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

[Dr Bruce Griffiths]

Didier Juges wrote:
> 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
>>> _______________________________________________
>>> time-nuts mailing list
>>> time-nuts at febo.com
>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>>
>>>   
>>>     
>>>       
>> 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
>
> _______________________________________________
> time-nuts mailing list
> time-nuts at febo.com
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>
>   
Didier

There is no virtually possibility of direct modulation working 
sufficiently well.
If an external modulator is unaffordable, then forget it.
All successful RF over fiber applications uses external modulators.

Integrated lasers + modulators cost somewhere in the region of $US1000 
or a perhaps a bit more.
The microwave InGaAs photodiode and matched amplifier at the other end 
has a similar cost.

End to end, the phase noise floor of around -130dBc/Hz at L band.
This may well be good enough.

Bruce