Page 335 - Optical Communications Essentials

Test and Measurement

Test and Measurement 325

methodologies for analyzing optical fibers and various passive and active com-
ponents are not covered here but can be found in the book by Derickson.

19.3. Optical Power Measurements
Optical power measurement is the most basic function in fiber optic metrology.
However, this parameter is not a fixed quantity and can vary as a function of
other parameters such as time, distance along a link, wavelength, phase, and
polarization.

19.3.1. Definition of optical power
To get an understanding of optical power, let us look at its physical basis and
how it relates to other optical quantities such as energy, intensity, and radiance.
■ As described in Chap. 3, light particles are known as photons, which have a
certain energy associated with them. The relationship between the energy E
of a photon and its wavelength λ is given by the equation E hc/λ, which is
known as Planck’s law. In terms of wavelength (measured in units of micro-
meters), the energy in electron volts (eV) is given by the expression E (eV)
1.2406/λ (µm). Note that 1 eV 1.60218 10 19 J (joules).
■ Optical power P measures the rate at which photons arrive at a detector; that
is, it is a measure of energy transfer per time. Since the rate of energy trans-
fer varies with time, the optical power is a function of time. It is measured in
watts or joules per second (J/s).
■ As noted in Chap. 6, radiance (or brightness) is a measure, in watts, of how
much optical power radiates into a unit solid angle per unit of emitting
surface.

Since optical power varies with time, its measurement also changes with time.
As shown in Fig. 19.2, which plots the power level in a signal pulse as a func-
tion of time, different instantaneous power-level readings are obtained depend-
ing on the precise time when the measurement is made. Therefore, two
standard classes of power measurements can be specified in an optical system.
These are the peak power and the average power. The peak power is the maxi-
mum power level in a pulse, which might be sustained for only a very short time.
The average power is a measure of the power level averaged over a relatively
long time period compared to the duration of an individual pulse. For example,
the measurement time period could be 1s, which contains many signal pulses.
As a simple example, in a non-return-to-zero (NRZ) data stream (see Chap. 16)
there will be an equal probability of 1 and 0 pulses over a long time period. In
this case, as shown in Fig. 19.2, the average power is one-half of the peak power.
If a return-to-zero (RZ) modulation format is used, the average power over a
long sequence of pulses will be one-fourth of the peak power since there is no
pulse in a 0 time slot and a 1 time slot is only half filled.

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