n3 = 1.3 pump output Inner cladding n = refractive index Core n2 1.4 Outer cladding Jacket Doped Core- Inner cladding Fig. 1: Structure of double clad doped fiber Given a doped double clad fiber shown in Fig. 1. The core has diameter: 5 micron meter; operational wavelength: 1.5 micron meter; NA: 0.20. (a) Calculate the V-number of the following optical fiber. Check if it is single mode or multi-mode operation in this case? (b) If the operational wavelength is changed to 1.2 micron meter, check if it is single mode or multi- mode operation in this case? (c) Calculate the NA of the "inner cladding"? (d) Explain how we can design a double clad fiber so that more pump laser can be launched into the inner cladding. (e) Explain how we can enhance the absorption of launched pump laser by the doped fiber core?