Compact Optical Sensor System Research Articles

Compact TDLAS based optical sensor for ppb-level ethane detection by use of a 3.34 μm room-temperature CW interband cascade laser

A mid-infrared ethane (C2H6) sensor based on a wavelength modulation spectroscopy (WMS) technique was developed using a thermoelectrically cooled (TEC), continuous-wave (CW) interband cascade laser (ICL) emitting at 3.34μm and a dense multi-pass gas cell (MPGC, 17×6.5×5.5cm3) with a 54.6m optical path length. A compact optical sensor system with a physical size of 35.5×18×12.5cm3 was designed and constructed. An ICL was employed for targeting a strong C2H6 line at 2996.88cm−1 at <100Torr gas pressure in the fundamental absorption band of C2H6. The sensor performance, including the minimum detection limit (MDL) and the stability were improved by reducing the effect of laser power drift by means of the 2f/1f-WMS technique. A MDL of ∼1.2 parts per billion (ppbv) for 2f-WMS and ∼1.0ppbv for 2f/1f-WMS were achieved, respectively, with a measurement time of 4s. The MDL was further improved from 299pptv (@108s for 2f-WMS) to 239pptv (@208s for 2f/1f-WMS), based on an Allan deviation analysis. The rise time (@0→100ppbv) and fall time (@100→0ppbv) were determined to be ∼64s and ∼48s, respectively, at a gas pressure of <100Torr for the C2H6 sensor operation.

An external cavity diode laser sensor

A simple compact optical sensor system consisting of a near-field external cavity coupled to a semiconductor laser is reported. In this device, a change of phase in the light fed back into the laser cavity by the perturbed external mirror modulates the output of the laser by effectively changing the laser facet reflectivity. Phase shifts of 9\cdot10^{-8} rad and 10-6rad were measured at 10 kHz and 100 Hz, respectively, using a 1-Hz bandwidth and a 98-percent reflector placed less than 10 μm from the laser facet. The sensitivity of these devices is shown to be limited by the intrinsic amplitude noise of the laser. The implementation of this sensor configuration is reported as an acoustic sensor, hydrophone, magnetic field, and current sensor as well as an accelerometer.