Practical collimation for single-mode, PM and multimode fibers.

Schäfter+ Kirchhoff ships all collimators prealigned and collimated for either a specific wavelength defined by the customer or a typical wavelength. The collimation is performed using professional collimating telescopes.

However, it might be necessary for a customer to readjust the collimating setting and collimate for another wavelength or simply readjust the collimation setting without having a professional collimating telescope at hand. 

The following describes some tricks and tips for the collimation adjustment of single-mode, PM or multimode fibers. Please note that single-mode and PM collimation is significantly different than multimode collimation. 

Practical collimation of a singlemode or PM fiber

The collimated beam diameter Øbeam is a function of the ­collimating focal length f’ and the ­numerical aperture NA of the ­single-mode fiber.

For a specified fiber NAe2, the beam diameter Øbeam (defined at its 1/e2-level) for a focal length f’ is given by:

{!{!{\emptyset_{beam}=2\cdot f'\cdot{\rm NA}_{e^2}}!}!}

Collimation comparing beam diameter sizes

To check and align the collimation setting of the fber collimator, couple a radiation source of appropriate wavelength into the fiber collimator. Direct the beam to a target about half a Rayleigh length zR away:

Here λ is the optical wavelength and Øbeam the collimated beam diameter (1/e2 level).
When correctly collimated, the laser spot diameter on a target about zR/2 away must have approximately the same diameter such as the beam directly behind the fiber coupler. Additionally, make sure that there is no focused spot between the fiber coupler and the target at zR/2.
Please note: For fiber collimators with a focal length f‘ > 30 mm it is best to use a shearing interferometer.

Practical Collimation of multimode fibers

Minimal beam diameter at the collimating lens

In case of a multimode fiber, a collimated beam has its smallest diameter right after the collimating lens. From this point on, the spot diameter increases linearly with distance to the lens. The divergence θ depends on the core diameter Øcore and the collimating focal length f‘: {!{!{\theta=\frac{\emptyset_{core}}{2f\prime}}!}!}
The collimated beam diameter is
{!{!{\emptyset_{beam}=2\cdot f\prime\cdot NA}!}!}
.NA is the multimode fiber NA.

Specific beam diameter in distance D

In some cases it might be more practical to focus the beam in a specific distance D from the fiber collimator, rather than to collimate the beam. A convinient distance D is: 
{!{!{D\approx2\cdot\ {f'}^2\frac{NA}{\emptyset_{core}}}!}!}
When focussing to this distance D, the core diameter Øcore is imaged to a spot that has approximately the same size such as the collimated beam right after the fiber collimator.