Study On 2?m Band Fiber Laser Based On Multi-channel Fiber Bragg Grating Filters

The 2 ?m band multi-channel fiber Bragg grating(FBG)is an excellent choice for filter components in the multi-wavelength thulium-doped fiber laser(TDFL)due to its outstanding filtering characteristics and good compatibility.The multi-channel FBG-based TDFL works at the eye-safe 2 ?m band,in which an atmospheric window and the strong absorption peaks of greenhouse gases exist.Also,due to its advantages of small size,high beam quality,strong immunity to interference,and fine compatibility with optical fibers,this TDFL is widely used in optical communication,laser therapy,fiber sensing,and laser processing.Under the supports of National Nature Science Foundation of China,the thesis is devoted to investigating the multi-channel FBG filter in the 2 ?m band,and the key technologies of switchable multi-wavelength and narrow-bandwidth TDFL,theoretically and experimentally.The main innovative achievements are listed as follows:1.A filter based on the phase-shifted chirped fiber Bragg grating(PS-CFBG)was proposed.The transmissivity,wavelength and bandwidth performance of the single-and multi-phase-shifted PS-CFBG with different grating parameters,including the phase-value,phase-shifted position,grating length,chirped coefficient and refractive index modulation amplitude,were discussed by simulation according to the transmission matrix method.By calculation,the transmission peak bandwidths of single-,dual-and tri-phase-shifted PS-CFBG were 0.0270 nm,0.0172 nm,and 0.0112 nm,respectively.In the experiment,the bandwidths of 0.09 nm and 0.05 nm were acquired for single-and dual-phase-shifted PS-CFBG,respectively.2.A filter based on the phase-shifted sampled fiber Bragg grating(PS-SFBG)was proposed.Using the transmission matrix method,the dependence of the transmission spectra of the PS-SFBG on different grating parameters,including the phase-value,phase-shifted position,grating length,duty cycle,sampling number and refractive index modulation amplitude,was simulated.By calculation,the bandwidth of the grade-0phase-shifted peak was as narrow as 0.0220 nm.Based on the theoretical analysis results,the PS-SFBG was fabricated experimentally,and the phase-shifted-peak bandwidths of the PS-SFBG were narrower than 0.09 nm.3.A continuous wave wavelength-tunable linear cavity TDFL with two sampled fiber Bragg gratings(SFBGs)was proposed and demonstrated.The output laser wavelength was tuned by applying strain to one SFBG.This proposed laser was stable operating with a significant 14.44 nm wavelength range due to the Vernier effect.The laser optical signal-to-noise ratios(OSNRs)of different wavelengths were higher than45 d B.The power and wavelength fluctuations were less than 0.460 d B and 0.03 nm in50 min,respectively.An 8.62% slope efficiency was achieved.4.A continuous wave switchable multi-wavelength ring cavity TDFL with a polarization-maintaining sampled fiber Bragg grating(PM-SFBG)was proposed and demonstrated.The four-wave mixing effect was introduced to suppress the wavelength competition existing in the gain medium.By adjusting the polarization state of light in the laser cavity,two six-wavelength operations at two orthogonal states of polarization and a ten-wavelength operation were obtained.The OSNRs were higher than 30 d B.Regardless of the operation mode,the TDFL exhibited high stability in 50 min.5.A compound-cavity-based wavelength-switchable single longitudinal mode(SLM)TDFL with a PM-SFBG was proposed and demonstrated.The polarization hole burning effect was introduced to switch the wavelength and suppress the gain competition.6 single-wavelength SLM operations and 9 dual-wavelength operations with different orthogonal polarizations were easily obtained and switched.For the single-wavelength operation,the maximum power and wavelength fluctuations were as low as 0.709 d B and 0.02 nm in 30 min,respectively.For the dual-wavelength operations,the maximum power and wavelength fluctuations were only 0.946 d B and0.03 nm,respectively.The OSNRs were higher than 54 d B.The frequency noise and linewidth of each lasing wavelength were measured through an unbalanced Michelson interferometry system.When the measuring time was 0.005 s,the linewidths for all six SLM lasing wavelengths were 1.08 k Hz,0.64 k Hz,0.60 k Hz,0.76 k Hz,0.97 k Hz,and0.60 k Hz,respectively.6.A wavelength-switchable SLM compound cavity TDFL employing an SFBG was proposed.The nonlinear polarization rotation effect was introduced to switch the wavelength and suppress the gain competition.The laser can be switched among three single-wavelength SLM operations and three dual-wavelength operations.For the single-wavelength operation,the maximum power and wavelength fluctuations in 50 min were ±0.404 d B and ±0.01 nm,respectively.For the dual-wavelength operations,the maximum power and wavelength fluctuations in 20 min were ±0.926 d B and ±0.03 nm,respectively.The OSNRs were higher than 49 d B.The frequency noise and linewidth of each lasing wavelength were measured.When the measuring time was0.005 s,the linewidths for all three SLM lasing wavelengths were 0.26 k Hz,1.19 k Hz,and 0.71 k Hz,respectively.