Diffractive Laser Beam Sampler
Laser welding and other types of material processing are very popular applications that utilize the growing industry of high-power lasers. The use of laser beam shaping can assist in optimizing the welding process for maximal throughput and process quality. However, high quality, high power laser processing requires constant control and metrology to ensure process stability. This is because in modern welding heads, the cost of optics and metrology systems can be as much as half of the laser cost!
Power meters and beam profiler cameras that are used to monitor laser beam characteristics often have a lower LDT than the level of power used in the process. A method of attenuation is therefore required to reduce the power for this purpose. For this application of high power laser monitoring and feedback, diffractive laser beam samplers are therefore a highly suitable solution.
What is a diffractive beam sampler?
Diffractive beam samplers are thin, flat windows that diffract a small, pre-defined fraction of the laser beam (0.15–1% typically) at a controlled angle. Most of the energy of the incident laser continues forward, unaffected, in the “zero order.” Simultaneously, a small amount of the beam is diffracted into a higher order, providing a sampled beam for measurement of power, although still with enough power to provide a stable reading when used with high power laser power meters.
The diffractive beam sampler also has a back-reflected order that is typically attenuated by a factor of 1/100, relative to the sampled order (a sampling of 0.001% of the input beam). This enables beam shape monitoring by a beam profiler even for high power laser beams. Since diffractive beam samplers are flat, thin windows with high LDT, they are often used to monitor high power lasers, where optical losses and wavefront distortions of the transmitted beam need to be kept to a minimum.
Why and where are laser beam samplers needed?
For multi-kW lasers used in welding, it is often difficult to sample both main beam power and get a beam profile at the same time, as cameras often have much higher sensitivity (and lower LDT) compared to power meters for high power ranges. This issue can be solved by using a single, diffractive sampler. A diffractive laser beam sampler is a Diffractive Optical Element (DOE) designed to split a sub-beam identical to the main beam, with a pre-determined small fraction of power, for example 0.2%. The main beam power can be sampled directly from the sample forward order with a power meter. While the reflected, sampled order (which is attenuated by another factor of ~100 compared to the forward order) can be used with a camera to monitor the beam profile. This enables real-time control of both power stability and beam profile.
Advantages of diffractive laser beam samplers
Diffractive laser beam samplers have many advantages over existing methods of power and beam profile monitoring. With some conventional methods, the laser beam is only measured at certain times. With others, the beam is deflected at 90 degrees from the optical axis. Still other methods rely on polarization of the laser beam. Here is a short list of the main advantages of diffractive laser beam samplers:
- Samplers are available for high-power lasers at a wide range of wavelengths with fused silica substrates for 0.193 nm to 3 µm and ZnSe for mid-IR CO2 lasers, with LDT suitable for use with high-power lasers.
- Diffractive beam samplers are a single element solution that do not require a module or large device and are not sensitive to alignment or centration.
- Samplers do not shift or tilt the main laser beam used for the process. They leave it completely undisturbed and do not add aberrations to the wavefront.
- Diffractive samplers are not sensitive to and do not change the polarization of the laser beam.
- Samplers can be made with large aperture optics for use with large input laser beams.
- Ultra-low, back-reflected sampling ratios down to 0.001% of the input beam enable use without additional filters and attenuators.
Holo/Or develops, designs and manufactures diffractive optical elements (DOEs) and micro-optical elements for any wavelength, from DUV to far IV, as well as software/automation solutions for the design.
For more information, contact Holo/Or.
(The original version of this item is a post on the Holo/Or website.)