Handbook for Sound Engineers

Transmission Techniques: Fiber Optics 477

5. What is the total length of the fiber optic cable?


6. What is the distance between transmitter and
   receiver?


7. Are there any physical obstacles that the cable must
   go through?


8. What are the tolerable signal parameters?


9. What is the acceptable SNR if system is analog?


10. What is the acceptable BER and rise/fall time if
    system is digital?

Once these parameters are established, the fiber optic

system can be designed.

15.11.3 Design Procedures

The procedures for designing a fiber optic system are as

follows:

1. Determine the signal bandwidth.


2. If the system is analog, determine the SNR. This is
   the ratio of output signal voltage to noise voltage,
   the larger the ratio the better. The SNR is expressed
   in decibels (dB). SNR curves are provided on
   detector data sheets.


3. If the system is digital, determine the BER. A
   typical good BER is 10^9. BER curves are provided
   on detector data sheets.


4. Determine the link distance between the transmitter
   and the receiver.


5. Select a fiber based on attenuation.


6. Calculate the fiber bandwidth for the system. This
   is done by dividing the bandwidth factor in mega-
   hertz per kilometer by the link distance. The band-
   width factor is found on fiber data sheets.


7. Determine the power margin. This is the difference
   between the light source power output and the
   receiver sensitivity.


8. Determine the total fiber loss by multiplying the
   fiber loss in dB/km by the length of the link in kilo-
   meters (km).


9. Count the number of FO connectors. Multiply the
   connector loss (provided by manufacturer data) by
   the number of connectors.


10. Count the number of splices. Multiply the splice
    loss (provided by manufacturer data) by the
    number of splices.


11. Allow 1 dB for source/detector coupling loss.


12. Allow 3 dB for temperature degradation.


13. Allow 3 dB for time degradation.


14. Sum the fiber loss, connector loss, splice loss,
    source/detector coupling loss, temperature degrada-
    tion loss, time degradation loss (add values of Steps
    8 through 13) to find the total system attenuation.


15. Subtract the total system attenuation from the
    power margin. If the difference is negative, the
    light source power receiver sensitivity must be
    changed to create a larger power margin. A fiber
    with a lower loss may be chosen or the use of fewer
    connectors and splices may be an alternative if it is
    possible to do so without degrading the system.


16. Determine the rise time. To find the total rise time,
    add the rise time of all critical components, such as
    the light source, intermodal dispersion, intramodal
    dispersion, and detector. Square the rise times.
    Then take the square root of the sum of the total
    squares and multiply it by a factor of 110%, or 1.1,
    as in the following equation:

(15-24)

15.11.3.1 Fiber Optic System Attenuation

The total attenuation of a fiber optic system is the differ-

ence between the power leaving the light source/trans-

mitter and the power entering the detector/receiver. In

Fig. 15-38, power entering the fiber is designated as PS

or source power. LC 1 is the power loss at the source to

fiber coupling, usually 1 dB per coupling. The power is

of that signal launched into the fiber from the light

source at the fiber coupling. LF 1 represents the loss in

the fiber between the source and the splice. Fiber optic

cable losses are listed in manufacturer’s spec sheets and

are in dB/km. LSP represents the power loss at the

splice. A typical power loss of a splice is 0.3 to 0.5 dB.

LF 2 represents the power loss in the second length of

fiber. LC 2 is the power loss at the fiber to detector cou-

pling. Finally, PD is the power transmitted into the

detector. Other power losses due to temperature and

time degradation are generally around 3 dB loss each.

Power at the detector is then generalized as

. (15-25)
Note: All power and losses are expressed in decibels
(dB).

15.11.3.2 Additional Losses

If the core of the receiving fiber is smaller than that of

the transmitting fiber, loss is introduced. The following

equation can be used to determine the coupling loss

from fiber to fiber:

(15-26)

System rise time 1.1 T 1

2

T 2

2

T 3

2

} TN

2

= +

PD PS–= LC 1 ++++LF 1 LSP LF 2 LC 2

Ldia 10–

diar

diat

©¹§·----------

2

= log