High Free Spectral Range Research Articles

Design and Optimization of Asymmetric Grating Assisted Slot Microring

In this paper, a slot microring with an asymmetric grating structure is proposed. Through the coupling between the grating and the slot microring, a high free spectral range or EIT-like effects with a high quality factor can be achieved in the same device. The grating is designed as an asymmetric structure to realize the modulation of the optical signal and the control of the resonance peak by changing the grid number, and the effect of different grating periods on the output spectrum is explored. The results show that changing the grating on slot sidewalls can increase or decrease the number of resonant peaks. By selecting a specific period of the gratings on both sides of the slot, the distance between adjacent resonance peaks can be increased to achieve modulation of the free spectral range. In this paper, depending on the grating period, we obtain a quality factor of 5016 and an FSR of 137 nm, or a quality factor of 10,730 and an FSR of 92 nm. The refractive index sensing simulation is carried out for one of the periods, which can achieve a sensitivity of 370 nm/RIU. Therefore, the proposed new structure has certain advantages in different sensing applications.

Designing of Optical Filter Based on Modelling Procedure of Multilayer Quadruple Combined Micro Ring Resonator (MQCMRR) Structure with 2 × 3 I/O Bus Waveguides

A new and advanced ring resonator model with its performance analysis as an optical filter is presented in this paper. The recommended model composites of four asymmetrical micro rings with two input and three output bus waveguides. This multilayer quadruple combined micro ring resonator (MQCMRR) structure is modelled in Z-domain by the application of delay lines and Masons gain principle. This model is confined in three layers by adopting series coupling as well as vertical coupling. Various performance parameters are evaluated like free spectral range (FSR), group delay (GD), dispersion (DP), full-width-half-maxima (FWHM), and Q-factor, by the analysis of spectral response in MATLAB environment. Design of optical couplers, as well as the study of field transmission through the rings and waveguides, are carried out by the finite-difference-time-domain (FDTD) method. Performance in optical communication system of this recommended ring resonator-based optical filter is investigated through a simulation program which is done by OptiSystem16.0 software. Data transmission performance is graphically shown by an eye diagram. For this proposed model high FSR and zero crosstalk is achieved at all output port. All the important consequences like large FSR, zero crosstalk, high data handling capability, perfect field transmission and exact open eye diagram certify the smashing performance of this architecture in the optical communication system as an optical add-drop filter.

Z-domain modeling approach of a multilayer sextuple-coupled micro-ring resonator with 1 × 5 input-output bus waveguides as optical filters.

This paper introduces the modeling approach of a new micro-ring resonator configuration composite of six asymmetrical rings with one input and five output bus waveguides. This multilayer sextuple-coupled micro-ring resonator (MSCMRR) is modelled in the z domain by practicing the delay line signal approach and Mason's gain formula. Both lateral coupling and vertical coupling are used to design this MSCMRR structure to confine it up to four layers. Performance analysis is carried out in a MATLAB platform with the graphical representation of performance parameters like free spectral range (FSR), group delay, dispersion, full width at half-maximum, etc. The optical couplers of the proposed model are also designed by the finite-difference time-domain (FDTD) method. Electric field propagation through the rings and the waveguides are also studied by FDTD. The proposed structure offers different values of high FSR with zero crosstalk at every output port that ensures the quality application of this proposed model as an optical add-drop filter in an optical communication network.

An FDTD analysis of serially coupled double ring resonator for DWDM

This paper presents a 2D FDTD analysis of a compact serially coupled double ring resonator filter. The design of the filter is done on the basis of a design procedure that decides the coupling coefficients for a given length of microring. The double ring resonator with radius of 1.65μm is designed and simulated using 2D FDTD. This device is giving a free spectral range (FSR) of the order of 63nm and crosstalk of nearly −15dB at resonance. The out of band rejection ratio is obtained as −30dB. Such a high FSR is required for DWDM systems to transmit data at multiple wavelengths between two resonant wavelengths of a given device. This designed device can accommodate nearly 50 channels where each channel is spaced by 1.2nm wavelength.

Narrow band wavelength selective filter using grating assisted single ring resonator

This paper illustrates a filter configuration which uses a single ring resonator of larger radius connected to a grating resonator at its drop port to achieve single wavelength selectivity and switching property with spectral features suitable for on-chip wavelength selection applications. The proposed configuration is expected to find applications in silicon photonics devices such as, on-chip external cavity lasers and multi analytic label-free biosensors. The grating resonator has been designed for a high Q-factor, high transmittivity, and minimum loss so that the wavelength selectivity of the device is improved. The proof-of-concept device has been demonstrated on a Silicon-on-Insulator (SOI) platform through electron beam lithography and Reactive Ion Etching (RIE) process. The transmission spectrum shows narrow band single wavelength selection and switching property with a high Free Spectral Range (FSR) ~60 nm and side band rejection ratio >15 dB.

Fabrication And Characterization Of Photonic Crystal Slow Light Waveguides And Cavities

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view and for its considerable potential for practical applications. Slow light photonic crystal waveguides, in particular, have played a major part and have been successfully employed for delaying optical signals(1-4) and the enhancement of both linear(5-7) and nonlinear devices.(8-11) Photonic crystal cavities achieve similar effects to that of slow light waveguides, but over a reduced band-width. These cavities offer high Q-factor/volume ratio, for the realization of optically(12) and electrically(13) pumped ultra-low threshold lasers and the enhancement of nonlinear effects.(14-16) Furthermore, passive filters(17) and modulators(18-19) have been demonstrated, exhibiting ultra-narrow line-width, high free-spectral range and record values of low energy consumption. To attain these exciting results, a robust repeatable fabrication protocol must be developed. In this paper we take an in-depth look at our fabrication protocol which employs electron-beam lithography for the definition of photonic crystal patterns and uses wet and dry etching techniques. Our optimised fabrication recipe results in photonic crystals that do not suffer from vertical asymmetry and exhibit very good edge-wall roughness. We discuss the results of varying the etching parameters and the detrimental effects that they can have on a device, leading to a diagnostic route that can be taken to identify and eliminate similar issues. The key to evaluating slow light waveguides is the passive characterization of transmission and group index spectra. Various methods have been reported, most notably resolving the Fabry-Perot fringes of the transmission spectrum(20-21) and interferometric techniques.(22-25) Here, we describe a direct, broadband measurement technique combining spectral interferometry with Fourier transform analysis.(26) Our method stands out for its simplicity and power, as we can characterise a bare photonic crystal with access waveguides, without need for on-chip interference components, and the setup only consists of a Mach-Zehnder interferometer, with no need for moving parts and delay scans. When characterising photonic crystal cavities, techniques involving internal sources(21) or external waveguides directly coupled to the cavity(27) impact on the performance of the cavity itself, thereby distorting the measurement. Here, we describe a novel and non-intrusive technique that makes use of a cross-polarised probe beam and is known as resonant scattering (RS), where the probe is coupled out-of plane into the cavity through an objective. The technique was first demonstrated by McCutcheon et al.(28) and further developed by Galli et al.(29).

Discrete and Fine Wavelength Tunable Thermo-Optic WSS for Low Power Consumption ${\rm C}{+}{\rm L}$ Band Tunability

Broadband wavelength tunability is demonstrated in a microring resonator (MRR)-based thermo-optic wavelength selective switch (TO-WSS) with low power consumption and low thermal crosstalk. Instead of direct tuning a single MRR spectrum, the discrete and fine wavelength tunability in a Vernier configured TO-WSS has been used to achieve single wavelength selection over a high free spectral range (FSR). With the observed discrete tunability (7.3 mW/FSR) and fine tunability (5 mW/nm) of the WSS, the power requirement for full band tuning can be reduced to 32.7% of that of a single ring tuning.

Broadband tunable bandpass filters using phase shifted vertical side wall grating in a submicrometer silicon-on-insulator waveguide

We propose the silicon-on-insulator (SOI) based, phase shifted vertical side wall grating as a resonant transmission filter suitable for dense wavelength division multiplexing (DWDM) communication channels with 100 GHz channel spacing. The gratings are designed and numerically simulated to obtain a minimum loss in the resonant cavity by adjusting the grating parameters so that a high transmittivity can be achieved for the resonant transmission. The resonant grating, which is designed to operate in the DWDM International Telecommunication Union (ITU) grid C band of optical communication, has a high free spectral range of 51.7 nm and a narrow band resonant transmission. The wavelength selectivity of the filter is improved through a coupled cavity configuration by applying two phase shifts to the gratings. The observed channel band width and channel isolation of the resonant transmission filter are good and in agreement with the ITU specifications.

Microwave photonic filter with tunability, reconfigurability and bipolar taps

A new microwave photonic filter structure realising fully programmable RF filtering that can tune the filter centre frequency and also reconfigure the filter shape, while exhibiting multiple taps and bipolar taps, is presented. It is based on a new arbitrary spectrum slicing technique using a multi-port programmable wavelength processor, based on liquid crystal on silicon pixels, which offers programmable finite impulse response filtering using software control. Experimental results demonstrate a high-order microwave filter with tunability, reconfigurability, bipolar taps, and with high free spectral range.

Widely Tunable, High-FSR, Coherence-Free Microwave Photonic Notch Filter

A new structure that can realize widely tunable microwave photonic notch filtering, high free spectral range (FSR), and coherence-free operation simultaneously, is presented. It is based on a Sagnac loop interferometer that comprises an off-loop-center electroabsorption modulator, and a wavelength dependent delay in the Sagnac loop. The structure is simple, and has no coherent interference or phase induced intensity noise limitations. Experimental results demonstrate both continuous and discrete widely tunable notch filters, high frequency and high FSR, and coherence-free low-noise operation with very deep notches in excess of 40 dB.

Novel synthesized photonic signal processor with hardware compression

A new grating-synthesized structure that realizes both a flat-top filter and a high rejection ratio response simultaneously is presented. The concept is based on an integrated grating synthesis approach in which the tap weights and delays are simultaneously realized in one unit. This enables multiple taps to be readily obtained with short sampling times, together with bipolar taps using a dual-output electrooptic modulator. Results demonstrate the realization of a flat-top filter response with high free-spectral range, high rejection ratio, and low passband ripple, and with compressed hardware requirements.

Quasicontinuous tunable fiber-ring laser applied as local oscillator in an absolute calibrated spectrometer for WDM systems

A special tunable fiber-ring laser is introduced. With this laser as local oscillator, a calibrated optical spectrometer was realized, which allows the determination of the wavelength of all transmitters of a wavelet division multiplexing (WDM) system in one sweep. The system is based on a fiber ring laser including a high finesse Fabry-Perot resonator with a high free spectral range and a 1-km-long dispersion shifted fiber. To achieve a high wavelength determination accuracy two auxiliary signals are generated: one with a Krypton filled absorption tube and another with a simple, silica based, periodical filter. With that, the wavelength of every transmitter of a WDM signal can be determined with an absolute frequency accuracy of better than /spl plusmn/0.8 GHz in a range of 1527-1562 nm in one sweep. The relative frequency accuracy is better than /spl plusmn/0.3 GHz.

The Axicon-Scanned Fabry–Perot Spectrometer

The optical properties of a thin axicon or prism of revolution can be combined with a Fabry–Perot etalon to produce a simple, but useful, photoelectric spectrometer. The spectrum can be scanned without disturbing the etalon solely by displacing the axicon. The system is particularly useful for the case where a pressure or mechanical scan of the etalon is difficult, i.e., low resolution and high free-spectral range. The system is also applicable to spectroscopy in the vacuum ultraviolet. A device based on these principles has been constructed and used in an experiment. Test results are presented. The etalon–axicon combination is shown to lend itself readily to the construction of a high-resolution, high-luminosity multichannel photoelectric analyzer.