Silicon Photonics Fundamentals — Optical Technology

Presentations in This Series

Waveguides, Couplers and WDM →
How a silicon wire guides light, and what the 220-nanometre platform makes cheap and what it makes hard: guided modes and group index, loss and bends, getting light on and off the chip, splitters and couplers, interferometers and rings, temperature and fabrication sensitivity, and the wavelength grids and multiplexers that put many channels on one fibre.
liveSOI · Guided modes · Group index · Grating couplers
Modulators and Detectors →
Putting data on light and taking it off again: the plasma-dispersion effect, depletion, injection and MOS phase shifters, Mach–Zehnder and ring modulators, travelling-wave electrodes and drive energy, keeping rings on resonance, GeSi and lithium-niobate alternatives, germanium photodiodes and APDs, and the receiver that turns photocurrent into bits.
livePlasma dispersion · MZM · Ring modulator · VπL
Lasers, Packaging and the Energy-per-Bit Budget →
Why silicon borrows its laser and the four ways to bring one; external laser modules, wall-plug efficiency and comb sources; fibre attach and packaging; putting electronics next to photonics; and the link budget from laser to detector, priced in picojoules per bit, with what published links actually report.
liveLaser integration · ELSFP · Wall-plug efficiency · Packaging

How to read this series. Deck 01 is the passive chip: waveguides, couplers, interferometers, rings and WDM, with a mode solver running in the browser. Deck 02 is the active devices: how silicon modulates light without a linear electro-optic effect, and how germanium detects it. Deck 03 is the system around them: the laser silicon cannot make, the package, and the link budget priced in energy per bit, with an interactive budget.

Where the numbers come from. This is mostly a literature series. Every number is cited on its slide to a paper, a standard (ITU-T, IEEE 802.3, OIF) or a datasheet, with a link, or computed live by a deck's interactive from stated inputs. Vendor claims are labelled as the vendor's own, and illustrative coefficients (in the deck 02 and 03 interactives) are labelled as illustrative. Research records are not product specifications; the slides say which is which.

Where it meets the other optical series. The data-centre series (pluggables and linear-drive optics, co-packaged optics, optical circuit switching, AI cluster networks) and the AI-processing series build on these devices and link this glossary. The Fourier Optics series use the same physics for computing rather than communicating: MZI meshes and on-chip Fourier optics (FOptInf 01), square-law detection (FOptInf 01), and a photonic link module that computes in transit (Compute in Transit).

Glossary: Concepts and Where They Are Explained

Every concept the decks rely on, with a short explanation and links to the slides that explain it. The other optical series link these entries by their anchors. Concepts the sister series already explain are linked, not repeated: Fourier Optics for Inference (the lens as a Fourier transformer, MZI meshes, the phase problem), Compute in Transit (the photonic link module, link energy per bit), LLM Inference Simulators (power) and FHE Accelerator Simulators (converter energy).

Platform and waveguides

Silicon photonics: Optical circuits made with CMOS wafers, lithography and etch: silicon waveguides (transparent at 1310 and 1550 nm), germanium detectors, modulators and heaters on one chip. Silicon does not emit light efficiently, so the laser usually comes from another material.Explained in: OptSiP 01 · why silicon photonics · OptSiP 03 · why silicon needs someone else's laser
Silicon-on-insulator (SOI) platform: The common platform: a 220 nm silicon layer on 2 µm of buried oxide. Silicon's index is about 3.48 at 1550 nm against oxide's 1.44, a contrast of about 2 that allows sub-micron waveguides and micrometre bends.Explained in: OptSiP 01 · the SOI platform and its index contrast
Guided mode and effective index: A field pattern that propagates along a waveguide unchanged, as exp(iβz). The effective index n_eff = β/k₀ lies between the cladding and core indices; about 2.5 for a 500 × 220 nm silicon strip at 1550 nm.Explained in: OptSiP 01 · guided modes · OptSiP 01 · interactive mode solver
Strip and rib waveguides: A strip (wire) is etched through the silicon: tightest bends, but its field touches the rough sidewalls. A rib is partly etched and leaves a slab: lower loss and room for electrical contacts, at the cost of larger bends.Explained in: OptSiP 01 · strip and rib waveguides · OptSiP 01 · loss, bends and group index · OptSiP 02 · phase-shifter cross-sections
Single-mode condition: The width and height below which a waveguide guides only its fundamental mode. Higher-order modes couple at bends and junctions, so routing guides are kept single-mode; for a 220 nm oxide-clad strip at 1550 nm the solver puts the TE cut-off near 0.5 µm width.Explained in: OptSiP 01 · guided modes · OptSiP 01 · interactive mode solver
Group index: n_g = n_eff − λ dn_eff/dλ: the index that sets pulse delay, filter free spectral ranges and resonance shifts. A silicon wire's own dispersion is strong: n_g ≈ 4.0 against n_eff ≈ 2.5 for 500 × 220 nm.Explained in: OptSiP 01 · loss, bends and group index · OptSiP 01 · interactive mode solver
Propagation and bend loss: Sidewall roughness scatters light from strip waveguides: 3.6 dB/cm reported for a single-mode strip, 0.27 dB/cm for a shallow ridge. Bends radiate; at a 2 µm radius a silicon strip loses about 0.013 dB per 90°.Explained in: OptSiP 01 · loss, bends and group index
Mode-field diameter and mode mismatch: The width of a fibre's guided mode: 10.4 µm at 1550 nm and 9.2 µm at 1310 nm for SMF-28 Ultra. A silicon wire's mode is about half a micrometre, so butting one onto the other loses about 20 dB without a coupler.Explained in: OptSiP 01 · getting light on and off the chip · OptSiP 01 · interactive mode solver
Grating coupler: A grating etched into the waveguide that diffracts light up to a fibre held above the wafer. It allows wafer-level testing and placement anywhere on the die, but is narrowband and polarisation-dependent; the best research result is −0.58 dB with a bottom mirror.Explained in: OptSiP 01 · getting light on and off the chip · OptSiP 03 · fibre attach and packaging
Edge coupler (inverse taper): A silicon taper narrowed until the mode spreads into a larger cladding guide that matches a fibre at the polished chip facet. Broadband and low-loss (0.36–0.66 dB per connection reported) but needs facet preparation and edge access.Explained in: OptSiP 01 · getting light on and off the chip · OptSiP 03 · fibre attach and packaging
Directional couplers, MMIs and Y-branches: Passive splitters. A directional coupler transfers sin²(κL) of the power across a narrow gap and is sensitive to gap and wavelength; a multimode interference (MMI) coupler splits by self-imaging and is more tolerant; a Y-branch splits adiabatically.Explained in: OptSiP 01 · splitters and directional couplers
Mach–Zehnder interferometer (MZI): Split, two arms, recombine: the bar port carries cos²(Δφ/2). With unequal arms it is a periodic filter with FSR = λ²/(n_gΔL); with a phase shifter in one arm, a switch or modulator.Explained in: OptSiP 01 · the MZI as a filter · OptSiP 02 · the Mach–Zehnder modulator
Ring resonator: A closed loop beside a bus waveguide that resonates when its round trip holds a whole number of wavelengths. All-pass rings notch the bus; add-drop rings route a wavelength to a second bus. Compact filters, modulators and WDM elements.Explained in: OptSiP 01 · ring resonators · OptSiP 02 · ring modulators
Free spectral range (FSR): The wavelength spacing between successive resonances or transmission peaks: λ²/(n_gL) for a ring of circumference L. About 9.5 nm for a 10 µm-radius silicon ring at 1550 nm.Explained in: OptSiP 01 · ring resonators · OptSiP 01 · the MZI as a filter
Q factor, linewidth and finesse: Q = λ/Δλ_FWHM, set by round-trip loss and bus coupling; a loaded Q of 10,000 at 1550 nm is a 0.155 nm linewidth. Finesse = FSR/Δλ_FWHM. Higher Q sharpens a filter or modulator but narrows its optical bandwidth and its tolerance to drift.Explained in: OptSiP 01 · ring resonators · OptSiP 02 · ring modulators · OptSiP 02 · interactive ring and MZ modulators
Thermo-optic effect: Silicon's index rises by 1.8×10⁻⁴ per kelvin at room temperature, moving a silicon-wire resonance by about 72 pm/K at 1550 nm: about 2 K detunes a Q = 10,000 ring by a full linewidth. It makes heaters effective phase shifters and makes rings need tuning.Explained in: OptSiP 01 · temperature and fabrication sensitivity · OptSiP 02 · keeping a ring on resonance
Fabrication variation: Nanometre changes in waveguide width and silicon thickness shift resonances: about 0.5 nm per nm of width and 1.3 nm per nm of thickness for a 500 × 220 nm strip (semi-vectorial estimate). Rings and filters are tuned after fabrication.Explained in: OptSiP 01 · temperature and fabrication sensitivity · OptSiP 01 · interactive mode solver

Wavelength multiplexing

Wavelength-division multiplexing (WDM): Several channels on one fibre, each on its own wavelength, combined by a multiplexer and separated by a demultiplexer. It multiplies bandwidth per fibre without faster electronics.Explained in: OptSiP 01 · WDM grids · OptSiP 01 · multiplexers · OptSiP 03 · how bandwidth scales
CWDM, LAN-WDM and DWDM grids: Standard channel plans. CWDM: 20 nm spacing, 18 wavelengths 1271–1611 nm (ITU-T G.694.2; CWDM4 uses 1271–1331 nm). LAN-WDM: 800 GHz near 1300 nm (100GBASE-LR4). DWDM: anchored at 193.1 THz with 12.5–100 GHz spacing (ITU-T G.694.1).Explained in: OptSiP 01 · WDM grids
Arrayed waveguide grating (AWG) and echelle grating: Integrated demultiplexers. An AWG's stepped-length arms tilt the phase front by wavelength and a slab focuses each wavelength to its own port, an integrated Fourier optic; an echelle uses a reflective grating in a slab. AWGs suit dense spacing, echelles coarse.Explained in: OptSiP 01 · multiplexers

Modulators

NRZ and PAM4: Intensity-modulation formats. NRZ (on–off keying) sends one bit per symbol on two levels; PAM4 sends two bits on four levels, halving the symbol rate for a given bit rate at the cost of eyes a third the height. 400GBASE-DR4 uses PAM4 at 53.125 GBd per lane.Explained in: OptSiP 02 · what a modulator has to do · OptSiP 02 · from photocurrent to bits
Extinction ratio and OMA: Extinction ratio: the power ratio between the high and low levels. Optical modulation amplitude (OMA): their difference, the quantity a receiver's decision actually uses. A modulator trades insertion loss against extinction ratio by where it is biased.Explained in: OptSiP 02 · what a modulator has to do · OptSiP 02 · interactive ring and MZ modulators · OptSiP 03 · the optical link budget
TDECQ: Transmitter and dispersion eye closure for PAM4: the IEEE 802.3 metric of how much optical power penalty a transmitter's eye costs a reference receiver with a reference equaliser. Defined in IEEE 802.3bs for 400GBASE PAM4 optics.Explained in: OptSiP 02 · what a modulator has to do · OptSiP 02 · ring modulators
Plasma-dispersion effect: Free electrons and holes lower silicon's refractive index and add absorption (Soref and Bennett): at 1550 nm, Δn = −(8.8×10⁻²²ΔN_e + 8.5×10⁻¹⁸ΔN_h^0.8). Silicon has no linear electro-optic effect, so this is how most silicon modulators work.Explained in: OptSiP 02 · the plasma-dispersion effect
Depletion, injection and MOS-capacitor phase shifters: Three ways to move carriers in a silicon waveguide. Reverse-biased pn depletion is fast but weak (VπL about 2 V·cm); forward-biased injection is strong but slow; a MOS capacitor (accumulation) is strong with a high capacitance.Explained in: OptSiP 02 · depletion, injection and MOS capacitors
Mach–Zehnder modulator (MZM): An MZI with high-speed phase shifters in its arms, driven push–pull and biased at quadrature. Broadband and temperature-tolerant, but millimetres long, so it needs travelling-wave electrodes and a driver that can swing volts.Explained in: OptSiP 02 · the Mach–Zehnder modulator · OptSiP 02 · interactive ring and MZ modulators
VπL: The voltage-length product needed for a π phase shift: the efficiency figure of a phase shifter. About 2 V·cm for silicon carrier depletion; 0.52 V·cm reported for an optimised design. Lower VπL means a shorter device or a smaller drive.Explained in: OptSiP 02 · the Mach–Zehnder modulator · OptSiP 02 · depletion, injection and MOS capacitors
Travelling-wave electrode: A transmission line along a long modulator, so the electrical signal travels with the light. Bandwidth is limited by RF loss and by velocity and impedance mismatch; the line is terminated, so the driver's power is burnt in the termination whatever the data.Explained in: OptSiP 02 · travelling-wave electrodes and driver energy
Capacitive energy per bit (¼CV²): A lumped capacitive modulator driven by random NRZ data draws CV² from the supply on a 0→1 transition, one bit in four on average, so it dissipates about ¼CV² per bit (Miller). A 10 fF ring at 1 V: 2.5 fJ/bit, before the driver.Explained in: OptSiP 02 · travelling-wave electrodes and driver energy · OptSiP 02 · interactive ring and MZ modulators
Ring modulator: A ring whose resonance is shifted by a pn junction: a small shift moves a sharp notch on or off the laser line. Micrometres across, femtofarads of capacitance and femtojoules per bit, but narrowband, temperature-sensitive and limited by photon lifetime at high Q.Explained in: OptSiP 02 · ring modulators · OptSiP 02 · interactive ring and MZ modulators
Thermal tuning and wavelength locking: Heaters next to rings and interferometers to correct fabrication offsets and follow temperature, held by a control loop that monitors a photodiode. Reported heater efficiencies are of order 12–25 mW per π phase shift; across many rings this static power is a large share of a link's energy.Explained in: OptSiP 02 · keeping a ring on resonance · OptSiP 03 · thermal tuning in the budget
Electro-absorption modulator (GeSi EAM): A modulator that changes absorption rather than index: germanium or GeSi near its band edge, under an electric field (Franz–Keldysh effect). Short and low in capacitance, but its wavelength window is fixed by the material.Explained in: OptSiP 02 · beyond plasma dispersion
Thin-film lithium niobate (and other added materials): Materials with a linear (Pockels) electro-optic effect, which silicon lacks, bonded or deposited onto a photonic chip: thin-film lithium niobate, barium titanate, polymers, plasmonics. They lower drive voltage or loss, at the price of new process steps.Explained in: OptSiP 02 · beyond plasma dispersion

Detection

Germanium photodiode: Germanium grown on silicon absorbs at 1310 and 1550 nm, where silicon is transparent, and makes the detector of a silicon photonic chip. Waveguide-coupled Ge diodes reach tens of GHz with responsivities near 0.8 A/W.Explained in: OptSiP 02 · germanium photodiodes
Responsivity and quantum efficiency: Photocurrent per watt of light, R = ηqλ/(hc): at most 1.25 A/W at 1550 nm and 1.06 A/W at 1310 nm for 100% quantum efficiency η.Explained in: OptSiP 02 · germanium photodiodes
Dark current, capacitance and bandwidth: A photodiode's limits: current with no light (noise), capacitance (which with the load sets an RC bandwidth) and carrier transit time across the absorber. Small waveguide detectors cut capacitance and transit time together.Explained in: OptSiP 02 · bandwidth, dark current and avalanche gain
Avalanche photodiode (APD): A photodiode with internal gain from impact ionisation, which improves sensitivity when the amplifier's noise dominates. Ge/Si APDs separate germanium absorption from silicon multiplication: 340 GHz gain–bandwidth product reported.Explained in: OptSiP 02 · bandwidth, dark current and avalanche gain
Transimpedance amplifier and receiver sensitivity: The TIA turns photocurrent into a voltage; its input noise and the detector's capacitance set the sensitivity: the smallest optical power (or OMA) that meets the target bit error ratio. The sensitivity is where a link budget ends.Explained in: OptSiP 02 · from photocurrent to bits · OptSiP 03 · the optical link budget
Direct and coherent detection: Direct (intensity) detection measures |E|² and suits short reaches; coherent detection mixes the signal with a local laser to recover amplitude and phase, carrying more bits per symbol at the cost of a second laser and DSP.Explained in: OptSiP 02 · direct and coherent detection

Lasers, packaging and energy

Indirect bandgap: In silicon the lowest conduction-band state and the highest valence-band state are at different crystal momenta, so an electron and hole can only recombine radiatively with a phonon's help. Radiative recombination is slow and efficient lasing is out of reach; III–V materials have direct gaps.Explained in: OptSiP 03 · why silicon needs someone else's laser
Laser integration options: Four ways to bring the laser: an external source fibred in; a packaged laser chip flip-chipped or attached; III–V bonded onto silicon and processed (heterogeneous); or III–V grown directly on silicon (monolithic). They trade coupling loss, thermal path, replaceability and process maturity.Explained in: OptSiP 03 · four ways to bring the laser
External laser source and ELSFP: Keeping the laser off the hot photonic die, in a front-panel module that feeds light by fibre: replaceable, cooler, and shared across many channels. OIF's External Laser Small Form Factor Pluggable (ELSFP) implementation agreement standardises such a module for co-packaged optics.Explained in: OptSiP 03 · external lasers and the ELSFP
Wall-plug efficiency: Optical power out divided by electrical power in. A III–V-on-silicon research DFB laser measured up to 13%; a comb-driven link analysis assumes 35% for its pump laser and 14.35% overall; vendors claim higher for high-power sources. Every dB of optical loss in a link is paid at the wall divided by this number.Explained in: OptSiP 03 · wall-plug efficiency · OptSiP 03 · interactive link budget
Multi-wavelength sources and combs: One source for many WDM channels: a laser array, a mode-locked quantum-dot comb laser, or a Kerr microcomb. They save lasers but split power between lines, so per-line power and line-to-line flatness enter the budget.Explained in: OptSiP 03 · multi-wavelength sources
Fibre attach and photonic packaging: Aligning fibres to couplers to a fraction of a micrometre and keeping them there: active alignment, V-groove fibre arrays, and the thermal and mechanical design around them. Packaging is far more expensive than electronic packaging and is often called the bottleneck.Explained in: OptSiP 03 · fibre attach and packaging
Electronic–photonic integration: Where the drivers and receivers sit: on the same die (monolithic, in a CMOS SOI process), on a separate electronic chip bonded face-to-face (3-D), or beside it on a substrate. Shorter connections cut parasitic capacitance and so energy per bit.Explained in: OptSiP 03 · putting electronics next to photonics
Optical link budget: The bookkeeping from laser to detector: laser power minus coupling, modulator, multiplexer, fibre and connector losses and penalties must exceed the receiver's sensitivity plus a margin. The required laser power follows from the losses, and the electrical power from the wall-plug efficiency.Explained in: OptSiP 03 · the optical link budget · OptSiP 03 · interactive link budget
Energy per bit: Total link power divided by data rate, in pJ/bit or fJ/bit: laser (via wall-plug efficiency), modulator driver, receiver, serialisers and clocking, thermal tuning, and any DSP. Published silicon links span from about 120 fJ/bit to a few pJ/bit; what is included matters as much as the number.Explained in: OptSiP 03 · what published links report · OptSiP 03 · interactive link budget
Bandwidth scaling: lanes, wavelengths and symbol rate: A link's bandwidth is fibres × wavelengths per fibre × bits per symbol × symbol rate. Each axis costs something different: fibres cost packaging, wavelengths cost lasers and filters, bits per symbol cost sensitivity, and baud costs electronics.Explained in: OptSiP 03 · how bandwidth scales

Explained in the sister series

The lens as a Fourier transformer · Mach–Zehnder interferometer (MZI) meshes · Integrated optical Fourier transforms · Coherent and incoherent light · Square-law detection and the phase problem · Photonic interconnect · Photonic link module · Link energy per bit · Power in photonic compute · Static and dynamic power · Walden figure of merit.