It is now possible to design Iasers using optical fibres. A fibre doped with Nd has been found to have a small signal gain coefficient g = 1.1 m-1. Assuming that the refractive index of the fibre is 1.5, derive an expression for and hence calculate the Fresnel reflection at the glass/air interface for a cleave at 90° to the fibre axis. Describe the requirements to obtain Iaser oscillation and calculate the minimum length of fibre required to produce Iaser oscillation where the reflectors are the cleaved fibre ends (ignore material Iosses). What is the frequency mode spacing for this length of Iaser? If it is possible to mode lock this Iaser, what would be the repetition rate and approximate pulse width of the output pulse when the line width of the Iaser is 10 GHz? State any assumptions made. Suggest a way in which this Iaser could be pumped.