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Old 04-26-2012, 08:56 PM #1
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Default Pulse Modulation with pulse width of 2ns to 20ns

I am building a laser diode power supply allowing pulse modulation with adjustable pulse width from 2ns to 20ns and repetition rate from 10kHz to 1MHz.
Because of modulation, optical feedback through photo-diode won't work and therefore are not necessary. Correct me if I am wrong.
The laser diode has a threshold current of 10mA and max current of 50mA, and my plan is to look for a power supply (PCB Circuit or bench) with current limit of 30mA. To achieve pulse width 2ns, the power supply must allow modulation up to 500MHz. A square current pulse would be sent from function generator with pulse width of 2ns and current from 0mA to 20mA (0mA because I want the optical power to be completely off) to the power supply.
Do you know the response time of a typical laser diode for injected current from 0mA to be 20mA above the threshould current? I am worried the response time may limit the rise time of the laser pulse (desired to be well below 2ns).
Please comment on the possibility of killing the laser diode. The function generator would produce some spikes, would the current limit from the power supply (either PCB or bench) be able to limit that to avoid damage of the laser diode. And since no regulator is used because modulation, what are the chances that the laser diode be damaged.


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Old 04-27-2012, 01:53 AM #2
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Default Re: Pulse Modulation with pulse width of 2ns to 20ns

You need a constant current power source for the diode or current spikes will likely toast the diode. Overvoltage usually isn't an issue for the diode as long as you stay under 2x Vf. Most classic laser diode driver circuits cannot tolerate the diode being removed from the circuit while the power is being applied so the normal method is to switch the power to the driver, and not switch the power from the driver to the diode. In this case if you use a constant voltage power supply set to the diode's Vf and a series resistor to limit current to the exact desired level, it should work fine, and you won't have to worry about a slow regulator or CC source mucking up the design, especially at high frequencies.

If you only need 10kHz to 1MHz rep rate why does the power source need to have 500MHz modulation capability?

I would recommend using a different approach. Since you want TTL type modulation (completely off to on) with a maximum current of 30mA I would simply things greatly and go with a constant voltage source and series resistor set to 30mA which feeds a transistor which gets interrupted/switched by your PWM modulator. Output then goes to the diode. This way the only frequency limits are the PWM controller you choose and the transistor/fets you choose. Standard low power red diodes are good to 1Ghz. At such high frequencies you need to think about parasitic and stray inductances and capacitances. Well, 1MHz isn't too bad but if you really meant you wanted 500MHz then most certainly. The hardest part would be picking the PWM IC and the switching transistor. Remember if you use BJTs and not FETs to compensate for the drop out voltage when setting the CV source and limiting resistor.
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Old 03-27-2013, 04:54 AM #3
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Default Re: Pulse Modulation with pulse width of 2ns to 20ns

Hai ,

I want to generate minimum pulse width of 20 ns upto 100ns(programmable).kindly suggest sime ideas how to generate...

Rajvel
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Old 03-27-2013, 10:44 AM #4
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Default Re: Pulse Modulation with pulse width of 2ns to 20ns

Quote:
Originally Posted by rajvel1988 View Post
Hai ,

I want to generate minimum pulse width of 20 ns upto 100ns(programmable).kindly suggest sime ideas how to generate...

Rajvel
Read the 1st post... The OP states how he produces those
short pulses....


Jerry
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Old 03-27-2013, 11:07 AM #5
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Default Re: Pulse Modulation with pulse width of 2ns to 20ns

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Originally Posted by lasersbee View Post
Read the 1st post... The OP states how he produces those
short pulses....


Jerry

Hai jerry,

Can u give some sample circuit to generate pwm pulse(minimum 20 ns i need)... because i am not clear what 1st code states..

Rajvel

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Old 03-29-2013, 01:12 AM #6
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Default Re: Pulse Modulation with pulse width of 2ns to 20ns

Why do you need such a short pulse?
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