WARNING: Communication Fibers can emit light!

Photo: Copyright (c) 1995 CTIO. All Rights Reserved.

Fiber Optic Light Pollution Alert

One would think that the manufacturers of communication fibers would make the jacket light-proof. This is not necessarily so!

Look at the photograph above. This is a view of an Arcon, taken by another Arcon. There are no sources of illumination other than the fibers. It is clear that the communication fibers which are connected to the Arcon are severe sources of light contamination. It should go without saying that this can be extremely serious in astronomical applications.

Note that light is emitted at the connectors and the area around them, as well as from the fibers. The emission is essentially at the transmitter, which is not a surprise.

Fortunately, only certain types of fibers emit light and it is relatively easy to shield against this kind of light contamination. This picture was taken when fibers were being used which are very transparent to the light traveling through them. Of course, since communication fibers use infrared light, no visible light is emitted, so this light contamination will not be obvious to the eye, but it is certainly obvious to Arcon!

All persons who work with optical communication fibers under low light conditions should be aware of this possibility. The following report is one I wrote several months ago which addresses the problem and its solutions. It is primarily intended for the use of CTIO personnel, but the information contained here may be useful for others who have encountered similar problems.

NOTE To CTIO Personnel working with Arcon!! There is a place for an LED on the Arcon Fiber Optic Link (FOL) module. This LED, if installed, can produce light within the Arcon which will leak out through cracks in the housing. This LED should only be installed when the FOL is being tested and is NEVER to be installed when the FOL is in normal use. If any of these LEDs are found installed in FOLs in the field, they should be removed immediately! In the picture above, light can be seen leaking out of the Arcon because one of these LEDs was inside, blazing away.

Notes on Light Emission from FOLs and Fiber Optic Cables

When we began to study the problem at CTIO, we discovered that the fibers we were using at that time (Belden type 225182) were emitting light over their entire length. The jacket of these fibers is white and transmits excellently in the near IR.

KPNO did not have this problem, as they happened to be using IR-opaque black AMP fibers (AMP LDD family, type example 501738-1) which do not transmit in the IR. CTIO was able to eliminate the most serious part of its contamination problem by switching to the same black AMP fibers.

Apart from the light emitted along the length of the fibers, the offending radiation is produced by the FOL transmitter, which communicates using an AT&T type 1261AAC fiber transmitter which contains an LED emitting at 850 nm. Most of the offending radiation comes from the plastic housing of the ST connector on the FOL transmitter and is caused by scattered light which does not make it into the fiber. Presumably there is some kind of lens in the transmitter in front of the LED which tries to focus as much energy as possible into the end of the fiber. Less than 10% of the light produced by the LED actually gets into the fiber, so the other 90% has to go somewhere.

Unfortunately, AT&T used a white plastic housing for its ST connector in early models of the 1261AAC. This housing transmits infrared light very well. The white plastic pipes the light throughout its entire body, which causes the connector to glow like a lighthouse in the infrared. This is the main source of light contamination from the FOL.

1. Be sure the LED is not installed in the FOL! The led site is at location J4, next to pin 1 of the TRAM connector.
2. Use the AMP cable everywhere and throw the Belden stuff away.
3. The FOL transmitters and receivers are supposed to be secured to the PCB with a pair of #5-40 screws on the transmitter and another pair on the receiver. Many FOLs were made without these screws. Installing them significantly reduces light emission from the 1261AAC. They probably should be installed if possible, since it makes the transmitter and receiver attachment to the FOL PCB somewhat more robust. Alignment problems sometimes make this difficult once the FOL has been manufactured, in which case, a layer of black electrical tape should be placed on the FOL covering the entire area on the bottom of the FOL, underneath the transmitter and receiver.
4. Installing a thin rectangular rubber gasket over the ST connectors on the front of the FOL transmit and receive modules eliminates the great majority of the light which escapes from the area where the connectors protrude from the Arcon. This gasket should be made of soft black rubber, 12 mm x 24 mm, 1 mm thick, with two circular 6.5mm dia. holes 12.75 mm apart. It is slipped over the ST transmit and receive connectors and left there permanently.
5. Current 1261 transmitters used to be made out of a white plastic, but are now made out of a grey material. This plastic is not entirely opaque to the offending illumination but the grey units emit less than 5% of the stray light emitted by the white units and are to be preferred if one has a choice.
6. Painting the plastic parts of both ST connectors on the FOL with an opaque black paint greatly reduces the light scattered inside the Arcon. This light could escape through other cracks in the Arcon and is best reduced to a minimum. I tried a simple black acrylic purchased at a local artist's supply store and found that it blocked the IR very well. To avoid flaking, I painted the exposed plastic parts of the ST connector which do not actually touch the ST cable connector. This eliminated nearly all the light emission, with only a small amount of residual leakage from the guide slot on the side of the ST connector, about which more later. Emission from this slot on the grey 1261 transmitters was very small.
7. It is important to be sure and block light from coming from underneath the 1261. If the plastic parts of the module are painted black before it is installed on the PCB, there will be no problem. In units where the transmitter has already been installed, the exposed parts of the 1261 should be painted. The area on the FOL PCB underneath both the transmitter and receiver should be covered with black tape, as mentioned previously. In addition, the gaps between the transmitter and receiver units and the FOL PCB should be sealed with something such as black epoxy, RTD or black tape.

The connectors must be made with the standard black plastic strain relief boot, which eliminates light leakage at the rear of the connector, as well as reducing strain on the cable. AMP ST connectors come in three types, with ceramic, stainless steel or plastic ferrules. The ceramic ferrules transmit slightly in the IR so the stainless steel ferrules produce slightly less scattered light. The black plastic ones are probably a little bit better still because the tips of the stainless steel ferrules are shiny. We have not yet tested the plastic ferrules to verify that they don't transfer IR. Even so, the difference between ferrules is a very small effect. We are continuing to use the ceramic tips because that is the type AMP uses in pre-made fibers with connectors.

Such light as sneaks around or through the ferrules may come out through the guide slot of the connector. This slot in an ST panel socket is not quite covered by the housing of the mating ST cable plug. A small exposed hole about 1.2 x .6 mm in size is left at the end of the guide slot when the plug is secured. When a cable is plugged into its socket it is possible to detect a spot of light emitted by this hole.

To completely eliminate the last trace of emission from the FOLs, a "boot" needs to be installed over the ST cable connector and slid up until it presses against the panel on which the mating connector is installed.

Roger Smith has figured out an arrangement using a small piece of tubing and an "O" ring which eliminates the last trace of light. The "O" ring is slipped over the ST cable connector until it is on the cable itself and then the tubing is slipped over the housing. After the connector is mated with the panel, the O-ring is pushed up over the strain relief until it presses the tubing against the panel. This takes care of everything.

I have been looking for a commercially available shroud which would do this job and be a bit more attractive and easier to use. I have not yet found one, though I have found a source that looks promising. In the meantime, one should install Roger's solution or something similar. Doing nothing for now is a reasonable approach for 99% of all uses. The effect of this hole is small enough so that if I didn't mention it, I doubt that anyone would ever notice. A boot will also help, of course, in cases where the gasket or black paint are not doing their job.