Based on the scalar four-photon scattering process, the quantum state of a lightwave at the output of fiber is derived by solving the nonlinear Schrödinger equation with a perturbation theory. The joint spectral function of two photons is achieved from the derived quantum state. The dispersion operator involves the third-order dispersion term in the case that the pump wavelength is close to the zero dispersion wavelength. Simulation results show the first-order approximation of our joint spectral function is in excellent agreement with the complicated exact solution. By analyzing the spectral property of the two-photon flux generated by four-photon scattering in photonic-crystal fibers, it is found that the sign of dispersion has very little influence on the spectrum except the slight modulation instability in the anomalous dispersion domain.
Switchable multiwavelength fiber laser outputs with a wide tuning range are experimentally observed in an ultralong cavity. Because of the long spooled single-mode fiber and filter effect of the cavity, multiwavelength lasers with the spacing of ~14.5 nm are obtained. The proposed fiber laser has the capacity of simultaneously emitting the three wavelengths. By means of adjusting the polarization controllers, the arbitrary single- and dual-wavelength operations are achieved in our laser.