Abstract: In this article we report recent modeling and design work indicating that mixtures of nanoparticles in liquids can be used as an alternative to conventional optical filters. The major motivation for creating liquid optical filters is that they can be pumped in and out of a system to meet transient needs in an application. To demonstrate the versatility of this new class of filters, we present the design of nanofluids for use as long-pass, short-pass, and bandpass optical filters using a simple Monte Carlo optimization procedure. With relatively simple mixtures, we achieve filters with <15% mean-squared deviation in transmittance from conventional filters. We also discuss the current commercial feasibility of nanofluid-based optical filters by including an estimation of today's off-the-shelf cost of the materials. While the limited availability of quality commercial nanoparticles makes it hard to compete with conventional filters, new synthesis methods and economies of scale could enable nanofluid-based optical filters in the near future. As such, this study lays the groundwork for creating a new class of selective optical filters for a wide range of applications, namely communications, electronics, optical sensors, lighting, photography, medicine, and many more.

Abstract: A widely tunable optoelectronic oscillator (OEO) based on a broadband phase modulator and a tunable optical bandpass filter is proposed and experimentally demonstrated. A tunable range from 4.74 to 38.38 GHz is realized by directly tuning the bandwidth of the optical bandpass filter. To the best of our knowledge, this is the widest fundamental frequency tunable range ever achieved by an OEO. The phase noise performance of the generated signal is also investigated. The single-sideband phase noise is below -120 dBc/Hz at an offset of 10 KHz within the whole tunable range.

Abstract: A type of light absorber made of continuous layers of metal and dielectric films is studied. The metal films can have thicknesses close to their skin depths in the wavelength range concerned, which allows for both light transmission and reflection. Resonances induced by multiple reflections in the structure, when combined with the inherent lossy nature of metals, result in strong absorption spectral features. An eigen-mode analysis is carried out for the plasmonic multilayer nanostructures which provides a generic understanding of the absorption features. Experimentally, the calculation is verified by a reflection measurement with a representative structure. Such an absorber is simple to fabricate. The highly efficient absorption characteristics can be potentially deployed for optical filter designs, sensors, accurate photothermal temperature control in a micro-environment and even for backscattering reduction of small particles, etc.

Abstract: Hybrid photovoltaic/thermal (PV/T) collectors would benefit from the use of fluid based optical filters as a means to separate the useful irradiance for the PV cell from those wavelengths which are more suited to heat generation. Nanoparticle based dispersions within a working fluid can be designed/tuned to serve as optical filters for this purpose. The advantage of this concept is that the thermal part of the system is separated, allowing the photovoltaic and thermal components to operate at significantly different temperatures. Additionally, by using a fluid filter, it is relatively easy to remove heat from the thermal side. This paper theoretically investigates the performance of nanoparticle-based and conventional thin film-based optical fluid filters within a concentrating hybrid PV/T system. General results are presented to demonstrate the impact to overall efficiency when a realistic (i.e., non-ideal) filter is used at a wide-range of operating conditions. The results demonstrate that nanoparticle based filters have a slightly lower overall efficiency compared to the conventional thin film filters due to their lower performance within the window of high transmittance to the PV cell. However, nanoparticle based filters achieve up to 4% higher thermal efficiencies as a result of their significantly reduced filter thickness demonstrating their potential as a favorable compact and lower cost design.

Abstract: A scheme to generate a flat optical frequency comb (OFC) with a fixed phase relationship between the comb lines is proposed and experimentally demonstrated based on a cascaded polarization modulator (PolM) and phase modulator. Because the PolM introduces more controllable parameters compared with the conventional intensity modulator, 9, 11, and 13 comb lines can be generated with relatively low RF powers, or 15, 17, and 19 comb lines can be obtained if high RF powers are applied. The experimentally generated 9, 11, and 13 OFCs have a flatness of 1, 1.3, and 2.1 dB, respectively. The scheme requires no DC bias to the modulators, no optical filter, and no frequency divider or multiplier, which is simple and stable.

Abstract: In this paper, an airborne vision-based navigation method for Unmanned Aerial Vehicle (UAV) accuracy landing is presented. In this method, a visible light camera integrated with a Digital Signal Processing (DSP) processor is installed on the UAV and a 940 nm optical filter is fixed in front of the camera lens. In addition, four infrared light-emitting diode (LED) lamps whose emission wavelengths are 940 nm are placed behind ideal landing site on the runway. In this way, the infrared lamps in the image are distinct even if the image background is complicated. In the image processing procedure, firstly maximum between-class variance algorithm and region growing algorithm are used to determine candidate infrared lamp regions in the images. Then Negative Laplacian of Gaussian (NLOG) operator is applied to detect and track centers of the infrared lamps in the images. The space position and attitude of the camera can be obtained according to the corresponding relationship between image coordinates and space coordinates of the infrared lamp centers. Finally, high precision space position of the UAV can be calculated according to the installation relationship between the camera and the UAV. Plenty of real flight and static precision experiments have proved the validity and accuracy of the proposed method.

Abstract: In this paper, we propose to implement an all-optical XOR gate for 160 Gb/s return-to-zero data signals using a single quantum-dot semiconductor optical amplifier (QD-SOA) assisted by a detuned optical filter (OF). These two elements are connected in series in a probe-dual pump configuration. By conducting numerical simulations, we thoroughly investigate and assess the impact of the critical performance parameters on the Q2-factor. The analysis of the obtained results against this metric enables us to specify the data signals peak power, QD-SOA small signal gain, current density, electron relaxation time from the excited state to the ground state and linewidth enhancement factor, and OF detuning, bandwidth and shape, for which the XOR logic is executed at the target data format and rate both with logical correctness and high quality. The confirmation of its design feasibility combined with its simplicity and ultrafast capability makes the XOR gate scheme promising for exploitation in all-optical signal processing and switching applications.

Abstract: We demonstrate a robust, compact and low-loss four-channel wavelength-division multiplexing (WDM) filter based on cascaded double-ring resonators (2RR) in silicon. The flat-top channel response obtained by the second-order filter design is exploited to compensate for the detrimental effects of local fabrication variations and their associated phase errors on the ring-based filter response. Full wafer-scale characterization of a cascaded, four-channel 2RR filter with channel spacing of 300 GHz shows an average worst-case insertion loss below 1.5 dB and an average worst-case crosstalk below -18 dB across the wafer, representing a substantial improvement over a first-order based ring (1RR) design. The robust 2RR filter design enables the use of a simple collective thermal tuning mechanism to compensate for global fabrication variations as well as for global temperature fluctuations of the WDM filter, the WDM laser source, or both. Highly uniform collective heating is demonstrated using integrated doped silicon heaters. The compact filter footprint of less than 50×50 μm2 per channel enables straightforward scaling of the WDM channel count to 8 channels and beyond. Such low-loss collectively tuned ring-based WDM filters can prove beneficial in scaling the bandwidth density of chip-level silicon optical interconnects.

Abstract: We present a simple method to generate spectrally uniform wideband chaos by injecting chaotic laser into a fiber ring resonator. The resonator is a single-coupler ring equipped with an optical filter and amplifier, which adjust the optical field circulating in the ring. The incoherent interference of the circulating fields produces wideband chaos with uniform power spectrum density distribution. We experimentally achieved a chaotic spectrum that extends over 26.5 GHz (limited by measurement bandwidth) and fluctuates within ±1.5 dB. In addition, tuning the filter frequency can control the spectral profile so as to meet different application needs.

Abstract: We present thermally tunable microring optical filters using p-i-p-type microheaters. The use of Vernier effect in the cascaded two microring resonators significantly enlarges the free spectral range (FSR) and the thermal tuning range with reduced power consumption. Heat generated by the p-i-p-type microheaters interacts directly with the microring waveguides, providing a means to effectively tune resonances without incurring excess loss. Experimental results reveal that the filter passband can be discretely shifted in the wavelength range from 1520 to 1600 nm by tuning one resonator with a power tuning efficiency value of 2.5 nm/mW. The passband can be also continuously shifted by simultaneously tuning both resonators with a power tuning efficiency value of 0.11 nm/mW. The rise (fall) time of the p-i-p microheater is measured to be 460 ns (1.1 μs) under a peak-to-peak driving voltage of 3.4 V.

Abstract: Many applications of optical frequency combs (OFCs) require manipulation and amplification of individual comb modes, e.g., arbitrary waveform generation, terahertz generation and telecommunications. Extracting individual comb modes can be a challenging task for OFCs with narrow comb mode spacings (100 MHz to 10 GHz) due to the limitations of conventional optical filters. Optical injection locking can address this problem, but-due to the relatively large bandwidth (1 to 10 GHz) required for simple (i.e., without the need for additional feedback loops) and stable locking-can struggle when processing OFCs with sub-GHz comb mode spacings. Here, we present an approach to optical injection locking which incorporates a dither-free phase locked loop that allowed for long-term locking to OFCs with comb spacings below the high power injection locking bandwidth. As a result, we achieved robust injection locking directly to a sub-GHz OFC (250 MHz in our experiments). Optimization of the optical injection power is carried out using detailed phase noise characterization. We achieved an Allan deviation for the frequency variation of the slave laser with respect to the injected comb mode (1 s gate time) down to 9.7 × 10-17 and 4.4 × 10-19 at 1 s and 1000 s averaging times respectively, and a phase error variance of 0.02 rad2 (integration bandwidth of 100 Hz to 500 MHz).

Abstract: We propose and experimentally demonstrate a compact notch microwave photonic filter (MPF) using two integrated microring resonators (MRRs) on a single silicon-on-insulator (SOI) chip. The free spectral ranges (FSRs) of two cascaded MRRs are 160 GHz and 165 GHz, respectively. Due to the vernier effect, the transmission spectrum of cascaded MRRs is a series of bimodal distribution whose interval is an arithmetic sequence. By locating the laser wavelength at the middle of different bimodal intervals and fine tuning it properly, both central frequency and bandwidth of the notch MPF can be tunable. In the experiment, the tunability of central frequency and 3-dB bandwidth are demonstrated from 2.5 GHz to 17.5 GHz and from 6 GHz to 9.5 GHz, respectively. The best rejection ratio of the notch filter is larger than 40 dB. This approach will allow the implementation of low-cost, very compact, and integrated notch MPFs in a silicon chip.

Abstract: We present here ultra-compact high-Q Fano resonance filters with displaced lattices between two coupled photonic crystal slabs, fabricated with crystalline silicon nanomembrane transfer printing and aligned e-beam lithography techniques Theoretically, with the control of lattice displacement between two coupled photonic crystal slabs layers, optical filter Q factors can approach 211 000 000 for the design considered here Experimentally, Q factors up to 80 000 have been demonstrated for a filter design with target Q factor of 130 000

Abstract: This paper is focused on the application of split ring resonators (SRRs) to the design of compact bandpass filters for terahertz surface waves on single-wire waveguides, the so-called planar Goubau lines (PGLs). Through equivalent circuit models, electromagnetic simulations, and experiments, it is shown that, while a pair of SRRs coupled to a PGL inhibits the propagation of surface waves along the line, introducing a capacitive gap to the PGL switches the bandstop behavior to a bandpass behavior. In order to highlight the potential application of the proposed structure to the design of practical higher order terahertz bandpass filters, two types of compact bandpass filters are designed and fabricated: 1) third-order periodic bandpass filters based on SRR/gap-loaded PGL and 2) coupled-resonator bandpass filters. It is shown that, while the frequency response of the both filter types can be controlled by altering the physical dimensions of the structure, a wider bandwidth can be achieved from the coupled-resonator filters. The design concept and simulation results are validated through experiments.

Abstract: We proposed a nanoplasmonic optical filtering technique based on complementary split-ring resonator structures. Interestingly, the proper plasmonic modes of the nanoring in the side-coupled arrangement can be selected and excited by the proposed structures. It is observed that the non-integer modes can be excited due to the presence of a metallic nano-wall as well as the integer modes. Furthermore, the numerical results indicate that the optical transmission spectrum of the investigated filter can be efficiently modified and tuned by manipulation either the position or the width of the employed nano-wall inside the metal-insulator-metal ring. The antinodes of the magnetic field of these modes, located on the symmetry plane of the proposed structures, can be manipulated by the position of the wall. Additionally, these modes, in particular the fundamental mode, are highly sensitive to the nano-wall dimensions. It indicates that the proposed nanofilter is a promising candidate as a tunable filter in nanophotonics applications.

Abstract: We report the fabrication of a superconducting nanowire single-photon detector (SSPD or SNSPD) with an ultralow dark count rate. By introducing optical band-pass filters at the input of the SSPD and cooling the filters at 3 K, the dark count rate is reduced to less than 1/100 at low bias. An SSPD with 0.1 cps dark count rate and 5.6% system detection efficiency at 1550 nm wavelength is obtained. We show that a quantum key distribution (QKD) over 300 km of fiber is possible based on a numerical calculation assuming a differential phase shift QKD protocol implemented with our SSPDs.

Abstract: A stable and switchable dual-wavelength polarization-maintaining erbium-doped fiber ring laser is proposed and demonstrated experimentally by using a novel filter, which is formed from dual-pass Mach-Zehnder interferometer incorporating a sagnac loop. By adjusting the polarization controllers, the output laser can be switched between single- and dual-wavelength. The wavelength spacing of the dual-wavelength can be tuned from 0.084 to 4.26 nm. Its 3-dB bandwidth and side mode suppression ratio are less than 0.015 nm and higher than 64 dB, respectively. In addition, the peak power fluctuation and wavelength shift are monitored to be less than 0.5 dB and 0.01 nm during an hour, respectively. The characteristics of the novel filter in experimental measurement accord with the result of theoretical analysis.

Abstract: The add drop filter (ADF) is one of the most significant devices for coarse wavelength division multiplexing (CWDM) systems to add and/or drop a required channel individually from multiplexed output channels without disturbing other channels. The important parameters of the ADF are coupling efficiency, dropping efficiency, passband width and Q factor. Photonic crystal (PC)-based optical devices have attracted great interest due to their compactness, speed of operation, long life period, suitability for photonic integrated circuits, and future optical networks. Here, an extensive overview of a photonic crystal ring resonator (PCRR)-based ADF using a different shape of ring resonator is presented, and its corresponding functional parameters are discussed. Finally, the designed circular PCRR-based ADF for an ITU-T G 694.2 CWDM system is presented. Approximately 100% of coupling efficiency and dropping efficiency, 114.69 of Q factor, and 13 nm of passband width is obtained through simulation, which outperforms the reported one.

Abstract: A system of microring resonators (MRRs) connected to an optical modified add/drop filter system known as a Panda ring resonator is presented. The optical soliton pulse of 60 GHz frequency band can be generated and used for Wireless Personal Area Network (WPAN) applications such as IEEE 802.15.3c. The system uses chaotic signals generated by a Gaussian laser pulse propagating within a nonlinear MRRs system. The chaotic signals can be generated via a series of microring resonators, where the filtering process is performed via the Panda ring resonator system wherein ultrashort single and multiple optical soliton pulses of 60 GHz are generated and seen at the through and drop ports, respectively. The IEEE 802.15.3c standard operates at the 60 GHz frequency band, and it is applicable for a short distance optical communication such as indoor systems, where the higher transmission data rate can be performed using a high frequency band of the output optical soliton pulses. The single and multi-soliton pulses could be generated and converted to logic codes, where the bandwidths of these pulses are 5 and 20 MHz, respectively. Thus, these types of signals can be used in optical indoor systems and transmission link using appropriate components such as transmitter, fiber optics, amplifier, and receiver.

Abstract: Optical filters with an ultranarrow and rectangular spectral response are highly desired for high-resolution optical/electrical signal processing. An all-fiber optical filter based on a fiber Bragg grating with a large number of phase shifts is designed and fabricated. The measured spectral response shows a 3 dB bandwidth of 650 MHz and a rectangular shape factor of 0.513 at the 25 dB bandwidth. This is the narrowest rectangular bandpass response ever reported for an all-fiber filter, to the best of our knowledge. The filter has also the intrinsic advantages of an all-fiber implementation.

Abstract: A novel approach to the generation of an optical frequency comb with a widely tunable center wavelength and comb spacing based on an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The OEO is implemented using a polarization modulator (PolM), a phase-shifted fiber Bragg grating (PS-FBG), and a photodetector (PD). The PolM is a special phase modulator that supports phase modulation along the two principal axes with opposite modulation indexes. The joint operation of the PolM, the PS-FBG, and the PD corresponds to a frequency-tunable microwave photonic bandpass filter. When the output from the PD is fed back to the PolM, the OEO starts to oscillate and the oscillation frequency can be tuned by tuning the center frequency of the microwave photonic bandpass filter through tuning the optical wavelength. The optical comb is then generated by tapping part of the optical signal from the PolM and sending it to a second PolM. The joint operation of the two PolMs generates an optical comb with the comb spacing tunable by tuning the center frequency of the microwave photonic filter. Through introducing a second wavelength into the OEO, a duplicated optical comb at the second wavelength is generated. An experiment is performed. An optical frequency comb with tunable frequency spacing from 6.6 to 15.3 GHz and a tunable center wavelength from 1500 to 1580 nm is generated.

Abstract: We report on a photonic system for generation of high quality continuous-wave (CW) sub-THz signals. The system consists on a gain-switching-based optical frequency comb generator (GS-OFCG), a two-optical-modes selection mechanism and a n-i-pn-i-p superlattice photomixer. As mode selection mechanism, both selective tunable optical filtering using Fabry-Perot tunable filters (FPTFs) and Optical Injection Locking (OIL) are evaluated. The performance of the reported system surpasses in orders of magnitude the performance of any commercially available optical mm-wave and sub-THz generation system in a great number of parameters. It matches and even overcomes those of the best commercially available electronic THz generation systems. The performance parameters featured by our system are: linewidth <;10 Hz at 120 GHz, complete frequency range coverage (60-140 GHz) with a resolution in the order of 0.1 Hz at 120 GHz (10-12 of generated frequency), high long term frequency stability (5 Hz deviation over one hour). Most of these values are limited by the measurement instrumentation accuracy and resolution, thus the actual values of the system could be better than the reported ones. The frequency can be extended straightforwardly up to 1 THz extending the OFCG frequency span. This system is compact, robust, reliable, offers a very high performance, especially suited for sub-THz photonic local oscillators and high resolution spectroscopy.

Abstract: We present new UV observations for NGC288, taken with the WFC3 detector on board the Hubble Space Telescope, and combine them with existing optical data from the archive to explore the multiple-population phenomenon in this globular cluster (GC). The WFC3's UV filters have demonstrated an uncanny ability to distinguish multiple populations along all photometric sequences in GCs, thanks to their exquisite sensitivity to the atmospheric changes that are tell-tale signs of second-generation enrichment. Optical filters, on the other hand, are more sensitive to stellar-structure changes related to helium enhancement. By combining both UV and optical data we can measure helium variation. We quantify this enhancement for NGC288 and find that its variation is typical of what we have come to expect in other clusters.

Abstract: We demonstrate the operation of a flexible optical filter based on guided mode resonances that operates in the visible regime. The filter is fabricated on a free standing polymeric membrane of 1.3 μm thickness and we show how the geometrical design parameters of the filter determine its optical properties, and how various types of filter can be made with this scheme. To highlight the versatility and robustness of the approach, we mount a filter onto a collimated fibre output and demonstrate successful wavelength filtering.

Abstract: A class of nano-scale wavelength-selective optical filters is proposed where the core of a metal-insulator-metal square ring is replaced with a split-ring core (SRC). The proposed resonator supports split-ring-resonator-like (SRR-like) resonant modes that are characteristics of the structure. These resonant modes are highly adjustable, via the gap size of the split-ring core, over a range of hundreds of nanometers. The proposed resonator can also incorporate tunable materials localized in the gap of the SRC or placed throughout the resonating path. By varying the refractive index (1 to 2) of the material in the gap of the SRC, first and second SRR-like modes can be tuned over ~200 and 300 nm, respectively. A circuit model based on transmission-line theory is proposed for the structure and used to derive the resonance conditions of the split-ring-resonator-like modes; the model compares favorably to the numerical results. The proposed resonator has the potential to be utilized effectively in integrated nano-scale optical switches and tunable filters.

Abstract: Infrared (IR) sensors employing optical readout represent a promising class of devices for the development of thermographic imagers. We demonstrate an infrared radiation detection principle based on thermally tunable one-dimensional (1D) photonic crystals acting as optical filters, integrated with organic and inorganic light emitting diodes (OLEDs and LEDs, respectively). The optical filters are composed of periodically assembled mesoporous TiO2 and SiO2 layers. Due to the thermal tunability of the transmission spectrum of the optical filter, the intensity of light passing through the filter is modulated by temperature. The tuned spectrum lies in the visible region and, therefore, can be directly detected by a visible-light photodetector. The thermal response of the luminance of the OLED-photonic crystal ensemble is 3.8 cd m–2 K–1. Furthermore, we demonstrate that the local temperature profile can be time and spatially resolved with a resolution of 530 by 530 pixel, thus enabling a potential application a...