Lasers & gain
Semiconductor lasers and the physics that runs them: threshold, efficiency, linewidth, noise, dynamics, and the device structures from FP to VCSEL.
26 articles · 225 glossary terms · 9 tools · 2 materials
Start here: Semiconductor lasers in 10 pages
From why a laser amplifies light to reading a datasheet and measuring the device yourself.
- 1Stimulated emissionGlossary
- 2Population inversionGlossary
- 3Optical gainGlossary
- 4Optical cavityGlossary
- 5Laser thresholdGlossary
- 6LIV curveGlossary
- 7Slope efficiencyGlossary
- 8LinewidthGlossary
- 9How to Read a Laser Diode DatasheetArticle
- 10Setting Up a Temperature-Controlled Laser Characterization BenchArticle
Tools
Laser characterization
T₀ Extraction Calculator
Fit the characteristic temperature T₀ and pre-exponential I₀ from threshold current vs. temperature data.
Laser characterization
Wall-Plug Efficiency Calculator
Compute wall-plug efficiency from LIV data point-by-point and identify the peak operating current.
PIC design
Bragg Wavelength Calculator
Convert between grating period and Bragg wavelength for DFB/DBR lasers and integrated photonic Bragg gratings.
Laser characterization
Junction Temperature Calculator
Extract junction temperature rise and thermal resistance from a DFB wavelength shift, or predict junction temperature from known R_th.
Optical coatings
Thin-Film Reflectance Calculator
Reflectance and transmittance of any dielectric layer stack by the characteristic-matrix method, s and p polarization at any angle, with presets for anti-reflection coatings, quarter-wave Bragg mirrors, laser facet coatings, and etalons, live spectrum and angle plots, and a 16-row validation table.
Resonators
Fabry-Perot Cavity Calculator
Free spectral range, finesse, linewidth, Q, photon lifetime, mirror loss, peak transmission, fringe contrast and intracavity power for a plane-parallel or confocal two-mirror cavity of any length, index, reflectivities and internal loss, with exact Airy transmission and reflection spectra, presets from etalons and laser diode chips to ULE reference cavities, guided experiments and a validation table.
Imaging and diffraction
Diffraction Grating Calculator
Diffraction angles of every propagating order, the Littrow angle, angular and linear dispersion, resolving power, spectrometer resolution for a given slit, free spectral range and beam width ratio for any groove density, wavelength, incidence and order, with a ray diagram, angle-versus-wavelength curves, presets, guided experiments and a validation table.
Laser characterization
Pulsed Laser Calculator
Pulse energy, peak power, duty cycle, photons per pulse, fluence and peak intensity of a pulsed laser from its average power, repetition rate, pulse duration and shape, and spot size, with presets from Q-switched to femtosecond sources, guided experiments and a validation table.
Laser characterization
Coherence Length Calculator
Coherence time and coherence length from a linewidth in frequency or wavelength for Lorentzian, Gaussian and rectangular spectra, the fringe visibility at a given path difference, the path difference where it halves and the OCT axial resolution, with a visibility plot, presets, guided experiments and a validation table.
Articles
- Wavemeter vs Optical Spectrum Analyzer: Measuring a Laser's Wavelength and SpectrumWhen to use a wavemeter and when to use an optical spectrum analyzer to measure a laser: what each reports, accuracy and resolution compared in picometers and megahertz, single-mode and multimode sources, side-mode suppression, tuning and mode hops, with worked numbers at 1550 nm.
- Types of Lasers: Gas, Solid-State, Fiber, Semiconductor and MoreReference to the main types of laser, grouped by gain medium: gas lasers (HeNe, CO₂, argon-ion, excimer), solid-state lasers (Nd:YAG, Yb:YAG, Ti:sapphire), fiber lasers, semiconductor lasers (Fabry-Perot, DFB, DBR, VCSEL, external-cavity, quantum cascade), dye lasers and parametric sources, with their wavelengths, how they operate in time, and what each is used for.
- Nonlinear Optics: An Overview of Second- and Third-Order EffectsWhat nonlinear optics is and where it matters in practice: the nonlinear polarization, second-order effects (harmonic generation, sum and difference frequencies, parametric amplification, the Pockels effect), phase matching and quasi-phase matching, third-order effects (Kerr, self-phase modulation, four-wave mixing, Raman, Brillouin), with worked numbers for PPLN and optical fiber.
- How to Measure Ultrashort Pulse Duration: Autocorrelation, FROG and SPIDERProcedures and choices for measuring femtosecond and picosecond laser pulses: the intensity autocorrelator and its calibration, the deconvolution factor and what it hides, checking against the spectrum with the time-bandwidth product, interferometric autocorrelation, and when FROG, SPIDER or d-scan is needed.
- Lidar: Time of Flight vs FMCWHow pulsed time-of-flight and FMCW coherent lidar measure distance, with the range, resolution and Doppler equations, worked numbers, the choice of 905 nm or 1550 nm, detectors, scanning methods, and the strengths of each approach.
- How to Measure Laser Beam Divergence: Two-Point and Focal-Plane MethodsProcedure for measuring the far-field divergence of a laser beam: the two-point method and why it underestimates inside the Rayleigh range, the focal-plane lens method that works on any bench, width definitions, half versus full angle, and a worked example.
- Fiber vs CO₂ vs Nd:YAG Lasers for Cutting, Welding and MarkingHow fiber, CO₂ and Nd:YAG lasers compare for materials processing: wavelength and absorption, focused spot size and depth of focus, beam delivery, efficiency, and which laser suits metal cutting, welding, marking, wood, acrylic, glass and copper, with worked numbers.
- Laser Diode Lifetime: How an MTTF Claim Is BuiltThe arithmetic behind a laser diode lifetime specification: the Arrhenius temperature acceleration and the current acceleration exponent, worked with realistic activation energies; how a few thousand hours of stress on a few dozen devices becomes a million-hour mean time to failure and what confidence that number carries; the failure criteria that define a failure; what burn-in catches and does not; and how to derate a laser from the claim to an operating point.
- Facet Coatings and What They Do to a Laser DiodeHow the front and back facet reflectivities of a Fabry-Perot laser diode set its mirror loss, threshold gain, slope efficiency, and the split of power between the two facets, worked through with computed numbers for uncoated, 10/90, 5/95, and 2/95 pairs on a 1 mm pump laser and a 300 µm telecom chip, and what the choice costs in facet intensity, feedback sensitivity, and monitor photodiode signal.
- Why L-I Curves KinkWhat a kink in a laser diode's light-current curve is, the lateral-mode and beam-steering mechanism that produces most of them in ridge lasers, how mode hops, polarization switches, and facet damage look different, how to read the signature, and how kink-free power is measured and specified.
- Light Sources for Co-Packaged OpticsWhy co-packaged optics keeps its lasers outside the package: the thermal, reliability, and yield arguments, the ELSFP and CW-WDM interface standards, the laser-count arithmetic of split-and-modulate architectures, quantum dot combs against Kerr microcombs, and a worked power budget from wall plug to modulator.
- Optical Feedback and Coherence Collapse in LasersHow much optical feedback a laser diode tolerates: the five Tkach–Chraplyvy regimes in dB, coherence collapse from about −39 dB, and the isolator budget.
- Measuring Laser Diode Junction Temperature and Thermal ResistanceTwo 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.
- Laser Diode Packages: TO-Can, C-Mount, CoS, and ButterflyA 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.
- How to Read a Laser Diode DatasheetA 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.
- How to Measure Laser Linewidth: Choosing the Right MethodA 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.
- DFB Laser Characterization: Full Parameter Extraction WorkflowComplete 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.
- Optical Spectrum Analyzers: Operating Principles and Measurement ConfigurationReference 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.
- How to Compute Wall-Plug Efficiency from an LIV CurveStep-by-step procedure for turning a laser diode's LIV sweep into its wall-plug efficiency curve: the definition, why the peak sits at a few times threshold, the three factors that set it, a worked example with numbers, the measurement errors that bias it, and typical peak values from telecom DFBs to pump diodes.
- Setting Up a Temperature-Controlled Laser Characterization BenchEquipment 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.
- Pulsed versus Continuous-Wave LIV Measurement: Self-Heating and MethodologyComparison of pulsed and continuous-wave measurement modes for semiconductor laser LIV characterization, with quantification of self-heating bias, duty cycle requirements, and equipment configuration.
- Threshold Current Extraction Methods ComparedComparison 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.
- How to Measure M² Beam Quality: Camera and Knife-Edge MethodsProcedure for measuring laser beam quality (M²) per ISO 11146 with a beam-profiling camera or a knife edge: second-moment (D4σ) widths and why baseline noise inflates them, the caustic scan through a focus, hyperbolic fitting, automated M² systems, and the dominant errors.
- Relative Intensity Noise (RIN): How to Measure ItMeasure a laser's relative intensity noise (RIN) in dB/Hz with a photodiode and spectrum analyzer: formula, shot-noise calibration, worked DFB example, errors.
- Slope Efficiency and Differential Quantum Efficiency from LIV MeasurementsProcedure 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.
- Characteristic Temperature Extraction from LIV MeasurementsProcedure 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.
Glossary
- ABCD matrix (ray transfer matrix)
- Absorption and emission cross section (σ)
- Active region
- Amplified spontaneous emission (ASE)
- Auger recombination
- Autocorrelation (pulse measurement)
- B-integral (nonlinear phase)
- Band filling (Burstein-Moss shift)
- Bandgap (energy gap, E_g)
- Beam diameter (1/e², FWHM and D4σ)
- Beam parameter product (BPP)
- Beam quality (M²)
- Beam waist (w₀)
- Bias tee
- Birefringent (Lyot) filter
- Broad-area laser diode
- Burn-in
- Carrier confinement
- Carrier density
- Carrier injection
- Carrier lifetime
- Carrier-envelope offset (CEO)
- Catastrophic optical damage (COD)
- Cavity dumping
- Characteristic temperature (T₀)
- Chirped mirror
- Chirped-pulse amplification (CPA)
- CO₂ laser
- Coherence collapse
- Coherence length
- Confinement factor (Γ)
- Continuous-wave (CW) vs pulsed lasers
- D-factor and MCEF
- Delayed self-heterodyne
- DFB laser array
- Differential gain
- Differential quantum efficiency
- Differential resistance
- Direct vs indirect bandgap
- Directly modulated laser (DML)
- Distributed Bragg reflector (DBR mirror)
- Distributed Bragg reflector (DBR) laser
- Distributed feedback (DFB) laser
- Dither locking (top-of-fringe lock)
- Double-clad fiber
- DPSS laser (diode-pumped solid-state laser)
- Dye laser
- Edge-emitting laser
- Effective index method
- Electroluminescence (EL)
- Epitaxy (MBE and MOCVD)
- Excimer laser
- External cavity laser (ECL)
- Fabry-Perot laser
- Fabry-Perot resonator (FP cavity)
- Facet (laser and waveguide facets)
- Far-field divergence
- Fast and slow axis
- Fiber laser
- Flip-chip bonded laser
- Fluence
- Forward voltage (V_f)
- Four-level laser
- Free-carrier absorption (FCA)
- Frequency chirp
- Frequency comb
- FROG (frequency-resolved optical gating)
- Gain bandwidth (gain spectrum)
- Gain compression factor (ε)
- Gain medium
- Gain saturation
- Gain switching
- Gallium nitride (GaN)
- Gas laser
- Gouy phase
- Ground state
- Group delay dispersion (GDD)
- Hakki-Paoli method
- Helium-neon laser
- Heterojunction
- High-harmonic generation (HHG)
- Homogeneous and inhomogeneous broadening
- Ideality factor
- III-V semiconductors
- Injection locking
- Interband cascade laser (ICL)
- Internal loss (α_i)
- Internal quantum efficiency (η_i)
- Intervalence-band absorption (IVBA)
- Junction temperature
- K-factor
- Kerr-lens mode locking (KLM)
- Large-mode-area (LMA) fiber
- Laser cooling
- Laser diode (semiconductor laser)
- Laser diode bar
- Laser machining
- Laser mirror
- Laser pointer
- Laser threshold
- Lattice matching
- Lidar
- Light-emitting diode (LED)
- Lineshape (Lorentzian, Gaussian and Voigt)
- Linewidth
- Linewidth enhancement factor (α)
- LIV curve
- LIV kink
- Longitudinal mode spacing
- Longitudinal modes
- Material gain
- Microchip laser
- Microcomb (Kerr soliton frequency comb)
- Mirror loss (α_m)
- Modal gain
- Mode hop
- Mode locking
- Mode matching
- Mode partition noise (MPN)
- Modulation bandwidth
- MOPA (master oscillator power amplifier)
- Multi-quantum well (MQW)
- Multiphoton microscopy
- Nd:YAG laser
- Neuromorphic photonics
- Non-radiative recombination (Shockley-Read-Hall, Auger and surface)
- Nonlinear optics
- Optical amplifier
- Optical atomic clock
- Optical cavity
- Optical feedback regimes (Tkach–Chraplyvy)
- Optical gain
- Optical parametric amplifier (OPA)
- Optical parametric oscillator (OPO)
- Optical phase-locked loop (OPLL)
- Optical pumping
- Optical tweezers
- Output coupler
- Peak power
- Petermann factor
- Phase noise and frequency noise
- Photoluminescence (PL)
- Photon lifetime (cavity lifetime)
- Photonic-crystal surface-emitting laser (PCSEL)
- Picosecond laser
- Pockels cell
- Population inversion
- Pound-Drever-Hall locking (PDH)
- Pulse compressor
- Pulse duration
- Pulse energy
- Pulse picker
- Pump combiner (tapered fiber bundle)
- Purcell effect
- Q-switching
- Quantum cascade laser (QCL)
- Quantum defect
- Quantum dot laser
- Quantum efficiency
- Quantum well
- Quantum-confined Stark effect (QCSE)
- Quasi-Fermi level
- Radiative recombination
- Rare-earth ions (as laser and amplifier dopants)
- Rate equations
- Regenerative amplifier
- Relative intensity noise (RIN)
- Relaxation oscillations
- Repetition rate
- Resonant saturable absorber mirror (RSAM)
- Resonator stability
- Ridge-waveguide laser
- Saturable absorber
- Saturation power (P_sat)
- Scanning Fabry-Perot interferometer
- Schawlow–Townes linewidth
- Self-injection locking
- Semiconductor saturable absorber mirror (SESAM)
- Separate-confinement heterostructure (SCH, GRINSCH)
- Series resistance
- Shunt resistance
- Side-mode suppression ratio (SMSR)
- Slope efficiency
- Small-signal gain
- Solid-state laser
- Spatial hole burning
- Spectral hole burning
- Spontaneous emission
- Spontaneous emission factor (β)
- Standing wave
- Stimulated emission
- Strained quantum well
- Submount
- Superluminescent diode (SLD)
- T2* (dephasing time)
- Tapered amplifier
- TEM00 mode
- Terahertz generation
- Thermal lensing
- Thermal resistance
- Thermal rollover
- Thermistor (NTC)
- Thermoelectric cooler (TEC)
- Thin-disk laser
- Threshold condition
- Threshold current (laser diode)
- Threshold current density (J_th)
- Thulium fiber laser
- Ti:sapphire laser
- Time-bandwidth product
- Tracking error
- Transparency current (I_tr)
- Transverse modes
- Tunable laser
- Ultrafast laser
- Ultrastable optical cavity
- Upper-state lifetime
- VECSEL (semiconductor disk laser)
- Vertical-cavity surface-emitting laser (VCSEL)
- Volume Bragg grating
- Wall-plug efficiency
- Wavelength locker
- Wavelength temperature coefficient
- Ytterbium-doped fiber
- β-separation line
Materials
Gallium arsenideGaAs
The direct-gap workhorse of the 780–1100 nm range: VCSELs, pump diodes, and high-power bars. A strong second-order nonlinearity keeps it interesting for integrated nonlinear optics.
Indium phosphideInP
The native platform of telecom light. Direct bandgap, lattice-matched quaternaries spanning 1.0–1.65 μm, and monolithic integration of lasers, amplifiers, modulators, and detectors on one substrate.