Fiber optics
A strand of glass thinner than a hair now carries all the internet traffic between cities and continents. The reason is simple: since the carrier is light —hundreds of terahertz—, the available bandwidth is enormous, and communications-grade glass is one of the most transparent materials there is.
01How the light gets trapped
A fiber is a glass core surrounded by a cladding with a slightly lower refractive index. When light reaches the boundary at a sufficiently shallow angle, it is totally reflected: nothing escapes. By repeating that reflection thousands of times per meter, the light travels kilometers confined in the core, even following the bends of the cable.
| Type | Core | Behavior and use |
|---|---|---|
| Step-index multimode | 50 to 100 µm | Many paths with very different lengths: the pulse broadens quickly. Short spans and low speed. |
| Graded-index multimode | 50 / 62.5 µm | The index decreases outward, so the rays that travel farther move faster and arrive almost together. Campus and building networks, up to a few hundred meters. |
| Single-mode | 8 to 10 µm | The core is so thin that only one mode propagates. It is the fiber for long links and for the whole internet backbone. |
All communications fibers have 125 µm of cladding; what changes is the core. That is why connectors and tools are interchangeable, and why it is so easy to make a mistake: a single-mode and a multimode fiber look identical from the outside. They are told apart by the cable color and by the markings, not by eye.
02Attenuation, windows and dispersion
| Window | Wavelength | Attenuation | Use |
|---|---|---|---|
| 1st | 850 nm | ≈ 3 dB/km | Multimode, short spans. Cheap emitters. |
| 2nd | 1310 nm | ≈ 0.35 dB/km | Single-mode, minimum dispersion. Metropolitan networks. |
| 3rd | 1550 nm | ≈ 0.20 dB/km | Minimum attenuation. Long distance and optical amplification. |
- Absorption: impurities in the glass, above all traces of water (OH groups), which produce attenuation peaks at certain wavelengths.
- Rayleigh scattering: light is deflected by microscopic irregularities in the glass. It is stronger at short wavelengths, and it sets the attenuation floor.
- Modal dispersion: multimode only. The different paths take different times and the pulse broadens.
- Chromatic dispersion: each color travels at a slightly different speed. It limits very high speeds in single-mode and is compensated with specific fibers and modules.
A 40 km single-mode link at 1550 nm, with 4 fusion splices and 2 connectors at each end. Transmitter of 0 dBm, receiver with a sensitivity of −28 dBm.
| Item | Loss |
|---|---|
| Fiber: 40 km × 0.20 dB/km | 8.0 dB |
| Splices: 4 × 0.10 dB | 0.4 dB |
| Connectors: 4 × 0.5 dB | 2.0 dB |
| Reserve margin for repairs and aging | 3.0 dB |
| Total | 13.4 dB |
Received power: 0 − 13.4 = −13.4 dBm, far above the −28 dBm of sensitivity. The link has 14.6 dB of margin: more than enough. With those numbers one could even use a cheaper transmitter or extend the distance.
Unlike radio, in fiber there is a maximum: if the received power exceeds the receiver's limit, it saturates and the error rate gets worse. This is what happens when connecting two devices with a short patch cord using long-distance modules. The solution is to insert an optical attenuator, not to leave it as it is because it “links anyway.”
03Emitters, receivers and multiplexing
- LED: cheap, wide spectrum, low power. Multimode and low speed only.
- Laser: narrow spectrum, high power and extremely fast switching. It is what makes long-distance single-mode and high speeds possible.
- PIN photodiode: simple, stable, the most widely used.
- Avalanche photodiode (APD): has internal gain and therefore more sensitivity, at the cost of more noise and of needing high voltage.
Both are direct applications of the devices covered in special semiconductors: the same LED and the same photodiode, taken to the extreme of speed and spectral purity.
Dozens of simultaneous channels can travel over the same fiber, each one in a different color, separated at the end by optical filters. It is what allows the capacity of an already installed fiber to be multiplied without touching the cable run: you swap the equipment at the ends and that is it. In dense systems, the channels are spaced less than a nanometer apart.
And at 1550 nm there are optical amplifiers that amplify the light directly, without converting it to an electrical signal. Thanks to them a submarine cable crosses an ocean with simple repeaters every few tens of kilometers.
04Termination, splicing and measurement
| Task | Typical loss | Details |
|---|---|---|
| Fusion splice | 0.02 to 0.10 dB | The fibers are stripped, cleaned and cleaved with a precision cleaver, and the fusion splicer aligns them and joins them with an electric arc. It is the permanent method. |
| Mechanical splice | 0.1 to 0.5 dB | Alignment with index-matching gel. Fast, for temporary repairs. |
| Connector (SC, LC, FC, ST) | 0.2 to 0.5 dB | Flat, PC, UPC or APC polish. APC connectors, green, have the end face at an angle to reduce reflection. |
- Power meter and light source: measures the total loss of the span. It is the measurement that certifies a link.
- OTDR: shows the profile along the fiber and locates the fault to the meter.
- Visual fault locator: a red laser that can be seen through the cladding; it is used to identify fibers and find nearby breaks.
- Inspection scope: for viewing the connector end face. The number one cause of problems is, simply, dirt.
- Never look into the end of a fiber or of an active connector: 1310 and 1550 nm light is invisible and can damage the retina before the blink reflex kicks in. Check with a meter, never with the eye.
- The fiber scraps from cleaving are slivers of glass a few microns thick: they embed in the skin, cannot be seen and do not come out. Pick them up with adhesive tape and discard them in a rigid container.
- The isopropyl alcohol used for cleaning is flammable, and the fusion splicer generates an electric arc.
05In the lab
Before touching the fiber: shine a laser beam into a stream of water falling from a container. The light follows the curve of the stream by total internal reflection. It is the historic demonstration of the principle, and it makes visible what will later happen inside the glass.
With a source and a power meter, measure the loss of a patch cord, of a spool and of a link with splices. Calculate the attenuation per kilometer and compare it with the fiber specification. Repeat after cleaning the connectors: the difference is often surprising.
Strip, clean, cleave and fuse two fibers. Measure the splice loss and repeat until it is below 0.1 dB. Note what happens when the cleave is angled or when dirt is present: the fusion splicer reports it and the loss confirms it.
Measure the received power while bending the fiber with ever smaller radii: macrobend loss appears. Then, with an OTDR, locate a splice and a forced bend along the spool, and check the reported distance with a tape measure.
06Common errors
| Symptom | Usual cause |
|---|---|
| High loss on a new link | Dirty connectors. It is the most common cause of all, and it is solved by cleaning. |
| Unstable link that gets worse when the cable is moved | Excessive bending, a crushed cable or a loose connector. |
| The equipment does not link even though plenty of light arrives | Receiver saturated: an attenuator is missing. Or different fiber types at each end. |
| Single-mode was mixed with multimode | They look the same from the outside. Read the cable markings and respect the colors. |
| High loss in a splice | Angled cleave, dirt, or worn electrodes in the fusion splicer. |
| The OTDR shows a huge peak at the start | Dead zone of the instrument itself: a launch cable is used to measure the first connector properly. |
| Total outage with no apparent cause | Cable cut by third-party construction work. The OTDR tells you how many meters away it is. |
07Self-assessment
What keeps the light inside the core?
Total internal reflection: the core has a higher refractive index than the cladding, and rays striking at an angle greater than the critical angle are reflected completely.
Difference between multimode and single-mode fiber.
Multimode has a large core (50 to 62.5 µm) and admits many paths, which broadens the pulse; single-mode has a core of 8 to 10 µm and admits only one, so it is suited to long distances and high speeds.
Why is 1550 nm used for long distance?
Because it is the window of minimum attenuation, around 0.20 dB/km, and because there are optical amplifiers that work in that band.
What is the numerical aperture?
The maximum angle at which light can be injected so that it stays guided. It depends on the difference in index between core and cladding.
Budget for a 20 km link at 1310 nm with 2 splices and 4 connectors.
20 × 0.35 = 7.0 dB of fiber; 2 × 0.1 = 0.2 dB of splices; 4 × 0.5 = 2.0 dB of connectors; plus 3 dB of margin: total 12.2 dB.
Can “too much” light reach the receiver?
Yes: above its maximum power the receiver saturates and the error rate increases. It is corrected with an optical attenuator.
What is an OTDR for, and what does its trace show?
To see the profile of the fiber along its length. The slope is the attenuation per kilometer, the steps are splices, the peaks are connectors or reflections, and the abrupt end indicates a break, along with its distance.
What is wavelength-division multiplexing?
Transmitting several channels over the same fiber, each one at a different wavelength, separating them with filters. It multiplies the capacity of an already installed fiber.
Why should you never look into the end of an active fiber?
Because 1310 and 1550 nm light is invisible and does not trigger the blink reflex, but it can damage the retina. The presence of signal is checked with a power meter.
What is the most frequent cause of excessive loss?
Dirt on the connector end faces. Before looking for any other explanation, inspect and clean.