Fiberoptic collimators come in many forms. They can be single mode 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. In this tutorial we will explore the many faces of “simple” fiberoptic collimators.
LENS TYPE
Almost all known lens types have been used to construct fiber optic collimators. These lenses include fiber lenses, ball lenses, aspherical lenses, spherical singlets and doublets, GRIN (GRaded INdex) lenses, microscope objectives, cylindrical lenses, no lens at all as in the case of thermally expanded core (TEC) fiber. Lens materials can vary from glass to plastic to silicon. By a large margin, most of the fiber optic collimators used today are made using GRIN lenses. GRIN lenses are small, easy to handle, relatively low cost, and competitive in optical performance. They do have limitations though. GRIN lenses rarely come in large size and their performance is marginal in the visible spectrum range.
SIZE DOES MATTER
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.
SPHERICAL OR CHROMATIC ABERRATION
Almost all polished lenses form flat and spherical surfaces which limit the lens’ ability to focus a Gaussian optical beam into a diffraction limited spot. The technical term for this behavior is spherical aberration. As a result, the coupling efficiency between collimator pairs made with spherical lenses are far from optimum. Aspherical lenses use a slightly modified surface contour to significantly reduce spherical aberration. They are ideal for most applications with single wavelength or relatively small bandwidth. When two beams with far different wavelengths come into a single lens, either spherical or aspherical, their focal distances are different, often called chromatic aberration. Achromatic doublets are frequently used to reduce the effect.
SINGLE MODE OR MULTIMODE
Single mode fibers produce almost mathematically perfect Gaussian beams. A collimated Gaussian beam behaves very differently from a multimode beam. Gaussian beams maintain collimation over a certain distance, often called the beam confocal parameter which varies from a fraction of a millimeter for very small collimators to meters for large beam collimators. The center of the beam confocal parameter is called the beam waist. It is very important to define the position of the beam waist to achieve optimum performance. The beam waist can be adjusted for a collimator with a given size. The longer the focal length, the larger the tuning range.
What is important to remember is that doubling the focal length would quadruple the tuning range and the beam confocal parameter. Multimode “collimators” on the other hand really never collimate the beam. They simply form an image of the fiber core at a certain distance. Therefore, it is critical to define the image distance between the pairing collimators.
PAIRING, TARGETING, OR LASER PIGTAILING
One can never overemphasize the importance of defining the exact application before purchasing collimators since the same collimator may be made very differently for different applications. Pairing requires the well defined working distance, the spacing between the pair. Targeting would have a defined target distance and sample size. For laser pigtailing one needs to define the beam diameter well for maximum coupling efficiency.
0 DEGREE OR 8 DEGREE
Single mode 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.
ALTERNATIVES
There are many creative methods to form collimated beams without using a separate lens. The most important category is fiber tip treatment. TEC fibers greatly reduce the beam divergence to maintain reasonable insertion loss with a small gap between a fiber pair. Similar results can be achieved by forming a convex surface on the fiber tip. These specially treated fibers can be used in micro-sized devices or arrays and bundles to create devices with very large channel counts. It is often difficult, however, to add optical anti-reflective coating on the fibers due to their fragile nature and the difficulty of cleaning them.
CONCLUSION
There are more things to consider than one might think when it comes to purchasing collimators . It is crucial for design engineers to understand the exact need of the system and study the similarity and difference between different types of collimators before making a choice.