# Laser pulse duration

1. Jan 12, 2013

### bhartish

I have pulse frequency as 2.5kHz. How to calculate laser pulse duration at FWHM (full width half maximum)

2. Jan 12, 2013

### Bobbywhy

Having only the pulse repetition frequency is not sufficient to find the pulse duration.

3. Jan 12, 2013

### AlexLAV

The pulse repetition rate and the pulse duration do not correlate, generally speaking. Base on the provided data one can say only that the pulse duration is less than 400 microsec :-) . The most reliable way is measurement of the pulse width by fast enough photodiode and oscilloscope. If the duration is less than dosens of picoseconds other devices are used. If you do not need high presicion you can estimate the pulsewidth taking into consideration the other laser parameters. The most important are: type of the laser, resonator length, pulse energy (or the average power), and the beam spot diameter.

4. Jan 13, 2013

### bhartish

Yes I have average power = 300W, beam spot diameter = 0.37mm, I am using Co2 laser Now can we find?

5. Jan 13, 2013

### Redbelly98

Staff Emeritus
Still no.

You can get the energy per pulse -- using the joules/sec and pulses/sec values you have. From there you could get the pulse width if you knew the peak power within a pulse.

Are you actually in a lab using an actual laser? Is there a spec sheet for the laser?

6. Jan 13, 2013

### bhartish

Yes I am in lab and I have used 750W peak power laser . I have specification sheet. But I don know peak power per pulse.

I have actually used three parameters in my experiment : Laser power (in terms of duty cycle), pulse frequency and scanning speed.

7. Jan 14, 2013

### sophiecentaur

A neat way of measuring the pulse duration would be to use a fast rotating mirror and look at the duty cycle of the dashed line that would be projected on a distant screen. This method would eliminate the problem of the limitations of frequency response of any detector you were using. If you sync the rotation of the mirrir, you should get a stable pattern on the screen.

It's not quite whether you have both peak and mean power information on your spec sheet. If you did have, then the duty cycle would be in the order of
(mean power) / (peak power) - depending on the profile of the pulse, of course.

8. Jan 14, 2013

### AlexLAV

Writing "type of the laser" I had in mind not only type of the active medium but also the mode of operation? i.e. free running or Q-switched or mode locking. I'm not familiar with gas lasers but I have asked one of my friend who worked with them. He answered typical pulse duration depended on type of ionizaton of the gas. Photo-ionized lasers produce typically 1 microsec pulse plus about 20 mus of bottom. Electro-ionized lasers provide 20 mus and more. There may be other variants as well.

I suppose 0.37 mm is the beamspot after focusing rather than behind the laser output coupler.

9. Jan 14, 2013

### AlexLAV

I dont understand - if the average power is 300 W while the peak one is 750 W, it should mean the pulse duration is about 400/2.5 mus. Am I wrong?

10. Jan 14, 2013

### sophiecentaur

That would work if the pulse were 'flat topped'.

11. Jan 14, 2013

### Redbelly98

Staff Emeritus
Sounds reasonable to me, at least for a ballpark value. Though the 750 W is from the spec sheet, not an actual measurement of the laser being used. And I'm not sure if the 300 W figure is from specs or a measurement.
But it should at least give a reasonable estimate. For Gaussian-shaped pulses, it gives a value that is 26% too low.

So we estimate the pulse width to be 150-200 μs. Anybody know if there are detectors this fast at 10.6 μm wavelengths?

12. Jan 14, 2013

### morrobay

So if the pulse is 400 microseconds, 400 x 10-6
and speed of light is 3 x 108meters/second
Then the length segment of the laser pulse is only 120,000 meters

Last edited: Jan 14, 2013
13. Jan 20, 2013

### Redbelly98

Staff Emeritus
Okay, but the pulse duration is not 400 μsec. That is what the OP would like to find out.