Please explain the source of energy to drive the fan.......
Errr...the N2 turbine that is driven by the hot gases exiting the combustion chamber?
Not quite. Consider the nomenclature:
On a two-spool machine (P&W, GE, CFMI, GP7000...) it's the final multi-stage turbine that drives N1 (fan rpm). Also called the LP turbine.
Upstream of that, at the combustor exit, is the HP turbine which is either 1 or 2 stages. It turns the HP compressor, a.k.a. N2.
In the R-R machine there's an intermediate turbine - and the gas flow though the turbine passes through N3(HP), then N2(IP), then N1(LP).