Using a photodiode in optical communications

In summary, the main reason why the photovoltaic mode is considered the most suitable choice for a long distance fibre optic transmission system is because it eliminates the dark current present in the photodiode when there is zero bias, allowing for a better signal-to-noise ratio for weak signals. This is especially important for long distance fiber optic communication where weak signals may be encountered. However, this choice sacrifices bandwidth compared to using a reverse biased photodiode.
  • #1
Henry
9
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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
 
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  • #2
Henry said:

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. :smile:

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?
 
  • #3
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.

Capture.PNG
 
  • #4
Capture.PNG
 
  • #5
Henry said:
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?)...
 
  • #6
Henry said:
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...
 
  • #7
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"?.
 
  • #8
Henry said:
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.
 
  • #9
Okay, well thank you very much for your help :smile:
 
  • #11
berkeman said:
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...
 

1. What is a photodiode and how does it work in optical communications?

A photodiode is a type of semiconductor device that converts light into electrical current. In optical communications, the photodiode is used to detect and receive light signals, converting them into electrical signals that can be transmitted and processed.

2. How is a photodiode different from other types of photodetectors?

A photodiode is different from other types of photodetectors, such as phototransistors or photomultiplier tubes, because it operates based on the principle of internal photoelectric effect. This means that when light is absorbed by the photodiode's semiconductor material, it creates electron-hole pairs that generate an electrical current. Other photodetectors may use different mechanisms, such as amplification or multiplication of the electrical signal.

3. What are the key factors to consider when choosing a photodiode for optical communications?

The key factors to consider when choosing a photodiode for optical communications include the wavelength of light it can detect, its responsivity (how efficiently it converts light into electrical current), its speed of response, and its noise characteristics. Other factors may also include the size and packaging of the photodiode, as well as its cost.

4. How is a photodiode used in a communication system?

In a communication system, the photodiode is typically placed at the receiving end of the optical link. It receives the modulated light signals and converts them into electrical signals, which are then amplified and processed by other components in the system. The photodiode may also be used in conjunction with other components, such as lenses or filters, to optimize its performance.

5. What are some common applications of using photodiodes in optical communications?

Some common applications of using photodiodes in optical communications include fiber optic communication systems, laser-based communication systems, and optical wireless communication systems. Photodiodes are also used in other optical technologies, such as barcode scanners, optical sensors, and medical imaging devices.

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