I wanted to measure Laser power for using it in a biological application. I can understand that as the distance of the laser is adjusted, the beam/spot size is also varied and subsequently power varies. In such cases, when the beam size is big and laser doesn't pass exactly on the center of sensor. So please suggest how to measure a laser power for larger beam.
Hello Sabarinathan,
I was used to use a pyroelectric sensor (PE50BB-DIF by OPHIR) suited for measurements of high-energy laser beams couple of years ago. The sensor was equipped with a removable filter useful to raise the threshold of damage to the sensor itself. It was possible to remove the filter to increase the sensitivity of the sensor, if you want to measure low energy beams.
Here is a link to the equipment I used: http://www.photonicsonline.com/doc/ophir-photonics-expands-line-of-pyroelectric-energy-sensors-0001
It gave me good performance and repeatability. It helped me to carry out the research described in the attached paper.
Hope it helps.
Regards,
Carmelo
Article Influence of laser beam profile on the generation of ultrasonic waves
So first of all the power does not vary with the spot size. The power is always the same. The intensity, which is the power per area, varies.
Anyway you identified the problem correctly. If your sensor is smaller then the spot size, then you will not measure the total power. However, you could simply use an appropriate lens to collimate or focus your laser beam on the sensor chip. You can also buy large area photodiodes to measure the full spot.
Besides the Pyroelectric sensor there exist also sensor heads which scatter the light internally on a porouse white surface till all the light hit the small sensor area within the sensor head.
Cheers
Christian
The answer of Christian is correct. But, what exactly you need to know for your "biological application"? The intensity or the energy density (in case your laser is pulsed)? The pyroelectric sensor is measuring enegy of a pulsed lasers only, but is not appropriate for CW lasers. For the latters, a calibrated large area photodiode is the most appropriate for low powers and "black body" sensors for higher powers.
As suggested by Christian, use a lens to reduce the beam size to fit the surface area of the sensor. BUT MAKE SURE that you have taken into account the REFLECTION LOSS from the lens surface. If it is not a coated lens, the reflection loss from the TWO lens surfaces together will be around 8%. For glass n1=1.5 and for air n2=1. R is given by [(n1-n2)/(n1+n2)]^2. If it is a anti-reflection coated lens, refer the manual to know the reflectivity of the surface.
There are two solutions to your problem.
1. Use bigger sensor expose area so that all your laser beam fits into the sensor surface.
2. Use a lens to focus on to the sensor (whatever you have).
You will be measuring the same laser power. However with second solution, you have to take into account lens losses
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