基于纯相位空间光调制器的近紫外飞秒多光束直写体布拉格光栅的研究

Femtosecond laser internal structuring of transparent materials presents unique abilities for localised refractive index modifications through nonlinear multiphoton absorption (MPA), thus allowing fabrication of various photonic devices with simplicity, flexibility and 3D capability. However, with commercial kilohertz Ti:Sapphire femtosecond laser systems, the refractive index modification process requires only pulses with sub-microjoule level energy, whereas, mili-joule pulse energy is generally available, resulting in low throughput with a light utilisation factor of < 0.1%. In order to increase the throughput and efficiency, multiple NIR beam parallel processing with a Spatial Light Modulator (SLM) is proposed. While for some applications, such as direct writing of Volume Bragg Gratings (VBGs) in PMMA and fused silica, multiple NUV beams are desired. However, presently, no SLM based on nematic liquid crystals operates in the UV near due to photodegradation of the liquid crystal or the polymer alignment layer with such high photon energy. By placing a thin BBO nonlinear crystal immediately after an SLM addressed with CGHs, the first order diffracted NIR components at 775 nm can be converted to parallel second harmonic NUV beams at 387 nm, avoiding this potential problem while simultaneously reducing the order of non-linear absorption for Δn structuring and also increasing processing efficiency and speed. In order to obtain high quality volume gratings, a series of optimisation techniques are adopted in this project. Filaments that affect the axial length of refractive index modification are carefully controlled to maximise their length while avoid nonlinear self-focussing. Writing asymmetry due to pulse front tilt is also studied and eliminated. Furthermore, uniformity of multiple beams is improved greatly by breaking symmetry of the multiple spots pattern. This novel technique not only can be applied to rapid fabrication of VBG, but also has potential applications in many other areas.

飞秒激光直写技术通过多光子吸收机制可以深入材料内部在介观尺度上实现方便、灵活的三维光子集成器件的微加工。但在实际加工中仍然存在效率低、速度慢的问题。通常使用的飞秒激光输出脉冲能量≥1mJ，而加工仅用微焦级能量，实际激光效率<0.1％。使用空间光调制器 (SLM)可以通过分光，实现近红外飞秒多光束直写。但是，当在PMMA和石英中加工体布拉格光栅(VBG)时，近红外多光束不能满足要求。而SLM不能够承受高峰值强度的紫外光照射，不能直接分解紫外光。本项目通过把BBO晶体放置于纯相位SLM之后，先进行光束分解，保持光束的偏振方向不受影响，再进行倍频得到紫外多光束，并对影响加工的因素，如对飞秒脉冲在材料内部成丝长度、倍频多光束能量分布均匀性以及波前倾斜效应进行控制，在PMMA和石英材料中直写VBG，可以显著提高加工效率和速度、简化工艺、同时获得高稳定性。这种独特的工艺也可应用于其它领域。

本项目使用飞秒激光直写技术在透明光学材料内部方便、灵活的实现介观尺度上的三维光子集成器件微加工。由于通常使用的飞秒激光输出脉冲能量≥1mJ,而加工仅用微焦级能量,实际激光效率<0.1%。使用空间光调制器 (SLM)可以通过分光,实现近红外飞秒多光束直写。但是,当在 PMMA 和 石英中加工体布拉格光栅(VBG)时,近红外多光束不能满足要求。而 SLM 不能够承受高峰值强度的紫外光照射,不能直接分解紫外光。在本项目中，通过把 BBO 晶体放置于纯相位 SLM 之后, 先进行光束分解,保持光束的偏振方向不受影响,再进行倍频得到紫外多光束,并对影响加工的因素,如对飞秒脉冲在材料内部成丝长度、倍频多光束能量分布均匀性等因素进行控制,在 透明材料中直写VBG,从而显著提高了加工效率和速度、简化工艺、同时获得高稳定性。这种独特的工艺也可应用于其它领域。