School of Engineering and Information Technology

MCT Detectors for the IR wavelengths

A consortium of researchers at Macquarie University (MQU), University of New South Wales (UNSW@ADFA), and Loyola University New Orleans (LU NO), together with Australian government security agencies (e.g., AFP), is working to develop highly sensitive laser-based forensic sensing strategies applicable to characteristic substances that pose chemical, biological and explosives (CBE) threats. We aim to optimise the potential of these strategies as high-throughput screening tools to detect prohibited and potentially hazardous substances such as those associated with explosives, narcotics and bio-agents.

Our central approach entails novel refinements of Spectroscopy implemented in the mid-infrared region of the electromagnetic spectrum between 4 and 12 micrometers. There we can take advantage of chemically specific molecular vibrations to unambiguously identify and detect compounds of interest in terms of each compound's molecular fingerprint for applications to forensic analysis and security screening.

Sensitive trace gas analysis techniques based on mid infrared spectroscopy are needed in many application areas ranging from explosive compounds (e.g. TATP, TNT, RDX, PETN) and biologically hazardous materials detection (e.g. Silane) to health monitoring (e.g. NO, CO2, NH3), and atmospheric monitoring (e.g. NOx, CO, CO2, H2O). The infrared region is of primary interest since the absorption features of a molecular IR spectrum are directly related to the given compound's structure, enabling one to obtain a ``molecular fingerprint" of a gas through spectral analysis.

In order to achieve an instrument noise floor limited by quantum (shot) noise, we need to develop a shot noise limited mid-infrared detector. Our laser system has an operating wavelength of 6.1μm, with a maximum power level of 25mW. The available laser power can conceivably drop to approximately 1mW of light exiting the cavity, before striking any infrared detector. The detector design needs to be fundamentally limited by the photon noise striking the detector at these power levels, if we are to achieve the highest sensitivity for our spectrometer.

A measurement of the noise characteristics of the detector module was carried out, to determine if the detector is shot noise limited. Laser light from a Daylight Solutions quantum cascade laser (QCL) was shone directly onto the detector, with the detector output coupled directly into an Agilent E4411B spec- trum analyser. The incident power was 25mW, measured with a Thorlabs S302C power meter. The measurement was taken from 1kHz to 10MHz, with a resolution bandwidth of 30kHz and a video/resolution bandwidth ratio of 0.01. Figure 9 shows traces for the noise power measured for the incident light, as well as for the detector with no light incident. A measurement of the spectrum analysers noise floor (with nothing connected) is also included, showing that the dark noise is about -88dBm. In the trace for the incident light, the DC component has effectively been removed by the high pass filter before the amplifier. If any shot noise can be seen by the detector, it would still be present in the measurement, since it is a noise signal across all frequencies. It can be seen that there is no difference between the incident signal and the dark noise of the detector, above 0.5MHz.

In our inital investigation of the detector module we have available and that was designed to operate with our quantum cascade laser we find that the detectors are not sensitive to the photon quantum noise. They do, however, have the RF bandwitdth to see the laser pulse.

Figure 7: Photoconductor geometry and bias. W×L is the detector area, and L×t is the electrical contact area. The detected signal is typically a voltage measured across the load resistor RL.

Figure 8: MCT detector responsivity curves. The wavelength of most relevance is 6μm, giving a responsivity of 65-75V/W.

Figure 9: Measured noise for the detector connected directly to the spectrum analyser, with and without an applied CW laser power of 25mW, at a resolution bandwidth of 30kHz.

Figure 10: Laser pulse observed on a CRO. The upper trace is the TTL pulse reference. The lower trace is the output from the detector.

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Other research projects for Opto-Electronics during 2011:

 Multiplexed, high-speed quantum communication over a quantum channel
 Robust Nonlinear Estimation for a Fabry-Perot Optical Cavity