Optical isolator
Passes light in one direction and blocks reverse propagation.
Open in the canvas →In the real world
An optical isolator is a one-way valve for light: it passes a beam in the forward direction and blocks anything coming back. Lasers need one because they are unusually vulnerable to their own reflected light. A few per cent returning into the cavity can destabilise the output power, broaden the linewidth, drive a diode into mode-hopping, or — with enough power — damage the facet outright. Every optic downstream reflects something, so on any serious laser bench the isolator goes in first.
What makes it possible is a genuinely unusual piece of physics. Almost everything in optics is reciprocal: reverse the direction of propagation and the light retraces its path exactly. A waveplate that rotates polarization one way on the way out rotates it back on the way in, so no arrangement of ordinary optics can distinguish forward from backward. The Faraday effect can. A magneto-optic material in a strong axial magnetic field rotates polarization by an angle fixed by the field direction, not by the direction the light travels — a beam going the other way is rotated the same absolute way, not back.
A standard isolator stacks three parts: an input polarizer, a 45° Faraday rotator, and an output polarizer set 45° from the input one. Forward, light is polarized, rotated 45°, and arrives aligned with the output polarizer — it passes. Backward, light entering the output polarizer is rotated a further 45° in the same absolute sense, reaching the input polarizer at 90° to it, and is rejected. The non-reciprocity is the whole mechanism; without it the return trip would simply undo the outward one.
Real devices reach 30–40 dB of isolation while costing 1–2 dB going forward. Because the Verdet constant and the required rotation both depend on wavelength, an isolator is specified for a particular one, and its performance falls off away from it.
In OpticalSetup
The isolator is modelled as what it does rather than how it does it: a directional gate. Rays travelling along the element's forward direction pass through untouched; rays with any backward component are removed from the trace entirely. Rotating the element sets which way is forward, so an isolator turned 180° blocks the beam it previously passed — the simplest way to see the element working.
Its use here is the same as on a bench. Put one right after a laser, aim a mirror or a partially reflecting surface downstream, and the return beam that would otherwise travel back into the source stops at the isolator instead. Because the sketch traces reflections as real rays, that back-propagating beam is genuinely there to be blocked rather than merely implied.
Note that despite living in the Polarization category, this element does not touch polarization at all. A beam's Stokes state is identical before and after it. That is a deliberate simplification, and it differs from a real isolator in a way worth knowing about — see below.
The Faraday mechanism is not modelled. There is no rotator and there are no internal polarizers, so the element does not polarize its output the way a real isolator does: light leaves in whatever state it arrived, whereas a real device emits light polarized along its output polarizer regardless of the input. If your setup depends on that, place an explicit polarizer after the isolator to represent it. Isolation is also perfect and instantaneous rather than the 30–40 dB a real device achieves, forward transmission is lossless rather than costing 1–2 dB, and there is no wavelength, temperature, or field dependence — a real isolator works properly only near the wavelength it was built for. Nothing outside the clear aperture is affected.