Using a photodiode in optical communications

[Henry]

Homework Statement

[/B]
As part of my coursework in a sensor systems module at university I was given the question:

State the most important reason why the photovoltaic (as opposed to photoconductive) mode would be the most suitable choice for a long distance fibre optic transmission system.

Homework Equations

From my lecturers notes, I could draw a two main answers to that although the question asks for the most important reason.

The Attempt at a Solution

My reasons would be:
There isn't a dark current present in the photovoltaic mode (i.e. zero bias).
There is no chance of an avalanche effect being experienced.
Any help would be greatly appreciated!

Many thanks,
Henry

 

[berkeman]

Homework Statement

[/B]
As part of my coursework in a sensor systems module at university I was given the question:

State the most important reason why the photovoltaic (as opposed to photoconductive) mode would be the most suitable choice for a long distance fibre optic transmission system.

Homework Equations

From my lecturers notes, I could draw a two main answers to that although the question asks for the most important reason.

The Attempt at a Solution

My reasons would be:
There isn't a dark current present in the photovoltaic mode (i.e. zero bias).
There is no chance of an avalanche effect being experienced.
Any help would be greatly appreciated!

Many thanks,
Henry

Welcome to the PF.

So by "photovoltaic" you mean a photodiode, and by "photoconductive" you mean a Light Dependent Resistor?

Photodiodes do have a small dark current (leakage), so I don't think that's an answer. What characteristics are important for fiber-optic communication?

 

[Henry]

Thank you!

No sorry, by photovoltaic I mean a no bias setup with a photodiode and by photoconductive I mean a reverse bias setup like in the attachment :). That's a slide from my lecture notes.

 

[Henry]

 

[berkeman]

Thank you!

No sorry, by photovoltaic I mean a no bias setup with a photodiode and by photoconductive I mean a reverse bias setup like in the attachment :). That's a slide from my lecture notes.

View attachment 114618

Hmm. So you are supposed to say why a receiver with no reverse bias on the photodiode is best for long-distance FO communication? That's counterintuitive to me, so I guess I should let others chime in. I know why all of my FO receivers use reverse biased photodiodes (can you think of that reason?)...

 

[berkeman]

View attachment 114619

Yeah, that higher speed of the reverse biased photodiode is what I was looking for. But I guess they are saying that the "no dark current" with no reverse bias is more important in FO detectors for very weak signals. I hadn't heard that before, but I suppose it's possible. Still, the bandwidth of an unbiased photodiode is so much worse...

 

[Henry]

Yeah I was a little confused when looking around because I saw a lot of people saying the same (i.e. that reverse bias is better). I'm a little confused to be honest, I might go and talk to my lecturer and see what he says, do you think he was implying weak signals by saying "long distance"?.

 

[berkeman]

do you think he was implying weak signals by saying "long distance"?.

That would be my guess.

It might be worth looking around some to see if you can find some typical FO receiver circuits used in the trans-Atlantic FO cables or other long-haul FO systems. I would think that high BW is so important that they would just deal with the dark current in their S/N calculations. After all, the dark current is mostly DC, so filtering it out should be possible.

 

[Henry]

Okay, well thank you very much for your help

 

[berkeman]

Maybe read through this Wikipedia article:

https://en.wikipedia.org/wiki/Fiber-optic_communication#Bandwidth.E2.80.93distance_product

It may offer some insights.

 

[berkeman]

Maybe read through this Wikipedia article:

I meant to post the link to the top of the article, but that link is a good place to start...