Mode Q factor and lasing spectrum controls for deformed square resonator microlasers with circular sides

Abstract

Deformed square resonators with the flat sides replaced by circular sides are proposed and demonstrated to enhance mode Q factors and adjust transverse mode intervals using the regular ray dynamic analysis and numerical simulations. Dual-transverse-mode emissions due to the ultrahigh-Q factors with different wavelength intervals are realized experimentally for AlGaInAs/InP circular-side square microlasers, and the stationary condition of the dual-mode emission is satisfied because the high-Q confined modes have totally different mode numbers.Furthermore, optical frequency combs are generated using the dual-mode lasing microlaser as a seeding light source by cascaded four-wave mixing in a highly nonlinear optical fiber. PACS: 42.55.Sa, 42.55.Px, 42.60.Da, OFC was traditionally generated using femtosecond mode-locked lasers [19, 25, and 26]. The generations of OFC in ultrahigh-Q microresonator have triggered intensive researches of four-wave mixing (FWM) in the microresonator under a continuous-wave laser pumping [27][28][29], and cascaded FWM in the nonlinear optical fiber has also been utilized for generating frequency combs via the pumping of two single-frequency lasers [30][31][32].Recently, square resonators have received a great deal of interest for single-mode lasing and enhancement of mode Q factors [33][34][35][36]. Dual-mode lasing with a tunable wavelength interval was demonstrated for square resonator microlasers with a patterned electrode and a vertex waveguide [33]. The large square resonators with a vertex waveguide can have high passive mode Q factors and result in dual-mode lasing simultaneously for the fundamental and first-order transverse modes, as they have near equal mode Q factors accounting the internal absorption loss. However, the mode Q factors are greatly limited by the inevitable radiation loss from the vertices of the square resonator [33,34]. In this letter, we introduce a novel circular-side square resonator (CSR) to modify the mode field patterns and enhance mode Q factors. The concave mirrors are widely used in conventional gas or solid laser systems for focusing the light beam [37][38][39], but are rarely applied to the polygonal microresonators.Here, dual-mode lasing due to the high-Q factors with changeable mode intervals is demonstrated by varying the radius of the circular-side. Frequency combs are obtained in nonlinear fiber using the dual-mode lasing microlaser as a seeding light source, which opens a way to realize OFC with a large line interval by simply adjusting the resonator form instead of the size.

Square microcavity semiconductor lasers

“…The p and q are different for the near high-Q WGMs, and therefore stable dual-mode lasing can be expected for the square microcavity laser. In the deformed square microcavity with circular-sides [56] or variable curvature, [57] the overlap of mode field intensities between even the near high-Q WGMs is less than that of the non-deformed square microcavities according to the numerical simulations. Thus, stable dual-mode lasing can also be achieved for the deformed square microcavity laser with an enhanced wavelength interval.…”

“…In order to further enhance the wavelength interval of the dual-transverse-mode square microcavity laser, we proposed and demonstrated deformed square microcavities with their flat sides replaced by circular sides. [56] Figure 4(a) shows a schematic diagram of a deformed square microcavity laser with the flat sides replaced by circular sides, where a is the side length of the square, δ is the deformation parameter, and r is the radius of the circular side. Considering the mode field distributions of the fundamental and first-order transverse modes as shown in Figs.…”

“…The mode Q factors are low around δ = 0.95 µm because there are no long-life light rays according to the light ray simulation. [56] Like the AlGaInAs/InP square microcavity laser, the circular-sided square microcavity laser was fabricated on the AlGaInAs/InP MQW epitaxial wafer with parameters similar to those used in the simulation (a = 16 µm and w = 1.5 µm). The inset in Fig.…”

“…Based on the dual-mode lasing nondeformed and deformed square microcavity lasers, the signal frequency can be extended from tens of GHz to the THz through mode beating. [55,56] We have demonstrated sub-THz wave generation based on the dual-mode square microcavity laser and the uni-travelling-carrier photodiode (UTC-PD), and the obtained signal frequency is mainly limited by the response bandwidth of the UTC-PD. [60]…”

Self-chaotic microlasers for random bit generation

“…By modifying the hexagon to a circular-side hexagonal resonator (CSHR), the Q factors of different transverse modes can be approximated, and then the multi-transverse modes can be realized. Recently, we realized dual-mode lasing with an adjusting interval in a square microcavity laser (Long et al, 2015) and demonstrated the enhancements of mode Q-factors and intervals in circular-side square microcavities (Weng et al, 2017). Circular sides were also applied to a hexagonal microcavity for enhancing mode Q factors and adjusting mode intervals (Xiao et al, 2017;Yang et al, 2019).…”