How should I select a collimating lens, given the beam divergence and required collimated beam diameter? Should I use a concave + convex combination to get the result if the divergence is very small?

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.

Is there a reason to prefer collimators with shorter focal length if everything I need is passing from fiber to free space, doing something on the beam with bulk optics, and then passing again to fiber? Since longer focal length ensures bigger beam radius, so less divergence, I don't get how I can chose the best collimator.

Special requirements may lead to the use of special lenses. For example, achromatic doublet lenses are used if beams with quite different wavelengths need to be handled, as otherwise proper collimation may not be achieved for all wavelengths. Aspheric lenses may be used in cases with large beam divergence from the fiber (i.e., for fibers with small mode radius) in order to eliminate spherical aberrations.

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Edmund Optics offers fiber-optic collimators for FC/PC, FC/APC and SMA connectors and different wavelength ranges around 350 nm to 1600 nm.

A shorter focal length is mainly useful to keep the whole setup smaller. Concerning the coupling efficiency, there is no fundamental reason to choose a larger or smaller focal length.

Fiber-optic collimators are available for different collimated beam sizes, which simply means different values of the focal length. Naturally, devices for larger collimated beams need to be both longer and larger in diameter. The largest fiber collimators are those for high-power multimode fibers as used in laser material processing or for pumping high-power lasers; they also need to be optimized for reliable operation at high optical power levels.

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.

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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 <$\theta_\rm{fiber}$>.

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Some fiber-optic collimators have adjustment screws for controlling the beam direction (by an integrated tilt adjustment) or possibly even for the fine longitudinal positioning (adjustment of focusing or working distance). Others don't have such adjustment options, and one may position and align the whole collimator with additional opto-mechanics.

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.

Many anti-reflective coatings consist of transparent thin film structures with alternating layers of contrasting refractive index with multiple layers, depending on the wavelength range and performance required by the application. Layer thicknesses are chosen to produce destructive interference in the light reflected from the interfaces, and constructive interference in the corresponding transmitted light.

Precision Optical’s expansive capabilities and arsenal of coating equipment makes us an ideal choice for your thin-film coating needs. Whether you’re requesting coatings on your components, or need a quick turn-around for substrates you already have, Precision Optical offers a variety of anti-reflective coatings in wavelength ranges that span the Ultraviolet (“UV”, 194nm to 400nm), Visible (“VIS”, 400nm to 600nm), Near-Infrared (“NIR”, 600nm to 1.5 microns), and Short Wave Infrared (“SWIR”, 1.5 microns to 2.5 microns).

When you select a product, just ask the manufacturer what lens is suitable for your fiber and needed collimated beam diameter. There are different technical routes for achieving optimal performance.

In principle, a fiber collimator can also be used “in reverse”, i.e., for launching a free-space beam into an optical fiber. However, it usually does not provide the required tools for fine adjustment (which are particularly required for single-mode fibers). That adjustment then has to be done e.g. with turning mirrors in the path of the input beam.

O/E Land's fiber collimators and focusers are available as fixed or adjustable versions, in fiber-pigtailed or receptacle form, also with customized designs.

The insertion loss of a single fiber collimator can be pretty small – of the order of 0.2 dB (i.e., a few percent) or even lower. It depends on various factors, such as anti-reflection coatings and dirt on the lens. It should not matter, however, whether a bare fiber or a connectorized fiber is used.

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There are also fiber launch systems with additional adjustment tools for different degrees of freedom, allowing the launch of a non-adjustable input beam.

Another application is the combination with a back-reflecting mirror and some additional optical element. For example, one may insert a Faraday rotator in order to obtain a fiberized Faraday mirror, or a quarter-waveplate for an effective half-waveplate reflector. In other cases, one may be using some optical filter or a saturable absorber.

Collimation definition in radiography

Schäfter+Kirchhoff offers a wide range of fiber collimators that can be used for collimating or in reverse as an incoupler. This includes the series 60FC collimators or series 60FC-SF collimators with super fine-focussing mechanism. For large beam diameters the series 60FC-T collimators can be used with integrated TILT mechanism. Special collimators 60FC-Q with integrated quarter-wave plate or made from amagnetic titanium are also available.

Different kinds of lenses can be used in collimators. For standard telecom fibers and in fact many others, one mostly uses GRIN lenses (gradient-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.

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The same kind of device can also be used for launching light from a collimated beam into a fiber, or for fiber-to-fiber coupling: light from the first fiber is collimated with a fiber collimator and then focused into the second fiber by another collimator. Basically, fiber collimators can be seen as the natural interface between fiber optics and free-space optics.

For calculations, the simpler case is that of a single-mode fiber. Here, the beam radius can be calculated with reasonably good accuracy using the following equation:

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.

The insertion loss for a pair of collimators, as used for fiber-to-fiber coupling, may be substantially higher than the sum of insertion losses of the two devices. Particularly for single-mode fibers, it is important to achieve good mode matching. Obviously, both collimators should have the same collimated beam size. Depending on the exact longitudinal positioning of the fibers in the collimators, some non-zero distance between the collimators may be ideal. This also allows one to insert additional optical elements, such as optical filters or polarizers.

The housing of the lens easily connects to fiber-optic cables either with FC/PC or with SMA connectors and enables Z-axis alignment.

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Coupling of light into single mode fibres can be made simple with the use of a PowerPhotonic fiber coupling micro lens array. We design and manufacture standard and custom optics in 1D and 2D arrays. All products are made in high grade fused silica and capable of both high efficiency and high power handling and our unique process minimises channel cross talk due to extremely low scatter. Lenses can spheric, aspheric or freeform due to our unique manufacturing process.

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How to make acollimated beam

CSRayzer provides different kinds of fiber collimators, which can be customized for high power, focusing distance, beam spot diameter, etc. Fixed focus collimators are also available.

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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.

For some fiber connectors with inclined fiber connection, this can be compensated by some tilt of the fiber fixture. Otherwise, the beam from the fiber will hit the lens at some angle. After the lens, the beam direction should nevertheless be in the direction of the fiber (assuming correct longitudinal and transverse positioning), but it will be somewhat offset from the center of the lens. That may also lead to increased insertion loss and to beam quality deterioration if some clipping, reflection or scattering occurs at the edge.

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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 (see Figure 1) 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). There are two different basic types of such devices, differing in how the fiber is mounted:

A shorter wavelength usually leads to a somewhat smaller mode size, but nevertheless to a lower beam divergence and to a smaller output beam radius. 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.

Our polarization-maintaining fiber collimator has a high extinction ratio, low insertion and high return loss. The unique processing and high-quality AR coating also enable this collimator to handle high optical powers.

Anti-Reflective Coatings (commonly referred to as “AR” or “ARC”) are a type of optical coating applied to the surface of optical elements to reduce reflection. In many optical systems, anti-reflective coatings are critical to prevent loss of light in the system. In more complex optical systems such as telescopes the reduction in reflections using anti-reflective coatings can improve the contrast of the image by elimination of stray light.

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