LIV kink

An ideal laser diode's L–I curve above threshold is a straight line whose slope is the slope efficiency. A kink is a visible departure (a step, a slope change, or a stutter), and it is never cosmetic: something about the lasing state reorganized at that current.

The usual culprits, in rough order of frequency:

Longitudinal mode hop. The lasing mode jumps to an adjacent cavity mode as gain peak and comb drift (thermally) at different rates. Output power steps slightly because the new mode couples differently; the wavelength simultaneously jumps by one mode spacing. Fabry–Pérot lasers do this chronically; DFBs are designed not to.

Lateral-mode change / beam steering. At high injection, spatial hole burning and thermal lensing reshape the lateral gain and index profiles; the fundamental lateral mode deforms or a first-order mode reaches threshold. In broad-area and some ridge lasers this appears as the beam steering sideways: the far field shifts while total power barely blinks. Any application with an aperture downstream (fiber, slit) experiences it as a coupled-power discontinuity.

Polarization or filamentation events in some structures; COD precursors at the aggressive end. A kink followed by irreversible slope loss is facet damage announcing itself.

Detection benefits from differentiation: kinks that hide in the raw L–I stand out sharply in $dL/dI$ versus $I$ (and companion structure appears in $dV/dI$). Production test extracts a kink-free power or maximum rated current with margin below the first event, since single-mode behavior (spectral or spatial) is unguaranteed beyond it.

During bring-up, distinguish true kinks from measurement artifacts: integrating-sphere reflections, temperature drift during slow sweeps, or current-source range changes all counterfeit slope structure. A kink that moves with heatsink temperature is modal; one that doesn't move is often the setup.