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 used. 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 and designed in such a way that it leads to a diffraction-limited beam with an M2<1.05.The focus position varies strongly with wavelength so that the coupler/collimator has to be refocussed/recollimated after any changes to the wavelength. Monochromats are not suitable for UHV applications.
Achromats are designed for coupling/collimating multiple wavelengths. They are additionally corrected for chromatic aberration so that there are certain wavelengths or wavelength ranges where the focal length does not vary significantlyand the coupler or collimator does not need to be readjusted. Achromats are not suitable for UHV applications.
Both exhibit an undisturbed Gaussian beam profile.
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.