Single-Core-Multi-Core 3D Photonics Chip Processing

A fan-in-fan-out module with multicore, few-mode fiber is a high-density fiber module where a multicore fiber positions at one end and multiple single-mode (SM) fibers at the other end, with a three-dimensional photonics chip in the middle to enable single-core-multiple-core conversion. The photonics chip utilizes stereoscopic waveguide technology to achieve efficient optical coupling of multicore and single-mode fibers with low insertion loss, low polarization dependent loss, and low crosstalk. It thus enables high-density mode-division multiplexed fiber optical communications and increased-capacity distributed fiber optic sensing applications.

Optical waveguides are the underlying structure of fan-in-fan-out 3D photonic chips. Their waveguide transmission characteristics can effectively eliminate beam divergence, maintaining high optical density and uniform waveguide modes over long transmission lengths. It is conducive to enhancing the interaction between light and waveguide materials and improving the basic optical properties of the substrate. Therefore, the fabrication of low-loss optical waveguide with flexible structures in different optical materials, and hence the realization of multifunctional high-performance waveguide optical devices has been a hot topic for research in photonics chips.

Optical crystal materials are ideal platforms for building multifunctional photonics chips because their diverse lattice structures can exhibit plentiful optical properties. Due to the limitations of the conventional waveguide fabrication process and the complexity of the crystal material structures, it is difficult to construct 3D optical waveguide structures in crystal materials. Nevertheless, the rapidly growing femtosecond laser direct-writing technology in recent years has provided an effective solution.

First, due to the nonlinear absorption mechanism, laser material modification is subject to the volume focused. By scanning, 3D structures that are geometrically complex can be achieved. Second, the material-independent nonlinear absorption process makes it possible to form fine structures such as optical devices in transparent materials. Femtosecond laser nanofabrication is caused by a phenomenon called laser-induced optical breakdown. During this process, the optical energy from a femtosecond laser is transmitted to the processed material, exciting many electrons and causing them to ionize and deliver the energy to the crystal. Subsequently, the material undergoes structural changes causing a permanent change in refractive index, even leaving a hole at the focal point. Using focused femtosecond laser beams interacting with the material, one can induce the refractive index changes in local regions in transparent medium material. Scanning the substrate material enables highly flexible, high-precision, and easy-to-operate three-dimensional nanofabrication, resulting in fabricating structurally flexible and fine photonics devices with channel-structure optical waveguide, allowing the optical properties and integrated characteristics of optical waveguides and crystal materials to be maximized.

This solution scheme uses a femtosecond laser to process optical crystals (including optical glass, lithium niobate crystals, sapphire, etc.) by making the femtosecond pulse tightly focused (using a microscope) inside the optical crystal with a power density of more than several terawatts per square centimeter. It can trigger complex and diverse processes such as simultaneous multiphoton absorption, avalanche, and collisional ionization, resulting in a highly localized modifications, while there is almost no energy deposition. Due to the extremely short pulses of the femtosecond laser, the thermal impact is negligible and hence the process can be called “cold processing”. No cracks are caused so to avoid damage to the optical crystal material. Meanwhile, the three-dimensional nanofabrication is achieved by controlling the position of the laser focus relative to the optical crystal with a high-precision (nano-scale precision) three-dimensional operating stage.