Dual-wavelength Fiber Lasers Based On Equivalent Phase Shift

Fiber lasers, as one important part of fiber technology, is always a fascinating topic for lots of advantages such as low noise, narrow linewidth, simple configuration, small volume, low cost and EMI resistance. Dual wavelength distributed feedback (DFB) fiber lasers, as a kind of fiber lasers, have wide application in communication, sensing, microwave generation, and many other areas. This paper mainly focuses on the method to achieve dual-wavelength DFB fiber laser based on equivalent phase shift (EPS) theory, and to prove this technology theoretically and experimentally through two dual-wavelength grating structure. It shows that this technology of EPS can achieve high performance DFB lasers with the precision of submicron instead of nanometer, which simplifies the laser fabrication and is suitable for mass production.First of all, the history of DFB fiber lasers is reviewed; the basic principles and the recent progress on multi-wavelength DFB fiber laser are also discussed. Then the basic theory of fiber Bragg grating is discussed in detail, and the fiber Bragg condition and the numerical analysis are deduced, such as coupled mode theory and transfer matrix method. Several special fiber gratings are mentioned and simulated, giving their reflection/transmission spectra and time delay characteristics, the methods and technologies of fiber grating fabrication are also introduce. Equivalent theory and EPS are then introduced. They are part of the theories which the fiber gratings design proposed in this paper based on. With the help of the analysis tool, the causation of equivalent phase shift in sampled fiber Bragg gratings is analyzed. The EPS methods can greatly reduce the fabrication cost as the precision is decreased from the micrometer to nanometer scale. Based on the EPS technology, the author proposed two dual-wavelength DFB fiber laser structure. Through simulation and discussion on the parameters of fiber grating, an equivalent double phase shift structure is designed and manufactured, which can achieve stable dual-wavelength lasing with 31pm spacing under the 980nm pump. Combined with trapping techniques, a dual-wavelength laser at 1557:355nm and 1557:753nm is realized on the imposition EPS structure.Given only normal uniform phase mask and sub-micron precision control in the fabricating procedure, those spectral response characteristics of phase-shifted fiber gratings which require certain complicated phase mask and nanometer precision control, are realized in hydrogen-loaded Erbium-doped fibers. Based on the EPS technology, two dual-wavelength DFB fiber lasers are realized in this paper. It solves the problems of complex production process, accurate phase control and other issues of traditional production methods, and has the advantage of simple manufacture and low-cost, which is greatly significant in the experimental production and practical application.