利用Kagome光纤实现多芯光子晶体光纤的输出合束

The beam combination of multi-core photonic crystal fiber by using the Kagome fiber

多芯光纤的输出光束只能在远场和焦点附近实现良好的同相位超模合束,这种超模传输特性大大影响了多芯光纤的应用范围.一种新型中空Kagome光纤为解决这一难题提供了可行的方案,利用中空Kagome光纤可以实现七芯光纤输出模式的整形合束.本文利用中心波长800 nm的钛宝石飞秒激光作为激光源,耦合入七芯非线性光子晶体光纤,得到700 nm至1050 nm的展宽光谱,并实现同相位超模输出.随后,将非线性展宽之后的宽谱七芯光束耦合至Kagome光子晶体光纤中,从Kagome光纤输出光斑呈高斯分布的模式传输,不再演变回七芯模式,耦合效率71%.实验还进一步验证此方法适用于不同结构的多芯光纤,为多芯光纤在高功率激光等领域的应用提供了参考.

Using multi-core photonic crystal fiber （PCF） has advantages of large-mode-area that can support high beam intensity and disperse heat. However, only when the beam profile in far-field and the focal point of in-phase super mode is Gaussian-shaped, the energy can be more concentrated as compared with other shapes. And this beam profile feature limits the applications of multi-core PCF. With the development of optics, there is a practical solution to improve the beam profile of multi-core PCF in which a Kagome fiber is used. This solution is to couple the in-phase super mode source （obtained from multi-core PCF） into Kagome fiber to achieve the beam combination of multi-core photonic crystal fiber, i. e. the beam profile remains to be Gaussian-shaped at any location in the optical field. The Kagome fibers have a novel hollow structure and thus will show some new properties, such as broad optical transmission bands with relatively low loss, no detectable surface modes, and high confinement of light at the core, and these features are suitable for beam combination. In this paper, a Ti：sapphire femtosecond pulsed fiber oscillator, with its center wavelength of 800 nm and output power of 550 mW, is used to pump a piece of seven-core nonlinear PCF, with an efficiency of 19%. EFL of the coupling lens is 18.40 mm and the NA is 0.15. Then the in-phase super mode source can be obtained from the 15 m multi-core PCF, with a broadband spectrum from 700 to 1050 nm. The beam profile of far field and the focal point of in-phase super mode is Gaussian-shaped and there is a seven-core-shaped pattern at nearfield and other locations in the optical field. In order to combine the beams of multi-core fiber, the in-phase super mode source is coupled into a piece of Kagome fiber, 10 cm long, by using the coupling lens whose EFL is 13.86 mm. Its coupling efficiency is 71~ and the output beam profile remains to be Gaussian-shaped at any locations in the optical field; this means that there is no seven-core-shaped pattern. It also transmits broadband spectrum with low loss. Moreover, this experiment also proves that the solution can be used for different multi-core PCFs and can have a higher coupling efficiency, 80%. Thus a reference can be given for high power applications of multi-core PCF, and inspiration may be given to some other frontier, fields in fiber optics.