Buried Waveguides Research Articles

3D adjoint-based marching scheme for optical propagation in inhomogeneous waveguides

This paper presents a three-dimensional adjoint-based marching scheme for long-range inhomogeneous optical waveguide devices. An efficient numerical propagation computation of an optical waveguide terminated by perfectly matched layers is available using our approach. It uses new transmission and reflection operators through a one-way formulation of the complex Helmholtz–PML equation and a design of the transverse and adjoint operators that calculates the coordinates through the biorthogonal of their eigenfunctions in each marching step. In addition, the validity and accuracy of our proposed method are demonstrated by analyzing a buried channel waveguide and a straight waveguide with varying refractive indices.

Broadband polarization-insensitive amplitude and phase modulators based on graphene-covered buried and ridge silicon waveguides

In this paper, four easy-to-fabricate graphene-based Si waveguide modulators are presented to overcome the strong polarization dependency of graphene-based modulators. The modulation features of two newly proposed structures, i.e. two graphene-based buried silicon waveguides in addition to two standard ridge silicon waveguides at the telecommunication wavelength of λ=1.55μm are studied. The results show that for certain widths of each waveguide (the height is constant), the amplitude and phase modulations clearly become polarization-insensitive. The amplitude modulation depths for both the TE and TM modes are equal for these optimized waveguides with the precision of 10−3 dB/μm. Moreover, in the proposed modulators, the maximum variations of the real parts of the effective mode indices (EMI) for both the TE and TM modes coincide with each other with an excellent precision (10−4). This precision value is much smaller than the standard criterion value for confirming a polarization-insensitive phase modulation. For proposed structures, the average modulation depth (MD) and maximum variation of the real parts of EMI are about 0.3 dB/μm and 0.02 respectively. Thus, it means to clearly imply small footprints for both amplitude and phase modulations. Furthermore, the performances of all the structures are studied for all optical telecommunication wavelengths. Even without making any changes to the structures designed at 1.55μm at appropriate wavelength intervals, the structures exhibit polarization-insensitive behaviors.

Low-Loss Buried-Heterostructure Optical Waveguide Based on Impurity-Free-Vacancy-Diffusion Quantum Well Intermixing

A new method for fabricating a high-quality buried-heterostructure optical waveguide using quantum well intermixing (QWI) has been demonstrated. By patterning a SiO2 thin film on top of a multiple quantum well (MQW) heterostructure, rapid thermal annealing (RTA) could induce laterally local QWI, resulting in a bandgap blueshift and a simultaneous decrease in the refractive index. Both lateral bandgap and index engineering could be attained along the MQW plane, which could be used for a buried-heterostructure optical waveguide. Two SiO2 strips with 3, 5 and 7 μm windows were fabricated for waveguide on a 1540 nm InGaAsP MQW sample. A 120 nm blueshift under the SiO2 area was observed, leading to the index contrast of 0.07. Far-field optical diffraction measurements were also performed to yield angles of 13.9°, 12.8° and 10.6°. A narrower window resulted in a narrower optical waveguide width and exhibited a larger diffraction angle, suggesting that QWI defined the buried optical waveguide. In addition, an electroabsorption modulator was also made by buried waveguide. A −10 dB low optical insertion loss and a 15 dB high extinction ratio in a 500 μm long waveguide were obtained, indicating that a buried heterostructure could be used for photonic devices and integration applications.

Mode, dispersion, and loss analysis of a graded-index germanium-doped silicon buried strip waveguide

The mode, loss, and dispersion characteristics of a buried strip waveguide have been calculated for coarse wavelength division multiplexing wavelengths. Process simulation has been used to form the graded-index core by germanium implantation in silicon. Quasivectorial finite-difference method has been used to calculate the number of propagating modes, effective indices, material absorption, mode confinement, and the dispersion parameters. The scattering loss has been determined from the Payne–Lacey model. The zero dispersion for higher-order modes occurs in the O-band, which is suitable for short-reach multimodal applications. The usable waveguide length and number of modes have been characterized and are limited by the mode with highest propagation loss/dispersion and lowest mode confinement, respectively.

Ultrafast laser inscribed waveguide lasers in Tm:CALGO with depressed-index cladding.

Depressed-index buried and surface channel waveguides (type III) are produced in a bulk 3.5 at.% Tm3+:CALGO crystal by femtosecond direct-laser-writing at kHz repetition rate. The waveguides are characterized by confocal microscopy and µ-Raman spectroscopy. Under in-band-pumping at 1679 nm (3H6 → 3F4 transition) by a Raman fiber laser, the buried channel waveguide laser with a circular cladding (diameter: 60 µm) generated a continuous-wave output power of 0.81 W at 1866-1947 nm with a slope efficiency of 71.2% (versus the absorbed pump power) and showed a laser threshold of 200 mW. The waveguide propagation losses were as low as 0.3 ± 0.2 dB/cm. The laser performance under in-band pumping was superior compared pumping at ∼800 nm (3H6 → 3H4 transition), i.e., the convetional pump wavelength. Vibronic laser emission from the WG laser above 2 µm is also achieved. The low-loss behavior, the broadband emission properties and good power scaling capabilities indicate the suitability of Tm3+:CALGO waveguides for mode-locked laser operation at ∼2 µm.

Design and analysis of a grating assisted hybrid mode and polarization division (de)multiplexer.

In this paper, a hybrid mode and polarization division (MDM-PDM) (de)multiplexer based on a buried strip waveguide, operating at 1550 nm, is proposed on the silicon-on-insulator platform. The proposed device can (de)multiplex three quasi-transverse electric and two quasi-transverse magnetic modes of a multimode waveguide concurrently. The multiplexing and demultiplexing operations are achieved by coupling the multimode waveguide modes with the fundamental modes of the single-mode waveguides. The couplings between the modes are realized by employing grating structures in between the multimode and single-mode waveguides. A 2.5D finite-difference time-domain simulation technique is used to examine the performance of the device. An insertion loss of >-0.76dB, return loss of <-11.23dB, and crosstalk of <-12.42dB are obtained for the proposed hybrid MDM-PDM (de)multiplexer. Moreover, the fabrication tolerance of the proposed device structure is studied by keeping the center-to-center distance between the waveguides constant and by varying the waveguide width.

BURIED WAVEGUIDE POLYMETHYLMETHACRYLATE MODELING FOR REFRACTIVE INDEX SENSOR APPLICATION USING FINITE ELEMENT METHOD

The purpose of this study is to obtain the optimum buried waveguide structure through modeling for refractive index sensor applications. The waveguide cladding material used as Polymethylmethacrylate (PMMA). The core cross-section size was 1 × 1 mm2. The simulation was carried out at a wavelength of 650 nm using the Finite Element Method (FEM). The parameter of the buried waveguide optimized in this model was the core refractive index and the thickness of the upper cladding to obtain a high propagation constant and good sensitivity to refractive index. Modeling was done for various core refractive index values ​​varied in the range of 1.52 to 1.59, which are the refractive index of various types of polymers. To optimize the sensitivity, the thickness of the upper cladding was varied between 0.125mm to 0.5mm. Besides, a simulation was also carried out for a waveguide without an upper cladding. The results show that the optimum waveguide is a waveguide without upper cladding using polyester as core material with a refractive index value of 1.57 and a sensitivity of 4.9 × 10-10rad /m. RIU.

Parametric analysis and comparative study of multimode waveguides on lithium niobate-on-insulator and silicon-on-insulator platforms

Multimode waveguides on lithium niobate-on-insulator (LNOI) and silicon-on-insulator (SOI) platforms are numerically investigated in buried, rib, and strip configurations. Performance of waveguides is compared in terms of waveguide cross-sectional area, dispersion, mode hybridization, and power confinement for both quasi-transverse electric and quasi-transverse magnetic modes. Tall waveguides with single mode in the horizontal direction, supporting higher order modes in the vertical direction, are analyzed. Also, wide waveguides with single mode in the vertical direction, supporting higher order modes in the horizontal direction, are studied. Designs that overcome mode hybridization are proposed, which are well suited for applications such as optical interconnects. LNOI waveguides were found to exhibit lower dispersion in all the configurations, with power confinement and physical dimensions comparable to those of SOI waveguides. The results are instrumental in design optimization of multimode components for on-chip mode-division multiplexing schemes and multiparameter sensing applications. Electro-optic effect is also illustrated in a buried multimode LNOI waveguide that is largely useful in modulation and switching applications.

Hybrid plasmonic graphene modulator with buried silicon waveguide

We propose a low-loss broadband modulator by combining plasmonic effect with graphene to realize highly efficient modulation on the SOI (Silicon-On-Insulator) platform. Due to a hybrid plasmonic waveguide including buried silicon waveguide and silver waveguides, the fundamental mode is mainly confined in two single-layer graphene sheets, and the electric field vector is parallel to the graphene sheets ensuring the high efficient light absorption of graphene. The specific hybrid structure makes it easy for graphene transfer. Also, non-coplanar structure of silicon waveguide and silver waveguides provides simplified fabrication process and fabrication tolerance. The simulation results demonstrated that modulation bandwidth of 346 GHz is obtained, and the 3 dB modulation length is only 9.493μm. In addition, competitively low propagation loss of 0.85 dB was realized of the hybrid modulator.

Refractive index change and thermo-optic effect in polydimethylsiloxane nanocomposites with oxide nanoparticles induced by proton beam writing

We studied polydimethylsiloxane nanocomposites with oxides such as TiO2, SiO2, and Al2O3 aiming at proton beam writing (PBW) of buried optical waveguides. Relatively high transmission was achieved for 1.0 wt% TiO2/PDMS composite without aggregation. The refractive index can be tunable by varied TiO2 concentration in nanocomposites. We evaluated refractive index changes by 1.0 MeV proton irradiation. Refractive index of TiO2/PDMS nanocomposite increased at low fluence of less than 100 nC/mm2 and then decreased at higher fluence. On the other hand, we observed monotonic increase of the refractive index as a function of fluence for PDMS and PDMS nanocomposites with SiO2 and Al2O3. We also compared thermo optic effect of PDMS nanocomposites. We evaluated buried optical waveguides in TiO2/PDMS nanocomposite written by PBW for near field patterns with single mode waveguide.

Femtosecond-laser-written Ho:KGd(WO4)2 waveguide laser at 2.1 μm.

We report on efficient laser operation of the first holmium monoclinic double tungstate waveguide laser fabricated by femtosecond direct laser writing. A depressed-index buried channel waveguide with a 60μm diameter circular cladding was inscribed in 5at.% Ho3+:KGd(WO4)2. It was characterized by confocal microscopy and μ-Raman and μ-luminescence spectroscopy, indicating well-preserved crystallinity of its core. Pumped by a thulium bulk laser, the holmium waveguide laser generated 212mW at 2055nm with a slope efficiency of 67.2%. The waveguide propagation losses were 0.94±0.2 dB/cm.

Laser-written depressed-cladding waveguides deep inside bulk silicon

Laser-written waveguides created inside transparent materials are important components for integrated optics. Here, we demonstrate that subsurface modifications induced by nanosecond pulses can be used to fabricate tubular-shaped “in-chip” or buried waveguides inside silicon. We first demonstrate single-line modifications, which are characterized to yield a refractive index depression of ≈2×10−4 compared to that of the unmodified crystal. Combining these in a circular geometry, we realized 2.9-mm-long, 30-μm core-diameter waveguides inside the wafer. The waveguides operate in a single-mode regime at a wavelength of 1300 nm. We use near- and far-field imaging to confirm waveguiding and for optical index characterization. The waveguide loss is estimated from scattering experiments as 2.2 dB/cm.

Fs-laser-written thulium waveguide lasers Q-switched by graphene and MoS2.

We report the generation of mid-infrared (~2 µm) high repetition rate (MHz) sub-100 ns pulses in buried thulium-doped monoclinic double tungstate crystalline waveguide lasers using two-dimensional saturable absorber materials, graphene and MoS2. The waveguide (propagation losses of ~1 dB/cm) was micro-fabricated by means of ultrafast femtosecond laser writing. In the continuous-wave regime, the waveguide laser generated 247 mW at 1849.6 nm with a slope efficiency of 48.7%. The laser operated at the fundamental transverse mode with a linearly polarized output. With graphene as a saturable absorber, the pulse characteristics were 88 ns / 18 nJ (duration / energy) at a repetition rate of 1.39 MHz. Even shorter pulses of 66 ns were achieved with MoS2. Graphene and MoS2 are therefore promising for high repetition rate nanosecond Q-switched infrared waveguide lasers.

Suspended mid-infrared waveguides for Stimulated Brillouin Scattering.

We theoretically investigate a new class of silicon waveguides for achieving Stimulated Brillouin Scattering (SBS) in the mid-infrared (MIR). The waveguide consists of a rectangular core supporting a low-loss optical mode, suspended in air by a series of transverse ribs. The ribs are patterned to form a finite quasi-one-dimensional phononic crystal, with the complete stopband suppressing the transverse leakage of acoustic waves, confining them to the core of the waveguide. We derive a theoretical formalism that can be used to compute the opto-acoustic interaction in such periodic structures, and find forward intramodal-SBS gains up to 1750 m-1W-1, which compares favorably with the proposed MIR SBS designs based on buried germanium waveguides. This large gain is achieved thanks to the nearly complete suppression of acoustic radiative losses.

Fs-laser-written erbium-doped double tungstate waveguide laser.

We report on the first erbium (Er3+) doped double tungstate waveguide laser. As a gain material, we studied a monoclinic Er3+:KLu(WO4)2 crystal. A depressed-index buried channel waveguide formed by a 60 µm-diameter circular cladding was fabricated by 3D femtosecond direct laser writing. The waveguide was characterized by confocal laser microscopy, µ-Raman and µ-luminescence mapping, confirming that the crystallinity of the core is preserved. The waveguide laser, diode pumped at 981 nm, generated 8.9 mW at 1533.6 nm with a slope efficiency of 20.9% in the continuous-wave regime. The laser polarization was linear (E || Nm). The laser threshold was at 93 mW of absorbed pump power.

Passive Q-Switching by Cr4+:YAG Saturable Absorber of Buried Depressed-Cladding Waveguides Obtained in Nd-Doped Media by Femtosecond Laser Beam Writing.

We report on laser performances obtained in Q-switch mode operation from buried depressed-cladding waveguides of circular shape (100 μm diameter) that were inscribed in Nd:YAG and Nd:YVO4 media by direct writing with a femtosecond laser beam. The Q-switch operation was realized with a Cr4+:YAG saturable absorber, aiming to obtain laser pulses of moderate (few μJ) energy at high (tens to hundreds kHz) repetition rate. An average power of 0.52 W at 1.06 μm consisting of a train of pulses of 7.79 μJ energy at 67 kHz repetition rate, was obtained from a waveguide realized in a 4.8 mm long, 1.1-at % Nd:YAG ceramics; the pulse peak power reached 1.95 kW. A similar waveguide that was inscribed in a 3.4 mm long, 1.0-at % Nd:YVO4 crystal yielded laser pulses with 9.4 μJ energy at 83 kHz repetition rate (at 0.77 W average power) and 1.36 kW peak power. The laser performances obtained in continuous-wave operation were discussed for each waveguide used in the experiments. Thus, a continuous-wave output power of 1.45 W was obtained from the circular buried depressed-cladding waveguide inscribed in the 1.1-at %, 4.8 mm long Nd:YAG; the overall optical-to-optical efficiency, with respect to the absorbed pump power, was 0.21. The waveguide inscribed in the 1.0-at %, 3.4 mm long Nd:YVO4 crystal yielded 1.85 W power at 0.26 overall optical efficiency. This work shows the possibility to build compact laser systems with average-to-high peak power pulses based on waveguides realized by a femtosecond (fs) laser beam direct writing technique and that are pumped by a fiber-coupled diode laser.

Integrated waveguides and deterministically positioned nitrogen vacancy centers in diamond created by femtosecond laser writing.

Diamond's nitrogen vacancy (NV) center is an optically active defect with long spin coherence times, showing great potential for both efficient nanoscale magnetometry and quantum information processing schemes. Recently, both the formation of buried 3D optical waveguides and high-quality single NVs in diamond were demonstrated using the versatile femtosecond laser-writing technique. However, until now, combining these technologies has been an outstanding challenge. In this Letter, we fabricate laser-written photonic waveguides in quantum grade diamond which are aligned to within micron resolution to single laser-written NVs, enabling an integrated platform providing deterministically positioned waveguide-coupled NVs. This fabrication technology opens the way toward on-chip optical routing of single photons between NVs and optically integrated spin-based sensing.

Buried Rib SiO2 Multimode Interference Waveguides for Optofluidic Multiplexing.

Multimode interference (MMI) waveguides can be used to create wavelength-dependent spot patterns which enables simultaneous analyte detection on a single optofluidic chip, useful for disease diagnostics. The fidelity of such multi-spot patterns is important for high sensitivity and accurate target identification. Buried rib structures have been incorporated into these SiO2-based waveguides to improve environmental stability. Through experiments and simulation, this letter explores design parameters for a buried MMI rib waveguide based on anti-resonant reflecting optical waveguides in order to produce high-fidelity spot patterns. Optimal rib heights and widths are reported in the context of available microfabrication etch technology and performance for an optimized biosensor is shown.

Computation of ion exchange buried microring resonator waveguide for THz communication applications

Abstract The ion exchange creates the optical buried waveguides, whereas it is the method that the glass doping occurs with the condition of existence of the mobile metal ions. The most common ion used is silver (Ag+) due to its broad variation in the refractive index. The proposed application presented in this work is to use the microring resonator (MRR) system based on the buried waveguides simulated by the ion exchange method. The passive MRRs are realized as the multi-wavelength transceiver, applicable in optical technologies such as the optical sensing, Terahertz (THz) communication and optical switching. The cross-sectional view of the simulated buried optical waveguide for the Ag+ concentrations of 0.8 mol/m3 is presented. The fundamental mode propagation is simulated using the time-domain traveling wave method, where the simulation of the optical power flow density and the electric field distributions of the buried waveguides were performed. The two coupled MRRs are used to generate tunable dual-wavelength with spacing in the range of Terahertz (THz). As results, the dual wavelengths in the range between 0.25 and 0.96 THz were generated at the throughput and drop port of the MRRs system.

Femtosecond-laser-written hexagonal cladding waveguide in Tm:KLu(WO_4)_2: µ-Raman study and laser operation

We report on the fabrication, µ-Raman characterization, and continuous-wave laser operation of a channel waveguide with a hexagonal optical-lattice-like cladding fabricated in monoclinic Tm:KLu(WO4)2 crystal by femtosecond direct laser writing. µ-Raman spectroscopy indicates preservation of the crystalline quality in the core region and an anisotropic residual stress field. When pumped by a Ti:Sapphire laser at 802 nm, the Tm:KLu(WO4)2 buried channel waveguide laser generated 136 mW at 1843.7 nm with a slope efficiency of 34.2% and a threshold as low as 21 mW, which are the record characteristics for femtosecond-laser-written Tm crystalline waveguide lasers. The variation of the output coupling resulted in discrete wavelength tuning of the laser emission from 1785 to 1862 nm. The propagation losses in the waveguide are ~1.2 ± 0.3 dB/cm.