Key issues in high power fiber lasers and amplifiers
|Keywords||All fiber Large mode area fiber Double cladding Diode pump coupler Multi-mode combiner Fiber laser Fiber amplification|
This work is financially supported by the National 863 High Technology Project （N0.2003AA312100）, PLA General Armament Department Pre-research Project （N0. 41501070408）, and Tianjin Science and Technology Emphases Project （N0.05YFGZGX02700）. Based on the investigation of the large mode area （LMA） double-clad fiber, side-pump coupler and multi-mode fiber （MMF） combiner, firstly, several key-problems of the high power double-clad fiber lasers and amplifiers are investigated theoretically and experimentally. The details are described as follows:1. The LMA double-clad fiber, side-pump coupler and MMF combiner are investigated.Utilizing the MCVD technics and the solution doping technology, we obtained the high performance home-made LMA double-clad fiber based on the investigation of the key technics. The effective absorption coefficient of LMA Ytterbium-doped double-clad fiber has been increased largely from 1dB/m to 4dB/m. The numerical aperture of the fiber core has been decreased from 0.15 to 0.07.The side-pump coupler based on the fusion taper-drawing is proposed for the first time. A side-pump coupler integrating all performances of high pump coupling efficiency（74%）, high signal coupling efficiency（95%）, high signal isolation（50dB for the signal between the signal input end and pump input end, 20dB for the signal between the signal output end and pump input end）, high peak-power damnification threshold（>15kW） together, is realized.Based on the taper-drawing and fiber fusing technics, the MMF combiners are studied. The 3x1、6x1 MMF combiners have been demonstrated with a max. 93% coupling efficiency which is adequate for the need of experiment.2. The relative theory of high power Yb3+-doped double-clad fiber laser and amplifier is investigated.Based on the all-spectrum method, the stable-state and evanescence-state rate equation have been established. Using the stable-state rate equation, we study the ion distributing of the upper enrage-level and characteristic of power output. The results indicate the ion numbers of the upper enrage-level is larger in the fiber center than fiber ends. Increasing the fiber length, the laser power improved linearly and then saturated. Based on the evanescence-state rate equation, the amplifying characteristic of different pulse is studied. The results indicate that there are different deformations for the different-figure pulse after amplified. The square pulse and supper- gauss pulse deformed mostly while the gauss pulse and hyperbolic secant pulse deformed less.3. All- fiber Yb3+-doped double-clad fiber laser is studied experimentally.Based on the hydrogen holding double-clad fiber and the phase mask, the FBG for the double-clad fiber laser is realized. Based on the cavity configuration with a FBG for the front mirror and the double-clad fiber back-end, we studied the output characteristic of laser with different fiber length. The experience shows that the laser output power, the pump threshold and the slope efficiency increase with the shorter fiber length. The experimental results are well accord with theoretical results. Based on the experience, a home-made all-fiber double-clad fiber laser with an output power up to 20W is realized.Utilizing the DBR configuration, anYb3+-doped double-clad fiber laser integrating high power （1.48W）, high stability and narrow line-width （0.21nm） together is obtained. The fiber laser wavelength is 1054.5nm. Based on the experience results, an Yb3+-doped double-clad fiber laser with 20W output power is obtained.4. All- fiber pulse Yb3+-doped double-clad fiber amplifier is studied theoretically and experimentally.Using the home-made Yb3+-doped fiber, the side pump coupler has been used in the master oscillation power amplification firstly, and the all fiber pulse fiber amplification has been designed and completed. From the experiments, the output pulse power at different the repetition frequency, and the pulse duration is compressed while the pulse laser have studied. And the correlation between the experiments and the theory is good.The two stage all-fiber Yb3+-doped double cladding fiber pulse amplification in which the side pump couplers have been used has been designed and studied experimentally. The high repetition frequency high power amplified pulse laser has been achieved, which the average output power 2.12W, the pulse duration 20ns, the repetition frequency 50 KHz. And the compact prototype has been made, too.5. All-fiber high-power narrow line-width Yb3+-doped double-clad fiber amplifier is studied experimentally.Using the highly Yb3+-doped fiber and FBG both made by ourselves, the narrow line-width fiber laser with the distributed Bragg reflector （DBR） structure has been studied experimentally. From the experiments, the stable output laser is obtained with center wavelength of 1052.746 nm, and maximal output laser power of 107.8 mW, and 20dB spectrum bandwidth of 0.078 nm, and optical SNR of 76dB, and slope efficiency of 46.3%.Using the side pump coupler, the first stage all-fiber narrow line-width Yb3+-doped double cladding fiber amplification has been studied experimentally. By optimizing amplifier parameters, such as the signal laser powers, and the pump power, and the fiber lengths, the amplified signal laser power exceeds 1.07W, and the gain is 18dB.In the all-fiber narrow line-width Yb3+-doped double cladding fiber amplifier, the exceeding 15W output amplified laser has been achieved firstly. In the two stages amplification experiment by using the MMF combiner and the side pump coupler, the maximum amplified laser power is 15.18W, and the gain is 12dB.