Physics applied to a context: fibre-optic broadband

How this internal works

• You research a context of your choice (technological, biological or astronomical) and present the physics in it, usually as a written report or a poster with notes.
• Achieved relates the key physics ideas to the context: what principle is at work and where.
• Merit explains how or why each idea applies, with cause and effect and the relevant equations used correctly.
• Excellence links the ideas together into one coherent account, so the reader sees how each piece of physics depends on the others to make the whole thing work.
• The physics must be different from what you use for the socio-scientific issue standard (AS 91527), so plan both contexts together.

Choosing a technology or context

• Choose something you are curious about that rests on at least three connected Level 8 ideas, so there is enough to link for Excellence.
• Check that good sources exist at your level: a manufacturer's datasheet, a university page or a textbook beats a single news story.
• Avoid contexts that are mostly engineering or economics with little physics, or ones so broad (all of renewable energy) that you cannot go deep.

Inside an optical fibre

• The Ultra-Fast Broadband rollout ran fibre past about 87% of New Zealanders by the end of 2022, with the signal carried as rapid pulses of laser light that stand for the 1s and 0s of digital data.
• A single-mode fibre has a glass core about 9 µm across inside a glass cladding 125 µm across, wrapped in a plastic coating.
• The core is doped to have a slightly higher refractive index than the cladding, so light meeting the boundary at a large angle is totally reflected and stays in the core.
• Because the reflection is total, almost no light leaks out, which is why a signal can cross kilometres of fibre while copper phone lines lose a broadband signal within a few kilometres.

Refraction and total internal reflection

• Light slows down in glass: the refractive index is n = c ÷ v, so in a core of n = 1.468 light travels at 3.00 × 10⁸ ÷ 1.468 = 2.04 × 10⁸ m s⁻¹.
• At a boundary, n₁ sin θ₁ = n₂ sin θ₂, with angles measured from the normal; going into a lower index, the ray bends away from the normal.
• At the critical angle θc the refracted ray would travel along the boundary (θ₂ = 90°), so sin θc = n₂ ÷ n₁.
• For any angle bigger than θc no refracted ray is possible and all the light reflects: total internal reflection, which only happens going from a higher index into a lower one.

Light and waves

Why infrared light

• Home fibre (GPON) sends data to the house at 1490 nm and back at 1310 nm, both in the near infrared, just beyond the red end of visible light (about 400 to 700 nm).
• Glass loses light by Rayleigh scattering from tiny variations in its structure, which grows as 1/λ⁴, so longer wavelengths scatter far less: light at 1550 nm scatters about 24 times less than at 700 nm.
• Beyond about 1600 nm the glass itself starts absorbing infrared, so the lowest-loss window sits around 1300 to 1600 nm.
• Each pulse is made of photons of energy E = hf = hc/λ, about 1.3 × 10⁻¹⁹ J at 1490 nm; a laser diode makes them when electrons drop across an energy gap of the same size, and a photodiode detects them when each photon frees an electron.

Try it: where infrared sits

Dispersion: why single-mode fibre

• In a wide multimode core, rays at different angles take different path lengths, so parts of one pulse arrive at different times and the pulse spreads out (modal dispersion).
• If pulses spread into each other, the receiver cannot tell a 1 from a 0, which limits how many pulses per second (bits per second) the link can carry over a given distance.
• A single-mode core is only a few wavelengths wide, so the light can only travel as one mode along the axis and modal dispersion disappears.
• A little spreading remains because the refractive index depends slightly on wavelength (chromatic dispersion), which is one reason lasers with a very narrow range of wavelengths are used.

Researching and evaluating sources

• Use at least three sources of different kinds and record each one fully (author or organisation, title, date, URL) as you go, not at the end.
• Judge each source: who wrote it and why, how recent it is, whether its numbers agree with other sources, and whether the physics is explained or just asserted.
• Rewrite everything in your own words and explain it; copying a diagram or a paragraph shows you found the idea, not that you understand it.

Try it: what each source is good for