCP). The coil must be narrow enough (<3 mm) to pass through the biopsy channel of the endoscope and sufficiently flexible to turn through 90 ◦ to enter the duct. Coils are fabricated as multi-turn electroplated conductors on a flexible base, and two designs formed on SU-8 and polyimide substrates are compared. It is shown that careful control of thermal load is used to obtain useable mechanical properties from SU-8, and that polyimide/SU-8 composites offer improved mechanical reliability. Good electrical performance is demonstrated and sub-millimetre resolution is obtained in 1 H MRI experiments at 1.5 T magnetic field strength using test phantoms and in vitro liver tissue.

Magnetic resonance imaging using linear magneto-inductive waveguides

Magneto-inductive waveguides are arrays of magnetically coupled, lumped element resonators, which support slow waves at radio frequency. Their use in internal magnetic resonance imaging (MRI), where they may provide an intrinsically safe method of signal detection and transmission, is described. A catheter-based receiver formed from a thin-film printed circuit mounted on a tubular scaffold using heat-shrink tubing is demonstrated, and its electrical response and imaging sensitivity are explained in terms of the excitation and propagation of magneto-inductive waves. The theoretical predictions are confirmed using the results of electrical measurement and 1H MRI at 1.5 T, and imaging is achieved over a total length greater than 1.5 m using a single receiver.