III-V-on-Si Membrane Distributed Reflector Lasers with Intermixed MQW DBRs

Introduction

In recent years, silicon photonics has gained significant traction, with many foundries supporting Si photonics options and various products being commercialized. The functions in Si photonics can be categorized into fiber I/O, modulation, detection, polarization management, and light sources. While most Si photonics foundries support Si-based modulators, there are limited studies on on-chip light sources due to the process difficulties in integrating III-V chips/wafers on Si. Conventionally, off-chip lasers with hermetically sealed packages under temperature control are used, leading to additional fiber-to-chip coupling loss and an increasing fiber I/O count, resulting in higher power consumption and larger chip/package sizes.

To address these challenges, NTT Device Technology Labs has developed on-Si lasers based on InP membranes. These lasers have two unique features: (1) a thin (typical thickness of 230 – 340 nm) InP-based film formed on SiO2/Si or Si waveguides, and (2) multiple quantum well (MQW) active regions embedded in InP slabs to form a buried heterostructure (BH). These features enable strong optical confinement in the active media, allowing for direct modulation of the lasers with relatively small operating energy.

The proposed III-V membrane lasers

NTT has demonstrated on-Si membrane lasers with various cavity lengths, ranging from 20 µm to 1000 µm. For datacenter applications, a cavity length of 100 – 150 µm is considered reasonable for obtaining mW-class output directly modulated lasers (DMLs). Previously, distributed reflector (DR) lasers or detuned distributed feedback (DFB) lasers were used as the optical cavities for these lengths. However, these cavities had issues with poor tolerance to fabrication processes and inefficient lasing.

To address these challenges, NTT has developed a new membrane laser cavity consisting of DR lasers with quantum well intermixed (QWI) distributed Bragg reflectors (DBRs). This structure solves the remaining problems of wavelength and phase control accuracy between two sections, as well as lasing efficiency.

Fig. 1 shows schematic diagrams of the III-V membrane lasers developed by NTT. Fig. 1(a) illustrates the previous DR laser, which consists of a DFB section with a uniform grating and a passive InP-based rear-DBR mirror. Fig. 1(b) shows the detuned DFB laser, where the Bragg wavelength of the rear section is shifted to act as a partial DBR mirror.

Fig. 1. Top-view schematic structure of (a) conventional DR, (b) detuned DFB, and (c) proposed DR lasers.

The proposed DR laser structure is shown in Fig. 1(c). It is similar to the conventional DR laser, but the BH structure is extended to the DBR section so that both sections have the same structure. Quantum well intermixing (QWI) is applied to the DBR section to shift the absorption/emission wavelength of the BH. This allows for easy wavelength and phase matching between the two sections since they consist of the same materials and structures, with the MQWs being intermixed in the DBR section.

Device fabrication was similar to NTT's previous work, with the main differences being the shape of the BH and the regions for Si ion implantation. The original BH shape was a simple rectangle aligned on the DFB section, but the new one was a longer rectangle across both DFB and DBR sections, with tapered front/rear edges to avoid unwanted reflections. Si ion implantation was applied to the DBR sections and the two tapered regions to obtain QWI.

Fig. 2 shows the threshold current versus lasing wavelength of the proposed DR lasers and conventional detuned DFB lasers fabricated on the same wafer. The new structure decreased the threshold current to about half that of the conventional one, with the minimum threshold current being 0.88 mA at a lasing wavelength of 1296.4 nm, compared to 2.1 mA for the detuned DFB laser. The active length of all twelve lasers was 100 µm.

Fig. 2. Threshold current versus lasing wavelength of proposed DR lasers and conventional detuned DFB lasers fabricated on the same wafer.

Fig. 3 shows the lasing spectra of six proposed DR lasers having different Bragg wavelengths, measured at a bias current of 20 mA. Since the edge of the BH was tapered, unwanted reflections at the edge were suppressed, and smooth lasing spectra were obtained from all six lasers. The side-mode suppression ratio ranged from 51 dB (shortest wavelength) to 43 dB (longest wavelength).

Fig. 3. Lasing spectra of six proposed DR lasers having different Bragg wavelengths.

Conclusion

NTT Device Technology Labs has demonstrated distributed reflector lasers that use quantum well intermixed distributed Bragg reflectors. This new structure reduced the threshold current to approximately half that of conventional detuned DFB lasers, with the minimum threshold current being 0.88 mA for an active length of 100 µm. By addressing the issues of wavelength and phase control accuracy, as well as lasing efficiency, these lasers offer a promising solution for on-chip light sources in silicon photonics applications.

Reference

[2] K. Takeda, T. Segawa, T. Fujii, S. Matsuo, and Y. Maeda, "III-V-on-Si Membrane Distributed Reflector Lasers with Intermixed MQW DBRs," NTT Device Technology Labs, Nippon Telegraph and Telephone Corp., Kanagawa, Japan, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.