Study On Fiber-Amplifier-Based Supercontinuum Source

The outstanding characterictics of supercontinuum: broad spectrum and high brigtness make it having important amplication prospects in the fields of spectroscopy, biology and medicine, hyperspectral imaging, photoelectric countermeasures and so on. High power supercontinuum can be generated effectively in a fiber amplifier; this method combines the power amplifying and nonlinear effects which makes the system simple and compact and the underlying physical mechanisms complicated. In this thesis, supercontinuum generation based on fiber amplifier is investigated theoretically and experimentally in an all-round way. Additionally, incoherent combining technology of supercontinuum sources is studied and beam quality evaluation method of supercontinuum is proposed.Based on dynamic rate equations and generalized nonlinear Schr?dinger equation, a theoretical model of supercontinuum generation in fiber amplifier is built. This model can simulate the dynamic processes including power amplification and nonlinear spectral and temporal evolution of pulse trains in the gain fiber. Thus the physical process of supercontinuum generation in fiber amplifier can be modeled.Take Ytterbium-doped fiber amplifier for example, the influences of fiber length and signal pulse duration on the superconinuum generation in fiber amplifier are studied theoretically an experimentally, and the requirements of extending the spectrum and augmenting the output power are concluded. The results indicate that longer gain fiber can lower the Raman effect threshold and lengthen the action distance of soliton self-frequency shift; thus the spectrum of supercontinuum is broader while the output power is lower. A setion of passive fiber spliced to the gain fiber can assist the spectral extension and smoothness. However, ultra-long fiber will transfer more power to spectral range longer than 2 μm which can bring large loss and lower the output power, so the lengths of gain fiber and passive fiber should be optimized to ensure the spectral width and the output power. When the signal is ultrashort pulse(femtosecond to picosecond), in the initial phase of spectral broadening self-phase modulation is the main nonlinear effect and the final output spectrum is narrow. When the signal pulse duration is longer than 10 ps, the spectrum of the signal light will be broadened by stimulated Raman scattering effect to anomalous dispersion region and then the spectrum will be extended to form a supercontinuum. The longer the pulse duration is, the wider the spectral range is.Hundred-watts level linearly-polarized supercontinuum source is achieved based on polarization-maintaining fiber amplifier. The influence of the splicing types of polarization-maintaining fibers on the polarization properties of supercontinuum is studied. Finally a supercontnnum with maximum power of 124.8 W is output from a large-mode-area polarization-maintaining fiber amplifier which is the reported highest output power of supecontinuum generated in polarization-maintaining fiber amplifiers. The spctral range of the supercontinuum is from 1060 nm to 1800 nm and the all-spectral polarization extinction ratio is 84.85%.Fiber broadband power combiners are designed and manufactured. Through theoretical study, the fabrication rules of fiber combiner which has high transmission efficiency in a wide spectral range are concluded as: the claddings of the input fibers should all be tapered to be covered by the core of the output fiber and the length of the taper transition region should be long enough(generally longer than 1 cm). Making use of the fabricated fiber broadband power combiner, the principle experiment is carried out and the result verifies the fesibility of the incoherent combination of supercontinuum sources. And then a combined supercontinuum source covering 1060 nm-1600 nm with >300 W output power is achieved. Finally, a fiber broadband power combiner with an output fiber of a core of 100 μm is designed and fabricated; based on this combiner a supercontinuum source is obtained with >200 W output power and the spectral range is from 1060 nm to 1900 nm. The supercontinuum power transmission efficiency is as high as 96.6% and the measured M2 factor is ~9 around 1064 nm.Three beam qulity evaluation factors of supercontinuum source are proposed based on the introduction of power spectral centroid. The spatial transmission model of narrowband laser is extended to deduce the propagation model of supercontinuum source. Using this model the proposed beam quality evaluation method of supercontinuum source is validated. The beam quality of supercontinuum sources with different power and spectral distributions obtaned experimentally are evaluated and compared. The results show that the three evaluation factors can reflect the influence of transmission mode and power spectrum on the beam quality of supercontinuum comprehensively. They are suitable for the evaluation and comparison of the beam quality of supercontinuum as a whole and can be used to guide the obtaining of supercontinuum source with good beam quality and its practical applications.