An optical continuous phase FSK modulation scheme with an arbitrary modulation index over long-haul transmission fiber link

This paper presents an optical continuous phase frequency shift keying (CPFSK) modulation scheme with an arbitrary modulation index. The detailed principle on the optical CPFSK generation is derived and analyzed, which includes the special case of the minimum-shift keying (MSK) with a modulation index h = 1/2. The differential detection and the coherent detection of CPFSK are also depicted. The performances of the four kinds of the optical CPFSK modulated system with a 40 Gb/s modulation rate whose modulation index are h = 1/2, h = 2/3, h = 3/4 and h = 1 are simulated via the spectral efficiency and the receiver sensitivity over fiber link respectively. In addition, comparison with the differential phase shift keying (DPSK) is taken. Through the calculation of the spectral efficiency of each modulation formats, CPFSK has higher spectral efficiency than DPSK with the same optical devices. The transmission performances of our CPFSK over the fiber link change better as the modulation index increases under the condition of the first order dispersion of the fiber link is completely compensated. Through simulations, a 1200 km transmission distance can be achieved with a modulation index h = 1.