The feasibility of achieving efficient coupling between an optical fiber and a periodically poled lithium niobate(PPLN)waveguide in an ideal optical system was theoretically analyzed using a finite element simulator.Divergence of single-mode polarization-maintaining fiber and lensed fiber in space was simulated.Direct end-face coupling simulations were conducted for a PPLN ridge waveguide with a cross-sectional size of 10 pm × 10 pm and a length of 20 mm,involving two different types of fibers.It is found that despite the lensed fiber achieving a coupling efficiency of up to 95％,its fundamental mode contribution is less than 5％.Given that PPLN frequency-doubling devices primarily rely on the laser's fundamental mode for operation,this renders the coupling efficiency of the lensed fiber relatively low in practical applications.In contrast,ordinary single-mode fiber exhibits a significantly higher fundamental mode contribution of 93.8％,demonstrating its superior performance.Therefore,this study selects ordinary single-mode fiber for encapsulation testing with PPLN ridge waveguides.The experimental results show that when the maximum pump power output from the fiber amplifier reaches 1.6 W,the calculated input pump power,after deducting the coupling losses between the input fiber and the waveguide,stands at 1.2 W.The fiber-to-waveguide coupling efficiency is 75％,slightly exceeding the currently known advanced value of 72％.At a temperature of 24.8℃,with an input power of 1.2 W at the fundamental wavelength of 1 560 nm,the maximum output of frequency-doubled light is 653 mW.The optical-to-optical conversion efficiency reaches 54.4％,with a normalized conversion efficiency of 20.2％/(W·cm2).
维普
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