Optical Link Loss Budget

A logical way to proceed with designing a fiber optic link involves analyzing the fiber optic link power budget, also called an optical link loss budget. Figure 1 illustrates the key required optical calculations for designing a fiber optic link. A practical link must tolerate some range of optical loss. Ideally, but not always, it should work back-to-back (i.e., with the shortest possible fiber). And of course, it should work with some longer length of fiber. The designer can often adjust any or all of these variables to create a product that meets the needs of a given application.

Figure 1 – Optical Link Loss Budget

The graphic shows a hypothetical link and its corresponding link budget. Start with the transmitter output power on the left side of the chart. The typical launch power is -12.5 dBm. However, the transmitter LED output power can vary by ±2 dB due to manufacturing variability of the LED itself. Therefore, the output power can be as high as -10.5 dBm or as low as -14.5 dBm. The block is shaded between these two values.

Further transmitter variations of ±2 dB result from the effects of temperature on the electronics and the electro-optics (e.g., LED or laser). Another potential ±2 dB of loss is due to variations in the optical coupling to the transmitter output. The effects of aging, typically 1-3 dB, should be included in the system’s design. The next factor involves the losses due to optical connectors that may be in the optical path. The graphic allows 2 dB for this factor. For this system, the loss due to the optical fiber itself amounts to 4 dB/km of length. Multiply this value times the actual length to determine the loss due to the fiber. considerations for temperature effects associated with most fibers usually yield ±1 dB.

The next factor, variation in loss at the receiver, requires a large-area detector to eliminate the effects of this parameter. Finally, a 3 dB safety margin should be built into all systems. At each step, any variation causes the shaded band to enlarge. On the right side of the chart the receiver has to cope with optical inputs as high as -5.5 dBm and as low as -31.5 dBm. Or stated differently, the receiver would need an optical loss range or optical dynamic range of 26 dB.

A discussion of the decibel is necessary to understand these link loss values. The decibel (dB) is a convenient means of comparing two powers. It is always a ratio between two numbers. For fiber optics, the ratio is usually the transmitter output power compared to the receiver input power. The equation to calculate a decibel is:

dB = 10 x log10(Power1/Power2)

The decibel describes all loss mechanisms in the optical path of a fiber optic link. For example, a given AM video link may tolerate a maximum of 9 dB of optical loss. How much light actually reaches the receiver? Table 1 describes the decibel to power conversion. According to the table, 12% of the optical power actually reaches the receiver, so 88% of the light output by the transmitter was lost somewhere along the way. If the link could tolerate 20 dB of optical loss, only 1% of the transmitter’s optical output would reach the receiver. To determine the amount of light reaching the receiver, take any two values that total the dB of optical loss in question. For example, 15 dB is the total of 10 dB and 5 dB. The corresponding power out for 15 dB is 3.2% according to Table 1. This value is also attainable by multiplying the corresponding percent values for the two dB readings, 10 dB and 5 dB, to get the desired result, e.g. 10% times 32% is 3.2%. Thus, 3.2% of the light actually reaches the receiver.

Table 1 - Decibel to Power Conversion

dB

Power Out as a % of Power In

% of Power Lost

Remarks

1

79%

21%

---

2

63%

37%

---

3

50%

50%

1/2 the power

4

40%

60%

---

5

32%

68%

---

6

25%

75%

1/4 the power

7

20%

80%

1/5 the power

8

16%

84%

1/6 the power

9

12%

88%

1/8 the power

10

10%

90%

1/10 the power

11

8%

92%

1/12 the power

12

6.3%

93.7%

1/16 the power

13

5%

95%

1/20 the power

14

4%

96%

1/25 the power

15

3.2%

96.8%

1/30 the power

16

2.5%

97.5%

1/40 the power

17

2%

98%

1/50 the power

18

1.6%

98.4%

1/60 the power

19

1.3%

98.7%

1/80 the power

20

1%

99%

1/100 the power

25

0.3%

99.7%

1/300 the power

30

0.1%

99.9%

1/1000 the power

40

0.01%

99.99%

1/10,000 the power

50

0.001%

99.999%

1/100,000 the power

When purchasing fiber optics, some customers often mistakenly feel they must over-specify the system to include transmitter optical output power, receiver sensitivity, and optical link loss budget. In most cases, the customer needs to specify only the optical link loss budget. An application that requires a 10 dB maximum optical link loss budget will operate the same using a transmitter with a 0 dBm output and a receiver with -10 dBm sensitivity or a system with a transmitter with a -10 dBm output and a receiver with -20 dBm sensitivity. By specifying only the required maximum optical loss, the most economical transmitter/receiver pair can be utilized. The only time that transmitter optical output power and receiver optical sensitivity need to be specified is when the transmitter and receiver are bought separately. In that case, the maximum optical link loss budget need not be specified.

Calculating Fiber Loss and Distance Estimates

Fiber Type
Wavelength
Attenuation per km
Connector Loss
Splice Loss
SM 9 micron
1310 nm
0.4 dB
0.75 dB
0.1 dB
SM 9 micron
1550 nm
0.3 dB
0.75 dB
0.1 dB

Estimate Total Link Loss

This calculation will estimate the total link loss through a particular fiber optic link where the fiber length, as well as the number of splices and connecters, are known. This calculation is simply the sum of all worst-case loss variavles in the link:

Link Loss = [fiber length (km) x fiber attenuation per km] +
  [splice loss x # of splices] +
  [connector loss x # of connectors] + [safety margin]

For Example: Assume a 40 km single mode link at 1310nm with 2 connector pairs and 5 splices.

Link Loss = [40km x 0.4dB/km] + [0.1dB x 5] + [0.75dB x 2] + [3.0dB] = 21.0dB

In this example, an estimated 21.0 dB of power would be required to transit across this link. Of course, it is very important to measure and verify the actual link loss values once the link is established to identify any performance issues.

Estimate Fiber Distance

This calculation will estimate the maximum distance of a particular fiber optic link given the optical budget and the number of connectors anc splices contained in the link.

Fiber Length = [Optical budget] - [link loss]
  [fiber loss/km]

 

Fiber Length = {[(min. TX PWR) - (RX sensitivity)]
  -[splice loss x # of splices]
  -[connector loss x # of connectors]
  -[safety margin]}
  / [fiber lost/km]

For example: Assume a Fast Ethernet Single mode link at 1310nm with 2 connector pairs and 5 splices.

Fiber Length = [(-8.0dB) - (-34.0dB)] - [0.1dB x5] - [0.75dB x 2] - [3.0dB]
[0.4dB/km]
   
Fiber Length = [(26.0dB) - (0.5dB)] - [1.5dB] - [3.0dB] = 52.5 km
[0.4dB/km]

In this example, an estimated 52.5 km distance is possible before dissipating the optical power to a value below the RX sensitivity. As always, it is very important to measure and verify the actual link loss values once the link is established to identify any potential performance issues. Actual maximum distances will vary depending on: