Low Waveguide Research Articles

Transmission Characteristics Analysis of a Twin-Waveguide Cavity

The transmission spectrum of a twin-waveguide cavity is systematically analyzed based on coupled mode theory, using the transfer matrix method (TMM). The results show that the traveling-wave transmission spectra of the twin-waveguide cavity is entirely determined by the coherent coupling effect involving the parameters of the effective refractive indices of the upper and lower waveguides, the coupling length Lc, and the ratio of the cavity length L to the coupling length (L/Lc). Filters with single, double, or triple-notch filtering could be obtained by choosing an appropriate L/Lc value. When the facet reflection is taken into consideration, the traveling-wave transmission spectrum is modified by the Fabry––Perot (FP) resonance, making it a standing-wave transmission spectrum. As a result, resonance splitting has been observed in the transmission spectrum of twin-waveguide resonators with high facet reflectivity. Further analysis shows that such an abnormal resonance phenomenon can be attributed to the destructive interference between the two FP resonance modes of the upper and lower waveguide through coherent coupling. In addition, narrow bandwidth amplification has also been observed through asymmetric facet reflections. Undoubtedly, all these unique spectral characteristics should be beneficial to the twin-waveguide cavity, achieving many more functions and being widely used in photonic integration circuits (PICs).

480 nm InGaN-based cyan laser diode grown on Si by interface engineering of active region

InGaN-based long wavelength laser diodes (LDs) grown on Si are highly desirable for expanding the applications in laser display and lighting. Proper interface engineering of high In-content InGaN multi-quantum wells (MQWs) is urgently required for the epitaxial growth of InGaN-based long wavelength LD on Si, because the deteriorated interfaces and crystalline quality of InGaN MQWs can severely increase the photon scattering and further exacerbate the internal absorption loss of LDs, which prevents the lasing wavelength of InGaN-based LDs from extending. In this work, a significantly improved morphology and sharp interface of the InGaN active region are obtained by using a graded-compositional InGaN lower waveguide (LWG) capped with a 10-nm-thick Al0.1Ga0.9N layer. The V-pits density of the InGaN LWG was one order of magnitude reduction from 4.8 × 108 to 3.6 × 107 cm-2 along with the root-mean-square surface roughness decreasing from 0.3 to 0.1 nm. Therefore, a room-temperature electrically injected 480 nm InGaN-based cyan LD grown on Si under pulsed current operation was successfully achieved with a threshold current density of 18.3 kA/cm2.

Double Fano sensing mechanism of waveguide coupled surface plasma

Based on the optical properties of waveguide-coupled surface plasmon resonance, a sensor of surface plasmon coupled waveguide resonance with double discrete states generated by the upper and lower waveguide structures combined with prisms and Au films, respectively, is proposed. In this paper, the sensing structure is studied numerically and analytically, and the transmission characteristics of surface equipartition excitations are analyzed and elaborated in-depth in combination with the reflection angle spectrum, as well as the generation and evolution mechanism of dual Fano resonance at different incidence angles at fixed wavelengths. The structural model of Fano resonance sensing based on angle modulation is established, and the sensing characteristics are analyzed to produce electromagnetic field enhancement at low-angle Fano. The mathematical model between structural parameters and spectral performance is established using back propagation (BP) neural network, and the particle swarm optimization (PSO) algorithm is used to find the input structural parameters corresponding to the optimal performance, The results show that the quality factor of resonance (FOM) and sensitivity is significantly improved; When it is used in the field of biosensing, the response curves of Fano with different angles of high and low to concentration are quite different, thus realizing high precision detection of different concentrations of solutions and reflecting excellent sensing performance.

Low threshold current density and high power InGaN-based blue-violet laser diode with an asymmetric waveguide structure.

Performance of InGaN-based blue-violet laser diodes (LD) with different waveguide structure were investigated by simulation and experimental methods. Theoretical calculation demonstrated that threshold current (Ith) can be reduced and slope efficiency (SE) can be improved by using an asymmetric waveguide structure. Based on the simulation results, a LD with 80-nm-thick In0.03Ga0.97N lower waveguide (LWG) and 80-nm-thick GaN upper waveguide (UWG) is fabricated with flip chip package. Under continuous wave (CW) current injection at room temperature, its optical output power (OOP) reaches 4.5 W at an operating current of 3 A and the lasing wavelength of 403 nm. The threshold current density (Jth) is 0.97 kA/cm2 and the SE is about 1.9 W/A.

Performance enhancement of an AlGaN-based deep-ultraviolet laser diode using a two-stepped doped lower waveguide

Subject of study. The performance of a deep-ultraviolet laser diode is improved using a two-stepped Si-doped lower waveguide. Method. The impact of the variations on AlGaN-based deep-ultraviolet laser diodes has been evaluated based on the simulated results after theoretical calculations. The two AlGaN-based ultraviolet laser diodes as a traditional device and the proposed device have been analyzed comparatively based on their performance within a nomination wavelength region range of 269–280 nm. Main results. Using a two-stepped Si-doped lower waveguide, the lasing threshold current decreases in the proposed device D2 in comparison to the traditional device D1. The operating threshold voltage of D1 is 13.8 V, and that of D2 is 4.24 V, respectively, with lasing threshold currents of 0.4 A and 0.002 A, respectively. Practical significance. The optimization of the design of AlGaN-based ultraviolet laser diodes following the perfect bulk aluminium nitride substrate in the traditional device is the crucial factor of the practical significance of the scientific field utilized in the proposed device. The performance of deep-ultraviolet laser diodes is improved when a suitably constructed two-stepped Si-doped lower waveguide substitutes the traditional lower waveguide. The noted improvements are the reduction in total optical loss with the improved optical confinement factor. These are primarily due to an increase in the hole injection current and a decrease in the electron leakage current.

Near-infrared broadband polarization beam splitter with an Au nanocube array

A near-infrared broadband polarization beam splitter (PBS) is proposed and numerically simulated, which is realized by a directional coupler assisted with an Au nanocube array. The fundamental transverse electric (TE) mode can be coupled to the output of the upper waveguide by exciting the surface plasmon polaritons (SPPs), while the fundamental transverse magnetic (TM) mode is output directly from the lower waveguide. The length of the coupling region of the PBS is only 1 μm. It has a working bandwidth of 210 nm in the range of 1420 ∼ 1630 nm, covering three bands: S-, C- and L-bands. The simulation shows that the extinction ratio of TE and TM polarization is 20.31 dB and 14.15 dB, respectively.

Wide-waveguide high-power low-RIN single-mode distributed feedback laser diodes for optical communication.

We present an 8-µm-wide 800-µm-long high-power, single-mode and low RIN DFB laser using a dual-waveguide structure. The introduced passive lower waveguide has weakenes the lateral optical confinement for the ridge waveguide, and thus reduces losses caused by the p-doped layers and maintains single mode stability of the laser. The fabricated laser exhibited an output power higher than 170 mW and a relative intensity noise (RIN) below -157 dB/Hz along with a side-mode suppression-ratio (SMSR) over 55 dB. The temperature tuning from -10°C to 60°C allows an 8.6 nm wavelength tunability with a variation coefficient of 0.12 nm/K. The relaxation oscillation frequency is around 8 GHz, and the linewidth is about 250 kHz at 100 mW output power for the fabricated laser. The characteristics of the proposed high-power laser, including high slope efficiency, single mode stability and low noise, make it a suitable candidate for optical communication.

Improving Output Efficiency of InGaN-Based MQW Green Laser Diodes by Modulating Indium Content of Quantum Barriers and Using Composite Lower Waveguide Layers.

Potential barriers between the waveguide layer and MQW active region may influence injection efficiency significantly, which is important in improving output characteristics of GaN-based green laser diodes (LDs). In this study, potential barriers and injection efficiency of LDs are investigated by simulation methods. It is found that different indium content in quantum barrier layers results in different potential barrier heights, leading to different recombination rates in upper and lower waveguide layers, and the injection efficiency can be modulated effectively. An eclectic choice of indium content can suppress recombination in two waveguide layers, improving the output characteristics of green LDs. Additionally, a composite lower waveguide layer structure is proposed to reduce the negative effect of potential barriers. High output power and low threshold current are achieved owing to the reduction in electron injection blockage and hole leakage effects.

Sensing performance of temperature insensitive microring resonators with double-layer U-shaped waveguide

An temperature insensitive microring resonator with double-layer U-shaped waveguide for sensing applications is proposed and investigated theoretically. The unique structure of microring resonator with double-layer U-shaped waveguide not only limits the light field in the ring, but also releases the evanescent field from the waveguide through the coupling of the upper and lower layers, thus enhancing the interaction between light and matter and improving the sensing ability. Then, through correctly selecting the cross-sectional dimensions of the waveguide, the bending radius, the spacing between the upper and lower waveguides and the shape of overlapping area, it is designed to have very low temperature dependence in aqueous solution, so as to obtain sensing accuracy and stability. The simulation results show that the sensitivity is 496 nm/RIU ± 5 pm/K. Within the temperature change range of 3.2 K, it can be considered that the measurement result is irrelevant to the change in ambient temperature. Compared with the other ring resonators, this resonator does not need extra temperature compensation equipment, active temperature controller, polymer cladding, or even extra processing of the measured data to eliminate the effect of temperature. Thanks to its excellent performance in temperature insensitive and sensing performance, the device is suitable for refractive index measurement in a complex environment. It might have broad application prospects in portable real-time monitoring equipment, for example, analyzing health status from the change of sweat composition, or detecting the content of microorganisms and other harmful substances in drinking water.

Effects of the Stepped-Doped Lower Waveguide and a Doped p-Cladding Layer on AlGaN-Based Deep-Ultraviolet Laser Diodes

Effects of the Stepped-Doped Lower Waveguide and a Doped p-Cladding Layer on AlGaN-Based Deep-Ultraviolet Laser Diodes

Calibration Alignment Sensitivity in Corneal Terahertz Imaging.

Improving the longitudinal modes coupling in layered spherical structure contributes significantly to corneal terahertz sensing, which plays a crucial role in the early diagnosis of cornea dystrophies. Using a steel sphere to calibrate reflection from the cornea sample assists in enhancing the resolution of longitudinal modes. The requirement and challenges toward applying the calibration sphere are introduced and addressed. Six corneas with different properties are spotted to study the effect of perturbations in the calibration sphere in a frequency range from 100 GHz to 600 GHz. A particle-swarm optimization algorithm is employed to quantify corneal characteristics considering cases of accurately calibrated and perturbed calibrated scenarios. For the first case, the study is carried out with signal-to-noise values of 40 dB, 50 dB and 60 dB at waveguide bands WR-, WR-, and WR-. As expected, better estimation is achieved in high-SNR cases. Furthermore, the lower waveguide band is revealed as the most proper band for the assessment of corneal features. For perturbed cases, the analysis is continued for the noise level of 60 dB in the three waveguide bands. Consequently, the error in the estimation of corneal properties rises significantly (around ).

Enhancing performance of GaN-based LDs by using GaN/InGaN asymmetric lower waveguide layers

The effects of GaN/InGaN asymmetric lower waveguide (LWG) layers on photoelectrical properties of InGaN multiple quantum well laser diodes (LDs) with an emission wavelength of around 416 nm are theoretically investigated by tuning the thickness and the indium content of InGaN insertion layer (InGaN-IL) between the GaN lower waveguide layer and the quantum wells, which is achieved with the Crosslight Device Simulation Software (PIC3D, Crosslight Software Inc.). The optimal thickness and the indium content of the InGaN-IL in lower waveguide layers are found to be 300 nm and 4%, respectively. The thickness of InGaN-IL predominantly affects the output power and the optical field distribution in comparison with the indium content, and the highest output power is achieved to be 1.25 times that of the reference structure (symmetric GaN waveguide), which is attributed to the reduced optical absorption loss as well as the concentrated optical field nearby quantum wells. Furthermore, when the thickness and indium content of InGaN-IL both reach a higher level, the performance of asymmetric quantum wells LDs will be weakened rapidly due to the obvious decrease of optical confinement factor (OCF) related to the concentrated optical field in the lower waveguide.

Mode Theory and Propagation of ELF Radio Wave in a Multilayered Oceanic Lithosphere Waveguide

Extremely low-frequency (ELF: below 30 Hz) wave penetration theory in the layered oceanic lithosphere is essential for studying submarine communications. In this article, we investigate the mode theory of ELF wave propagation in a layered oceanic lithosphere waveguide. Based on the real situation of the oceanic lithosphere approximated by the stratified prototype, in which a low-conductivity layer lies between two overlying and underlying high-conductivity regions, a theoretically sound physical model of oceanic lithosphere waveguide is built. The modal equations for transverse magnetic (TM) and transverse electric (TE) polarized guided modes are delivered using surface impedance boundary conditions. By evaluating the reflection coefficients of the upper and lower waveguide boundaries, formulas for the modal equation's roots are derived. Analytical expressions for propagable mode parameters, such as the phase velocity, attenuation rate, excitation factor, and the field strength for different components, are obtained, and the cutoff frequency for high-order modes is given. Computations show that the field component curves exhibit typical peaks and nulls associated with multiple-mode propagation in the waveguide region, with each mode traveling at different phase velocities and attenuation rates. Moreover, it is found that all the high-order modes are evanescent below the cutoff frequency.

The influence of material quality of lower InGaN waveguide layer on the performance of GaN-based laser diodes

We investigated the influence of surface morphology, V-defects and residual impurities of InGaN lower waveguide layer on the performance of GaN-based laser diodes. It is found that the surface roughness is one of key factors to affect the L-I and I-V characteristics of lasers. A smooth interface/surface is critical to reduce the threshold current and resistance of the laser. Reducing the V-defects and impurities in InGaN waveguide layer is also expected to raise current injection efficiency and reduce series resistance of lasers.

Effect of stress and groove geometry on the performance of an IBG filter with Si3N4-filled corrugations

We design and estimate the performance of an integrated Bragg grating (IBG) filter that has Si3N4-filled periodic grooves on a silicon strip waveguide. Also, the entire IBG filter is covered with Si3N4 cladding. The refractive index variation along the length of the filter is achieved by the periodic Si3N4-filled corrugations. We consider different geometries of grooves that can be other than the ideal rectangular shape as a result of fabrication limitations. Moreover, we also include the stress-optic effect, which arises because of the different thermal expansion coefficients and lattice constants of the core and cladding materials, to estimate filter performance. We categorically estimate the impact of groove geometry and the stress-optic effect on the filter performance—namely, resonance wavelength, bandwidth, extinction ratio, and side-lobe suppression ratio. Also, the effects of varying the waveguide width, cladding thickness, and corrugation depth on the optical performance of the proposed filter is studied. Our numerical studies reveal that a larger magnitude of stress can be found for lower waveguide width, lower cladding thickness, lower corrugation depth, and rectangular corrugation shape. The estimated changes in the resonance wavelength, bandwidth, extinction ratio, and side-lobe suppression ratio due to the stress effect and geometrical variation of the groove for the IBG filter designed with a waveguide width of 300 nm (600 nm), cladding thickness of 100 nm (400 nm), and rectangular corrugation depth of 10 nm (50 nm) are 1891 pm (1593 pm), 21% (3.8%), 27.2% (5.5%), and 26.9% (5.4%), respectively. The significant performance variation in the IBG filter due to stress and groove shapes emphasizes the need to address these factors to estimate the performance of the proposed IBG filter.

Solution-processed PEDOT:PSS:GO/Ag NWs composite electrode for flexible organic light-emitting diodes

Flexible organic light emitting diodes (OLEDs) have attracted considerable attention for the reason of light weight, high mechanical flexibility in display and lighting. The most widely used transparent anode indium tin oxide (ITO) is unsuitable for flexible OLEDs because of its easy cracking upon bending. In this paper, we proposed a simple two steps solution processing method to fabricate flexible PEDOT:PSS:GO/Ag NWs composite electrodes. The optimized PEDOT:PSS:GO/Ag NWs composite electrode exhibits an optical transmittance of 88.7% at a wavelength of 550 nm and a low sheet resistance of 17 Ω/sq, which arecomparable to that of ITO. With PEDOT:PSS:GO/Ag NWs composite electrodes, the turn on voltage, current density and maximum brightness of OLEDs based on composite electrode were 2.1 V, 6.2 cd/A and 22894 cd/m2, respectively, which were superior to that OLED based on ITO anode. The enhanced performance of OLEDs based on composite anode mainly attributed to the lower sheet resistance, smoother surface of the composite anode and the far surface plasma resonance (Far SPR) effect, a lower waveguide optical loss because of the introduction of Ag NWs in the electrode.

Improving optical and electrical properties of InGaN-based green laser diodes by graded-compositional waveguide structure

A graded-compositional waveguide structure is proposed to improve the optical field distribution of InGaN-based green laser diodes (LDs). It is found that the optical field leaking into the GaN substrate is suppressed obviously when using the graded-compositional upper and lower waveguide. Owing to the enhancement of refractive index contrast between the waveguide layer and the cladding layer, the optical field confinement factor increases, and the optical field is more concentrated near the active region. Besides, higher output power and lower threshold current are achieved due to reduced electron leakage and increased hole injection. This work provides a theoretical design method for obtaining high-performance InGaN-based green LDs. • A new structure with graded-compositional upper and lower waveguide is proposed. • The optical field leaking into GaN substrate is obviously reduced. • High output power and low threshold current are achieved. • The photoelectric performance of green LDs is significantly improved.

Performance improvement of GaN-based blue and ultraviolet double quantum well laser diodes by using stepped-doped lower waveguide

We have proposed a new stepped-doped lower waveguide (LWG) and systematically investigated its effect on GaN-based blue and ultraviolet laser diodes (LDs) through the simulation software LASTIP. It is found that increasing doping concentration of the LWG is beneficial to decrease the threshold current and raise the output power. Furthermore, when a suitably designed stepped-doped LWG layer replaces the homogeneously heavily doped LWG layer, the performance of the blue and ultraviolet LDs is further improved. It is mainly attributed to the decrease of electron leakage current and the increase of hole injection current. In addition, the stepped-doped LWG improves ultraviolet LDs characteristics more significantly than the blue LDs, which is due to the reduction of the total optical loss.

Hybrid III-V-on-SOI optical spot size converter by self-aligned selective undercut dry etching of Si.

A technology called self-aligned selective undercut dry etching processing has been demonstrated for fabricating a highly efficient hybrid optical spot size converter (SSC) on a Si-on-insulator (SOI) template. The process was based on a bonded wafer between the upper InP-based multiple quantum well heterostructure and the lower SOI substrate. After defining the mask on the upper InP-based ridge waveguide, CF4/O2 dry reactive ion etching was then used for selective undercut etching of the Si material from the surrounding materials, forming a vertical waveguide coupler of the optical SSC. The lower waveguide, whose dimension is even smaller than the upper one, can thus be vertically self-aligned to the top ridge via an independent processing step. A laterally tapered waveguide ranging from 0.3 to 3 µm in width on the upper InP waveguide was fabricated. The phase-matching condition of the vertical coupler leads to a length of 45 µm and extracts 88% conversion efficiency. The selective undercut etching processing in III-V/SOI material provides a vertical self-alignment scheme for realizing compact and submicron scale heterogeneous integration in a Si photonics template.

Different influence of InGaN lower waveguide layer on the performance of GaN-based violet and ultraviolet laser diodes

Abstract Two Series of InGaN/GaN quantum well laser diodes (LDs) with different InxGa1-xN lower waveguide (LWG) layers are investigated. For violet LDs with lasing wavelength of 405 nm, the threshold current and the output power are improved when the indium content of LWG increases from 0 to 6%. It is mainly attributed to the increase of optical confinement factor (OCF) and the decrease of the total optical loss (TOL) which are caused by the increasing difference of the refractive indices between InGaN and GaN (and AlGaN in cladding layer). However, for ultraviolet LDs lasing at 377 nm, the InxGa1-xN LWG is not beneficial to the emission performance due to the decrease of OCF. Furthermore, when the indium content of LWG reaches a higher level, the performance of both violet and ultraviolet LDs will be weakened due to the obvious increase of carrier leakage current related to the enhancement of piezoelectric polarization effect.