Buried Waveguides Research Articles

Highly coherent mid-infrared wideband supercontinuum generation by a silica cladded silicon nitride core buried waveguide

Abstract Optical waveguides with semiconductor cores are drawing considerable research interest in the domain of supercontinuum (SC) generation in recent times. In this work, we design a square-core silicon nitride buried waveguide with a silica-clad, aiming for a wideband spectrum generation in the mid-IR region when operated at the standard telecommunication wavelength of 1550 nm. Among different such silicon nitride square-core buried waveguides, we propose a typical design with dimensions of 400 nm × 400 nm along its height and width, capable of producing a highly coherent broadband intensity spectrum ranging from 810 nm to 5441 nm after propagating through just a few millimeters of the waveguide. The group velocity dispersion maintains minimal value over a broad wavelength range in the mid-IR region, while the nonlinear coefficient is estimated to be sufficiently high. The nonlinear pulse propagation through such a waveguide leads to achieving an SC spanning over 2.76 octaves, sufficiently broader than previously reported silicon nitride-based waveguides. Furthermore, our calculations confirm the highly coherent nature of the generated SC. To the best of our knowledge, this is the first report of SC generation maintaining a high degree of coherence over such a wide wavelength range in the mid-IR zone using a square-core silicon nitride buried waveguide.

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.

Coherent Broadband Spectrum Generation in Rectangular Silicon Core Buried Waveguide Operated at Telecommunication Wavelength

Coherent Broadband Spectrum Generation in Rectangular Silicon Core Buried Waveguide Operated at Telecommunication Wavelength

Laser emission from buried depressed-cladding waveguides inscribed in Nd:YAG ceramics by picosecond-laser beam writing

Buried, depressed-cladding waveguides were inscribed in a 1.1-at.% Nd:YAG ceramic by direct writing with a picosecond-laser beam. Laser emission performances were investigated under the pump with fiber-coupled diode lasers. For quasi-continuous-wave pumping, a circular waveguide with 100 μm diameter yielded laser pulses at 1.06 μm with 2.4 mJ energy for 12.8 mJ pump pulse energy at 807 nm (corresponding to an overall optical-to-optical efficiency of 0.19); the slope efficiency was 0.22. Continuous-wave output power of 0.35 W at 1.06 μm with overall optical-to-optical efficiency of 0.09 was obtained from the same waveguide; the slope efficiency amounted to 0.12. Furthermore, the circular waveguide yielded laser pulses at 1.32 μm with 0.4 mJ energy (at an overall optical-to-optical efficiency of 0.03); the slope efficiency amounted to 0.03.

Experimental Demonstration of an Easy-to-Fabricate 1.3-μm Directly Modulated DFB Laser With Improved Beam Profile

The integration of a directly modulated laser with a spot-size-converter can greatly reduce packaging requirements thereby reducing costs and promoting applications of these devices. However, most of the reported spot-size-converters integrated directly modulated laser need complex fabrication process which will increase cost and may degrade modulation bandwidth. We report a 1.3-μm directly modulated laser with ridge waveguide distributed feedback region and buried waveguide spot-size-converter region to improve beam profile and increase coupling efficiency with low fabrication cost. The core layer with high refractive index is simply tapered into a 500-nm-wide waveguide before regrowth and after the regrowth process, the buried heterojunction spot-size-converter is formed. The ridge waveguide distributed feedback with conventional high reflection coating is replaced by active-distributed-reflector is applied to prevent the aluminium-containing multi quantum wells from the risk of oxidation and further reduce fabrication cost. The fabricated device realized a nearly circular beam profile and more than 70% improvement of power coupling into a 5-μm lensed-fiber compared to normal ridge waveguide lasers. The fabricated laser also demonstrated a 3-dB modulation bandwidth of ∼19 GHz with good single mode stability. Key parameters are extracted to prove that integration of this spot-size-converter does not degrade the modulation performance and the modulation bandwidth is limited by the material gain as the estimated D factor of the tested device is 2.32 GHz/mA close to that extracted from the material parameters.

Fabrication of an erbium–ytterbium-doped waveguide amplifier at communication wavelengths for integrated optics applications

Erbium–ytterbium-doped waveguide amplifiers provide a considerable gain at telecom wavelengths, low noise, nonlinearity, and compatibility with optical networks, making it an outstanding amplification module for telecommunication systems. This study reports on the fabrication of an optical waveguide amplifier for integrated optics. The signal can be amplified by using rare-earth dopings such as erbium (Er), which works at telecommunication wavelengths, i.e., 1.55 μm. Er-doped phosphate glass waveguides can be deposited using the sol–gel method, which is convenient for preparing active films on several substrates. The Er concentration was 1–2 × 1020/cm3. The confinement and the gain of the waveguide can be increased by reducing the width of the waveguide to 0.5 μm. In such a case, more than 1dB net gain can be achieved without additional pump power. The other material used as a dopant in optical amplifiers is ytterbium (Yb). For Er energy levels, a more significant pump intensity is necessary for inversion due to the limited absorption cross-section. This issue is solved by including a substance with a large absorption cross-section that transfers energy to Er. The Ag–Na ion exchange process is then used to fabricate the buried waveguide. In such a process, ions trade between the core material and the molten salt. Then, the waveguide is immersed in the molten salt. The fabricated waveguide has low loss, and a net gain of around 2 dB at a wavelength of approximately 1.55 μm in Er:Yb:Al: phospho silicate glass is achieved. The focus of the research is on the fabrication procedure (materials and methods) of the waveguide.

Efficient Optical Waveguiding Enabled by Focused Proton Beam Writing in Nd:YCOB Crystal

We report on microfabrication and optical characterization of buried channel waveguides defined in Nd:YCOB crystal by focused proton beam writing (PBW). In the fabrication process, the focused proton beam irradiation creates a local material modification region with geometrically symmetric positive index changes at the end of the proton trajectory, where efficient optical waveguiding can be locally supported within a fiber-like channel structure. The impact of the proton fluence (with different values ranging from 1015 to 1016 cm−2) on the optical waveguiding performance is well studied. The experimental results of the optical waveguide properties are in fairly good agreement with the simulation results.

Fabrication and Characterization of Core Offset Waveguide Optical Sensor for Refractive Index Measurement

<p>Refractive index is important parameter in various fields, such as environmental, chemical, industrial, and medical. Measurement of refractive index can be done using optical sensor based buried waveguide core offset. This research aims to fabricate and characterize core offset buried waveguide based on <em>Polymethyl Methacrylate </em>(PMMA). Fabrication of buried waveguide was done by creating a cladding of waveguide made from PMMA which has a refractive index of 1.4908 using computer numerical Control machine (CNC). The material used as waveguide core was unsaturated polyester resin (UPR). Characterization was performed to determine the spectrum of input intensity and output intensity. Characterization were done by dipping the waveguide sample in to a container containing glucose solution with concentration of 12%, 14%, 16%, 18%, 20%, and 24%. Waveguide was connected to Polymer Fiber Optic (POF). One of the of POF as an input is connected to light emited diode with wavelength of 470 nm and one of the end other POF as an output was connected to Spectrometer Ocean Optic USB4000. The result showed that the total loss at concentration of 12%-24% is -4.62 dB, -5,70 dB, -6,01 dB, -6,49 dB, -6,15 dB, -6,16 dB, respectively. Sensor works well at a solution concentration of 16%-20% with a sensitivity value of 19.62 dB/RIU and correlation factor of 92.76%.</p><p> </p>

Programmable Parallel Optical Logic Gates on a Multimode Waveguide Engine

Optical logic gates have been proposed and demonstrated on a function programmable waveguide engine constructed using buried silicon nitride waveguides in polymer and a set of thermal electrodes. The device can perform logic AND or OR operations for the input signals A and B, each containing two bits of information, in parallel. The input signals, in the form of binary current values in the electronic domain, are applied to a subset of thermal electrodes, while the computed logic states are converted to optical intensity variations at the single-mode waveguide outputs. The rest of the electrodes work as weights to define the device function, either AND or OR, by adjusting the light interference in the multimode waveguide through thermo-optic effect. Simulations were first performed to reveal the nonlinear response of the received light intensity with respect to the applied current, thus allowing complex and effective manipulation of the light field on the waveguide engine. After chip fabrication and system integration, 65,536 experiments were performed automatically. The data are fed into a sorting program to find the valid settings that satisfy the respective truth table out of the 283,852,800 possible input/weight/output combinations. Four cases of operations for the AND and OR gates are presented in the end, with different bar and contrast values. This simple, low-cost yet powerful engine may be further developed for applications in on-chip photonic computing and signal switching.

Generation of high-frequency pulse train by designing a buried SOI waveguide

Silicon photonics has attracted tremendous research interest in recent times because of its compact size, easy-to-build nature, and high values of nonlinearity. Though the study of pulse interaction inside a planer SOI waveguide is done earlier, the confinement of light should be better in the case of a buried core waveguide. Such a buried rectangular SOI waveguide is designed here, which possesses a small value of group velocity dispersion (β2) ∼2.432 (ps2/m) and a quite high value of nonlinear coefficient (γ) of ∼ 441.19 (W.m)−1. When the propagation of a pair of pulses of lower repetition rate (∼GHz) is investigated through it, we have found that the pulse pair interact with each other as they overlap and generate a high-frequency (∼THz) pulse train. The effect of timing jitter on high-frequency generation is also studied. The study of interactions of pulse pairs inside an SOI waveguide helps us to explore the potential applications of such devices as tunable high frequency (THz) pulse generators and many more.

Generation of Parabolic pulse by nonlinear pulse reshaping inside a Silicon on Insulator (SOI) Waveguide

In recent times, silicon photonics attracts a lot of attraction of researchers. It is the technology that converts many functions of the circuit on the thumb-size chip. Nowadays, various types of technology platforms are being used to design photonic integrated circuits using various materials such as high index glass, semiconductors, polymers, and silicon. In our work, we have used silicon to design rectangular silicon on Insulator (SOI) buried waveguide. This type of waveguide has shown great potential in the field of pulse reshaping. We have used the effective index method for the calculation of group velocity dispersion and nonlinearity. Though the two-photon absorption and free-carrier generation contribute significantly to loss parameters, the highly nonlinear buried waveguide is found to be capable of reshaping super-Gaussian pulse input into a parabolic shape. Moreover, the values of input chirp, pulse width, and peak power are further optimized for the generation of a high-quality parabolic pulse at shorter lengths. The length required for pulse reshaping is much less when compared to optical fibers. Thus, our design waveguide has potential in the domain of pulse generation, signal processing, and many more.

Waveguide scattering antennas made by direct laser writing in bulk glass for spectrometry applications in the short-wave IR.

A buried straight waveguide perturbed periodically by six antennas composed of submicronic cylinder voids is entirely fabricated using ultrafast laser photoinscription. The light scattered from each antenna is oriented vertically and is detected by a short-wave IR camera bonded to the surface of the glass with no relay optics. The response of each antenna is analyzed using a wavelength tunable laser source and compared to simulated responses verifying the behavior of the antenna. These results show the good potential of the direct laser writing technique to realize monolithic embedded detectors by combining complex optical functions within a 3D design. A wavelength meter application with a spectral resolution of 150 pm is proposed to demonstrate this combination.

1 × 4 Wavelength Demultiplexer C-Band Using Cascaded Multimode Interference on SiN Buried Waveguide Structure

Back reflection losses are a key problem that limits the performances of optical communication systems that work on wavelength division multiplexing (WDM) technology based on silicon (Si) Multimode Interference (MMI) waveguides. In order to overcome this problem, we propose a novel design for a 1 × 4 optical demultiplexer based on the MMI in silicon nitride (SiN) buried waveguide structure that operates at the C-band spectrum. The simulation results show that the proposed device can transmit four channels with a 10 nm spacing between them that work in the C-band with a low power loss range of 1.98–2.35 dB, large bandwidth of 7.68–8.08 nm, and good crosstalk of 20.9–23.6 dB. Thanks to the low refractive index of SiN, a very low back reflection of 40.57 dB is obtained without using a special angled MMI design, which is usually required, using Si MMI technology. Thus, this SiN demultiplexer MMI technology can be used in WDM technique for obtaining a high data bitrate alongside a low back reflection in optical communication systems.

Laser-Induced Erasable and Re-Writable Waveguides within Silver Phosphate Glasses.

Femtosecond direct laser writing is a well-established and robust technique for the fabrication of photonic structures. Herein, we report on the fabrication of buried waveguides in AgPO3 silver metaphosphate glasses, as well as, on the erase and re-writing of those structures, by means of a single femtosecond laser source. Based on the fabrication procedure, the developed waveguides can be erased and readily re-inscribed upon further femtosecond irradiation under controlled conditions. Namely, for the initial waveguide writing the employed laser irradiation power was 2 J/cm2 with a scanning speed of 5 mm/s and a repetition rate of 200 kHz. Upon enhancing the power to 16 J/cm2 while keeping constant the scanning speed and reducing the repetition rate to 25 kHz, the so formed patterns were readily erased. Then, upon using a laser power of 2 J/cm2 with a scanning speed of 1 mm/s and a repetition rate of 200 kHz the waveguide patterns were re-written inside the glass. Scanning electron microscopy (SEM) images at the cross-section of the processed glasses, combined with spatial Raman analysis revealed that the developed write/erase/re-write cycle, does not cause any structural modification to the phosphate network, rendering the fabrication process feasible for reversible optoelectronic applications. Namely, it is proposed that this non-ablative phenomenon lies on the local relaxation of the glass network caused by the heat deposited upon pulsed laser irradiation. The resulted waveguide patterns Our findings pave the way towards new photonic applications involving infinite cycles of write/erase/re-write processes without the need of intermediate steps of typical thermal annealing treatments.

Integrated opto-mechanical cantilever sensor with a rib waveguide.

An integrated opto-mechanical cantilever sensor with a rib waveguide is reported in this paper. The device consists of a rib waveguide cantilever with buried waveguides on silicon. The rib cantilever is introduced to match better with the buried waveguide, further for increasing the interface coupling efficiency. With this configuration, single-mode operating can be achieved in transverse direction without decreasing the width of optical waveguide cantilever. The system sensitivity is 1.1 μm-1 which is increased by about 21%, compared with the conventional structure.

A Study of Wave Confinement and Optical Force in Polydimethlysiloxane-Arylazopyrazole Composite for Photonic Applications.

A refractive index of dielectrics was modified by several methods and was known to have direct influence on optical forces in nanophotonic structures. The present contribution shows that isomerization of photoswitching molecules can be used to regulate refractive index of dielectrics in-situ. In particular, spectroscopic study of a polydimethylsiloxane–arylazopyrazole (PDMS–AAP) composite revealed that refractive index of the composite shifts from 2.0 to 1.65 in trans and cis states, respectively, of the embedded AAP. Based on this, a proposition is made for a waveguide structure, in which external UV/Vis source reversibly regulates the conformation of the PDMS–AAP core. Computational study is performed using Maxwell’s equations on buried waveguide structure. The simulation, implemented in PYTHON, sequentially utilizes empirical refractive indices of the composite in the isomeric states in lieu of regulation by a source. The simulation revealed highly confined wave propagations for injected signals of 340 and 450 nm wavelengths. It is observed that the cis state suppresses higher order mode when propagating UV wavelength but allows it for visible light. This modal tuning demonstrated that single mode can be selectively excited with appropriate waveguide dimensions. Further impact of the tuning is seen in the optical force between waveguide pair where the forces shift between attractive and repulsive in relation to the isomeric state of the PDMS–AAP core. These effects which stem from the adjustment of refractive index by photoisomerization suggests that in-situ regulation of index is achievable by successful integration of photoswitching molecules in host materials, and the current PDMS–AAP composites investigated in this study can potentially enhance nanophotonic and opto-mechanical platforms.

Enhanced Raman Spectra in Femtosecond Laser Inscribed Yb:YVO4 Channel Waveguides

The femtosecond laser writing with double-line technique was employed to fabricate buried channel waveguides with different widths in Yb:YVO4 crystal. Model profiles of the waveguides were captured using the endface coupling setup at the wavelength of 633 nm under TE and TM polarization. Furthermore, the confocal micro-Raman spectra in bulk and waveguide areas were studied at the wavelength of 633 nm. The enhanced Raman intensity were performed in waveguide areas.

Reflective Electroabsorption Modulators for Beyond 25 Gb/s Colorless Transmissions

We present a complete characterization of reflective electroabsorption modulators (EAMs) monolithically integrated with semiconductor optical amplifiers (SOAs), components that are capable of operating beyond 50 Gb/s in the C-band. The devices are based on GaInAsP multiple quantum wells on InP substrate, leveraging semi-insulating buried heterostructure waveguide definition and butt-joint integration technologies. Different device configurations, based on 80- and 150-μm long EAMs, are fabricated and characterized in both static and dynamic modes. The frequency response of the 80 μm long EAM is still flat at 26.5 GHz (setup upper limit) whereas the 150 μm long EAM exhibits a 3-dB cutoff bandwidth of 23 GHz. A zero-chirp is achieved for EAM reverse bias voltages between -1.2 and -1.5 V depending on the wavelength. Under large-signal modulation, the frequency chirp induced by the shorter EAM is almost half that of the longer EAM, with their respective peak values being +1.5/-2 and +3.2/-3.7 GHz (rising/falling edges) at 1545 nm (-1.3 V bias, 2.6 V voltage swing). We obtained high dynamic extinction ratios of ~14.5 and ~8 dB from the longer and the shorter EAMs, respectively, when they are operated at 25 Gb/s using non-return-to-zero coding. Finally, we achieved 12 and 16 km colorless transmissions in the C-band (between 1530 and 1545 nm) over a standard single-mode fiber without equalization using the 150- and the 80 μm EAMs, respectively, with 4.5 and 2.5 dB dispersion penalties at a bit error rate of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> .

Core depth determination of a slightly buried Si-wire waveguide with a reduced bend loss

The effects of adjusting the distance between a core and an air-cladding interface on the propagation properties are investigated in a bent Si-wire waveguide. The eigenmode analysis is carried out using the imaginary-distance beam-propagation method based on Yee’s mesh in the cylindrical co-ordinate system. The pure bend loss (PBL) is evaluated as a function of core depth for several core aspect ratios. Since a waveguide bend causes polarization coupling, the polarization crosstalk (PCT) is also evaluated by the propagation analysis with the aid of the FDTD method. It is found that a core depth with a minimized PBL exists for the TM mode when the air-cladding interface is placed at the position where the field amplitude decays to approximately 5% of its peak, with a PCT being less than −30 dB.

Room temperature quantum cascade lasers with 22% wall plug efficiency in continuous-wave operation.

We report the demonstration of quantum cascade lasers (QCLs) with improved efficiency emitting at a wavelength of 4.9 µm in pulsed and continuous-wave (CW) operation. Based on an established design and guided by simulation, the number of QCL-emitting stages is increased in order to realize a 29.3% wall plug efficiency (WPE) in pulsed operation at room temperature. With proper fabrication and packaging, a 5-mm-long, 8-µm-wide QCL with a buried ridge waveguide is capable of 22% CW WPE and 5.6 W CW output power at room temperature. This corresponds to an extremely high optical density at the output facet of ∼35 MW/cm2, without any damage.