A laser link that aims itself.
The third job for the same ₹2,500 stage. After writing patterns and aligning fibres to chips, XY Lab holds a fibre behind a lens and keeps a two-way laser link locked on target between rooftops up to a kilometre apart — while the buildings sway. Step through it below, or grab the scene and look around.
Two roofs, one invisible beam
where A’s fibre tip is
Ring on dot = linked. The circle is the fibre core.
Same stage. This time it moves the fibre.
One fibre tip on the stage. Nothing else.
- 01Fibre-network transceiver (SFP). A mass-produced plug-in module with its own laser and light detector, made for fibre-optic networks. It stays on the fixed frame.
- 02Media converter. Turns the SFP into an ordinary Ethernet port, so a camera, a Raspberry Pi or a laptop plugs straight in.
- 03Optical fibre. A short patch cord from the SFP. Its free end is the only thing on the stage — it weighs a few grams.
- 04XY Lab stage. The same one-piece printed stage, stood on its back so its slide holder faces the lens. A small post on the holder carries the fibre tip; the flexures bend to move it sideways and up-down in steps of tens of nanometres. Nothing slides.
- 05Lens. Fixed one focal length in front of the tip. It turns the light from the tip into a straight beam, and focuses the incoming beam back to a point.
Both ends talk and listen at the same time.
The link uses a matched pair of BiDi transceivers. Each sends on one wavelength and listens on the other, through a single fibre. A tiny filter inside the module keeps the outgoing and incoming colours apart.
So the two directions share one beam, travelling opposite ways at once — like a two-lane road, not a walkie-talkie. The video only goes from A to B, but the network needs the return lane to confirm the link is alive.
And because light retraces its path, the stage only has to do one thing: put A’s fibre tip exactly where B’s light lands. A’s own beam then points straight back at B.
Both wavelengths are invisible infrared; the colours here are for explanation only.
Why a nanometre stage, for a kilometre link.
| Quantity | Value | At 1 km |
|---|---|---|
| Single-mode fibre core | ≈ 9 µm | the target the light must hit |
| Pointing tolerance (core ÷ focal length) | ± 45 µrad | ≈ ± 4.5 cm |
| Smallest XY Lab increment, resolved optically | < 20 nm | ≈ 0.2 mm of beam movement |
| XY Lab travel, about 3 × 3 mm | ± 15 mrad | ≈ ± 15 m of correction |
| Light travel time | 3.3 µs | the link adds almost no delay |
| Correction speed (geared motors) | up to ≈ 20 Hz | drift, sway and bumps |
Stage figures are measured on XY Lab; link figures are calculated targets for a design that has not yet been built.
What this is not — yet.
- Not a measurement. The stage exists and is measured; the link on this page is simulated. The kilometre is the goal we are building towards, starting at tens of metres.
- Slow disturbances only. The stage corrects drift, sway and bumps up to about 20 Hz. The fast shimmer of hot air over a kilometre is faster than that and is not corrected.
- Weather wins. Fog and heavy rain block every laser link. A radio link as backup covers those hours.
- The fixed end must stay still. For long rooftop links, the second terminal gets its own stage too.
- Eye safety first. Low power, beams above head height and away from flight paths.
Want a link like this, or a stage to try it?
Write to us about campus or rural links, research collaborations, or building your own terminal on an XY Lab stage.
© 2026 F²R — Fiction to Reality. Application notes AN-01 · Step-and-repeat lithography, AN-02 · Fibre-to-chip alignment, AN-03 · Free-space laser link / F2R-XY-01. The 3D scene is an illustration: the stage is the XY Lab model, but distances, beam widths, angles and the holder’s travel are exaggerated so the mechanism can be seen. Beam colours are false colour.