Pulsed light question

[ozgur85]

Hello everyone:smile:

May i ask you guys sth? I had someone make a pulse generator for my laser modules. The circuit can pulse the lasers at 5 hz or 60 hz.

The question is, when i run the lasers at continuous wave, the light is strong. It's also strong at 5 hz. But at 60 hz, the light gets "a little" dimmer. I asked the guy who made the circuit for me and he said it was normal.

What do you guys think? Lasers are 5 mW. Do you think at 60 hz, the lasers' power is also 5 mW at each flash? Or because the light is a little dimmer, the power is also a little low? Or is it just our eyes that can't see the strong flashes because it's 60hz?

Thank you very much already!

 

[D.A.(Tony)Stewart]

It may be that the temperature rise at faster pulse rates, lowers the laser efficacy and the internal voltage of the diode also drops with rising temperature. So a dimmer output is an indication your driver is pulsing beyond its rated current DC and at risk of burning out.

Verify by monitoring voltage on a 50mV current shunt and compare with specs . Your driver may also have a surge from a charged capacitor in a ( not so ) " Constant Current" sink.

This is normal temperature effect for LEDs and lasers when pulsed beyond their continuous rated DC current, even though the voltage will rise with current due to bulk resistance (ESR) instantaneously. With very low duty cycles, some laser diodes are designed for 0.1% duty cycles and high currents producing 75W peaks in a device only rated for 75mW of thermal power. Efficacy starts at <10% of thermal power in and reduces with rising temperature above threshold.

The eye will perceive a brighter result with faster rep. Rates or or pulse width only if the pulse width is much shorter than the eye response time, which varies.

For relatively long fixed times > 1-10ms , the rep rate will not increase the apparent brightness, but reduce the perception of flicker.

Flicker

Flicker is more sensitive on moving objects and more sensitive on peripheral vision, so 60Hz might start to appear continuous but if moving such as car tail LED lights, a much faster rep. Rate is needed to appear continuous such as 3 kHz.

 

[ozgur85]

Thank you very much

So you say the current sent to lasers at 60 hz is too much and the lasers are at risk of burning:-(

I should talk with the guy as soon as possible in that case..

Thank you

 

[D.A.(Tony)Stewart]

It depends on the pulse current levels x V of thermal Power. Can you verify with a scope?

 

[ozgur85]

Unfortunately i have little experience with testing. I don't want to do sth wrong. So i will go to the guy who built the driver and ask him if there is a possibility of sending too high currents at 60 hz. And want a good explanation about why lasers get a little dim when pulsing at 60 hz..

He is on holiday now and returning on Friday. I couldn't wait and wanted to ask you

Thank you

- - - Updated - - -

I couldn't wait and called him

He said there is no possibility of sending too much current to the lasers. The driver has no control of the current. The laser draws what it needs and the driver doesn't get involved he said.

I asked him that normally each laser draws about 20mA. But at 60 hz, the required current is more, for example 40 mA. I asked him can we be sure the laser gets the required 40 mA current when pulsing at 60 hz. He said let's test it..

Do you have any ideas?

Thank you:smile:

 

[FvM]

I fear there's most of the necessary information missing:
- laser type (apparently CW laser diode) and specification
- pulse duration

 

[Audioguru]

Our eyesight sees a light with a duration of 30ms or longer as having the same brightness as a steady continuous light at the same level. But pulses shorter than 30ms appear to be dimmed. That is why Pulse Width Modulation is used for LED dimmers as seen on the rear of many cars and buses today.

A bright light pulsing at 60Hz appears to be steady but if you move your eyes you can see its rapid blinking.
Avoid blinking at 15Hz to 20Hz because it causes epileptic seizures in some people.

 

[ozgur85]

Thank you all

I'm using CW red laser. 5 mW. Duty cycle is %50. I talked with the guy. He explained that the dimming is due to %50 duty cycle. I guessed that way but wanted to ask you anyway.

Thank you

 

[HTA]

Does the laser diode has a monitor diode? If so you need a driver which runs in the APC mode(automatic power control). For a better understandung of the basics on how to drive laser diodes see: https://www.ichaus.de/upload/pdf/EuroPhotonics_H89_2004_Driving_Diode_Lasers.pdf .

Enjoy your design work!

 

[D.A.(Tony)Stewart]

Integrated diodes are part of IC-Haus laser product line.

The advantage is when the diode is bonded to the laser, you can regulate the max current with diode voltage which represents laser temp.

 

[Audioguru]

With a duty cycle of 50% then 5Hz blinks have a duration of 100ms so your vision sees it with the same brightness as if it is continuous.
But blinks at 60Hz (also at 50% duty cycle) have a duration of only 8.3ms so they are less than 30ms and appear to be dimmed.