Glossary entry

Slope efficiency

The local derivative of optical output power with respect to drive current in the lasing region of a laser diode LIV curve. Units are W/A or mW/mA.

Slope efficiency $\eta_s$ is defined as

\eta_s \;=\; \left. \frac{d P_\text{out}}{d I} \right|_{I > I_\text{th}},

with units of W/A or mW/mA. It is determined by least-squares linear regression in the lasing region of the LIV curve (typically $I \geq 1.2 \, I_\text{th}$ , below the onset of thermal rollover).

Slope efficiency depends on collection geometry. Single-facet measurements report the power emitted from one facet of the device; integrating-sphere measurements report the total emitted power. For symmetric uncoated Fabry–Pérot devices, the front-facet value is approximately half the total. For HR/AR-coated devices, the front-facet value is the asymmetric majority.

Typical single-facet values for uncoated edge-emitters:

System	Wavelength	$\eta_s$
InGaAsP / InP	1310 / 1550 nm	0.1–0.3 W/A
InGaAs / GaAs	980 nm	0.4–0.8 W/A
AlGaAs / GaAs	808 nm	0.3–0.7 W/A
GaN / InGaN	405 nm	0.5–1.5 W/A

Conversion to dimensionless differential quantum efficiency requires multiplication by $q \lambda / (h c)$ . Extraction methodology is covered in Slope Efficiency and Differential Quantum Efficiency from LIV Measurements.