[time-nuts] OT: transfer of 3 GHz via fiber optic

[Didier Juges]

Dr Bruce Griffiths wrote:
> Didier Juges wrote:
>   
>> Bruce,
>>
>> I have read about this, noise performance also is not good for analog 
>> transmissions, causing very limited dynamic range.
>>
>> That's probably why they use either FM or digital coding in just about 
>> all applications.
>>
>> I just did not think the jitter would be so bad, even with a laser 
>> transmitter.
>>
>> In my case, we are just trying to send a reference signal, but it must 
>> be clean, so unless a clean-up PLL is used, forget about fiber.
>>
>> Thanks,
>>
>> Didier
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>>   
>>     
> Didier
>
> I presume you are directly modulating the laser.
> This is hopeless you get chirping and all sorts of other effects that 
> are virtually impossible to tame.
> An external electrooptic (LiNb03) modulator is the only way that 
> actually works and has a halfway decent performance.
>
> Bruce
>
>   
Bruce,

I am talking about an integrated 4 GB/s transceiver from Avago: 
AFCT-57R5. It is a small plug-in module, so they probably directly 
modulate the laser. I understand when the laser turns on, there are 
probably a bunch of transient effects until the beam is stabilized. An 
external modulator would alleviate these problems, but the cost is most 
likely out of the question for this market.

Both the transmitter and the receiver each have *average* jitter 
specifications of about 60 pS, from logic level signals.

So even with a better modulator, the receiver still would create way too 
much jitter. It is probably a pin diode. I guess the answer would be an 
heterodyne receiver?

Probably performance would be much better if it were not measured at the 
end of 10 kM of monomode fiber, but they don't say. I guess they are 
designed for an application where it's 10kM or you don't need it.


Didier