Fiber & telecom

Optical fiber, links, and networks: propagation effects, amplification, WDM, modulation formats, and the components that connect it all.

2 articles · 82 glossary terms · 1 tool · 1 material

Tools

Optical alignment

Fiber Coupling Efficiency Calculator

Estimate Gaussian beam to single-mode fiber coupling efficiency from beam waist, fiber MFD, and alignment offsets.

Articles

Jul 26, 2026Reference · Optical networking
WDM Mux/Demux Technologies: Thin-Film Filters vs AWG vs FBG
The three ways to split wavelengths: cascaded thin-film filters, arrayed waveguide gratings, and fiber Bragg gratings with circulators. Compared on loss, channel count, isolation, temperature behavior, and cost.
Jul 26, 2026Reference · Optical networking
Optical Amplifiers Compared: SOA vs EDFA vs Raman
The three amplifier technologies compared on the numbers that decide between them: gain, noise figure, saturation power, bandwidth, and dynamics. With the use cases where each is the only sensible answer.

Glossary

Acceptance angle: The maximum incidence angle at which light can enter an optical fiber or waveguide and still be guided. Geometrically related to the numerical aperture.
Amplified spontaneous emission (ASE): Broadband incoherent optical power produced by spontaneous emission events that are subsequently amplified by stimulated emission in an optical amplifier. The fundamental noise source in optical amplifiers.
Apodization: Smooth variation of a periodic structure's strength along its length to control the spectral response. The standard technique for sidelobe suppression in gratings, filters, and resonators.
Arrayed waveguide grating (AWG): The integrated-optic wavelength (de)multiplexer: two star couplers joined by a waveguide array with constant length increments, imaging each wavelength onto a different output port. The standard mux/demux of DWDM systems.
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.
Bend loss: Optical attenuation caused by physical bending of a fiber or waveguide. Light radiates away when the bend curvature is sharp enough that the guided mode can no longer follow the bend trajectory.
Birefringence: The optical property of a material whose refractive index depends on the polarization of the propagating light. Produces polarization mode splitting in fibers and waveguides.
Bit error rate (BER): The fraction of received bits that are decoded incorrectly. The fundamental performance metric of any digital optical communication link.
Brillouin scattering: Inelastic scattering of light from acoustic phonons or material density waves, producing a small frequency shift (10s of GHz). Used in distributed strain sensing and a power limit in narrow-linewidth fiber transmission.
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).
Chromatic dispersion: The wavelength dependence of the group velocity in an optical medium. Causes pulse broadening and frequency-dependent phase shift in optical communication.
Co-packaged optics (CPO): An architecture where optical transceivers are placed directly on the same package as a networking ASIC, eliminating long electrical traces between switch chip and optics. Proposed solution for next-generation high-bandwidth interconnect power scaling.
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.
Cross-phase modulation (XPM): Nonlinear phase shift induced on one optical wave by the intensity of a different co-propagating wave through the Kerr effect. A primary source of inter-channel impairment in WDM transmission.
Cutoff wavelength: The wavelength below which a fiber or waveguide supports more than one guided mode. Sets the single-mode operating range for any guiding structure and is the central design parameter for single-mode fibers.
CWDM vs DWDM: The two wavelength-multiplexing grids: CWDM spaces channels 20 nm apart so lasers can run uncooled; DWDM packs channels at 100/50 GHz (or tighter) in the C-band where amplifiers and coherent systems live.
Decibel (dB, dBm): A logarithmic unit for expressing ratios of power, with dBm specifically denoting power referenced to 1 milliwatt. The standard unit for optical power and loss in photonics and telecommunications.
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.
Dispersion compensation: Undoing accumulated chromatic dispersion so pulses arrive intact. Historically done with negative-dispersion fiber or chirped gratings; now handled almost entirely in DSP at coherent receivers.
Dispersion-shifted fiber (DSF): A single-mode fiber whose zero-dispersion wavelength has been shifted from the natural 1310 nm of standard SMF to approximately 1550 nm. Designed to operate at minimum dispersion in the EDFA-amplified C-band.
Effective area (A_eff): An equivalent transverse cross-sectional area of a guided optical mode, used to compute nonlinear interaction strength in fibers and waveguides. Differs from physical core area for non-uniform modes.
Electro-absorption modulator (EAM): An optical intensity modulator that uses voltage-controlled absorption in a semiconductor (typically via the quantum-confined Stark effect) to attenuate transmitted light. Compact and used in low-power telecom transmitters.
Erbium-doped fiber amplifier (EDFA): An optical amplifier using erbium-doped silica fiber pumped at 980 nm or 1480 nm. The dominant amplifier technology in the telecom C and L bands.
Extinction ratio (ER): The ratio of the maximum to minimum transmitted optical power of a device or signal, in dB. Quantifies on/off contrast for modulators, filters, and switches.
Eye diagram: The overlay of many consecutive bit periods of a received optical signal, displayed on a fast sampling oscilloscope. Visualizes signal quality, jitter, and noise simultaneously.
Fiber Bragg grating (FBG): A periodic refractive-index modulation written into a fiber core, reflecting a narrow band around the Bragg wavelength and transmitting the rest. The fiber-native filter, sensor, and laser mirror.
Fiber connectors: PC, UPC, APC: The three polish geometries of mated fiber connectors. PC and UPC polish the end-face flat-convex to increasing surface quality; APC angles it 8° so residual reflections leave the core. Polish sets return loss; the connector body (LC, SC, FC) is a separate choice.
Forward error correction (FEC): Channel coding that adds redundancy to transmitted data so the receiver can detect and correct errors without retransmission. Universal in modern optical communication; effectively part of every modern coherent transceiver.
Four-wave mixing (FWM): A third-order nonlinear process in which three input optical waves generate a fourth wave at a new frequency. The basis of wavelength conversion, parametric amplification, and a source of inter-channel crosstalk in WDM systems.
Free-space optics (FSO): Optical communication or measurement using light propagating through atmosphere or vacuum rather than guided through a waveguide. Encompasses optical wireless links, deep-space communications, lidar, and many laboratory measurement systems.
Frequency chirp: The shift of a laser's instantaneous optical frequency during modulation, caused by carrier-density–dependent refractive index. Chirp interacting with fiber dispersion is the classic reach limit of directly modulated links.
Fusion splicing: Joining two fibers permanently by aligning their ends and melting them together with an electric arc. The lowest-loss, lowest-reflection fiber joint, and the default for any connection that never needs to be undone.
Graded-index fiber: An optical fiber whose core refractive index varies smoothly with radial position, typically following a near-parabolic profile. Equalizes the group velocity of different modes, dramatically increasing the bandwidth-distance product of multimode fiber.
Group velocity dispersion (GVD): The wavelength dependence of group velocity that causes optical pulses to broaden in time as they propagate through a dispersive medium. The fundamental mechanism underlying chromatic dispersion in fibers.
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.
Insertion loss (IL): The optical power lost by inserting a component into an otherwise-lossless signal path, in dB. Positive value indicates power reduction.
Kerr effect: An intensity-dependent change in refractive index in an optical medium. The third-order nonlinear optical effect underlying self-phase modulation, four-wave mixing, and ultrafast optical switching.
Linear optical sampling (LOS): An optical measurement technique that samples a high-speed optical waveform by coherent beating with a short-pulse local oscillator, producing time-resolved waveforms at sampling rates well below the signal bandwidth. The standard technique for visualizing waveforms above 100 GHz.
Link budget: The accounting of optical power, gain, and loss across a transmission link, used to verify that the received signal exceeds the receiver sensitivity with sufficient margin. The basic design tool for any point-to-point optical link.
Mach–Zehnder modulator (MZM): An optical intensity modulator based on a Mach–Zehnder interferometer with electro-optic phase shifters in one or both arms. The dominant external modulator in telecom and PIC applications.
Macrobend loss: Optical loss caused by deliberate, controlled bending of a fiber at radii of millimeters or more. The dominant bend loss mechanism in fiber routing, splicing trays, and equipment cabling.
Microbend loss: Optical loss from microscopic random perturbations along the fiber axis, typically caused by external pressure or thermal expansion mismatch. Distinct from controlled macrobends; sets a fundamental cable manufacturing constraint.
Modal dispersion: Pulse broadening in multimode fiber caused by different propagation speeds of different guided modes. The dominant bandwidth-distance limit in multimode fiber transmission.
Mode field diameter (MFD): The full-width diameter at which the optical field amplitude in a single-mode fiber falls to 1/e of its peak value, equivalently the diameter at which intensity falls to 1/e². The standard parameter for fiber coupling calculations.
Mode scrambler: A device that perturbs an optical fiber to redistribute light among the fiber's guided modes, achieving a uniform mode-power distribution. The standard fiber test equipment for ensuring repeatable multimode fiber measurements.
Modulation bandwidth: The frequency at which a device's small-signal response to modulation falls 3 dB below its low-frequency value. For directly modulated lasers it is set by the relaxation-oscillation resonance; for detectors and modulators, by transit time and RC limits.
Modulation format: The scheme by which information is encoded onto an optical carrier. Classified by which optical-field parameters (amplitude, phase, polarization) carry the data and how many distinct symbol states are used.
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.
Non-zero dispersion-shifted fiber (NZ-DSF): A single-mode fiber with small but non-zero chromatic dispersion in the EDFA-amplified C-band (typically 4 – 8 ps/(nm·km) at 1550 nm). The dominant fiber type for terrestrial long-haul WDM transmission.
NRZ (Non-Return-to-Zero): A binary modulation format where the optical intensity holds at one of two levels (logical 0 or 1) for the entire symbol period. The simplest and historically dominant format in optical communications.
Numerical aperture (NA): The sine of the maximum acceptance half-angle for light entering an optical fiber, lens, or waveguide. Quantifies light-gathering or focusing capability.
Optical amplifier: A device that amplifies an optical signal directly in the optical domain, without conversion to electrical. The three principal types are erbium-doped fiber amplifiers (EDFAs), semiconductor optical amplifiers (SOAs), and Raman amplifiers.
Optical circulator: A three- or four-port non-reciprocal device that routes light sequentially: port 1 → port 2, port 2 → port 3, port 3 → port 4 (or back to port 1 for three-port). Used in bidirectional optical systems and Bragg grating filters.
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.
Optical isolator: A non-reciprocal optical device that transmits light in one direction and blocks light in the reverse direction. Essential for protecting lasers from back-reflections that destabilize their operation.
Optical return loss: The ratio of power launched into a component or link to the power reflected back, in dB. Larger is better. This budget keeps reflections from destabilizing lasers and corrupting signals.
Optical signal-to-noise ratio (OSNR): The ratio of optical signal power to optical noise power in a specified reference bandwidth, in dB. Standard performance metric for optical communication links.
Optical soliton: A pulse that propagates without changing shape because nonlinear self-phase modulation exactly balances chromatic dispersion. The natural pulse shape for ultra-long-haul transmission and microresonator frequency combs.
Optical time-domain reflectometry (OTDR): A measurement technique that maps optical loss along a fiber by sending pulses and recording the time-resolved Rayleigh backscatter. The standard field instrument for fiber installation, splice characterization, and fault location.
Optical transceiver / transponder: A module that combines an optical transmitter and receiver in a single pluggable form-factor. A transceiver has electrical signal interfaces; a transponder additionally performs forward error correction and digital signal processing.
PAM4: Four-level pulse amplitude modulation, encoding 2 bits per symbol. The dominant modulation format for 50G+/lane data center optical interconnect, doubling bit rate per Hz of optical bandwidth vs NRZ.
Passive optical network (PON): The fiber-access architecture in which one head-end transceiver serves dozens of subscribers through a purely passive optical splitter. No powered equipment sits between office and user. The physical layer of most FTTH.
Pluggable optics: Optical transceiver modules in standardized, hot-swappable form factors (SFP, QSFP, OSFP, etc.) that plug into networking equipment via a defined electrical interface. The architectural standard for all current datacom and most telecom optical interfaces.
Polarization extinction ratio (PER): The power ratio between the principal polarization axis and the orthogonal axis of a polarized light source or polarization-maintaining fiber, in dB. Quantifies polarization purity.
Polarization mode dispersion (PMD): Pulse broadening arising from random birefringence variations along a fiber, which produce a polarization-dependent group delay. A secondary but cumulatively significant limit on long-haul transmission.
Polarization-dependent loss (PDL): The maximum-to-minimum optical loss of a device measured across all input polarization states. A standard specification for optical components and a system impairment in long links.
Polarization-maintaining fiber (PMF): An optical fiber that preserves the linear polarization state of light launched along one of two principal axes. Used where polarization stability is essential.
QAM (Quadrature Amplitude Modulation): A coherent modulation format encoding data in both amplitude and phase of the optical field. Higher-order variants (16-QAM, 64-QAM, 256-QAM) carry 4, 6, or 8 bits per symbol and enable highest-capacity optical transmission.
Raman scattering: Inelastic scattering of light by optical phonons or molecular vibrations, producing frequency-shifted output. Used for spectroscopy, Raman amplification, and distributed sensing.
Rayleigh scattering: Elastic scattering of light by particles much smaller than the wavelength. Produces the fundamental attenuation floor of optical fibers and the dominant backscatter in OTDR measurement.
Saturation power (P_sat): The optical power at which the gain of an amplifier drops to half its small-signal value. The fundamental scaling parameter for amplifier output power.
Self-phase modulation (SPM): An intensity-dependent phase shift acquired by an optical wave propagating in a Kerr nonlinear medium. The dominant intra-channel nonlinear effect in optical fiber communication.
Semiconductor optical amplifier (SOA): An optical amplifier based on a semiconductor gain medium driven by injection current. Provides broad gain bandwidth in a compact form factor; used for amplification, switching, and wavelength conversion.
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.
Single-mode vs multimode fiber: The fundamental classification of optical fiber: single-mode supports only the LP01 fundamental mode, multimode supports many guided modes. The distinction drives core size, application class, source compatibility, and cost.
Step-index fiber: An optical fiber with uniform high refractive index in the core and a sharp transition to uniform lower refractive index in the cladding. The simplest fiber profile and the standard for single-mode telecom fiber.
Telecom wavelength bands: The six standardized wavelength bands used in optical telecommunications: O, E, S, C, L, and U. Each is named for its spectral position and defined by ITU-T G.692.
Tunable laser: A laser whose output wavelength can be deliberately and continuously varied across a range, by mechanical, thermal, or electrical control. The standard signal source for WDM testing, coherent communications, and broadband spectroscopy.
V number: Dimensionless parameter determining the number of guided modes in a step-index optical fiber. Single-mode operation requires V < 2.405.
Variable optical attenuator (VOA): A device that introduces a controllable, wavelength-flat optical loss into a signal path. Used to set absolute power levels, equalize channels in WDM systems, and protect downstream optics.
Wavelength division multiplexing (WDM): Carrying multiple independent data channels on a single optical fiber by assigning each channel a different wavelength. The foundational architecture of long-haul and metro optical networks.
Zero-dispersion wavelength: The wavelength at which a fiber's chromatic dispersion crosses zero, near 1310 nm in standard single-mode fiber. Pulse spreading is minimal there. Unfortunately, four-wave mixing is maximal there too.

Materials

Fused silicaSiO₂

The quiet foundation of all of photonics: the fiber that spans oceans at 0.2 dB/km, the cladding on every integrated platform, and the substrate under most free-space optics.