Long-period Waveguide Gratings Research Articles

Liquid-crystal clad tunable wavelength de-multiplexer and coupler/filter in silicon nitride rib waveguide structure

We report long-period waveguide grating (LPWG) based tunable wavelength de-multiplexer and tunable coupler/filter in liquid crystal (LC) clad rib waveguides on silicon nitride on insulator platform. High birefringence and electrically tunable refractive index of the MLC-9200-000 LC have been used to achieve a tuning range covering most of the S, C and L bands of the optical communication system. The structure consists of two parallel rib waveguides with an elevated slab. We have considered a new concept of injecting power into a higher-order slab mode. The power from the slab mode is coupled to the rib modes using LPWGs inscribed on the sidewalls of the ribs. The structure works as a tunable wavelength de-multiplexer when the grating periods in both ribs are different. The wavelength tunability of the de-multiplexer is achieved by an external voltage applied across the liquid crystal cell. Our numerical results show wavelength tuning of the de-multiplexer in the range 1.437 - 1.627 μm. The structure works as an electrically tunable coupler/filter when the grating period of both the gratings are the same. We numerically demonstrate the tuning of the coupler in the wavelength range 1.487 - 1.572 μm and that of the filter in the range 1.484 - 1.577 μm with 20-dB rejection.

Nonlinear mode coupling in graphene-buried optical waveguides

The photothermal effect of graphene, which refers to the effect of converting light absorbed by graphene into heat, offers an effective physical mechanism for the realization of all-optical control devices. In this paper, we explore this physical mechanism for the study of nonlinear mode-coupling effects with three graphene-buried waveguide structures: a graphene-buried long-period waveguide grating, a symmetric directional coupler with graphene buried in two cores, and a symmetric directional coupler with graphene buried in one core. We establish physical models for these graphene-buried waveguide structures based on the coupled-mode theory and experimentally implement these structures with polymer waveguides. Our experimental results agree well with the theoretical analyses. The nonlinear mode-coupling effects generated in the graphene-buried waveguide structures show similar characteristics as those achieved with Kerr nonlinearity, but the input powers required in our experiments are much lower (only several tens of milliwatts), which can be delivered by common continuous-wave lasers. The graphene-buried waveguide platform makes feasible the generation of strong nonlinear mode-coupling effects at low powers and offers much flexibility for nonlinearity engineering, which can greatly facilitate the investigation of nonlinear mode-coupling effects in different waveguide structures for practical applications.

Electro-optic tunable optical filter based on long-period waveguide grating in lithium niobate on insulator with absorption ribbons.

We propose an electro-optic tunable optical filter based on sidewall long period waveguide grating (LPWG) in lithium niobate on insolator (LNOI). The operation of our proposed filter is based on the mode coupling, filtering, and absorption achieved, respectively, with two corrugated sidewall LPWGs, a tapered waveguide, and two metal ribbons. Our typical fabricated devices achieved a 16.32-dB rejection band and an EO tuning efficiency of ∼0.344 nm/V. Our proposed LPWG and filter are compact and could be integrated with other LNOI waveguide devices to realize more sophisticated functions for on-chip optical signal processing.

All-Optical Tunable Filter With a Graphene-Buried Long-Period Waveguide Grating

All-Optical Tunable Filter With a Graphene-Buried Long-Period Waveguide Grating

A Long-Period Waveguide Grating Sensor for Accurate Simultaneous Detection of Dual Analytes

To the best of our knowledge, we are the first to report the existence of two dispersion turning points (DTP) in an optimized planar waveguide long-period grating (LPG) sensor for the lowest order cladding mode. Using numerical simulation, we investigate the potential of the sensor structure for bulk refractive index (RI) and surface sensing, with the goal of predicting its possible application as an integrated photonic biosensor. The proposed sensor exhibits an exceptionally high RI sensitivity ~7500 nm/refractive index unit (RIU) (~10000 nm/RIU) near lower (higher) DTP. The surface sensitivity, which is incredibly high in this form of structure, is found to be 6.5 nm/nm (7 nm/nm) near lower (higher) DTP for the RI relevant for biosensing. Utilizing the sensing characteristics of both the DTP, we propose a dual-slot biosensor by cascading two LPGs having different grating periods. The proposed biosensor shows two independent dual-resonance phenomena and is, therefore, capable of simultaneous detection of dual analytes. Utilizing the proposed dual-slot biosensor, we present simulation results for specific detection of Hepatitis B antigen and DNA hybridization simultaneously. The proposed biosensor will reduce the sensor cost as well as detection time since a single source and detector are required to sense two different analytes at once.

Tunable narrow band add-drop filter design based on apodized long period waveguide grating assisted co-directional coupler.

Tunable add/drop filter based optical interconnects are an integral part of data centers as well as optical communications. Although add/drop filters based on ring resonators and waveguide Bragg gratings are well developed, long period waveguide grating (LPWG) based add/drop filters have little been investigated so far. In this article, we propose an apodized LPWG assisted co-directional coupler for narrow band add/drop filtering by combining silicon (Si) waveguide with titanium dioxide (TiO2) waveguide geometry. The proposed structure has been analyzed by combining the finite element method (FEM) and transfer matrix method (TMM), showing a good side lobe suppression ratio (SLSR) equal to 25.7 dB and an insertion loss of 0.6 dB. Owing to the high group index difference of Si and TiO2 waveguides, a narrow band response of 1.4 nm has been achieved with 800µm long LPWG. The opposite thermo-optic coefficients of Si and TiO2 ensures a good thermal tunability of the central wavelength. Considering a thin metallic heater of titanium nitride (TiN) the thermal tuning efficiency is found to be 0.07 nm/mW. Further, two LPWGs have been cascaded to realize a tunable dual channel filter with a minimum channel spacing of 185 GHz and a channel crosstalk better than 20 dB, showing its potential application towards dense wavelength division multiplexing.

Tunable Filters Based on Cascaded Long-Period Polymer Waveguide Gratings

Long-period waveguide grating based filters have attracted attention due to their flexible fabrication, a variety of materials and structures, low back reflection, low insertion loss, and excellent performance in the tuning range and temperature sensitivity. To our knowledge, for the first time, a two-segment polymer long-period waveguide grating was cascaded to implement a filter with a narrower bandwidth. Experimental results showed that the device had a maximum extinction ratio of 24 dB at 1 577 nm, and the 12 dB bandwidth was 10 nm. The temperature sensitivity of the fabricated device was 1.79 nm/°C.

Integrated Width-Modulated SiN Long Period Grating Designed for Refractometric Applications

A novel integrated photonic structure based on Long Period Waveguide Gratings (LPWGs) relying on channel-width modulation is proposed and tested for refractometric applications. These LPWGs have been fabricated through a Silicon Nitride process and experimentally characterized in terms of both surface and bulk sensitivities. Surface sensing configuration is first achieved by propagating one of the coupling optical modes into an epoxy-based negative photoresist SU8 cladding region that is in contact with the analyte via its outer surface. We subsequently show that the proposed LPWGs cladding layer can be advantageously replaced by a gas-porous polymeric bulk layer such as Styrene-co-AcryloNitrile (SAN) as the cladding region to be directly sensed to anticipate future gas sensing applications. Here, bulk sensing is optimized by increasing the analyte's influence on the modal propagation constants as it is demonstrated to be currently the most promising solution to effectively enhance the figure of merit of long period gratings of given lengths. Using varying water-glycerol mixtures, the surface sensitivity of these LPWGs has been measured at up to 240 nm per RI unit (RIU) that is in agreement with simulation. In addition, the bulk sensitivity has been indirectly estimated to be <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 1900 nm/RIU via temperature measurements, which corroborates simulation results, thereby paving the way towards gas sensing applications.

Liquid Crystal Based Rib Waveguide

We report electrically controlled liquid-crystal-core rib waveguide on fused silica substrate, in which the modal characteristics of waveguide can be tuned over a wide range of wavelengths. The coplanar electrode structure is proposed for controlling the LC molecules direction in the waveguide. Electrically controlled refractive index contrast and profile of waveguide is used for single and dual mode operations of the waveguide. We experimentally demonstrate transition from single mode to dual mode operation by externally applied voltage. In single mode regime the externally applied voltage has been used to tune the mode field diameter of the modes from 11.6 to 9.2 μm along the horizontal direction and 7.8 to 6.4 μm along the vertical direction. In the dual mode regime, long period waveguide grating (LPWG) based mode converter has been numerically demonstrated, and more than 99.9% power conversion from TE00 mode to TE01 mode has been obtained. Further, we show electrically tunable wavelength filters using LPWG in an LC core rib waveguide. We have obtained a tuning range of 140 nm for such electrically tunable wavelength filters. The proposed liquid crystal based rib waveguide has the possibility of on-chip low-cost fabrication.

Ultra-short embedded long-period waveguide grating for broadband mode conversion

We propose a novel embedded long-period waveguide grating (LPWG) for the implementation of a broadband mode conversion. The proposed LPWG is formed in the inner of waveguide where the two coupled modes have the largest field overlap and, hence, features an ultra-short length and a wide bandwidth. Besides, the bandwidth of the proposed LPWG is further increased by manipulating its phase-matching curve. Our proof-of-principle device, fabricated with polymer for the conversion between the E11 and E21 modes, contains only two periods and achieves a mode conversion efficiency larger than 98.2% (99.2%) for the x (y) polarization over the C + L band. The proposed LPWG could find applications as an ultra-compact and broadband mode converter in mode-division-multiplexing system.

Fabrication and characterization of annealed proton exchanged long period waveguide grating in x-cut LiNbO3

In this paper, a long-period waveguide grating was fabricated in x-cut lithium niobate substrate by patterned annealed proton exchange waveguide fabrication process. The waveguide mode characteristic was evaluated using a charge-coupled device (CCD) camera. It shows that the waveguide is single mode transmission at a wavelength of 1 550 nm. The transmission spectra of the long period waveguide gratings were measured by optical spectrum analyzer (OSA) and show an extinction ratio of ~17 dB and a 3 dB bandwidth of ~10 nm at the resonant wavelength. The resonant wavelength moves toward to the long wavelength direction as the waveguide width-difference increases in the same period, and also shifts toward to the long wavelength direction with the increase of the period in the case of the same waveguide width-difference. The method of fabricating a long period waveguide grating based on a patterned annealed proton exchange technique simplifies the fabrication process, and at the same time, reduces the fabrication cost.

Liquid crystal-based widely tunable integrated optic wavelength filters

We report widely tunable wavelength filters using liquid crystal (LC)-based long-period waveguide gratings (LPWGs). The high birefringence and electrically tunable refractive index of the LC have been used to achieve a tuning range covering most of the S, C, and L bands of an optical communication system. E7 LC has been used as a cover layer in an LPWG rib waveguide structure. The LPWG structure couples light from the fundamental mode to the slab modes of the waveguide. This results in transmission dips at resonance wavelengths. The mode constants of the coupling modes can be changed by controlling the refractive index of the LC and hence the resonance wavelengths by using the externally applied electric field. We have carried out numerical simulations to analyze the performance of E7 LC-based LPWGs on various substrates such as ion-exchanged BK7 glass and proton-exchanged LiNbO3 and silicon-on-insulator and silicon nitride substrates. The potential applications of the device are electrically tunable wavelength filters for optical communication systems.

Temperature insensitive long period waveguide gratings in rib waveguide

We propose long period waveguide gratings (LPWGs) in a rib structure in which the coupling takes place between the fundamental mode and the higher order modes (slab modes). Such an LPWG avoids the need of finite over cladding. The structure is inherently temperature insensitive as the involved modes have light confinement in the same material in contrast to the LPWGs, which use the over cladding of different material. Moreover, such a structure has an added advantage of less critical and easier fabrication process than the conventional LPWGs, in which the choice and deposition process of over cladding layer imposes several limitations. We demonstrate formation of such rib LPWGs in ion exchanged BK7 glass by laser ablation method and show their applications as wavelength filters and refractive index sensors. In particular we demonstrate temperature insensitive wavelength filter having the center wavelength of 1557.5 nm, an extinction ratio of ˜ 12 dB and 3-dB bandwidth of 3.8 nm. We also show a refractive index sensing characteristics of the structure with a sensitivity of 246 nm/RIU as an additional application.

Highly Sensitive Refractive Index Sensor Based on Polymer Long-Period Waveguide Grating With Liquid Cladding

We propose a novel structure and unique sensing mechanism bio-chemical sensor which is fabricated by a polymer long-period waveguide grating with the detection liquid directly as the waveguide cladding. Quantitative detection is realized from analyzing the output absorption spectrum and resonant wavelength shift related to the liquid detection concentration. The proposed polymer long-period waveguide grating based liquid refractive-index sensor is developed experimentally, the high sensitivity of 1.01 × 104 nm/RIU is achieved, and the temperature stability coefficient is 1.47 nm/°C. Theoretically and experimentally, this work has been demonstrated to have potential application in chemical and biological detections and may provide an important technical support for solving today’s increasingly serious civil problems such as food safety and drug safety, which will also have the important scientific significance and application prospects.

Three-dimensional long-period waveguide gratings for mode-division-multiplexing applications.

We propose a three-dimensional (3D) long-period grating structure that has a controllable grating width and depth and can be formed at any chosen position on the surface of a waveguide core with a single photolithography process. The process relies on the partial etching of small structures on the surface of a polymer waveguide through a waveguide mask with narrow apertures that define the grating pattern. The 3D grating structure allows the design of mode converters for any nondegenerate guided modes of a waveguide, regardless of their symmetry properties, and thus relaxes the design constraint of conventional two-dimensional waveguide gratings. To show the flexibility of the 3D grating structure, we present several mode converters fabricated with this structure. The mode-conversion efficiencies achieved are higher than 90% at the resonance wavelengths. In addition, we demonstrate a three-mode multiplexer by integrating a grating-based mode converter with two asymmetric directional couplers. The proposed grating structure together with the fabrication process can greatly facilitate the development of grating-based devices, especially for MDM applications.

Ultra-broadband mode converters based on length-apodized long-period waveguide gratings.

We propose an ultra-broadband mode converter based on the structure of a length-apodized long-period grating, where π-phase shifts are introduced at strategic locations of the grating profile. Using a 3-section length-apodized grating structure, we design and fabricate an LP01-LP11a and an LP01-LP11b mode converter with a sidewall grating and a surface grating formed along a polymer channel waveguide, respectively. The fabricated LP01-LP11a and LP01-LP11b mode converters provide a conversion efficiency higher than 99% over a bandwidth of ~120 nm and ~150 nm, respectively, or a conversion efficiency higher than 90% over a bandwidth of ~180 nm and ~300 nm, respectively. The transmission characteristics of these devices are weakly sensitive to polarization and temperature variations. These mode converters can find applications in ultra-broadband mode-division-multiplexing transmission systems based on few-mode fibers and the design principle can be applied to general grating-based mode-coupling devices for a wide range of applications.

Tunable wavelength filters using polymer long-period waveguide gratings based on metal-cladding directly defined technique**Project supported by the National Natural Science Foundation of China (Grant Nos. 61575076, 61475061, and 61405070), the Fundamental Research Funds for the Central Universities, China (Grant No. JCKY-QKJC08), the Science and Technology Development Plan of Jilin Province, China (Grant Nos. 20130522151JH, 20140519006JH, and 20160520091JH), and the China

In this work, long-period waveguide grating-based tunable wavelength filters using organic–inorganic grafting poly(methyl methacrylate) (PMMA) materials are designed and fabricated by metal-cladding directly defined technique. The thermal stabilities and optical properties of the organic–inorganic grafting PMMA core materials are analyzed. Structures and performance parameters of the waveguide gratings and self-electrode heaters are designed and simulated. The contrast of the filter is about 15 dB and the resonant wavelength can be tuned by different electric powers applied to the metal-cladding self-electrode heaters. The temperature sensitivity is 3.5 nm/°C and the switching time is about 1 ms. The technique is very suitable for realizing the optoelectronic integrated wavelength-division-multiplexing systems.

基于LiNbO3的长周期波导光栅可调谐耦合器的设计

基于LiNbO₃的长周期波导光栅可调谐耦合器的设计

Ultra-compact, broadband tunable optical bandstop filters based on a multimode one-dimensional photonic crystal waveguide.

In this paper, ultra-compact, broadband tunable optical bandstop filters (OBSFs) based on a multimode one-dimensional photonic crystal waveguide (PhCW) are proposed and systematically investigated. For the wavelengths in the mini-stopband, the input mode is coupled to a contra-propagating higher order mode by the PhCW and then radiates in a taper, resulting in a stopband at the output with low backreflection at the input. Three-dimensional finite-difference time-domain method is employed to study the OBSFs. The influence of main structural parameters is analyzed, and the design is optimized to reduce the back-reflection and band sidelobes. Using localized heating, we can shift the stopband and tune the bandwidth continuously by cascading the proposed structures. Due to the strong grating strength, our device provides a more compact footprint (40 μm × 1 μm) and much broader stopband (bandwidth of up to 84 nm), compared to the counterparts based on microrings, long-period waveguide gratings, and multimode two-dimensional PhCWs.

Mode converters based on cascaded long-period waveguide gratings.

We propose a mode-conversion platform based on cascaded long-period waveguide gratings. We demonstrate two specific devices by cascading a surface-corrugated grating and a sidewall-corrugated grating. One device allows conversion among the LP₀₁, LP_11b, and LP_11a modes in a cyclic manner. The maximum conversion efficiency of a typical experimental device fabricated with polymer materials is about 94% at 1540 nm. The other device allows conversion between the LP₀₁ and LP_21a modes. The maximum conversion efficiency of a typical experimental device is estimated to be higher than 90% at 1550 nm. The performance of both devices is polarization-insensitive. The proposed platform could be developed into various mode converters for mode-division-multiplexing systems and other mode-sensitive applications.