Lens Types

Lens Types for fiber optic components

The coupling lenses provided by Schäfter+Kirchhoff are corrected for spherical aberration and are ­optimized for the diffraction-­limited focusing or collimation. Three different kinds of optics are available:
  • Type A (aspheres)
  • Type M (laser mono­chromats or achromat),
  • Type RGBV (apochromat)

Lens Types

Asphere

Asphere

Aspheres are designed for single wavelength applications and are corrected for spherical aberration. The focus position varies strongly with wavelength so that the coupler/collimator has to be refocussed/recollimated after any changes to the wavelength. The aspheres used are all glass aspheres. This lens type is suitable for UHV applications.
Limited performance as collimators
Due to the manufacturing process of molded aspheres, aspheres used as a collimating lens  show a fine structure (concentric rings) or worse in the beam profile. As a result the beam profile is no longer Gaussian. The lens performance as a collimator is limited and alternatives such as monochromats or achromats should be consideres. However, aspheres can be used as coupling or focus optics without any restriction.
Monochromat or achromat

Laser monochromat or achromat

Monochromats are designed for coupling/collimating single wavelengths. They are corrected for spherical aberrations. For focal lengths < 20mm they are usually designed in such a way that it leads to a diffraction-limited beam with an M2<1.05 when combined with high quality single-mode fibers/lasers. The focus position varies strongly with wavelength so that the coupler/collimator has to be refocussed/recollimated after any changes to the wavelength. Monochromats exhibit an undisturbed Gaussian beam profile and are not suitable for UHV applications.
Achromats are designed for coupling/collimating multiple wavelengths.They are corrected for spherical aberrations. For focal lengths < 20mm they are usually designed in such a way that it leads to a diffraction-limited beam with an M2<1.05 when combined with high quality single-mode fibers/lasers. They are additionally corrected for chromatic aberration so that there are certain wavelengths or wavelength ranges where the focal length does not vary significantly and the coupler or collimator does not need to be readjusted. Achromats exhibit an undisturbed Gaussian beam profile and are not suitable for UHV applications.
For focal lengths > 20 mm there is often a trade-off between achromatic characteristics and monochromatic performance. For an optimum beam quality, the performance of each optics needs to be evaluated e.g. with respect to the individual wavelengths or fiber NAs. Please refer to the Black Boxes for more information or contact Schäfter+Kirchhoff.
If you have the choice between achromat and monochromat and you have a monochromatic application, the monochromat should be preferred.
Apochromat

Apochromat

RGBV optics (achromats and even apochromats) are designed for optimum multiple wavelength coupling/collimation by minimizing the chromatic focal shift for all wavelengths from 400 to 660 nm. They are corrected for spherical aberrations and designed in such a way that it leads to a diffraction-limited beam with an M2<1.05. A recollimation for wavelengths 400 to 660 nm is not necessary. By minimizing the chromatic focal shift the polychromatic beams are focused at the fiber end-face onto a common point reducing otherwise significant coupling losses. It is not suitable for UHV applications.