Fibre Optic Brief

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 which I shall not go into any great detail on the physics, however I will share some information which may be of interest and use.

The 'light' can be at different wave lengths dependent on materials. These can go simultaneously and independently down a single fibre so you can increase the capacity without having to put in multiple fibres. The 'light' has a wavelength of approximatively 1200 to 1600nm spacing and is usually 20nm referred to as CWDM (course wave length division multiplexing), a more expensive 10nm spacing can also 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 this 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.

At one time there were a lot of different fibres and connectors used, but for the broadcast industry the most common cable Single Mode (SM), which 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 and the difference between a short cable of around 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).

However, there is one major difference with video SFPs and data types and this is due to the coding used. The data in both cases starts off as an NRZ (non return to zero) which must be encoded for fibre or ac coupled links. Ethernet uses a coding which results in a 25% increase in data rate, which is then scrambled for copper to reduce EMC, but not scrambled for fibre. SMPTE just scrambles and this results in difficulties with certain patterns so called Pathological. Data types will error with these – for net results you have to use a video type which costs five times as much. There is negligible internal difference in the SFPs and it is put down to quantity. This is regrettable as SMPTE did not recognise this and code such as 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) varies 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.

2G-MSG
This note is to bring the users attention that Multi Source Agreement MSA has limitations.
Volamp has had a 12G in the 4x3G formate for some time and these were all basically 3G products on different wavelengths which are combined (multiplexed) at the transmitter and demultiplexed at the receiver.
The fibre to copper transceivers usually incorporate SFPs (Small Form-factor Plugable) - these are now becoming available at 12G.

In an effort to standardise the manufacturers of these devices agreed a common specification so that they would be interchangeable.
SFPs produced to this specification are mechanically interchangeable. SFPs with DOM/DDM (Digital Optical/Diagnostics Monitor can also be monitored for many parameters by reading from standard registers. This includes transmit wavelength, receive power, information on vendor and many more parameters.

BUT MSA have not standardised the very basic parameters of transmit power, sensitivity and overload.
From the same SFP manufacturer you can have a maximum power output which exceeds the overload specification which can cause errors. This applies to HD as well as 12G. Different manufacturers again have varying specifications. The 12G SFPs also can incorporate re-clocking.

Volamp uses SFPs which are gauranteed to have a budget of 10db and will not overload.

William Saich, Volamp

www.volamp.com