A question about laser driver's automatic power control~~

hi guys:
Now I am designing a laser driver's automatic power control circuit which is called APC in short .I have read some articles about laser driver circuit.But I don't understand is:

Suppose the temperature doesn't change and I(bias)=30mA,I(mod_pp)=10mA,
I(bias)=bias current for laser diode.
I(mod_pp)=modulation peak-to-peak current for laser diode.
how the laser driver keeps constant power while the amplitude of I(mod) is changing???

Can anyone know or tell me some article about this,thanks very much!

Re: A question about laser driver's automatic power control~

newhand said:

hi guys:
Now I am designing a laser driver's automatic power control circuit which is called APC in short .I have read some articles about laser driver circuit.But I don't understand is:

Suppose the temperature doesn't change and I(bias)=30mA,I(mod_pp)=10mA,
I(bias)=bias current for laser diode.
I(mod_pp)=modulation peak-to-peak current for laser diode.
how the laser driver keeps constant power while the amplitude of I(mod) is changing???

Can anyone know or tell me some article about this,thanks very much!

Click to expand...

Actually in APC, the system try to adjust the average power to be constant.

iavg= K* (ibias + imon/2) . k= optical to electrical ratio

When u use monitor photodiode for optical to elctrical convertion, the current will convert to voltage by using resistor Rpd( see attachment). when optical power is change, so does the voltage across the Rpd. But it will regulate to same voltage as
Abias in negative feeback by using the opamp to adjsut ibias to mantain average
power constant

Hope it helps

Surianova

Re: A question about laser driver's automatic power control~

Surianova:
Thanks for your explanation.
Actually,I have designed a circuit like your attachment.Now I am confused about my simulation result.
At first,I set I(bias) by modifying the resistance of Rpd,suppose I(bias)=50mA and I(PD)=480uA;then I set I(mod)=20mA,and get such simulation result:
I(LD) is varying between 50mA and 70mA,
I(PD) is keep constant at 480uA.

I(LD):the current of Laser Diode
I(PD):the current of PhotoDiode

It seems that the circuit has performed the automatic power control since the current through the PhotoDiode is constant.However,we know that I(pd)=αI(LD).
(α:the conversion factor from Laser diode current to Photodiode current.)
So why I(PD) keeps constant when I(LD) is varying???

Re: A question about laser driver's automatic power control~

newhand said:

Surianova:
Thanks for your explanation.
Actually,I have designed a circuit like your attachment.Now I am confused about my simulation result.
At first,I set I(bias) by modifying the resistance of Rpd,suppose I(bias)=50mA and I(PD)=480uA;then I set I(mod)=20mA,and get such simulation result:
I(LD) is varying between 50mA and 70mA,
I(PD) is keep constant at 480uA.

I(LD):the current of Laser Diode
I(PD):the current of PhotoDiode

It seems that the circuit has performed the automatic power control since the current through the PhotoDiode is constant.However,we know that I(pd)=αI(LD).
(α:the conversion factor from Laser diode current to Photodiode current.)
So why I(PD) keeps constant when I(LD) is varying???

Click to expand...

Hi!


1. I(PD) is constant because this value is extarcted using low pas filter.
You need to put capacitor around 0.1 uF at the monitor photodiode to get the average current and for APC loop compensation purpose. That why you get I(PD) constant while the I(LD) still varying. And monitor photodiode will contribute 50 pf and normally use the low speed photodiode because you just want to get the DC current. Anyway, I(PD) should be average value of Ibias + Imon.


2. I(LD) is the actual current going into Vcsel diode, it is Ibias + Imon.

Hope this help you

Surianova

Re: A question about laser driver's automatic power control~

1. I(PD) is constant because this value is extarcted using low pas filter.
You need to put capacitor around 0.1 uF at the monitor photodiode to get the average current and for APC loop compensation purpose. That why you get I(PD) constant while the I(LD) still varying. And monitor photodiode will contribute 50 pf and normally use the low speed photodiode because you just want to get the DC current. Anyway, I(PD) should be average value of Ibias + Imon.


2. I(LD) is the actual current going into Vcsel diode, it is Ibias + Imon.

Hope this help you

Surianova

Click to expand...

Hi Surianova:
In the above passage,you wrote"I(PD) should be average value of Ibias + Imon",I am wondering output is changing depending on the input.When we use different input,such as 101010...,or 111000...,or something like that.
(1:I(LD)=Ibias+Imon;0:I(LD)=I(bias))
Because the average value of Imon is different, so can I(PD),the average value of Ibias+Imon, be different?
If what I just said is true,the average power differs depending on the input,and the average power seems not to make sense.I have look up this question in many article,but still not to get an answer.
Thanks again for concentrating on my problem.

Re: A question about laser driver's automatic power control~

[/quote]

Hi Surianova:
In the above passage,you wrote"I(PD) should be average value of Ibias + Imon",I am wondering output is changing depending on the input.When we use different input,such as 101010...,or 111000...,or something like that.
(1:I(LD)=Ibias+Imon;0:I(LD)=I(bias))
Because the average value of Imon is different, so can I(PD),the average value of Ibias+Imon, be different?
If what I just said is true,the average power differs depending on the input,and the average power seems not to make sense.I have look up this question in many article,but still not to get an answer.
Thanks again for concentrating on my problem.[/quote]

Actually the current will not be almost constant if without the big capacitor there(0.1uF), but with that capacitor, able to provide longer RC delay. The current will not change much eventhough the input sequence is change.