Laterally-distributed fiber Bragg gratings for fiber laser and fiber sensing applications

光纤布拉格光栅(FBG)自40年前在加拿大问世以来，在通信、光纤激光器和光纤传感器等领域得到了广泛的应用。为了实现更多的功能，人们在FBG结构中引入了许多设计，如采样FBG、相移FBG和啁啾莫尔FBG等，但这些设计都是沿光纤轴线变化的。随着高功率光纤激光器和传感器的发展，人们不仅需要更复杂的功能来控制波长，还需要控制偏振和模式含量等。在这种授权应用中，我提出了二维FBG，其中附加的FBG结构被设计在横向上，可以称为横向分布的FBG。随着现有的光刻技术可以在相位掩模上刻蚀出小至300 nm的沟槽，在光纤芯的尺寸内拥有许多波纹相位掩模成为可能；例如，对于电信光纤来说，8微米的波纹相位掩模，对于大模面积的光纤，20微米。利用这种增加的自由度，横向分布的FBG可以实现更多的功能。例如：1)如果在纤芯中制作多个不同波长的pi相移FBG，但由于每个波长的pi相保持不变，因此可以实现稳定的多波长激光。2)如果中心FBG为LP01模波长，两侧FBG为LP11模波长，则在输出光纤布拉格主要反射LP01波长的情况下，该器件可用于降低高次模。3)如果中心FBG和两侧FBG在三个不同的波长下制作，则可以同时测量应变、温度和弯曲等多个参数。这些只是几个例子。文中将介绍横向分离FBG的概念和设计，并描述其在光纤激光器和传感器中的应用。

Fiber Bragg grating (FBG) has been widely used in telecommunication, fiber laser and fiber sensors since its invention in Canada about 40 years ago. In order to achieve more functions, many designs have been introduced into the FBG structure, such as sampled FBG, phase-shifted FBG and chirped Moire FBG etc. However those designs all vary the FBG structure along the optic fiber axis. With the advancement of high power fiber lasers and sensors, more sophisticated functions are required to control not only the wavelength, but also polarization and mode content etc. In this grant application, I propose two dimensional FBGs, in which additional FBG structures are designed in lateral direction that can be called laterally-distributed FBG. With the available photolithography technology that can etching a groove as small as 300 nm on a phase mask, it becomes feasible to have many corrugated phase masks within the dimension of the fiber core; for example 8 micrometer for telecommunication fiber and 20 micrometer for large-mode-area fiber. With this added degree of freedom, the laterally-distributed FBG can achieve much more functionality. For example: 1) If several pi-phase shifted FBGs of different wavelengths are made in the core but laterally-separated, it can realize stable multi-wavelength lasing since the pi-phase for each wavelength is preserved. 2) If the central FBG is made at the LP01 mode wavelength and two side FBGs at LP11 mode wavelength, the device can be used to reduce the high order mode if the output coupler FBG reflects mainly LP01 wavelength. 3) If the centre FBG and two side FBGs are made at three different wavelengths, it can be used to measure multiple parameters such as strain, temperature and bending simultaneously. Those are just a few examples. In this proposal the concept and designs of the laterally-separated FBGs will be presented and their applications in fiber lasers and sensors will be described.