Padmanabhan Radhakrishnan

Fabrication and characterization of dye-doped polymer optical fiber as a light amplifier

The fabrication and characterization of a Rhodamine 6G-doped polymer optical fiber amplifier have been carried out. Two different schemes were employed to characterize the optical fiber: the stripe illumination technique to study the fiber as a gain medium and another technique to study its performance as an amplifier. We observed a spectral narrowing from 42 to 7 nm when the pump energy was increased to 6 mJ in the stripe illumination geometry. A gain of 18 dB was obtained in the amplifier configuration. The effects of pump power and dye concentration on the performance of the fiber as an amplifier were also studied.

Multimode laser emission from dye doped polymer optical fiber

Multimode laser emission is observed in a polymer optical fiber doped with a mixture of Rhodamine 6G (Rh 6G) and Rhodamine B (Rh B) dyes. Tuning of laser emission is achieved by using the mixture of dyes due to the energy transfer occurring from donor molecule (Rh 6G) to acceptor molecule (Rh B). The dye doped poly(methyl methacrylate)-based polymer optical fiber is pumped axially at one end of the fiber using a 532 nm pulsed laser beam from a Nd:YAG laser and the fluorescence emission is collected from the other end. At low pump energy levels, fluorescence emission is observed. When the energy is increased beyond a threshold value, laser emission occurs with a multimode structure. The optical feedback for the gain medium is provided by the cylindrical surface of the optical fiber, which acts as a cavity. This fact is confirmed by the mode spacing dependence on the diameter of the fiber.

Filamentation characteristics of focused fs pulses in atmosphere

We present the experimental investigations on the filament characteristics of sharply focused fs pulses (800 nm, 45 fs, 1 kHz) in air. Pulses with input powers in 3-12.2 PCr range were focused using three different focusing geometries f/#10, f/#15 and f/#20 corresponding to numerical apertures (NA) of 0.05, 0.033 and 0.025, respectively. The dynamics of filaments were observed via direct imaging of the entire reaction zone. The length of the filament has decreased with increasing NA from 0.025 to 0.05, while, the filament width has increased. For a given focusing geometry, the filament length and width increased with increasing power. However with higher NA, the length and width were observed to saturate at higher input powers. With the highest NA of 0.05 and higher input powers used in the current study, the presence of coherently interacting multiple filaments either resulting in a fusion or exchange of power.

Optical engineering by the nanocomposites of ZnO-CdS/TiO2

In this article we present the bandgap engineering and spectral properties of ZnO-CdS and ZnO-TiO 2 nanocomposites prepared by colloidal chemical synthesis. The optical bandgap (E g ) of the material is tunable between 2.62 and 4.12 eV. Emission peaks of ZnO-CdS change from 385 to 520 nm and those of ZnO-TiO 2 change from 340 to 385 nm almost in proportion to the changes in E g . It is possible to obtain a desired luminescence color from UV to green by simply adjusting the composition. ZnO-CdS and ZnO-TiO 2 are potential nanocomposite materials for the tunable light emission.

Bulk second-harmonic generation from thermally evaporated indium selenide thin films

We investigate bulk second-order nonlinear optical properties of amorphous indium selenide thin films fabricated by thermal evaporation. Such films are shown to exhibit strong and photostable second-harmonic generation (SHG). We report strong thickness dependence of the second-harmonic signals as characterized by the Maker-fringe method. The absolute value of the nonlinear susceptibility tensor of the film is addressed by analyzing the interference of SHG signals from the film and the glass substrate. The value of the joint non-diagonal component of the susceptibility is found to be 4 pm/V, which is comparable to that of widely used second-order nonlinear materials.

Supercontinuum emission from water using fs pulses in the external tight focusing limit

From the initial observation of self-channeling of high-peak power femtosecond (fs) laser pulses in air, propagation of intense ultrashort laser pulses in different media has become one of the most investigated research areas. The supercontinuum emission (SCE), a spectral manifestation of the spatio-temporal modifications experienced by a propagating ultrashort laser pulse in a nonlinear medium, has many practical applications. However, the extent of blue shift of SCE is reported to be constant due to the phenomenon of intensity clamping. To further explore the recently observed regime of filamentation without intensity clamping, we measured the evolution of spectral blue shift of SCE resulting from the propagation of fs pulses (800 nm, 40 fs, 1 kHz) in distilled water under different focusing geometries. The efficiency of SCE from tight focusing (f/6) geometry was always higher than the loose focusing (f/12) geometry for both linear and circular polarized pulses. The blue edge of the SCE spectrum (λmin) was found to be blue shifted for f/6 focusing conditions compared to f/12 focusing geometry. The lower bound of the intensity deposited in the medium measured from the self-emission from the filament demonstrated the existence of intensities ~ 6x1013 Wcm-2, far beyond the clamping intensities achieved erstwhile.

A Simple and Novel Integrated Opto-Electronic System for Blood Volume Pulse Sensing and Heart Rate Monitoring

A high sensitivity, low power, low cost sensor has been developed for sensing the blood volume pulse using transmission mode photoplethysmography (PPG) from the finger tip. It uses standard light emitting diode (LED) and phototransistor as emitter and detector, respectively. The sensor probe head is made of a material called delrin which is an acetal polymer having high dimensional stability and biocompatibility. A peripheral interface controller (PIC) microcontroller is used for the implementation of the measurement protocol. The wavelet denoising algorithm is used to suppress the noise component in the PPG signal. For heart rate (HR) estimate comparison, PPG signals are evaluated by comparing their beat-to-beat estimates with the corresponding R-R intervals from an electrocardiogram (ECG). The main advantages of the proposed approach are the reduction in cost, dimensions and power consumption. The probe can be well tolerated by the subject and is self-contained and portable.

Chemically Tapered Multimode Optical Fiber Probe for Fluoride Detection Based on Fluorescence Quenching of Curcumin

This paper reports a simple fluorescence-based tapered fiber optic probe for fluoride ion having a detection range of 2.08 × 10-6-2.005 × 10-4 M. The performance of the tapered probe is evaluated with respect to the probes that consist of combinations of bare uncladded multimode optical fibers. The effect of fluorescence quenching of a natural dye curcumin in the presence of fluoride ion is used in the implementation of the probes. The probe effectively uses multiple mechanisms for the excitation and collection of fluorescence from the medium enabling higher sensitivity compared with conventional spectrophotometry especially at very low concentrations of fluoride.

Supercontinuum Emission from Water using 40 fs Pulses in the External Tight Focusing Limit

We present our results from the measurements of Supereonlinuum emission (SCE) resulting from the propagation ol” tightly foe used 40 femtosecond laser pulses through distilled water. The e fleet of linearly polarized (LP) and circularly polarized (CP) light pulses on the SCE: in different external focal geometries (f/6 & f/12) is studied in detail. A considerable shift in the minimum wavelength of SCF under tighter focusing limit is observed.

Supercontinuum Emission from Focused Femtosecond Laser Pulses in Air

We present our experimental results from the measurements of Supercontinuum emission (SCE) from air resulting from propagation of tightly focused femtosecond (40 fs) laser pulses. The effect of linearly polarized (LP) and circularly polarized (CP) light pulses on the SCE in two different external focal geometries (f/6, f/15) is presented. A considerable shift in the minimum wavelength of SCE is observed with external tighter focusing.