The last lesson left us with a problem of arithmetic: the Kerr effect lets light switch light, but only at gigawatts per square centimetre. From the late 1970s to the 2000s, some of the best physicists and engineers alive attacked that arithmetic from every angle, and this lesson is their parts catalogue. The strategies all reduce to one of three moves: borrow a stronger nonlinearity from semiconductor carriers instead of bound electrons; stretch the interaction over kilometres of fibre so a feeble nonlinearity has time to add up; or trap the light in a resonant box a wavelength across until modest powers become enormous intensities. Each move works. Each exacts a price — in speed, in size, or in fragility — and the pattern of those prices is the real syllabus here. Read this catalogue the way a sceptical engineer would: not "does it switch?" (they all switch) but "what did the switching cost, and what did it break?"
A semiconductor optical amplifier (SOA) is a laser diode with the mirrors removed: a
current-pumped stripe of gain material that amplifies whatever passes through. Its population of
excited carriers is a shared, exhaustible resource — and that is exploitable. Send in a strong
"control" pulse and it eats the available gain; a weak probe arriving at the same moment finds the
larder bare and emerges dimmed. That is cross-gain modulation (XGM): control on →
probe off, an inverting gate for free. Subtler and better: the depleted carriers also change the
refractive index (carrier density and index are locked together in a semiconductor), so the control
writes a phase onto the probe — cross-phase modulation (XPM). Put one SOA
in each arm of a
The price is time. Depleting carriers is fast (picoseconds); replenishing them waits on the injection current, with recovery times of tens to hundreds of picoseconds. Drive the gate faster than it recovers and each bit sees a gain that depends on what the previous bits were — the dreaded pattern effect, a data-dependent error source with no analogue in a clocked CMOS gate. Carrier devices bought their sensitivity by storing energy in matter, and stored energy has a memory.
If you insist on the instantaneous Kerr nonlinearity of silica — femtosecond response, no
carriers, no pattern effects — you must compensate for
The switching condition is exactly the arithmetic of last lesson:
The third strategy attacks intensity directly. Recall from
Both numbers multiply your nonlinearity. A photon entering the cavity recirculates for a lifetime
| Device | Nonlinearity | Switching energy | Response / recovery | The catch |
|---|---|---|---|---|
| SOA-MZI gate | carriers (XGM/XPM) | ~10–100 fJ | ps / 25–100 ps | pattern effects; amplified noise |
| Fibre NOLM | silica Kerr (χ⁽³⁾) | ~1–10 pJ | fs / fs | metres-to-kilometres of fibre |
| TOAD (SOA-in-loop) | carriers | ~100 fJ | ps / ~100 ps | SOA recovery returns |
| Photonic-crystal cavity | carriers, cavity-enhanced | ~1–10 fJ | ~10 ps / ~10–100 ps | thermal drift; narrowband |
| Microcavity "optical transistor" | cavity QED | ~single photons (aJ) | ns-scale | cryogenics, vacuum, one-off |
| CMOS gate (for shame) | electrons | ~0.1–1 fJ | ~10 ps | none of the above |
Squint at the table and a law emerges: fast and weak, or strong and slow. The instantaneous Kerr response is too feeble to use compactly; every nonlinearity strong enough to switch at femtojoules stores its state in carriers or cavities, whose relaxation time then caps the speed. Nothing in the catalogue is simultaneously as small, as fast, as cheap and as well-behaved as the transistor it aims to replace — each entry wins one column and concedes the rest. Whether any of them could anchor a computer is not a question about a single switching event at all, and the next lesson makes that precise.
How far can "trap the light" be pushed? To the very end, it turns out. In cavity quantum electrodynamics, a single atom parked in a small enough, good enough cavity couples so strongly to the field that one photon saturates it — the ultimate nonlinearity, at the ultimate energy. In 2013, groups at MIT and the Max Planck Institute demonstrated exactly this: a cavity-trapped atomic gas in which a single stored "gate" photon toggled the cavity's transparency for hundreds of subsequent photons — a genuine optical transistor with gain, switched by one quantum of light. It is a magnificent rebuttal to any claim that optical switching is fundamentally energy-hungry. It is also an apparatus of lasers, magnetic traps and ultra-high vacuum occupying an optical table, operating at kilohertz-to-megahertz rates, at microkelvin temperatures. As a physics result: profound — nonlinear optics has no floor above the single photon. As a computing technology: a reminder that "possible" and "practical" are separated by the roughly fifteen orders of magnitude between one optical table and one square millimetre of silicon. These single-quantum tools found their true calling elsewhere — in the quantum computing module at the end of this course.
Every device on this page has a landmark paper with an eye-catching contrast ratio, and it is easy
to read "10 dB switching at 40 Gb/s" as "logic gate achieved". Resist. A demonstration typically
shows one switching event, driven by a strong, clean, lab-groomed control pulse, measured
with a sensitive receiver, under conditions (bias, temperature, polarization, timing) tuned by a
graduate student in real time. A logic gate in a computer must instead be driven by the
battered output of another identical gate, must feed several more, must not care that its
input is 3 dB low, and must do this
We now own a drawer full of working optical switches spanning six orders of magnitude in energy
and nine in size. The question that decides the fate of optical computing is not whether any of
them works — it is what a technology must provide beyond switching to be the substrate of
a computer: gain, restoration, isolation, cascadability. Those criteria — Keyes' famous checklist,
sharpened by Miller for optics — are the