lowcoherence

Low coherence is a term used to describe light, waves, or signals that exhibit a low degree of temporal or spatial coherence. Coherence describes the ability of a wave field to maintain a stable phase relationship with itself over time or space. Low coherence results from rapid phase variations or a broad spectrum, yielding short coherence length or time and reducing interference visibility.

In optics, the coherence length l_c roughly equals c divided by the spectral bandwidth Δν (or l_c ≈ λ^2 / Δλ for fractional bandwidth). A broad spectrum light source, such as white light or a supercontinuum, has a short coherence length, and interference is observable only for path differences smaller than l_c. By contrast, lasers with narrow linewidths can have long coherence lengths, enabling high-contrast fringes over larger path differences.

Causes include multiple independent emitters, rapid phase fluctuations, or deliberately broad spectral content. In practice, low coherence is advantageous for depth-resolved interferometry because only nearby optical paths interfere; this underpins low-coherence or white-light interferometry and optical coherence tomography.

Quantification relies on the degree of coherence g12 and fringe visibility. Temporal coherence relates to spectral content; spatial coherence relates to source size and geometry.

Beyond optics, coherence is used in signal analysis to describe the correlation between signals. Low coherence in this context indicates weak phase or frequency relationship, aiding separation of signals in methods such as coherence-based filtering or blind source separation.