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.
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.

AR Coating, Transmission and Working Range

The AR coating does not necessarily define the wavelength range in which an optics can be used. When it comes to short wavelengths (UV) or longer wavelengths (IR), an optics internal or glass transmission plays a key role, as it can significantly reduce the amount of light passing through the optics. When it comes to doublet optics also the cement between the single lenses must be considered.


This is why we have introduced the working range, which considers cement and both glass transmission as well as AR coating. Please note that all optics are always AR coated for the complete working range (unless described as uncoated). The optics used in our products are the same as before, we only want to describe them more accurately.


When it comes to glass transmission the threshold is 2% internal transmission loss, unless explicitly specified otherwise. Details about the AR coating (rest reflectivity over wavelength) can be found in the AR coating plot, details about the internal glass transmission (transmission over wavelength) in the transmission plot. Both plots can be found in the downloads section of the respective product webpage.