Channel Waveguides Research Articles

Phase composition of channel proton-exchanged waveguides in different near-congruent LiNbO3

Micro-Raman spectroscopy and method of Raman data calibration are proposed for the quantitative study of the proton-exchanged LiNbO3 channel waveguides, which contain the different HxLi1–xNbO3 phases, depending on the fabrication conditions. The spectroscopic parameters, phase composition and electro-optic properties of these waveguides have been found to be depending on the small variation of stoichiometry in the near-congruent lithium niobate substrates used for waveguides fabrication.

Photonics integrated circuits using AlxGa1−xN based UVC light-emitting diodes, photodetectors and waveguides

We report on a study of UVC photonics integrated circuit consisting of monolithically integrated AlxGa1−xN multiple quantum wells based light-emitting diodes, detectors and channel waveguides on sapphire substrates. The waveguide stack consisted of a 1.5 μm thick n-Al0.65Ga0.35N waveguide over an AlN (3.5 μm thick) clad layer. Using the integrated devices, we estimated the multi-mode ridge waveguide losses to be 23 cm−1 at λemission ∼ 280 nm. We also measured that approximately 80% of the guided light was confined in the n+-Al0.65Ga0.35N layer, 7% in the underlying AlN cladding and the remaining 13% in the double-side polished sapphire substrate.

Non-Hermitian disorder in two-dimensional optical lattices

In this paper, we study the properties of two-dimensional lattices in the presence of non-Hermitian disorder. In the context of coupled mode theory, we consider random gain-loss distributions on every waveguide channel (on site disorder). Our work provides a systematic study of the interplay between disorder and non-Hermiticity. In particular, we study the eigenspectrum in the complex frequency plane and we examine the localization properties of the eigenstates, either by the participation ratio or the level spacing, defined in the complex plane. A modified level distribution function vs disorder seems to fit our computational results.

Efficient 671 nm red light generation in annealed proton-exchanged periodically poled LiNbO3 waveguides

We report efficient generation of 671 nm red light based on quasi-phase-matched second harmonic generation of 1342 nm in LiNbO3 waveguides. The design method and fabrication process of the high-quality annealed proton-exchanged periodically poled channel waveguides were presented. A continuous-wave 1.71 mW red light was obtained with a single-pass conversion efficiency of 47%·W-1·cm-2, which is 88% that of the theoretical value. While for 1 mW quasi-continuous-laser input, the corresponding peak power being 2 W, the conversion efficiency reached up to 60%. Our results indicate that the annealed proton-exchanged periodically poled LiNbO3 waveguide is promising for high-efficiency and low power consumption nonlinear generation of visible light.

Лазерная модификация титановой пленки на поверхности оптических волноводов в ниобате лития

The availability of thin titanium film laser modification for precise loss control in lithium niobate optical waveguides was demonstrated. A simple ray model of the interaction between a nanosized metal cover on the top surface of a Ti-indiffused channel waveguide and orthogonal optical waveguide modes with changes in optical losses up to 0.95 and 1.05 dB/mm for TE- and TM-polarized modes respectively in case of 5 nm titanium film covering was suggested. The consistency between obtained theoretical losses and results of optical film modification by a 976 nm laser beam with the threshold intensity up to 1 kW/mm2 was demonstrated. The suggested method was applied for precise balance of optical field intensities in arms of Mach-Zehnder optical modulator; an associated increase of the extinction ratio from 30 to 48 dB was achieved.

Two-dimensional apodized grating coupler for polarization-independent and surface-normal optical coupling

A four-port grating coupler with etched square holes is proposed and demonstrated for surface-normal and polarization independent coupling. Benefiting from the perfectly vertical coupling and 1 × 4 power splitting/combing scheme, efficient polarization independent operation was achieved. The coupling efficiency was further improved by grating apodization. According to the simulations, the proposed 2D apodized grating coupler can achieve a coupling efficiency (CE) of 64.5% (−1.9 dB), a low upward back-reflection of 8.5% (−11 dB), and polarization-dependent loss (PDL) lower than 0.04 dB across the wavelength range of 1520–1620 nm. A CE of 56.3% (−2.5 dB) was experimentally measured, with PDL below 0.3 dB within the C-band and lower than 0.5 dB within the L-band. Waveguide delay lines were used to compensate the waveguide channel phase differences at the output port.

Carbon nanotube Q-switched Yb:KLuW surface channel waveguide lasers

A channel waveguide (WG) buried immediately below the surface of a Yb:KLuW crystal is used as a laser gain medium for passive Q -switching by both evanescent- and direct-field interactions with single-walled carbon nanotubes (SWCNTs) near 1040 nm. The SWCNTs used as saturable absorbers (SAs) are deposited on top of the half-ring-type channel WG fabricated via femtosecond direct laser writing. The Q -switched WG laser delivers 88.5 ns pulses at a 1.16 MHz repetition rate with a maximum average output power of 680 mW. For the two different interaction schemes with SWCNT-SAs, the pulse characteristics, depending on the output coupling ratio and absorbed pump power, are experimentally investigated and compared to the results of theoretical analyses of the SA Q -switched operation.

Lithium niobate waveguides with high-index contrast and preserved nonlinearity fabricated by a high vacuum vapor-phase proton exchange

Highly confining waveguides ( Δ n e > 0.1 ) without a degraded nonlinear coefficient and low propagation losses have been fabricated in lithium niobate (LN) by a new process that we called high vacuum vapor-phase proton exchange (HiVac-VPE). Index contrast, index profile, nonlinearity, and crystallographic phases are carefully investigated. Original analysis of index profiles indicates that the waveguides contain sub-layers whose depths depend on the exchange durations. Propagation behavior, propagation losses, and second-harmonic generation response of HiVac-VPE channel waveguides are investigated at telecom wavelength. The results recommend HiVac-VPE as a very promising technique for fabricating efficient nonlinear photonic integrated circuits in LN crystals.

In-depth structural analysis of swift heavy ion irradiation in KY(WO4)2 for the fabrication of planar optical waveguides

Rare-earth ion doped KY(WO4)2 is a well-known active laser crystal, due to its excellent gain characteristics and its relatively high nonlinear refractive index. As these properties are of great benefit to applications in integrated photonics, a study has been done into the fabrication of high refractive index contrast slab waveguides in KY(WO4)2 as a first step towards the fabrication of channel waveguides. When properly choosing the fluence and annealing parameters, ion irradiation with 12 MeV carbon ions produces a step-like damage profile. Confocal Raman microscopy, X-ray diffraction and transmission electron microscopy are used in this work to study the structural damage induced by ion irradiation. The characterization indicates damage to the crystal structure due to the ion irradiation that increases as a function of both depth and ion fluence till the threshold for amorphization is achieved. Successive annealing steps of the irradiated crystals at different temperatures show partial repair of the crystalline structure when the irradiation did not fully amorphize the material. When the threshold of amorphization was reached, annealing further increases the damage induced by the irradiation. By tuning the irradiation fluence, a high-refractive index contrast slab waveguide in KY(WO4)2 produced by ion irradiation was demonstrated.

Energy-transfer upconversion and excited-state absorption in KGdxLuyEr1-x-y(WO4)2 waveguide amplifiers

We perform a systematic spectroscopic study in channel waveguides of potassium gadolinium lutetium double tungstate doped with different Er3+ concentrations. Transition cross sections of ground-state absorption (GSA) and excited-state absorption (ESA), as well as stimulated emission (SE) at the pump wavelength around 980 nm are determined. ESA is directly measured by the pump-probe technique. Evaluation of GSA and ESA spectra indicates that ESA may be diminished by an appropriate choice of pump wavelength near 980 nm. Besides, GSA and SE at the signal wavelength around 1.5 µm are measured and the wavelength-dependent gain cross section as a function of excitation density is determined. Non-exponential luminescence decay curves from the 4I13/2 and 4I11/2 levels are analyzed and the probabilities of the energy-transfer-upconversion (ETU) processes (4I13/2, 4I13/2) → (4I15/2, 4I9/2) and (4I11/2, 4I11/2) → (4I15/2, 4F7/2) are quantified. Despite the large interionic distance between neighboring rare-earth sites in potassium double tungstates, the probability of ETU is comparatively large because of the large cross sections of the involved transitions. A rate-equation analysis of the influence of ETU and ESA on gain at ∼1.5 µm is performed, revealing that ETU from the 4I13/2 amplifier level strongly limits the gain when the doping concentration increases above ∼6at.%. The calculated maximum achievable internal net gain per unit length amounts to ∼15 dB/cm for an optimized Er3+ concentration of ∼4 × 1020 cm−3 and a launched pump power of 300 mW at a pump wavelength of 984.5 nm, in reasonable agreement with recent experimental results.

All-acoustic signal modulation and logic operation via defect induced cavity effects in phononic crystal coupled-resonator acoustic waveguides

A coupled resonant acoustic waveguide (CRAW) in a phononic crystal (PnC) was engineered to manipulate the propagation of ultrasonic waves within a conventional phononic bandgap for wavelength division multiplexing. The PnC device included two, forked, distinct CRAW waveguide channels that exhibited strong frequency and mode selectivity. Each branch was composed of cavities of differing volumes, with each giving rise to deep and shallow ‘impurity’ states. These states were utilized to select frequency windows where transmission along the channels was suppressed distinctly for each channel. Though completely a linear system, the mode sensitivity of each CRAW waveguide channel produced apparent nonlinear power dependence along each branch. Nonlinearity in the system arises from the combination of the mode sensitivity of each CRAW channel and small variations in the shape of the incident wavefront as a function of input power. The all-acoustic effect was then leveraged to realize an ultrasonic, spatial signal modulator, and logic element operating at 398 and 450 kHz using input power.

Investigation of characteristics of waveguide channels system formed in PDLC with inhomogeneity of the amplitude-phase distribution of forming field

This work presents the results of modeling the influence of amplitude, phase and amplitude-phase heterogeneities of the forming field on the spatial distribution of the refractive index profile and, accordingly, the mode composition of radiation capable of propagating in waveguides formed by the holographic method in the photopolymer-liquid crystalline compositions. The spatial distribution of the refractive index is found by solving the system of kinetic equations. The mode composition of the radiation is determined by solving the dispersion equation for TE-modes.

Distributed Bragg deflector coupler for on-chip shaping of optical beams.

In integrated optical circuits light typically travels in waveguides which provide both vertical and horizontal confinement, enabling efficient routing between different parts of the chip. However, for a variety of applications, including on-chip wireless communications, steerable phased arrays or free-space inspired integrated optics, optical beams that can freely propagate in the horizontal plane of a 2D slab waveguide are advantageous. Here we present a distributed Bragg deflector that enables well controlled coupling from a waveguide mode to such a 2D on-chip beam. The device consists of a channel waveguide and a slab waveguide region separated by a subwavelength metamaterial spacer to prevent uncontrolled leakage of the guided mode. A blazed grating in the waveguide sidewall is used to gradually diffract light into the slab region. We develop a computationally efficient strategy for designing gratings that generate arbitrarily shaped beams. As a proof-of-concept we design, in the silicon-on-insulator platform, a compact ×75 Gaussian beam expander and a partial beam deflector. For the latter, we also demonstrate a prototype device with experimental results showing good agreement with our theoretical predictions. We also demonstrate via a rigorous simulation that two such couplers in a back-to-back configuration efficiently couple light, suggesting that these devices can be used as highly directive antennas in the chip plane.

“Shutdown” of the Proton Exchange Channel Waveguide in the Phase Modulator under the Influence of the Pyroelectric Effect

It is shown that the termination of the channeling of the fundamental radiation mode in the waveguide can be observed upon heating of an optical integrated circuit based on proton exchange channel waveguides formed in a lithium niobate single crystal. This process is reversible, but restoration of waveguide performance takes tens of minutes. The effect of the waveguide disappearance is observed upon rapid heating (5 K/min) from a low temperature (minus 40 °C). This effect can lead to a temporary failure of navigation systems using fiber optic gyroscopes with modulators based on a lithium niobate crystal.

Design and Optimization of Proton Exchanged Integrated Electro-Optic Modulators in X-Cut Lithium Niobate Thin Film

In this study, we designed, simulated, and optimized proton exchanged integrated Mach-Zehnder modulators in a 0.5-μm-thick x-cut lithium niobate thin film. The single-mode conditions, the mode distributions, and the optical power distribution of the lithium niobate channel waveguides are discussed and compared in this study. The design parameters of the Y-branch and the separation distances between the electrodes were optimized. The relationship between the half-wave voltage length production of the electro-optic modulators and the thickness of the proton exchanged region was studied.

Ultrashort pulse propagation through depressed-cladding channel waveguides in YAG crystal: Spatio-temporal characterization

Inscription of optical waveguides by direct femtosecond laser irradiation has become a very versatile tool for the development of integrated photonic devices, such as waveguide lasers, frequency converters or photonic lanterns, among many others. The potential application of such devices for the control and manipulation of ultrashort pulses requires the precise knowledge of the temporal distortions that may be induced in the pulse propagation. Currently, research in this topic is scarce, and to our knowledge there is no previous experimental study on the spatio-temporal characterization at the output of waveguides inscribed inside crystals. Here, we have firstly fabricated depressed-cladding waveguides with different modal behavior in YAG crystal by direct femtosecond laser irradiation. Then, we implemented an experimental method based on the fiber coupler assisted spectral interferometry technique, that allows obtaining: (1) the temporal dispersion of a pulse at the output of an inscribed waveguide and, (2) full spatio-spectral and spatio-temporal characterization of the output of single-mode and multi-mode waveguides. Our results suggest that the main contribution to the pulse dispersion is due to the material dispersion. Moreover, we found that multimodal waveguides may induce an appreciable inhomogeneity in the temporal features of the pulses that needs to be taken into account in the design of complex devices.

Multipurpose Polymer Bragg Grating-Based Optomechanical Sensor Pad.

Flexible epoxy waveguide Bragg gratings are fabricated on a low-modulus TPX™ polymethylpentene polyolefin substrate for an easy to manufacture and low-cost optomechanical sensor pad providing exceedingly multipurpose application potentials. Rectangular EpoCore negative resist strip waveguides are formed employing standard UV mask lithography. Highly persistent Bragg gratings are inscribed directly into the channel waveguides by permanently modifying the local refractive indices through a well-defined KrF excimer laser irradiated +1/-1 order phase mask. The reproducible and vastly versatile sensing capabilities of this easy-to-apply optomechanical sensor pad are demonstrated in the form of an optical pickup for acoustic instruments, a broadband optical accelerometer, and a biomedical vital sign sensor monitoring both respiration and pulse at the same time.

Fabrication of low loss channel waveguide in tungsten-tellurite glass by 11 MeV carbon ion microbeam for telecom C band

Channel waveguides were directly written in Er: TeO2W2O3 glass using 11 MeV C4+ ion microbeam with fluences in the range of 1·1014–5·1016 ion/cm2. The channel waveguides supported a single guided mode at λ = 1540 nm. Propagation losses of the as-irradiated channel waveguides were around 14 dB/cm. A 30-min thermal annealing at 150 °C in air reduced propagation losses at λ = 1400 nm to 1.5 dB/cm. This method produced channel waveguides with confinement and propagation losses comparable to or better than other current methods, such as MeV energy focused proton or helium ion beam writing.

Universal deep learning representation of effective refractive index for photonics channel waveguides

An optical waveguide is the fundamental element in a photonic integrated circuit. This paper establishes a universal deep learning representation for the effective refractive index of an optical channel waveguide for the entire and usual parameter space for applications in photonics. The deep learning model is able to make precise predictions for wide spectrum optical wavelengths, dielectric materials of refractive indices varying from 1.45 to 3.8, and a wide range of feasible geometrical parameters of the waveguides. The deep learning model consists of fully connected feedforward neural networks, and rigorous optimization of neural network architecture is carried out. Deep learning models with two and three hidden layers provide rapid convergence with a minimal number of training data points and offer unprecedented precisions that are a few orders better in magnitude than the conventional interpolation techniques.

CNOT-based quantum swapping of polarization and modal encoded qubits in photonic Ti:LiNbO3 channel waveguides

A photonic circuit aimed at implementing a CNOT-based quantum swapping of the modal and polarization qubits, is designed and simulated, using Ti:LiNbO3 indiffused channel waveguides. A few on-chip optical elements such as the polarization-sensitive odd-mode analyzers and combiners, along with the electrically controllable directional couplers and polarization converters, are incorporated within the circuit and together serve to realize the operation of a deterministic single-photon quantum swap gate. Entanglement generation is realized on-chip by means of two concurrent spontaneous parametric down-conversion processes in the twice periodically poled Ti:LiNbO3 waveguide, placed at the input section of the circuit. The constituent elements of the circuit are characterized separately using numerical beam propagation method in RSoft program package. Two poling periods of $$\varLambda_{1} = 11.762\,\upmu{\text{m}}$$ and $$\varLambda_{2} = 23.667\,\upmu{\text{m}}$$ are calculated for the dual periodically perturbed structure in the circuit. The directional couplers operate at $$V_{DC} = 31.6$$ V. Maximum polarization conversion efficiency is achieved by applying a constant switching voltage of $$V_{EO} = 4.52$$ V.