Detection & noise

Photodetectors and the noise floors beneath every measurement: responsivity, NEP, shot and thermal noise, receiver architectures.

3 articles · 29 glossary terms

Articles

Sep 7, 2026Reference · Optical networking
What the Coherent DSP Actually Does
A function-by-function account of the digital signal processing behind coherent optical links: chromatic dispersion compensation as a static filter, adaptive polarization demultiplexing, carrier frequency and phase recovery, timing, and FEC, and why shedding parts of this chain is what coherent-lite trades for power.
May 20, 2026Procedure · Detector characterization
Photodetector Characterization: Responsivity, NEP, and Bandwidth
Procedures for measuring the three primary parameters of photodetectors used in photonics (wavelength-dependent responsivity, noise-equivalent power, and 3-dB electrical bandwidth), with focus on the standard measurement configurations for InGaAs PIN photodiodes.
May 11, 2026Procedure · Laser characterization
Relative Intensity Noise Measurement of Semiconductor Lasers
Procedure for measuring relative intensity noise (RIN) of a semiconductor laser using direct detection and an electronic spectrum analyzer, including shot-noise calibration, detector and electronic noise subtraction, and common measurement errors.

Glossary

Avalanche photodiode (APD): A photodetector with internal carrier-multiplication gain via impact ionization in a high-field region. Provides higher sensitivity than PIN at the cost of multiplication noise and bandwidth-gain tradeoff.
Balanced detection: A photodetection scheme using two photodiodes with the difference of their photocurrents taken as the output. Cancels common-mode signals and noise, doubling sensitivity for coherent detection.
Burst-mode receiver: An optical receiver designed to recover data from sequential bursts of optical signal from different transmitters, with each burst arriving at unpredictable amplitude and phase. The standard receiver for passive optical networks (PON).
Carrier lifetime: The average time a free electron or hole exists in a semiconductor before recombining. Sets the speed of carrier-mediated devices (lasers, photodetectors, modulators) and the steady-state carrier population at given injection.
Coherent detection: An optical receiver architecture that recovers both amplitude and phase of the optical signal by interfering it with a local oscillator laser. Enables higher-order modulation formats and digital impairment compensation.
Coherent DSP: The digital signal processor that makes coherent optical detection practical: it compensates chromatic dispersion, separates the two polarizations, tracks the laser frequency and phase, recovers timing, and decodes the forward error correction, all in the electronic domain after the optical front end.
Dark current (I_d): The current flowing through a photodetector in the absence of any incident light. Sets the noise floor and minimum detectable signal.
Direct detection: An optical receiver architecture in which the photodetector responds to the optical power (intensity) of the received signal. The simplest and most widely deployed detection scheme in optical communication.
FMCW lidar: Coherent ranging with a chirped continuous-wave laser: distance comes from the beat frequency, velocity from the Doppler shift, and sunlight and other lidars are rejected by the coherent gate.
Germanium-on-silicon photodetector: A photodiode using a germanium absorber layer epitaxially grown on a silicon substrate, integrated with silicon photonic waveguides. The standard receiver element for silicon photonic integrated circuits at telecom wavelengths.
Heterodyne detection: A coherent detection technique in which the local oscillator is offset in frequency from the signal, producing a beat at an intermediate frequency (IF) for subsequent electronic demodulation.
Ideality factor: A dimensionless parameter $n$ in the diode equation that quantifies the deviation of a real p-n junction's I-V relationship from the ideal thermal-emission limit. Reveals the dominant recombination mechanism in the junction.
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.
Noise figure (NF): The factor by which an optical amplifier degrades the input signal-to-noise ratio, in dB. The figure of merit for amplifier quality.
Optical hybrid (90° hybrid): A four-port (2×4) coupler that combines a signal and a local oscillator with relative phase shifts of 0°, 90°, 180°, 270° at its outputs. The standard front-end element of coherent optical receivers.
Phase noise and frequency noise: The spectral-density description of an oscillator's phase fluctuations. It is the complete characterization of laser coherence; a single linewidth number is a lossy summary of it.
Photodiode: A semiconductor diode designed to convert incident photons into electrical current via the photoelectric effect. The fundamental optical receiver element.
PIN photodiode: A photodetector with an undoped (intrinsic) absorber region sandwiched between p-type and n-type contacts. The standard linear photodetector for optical communication and measurement.
Quantum efficiency: The probability that an absorbed or generated photon successfully produces a measurable carrier (or vice versa). The fundamental efficiency parameter for any photonic device.
Quantum photonic integrated circuit (QPIC): A photonic chip that generates, manipulates, and detects quantum states of light: pair sources in ring resonators, programmable interferometer meshes, and single-photon detectors on one platform.
Receiver sensitivity: The minimum optical power at which a receiver holds the target error ratio, quoted either as average power or as OMA. The receiving end of every link budget: transmitter power minus channel loss must land above it, with margin.
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.
Responsivity (R): The ratio of photocurrent generated by a photodetector to incident optical power. Measured in A/W; sets the fundamental signal level for any optical receiver.
Shot noise: The fundamental noise floor of photodetection caused by the discrete nature of photons (or electrons). The Poisson-statistics fluctuation that sets the ultimate sensitivity limit of any optical measurement.
Shunt resistance: Parallel resistance across a diode that provides a leakage current path bypassing the junction. Sets the dark current floor of photodetectors and the leakage floor of laser diodes.
Signal-to-noise ratio (SNR): The ratio of signal power to noise power, characterizing the quality of a measurement or communication channel. Determines achievable bit error rate, dynamic range, and detection sensitivity.
Superconducting nanowire single-photon detector (SNSPD): A current-biased superconducting nanowire that clicks on single photons with efficiency beyond 90%, jitter below 20 ps, and dark counts below 1 Hz, at the cost of a cryostat.
Thermal noise: The Johnson-Nyquist noise arising from the random thermal motion of charge carriers in any resistive element. Sets the noise floor of the photodetector's electrical readout circuit.
Uni-traveling-carrier photodiode (UTC-PD): A photodiode in which only electrons cross the depleted collector, while holes are absorbed as majority carriers in a doped layer and never travel at all. The design removes the hole-transport bottleneck of PIN photodiodes, giving detectors that stay linear at high photocurrent with bandwidths beyond 300 GHz.