Noncoherent

Noncoherent describes the absence of a fixed phase relationship between waves, oscillations, or signals. In science and engineering, the term is used for sources, processes, or receivers that do not rely on phase information to operate or to convey information.

In optics, noncoherent light has random phase relationships over time or space, giving it a short coherence length and coherence time. Incandescent lamps, LEDs, and other broadband or thermal sources typically emit noncoherent light. Noncoherent detection and imaging rely on intensity measurements rather than measuring phase, often using direct detection with a photodetector and no local oscillator. This is in contrast to coherent light, such as that produced by lasers, which can produce stable interference and carry phase information over longer distances.

In communications and signal processing, noncoherent refers to receivers or modulation methods that do not estimate or track the carrier phase. Noncoherent receivers use envelope or energy detection, offering simpler and more robust operation in the presence of phase noise or movement, but usually at the cost of lower spectral efficiency or higher error rates for a given signal-to-noise ratio. Common examples include on-off keying (OOK) and other noncoherent modulation schemes used in optical and RF links.

Noncoherent techniques also appear in radar and imaging contexts where phase information is difficult to obtain or is not required, such as certain forms of direct-detection radar or speckle imaging. The choice between coherent and noncoherent approaches depends on trade-offs among sensitivity, receiver complexity, bandwidth, and the role of phase information in performance.