Single mode lasing in transversely multi‐moded PT‐symmetric microring resonators

Abstract

Single mode lasing is experimentally demonstrated in a transversely multi-moded InP-based semiconductor microring arrangement. In this system, mode discrimination is attained by judiciously utilizing the exceptional points in a parity-time (PT) symmetric double microring configuration. The proposed scheme is versatile, robust to small fabrication errors, and enables the device to operate in a stable manner considerably above threshold while maintaining spatial and spectral purity. The results presented here pave the way towards a new class of chip-scale semiconductor lasers that utilize gain/loss contrast as a primary mechanism for mode selection.Integrated photonic laser systems with larger cross sections are desirable for many applications since they allow for higher energies within the cavities while managing the thermal load and keeping the impact of optical nonlinearities under control. Unfortunately, however, merely enlarging the transverse dimensions of the waveguides inevitably gives rise to competing higher-order spatial modes. This, in turn, compromises the spectral and spatial fidelity of the laser and limits the power allocated within a specific mode [1]. These limitations exist on all scales, and may even be exacerbated in chip-scale semiconductor lasers, where the large gain bandwidths of the active media already pose a challenge in promoting single-mode operation [2].So far, utilizing intra-cavity dispersive elements has been the primary approach for longitudinal mode selection [3], while tapering along the direction of propagation, engineering the refractive index in the cross section, as well as evanescent filtering are some of the extensively explored techniques to enforce single spatial mode operation in such arrangements [4][5][6][7]. Yet, in spite of their success, they are not always compatible with on-chip microcavity structures and in most cases are quite sensitive to small fabrication tolerances. In this respect, it would be desirable to explore alternative avenues to address these issues.Lately, the selective breaking of parity-time (PT) symmetry has been proposed as a viable strategy for obtaining single transverse mode operation [8]. It is suggested that by pairing an active resonator/waveguide with a lossy but otherwise identical partner, it is possible to enforce single-spatial-mode performance even in the presence of strong mode competition in multi-moded laser/amplifier configurations. In general, a structure is considered to be PT-symmetric if it is invariant under the simultaneous action of the (space) and (time) inversion operations [9]. Despite 2 having a non-Hermitian representation, such a system may still support an entirely real spectrum. While originally developed in the context of quantum mechanics [9,10], such notions have recently attracted considerable attention in different areas of optics such as photonic lattices, microresonators, gratings, and lasers [11][12][13][14][15][16][17][18][19][20][21][22]. In optical settings, a structure is PT symmetric if the real part...

Dual-wavelength single-frequency laser emission in asymmetric coupled microdisks

“…In some symmetric coupled microcavity system such as identical coupled microrings2528, the coupling usually occurs between the same transverse WGMs. Therefore, there is no difference in the lasing spectra when one single cavity is pumped selectively.…”

“…Recently, controlling the gain and loss elements has been proved to be an alternative approach for the spectral modulation in microcavity laser systems789101112. This type of spectral modulation introduces a unique phenomenon, namely parity-time (PT) symmetry, which has been studied extensively in recent works including both physical fundamentals and practical applications131415161718192021222324252627282930. The eigenfrequency spectrum of the PT symmetry system remains real even if the gain and loss are introduced.…”

Unidirectional reflectionless light propagation at exceptional points

“…In the presence of balanced gain and loss, the rhombic ring structure is under reflection PT-symmetry, which induces asymmetric reflection and reciprocal transmission. The optical gain and loss in the coupledresonator optical systems can be experimentally realized using InGaAsP quantum wells and chrome or graphene layers, respectively, on top of the resonator [63][64][65][66][67].…”

Power characteristics of planar index-antiguided waveguide lasers with transverse mode competition

“…3,4 These LMA lasers are mostly multimoded (MM) and require some level of mode discrimination to achieve single fundamental mode operation. Among various means to achieve this goal, [4][5][6][7][8][9] index antiguiding (IAG) is a relatively simple approach, where the negative index step between the core and the cladding imposes higher loss for higher order modes. 10,11 Robust single fundamental mode oscillation has been reported both in IAG fibers with diameter up to 400 µm 12,13 and IAG planar waveguides with 200-µm core width.…”