Fiber Laser Lenses
A fiber laser is a solid-state laser that utilizes a monolithic design for high efficiency, single-mode output, and high beam quality. The Fiber laser light is created by banks of diodes, where the light is channeled and amplified through fiber optic cable in a similar way to that used for data transfer. The laser is guided within the fiber core, and because its interaction length is so great, it experiences a very high amplification. When the amplified light exits the fiber cable, it is straightened (collimated) and then focused by a lens onto the material to be cut. The use of fiber lasers in the metal processing industry for cutting applications is becoming increasingly popular due to the advantages versus other laser technologies. Fiber lasers do an excellent job cutting metals thinner than 3 mm (0.12 in). Thin metal processing times are faster than CO2 lasers with comparable edge quality.
The output optical assembly of a fiber optic beam delivery system consists usually of a collimating lens, a focusing lens, and a protective window. The optical lenses featured in the fiber laser cutting head are made of fused silica and are specially coated to minimize absorption and maximize transmittance of the 1064nm wavelength laser beam onto the metal sheet.
The collimating lens captures the highly divergent output from the optical fiber and creates a parallel (collimated) beam with reduced divergence enabling moderate propagation distances.
It is often necessary to transform the light output from an optical fiber into a free-space collimated beam. In principle, a simple collimation lens (see Figure 1) is sufficient for that purpose. However, the fiber end has to be firmly fixed at a distance from the lens which is approximately equal to the focal length. In practice, it is often convenient to do this with a fiber collimator (fiber-optic collimator). There are two different basic types of such devices, differing in how the fiber is mounted:
Some can be directly attached to bare fibers. This is the cheapest and most compact solution, but such a fiber collimator is more or less permanently attached to fiber.
Other fiber collimators have a mechanical interface to a fiber connector, e.g. of FC or SMA type; they are not for use with bare fibers. One can easily attach and remove such a collimator from a connectorized fiber.
The same kind of device can also be used for launching light from a collimated beam into a fiber, or for fiber-to-fiber coupling: light from the first fiber is collimated with a fiber collimator and then focused into the second fiber by another collimator. Basically, fiber connectors can be seen as the natural interface between fiber optics and free-space optics.
Similar to other laser systems for metal processing applications, the main role of the focusing lens is to concentrate the energy of the laser beam output to spot at a specific distance (focal length) – depending on the application. The focal length – defined by the radius of curvature of the lens – is the most important feature of a focusing lens. The first lens (collimator lens – L1) takes the rapidly diverging beam from the Fiber exit, and straightens, or collimates, it. This lens should be placed at a distance exactly equal to its focal length from the fiber exit face. If this is not done, the beam will not be collimated, and the imaging convention described below is void. The second lens (focusing lens – L2) acts as an objective and focuses the beam to form an image of the fiber face.
The contamination of the focusing lens is one of the prime reasons for poor laser performance and potential downtime of laser operations. No nozzle system is perfect and some debris, fume or back spatter can occasionally reach the lens.
This situation occurs for many high-power laser systems, including fiber lasers. Collimating lenses, as well as focusing lenses present in fiber laser cutting heads, are very sensitive to any type of contamination. For this reason, the majority of fiber laser systems employ protective windows to protect the focusing lens against contamination. The protective window is positioned in front of the focusing lens and serves as a barrier between the lens and the metal sheet. Protective windows are the most consumed optical element in fiber laser systems.
So-called protective windows are the last optics to be used before the working area;
they protect high-quality laser optics from material splatter.
In the majority of cases, protective windows are available inexpensively with few demands regarding the surface figure and surface quality. The windows are coated with an anti-reflex coating for the laser beam and,upon request, can also be AR coated for the pilot laser in the visible wavelength range.
This article is only for learning without any commercial value, part of the point of view is extracted from https://www.ophiropt.com/