Tham khảo tài liệu 'frontiers in guided wave optics and optoelectronics part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | High Power Tunable Tm3 -fiber Lasers and Its Application in Pumping Cr2 ZnSe Lasers 425 Fig. 21. The fluorescence spectrum of 3F4 3H6 transition in Tm3 -doped ZBLAN fiber Several kinds of wavelength tuning techniques in Tm3 -doped fiber lasers 1. Fiber-length tuning. Due to the quasi-four-level system feature the Tm3 -doped fiber laser can be wavelength tuned by changing the fiber length. When the fiber length is elongated re-absorption of the signal light will increase leading to red-shift of laser wavelength. This tuning method is simple and convenient for manipulating but the tuning range is limited. The broadest tuning wavelength spanning is less than 100 nm 54-55 . The most dominated shortcoming of this wavelength tuning technique is that laser wavelength cannot be tuned continuously. In the tuning process the replacement of fiber requires re-adjusting the laser cavity complicating the tuning work. This tuning method has little potential in practical applications. 2. Birefringence-tuning. This wavelength tuning method is based on changing the birefringence characteristic of the signal light in the cavity. By using a birefringence filter the Tm3 -doped fiber laser has been tuned over a 200 nm spectral range 56 . Although this method can provide a wide tuning range the tuning laser configuration is rather complicated and very inconvenient for tuning. Besides this technique is confined by the free-spectral range of the birefringence filter. Therefore this method is far from practical application. 3. Temperature-tuning. Due to the circumstance-field impact the ground-state level of Tm3 ions is Stark splitted into many sub levels. As one of the Stark sub levels the lower laser level has a population distribution significantly influenced by the circumstance temperature according to the Boltzman distribution . This leads to the wavelength shift with temperature. Electrical oven has been used to heat the Tm3 -doped fiber laser for wavelength tuning 57 and a tuning 426
