A closed-loop formula is derived for the nonlinear crosstalk degradation due to self-phase modulation and cross-phase modulation in a wavelength-division multiplexing system. The crosstalk is investigated in differential binary phase-shift keying (DBPSK) and differential quadrature phase-shift keying (DQPSK) schemes. The system under consideration is standard single-mode fiber with a single-span distributed Raman amplifier. The backward DBPSK shows a better performance with minimum crosstalk performance of 71%, as compared to backward DQPSK.