Active sources such as lasers and amplifiers are essential components for photonic integrated circuits (PICs) serving a myriad of applications. Silicon nitride (Si3N4) photonics, due to its low propagation loss (~0.1 dB/cm), wide transparency window (~400 nm to 2.35 µm) and good CMOS compatibility, is found to have increasing number of applications in the fields such as microwave photonics, nonlinear photonics, bio-sensing, and in technology towards autonomous driving like lidar and gyroscopes. In most applications, integration of active components onto the passive Si3N4 platform is required. The integration of III-V semiconductor optical amplifiers (SOA's) and Si3N4 by butt-coupling has led to external cavity lasers of exceptional performance. However, sophisticated assembly and packaging steps with high precision alignment are required, which significantly limits cost-effective scaling of manufacturing.
Monolithic integration of rare-earth-ion doped (RE3+) materials such as Al2O3, Ta2O5 and Te2O3 with Si3N4 show a promising approach to achieve wafer-scale active components. As compared with III-V semiconductors, RE3+ doped materials have a longer excited-state lifetime (0.1-10 ms) and less refractive index change (~10-6) induced by the excitation of the doped ions, which are beneficial for thermally and spatially stable gain.
These materials can be doped with different ions such as Nd3+, Yb3+, Er3+, Tm3+ and Ho3+ to achieve emission at different wavelengths by commonly used deposition techniques. Demonstrations have been carried out for high-speed and high internal net gain optical amplifiers and lasers. The proposed integration methodologies involve either the direct deposition of the active material onto Si3N4 elements or the sputtering into SiO2 trenches within the Si3N4 platform. Unfortunately, this inevitably increases the overall propagation loss of the photonic circuit as the active layer remains on the cladding.
To overcome this limitation, Prof. Sonia García-Blanco's team from the MESA+ Institute for Nanotechnology of the University of Twente, The Netherlands, has developed a waveguide amplifier monolithically integrated onto the Si3N4 platform via a double-layer photonic platform that supports wafer-scale manufacturing. It was published in Photonics Research, Vol. 8, Issue 10, 2020 (Jinfeng Mu, Meindert Dijkstra, Jeroen Korterik, et al. High-gain waveguide amplifiers in Si3N4 technology via double-layer monolithic integration[J]. Photonics Research, 2020, 8(10): 10001634).
Schematic of optical waveguide amplifier die via monolithic double-layer integration platform. The cyan part indicates Si3N4 platform and the green part indicates the rare-earth-ion doped gain medium.
In this approach, the active layer, i.e., Al2O3:RE3+, and the Si3N4 waveguides are located in two individual layers separated by a thin SiO2 film. The transfer of modes between the two layers occurs via vertically tapered adiabatic couplers that exhibit low loss, broadband behavior and high tolerance to overlay errors.
Based on the developed amplifier technology, Prof. Sonia García-Blanco believes that rare-earth ion doped materials and in particular, rare-earth ion doped Al2O3, can provide high net amplification to the Si3N4 platform, paving the way to the development of different active devices (i.e., amplifiers and lasers) monolithically integrating in this integrated photonics passive platform.
The work is impressive, particularly showing the compact spiral with gain on the double-layer active-passive integration platform. Moreover, the quality of fabrication and device performance are all excellent. Thus, this hybrid integration is important and attractive in the photonic field.
