Bragg Waveguide Research Articles

Hollow-core terahertz Bragg waveguide based on a helical support structure.

A helical structural support scheme is proposed to mechanically support dielectric layers in a hollow-core terahertz Bragg waveguide by means of an axially rotating wrap-around strip structure. The helical-strip supported waveguide samples are fabricated using 3D printing technology, and the waveguide samples are experimentally tested using a terahertz time-domain spectroscopy system. The results show that choosing a suitable helix period can obtain loss characteristics close to those of an ideal Bragg waveguide, with a low transmission loss of less than 0.097dB/m in the range of 0.57-0.58THz.

Structural polymorphism, electronic, and optical properties of NaSbS2: A computational approach toward eco-friendly and emerging semiconductor

In this work, the structural polymorphism and electro-optical characteristics of NaSbS2 ternary chalcogenide are realized using density functional theory (DFT). All of the NaSbS2 polymorphs are energetically stable based on the formation energy. The optimized lattice parameters of the monoclinic and triclinic structures and direct electronic bandgap (1.695 eV) of monoclinic structure are consistent with previous studies. Besides, the trigonal and triclinic structures are indirect bandgap (0.916 and 2.015 eV) semiconductors observed from electronic band structure. The higher degree of ionicity between Na and S atoms while strong covalency in the Sb-S bonds are confirmed from the charge and bond analysis. The lower value of electron-hole effective masses indicates higher carrier mobility leading to outstanding electrical conductivity. It was suggested that the polymorphs would be extremely desired to design Bragg reflectors and waveguides based on the reported high value of the real part of the dielectric constant and refractive index. These polymorphs will contribute significantly to being used as absorbers, as evidenced by their high absorption coefficient and photoconductivity and lower transmittance. All the polymorphs reflect significant optical anisotropy and birefringence, rendering them excellent for usage in the fabrication of waveguides, polarizers, and photonic devices. Thus, the projected good features of NaSbS2 polymorphs would play an essential role in the development of lead-free photovoltaic devices, including solar cells and other optoelectronic devices, and would be able to provide practical information for future theoretical and experimental studies.

Plug‐and‐Play Fiber‐Coupled Quantum Dot Single‐Photon Source via Photonic Wire Bonding

AbstractThe collection of single‐photon emission from a quantum dot (QD) in a Bragg waveguide through a photonic wire bond (PWB) via free‐space resonant frequency pumping at 1.6 K is demonstrated. The in‐fiber single photons show a small multiphoton contribution, quantified by a low second order photon autocorrelation value of (background‐corrected) or (raw data). The decay time of the QD is measured to be ps. The PWB obviates the need for in‐cryostat alignment of the single‐photon source with an optical fiber and thus offers a route to scalable integration of quantum photonic devices in a cryogenic environment. Uniquely, the approach combines the QD‐waveguide technique, enabling resonant driving of individual QDs without the need for cross‐polarization filtering, and the PWB for deterministic, alignment‐free coupling of single‐photon sources to optical fibers.

High Peak Power Ultrashort Pulsed Passively Q-Switched Mode-Locked Bragg Laser Diode

We demonstrate a Bragg waveguide laser diode with an unbiased saturable absorber exhibiting passive Q-switched mode-locking with two AlInGaAs quantum wells. The widths of the observed passively mode-locked pulses ranged from 250 fs to 1640 fs under a Q-switching envelope. The frequency response showed the fundamental peak and smallest linewidths, for Q-switched operation, to range from 0.35 GHz to 1.15 GHz and 1.4 MHz to 9.1 MHz respectively. At 25 °C the devices demonstrated the highest peak power of ~10.1 W @ ~791 nm and highest peak to average power ratio of ~2800.

Research on anti-resonant terahertz hollow waveguides with cascaded bridges

A low loss terahertz waveguide based on anti-resonant parameters is proposed to construct a periodic support structure of an arch bridge in Bragg air layer. Simulation results show that terahertz waves can be transmitted with low loss in the anti-resonant band. After numerical simulation to determine the parameters of the waveguide, the waveguide samples are manufactured by 3D printing technology and experimental tests are carried out. Experimental results demonstrate that the terahertz wave within the 0.84–0.887 THz band can be transmitted with a low loss of only 0.245dB/m, which is nearly ten times lower than that of the conventional multi-support structure terahertz Bragg waveguide, and comparable to that ofthe ideal air-core Bragg waveguide.

Cascaded terahertz hollow-core Bragg waveguide: numerical design and experimental demonstration.

A THz hollow-core Bragg waveguide with discontinuous support bridges in both radial and axial directions is proposed. The influence of the support bridges on the transmission loss of the waveguide is demonstrated numerically. The proposed waveguide shows confinement loss two orders of magnitude lower than that of the Bragg waveguide with conventional support bridges. A waveguide sample is fabricated by 3D printing technology, and the experimental results show that the transmission loss is in agreement with that of the simulation results. It is also demonstrated that the transmission loss of the fabricated waveguide is mainly determined by the large absorption loss of the waveguide material used in the experiment.

Design and Analysis of Si/CaF<sub>2</sub> Near-Infrared (λ∼1.7µm) DFB Quantum Cascade Laser for Silicon Photonics

We have designed a near-infrared wavelength Si/CaF2 DFB quantum cascade laser and investigated the possibility of single-mode laser oscillation by analysis of the propagation mode, gain, scattering time of Si quantum well, and threshold current density. As the waveguide and resonator, a slab-type waveguide structure with a Si/CaF2 active layer sandwiched by SiO2 on a Si (111) substrate and a grating structure in an n-Si conducting layer were assumed. From the results of optical propagation mode analysis, by assuming a λ/4-shifted bragg waveguide structure, it was found that the single vertical and horizontal TM mode propagation is possible at the designed wavelength of 1.70 μm. In addition, a design of the active layer is proposed and its current injection capability is roughly estimated to be 25.1 kA/cm2, which is larger than required threshold current density of 1.4 kA/cm2 calculated by combining analysis results of the scattering time, population inversion, gain of quantum cascade lasers, and coupling theory of a Bragg waveguide. The results strongly indicate the possibility of single-mode laser oscillation.

Design of Monolithic 2D Optical Phased Arrays Heterogeneously Integrated with On-Chip Laser Arrays Based on SOI Photonic Platform.

In this work, heterogeneous integration of both two-dimensional (2D) optical phased arrays (OPAs) and on-chip laser arrays based on a silicon photonic platform is proposed. The tunable multi-quantum-well (MQW) laser arrays, active switching/shifting arrays, and grating antenna arrays are used in the OPA module to realize 2D spatial beam scanning. The 2D OPA chip is composed of four main parts: (1) tunable MQW laser array emitting light signals in the range of 1480-1600 nm wavelengths; (2) electro-optic (EO) switch array for selecting the desired signal light from the on-chip laser array; (3) EO phase-shifter array for holding a fixed phase difference for the uniform amplitude of specific optical signal; and (4) Bragg waveguide grating antenna array for controlling beamforming. By optimizing the overall performances of the 2D OPA chip, a large steering range of 88.4° × 18° is realized by tuning both the phase and the wavelength for each antenna. In contrast to the traditional thermo-optic LIDAR chip with an external light source, the overall footprint of the 2D OPA chip can be limited to 8 mm × 3 mm, and the modulation rate can be 2.5 ps. The ultra-compact 2D OPA assembling with on-chip tunable laser arrays using hybrid integration could result in the application of a high-density, high-speed, and high-precision lidar system in the future.

Research Progress of Wide Tunable Bragg Grating External Cavity Semiconductor Lasers.

In this paper, we review the progress of wide tunable Bragg grating external cavity semiconductor lasers (BG-ECSLs). We concentrate on BG-ECSLs based on the wide tunable range for multicomponent detection. Wide tunable BG-ECSLs have many important applications, such as wavelength-division multiplexing (WDM) systems, coherent optical communications, gas detection and atom cooling. Wide tunability, narrow linewidth and a high side-mode suppression ratio BG-ECSLs have attracted much attention for their merits. In this paper, three main structures for achieving widely tunable, narrow linewidth, high side-mode suppression ratio BG-ECSLs are reviewed and compared in detail, such as the volume Bragg grating (VBG) structure, fiber Bragg grating (FBG) structure and waveguide Bragg grating (WBG) structure of ECSLs. The advantages and disadvantages of different structures of BG-ECSLs are analyzed. The results show that WBG-ECSLs are a potential way to realize the integration, small size, wide tuning range, stable spectral output and high side-mode suppression ratio laser output. Therefore, the use of WBG as optical feedback elements is still the mainstream direction of BG-ECSLs, and BG-ECSLs offer a further new option for multicomponent detection and multi-atoms cooling.

Design and investigation of terahertz hollow-core Bragg waveguide with axial periodic bridges

A terahertz (THz) hollow-core Bragg waveguide constructed by cascading waveguide units with supporting bridges (SPBs) on different air rings is proposed. The influence of the SPBs on the transmission loss of the waveguide is demonstrated numerically. Results show that the SPBs in the first air ring play the most important role in the transmission loss. By reasonably selecting the length ratio of the waveguide units, the waveguide absorption loss can be reduced to less than 0.4 dB / m in a wide bandwidth range of 0.53 to 0.73 THz, which is at least 2 orders lower than the material absorption loss.

Polymer Optical Waveguide Grating-Based Biosensor to Detect Effective Drug Concentrations of Ginkgolide A for Inhibition of PMVEC Apoptosis.

In this work, we successfully developed a fluorinated cross-linked polymer Bragg waveguide grating-based optical biosensor to detect effective drug concentrations of ginkgolide A for the inhibition of pulmonary microvascular endothelial cell (PMVEC) apoptosis. Fluorinated photosensitive polymer SU-8 (FSU-8) as the sensing core layer and polymethyl methacrylate (PMMA) as the sensing window cladding were synthesized. The effective drug concentration range (5–10 µg/mL) of ginkgolide A for inhibition of PMVEC apoptosis was analyzed and obtained by pharmacological studies. The structure of the device was optimized to be designed and fabricated by direct UV writing technology. The properties of the biosensor were simulated with various refractive indices of different drug concentrations. The actual sensitivity of the biosensor was measured as 1606.2 nm/RIU. The resolution and detection limit were characterized as 0.05 nm and 3 × 10−5 RIU, respectively. The technique is suitable for safe and accurate detection of effective organic drug dosages of Chinese herbal ingredients.

Strong and Short Bragg Waveguide Gratings With Trapezoidal-Shaped Grooves

In this study, we evaluate strong Bragg waveguide gratings formed of trapezoidal-shaped grooves, as typically produced in surface-etched structures, and we propose optimal groove shapes. We begin by validating our modelling approach, which is based on the finite-difference time-domain method, through comparisons with coupled mode theory applied to rectangular gratings. We then compute the reflectance, transmittance and loss of trapezoidal gratings as a function of Bragg order, and of the top and bottom widths of the grooves. Simulations reveal that the highest reflectance peaks are achieved for widths that are about λ/8, regardless of the Bragg order. We find that the characteristics of trapezoidal-grooved gratings are strongly dependent on the bottom width of the grooves, and that a very high reflectance can be achieved by carefully tailoring this width. We also calculate the coupling coefficients of infinitely long optimized structures, finding that they decrease with increasing Bragg order. Our results are of interest in the design of strong short gratings etched in the surface of semiconductor waveguides.

Direct UV written waveguides and Bragg gratings in doped planar silica using a 213 nm laser

In this paper, the first demonstration of simultaneous UV written Bragg gratings and waveguides in germanium and boron‐doped planar silica is presented using a 5th harmonic solid‐state nanosecond laser operating at 213 nm wavelength. The fabrication of high‐quality gratings by using a high peak power density, yielding sufficient uniformity and without any surface damage is demonstrated. The photosensitivity of the doped silica layer is investigated by measuring the local effective refractive index of the optical modes. The written gratings are used to measure grating refractive index modulation, grating detuning bandwidth and the waveguide propagation loss with a minimum value of 0.28 ± 0.07 dB cm−1. This paper shows that this new generation of pulsed UV lasers is a promising alternative for conventional longer wavelength CW laser sources used in small spot direct grating writing in doped silica.

Passively Mode-Locked Bragg Lasers With 64 GHz Sub-300 fs Pulses at 785 nm

We have demonstrated a monolithic semiconductor passively mode-locked Bragg waveguide laser (PMBRL) based on a two quantum well active region. The lasers showed a threshold current of ~33 mA at a saturable absorber reverse bias of 0 V in continuous wave operation (CW), with emission wavelength of ~785 nm. A systematic study of a two section PMBRLs showed that these devices can generate pulse widths down to 211 fs at a 64 GHz repetition rate with a time-bandwidth product of 0.32. This range of pulse widths sets a new record for quantum well based devices.

Substrate integrated Bragg waveguide: an octave-bandwidth single-mode hybrid transmission line for millimeter-wave applications.

We demonstrate an air-core single-mode hollow hybrid waveguide that uses Bragg reflector structures in place of the vertical metal walls of the standard rectangular waveguide or via holes of the so-called substrate integrated waveguide. The high-order modes in the waveguide are substantially suppressed by a modal-filtering effect, making the waveguide operate in the fundamental mode over more than one octave. Numerical simulations show that the propagation loss of the proposed waveguide can be lower than that of classic hollow metallic rectangular waveguides at terahertz frequencies, benefiting from a significant reduction in Ohmic loss. To facilitate fabrication and characterization, a proof-of-concept 20 to 45 GHz waveguide is demonstrated, which verifies the properties and advantages of the proposed waveguide. A zero group-velocity dispersion point is observed at near the middle of the operating band, which is ideal for reducing signal distortion. This work offers a step towards a hybrid transmission-line medium that can be used in a variety of functional components for multilayer integration and broadband applications.

Bragg waveguide of rectangular cross-section

A method for calculating parameters of rectangular waveguides using separation of variables is presented. The method makes it possible to calculate multilayer Bragg waveguides with an arbitrary number of layers and an arbitrary composition as photonic crystal structures with defect. A result of the numerical calculation of the dispersion diagram of such a structure is given. This result is in good agreement with the earlier published data on the study of similar structures.

Single-mode Bragg ring laser diodes.

We have designed and fabricated a monolithic semiconductor ring laser based on a Bragg waveguide structure. Through careful control of the waveguiding, we have overcome the inherent "leaky" nature of this waveguide mode and demonstrated a ring laser lasing in the Bragg mode. Best behavior was obtained from lasers with a diameter of 400 µm, where they exhibited output power ${ \gt }{1}\;{\rm mW}$>1mW, in continuous wave (CW) operation. A tangent waveguide provided access to the ring cavity using two ports through evanescent coupling. To meet the stringent waveguiding requirements imposed by the Bragg structure, a two-step etching process, consisting of a shallow-etched coupler and a deep-etched bend section of the ring, was developed in order to reduce the bend and scattering losses. The laser showed a threshold current density of ${\sim}{2.2}\;{{\rm kA/cm}^2}$∼2.2kA/cm2 in CW operation with single longitudinal mode operation with a signal-to-noise ratio of 30 dBm obtained at 1.5 ${I_{\rm th}}$Ith. Broadband phase-matching of $\chi ^{(2)}$χ(2) nonlinearity is observed, offering self-pumped parametric C-band conversion ${ \gt }{40}\;{\rm nm}$>40nm with efficiency of ${142}\% \;{{\rm W}^{ - 1}}\;{{\rm cm}^{ - 2}}$142%W-1cm-2.

Photonic Bragg waveguide platform for multichannel resonant sensing applications in the THz range.

In this paper, we study a photonic Bragg waveguide sensor for resonant sensing applications in the THz range. In order to enhance the resolution and detectivity of the sensor, we modify the relatively broad transmission spectrum of the Bragg waveguide with spectrally narrow transmission dips by creating a geometrical defect in Bragg reflector and causing anti-crossing phenomenon between the core-guided mode and defect mode. The spectral position of the resonant dip is highly sensitive to the thickness variation in the vicinity of the waveguide core. By designing and manufacturing a Bragg waveguide which includes several sections with different defect layer thicknesses, we can interrogate more than one sample simultaneously and thereby realize multichannel resonant sensing by directly tracking the independent resonant dips. Furthermore, we demonstrate the waveguide platform for online monitoring of the thickness variation of lactose powders, which is captured on the waveguide core via a centrifugal force using a home-built rotating setup. Additionally, we also demonstrate the waveguide for fingerprint detection of powder analytes, which further enriches the sensing scenario of the sensing platform. Finally, we discuss the advantages and the spectral tailoring flexibility of the THz Bragg waveguides sensors for future implementations.

Parity-Time Symmetry in Laterally Coupled Bragg Waveguides

We investigate theoretically, in the frame of Coupled-Mode Theory (CMT), a system made of two laterally coupled, perfectly phase-matched, index-coupled Bragg waveguides, the modal gain of the amplifying one compensating exactly the modal losses of the other. Reverting to the supermode eigenbasis of the underlying “naked” (non-periodic) Parity-Time Symmetric (PTS) directional coupler, we derive analytically the scattering parameters of the whole structure. We also consider a system made of two purely gain-coupled Bragg waveguides, either in phase or in anti-phase. In each case, we comment upon the gain requirement for achieving a laser oscillation.

Additive manufacturing of resonant fluidic sensors based on photonic bandgap waveguides for terahertz applications.

A hollow-core Bragg waveguide-based resonant fluidic sensor operating in the terahertz frequency band is studied. A fused deposition modeling 3D printing technique with a Polylactic Acid filament is employed to fabricate the sensor where the liquid analyte is flowing in the microfluidic channel integrated into the waveguide cladding. The fluidic channel supports a resonant defect state which is probed spectrally using the core-guided mode of the Bragg waveguide. Continuous-wave terahertz spectroscopy is used to characterize the fluidic sensor. The measured signal amplitude shows a dip in the transmission spectrum, while the measured phase shows a sharp change in the vicinity of the anticrossing frequency whose spectral position depends strongly on the real part of the analyte refractive index. The sensor spectral response is further optimized by tailoring the waveguide length and position of the defect layer. Consistent with the results of numerical modeling, the measured sensor sensitivity is ~110 GHz/RIU, while the sensor resolution ~0.0045 RIU is limited by the parasitic standing waves in the spectrometer cavity. We believe that the proposed fluidic sensor opens new opportunities in applied chemical and biological sensing as it offers a non-contact measurement technique for monitoring refractive index changes in flowing liquids.