OTDR travel time

[alamwte]

Hello,

I am searching about OTDR (Optical Time Domain Reflectometer) system.

It shows optical length in fiber. It has 3 main parts ; laser diode, photodetector and a proccessor.
First ligth energy is injected by OTDR into the fiber through a laser diode and pulse generator. In the measurement the returning ligth energy is seprated from injected pulse using a Coupler and fed to the photodiode. Then it is converted to electrical energy and the proccessor measure the travel time between outgoing pulse from laser diode and incoming pulse into photodiode. Then calculate the length by knowing the speed of ligth in fiber.

Now what I can not undrestand is that how OTDR measures the travel time of ligth. How it knows the time of injected pulse's time and returning pulse?

Thanks,

[ve7xen]

It knows the time of the pulse because it generated it...it's strong enough through the coupler to be measured by the photodiode too.

Then you use a fast ADC to sample the photodiode, but the timing requirements aren't *that* crazy. Propagation in fibre is about 5ns/m so you shouldn't need to do anything faster than 200MHz or so. Clever engineers don't need that.

You can probably build a poor man's version with just the optical components, a pulse generator, a couple fast comparators, and an A->B interval timer.

Dec 1988 issue of HP Journal has lots on OTDR. https://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1988-12.pdf

[alamwte]

I got it , Thank you so much

[sunandwavs]

Hi,

I read the HP pdf on the concept of an OTDR. If I understand it correctly, a circuit injects infrared light pulses into a cable. At the same time recording time-hashes in nanoseconds, which are then logically interpreted to make several determinations. Where can I look for plans or diagrams to build a simple OTDR?

[Terry Bites]

Here's a reference design from TI which details the practical aspects www.ti.com/lit/ug/tiduaz1/tiduaz1.pdf?ts=1686547431497
It may be far too complex for an experimental design, but parts of it maybe useful to you.

If you're starting from scratch, then the optics will be the hard part of this project. To transmit and recieve along the same fiber you need an optical splitter or circulator.
Prices often in the $500s +!  You can also find them on aliexpress for about $20, they may worth a try in a DIY experiment.
For a cheap laser source you might try hacking a tranciever module see "sfp-experimenter"