Tunable Fiber Laser Based On Mach-Zehnder Interferometer

Recently, tunable fiber lasers have attracted researchers attention because it plays important roles in high-resolution spectroscopy, optical communication and optical sensing, To date, various methods have been proposed to achieve wavelength tuning, including using Fabry-Perot filter or acoustic-optical modulator. However, these common tunable devices always have complex structures and great insertion losses. If the laser cavity consists entirely of optical fiber and fiber-based components, the laser could, in principle, have lower loss and simple structure. To obtain an all-fiber laser, all-fiber devices are applied, such as fiber gratings, but most of fiber gratings produced on the market are used in communication, so it is very difficult to buy 1064nm fiber gratings. In recent years. Mach-Zehnder interferometer has attracted much attention because of its simple structure, low cost and all-fiber structure.In this paper, a continuoully tunable all-fiber laser baded on single-pass Mach-Zehnder interferometer and a tunable passive mode-locked fiber laser based on double-pass Mach-Zehnder interferometer are studied experimently, the main works are as follows:1. Begining with the Maxwell equations, we derived the optical pulse propagation in optical fiber--generalized nonlinear Schrodinger equation, explained the theory of Mach-Zehnder interferometer, and introduced basic methods solving the light pulse transfers in a fiber transmission--split-step Fourier transform method.2. A tunable double-clad Yb-doped fiber laser, which utilizes single-pass Mach-Zehnder interferometer which consists of two 3 dB couplers as the tuning component, is proposed. When pump power is 1.8 W, a continuously tunable lasing wavelength covering the range of from 1065 nm to 1108 nm has been realized. The lasing output power is more than 9 mW with the laser side mode suppression ratio higher than 42 dB.3. In order to improve the tunability of the laser, and expand it’s application, we studied a tunable Yb-doped double-clad fiber laser again using double-pass Mach-Zehnder interferometer instead of the single-pass Mach-Zehnder interferometer. The laser is continuously tunable from 1064 nm to 1104 nm. By adjusting polarization controller which is placed in one arm of the double-pass Mach-Zehnder interferometer, the center of the wavelength and the spacing between wavelengths are both to be tunable. Laser side mode suppression ratio is 45 dB with 3 dB spectral width of less than 0.2 nm. The laser slope efficiency of 1068 nm,1082 nm and 1098 nm are 23%,32% and 26%, respectively. By further adjusting the state of polarization controller and pump power, multi-wavelength laser output and tunable multi-wavelength are both observed.4. Using split-step Fourier transform method, we numerically studied the whole process of the formation of mode-locked laser besed on carbon nanotube saturable absorber and studied the effection of the parameters such as small signal gain on mode-locked pulses. In experiment, we achieved stable self-starting mode-locked laser output, then achieved the all-fiber tunable mode-locked fiber laser by using of carbon nanotubes as saturable absorber and using the single-pass Mach-Zehnder interferometer as the tuning device, laser repetition rate is 6.49 MHz, pulse width is about 1 ps.3 dB spectral width is 3.3 nm, when the time-bandwidth product is 0.412, the adjustable range is from 1549.6 nm to 1566.3 nm.