Experimental Optics

Contact:Helena Kämmer (helena.kaemmer@uni-jena.de)

Last edition: Helena Kämmer, January 2017

Optical Time Domain Reflectometry(OTDR)

Contents1 Overview 3

2 Safety issues 3

3 Theoretical background 43.1 Optical fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.2 OTDR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

4 Description of the equipment in use 64.1 General setup description . . . . . . . . . . . . . . . . . . . . . . . . . . . 64.2 Preparation of the glass fiber . . . . . . . . . . . . . . . . . . . . . . . . . 74.3 Coupling light into the fiber . . . . . . . . . . . . . . . . . . . . . . . . . . 8

5 Goals of the experimental work 10

A Preliminary questions 11

B Final questions 11

Optical Time Domain Reflectometry (OTDR)

1 OverviewThe lab is aimed for learning something about a versatile tool for measuring the character-istics of optical fibers. Optical Time Domain Reflectometry cannot only measure the lengthof a fiber, but also many properties of optical networks. OTDR can provide informationabout attenuation of the fiber, the splice quality inside the network and connection lossesbetween two fibers. Furthermore, the Optical Time Domain Reflectometry can detect andlocalize a break in optical networks.

2 Safety issues

Figure 1: Laser safety

The laser system used is classified according to DIN IEC 60825-1 as Class 3B emittinglight in the NIR range at 905 nm up to 40 mW average power with a pulse duration of 100 nsat a repetition rate of 2 kHz. Thus, the maximum peak power is 200 W. It is recommendedto use the appropriate laser safety goggles in addition with protective sides against scatteredlaser light caused by additional optics during the measurements. Operation of the laser isonly permitted under supervision.

Additional care has to be taken by using an optical fiber. The striped fiber consists ofglass and has sharp edges when it is broken or cleaved. Thus, the fiber ends can easily passthrough the skin or other parts of the body. Remaining parts of the fiber after cleaving haveto be put in a special container by using tweezers.

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Optical Time Domain Reflectometry (OTDR)

3 Theoretical background

3.1 Optical fibersAn optical fiber is a cylindrical waveguide consisting of a central core with a refractive indexncore surrounded by cladding with a slightly lower refractive index nclad (see Fig. (2)). Thelight is guided in the core due to total internal reflection between the boundary of the coreand the cladding [2, chapter 8]. Light rays with a greater inclination to the fiber axis than thecritical angle are guided in the core, for smaller angles the rays are refracted in the cladding,lose power, and are not guided. This angle defines the Numerical Aperture

NA = sin(θa) =

√n2

core − n2clad (1)

with the acceptance angle θa of the optical fiber. The NA is characterized by the refractiveindex difference between the core and the cladding.

nclad

ncoreθa

Figure 2: Light guiding in an optical fiber

Optical fibers can be divided into step index, graded index and microstructured fibers.The common fiber types in telecommunication are the step index and the graded indexfiber. Furthermore, the fibers can be differed by single mode and multimode fibers. Opticalfibers are made of low loss materials like fused silica. The losses are in the range of 0.1 to0.3 dB/km. Therefore, fibers are very well suited for long distance communication networks.

3.2 OTDR

Figure 3: Principle setup for the Optical Time-Domain Reflectometry

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Optical Time Domain Reflectometry (OTDR)

Optical Time Domain Reflectometry (OTDR) is a distributed characterization techniquefor optical fibers [3,4]. OTDR is extensively used in telecommunications to check the in-tegrity of fiber links and to locate possible defects. OTDR exploits the Rayleigh scatteringnaturally taking place in the core of optical fibers. As the light propagates down a fiber,small material imperfections scatter the incoming light in all directions at every point alongthe fiber. A small amount of this scattered light propagates back and can be detected atthe same point at which the light was launched, as illustrated in Fig (3). This fact providesOTDR with one of its most unique and appreciated features: only access to a single fiber endis required to carry out the measurement. This brings obvious advantages when monitoringtens-of-kilometers-long communication links.

If the light launched into the fiber is pulsed, then the scattered light coming from differentsections of the fiber can be identified by its delay when arriving at the fiber end. This yieldsthe possibility of carrying out spatially resolved distributed fiber measurements. OTDR isuseful to measure reflective events (such as fiber interconnections or fiber ends) as well asattenuation or fiber defects.

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Optical Time Domain Reflectometry (OTDR)

4 Description of the equipment in use

4.1 General setup descriptionThe complete setup for measuring the optical time domain reflectometry is illustrated inFig. 4. The setup is mounted on two optical rails (1) with scales for positioning of thecomponents. (2) is the laser diode with the corresponding power supply (3). The laser beamis collimated by microscope objective (4) and passes a polarizing beam splitter (5). Onepart of the laser beam goes through a imaging lens (6) to a reference detector (7) consistingof a PIN-photodiode. The other part passes the beam splitter and a quarter waveplate (8).The combination of both is working as an optical diode. It turns the polarization of the lasertransmitted through the PBS by 90° when passing the plate twice. Back-reflected light istherefore steered by the PBS towards the measurement detector. Afterwards the laser beamis coupled by a microscope objective (9) into the fiber, which is mounted in an XY holderallowing — in combination with the small translation stage of the fiber holder (10) — a fineadjustment of the laser beam with respect to the glass fiber. The second end of the fiber isfixed in another fiber clamp (11), so the output light can easily be measured. All reflectedfrom the PBS is collected by an imaging optic (12) and focused onto the signal photodetector(13), which helps to magnify the signal. The focal length of the convex lens is 60 mm. ThePIN-photodiode is mounted in a housing with a BNC-socket for the connection via theprovided BNC T-connector to 50 Ω terminating resistor and BNC cable to oscilloscope.Additional to the optical fiber (14), a fiber striper (15) to remove the cladding of the fiber,and fiber cleaver (16) for breaking the fiber to get a plane fiber end are enclosed.

Figure 4: OTDR lab setup

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Optical Time Domain Reflectometry (OTDR)

4.2 Preparation of the glass fiberThis chapter describes how to prepare the glass fiber ends for the experiments. Only ade-quately prepared fiber ends are useful for performing the measurements.

Removing the plastic coating

First of all the plastic coating of the fiber has to be removed. For this purpose the so calledMiller Pliers (15) are used. To remove the plastic coating insert the fiber end into the V-blades of the Miller Pliers and smoothly peel the coating off from the last 40-50 mm of thefiber. The Miller Pliers are pre-adjusted for the provided fiber diameter and there is normallyno need to change the adjustment.

Cleaving the fiber

Figure 5: Description of the Cleaver

In Fig. 5 the cleaver is illustrated which is used for breaking the fiber. Push section Athat holds the optical fiber to lift the fiber holding section B as shown in Fig. 6 at the right.Set the optical fiber where the coating has been removed along the groove for placing thefiber section, and position the end of the coating onto the scale marked on the leaf spring,corresponding the length of about 15 to 20 mm.

CAUTION: Do not touch the blade to the leaf spring without a fiber in place.

Release section A to fix the fiber on the leaf spring. The leaf spring shall remain horizontalat this time. After clamping the fiber, place your finger at the end of the leaf spring and holdthe fiber firmly against the leaf spring. While keeping the fiber tight, press section A torelease torsion that may be on the fiber. Release section A for fixing the fiber. With the fiberin position, press section B and lightly score the fiber. Do not press the blade housing intothe fiber. The tip of the blade should only touch the fiber. Afterwards, release section B.

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Optical Time Domain Reflectometry (OTDR)

Figure 6: Cleaving procedure

The optical fiber will be cleaved at the point where the blade scored the optical fiber. Tocomplete the cleave, bend the leaf spring while keeping tension on the fiber, but do not twistthe leaf spring or overbend it.

After cleaving the remaining parts of the cleaved fiber must be disposed in an appropriatecontainer to insure no contact to the human body, because the fiber pieces can enter throughskin or other parts of the body.

4.3 Coupling light into the fiberFigure 7 illustrates the procedure for coupling light into the fiber core. The focus of thelaser light is located approximately 1 mm behind the microscope objective, the place whereone fiber end has to be positioned. The location of the focus behind the objective can alsobe inspected with the IR-converter card. The IR card should show a homogeneous roundspot centered at the optical axis. If the position of the spot escapes off-axis the microscopeobjective has to be adjusted for its X and Y position.

Before the next step is performed the laser should be switched off! The translation stagewith the fiber holder (10) is placed behind the focusing unit. The distance between the fiberholder and the microscope objective should be approximately 5 mm while the slide of thestage is moved to its center position by its fine screw.

Now the fiber is inserted into the setup. This is done by flipping the magnetic fiber holder

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Optical Time Domain Reflectometry (OTDR)

Figure 7: Setup for Coupling light into the fiber

to its open position and positioning the prepared fiber tip of the input fiber end of the reelinto the v-groove of the holder. The bare fiber tip should show about 3 to 4 mm over theedge of the fiber holder. Then the clamp of the holder is flipped back smoothly to avoid abreak of the fiber.

The other end of the fiber reel has to be prepared as described above and fixed at thefiber holder on the carrier (11). The reference detector in its holder (7) equipped with themounted pin hole is placed directly after the fiber holder. To avoid any scattered enteringthe detector the fiber tip can even be inserted in the pin hole. This way the signal comingout of the fiber can be detected and controlled.

Connect the photo detector with the provided BNC cable directly to a 100 MHz oscillo-scope. After switching on the laser, the system is ready for coupling the laser light into theglass fiber.

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Optical Time Domain Reflectometry (OTDR)

5 Goals of the experimental work1. Build up the setup on the rails.

2. Characterize the Multi-Mode fiber

• Align the input coupling of the Multi-Mode fiber.

• Measure the length of the fiber in Transmission and Reflection.

• Bend the fiber end with a small curvature radius. How does the reflected signalchange?

3. Change the objectiv and compare the results for the Multi-Mode fiber and one Single-Mode fiber. Wich one is the best and why?

4. Change the fiber spool, re-align the fiber coupling, and also measure OTDR charac-teristics. Repeat this for all fiber spools. What are the differences in the oscilloscopetrace between the fibers?

5. Characterize the fiber spool wich contains some "’event"’. What kind of "’event"’ islocated in the fiber and where?

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Optical Time Domain Reflectometry (OTDR)

A Preliminary questions• What is the NA of a fiber?

• How is the light guided inside the fiber?

• What is a single mode fiber?

• What is a multimode fiber?

• What is the difference between a step index and a graded index fiber?

• What is a polarizer?

• Why is a quarter waveplate used in the setup?

• What is the wavelength of the laser?

• How long is the laser pulse?

• What is the repetition rate of the laser?

• Explain how the attenuation at fiber interconnections can be measured in an OTDRtrace.

• Explain how the fiber attenuation is represented in an OTDR trace.

• Explain how the fiber length can be calculated from an OTDR trace.

• How many percent are reflected from the fiber end face?

B Final questions• What are the differences in the OTDR trace between a splice and a connector?

• Behind a reflective event the OTDR has a blind region, why? How can this blindregion be reduced?

• Even though most of the optical communication systems nowadays operate around1550 nm, why is a wavelength around 1310 nm preferred for OTDR measurements?

• Calculate the spatial resolution of our OTDR measurement. How can it be improved?

• Which are the limitations set on the repetition rate of the pulse train in an OTDRmeasurement?

• How is the longest lengths you can measure with the repetition rate of the currentsetup?

• What happened when the fiber is strongly bended?

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Optical Time Domain Reflectometry (OTDR)

References1. C. Yeh, Handbook of Fiber optics, Academic Press, San Diego 1990

2. B.E.A. Saleh, M.C. Teich, Fundamentals of photonics, Wiley-Interscience 2007

3. M. K. Barnoski, M. D. Rourke, S. M. Jensen, and R. T. Melville: "‘Optical timedomain reflectometer"’; Applied Optics, Vol. 16, p. 2375, 1997;

http://www.opticsinfobase.org/ao/abstract.cfm?URI=ao-16-9-2375

4. D. L. Philen, I. A. White, J. F. Kuhl, and S. C. Mettler: "‘Single-Mode Fiber OTDR:Experiment and Theory"’; IEEE Transactions on Mircrowave Theory and Techniques;Vol. MIT-30, p. 1487, 1982;

http://ieeexplore.ieee.org/xpls/abs_all.jsp?isnumber=25109&arnumber=1131282&count=58&index=12

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