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The most important task in the design of fiber optic link is to determine the maximum range of the optical transmission path, being in fact the balance of optical power in the link. Balance of power is a comparison of the power at the input of the optical link with the losses in fiber optic cables and other path components. This will help to find the optimal parameters of transmitting and receiving devices to ensure proper signal transmission.
To carry out the balance of power it is necessary to discuss the concept of decibel (dB) and decibel referenced to one milliwatt (dBm) - the units used for measuring attenuation of optical cables and sensitivity of photo-detectors.
Decibel is a logarithmic unit used in comparing two levels of the same physical parameter, where one of them is the reference level. For optical fibers the ratio of the optical power at the input of the fiber to that at its output is given in decibels. Decibel referenced to one milliwatt (dBm) is the ratio of the input power to reference power of 1 mW.
The definitions of signal levels given in dB and dBm:
The definitions of signal levels given in dB and dBm:
 |  |
| where: P1 - input power P2 - output power | where: P1 - input power |
In order to determine the relevant parameters of the transmitter/receiver, one should assess:
Ptx1 - Prx1 < Pl - Pg + Pm
Ptx2 - Prx2 > Pl - Pg + Pm
where:
- Ptx1; Ptx2 is the range of optical power (transmitter) at the input of the optical link, given in dBm.
- Prx1; Prx2 is the range of sensitivity of the photo-detector (receiver) at the output/end of the optical link, given in dBm.
- Pl - total loss of the passive components of the optical link, i.e. of the fiber and optical connectors/splices, given in dB.
- Pg - total gain of optical amplifiers in the path, given in dB.
- Pm - safety margin taking into account the aging of the transmitter/receiver elements, the effect of temperature on the electronics and optics, additional power loss in optical fibers due to bending, etc., given in dB. Typical safety margin is within 3 and 6 dB.
The designer, knowing the parameters of the link (lengths of runs) and optical components used, is able to calculate the loss of signal for any point of the path. This can be best seen in the example shown below:
| Diagram of an optical link, where: N - transmitter W - amplifier O - receiver Z - connector S - splice | |
Aside from the transmitter (N) and receiver (O), the optical link contains optical amplifier (W), 4 optical connectors (Z) and 5 splices (S). The following table gives for each item its attenuation or gain. With these values one can appropriately choose the sensitivity of the receiver:
| Tx power: Connector loss: Splice loss: Amplifier gain: Fiber optic loss: | 3 dBm 0.15 dB 0.15 dB 10 dB 0.2 dB/km |
NOTICE: Calculations are performed for III transmission window (fiber attenuation of 0.2 dB km).
The total loss of optical link depends on its length and the number and quality (attenuation) of optical connectors and splices. The total attenuation of the link is the sum of:
| fiber optic loss: attenuation of connectors: attenuation of splices: | (30 km + 50 km) x 0.2 dB/km = 16 dB 4 x 0.15 dB = 0.60 dB 5 x 0.15 dB = 0.75 dB |
Pl = 16 dB + 0.60 dB + 0.75dB = 17.35 dB
The total gain of the link is in this case equal to the amplification of the optical amplifier (W).
Pg = 10 dB
When designing a link, it should be taken into account the aging of the electro-optical element of the tranmitter (typically 1 dB ... 3 dB), and the effect of temperature on electronic and electro-optical devices (typically +/- 2dB). A good safety margin is 6 dB.
Pm = 6 dB
To select the receiver's sensitivity at the end of the optical path it is sufficient to rearrange and solve the equation:
Ptx - Prx < Ps - Pg + Pm
Prx > Ptx - Ps + Pg - Pm
Prx > 3 dBm - 17.35 dB +10 dB - 6 dB
Prx > -10.35 dB
Prx > Ptx - Ps + Pg - Pm
Prx > 3 dBm - 17.35 dB +10 dB - 6 dB
Prx > -10.35 dB
The receiver should provide a sensitivity better than -10.35 dBm.
Example of optical link design based on ULTIMODE devices
The example optical path employs devices from ULTIMODE series - Ethernet media converters ULTIMODE M-403M L11041. These reliable media converters enable transmission of Fast Ethernet data stream in two single-mode optical fibers at distances up to 40 km. They have two SC connectors used to transmit (Tx) or receive (Rx) the signal. Optical signal is transmitted and received in the second transmission window of 1310 nm.
ULTIMODE media converters are widely used for the transmission of Ethernet signals over long distances and in areas exposed to harsh weather conditions. Optical fibers provide full electrical insulation and resistance to electromagnetic interference. Media converters ULTIMODE M-403M L11041 are primarily used to connect building LANs (e.g. of scattered buildings of one corporation). Below there is a sample application of ULTIMODE converters.
With the basic parameters of the devices (output power of the transmitter and sensitivity of the receiver), one can calculate the maximum distance of transmission over typical single-mode fiber.
The parameters necessary for calculation:
| Tx power (Ptx1 ... Ptx2) Rx sensitivity (Prx1 ... Prx2) | -5 ....... 0 dBm -25 ... -7 dBm |
Additional assumptions:
- Attenuation of connectors/splices (Ps): 1 dB;
- Attenuation of a single-mode fiber (usually between 0.33 - 0.42 dB/km at 1310 nm): 0.4 dB/km;
- Adopted margin of safety (Pm): 5 dB;
- No signal regeneration in optical path.
The minimum distance (for minimum transmitter power) in the second transmission window is:
(Ptx1 - Prx1 - Pm - Ps)/0.4 = (-5 + 25 - 5 - 1)/0.4 = 35 km
The maximum distance (for maximum transmitter power) in the second transmission window is:
(Ptx2 - Prx1 - Pm - Ps)/0.4 = (0 + 25 - 5 - 1)/0.4 = 47.5 km
(Ptx1 - Prx1 - Pm - Ps)/0.4 = (-5 + 25 - 5 - 1)/0.4 = 35 km
The maximum distance (for maximum transmitter power) in the second transmission window is:
(Ptx2 - Prx1 - Pm - Ps)/0.4 = (0 + 25 - 5 - 1)/0.4 = 47.5 km
With the given assumptions, the transmission via single-mode fiber is guaranteed at least over 35 km distance.
The input power depends to some extent on operating temperature and aging of the laser diode. The MTBF of the diodes employed in ULTIMODE optical devices is at least 100 000 hours, i.e. 11 years.
