will propose a master’s thesis entitled,

Low noise double-beam laser absorption

spectroscopy for ammonia sensing 

On

Thursday, August 14 at 9 a.m. 
Ben T. Zinn Combustion Laboratory 107

 

Abstract
This research focuses on adapting an active noise-cancellation circuit for double-beam laser absorption spectroscopy to improve ammonia concentration measurements. By suppressing common-mode laser intensity noise, the circuit enhances absorption sensitivity without requiring complex modulation techniques. 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 laser 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, and prototyped on a breadboard. The circuit was tested using a water vapor absorption setup and benchmarked against both a DAS sensor and a commercial double-beam noise cancellation module from MKS-Newport. Results showed a clear improvement in NEA over standard DAS, with performance approaching that of the commercial system. Optimal beam-splitting ratio and frequency response (>10 kHz) were characterized.

The next phase of the project will focus on adapting the circuit for ammonia diagnostics using extended InGaAs photodetectors, which operate at the near-infrared wavelengths relevant to ammonia absorption. The circuit will then be transferred to a printed circuit board and housed in a Faraday cage to minimize electronic noise and improve system performance. Two experimental demonstrations are planned to validate the sensor’s performance. The first is a compression-driven test in a gas cell, initially at low pressure, to track transient ammonia absorbance while collisional broadening increases as the cell fills with ambient air. The second is a shock tube experiment, where time-resolved measurements of ammonia absorption will be used to determine its pyrolysis rate across a range of temperatures and pressures.

Committee

• Prof. Adam Steinberg – School of Aerospace Engineering (advisor)
• Prof. Wenting Sun– School of Aerospace Engineering (advisor)
• Dr. Shawn Wehe – Principal Research Engineer, School of Aerospace Engineering