A laser lab on an optical table

A top-down view of an optical table, with holes every 25 mm. Each beam is drawn as a ribbon as wide as the beam itself, made a few times wider so you can see it, and as bright as its power on a logarithmic scale: a 1 mW beam and a 1 µW reflection are both visible. The colour is the colour of the wavelength; infrared and ultraviolet beams are drawn in a dim false colour.

Detectors show what reaches them. A screen shows the light falling on it; a camera adds noise and saturation; a power meter reads watts; a spectrometer spreads the light by wavelength. Click one and its reading opens in the Inspect tab. Pin it to keep its card on the table while you work.

Interference

• Michelson interferometer: Split a beam, send it down two arms and bring it back: fringes that move by nanometres.
• Mach–Zehnder interferometer: Two splitters, two mirrors, two outputs that share the light between them.
• Fabry–Pérot cavity: Two facing mirrors pass light only when it fits between them a whole number of times.
• Beat note between two lasers: Two lasers of slightly different colour make a photodiode flicker at their difference.

Diffraction

• Young’s double slit: Two slits a quarter of a millimetre apart, and fringes on a screen a metre away.
• Single-slit diffraction: A tenth of a millimetre of slit spreads a laser into a row of fading spots.
• Airy disc from a pinhole: A round hole makes a bright disc ringed by faint circles.

Polarisation

• Malus’s law and three polarisers: Crossed polarisers block everything, until a third one goes between them.
• Brewster’s angle: At one angle, glass reflects no light polarised in the plane of incidence.
• Wave plates and circular light: A quarter-wave plate at 45° turns linear light circular.

Rays and beams

• A prism’s spectrum: White light through glass fans out into its colours.
• Imaging with a lens: Rays from an arrow meet again behind a lens, upside down.
• Focusing a laser beam: A lens squeezes a laser to a waist tens of micrometres across, and no smaller.
• Beam expander: Two lenses a focal length apart make a beam four times wider.
• Scattering and fluorescence: A green beam through milky water and a dye: scattered light, and orange glow.

Lasers

• Build a HeNe laser: A discharge tube between two mirrors lases only when they are aligned and stable.
• Passive Q-switching: A saturable absorber turns a continuous Nd:YAG laser into a train of nanosecond pulses.
• Passive mode-locking: A fast saturable absorber locks a laser’s modes into picosecond pulses, c/2L apart.

Open the Parts tab and click a part to pick it up, or drag it onto the table. Select a part to see its settings, move it with the mouse or type its position, and turn it. Real optics sit on kinematic mounts with fine screws, and so do these: “Fine adjustment” tips and tilts a mirror by microradians and moves it by nanometres on a piezo, which is how you walk a beam into a pinhole or scan an interferometer.

Hover over a beam to read its power, width, wavelength and polarisation at that point. The Room tab sets the table’s isolation, air currents and temperature drift, and how the beams are drawn.

Everything you build is saved in your browser. Share copies a link that holds the whole bench, so whoever opens it sees exactly what you see; Save writes it to a file.

Keys	What they do
Click a part in the Parts tab	Pick it up; click the table to put it down
Drag a part	Move it, snapping to 5 mm (hold Alt to move freely)
Shift-click	Add a part to the selection, or take it out
R, Shift+R	Turn the selection 15° anticlockwise or clockwise
[ and ]	Turn it by 1°
Arrow keys	Move it 1 mm, or 0.1 mm with Shift
Ctrl+D	Duplicate
Delete	Remove the selection
Ctrl+Z, Ctrl+Y	Undo and redo
F	Fit the table to the screen
Escape	Drop what you are holding, or clear the selection

Each beam is a Gaussian beam: a narrow bundle with a width $w$, a wavefront curvature and a direction. Its size and curvature live in one complex number, the beam parameter $q$, and every lens, mirror and stretch of free space changes it by the ABCD rule

A beam carries its polarisation as a Jones vector and its phase as the optical path it has travelled. At a splitter it divides into two beams that remember they came from the same laser. Where beams from one laser meet on a detector, their fields are added, not their powers:

That sum is what makes fringes. Beams from different lasers, or from a lamp, add as powers unless their wavelengths are close enough to beat.

Slits, pinholes, gratings and other apertures need more than beams: the lab replays the light through them by Fresnel propagation on a grid, using the Fourier transform, and the pattern on the screen comes from that calculation, not from a textbook formula.

When mirrors around a gain medium send light back on itself, the lab finds the closed loop, works out the cavity’s mode from its round trip, and balances saturated gain against the losses. If the gain wins it lases and a beam comes out of the output coupler; tilt a mirror by too much and it stops.