Fully Integrated Hybrid External Cavity Lasers via Micro-Transfer-Printing

Introduction

Photonic integrated circuits have gained significant traction due to the mature CMOS microelectronics industry and the availability of silicon-on-insulator (SOI) and silicon nitride (SiN) platforms. However, the lack of efficient light sources in silicon due to its indirect bandgap necessitates the integration of III-V materials, such as InP-based gain materials, for applications in the telecommunication wavelength domain. One of the most efficient approaches to integrate InP gain materials into SOI and SiN waveguides is edge-coupling, which provides broad band, polarization-agnostic low coupling losses of less than 1 dB.

This tutorial article presents the first demonstration of a fully integrated hybrid external cavity laser (HECL) in edge-coupling configuration achieved through micro-transfer-printing (μTP). The HECL exhibits single-mode lasing operation with a high side-mode suppression ratio (SMSR) over a wide range of driving currents, making it a promising solution for low-cost and high-volume manufacture of photonic integrated circuits.

Micro-Transfer-Printing Integration

Direct growth of InP-based materials on silicon is extremely challenging due to lattice constant mismatches and compatibility issues. Micro-Transfer-Printing (μTP) offers a promising solution by enabling high-throughput, parallel, and scalable transfer of devices or dies of material to desired locations on any platform with sub-micron accuracy and no waste of III-V material.

Figure 1 – a) Schematics of the coupling between the transfer-printed RSOA and SU8 polymer waveguide and tapers to push light into the SiN waveguide, b) Optical microscope image of the chip showing the transfer-printed RSOA onto the SiN, coupled to SU8 polymer waveguides and SiN waveguides and optical resonators (1D PhCs).

In this work, etched-facet InP reflective semiconductor optical amplifiers (RSOAs) were undercut, released from their substrate, and heterogeneously integrated by μTP onto SiN chips pre-patterned with trenches, SU8 polymer waveguides, SiN waveguides, and SiN optical resonators (1D photonic crystals) via electron beam lithography (EBL). The lateral alignment of the emitting facet of the RSOAs was ensured by μTP accuracies lower than 0.5 μm, and a high adhesion yield of over 90% was achieved using a thin vapor-coated BCB layer.

HECL Operation and Results

Figure 2 – a) Single-mode lasing spectra of one of the measured HECL at different driving currents in CW, b) Single-mode lasing spectra of the HECL in (a) with pulsed driving currents (20 kHz and 0.1% duty cycle).

Electrical and optical characterization of the fully integrated HECLs based on μTP RSOAs coupled to SiN optical resonators was performed to evaluate the laser operation. Figure 2 shows the measured lasing spectra of one of the HECLs in both continuous wave (CW) and pulsed (20 kHz, 0.1% duty cycle) operation.

The HECL exhibits single-mode lasing operation with an SMSR in the range of 30-40 dB, from 40 mA to 65 mA of driving currents in CW operation. The lasing wavelength shifts with increasing driving current due to heating of the gain and chip during operation. In pulsed operation, where heating contributions are greatly reduced, the lasing wavelength stabilizes over the full driving current range investigated, demonstrating the potential for stable single-mode operation.

Conclusion

This work presents the first demonstration of a fully integrated HECL in edge-coupling configuration achieved through micro-transfer-printing. The HECL achieves single-mode operation over a wide range of driving currents, with an SMSR in the range of 40 dB. The integration approach using μTP eliminates the need for expensive and time-consuming active alignment, unlocking the possibilities of low-cost and high-volume manufacture of photonic integrated circuits.

The successful demonstration of fully integrated HECLs via μTP paves the way for further advancements in photonic integrated circuit technology, enabling the realization of high-performance, cost-effective, and scalable photonic devices for various applications, including telecommunications, sensing, and optical computing.

Reference

[1] F. Atar, Y. Arafat, G. Paikkath, A. Vorobev, B. Corbett, L. O'Faolain, S. Iadanza, "First Demonstration of a Fully Integrated Hybrid External Cavity Laser in Edge-Coupling Configuration via µTransfer-Printing," Centre for Advanced Photonics & Process Analysis, Munster Technological University, Cork, Ireland; Photonics, Tyndall National Institute, Cork, Ireland; Laboratory of Nano and Quantum Technologies, Paul Scherrer Institut, Villigen, Switzerland; Laboratory of Integrated Nanoscale Photonics and Optoelectronics, Ecole Polytechnique Federale de Lausanne, Lausanne, Switzerland, 2024.