Glossary entry

Threshold current

The drive current at which a semiconductor laser transitions from spontaneous to stimulated emission. Below threshold, the device emits as an LED; above threshold, output power scales nearly linearly with current.

The threshold current $I_\text{th}$ marks the operating point at which optical gain in the laser cavity equals total cavity loss. Below threshold, photon density is set by spontaneous emission and the LIV curve exhibits weak superlinear dependence on current. Above threshold, stimulated emission dominates and output power scales nearly linearly with current.

Several extraction methods are in standard use and produce numerically different $I_\text{th}$ values from the same LIV curve:

Two-segment linear fit. Independent linear fits to the LIV above and below threshold; the intersection is $I_\text{th}$ . Most common method for clean LIV data.
Second-derivative peak. $I_\text{th}$ taken at the maximum of $d^2 P / d I^2$ . Mathematically well-defined but sensitive to noise.
First-derivative inflection. $I_\text{th}$ taken at the inflection point of $\ln P$ versus $I$ .
Fixed power threshold. $I_\text{th}$ taken at the current that produces a fixed reference output power. Simplest method; device-dependent.

The extraction method must be held constant across any dataset where $I_\text{th}$ values are compared.

Typical values:

Device	Wavelength	$I_\text{th}$
Telecom DFB laser	1310 / 1550 nm	5–20 mA
9xx nm pump diode (high power)	940–980 nm	100 mA – 1 A
VCSEL	850 nm	0.5–5 mA
Quantum cascade laser	4–10 μm	100 mA – 2 A (pulsed)