Font Size: a A A

Yb-doped Double-clad Doped Fiber Amplifier And Laser Beam Of Coherent Synthesis Of Basic Research

Posted on:2008-06-13Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z C GuoFull Text:PDF
GTID:1118360218455642Subject:Optics
Abstract/Summary:PDF Full Text Request
From the day of the birth of laser, the laser system with high output power and with high quality has been a constant goal of man's pursuit, due to it's significant application value in many fields such as science research, industrial production, military equipments and etc. In recent years the laser beam coherent combination (or called coherent addition technology) has been a vital approach to realize the goal and has been a reseach highlight. In this paper, we undertook fundamental research of this technology by use of Yb3+-doped double clad fiber, combining the technology with the cladding-pump technology. Through our work we got a series of results, which laid an important foundation for further work.The main contents in this paper are as follows:1. We reviewed the history and the research current situation of the technology, and introduced the latest domestic and international proceedings of the technology. We introduced and discussed the technology's main catalogues and their individual key characteristics of accomplishing coherent output. Then we introduced the cladding-pump technology about double clad fibers, and pointed out that the Yb3+-doped double clad fiber laser has its irreplaceable advantages in the research and application of laser beam coherent combination due to its inherent properties such as compact structure for convenient addition, high beam quality and etc., from which the paper's conception is.2. Based on the model of Yb3+-doped double clad fiber amplifier including amplified spontaneous emission(ASE) brought forth by D.C.Jones, etc., we deduced the solution of the amplifier's amplified signal light, launched pump light, ASE and number density in upper energy level under the stability condition, and then we simulated the effect of the amplifier's structure parameters on its output characteristics. We obtained the distribution along the fiber of launched pump light, amplified signal light, ASE forward and backward. We obtained the relation between amplified signal light and launched pump light. We obtained the spectra of the amplifier's output and ASE suppression.3. Built on the project of a self Fourier cavity to achieve coherent combination brought forth by Corcoran, etc., we used the parameters of a certain type of Yb3+-doped double clad fiber in our laboratory and the operation wavelength to design a one-dimensional self Fourier cavity and simulate the amplitude distribution of the output of all the fiber lasers on the input plane of the cavity.4. We studied a large-mode-area(LMA) Yb3+-doped double clad fiber amplifier. As for backward pump way, we demonstrated that there exists energy shift from forward launched pump light to amplified signal light and ASE whether with signal light or without signal light, higher output power of the amplifier requires a backward pump way. The introduction of signal light enhances the light to light conversion efficiency of the system through the suppression of ASE. The coherence of the parallel master oscillator power amplifier (MOPA) project of achieving laser beam coherent combination requires a narrow linewidth of every path amplifier output , however, the laser output with a narrow linewidth under the condition of high power is subject to nonlinear effect in the fiber. After the trade-off between these two factors above, we developed a Yb3+-doped double clad fiber amplifier whose output both preserves excellent spectrum properties of the signal light as well as suppresses effectively stimulated Brillouin scattering(SBS). The linewidth of the amplifier output is around 0.027nm, corresponding to a coherent length of about 4cm, which is enough to coherent operation; corresponding to a frequency spectrum width of 7.2GHz, which is adequate to suppress the SBS(about 100MHz frequency spectrum width ).5. We investigated the polarization properties of Er3+/Yb3+ co-doped double clad fiber amplifier. We studied the variation and the stability of the polarization state of double clad fiber amplifier representing the polarization state by use of the parameter pair of elliptical long-axis azimuth and polarization degree. To ensure good polarization characteristics-high polarization degree and high stability of polarization state of a double clad fiber amplifier, a polarizer has to be inserted in the control loop. In combination with the use of a polarization controller, we can ensure high polarization degree and control polarization orientation. A polarization maintaining fiber is recommended to be used to further improve the amplifier's polarization stability in the loop of signal light and in the amplified gain media. We had nearly uniform conclusions whether for continuous wave operation or for pulse mode-locked operation; the difference lies in that under the condition of pulse mode-locked operation, the polarization degree descends, while the stability of polarization state improves slightly.6. We achieved partly coherent combination in two Yb3+ -doped double clad fiber lasers, combining the improved Michelson cavity configuration and cladding-pump technology and bringing forth adding a delay line structure in one branch arm to optimize the arm length difference of the cavity. Through preliminary experiment, we concluded that coherent combination cannot be realized if the arm length difference of the two gain fibers is too large to have no overlapped spectrum gain area; the output mirror with low reflectivity can enhance output power of the laser system, however, excessive low relectivity can not offer enough feedback to ensure the two arms lasing at the same time, so the selection of the reflectivity of the output mirror requires a trade-off between the two factors; we used three methods including polarization state, spectra and power incremental ratio, mutually verifying, to describe the coherence of the combining light and demonstrated that we have achieved partly coherent combination in two Yb3+ co-doped double clad fiber lasers in the configuration, merely with low combination efficiency; under high power operation the coherence of the combining light will reduce. In combination with the simple estimate of Michelson cavity to obtain coherent combination in the theory part, we could know that this combination method does not suit high power operation and the coherent combination of an array of multitudes of lasers in spite of high coherent combination efficiency.7. We studied multi-wavelength convertible Yb3+-doped double clad fiber laser. With a phase mask, we fabricated the fiber Bragg gratings directly in a multimode Yb3+-doped double-clad fiber as the frequency selective mirror and obtained the results of multi-wavelength output which include: single wavelength output (1060.8 nm), dual wavelength output (1060.6 nm and 1061 nm), triple wavelength output (1059.2 nm, 1060.4 nm and 1061 nm), four wavelength output (1060.7 nm, 1061 nm, 1061.2 nm, 1061.4 nm). Each linewidth above is less than 0.02nm, with a sidemode suppression ratio higher than 20dB. The highest output power of the laser is 6.12W, with the slope efficiency of 40%.8. We designed and achieved a high power Yb3+-doped double clad fiber laser based on wholly domestic components. The laser was end pumped, with a linearity resonator cavity architecture, with direct output from the back end of the fiber. The output power is 12W, with the slope-efficiency of 76.9%. And the fluctuation of output power is less than 0.5% under long-time high-power operation.
Keywords/Search Tags:laser beam coherent combination, fiber amplifier, self-Fourier cavity, improved Michelson cavity, parallel MOPA, polarization state, polarization degree, power incremental ratio, double clad fiber, cladding-pump technology
PDF Full Text Request
Related items