Single-mode Filter Research Articles

Optical sensing in single-mode filters base on surface plasmon H-shaped cavities

Three novel refractive index (RI) sensors based on one, two, and three H-shaped cavities (HCs) and metal–insulator–metal (MIM) waveguides are proposed in this paper. By coupling more than one HC (two and three), the Q-factor and then the FOM value of the designed RI sensors increase. A well-known method for increasing the sharpness transition between the passband and stopband of a spectrum is coupling cavities together. Fortunately, the proposed HC has such a property that increases the FOM value. The maximum obtained FOM of the proposed sensors using one, two, and three HCs reached 69.5, 100.19, and 108.36 RIU-1, respectively. Three proposed plasmonic structures which are used to design RI sensors have single-mode spectra in a wide wavelength range of 500 to 2000 nm. A single-mode spectrum is easier to cope with in circuits with higher complexity such as RI sensors. The metal and insulator materials that are used in the designed structures are silver and air, respectively. The finite-difference time-domain (FDTD) method is used for the numerical investigation of the proposed RI sensors. To verify FDTD simulations, the basic rectangular cavity which is used to design HC is analyzed using an analytical method. In summary, the benefits of the proposed RI sensors are having single-mode spectra in a wide wavelength range and also their simple structures. The proposed topology easily be redesigned and adapted with different numbers of HCs without disturbing its single-mode spectrum to achieve various FOM values. Furthermore, considering the fact that the proposed cavity is based on rectangular shapes, it has a simple fabrication process. Taking into account the mentioned advantages of the proposed structures, they can be used in integrated optical circuits for optical communication purposes.

Reconfigurable and scalable 2,4-and 6-channel plasmonics demultiplexer utilizing symmetrical rectangular resonators containing silver nano-rod defects with FDTD method

Reconfigurable and scalable plasmonics demultiplexers have attracted increasing attention due to its potential applications in the nanophotonics. Therefore, here, a novel method to design compact plasmonic wavelength demultiplexers (DEMUXes) is proposed. The designed structures (two, four, and six-channel DEMUXes) consist of symmetrical rectangular resonators (RRs) incorporating metal nano-rod defects (NRDs). In the designed structures, the RRs are laterally coupled to metal–insulator-metal (MIM) waveguides. The wavelengths of the output channels depend on the numbers and radii of the metal NRDs in the RRs. The results obtained from various device geometries, with either a single or multiple output ports, are performed utilizing a single structure, showing real reconfigurability. The finite-difference time-domain (FDTD) method is used for the numerical investigation of the proposed structures. The metal and insulator used for the realization of the proposed DEMUXes are silver and air, respectively. The silver’s permittivity is characterized by the well-known Drude model. The basic plasmonic filter which is used to design plasmonic DEMUXes is a single-mode filter. A single-mode filter is easier to cope with in circuits with higher complexity such as DEMUXes. Also, different structural parameters of the basic filter are swept and their effects on the filter’s frequency response are presented, to provide a better physical insight. Taking into account the compact sizes of the proposed DEMUXes (considering the six-channel DEMUX), they can be used in integrated optical circuits for optical communication purposes.

Tunable single‐mode bandpass filter based on metal–insulator–metal plasmonic coupled U‐shaped cavities

A compact plasmonic bandpass filter based on metal-insulator-metal plasmonic waveguides and a coupled U-shaped cavity is proposed in this study. Air and silver are used as insulator and metal materials and silver is characterised by the Drude model in the simulations. By tuning the height of the designed cavity, it is found that the resonance wavelength can be easily adjusted. Furthermore, the quality factor of the designed filter can be increased by increasing the cavity numbers. One, two or three cavities can be used to generate single-mode filters. Finally, by sweeping different structural parameters, the effect of them on transmittance spectrum is investigated. All results are obtained by the finite-difference time domain method. The proposed structure due to its remarkable properties such as being a single-mode filter, having a compact size, tunablity of the resonance frequency and its variable quality factor can be used in complex integrated optical structures such as demultiplexers and so on.

Realization of single-mode plasmonic bandpass filters using improved nanodisk resonators

In this paper, symmetric and asymmetric plasmonic bandpass filter (BPF) topologies based on the metal–insulator–metal (MIM) configuration are proposed. These filters are numerically investigated using finite difference time domain (FDTD) method. The metal and dielectric used for the realization of the filters are silver and air, respectively. The real and imaginary parts of silver’s permittivity used in numerical simulation are based on the Drude–Lorentz and Palik models. Both structures are composed of two waveguides, a nanodisk, and parenthesis-shaped adjunctions. The inclusion of symmetrical and nonsymmetrical adjunctions results in single mode filters with higher transmission peaks compared to the original nanodisk-based filter. To provide better physical insight, various structural parameters of the filter are changed and their effects on filter’s response are presented. It is observed that the resonance mode of proposed BPFs can be tuned by changing the nanodisk resonator radius. Such structures can be employed in various plasmonic devices such as multiplexers and demultiplexers for optical communication purposes.

Compact narrow-linewidth integrated laser based on a low-loss silicon nitride ring resonator.

We design and demonstrate a compact, narrow-linewidth integrated laser based on low-loss silicon nitride waveguides coupled to a III-V gain chip. By using a highly confined optical mode, we simultaneously achieve compact bends and ultra-low loss. We leverage the narrowband backreflection of a high-Q microring resonator to act as a cavity output mirror, a single-mode filter, and a propagation delay all in one. This configuration allows the ring to provide feedback and obtain a laser linewidth of 13kHz with 1.7mW output power around 1550nm. This demonstration realizes a compact sub-millimeter silicon nitride laser cavity with a narrow linewidth.

A Triple-mode Wideband Bandpass Filter Using Single Rectangular Waveguide Cavity

A triple-mode wideband bandpass filter (BPF) is proposed. It consists of a single rectangular waveguide cavity with two small metal rods to be inserted into the cavity from input/output feeding ports. As such, one TM110 mode, one TM210 mode, and one TM130 mode can be simultaneously excited in this cavity to form up a wide frequency passband. The fractional bandwidth is 58% at center frequency of 5.5 GHz with low insertion loss (<0.5 dB). Furthermore, this design approach is good for miniaturization compared with traditional single-mode filter. For validation, a proposed filter is fabricated using silver-plated aluminum metal cavity. Both theoretical and measured results are presented with good agreement.

One-Wave Optical Phase Conjugation Mirror by Actively Coupling Arbitrary Light Fields into a Single-Mode Reflector.

Rewinding the arrow of time via phase conjugation is an intriguing phenomenon made possible by the wave property of light. Here, we demonstrate the realization of a one-wave optical phase conjugation mirror using a spatial light modulator. An adaptable single-mode filter is created, and a phase-conjugate beam is then prepared by reverse propagation through this filter. Our method is simple, alignment free, and fast while allowing high power throughput in the time-reversed wave, which has not been simultaneously demonstrated before. Using our method, we demonstrate high throughput full-field light delivery through highly scattering biological tissue and multimode fibers, even for quantum dot fluorescence.

Suppression of high-radial-order whispering-gallery modes and directional emission of anisotropic cylindrical microcavity

Suppression of high-radial-order whispering-gallery modes in a cylindrical microcavity made of electric anisotropic medium was studied by the finite-difference time-domain method. While the electric anisotropy of the microcavity medium increases, the quality factors of high-radial-order whispering-gallery modes decrease more quickly to zero than those of first-radial-order whispering-gallery modes. Thus, for an anisotropic cylindrical microcavity with large enough anisotropy, the spectrum consists of only those resonant peaks corresponding to first-radial-order whispering-gallery modes. This novel characteristic results in widening of the free spectral range of the cylindrical microcavity. Therefore, the anisotropic cylindrical microcavity has potential applications in single-mode low-threshold microlasers and narrow-linewidth wavelength-selective filters.

Theoretical analysis of multimode fiber amplifiers and lasers containing spatial filters

By the use of the coupling theory of the modes and the multimode rate equations, the performances of the multimode fiber amplifiers or lasers containing a spatial filter have been discussed. After the rate equations applicable to amplifiers are solved, the beam quality factors of the output from a multimode fiber amplifier with and without a single mode filter have been evaluated and compared. By the use of the analytical expression of the threshold pump power applicable to the multimode fiber lasers with the spatial filter and doped with quasi two-energy-level ions, and two example fibers had been selected which may guide 4 and 9 transversal modes, respectively, the threshold pump power for each transversal fiber mode has been evaluated. The results indicate that the single mode filter, does introduce a very strong discrimination for the modes between the high order modes and the fundamental mode. This should considerably favor the operation of the fundamental mode in the lasers with a homogeneously broadened active medium.

Dual-Mode Microring Reflection Filters

New high-order microring filter architectures based on the coupling between counterpropagating modes in the microring resonators are proposed and analyzed. The dual-mode nature of the resonators is exploited to maximize the spectral response order, thereby enabling novel reflection filters to be realized with a superior performance than conventional single-mode microring filters in terms of flat passband and steep skirt rolloff. Filter topologies for two general classes of devices, the serial and parallel reflection filters, will be presented along with general techniques for synthesizing their spectral responses. It is shown that a serial reflection filter consisting of N microring resonators can realize a filter response order of up to 2(N - 1), while the parallel reflection type can be used to realize ldquosuperflatrdquo filters, which exhibit a much flatter bandpass response than conventional maximally flat filters having the same number of cavities.

Dielectric resonators and filters

High-performance DRs and filters are widely used in wireless and satellite communication systems due to their superior characteristics, such as high unloaded Q, excellent temperature stability, and smaller size compared to their air-filled counterpart. Temperature-stable high-Q dielectric materials with a wide range of dielectric constants from 20 to 100 are available. The properties of a DR such as resonant frequency and unloaded Q can be accurately determined by full-wave EM simulations. TEoi single-mode filters offer the advantages of design simplicity and flexibility in layout options over HEn dual-mode filters, while dual-mode filters have significiint advantages in the mass and volume of the products. Mixing the TEoi and HE\\ modes, dielectric loaded resonator filters achieve the advantages of dual-mode HEn DR filters and the excellent spurious performances of the TEqi mode ring resonator filters.

Transmission behaviors of single mode hollow metallic waveguides dedicated to mid-infrared nulling interferometry

This paper reports the characterization of hollow metallic waveguides (HMW) to be used as single-mode wavefront filters for nulling interferometry in the 6-20microm range. The measurements presented here were performed using both single-mode and multimode conductive waveguides at 10.6microm. We found propagation losses of about 16dB/mm, which are mainly due to the theoretical skin effect absorption in addition to the roughness of the waveguide's metallic walls. The input and output coupling efficiency of our samples has been improved by adding tapers to minimize the impedance mismatch. A proper distinction between propagation losses and coupling losses is presented. Despite their elevate propagation losses, HMW show excellent spatial filtering capabilities in a spectral range where photonics technologies are only emerging.

Experimental demonstration of waveguide-coupled round-cornered octagonal microresonators in silicon nitride

We report laterally waveguide-coupled octagonal microresonators with designed round resonator corners in silicon nitride. We demonstrate nearly single-mode add-drop filter characteristics, when 50-microm-size round-cornered octagonal microresonators are used, with an optimum coupling efficiency exceeding 96%, a finesse of approximately 30, and a Q of approximately 6400. Our experiments also reveal two unforeseen phenomena in certain microresonator shapes: a pronounced drop in coupling efficiency and resonance notches in the drop spectrum. These device characteristics hint at the recently postulated multiple orbit interference in waveguide-coupled polygonal microresonators.

Simulation of cross-talk reduction in multimode waveguide-based micro-optical switching systems by use of single-mode filters.

We propose a method to reduce cross talk by using single-mode filters, namely, extracting the fundamental mode from a multimode optical network (EFMON). The EFMON effect is evaluated for a three-step switching system consisting of cascade electro-optic (EO) waveguide prism deflector (WPD) micro-optical switches (MOSs) and a multimode waveguide network. The WPD MOS is optimized for single-mode operation in lead lanthanum zirconate titanate thin films as EO slab waveguides with a driving voltage of 12 V, a length of 310 microm, and a channel distance of 20 microm. Beam propagation method simulation reveals that mode disturbance, higher-order modes, and cross talk are accumulated by switching steps. A single-mode filter for EFMON in the output region of the switching system reduces cross talk to below -20 dB at a propagation length of 2000 microm in the single-mode filter and below -30 dB at 20000 microm.

Mode locking of external-cavity semiconductor lasers with saturable Bragg reflectors

We demonstrate the mode locking of external-cavity semiconductor lasers by using a saturable Bragg reflector as an external reflector. Output pulses of 1.9 ps were generated from the semiconductor lasers without dispersion compensation. By coupling the output to a standard single-mode filter with a length of 35 m to compensate for the linear chirp, we have achieved mode-locked pulse durations as short as 880 fs.

Triple mode dielectric resonator hybrid reflection filters

A new class of dielectric resonator filter is described. These devices use the triply degenerate TE11δ mode in a cubic dielectric resonator. A simple theory is used to show the similarity between this mode and the TE10δ mode in a cylindrical resonator. The cubic resonator is used in a hybrid reflection mode filter where the even and odd subnetworks of a bandpass filter are connected to two ports of a 3 dB quadrature hybrid. The transmission response through the other two ports is then the reflection coefficient of the bandpass filter. The design approach only requires the realisation of simple one port ladder networks and is ideal for realising complex filters using triple mode resonators. A design example is presented for a degree 6 elliptic function bandstop filter operating in the 850 MHz cellular radio band. A threefold size reduction is achieved when compared to single mode filters with similar electrical performance.

A simple laterally tapered waveguide for low-loss coupling to single-mode fibers

Low-loss coupling between semiconductor photonic devices and single-mode fibers is achieved using a simple InP/InGaAsP tapered waveguide. The proposed simple structure has a small and nearly square guiding core at its output facet. In this structure, the output field has a non-Gaussian profile, but low-pass filter coupling can be achieved by optimizing the design of the guiding core sizes. The waveguide is composed of a laterally tapered InGaAsP guiding layer and an InP cladding region on an InP substrate, facilitating integration of the waveguide with active devices using conventional processes. The waveguide is shown to have a total insertion loss of 2.6 dB, including a coupling loss of 0.9 dB and large +or-2.5- mu m misalignment tolerance in lateral and vertical directions with single-mode filters.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Bias reduction by polarisation dispersion in the fibre-optic gyroscope

In a ‘reciprocal architecture’ FOG a single-mode filter is used to reduce bias drift resulting from changing fibre birefringence. We show that if a birefringent element is combined with this filter the bias is reduced if the dispersion in the element exceeds the source coherence length. This significantly relaxes the requirements for mode filter extinction.

Phase error bounds of fibre gyro with imperfect polariser/depolariser

In fibre-optic gyroscopes a drifting zero-offset due to changing fibre birefringence may be reduced by inserting a single-mode polarising filter and a depolariser. For a real gyro, in which these components and their alignment are imperfect, bounds are derived for the remaining zero-offset. A simple experimental procedure permits the measurement of that offset.

Fibre-optic gyro for sensitive measurement of rotation

A sensor is described which allows the detection of rotation with respect to an inertial frame. The sensor is based on the Sagnac effect, which is measured with a fibre-optic ring interferometer. The fibre gyro is characterized by the following features: a phase modulation scheme to obtain a high sensitivity; a single-mode filter to achieve low drift; depolarized light in order to get a constant scale factor; light with short coherence length to avoid noise due to backscattering.