Filter
Transmits a selected wavelength band, blocks one band as a notch, or attenuates light as a neutral-density filter, to isolate spectral signals and control beam power.
Open in the canvas →In the real world
Optical filters reject unwanted wavelengths by one of two physical mechanisms. Absorptive filters — colored or doped glass, or a dye suspended in a polymer — remove light by genuine absorption: photons in the rejected band are converted to heat inside the material. Interference filters instead use the same multilayer dielectric-coating physics as a dichroic mirror, engineered so the rejected band destructively interferes in transmission — which usually means it reflects back out rather than being absorbed. A neutral-density (ND) filter is the wavelength-flat special case of an absorptive or partially-reflective metallic coating, meant to attenuate intensity uniformly across the visible band rather than reject a specific color.
A notch filter (band-stop filter) is the inverse of a bandpass: it rejects one narrow band and transmits everything on either side. Its classic job is removing a laser line while keeping the light around it — the Rayleigh-scattered pump in Raman spectroscopy, many orders of magnitude stronger than the Raman signal a few nanometres away, or excitation light leaking into a fluorescence or multiphoton detection path. Holographic and multilayer-dielectric notch filters reach optical densities of 6 or more in the stop band with edges only a few nanometres wide.
Absorptive and interference designs behave very differently under high power: an absorptive filter converts the rejected light to heat and can be damaged or even cracked if that exceeds its thermal budget, while an interference filter's rejected light reflects back toward the source — a real hazard when placed near a laser cavity, since that reflection can re-enter the gain medium.
In OpticalSetup
One element models five filter types, chosen from the Type menu. The first four are idealized hard-edged filters — the same step-function model the dichroic mirror uses — that transmit some wavelengths completely and remove the rest. The fifth is wavelength-flat.
Modes
- Bandpass (the default) — transmits only the band from Band center − Band width/2 to Band center + Band width/2 and removes everything else. It is used to pick one laser line out of several, to isolate a fluorescence emission band, or to cut a narrow slice out of a supercontinuum.
- Longpass — transmits every wavelength at or above Cutoff and removes everything shorter. It is used to block the excitation light in front of a fluorescence detector, or to keep the long-wavelength output of a frequency-conversion stage while discarding the pump.
- Shortpass — the mirror image of the longpass: transmits every wavelength at or below Cutoff and removes everything longer. It is used to strip a residual near-infrared fundamental from a frequency-doubled beam, or to pass multiphoton signals that come out bluer than the laser exciting them.
- Notch — the inverse of the bandpass: set by the same Band center and Band width, it removes that band and transmits everything on both sides of it. It is used to take out a single laser line while keeping the light around it, as in Raman spectroscopy or when stray excitation light leaks into a detection path.
- Neutral density — attenuates every wavelength by the same Transmission fraction (0–1) and leaves the spectrum's shape alone. It is used to bring a beam's power down for a detector or camera without changing its colour.
On the canvas, the purple knob tunes the setting that matters for the current type: the band center for bandpass and notch, the cutoff for longpass and shortpass, and the transmission for neutral density.
Broadband light
For a broadband or supercontinuum beam, the transmitted spectrum is the exact overlap between the beam's band and the passband, so a wide beam through a narrow bandpass filter correctly comes out both dimmer and spectrally narrowed, and a notch leaves a gap in the spectrum with both sides intact. This also holds after dispersive glass or a prism has split the beam into wavelength samples: each sample carries its own slice of the spectrum, and the filter cuts inside that slice, so a 1 nm bandpass passes 1 nm of light rather than a whole sample, and a 1 nm notch removes just 1 nm. A notch splits a broadband beam into two beams, one carrying the light below the band and one the light above it, each with a sharp edge at the band, so glass or another filter further on never finds light inside the removed band. A detector puts the two sides back together, so a pulse is timed from the whole spectrum that passed, gap included. A pulse's duration after the filter is worked out from what passes — the transform of the surviving spectrum with the chirp it carries (see the pulsed laser) — and the passed band is kept however thin it is, so it still reaches the optics after the filter. A neutral-density filter changes only the power, so it leaves the pulse duration alone.
Rejected light simply vanishes rather than reflecting — this matches the physical picture of an absorptive colored-glass filter, but not a reflective interference filter (for a component that reflects its rejected band instead, use the Dichroic mirror; its band reflector type is a notch that sends the blocked band back out as a beam). The passband edge is a hard step with no transition slope, no per-wavelength optical density curve, and no angle dependence; a notch blocks its band completely, where a real one leaks a stop-band transmission of 10−OD. The neutral-density mode is perfectly grey at every wavelength — real ND filters have some spectral ripple — and there's no damage-threshold or thermal modeling for either absorptive heating or reflected back-power.