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sending 3 intensity levels correspond to 3 dicrete voltage levels from laser???

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chaththa19

New Member
Hi all,:):confused::confused::confused:

Im trying to convert a HDB3 line coded electrical signal to optical signal and transmit over fiber and regenerate HDB3 electrical signal at reciever.Since HDB3 use 3 signals levels (+V,0,-V) I converted them in to 3 positive voltage levels.(1V,2V,0V).I fed it to a laser.

at reciever im using IF-D91 detector.but I couldnt obtain 3 voltage levels at reciever.Im so upset with the detector...please help me...my data rate is 3Mbps.

I used IF E96 LED as source before using laser.The variation at reciever is sooo little.below 0.2mV...is there a better detector?..do i need to use a special components with D91?


thankz:confused::confused::confused:
 

MikeMl

Well-Known Member
Most Helpful Member
I dont have a clue what HDB3 is, but I have a comment about the laser.

What makes you think that the optical output of a laser is linearly proportional to the applied voltage? I suggest you already have good evidence that it is not!
I dont think there is anything wrong with your receiver; rather, you need to find a method of modulating your laser:rolleyes:
 

Pommie

Well-Known Member
Most Helpful Member
You can't amplitude modulate a laser and expect good results. You need to use some kind of digital encoding but I don't think you will get 3Mbps without some advanced hardware.

Mike.
 

tunedwolf

Well-Known Member
Fibre Optic transmission uses a coding technique of one sort or another for communication. Lasers are not amplitude modulated, rather, they are switched on and off at very high data rates to represent bit patterns.

The receiver uses a device, commonly a photodiode, to receive the beam. Both ends of the fibre must be cut, polished and precisely matched to the interface and connector type, otherwise your optical link-loss will be huge and will most likely fail over anything but the shortest of links.

Terminating fibre is a tricky prospect without the right tooling and experience. The type of fibre whether multi-mode or single-mode must match the laser/ receiver pair exactly, any misalignment means severe signal loss. The wavelength of the laser will also play a part depending on several factors.

Remember that unless you are planning on using smoke signals, communication is a two way thing, to that end, you actually require two lasers, two receivers and two fibre links, one set for each direction.

If your link distance is reasonable, say less than 100M, you would be better to buy a couple of cheap FDDI transceviers and modify those to suit your application. Cheap multi-mode fibre can then be used between them. You can either buy a small termination starter type kit and do it yourself, or better still, buy your fibre from a half decent networking company, and get them to terminate your fibre for you.
Cheap in this context is a relative concept. Fibre-optic communication is expensive.

I don't want to pour cold water on your enthusiasm, but rolling your own transceivers capable of anything near 3Mbps is a non starter. Put your time and energy into your project in a sensible way, and use purpose built and tested transceivers.

rgds


I forgot to mention that as you have gotten as far as the optical link in your project, I am assuming that you have already taken care of the protocol translation, but if you haven't, then you will also have to take this into account in your design as well.
HD3B uses clock recovery for example, so you would need to take account of that etc etc.
 
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thepaan

Member
I'd listen to that dude above me if I were you but, in case you are not, I offer this pointer.

I suggest creating your 3 intensities by modifying the forward current instead of the forward voltage. Be careful you do not apply so much current as to burn out your emitter. You will get a ballpark from the spec sheet but for an exact number you will have to measure the voltage past the LED in an active circuit.

Example Luxeon Rebel(red):

40 lumens @ 350mA (Voltage regulated by the LED to around 2.9)
85 lumens @ 700mA (Voltage regulated by the LED to around 3.4)
 
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