Protection is an important issue of resilience in elastic optical networks (EONs). Not only for working lightpaths, but also routing, modulation and spectrum assignment (RMSA) for backup paths is essential. In protection scheme, shared backup path protection (SBPP) is usually discussed now. SBPP allows backup lightpaths to share spectrum resource so that the amount of allocated frequency slots can be reduced. Spectrum usage critically affects blocking performance due to the limited resource in spectrum. For this reason, we aim at minimizing spectrum usage as our objective. For RMSA, many heuristic algorithms pursue the shortest path or minimum-hopped path and neglect the balance between number of required frequency slots and maximum transmission distance (MTD) of modulation levels. Therefore, we propose a RMSA algorithm, namely, lowest cost first and shortest path algorithm (LCF) to derive the RSA solutions for both working and backup lightpaths. However, among RSA solutions satisfying MTD constraint, the one with least vacant resource consumption might be neither the minimum-hopped path nor the shortest path. Hence we propose an enhanced RMSA algorithm called minimum usage algorithm (MU) to pre-calculate all paths satisfying MTD constraint and then find the one with minimum resource usage. Simulation results show that both LCF and MU can achieve great improvement on blocking performance.