The monolithic integration of active materials providing optical gain and optical non-linearities is instrumental to increase the functionality of integrated photonic circuits. A double layer integration scheme has been experimentally demonstrated, exhibiting transition losses between the two materials of less than 0.2 dB. Furthermore, a single layer integration scheme is proposed, which decreases the number of process steps, reducing fabrication cost, and also decreasing the sensitivity to fabrication tolerances. In this paper, we will overview our recent developments on the monolithic integration of both, Al2O3 and TiO2 onto the passive Si3N4 and Al2O3 platforms for the development of on-chip amplifiers and lasers.
Amorphous Al2O3 is an attractive material for integrated photonics, providing both active and passive functionalities. Al2O3 exhibits high solubility for rare-earth ions with moderate quenching of luminescence, a wide transparency window (150-7000 nm) and low propagation loss. It is therefore a very attractive material for visible, near- and mid-IR on-chip active devices.
We have developed two different integration procedures to integrate Al2O3 onto passive photonic platforms. A double photonic layer integration scheme permits the low-loss integration of rare-earth ion doped Al2O3 onto the Si3N4 photonic platform. A single photonic layer integration scheme, based on the photonic damascene process, permits the creation of active and passive regions at the same level on a wafer, with the consequent reduction of the number of fabrication steps compared to the vertical integration of two materials. On-chip amplifiers on Si3N4 with more than 10 dB of net gain at 1550 nm as well as the realization of narrow linewidth lasers on active-passive Al2O3 for label-free sensing applications will be discussed.
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.
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.
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.