No abstract

A high-power ultraflat near-infrared supercontinuum (SC) is generated in a section of photonic crystal fiber (PCF) pumped by an amplified spontaneous emission (ASE) source instead of continuous-wave (CW) and pulsed lasers. A low-power ASE seed at 1 m is amplified to be 90.9 W by two fiber amplifiers and then emitted from a 10-m-core fiber. Using this ASE source to pump a section of 100-m-long PCF, a 49.5-W near-infrared SC is obtained, and the 5-dB spectral bandwidth is 760 nm, covering from 1062 to 1822 nm. This is the reported highest power of ASE-pumped SC source. A comparative experiment is taken with a 122-W CW laser at 1090 nm to pump the same PCF. A 56.2-W SC source is generated with 5-dB spectral width of 605 nm from 1082 to 1687 nm. The conversion efficiency to SC is higher, and the spectrum is broader and flatter using the ASE source as the pump. Conclusively, pump incoherence can aid the SC generation and spectral flatness.

The theoretical research of supercontinuum (SC) generation in a fiber amplifier system has been seldom reported. For the purpose of further understanding the mechanism of SC generation in fiber amplifiers, we propose a combined numerical model of the laser rate equations and the generalized non-linear Schrödinger equation to simulate the amplification of 1060 nm picosecond pulses and their spectral broadening in an ytterbium-doped fiber amplifier. The calculation results of this model are compared with the experimental results under the same conditions and a good agreement is achieved. We find that the pulse is gain amplified initially, and then dominated by stimulated Raman scattering in the normal dispersion region. In anomalous dispersion region, modulation instability, higher-order soliton fission and soliton self-frequency shift dominates the spectral broadening. It is found numerically and experimentally that the length of the gain fiber and the 976 nm pump power are the most imperative parameters to control the output power, spectral range and flatness of the SC. The pulse width of signal pulse also plays a part in influencing SC generation. The results verify that our model is promising for analyzing the physical processes of pulse evolution and SC generation in a fiber amplifier system.

A 101 W all-fiber supercontinuum soure is demonstrated by seeding a piece of photonic crystal fiber with a high-power pulse-repetition-rate-tuning picosecond fiber laser. By performing a series of comparative experiments, influences of the pump pulse repetition rate and the photonic crystal fiber length on the supercontinuum generation process are investigated in detail. How to improve the power level of the supercontinuum soure is analyzed and discussed. The research results will make a contribution to the further development of high-power supercontinuum soure.

We numerically calculated the coherence properties of the supercontinuum based on the definition of complex degree of mutual coherence, and obtained the spectral broadening of the pump pulse and the coherence properties of the generated supercontinuum under different pump powers. The results showed that the dominant underlying mechanisms of the spectral broadening are soliton self-frequency shift and dispersive wave generation when the pump wavelength located in the anomalous dispersion region of photonic crystal fiber. While the coherence properties are affected by the modulation instability which amplifies the stochastic noise of the pump pulse, thus the generated frequency components have stochastic phases and amplitudes, which induces the degradation of the coherence of the supercontinuum. The gain of modulation instability increases with the increasing of the pump power, and the effect of the noise on the supercontinuum will become greater which degrades the coherence properties. Low pump power is required to obtain high coherent supercontinuum, and suitable pump power should be adjusted to get wide bandwidth and high coherent supercontinuum.