Choosing a Fiber Collimation Lens
Aspheric lenses are commonly chosen to couple incident light with a diameter of 1 - 5 mm into a single mode fiber. A simple example will illustrate the key specifications to consider when trying to choose the correct lens.
Collimated Beam Diameter Prior to Lens: Ø3 mm
The specifications for the P1-630A-FC-2, 630 nm, FC single mode patch cable indicate that the mode field diameter (MFD) is 4.3 μm. This specification should be matched to the diffraction-limited spot size given by the following equation:
Here, f is the focal length of the lens, λ is the wavelength of the input light, and D is the diameter of the collimated beam incident on the lens. Solving for the desired focal length of the collimating lens yields
Thorlabs offers a large selection of mounted and unmounted aspheric lenses to choose from. Scanning through the list of options under the Selection Guide Tab, you'll note that the aspheric lens with ta focal length that is closest to 16 mm has a focal length of 15.29 mm (Item# C260TME-B). This lens also has a clear aperture that is larger than the collimated beam diameter. Therefore, this aspheric lens is the best option given the initial parameters (i.e., a P1-630A-FC-2 single mode fiber and a collimated beam diameter of 3 mm). Remember, for optimum coupling the spot size of the focused beam must be less than the MFD of the single mode fiber. As a result, if an aspheric lens is not available that provides an exact match, then choose the aspheric lens with a focal length that is shorter than the calculation above yields. Alternatively, if the clear aperture of the aspheric lens is large enough, the beam can be expanded before the aspheric lens, which has the result of reducing the spot size of the focus beam.