Fiberoptics Measurements
Power Measurements
dBm to mW Conversion
Watts are the basic units of optical power measurement. In
fiberoptics, decibel units are the logarithmic transformations of watts
and submultiples of watts. Decibel units are used in fiberoptics
because they provide a convenient means of condensing power measurement information that has a wide dynamic range. (See Table 1.)
Since fiberoptic power levels cover many orders of magnitude, the
logarithmically compressed decibel scale is commonly used.
Decibel power is defined as:
10mW = +10dBm
5mW = +7dBm
1mW = 0dBm
0.5mW = -3dBm
0.1mW = -10dBm
0.05mW = -13dBm
10µW = -20dBm
5µW = -23dBm
dB = 10 log
1µW = -30dBm
( PP
SIGNAL
REFERENCE
)
100nW = -50dBm
When the reference power is 1 milliwatt, then dBm is defined as:
10nW = -50dBm
1nW = -60dBm
dBm = 10 log
100pW = -70dBm
10pW = -80dBm
( P1mW )
SIGNAL
When the reference power is 1 microwatt, then dBµ is defined as:
1pW = -90dBm
dBµ = 10 log
Table 1.
( P1µW )
SIGNAL
Since the decibel is a ratio, it must either have a mutually agreed
reference power (such as 1 milliwatt or 1 microwatt), or be understood to represent the power difference between two signals. For
example, to express the loss of an optical component where the input
power is PIN and the out power is POUT:
Light Source
Under Test
Sensor
Figure 1. Power Measurements
Optical
Power Meter
Loss (dB) = dBm (PIN) - dBm (POUT)
or
Loss (dB) = dBµ (PIN) - dBµ (POUT)
Power measurements are made by converting light from a laser
diode or LED, for example, into an electrical signal through an opticalelectrical converter or detector. (See Figure 1.)
Fiberoptics Measurements
Fiberoptics communications wavelengths range from 650 nm to
1550 nm.
You should select your detector according to the wavelength you
wish to measure. Fiber measurements in the wavelength range of 360
to 1100 nm require silicon detector heads. Measurements up to
1800 nm require germanium or indium gallium arsenide sensor heads.
There are two methods for making measurements of a fiberoptic
laser diode or LED. One way, suitable for low power light sources, is
to connect the fiber and its attached laser diode to the power meter.
The alternate method, which is particularly useful for high power light
sources, is to use a miniature integrating sphere.The sphere attenuates
the light intensity by several orders of magnitude, and thus permits
direct measurement of power output.
Open after
P1 measurement
is made
P1
Stabilized
Light Source
Optical loss of fiber
being measured
(Connection loss not included)
P2
Power = P2 - P1 (dB)
Figure 2. Cutback method
Cable under test
Light
Source
Fiber Attenuation Measurements
Figure 2 and 3 illustrate two methods for measuring cable loss.
The cutback technique uses just one fiber for measurement but
requires that you cut off an end piece of the fiber during the measurement process.The dB/km loss factor is the difference in dB power
measurements divided by the length of the cut-off piece of fiber.
In the cable substitution method, the dB/kilometer loss factor can
be established by comparing the transmission of a short test cable
with the transmission through that of a known longer length cable.
Optical
Power
Meter
Optical
Adapter
Power
Meter
2 Meter reference cable
Light
Source
Power
Meter
(Cable 1 and 2 are identical types but differ in length)
Figure 3. Cable substitution method
Splice/Interconnect Loss Measurements
To measure the loss derived by the insertion of a fiberoptic connector, first measure the cable output. (See Figure 4A.) Then insert a
connector or splice and take a second measurement. (Figure 4B) The
difference between the two measurements is the answer. UDT
Instruments power meters make this calculation automatically for you.
Stabilized
Light Source
Power
Meter
P1
Figure 4A. Optical connection loss
Stabilized
Light Source
Connector
Or Splice
Figure 4B. Optical loss in dB = P1 - P2
Power
Meter
P2