Optical 101 - EEs Talk Tech #9

Dark fiber, undersea cables, and the basics of optical communication

Daniel Bogdanoff and Mike Hoffman sit down with Stefan Loeffler to discuss the world of optical communication.

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Optical and electrical - 0:00

Stefan was at OFC 1:00
What is optics? 1:21
What is optical communication? 1:30
There's a sender and a receiver
Usually we use a 9 um fiber optic cable
But sometimes we use air as a medium 1:50

How Stefan got into optical 2:00

The transmitter is usually a laser 3:35
Why LEDs don't work for optical 3:50

Optical alignment is a challenge 5:00
Alignment is often done by robotics

How is optical different from electrical? 6:30

Photodiodes are receivers, and use a transimpedance amplifier
It's "electrical in, electrical out"

Optical used to be binary, but now it's QAM 64 7:50

Why do we have optical? 8:20
A need for long distance communication led to optical
Communication lines used to follow train tracks, and there were huts every 80 km. So, signals could be regenerated every 80 km.

In the 1990s, a new optical amplifier was introduced. 9:50

Reamplifcation vs regeneration 10:00

.1 dB per km loss in modern fiber optic cable 11:20
This enables undersea communication
Undersea has to be very reliable

How does undersea communication happen? 12:30
Usually consortiums.

AT&T was originally a network provider

What' is dark fiber? 14:30
Fiber is cheap, installation and right-of-way is expensive 15:00

What if fiber breaks? 16:00

Dark fiber can be used as a sensor 16:30
(change in refractive index)

Water in fiber optic line is bad
Anchors break fiber optic cable 17:30

Fiber optic cable can be made out of a lot of different things 18:15

Undersea fiber has to have some extra slack in the cable 18:30
Submarines inspect fiber optic cable 19:30

You can find breaks in the line using OTDR 21:00
"Optical time domain reflectometry"

A "distributed reflection" means a mostly linear loss. The slope tells you the loss rate.

Refractive index in fiber is about 1.5

Latency and delay 23:00
The main issue is the data processing, not the data transmission

A lot of optical engineers started in RF engineering 24:00

Environmental factors influence the channel 25:00
Factors include temperature, pressure, and bends

Recently, thunderstorms effected the fiber channel

Distributed fiber sensing for drilling 26:15

Polarization in fiber 27:00
Polarization multiplexing
Uses 194 THz, 50 nm windows

Future challenges for optical 28:25
It's cost driven. Laying fiber is expensive, and when alll dark fiber is being used, you have to increase bandwidth on existing fiber.

Shannon relation 30:00

Predictions 31:10

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