Differential Diode Laser Sensor for High-Purity Oxygen

A compact portable sensor for determining the purity of oxygen concentrations near 100 percent is proposed based on differential absorption of two beams from a diode laser. One beam passes through a cell containing the sample of gas to be analyzed and the second beam passes through a reference cell containing a known concentration of high-purity oxygen. An autobalanced detection system will be used for measuring the difference in photocurrents of the transmitted beams. Common mode noise such as laser intensity noise will be rejected to a high degree. The system should not be subject to drift because it would be possible to lock the laser wavelength to the oxygen line using the reference cell. We estimate that the proposed sensor concept should enable an accuracy of 0.05 percent to be achieved with a cell length of less than 10 cm. The sensor can be made rugged with a small footprint using microelectronics for laser control and signal processing. The proposed effort will test the feasibility of this sensor concept by seeking to demonstrate measurements of the desired accuracy using a breadboard system.