driver circuit for 100mW red laser diode to produce a 25ns pulse

[MedicMamat]

:razz:hi

need a driver circuit for a 100 mW red laser diode (laser pointer) to produce a single 25ns pulse. the repetition doesnt have to be high.

your suggestions would be very much appreciated.

thank you in advance

 

[keith1200rs]

I am not sure a laser pointer diode is suitable for such pulse mode driving. Normally an avalanche transistor would be used to provide a fast, high current pulse to the diode for example.

You could consider a MOSFET drive maybe.

Keith

 

[MedicMamat]

thanks Keith

as it is used for just a single shot short pulse. i was just wondering if it would work and hope to give it a try. laser pointer seems to work at 400mA, and for such a short pulse it should be ok run with 1A current.

what model of MOSFET would you recommend for that?

 

[keith1200rs]

Look for a small MOSFET otherwise you won't get the speed. So, you need to look for ones with quite low Ids max, maybe just a few hundred mA. Then look at the allowed peak current. I have found 1A devices with pulse ratings of 40A or more in the past.

Keith

---------- Post added at 16:38 ---------- Previous post was at 16:36 ----------

P.S. the other option is avalanche transistors but there are very few around now. Zetex still make some I think.

 

[MedicMamat]

thanks Keith
i found two two items: a Mosfet http://www.vishay.com/docs/70778/70778.pdf and http://www.vishay.com/docs/70657/70657.pdf
and BJT http://www1.elfa.se/data1/wwwroot/assets/datasheets/avDSI_HF-Transistor-BLY87a_EN.pdf , that look fast and current seems to be alright.

which one would you suggest me choose? considering, in BJTs there is something called storage time that intrinsic to it blocking the speed.

thanks

 

[keith1200rs]

They seem unusual choices. Normally you wouldn't worry about the storage or turn-off time. I'm am more used to driving high power lasers (10W to 100W) for short pulses and you would normally charge a capacitor first then discharge it into the laser. The pulse ends when the capacitor is discharged. Then the capacitor slowly charges for the next pulse. 'slowly' can be 100us.

Keith

 

[FvM]

The assumption, that laser diodes designed for continuous operation can take short pulses of higher current may be wrong. Laser diodes can be damaged by other kinds of overload than just thermal one. You have to strictly refer to the laser diode data sheet. If you have a cheap diode, it may O.K. to find it out by trial...

 

[keith1200rs]

I am sure you are right. I have seen CW lasers blow in a few ns of overdrive above their maximum ratings.

Keith

 

[emran_shd]

@ FVM
"The assumption, that laser diodes designed for continuous operation can take short pulses of higher current may be wrong."
But Can Pulsed Laser Diode be used with short pulses of current higher than the max. rated pulse current?

@ Keith
While driving the pulsed laser diode using Avalanche Transistor, there is always a reverse current peak at the end of the current pulse as in the figure below. Can this reverse peak damage the laser diode? If yes, how can it be removed?



Also Avalanche Transistor driver is supposed to be used for low ohmic load requiring high operating current. For high ohmic load, i have observed that the pulse shape distorts.Then what to do if it is required to drive a low power laser diode with operating current as low as 50mA and pulse width <10ns while it is required to put a high value resistor in series with the load to limit the current.

Finally, If a continuous laser diode is designed to dissipate more heat than the pulsed laser diode, should continuous laser diode be preferred for producing low optical power and high repetition rate pulses?

 

[FvM]

But Can Pulsed Laser Diode be used with short pulses of current higher than the max. rated pulse current?

At least not safely. In case of an incomplete specification, you may want to extrapolate it, but are you sure to understand the laser device physics?

While driving the pulsed laser diode using Avalanche Transistor, there is always a reverse current peak at the end of the current pulse.

I'm not sure about "always". Depends on the circuit, I think. You should least keep the laser diode's reverse voltage specification.

 

[HTA]

You may take a look at laser driver applications (iC-Haus Homepage - keyword: Laser Drivers Application Notes ) and newer devices like the iC-NZP/NZN are fast enough for 25ns pulses up for more then 100mW laser diodes.