Low Noise Double-Beam Absorption Spectroscopy
Author(s)
Preve, Federico
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Abstract
This thesis focuses on adapting an analog noise-cancellation circuit for double-beam laser absorption spectroscopy to improve accuracy in ammonia concentration measurements. By suppressing common-mode laser intensity noise, the circuit enhances absorption sensitivity without requiring complex modulation techniques. In this approach, a tunable diode laser beam is split into two paths: a signal beam that passes through an absorption cell and a reference beam that bypasses any absorbing medium. The two beams are independently detected and their corresponding photocurrents are fed into an electronic circuit designed to suppress the common-mode noise on laser intensity, thereby enhancing the diagnostic technique’s accuracy. This architecture remains cost-effective and has the potential to achieve shot-noise-limited performance.
A circuit originally designed by Philip Hobbs was adapted, simulated in Analog Devices LTspice, prototyped on a breadboard and ultimately implemented on a printed circuit board. Performance was evaluated using a water-vapor absorption setup and compared against both a standard DAS sensor and a commercial double-beam noise-cancellation module from MKS–Newport. Results show a clear improvement in NEA over standard DAS, with performance approaching that of the commercial system. Finally, extended-range InGaAs photodetectors were integrated to enable ammonia detection and the circuit was used to record room-temperature ammonia absorption line shapes and time-resolved absorption measurements of ammonia oxidation under post-reflected shock conditions in shock-tube experiments.
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Date
2025-12
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Text
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Thesis (Masters Degree)