Optical 101 – #9

Optical communication 101 – learn about the basics of optics! Daniel Bogdanoff and Mike Hoffman interview Stefan Loeffler.

Video Version (YouTube):

Audio version:

Discussion overview:

Similarities between optical and electrical

Stefan was at OFC
What is optics? 1:21
What is optical communication? 1:30
There’s a sender and a receiver (optical telecommunication)
Usually we use a 9 um fiber optic cable, but sometimes we use lasers and air as a medium

The transmitter is typically a laser
LEDs don’t work for optical

Optical fiber alignment is challenging, and is often accomplished using robotics

How is optical different from electrical engineering?

Photodiodes act receivers, use a transimpedance amplifier. It is essentially “electrical in, electrical out” with optical in the middle.

Optical used to be binary, but now it’s QAM 64

Why do we have optical communication?
A need for long distance communication led to the use of 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.

Optical amplifier test solutions

Signal reamplifcation vs. signal regeneration

There’s a .1 dB per km loss in modern fiber optic cable 11:20
This enables undersea fiber optic communication, which has to be very reliable

How does undersea communication get implemented?
Usually by consortium: I-ME-WE SEA-ME-WE

AT&T was originally a network provider

What is dark fiber (also known as dark fibre)?
Fiber is cheap, installation and right-of-way is expensive

What happens if fiber breaks?

Dark fiber can be used as a sensor by observing the change in its refractive index

Water in fiber optic line is bad, anchors often break fiber optic cable 17:30

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

Undersea fiber has to have some extra slack in the cable
Submarines are often used to inspect fiber optic cable

You can find breaks in the line using OTDR – “Optical time domain reflectometry”

A “distributed reflection” means a mostly linear loss. The slope of the reflection tells you the loss rate.

The refractive index in fiber optic cable 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, these include temperature, pressure, and physical bends
Recently thunderstorms were found to have an effect on the fiber channel

Distributed fiber sensing is used drilling

Polarization in fiber, polarization multiplexing techniques
Currently, we’re using 194 THz, which gives 50 nm windows

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

Shannon relation 30:00

Predictions 31:10

Watch the previous episode here!


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