Research On Power Flattening Mechanism Of Multiwavelength Brillouin Erbium-doped Fiber Lasers

Multi-wavelength Brillouin erbium-doped fiber lasers have shown great application value in many fields due to their advantages of narrow linewidth,stable output,fixed wavelength interval,low cost,wide tuning range,and high coherence.For example,it provides carrier signals as multi-channel light sources to improve system capacity and transmission efficiency in optical fiber communication systems;Using high-order Stokes to realize high-sensitivity optical fiber sensor in the optical fiber sensing system;High-frequency and ultra-highfrequency microwave signals are generated by beat frequency in the microwave photonics;The multi-wavelength output power flatness not only affects the bandwidth capability and cascading capability of the WDM system,but also affects the sensitivity of the optical fiber sensor and the quality factor of the microwave photonic system.Therefore,it is necessary to carry out targeted design to improve the flatness.In previous studies,some researchers have used fourwave mixing or Rayleigh scattering to achieve 3 d B multi-wavelength power flattening,but further exploration is still needed for multi-wavelength Brillouin erbium-doped fiber lasers with higher flatness.Therefore,we propose and study in detail a tunable flat multi-wavelength Brillouin erbium-doped fiber laser based on a Sagnac loop with an unpumped erbium-doped fiber.We proposed to use a Sagnac loop with an unpumped erbium-doped fiber based on the existing multi-wavelength Brillouin erbium-doped fiber laser,and combining the absorption effect of the unpumped erbium-doped fiber and the birefringence beat-length effect of the Sagnac loop to improve the EDFA gain spectrum,we achieved multi-wavelength Brillouin Stokes output power flat and completed multi-wavelength Brillouin Stokes output spectrum flatness analysis.This tunable and flat multi-wavelength Brillouin erbium-doped fiber laser scheme based on a Sagnac loop with unpumped erbium-doped fiber further improves the power flatness of the output laser,it is expected to propose new multi-wavelength fiber laser solutions for the fields of optical fiber communication and optical fiber sensing.(1)The application and realization method of multi-wavelength erbium-doped fiber lasers,and the output power flattening method of multi-wavelength erbium-doped fiber lasers are briefly described.Finally,the main research contents of this paper are introduced.(2)The mechanism of stimulated Brillouin scattering and the working principle of erbiumdoped fiber amplifiers are described,including the amplification mechanism,the uniform broadening effect of erbium-doped fibers and the basic structure.Finally,the output power flattening mechanism of multi-wavelength Brillouin-doped fiber lasers is described.(3)Three different structures of multi-wavelength Brillouin fiber lasers are simulated respectively.By deriving the theoretical model of the multi-wavelength Brillouin fiber laser,the simulation shows that the number of wavelengths of the three laser structures is mainly determined by the EDFA pump power,and the total output power and Brillouin gain are consistent with the variation trends of the two pump powers.A tunable multi-wavelength Brillouin-doped fiber loop laser scheme is proposed.Multi-wavelength output by combining Brillouin nonlinear gain and erbium-doped fiber linear gain.A tunable range from 1565 nm to1535 nm was obtained by adjusting the power of the erbium-doped fiber amplifier and the center wavelength of the TLS pump.Finally,the influence of EDFA power on the number of multi-wavelength Brillouin Stokes is analyzed.(4)A multi-wavelength Brillouin Stokes output power flattening scheme based on a Sagnac loop with unpumped erbium-doped fiber is proposed.We proposed to use a Sagnac loop with an unpumped erbium-doped fiber,and combining the absorption effect of the unpumped erbium-doped fiber and the birefringence beat-length effect of the Sagnac loop to improve the EDFA gain spectrum.We multi-wavelength obtain output power flattening effect of ±0.315 d B to ±1.38 d B at the laser transmission port The multi-wavelength output of the tunable flat range is 33.0 nm,covering almost the entire C-band.The optical signal-to-noise ratio in different experiments ranged from 16.26 d B to 18.97 d B,and the overall average optical signal-to-noise ratio was calculated to be about 17.51 d B.Peak power fluctuations from-18.01 d Bm to-18.75 d Bm.If the Sagnac loop with unpumped erbium-doped fiber is used in other structures of multiwavelength Brillouin fiber lasers,it is expected to obtain a larger number of flat Stokes output.