# Question about the Peak and Average Power of a Laser

• Irishdoug
In summary: If you can identify what make/model was used, you could look for a data-sheet on the manufacturer's website.
Irishdoug
Homework Statement
I am reading a paper and want to calculate the average power and peak power of the laser they have used in it.
Relevant Equations
Pav = Energy*rep.rate / Ppeak = Energy/pulse duration
Hi, I am reading paper that I have to give a quick presentation on. I want to calculate the average power and peak power of the laser used, a Ti:Saphire laser.

I have the wavelength they are lasing at (450nm), the repetition rate 81Mhz and the pulse duration (pulse width) of 100fs.

I have done the following calculation: ##E = \frac{hc}{\lambda} = \frac{6.626x10^{-34}c}{450x10^{-9}} = 5.372x10^{-19}J##

##P_{peak} = \frac{5.372x10^{-19}J}{100x10^{-15}} = 4.417x10^{-6} J/s##
##P_{av} = Energy*Repetition Rate = 5.372x10^{-19}J (81x10^{6}Hz) = 3.6x10^{-11}J = ~0.04nJ/s##

I am unsure as to my calculations as the numbers seem incredibly small. I was expecting mW and W at least. The laser is being used in biophotonics so maybe these low powers are required, but I am unsure as I have no experience in this area. Thankyou.

Irishdoug said:
Relevant Equations:: Pav = Energy*rep.rate / Ppeak = Energy/pulse duration

Hi, I am reading paper that I have to give a quick presentation on. I want to calculate the average power and peak power of the laser used, a Ti:Saphire laser.

I have the wavelength they are lasing at (450nm), the repetition rate 81Mhz and the pulse duration (pulse width) of 100fs.

I have done the following calculation: ##E = \frac{hc}{\lambda} = \frac{6.626x10^{-34}c}{450x10^{-9}} = 5.372x10^{-19}J##

##P_{peak} = \frac{5.372x10^{-19}J}{100x10^{-15}} = 4.417x10^{-6} J/s##
##P_{av} = Energy*Repetition Rate = 5.372x10^{-19}J (81x10^{6}Hz) = 3.6x10^{-11}J = ~0.04nJ/s##
A few points to think about which might help...

“Pav = Energy*rep.rate / Ppeak = Energy/pulse duration”
can't be correct because "Energy*rep.rate / Ppeak" does not have the dimensions of power (it has no dimensions).

In your equation, what do you mean by ‘Energy’? Is it the energy in a single pulse? I think that you have used Energy = ##hc/\lambda##. This is the energy in a single photon; but there are many photons in a pulse.

(To get the energy in a pulse, you would need to multiply ##hc/\lambda## by the number of photons in the pulse, This would make your answer much bigger!)

There are two ‘average powers’ and it is not clear to which you refer:
a) the average power during a single pulse; this depends on the shape of the pulse and the peak power during the pulse;
b) the average power-output over some extended time-period (during which there are many sequential pulses and time-gaps between each).

The data (wavelength, pulse repetition rate and the pulse duration) are not sufficient to calculate any type of power. You need additional data.

You have arithmetic mistakes.

Other points:
- the symbol for megahertz is MHz, not Mhz;
- if you want a multiplication sign inside a Latex expression, use \times rather than ##x##. This improves readability/appearance.

Irishdoug
I apologies. The dash was meant to separate the
Steve4Physics said:
A few points to think about which might help...

“Pav = Energy*rep.rate / Ppeak = Energy/pulse duration”
can't be correct because "Energy*rep.rate / Ppeak" does not have the dimensions of power (it has no dimensions).

In your equation, what do you mean by ‘Energy’? Is it the energy in a single pulse? I think that you have used Energy = ##hc/\lambda##. This is the energy in a single photon; but there are many photons in a pulse.

(To get the energy in a pulse, you would need to multiply ##hc/\lambda## by the number of photons in the pulse, This would make your answer much bigger!)

There are two ‘average powers’ and it is not clear to which you refer:
a) the average power during a single pulse; this depends on the shape of the pulse and the peak power during the pulse;
b) the average power-output over some extended time-period (during which there are many sequential pulses and time-gaps between each).

The data (wavelength, pulse repetition rate and the pulse duration) are not sufficient to calculate any type of power. You need additional data.

You have arithmetic mistakes.

Other points:
- the symbol for megahertz is MHz, not Mhz;
- if you want a multiplication sign inside a Latex expression, use \times rather than ##x##. This improves readability/appearance.
Hi, sorry I stupidly used / to separate the equations. They should read ##P_{av} = Energy \times rep.rate ## ## P_{peak} = \frac{E}{pulse width}##.

##P_{av}## is the energy over a period, whereas ##P_{peak}## is the energy in one single pulse. What is the additional data that I would require?

But yes I didn't take into account there are many photons in a pulse! Thankyou.

Irishdoug said:
##P_{av}## is the energy over a period, whereas ##P_{peak}## is the energy in one single pulse. What is the additional data that I would require?
You are mixing-up 'power' and 'energy' which are not the same thing. Take care to use the symbols 'E' for energy and 'P' for power.

The average power output of a laser and the energy per pulse are measured by the manufacturer. The values will be different for different makes and models. You can't calculate one without the other.

If you can identify what make/model was used, you could look for a data-sheet on the manufacturer's website. Or you could directly contact the manufacturer.

By the way, it's 'sapphire' not 'saphire'. (A very common spelling mistake!)

Last edited:

## 1. What is the difference between peak power and average power of a laser?

The peak power of a laser refers to the maximum power output of the laser beam at a specific point in time, while the average power refers to the average power output over a certain period of time. Peak power is typically much higher than average power, as it represents the most intense burst of energy from the laser.

## 2. How are peak power and average power measured?

Peak power is typically measured using a fast detector that can capture the highest intensity of the laser beam, while average power is measured using a slower detector that can integrate the power over a longer period of time. Both measurements are important in understanding the capabilities and performance of a laser.

## 3. What factors can affect the peak and average power of a laser?

The peak and average power of a laser can be affected by factors such as the type of laser, its design and construction, the quality of the components used, and the environmental conditions in which it is operated. Additionally, the power output can also be controlled by the operator through adjustments to the laser's settings and parameters.

## 4. Why is it important to know the peak and average power of a laser?

Knowing the peak and average power of a laser is important for several reasons. It helps determine the laser's capabilities and limitations, allows for proper safety precautions to be taken, and ensures that the laser is being used efficiently and effectively for its intended purpose.

## 5. How can the peak and average power of a laser be increased?

The peak and average power of a laser can be increased by using higher quality components, optimizing the laser's design and construction, and implementing advanced techniques such as mode-locking or Q-switching. However, it is important to note that increasing the power output of a laser also comes with potential risks and safety considerations.

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