Instytut Fizyki Doświadczalnej Wydział Matematyki, Fizyki i Informatyki UNIWERSYTET GDAŃSKI
I. Background theory. 1. Band structure of semiconductors. 2. The theory of operation of a semiconductor (diode) laser. 3. The law of refraction, law of reflection, total internal reflection of light. 4. Geometric picture of the propagation of light in optical fibre. 5. Fibre modes; numerical aperture. 6. Types of fibre: a) step ; b) gradient ; c) single-mode ; d) multi-mode. 7. Optical signal attenuation in optical fibres and its causes : a) absorption ; b) Rayleigh scattering ; c) mode coupling ; d) bending of fibre ; e) losses at optical fibre connections. II. Experimental tasks. 1. Refer to the measurement system shown in Picture 1. and procedures for the preparation of fibre-optic measurements described in Appendices A and B. Picture 1. Measuring system for investigating optical fibre properties: 1 - power supply with amplifier; 2 - laser; 3 - optical fibre; 4 - optical bench; 5 - laser mounting bracket; 6,7 adjustable brackets with lenses; 8,9 magnetic optical fibre mounting brackets; 10 photodiode mounting bracket; 11 optical fibre insulation stripper; 12 optical fibre cutting tool. Instytut Fizyki Doświadczalnej 1.
2. Prepare the optical fibre for measurement according to the instructions in Appendix A. 3. Set the correct geometry of the measuring system according to the instructions in Appendix B. 4. Investigate the physical properties of the fibre, guided by Appendix C. a) Measure the transit time of light though the fibre. b) Determine the angle at which the intensity of light falling on the photodiode is at its maximum and the angle at which light intensity value is 5% of the maximum value. c) Calculate the numerical aperture and discuss errors. d) Measure the transit time of light without the fibre. 5. Prepare measurement results from 4 (a) - 4 (c): Present a chart of the light transit time without fibre and with fibre ; Determine the refractive index of the fibre, discuss errors ; Calculate the speed of light in optical fibre. 6. Determine the profile of the laser beam. 7. Present a graph with a graphical profile of the laser beam together with measurement errors. III. Apparatus. 1. Semiconductor laser with power supply. 2. Optical bench with a set of lenses. 3. Multimode fibre (length 1000 m). 4. Photodiode. 5. Oscilloscope. 6. IR converter. 7. Tools for preparing fibres : a) fibre insulation stripper ; b) optical fibre cutting tool. IV. Literature. 1. J. A. Buck Fundamentals of Optical Fibers, Wiley-Interscience, Hoboken 2004. 2. W. S. C. Chang Principles of Lasers and Optics, Cambridge University Press, 2005. 3. P. Halley Fiber Optic Systems, John Wiley & Sons, Chichester 1987. 4. PHYWE Laboratory Experiments Physics, Optics, Fibre Optics 2.6.10 00, 2010. 5. Ch. Kittel Introduction to Solid State Physics, 8 th Edition, John Wiley & Sons, 2004. Instytut Fizyki Doświadczalnej 2.
Appendix A Preparing optical fibre for measurements Use the stripper (Picture 2 (a)) to remove the insulation from the ends of the fibre. The stripper is set exactly to the thickness of the fibre core, so you should tighten the jaws at a distance of about 30 mm from the end of the fibre and then, moving the tool, remove the insulation. a b Picture 2. Tools for the preparation of fibres : (a) fibre insulation stripper, (b) tool for cutting fibre: 1 - notch to place the fibre; 2 optical fibre restraint lever handle; 3 - blade; 4 - sliding arm. The end of the fibre with the insulation removed should be placed in the tool for cutting fibre (Picture 2(b)) in a special notch (1). Secure the fibre using the lever (2) such that the end of the insulation is between 5 mm and 10 mm on the distance scale. The blade (3) should be lightly pressed against the isolated fibre, ensuring that the fibre remains in the notch (1). If too much pressure is applied, you will cut the fibre core. In this case, repeat the procedure. After cutting, release the blade (3) so as to have returned to the starting position. Paying constant attention that the fibre remains in the notch (1), hold the device with one hand while using the thumb of the other hand to press the fibre into the notch (1) in the arm (4). Slide out the arm (4) until the moment the fibre breaks. Instytut Fizyki Doświadczalnej 3.
Appendix B Ensuring proper geometry of the measuring system Place the ends of the fibre, in special notches in the magnetic mounts - (8) and (9) in Picture 1. Turn on the laser (1). To do this, turn the key to "on" and then increase the current to maximum (227 ma) using the arrows on the power supply panel ((1) in Picture 1). Take the utmost care when working with lasers! The laser emits a beam of light with a wavelength of 850 nm (invisible to the human eye) with a power of 50 mw. In order to protect your eyes, always wear protective glasses. Set the power supply to a gain of 50 and a rectangular signal modulation. Turn on the oscilloscope. The signal from the photodiode is sent through the amplifier in the power supply (1) and should be sent to channel 2 of the digital oscilloscope. A modulating signal from the amplifier (1) should be sent to channel 1 of the oscilloscope. The time base on the oscilloscope should be set to 500 μs. Set the voltage for the first channel of the oscilloscope to 200 mv/division and 50 mv/division for the second channel. The lens (6) should be placed approximately 1 mm from the laser (2) and the following lens (7) approximately 5 cm from lens (6) and about 1 cm from the end of optical fibre mounted in the bracket (8). The shape and power of the beam can be observed using a infrared converterplate covered with a layer of luminescent material excited by infrared radiation. Using the adjustment screws in the lens mounts (6) and (7), focus the beam at the tip of the fibre placed in the holder (8). Light coming from the optical fibre mounted in a holder (9) should be collected by the photodiode (10), completely flush with the end of the rotating part of the optical bench (4) - about 10 cm from the end of the fibre. After recording the signal from the photodiode with the oscilloscope (Channel 2) the set-up should be fine-tuned using the micrometer screws in each bracket (6), (7) and (8) so as to achieve maximum signal intensity (the amplitude of the second channel should be about 140 mv). Instytut Fizyki Doświadczalnej 4.
Ad. Ref. II. 4. (a) Appendix C Tips for taking measurements When performing measurements, refer to the system shown in Picture 1. On the laser power supply (1), select a rectangular modulating signal (LED off). The time-base on the oscilloscope should beset to 5 μs. Measurements should be made using the rapid measurement function (Quick Meas.). Data from the oscilloscope should be recorded on a portable USB drive in the form of an ASCII file using the Save function. Ad. Ref. II. 4. (b) The rotating part of the optical bench (4) should be set to an angle Φ 1, at which the light intensity (measured by the voltage on channel 2 of the oscilloscope) incident on the photodiode is at a maximum. Determine the angle Φ 2, at which the light intensity is equal to 5 % of the maximum intensity (angle Φ 1 ). Measure the voltage on the photodiode when no light falls on it and the take this value into account when determining the appropriate angles. Ad. Ref. II. 4. (c) In order to perform measurements, modify the set-up in Picture 1. The bracket with the fibre (7) should be removed and the laser inserted at this point. The magnetic holder (9) should be lifted up and removed from the fibre. Aim the laser beam exactly perpendicularly onto the photodiode. Select a rectangular modulating signal (LED off) on the laser power supply. Set the oscilloscope time base to 5 μs. Take measurements using the rapid measurement function (Quick Meas.). Ad. Ref. II. 5 In order to perform measurements, make use of the modified system described in part Ad. Ref. II. 4. (c). Select a rectangular modulating signal (LED "off" enabled) on the laser power supply (1). Using the micrometer screws in the laser mount (5), tune the system so as to obtain an average voltage on the photodiode i.e. about 45 mv at a gain of 50 on the laser power supply (1). By adjusting the arms of the optical bench (4) on both sides between the angles of ± 14 o (in steps of 2 o ), measure the average voltage (Quick meas. on the oscilloscope) across the photodiode to obtain a profile of the laser beam. After completing the measurements, reduce the current to the power supply (1) to 0 ma and turn off the laser by turning the key to the off position. Instytut Fizyki Doświadczalnej 5.