Different types of sensors can be used for laser power measurement.

The measurement of laser optical power is accomplished by converting the optical power into a measurable current or voltage through a sensor. The physical principle of the sensor will ultimately determine the function of the instrument. Different sensors have different characteristics and application areas. Let's look at the characteristics of two common sensors:

In the communications field, the rapid propagation of light beams requires rapid and accurate measurement; in applications such as laser-accurate laser cutting and welding, laser power fluctuations will affect the quality of product processing, making power stability an important indicator. Therefore, a tool for fast feedback of power changes is particularly important.

The integrating sphere compensates for the above-mentioned shortcomings of the photodiode and thermopile. The integrating sphere is a passive device. It is a hollow sphere with several openings (ports), and the inside is uniformly coated with polymer or metal coating. The opening is used for the input and output of the laser beam.
 

working principle

Integrating sphere  lasers

The inner surface of the hollow sphere is made of highly reflective materials, which can highly scatter the incident light. The incident laser beam will undergo multiple strong diffuse reflections, and finally be uniformly scattered across the entire sphere.

The hollow sphere is made of a special polymer suitable for 250nm-2.5μm. Although the price of aluminum spheres coated with barium sulfate (BaSO4) is cheap, the color will turn yellow over time, so it is not suitable for accurate laser power measurement. For the measurement in the wavelength range of 700nm-20μm, the metal surface is plated with gold. Many high-power lasers have a spectral range of 700nm-20μm, so copper and aluminum are two good thermal substrate materials.

The photodiode is fixed on the wall of the integrating sphere. It only receives part of the laser power entering the integrating sphere. The received laser has the following important changes:

1. The power density is completely uniform.

2. The irradiation is unpolarized light, even if the incident radiation is polarized light.

3. The power received on the sensor has been greatly weakened.

It can be seen that the integrating sphere and photodiode can be combined into a laser power sensor, which can be as sensitive as a photodiode and can detect considerable power. Integrating spheres of different sizes correspond to different system sensitivities. In addition, the detector at this time is no longer restricted by the uniformity and polarization of the power density, nor by the incident angle and position of the incident light.

Because the size of the photodiode itself is not a limiting factor, the integrating sphere can be used to measure larger beam diameters. Since the inner surface area of the integrating sphere is at least 20 times larger than its input aperture, the power density received by its inner wall is much smaller than the power density received by the surface of the thermopile. The inner surface material of the integrating sphere can withstand higher power density and will not change over time.

Other measurement ports located on the integrating sphere can provide convenience for further applications. For example: the optical fiber interface can be used for simultaneous measurement of laser spectra.

Applications

     For example: we can use a copper gold-plated integrating sphere with an inner diameter of 100mm with a water cooling system to measure the real-time power fluctuation of a 5kW disc laser used for material processing. Since the laser of this power level will cause the integrating sphere to heat up, the photodiode is not installed on the integrating sphere. The temperature change of the photodiode will cause the measurement to be inaccurate. Therefore, through the SMA fiber interface of the integrating sphere, use the fiber to connect with the optical power meter (OPM150 optical power meter). The entire system (integrating sphere-fiber-OPM150 optical power meter) is calibrated as a whole to ensure accurate power measurement. The optical power meter is powered and controlled by USB, which greatly reduces the use of cables (only one USB cable and two water pipes are required).

Through this system, it can be seen that the laser is very stable at 2500W. When the power is increased to the maximum power (5000W), a long-term fluctuation of about 1.5% will be seen.

OPM150

In addition, it can be seen that the output power has a rapid fluctuation of about 0.7%. Note that this fluctuation is on a time scale that cannot be detected with a thermopile detector.

Power in 2.5kW 和5kW test results

Power in 5kW test results

The sensor composed of integrating sphere and photodiode presents a nearly perfect laser power measurement sensor. For high-power laser measurements, this combination allows the operator to see laser power fluctuations that the thermopile detector cannot capture. These fluctuations include: fluctuations during CW mode operation, transient and overshoot fluctuations when starting the laser, and short-term drop fluctuations during operation.

In addition, since the measurement is not restricted by the divergence of the beam, the integrating sphere can be used for measurement based on laser applications, such as the transmission of refracting objects and scattering objects. For example: the integrating sphere can measure the transmission of laser weldable plastic materials to determine the better operating parameters of the welding laser.

References:

    [1]: http://bildungsserver.hamburg.de/physik/unterricht/experimente/2584640/00522-thermosaeule/

 [2]:“Pin-Photodiode”from Kirnehkrib-own work. Licenced under CC BY-SA3.0 by WikimediaCommons- https://commons.wikimedia.org/wiki/File:Pin-Photodiode.png#/media/File:Pin-Photodiode.png

    [3]: L.P.Boivin, Appl. Opt. 21(5), pp 918-923, 1982

The German Artifex company was established in 2006. It designs and manufactures industrial and scientific measurement and detection systems such as optical power meters, transimpedance amplifiers, gated integrating amplifiers, LIV laser diodes and LED characteristic test systems, integrating spheres, laser diode drivers and other products , To provide light source detection solutions for industrial lasers, communications and other industries. All kinds of Artifex products are represented by Lianguangke. If you have any requirements, please call us for details. We will serve you wholeheartedly.