It is often necessary to transform the light output from an optical fiber into a free-space collimated beam. In principle, a simple collimation lens is sufficient for that purpose. However, the fiber end has to be firmly fixed at a distance from the lens which is approximately equal to the focal length. In practice, it is often convenient to do this with a fiber collimator (fiber-optic collimator). They can be singlemode or multimode. Their diameters can be as small as the fiber itself, for example 125 um, or as large as tens or hundreds of millimeters. Their basic structure, however, consists of a lens and an optical fiber. A lens can collimate the output from a fiber, or launch a collimated beam into the fiber.
Types of Lens
Different kinds of lenses can be used in collimators: include fiber lenses, ball lenses, aspherical lenses, spherical singlets and doublets, GRIN (GRaded INdex) lenses, microscope objectives, cylindrical lenses. For standard telecom fibers and in fact many others, one mostly uses GRIN lenses (graded-index lenses), as these are relatively cheap and small. However, they are less suitable for larger beam diameters, e.g. of more than a few millimeters. In such cases, one tends to use conventional singlet or doublet lenses, which may be of spherical or sometimes aspheric type. This is needed, for example, when a collimated beam needs to be transmitted over a large distance, such as in free-space optical communications, where a long Rayleigh length is required.
Size of the Collimated Beam
The beam radius of the obtained collimated beam depends on the circumstances. In some cases, the beam diameter is as small as the fiber diameter, e.g. 125 μm; the Rayleigh length can then be less than 1 cm. In other cases, one needs beam diameters of several millimeters or even more. For calculations, the simpler case is that of a single-mode fiber. Here, the beam radius can be calculated with fairly good accuracy using the following equation:
This assumes that the beam profile of the fiber mode has an approximately Gaussian shape, so that we can apply the corresponding formula for the beam divergence half-angle θfiber. It is also assumed that the distance between fiber end and lens is close to the focal length f of the lens. If the distance is too small, the beam will diverge, and for too large distances it converges to a focus at some distance. It can be useful to get slightly into that latter regime, where a beam focus (with a beam diameter slightly below that at the collimator) is reached in a suitable working distance. The longer the focal length, the less critical is the longitudinal positioning. A smaller fiber mode size often leads to a larger collimated beam! Note that a smaller mode size of the fiber implies a larger beam divergence and thus a larger collimated beam for a given focal length. This also means that a shorter wavelength, which usually leads to a smaller mode size, leads to a larger output beam. This holds even more if the fiber gets into the multimode regime for sufficiently short wavelengths. For such reasons, a visible pilot beam for an infrared beam, for example, may not accurately show the size of the infrared beam. Also, the correct fiber positioning for collimation may depend on the wavelength, particularly if no achromatic lens (see below) is used. For multimode fibers, the beam divergence at the output (and thus the collimated beam size) depends on the launch conditions, and possibly even on the condition (e.g. bending) of the fiber. Generally, the beam divergence angle will be larger than according to the estimate for the single-mode fiber – possibly even much larger. While GRIN lenses are perfect for small telecom devices, they are not suitable for generating large optical beams such as those used in Free Space Optic (FSO) communication applications where beam size can vary from a few millimeters to tens of millimeters. For beam size of 1 mm to 5 mm, aspherical lenses are ideal largely due to their excellent ability to correct spherical aberration. For beam sizes larger than a few millimeters, a spherical singlet or doublet may be a better choice since they are readily available and low cost.
Theoretical Approximation of the Divergence Angle
Angled fiber ends are often used to suppress back-reflections from the fiber end face into the core, i.e., to maximize the return loss. Unfortunately, the angle leads to some deflection of the output beam. Singlemode fibers are often polished at an 8 degree angle to reduce back reflection (increase return loss). The price to pay there is that the beam is slightly off-centered. It is possible, however, to correct this with specially designed fiber ferrule and alignment fixtures. Almost all multimode fibers are polished at 0 degree due to the fact that system return loss requirements are much lower. This divergence angle is easy to approximate theoretically using the formula shown below as long as the light emerging from the fiber has a Gaussian intensity profile. This works well for single mode fibers, but will underestimate the divergence angle for multimode fibers where the light emerging from the fiber has a non-Gaussian intensity profile. The divergence angle (in Degrees) θ ≈ (D/f)(180/3.1415927) where D and f must be in the same units. θ is Divergence Angle, D is Mode-Field Diameter (MFD) and f is Focal Length of Collimator Example Calculation: When the SF220SMA-A collimator is used to collimate 515 nm light emerging from a 460HP fiber with a mode field diameter (D) of 3.5 µm and a focal length (f) of approximately 11.0 mm (not exact since the design wavelength is 543 nm), the divergence angle is approximately given by θ ≈ (0.0035 mm / 11.0 mm) x (180 / 3.1416) ≈ 0.018°. When the beam divergence angle was measured for the SF220SMA-A collimator a 460HP fiber was used with 543 nm light. The result was a divergence angle of 0.018°.
We offer four gradient index (GRIN) fiber collimators that are aligned for either 1310 nm or 1550 nm and have either FC connectorized or unterminated fibers. Our GRIN collimators feature a Ø1.8 mm clear aperture, are AR-coated to ensure low back reflection into the fiber, and are coupled to standard Corning SMF-28 single mode fibers. These graded-index (GRIN) lenses are AR coated for applications at 1300 or 1560 nm that require light to propagate through one fiber, then through a free-space optical system, and finally back into another fiber. They are also useful for coupling light from laser diodes into fibers, coupling the output of a fiber into a detector, or collimating laser light.
Our pigtailed ferrules have broadband AR coatings centered at either 1310 nm or 1550 nm and are available with either a 0 or 8 degree angled face. These pigtailed ferrules include 1.5 meters of SMF-28e fiber.