Page 156 - Optical Communications Essentials

Passive Optical Components

146 Chapter Nine

Thus, in decibels, the excess loss for a 2 2 coupler is
 P 
Excess loss = 10 log  0  × (9.2)
P + P 
 1 2
The insertion loss refers to the loss for a particular port-to-port path. For example, for
the path from input port i to output port j, we have, in decibels,

 P 
Insertion loss 10 log  i  (9.3)
 P 
 j 
Another performance parameter is crosstalk, which measures the degree of isolation
between the input at one port and the optical power scattered or reflected back
into the other input port. That is, it is a measure of the optical power level P 3 shown
in Fig. 9.1:

 P 
Crosstalk 10 log  3  (9.4)
 P 0 

Example A 2 2 biconical tapered fiber coupler has an input optical power level of
P 0 200µW. The output powers at the other three ports are P 1 90µW, P 2 85µW,
and P 3 6.3nW. From Eq. (9.1), the coupling ratio is

Coupling ratio = 85 × 100% = 48 6. %
90 + 85
From Eq. (9.2), the excess loss is

 200 
.
Excess loss = 10 log   = 058 dB
 90 + 85 
By using Eq. (9.3), the insertion losses are

200
Insertion loss (port to port ) 1 10 log 3.47 dB
0
90
200
Insertiion loss (port to port ) 2 10 log 3.72 dB
0
85
The crosstalk is given by Eq. (9.4) as
 63 . × 10 −3 
Crosstalk = 10 log   =− 45 dB
 200 

Table 9.1 lists some typical power-splitting ratios for a fiber optic coupler
operating at either 1310 or 1550nm. The first number in the designation PP/SP
is for the throughput channel (primary power), and the second number is for
the coupled channel (secondary power). Typical excess losses are less than 0.1dB.

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