Bias of the diodes in RF through Fiber transmission

[DDavid]

Hi guys,

I am trying to built Optical link.
From one side I enter with RF signal (analog):

RF Input -- [RF AMP] -- [Diode Laser] -- optic fibre

The other side:

Optic fibre -- [Photo Diode] -- [RF AMP] -- RF output

My question it's about bias of the diodes. How to build it correct!!! What specific parameters I must consider.
It my first design with optical devices so please help me!!!

Regards
David

 

[flatulent]

Re: RF thro Fiber

It all depends on the type of modulation on the RF. If it is amplitude type modulation you have to bias the diode so that the RF swings the bias up and down but not down to zero current. If the modulation is FM or PM then you can use RF square waves with any bias you want on the diode.

 

[DDavid]

Re: RF thro Fiber

Hi,

I am using AM modulation, next configuration:

Do I make a mistake???

David

 

[flatulent]

Re: RF thro Fiber

You should use the op amp method as a transconductance amplifier.

 

[DDavid]

Re: RF thro Fiber

Hi,

Op amp??? But I am work with RF. Which op amp can work with RF (my freq range is 500÷2500MHz).
I am Sorry if my question is stupid. I am never design optics before this.
I am recognize papers about transimpedance ...
Please help me!!!

David

 

[icfarmer]

Re: RF thro Fiber

i recommend a company--Maxim, who has many products for O/E and E/O conversion.the web site is www.maxim-ic.com. good luck!

 

[DDavid]

Re: RF thro Fiber

I am not understand can opertional amplifier serve with RF????
Because in next circuit suggest work with opertonal amp.

 

[flatulent]

Re: RF thro Fiber

At the frequency range you want your options are very limited.

Try your original receiving circuit with a bypass capacitor on the diode side of the inductor. You need a reservoir of charge to pass through the diode when photons enter it.

With just the inductor the current is constant because that is the property of the inductor in a series circuit.

 

[biff44]

Re: RF thro Fiber

Do you expect to get useful responses without tell the RF frequency, dynamic range, linearity, etc????

 

[DDavid]

Re: RF thro Fiber

Hi biff44,

Thank you for replying

I am interest in next parameters:
RF range: 800÷2200Mhz,
Spur free dynamic range [SFDR]: 100 [dB/Hz]
IIP3(input): 10 [dBm]
Input Noise Floor: -130 [dBm-Hz]

Regards

David D.

 

[DDavid]

Re: RF thro Fiber

Guys
I think I know how to do it
But I need opertional amp that work in my freqeuncy range (until 2.2GHz)
If some one recognized this kind of chip please!!!!

David

 

[biff44]

Re: RF thro Fiber

Jeez, I don't know about that noise floor. Laser diodes have relative intensity noise (RIN) that the photodiode detector picks up too, so that forms a fairly flat noise floor. You would want to use the best laser diode you could find, Distributed feedback (DFB) modified quantum well blah blah blah diode, which would have better RIN noise. Also, realize that the light vs diode current is not that linear!

People do make RF FO links commercially, here is one:
http://www.globalspec.com/FeaturedP...hotonicSystems/RFFiber_Optic_Receiver/27597/0

Here are some transimpedance amps:

**broken link removed**

 

[DDavid]

Re: RF thro Fiber

I know that I can buy thus device actually I did it.
But know I want to build it by myself,
Laser [tx fo] as already working pretty good.
But with Rx i had some problems, but know I understand there is no escape from using Transimpedance Amplifiers (op amp)
It's correct?

David

 

[biff44]

Re: RF thro Fiber

You use the transimpedance amplifier at the photodetector output. The diode turns photons into electrons, so need to convert the current flow to an output voltatge (transimpedance amp).

Since you are only interested in driving the laser diode with 800 MHz to 2.5 GHz, you are not concerned about DC coupling at the RF port. This makes things easy. I am not an expert driving laser diodes, but basically you would need a bias tee, with one port as the RF input (capacitively blocked), one port to the power supply current source (to supply a steady DC bias current), and the third port connected to the laser diode. I think you need some temeperature compensation circuitry in the current source to keep the quiescent operating point constant.

You need the DC bias current source for the laser diode to be low noise, since fluctuation in DC bias = fluctuations in light output = amplitude noise at the photodetector.

To drive the RF port of the bias tee, I think all you need is a common RF amplifier with some gain and good output power.

 

[xxargs]

Re: RF thro Fiber

Hi biff44,

Thank you for replying

I am interest in next parameters:
RF range: 800÷2200Mhz,
Spur free dynamic range [SFDR]: 100 [dB/Hz]
IIP3(input): 10 [dBm]
Input Noise Floor: -130 [dBm-Hz]

Regards

David D.

Hmm...

for 100 dB SFDR and input -130 dB/Hz noise floor - you need IIP3 input handle of least 20 dBm...

 

[expatriated]

Re: RF thro Fiber

Hi my friend...
with respect to specific parameters....
The most important parameters is the threshold current of the laser which higher than it, the laser begin to lase. so you must know the value of the threshold current. second is the value of the power to be transmit and the type of modulation.