Comparison of macrobend losses in Single Mode Fibre (SMF) and Multimode Fibre (MMF)

Abstract

A comparison of macrobend losses in Single Mode Fibre (SMF) and Multimode Fibre (MMF) is presented. The increased demand on information has seen optical fibres slowly replacing the use of copper coaxial cables for signal transmission, and this, largely due to the high bandwidth and speed of transmission, large carrying capacity and security offered by optical fibres. Signal attenuation however remains a limitation to efficient and quality signal transmission and macrobending is one of the loss mechanisms that contributes to this signal attenuation. Appreciating the responses of the different optic fibres in use to macrobending is therefore instrumental in ensuring macrobending contribution to overall attenuation in optic fibres is minimum at all times. In this study the macrobend losses trends with bending diameter were investigated for the SMF and MMF in similar experimental setups. To do this improvisions were done for a light source using 555 timer astable multivibrator powering a green LED, a light detector using an LDR in a Wheatstone bridge circuit and optic fibres off-cuts. Optic power was measured at the input and output points of the optic fibres using the light detector in bend and straight conditions for determination of signal attenuation in each case. The experimental measurement results show that macrobending losses are more pronounced in a SMF than in a MMF. Characteristic values for macrobending loss are 1268 dB/km for SMF compared to 1155 dB/km for MMF for bending diameter value 4.19 cm. In the SMF macrobending loss trend showed an exponential variation which agrees with work by previous authors. The contribution of macrobending loss in multimode fibre attenuation was however minimal and therefore difficult to quantify. Results also showed that MMF have a higher overall signal attenuation than SMF which justifies the use of SMF in long distance signal transmission. Research findings confirm the significant contribution of macrobending to optic fibre signal attenuation hence the need for careful iv consideration in installations for optimal operation of optic fibres. This would mean the choice of optic fibre in any particular instance should consider the high attenuation in MMF not neglecting the insignificant macrobending contribution of MMF to attenuation at any preferred wavelength in order to obtain required transmission characteristics.