# Optical Receivers **URL:** \ **Category:** Factual Questions\ **Created:** [July 30, 2002, 2:06am UTC](https://boards.straightdope.com/t/optical-receivers/121470 "2002-07-30T02:06:00Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [July 30, 2002, 2:06am UTC](https://boards.straightdope.com/t/optical-receivers/121470/1 "2002-07-30T02:06:00Z")
I don’t really have a clue how such things work. I gather that they somehow convert the light they receive from the fiber optic cable into an electrical signal. I’m particularly interested in getting a simple explanation (if possible) of how they’re able to separate thousands of simultaneously arriving light signals, and conversely, how they generate the thousands of separate output signals. Thanks. KG ---
**Author:** ![SPOOFE](https://avatars.discourse-cdn.com/v4/letter/s/c5a1d2/32.png) [@SPOOFE](https://boards.straightdope.com/u/SPOOFE)\ **Post date:** [July 30, 2002, 4:30am UTC](https://boards.straightdope.com/t/optical-receivers/121470/2 "2002-07-30T04:30:34Z")
Hmm… well, I checked the [Howstuffworks.com](http://www.howstuffworks.com/fiber-optic.htm) article on it, but it doesn’t really explain how the optical signal is transferred to digital. I’m betting that there are probably two types of light “pulses” that the fiber optic cable sends… one that represents a 0, and the other that represents a 1. ---
**Author:** ![rjk](https://avatars.discourse-cdn.com/v4/letter/r/ed655f/32.png) [@rjk](https://boards.straightdope.com/u/rjk)\ **Post date:** [July 30, 2002, 5:40am UTC](https://boards.straightdope.com/t/optical-receivers/121470/3 "2002-07-30T05:40:22Z")
The transmitter is a semiconductor laser (pretty much like an LED, except that it sends all its light in one direction) that switches on and off to send pulses of light along the fiber. The detector is a phototransistor that switches current on or off depending on the light hitting it. As **SPOOFE** says, the pulses represent ones and zeroes, which are grouped together to represent numbers, which in turn represent whatever kind of data you’re sending. ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [July 30, 2002, 5:48am UTC](https://boards.straightdope.com/t/optical-receivers/121470/4 "2002-07-30T05:48:44Z")
[http://www.olson-technology.com/ds/LaserPlus.pdf](http://www.olson-technology.com/ds/LaserPlus.pdf) ---
**Author:** ![rjk](https://avatars.discourse-cdn.com/v4/letter/r/ed655f/32.png) [@rjk](https://boards.straightdope.com/u/rjk)\ **Post date:** [July 30, 2002, 6:02am UTC](https://boards.straightdope.com/t/optical-receivers/121470/5 "2002-07-30T06:02:43Z")
And of course I skipped over the main question in the OP. :smack: The fiber itself doesn’t have anything to do with keeping channels separate; it just pushes bits. Software at the transmitting end splits data to be sent into ‘packets’ of a limited size before it’s passed on to the transmitter. Each packet has extra data tacked on to identify the channel it belongs to, usually along with a sequence number or timing info in case a packet is lost and needs to be sent again. Packets from different data streams are queued up and sent through the fiber as capacity allows. The receiver sorts them by channel and makes sure those for each channel are passed on in order. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [July 30, 2002, 7:56am UTC](https://boards.straightdope.com/t/optical-receivers/121470/6 "2002-07-30T07:56:40Z")
Each fibre can carry multiple simultaneous data streams using a variety of techniques like the way that DSL connection over copper cables doesn’t interfere with the voice traffic on the same wires because it’s transmitted at a much higher carrier frequency; with optical networks, it’s possible to multiplex by using different light wavelengths too. ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [July 30, 2002, 12:53pm UTC](https://boards.straightdope.com/t/optical-receivers/121470/7 "2002-07-30T12:53:00Z")
Thanks for your answers. It seems that there are two ways that the “simultaneity” aspect is carried out: - multiplexing using various wavelengths (a hardware solution) - attaching identifying data to packets (a software solution) I am surprised that the software solution can keep up with the hardware solution. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [July 30, 2002, 1:03pm UTC](https://boards.straightdope.com/t/optical-receivers/121470/8 "2002-07-30T13:03:14Z")
In reality all of the solutions can be in use at the same time; multiplexing (carrier frequency as well as optical wavelength) enables you to push simultaneous and continuous data streams down the same physical line, packet switching enables you to send mixed data down the same stream within that line.