Abstract
Integrated photonics is attracting considerable attention and has found many applications in both classical and quantum optics, fulfilling the requirements for the ever-growing complexity in modern optical experiments and big data communication. Femtosecond (fs) laser direct writing (FLDW) is an acknowledged technique for producing waveguides (WGs) in transparent glass that have been used to construct complex integrated photonic devices. FLDW possesses unique features, such as three-dimensional fabrication geometry, rapid prototyping, and single step fabrication, which are important for integrated communication devices and quantum photonic and astrophotonic technologies. To fully take advantage of FLDW, considerable efforts have been made to produce WGs over a large depth with low propagation loss, coupling loss, bend loss, and highly symmetrical mode field. We summarize the improved techniques as well as the mechanisms for writing high-performance WGs with controllable morphology of cross-section, highly symmetrical mode field, low loss, and high processing uniformity and efficiency, and discuss the recent progress of WGs in photonic integrated devices for communication, topological physics, quantum information processing, and astrophotonics. Prospective challenges and future research directions in this field are also pointed out.
Highlights
Photonic integrated circuits have shown the potential to allow integrating passive and active optical components on one chip in a scalable manner and have been identified to support a plethora of applications, such as data communication, sensing, astrophotonics, quantum information processing, and national security.[1,2,3,4] Photonic circuits are indispensable components in modern optical communication networks and lie at the heart of integrated photonic devices
Though great progress has been made in developing improved techniques and achieving device applications, besides the aforementioned issue, there exist some potential challenges for the practical applications of fs laser writing WGs
Due to the relatively small refractive index change induced by fs lasers, reducing the bend loss is still a great challenge, and the effective working radius with reasonable performance is generally greater than 20 mm
Summary
Photonic integrated circuits have shown the potential to allow integrating passive and active optical components on one chip in a scalable manner and have been identified to support a plethora of applications, such as data communication, sensing, astrophotonics, quantum information processing, and national security.[1,2,3,4] Photonic circuits are indispensable components in modern optical communication networks and lie at the heart of integrated photonic devices. The transverse writing geometry with the writing direction perpendicular to the laser propagation axis can realize maximum degrees of processing flexibility, these effects intrinsically lead to an elliptical cross-section in WGs and usually cause high asymmetry in the mode field of the written WGs along with high coupling loss and propagation loss.[15,27,28] It is a great challenge to maintain the WG circularity, symmetry, and mode-field profiles, which would limit the coupling loss, propagation loss, and 3D capability of the FLDW technique These issues will be more serious for the deep WGs in glass. We will review the state-of-the-art progress of photonic applications in these fields with WGs written by fs lasers
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