Ultra-short Length Of Erbium Ytterbium Co-doped Superfluorescent Fiber Source

The length of active fiber used in the conventional Er³⁺-doped superfluorescent fiber source(ED-SFS) is usually more than several meters.Nonlinear effects in the fiber will be aggravated with increasing the fiber length.Such long length affects the performance of ED-SFS and is not convenient for optical integration.In this paper, our research focuses on the compact Er³⁺-Yb³⁺-codoped phosphate glass superfluorescent fiber source(EYD-SFS) in which the active fiber length can be controlled into～10cm.This type of EYD-SFS shortens the size of fiber device and reduces the nonlinear effects in the fiber,so it will benefit optical integrated application in the future.Firstly,we introduce the two-step rod-in-tube method for fiber preform fabrication.Take the preform diameter of 16 mm and the standard fiber diameter (125μm) for instance.The calculation shows that,if the single-mode Er³⁺-Yb³⁺-codoped phosphate glass fiber(EYDPF) with single cladding is required, we should bore an aperture on the fiber preform with the diameter of 2.21 mm and the offset distance of 3.4 mm everytime.After finishing the fiber preform,we try to fabricate EYDPF using the fiber pulling machine which is designed by ourselves. The single cladding EYDPFs with standard diameter are fabricated and the error can be controlled into±5μm.Then,we carry out the theoretical research on the compact EYD-SFS.The theorectial model is provided based on the propagation and rate equations method. We consider not only the cooperative up-conversion(UC),cross-relaxation(CR), forward energy transfer(FET),cumulative energy transfer(CET) and double-energy transfer(DET) processes,but also the influence of fiber temperature in the model. The simulated result indicates that the double-pass forward(DPF) configuration is an optimum basic configuration.Take the DPF configuration for example.When we fixed the fiber length as 14.6 cm,signal reflectivity as 0.99,Er³⁺ concentration as 2×10²⁶ ions/m³ and Yb³⁺ concentration as 0.8×10²⁷ ions/m³,the intrinsic thermal coefficient(ITC) is calculated to be 1.59 ppm/℃.Moreover,the influences of nonlinear transition processes(cooperative UC,CR,DET and CET),Yb³⁺ adulteration and glass materials on ITC have been calculated theoretically.We also optimize the performance of compact EYD-SFS.The simulation indicates that when the pump power is fixed as 330 mW,to achieve the optimum EYD-SFS performance, the Er³⁺ concentration,Yb³⁺ concentration and signal reflectivity should be 2×10²⁶ ions/m³,8×10²⁶ ions/m³ and 0.99,respectively.To enhance the output power and bandwidth,the optimum fiber length should be 14—16 cm,while eliminating fiber length can improve mean wavelength stability.Finally,in our experiment the EYDPF is 10.4-cm-long,and at 1.68 W pump power an output power of 16.85 mW,a mean wavelength of 1540.9 nm,a bandwidth of 18.4 nm,and a slope efficiency of 18.6%have been obtained.Meanwhile,the output power,mean wavelength and output bandwidth have also been investigated in a 15.2 cm EYDPF.There is a good agreement between theoretical results and experimental data.From a 14.6 cm EYDPF,the fluctuation of mean wavelength can reach to 16.7 ppm under room temperature(20—23℃) during 5 hours in our experiment.The fluctuation is caused by pump power,pump wavelength and fiber temperature.According to measurement,the influences of pump power and pump wavelength are±7.82 ppm and±0.016 ppm,respectively.Because it is difficult to measure the effect of fiber temperature experimentally,we adopt theoretical ITC value(1.59 ppm/℃) and the mean wavelength fluctuation is calculated to be 20.4 ppm which agrees well with the experimental results.