Fall 2011
Elec4620 Photonics and Optical Communications
Dr. Andrew W. Poon
Experiment 2.
Multimode fiber attenuation
Objectives
• In this second fiber-optic experience, you will measure the fiber attenuation per
unit length of a multimode optical fiber. The technique is known as the “cutback
method.”
• You will investigate how the light launching condition can affect this
measurement. You will learn about the technique of “mode scrambling” in order
to generate a desirable mode field distribution in the multimode fiber.
Background – “Cutback method”
We will learn in lectures the details of (i) fiber loss mechanisms, and (ii) fiber mode
patterns.
The amount of optical power remains in a fiber after the light propagating a fiber
length of L is given as follows,
Pf = Pi · 10γL/10
(1)
where Pi is the input optical power and γ is the fiber attenuation given in decibels per
kilometer (dB/km). The fiber length is typically given in kilometers.
The fiber attenuation is one of the most important fiber specifications and its
measurement is relatively straightforward. The commonly adopted method of fiber
attenuation measurement is called the “cutback method.” The method can be performed
in two steps:
1. Launch the laser light into a long length of fiber, and measure the fiber output
power.
2. Cut off a length of the fiber, and again measure the power transmitted through a
short length of fiber.
This way we can measure the fiber loss while preserving the light input coupling.
Therefore, the measurement obtained is not affected by the loss occurs as the light is
launched into the fiber.
The transmission through the fiber is given as
T = Pf/Pi
(2)
A logarithmic unit for loss in decibels (dB) is often used as follows
Loss (dB) = -10 log (Pf/Pi)
(3)
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The minus sign allows the loss to be expressed as a positive number. Note that the dB
unit allows different losses to be summed and subtracted from an initial power that is
also in logarithmic unit.
Optical powers are often expressed in the logarithmic unit of “dBm,” which means
“dB relative to 1 mW of optical power.” e.g. 0 dBm = 1 mW, 3 dBm = 2 mW, and –
10 dBm = 100 μW. When losses in dB are subtracted from powers in dBm, the result is
in dBm. e.g. an initial power of +3 dBm minus a loss of 3 dB results in a final power of
0 dBm. This means “an initial power of 2 mW with a 50 % loss results in a final power
of 1 mW.”
The fiber attenuation, γ, in dB/km, is given by
γ(dB/km) = (1/L) [-10 log (Pf/Pi)]
(4)
Practical issues with laser launching
The measured loss will depend on how the light is launched into the fiber. This is
particularly the case for multimode fibers. The laser launching can result in an
“overfilled launch” or an “underfilled launch.”
Overfilled launch – many high-order modes are excited. These modes are less confined
by the fiber core and have a relatively large portion of the mode field penetrate into the
cladding (in the form of evanescent field). Therefore, high-order modes are more “lossy”
compared with low-order modes.
Underfilled launch – mostly low-order modes are excited. These modes are more
confined by the fiber core and only have a relatively small portion of the mode field
penetrated into the cladding (the evanescent field has a shorter penetration depth). Thus,
low-order modes have less loss.
A good repeatable measurement of a multimode fiber should involve a stable mode
distribution after a short distance of fiber. In order to attain a stable mode distribution,
we use a mode scrambler to induce coupling (power exchange) between modes (by
micro-bending the fiber). This helps generate an approximation of a stable mode
distribution.
New equipment and tools
• Fiber coupler with a microscope objective lens
• Tilt platform
• Mode scrambler
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Experiment – Cutback measurement
Figure 1 shows the experimental setup.
1. Prepare both ends (stripping and cleaving) of a fiber spool and note length marked on
the label. Please update the length record after your experiment.
2. Place one cleaved fiber end into the fiber holder and insert the holder into the postmounted fiber positioner.
3. Post-mount the detector head of the optical power meter. Align the detector head
with the fiber end so that you will be able to collect and measure all the output power.
4. Mount the fiber coupler onto the tilt platform (Fig. 2). The mounted coupler on the
tilt platform is shown in Fig.3.
5. Use the fiber coupler to launch the HeNe gas laser beam (633 nm wavelength) into
the fiber. In order to do this, you want to first align the coupler and the laser so that
the laser beam shines along the axis of the fiber coupler and project a circular spot at
the far end. Insert the other fiber end into the fiber holder and insert the holder into
the fiber coupler. Carefully align the fiber to maximize the amount of light launched
into the fiber, using the power meter to monitor the launched power.
Tips: The focal length of the objective lens is only a few mm, and the coupled power
can be particularly sensitive within an mm range near the focus. So it will be smart to
first outline your plan to fine-tune the setup. Note from Figs. 2 & 3 that there are a
number of adjustable dimensions for the focusing and the fiber tilting.
Tips: For a reasonable measurement, the minimum output power from the fiber
should be about 200 μW (i.e. ~ 20 times higher than in Lab1 which has no focusing
lens).
6. Mount the mode scrambler (Fig. 4) near the launch end of the fiber. Rotate the
adjustment knob to fully open the slot with corrugations. Place the fiber between the
two corrugated surfaces. These corrugations will micro-bend the fiber and thus mix
the mode power distribution. Now rotate the knob until the corrugated surfaces just
touch the fiber. Examine the output pattern of the fiber. Rotate the knob slightly
further and notice the changes in the output pattern as the fiber bending is changed.
Caution: You can break the fiber if you “squeeze” the slot too tight. Besides,
excessive bending also causes excess fiber losses, which will reduce the available
power for your measurement.
7. To attain an approximation of the stable mode distribution, carefully rotate the
scrambler knob until the fiber output pattern gives you approximately the fiber NA.
(Recall from Lab1 that we could estimate the fiber NA using the output pattern at a
distance.) Sketch or capture the mode field pattern of this multimode fiber using your
camera and briefly discuss the pattern structures.
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Caution: Once you obtain the approximate fiber NA, this launching and mode
scrambling should NOT be changed again for the rest of the measurement. (Why?)
8. Measure the fiber output power. Be sure you record on your notebook the exact
length of the fiber. This information should already be labeled on the fiber spool.
9. Cut off the fiber ~ 1 m after the mode scrambler and make sure the new end can
reach the detector head without stress. Measure and record on the fiber spool the
exact fiber length you have removed. Cleave the broken end and measure the output
from the “cutback” segment.
10. Calculate the fiber attenuation and compare this with the attenuation in the fiber
specification. Your value is likely to be higher than the specification. Why? In your
report, discuss possible uncertainty in your measurement and possible improvement
to the experiment.
Fig. 1. Experimental setup for cutback measurement
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Fig.2. Tilt platform
Fig.3. Fiber coupler assembly
Fig.4 Mode scrambler
*Pre-Lab question regarding the cutback method
What are the three quantities to be measured in this experiment in order to extract the
fiber attenuation γ from equation (4)? Briefly discuss how you may proceed to measure
each of these three quantities. Comment on why you need to cutoff the fiber downstream
from the mode scrambler, and why the mode scrambler should NOT be adjusted once the
desired fiber output pattern is obtained. Have you got more idea about the principle of
“cutback” method?
[**Make sure you write down your answer on your notebook before the lab session, and
discuss your answer with the TA or the instructor before you proceed with the
experiment.]
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