Spatial hole burning

Spatial hole burning (SHB) is gain saturation with geography. A lasing mode does not fill the cavity uniformly (it has nodes, antinodes, and an envelope), so it burns carriers down hardest where its intensity is highest. The result is a gain landscape with "holes" at the mode's maxima and surplus gain wherever that mode is weak. Any competing mode whose intensity pattern overlaps the surplus sees more gain than the lasing mode itself. The laser's single-mode discipline starts to erode.

Three length scales matter, and they behave differently:

Standing-wave scale (~λ/2n ≈ 230 nm at 1550 nm). The interference pattern of forward and backward waves would burn a sub-micron carrier grating, but carrier diffusion (diffusion lengths of ~1–2 µm in III–V active regions) largely washes this fine structure out in conventional edge emitters. This is why the textbook standing-wave argument, taken alone, overstates multimode behavior in FP lasers. (In VCSELs, with gain segmented into quantum wells pinned at particular standing-wave positions, the axial pattern matters more.)

Cavity-envelope scale (longitudinal SHB). Diffusion cannot flatten carrier variations over hundreds of microns. In a DFB laser, the grating concentrates optical power (for a high-κL design, near the cavity center), so the center saturates deeply while the ends retain gain. The resulting non-uniform carrier density perturbs the effective index along the cavity (through the same carrier–index coupling quantified by the linewidth enhancement factor), which detunes the grating locally, reshapes the mode, and erodes the margin to the side modes. Longitudinal SHB is a primary reason DFB single-mode yield and SMSR degrade at high power, and a classic source of kinks: the mode profile reorganizes at some current, the output slope steps, and the wavelength may jump. Design countermeasures are largely exercises in SHB control: moderate κL (~1–1.5), quarter-wave-shifted gratings with intentional profile management, chirped or sampled gratings.

Lateral scale (lateral SHB). Across the stripe of a ridge or broad-area laser, the fundamental lateral mode is brightest at the center, so the center depletes first. The carrier profile that remains (higher at the edges) both raises the gain seen by higher-order lateral modes and, via the carrier-dependent index, builds a lensing profile that reshapes the modes themselves. The observable consequences arrive in sequence as current rises: far-field broadening, beam steering, higher-order-mode onset, and in broad-area devices filamentation. Kink-free power specifications are, to first order, lateral-SHB specifications.

SHB is distinct from spectral hole burning (depletion of carriers at a particular energy within the gain spectrum, relevant on ultrafast timescales). The two share initials and the general logic of localized saturation, but not physics or time scales.

Experimentally, SHB announces itself as the correlated signature behind several separate measurements: an L–I kink, a simultaneous far-field change, an SMSR dip, or a wavelength step. One underlying carrier redistribution produces several symptoms. The LIV kink entry covers the diagnostic side.