Derrick Peterman, PhD
Beam profiling characterizes the size, shape, quality, and focal position of a laser beam. Whether the application is a few milliWatts in power or several kWatts, beam profiling is used to test optical designs, verify laser performance, or monitor the performance of laser systems.
Sensors used in most beam profiling instrumentation are very sensitive, saturating at power levels of ~1 microWatt per cm2, much lower than the irradiance of even the lowest power lasers. Thus, beam profiling is a lot about carefully attenuating the power level so the beam can be analyzed. This is critical because lasers designed for cutting sheet steel will also have no trouble cutting through a beam profiler if the beam power isn't attenuated!
The good news is there are reliable methods to reduce the beam power to levels required while preserving the beam profile integrity. What is required is some understanding about the optics involved and some back-of-the-envelope calculations.
Neutral density filters are commonly used to attenuate the beam in profiling applications. But at high enough powers, they can thermally lens, distorting the beam profile. Thermal lensing occurs when local heating of the absorptive neutral density filter changes the local index of refraction of the material. The temperature induced change of the index of refraction creates a thermal lens which makes the beam appear larger than it true size. This effect occurs with the beam power per unit area is 5 Watts per cm2 or larger.
To avoid thermal lensing, a reflective rather than an absorptive attenuator must be used. Typical reflective attenuators involve a beam splitter or using the front surface reflection from a wedge optic, which reflects 4% from the front surface. Lower percentages of reflection are achieved with anti-reflection coatings. Laser grade optics should be used in reflective beam attenuation schemes to maintain the profile integrity.
When using front surface reflections to attenuate the beam, usually two reflective optics are used, mounted so the beam strikes the surfaces at opposing right angles to preserve the beam polarization. Once the beam power is reduced to the point where thermal lensing is no longer an issue, absorptive neutral density filters are then used further down the optical beam path to reduce the beam power even more.