Integrated photonics

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• Jul 26, 2026Reference · Materials
  Thermo-Optic Coefficients of Photonic Materials: Reference Table

  dn/dT values for silicon, silicon nitride, silica, InP, GaAs, lithium niobate, and polymers at telecom wavelengths. Includes the resonator-shift formula, athermal design logic, and the caveats that make coefficients composition-dependent.
• Jul 26, 2026Reference · PIC characterization
  Edge Coupler vs Grating Coupler: Choosing Fiber I/O for a Photonic Chip

  The decision every PIC project makes once: edge coupling or grating coupling. Loss, bandwidth, polarization, wafer-level test, packaging, and the hybrid strategy most real programs land on.
• May 15, 2026Procedure · PIC characterization
  Active Fiber Alignment to Edge Couplers on Photonic Integrated Circuits

  Procedure for alignment of optical fiber to edge coupler structures on photonic integrated circuits using lensed fibers and inverse-taper couplers, with discussion of butt-coupling, anti-reflection cleaving, and the distinct failure modes compared to grating coupler alignment.
• May 10, 2026Procedure · PIC characterization
  Loaded and Intrinsic Q Factor Extraction from Ring Resonator Transmission Spectra

  Procedure for extracting loaded and intrinsic quality factors of integrated ring resonators from measured transmission spectra, including fit methodology, the all-pass / add-drop distinction, and resolution of the coupling regime ambiguity.
• May 9, 2026Procedure · PIC characterization
  Waveguide Propagation Loss by the Cutback Method

  Procedure for measuring optical propagation loss in integrated photonic waveguides using the cutback method, including paired-device variants used in foundry process control, fit methodology, and uncertainty analysis.
• May 5, 2026Procedure · PIC characterization
  Active Fiber Alignment to Surface Grating Couplers

  Procedure for obtaining initial optical coupling between a single-mode fiber and a surface grating coupler on a silicon photonic integrated circuit, including equipment requirements, search algorithm, expected losses, and common failure modes.

Glossary

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.

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.

Bragg condition

The wavelength selection rule for a periodic refractive-index structure. Determines the wavelength of constructive backward reflection or out-of-plane diffraction in DFB lasers, fiber Bragg gratings, and grating couplers.

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.

Confinement factor (Γ)

The fraction of the optical mode intensity that overlaps with the active (gain or absorbing) region of a waveguide. Determines modal gain and effective material loss in any guided-wave device.

Cutback method

A measurement technique for waveguide propagation loss in which the transmitted optical power is measured at multiple known waveguide lengths and the loss extracted from the slope of transmission versus length.

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.

DFB laser array

A monolithic photonic integrated circuit containing multiple DFB lasers at different design wavelengths, integrated on a single substrate. The standard implementation of multi-wavelength laser sources for WDM and tunable laser applications.

Die-level test

Characterization of individual photonic chips after wafer dicing but before packaging. Captures parameters that require polished facets or specialized environmental conditions inaccessible at the wafer level.

Directional coupler

A four-port optical device that transfers power between two parallel waveguides via evanescent-field coupling. Used as a power splitter, combiner, and wavelength-selective filter in integrated photonics.

Distributed Bragg reflector (DBR mirror)

A multi-layer dielectric mirror that reflects via constructive interference from many quarter-wave layer interfaces. The high-reflectivity mirror used in VCSELs, DBR lasers, and many fiber and free-space optics.

Edge coupler

A chip-facet structure that expands the sub-micron waveguide mode to approach the mode size of an optical fiber, allowing in-plane coupling at the die edge. The broadband, polarization-tolerant alternative to a grating coupler.

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.

Effective index (n_eff)

The ratio of the free-space propagation constant to the guided-mode propagation constant of a waveguide. Quantifies how slowly the guided phase advances compared to plane-wave propagation in vacuum.

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.

Electro-optic effect (Pockels effect)

A linear change in refractive index produced by an applied electric field in a non-centrosymmetric crystal. The underlying mechanism of fast electro-optic modulators.

Evanescent wave

A non-propagating electromagnetic field that exponentially decays from a boundary, present in the lower-index medium during total internal reflection or outside a guided mode's confinement region. Underlies near-field optics, fiber sensors, and integrated photonic coupling.

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.

Filling factor

The fraction of a periodic structure occupied by one of the two refractive-index regions. For grating couplers, the fraction of the period occupied by the etched groove vs the unetched ridge.

Finesse

The ratio of the free spectral range to the FWHM linewidth of a single resonance. Quantifies resonator sharpness, set by round-trip losses.

Flip-chip bonded laser

A semiconductor laser die mounted onto a carrier substrate with its active region facing the carrier, bonded by solder bumps or conductive epoxy. The standard hybrid-integration method for adding III-V lasers to silicon photonic systems.

Free spectral range (FSR)

The frequency or wavelength spacing between adjacent resonances of an optical cavity. Set by the round-trip group delay.

Free-carrier absorption (FCA)

Absorption of photons by free electrons and holes in a semiconductor, through intraband transitions that conserve energy and momentum via phonon or impurity scattering. The companion loss mechanism to plasma dispersion.

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.

Grating coupler

A periodic structure etched into the surface of a photonic integrated circuit waveguide, used to diffract light between the in-plane waveguide mode and a near-vertical free-space mode for fiber I/O.

Group index (n_g)

The ratio of the speed of light in vacuum to the group velocity of a guided mode. Governs pulse propagation, resonator free spectral range, and dispersion.

Heterogeneous integration

The combination of multiple optical material systems on a single chip, typically by bonding or transfer-printing dissimilar materials onto a common substrate. The standard architecture for combining silicon photonic passives with III-V lasers.

Inverse taper

A waveguide geometry that narrows toward the chip facet to expand the optical mode for fiber coupling. The standard silicon photonic edge coupler design.

Lensed fiber

An optical fiber with a precision micro-lens formed or attached at its tip that focuses the emerging beam to a smaller spot than the fiber mode field. The standard input for coupling between fibers and small-mode chip waveguides or laser facets.

Lithium niobate on insulator (LNOI)

A photonic integration platform consisting of a thin film of lithium niobate bonded to a silicon-oxide-on-silicon substrate. Combines the strong Pockels effect of LiNbO3 with dense waveguide integration.

Mach–Zehnder interferometer (MZI)

A two-arm interferometer that splits light, applies a phase difference between the arms, and recombines them. The most common interferometric building block in integrated photonics.

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.

Modal gain

The effective gain experienced by a guided optical mode, equal to the material gain multiplied by the optical confinement factor. The directly-relevant gain in laser threshold analysis.

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 mismatch loss

Optical power loss arising from imperfect spatial overlap between two propagating modes. Dominates fiber-to-fiber, fiber-to-PIC, and laser-to-fiber coupling in misaligned or geometrically dissimilar systems.

Multimode interference (MMI) coupler

An integrated-photonic power-splitting element that uses self-imaging in a wide multimode waveguide section to distribute light between input and output ports. The standard 1×N and N×N power splitter in modern PIC platforms.

Photonic crystal

A structure whose refractive index repeats with a period comparable to the wavelength, opening photonic band gaps where light cannot propagate. The route to wavelength-scale cavities with extreme Q-to-volume ratios.

Photonic integrated circuit (PIC)

A chip that integrates two or more photonic functions (waveguides, modulators, photodetectors, sometimes lasers) on a single semiconductor substrate. Analogous to electronic integrated circuits.

Pigtailing

Permanent attachment of an optical fiber to a laser, detector, or photonic chip, typically by active alignment followed by laser welding, epoxy bonding, or solder reflow. The standard packaging operation that converts an unpackaged optical component into a fiber-coupled device.

Plasma dispersion effect

The change in refractive index and absorption of a semiconductor produced by changes in free-carrier density. The dominant electro-optic mechanism in silicon photonic modulators.

Pockels effect (linear electro-optic effect)

The change in refractive index linearly proportional to an applied electric field, occurring only in crystals without inversion symmetry. The basis for the fastest commercial optical modulators.

Polarization splitter-rotator (PSR)

The integrated component that splits incoming TE and TM light and rotates the TM component into TE. It lets a polarization-blind fiber input drive a chip that only behaves well in one polarization.

Propagation constant (β)

The spatial rate of phase advance of a guided mode along its propagation direction. The fundamental wave parameter from which effective index, group index, and dispersion are all derived.

Propagation loss

The optical power attenuation per unit length in a waveguide, expressed in dB/cm or cm⁻¹. The standard figure of merit for waveguide quality in photonic integrated circuits.

Quality factor (Q)

A dimensionless figure of merit describing how lightly damped an optical resonator is. Defined as 2π times the ratio of stored energy to energy dissipated per oscillation cycle, equivalently as resonance frequency divided by linewidth.

Quantum-confined Stark effect (QCSE)

The electric-field-induced shift of the band-edge absorption in a quantum well, used as the gain mechanism in electro-absorption modulators.

Ring resonator

A closed-loop waveguide evanescently coupled to one or two bus waveguides, resonant at wavelengths where the round-trip optical path equals an integer number of wavelengths. The workhorse filter, modulator, and sensor element of integrated photonics.

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.

Silicon nitride photonics

The low-loss integrated photonics platform: Si₃N₄ waveguides clad in oxide, with moderate index contrast, no two-photon absorption at telecom wavelengths, and transparency from the visible through the near-IR.

Silicon-on-insulator (SOI)

The wafer platform of silicon photonics: a thin single-crystal silicon device layer separated from the silicon handle wafer by a buried oxide. High index contrast in a CMOS-compatible material system.

Spot-size converter (SSC)

A waveguide structure that adiabatically transforms the optical mode size between two different cross-sections, used to match an external fiber, laser facet, or another integrated waveguide.

Strip vs rib waveguide

The two standard cross-sections of integrated waveguides: fully etched strip for tight confinement and small bends; partially etched rib for lower loss and the slab access that electrical devices need.

TE and TM polarization

Transverse electric (TE) and transverse magnetic (TM) are the two principal polarization states of a guided mode in a waveguide. Many photonic devices respond very differently to the two.

Thermo-optic phase shifter

A resistive heater over a waveguide that tunes optical phase through the temperature dependence of refractive index. Slow but simple and lossless at DC, and the trim-and-bias workhorse of photonic integrated circuits.

Two-photon absorption (TPA)

A nonlinear absorption process in which two photons are simultaneously absorbed to excite an electron across a bandgap larger than either photon's individual energy. Limits high-power operation of silicon photonic devices at telecom wavelengths.

Wafer-level test

Electrical and optical characterization of photonic or optoelectronic devices while they are still on the un-diced wafer, before any singulation or packaging. The dominant characterization mode in modern silicon photonic foundries.

Materials