Into the “light” Fibre Optic Brief from Bill Saich

   Fibre optic links have now been in use for many years in the commercial/military field. Primarily in the Military field for security as it does not radiate and the commercial field for its greater link spans and high data rates. As it is now becoming increasingly used in the broadcast industry this article is intended to give an understanding of the various items and terms used. There are many excellent articles which cover how light propagates down fibres, I shall not go into any great detail on the physics, but just give information which may be of interest and use.

   The “Light” can be at different wave lengths dependant on materials. These can go simultaneously and independently down a single fibre so we can increase the capacity without having to put in multiple fibres. The “light” has a wavelength of approximatively 1200 to 1600 nm spacing is usually 20nm referred to as CWDM (course wave length division multiplexing), a more expensive 10nm spacing can be used DWDM (dense).  This “light” generation requires laser transmitters. Some Ethernet is at 850nm which can then use LEDs and is visible.

   It is also possible to transmit in one direction at one wavelength and a different wavelength in the other “BIDI.” So, you get full duplex with one fibre. An example of CWDM and BIDI is in the Volamp Camlinx camera back systems.  A BIDI for the TDM control and CWDM for 4 video channels. 

  At one time there was a lot of different fibres and connectors used, but for the broadcast industry the most common cable Single Mode (SM) has a core of 9 microns hair is about 70. The data world is normally 62.5 microns Multi Mode, both fibres are 125-micron diameter external. The attenuation down SM fibre is very low . The difference between a short cable say, 100m and 1Km will not normally be a problem dirt or damaged ends will be far more likely to be the culprit. The transmitters / receivers are now much more simplified and readily available as a module (SFP)


         But there is one major difference with video SFPs and data types which is due to the coding used.  The data in both cases starts of as NRZ (non return to zero) this must be encoded for fibre or ac coupled links Ethernet uses a coding which results in a 25% increase in data rate, this is then scrambled for copper to reduce EMC ,but not scrambled for fibre. SMPTE just scrambles, but this results in difficulties with certain patterns so called Pathological. Data types will error with these .net result you have to use a video type which cost 5 times as much. There, is negligible internal difference in the SFPs, it is put down to quantity. It is regrettable SMPTE did not recognize this and code such that the higher rates can be implemented with data SFPs. which are readily available. WARNING due to lack of standardization with video SFPs it is possible that a high output transmitter will overload a receiver, also the flux budget difference between output power and sensitivity) verifies from the SFP manufacturers. A different SFP at each end of the link can be a problem with overload or low flux budget. MSA (multi source agreement) ensures physical compatibility, but not optical connectors

     For the simple non rugged connections then the LC connector as standardised in the data industry is the most common, although you may come across SC (LC’s big brother) or ST. Over the last few years there have been many different types introduced (100).

    The broadcast and military require a rugged connector, this has been implemented using LC connectors or fibre ferrules in rugged housings. The, big advantage of the LC approach is that it is inexpensive and readily repairable.

     A camera can also be powered by using cable which also has copper conductors (hybrid). The higher the voltage the greater the distance. If the voltage is under the low voltage directive then this will avoid power interlocks generally using a couple of copper conductors. Volamp pioneered using a voltage of about 50v dc. Which complies with LVD.

The Hybrid connectors are relatively expensive and some suppliers are restrictive regarding manufacture and repair. The two most used camera connectors are the LEMO with ferrules and Neutrik based on LC’s.. Volamp has introduced a hybrid system in which the fibre is an inexpensive rugged connector also based on LC and the cable is Y split with a separate power connector. The result is a relatively inexpensive, rugged cable.

  These are all butt connector and must be incredibly accurately aligned and kept clean as core of fibre (9 microns for SM) is about one eight the diameter of hair.

  An alternative is expanded beam connectors, the effective diameter is increased by lenses. These are very expensive most likely seen on military equipment.

The angular error is now critical. They will have a higher insertion loss, but it is consistent and they are easy to clean. This is written from a practical viewpoint to help understand fibre links I hope it helps all GBIN readers.

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