Laser Pulse Duration @ 2.5kHz FWHM

  • Context: Undergrad 
  • Thread starter Thread starter bhartish
  • Start date Start date
  • Tags Tags
    Laser Pulse
Click For Summary

Discussion Overview

The discussion revolves around calculating the laser pulse duration at full width half maximum (FWHM) given a pulse frequency of 2.5 kHz. Participants explore various factors influencing pulse duration, including average power, peak power, and laser type, while addressing measurement techniques and assumptions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • Some participants note that having only the pulse repetition frequency is insufficient to determine pulse duration.
  • One participant suggests that the pulse duration is less than 400 microseconds and emphasizes the importance of measuring pulse width with appropriate equipment.
  • Another participant mentions that energy per pulse can be calculated using average power and pulse frequency, but peak power is also needed for accurate pulse width determination.
  • A method involving a fast rotating mirror is proposed for measuring pulse duration, which could bypass limitations of conventional detectors.
  • Participants discuss the influence of laser type and mode of operation on pulse duration, with references to different ionization methods in gas lasers.
  • There is a calculation presented suggesting that if average power is 300 W and peak power is 750 W, the pulse duration could be estimated, but this is contested based on pulse shape considerations.
  • One participant estimates the pulse width to be between 150-200 microseconds, while another questions the accuracy of the 400 microseconds figure.
  • There is a mention of the segment length of the laser pulse based on pulse duration and the speed of light, but this is noted as not directly answering the original question about pulse duration.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the exact pulse duration, with multiple competing views and calculations presented. The discussion remains unresolved regarding the precise determination of pulse duration.

Contextual Notes

Participants highlight the need for additional parameters such as peak power and laser mode of operation, which are not fully defined in the discussion. The calculations presented depend on assumptions about pulse shape and measurement accuracy.

bhartish
Messages
26
Reaction score
0
I have pulse frequency as 2.5kHz. How to calculate laser pulse duration at FWHM (full width half maximum)
 
Science news on Phys.org
Having only the pulse repetition frequency is not sufficient to find the pulse duration.
 
bhartish said:
I have pulse frequency as 2.5kHz. How to calculate laser pulse duration at FWHM (full width half maximum)

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

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.
 
bhartish said:
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.

I don't 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
That would work if the pulse were 'flat topped'.
 
  • #11
AlexLAV said:
I don't 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?
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.
sophiecentaur said:
That would work if the pulse were 'flat topped'.
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
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:
  • #13
morrobay said:
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
Okay, but the pulse duration is not 400 μsec. That is what the OP would like to find out.
 

Similar threads

Undergrad Interference of two ultra short laser pulses
  • · Replies 15 ·
Replies
15
Views
3K
Graduate Fluorescence of a laser vs its pulse duration
  • · Replies 1 ·
Replies
1
Views
1K
Graduate Modes and Q-factor(s) of a random laser
  • · Replies 2 ·
Replies
2
Views
10K
Graduate Can a continous wave laser be converted to a more powerful pulsed laser?
  • · Replies 23 ·
Replies
23
Views
4K
Graduate Relative efficiency of SHG vs SFG
  • · Replies 6 ·
Replies
6
Views
2K
Graduate Understanding Waveform & Autocorrelation in Pulse Lasers
  • · Replies 2 ·
Replies
2
Views
10K
Undergrad Laser pulse: spectral width vs time
  • · Replies 5 ·
Replies
5
Views
3K
Graduate Confirming the pulse-bandwidth-product for fs laser pulses
  • · Replies 1 ·
Replies
1
Views
2K
Graduate Relation of laser pulse length and the Output Coupler trasmissivity
  • · Replies 5 ·
Replies
5
Views
8K
Graduate Laser rangefinder by phase-shift
  • · Replies 10 ·
Replies
10
Views
5K