Photonics & Lasers

Start here: ten words every newcomer needs

Overheard on the job

Real-sounding lines from the floor, translated into plain English.

• Ti:sapph came up CW after the power blip, kick it and check for CW breakthrough before the users get here.

  TranslationThe titanium-sapphire laser restarted without pulsing after a power interruption; perturb the cavity to start mode locking and check the spectrum for a leftover continuous-wave spike.

• M-squared is 1.6 at full pump, that's the thermal lens, the cavity was designed for half the power.

  TranslationBeam quality degrades at full pump because heating in the gain crystal acts as a lens the resonator design did not account for.

• Walk the beam through both irises, then peak up the fiber launch on the power meter.

  TranslationAlign the beam with two steering mirrors against two aperture targets, then fine-tune for maximum power coupled into the fiber.

• Regen's dumping a round trip early, the Pockels cell timing drifted and the energy's down a third.

  TranslationThe regenerative amplifier is releasing its pulse too soon because its electro-optic switch is mistimed, so output pulse energy has dropped.

• Backward power jumped and the isolator's hot, we hit SBS, broaden the seed before you try that again.

  TranslationA surge of light traveling backward shows stimulated Brillouin scattering has started; widen the seed laser's linewidth to raise the threshold before increasing power.

• Doubler's peaked but there's IR bleed-through on the meter, and those goggles are OD 2 at 1064.

  TranslationThe frequency-doubling stage is optimized, but leftover infrared is leaking into the beam, and the safety eyewear in use is far too weak at that infrared wavelength.

• There's a ghost off the lens focusing right on the SESAM, that's why it keeps getting burn spots.

  TranslationA stray back-surface reflection is being focused onto the saturable absorber mirror and damaging it repeatedly.

How lasers make light: gain, pumping, inversion, threshold and the common laser types.

• ASEalso amplified spontaneous emission
  Amplified Spontaneous Emission: random spontaneous photons amplified in a gain medium, a broadband noise background.
• CWalso continuous wave, CW mode
  Continuous Wave: laser operation with a steady output beam rather than pulses.
• Four-level laseralso four-level system
  A laser whose lower laser level sits above the ground state and empties quickly, so inversion is easy to reach.
• Gain mediumalso active medium, laser medium
  The material inside a laser or amplifier that amplifies light once it is pumped: a crystal, glass, gas, semiconductor or doped fiber.
• Gain saturationalso saturation
  The drop in an amplifier's gain as the signal grows strong enough to deplete the stored inversion.
• Kinkalso LI kink
  A sudden bend or step in a laser diode's power-versus-current curve, usually a sign of mode switching.
• Lase
  To produce laser light, as in the cavity is lasing or it won't lase below 2 amps. Shop talk
• Laser diodealso diode laser, LD
  A semiconductor laser driven by electric current, used as a light source in its own right and to pump other lasers.
• Lasing thresholdalso threshold, threshold current
  The pump level at which gain exactly balances resonator losses and laser output begins.
• Population inversionalso inversion
  The condition where more atoms occupy the upper laser level than the lower one, which a medium needs to give gain.
• Pumpalso pumping, pump light
  The energy source that excites a gain medium, or the act of supplying it, usually with diodes, flashlamps or current.
• Slope efficiencyalso differential efficiency, slope
  The rate at which laser output power rises per unit of extra pump power above threshold.
• Smilealso bar smile
  The slight bowing of the emitter line on a laser diode bar, which spoils fast-axis collimation.
• Stimulated emissionalso induced emission
  Emission of a photon by an excited atom, triggered by an incoming photon and identical to it in color, phase and direction.
• Ti:sapphalso Ti:sapphire, Ti:Al2O3
  Titanium-doped sapphire, the broadband tunable laser crystal behind most femtosecond oscillators and amplifiers. Shop talk
• YAGalso Nd:YAG, YAG laser
  Shorthand for yttrium aluminum garnet, and in practice for the Nd:YAG laser at 1064 nm. Shop talk

Cavities, modes and the numbers that describe a beam's size, spread and quality.

• Beam waistalso waist, w0
  The narrowest point of a focused or propagating laser beam, where its wavefront is flat.
• Divergencealso beam divergence, far-field angle
  How fast a laser beam spreads with distance far from its waist, given as an angle in milliradians.
• Doughnut modealso donut mode, TEM01*
  A ring-shaped laser mode with a dark center, usually a sign of misalignment or overheating in a TEM00 laser. Shop talk
• Finessealso cavity finesse
  The sharpness of an optical cavity's resonances: free spectral range divided by the resonance linewidth.
• Free spectral rangealso FSR, mode spacing
  The frequency spacing between neighboring resonances of a laser cavity, etalon or interferometer.
• Gaussian beamalso Gaussian, TEM00 beam
  A beam whose intensity falls off from the center as a bell curve, the shape of an ideal single-mode laser.
• Hot spotalso hotspot
  A small region of a beam with much higher intensity than the rest, a common cause of optic damage. Shop talk
• Longitudinal modealso axial mode
  One of the discrete frequencies at which a standing wave fits a whole number of half-wavelengths into the resonator.
• M-squaredalso M2, M²
  The beam quality factor: how many times worse a beam focuses and diverges than an ideal Gaussian of the same wavelength.
• Mode hopalso mode hopping, mode jump
  A sudden jump of a laser's output frequency from one cavity mode to a neighboring one.
• Output coupleralso OC, partial reflector
  The partially transmitting resonator mirror through which the laser beam leaves the cavity.
• Rayleigh rangealso Rayleigh length, zR
  The distance from a beam's waist to where its cross-sectional area has doubled, a measure of depth of focus.
• Resonatoralso cavity, optical cavity
  The arrangement of mirrors that sends light back and forth through the gain medium to build up laser oscillation.
• TEM00also fundamental mode, single transverse mode
  The fundamental transverse mode of a laser: a single round spot with a Gaussian intensity profile.
• Thermal lensingalso thermal lens
  The lens effect a heated gain medium or optic creates as its center gets hotter than its edges.
• Top-hatalso flat-top, top hat profile
  Describes a beam with flat, uniform intensity across its width and steep edges, instead of a Gaussian peak. Shop talk

Q-switched and mode-locked lasers, pulse measurement and the slang of ultrafast labs.

• Autocorrelatoralso AC
  An instrument that measures ultrashort pulse duration by overlapping a pulse with a delayed copy of itself.
• Chirpalso frequency chirp
  A change in instantaneous frequency across a pulse, so its colors arrive spread out in time.
• CPAalso chirped pulse amplification
  Chirped Pulse Amplification: stretching a short pulse before amplifying it, then recompressing it afterwards.
• CW breakthroughalso CW spike
  A narrow continuous-wave spike in a mode-locked laser's spectrum, showing part of the power is not pulsing. Shop talk
• Double pulsing
  A fault where a pulsed laser emits two pulses where it should emit one, from too much gain or bad switch timing. Shop talk
• Femtoalso femtosecond, fs
  Clipped form of femtosecond, for lasers and pulses lasting from a few to a few hundred quadrillionths of a second. Shop talk
• Fluencealso energy density
  Pulse energy per unit area, usually in J/cm squared, the quantity that sets ablation and optical damage.
• Kickalso kick-start
  To start a Kerr-lens mode-locked laser by briefly disturbing the cavity so a pulse can form. Shop talk
• Lock
  The stable locked state of a laser, either mode-locked pulsing or frequency-locked to a reference. Shop talk
• Mode lockingalso modelocking, ML
  Fixing the phases of many longitudinal modes so they add up into a train of ultrashort pulses.
• Peak poweralso peak, Ppk
  The highest instantaneous power reached during a laser pulse, roughly pulse energy divided by pulse duration.
• Pedestal
  A low, broad background of light under or around an ultrashort pulse, which carries energy outside the main peak.
• Pockels cellalso PC
  An electro-optic crystal that changes a beam's polarization when high voltage is applied, used as a fast optical switch.
• Pulse durationalso pulse width, pulse length
  How long a laser pulse lasts, normally given as the full width at half maximum of its intensity.
• Q-switching
  Producing short, intense laser pulses by holding cavity losses high while energy builds up, then suddenly dropping them.
• Regenalso regenerative amplifier
  Clipped form of regenerative amplifier, the cavity that traps a seed pulse for many passes of gain before releasing it. Shop talk
• Rep ratealso repetition rate, PRF
  Clipped form of repetition rate: how many pulses per second a pulsed laser emits. Shop talk
• Saturable absorberalso bleachable absorber
  A material whose absorption drops at high intensity, used to start and sustain Q-switching or mode locking passively.
• Shotalso pulse
  A single laser pulse, especially from a high-energy or low-repetition-rate system. Shop talk
• Transform-limitedalso Fourier-limited, bandwidth-limited
  Describes a pulse that is as short as its spectral bandwidth allows, with no leftover chirp.

Frequency doubling, phase matching, parametric sources and intensity-driven effects.

• Bleed-throughalso leak-through
  Residual unwanted wavelength, such as leftover infrared pump, leaking past filters into the useful beam. Shop talk
• Doubleralso SHG crystal, doubling crystal
  Lab slang for the frequency-doubling crystal or module that turns an infrared laser into its second harmonic. Shop talk
• OPOalso optical parametric oscillator
  Optical Parametric Oscillator: a tunable source that splits pump photons into two longer-wavelength beams in a nonlinear crystal.
• Ovenalso crystal oven, crystal heater
  The temperature-controlled mount that holds a nonlinear crystal at its phase-matching temperature. Shop talk
• Phase matchingalso PM
  Keeping interacting waves in a nonlinear crystal in step so the generated light builds up instead of canceling.
• Quasi-phase matchingalso QPM
  Phase matching by periodically flipping a crystal's nonlinear orientation instead of relying on birefringence.
• Self-phase modulationalso SPM
  Spectral broadening of an intense pulse caused by its own intensity changing the refractive index it travels through.
• SHGalso second harmonic generation, frequency doubling
  Second Harmonic Generation: converting light to exactly double its frequency, half its wavelength, in a nonlinear crystal.
• Supercontinuumalso SC, white-light laser
  Extremely broad, spatially coherent white-light spectrum created by sending intense pulses through a nonlinear fiber.
• Walk-off
  The drifting apart of interacting beams in a crystal, either sideways from birefringence or in time from velocity mismatch.

Doped fibers, amplifiers, gratings and the limits that high-power fiber systems run into.

• Double-clad fiberalso DCF, double-clad
  Fiber with a small doped core for the laser light inside a much larger inner cladding that guides multimode pump light.
• EDFAalso erbium amplifier, erbium-doped fiber amplifier
  Erbium-Doped Fiber Amplifier: the optical amplifier that boosts signals near 1550 nm without converting them to electronics.
• Fiber Bragg gratingalso FBG, Bragg grating
  A periodic index pattern written into a fiber core that reflects a narrow band of wavelengths and passes the rest.
• Fiber fusealso fuse effect
  A glowing damage wave that runs back up a fiber toward the laser, leaving a trail of bubbles in the core.
• Fiber laseralso fibre laser
  A laser whose gain medium is an optical fiber with a rare-earth-doped core, pumped by diodes.
• LMAalso large mode area fiber, LMA fiber
  Large Mode Area: fiber with an enlarged core that spreads light out to raise nonlinear and damage limits.
• MOPAalso master oscillator power amplifier, MOFA
  Master Oscillator Power Amplifier: a low-power seed laser followed by amplifier stages that bring it to full power.
• Photodarkeningalso photo-darkening
  A slow, permanent rise in background loss in rare-earth-doped fiber under pumping, which erodes output power.
• PM fiberalso polarization-maintaining fiber, PMF
  Polarization-maintaining fiber: fiber with built-in stress that keeps linearly polarized light in its launch orientation.
• Pump combineralso tapered fiber bundle, TFB
  A fused fiber component that merges the outputs of many pump diodes into the inner cladding of a double-clad fiber.
• SBSalso stimulated Brillouin scattering, Brillouin
  Stimulated Brillouin Scattering: narrow-linewidth light scattering backward off sound waves it creates in fiber.
• Seedalso seed laser, seeder
  The low-power laser whose output is amplified in a MOPA or amplifier chain, setting its timing and spectrum. Shop talk

Linewidth, coherence and the dispersion effects that shape spectra and pulses.

• Chromatic dispersionalso dispersion, CD
  The change of refractive index with wavelength, which separates colors in prisms and spreads pulses in time.
• Coherence length
  The path difference over which a light source keeps a fixed phase relationship and can still form interference fringes.
• Etalonalso Fabry-Perot etalon, FP
  A pair of parallel reflecting surfaces that transmits a comb of sharp frequency peaks, used to select or measure wavelengths.
• GVDalso group velocity dispersion, beta2
  Group Velocity Dispersion: how strongly a material's group velocity changes with wavelength, which stretches short pulses.
• Linewidthalso laser linewidth, spectral linewidth
  The spectral width of a single-frequency laser's output, usually given in Hz, kHz or MHz at half maximum.
• Specklealso laser speckle, speckle pattern
  The grainy pattern of bright and dark spots seen when coherent laser light scatters off a rough surface.

Optics, mounts, safety and the hands-on alignment habits of working laser labs.

• AR coatingalso antireflection coating, AR
  Antireflection coating: thin dielectric layers that cut surface reflections from about 4 percent to a fraction of a percent.
• Beam dumpalso dump, beam stop
  A device that absorbs an unwanted or terminated laser beam completely and safely, without reflecting it.
• Beam expanderalso telescope
  A pair of lenses arranged as a telescope that enlarges a collimated beam's diameter and cuts its divergence.
• Beam profileralso profiler
  An instrument, usually a camera or scanning slit, that measures a laser beam's intensity distribution and width.
• Breadboardalso optical breadboard
  A rigid, flat plate drilled with a grid of threaded holes on which optics are mounted, smaller than a full optical table.
• Burn paperalso zap paper
  Paper that marks or darkens where a laser pulse hits it, giving a quick record of beam position and shape. Shop talk
• Class 4also Class IV, class four
  The highest laser hazard class, for beams that can injure eyes and skin even by diffuse reflection and can start fires.
• Clipping
  A beam partly cut off by an aperture, mount or edge, causing power loss, diffraction rings and hot spots. Shop talk
• Damage thresholdalso LIDT, laser damage threshold
  The maximum fluence or power density an optic can take before it is permanently damaged.
• Friedalso toasted, cooked
  Lab slang for a laser diode, optic, coating or detector that has been destroyed by too much light or current. Shop talk
• Ghostalso ghost reflection, ghost beam
  A weak stray beam from a back-surface reflection that travels off in an unexpected direction or focuses somewhere it should not. Shop talk
• Glint
  A brief, bright reflection of a laser beam off a shiny object such as a tool, screw head, mount or jewelry. Shop talk
• Gogglesalso laser goggles, laser safety eyewear
  Lab shorthand for laser safety eyewear, the filtering glasses or goggles rated by optical density at specific wavelengths. Shop talk
• HeNealso helium-neon laser, He-Ne
  A helium-neon gas laser, usually red at 632.8 nm, the classic low-power alignment and reference laser. Shop talk
• IR cardalso viewer card, detector card
  A small card coated with phosphor that glows visibly where an invisible infrared beam hits it, used for alignment. Shop talk
• Kinematic mountalso kinematic
  An adjustable optic holder that uses three-point contact and fine screws to set tip and tilt repeatably.
• Lambda over tenalso lambda/10, tenth-wave
  Shorthand for an optic flat to one tenth of a wavelength, used to mean good-quality laser optics. Shop talk
• ODalso optical density, OD rating
  Optical Density: a logarithmic measure of how strongly a filter or laser eyewear blocks light at a given wavelength.
• Optical isolatoralso isolator, Faraday isolator
  A one-way valve for light that passes a beam forward and blocks reflections from returning to the laser.
• Peak upalso peak, tweak up
  To fine-tune alignment knobs while watching a power meter or signal until the reading reaches its maximum. Shop talk
• Periscope
  A pair of stacked mirrors that changes a laser beam's height above the table, and optionally rotates its polarization.
• Pickoffalso pick-off, beam sampler
  An optic, usually an uncoated wedge, that samples a small fraction of a beam and sends it to a diagnostic. Shop talk
• Scratch-digalso S-D, scratch and dig
  The surface quality rating for optics, given as two numbers such as 20-10 for the allowed scratches and pits.
• Walk the beamalso beam walking, walking the mirrors
  To align a beam onto a path using two steering mirrors adjusted in turn against two targets or irises. Shop talk
• Waveplatealso wave plate, retarder
  A birefringent plate that shifts the phase between two polarization components to rotate or convert polarization.

Frequently asked questions

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