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# LAB 4 - OPTICAL FIBERS

By Donald Cook,2014-05-07 12:23
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LAB 4 - OPTICAL FIBERS

LAB 10 - OPTICAL FIBER

FALL 2009

Objective To measure the basic quantities which characterize optical fiber.

PRELAB

1) Read the data sheet provided on the website (f625.pdf) for the Corning 62.5/125?m fiber and

i) What is the numerical aperture and what is index difference ?n calculated using it?

ii) What is the measured attenuation at 850 nm and 1.5 ?m?

iii) Some micrometer stages in lab have 50 turns/inch. How far does the micrometer move in 1

turn (express in microns.)?

PART I NUMERICAL APERTURE FROM A FAR FIELD MEASUREMENT

Discussion By examining the angular spread of the light output of a fiber, the numerical aperture (NA) can be

measured. In the paraxial regime, (sin(?) ? ?), the divergence of the output of a fiber is linear in z where z

is the distance where the output pattern is measured. By measuring the transverse location (x, x etc. on the 12

figure) of the 5% value of the on-axis intensity as a function of z, the divergence angle can be measured and

thus the NA can be determined. The 5% criteria is an established standard for graded index fiber.

Caution! HOT!!!Caution! HOT!!!Camera Camera fiber fiber w/o Lens w/o Lens cablecableon Translation on Translation HP ScopeStageHP ScopeStage

Ocean Optics FiberOcean Optics Fiber Coupled Light sourceCoupled Light source

Cable holder Cable holderMonitorMonitor

+12 Volts+12 Volts

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Experiment

1) Set up the above system with the CCD camera on a micrometer translation stage without a lens. Use 50-

micron core patchcord as the fiber cable for this part. Start with the distance between the fiber facet and

CCD plane at about 5 mm.

2) Set up the scope with TV trigger to display the middle of the output from the fiber. The CCD has

automatic blanking to protect against CCD saturation. However, if you do happen to saturate the CCD

(you will see a flat top on the signal), cut down on the light level launched into the fiber.

4) Calibrate the CCD sensor by the following steps (this is the same procedure you have used in at least two

previous labs).

a) Display 1 horizontal frame of the fiber output on the scope.

b) Measure the total time for the whole trace, not including the blanking signal (this should be

approximately, but not exactly 50 ?s). Your TA will explain this to you.

c) Knowing the CCD sensor is 6.4 mm across, determine the scaling factor of the scope (?s/mm) This

should be on the order of ~8 ?s/mm.

5) Turn off the room lights and measure the width where the value of the far field drops to 5% of the

maximum (which should be in the middle of the fiber) using the ?v ??t feature of the HP scope (see figures

below). If your signal is noisy, pick points in the center (mean) of the noise distribution.

6) Back off the translation stage a 1/2 turn (The actuator moves 500 microns per complete turn. The scale on

the vernier is in units of 10 microns. So, you will be moving the CCD plane by 250 microns.). Record the

distance traveled and the 5% value of the far field. Continue moving the CCD plane in steps of 250

microns, recording the 5% value and distance traveled, until the image of the fiber output is too large to

determine the base line. You should get about 5 to 10 data points.

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PART II CORE DIAMETER FROM NEAR FIELD MEASUREMENT

Discussion By imaging the near field (i.e. the end of the fiber) when it is illuminated, the core diameter can be

determined. In order the accurately measure this, the imaging system needs to be calibrated.

Experiment

1) Do not hesitate to enlist the help of the TA if you run into trouble during this part. Use the blue fiber patch

cable that is provided and an Ocean Optics fiber coupled white light source for the imaging experiment.

CAUTION: THIS LIGHT SOURCE CAN GET VERY HOT!!! You should image to a CCD array and

view the result on a TV monitor.

2) Using a single fast lens, set up an imaging system that produces a spot size that is on the order of a

millimeter (Hint: M ~ 5). Calibrate the setup by placing a known ruled pattern (10 line pairs/mm) at the

fiber facet. Be sure that the side with the black lines is butted carefully up against the fiber end. If the

image is properly projected, you should see an image of the fiber facet overlapped by the grating. You may

have to use a pair of polarizers so that the CCD is not saturated. If the lines are not in focus when the core

is in focus, you may have the ruled grating in backwards.

3) Determine the diameter of the fiber core from your setup using the image on the TV monitor.

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PART III MODES OF AN OPTICAL FIBER

Discussion Single mode optical fibers propagate only one mode, usually the LP mode. LP in this case stands for 01

linearly polarized. It is also possible to have a highly multimode fiber, which can propagate hundreds of

LP modes simultaneously. We will investigate a slightly multimode fiber, and try to excite single mode nmoperation through the use of strain-induced mode filtering. The modes we excite can be any of LP, LP, 0111LP, LP, … 0212

Experiment

1) You will be provided a fiber-coupled HeNe setup through a partially multimode fiber. Analyze and try to

understand the coupling setup and try to optimize the coupling into the fiber. If you have trouble, see the

TA.

2) Investigate the output field pattern on a white piece of paper. You may need to turn the room lights off.

What mode are you exciting? Is it one mode or multiple modes?

3) You may also try to bend the fiber with your fingers to excite a different (or single) mode. Which mode

did you excite? You may have to keep trying.

PART IV FIBER OPTIC IMAGING SYSTEMS

Discussion This week we are focusing on optical fibers as light guides. If we have an array of them, we can use it to

relay an image with each fiber corresponding to a ‘pixel.’ The fiber bundle, or image relay, that you will be

using this week is rigid. However, the fiber bundle can also be housed in loose tubing. When

manufacturing these light guides, it is very important that the relative position of each fiber in the array be

precisely tracked. Otherwise, the image will appear distorted.

Light SourceLight Source BeamsplitterBeamsplitter 1-inch Lens 1-inch Lens ObjectObject (Virtual Image Mode)(Virtual Image Mode)

ViewView Image RelayImage Relay

Beam BlockBeam Block

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Experiment

1) Set up the above fiber optic imaging system. This is only a recommended configuration, so you can try

something different if you prefer.

2) Using a lens to magnify the image formed at the end of the fiber, try to use the calibrated ruled grating (10

lp/mm) to determine the resolution of the image relay. How is the resolution related to the fiber core

diameter? You may have to shine the light in from the back when using the ruled grating.

3) Use the imaging system to observe images of other objects. Does light intensity play a role in image

quality?

4) Repeat using coherent illumination from a laser pointer. Be careful not to stare into the laser beam.

QUESTIONS FOR WRITE-UP

1) Plot ?z (distance fiber travels) vs. ?x (5% value of the far field) and determine the angle of divergence ?

from the plot and thus the numerical aperture NA of the fiber. What are the principle sources of error for

this part?

2) Are the conditions of a far field approximation satisfied for the Numerical Aperture measurement? Explain.

Why is the output field distribution of a fiber (assuming a temporally and spatially coherent source of light)

not a Bessel function (diffraction from a circular aperture)?

3) Determine the core diameter of the blue fiber patchcord. Comment on any sources of error.

4) How are the near field image recorded for Part II and the far field measurement recorded for Part I related?

Can you use one measurement to determine the other? Explain.

5) How do coherent and incoherent fiber optic imaging systems compare?

6) What is the resolution of the fiber optic imaging system? What sets this limitation?

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