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

Efficiently coupling collimated laser diode light into an optical fiber is best achieved using an aspheric lens. This method operates smoothly, ensuring success when key considerations are taken into account. Learn more in this blog article.

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: EFL_lens = Øbeam/2 x NA_fiber

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 “EFL_lens = Øbeam / 2 x NA_fiber” 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.

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FAQ: Aspheric lenses for fiber coupling

Why are aspheric lenses suitable for coupling laser light into an optical fiber?

Aspheric lenses can focus a collimated laser beam efficiently into the small core of an optical fiber. Their compact design makes them well suited for applications where precise beam control and efficient coupling are required.

How can I calculate the required focal length of the coupling lens?

The minimum effective focal length can be calculated by dividing the beam diameter by twice the numerical aperture of the fiber: EFL = beam diameter / (2 × fiber NA). The beam diameter and numerical aperture should be specified using comparable measurement criteria.

Why must the definitions of beam diameter and numerical aperture match?

Beam diameter may be specified using FWHM, 1/e², or the diameter containing 99% of the optical power. Since the fiber’s numerical aperture is commonly defined for 99% of the energy, using a beam diameter based on the same criterion provides a more reliable basis for selecting the lens.

What happens if the focal length is too short or too long?

If the focal length is below the calculated minimum, not all of the beam power can be effectively coupled into the fiber. A focal length significantly above the minimum results in a larger laser spot at the far end of the fiber.