Discussion in 'General Electronics Chat' started by CantataAndAria, Nov 23, 2014.

1. ### CantataAndAria Thread Starter New Member

Nov 14, 2014
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this is the datasheet for a laser. It says the Output Power is 5mW at Operating Current (which is 20-29mA). However the Forward Voltage is 1.2-1.5V.
According to what we know, Pout=VI.. It doesn't seem right.
Can anyone explain this to me？
I'm trying to design a Laser Drive Circuit, I guess the most import parameter here is Operating Current?

2. ### kubeek AAC Fanatic!

Sep 20, 2005
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Output power is not the same thing as input power. The output power is the optical power of the beam, and is mainly used to categorize the lasers safety.
So lets say the diode has 1.3Vf at 25mA, then the input power is 32.5mW and if the outptu power stays at 5mW, then the efficiency of the diode is roughly 15%.

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3. ### paulktreg Distinguished Member

Jun 2, 2008
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Some power will be lost as heat.

4. ### #12 Expert

Nov 30, 2010
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You are right. Operating current is the important variable. Design a constant current source in the correct range and your laser will survive.

5. ### GopherT AAC Fanatic!

Nov 23, 2012
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No,
Pout = VI * (efficiency co-efficient)

Efficiency is normally dependent on drive current and is not a constant a perfect constant.

Note the QUANTUM EFFICIENCY spec (0.5 mW output per mA input).

Also, the Pout listed is MINIMUM of 5 mW. You will likely get much more as a threshold current of 10 mA should yield 5 mW alone.

Last edited: Nov 23, 2014
6. ### ian field Distinguished Member

Oct 27, 2012
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In other words - don't forget the heatsink!

7. ### CantataAndAria Thread Starter New Member

Nov 14, 2014
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Hi, does anyone know how to set Forward Voltage? For example, in this diagram, is the Forward Voltage 2.5V on the right side? or is it the voltage drop across Laser Diode?

8. ### GopherT AAC Fanatic!

Nov 23, 2012
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Forward voltage is drop across the laser diode. Your mosfet should be controlling the current so the rest of the 2.5 V total.

That is 2.5 V source = voltage drop across the laser + voltage drop across the mosfet.

This is not a good current mirror as far as I know, and mentioned above, so you may cook your laser diode.

9. ### CantataAndAria Thread Starter New Member

Nov 14, 2014
11
0
Hi, I drew a new one, it's a very simple structure. but the problem is I don't know how to control the Forward Voltage across Laser Diode.....

Sep 20, 2005
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11. ### GopherT AAC Fanatic!

Nov 23, 2012
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On e you control the current, the forward voltage will happen on its own. Like an LED or regular diode.

12. ### ian field Distinguished Member

Oct 27, 2012
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If the OP was using a bipolar current mirror - it would be advisable to use a monolithic transistor array for the best possible Vbe matching.

MOSFET VGSthr specs are all over the place from one device to another.

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