# Optical photodectors question

1. Apr 11, 2006

### dervast

Hi to everyone... I had a silly question to ask... I know that the light in a fiber travels faster than a electromagnetic signal in a wire. I was wondering how photodetectors work because we all know that they convert the ligh to electromagnetic signals. But if the light travels faster than the e/m signal in a wire then i think that inside the photodetector some sort of information will be lost because we cant produce electricity that runs faster than the light..

2. Apr 11, 2006

### Averagesupernova

Think about this dervast: Suppose we use a speaker and a mic to go from electrical signal to audio and back to electrical. There is a HUGE difference in the signals velocities in this example yet we don't expect to lose any information.

3. Apr 11, 2006

### Staff: Mentor

First of all, the speed of light in a fiber is comparable to the speed of an electrical signal travelling down a wire. A signal propagating down a wire is just another example of E&M. And as supernova says, think of other situations where the propagation speed of information changes, but the information transfer rate does not. Like, what about when you use a flashlight to send Morse Code information to somebody else? The light carrying the information is zipping right along, but the information content is something like one word per second....

4. Apr 12, 2006

### dervast

I still cant understand the example with morse code wasnt succesful i think.. What i need to learn is how we dont lose information when light travels faster than a signal inside a copper.. For example lets assume that light travels in the theoritical spee of 3*10^8 m/s and we need to convert it to an electric signal.. How we will not lose information..? We can create an electric signal in a copper that runs as fast as 3*10^8

5. Apr 12, 2006

### Staff: Mentor

The propagation velocity of EM radiation is c in a vacuum or air, and is slower in channels where the EM is immersed in a media that has an epsilon and/or mu that is different from the vacuum values of epsilon0 and mu0. Vor example, twisted pair that you might use for Ethernet comm may have a prop velocity of 70% of c or so. Typical multi-mode optical fiber is also in the 70% of c range.

For your question, you have to think about the *modulation* technique being used for the information transfer, and how that compares to the carrier frequency. In my example of morse code with a flashlight, the carrier is light in the air, so that's in the THz or wherever. The morse information is being amplitude modulated (light on or off, 100% modulation depth) at human speed, so say 5Hz. The carrier frequency will support much higher datarates, so if you make an electronic AM modulator device for the light, you could get up into the MHz or GHz for information transfer, but not up to the THz carrier frequency.

Going the other way, you would have a problem if the modulation frequency of one channel was faster than the modulation frequency of a channel that followed it. For example, you can't use human speed Morse Code to transfer all the information that you get from an optical channel that is running AM in the MHz. You can always go to a faster and faster modulation scheme and not lose info, but not the other way around. So for your original question, the important thing is how fast the modulation frequency (and hence the information data rate) is for each channel (twisted pair versus fiber), not what the propagation velocity and carrier frequencies are. Make sense?

6. Apr 12, 2006

### Staff: Mentor

Oh, and the carrier frequencies that you use on twisted pair or coax are way lower than the THz optical frequencies. Since you will only be able to modulate your EM information up around the carrier frequency, you can get higher datarates from optical data transmission than from TP or coax transmission. It all goes with the carrier frequency and how good the technology you have is for optical modulation.

7. Apr 12, 2006

### dervast

Thx a lot ... but can u explain what i have quoted? Why cant u transfer information at thz?

8. Apr 12, 2006

### Staff: Mentor

I don't know what the maximum modulation frequency is so far for optical modulation, but I don't think we have the technology yet to do it. Maybe the research labs are getting close? Since the information is going to have to originate in electical signals from digital channels (runniung in the GHz probably), you'll need to have a technique of combining many parallel data streams in the GHz to make the THz modulation of the light work. I dunno -- maybe google "THz modulation" and see what pops up. Maybe they're already there....

9. Apr 12, 2006