Glossary entry

Differential quantum efficiency

The dimensionless ratio of photons emitted per electron injected above threshold in a semiconductor laser. Bounded by unity for single-facet collection.

External differential quantum efficiency $\eta_d$ is defined as the ratio of incremental photons emitted to incremental electrons injected above the threshold current:

\eta_d \;=\; \frac{(P_\text{out} - P_\text{out}(I_\text{th})) / h\nu}{(I - I_\text{th}) / q} \;=\; \frac{q \lambda}{h c} \, \eta_s,

where $\eta_s$ is the slope efficiency, $q$ is the electron charge, $\lambda$ is the emission wavelength, $h$ is Planck's constant, and $c$ is the speed of light.

The conversion factor $q \lambda / (h c)$ at common wavelengths:

Wavelength	$q \lambda / (h c)$
850 nm	0.686 A/W
980 nm	0.790 A/W
1064 nm	0.858 A/W
1310 nm	1.057 A/W
1550 nm	1.250 A/W

Single-facet $\eta_d$ is bounded by the internal quantum efficiency $\eta_i$ times the mirror loss fraction; for typical uncoated symmetric Fabry–Pérot devices the front-facet $\eta_d$ ranges from $\sim 20$ % to $\sim 50$ %. Two-facet integrating-sphere measurements can exceed 50% and approach $\eta_i$ in low-loss devices.

Internal quantum efficiency $\eta_i$ (the fraction of injected carriers that produce stimulated photons inside the cavity, before facet escape) is extracted from inverse-length measurements over a set of devices with varying cavity lengths and is not derivable from single-device LIV.