In this post we want to show through pictures and drawings the process of assembling a bare fibre into a protective jacket and ended with commercial connectors for spectroscopy purposes. In particular, we have prepared a 10m, 50μm core fibre with SMA connectors and protected with a square-locked stainless steel tube.
A very descriptive document is the instructions sheet 408-9863 distributed by Tyco Electronics: OPTIMATE FSMA Fiber Optic Connector Types 905 and 906 in PDF format. The document describes the assembly process fot their SMA connectors types 906 and 905 for data and telecommunications applications. It is basically the same process we use for linking telescopes to spectrographs were we take special care to reduce the focal ratio degradation.
The User Guidelines from Polymicro contain salso very useful information about the handling of fibres.
- Fibre: Silica Optical Fiber Polymicro, Step Index, Numerical Aperture 0.22, Core 50 µm, Clad 70 µm, Buffer 90 µm. Ref. FBP050070085. Other characteristics at Polymicro Technologies. Price ~3.5 $/m
- Tube: Square-locked stainless steel tube from Kientec Systems model SLP-40N (internal diameter 4 mm) with PVC sheath. Price ~4.68$/m
- Connectors: SMA-905 connectors from Kientec Systems model 504-R-Z-127-A3-3, ferrule type: zirconia, ferrule ID 127 µm. Price ~56$/10 pieces
- Adhesive: Epoxy two components, transparent, 2 minutes curing, from UHU profi-shop. Product UH45705. Price ~6€/35 gr
- Complete polishing kit from Thorlabs
or separated tools like:
- Sapphire-tipped Scribe Tool from AMP Netconnect or Diamond Scribe from Thorlabs
- Crimper Hand Tool from AMP Netconnect
- Polishing Bushing (Disc) with SMA from Thorlabs
- Polishing Sheets 3 µm and 0.3 µm from AMP Netconnect or Thorlabs
- Polishing Plate from Thorlabs
- Miscellanea: scissors, alcohol (propanol, isopropanol or methanol), acetone, water, microscope, optical tissue
Considerations before purchasing
- Fibre: Best optical fibres for linking telescopes to spectrographs are multimode step index fibre with high UV transmission. We advice to buy fibres protected with polyimide jacket because you can leave it into the connector making it more robust against strenghts. Always order some more fibre, e.g. 1 meter longer than required in order to operate it with some comfort.
- Connector: Check carefully both, the diameter of the fibre including the buffer/jacket and the inner diameter of the connector ferrule. Remember that the standard ferrule diameter is 125 µm, therefore you can accommodate fibres with buffer/jackets of up to 125 µm diamter, or with cladding of up t0 125 µm if the jacket is removed (see Polymicro document here on how to remove the jacket). The rear part (boot type or tubing) has to fit in the protective tube. If the fit between connector and tube is too loose you may need to use some crimping tool.
- Jacket/Tube: the stainless steel tubing is more expensive than the plastic one but it offers the highest protection against accidents, forced loops and light pollution.
- Abrasive sheets. After experience in polishing, we found that we need basically 2 abrasive sheets, the 3 and 0.3 μm. For rough lapping (reducing the excess of glue, for example), we use very fine sandpaper (P240 or higher, grain < 60 μm)
- Preparing the mettalic jacket. Use a drill machine provided with a grinding corundum weel to cut the mettalic jacket to the required length first, and to polish and round both ends second.
- Tubing the fibre. Fibres with polyimide jacket may be slided directly inside the metallic jacket. We found the contact of the fibre against the metal does not break the fibre. The fibre is usually delivered rolled in a fibre wheel. Place the wheel on a rod or long screwdriver for un-wheeling. If the fibre length is below 4 meters, you may try to push it gently, with the hand, inside the tube. For that, lay the tube straight on the floor. For longer fibres, you can try by vertically hanging the tube. If the high of the building is not enough, you should insert first a guide into the tube before you pass the fibre. The guide can be a steel wire used for hanging the curtains at home. In our case, we used an old fibre plastic cable with an standard 3mm diameter. It is stiff enough to pass up 20 m! Once the guide is tubed, we glued it to the fibre end with a drop of any fast cyanoacrylate adhesive. When pulling the guide, do not pull to fast to avoid breakage. Leave at least 20 cm of fibre out of the tube.
- Manipulating the tube with the fibre inside. Once you passed the fibre and if the jacket is long (>10m), do not roll up the jacket in a wheel but fold it in halves: the tube lies straight on the floor, take a tube end bring together the tube ends. Use masking tape to hold the ends. Next, bring the first folded tube to the ends, etc. The reason is to avoid the differential length between the fibre and the tube when rolled. In this case the fibre “shrinks” with respect to the tube. After assembling connectors and straighting the cable, the fibre is “longer” and adopts a serpent shape inside the tube creating a high focal ratio degradation.
- Gluing the connector to the jacket and to the fibre. First, clean the fibre ends with acetone. Pass the connector into the fibre (Figure). Check that the rear side of the connector enters properly into the tube.
Before you prepare the glue, be sure you have all material ready, especially the tissue paper in case the glue goes in non desirable places. Prepare the glue. Remember that you have just 2 minutes to use the glue before polymerization! Put a drop of glue around the rear side of the connector and slide it into the tube. Be sure you do not glue the nut of the connector! Do the same operation for the other fibre end. Check that the fibre ends slide free inside the ferrules, i.e. that the glue has not touch the fibre. If yes, you can still proceed but ensure that the fibre will not be longer than the tube.
The previous operation should be done inside the 2 minutes. So, at this stage do not try to glue the fibre with respect to the ferrule. It is better to prepare a new glue for this operation.
In order to glue the fibre to the ferrule, put a small drop of glue just at the ferrule end and the fibre. Take the fibre with your fingers and slide the fibre into the ferrule in and out by around 3 millimeters in order to moist with glue the inner side of the ferrule.
Immediately after, apply fo a few seconds some heat with a hairdryer. This will increase the fluidity of the glue and you will obtain an homogenous distribution of the glue around the fibre. When properly applied, the glue will take the shape of a rounded cone around the fibre. The glue will hard inside 5 minutes but we advice to wait at least 5 hours tobe sure that the glue is hard enough before to cleave and polish the fibre.
- Cleavage of the fibre. Once the glue is hard, you can proceed to cut the fibre (Figure).
In order to avoid axial breakages (breakages which go along the fibre axis and go inside the connector), use a cleavage tool with a sapphire or diamond blade as the one described in Material section. If you do not have it, you can still try with good sharped scisors, but DO NOT cut the entire fibre, just make a mark on the polyimide. To brake the fibre, take it with your fingers, slightly tilt it and pull it out from the ferrule until it breaks.
Estimate the thickness of the glue in front of the ferrule. In the case the thickness of the polymerized glue is bigger than 1 mm, proceed to remove it, otherwise you can directly polish it as explained below. To remove the excess of glue, use a very fine sandpaper (P240 or higher, grain < 60 μm).
- Lapping. Place the sandpaper on a glass plate or a very flat surface. Moist it.
Tilt a bit the connector and gently slide the ferrule against the sandpaper along the tilt. DO NOT come back with the tilted connector to avoid internal brekage of the fibre into the ferrule. Turn the ferrule around its axis of a quarter of turn and lap again.
At this stage, you can check with a microscope objective (10 X is optimal) the state of the fibre. Check that the fibre is not broken. For that illuminate the fibre from the other end.
Repeat the lapping until the glue thickness is around 0.3 mm (~ 3 times the fibre diameter). With a water moisten paper tissue, clean thoroughly the fibre end.
- Coarse polishing. Before you polish the fibre, you have to flat the excess of glue on the ferrule
For that you can use the 3 μm fibre abrasive paper without using the connector support. Place the abrasive paper on the glass plate and proceed to lap the fibre maintaining the ferrule perpendicular to the plate. Go on until you leave a fine layer of glue on the ferrule (around 100 µm, diameter of the fibre). Clean the fibre end with water.
Insert the fibre connector into the SMA polishing support. Do not screw the nut until the end but leave a bit of play to start the polishing. This will avoid breaking accidentally the fibre end when the excess of glue is still thick. Take the connector and not the support and begin to polish. Follow an 8 shape movement as shown in the Figure.
Control the thickness of the glue and when it is thinner as around 50 μm (half of the diameter of the fibre), you can firmly screw the connector against the polishing tool. Check that you did not break the fibre along the axis (fissure).
Take the polishing tool with your hand and proceed to polish by doing the 8 shape movement. Control the polishing quality and check that the fibre has not been broken. You can accept chips on the edges of the fibre including the cladding but confirm that the core is totally free.
If you are not going to glue the fibre end against a lens or glass plate, you can leave a thin layer of glue, the fine polishing will be easier. If not, keep lapping until, ideally, you almost reach the ferrule. If there is still a fine layer of glue, the fibre is still out of the connector and when you pass to the fine polishing you will not be disturbed by the ferrule. If you have reached the ferrule, of course, you are not done, but you have to finish the fine polishing with some precautions.
- Fine polishing. Before you pass to the fine polishing (0.3 µm), clean thoroughly the ferrule and polishing tool.
This is important to avoid contamination of the fine abrasive pad with thick grains which can make undesirable scratches on the fibre. Dismount the connector and clean with a moisten tissue the inner wall of the tube and inside the groves of the polishing tool.
Put some drops of water on the abrasive and polish in the usual 8 shape way. Check the polishing quality with the microscope. If you notice that you have only polished the ferrule and not the fibre, it means that you went too strong with the coarse polishing and the fibre is hided inside the ferrule.
There are several possibilities:
- You keep polishing with the fine abrasive until you reach the fibre end
- You come back with the 3 µm polishing grain and polish smoothly to expose the fibre
- You polish with a cloth pad and liquid cerium oxide fluid
The last method requires additional material like the oxide oxide suspension (Buehler polishing products). If you can procure it, place on the glass plate a clean, smooth and plain piece of cloth. Moisten it with water and apply some drops of oxide oxide. Polish with the normal 8 shape movement.Following pictures provide indications on the quality of the polishing.
Example of a 50 μm polished fibre. Click on the picture to enlarge. In this particular case, the fibre protrudes from the ferrule by about 30 μm. Note also that the polyimide jacket has a diameter of 90 μm and the ferrule 125. The decentering is extreme!
If you have foreseen to glue a mini-lens in front of the fibre or glue it against a glass plate. You do not need an extreme polishing quality. Usually the residual digs and scratches vanish when you apply the glue.
Carlos Guirao and Gerardo Avila
Assembling a fibre with SMA connectors by CAOS group is licensed under a Creative Commons Attribution-Non-Commercial-No Derivative Works 3.0 Germany License.
Based on a work at spectroscopy.wordpress.com.