ipment. Cost has also reduced as fibre quality has improved, which in turn is due to improvements in manufacturing technique. For example, hydroxide ions (OH-) absorb particular wavelengths. 15 Multiplexing gain is studied in Chapter 2. 16 Fabry-Perot laser diodes are a type of diode laser combined with a mirror system. They are generally slower and noisier than distributed feedback (DFB) lasers. DFB lasers use diffraction gratings rather than mirrors to create resonance and oscillation in the cavity. They produce a finer range of wavelengths than a Fabry-Pero cavity. Making Telecoms Work Reducing OH ion content is allowing wider bandwidths to be supported allowing up to 16 wavelengths to be supported on a single fibre pair. Making fibre that bends easily in a relatively tight radius and the use of simpler lower-cost connector and terminating devices have helped make domestic fibre to the home more economic. The choice of technologies within the access network include point-to-point Ethernet, wavelength division multiplexing passive optical networks (WDM PON) and time division multiplexed passive optical networks (TDM PON). Point-to-point Ethernet delivers dedicated bandwidth to individual users, TDM PON pro- vides shared bandwidth up to 64 users, WDM provides dedicated bandwidth to each user by allocating a specific wavelength. As the performance of photonic integrated circuits and temperature-independent optical components improve, all three options become more efficient. Both TDM and WDM systems require demultiplexers and space needs to be found within splitter sites to accommodate these. In practice, physical access costs remain a dominant cost component. Allowing telecom operators to string fibre on telegraph/utility poles, a proposal presently being actively pursued in some markets, would substantially change fibre access economics though the data pipe still needs to be coupled to the user over the final few metres into the home or office. So first let's look at the present and future evolution of optical d