Study on optical transmission amplification characteristics of high-power large mode field fiber.

In order to overcome the limitations of nonlinear effect, optical damage and other physical factors on the expansion of fiber laser power, large mode field fibers are widely used to break these limitations. However, with the increase of fiber core diameter, higher-order modes will be excited to cause the decline of laser beam quality, and the transverse mode will also affect the spectrum and time waveform of small broadband pulses. Therefore, obtaining high power and high beam quality fidelity pulse output is the key technology of large mode field fiber laser system. In this thesis, theoretical and experimental studies are carried out on the influence of transverse mode characteristics of large mode field fiber on the time spectrum characteristics of small broadband pulses and the near-field distribution of output. The influence of transverse mode interference on amplitude frequency effect of phase modulated pulse is demonstrated for the first time, and the transmission and amplification characteristics of small broadband pulse and mode field are studied. Based on this, an all fiber amplifier system with mJ level near fundamental mode output is designed.

Firstly, the analytical and numerical methods of mode field distribution in large mode field passive optical fiber are studied. Based on this theoretical model, it is concluded that the mode field distribution of active optical fiber can be approximated to the eigenmode field distribution of passive optical fiber when the gain of active optical fiber is less than 100dB/m. Based on the characteristics of mode field transmission and distribution, a theoretical model of transverse mode interference is established. The effects of inter mode interference on the near field distribution and the time spectrum characteristics of phase modulated pulses in large mode field fiber amplifiers are studied for the first time. At the output end face of a large mode field fiber amplifier, the phase difference between modes corresponding to different wavelengths is different and the polarization of higher-order modes at different end face positions is different. 

After multiple mode interference, the spectral transfer functions at different positions of the fiber end face are different, which makes the phase modulated pulses output by the fiber have spatio-temporal distribution characteristics. Due to the orthogonality of the modes, the statistical result of the pulse modulation of the whole end of the fiber is zero. It is the first time to analyze the effect of multimode interference on the time-frequency spectrum characteristics of phase modulated pulse. It is concluded that the more modes, the smaller the modulation period of the spectral transfer function, and the greater the impact on the amplitude frequency effect of phase modulated pulse. This result has a beneficial guiding role for experimental measurement, and puts forward higher requirements for mode control in the design of all fiber system with small broadband fidelity transmission amplification. Combined with the rate equation and Ginzburg Landau equation, the change of time spectrum characteristics in the process of broadband pulse amplification is analyzed. It is concluded that the main factor causing the pulse distortion is the gain saturation effect, which can be effectively suppressed by pulse pre shaping or using Gaussian pulses. However, the nonuniformity of its gain does not greatly affect the amplitude frequency effect of phase modulated pulses, which is about 3%.

The theoretical calculation results show that when the power of the signal light is amplified to several MW high power, the high power laser will cause mode field distortion and self focusing characteristics, and the high-order mode cannot improve the self focusing threshold of the fiber. For the mode control technology of step large mode field optical fiber, theoretical analysis and numerical calculation results show that gain mode filtering is a simple and effective mode control method that can reduce the distortion of broadband pulse. The use of mode field adapter can effectively avoid the excitation of higher-order modes and the effect of inter mode interference, and reduce loss. For bending mode selection, a new tapered fiber winder is proposed based on the principle of mode matching.

Finally, based on the theoretical and experimental results of this paper, an all fiber amplifier system with mJ level near fundamental mode output is designed, and preliminary experimental verification is carried out.