Wavelength-selective Coupling Research Articles

Multifunctional on-chip directional coupler for spectral and polarimetric routing of Bloch surface wave

Abstract Integration of multiple diversified functionalities into an ultracompact platform is crucial for the development of on-chip photonic devices. Recently, a promising all-dielectric two-dimensional platform based on Bloch surface waves (BSWs) sustained by dielectric multilayer has been proposed to enable various functionalities and provide novel approach to photonic devices. Here, we design and fabricate a multifunctional directional coupler to achieve both spectral and polarimetric routing by employing asymmetric nanoslits in a dielectric multilayer platform. Due to the dispersion property of BSWs, the directional coupling behavior is sensitive to wavelength and polarization. We demonstrate numerically and experimentally the wavelength selective directional coupling of TE BSW mode with an intensity ratio of the BSW excitation in opposite directions reaching 10 dB. Polarization selective directional coupling is also achieved at specific operating wavelength due to different response to a nanoantenna for TE and TM BSWs. The proposed two-dimensional photonic device opens new pathway for a wide range of practical applications such as molecular sensing, imaging with different polarization, and spectral requirements.

Time-Resolved FDTD and Experimental FTIR Study of Gold Micropatch Arrays for Wavelength-Selective Mid-Infrared Optical Coupling.

Infrared radiation reflection and transmission of a single layer of gold micropatch two-dimensional arrays, of patch length ∼m and width ∼m, have been carefully studied by a finite-difference time-domain (FDTD) method, and Fourier-transform infrared spectroscopy (FTIR). Through precision design of the micropatch array structure geometry, we achieve a significantly enhanced reflectance (85%), a substantial diffraction (10%), and a much reduced transmittance (5%) for an array of only 15% surface metal coverage. This results in an efficient far-field optical coupling with promising practical implications for efficient mid-infrared photodetectors. Most importantly we find that the propagating electromagnetic fields are transiently concentrated around the gold micropatch array in a time duration of tens of ns, providing us with a novel efficient near-field optical coupling.

Wavelength-Selective Coupling and Decoupling of Polymer Chains via Reversible [2 + 2] Photocycloaddition of Styrylpyrene for Construction of Cytocompatible Photodynamic Hydrogels.

Reversible photocycloaddition reactions have previously been employed in chemical cross-linking for the preparation of biomaterial scaffolds. However, the processes require activation by high-energy UV light, rendering them unsuitable for modification in biological environments. Here we demonstrate that the [2 + 2] photocycloaddition of styrylpyrene can be activated by visible light at λ = 400-500 nm, enabling rapid and effective conjugation and cross-linking of poly(ethylene glycol) (PEG) in water and under mild irradiation conditions (I = 20 mW cm-2). Notably, the reversion of the cycloaddition can be triggered by low-energy UV light at λ = 340 nm, which allows for efficient cleavage of the dimer adduct. Using this wavelength-gated reversible photochemical reaction we are able to prepare PEG hydrogels and modulate their mechanical properties in a bidirectional manner. We also demonstrate healing of the fractured hydrogel by external light triggers. Furthermore, we show that human mesenchymal stem cells can be encapsulated within the gels with high viability post encapsulation. This photochemical approach is therefore anticipated to be highly useful in studies of cell mechanotransduction, with relevance to disease progression and tissue regeneration.

High-performance fibre-optic humidity sensor based on a side-polished fibre wavelength selectively coupled with graphene oxide film

A high-performance humidity sensor based on a side-polished single-mode fibre (SPF) coated with graphene oxide film was fabricated with a wheel side-polishing technique and spontaneous evaporation. The wavelength-selective coupling gave rise to a loss dip in the transmitted spectrum when a phase match condition was satisfied at a given resonant wavelength. The swelling effect of GO under high RH, which was confirmed by XRD and a spectroscopic ellipsometer, could increase the GO film thickness and led to a redshift at resonant wavelength. Experimental results show that the GO-film-coated SPF (GFC-SPF) is capable of working in two modes: tracing-wavelength mode and intensity variation mode. For the two working modes, the GFC-SPF has high sensitivity, good reversibility, good repeatability and excellent linearity. For the tracing-wavelength mode, a very high sensitivity of 0.145nm/RH% and a high linear correlation of 99.6% were achieved in a low RH range of 32%–85%, while an ultrahigh sensitivity of 0.915nm/RH% and a linear correlation of 98.7% were achieved in a high RH range of 85%–97.6%. For the intensity variation mode, a high sensitivity of 0.427dB/RH% and an excellent linearity correlation of 99.8% were achieved in a wide RH range of 58.2%–92.5%. Such a high performance allows GFC-SPF to have widespread potential applications, such as in the fields of biology, chemical processing and food processing.

Dual-Core Photonic Crystal Fiber for Use in Fiber Filters

An asymmetrical dual-core photonic crystal fiber (DC-PCF), which possesses all circular air holes, is proposed. By setting appropriate geometrical parameters, the wavelength-selective coupling property is realized, and a compact optical filter with a short length of 1.83 mm based on the DC-PCF is designed. The spectral transmission characteristics of the filter are investigated by the beam propagation method. The results demonstrate that the optical filter possesses a bandwidth of ~58 nm and small sidelobes. The proposed optical filter could be used in the integrated optical systems.

Lens-Less Coupling of Cavity-Resonator-Integrated Guided-Mode Resonance Filter

A cavity-resonator-integrated guided-mode resonance filter (CRIGF) consists of a grating coupler (GC) and a pair of distributed-Bragg reflectors (DBRs) on a thin-film dielectric waveguide, and provides a narrowband reflection spectrum for an incident small-diameter beam. A lens-less direct coupling of CRIGF to a single-mode optical fiber is theoretically and experimentally studied. A field-profile matching was discussed with alignment tolerance between CRIGF and the fiber. Design, fabrication, and characterization of CRIGF were described for an operation wavelength of 1540 nm. The aperture size and a nonuniform fill factors of the GC were investigated in order to realize high-efficiency coupling with an incident Gaussian beam of a 10.5-μm diameter. GeO2:SiO2 guiding core and Si-N cladding layers were deposited on a SiO2 glass substrate, and GC and DBRs were formed in the cladding layer by the electron-beam direct writing lithography. The lens-less direct coupling was demonstrated with wavelength selective coupling of the maximum efficiency of 50% and full-width at half-maximum of 1.3 nm.

Vertical plasmonic resonance coupler.

Efficient wavelength-selective coupling of lights between sub-wavelength plasmonic waveguides and free space is theoretically investigated. The idea is based on a new type of vertical resonance coupling devices built on plasmonic metal/insulator/metal (MIM) waveguides. The device structure consists of a vertical grating coupler in a resonance cavity formed by two distributed Bragg reflectors (DBRs). With the metal loss included, maximum coupling efficiency around 50% can be obtained at the 1550 nm wavelength with a filtering 3 dB bandwidth around 20 nm (7 nm for the lossless case), demonstrating the feasibility of the idea for achieving high efficiency wavelength-selective vertical coupling through optical resonance. By utilizing this coupler, a plasmonic add-drop device is proposed and theoretically demonstrated. This kind of compact wavelength selective coupling devices shall have the potential to open up a new avenue of photonics circuitry at nanoscale.

Bending dual-core photonic crystal fiber coupler

In this paper, we systematically study a designed structure of a bending dual-core photonic crystal fiber (PCF). We propose the controllable wavelength-selective coupling PCF. This coupler allows highly accurate control of the filtering wavelength. The different wavelengths can be selected by controlling the bending radius of the fiber. Coupling characteristics of novel bending wavelength-selective coupling PCF are evaluated by using a vector finite element method and their application to a multiplexer demultiplexer (MUX–DEMUX) based on the novel coupler is investigated. When the fiber length is 4168μm, the bending radius of PCF couplers for 1.48/1.55μm, 1.3/1.55μm, 0.98/1.55μm, and 0.85/1.55μm is calculated, respectively, and the beam propagation analysis is performed. Different from the traditional wavelength-selective coupling PCF, the dual-core PCF is bent and it can realize the separation of multiple wavelengths.

Realization of a broadband terahertz wavelength-selective coupling based on five-core fibers

We propose a novel kind of wavelength-selective coupling for the terahertz range based on solid five-core fiber (FCF). The performances of coupling, propagation characteristics, and confinement loss properties are numerically investigated by using a full vector beam propagation method (BPM). Simulation results show that it is possible to realize a broadband wavelength-selective coupling. The coupling length can reach 1.913 cm, and the confinement loss is better than 1.965 × 10−4 cm−1. Furthermore, a parameter, the power difference, is defined, and it numerically demonstrates the working performance of the wavelength-selective coupler; that is, when the power difference is better than − 15 dB, the frequency located in the range of 0.76 THz–1.00 THz is separated relatively well from the frequency of 0.3 THz. Finally, the effect of the structural parameter on the working performance of the coupler is also investigated. We show that the performance optimization is possible by appropriately tuning the core diameter, and the tunabilities of frequency and bandwidth are possible by appropriately tuning the pitch. The wavelength-selective coupler is of potential application for optical fiber sensing and communication in terahertz wavelength division multiplexer fields.

Design and characteristics of a novel narrow-band filter with the dual-core photonic crystal fiber

A novel narrow-band filter based on the dual-core doped photonic crystal fiber is proposed. By optimizing structure parameters of the photonic crystal fiber, the index curves of the fundamental modes for the two cores are crossed at 1.55 μm and the wavelength-selective coupling is achieved. The characteristics of the filter are numerically simulated by the full-vector beam propagation method. And the simulation results show that the coupling length and the bandwidth of the filter are 16.4 mm and 9 nm at −3 dB loss, respectively. It demonstrates the advantages of the filter with a small size, a low transmission loss and the narrow-band width.

双芯光子晶体光纤中的波长选择性耦合及其应用研究

We investigate the coupling characteristics of photonic crystal fiber composed of two cores with different index profiles. The index curves of the fundamental modes of the two cores can cross at a desired wavelength by choosing appropriate index profiles of the cores. As a result, wavelength-selective coupling is achieved. The designed coupler can be applied as an optical fiber bandstop filter with 26-nm bandwidth.

Photonic crystal fiber interferometer composed of a long period fiber grating and one point collapsing of air holes

We present an all-fiber interferometer fabricated with a single piece of an endless single-mode photonic crystal fiber (PCF) by an electric arc discharge. By forming a long period grating (LPG) at a point and collapsing the air holes at another point along the PCF, the simple but effective interferometer could be implemented. The LPG made a strong wavelength selective mode coupling between the core and cladding modes in the interesting wavelength range, while the air-hole collapse induced wavelength independent mode couplings. By cascading them, we could implement the all-fiber interferometer. As a potential application of the proposed all PCF interferometer, strain sensing is experimentally demonstrated.

Polarization coupling in a highly birefringent photonic crystal fiber by torsional acoustic wave

We demonstrate and analyze the acousto-optic coupling between two optical polarization modes of the LP(01) mode propagating in a highly birefringent photonic crystal fiber. The coupling is realized based on wavelength selective acousto-optic coupling by traveling torsional acoustic wave in an all-fiber tunable polarization filter configuration. The dispersion properties of the torsional acoustic wave in the photonic crystal fiber and the influence of axial non-uniformity in the modal birefringence on the filter transmission are discussed in detail.

Short Pulse Propagation in Wavelength Selective Index-Guided Photonic Crystal Fiber Coupler

Dual core photonic crystal fiber (PCF) couplers that support femtosecond (fs) pulses have been presented in this paper. The couplers are dispersion managed and relatively short. A vector finite-element-based model has been developed to precisely evaluate coupling coefficients and higher order dispersions for wavelength selective applications of these PCFs. Material dispersion of silica glass along with the waveguide dispersion has also been taken into account. Cladding has been designed innovatively to manage coupling length and dispersion. Dispersion demonstrates the wavelength region where it can support short duration pulses, and the propagation of 100 fs pulses has been shown at 1550 and 1310 nm wavelengths, respectively, incorporating the higher order dispersions and wavelength selective coupling behavior.

Wavelength-selective coupling of dual-core photonic crystal fiber with a hybrid light-guiding mechanism

A novel class of wavelength-selective coupling photonic crystal fiber (PCF) that operates by a hybrid light-guiding mechanism has been proposed. Different from the traditional PCF coupler operating principle, this fiber coupler shares properties of both the total internal reflection index-guided and the photonic bandgap mechanism. This coupler allows highly accurate control of the filtering wavelength; both bandstop and bandpass filters can be easily implemented. The spectral transmission results demonstrate that the bandpass characteristic of the coupler is very narrow and free of sidelobes. Moreover, the operating wavelength and the coupling length can be continuously tuned by changing the refractive index of the filling material. This research gives a physical insight into the propagation mechanism in the PCF coupler and is crucial for future applications of the proposed device.

Improved sensing performance of D-fiber/planar waveguide couplers

Wavelength selective coupling is demonstrated between the core of a D-shaped optical fiber and a multimode planar waveguide. The fabrication process consists of wet chemical etching of the D-fiber and spin coating or molding to produce the planar waveguide. This fabrication process is shown to produce weak coupling and long interaction length, which exhibits transmission dips with narrow wavelength linewidths. A comb filter is demonstrated with peak separations of 12nm, transmission dips of -20dB, and linewidths of 0.25nm. High sensitivity is demonstrated by showing shift in the transmission dips of -3.16 nm/degree C.

Wideband tunable wavelength-selective coupling in asymmetric side-polished fiber coupler with dispersive interlayer

We demonstrate tunable highly wavelength-selective filter based on a 2 x 2 asymmetric side-polished fiber coupler with dispersive interlayer in one of the coupling arms. The asymmetric fiber coupler is made of two side-polished fibers using identical single-mode fibers and one of the polished fibers is further chemically etched at the central evanescent coupling region to gain closer to the core. An optical liquid with different dispersion characteristics than that of silica fiber is used to fill up the etched hollow and therefore the propagation constant for the polished fiber with dispersive liquid becomes more dispersive and crosses with that of another untreated polished fiber. The location of the cross point and the cross angle between two propagation constant curves determine the coupling wavelength and coupling bandwidth as well as channel wavelength separation, respectively. The coupling wavelength can be tuned at least wider than 84 nm (1.326-1.410 microm) under index variation of 0.004 and with coupling ratios of higher than 30 dB.

Photonic band gap material for integrated photonic application: technological challenges

We report on the fabrication of two-dimensional photonic crystals (PCs), aimed at operating in compact photonic systems, for optical communications. The targeted device is an optical add drop multiplexer (OADM) whose operation relies on the wavelength-selective and directional coupling of two optical waveguides. The microstructures are fabricated in an InP-based technology by means of a high resolution electron beam patterning generator (HR-EPBG), silicon nitride mask transfer and deep reactive ion etching (DRIE). Successful fabrication of arrays of holes with nano-scale dimensions is demonstrated with aspect ratios of the order of seven for devices operating in the long wavelength windows.

Wavelength-selective coupling between vertically integrated thin-film waveguides via supermode by a pair of grating couplers

Grating coupling between vertically integrated waveguides via supermode was discussed and demonstrated. Two thin-film waveguides with grating couplers were stacked on a substrate. A guided wave in one waveguide is converted by a grating coupler to a supermode propagating contradirectionally, and then converted by another grating coupler to a guided wave in the other waveguide. The coupling shows high wavelength selectivity, and the wavelength-division add/drop multiplexing function can be combined. A demonstrator was designed and fabricated. Theoretically predicted coupling efficiency was almost 100% with wavelength selectivity of 2 nm, while the experimentally obtained value was 40% in the efficiency and 1 nm in the selectivity.

Wavelength-selective coupling among three vertically integrated optical waveguides by grating couplers

Selective coupling between two specified optical waveguides in a multistory device via radiation modes was proposed and discussed. Three single-TE-mode waveguides consisting of a Corning 7059 glass guiding core layer and a SiO/sub 2/ waveguide-separation layer were stacked on a Si substrate with grating couplers. The coupling length was 0.5 mm. Contradirectional couplings from the first story to the second story at a wavelength of 814 nm and from the first story to the third story at 819 nm with sharp wavelength dependence of 0.5-nm full-width at half-maximum (FWHM) were experimentally demonstrated.