Tutorials, characterization procedures, and methodology references for photonic engineering.
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.
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.
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.
What back-reflections actually do to a laser: the Tkach–Chraplyvy feedback regimes, why coherence collapse destroys linewidth and RIN, which devices are sensitive and why, and the isolation budget that keeps links stable.
Two bench methods for finding the actual temperature of a laser chip: the DFB wavelength-shift method and the forward-voltage method. Includes the pulsed-vs-CW logic, worked numbers, and thermal resistance extraction.
A working guide to the standard laser diode packages. What each is for, thermal paths, monitor and TEC contents, the wiring-variant traps, and how to choose for a bench experiment versus a product.
A working engineer's walkthrough of a laser diode datasheet: what threshold, slope efficiency, kink-free power, SMSR, RIN, and tracking error actually promise, the conditions-column traps, and the specs that are missing when a vendor hopes you won't ask.
A selection guide for the four standard linewidth measurements: DSH, heterodyne, frequency-noise discriminator, and coherent receiver. Covers the fiber-length math, what each method actually reports, and the mistakes that skew results.
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.
How single-layer anti-reflection coatings work and how to design one: the quarter-wave thickness condition, the ideal-index rule, worked examples on glass and silicon, and when a single layer isn't enough.
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.
Complete characterization workflow for a distributed-feedback (DFB) semiconductor laser, integrating LIV, spectral, temperature-dependent, and noise measurements to extract the parameters required for telecom and sensing applications.
Reference for optical spectrum analyzers (OSAs) including diffraction grating, Fabry-Pérot, and Fourier-transform variants, resolution bandwidth and dynamic range specifications, and the standard measurement configurations for laser source characterization, OSNR measurement, and EDFA gain characterization.
Procedure for computing wall-plug efficiency (electrical-to-optical conversion efficiency) of a semiconductor laser from LIV data, with discussion of peak operating point, comparison with internal and external quantum efficiencies, and typical values across device classes.
Equipment selection, thermal mounting, electrical configuration, and noise mitigation for a benchtop laser diode characterization setup, with focus on the configuration choices that determine measurement quality.
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.
Comparison of pulsed and continuous-wave measurement modes for semiconductor laser LIV characterization, with quantification of self-heating bias, duty cycle requirements, and equipment configuration.
Comparison of the four standard methods for extracting threshold current from a semiconductor laser LIV curve, applied to a single dataset, with discussion of when each method is preferred and the systematic differences between them.
Procedure for measuring the M² beam quality parameter of a laser beam using the knife-edge method per ISO 11146-1, including hyperbolic fitting, the D4σ width definition, and the dominant sources of measurement error.
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.
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.
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.
Procedure for extracting slope efficiency and external differential quantum efficiency from laser diode LIV data, with discussion of measurement geometry, facet-collection considerations, and common extraction errors.
Procedure for extracting the characteristic temperature T₀ of a semiconductor laser diode from temperature-dependent threshold current measurements, with worked example and discussion of extraction errors.
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.