B

Ballast resistor

This mechanism is placed in series with a laser to promote a stable electrical discharge and to prevent the laser from acting as an oscillator.

Bandpass

The range of frequencies that will pass through a filter.

Bandpass filter

A filter that ideally passes all frequencies between two non-zero finite limits and that bars all frequencies not within the limits.

Beam

A collection of rays that may be parallel, convergent, or divergent.

Beam diameter

Along any specified line that (a) intersects the beam axis and (b) lies in any specified plane normal to the beam axis, the distance between two exactly opposite points at which the irradiance is a specified fraction (e.g., 1/e²) of the beam’s peak power.

Beam divergence

The tendency of a laser beam to expand in diameter as it moves away from the source, measured in milliradians (mrad) at specified points.

Beam expander

An optical system designed to increase the diameter of a laser beam. Term generally refers to a telescope that transforms a collimated input beam with a small diameter into a collimated output beam with a larger diameter, thereby reducing the divergence of the beam.

Beam noise

Undesirable, unpredictable fluctuations in the output of a laser module. Potential sources for noise include the laser diode drive circuit or any number of external interferences. Laser modules that operate in automatic power control mode are designed to compensate for and reduce low-frequency power fluctuations.

Beamsplitter

An optical device (e.g., a partially reflecting mirror) for splitting a single beam into two or more individual beams.

Beam waist

With Gaussian laser output, the point at which the curvature of the beam is zero. This corresponds to the point at which the beam diameter is at a minimum. For lasers with elliptical beam shapes, the beam diameter is given for both the major and the minor axes.

Bias current

Bias current is the steady current applied to a laser diode to overcome the threshold current of the laser diode.

Blink reflex

See aversion response.

Brewster’s windows

The transmissive end (or both ends) of the laser tube, made of transparent optical material and set at Brewster’s angle in gas lasers to achieve zero-reflective loss for one axis of plane polarized light. They are non-standard on industrial lasers but essential if polarization is desired.

Brightness

The visual sensation of the luminous intensity of a light source. The brightness of a laser beam is most closely associated with the radiometric concept of radiance.