Laser direct lithography of large-area three-dimensional integrated photonics: Technological challenges and advances

Integrated optics have been stuck in two-dimensional (2D) topologies for decades until the femtosecond laser direct writing (FLDW) technique enables direct lithography of three-dimensional (3D) geometries and nanoscale structures with rapid prototyping and large-scale manufacturing capabilities in a variety of transparent substrates. The 3D capability of FLDW makes diverse lightwave remapping geometries possible, thereby realizing efficient interconnection of optical systems at different spatial scales, offering a 3D integrated optics footprint capable of scaling a benchtop optical system down to a 3D glass chip. This work summarizes the history and important milestones in developing FLDW waveguides. Basically, all revolutionary improvements in waveguide key performance, including low propagation loss and small bending radius, were accompanied by the discovery and development of new mechanisms for laser-induced refractive index modification. At the same time, advanced laser beam shaping methods for tightly focused spatiotemporal fields have been technically grafted onto the fine control of laser-matter interaction in FLDW, notably achieving variable cross-section, arbitrary refractive index and mode field distribution, thus providing new degrees of freedom beyond the limitations of traditional 2D planar waveguides for more complex photonics circuit design. In this work, we present a comprehensive review of the field, encompassing fundamental mechanisms (such as refractive index modification) as well as key technological advances that enable true 3D integration. On basis of this, we summarize the basic integrated waveguide components fabricated by FLDW and point out the prospective challenges and future research directions. Tentative routes towards large-area, ultra-broadband, hybrid multifunctional, all-optical system integration in 3D glass chips are also suggested.