Experimental Optics Contact: Helena Kämmer (helena.kaemmer@uni-jena.de) Last edition: Helena Kämmer, January 2017 Optical Time Domain Reflectometry (OTDR)
Contents 1 Overview 3 2 Safety issues 3 3 Theoretical background 4 3.1 Optical fibers................................. 4 3.2 OTDR..................................... 4 4 Description of the equipment in use 6 4.1 General setup description........................... 6 4.2 Preparation of the glass fiber......................... 7 4.3 Coupling light into the fiber.......................... 8 5 Goals of the experimental work 10 A Preliminary questions 11 B Final questions 11
1 Overview The lab is aimed for learning something about a versatile tool for measuring the characteristics of optical fibers. Optical Time Domain Reflectometry cannot only measure the length of a fiber, but also many properties of optical networks. OTDR can provide information about attenuation of the fiber, the splice quality inside the network and connection losses between two fibers. Furthermore, the Optical Time Domain Reflectometry can detect and localize 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 emitting light in the NIR range at 905 nm up to 40 mw average power with a pulse duration of 100 ns at a repetition rate of 2 khz. Thus, the maximum peak power is 200 W. It is recommended to use the appropriate laser safety goggles in addition with protective sides against scattered laser light caused by additional optics during the measurements. Operation of the laser is only permitted under supervision. Additional care has to be taken by using an optical fiber. The striped fiber consists of glass and has sharp edges when it is broken or cleaved. Thus, the fiber ends can easily pass through the skin or other parts of the body. Remaining parts of the fiber after cleaving have to be put in a special container by using tweezers. 3
3 Theoretical background 3.1 Optical fibers An optical fiber is a cylindrical waveguide consisting of a central core with a refractive index n core surrounded by cladding with a slightly lower refractive index n clad (see Fig. (2)). The light is guided in the core due to total internal reflection between the boundary of the core and the cladding [2, chapter 8]. Light rays with a greater inclination to the fiber axis than the critical 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(θ a ) = n 2 core n 2 clad (1) with the acceptance angle θ a of the optical fiber. The NA is characterized by the refractive index difference between the core and the cladding. θ a n clad n core 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 index fiber. Furthermore, the fibers can be differed by single mode and multimode fibers. Optical fibers are made of low loss materials like fused silica. The losses are in the range of 0.1 to 0.3 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 4
Optical Time Domain Reflectometry (OTDR) is a distributed characterization technique for optical fibers [3,4]. OTDR is extensively used in telecommunications to check the integrity of fiber links and to locate possible defects. OTDR exploits the Rayleigh scattering naturally 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 along the fiber. A small amount of this scattered light propagates back and can be detected at the same point at which the light was launched, as illustrated in Fig (3). This fact provides OTDR with one of its most unique and appreciated features: only access to a single fiber end is required to carry out the measurement. This brings obvious advantages when monitoring tens-of-kilometers-long communication links. If the light launched into the fiber is pulsed, then the scattered light coming from different sections of the fiber can be identified by its delay when arriving at the fiber end. This yields the possibility of carrying out spatially resolved distributed fiber measurements. OTDR is useful to measure reflective events (such as fiber interconnections or fiber ends) as well as attenuation or fiber defects. 5
Optical Time Domain Reflectometry (OTDR) 4 Description of the equipment in use 4.1 General setup description The complete setup for measuring the optical time domain reflectometry is illustrated in Fig. 4. The setup is mounted on two optical rails (1) with scales for positioning of the components. (2) is the laser diode with the corresponding power supply (3). The laser beam is collimated by microscope objective (4) and passes a polarizing beam splitter (5). One part of the laser beam goes through a imaging lens (6) to a reference detector (7) consisting of 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 laser transmitted through the PBS by 90 when passing the plate twice. Back-reflected light is therefore steered by the PBS towards the measurement detector. Afterwards the laser beam is coupled by a microscope objective (9) into the fiber, which is mounted in an XY holder allowing in combination with the small translation stage of the fiber holder (10) a fine adjustment of the laser beam with respect to the glass fiber. The second end of the fiber is fixed in another fiber clamp (11), so the output light can easily be measured. All reflected from 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. The PIN-photodiode is mounted in a housing with a BNC-socket for the connection via the provided 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 6
4.2 Preparation of the glass fiber This chapter describes how to prepare the glass fiber ends for the experiments. Only adequately 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 called Miller 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 the fiber. The Miller Pliers are pre-adjusted for the provided fiber diameter and there is normally no 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 A that 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 the fiber 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 horizontal at this time. After clamping the fiber, place your finger at the end of the leaf spring and hold the fiber firmly against the leaf spring. While keeping the fiber tight, press section A to release torsion that may be on the fiber. Release section A for fixing the fiber. With the fiber in position, press section B and lightly score the fiber. Do not press the blade housing into the fiber. The tip of the blade should only touch the fiber. Afterwards, release section B. 7
Figure 6: Cleaving procedure The optical fiber will be cleaved at the point where the blade scored the optical fiber. To complete the cleave, bend the leaf spring while keeping tension on the fiber, but do not twist the leaf spring or overbend it. After cleaving the remaining parts of the cleaved fiber must be disposed in an appropriate container to insure no contact to the human body, because the fiber pieces can enter through skin or other parts of the body. 4.3 Coupling light into the fiber Figure 7 illustrates the procedure for coupling light into the fiber core. The focus of the laser light is located approximately 1 mm behind the microscope objective, the place where one fiber end has to be positioned. The location of the focus behind the objective can also be inspected with the IR-converter card. The IR card should show a homogeneous round spot centered at the optical axis. If the position of the spot escapes off-axis the microscope objective has to be adjusted for its X and Y position. Before the next step is performed the laser should be switched off! The translation stage with the fiber holder (10) is placed behind the focusing unit. The distance between the fiber holder and the microscope objective should be approximately 5 mm while the slide of the stage 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 8
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 reel into the v-groove of the holder. The bare fiber tip should show about 3 to 4 mm over the edge of the fiber holder. Then the clamp of the holder is flipped back smoothly to avoid a break of the fiber. The other end of the fiber reel has to be prepared as described above and fixed at the fiber holder on the carrier (11). The reference detector in its holder (7) equipped with the mounted pin hole is placed directly after the fiber holder. To avoid any scattered entering the detector the fiber tip can even be inserted in the pin hole. This way the signal coming out of the fiber can be detected and controlled. Connect the photo detector with the provided BNC cable directly to a 100 MHz oscilloscope. After switching on the laser, the system is ready for coupling the laser light into the glass fiber. 9
5 Goals of the experimental work 1. Build up the setup on the rails. 2. Characterize the Multi-Mode fiber Align the input coupling of the Multi-Mode fiber. Optical Time Domain Reflectometry (OTDR) Measure the length of the fiber in Transmission and Reflection. Bend the fiber end with a small curvature radius. How does the reflected signal change? 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 characteristics. Repeat this for all fiber spools. What are the differences in the oscilloscope trace between the fibers? 5. Characterize the fiber spool wich contains some " event". What kind of " event" is located in the fiber and where? 10
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 OTDR trace. 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 blind region be reduced? Even though most of the optical communication systems nowadays operate around 1550 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 OTDR measurement? How is the longest lengths you can measure with the repetition rate of the current setup? What happened when the fiber is strongly bended? 11
References 1. 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 time domain 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 12