Photoacoustic Effects

Arises with the use of very short-duration high-energy laser pulses, at pulse durations typically below 10 microseconds. Significant amounts of energy are absorbed and a rapid expansion occurs in the tissue, generating an acoustic shock wave that causes mechanical disruption to cellular structures.

Photochemical Effects

Effects that occur from long exposure durations at incident power levels insufficient to cause damaging photothermal effects. It is an energy dependent process (a function of the total quantity of radiation absorbed rather than its rate of absorption).

Photon

In quantum theory, the elemental unit of light, having both wave and particle behavior. It has motion, but no mass or charge.

Photothermal Effects

The damage mechanism for acute laser injury (i.e. for injury immediately following exposure). The radiation incident at the surface is absorbed in the underlying tissue, increasing the temperature of the tissue to the level at which damage can occur, and laser burns result. It is a power dependent process (a function of the RATE at which energy is absorbed rather than the total quantity of energy involved).

Plasma

In laser welding, a metal vapor that forms above the spot where the beam reacts with the metal surface. Also used to describe the laser tube (plasma tube, discharge tube) which contains the completely ionized gas in certain lasers.

Polarization

Restriction of the vibrations of the electromagnetic field to a single plane, rather than the innumerable planes rotating about the vector axis. This prevents optical losses at interfaces between the lasing medium and optical elements. Various forms of polarization include random, linear (plane), vertical, horizontal, elliptical, and circular. Of two polarization components (so-called), S and P, the P component has zero losses at Brewster’s angle. See Brewster Windows. λ = wavelength E = electric vector H = magnetic vector.

Population Inversion

When more molecules (atoms, ions) in a laser are in a metastable state than in the ground state (a situation needed for sustaining a high rate of stimulated emissions), a “population inversion” is said to exist. Without a population inversion, there can be no lasing action.

Power Meter

An accessory used to monitor laser beam power at the rear reflector, tune the beam for optimum power, or monitor power delivered to the work station.

Power Ramping

A controlled change in the power level of a laser beam, either linearly, as up a ramp, across, and down again—or in several discrete steps. Useful for smooth completion of circular welds and for preventing fractures from rapid cooling.

Pulse Duration

The duration of a laser pulse, typically measured as the time interval between the half-power points on the leading and trailing edges of the pulse.

[box]Typical units:
Microsecond (µs) = 10-6s
Nanosecond (ns) = 10-9s
Picosecond (ps) = 10-12s
Femtosecond (fs) = 10-15s[/box]

Pulse Energy

The power of a single, brief emission from a laser programmed for pulsed behavior rather than continuous operation. Pulse energy can be several times greater than CW emission.

Pulse Tail

Pulse decay time, which can be shortened (by using a special mixture of gases) to allow for fast repetition of laser pulses within a given length of time.