Channel Waveguides Research Articles

Holographically fabricated out-of-plane blazed gratings and channel waveguides in silica for integrated free-space beam delivery.

Grating couplers are widely used in integrated optics to generate free-space beams and facilitate localized interactions with systems such as atom or ion traps. However, etched devices often exhibit small-scale inconsistencies; exacerbated by the high index contrast of the devices, this can lead to phase errors, limiting devices to a sub-millimeter scale. Here we present the first demonstration, to our knowledge, of tilted, out-of-plane blazed gratings in planar silica fabricated by UV inscription using a 213 nm laser. Our devices deliver collimated and focusing beams into free space from a waveguide input without the need for additional optics such as beam expanders.

Sub-picosecond mode-locked laser operation in a femtosecond-laser-inscribed Yb:CaF2 channel waveguide

We present the successful demonstration of both Q-switched mode-locked (QSML) and continuous-wave mode-locked (CWML) operation in a femtosecond-laser-inscribed Yb:CaF2 double-track waveguide (WG) structure. A semiconductor saturable absorber output coupler (SOC) was used as the mode locker with fine tuning of the intracavity group delay dispersion (GDD) achieved through the Gires-Tournois interference (GTI) effect induced by an air gap. To ensure sufficient fluence on the saturable absorber, the cavity was extended to 50 cm by inserting two lenses between the SOC and WG, resulting in a repetition frequency of ∼300 MHz. In the QSML regime, the laser exhibited an amplitude modulation period of 65 kHz within Q-switched pulses of 3-µs duration. Notably, in the purely CWML regime, the laser generated a maximum output power of 51 mW near 1036 nm with a pulse width of 979 fs.

A low quantum defect and power scalable Yb-doped glass waveguide laser

We present a continuous wave laser action in ytterbium (Yb)-doped phosphate glass buried channel waveguides. The 25 mm long waveguides were fabricated by the standard ion exchange method. When pumped with a 976 nm laser diode, the waveguide laser emits at 982.4 nm, yielding an ultralow quantum defect of 0.66 %. The impact of coupling and pump power variation on the laser emission wavelengths is demonstrated. Furthermore, the results of pulsing using a monolayer graphene as a saturable absorber (SA) are presented and a pulse train with a repetition rate of 1 MHz and a pulse width of 700 ns was obtained.

Physical coherent cancellation of optical addressing crosstalk in a trapped-ion experiment

Abstract We present an experimental investigation of coherent crosstalk cancellation methods for light delivered to a linear ion chain cryogenic quantum register. The ions are individually addressed using focused laser beams oriented perpendicular to the crystal axis, which are created by imaging each output of a multi-core photonic-crystal fibre waveguide array onto a single ion. The measured nearest-neighbor native crosstalk intensity of this device for ions spaced by 5 µm is found to be ∼ 10 − 2 . We show that we can suppress this intensity crosstalk from waveguide channel coupling and optical diffraction effects by a factor > 10 3 using cancellation light supplied to neighboring channels which destructively interferes with the crosstalk. We measure a rotation error per gate on the order of ϵ x ∼ 10 − 5 on spectator qubits, demonstrating a suppression of crosstalk error by a factor of > 10 2 . We compare the performance to composite pulse methods for crosstalk cancellation, and describe the appropriate calibration methods and procedures to mitigate phase drifts between these different optical paths, including accounting for problems arising due to pulsing of optical modulators.

A route to an all direct laser written integrated FTS (SWIFTS) in the 3.39–4.1 μm range

This work aims to present the building blocks for an all direct laser written integrated spectrometer in the mid-infrared (3.39–4.1 μm range) based on the SWIFTS (Stationary Wave Integrated Fourier Transform Spectrometer) principle. In a SWIFTS-Gabor configuration, the light from the source interferes with itself in the middle of a channel waveguide, creating a stationary wave. This interferogram is then sampled through scattering centers placed on top of the waveguide that radiate the light they extract onto a detector placed directly on the sample. Finally, the spectrum of the source is retrieved through a Fourier transform. To implement a SWIFTS, two main photonic functions are required: the waveguide and the scattering centers. In this work, both functions have been created using Direct Laser Writing (DLW), a versatile technique, allowing to easily access 3D configurations and to reduce fabrication time. DLW focuses a femtosecond laser onto a sample so as to locally change the crystal lattice, resulting in its structural modification. The waveguides presented here are surface half-circular cladding structures made through type II modifications in the sample, and the scattering centers are surface damage tracks (also referred to as grooves). These surface tracks are creating dielectric discontinuities in the evanescent part of the stationary wave, resulting in the light being radiated outside the waveguide. All of these are made in a z-cut lithium niobate substrate for future implementation of the electro-optic effect. We demonstrate that we have functional waveguides in the mid-IR and that our grooves are extracting the stationary wave as expected, showing promising results for future implementation of a complete mid-IR SWIFTS using DLW.

Metalorganic Vapor‐Phase Epitaxy of +c/−c GaN Polarity Inverted Bilayer for Transverse Quasi‐Phase‐Matched Wavelength Conversion Device

Photon‐pair generation based on optical parametric down‐conversion has attracted for the application as a light source for quantum information. Highly efficient wavelength‐conversion devices require a polarity‐inversion structure when using nitride semiconductors. A transverse quasi‐phase‐matching (QPM) polarity‐inverted GaN bilayer channel waveguide device is suitable for efficient wavelength conversion. This study designed a cross‐section device to satisfy the modal dispersion phase‐matching condition between the TM02 mode pump light and the TM00 mode signal/idler light. Moreover, an AlN oxidation interlayer fabricates the Ga‐polar/N‐polar (+c/−c) GaN layers via metalorganic vapor‐phase epitaxy (MOVPE). A 145 nm thick film layer with a macro‐step‐free surface is grown by optimizing the −c‐GaN growth conditions and reducing the substrate off‐angle to 0.2°. Next, the AlN layer is oxidized in an electric furnace and MOVPE is used to regrow a 1500 nm thick +c‐GaN layer. A macrosteps‐free surface can be achieved by reducing the off‐angle to 0.2° and optimizing the −c‐GaN growth conditions to avoid hillock formation. These results pave the way for improving the efficiency of GaN transverse QPM wavelength‐conversion devices.

Two-step scattering theory method for designing metamaterials

Abstract A two-step method for designing an acoustic metamaterial with desired properties is proposed. At the first step, the scattering matrices of its elements are calculated using the Lippmann-Schwinger type equation. For scatterers of small wave size, the problem is reduced to determining the phases of several scattering coefficients. At the second step, the particular design of metamaterial elements is determined. This problem can be considered an inverse scattering problem or a problem of optimizing the geometric parameters of a metamaterial element. The second approach turns out to be more practically applicable. As an example, a metamaterial in the form of a reflective layer with a narrow waveguide channel is considered, and the design of its elements is determined with the proposed method.

Design of Horizontally Stacked AlN and Dielectric Cores Transverse Quasi‐Phase‐Matched Channel Waveguide for Squeezed Light Generation

AlN is a promising candidate for photonic integrated circuits. In this study, horizontally stacked transverse quasi‐phase‐matched (QPM) waveguides with an AlN center core and Si3N4, Ta2O5, and TiO2 side cores were designed to generate squeezed light at 920 nm. The squeezing level was calculated by considering the propagation loss at the wavelengths of the squeezed light and pump light. Using TiO2 for the side cores enhanced the electric field amplitude of the pump light in the AlN center core, and a high nonlinear coupling coefficient was obtained. Owing to the small propagation loss of the AlN waveguide on sapphire and high nonlinear coupling coefficient of the transverse QPM with TiO2 side cores, a squeezing level of over 8.1 dB was expected with a 1 cm long waveguide and pump power of 70 mW.

Reconfigurable topological wave routing based on tunable valley kink states and valley-polarized chiral edge states.

Valley kink states and valley-polarized chiral edge states, whose topologically protected one-way propagation property provides a promising solution for manipulating light waves, have recently attracted considerable attention in topological photonics. However, it remains a great challenge to realize flexibly tunable dispersion for two different topological states and to develop a dynamically controllable topological photonic platform for switching topological wave routing. In this work, we propose a reconfigurable topological wave routing structure in the telecommunication frequency range, where phase-change material Sb2S3 cylinders with tunable refractive index are embedded into each topological channel to dynamically tune the dispersion of topological edge states. Via switching the phase states of Sb2S3 between amorphous and crystalline, we numerically demonstrate some unique applications of the proposed topological photonic crystals, such as topological optical switches, dual-channel selective transport, and controllable multi-channel intersection waveguides. More importantly, by digitally encoding each waveguide channel without the requirement of controlling each unit cell in the bulk domain, the proposed topological photonic platform provides a convenient and easy-to-implement solution for achieving dynamically reconfigurable topological wave routing propagation. Besides, the unique features of immunity against bending interface with disorders demonstrate the robustness of the topological wave propagation. Our proposed topological photonic platform has potential applications for designing intelligent photonic devices and opens up an avenue for advanced integrated photonic systems with reconfigurability.

Characterization of noise spectra in low-jitter GHz mode-locked fs-laser-inscribed waveguide lasers

We demonstrate low-noise, continuous-wave mode-locking of a diode-pumped Yb:KLuW waveguide laser operating at fundamental repetition rates in the GHz range. Utilizing a femtosecond-laser-inscribed Yb:KLuW channel waveguide laser with a tunable cavity length of a few centimeters, we successfully generate femtosecond pulses near 1030 nm, employing either single-walled carbon nanotubes or semiconductor saturable absorbers. Although timing noise is one of the most critical factors in the implementation of high-repetition-rate laser sources for various applications, investigations into timing noise characteristics in waveguide lasers remain limited. This study fills this gap by presenting timing jitter measurements for GHz mode-locked waveguide lasers with diverse cavity parameters. To quantitatively analyze noise spectral characteristics, including relaxation oscillation processes induced by intensity noise features, we introduce a theoretical modeling approach. The present investigation directly enables the provision of strategies for further suppression of timing jitter and designing high-performance, low-noise mode-locked lasers.

Optical Amplification at 1.5µm in ErIII Coordination Polymer-Doped Waveguides Based on Intramolecular Energy Transfer.

9,9-bis (diphenylphosphorylphenyl) fluorene (FDPO) and dibenzotetrathienoacene (DBTTA), are synthesized as the neutral and anionic ligands, respectively, to prepare the ErIII coordination polymer [Er(DBTTA)3(FDPO)]n. Based on the intramolecular energy transfer, optical gains at 1.5µm are demonstrated in [Er(DBTTA)3(FDPO)]n-doped polymer waveguides under excitations of low-power light-emitting diodes (LEDs) instead of laser pumping. A ligand-sensitization scheme between organic ligands and Er3+ ions under an excitation of an ultraviolet (UV) LED is established. Relative gains of 10.5 and 8.5dBcm-1 are achieved at 1.53 and 1.55µm, respectively, on a 1-cm-long SU-8 channel waveguide with a cross-section of 2×3µm2 and a 1.5-µm-thick [Er(DBTTA)3(FDPO)]n-doped polymethylmethacrylate (PMMA) as upper cladding. The ErIII coordination polymer [Er(DBTTA)3(FDPO)]n can be conveniently integrated with various low-loss inorganic waveguides to compensate for optical losses in the C-band window. Moreover, by relying on the intramolecular energy transfer and UV LED top-pumping technology, it is easy to achieve coupling packaging of erbium-doped waveguide amplifiers (EDWAs) with pump sources in planar photonic integrated chips, effectively reducing the commercial costs.

Buried annealed proton-exchanged waveguide in periodically-poled MgO:LiTaO3 fabricated by surface-activated bonding for high-power wavelength conversion

Watt-class UV laser lights with symmetric spatial modes are suitable for high-resolution industrial applications. Channel waveguides with a guided mode of several ten microns diameter in MgO:LiTaO3 enhance integration capability while avoiding photorefractive damages during high-power wavelength conversion. We focused on buried waveguides fabricated by proton exchange, surface-activated bonding (SAB), and proton diffusion processes in periodically-poled (PP) MgO:LiTaO3. In this work, the mode profiles were simulated using the proton diffusion coefficients estimated by secondary ion mass spectrometry. Using the simulation results, the buried waveguides with a mode diameter larger than 30 μm were fabricated. By adopting designed PP structures, second harmonic generation (SHG) devices at a wavelength of 532 nm were fabricated. The nonlinear coupling coefficient was estimated to be 0.15 W−1/2 cm−1. Compared to the conventional annealed proton-exchanged waveguide SHG device without SAB, symmetric guided modes were obtained while maintaining the nonlinear coupling coefficient.

Investigation of the effect of the applied voltage to the control electrodes of a lithium niobate phase modulator on the intensity distribution at the ends of channel waveguides and on parasitic amplitude modulation

Investigation of the effect of the applied voltage to the control electrodes of a lithium niobate phase modulator on the intensity distribution at the ends of channel waveguides and on parasitic amplitude modulation

Quantum Key Distribution with Displaced Thermal States.

Secret key exchange relies on the creation of correlated signals, serving as the raw resource for secure communication. Thermal states exhibit Hanbury Brown and Twiss correlations, which offer a promising avenue for generating such signals. In this paper, we present an experimental implementation of a central broadcast thermal-state quantum key distribution (QKD) protocol in the microwave region. Our objective is to showcase a straightforward method of QKD utilizing readily available broadcasting equipment. Unlike conventional approaches to thermal-state QKD, we leverage displaced thermal states. These states enable us to share the output of a thermal source among Alice, Bob, and Eve via both waveguide channels and free space. Through measurement and conversion into bit strings, our protocol produces key-ready bit strings without the need for specialized equipment. By harnessing the inherent noise in thermal broadcasts, our setup facilitates the recovery of distinct bit strings by all parties involved.

Refractive index tuning and birefringence effect in crosslinked siloxane-acrylate copolymers for optical waveguide applications

Tunability of refractive index of polymeric waveguide material could be made by controlling free volume as well as polarizability of constituent materials. Two acrylate-co-polysiloxane series of different crosslinking network were synthesised by utilizing different crosslinker and monomer feeding ratio. Using Claussius-Mossotti/Lorentz-Lorentz (CM/LL) model, fractional free volumes were calculated showing a decrease in the experimental (nexp) compared to theoretical (ni) refractive indices. Curing agent with asymmetrical configuration induce a crosslink network of higher birefringence. They were respectively fabricated into multimode channel waveguide whose core-clad refractive index differences (Δn) show values in the range 0.0049–0.0408.

Phase Composition of HiVac-VPE Lithium Niobate Optical Waveguides Identified by Spectroscopic Investigations.

High-index contrast lithium niobate waveguides, fabricated by the High Vacuum Vapor-phase Proton Exchange (HiVac-VPE) technique, are very promising for increasing both the optical nonlinear and electro-optical efficiencies of photonic integrated devices. So as to play this role effectively, it is mandatory to know the crystallographic phase composition of waveguides and the position of protonated layers for appropriate tailoring and optimization based on the intended applications. In addition, the estimation of structural disorder and electro-optical properties of the waveguides are also of high interest. Benefiting from Raman spectroscopy, IR reflection, IR absorption, and UV-VIS absorption, the HxLi1-xNbO3 phase compositions, as well as the structural disorder in waveguides, were determined. Based on experimental data on the shift of the fundamental absorption edge, we have quantitatively estimated the electro-optic coefficient r13 in as-exchanged waveguides. The electro-optical properties of the waveguides have been found to be depending on the phase composition. The obtained results allow for reconsidering the proton exchange fabricating process of photonic nonlinear devices and electro-optic modulators based on high-index contrast channel waveguides on the LiNbO3 platform.

Mitigating waveguide loss in Ge–Sb–Se chalcogenide glass photonics

Minimizing propagation loss within waveguides remains a central objective across diverse photonic platforms, impacting both linear lightwave transmission and nonlinear wavelength conversion efficiencies. Here, we present a method to mitigate waveguide loss in Ge28Sb12Se60 chalcogenide glass, a material known for its high nonlinearity, broad mid-infrared transparency, and significant potential for mid-IR photonics applications. By applying a sacrifical oxide layer to eliminate etching residues and a subsequent waveguide thermal reflow to smooth lithography-induced line edge roughness, we successfully reduced the waveguide loss down to 0.8 dB cm−1 at 1550 nm wavelength. This represents the best result in small-core and high-index-contrast Ge28Sb12Se60 channel waveguides. Our approach paves the way for low-loss, on-chip chalcogenide photonic devices.

Helical hollow channel waveguide in YAG fabricated by femtosecond laser enhanced wet etching.

Three-dimensional optical waveguides with hollow channels have many advantages, such as strong mode confinement and excellent dispersion control ability. Femtosecond laser enhanced wet etching is widely used to fabricate hollow channel waveguides in transparent dielectric materials. We propose a method for fabricating hollow channel waveguides in YAG using femtosecond laser enhanced wet etching with a simpler fabrication process and shorter etching time compared with the previous work. After 90 h of etching, a series of helical hollow channel waveguides with a length of 5 mm and a radius of 32 µm were successfully fabricated. At a pitch of 3 µm, the waveguide exhibited a loss (including coupling loss and transmission loss) as low as 0.68 dB at 1030 nm. The helical hollow channel waveguide also exhibited exceptional isotropic light confinement capability and remarkable supercontinuum-generating properties. Moreover, helical hollow channel waveguides with a radius of 2 µm were successfully fabricated. According to simulations, waveguides of such size can effectively control dispersion. Our work presents, to our knowledge, a novel approach to fabricating hollow channel waveguides with arbitrary lengths using femtosecond laser-enhanced wet etching.

Temperature-assisted electromagnetic surface modes in graphene-based temperature sensitive metafilms

Graphene has the potential to manipulate surface modes in frequency bands from THz to mid-IR regions. Typically, due to single-atom thickness and low charge-carrier density, the thermal response of graphene is ineffective. Temperature-sensitive materials (TSMs) can play an active role in enhancing the thermal response of graphene-based devices. In the present work, graphene-based temperature-sensitive metafilms have been proposed for thermally tunable propagation of electromagnetic surface modes. A detailed analytical and numerical solution for temperature-dependent electromagnetic surface (even and odd) modes supported by the graphene-based temperature-sensitive metafilm has been studied. The Kubo’s formulation has been used to model optical conductivity (σ g ) while the hybrid Drude’s model is implemented to realize the indium antimonide (InSb) as temperature-sensitive material. To simulate the metafilm, the waveguide modal analysis approach was implemented, while the realization of the graphene sheets was achieved by the use of impedance boundary conditions (IBCs). The propagation characteristics for even/odd surface modes were analyzed under different values of temperature (T), chemical potential (µ c ), and thickness of metafilm (d). Further, the numerical results for even and odd surface modes under two phases of InSb [Insulator phase (T = 200 K) and metallic phase (T = 300 K)] were compared under different values of chemical potential (µ c ) and TSM film thickness (d). It is concluded that the propagation characteristics of surface modes are sensitive to the external temperature and can be tailored by tuning the temperature, chemical potential (µ c ), and TSM film thickness (d). Moreover, the degeneracy of the even and odd modes can be controlled by varying the temperature and TSM film thickness. The work is suitable for designing temperature-assisted dual channel waveguides, THz optical switches, THz optical logic designs, and flexible thermal-optical sensors.

Coupling Nanowire Quantum Dots to Optical Waveguides by Microsphere-Induced Photonic Nanojet

Silica-on-silicon is a major optical integration platform, while the emergent class of the integrated laser-written circuits’ platform offers additionally high customizability and flexibility for rapid prototyping. However, the inherent waveguides’ low core/cladding refractive index contrast characteristic, compared to other photonic platforms in silicon or silicon nitride, sets serious limitations for on-chip efficient coupling with single photon emitters, like semiconductor nanowires with quantum dots, limiting the applications in quantum computing. A new light coupling scheme proposed here overcomes this limitation, providing means for light coupling >50%. The scheme is based on the incorporation of an optical microsphere between the nanowire and the waveguide, which is properly optimized and arranged in terms of size, refractive index, and the distance of the microsphere between the nanowire and waveguide. Upon suitable design of the optical arrangement, the photonic nanojet emitted by the illuminated microsphere excites efficiently the guided eigenmodes of the input channel waveguide, thus launching light with high-coupling efficiency. The method is tolerant in displacements, misalignments, and imperfections and is fabricationally feasible by the current state of art techniques. The proposed method enables the on-chip multiple single photon emitters’ integration, thus allowing for the development of highly customizable and scalable quantum photonic-integrated circuits for quantum computing and communications.