fiber collimators (original) (raw)

Definition: devices for collimating the light coming from a fiber, or for launching collimated light into the fiber

Alternative term: fiber-optic collimators

Category: fiber optics and waveguides

• optics
  • fiber optics
    * fibers
    * fiber connectors
    * fiber-optic adapters
    * fiber couplers
    * fiber-optic pump combiners
    * fiber bundles
    * fiber endface inspection
    * cleaving of fibers
    * fiber cleavers
    * fiber joints
    * fiber splices
    * fiber Bragg gratings
    * fiber cables
    * fiber coatings
    * fiber strippers
    * fiber recoaters
    * fiber coils
    * fiber collimators
    * fiber launch systems
    * fiber lenses
    * fiber loop mirrors
    * fiber patch panels
    * fiber shuffles
    * fiber-optic attenuators
    * fiber-optic plates
    * fiber-optic tapers
    * (more topics)

Related: beam collimatorsfibersfiber connectorscollimated beamsinsertion lossfiber launch systems

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Contents

What is a Fiber Collimator?

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:

• Some can be directly attached to bare fibers. This is the cheapest and most compact solution, but such a fiber collimator is permanently attached to a fiber.
• Other fiber collimators have a mechanical interface to a fiber connector, e.g. of FC or SMA type; they are not for use with bare fibers. One can easily attach and remove such a collimator from a connectorized fiber.

Commercially offered collimators may offer several directional adjustments, e.g. through knobs or screws. There are also more complex fiber launch systems.

There are also fiber lenses in the form of lensed fiber endfaces, but these are usually not used for collimating a fiber output to a larger diameter; instead, they typically focus light further down.

Applications of Fiber Collimators

The original purpose of fiber collimators is collimating light coming from a fiber.

Figure 1: A lens can collimate the output from a fiber, or launch a collimated beam into the fiber.

The same kind of device can also be used “in reverse” for launching light from a collimated beam into a fiber. However, it may 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.

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

Fiber couplers are also used 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.

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

Basically, fiber collimators can be seen as the natural interface between fiber optics and free-space optics.

Size of the Collimated Beam

Different types of fiber collimators can provide a wide range of beam radii, suitable for different applications. In some cases, the collimated 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 reasonably good accuracy using the following equation: {w_{\rm collimated}} = f \cdot \theta_{\rm fiber} \approx \frac{f \cdot \lambda}{\pi \: w_{\rm fiber}}$$

where ($w_{\rm fiber}$) is the mode radius in the fiber. 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 ($\theta_\rm{fiber}$).

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.

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.

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.

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.

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.

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.

Used Types of Lenses

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.

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) to minimize spherical aberrations.

Insertion Loss

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.

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, if the mutual alignment is not perfect. 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.

Use with Angled Fiber Ends

Angled fiber endfaces 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.

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.

Tutorials

See our tutorial on Passive Fiber Optics, part 13: Fiber Accessories and Tools.

Frequently Asked Questions

What is a fiber collimator?

A fiber collimator is an optical device used to transform the diverging light from an optical fiber into a free-space collimated beam. It consists of a lens that holds the fiber end at its focal point, often within a pre-aligned mechanical housing.

What are the main applications of fiber collimators?

Their primary use is collimating a fiber's output. They are also used in reverse to launch light into a fiber, in pairs for fiber-to-fiber coupling, or combined with other elements like a Faraday rotator to create fiber-optic components. They act as an interface between fiber optics and free-space optics.

How is the size of the collimated beam determined?

The radius of the collimated beam depends on the lens's focal length ($f$) and the fiber's output beam divergence ($\theta_{\rm fiber}$). For a single-mode fiber, it can be calculated as ($w_{\rm collimated} \approx f \cdot \lambda / (\pi \cdot w_{\rm fiber})$), where ($\lambda$) is the wavelength and ($w_{\rm fiber}$) is the fiber's mode radius.

What types of lenses are used in fiber collimators?

GRIN lenses are common for small beam diameters as they are compact and inexpensive. For larger beams or demanding applications, conventional singlet or doublet lenses, sometimes with aspheric surfaces or achromatic designs, are used to minimize aberrations.

Can a collimator be used with both single-mode and multimode fibers?

While the principle is the same, performance differs. With a single-mode fiber, the output beam is well-defined. With a multimode fiber, the collimated beam size and quality depend heavily on the light launching conditions into the fiber.

How does an angled fiber connector (APC) affect a fiber collimator?

An angled fiber end, used to reduce back-reflections, causes the light to exit the fiber at an angle. This leads to the collimated beam being offset from the center of the lens, which can increase insertion loss or degrade the beam quality.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 63 suppliers for fiber collimators. Among them:

⚙ hardware

Schäfter+Kirchhoff offers a wide range of fiber collimators for collimating or reverse as incouplers. All have a high pointing stability:

• Series 60FC as our standard small fiber coupler
• Series 60FC-SF with fine-focussing meachnism for precise adjustments
• Series 60FC-T with TILT mechanism for large beam diameters
• Special 60FC-Q with integrated quarter-wave plate

All couplers/collimators are also available in amagnetic titanium.

⚙ hardware

Mid-IR focusing and collimating objectives are designed to use with infrared fiber optic cables and bundles. The housing of the lens easily connects to fiber optic cables either with FC/PC or with SMA connectors and enables Z-axis alignment. Thread adaptors are suitable to mount lens objectives to Thorlabs optical components with SM1, RMS and S-Mount.

Mid-IR lenses are available with anti-reflection (AR) coating for two spectral ranges:

• 3 — 5 µm for chalcogenide fiber (CIR) cables
• 8 — 12 μm for polycrystalline IR fiber (PIR) cable

⚙ hardware

Edmund Optics offers fiber-optic collimators for FC/PC, FC/APC and SMA connectors and different wavelength ranges around 350 nm to 1600 nm. Fiber optic collimators can be used in pairs to couple the input and output light of optical devices. Typical applications include the use with fiber coupled lasers and pigtailed receptacles, as well as communications and data transfer.

⚙ hardware

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.

⚙ hardware

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.

⚙ hardware

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.

⚙ hardware

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.

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