Jinfeng Mu

Low-Loss Highly Tolerant Flip-Chip Couplers for Hybrid Integration of Si 3 N 4 and Polymer Waveguides

In this letter, low-loss and highly fabrication-tolerant flip-chip bonded vertical couplers under single-mode condition are demonstrated for the integration of a polymer waveguide chip onto the Si₃N₄/SiO₂ passive platform. The passively aligned vertical couplers have a lateral misalignment between polymer and Si₃N₄ waveguide cores of ±1.25 μm. Low-loss operation has been experimentally demonstrated over a wide spectral window of 1480-1560 nm, with measured coupler losses below 0.8 dB for Si₃N₄ taper angles below 1.2°, in good agreement with the calculated values. Furthermore, thermal shock test results show less than 0.1 dB degradation, indicating a robust coupling performance.

Design and length optimization of an adiabatic coupler for on-chip vertical integration of rare-earth-doped double tungstate waveguide amplifiers

The integration of rare-earth doped double tungstate waveguide amplifiers onto passive technology platforms enables the on-chip amplification of very high bit rate signals. In this work, a methodology for the optimized design of vertical adiabatic couplers between a passive Si3N4 waveguide and the on-chip amplifier is proposed. The methodology shows high efficiency and tolerance in the adiabatic coupler design. The calculated losses of the adiabatic coupler are as low as 0.5 dB for 0.98 μm and 0.14 dB for 1.55 μm. The length of the taper is quantitatively optimized with an adiabatic transfer.

Design and fabrication of adiabatic vertical couplers for hybrid integration by flip-chip bonding

Rare-earth ion doped crystalline potassium double tungstates, such as KY(WO4)2, KLu(WO4)2 and KY(WO4)2, exhibit many properties that make them promising candidates for the realization of lasers and amplifiers in integrated photonics. One of the key challenges for the hybrid integration of different photonic platforms remains the design and fabrication of low-loss and fabrication tolerant couplers for transferring light between different waveguides. In this paper, adiabatic vertical couplers realized by flip-chip bonding of polymer waveguides to Si3N4 devices are designed, fabricated and tested. An efficient design flow combining 2D and 3D simulations was proposed and its validity was demonstrated. The vertical couplers will ultimately be used for the integration of erbium doped KY(WO4)2 waveguides with passive platforms. The designed couplers exhibit less than 0.5 dB losses at adiabatic angles and below 1 dB loss for ±0.5 μm lateral misalignment. The fabricated vertical couplers show less than 1dB losses in average for different adiabatic angles of Si3N4 tapers, which is in good quantitative agreement with the simulations.

A novel polishing stop for accurate integration of potassium yttrium double tungstate on a silicon dioxide

Rare-earth ion doped potassium yttrium double tungstate, RE:KY(WO4)2, is a promising candidate for the realization of on-chip lasers and amplifiers. Two major bottlenecks difficult the realization of compact, high-contrast devices. Firstly, the crystal can only be grown on a lattice matched substrate, leading to a low (<2×10-2) refractive index contrast between core and cladding. Secondly, the required thickness for the high-index contrast waveguides, ~1 μm, makes a lapping and polishing approach very challenging. In this work we propose a novel polishing stop that will permit to accurately control the final thickness of the KY(WO4)2 waveguide within a few tens of nanometers. A 1 mm thick KY(WO4)2 substrate is flip-chip bonded with an adhesive layer onto a SiO2 substrate. Afterwards a low temperature pulsed laser deposited (PLD) Al2O3 layer - with the desired final thickness of the KY(WO4)2 waveguide core - is deposited on top of the assembly. The sample is then thinned using a multistep lapping and polishing procedure. Earlier work with a polishing stop made from SiO2, showed a decrease of the polishing speed with a factor 3-4, allowing the termination of the process within a tolerance of a few tens of nanometers.

Ultra-low-loss and broadband mode converters in Si3N4 technology

Si3N4 grown by low pressure chemical vapor deposition (LPCVD) on thermally oxidized silicon wafers is largely utilized for creating integrated photonic devices due to its ultra-low propagation loss and large transparency window (400 nm to 2350 nm). In this paper, an ultra-low-loss and broadband mode converter for monolithic integration of different materials onto the passive Si3N4 photonic technology platform is presented. The mode size converter is constructed with a vertically tapered Si3N4 waveguide that is then buried by a polymer or an Al2O3 waveguide. The influence of the various design parameters on the converter characteristics are investigated. Optimal designs are proposed, in which the thickness of the Si3N4 waveguide is tapered from 200 nm to ~40 nm. The calculated losses of the mode converters at 976 nm and 1550 nm wavelengths are well below 0.1 dB for the Si3N4-polymer coupler and below 0.3 dB for the Si3N4-Al2O3 coupler. The preliminary experimental results show good agreement with the design values, indicating that the mode converters can be utilized for the low-loss integration of different materials.

Fabrication of high-contrast waveguide amplifiers in erbium doped potassium double tungstates

High-contrast waveguides in crystalline potassium double tungstates pave the road towards compact and efficient on-chip amplifiers. In this work, the design and fabrication of erbium doped high contrast potassium double tungstates waveguides will be described.