592
14 Basic Measurement and Monitoring Techniques
To carry out the attenuation tests, the connector of the short-length launching
fiber is attached to the connector of the receiving system and the launch power level
P 1 (λ) is recorded. Next, the cable assembly to be tested is connected between the
launching and receiving systems, and the received power level P 2 (λ) is recorded.
The attenuation of the cable in decibels is then
A = 10 log
P 1 (λ)
P 2 (λ)
(14.3)
This attenuation is the sum of the loss of the cabled fiber and the connector between
the launch connector and the cable.
Example 14.2 The insertion-loss technique also can be used for measuring the loss
through an optical device that has fiber flyleads attached. Suppose an optical filter
with attached flyleads is inserted into the link in Fig. 14.6a. Consider the case when
the power at the photodetector prior to inserting the filter is P 1 = 0.51 mW and the
power level with the optical filter in the link is P 2 = 0.43 mW. What is the insertion
loss of the device?
Solution From Eq. (14.3)
Insertion loss = 10 log P 1 /P 2 = 10 log 0.51/0.43 = 0.74 dB.
14.5 Concept of Eye Diagram Tests
The use of an eye diagram is a traditional technique for quickly and intuitively
assessing the quality of a received signal. Modern bit-error rate (also called biterror ratio) measurement instruments construct such eye diagrams by generating a
pseudorandom pattern of ones and zeros at a uniform rate but in a random manner.
When the pulses in this pattern are superimposed simultaneously, an eye pattern
as shown in Fig. 14.7 is formed [10, 11]. The word pseudorandom means that the
generated combination or sequence of ones and zeros will eventually repeat but that
it is sufficiently random for test purposes. A pseudorandom binary sequence (PRBS)
comprises sequences of 2
N different N-bit long combinations. For example, these can
be four different 2-bit long combinations, eight different 3-bit long combinations,
sixteen different 4-bit long combinations, etc. up to a limit set by the instrument.
These combinations are randomly selected. The PRBS pattern length is of the form
2
N – 1, where N is an integer. This choice assures that the pattern-repetition rate is
not harmonically related to the data rate. Typical values of N are 7, 10, 15, 20, 23,
and 31. After this limit has been reached, the data sequence will repeat.
Ideally, if the signal impairments are small, the received pattern should look like
that shown in Fig. 14.7. However, time-varying signal impairments in the transmission path can lead to amplitude variations within the signal and timing skews
between the data signal and the associated clock signal. Note that a clock signal,
14 Basic Measurement and Monitoring Techniques
To carry out the attenuation tests, the connector of the short-length launching
fiber is attached to the connector of the receiving system and the launch power level
P 1 (λ) is recorded. Next, the cable assembly to be tested is connected between the
launching and receiving systems, and the received power level P 2 (λ) is recorded.
The attenuation of the cable in decibels is then
A = 10 log
P 1 (λ)
P 2 (λ)
(14.3)
This attenuation is the sum of the loss of the cabled fiber and the connector between
the launch connector and the cable.
Example 14.2 The insertion-loss technique also can be used for measuring the loss
through an optical device that has fiber flyleads attached. Suppose an optical filter
with attached flyleads is inserted into the link in Fig. 14.6a. Consider the case when
the power at the photodetector prior to inserting the filter is P 1 = 0.51 mW and the
power level with the optical filter in the link is P 2 = 0.43 mW. What is the insertion
loss of the device?
Solution From Eq. (14.3)
Insertion loss = 10 log P 1 /P 2 = 10 log 0.51/0.43 = 0.74 dB.
14.5 Concept of Eye Diagram Tests
The use of an eye diagram is a traditional technique for quickly and intuitively
assessing the quality of a received signal. Modern bit-error rate (also called biterror ratio) measurement instruments construct such eye diagrams by generating a
pseudorandom pattern of ones and zeros at a uniform rate but in a random manner.
When the pulses in this pattern are superimposed simultaneously, an eye pattern
as shown in Fig. 14.7 is formed [10, 11]. The word pseudorandom means that the
generated combination or sequence of ones and zeros will eventually repeat but that
it is sufficiently random for test purposes. A pseudorandom binary sequence (PRBS)
comprises sequences of 2
N different N-bit long combinations. For example, these can
be four different 2-bit long combinations, eight different 3-bit long combinations,
sixteen different 4-bit long combinations, etc. up to a limit set by the instrument.
These combinations are randomly selected. The PRBS pattern length is of the form
2
N – 1, where N is an integer. This choice assures that the pattern-repetition rate is
not harmonically related to the data rate. Typical values of N are 7, 10, 15, 20, 23,
and 31. After this limit has been reached, the data sequence will repeat.
Ideally, if the signal impairments are small, the received pattern should look like
that shown in Fig. 14.7. However, time-varying signal impairments in the transmission path can lead to amplitude variations within the signal and timing skews
between the data signal and the associated clock signal. Note that a clock signal,