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Govind P Agrawal

Publications and source records attributed to Govind P Agrawal.

18 recordsLinked to original sources
PubMed2006

Evaluation of porous carrier-based floating orlistat microspheres for gastric delivery.

The purpose of this research was to prepare floating microspheres consisting of (1) calcium silicate as porous carrier; (2) orlistat, an oral anti-obesity agent; and (3) Eudragit S as polymer, by solvent evaporation method and to evaluate their gastro-retentive and controlled-release properties. The effect of various formulation and process variables on the particle morphology, micromeritic properties, in vitro floating behavior, percentage drug entrapment, and in vitro drug release was studied. The gamma scintigraphy of the optimized formulation was performed in albino rabbits to monitor the transit of floating microspheres in the gastrointestinal tract. The orlistat-loaded optimized formulation was orally administered to albino rabbits, and blood samples collected were used to determine pharmacokinetic parameters of orlistat from floating microspheres. The microspheres were found to be regular in shape and highly porous. Microsphere formulation CS4, containing 200 mg calcium silicate, showed the best floating ability (88% +/- 4% buoyancy) in simulated gastric fluid as compared with other formulations. Release pattern of orlistat in simulated gastric fluid from all floating microspheres followed Higuchi matrix model and Peppas-Korsmeyer model. Prolonged gastric residence time of over 6 hours was achieved in all rabbits for calcium silicate-based floating microspheres of orlistat. The enhanced elimination half-life observed after pharmacokinetic investigations in the present study is due to the floating nature of the designed formulations.

Administration, Oral↗
PubMed2006

Raman response function for silica fibers.

The commonly used Lorentzian form of the Raman response function for studying propagation of ultrashort pulses in silica fibers does not properly account for the shoulder in the Raman gain spectrum originating from the Boson peak. We propose a more accurate form of this response function and show that its predictions for the Raman-induced frequency shift should be in better agreement with experiments.

Journal Article↗
PubMed2006

Silicon waveguides for creating quantum-correlated photon pairs.

We propose to use four-wave mixing inside silicon waveguides for generating quantum-correlated photon pairs in a single spatial mode. Such silicon-based photon sources not only exhibit high pair correlation but also have high spectral brightness. As the proposed scheme is based on mature silicon technology, it has the potential of becoming a cost-effective platform for on-chip quantum information processing applications.

Journal Article↗
PubMed2006

Do solitonlike self-similar waves exist in nonlinear optical media?

We show analytically that bright and dark spatial self-similar waves can propagate in graded-index amplifiers exhibiting self-focusing or self-defocusing Kerr nonlinearities. The intensity profiles of the novel waves are identical with those of fundamental bright or dark spatial solitons supported by homogeneous passive waveguides with the same type of nonlinearity. Thus, we reveal a previously unnoticed connection between spatial solitons and self-similar waves. We also suggest that the discovered self-similar waves can be used in a promising scheme for the amplification and focusing of spatial solitons in future all-optical networks.

Journal Article↗
PubMed2006

Photon-pair generation by four-wave mixing in optical fibers.

We present a theory to quantify a fundamental limit on correlated photon pairs generated through four-wave mixing inside optical fibers in the presence of spontaneous Raman scattering (SpRS). Our theory is able to explain current experimental data. We show that if correlated photon pairs are generated with polarization orthogonal to the pump the effect of SpRS is significantly reduced over a broad spectral region extending from 5 to 15 THz.

Journal Article↗
PubMed2006

Dispersion tailoring and soliton propagation in silicon waveguides.

The dispersive properties of silicon-on-insulator (SOI) waveguides are studied by using the effective-index method. Extensive calculations indicate that an SOI waveguide can be designed to have its zero-dispersion wavelength near 1.5 microm with reasonable device dimensions. Numerical simulations show that soliton-like pulse propagation is achievable in such a waveguide in the spectral region at approximately 1.55 microm. The concept of path-averaged solitons is used to minimize the impact of linear loss and two-photon absorption.

Journal Article↗
PubMed2006

A novel calcium silicate based microspheres of repaglinide: in vivo investigations.

The objective of the present investigation was to evaluate gastro-retentive performance and pharmacokinetic parameters of optimized floating microspheres (RgFMCS4) consisting of (i) calcium silicate (CS) as porous carrier; (ii) repaglinide (Rg), an oral hypoglycemic agent; and (iii) Eudragit S (ES) as polymer. The optimized formulation demonstrated favorable in-vitro-floating and drug release characteristics. The gastro-retentive behavior of this optimized formulation was compared with non-floating microspheres (RgNFM) prepared from the identical polymer. Stability test of (99m)Tc-labeled formulations were carried out using appropriate standard buffer solutions of pH 2.0, 6.8 and 7.4. The organ distribution study was performed in albino rats in order to measure labeling efficiency of the formulation with (99m)Tc. The gamma scintigraphy of the formulations was carried out in albino rabbits to monitor the transit of RgFMCS4 and RgNFM in the gastrointestinal (GI) tract. Prolonged gastric residence time (GRT) of over 6 h was achieved in all animals for calcium silicate based floating microspheres of Rg. Rg loaded optimized formulation was orally administered to albino rabbits and blood samples were used to determine pharmacokinetic parameters of Rg from floating microspheres, which were compared with pharmacokinetic parameters of the marketed tablet formulation. The relative bioavailability of Rg loaded floating microspheres was found to be increased about 3.17 times in comparison to that of the marketed tablet. The enhanced bioavailability and eliminated half-lives of Rg formulation observed in the present study are attributed to the floating nature of the designed formulations.

Animals↗
PubMed2006

Changes in the spectrum, in the spectral degree of polarization, and in the spectral degree of coherence of a partially coherent beam propagating through a gradient-index fiber.

Expressions are derived for the cross-spectral density matrix of an electromagnetic Gaussian Schell-model beam propagating through a paraxial ABCD system. Using the recently developed unified theory of coherence and polarization of electromagnetic beams and the ABCD matrix for gradient-index fibers, we study the changes of the spectral density, of the spectral degree of polarization, and of the spectral degree of coherence of such a beam as it travels through the fiber. Effects of material dispersion are also considered.

Journal Article↗
PubMed2006

Controlled release calcium silicate based floating granular delivery system of ranitidine hydrochloride.

The objective of the present investigation was to prepare and evaluate floating granular delivery system consisting of (i) calcium silicate (CS) as porous carrier; (ii) ranitidine hydrochloride (RH), an anti-ulcer agent; and (iii) hydroxypropyl methylcellulose K4M (HPMC) and ethylcellulose (EC) as matrix forming polymers. The effect of various formulation and process variables on the particle morphology, particle size, micromeritic properties, percent drug content, in vitro floating behavior, and in vitro drug release from the floating granules was studied. The scanning electron microscopy (SEM) of granules revealed that that more pores of CS in secondary coated granules (SCG) were covered by the polymer film than those in primary coated granules (PCG). The formulation demonstrated favorable in vitro floating and drug release characteristics. The in vivo evaluation for the determination of pharmacokinetic parameters was performed in albino rats. Higher plasma concentration was maintained throughout the study period from the floating granules of RH. The enhanced bioavailability and elimination half-life observed in the present study may be due to the floating nature of the dosage form. The results suggested that CS is a useful carrier for the development of floating and sustained release preparations.

Animals↗
PubMed2005

Broadly tunable femtosecond parametric oscillator using a photonic crystal fiber.

Pulses as short as 460 fs and a tuning range as wide as 200 nm around 1 microm have been achieved from a photonic-crystal-fiber-based parametric oscillator. The ring cavity with only 65 cm of photonic crystal fiber is synchronously pumped with a tunable passively mode-locked Yb-doped fiber laser. Widely extended tunability is achieved by using the modulation instability gain in normal dispersion as the result of high-order dispersion in the photonic crystal fiber.

Journal Article↗
PubMed2005

Pump-noise transfer in dual-pump fiber-optic parametric amplifiers: walk-off effects.

Transfer of intensity noise from pumps to signal in dual-pump fiber-optic parametric amplifiers is simulated numerically for a realistic configuration in which both pumps are amplified and filtered before they enter the fiber. The walk-off effects induced by different group velocities of pumps, signal, and idler are fully taken into account. It is found that the optical signal-to-noise ratio can be as low as 15 dB when the amplifier length is close to 0.5 km, but it can be improved by 3 dB or so by use of longer fibers.

Journal Article↗
PubMed2005

Intrapulse depolarization in optical fibers: a classical analog of spin decoherence.

We show that the combination of cross-phase modulation and polarization-mode dispersion inside optical fibers leads to a novel phenomenon of intrapulse depolarization manifested as different random states of polarization across the pulse profile. Such polarization evolution of optical pulses is directly analogous to the phenomenon of spin decoherence in semiconductors or pseudospin relaxation in atoms.

Journal Article↗
PubMed2004

Vector soliton fission.

We investigate the vectorial nature of soliton fission in an isotropic nonlinear medium both theoretically and experimentally. As a specific example, we show that supercontinuum generation in a tapered fiber is extremely sensitive to the input state of polarization. Multiple vector solitons generated through soliton fission exhibit different states of elliptical polarization while emitting nonsolitonic radiation with complicated polarization features. Experiments performed with a tapered fiber agree with our theoretical description.

Journal Article↗
PubMed2004

Effects of polarization-mode dispersion on fiber-based parametric amplification and wavelength conversion.

We present a vector theory of four-wave mixing in optical fibers and use it to discuss the effect of polarization-mode dispersion (PMD) on the performance of parametric amplifiers and wavelength converters. We show that PMD distorts the gain spectrum and makes it less uniform than that expected in the absence of residual birefringence. PMD also induces large fluctuations in the amplified or wavelength-converted signal.

Journal Article↗
PubMed2004

Asymmetric incoherent vector solitons.

We introduce a wide class of partially coherent, partially polarized solitons whose second-order statistical properties include a certain type of reflectional symmetry to be referred to as the specularity. We describe generic properties of such solitons in any noninstantaneous saturable nonlinear media. We derive closed form analytical expressions for the intensity profiles as well as the degree of polarization of specular and antispecular partially coherent solitons supported by logarithmically saturable nonlinear media.

Journal Article↗
PubMed2003

Statistics of polarization-dependent gain in fiber-based Raman amplifiers.

We develop an analytic model for finding the statistics of polarization-dependent gain (PDG) in fiber-based Raman amplifiers. We use it to find an analytic form for the probability distribution of PDG and study how the mean PDG and the variance of PDG fluctuations depend on the PMD parameter. We show that mean PDG as well as PDG fluctuations are reduced by approximately a factor of 30 in the case of backward pumping.

Journal Article↗
PubMed2003

Nonlinear theory of polarization-mode dispersion for fiber solitons.

We consider the evolution of optical solitons inside a nonlinear dispersive fiber with random birefringence, causing polarization-mode dispersion. We convert the pair of coupled nonlinear Schrödinger equations satisfied by the orthogonally polarized components into a Fokker-Planck equation using the collective-variable approach. We solve this equation and derive expressions for the probability density functions associated with the differential group delay and the pulse width in the limit of large propagation distances.

Journal Article↗