Glossary entry

Noise equivalent power (NEP)

The optical input power that produces a signal equal to the detector's noise in a 1 Hz bandwidth. The standard figure of merit for photodetector sensitivity.

NEP is the input optical power that produces a photocurrent equal to the detector's noise current in a 1 Hz electrical bandwidth:

\text{NEP} \;=\; \frac{\sigma_I}{\mathcal{R}} \quad [\text{W} / \sqrt{\text{Hz}}],

where $\sigma_I$ is the RMS noise current per $\sqrt{\text{Hz}}$ and $\mathcal{R}$ is the responsivity.

Total noise current sums contributions from shot noise on dark current and the amplifier:

\sigma_I^2 \;=\; 2 e I_d \;+\; \frac{4 k T}{R_L} \;+\; i_{n,\text{amp}}^2 \;+\; M^2 F(M) \cdot 2 e (I_{ph} + I_d^{*}) \quad [\text{A}^2/\text{Hz}],

where $I_d$ is dark current, $R_L$ is the load (or transimpedance) resistance, $i_{n,\text{amp}}$ is the amplifier input-referred current noise, $M$ is APD gain ( $M = 1$ for PIN), $F(M)$ is the APD excess noise factor, and $I_d^*$ is the multiplied portion of the dark current.

At low light levels, NEP is dominated by dark current and amplifier noise (independent of signal). At high light levels, signal shot noise dominates and NEP is meaningless — the signal is well above noise by definition.

Typical NEP values at 1550 nm:

Detector	NEP
InGaAs PIN, 50 μm, transimpedance amp	$1 - 5 \times 10^{-14}$ W/ $\sqrt{\text{Hz}}$
InGaAs PIN, 1 mm, large-area, low-noise amp	$0.5 - 2 \times 10^{-13}$ W/ $\sqrt{\text{Hz}}$
InGaAs APD, $M = 10$	$5 - 20 \times 10^{-15}$ W/ $\sqrt{\text{Hz}}$
Single-photon InGaAs APD (Geiger mode)	quantum-limited; quoted as dark count rate instead
Cooled superconducting nanowire (SNSPD)	$< 10^{-18}$ W/ $\sqrt{\text{Hz}}$ effective
Si PIN, large-area, $-30$ °C	$1 - 10 \times 10^{-15}$ W/ $\sqrt{\text{Hz}}$

NEP scales with $\sqrt{B}$ for a measurement bandwidth $B$ : minimum detectable power $P_\text{min} = \text{NEP} \cdot \sqrt{B}$ . A receiver with $10^{-13}$ W/ $\sqrt{\text{Hz}}$ at 10 GHz bandwidth has minimum detectable power $\approx 10^{-8}$ W = $-50$ dBm.

For telecom receivers, the relevant figure is usually input-referred sensitivity at a target bit error rate — typically $-20$ to $-30$ dBm for direct-detection 10–25 Gb/s — rather than NEP directly. NEP remains the standard metric for instrumentation photodetectors (lock-in amplifiers, FTIR spectrometers, low-light imaging).