Fiber collimator

A fiber collimator is a lens and a fiber end held in a common housing so that the fiber tip sits at the lens's focal point. Light leaving the fiber comes out as a collimated beam, and a collimated beam entering the lens is focused back into the fiber, so collimators are used in pairs to open a free-space gap in a fiber path, where isolators, filters, polarizers and switches can be placed. The general theory of collimation, including the beam size for a given lens and the best waist for a given distance, is covered in that entry; this entry covers the packaged device. Telecom collimators for 1310 and 1550 nm usually combine a GRIN lens or a spherical "C-lens" with a glass ferrule and give beams about 0.3–0.5 mm in diameter; laboratory collimators use aspheric lenses or multi-element designs and give beams from about 1 mm to several millimetres, sometimes with a connector receptacle and an adjustable focus.

Construction

The fiber is epoxied into a glass or ceramic ferrule, the ferrule end is polished, and the lens is aligned to it in a glass tube, then fixed with adhesive or solder and slid into a steel sleeve. The ferrule end and the facing lens surface are usually polished at an angle, commonly 8°, so that reflections from those surfaces do not return into the fiber core; the same principle as the angled connectors described under fiber connectors. Antireflection coatings on the lens and a small air gap complete the assembly. Collimators are specified by wavelength band, beam diameter, working distance (the lens-to-lens spacing for a pair), insertion loss for a matched pair, return loss and pointing angle, the angle between the beam and the housing axis.

Beam size and pointing

For a single-mode fiber with mode radius $w_f$, a lens of focal length $f$ gives a beam of waist radius $w = \lambda f / (\pi w_f)$ and divergence half-angle $w_f/f$. With $w_f = 5.2$ µm (half the mode field diameter of standard fiber at 1550 nm), a 0.5 mm diameter beam needs $f \approx 2.6$ mm, and its divergence is about 2.0 mrad. The Rayleigh range of that beam is

$$z_R = \frac{\pi w^2}{\lambda} \approx 127\ \text{mm},$$

so a pair with the waist midway works over a few hundred millimetres at most, as the collimation entry discusses; the procedure for choosing the lens and setting the waist is in How to collimate a laser beam. The same short focal length makes the beam direction sensitive to the fiber position: a lateral offset $\Delta x$ of the fiber from the lens axis tilts the output by $\Delta x/f$, about 0.38 mrad per micrometre here. Angled polishing also tilts the beam relative to the lens axis, which is why pointing angle is part of the specification and why collimators are aligned in holders with angular adjustment.

Coupling loss of a collimator pair

Two identical collimators facing each other couple with an efficiency set by the overlap of the two Gaussian beams. For waist radius $w$, an angular misalignment $\theta$ gives

$$\eta_\theta = \exp\!\left[-\left(\frac{\pi w \theta}{\lambda}\right)^2\right],$$

and a lateral offset $d$ gives $\eta_d = \exp[-(d/w)^2]$. For $w = 0.25$ mm at 1550 nm, a tilt of 0.5 mrad costs 0.28 dB and 1 mrad costs 1.1 dB, while a lateral offset of 25 µm costs only 0.04 dB and 50 µm costs 0.17 dB. Larger beams tolerate lateral offset better and angular error worse, so expanded-beam connectors and long free-space paths need tighter angular alignment, and compact telecom packages choose small beams that tolerate the angular error of epoxy joints and temperature changes.

Where fiber collimators are used

• Free-space sections inside fiber components: optical isolators, thin-film WDM filters, variable attenuators, circulators and MEMS switches.
• Laboratory benches, where a collimator connects a fiber-coupled laser or detector to free-space optics.
• Expanded-beam connectors for harsh environments, where the larger beam reduces sensitivity to dust.
• Free-space optical links and sensors, often with a larger telescope after the collimator.

Pitfalls

A collimator is designed for one wavelength band; chromatic focal shift in singlet and GRIN lenses means a unit aligned at 1550 nm is slightly defocused at 1310 nm or in the visible, and achromatic multi-element collimators are used for broadband sources. Epoxy in the beam path limits power handling, and dirt on the ferrule end or lens concentrates heat. Mismatched pairs, with different focal lengths or beams, add mode-mismatch loss even when perfectly aligned. Polarization-maintaining collimators need the fiber stress axes keyed to the housing.

Common questions

How far apart can two fiber collimators be?

For identical collimators the maximum practical spacing is about twice the Rayleigh range of the beam, with the waist placed midway: roughly 250 mm for a 0.5 mm beam at 1550 nm, and metres for beams of a few millimetres.

What is the difference between a GRIN and an aspheric fiber collimator?

A GRIN lens is a flat-faced rod whose index falls from the axis outward; it is small, easy to angle-polish and bond, and suits telecom packaging. An aspheric lens gives better wavefront quality and larger beams and is common in laboratory collimators.

References: B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics, 3rd ed. (Wiley, 2019); A. E. Siegman, Lasers (University Science Books, 1986); W. B. Joyce and B. C. DeLoach, Appl. Opt. 23, 4187 (1984).