(a) A 1550mm diode laser is constructed from a InGaAs chip 1mm in length (the end faces act as mirrors). Assuming that the light travels with refractive index similar to that of the material (n=3.9), what is the approximate wavelength separation between longitudinal modes (in nm)? [4 marks] (b) A circular fiber, designed for single-mode at 1.3 um wavelength, is to be operated so that the first two modes (and no more) will be transported. What wavelength should be used? [3 marks]