Raman Lasing in Multimode Silicon Racetrack Resonators

Abstract

Integrated Raman lasers have been well explored using silica and silicon platforms for decades. A well‐known equation with negligible nonlinear losses is employed for predictions of Raman lasing threshold powers in critically coupled cavities. However, nonlinear losses are known to be highly detrimental to silicon devices. Herein, for the first time, including the effects of linear loss, nonlinear losses, and cavity design, a new general equation that predicts the onset of Raman lasing in a cavity is derived and validated experimentally. Generally, a cavity with a small effective area, a short length, and high quality factors (Qs) at both pump and Stokes wavelengths can lase at relatively low pump power. This theory is verified by the experimental results with sub‐milliwatt threshold powers in 2.8 mm long multimode cavities with different Qs at different pump wavelengths. The derived Raman gain coefficients from the measurements follow the scaling rules of Raman gain. This work advances the understanding of Raman lasing in high‐Q multimode cavities. It also shows that Raman lasing at O/S‐band is possible in racetrack resonators without needing any reverse bias and the broad operation wavelength is promising for single‐chip silicon devices operating at all communication bands.

Broadband high-Q multimode silicon concentric racetrack resonators for widely tunable Raman lasers

“…Generally, the laser output power decreases with the increase of the pump wavelength because the Raman gain coefficient is inversely proportional to the Stokes wavelength, and also because the longer wavelength has a larger free-carrier absorption coefficient 68 . Currently, the minimum and maximum lasing wavelengths at 1325.5 and 1840.7 nm are limited by the pump wavelengths from the tunable lasers which have minimum and maximum wavelengths of 1240 and 1680 nm.…”

“…The higher output power at the longer wavelength can be probably explained by the slightly higher Q i factors at the C band (Fig. 2f ) resulting in lower Raman thresholds for the C-band wavelengths 68 . Overall, for this multimode concentric racetrack resonator, the Raman lasing outputs with C-band pumps are more efficient than those with O-band pumps.…”

“…The tunable Raman lasers listed all are tunable in discrete steps equal to the FSR of the resonator. For example, we recently reported silicon Raman lasing at O/S-band using a multimode racetrack resonator 68 . But that silicon Raman laser is not widely tunable, i.e., the pump wavelength cannot be continuously tuned across all FSRs because of the reduction in Q factors from coupling to higher-order modes at certain wavelengths.…”

Integrated Lasers on Silicon at Communication Wavelength: A Progress Review

“…The racetrack ring resonator has the advantage of higher output power contributed by its larger cavity size compared with PhC nanocavity. [151] While the advantage of the PhC nanocavity is ultra-low threshold power attributed to its compact volume compared with a racetrack ring resonator. [151] In 2013, Takahashi et al [145] reported the first nanocavity Si Raman laser, with the size in micrometer scale and lasing threshold of a microwatt.…”

Low-Threshold Continuous-Wave Anti-Stokes Raman Lasing in Silicon Racetrack Resonators

“…Stimulated Raman scattering (SRS) in optical waveguides has attracted much interest − because of its potential applications in wavelength conversion , and on-chip Raman spectroscopy. − It also improves the understanding of the optical physics of high-quality-factor ( Q ) cavities in terms of Stokes Raman lasers and anti-Stokes Raman lasers. − Recent interest in integrated Raman lasers has extended to the use of machine-learning methods to design silicon Raman lasers with low threshold powers . Nonlinear wavelength conversion is widely used to produce wavelength differences from a pump laser at either longer or shorter wavelengths and typically relies on nonlinear parametric conversion. − SRS-based wavelength conversion offers additional advantages of high efficiency that can enable cascaded Raman lasing for wavelength conversion spanning a large wavelength range. ,, A SRS wavelength converter can convert the pump wavelength to both the Stokes wavelength and the anti-Stokes wavelength via the SRS and coherent anti-Stokes Raman scattering (CARS) covering a wide wavelength conversion range .…”