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S.M. Lord- R.A. Sherry

Background:

The invention of low loss optical fiber revolutionized the way data is communicated and processed. Optical fiber has found applications not only in communications but also in sensor technology. Advances in different types of fiber have led to specialized applications of specialized fibers. This experiment will be to investigate the two major classification of optical fiber: single mode versus multi-mode fiber and the different characteristics that makes each more suitable to a particular application than the other.

Part A: Fiber Cleaving and Core/Cladding Observation

A 1 CLEAVING FIBER

• Obtain a sample piece of F-MLD multi-mode fiber provided.
• Use the No-Nik fiber stripping tool to remove the fibers polymer coating over a length of approximately two inches. Refer to the instructions included with the fiber stripper for correct use.
• Use the T&B Fiber Cleaver to cleave the end just prepaired. Refer to the instructions for using the T&B Fiber Cleaver before attempting to cleave the fiber.
• Gently wipe the fiber ends with KimWipes to remove any excess coating flakes. not allow anything to contaminate or scratch the freshly cleaved fiber ends.

A2 Checking the Cleave

• Insert the cleaved end of the multi-mode fiber into the small rubber tubing of the fiber adapter.
• Slide the tube and fiber into the adapter while pressing the button on the side of the adapter. Adjust the fiber until it just barely extends beyond the nylon tip of the adapter.
• Insert the nylon tip of the adapter in to the hole on the bottom of the Panasonic hand held microscope. Have your lab partner shine a flashlight in to the other end of the fiber.
• Adjust the focus of the microscope until you see an image of the core and cladding that looks similar to one of the examples shown in Figure 5.1 below
• .

Figure 5.1 Examples of Good and Bad Fiber Cleaves for Multi-mode Fiber

• If the cleave looks smooth like the example on the left of Figure 5.1, then the cleave is a clean one and will work just fine. A bad cleave will look similar to that on the right of Figure 5.1. If you notice obvious cracks or other types of defects in the core of the fiber, you should perform the steps of part A1 to recleave the fiber end.
• NOTE: Sometimes the apparent defects are simply coating debris that remained from stripping. Remove the fiber from the adapter and wipe the fiber end gently with a Kim
• Wipe and repeat steps 1-4 of part A2.
• Obtain a pre-cleaved sample of single mode fiber and repeat the steps for viewing the cleaved fiber in the hand held microscope. You should notice drastic differences in the core/cladding diameters. Why is this?

Figure 5.2 Core/Cladding Diameters of Multi-mode and Single Mode Fibers

PART B: Single Mode Interferometric Sensors

Interferometry is the study of wave interference. It is a useful tool in measurement by relating shifts in interference patterns to quantities of change in a physical phenomena. Optical fibers are a perfect medium with which to make interferometric measurements. The basic principle is based upon the speed at which light travels through two equal length 'arms' of fiber, one of which is a reference and the other is the actual sensor exposed to the physical phenomena to be observed. Physical conditions can easily change the length of the sensor arm as well as the speed at which the light propagates through it, thus creating interference patterns as the light from each arm is superimposed on the same focal plane. In this part of the experiment you will implement a fiberoptic sensor using the setup shown in Figure 5.3.

Figure 5.3 Interferometric Sensor Setup

B 1 ALIGNMENT

• Turn the HeNe on and observe the path of the light through the setup. Be sure you understand where the HeNe light starts and where it ends up. Explain to a lab TA what is happening to the light in the various sections of the setup as shown in Figure 5.3.
• There are two beams that leave the beamsplitter cube; one goes in direction A and the other in direction B. For now you can ignore direction A and observe the image of beam B on the white posterboard on the wall.
• You should notice two distinct red spots on the wall. Make adjustments to the focus of both the F-916 Fiber Couplers until each spot is approximately 6 inches in diameter.

Figure 5.4 F-925 Grin Rod Coupler and F-916 Fiber Coupler

• To obtain the fringes first align the spots so they are nearly on top of one another. Then make fine adjustments to one of the couplers, slowly moving the spots apart. Continue making X and Y adjustments on the F-916 Coupler until you can see an interference pattern of concentric circles or 'fringes'. It should look similar to a bullseye pattern.
• NOTE: You can adjust the size of the fringes by focusing or expanding the two beams. As you change the focus, you will also have to make X and/or Y adjustments to keep the center of the bullseye pattern in view.

B2 SENSING, PHYSICAL PERTURBATION

• Without touching it, bring your hand within a few inches of Fiber 1 which is shown in Figure 5.3, and observe what happens to the fringes. Comment on the sensitivity of the fiber.
• Now lay the fiber across a rubber eraser the and gently apply and release pressure on the fiber with another eraser. Observe what happens to the fringes.
• Name some other types of stimuli that you think the glass fiber could 'sense'? Remember the actual sensing occurs because of the physical change in the length of the fiber or the speed of the light propagating through it.

EE34 I LAB#5

S.M. Lord- R.A. Sherry

April 1, 1997

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last updated 7/9/97