From: Rapunzel 19/03/2000 13:09:00 Subject: Flexible glass post id: 48182 Since most of the glass I come across is rigid and brittle, I am interested in the process by which glass is made flexible for things like fibreglass and fibre optic cable cores. Do they put additives into the glass? What's the difference on a molecular level? From: Rapunzel 19/03/2000 13:13:00 Subject: re: Flexible glass post id: 48184 Oh, and while on this subject: Are the repeater stations which fibre optic cable apparently requires at regular intervals (source written in 1997 says approx. every 100km for long-distance optical cable) just signal amplifiers, or are they more sophisticated than that? From: pete the Pom 19/03/2000 13:44:00 Subject: re: Flexible glass post id: 48188 Hi Rapunzel, Fibre optics are made from very pure glass, and supported by polymers. The repeater stations are, in land use, placed about every 3 to 10 kilometres in telecoms applications, I don't know how closely they are placed in sub-sea applications. The repeaters simply recieve the signal, do some cleaning up to remove noise, and re-transmit it. As for other flexible applications, I have no knowledge. Pete the Pom. From: Chris W (Avatar) 19/03/2000 14:09:00 Subject: re: Flexible glass post id: 48194 There a couple of different methods of manufacture for the glass fibre. The cheaper of these allows light to travel for one end to the other by multiple possible paths (multi-mode). Some of these paths are longer than others. A result of this is that narrow signals sent down the fibre will be smeared a little along the time axis by the time they reach the other end (modal dispersion). For analogue transmission this may not be a problem, but for digital signals the cumulative distortion can destroy the integrity of the message. The repeaters take the distorted signal, reconstruct it, and send it down the next segment. Repeaters are placed at intervals that allow the signals to be reconstructed before they are permanently lost. I have always assumed that the brittle nature of glass is a function of its crystal structure and the defects in it. In the 50+ micrometres across a fibre core there won't be much of a crystal structure to contain defects, and the manufacturing process is specifcally designed to minimise defects. The core is clad in various polymer (I guess) compounds which further protect the fragile fibre. The whole lot is wrapped in coverings that permit the cable to be used in normal circumstances without fracturing the core: pulled through ducts, transported, secured etc. Fibre optic cables have some limits on use, such as a minimum radius for any curvature, which are designed to limit losses in transmission and chance of core fracture. Some good stuff http://www.fibreoptic.com.au/tech/intro.htm" From: pete the Pom 19/03/2000 15:44:00 Subject: re: Flexible glass post id: 48199 Hi Chris, The bending radius of fibre optic cables is, as you say, very important, to avoid damage to the core fibre. However, it only really affects the cable runs inside exchanges etc., as the bending radius is usually of the order of 500 mm or less. In long, cross country runs, that does not cause any complications. There the main problem is to guard against mechanical damage. A rapidly growing area for fibre optic is in instrumentation as low voltage signals are easily currupted, whilst light signal are not. Pete the Pom. From: pete the Pom 19/03/2000 15:47:00 Subject: re: Flexible glass post id: 48200 Oh! and remember that copper cables also have radius limits, in order to limit heat build in the cable. Pete the Pom. From: Alan™ 19/03/2000 18:58:00 Subject: re: Flexible glass post id: 48240 I don't have all the details about the chemical structure of fibreglass around just at the moment. Maybe I'll find it tonight. But please remember for starters that glass is amorphous (without crystal structure), although some glasses have microcrystals after they've been hardened. From: bob s 20/03/2000 8:08:00 Subject: re: Flexible glass post id: 48329 Glass fibre is flexible simply because it is thin. The thinner you make fibre of any substance the smaller the radius to which it can be bent without breaking it. You have tensile stress on the outside of the bend and compressive stress on the inside. Thin fibres can be bent more sharply without the tensile stress exceeding the tensile strength. From: Ken 21/03/2000 0:05:00 Subject: re: Flexible glass post id: 48624 I thinks its flexiable mainly because its thin and proably had a polymerised structure of some sort. As for glass being non-crystaline this is usually true soon after the glass is formed but over time at room temp it can devitrify. Thousands of small microscopic to several mm sized sphelurites (spherical crystal clusters) grow over thousands of years within glass as it adopte a more stable atomic state. Glassy lavas such as rhyolite, and some very old windows show devitrification. I reckon that over time this could possibly stuff up all the optic cables, or maybe this problem is known and the glass is doped with something to stop the spheres growing. From: Alan™ 21/03/2000 10:17:00 Subject: re: Flexible glass post id: 48667 I was going to get back on this one. I haven't been able to find any references that the chemical formulas for fibreglass and ordinary glass. My gut feel is still that they are different, chemically. From: Dr Paul (Avatar) 21/03/2000 12:22:00 Subject: re: Flexible glass post id: 48684 Hi all, an extra question on this. Is the refrective index of the polymer higher than that of the glass used for the fibre. It should be , to allow a higher proportion of light reflected off the surface of the walls of the fibre. The reason optical fibres are replacing the "good old copper wires" is not heating or some other similar effect, but the transmission properties of sending an electrical signal along a conductor. As the electrical signal passes along the conductor, a small amount of the electricity is lost as a leak through the side of the copper. That is there is a transverse loss. Over some distance, this leak reduces the overall intensity of the electrical signal until it is not detectable. With optical fibres, there is a much lower amount of leak loss, which is improved if the covering of the fibre is tuned to increase the total internal reflection at the wall of the fibre. The problem with copper transmission lines was encountered and numerically solved when a cable was to be laid across the Atlantic. Interestingly, the same transmission properties that occur with copper wires also occur with nerve axons. Fortunately for us there are pump proteins to enhance the maintenance of electrical signals and on long axons, myelin sheaths reduce the transverse losses of electrical potentials. Hope this helps Paul From: Rapunzel 21/03/2000 13:55:00 Subject: re: Flexible glass post id: 48710 The core has a higher refractive index than its cladding. Light needs to be travelling from a more optically dense medium to a less optically dense medium for total internal reflection to occur. From: Grant¹ 21/03/2000 20:02:00 Subject: re: Flexible glass post id: 48850 Another disadvantage of copper cable is the degredation of the signal due to capacitive & inductive effects & it's susceptability to external interference. This forum is un-moderated. The views and opinions expressed are those of the individual poster and not the ABC. The ABC reserves the right to remove offensive or inappropriate messages.