Fiber Optics V 1/27/2012 Laboratory exercise The purpose of the present laboratory exercise is to get practical experience in handling optical fiber. In particular we learn how to cleave the fiber and couple light therein. We perform measurements of basic optical fiber properties as numerical aperture and attenuation spectra. Finally also study performance of the fiber Bragg grating which is one of the most widely used fiber optics based components. Contents of this manual Preparation exercises Report guidelines Aligning and coupling light into fibers White light spectroscopy of fibers Preparation for the lab Read in Fundamentals of Photonics 2nd edition about Input Couplers, p 314 Fiber Optics, p 326 331 Attenuation, p 348 351 Read these instructions and complete the preparation assignments given below. Feedback Comments on these instructions are greatly welcomed at dmitry.khoptyar@fysik.lth.se Report A compulsory written report is due within one week after the date for laboratory. The report is expected to be carefully written and present the results you find during the laboratory. It should include all the relevant details of the experimental procedure necessary to reproduce the experiment. The report may not contain more than 10 pages and 3000 word including appendices, but excluding the front page. A report can be written in many ways. Suggested topics and contents are: Purpose Describe the experimental purpose of the laboratory. What is the key problem? Try also to put it in a bigger context. Theory Give a short summery of the theory if needed. Experimental Setup Describe the equipment used and how it works. Use images to explain. Method State clearly how you performed the measurements. Results Present the results, preferably with the use of tables and figures. For all results reported, include an estimate of its accuracy. Discuss the physics behind your findings. Discussion Discuss your work, does it agree with theory or not? Did you have any problems and how did they affect your results. 1
Conclusions Draw conclusions and summarize your work. References List the references used in the report with precise links to the text. Appendices Present for example printouts or source code if necessary. Do not forget to: Write academically. Avoid sentences including we or I and keep an objective tone. Make a separate front page including: Title, date, name of the authors with email addresses, name of supervisor and word count. Use standard margins and a font size equivalent to times new roman 12pt. Use page numbers. Tables should be numbered and include a caption. Units are to be given in the table header. Figures should also be numbered and include captions. All figures axis need labels and units. All equation should be numbered for easy identification. Appendices should be numbered. When you send you report electronically please include your names in the name of the file. 2
Preparations exercises 1. Numerical aperture. For the fiber depict in Figure 1, the numerical aperture (NA) equals 0.27 and the refractive index of the core is 1.5. a) What is the refractive index of the cladding? b) If the fiber is immersed in water, what will its NA be? Figure 1. The maximum acceptance angle for a fiber is given by the angle. 2. Multi mode fiber. Given that the fiber in exercise 1 has a step index profile and that the core diameter is 62.5 m, calculate the approximate number of modes that can be transmitted for the wavelength 633 nm. 3. Single mode fiber. What is the shortest wavelength for which a 4 m core diameter fiber with a NA of 0.11 still is a single mode fiber? 4. Focusing optics. A He Ne laser has a wavelength of 633nm and a beam diameter of 10 mm. a) Using a one inch single lens with a focal length of 40mm, what is its NA? b) Calculate the focal spot size of the laser beam focused by this lens c) Calculate the NA and focal spot size of the same laser bean focused by microscope objective with the focal length of 16 mm. d) Which of the lenses is more suitable for coupling of this laser beam into a single mode fiber with a core diameter of 4 m and a NA=0.11? Figure 2. Lens with a sin Hint: The minimal focal spot size of a Gaussian beam is given by a relation 1.22 / is due to diffraction. Here d is the spot size diameter and λ is the light wavelength (cf. Figure 2). 5. Fiber losses. Perform the following calculations for a 10 meter long fiber with a core refractive index of 1.46. a) Calculate the transmission through the fiber assuming that the losses occur only due to reflections at the input/output interfaces. Express the attenuation in db. b) What are the total loses in the fiber assuming that the fiber attenuation coefficient (α) is 15 db/km? Figure 3. A positive lens focusing a beam with a waist D into a spot with diameter d Answers: 1. a) 1.475; b) 0.27 2. 2843 modes 3. 575 nm 4. a) 0.12; b) 6.4 m; c) 0.3 and 2.6 m; d) Neither of them: either the NA is not matched (too big) or the spot size is too large. 5a) 0.93 or 0.31 db; b) 0.47 db; 3
Laboratory assignment Task I. Aligning and coupling light into fibers In this part you will study how to cut a fiber and couple light into it. Two different fibers are used, a single mode fiber from Newport and a multi mode fiber from Ericsson. Specifications are given in Table 1. Initially you will need to prepare one single mode sample fiber. It should be of a length about 1 meter. A multimode sample fiber is already prepared for you. Manufacturer Newport Ericsson Ericsson Type Single mode Multi mode Graded Index Core/cladding 4/125 100/140 62.5/125 Ø [µm] Material quartz/quartz glass/glass quartz/quart z Outer Ø [µm] 400 300 300 Attenuation @ 2.2 15 2.8 850nm [db/km] Numerical 0.11 0.31 0.27 Aperture Price [SEK/m,1988] 5 40 1 Table 1 Specifications for the fibers used in the alignment laboratory section Terminating a fiber There are various ways of handling fiber termination. The most common are: to prepare a fiber connector (for temporal fiber to fiber coupling), splice the fibers (for permanent coupling) or prepare high quality bare fiber tip facet (for e.g. direct source or detector coupling). In either case one needs to achieve a flat high quality facet surface, in order to avoid noticeable distortion of the wave front entering or exiting the fiber. In this exercise we cleave the fiber using a diamond blade. The small scratch that diamond blade leaves on the fiber cladding makes the fiber very fragile and enables it to brake as soon as minor stress is applied. This procedure in fact is similar to cutting usual window glass with diamond glass cutter and in many cases results in a nice flat tip facet surface that does not distort wave front. This cutting/cleaving procedure is most suitable for thin fibers. The thicker fibers may require special polishing procedure. To cut the fiber do the following: Carefully remove the outer plastic coating (you may use your nails) from the fiber for a distance of about 5 cm. In order to simplify the pilling process place the fiber in acetone for a minute or two this softens and deteriorates the plastic. Place the fiber in the cutter. Attach and stretch the fiber. Cut the fiber by pressing the switch on the cutter. Remember to cut both sides. Inspect the result (fiber tip facet) under a stereo microscope. Once you have cut both ends of the two fibers, you may proceed with aligning light into them. 4
Focusing optics Using the setup depicted in Figure 4, align light into the two fibers. A suggestion is to start with the multi mode fiber, since it has larger core and thus is easier to couple light into. At this point, however, you will need to consider which focusing optics to use with your fiber. You have two alternatives available, either a one inch single lens with a focal length of 40mm, or a microscope objective with a focal length of 16mm. How do you choose the focusing optics, and on which criteria? Hint: There are two main criteria to fulfill. Figure 4 The alignment setup consisting of a He Ne laser, two turning mirrors, the focusing optics and an x y z translation stage for the fiber Aligning procedure Alignment can be somewhat tedious procedure that generally requires some experience. The reasonable sequence of steps to get to good result is outlined in Fig 5. There the trick is to always monitor how the fiber tip is positioned with respect the focus and gradually bring it there. Using this method align both multi mode and single mode fibers. Figure 5. Fiber alignment procedure. Start with positioning the fiber approximately in the center of the beam behind the focus. Adjust transverse position of the fiber to get maximum coupling. Gently move the fiber towards the focus until the signal decreases. Readjust in transverse plane to get the maximum signal. Repeat the procedure until the fiber tip is in the focus and the coupling is maximal. Coupling efficiency Using the photodiode connected to the ampere meter measure signal directly from the laser and that the output of your fiber. Estimate and compare coupling efficiency for the single mode and multimode fiber. How the coupling efficiency is dependent upon the choice of coupling optics. Elaborate and present your conclusions in the report. 5
Numerical aperture measurements Measure the numerical aperture of your optical system, fiber plus focusing optics, by determining angle at which the light is exiting the fiber. What is limits NA in your case, the NA of the fiber or the NA of focusing optics? What happens if NA of the focusing optics is larger than NA of the fiber? Mode patterns Study the mode pattern from both the multi mode and single mode fiber. How do they differ? What happens to the mode pattern if we gently disturb (e.g. touch) the fiber? Finally try to bend the fiber to the point where it almost breaks (mammal bending radius XXX). Observe and discuss what happens to the mode pattern of the SM and MM fibers in this case. What do you observe at fiber bend? Discuss during the lab and review the discussion in your report. Figure 6 Mode patterns in a cylindrical wave-guide. Adopted from ref 1. Task II. White light spectroscopy of optical fibers The attenuation in optical fibers is generally determined by interplay of absorption and scattering and strongly dependent of the wavelength. A typical attenuation spectrum for silica fiber in the near infra red spectra region is shown in Figure 7. Figure 7 Typical attenuation spectra in silica fibers. In this experiment we investigate attenuation spectra of MM optical fiber in the spectra region around 1500nm. The fiber to be used is a graded index multi mode fiber (Ericsson AB, Sweden) with the length of approximately 100 meters. More detailed fiber specifications are given in Table 2. Manufacturer Ericsson Type Graded Index Core/cladding Ø [µm] 62.5/125 Material quartz/quartz Outer Ø [µm] 300 Attenuation @ 850nm [db/km] 2.8 Numerical Aperture 0.27 6
Price [SEK/m,1988] 1 Table 2 Specifications for the fiber used for the white light spectroscopy. In a nut shell the experimental procedure is as follows. In order to evaluate the attenuation spectra of the optical fiber we compare optical power spectra form the spectrally broadband source before and after the fiber. If the and are power spectra before and after the fiber, respectively than fiber transmission spectra is given by: 1 In practice in order to perform the measuring accurately two issues has to be taking care of. First, while performing the measurements we have to avoid influence of (spectrally dependent) absorption of all the setup components except the tested fiber. In order to do it and and are actually measured using two different pieces of the same (tested) fiber of the different length. In this way power attenuation caused by the setup components are equally present in both it and spectra and cancel out in (1). Secondary, in order to perform correct attenuation measurement one has to ensure that all the modes of the MM fiber are uniformly excited. In order to achieve this one can use a long piece of the same fiber prior to the tested fiber piece, co called equi mode fiber. After propagation of the few hundred meters in this equi mode fiber all the modes become coupled to each other and eventually uniformly excited. The experimental setup used to measure fiber attenuation spectra is depicted in the Figure 8. The halogen lamp (1) is coupled to equi mode mode fiber (2) using the coupling lens (2). The equi mode mode is coupled to tested (or reference) fiber (6) using the fiber optics connectors (4). The output of the tested fiber is coupled to the StellarNet CCD fiber optics spectrometer (7). Figure 8. Experimental setup for measuring attenuation spectra of optical fiber. After measuring fiber transmission spectra we can calculate the fiber attenuation coefficient as follows: 10, Here and are lengths of the tested and reference fibers, respectively. Measurement procedure To register the power spectra in this experiment we use fiber optic grating spectrometer equipped with a CCD chip. As a light source we use a halogen lamp that is focused into the fiber by the lens. The spectrometer is connected to a computer and controlled by the SpectraWiz spectroscopy software. 7
Measure power spectra of the 1m reference and test fiber and save the acquired spectra. While performing power spectra measurements make sure not to saturate the spectrometer CCD chip (the maximum count rate is XXX counts). Remember also to correct measured spectra for the CCD dark current and background illumination. Evaluation in Matlab Calculate the attenuation coefficient, expressed in db/km, as a function of wavelength. Plot the result and discuss why the spectrum has this particular shape. What is the main origin for the attenuation in this wavelength range? Task III The Fiber Bragg Grating (FBG) Introduction Fiber Bragg Grating (FBG) is a very typical fiber optic component that widely used in various applications as for example band stop filter, as a part of the signal multiplexing devices as well as in fiber optic sensors of temperature and stress. In essence FBG (shown in fig 9) is an optical fiber with a periodically modulated core refractive index. The tiny fraction of light propagating through the FBG is reflected on each abrupt change of the refractive index. Even each single reflection is very small the collective effect of multiple reflections is very large provided that the reflections are in phase with each other or in resonance that happens at the particular wavelength. The principle is exactly same as in usual reflection or transmission diffraction grating. In this way FBG acts as band stop filter that reflects light in the particular wavelength range and whereas light at other wavelength remain intact. The resonant wavelength for which full reflection occurs is called Bregg wavelength and given by 2 Λ where n is the average value of the core refractive index and Λ is a FBG period (see Fig 9). 8
Figure 9. Fiber Bragg Grating. Periodic modulation of the core refractive index causes light reflection at the resonance (so called Bragg) wavelength. This results in the sharp spectral features in the transmitted and reflected spectra. In this part of the lab we experimentally investigate operation of FBG by using wavelength tunable semiconductor laser diode. By varying diode laser driving current we can tune its emission wavelength. We can use tunable laser output for monitoring wavelength dependent transmission and reflection of the FBG. Attention! FBG is very fragile, handle it with care! SAFETY WARNING: Make sure never to look into the output of any of the fibers used in the lab. The diode laser used in the experiment has power of ca. 10 mw and wavelength of 1550nm. This implies that although laser output is invisible is sufficiently strong to cause severe damage to your eyes. SEVERE EYE DAMAGE INVOLVING A PERMANENTLY DESTROYED RETINA AND BLINDNESS MAY RESULT It is crucially essential for your safety that during this lab you follow the following rules: Never direct the fiber output into your and other people eyes. Do not turn on laser diode before lab assistant instructed you how to use the setup and checked the interconnections. Never turn on lasers before checking that all fibers are either appropriately connected or terminated to prevent uncontrollable emission. Always turn off the lasers when you making any rearranges in the setup. 9
Figure 10. The schematic of the experimental setup. Tasks 1. The schematic diagram of the experimental setup is shown in Figure 10. Identify different setup components and instruments on the optical table, and discuss their purpose with the lab instructor. 2. Activating the laser diode. Turn on temperature controller (TED200C). Set the laser diode temperature according to the guidelines on the controller. Keep the IR card in front of the laser pigtail when you partner turns on the laser by pressing Laser on button and slowly increase laser current. Observe the effect. Turn the laser off. 3. Splitters. a. Coupe splitter to the laser diode. Start laser diode and control that you have output form the both arms of the splitter. Connect the photodiode (PD2) to one of the splitter outputs to block any uncontrollable emission. The signal from this photodiode is used to monitor laser diode power output. b. Slowly increasing diode laser driving current and monitor signal from the reference photodiode (PD2) in the program Diode Laser Control while. Verify that the laser emission threshold occurs at the same current as before. c. The program uses the measured laser driving current and appropriate calibration settings in order calculate laser emission wavelength (that is dependent on the driving current). Determine what are tuning wavelength range for your laser. 4. Circulators a. Investigate operation of the fiber optic circulator by sequentially coupling the circulator ports to the free output of the splitter and monitoring the output(s) at the remaining circulator ports. b. Determine which color of the circulator pigtail corresponds to which port number. The circulator ensures the transmission form port 1 to port 2, and from port 2 to 3 whereas it blocks transmission from port 2 to 1 and from 3 to 2. c. Couple the splitter to port 1 of the circulator. 5. Fiber Bragg Grating Experiment a. Couple FBG till port 2 of the circulator and connect photodiode 1 (PD1) after the FBG. The photodiode is used to monitor transmission through the FBG. b. Set the LD driving current just above the laser threshold. Initial scanning of the driving current by pressing Scan button of the program. While scanning, the 10
program preforms continuous monitoring of the two photo diodes and plots their outputs as well as their ratio as a function of the laser wavelength (determined from the laser driver current). Discus and protocol your observations. c. Now place photo diode 1 to the port 3 of the circulator. This enables you to monitor reflection from the FBG. Determine the wavelength at which the maximum FBG reflection occurs. Determine FWHM (full width at half maximum) of the reflection peak. Save the measurements and present them as plot in your lab report d. Try to slightly (!) disturb the grating by e.g. gently touching it with the finger or just blowing on it. What is the effect on reflection spectra do you observe? Document and explain your observations! e. The average refractive index of the FBG core is n = 1.47 What is the FBG period? References http://www.rp photonics.com/fibers.html 11