Anil K. Patnaik

Slow light propagation in a thin optical fiber via electromagnetically induced transparency

We propose a configuration that utilizes electromagnetically induced transparency (EIT) to tailor a fiber mode propagating inside a thin optical fiber and coherently control its dispersion properties to drastically reduce the group velocity of the fiber mode. The key to this proposal is that the evanescent field of the thin fiber strongly couples with the surrounding active medium, so that the EIT condition is met by the medium. We show how the properties of the fiber mode are modified due to the EIT medium, both numerically and analytically. We demonstrate that the group velocity of the modified fiber mode can be drastically reduced

Multiorder coherent Raman scattering of a quantum probe field

(\ensuremath{\approx}44\mathrm{}\mathrm{m}/\mathrm{s}\mathrm{e}\mathrm{c})

Sensitivity, stability, and precision of quantitative Ns-LIBS-based fuel-air-ratio measurements for methane-air flames at 1-11 bar.

using the coherently prepared orthohydrogen doped in a matrix of parahydrogen crystal as the EIT medium.

Gas-Phase Temperature Measurements in Reacting Flows using Fiber-Coupled Picosecond Coherent Anti-Stokes Raman Scattering Spectroscopy

We study the multiorder coherent Raman scattering of a quantum probe field in a far-off-resonance medium with a prepared coherence. Under the conditions of negligible dispersion and limited bandwidth, we derive a Bessel-function solution for the sideband field operators. We analytically and numerically calculate various quantum statistical characteristics of the sideband fields. We show that the multiorder coherent Raman process can replicate the statistical properties of a single-mode quantum probe field into a broad comb of generated Raman sidebands. We also study the mixing and modulation of photon statistical properties in the case of two-mode input. We show that the prepared Raman coherence and the medium length can be used as control parameters to switch a sideband field from one type of photon statistics to another type, or from a nonsqueezed state to a squeezed state and vice versa. We demonstrate that an even or odd coherent state of the quantum probe field can produce a multipartite entangled coherent state. We show that the concurrence reaches its maximal value at an optimal medium length that is determined by the magnitude of the Raman coherence and the orders of the Raman sidebands.

Development of an All-Fiber-Coupled, Pulsed, Ultraviolet, Laser-Induced-Fluorescence (UV-LIF) Detection System for OH Radicals in Practical Combustion Devices

Nanosecond laser-induced breakdown spectroscopy (ns-LIBS) is employed for quantitative local fuel-air (F/A) ratio (i.e., ratio of actual fuel-to-oxidizer mass over ratio of fuel-to-oxidizer mass at stoichiometry, measurements in well-characterized methane-air flames at pressures of 1-11 bar). We selected nitrogen and hydrogen atomic-emission lines at 568 nm and 656 nm, respectively, to establish a correlation between the line intensities and the F/A ratio. We have investigated the effects of laser-pulse energy, camera gate delay, and pressure on the sensitivity, stability, and precision of the quantitative ns-LIBS F/A ratio measurements. We determined the optimal laser energy and camera gate delay for each pressure condition and found that measurement stability and precision are degraded with an increase in pressure. We have identified primary limitations of the F/A ratio measurement employing ns-LIBS at elevated pressures as instabilities caused by the higher density laser-induced plasma and the presence of the higher level of soot. Potential improvements are suggested.

Picosecond Laser-Based Fiber-Coupled CARS Spectroscopy for Gas-Phase Thermometry

The primary objective of this research effort was to investigate the transmission of highpower picosecond (ps) laser beams through various fibers for a fiber-coupled ps-CARS system. In particular, the damage threshold, nonlinearity, fiber length, and spatial beam profile through various fibers was investigated. It has been experimentally determined that an approximate increase in signal of 100 to 1000 times is possible by designing a system for ps fiber-based CARS. The other unique feature of the picosecond CARS system that makes it ideal for fiber coupling is the requirement of only ~200 μJ of energy for the pump and probe beams as compared to 25 mJ of energy per laser beam generally required in nanosecond laser-based CARS. A proof-of-principle demonstration of fiber-based nitrogen CARS in a collinear configuration was performed using multimode step-index fiber.

Engineering the dispersion of a fibre mode via molecular coherence

An all-fiber-coupled, pulsed, ultraviolet, laser-induced-fluorescence (UV-LIF) detection system for hydroxyl radicals (OH) is developed for use in practical combustion devices that operate in harsh environments. The system is designed to transmit the required excitation-pulse energy through long optical fibers (up to 10 m) for both point and planar LIF (PLIF). First, we investigate the fundamental transmission characteristics of nanosecond (ns)-duration, high-power laser pulses at a 283-nm OH-excitation wavelength for state-of-the-art, commercial, UV-grade optical fibers. Detailed studies carried out on the fibers include 1) damage threshold, 2) optical-transmission stability under long-term, high-power, UV laser irradiation, 3) beam quality at the output, and 4) nonlinear effects during beam propagation. Based on these studies, an all-fiber-coupled ns-UV-LIF excitation/detection system for OH is developed. Singlelaser-shot PLIF imaging of OH in flames is also demonstrated using fiber-based excitation. Development of such fiber-based diagnostics and imaging systems constitutes a major step forward in transitioning laser-diagnostic tools from research laboratories to practical combustion facilities.

All Fiber-Coupled OH Planar Laser-Induced-Fluorescence (OH-PLIF)-Based Two-Dimensional Thermometry

Fiber-based, picosecond coherent anti-Stokes Raman scattering (ps-CARS) spectroscopy employing multimode step-index fiber for gas-phase thermometry in reacting flows is demonstrated. The effects of fiber delivery on the spectral and temporal profiles of the highpeak-power, narrow-band pump and broadband Stokes pulses and, in turn, on the CARS signal are investigated. The proof-of-principle experiment shows significant promise for fiber-based ps-CARS for temperature and species-concentration measurements in harsh chemical environments of practical combustors and gas-turbine-engine test facilities.

Time-resolved correlated measurement of laser-induced-breakdown spectroscopy and electron number density: application to high-pressure hydrocarbon flames

Abstract Propagation of a fibre mode inside a thin optical fibre surrounded by a molecular solid of ortho-H2 doped in a matrix of para-H2 crystal is considered. The evanescent fields of the fibre modes interact with the molecular solid which behaves as an electromagnetically induced transparency medium. It is shown that the dispersion behaviour and hence the group velocity of a weak (probe) field fibre mode can be manipulated by coherently controlling the dispersion of the surrounding medium via another strong (control) evanescent field of the thin fibre, using a low density crystal surrounding the fibre. For a set of experimental parameters of the medium, fibre and fields, the group velocity of the fibre mode could be drastically reduced to about a few hundreds of ms−1.

High-speed imaging of OH radicals in flames using fiber-coupled UV-PLIF

Two-color, planar laser-induced fluorescence (PLIF)-based two-dimensional (2D) thermometry techniques for reacting flows, which are typically developed in the laboratory conditions, face a stiff challenge in their practical implementation in harsh environments such as combustion rigs. In addition to limited optical access, the critical experimental conditions (i.e., uncontrolled humidity, vibration, and large thermal gradients) often restrict sensitive laser system operation and cause difficulties maintaining beam-overlap. Thus, an all fiber-coupled, two-color OH-PLIF system has been developed, employing two long optical fibers allowing isolation of the laser and signal-collection systems. Two OH-excitation laser beams (∼283 nm and ∼286 nm) are delivered through a common 6 m long, 400 µm core, deep ultraviolet (UV)-enhanced multimode fiber. The fluorescence signal (∼310 nm) is collected by a 3 m long, UV-grade imaging fiber. Proof-of-principle temperature measurements are demonstrated in atmospheric pressure, near adiabatic, CH4/O2/N2 jet flames. The effects of the excitation pulse interval on fiber transmission are investigated. The proof-of-principle measurements show significant promise for thermometry in harsh environments such as gas turbine engine tests.