Master Oscillator Power-Amplifier. Compared with traditional solid and gas lasers, fiber lasers have the following advantages: high conversion efficiency (light-to-light conversion efficiency over 60%), low laser threshold; simple structure, working material is flexible medium, easy to use; high beam quality ( It is easy to approach the diffraction limit); the laser output has many spectral lines and a wide tuning range (455 ~ 3500nm); small size, light weight, good heat dissipation effect and long service life.
However, due to the relatively low output power, its application range has been greatly limited. With the gradual maturity of double-clad fiber and high-power semiconductor laser (LD) manufacturing technology, the output power of fiber lasers has been greatly improved, and its application range has also been greatly expanded. Ultrashort pulse lasers with high power and high beam quality have attractive application prospects in the fields of optical fiber communication, medical, military and biology, and have become one of the current research hotspots.
There are two main ways to obtain ultrashort pulse laser in optical fiber: mode-locking technology and Q-switching technology. Mode-locked pulsed fiber lasers mainly use various factors to modulate the oscillating longitudinal modes in the cavity. When each longitudinal mode has a definite phase relationship and the phase difference between any adjacent longitudinal modes is constant, coherent superposition can be achieved to obtain ultrashort pulses. , the pulse width can reach the order of sub-picosecond to sub-femtosecond. The Q-switched pulsed fiber laser is to insert a Q-switching device in the laser resonator, and realize the pulsed laser output by periodically changing the loss in the cavity, and the pulse width can reach the order of 10-9 s. Using Q-switched or mode-locked technology, very high peak power can be obtained, but the pulse energy obtained by a single Q-switched or mode-locked laser is often very limited, which limits its scope of application. In order to further improve the pulse energy, it is necessary to use amplification technology, that is, the use of main oscillator power amplification (MOPA) structure. The high-energy pulsed laser obtained in the fiber with this structure has the same wavelength and repetition frequency as the seed light source, and the shape and width of the time-domain pulse are almost unchanged. The seed light source with a certain repetition frequency and pulse width is selected as the main oscillator, and the required high-energy pulsed laser output can be obtained after power amplification. Therefore, it is an ideal choice to use the main oscillation power amplification technology to achieve high pulse energy and high average output power.