Yutong Feng

Progress towards non-intrusive optical measurement of gas turbine exhaust species distributions

We report on the development of three systems intended to provide fast, non-intrusive measurement of cross-sectional distributions of pollutant species within gas turbine exhaust flows, during ground-based testing. This research is motivated by the need for measurement systems to support the introduction of technologies for reducing the environmental impact of civil aviation. Tomographic techniques will allow estimation of the distributions of CO2, unburnt hydrocarbons (UHC), and soot, without obstruction of the exhaust, bypass or entrained flows, from measurements made in a plane immediately aft of the engine.

Implementation of non-intrusive jet exhaust species distribution measurements within a test facility

We report on the installation and commissioning of two systems for the measurement of cross-sectional distributions of pollutant species in jet exhaust, within the engine ground test facility at INTA, Madrid. These systems use optical tomography techniques to estimate the cross-sectional distributions of CO2 and soot immediately behind the engine. The systems are designed to accommodate the largest civil aviation engines currently in service, without obstruction of the exhaust or bypass flows and with negligible effect upon the entrained flow behavior. We describe the physical construction and installation status of each system. In the case of the CO2 system, we examine the challenges of achieving the structural rigidity necessary for adequate suppression of pointing error within 126 laser-based transmittance measurements, each utilizing a 7 m overall path length. We describe methods developed for efficient implementation of co-planarity and 4-degree-of-freedom alignment of individual paths within this beam array. We also present laboratory performance data for three alternative optical designs that differ in their approach to the management of pointing error and turbulence-induced beam wander and spread. The FLITES soot monitoring capability is based on laser induced incandescence (LII) and uses a short-pulse fiber laser and two CCD cameras, in an autoprojection arrangement. We describe the measurement geometry currently being implemented in the test cell and discuss optical design issues, including once again the effect of the plume itself.

Absorption measurement errors in single-mode fibers resulting from re-emission of radiation

We investigate errors in small-signal absorption spectra that result from re-emission in single-mode fibers with overlapping absorption and emission spectra. Experiments on Er-doped fibers and simulations of Er- and Yb-doped fibers show that the re-emission can severely distort the spectrum, especially the peak, under common measurement conditions, and underestimate the absorption by well over 10% already at 30-dB peak absorption, if only the source or the detector is spectrally filtered. Re-emission can then be the dominant source of errors. The error increases for higher absorption and higher fiber-NA. For sufficiently high NA, a significant error remains even in the limit of zero absorption and reaches 5% at the peak of a 0.46-NA Yb-doped fiber. Furthermore, in contrast to the high-absorption case, the error is larger at longer wavelengths than at the peak. Simultaneous filtering of both source and detector with 0.1-nm bandwidth reduces the re-emission error to ~1% or less up to 90-dB absorption. Then, detector noise or saturation errors are likely to dominate and render re-emission errors insignificant. A standard amplifier model is well suited to the simulations of the rich physics of single-mode-fiber absorption measurements.

Multimode-pumped Raman amplification of a higher order mode in a large mode area fiber

We report the first demonstration of Raman amplification in a fiber of a single Bessel-like higher order mode using a multimode pump source. We amplify the LP08-mode with a 559-µm2 effective mode area at a signal wavelength of 1115 nm in a pure-silica-core step-index fiber. A maximum of 18 dB average power gain is achieved in a 9-m long gain fiber, with output pulse energy of 115 µJ. The Raman pump source comprises a pulsed 1060 nm ytterbium-doped fiber amplifier with V-value ~30, which is matched to the Raman gain fiber. The pump depletion as averaged over the signal pulses reaches 36.7%. The conversion of power from the multimode pump into the signal mode demonstrates the potential for efficient brightness enhancement with low amplification-induced signal mode purity degradation.

Data for: Multi-Wavelength Diode-Pumping of Fiber Raman Laser

The data for the figures in the conference paper:Hong, S., Feng, Y., & Nilsson, J. (2018). Multi-wavelength diode-pumping of fiber Raman laser. In Conference on Lasers and Electro-Optics: Science and Innovations 2018 Optical Society of America. DOI: 10.1364/CLEO_SI.2018.SM1K.6A fiber Raman laser pumped by two wavelength-combined multimode diode lasers at 950 nm and 976 nm generates up to 23 W of output power at a single wavelength (1020 nm) with 51% slope efficiency.

Data for EuroPhoton 2016 conference paper "Simulation on thermal load gradient mitigation with auxiliary multi-seeds amplification in fiber amplifier"

A technique that employs auxiliary multi-seeds for mitigating the inhomogeneous of thermal load in Yb-doped double cladding amplifier is presented and verified in simulation. The results shows this technique can reduce the thermal load gradient by a significant ratio, thus has potential application in high power amplifiers.

High-Sensitivity In Situ Soot Particle Sensing in an Aero-Engine Exhaust Plume Using Long-Pulsed Fiber-Laser-Induced Incandescence

A method to produce spatially resolved images of the distribution of absorbing particles in the exhaust plume of a modified helicopter gas turbine engine is presented. Over a small region of the plume, in situ sensing of soot particles by laser-induced incandescence (LII) is demonstrated using fiber lasers with higher power (~10 W), longer pulse duration (>100 ns), and higher pulse repetition rates (>10 kHz) than the conventional LII. The sensitivity of the method is illustrated by the detection of ambient absorbing particles in background conditions with engine at rest. With a running engine, single-beam images are obtained in 0.01 s. The feasibility of using long-pulsed fiber lasers for soot particle concentration measurement is investigated using a representative laboratory system. The time-resolved LII behavior and the measurement linearity are investigated, demonstrating the suitability of using fiber lasers for soot particle measurement for aero-engine emissions. Results for normalized soot concentration are compared with extractive measurements illustrating good correlation across a range of engine speeds. This paper is the first step toward the development of a non-intrusive system for the measurement of 2-D soot concentration in the cross section of an aero-engine exhaust plume.

Data for ASSL conference paper "Fiber absorption measurement errors resulting from re-emission of radiation"

We show that errors in the absorption measured in rare-earth-doped fibers can exceed 50% and severely distort the spectral shape. This is a result of re-emission in fibers with overlapping absorption and emission spectra.

Fiber absorption measurement errors resulting from re-emission of radiation

We show that errors in the absorption measured in rare-earth-doped fibers can exceed 50% and severely distort the spectral shape. This is a result of re-emission in fibers with overlapping absorption and emission spectra.

Thermally induced distortions of the temporal phase of optical pulses in phosphorous-doped silica fibers

We study the phase distortion induced in optical pulses due to thermal effects in Sm doped silica fibers. We experimentally measure the phase change accurately using an interferometric technique.