Glossary entry

Relative intensity noise (RIN)

The power spectral density of relative optical power fluctuations of a laser source, normalized to the average optical power. Units are 1/Hz, conventionally expressed in dB/Hz.

Relative intensity noise (RIN) characterizes the random fluctuations of a laser's output power, normalized so that the result is independent of absolute power level:

\text{RIN}(f) \;=\; \frac{S_{\delta P}(f)}{\langle P \rangle^2},

where $\langle P \rangle$ is the average optical power and $S_{\delta P}(f)$ is the one-sided power spectral density of the power fluctuation $\delta P(t) = P(t) - \langle P \rangle$ . Units are 1/Hz; values are conventionally reported in dB/Hz as $10 \log_{10}(\text{RIN})$ .

The RIN spectrum of a typical semiconductor laser shows characteristic frequency-dependent structure:

Frequency range	Behavior
$< 100$ kHz	Elevated $1/f$ region (carrier and current noise)
100 kHz to relaxation oscillation	Approximately flat floor
At relaxation oscillation frequency	Peak (carrier-photon resonance)
Above relaxation oscillation	Roll-off $\sim f^{-4}$

Typical floor RIN values:

Source	Wavelength	Floor RIN
DFB telecom laser	1310 / 1550 nm	$-150$ to $-160$ dB/Hz
Fabry–Pérot multi-mode laser	1310 nm	$-130$ to $-145$ dB/Hz
VCSEL	850 nm	$-130$ to $-145$ dB/Hz
Solid-state Nd:YAG	1064 nm	$-150$ to $-170$ dB/Hz

The shot-noise limit at detected DC photocurrent $I_\text{dc}$ corresponds to a RIN floor of $\text{RIN}_\text{shot} = 2 q / I_\text{dc}$ . For $I_\text{dc} = 1$ mA, this is $-155$ dB/Hz; for higher detected currents the shot-noise floor decreases. Laser RIN is bounded below by the shot-noise limit for the chosen detection power.

RIN matters for analog optical links (where it sets the SNR floor independent of received power), for coherent communication (where it contributes to phase noise via amplitude-to-phase coupling), and for any system using lasers as a precision reference. Measurement procedure is in Relative Intensity Noise Measurement of Semiconductor Lasers.