n to occur. The critical angle is a function of the refractive indices of the core and the cladding. Making Telecoms Work Table 2.2 Optical band designations and relative frequencies and wavelengths Description Frequency THz Wavelength O band Original (second-generation window) 238 to 220 1260 to 1360 nm E band Extended 220 to 205 1360 to 1460 nm S band Short wavelength 205 to 195 1460 to 1530 nm C band Conventional ('erbium window') 195 to 191 1530 to 1565 nm Third generation window L band Long wavelength 191 to 185 1565 to 1625 nm Third-generation window U band Ultralong wavelength 185 to 179 1625 to 1675 nm The second telecom window was originally used for long-haul transmission. Long-haul transmissions are now all based on the third window. C and L bands between them have over 11.5 THz of bandwidth available. In the longer term, evolved semiconductor optical ampli- fier and Raman amplifier technologies will allow additional bandwidth to be economically accessed. As with radio communication there is a choice of channel spacing within each band. In the optical domain this is referred to as wavelength division multiplexing and can be used to increase the capacity-distance product. Similarly different modulation schemes can be deployed including coherent transmission (and detection). Modulation was typically (is typically still in legacy systems) on off keying (OOK) with direct detection. The combination of these techniques can achieve data rates up to 10 Gbit/s per channel but any increase beyond that requires more complex modulation for example dif- ferential phase shift keying with coherent detection. Coherency requires an ability to control the phase of the modulated signal, which requires an optical phase-lock loop and then demod- ulation of the signal in the receiver, which requires coherent detection. Essentially, these are techniques now widely deployed in mobile broadband and point-to-point microwave systems and in copper and cable but now extended into the optical domain. Wavelength division multiplexin