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

光纤布拉格光栅（FBG）自40多年前在加拿大发明以来，已广泛应用于通信、光纤激光器和光纤传感器等领域。为了实现更多的功能，人们在FBG结构中引入了许多设计，如取样FBG、相移FBG和啁啾莫尔FBG等，但这些设计都是沿光纤轴向沿着改变FBG结构。随着高功率光纤激光器和传感器的发展，需要更复杂的功能来控制不仅是波长，但也偏振和模式内容等。在本申请中，我提出了二维FBG，其中附加的FBG结构设计在横向方向，可以被称为横向分布的FBG。利用可以在相位掩模上蚀刻小至300 nm的凹槽的可用光刻技术，在光纤芯的尺寸内具有许多波纹相位掩模变得可行;例如，对于通信光纤为8微米，对于大模面积光纤为20微米。通过这种增加的自由度，横向分布的FBG可以实现更多的功能。举例来说：1）如果在纤芯中制作几个不同波长的π相移FBG，但横向分开，则可以实现稳定的多波长激射，因为每个波长的π相位被保留。2)如果中心FBG制作在LP 01模波长处，两侧FBG制作在LP 11模波长处，则如果输出耦合器FBG主要反射LP 01波长，则该装置可以用于减少高阶模。3)如果中心光纤光栅和两侧光纤光栅在三个不同的波长，它可以用来同时测量多个参数，如应变，温度和弯曲。这些只是几个例子。在本提案中，将介绍横向分离光纤光栅的概念和设计，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.