Which aspheric lens can I use to couple the light from my laser diode into my fiber?

Utilizing an aspheric lens proves highly effective in collimating the divergent output beam from your laser diode. Important general and specific aspects for their selection are detailed in our blog titled "I want to collimate the light of a laser diode – how do I find the right aspheric lens?" Additionally, for the effective coupling of collimated laser diode light into an optical fiber, an aspheric lens is the component of choice. This process works seamlessly, provided you observe a few crucial points in conjunction with the general aspects outlined in the previous blog.

What focal length do I need?

We assume here that the laser beam is already collimated to a certain beam diameter, i.e. it no longer diverges. In order to couple this collimated beam as well as possible into the core of an optical fiber, the effective focal length (EFL) must be sufficient. Otherwise, too much optical energy ends up in the fiber’s cladding or is even lost completely.

The appropriate focal length is calculated with sufficient accuracy by dividing the beam diameter by twice the numerical aperture of the fiber into which the light is to be coupled: EFLlens = Øbeam/2 x NAfiber

 99%, 1/e² or FWHM?

Again, attention should be paid to how both the numerical aperture of the fiber and the beam diameter are defined. For good coupling, the definition criteria should match. For example, the numerical aperture of the fiber is usually defined for 99% of the optical energy, so in this case the value of the beam diameter should be equally defined for 99%. If you only have a 50% or FWHM (Full Width, Half Maximum) value for the beam diameter, you can approximate the 99% value for a Gaussian beam profile by multiplying it by a factor of 2.576. From the 1/e² value of a Gaussian beam profile, you get the 99% value by multiplying by the factor 1.515.

Larger, but not much larger than the minimum focal length

The formula “EFLlens = Øbeam / 2 x NAfiber” provides the minimum focal length for the aspherical lens. Falling below this threshold means that not all the power of the beam will be effectively coupled into the fiber. It is essential to avoid a significantly higher focal length than this minimum value, as an increased focal length leads to a larger diameter of the laser spot exiting the far end of the fiber. 

Click below for an overview of our portfolio of precision molded aspheric lenses: standard, infrared and mounted in stainless steel holders:

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