VOSCOM Fiber Optics Blog

When implementing a fiber optic transmission system, several factors need to be considered. Included are: fiber optic transmitter output, fiber optic sensitivity, fiber losses and bandwidth, losses from splices and terminations; over all distances – see figure (1).

Figure 1. Optical System Design Consideration

1. Determine the correct optical transmitter and optical receiver based upon the signals to be transmitted – video, audio (bal/unbal 600, unbal 10k or 47k Ohm) data (over the Coax as Coaxitrontm and Proteustm , Manchester, Biphase, RS-232, RS-422, RS-485, Sensornettm, Ethernet, TTL, Open Collector or Contact Closure);

2. Determine the operating power available at remote locations (24VAC, 12VDC) and at the control rooms (120V, 240V);

3. Determine the type (multimode or singlemode) and number of available fiber strands and optical connectors installed on the cable (ST or FC);

4. Calculate the total optical loss (in dB) in the system from end to end; include cable, splices, patch panels and connector losses. Parameters should be available from the manufacturers of the optical cables, fiber accessory hardware (patch, splices) and/or the sub contractor who installed the cable network;

5. Compare the loss figure obtained with the optical loss budget of the electronics as per the manufacturer’s specifications. In order to account for ageing of light sources, add a safety margin factor of 3 dB to the entire system.

6. When transmitting multiple high bandwidth signals, as in video multiplexers, compare with the available bandwidth in the fiber, to ensure the signal will be transmitted.

Example # 1: we have a requirement for transmission of 1 video signal with bi-directional PTZ data, there is one fiber available and the distance required is 13,000ft or a little less than 4Km (1Km = 3381ft.). As the signal is carried through several buildings, there are a total of 6 patch panels from end to end. Let us assume the loss of the video/data link is 13dB and the center carrier is 70 MHz. The specifications indicate that the video signal is transmitted @ 1300nm and fiber loss and bandwidth are specified @ 1dB and 500 MHz, respectively. NTK3631 Series transceivers are specified.

Distance required is 13,000ft. or 4Km

Calculation # 1 – as to loss budget

Fiber rated @ 1dB loss per Km x 4 Km, loss in cable = 4dB
Connector loss is system @ 1dB/connector x 2 = 2dB
Patch Panel loss in system @ 0.5dB/adapter x 6 = 3dB
Safety Margin = 3dB
Total Loss = 12dB
Since the 3631 Series link budget of 13dB is better than the 12dB the system requires, the conclusion is
that, based on loss, video/data signals may be transmitted on the 1 fiber.

Calculation # 2 – as to available bandwidth
Fiber optic Bandwidth per Km @ 1300nm = 500MHz
Divided by Bandwidth of 3631 Series System = 70MHz
Total Distance System may Transmit = 7 Km
Since the video/data signals use 70MHz, one simply divides the available fiber bandwidth of 500 MHz by
the system bandwidth of 70MHz. The result indicates that, based on bandwidth, the video/data signals may
be transmitted on the existing fiber.

Example # 2: we have a requirement for transmission of 4 video signals, there is but one (1) fiber available and the distance required is 10,000ft or a little less than 3Km. As the signal is carried between two buildings, there are two (2) patch panels – one at each end. Let us assume the loss budget of the video multiplexer is 10dB and carrier (combined bandwidth) of the 4 video signals is 150 MHz. The specifications indicate that video signals are transmitted @ 1300nm and fiber loss and bandwidth are specified @ 1dB and 500 MHz respectively. NTK3644 Series transmitters and receivers are specified.

Distance required is 10,000ft or 3Km

Calculation # 1 – as to loss budget

Fiber rated @ 1dB loss per Km x 3 Km, loss in cable = 3dB
Connector loss is system @ 1dB/connector x 2 = 2dB
Patch Panel loss in system @ 0.5dB/adapter x 2 = 1dB
Safety Margin = 3dB
Total Loss = 9dB
Since the 3644 Series link budget of 10dB is better than the 9dB the system requires, the conclusion is that,
based on loss, 4 video signals may be transmitted on the existing fiber.

Calculation # 2 – as to available bandwidth
Fiber Bandwidth per Km @ 1300nm = 500MHz
Divided by Bandwidth of 3644 Series System = 150MHz
Total Distance System may Transmit = 3.33 Km

When fiber is rated @ 500MHz per Km, it does not mean that for every new Km, there is an additional 500MHz. Since the 4 video signals use 150MHz, one simply divides the available fiber bandwidth of 500 MHz by the system bandwidth of 150MHz. The result indicates that, based on bandwidth, the 4 video signals may be transmitted on the 1 fiber. If, after performing the calculations, it is discovered that the loss is greater or bandwidth is inadequate, it will be necessary to consider the installation of additional fiber.

Typical Applications

The following chart of typical applications includes the reasons, advantages and benefits when fiber optic transmission systems are installed over copper twisted pair or coaxial systems.