Longitudinal Modulation Of Refractive Index Research Articles

Ultra-compact mode converter based on longitudinal refractive index modulation

Mode converter (MC) is a crucial device in optical information processing based on mode division multiplexing. Recently, MCs based on metasurface have attracted much attention due to their compact footprints. However, this type of MCs is usually achieved by introducing refractive index modulation in the transverse direction. As a result, many interfaces are unavoidable, which is the primary origin of high loss and narrow bandwidth. In this paper, we propose a new mechanism of ultra-compact, high efficiency, and broadband MC based on longitudinal refractive index modulation. The proposed dielectric modulation only introduces a few longitudinal pixels in the waveguide, resulting in weak scattering and weak resonance refractive index modulation with low crosstalk, and large bandwidth. By adopting the inverse design algorithm, the specified TE0-TE1 and TE0-TE2 mode conversions are obtained with superior performances including the ultra-compact footprint, high conversion efficiency, and low crosstalk over a wide bandwidth. This method could be extended to other desired mode conversions and also has excellent potential in optical integrated circuits and optical information processing.

Bragg solitons in topological Floquet insulators.

We consider a topological Floquet insulator consisting of two honeycomb arrays of identical waveguides having opposite helicities. The interface between the arrays supports two distinct topological edge states, which can be resonantly coupled by additional weak longitudinal refractive index modulation with a period larger than the helix period. In the presence of Kerr nonlinearity, such coupled edge states enable topological Bragg solitons. Theory and examples of such solitons are presented.

Parity-time symmetry in a single-loop photonic system

Non-Hermitian photonics based on parity-time (PT) symmetry has been considered an effective solution to achieve mode selection for optical or microwave single-mode oscillation. A PT-symmetric system is usually implemented using two mutually coupled elements with identical geometry or a single element based on precise longitudinal refractive index modulation. In this study, we propose a novel scheme to achieve PT symmetry based on polarization mode manipulation in a single optical loop. By controlling the polarization states of two light waves that are propagating along the clockwise (CW) and counter-clockwise (CCW) directions in the single optical loop, two equivalent mutually-coupled loops with identical geometry and a balanced gain and loss are formed. Based on this concept, a wavelength-tunable single-mode fiber laser with a sub-kHz linewidth is realized. The tunability is achieved by thermally tuning the wavelength of an integrated microdisk resonator (MDR) incorporated in the optical loop. Experimental results show that a continuously tunable single-mode laser with a sub-kHz linewidth of 640 Hz and a wavelength tuning range from 1552.953 to 1554.147 nm is realized. The key advantages of using a single optical loop to implement PT symmetry include greatly simplified implementation and highly improved stability. The demonstration opens new avenues to implement high-performance PT-symmetric systems for applications in photonic and microwave photonic systems.

Channel Optical Waveguides with Spatial Longitudinal Modulation of Their Parameters Induced in Photorefractive Lithium Niobate Samples

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Light tunneling inhibition in longitudinally modulated Bragg-guiding arrays

We consider a waveguide array in which defect channels with reduced refractive index are spaced periodically and the light guiding is achieved because of Bragg reflection. We show that tunneling between defects can be inhibited using out-of-phase longitudinal modulation of refractive index exclusively in defect channels. Tunneling inhibition is almost perfect for strongly localized modes with propagation constants close to the gap center.

Light tunneling inhibition in array of couplers with longitudinal refractive index modulation

We consider light tunneling inhibition in a periodic array of optical couplers due to the specially designed longitudinal and transverse modulation of the refractive index. We show that local out-of-phase longitudinal modulation of refractive index in channels of directional couplers in combination with the global refractive index modulation between adjacent couplers allow simultaneous suppression of both local and global energy tunneling inside each coupler and between adjacent couplers. This enables the localization of light in single waveguide despite the remarkable difference of corresponding local and global energy tunneling rates.

Switching management in couplers with biharmonic longitudinal modulation of refractive index

We address light propagation in couplers with longitudinal biharmonic modulation of refractive index in neighboring channels. While simplest single-frequency out-of-phase modulation allows suppression of coupling for a strictly defined set of resonant frequencies, the addition of modulation on multiple frequencies dramatically modifies the structure of resonances. Thus, additional modulation on a double frequency may suppress the primary resonance, while modulation on a triple frequency causes fusion of primary and secondary resonances.

Coupled-wave analysis of apodized volume gratings

This work presents the use of longitudinal refractive index modulation (apodization) in photosensitive glass for improved sidelobe suppression in volume holographic optical elements. We develop a numerical model for both uniform and apodized volume holograms based on rigorous coupled-wave analysis. We validate the model by comparison with a transmissive 1.55- mum uniform volume grating in photothermorefractive glass. We then apply our numerical model to calculate the spectral response of apodized gratings. The numerical results demonstrate that apodization of the refractive index modulation envelope improves spectral selectivity and reduces first and second-order side-lobe peaks by up to 33 and 65 dB, respectively. We suggest a method for creating apodization in volume holograms with approximately Gaussian spatial refractive index profile.