【正文】 re, they travel both further and faster ? as a result, they arrive at the output at almost the same time as the waves with shorter trajectories ? Anything that might cause scattering in the core must be minimised ? Cu, Fe, V are all reduced to parts per billion ? H2O and OH concentrations also need to be very low ? Variations in the diameter of the fibre also cause scattering ? this variation is now 1181。m over a length of 1km ? To avoid dispersion of different wavelengths, lasers are used as the light sources ? many data channels are possible using wavelength division multiplexing (WDM) Optic: 49 ? A convenient fact is that pound semiconductor lasers can emit IR light close to the wavelength where the fibre absorbs least ? Referring back to the system diagram, it would be advantageous to integrate the encoder and transmitter ? so the circuits and the light emitter can be integrated ? This is why there is so much interest in getting light out of porous silicon or Si pounds ? where thin strands of material exhibit quantummechanical effects which adjust the Si band structure to facilitate efficient light emission Optic: 50 Optic: 51 Lasers ? LASER stands for Light Amplification by the Stimulated Emission of Radiation ? The key word here is “stimulated” ? All of the light emission we have mentioned so far is spontaneous ? it happened just due to randomly occurring “natural” effects ? Stimulated emission refers to electron transitions that are “encouraged” by the presence of other photons ? Einstein showed that an incident photon with E ≥ Eg was equally likely to cause stimulated emission of light as to be absorbed Optic: 52 ? The emitted light has the same energy and phase as the incident light (= coherent) ? Under normal circumstances, there are few excited electrons and many in the groundstate, ? so we get predominantly absorption ? If we could arrange for more excited than nonexcited electrons, then we would get mostly stimulated emission equally likely as Optic: 53 ? Since we get more photons out than we put in, this is optical amplification ? hence lAser ? this system was first used to amplify microwaves for munications (maser) ? Such a condition is called a population inversion ? This stimulated emission is what gives the laser its coherent output ? which is what makes it useful for holography, for example ? Clearly, random spontaneous emission “wastes” electron transitions by giving incoherent output ? so we minimise them by using transitions for which the spontaneous emissions are of low probability ? socalled metastable states Optic: 54 ? The energy levels of a laser material therefore look like: ? Ruby is a mon laser material, which we saw was Al2O3 (sapphire) with Cr3+ impurities Optic: 55 ? So all we need to make a laser is to achieve ? (i) a population inversion ? (ii) enough photons to stimulate emission ? The first is achieved by filling the metastable states with electrons generated by light from a xenon flash lamp ? The second condition is achieved by confining the photons to travel back and forth along the rod of ruby using mirrored ends ? next slide ? The ruby laser has an output at nm Optic: 56 ://www.repairfaq.org/sam/laseroif Optic: 57 ? In order to keep the coherent emission, we must ensure that the light which pletes the round trip between the mirrors returns in phase with itself ? Hence the distance between the mirrors should obey 2L = N? ? where N is an integer, ? is the laser wavelength and L is the cavity length ? Semiconductor lasers work in just the same way except that they achieve the population inversion electrically ? by using a carefully designed band structure Optic: 58 ? Some laser characteristics are given in the following table: Callister Optic: 59 Summary ? We have looked at how the electronic structure of atoms and their bonding leads to varying optical behaviours in materials ? In particular, properties such as absorption and emission are closely related to the electrons ? Applications of this knowledge include ? antireflective coatings for lenses ? fibreoptic munications ? lasers Optic: 60 THE EN