Vineetha Kalavally

LED Based Indoor Visible Light Communications: State of the Art

Visible Light Communication (VLC) is an emerging field in Optical Wireless Communication (OWC) which utilizes the superior modulation bandwidth of Light Emitting Diodes (LEDs) to transmit data. In modern day communication systems, the most popular frequency band is Radio Frequency (RF) mainly due to little interference and good coverage. However, the rapidly dwindling RF spectrum along with increasing wireless network traffic has substantiated the need for greater bandwidth and spectral relief. By combining illumination and communication, VLC provides ubiquitous communication while addressing the shortfalls and limitations of RF communication. This paper provides a comprehensive survey on VLC with an emphasis on challenges faced in indoor applications over the period 1979-2014. VLC is compared with infrared (IR) and RF systems and the necessity for using this beneficial technology in communication systems is justified. The advantages of LEDs compared to traditional lighting technologies are discussed and comparison is done between different types of LEDs currently available. Modulation schemes and dimming techniques for indoor VLC are discussed in detail. Methods needed to improve VLC system performance such as filtering, equalization, compensation, and beamforming are also presented. The recent progress made by various research groups in this field is discussed along with the possible applications of this technology. Finally, the limitations of VLC as well as the probable future directions are presented.

Raman Amplification in Silicon-Nanocrystal Waveguides

The strength of Raman interaction between optical fields propagating through a silicon-nanocrystal waveguide is known to significantly differ from that in bulk silicon and silicon-on-insulator waveguides. Here we present the first theoretical study of continuous-wave Raman amplification in silicon-nanocrystal waveguides with improved mode confinement. By calculating numerically the mode-overlap factors and effective refractive indices of the pump and Stokes fields, we analyze how the maximal Stokes intensity and the optimal waveguide length depend on the cross-section parameters of the composite, density of silicon nanocrystals, and input conditions. In particular, we demonstrate that the maximal Stokes intensity peaks at certain waveguide height and volume fraction of silicon nanocrystals for fixed input intensities, and at certain waveguide width for fixed input powers. These features enable simple performance optimization of Raman amplifiers and lasers based on silicon nanocrystals.

Analytical Study of RIN Transfer in Pulse-Pumped Raman Amplifiers

Relative intensity noise (RIN) is one of the most significant artifacts that ultimately limits the use of broadband Raman amplifier applications in optical communications systems. Recently, as a way to alleviate some of the detrimental features of continuously pumped Raman amplifiers, pulsed pumping was introduced. In this paper, we study the RIN transfer in pulse-pumped fiber Raman amplifiers. An analytical expression describing RIN transfer from a pump with an arbitrary pulse shape has been derived for co- and counter-pumping configurations. The dependence of RIN transfer on pump modulation frequency, depth of modulation, and duty cycle in a typical 80-km fiber span has been analyzed for pumps with sinusoidal and rectangular profiles. We show that in the case of rectangular pulsing of pump, RIN transfer in counter-pumping regime is substantially enhanced compared with that of continuous wave (CW) pumping due to the presence of resonant peaks at the pump intensity harmonics. In the co-propagation regime, RIN transfer for pulsed pumping does not exhibit resonant peaks and is generally lower than that for CW pumping. These features are reflected on the Q-penalty and may have detrimental effects on the system performance.

Resonant characteristics and sensitivity dependency on the contact surface in QCM-micropillar-based system of coupled resonator sensors

We report the characteristics and sensitivity dependence over the contact surface in coupled resonating sensors (CRSs) made of high aspect ratio resonant micropillars attached to a quartz crystal microbalance (QCM). Through experiments and simulation, we observed that when the pillars of resonant heights were placed in maximum displacement regions the resonance frequency of the QCM increased following the coupled resonance characteristics, as the pillar offered elastic loading to the QCM surface. However, the same pillars when placed in relatively lower displacement regions, in spite of their resonant dimension, offered inertial loading and resulted in a decrease in QCM resonance frequency, as the displacement amplitude was insufficient to couple the vibrations from the QCM to the pillars. Accordingly, we discovered that the coupled resonance characteristics not only depend on the resonant structure dimensions but also on the contact regions in the acoustic device. Further analysis revealed that acoustic pressure at the contact surface also influences the resonance frequency characteristics and sensitivity of the CRS. To demonstrate the significance of the present finding for sensing applications, humidity sensing is considered as the example measurand. When a sensing medium made of resonant SU-8 pillars was placed in a maximum displacement region on a QCM surface, the sensitivity increased by 14 times in comparison to a resonant sensing medium placed in a lower displacement region of a QCM surface.

Multipath Interference in Pulse-Pumped Fiber Raman Amplifiers: Analytical Approach

We present an analytical study of multipath interference (MPI) in pulse-pumped fiber Raman amplifiers (FRAs), caused by the internal reflections from the fiber facets and the effect of Rayleigh backscattering. For the first time, to the best of our knowledge, we derive a generalized expression for the MPI-induced noise on signal in counter-pumped FRAs. The obtained result takes into account four major sources of MPI, suggests their physically transparent classification, and enables us to predict the FRAs noise performance for an arbitrary modulated pump. Using the derived expression, we characterize the MPI-induced noise in a typical 80-km span of single-mode optical fiber. Specifically , we examine the noise enhancement in the pulse-pumped amplifier as compared to the amplifier operating in the continuous-wave regime, and analyze its dependence on the on-off gain and the pump-pulse duty cycle. We also estimate the impact of the MPI-induced noise transfer on the amplifiers performance, and discuss the optimal choice of pump modulation for different values of Q-penalty. The results of our study are important for the design optimization of FRAs with time-division-multiplexed pumping.

Improving Lighting Quality and Capacity of OFDM-Based WDM-VLC Systems

Wavelength division multiplexing (WDM) systems in visible light communication are able to increase the data rates by independently modulating colored light sources-generally red, green, and blue. We analytically model such a system using orthogonal frequency division multiplexing modulation and show that the main limitation in achieving higher capacity and illuminance is the red channel. We propose alternate WDM techniques-one which uses an additional red light emitting diode (LED) to offset this effect and another which uses an unmodulated amber LED. By evaluating these systems, we show that increased capacity, higher illuminance range, as well as superior light quality could be achieved by the latter.

Engineering optical nonlinearities in silicon–nanocrystal waveguides

The strength of the nonlinear optical effects in silicon–nanocrystal waveguides can be varied as required for applications by altering the geometry of the waveguide and the composition of its nonlinear core. By studying theoretically the geometric and composition dependencies of the mode overlap factors responsible for the nonlinear interaction between the pump and Stokes optical fields, which are separated by the Raman shift in silicon, we demonstrate that this strength can be varied in a wide range, thus offering broad opportunities for engineering optical nonlinearities in silicon–nanocrystal waveguides. The numerically calculated mode overlap factors are useful in modeling light propagation through nonlinear silicon–nanocrystal waveguides governed by the recently derived generalized nonlinear Schrodinger equations.

Experimental characterization of TDM-pumped distributed Raman amplifier with commercial laser diode controller

In this paper, we perform the experimental characterization of a distributed Raman amplifier (DRA) employing time division multiplexed (TDM) pumping. We measure the response of a commercially available laser diode controller (LDC) to typical pump modulation parameters used in a TDM pumping scheme. These parameters are then optimized for the restricted response of the LDC. Using the optimal parameters, we present the noise characterization of an 8-channel wavelength division multiplexed (WDM) transmission system, counter-pumped using four pulsed semiconductor laser diodes that are time division multiplexed. Specifically, we show that for similar gain profiles and a typical 75-km single mode fiber span, the effective noise figure variation of the WDM channels is lesser by 0.8 and 0.5 dB for TDM-pumped DRA compared with continuous-wave-pumped DRA for 5 and 9 dB on-off gain, respectively.

Novel directions in Raman amplifier research

Raman amplifiers provide a simple and yet powerful platform for optical amplification needs in modern optical communication networks. This paper reviews recent progress in Raman amplifier research and highlights some non-conventional future of the field. Although the low-noise and wavelength-independent gain properties of Raman amplifiers have been long exploited in optical communication networks, researchers strive to tap into the huge potential of non-linear optics to improve reach and performance. Alternative approaches used to enhance the quality of Raman amplified networks such as the use of novel pumping schemes that can ensure uniformity in gain and noise performance are discussed. Finally, the advances in research on Raman amplification in monolithically integrated silicon waveguides which should fuel wide deployment in the coming years are presented.

QCM Coupled Resonating Systems Under Vacuum: Sensitivity and Characteristics

We present the underlying physics in the resonance frequency characteristics of a bare quartz crystal microbalance (QCM) and a QCM-micro pillar based coupled resonator sensor (QCM-CRS) when they were subjected to pressure changes in a vacuum chamber. The QCMs experienced an increase in resonance frequency with decrease in pressure due to an inverse mass loading effect introduced on the QCM surface. However, when the pressure (below atmospheric pressure) dominated the mass loading, the resonance frequency of QCMs decreased. A sensitivity increase of 11 times to the pressure changes was observed for the case of QCM-CRS made of resonating micropillars attached around the central region of the electrode. In addition, the resonance frequency shift was linear for pressure changes between 31 and 71 kPa, demonstrating the possibility of employing the QCM-CRS as a vacuum detector element. The proposed sensor element is envisioned to have wide industrial applications ranging from detecting vacuum in chambers used in coating systems to crack detection in closed chambers.