Anand Gopinath

Numerical techniques for modeling guided-wave photonic devices

Accurate modeling of photonic devices is essential for the development of new, higher performance optical components required by current and future high-bandwidth communications systems. This paper reviews several key techniques for such modeling, many of which are used in commercial design tools. These include several mode-solving techniques, the beam propagation method, the method of lines, and the finite-difference time-domain technique.

9.4T human MRI: preliminary results.

This work reports the preliminary results of the first human images at the new high‐field benchmark of 9.4T. A 65‐cm‐diameter bore magnet was used together with an asymmetric 40‐cm‐diameter head gradient and shim set. A multichannel transmission line (transverse electromagnetic (TEM)) head coil was driven by a programmable parallel transceiver to control the relative phase and magnitude of each channel independently. These new RF field control methods facilitated compensation for RF artifacts attributed to destructive interference patterns, in order to achieve homogeneous 9.4T head images or localize anatomic targets. Prior to FDA investigational device exemptions (IDEs) and internal review board (IRB)‐approved human studies, preliminary RF safety studies were performed on porcine models. These data are reported together with exit interview results from the first 44 human volunteers. Although several points for improvement are discussed, the preliminary results demonstrate the feasibility of safe and successful human imaging at 9.4T. Magn Reson Med, 2006.

High field magnetic resonance

This chapter deals with data and concepts relevant to high magnetic fields with the primary focus on efforts related to probing brain function and neurochemistry utilizing imaging and spectroscopy capabilities. One of the most important accomplishments in magnetic resonance imaging (MRI) research over the past years is the introduction of methods that can map the areas of altered neuronal activity in the brain, that is, functional MRI or fMRI. The most commonly used method of fMRI is based on blood oxygen level dependent (BOLD) contrast which is sensitive to the presence of deoxyhemoglobin. In an fMRI experiment, images are collected subsequent to signal excitation and echo formation, either by a gradient reversal or application of a refocusing radio frequency (RF) pulse. During the delay after excitation and before echo formation, it is possible to apply a pair of gradient pulses with opposing or same polarity depending on whether the experiment is a gradient recalled echo or a spinecho experiment, respectively.

Vector analysis of optical dielectric waveguide bends using finite-difference method

A full-vectorial analysis of optical dielectric waveguide bends using the finite-difference method has been developed. The formulation was based on the transverse electric field components, E/sub r/ and E/sub z/. To set up the boundary conditions at each dielectric interface, the continuity of E/sub /spl theta//, H/sub /spl theta//, and the tangential component of the electric field, and the discontinuity of the normal component of the electric field were satisfied. The finite-difference scheme was modified to satisfy these boundary conditions. The results of the analysis using the current method is compared with previous results. The optimal offset for 90/spl deg/ bends was obtained, and the losses for these optimal bend structures are also presented.

WKB analysis of bend losses in optical waveguides

A more complete Wentzel-Kramers-Brillouin (WKB) analysis of bend losses is given for a circularly curved waveguide. Using the WKB approximation with a conformal transformation of a curved optical waveguide, is shown to give more accurate bend loss results.

Single-mode vertical cavity surface emitting laser by graded-index lens spatial filtering

4.5 mW of power in a single spatial mode has been obtained from a vertical cavity surface emitting laser (VCSEL) in an external cavity setup, using a graded index (GRIN) lens with a spatial filtering high reflecting aperture deposited on its endface. The spatial filter on the GRIN lens endface forces a larger single transverse mode in the VCSEL than is obtained without it. A brightness of 5.1×105 W/cm2 str is demonstrated, which is 91% of the maximum achievable.

Polarization-insensitive quantum-well semiconductor optical amplifiers

Theoretical modeling and fabrication of polarization-insensitive semiconductor optical amplifiers that use a multi-quantum-well structure as the gain media are reported. Polarization insensitivity of gain is achieved through the introduction tensile strain into the quantum wells. Gain calculations, using the k/spl middot/p method, were performed to obtain the required amount of tensile strain to obtain polarization insensitivity over a wide energy spectrum. Fabricated amplifiers show a polarization-insensitive (<1 dB) spectral width of 10 nm at 1300 nm in the InGaAsP/InP system, 15 nm at 1300 nm in the AlInGaAs/InP system, and 40 nm at 1550 nm in the AlInGaAs/InP system.

Single-mode vertical-cavity surface-emitting laser

Summary form only given. Numerous approaches have attempted to increase the single-transverse-mode output power of vertical-cavity surface-emitting lasers (VCSELs). These include external cavity, lossy or anti-guiding, and filtering approaches. For the purpose of commercial production, a monolithic, continuous wave (cw) device incorporating a lossless filter approach is desirable.

Magneto-optical garnet films made by reactive sputtering

Reactive radio frequency (RF) sputtering was used to grow cerium-doped yttrium iron garnet (YIG) films on magnesium oxide substrates. Magnesium oxide has been proven to be a good buffer material for semiconducting substrates. Reactive sputtering was not effective for cerium doping because the cerium target reacted with the oxygen in the sputtering gas. The films were amorphous as deposited. Stoichiometric compositions yielded smooth, polycrystalline garnet films on annealing. The effects of fluctuations in the yttrium-to-iron ratio were studied. Separate iron and yttrium targets were cosputtered in order to tailor the composition systematically along the iron-yttrium binary phase diagram. Oxygen content was found to be important in the formation of garnet and in the prevention of secondary phases. The garnet films had strong in-plane magnetizations and small coercive fields, which have promise for waveguide and switch devices, respectively.

Optical amplification at 1534 nm in erbium-doped zirconia waveguides

We have developed planar waveguides with net gain in erbium-doped zirconia. Ion-beam sputtering was used to deposit amorphous high-refractive-index zirconia films, which were fabricated into single-mode waveguides. By adjusting oxygen flow rates while sputtering, and annealing the films after deposition, waveguide losses were reduced to 0.45 dB/cm at 1534 nm. Erbium in the zirconia, added by co-sputtering, had a wide, 54-nm full-width at half maximum emission band centered at 1538 nm, which offers potential advantages for wideband amplification in wavelength division multiplexing systems. When pumped with 36 mW at 980 nm, a 6.5 cm long, 8.8 /spl times/ 10/sup 19/ cm/sup -3/ doped waveguide produced 2.95 dB of optical amplification at 1534 nm. This was enough to overcome the waveguide loss and produce a small amount of net gain. With a higher pump power, substantial net gain appeared to be possible. These results show that wide-bandwidth erbium-doped optical amplifiers should be possible in zirconia.