Showing papers on "Optical filter published in 2013"

Silicon-Organic Hybrid Electro-Optical Devices

Abstract: Organic materials combined with strongly guiding silicon waveguides open the route to highly efficient electro-optical devices. Modulators based on the so-called silicon-organic hybrid (SOH) platform have only recently shown frequency responses up to 100 GHz, high-speed operation beyond 112 Gbit/s with fJ/bit power consumption. In this paper, we review the SOH platform and discuss important devices such as Mach-Zehnder and IQ-modulators based on the linear electro-optic effect. We further show liquid-crystal phase-shifters with a voltage-length product as low as VπL = 0.06 V·mm and sub-μW power consumption as required for slow optical switching or tuning optical filters and devices.

Ultra-High-Contrast and Tunable-Bandwidth Filter Using Cascaded High-Order Silicon Microring Filters

Abstract: High-contrast optical filtering is demonstrated using cascaded silicon microrings. We report an experimental measurement of a record 100 dB pass-band to stop-band contrast, tunable 12–125 GHz passband full-width at half-maximum, band-center insertion loss ripple ${ , and a group delay ripple ${ , using transverse electric polarized light.

Investigation of the graphene based planar plasmonic filters

Abstract: We investigate numerically the edge modes supported by graphene ribbons and the planar band-stop filter consisting of a graphene ribbon lateral coupled a graphene ring resonator by using the finite-difference time-domain method. Simulation results reveal that the edge modes can enhance the electromagnetic coupling between objects indeed and this structure realizes perfect, tunable filtering effect. Successively, the channel-drop filter is constructed. Especially, the proposed structures can be designed and the size of the ring is changed by creating non-uniform conductivity patterns on monolayer graphene. Our studies will benefit the fabrication of the planar, ultra-compact devices in the mid-infrared region.

Feasibility of nanofluid-based optical filters

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.

Si3N4 ring resonator-based microwave photonic notch filter with an ultrahigh peak rejection

Abstract: We report a simple technique in microwave photonic (MWP) signal processing that allows the use of an optical filter with a shallow notch to exhibit a microwave notch filter with anomalously high rejection level. We implement this technique using a low-loss, tunable Si3N4 optical ring resonator as the optical filter, and achieved an MWP notch filter with an ultra-high peak rejection > 60 dB, a tunable high resolution bandwidth of 247-840 MHz, and notch frequency tuning of 2-8 GHz. To our knowledge, this is a record combined peak rejection and resolution for an integrated MWP filter.

A reconfigurable camera add-on for high dynamic range, multispectral, polarization, and light-field imaging

Abstract: We propose a non-permanent add-on that enables plenoptic imaging with standard cameras. Our design is based on a physical copying mechanism that multiplies a sensor image into a number of identical copies that still carry the plenoptic information of interest. Via different optical filters, we can then recover the desired information. A minor modification of the design also allows for aperture sub-sampling and, hence, light-field imaging. As the filters in our design are exchangeable, a reconfiguration for different imaging purposes is possible. We show in a prototype setup that high dynamic range, multispectral, polarization, and light-field imaging can be achieved with our design.

Frequency agile microwave photonic notch filter with anomalously high stopband rejection

Abstract: We report a novel class microwave photonic (MWP) notch filter with a very narrow isolation bandwidth (10 MHz), an ultrahigh stopband rejection (>60 dB), a wide frequency tuning (1-30 GHz), and flexible bandwidth reconfigurability (10-65 MHz). This performance is enabled by a new concept of sideband amplitude and phase controls using an electro-optic modulator and an optical filter. This concept enables energy efficient operation in active MWP notch filters, and opens up a pathway toward enabling low-power nanophotonic devices as high-performance RF filters.

Metal?insulator?metal light absorber: a continuous structure

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.

Wideband tunable optoelectronic oscillator based on a phase modulator and a tunable optical filter.

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.

Photovoltaic/thermal system performance utilizing thin film and nanoparticle dispersion based optical filters

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.

Double-layer Fano resonance photonic crystal filters

Abstract: We report ultra-compact surface-normal high-Q optical filters based on single- and double-layer stacked Fano resonance photonic crystal slabs on both Si and quartz substrates. A single layer photonic crystal filter was designed and a Q factor of 1,737 was obtained with 23 dB extinction ratio. With stacked double-layer photonic crystal configuration, the optical filter Q can increase to over 10,000,000 in design. Double-layer filters with quality factor of 9,734 and extinction ratio of 8 dB were experimentally demonstrated, for a filter design with target Q of 22,000.

Tunable Microwave and Sub-Terahertz Generation Based on Frequency Quadrupling Using a Single Polarization Modulator

Abstract: Frequency quadrupling for tunable microwave and sub-terahertz generation using a single polarization modulator (PolM) in a Sagnac loop without using an optical filter or a wideband microwave phase shifter is proposed and experimentally demonstrated. In the proposed system, a linearly polarized continuous wave from a tunable laser source (TLS) is split into two orthogonally polarized optical waves by a polarization beam splitter (PBS) and sent to the Sagnac loop traveling along the clockwise and counter-clockwise directions. A PolM to which a reference microwave signal is applied is incorporated in the loop. The PolM is a traveling-wave modulator, due to the velocity mismatch only the clockwise light wave is effectively modulated by the reference microwave signal, and the counter-clockwise light wave is not modulated. This is the key point that ensures the cancelation of the optical carrier without the need of an optical filter. Along the clockwise direction, the joint operation of the PolM, a polarization controller (PC), and a polarizer corresponds to a Mach-Zehnder modulator (MZM) with the bias point controlled to suppress the odd-order sidebands. The optical carrier is then suppressed by the counter-clockwise light wave at the polarizer. As a result, only two ±2nd-order sidebands are generated, which are applied to a photodetector (PD) to generate a microwave signal with a frequency that is four times that of the reference microwave signal. A theoretical analysis is developed, which is validated by an experiment. A frequency-quadrupled electrical signal with a large tunable range from 2.04 to 100 GHz is generated. The performance of the proposed system in terms of stability and phase noise is also evaluated.

Pulse-Shaping With Digital, Electrical, and Optical Filters—A Comparison

Abstract: We investigate the performance of sinc-shaped QPSK signal pulses generated in the digital, electrical, and optical domains. To this end an advanced transmitter with a digital pulse-shaper is compared to analog transmitters relying on pulse-shaping with electrical and optical filters, respectively. The signal quality is assessed within a single carrier setup as well as within an ultra-densely spaced WDM arrangement comprising three channels. An advanced receiver providing additional digital filtering with an adaptive equalization algorithm to approximate an ideal brick-wall Nyquist filter has been used for all schemes. It is found that at lower symbol rates, where digital processing is still feasible, digital filters with a large number of filter coefficients provide the best performance. However, transmitters equipped with only electrical or optical pulse-shapers already outperform transmitters sending plain unshaped NRZ signals, so that for higher symbol rates analog electrical and optical techniques not only save costs, but are the only adequate solution.

All-Optical XOR Gate Using Single Quantum-Dot SOA and Optical Filter

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.

Generation of a flat optical frequency comb based on a cascaded polarization modulator and phase modulator.

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.

Compact Notch Microwave Photonic Filters Using On-Chip Integrated Microring Resonators

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.

SFDR enhancement in analog photonic links by simultaneous compensation for dispersion and nonlinearity.

Abstract: A method to improve the spurious-free dynamic range (SFDR) of analog photonic links has been proposed and experimentally demonstrated, which only consists of a phase modulator (PM), a polarizer and an optical filter. Such structure could compensate for the chromatic dispersion and the nonlinearity of the modulator simultaneously. In addition, by adjusting the states of polarization (SOPs) launching into the PM and the polarizer, the proposed scheme could also be reconfigured to mitigate the second harmonic nonlinearity induced by the photodetector. Experimental results show that the suppressions of the second-order and third-order intermodulation distortions (IMD2 & IMD3) are larger than 14-dB and 25.4-dB, respectively. Furthermore, the SFDR can achieve ~110-dB·Hz4/5 for 40-km fiber transmission, which is 26-dB higher than that of the link without compensation.

A controllable noise-like operation regime in a Yb-doped dispersion-mapped fiber ring laser

Abstract: We report the generation of tunable high-energy noise-like pulses with a super-broadband spectrum from a Yb-doped dispersion-mapped fiber ring laser. Self-starting noise-like operation can be maintained over a relatively large range of pumping powers (4–13 W). The corresponding output power varies from 0.1 to 1.45 W. The maximum 3 dB spectral bandwidth of the noise-like pulses is about 48.2 nm while the output energy is as high as 47 nJ, limited by optical damage of the components. The central wavelength of the noise-like pulses can be tuned easily over ~12 nm. The bandwidth and duration of the generated wave packets can also be controlled. The use of a negative dispersion-delay line and spectral filter are found to be important for generating such high-power noise-like operation. Experimental results are in good agreement with theoretical simulations.

A Compact All-Silicon Temperature Insensitive Filter for WDM and Bio-Sensing Applications

Abstract: We propose a compact, temperature-insensitive, and all-silicon Mach-Zehnder interferometer filter that uses the polarization-rotating asymmetrical directional couplers. Temperature sensitivity of the filter is for a wavelength range of 30 nm. The device achieves a reduced footprint by making use of different polarizations, which is made possible by the asymmetric directional couplers that act both as a splitter/combiner and as a polarization rotator. Simulation of the device shows that it can also be useful for gas sensing and bio-sensing applications with three times larger response to cladding changes while keeping a thermally robust behavior.

Fabrication-Tolerant Four-Channel Wavelength-Division-Multiplexing Filter Based on Collectively Tuned Si Microrings

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.

Thin-film tunable filters for hyperspectral fluorescence microscopy

Abstract: Hyperspectral imaging is a powerful tool that acquires data from many spectral bands, forming a contiguous spectrum. Hyperspectral imaging was originally developed for remote sensing applications; however, hyperspectral techniques have since been applied to biological fluorescence imaging applications, such as fluorescence microscopy and small animal fluorescence imaging. The spectral filtering method largely determines the sensitivity and specificity of any hyperspectral imaging system. There are several types of spectral filtering hardware available for microscopy systems, most commonly acousto-optic tunable filters (AOTFs) and liquid crystal tunable filters (LCTFs). These filtering technologies have advantages and disadvantages. Here, we present a novel tunable filter for hyperspectral imaging—the thin-film tunable filter (TFTF). The TFTF presents several advantages over AOTFs and LCTFs, most notably, a high percentage transmission and a high out-of-band optical density (OD). We present a comparison of a TFTF-based hyperspectral microscopy system and a commercially available AOTF-based system. We have characterized the light transmission, wavelength calibration, and OD of both systems, and have then evaluated the capability of each system for discriminating between green fluorescent protein and highly autofluorescent lung tissue. Our results suggest that TFTFs are an alternative approach for hyperspectral filtering that offers improved transmission and out-of-band blocking. These characteristics make TFTFs well suited for other biomedical imaging devices, such as ophthalmoscopes or endoscopes.

Tunable Vernier Microring Optical Filters With $p\!-\!i \!-\!p$ -Type Microheaters

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.

Metamaterial-Inspired Bandpass Filters for Terahertz Surface Waves on Goubau Lines

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.

A snapshot multispectral imager with integrated tiled filters and optical duplication

Abstract: Although the potential of spectral imaging has been demonstrated in research environments, its adoption by industry has so far been limited due to the lack of high speed, low cost and compact spectral cameras. We have previously presented work to overcome this limitation by monolithically integrating optical interference filters on top of standard CMOS image sensors for high resolution pushbroom hyperspectral cameras. These cameras require a scanning of the scene and therefore introduce operator complexity due to the need for synchronization and alignment of the scanning to the camera. This typically leads to problems with motion blur, reduced SNR in high speed applications and detection latency and overall restricts the types of applications that can use this system. This paper introduces a novel snapshot multispectral imager concept based on optical filters monolithically integrated on top of a standard CMOS image sensor. By using monolithic integration for the dedicated, high quality spectral filters at its core, it enables the use of mass-produced fore-optics, reducing the total system cost. It overcomes the problems mentioned for scanning applications by snapshot acquisition, where an entire multispectral data cube is sensed at one discrete point in time. This is achieved by applying a novel, tiled filter layout and an optical sub-system which simultaneously duplicates the scene onto each filter tile. Through the use of monolithically integrated optical filters it retains the qualities of compactness, low cost and high acquisition speed, differentiating it from other snapshot spectral cameras based on heterogeneously integrated custom optics. Moreover, thanks to a simple cube assembly process, it enables real-time, low-latency operation. Our prototype camera can acquire multispectral image cubes of 256x256 pixels over 32 bands in the spectral range of 600-1000nm at a speed of about 30 cubes per second at daylight conditions up to 340 cubes per second at higher illumination levels as typically used in machine vision applications.

Generation of flat-spectrum wideband chaos by fiber ring resonator

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.

Optical Nanofilters Based on Meta-Atom Side-Coupled Plasmonics Metal- Insulator-Metal Waveguides

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.

Coupled double-layer Fano resonance photonic crystal filters with lattice-displacement

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

Superconducting Nanowire Single-Photon Detector with Ultralow Dark Count Rate Using Cold Optical Filters

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.