Detection of coherent light in an incoherent background [for IRST]

Detection of a laser emission in daylight clutter in infrared search and track systems is usually carried out by measuring the energy from the scene in the instrument field of view. When the emitter is at a considerable distance or the reflected sunlight is intense, the target cannot be detected by this method. This paper describes a more sensitive method based on detecting the coherence properties of the incident radiation. The system uses both optical and digital signal processing. The optical section is an interferometer to measure the self-coherence function of the incident radiation to give an interferogram, and uses a narrow band filter to produce the sine envelope. The photodetector only detects power, which is the square of the modulus of γ. So the region between the main lobe and the first sidelobe of the envelope appears as a minimum rather than as a zero crossing where the envelope changes sign. The enclosed carrier undergoes a π phase shift at this point, which we use to define the point more accurately. The post detector section comprises signal amplification with a low pass filter, A/D conversion and a demodulation MATLAB algorithm, giving phase and instantaneous frequency signals as a function of optical path difference