Glossary entry

Wall-plug efficiency

The ratio of optical output power to electrical input power at a specified operating point, capturing total electrical-to-optical conversion efficiency.

Wall-plug efficiency (WPE), also called power conversion efficiency, is defined as

\eta_\text{WPE} \;=\; \frac{P_\text{optical}}{P_\text{electrical}} \;=\; \frac{P_\text{out}}{I \cdot V},

where $P_\text{out}$ is the emitted optical power, $I$ is the drive current, and $V$ is the voltage across the device terminals.

WPE captures the total conversion of electrical input to useful optical output at a single operating point, including contributions from threshold current (which produces no above-threshold output but consumes power), differential quantum efficiency in the lasing region, and series resistance losses in the device.

WPE varies with drive current: at threshold it is zero (no output), then rises with increasing current as more electrons contribute to stimulated emission, then falls at high currents due to self-heating and series-resistance losses. Maximum WPE typically occurs at $I \sim 2{-}5 \, I_\text{th}$ .

Typical peak WPE for well-designed devices:

Device	Wavelength	Peak WPE
Single-mode telecom DFB	1310 / 1550 nm	5–15%
High-power 9xx nm pump	940–980 nm	50–70%
VCSEL	850 nm	30–50%
Edge-emitting semiconductor (laboratory)	various	10–40%
Best-in-class research devices (9xx nm)	940–980 nm	$>$ 70%

WPE is distinct from slope efficiency and differential quantum efficiency, which describe only the marginal conversion above threshold and do not capture the threshold-current and voltage-drop contributions.