Comparison of Small Fibre Connectors for High-Power Transmission SPIE Photonics West Conference in San Francisco, USA January 2010

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Stuart Campbell, Ola Blomster, Magnus Pålsson

OPTOSKAND

ABSTRACT Small fibre connectors capable of handling medium powered lasers are available on the market from multiple suppliers. Typical connector types are the SMA905 and LD80. The capability to handle power losses, for example radiation falling outside the fibre core is, due to the small size and restrictive design, limited. A new type of SMA fibre connector, designed for high-power loss capability will be presented. The basic principle is to strip off the losses in terms of radiation rather than being absorbed in the fibre connector. The radiation is instead absorbed in the female connector housing or within the laser housing, where it can easily be cooled away. In this paper both the principles and measurement of power capability are presented. Furthermore, in order to give a perspective of the available high-power SMA fibre connectors on the market today, a comparison between the best competitive products is presented. Keywords: SMA, high-power lasers, power handling.

1. INTRODUCTION When considering fibre launching a laser beam there are a number of conditions to consider when deciding upon the ideal connector within which to house the fibre. For lasers of medium power, in the region of 100-500 W, typical connectors on the market today are the different realisations of the SMA905 type. These SMA905 connectors are well suited to carrying ideally launched “clean” laser beams, when there is insignificant radiation falling outside of the fibre core or numerical aperture (NA). If however, light is launched outside the core or the NA, the design of the SMA905 is very restrictive in allowing the implementation of methods by which this unwanted light can be removed. A small metal housing and a fibre-to-metal interface produces problems that severely restrict power handling capabilities. There are a number of different versions which have tried to solve the problem of how to deal with power losses and still remain within the standard of SMA905. This report will show that is possible to achieve a major performance improvement by deviating slightly from the standard by making a new z-plane. The fibre end plane is moved forward by just over 25mm and supporting the fibre is a fused silica capillary, which is transparent for NIR / visible light. The extra length is used to “clean” the beam by removing any radiation, which ends up outside the core when launching the beam into the fibre. There are two loss mechanisms when coupling a laser beam into the fibre. The first is overfilling the core, focusing the laser beam down to a diameter which is larger than the fibre core. A variation of this loss is to misalign the spot with respect to the core of the fibre, also causing losses. The second is overfilling the numerical aperture of the fibre and entering the fibre with a larger angle than what is accepted by the fibre. This report will show the performance of the Optoskand SMA0.5 regarding overfilling and misaligning losses in the fibre connector and a comparison between the best competitive products. * [email protected]; phone +46 31 706 27 65; www.optoskand.se

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2. BEAM DEFINITION Equipments for measuring the diameter and divergence of laser beams typically measure an 86% value of radiation. This of course differs from the 98% value, which is the interesting value when transporting light trough fibres. The power measurement is usually carried out with 98% or 100% values. It is important however to keep this in mind when comparing values and examining results. In this report only the 98% beam diameter will be discussed.

Figure 1: Definition of BPP (86%) and BPP (98%)

In free space the numerical aperture is defined as the sine of the travelling angle of the beam NA = sin a. For this paper it is the 98% angle that defines the numerical aperture. The maximum numerical aperture of light guided by a fibre is defined as the square root of the core refractive index squared minus the cladding refractive index squared NA = (ncore2-nclad2)0.5.

3. FIBRE INCOUPLING Ideally, when coupling a laser beam into a fibre, the beam should be within the numerical aperture of the fibre and the focal spot imaged onto the fibre facet should be smaller than the fibre core. When fitting the spot into the core, the spot should fit in with some margins. If the spot-core ratio is too small, there will be an unnecessary loss in beam quality and if the spot-core ratio is too large, there will be power losses in the fibre connector, in the form of light inside the cladding. The angle of the laser beam focused onto the fibre facet should be below the acceptance angle for the fibre. When designing the optical system for fibre coupling, the angle of the beam should not be larger than the smallest possible acceptance angle. All fibres tested in this paper have a core diameter of 200 µm and a numerical aperture of 0.22 ± 0.02.

Figure 2: Laser beam focused into a fibre

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3.1 Losses by overfilling the fibre core Launching a laser beam into the fibre when the spot diameter (ds) is larger than the core diameter (dco) will result in light travelling inside the cladding material. This light is within the acceptance angle of the fibre, but the light will remain in the cladding material and leak along the fibre length. To avoid the potentially damaging power losses along the fibre length, a mode stripper can be used to remove the light in the input fibre connector. The light leaving the output end of the fibre cable will then be confined to the core and well defined.

Figure 3: Overfilling the core of the fibre.

3.2 Losses by overfilling the NA of the fibre Launching a laser beam into a fibre where the angle exceeds the acceptance angle of the fibre means that the fibre will not be able to guide the light and will leak power into the cladding. This will result in power losses in the fibre and it is difficult to specify the power distribution along the length of the fibre. It is possible to guide the laser beam in the cladding avoiding the fibre leaking power provided there is a coating outside the cladding that allows the laser beam to be guided. To guide the power inside the cladding is not preferable as this puts demands on the guiding material and it is difficult to remove the cladding modes at the output end of the fibre. Working with mode stripping to remove high NA light inside the cladding is not easy when working with larger angles than the acceptance angle. The mode stripper needs to be very long to successfully remove all the light in the cladding. It is strongly recommended not to overfill the NA of the fibre when focusing the light.

Figure 4: Overfilling the NA of the fibre.

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4. SMALL CONNECTOR DESIGN The Optoskand SMA0.5 differs from the standard SMA905. The difference in form is slight but noticeable. The difference in material is significant and extremely important.

Figure 5: SMA905 (top) compared to SMA0.5.

4.1 SMA905 design The designs of all “high-power” SMA905 (HP905) connectors currently available are essentially the same. A metal ferrule is held tightly around the cladding of a fibre optic with a polished (or possibly cleaved) end surface. Low power SMA905 connectors have stainless steel ferrules, but since high-power SMA905 connectors are prone to overheating, many have replaced the steel with copper (or copper alloy) to exploit the thermal conduction properties of that material. The ferrule design creates several problems when considering launching a high-power beam. The metal-glass interface of the front of the connector produces difficult issues for cleaning the fibre facet. Particles of dirt can gather in the join, a short-cut to contamination and fibre damage. The fibre must be held in the ferrule, typically by glue. This is another source for contamination, and if the temperature of the ferrule exceeds the limit for damaging the glue then the fibre will become free and likely contaminated by the melted/burnt glue. To avoid these problems, many manufacturers have produced connectors with a chamfer around the fibre, allowing the fibre end to protrude from the central part of the ferrule but still be protected by the outer part of the ferrule. This allows for easier cleaning of the fibre, but leaves it more vulnerable to external damage and overheating.

Figure 6: Cross section sketch of the internal chamfer common to High power SMA905 connectors.

Some HP905 manufacturers produce connectors that have a modestripping feature on the fibre. The restrictive size and design of the HP905 forces the position of this section of modestripped fibre to lie at least 25 mm behind the entrance facet to allow for cooling of the modestripped radiation which has been absorbed in the connector. If any radiation is launched into the cladding it can leak out along this first section, heating the glue and the ferrule which will eventually cause damage. 4.2 SMA0.5 design The Optoskand design maintains the SMA905 ferrule dimensions exactly and continues to use the same locking thread principle also. The design difference is the fibre end plane, which in the Optoskand fibre is extended out from the ferrule by just over 25 mm. This extension takes the form of a capillary tube manufactured in fused silica of low OH grade (160°C) at low losses (22.4 W) caused leakage of glue from the metal part of the connector which caught fire and contaminated the fibre facet. The eventual failure of the Optoskand SMA0.5 fibre (199.8W loss) was less dramatic and much cleaner. No debris was emitted by the failure and all optics in the system were undamaged. The connector’s performance is improved further by water cooling the house, a cooling technique that will not result in water damage to optics or lasers in the unlikely event that the fibre was to fail.

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7.2 Conclusions The Optoskand SMA0.5 connector is proven to have extraordinary power handling capabilities compared to competitive “high-power” SMA905 connectors. The connector can handle extreme losses outside of the fibre core compared to the transmission power it was designed for. It can run hotter and for longer than any competitor and when it does finally fail it causes minimal damage. Of the two types of launch losses discussed earlier, only spot-overfill was discussed in this report. It is strongly recommended to avoid overfilling the NA of the fibre by designing suitable focusing optics. Tests have been carried out at Optoskand which show that the SMA0.5 has superior ability to handle radiation outside the NA compared to the competitors. Fresnel losses were mentioned previously in this report as being unavoidable without anti-reflection coating the fibre facet. Currently in development at Optoskand is a quartz-block version of the SMA0.5. The Optoskand quartz-block design results in a more robust solution in systems with high power density on the fiber end surface. The antireflection coating on the quartz-block provides nearly 8% extra light transmitted trough the fiber cable. The fiber end plane is put in the same position as for the standard versions, so the SMA0.5Q makes both connector versions compatible in the same fiber holder.

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Article authors: Stuart Campbell*, Ola Blomster, Magnus Pålsson Optoskand AB Krokslätts Fabriker 27, SE-431 37 Mölndal, Sweden * [email protected]; phone +46 31 706 27 65; fax +46 31 706 27 78; www.optoskand.se

Authors: Stuart Campbell, Ola Blomster, Magnus Pålsson ”Comparison of Small Fibre Connectors for High-Power Transmission”, Conference 7578 - Proceedings of SPIE Volume 7578, SPIE Paper number 7578-61 Copyright 2010 Society of Photo-Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.