Laser beam profiler
Since there are many types of lasers—ultraviolet, visible, infrared, continuous wave, pulsed, high-power, low-power—there is an assortment of instrumentation for measuring laser beam profiles.
The choice of definition can have a large effect on the beam width number obtained, and it is important to use the correct method for any given application.
It is difficult to obtain excellent beam quality and high average power (100 W to kWs) due to thermal lensing in the laser gain medium.
Power-in-the-bucket and Strehl ratio are two attempts to define beam quality as a function of how much power is delivered to a given area.
It is also possible to derive the beam divergence from the complex amplitude profile of the beam in a single plane: spatial Fourier transforms deliver the distribution of transverse spatial frequencies, which are directly related to propagation angles.
The metric for astigmatism is the power of cylindrical lens needed to bring the focuses of the horizontal and vertical cross sections together.
The typical kinematic tip-tilt mount drifts by around 100 μrad per day in a laboratory environment (vibration isolation via optical table, constant temperature and pressure, and no sunlight that causes parts to heat).
Fortunately, this is usually not a great concern for most laboratory laser systems and the frame rates of CCDs are fast enough to capture the beam wander over the bandwidth that contains the greatest noise power.
The rms deviation of the centroid data gives a clear picture of the laser beam pointing stability.
[11] Beam wander is caused by: It is to most laser manufacturers' advantage to present specifications in a way that shows their product in the best light, even if this involves misleading the customer.
By measuring the intensity curve in several directions, the original beam profile can be reconstructed using algorithms developed for x-ray tomography.
The measuring instrument is based on high precision multiple knife edges each deployed on a rotating drum and having a different angle with respect to beam orientation.
Scanned beam is then reconstructed using tomographic algorithms and provides 2D or 3D high resolution energy distribution plots.
Unlike other camera based systems this technology also provides accurate power measurement in real time Scanning-slit profilers use a narrow slit instead of a single knife edge.
This creates capability of accurate measurement from a micron to over 10 millimeters with adaptable resolution over a wide spectrum range, practically if a single-surface detector exists for a certain wavelength region, then using this technology an image-like profile could be derived.
The most popular cameras used are silicon CCDs that have sensor diameters that range up to 25 mm (1 inch) and pixel sizes down to a few micrometres.
The advantages of the CCD camera technique are: The disadvantages of the CCD camera technique are: The D4σ width is sensitive to the beam energy or noise in the tail of the pulse because the pixels that are far from the beam centroid contribute to the D4σ width as the distance squared.
To reduce the error in the D4σ width estimate, the baseline pixel values are subtracted from the measured signal.
Placing the largest attenuator last before the CCD sensor will result in the best rejection of ghost images due to multiple reflections.
These filters work fine to about 5 W average power (over about 1 cm2 illumination area) before heating causes them to crack.
The angle of the wedge is typically selected so that the second reflection from the surface does not hit the active area of the CCD, and that no interference fringes are visible.
Wedges have the disadvantage of both translating and bending the beam direction — paths will no longer lie on convenient rectangular coordinates.
The glass must be thick enough so that the beam does not overlap with itself to produce interference fringes, and if possible that the secondary reflection does not illuminate the active area of the CCD.
The Fresnel reflection of a beam from a glass plate is different for the s- and p-polarizations (s is parallel to the surface of the glass, and p is perpendicular to s) and changes as a function of angle of incidence – keep this in mind if you expect that the two polarizations have different beam profiles.
To prevent distortion of the beam profile, the glass should be of optical quality — surface flatness of λ/10 (λ=633 nm) and scratch-dig of 40-20 or better.
A half-wave plate followed by a polarizing beam splitter form a variable attenuator and this combination is often used in optical systems.
A laser beam profiler with a 5.6 μm pixel size would adequately sample the spot at 56 locations.
Low-cost beam profilers have opened up a number of new applications: replacing irises for super-accurate alignment and simultaneous multiple port monitoring of laser systems.
The laser beam profiler's effective aperture size is three orders of magnitude smaller than that of an iris.
The output beam profile is often a strong function of pump power due to thermo-optical effects in the gain medium.
