- Wavelengthalso λ
- The colour of the light a laser emits, given in nanometres or micrometres. It decides what a material absorbs, so it is the first thing that makes a laser suitable or unsuitable for a job: 1.06 µm from a fiber or Nd:YAG source, 10.6 µm from a CO2 source and 355 nm from a tripled source behave completely differently on the same plastic.
- Beam quality factoralso M squared, M²
- A dimensionless number comparing a real beam to an ideal diffraction-limited one, which has M² = 1. It is the ratio of the real beam's waist-times-divergence product to that ideal. A higher M² cannot be focused as small, so a change in M² shows up as a wider cut or a weaker weld before anything else does.
- Beam parameter productalso BPP
- The beam waist radius multiplied by its half divergence angle, usually quoted in mm·mrad. It is the beam-quality figure the fiber laser industry normally states, and it is what a cutting head's optics were selected around.
- Transverse modealso single-mode, multimode
- The spatial shape of the beam across its cross-section. A single-mode beam has one clean intensity profile and the best focusability; a multimode beam carries several and focuses to a larger spot with a flatter top, which some thick-section cutting and welding prefers.
- Focal spot size
- The diameter of the beam where it is smallest, at the work. It is set by the wavelength, the beam quality and the optics in the head, not by the power setting.
- Depth of focusalso Rayleigh range
- The distance along the beam over which the spot stays close enough to its smallest size for the process to work. A shorter depth of focus makes a machine less tolerant of sheet flatness and standoff variation.
- Collimation
- Turning a diverging beam into one whose rays are close to parallel, so it can be carried a distance and then focused deliberately.
- Average power
- The optical power delivered over time, in watts. On a pulsed system it is the figure quoted on the nameplate, and on its own it says little about what each pulse does.
- Peak power
- The power inside a single pulse at its maximum, which on a short-pulse system can be many orders of magnitude above the average power. It is what allows material removal with very little heat.
- Pulse energy
- The energy carried by one pulse, in joules or millijoules. Average power divided by repetition rate gives it, which is why raising the rate on a fixed-power source lowers the energy in each pulse.
- Pulse durationalso pulse width
- How long a pulse lasts. The ranges have their own names and their own equipment: nanosecond, picosecond and femtosecond. Shorter pulses put less heat into the surrounding material for the same removed volume.
- Repetition ratealso PRF, pulse repetition frequency
- How many pulses a second the source emits, in hertz, kilohertz or megahertz.
- Duty cycle
- The fraction of time a modulated source is actually emitting, given as a percentage. Relevant to marking and coding systems whose output is gated by the production line rather than run continuously.
- Fluencealso energy density
- Energy per unit area, usually J/cm². It is the figure that decides whether a surface is marked, ablated or damaged, and it is why the same source can be safe on one optic and destructive on another.
- Irradiancealso power density
- Power per unit area, usually W/cm². The continuous-wave counterpart of fluence, and the figure that matters on a source that runs continuously rather than in pulses. The same power through a smaller spot is a completely different process.
- Chirp
- A pulse whose instantaneous frequency changes across its duration, so its colours do not arrive together. Ultrashort systems chirp a pulse deliberately to make it safe to amplify, then remove the chirp again at the output.
- Group delay dispersionalso GDD
- The wavelength dependence of how long light takes to pass through something, in fs². It is why an ultrashort pulse gets longer as it travels through glass, and why those systems contain elements whose whole job is to cancel it.
- Mode-locking
- Holding a fixed phase relationship between a laser's longitudinal modes so the output becomes a regular train of ultrashort pulses at the cavity round-trip rate. A system that has stopped mode-locking still emits light, which is why the symptom is usually described as wrong output rather than no output.
- Q-switching
- Holding loss high inside the resonator while energy builds in the gain medium, then releasing it quickly, which produces short pulses of very high peak power. The standard way a nanosecond industrial or scientific source is built.
- Harmonic generationalso SHG, THG, FHG, frequency doubling
- Converting light to a shorter wavelength in a nonlinear crystal: second harmonic halves the wavelength, third and fourth divide it further. It is how a 1064 nm source becomes a 532, 355 or 266 nm source, and conversion is never complete, so the output is a mixture that has to be separated.
- Optical parametric oscillatoralso OPO
- A nonlinear device in a resonant cavity where one pump photon becomes two of lower energy, called signal and idler. Changing the crystal angle or temperature tunes the output, which is why an OPO is used where a fixed wavelength will not do.
- Optical parametric amplifieralso OPA
- The same nonlinear process used as a single-pass amplifier for an existing signal rather than inside a cavity. Common in ultrafast systems for tunable output.
- Seed laseralso oscillator
- The small, well-behaved source that defines the pulse a larger system then amplifies. Its properties set what the whole chain can produce.
- Master oscillator power amplifieralso MOPA
- An architecture where pulse shape and timing are set by a low-power oscillator and the power comes from separate amplifier stages. It lets pulse duration be varied independently of repetition rate, which fixed-pulse sources cannot do.