Measurement of Optical Emission from Gesn Waveguides

The development of Si-based lasers has been a goal of the photonics industry for many years. A number of methods have been explored for achieving lasers on Si platforms, and a few attempts, such as the Si Raman laser, have achieved marginal success, but typically suffer from low efficiency. The silicon photonics problem requires the realization of high-efficiency lasers on CMOS-compatible platforms.

One possible solution to this problem is to employ the use of direct band gap materials that can be grown on Si. Of course, the epitaxial growth of such materials has been extremely difficult, and the efforts have mostly concentrated on the use of III-V materials like InGaAs. These materials typically suffer from high defect densities when grown directly on silicon, making them unsuitable for use in lasers. Additionally, the introduction of Group III and Group V materials into a CMOS fabrication facility can cause severe problems in device production due the electronic properties of these atoms, which are typically used as dopants in Si.

GeSn alloys present a possible path forward for the realization of Si-based lasers, as these materials are grown directly on Si or Ge-buffered Si and have a band structure that is more suited for efficient optical emission. A laser based on tensile strained Ge grown directly on Si has been demonstrated at room temperature, but the efficiency is limited by the indirect nature of the Ge band gap. More recently, demonstration of lasing at low temperature in GeSn (12.6% Sn) was achieved using optical pumping of a GeSn waveguide. While this represents a first step to the actualization of GeSn lasers, room temperature operation is a necessary condition for widespread adoption and incorporation into everyday devices.

The approach for demonstration of lasing in GeSn and Ge-on-Si has typically relied on measuring the optical emission spectrum at very high resolution to see the narrowing of the emission linewidth and the presence of cavity modes while optically-pumping a waveguide with a pulsed laser via topside illumination. We have designed a new system for optically pumping waveguides by coupling into the waveguide edge, which increases the absorption length by orders of magnitude, thereby increasing optical emission intensity and relaxing the constraints under which lasing is achieved. We employ a CW laser and erbium-doped fiber amplifier at 1550 nm as the pump source, which serves to decrease the cost of the experiment and increase the pump power. The edge emission from the waveguide is sent into a Fabry-Perot cavity with an InGaAs detector, and the optical pump power is increased until coherence is achieved, which results in the observance of modes in the Fabry-Perot cavity. This measurement can be performed with much lower pump power than a measurement using a high-resolution spectrometer. In this work, we demonstrate the use of this method for measuring the optical emission from GeSn waveguides with Sn concentrations of 4-7%.