Laser beam quality can be measured using the M2 factor, representing the beam's deviation from a perfect Gaussian profile. A laser with a perfectly Gaussian beam profile has an M2 value of 1 . However, real-world systems deviate from absolutely ideal behavior. The closer the M² factor of a laser beam to 1 the closer it is to being a single mode TEM00 beam, and the better it can be focused to a small spot [2][3].
A beam from a laser can either directly be collimated using a lens of a specific focal length e.g. or it can be fiber-coupled and then collimated to the desired beam parameter. The advantages of fiber-coupling a laser first, is that the laser can be separated spatially from the sensitive environment of an instrument or experiment and that the fiber cable serves as a mode filter, essentially emitting an almost perfect Gaussin beam [1]. It also reduces the laser safety class of the complete system.
At Schäfter+Kirchhoff GmbH, we provide fiber couplers for coupling into fibers as well as a series of fiber collimators for collimation of the fiber output.
Maximum coupling efficiency is achieved for an ideal Gaussian laser beam (M2 = 1, no astigmatism) when the convergence of the focused, circular beam equals the effective NAe2 of the fiber. Then the laser spot on the fiber end face equals the mode field diameter MFD of the single-mode fiber. After passing through the fiber, the fiber output is then collimated using a fiber collimator.
Schäfter+Kirchhoff offers a wide range of different fiber collimators. The first parameter to consider when collimating a laser beam is how large the desired final collimated beam diameter should be. The final beam diameter and the NA of the fiber cable determine the focal length of the collimating optics to be used.
The second parameter to consider is the type of optics within the fiber collimator: asphere, monochromats, achromats, or apochromats, equipped with many different AR coatings . Achromat and apochromat optical components are better suited for where broadband light is being transmitted, whereas aspheres and monochromats are optimized for single wavelengths.
Schäfter+Kirchhoff fiber collimators are designed to be long-term stable and have excellent pointing stability. The focus position can either be adjusted by an eccentric key or manually using a super fine-focussing mechanism. They are suitable for use in avionics, quantum optics and ultra-stable instrumentation.
