The first decision to be made is: do we want to measure power or average power for pulsed lasers or pulse energy? Different sensors are used for these measurements. The power of a CW laser or the average power of a pulsed laser will be measured by a thermal sensor or a photodiode-based sensor, while the energy of each pulse of a pulsed laser will be measured by a pyroelectric head.
Since the measurement requires us to apply the laser beam to the sensor, we need a sensor that will stand up to the laser. Laser power density is the ratio of the laser power over the area of the beam cross section. It is measured in W/cm2 . One of the sensor characteristics is its damage threshold, also expressed in W/cm2. If a laser beam with a power density greater than the damage threshold of the sensor is applied to the sensor, it will damage it. The damage can be cosmetic, affecting only the appearance of the detector, or functional, having a negative effect on the sensor accuracy. In order to calculate the beam density, we need to know the beam maximum power and its diameter. The third piece of data we need is the laser wavelength, in order to match it to the sensor absorber characteristics.
We need to select a sensor whose power rating is equal to or higher than the maximum power we will measure, whose aperture is larger than the laser beam diameter so that it will contain the whole beam, and whose damage threshold is higher than the maximum expected laser power density. The sensor absorber should be selected so it matches the wavelength for higher absorption.
Secondary aspects of the measurement system are its dynamic range, its response time, and its cooling. The dynamic range should include the minimum and maximum powers we expect to measure. The response time of most sensors is optimized, but it should be verified to check that it matches our measurement needs. The requirement for cooling the head should also be verified, A water cooled head will need an available water supply and appropriate water connectors.
It may happen that we will find no sensor that matches all our criteria since some are conflicting, like high power and fast response, or high power and low sensitivity. In that case it is best to consult with the manufacturer’s salesperson for advice.
The methodology described above is also true for pulsed lasers, but we have to consider some additional parameters.
The maximum pulse energy, the frequency, and the pulse width play an important part in the sensor selection.
For average power measurement, we will use exactly the same calculations as above, but we will also calculate the energy density of the beam, and find a sensor with a superior energy damage threshold.
For pulse energy measurement we will be using a pyroelectric sensor. The basic two sensors are metallic and black. The metallic sensor will have a higher sensitivity, a lower damage threshold and a narrower spectral coverage than the black coated sensor.
The damage threshold of a pyro head is dependant on the pulse width. A shorter pulse will damage the sensor at a lower energy density for the same beam diameter. One way to bypass this issue is to fit a diffuser on the sensor. The diffuser function is to attenuate and widen the beam, decreasing its energy, increasing its diameter and thus reducing its energy density.
The data on damage threshold is normally given in graphs of damage energy density vs. pulse width. The graphs contain information for various coatings. We have to select a sensor whose damage threshold is above our energy density.
Pyroelectric sensors have maximum permitted frequencies and pulse widths. The sensor should be able to measure the maximum frequency and pulse width required by the application.
As you have seen above, there are a number of criteria that have to be met to pick the correct sensor for your application. Ophir Optronics has developed a simple laser sensor finder that when given all the laser parameters will output a list of the matching heads.