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A photonic scheme for frequency-quadrupled microwave signal generation with full 360 ° arbitrarily tunable phase shift is proposed and experimentally demonstrated. Two coherent optical components are generated by biasing a Mach-Zehnder modulator at maximum transmission point to suppress odd-order sidebands. Then, two second order sidebands together with the optical carrier are sent to a programmable filter to reject the optical carrier and introduce a phase shift to one of the second order sidebands. A frequency-quadrupled microwave signal with tunable phase is finally obtained by beating the two sidebands at a photodetector. Tunable phase shift of full 360 ° with a microwave frequency from 30 to 42.4 GHz is experimentally demonstrated. The proposed approach has potential applications in the broadband array signal processing system. [ABSTRACT FROM PUBLISHER]

This study has developed an independently-addressed tunable cat's eye retroreflector array for an optical identification system. Switchable cat's eye retroreflectors are achieved by incorporating polymer dispersed liquid crystal film and indium tin oxide (ITO) electrodes into the cat's eye structure. Experiments show that the gap between the front and back units of the cat's eye retroreflector plays a role in the optical performance of the device. The second-generation device is used to realize the optical ID tag. By controlling the ON-OFF state of each pixel, the tunable optical device can generate a dynamic image which carries temporal and spatial codes for the purpose of identification. The ID tag (cat's eye retroreflector array) can operate at two orientations. [ABSTRACT FROM PUBLISHER]

Using ZnSe nanocrystals (NCs) as starting material, Ag-doped or Cu-doped ZnCdSe ternary NCs were prepared by hydrazine-promoted sequential cation exchange in aqueous media. The composition of the NCs can be flexibly controlled by varying the amount of intermediate Ag or Cu cation addition, thus changing the emission of the ternary NCs while preserving the NC size. According to Vegard’s law, the as-prepared ternary NCs possess an alloyed structure. In addition, the ternary NCs obtained have a high quantum yield, strong stability and a broad optical tuning range. [ABSTRACT FROM AUTHOR]

A structure for multiband-UWB waveforms generation is proposed using an N tap microwave photonic filter with positive and negative taps. The system is based on phase inversion in electro-optical modulators and the sliced broadband source optical signal by means of a structure of AWGs, which permits us to obtain a high number of taps. System performance is experimentally shown by the generation of a uniform waveform. Capabilities as reconfigurability and frequency tuning have been also experimentally demonstrated. The flexibility of the system allows us to obtain multiband UWB waveforms, which fulfill the FCC spectral requirements. [ABSTRACT FROM PUBLISHER]

We have characterized the process of radio-frequency (RF) frequency mixing and signal down-conversion in radiation pressure driven optomechanical RF oscillators. We have studied the dependence of the optomechanical mixing process on RF frequency, optical pump power and wavelength detuning and verified the existence of a linear regime where down-converted power is proportional to the RF input power. These outcomes show that optomechanical oscillator (OMO) has the potential to be used as frequency down-converter in RF over fiber links and photonic RF receivers. We have verified the fidelity of the optomechanical down-conversion process and demonstrated the first optomechanical voice down-conversion from an RF carrier. [ABSTRACT FROM PUBLISHER]

A novel high-resolution microwave photonic signal processor is presented, which can realize single bandpass response, narrow passband characteristic, and widely tuned center frequency with shape-invariance. It realizes the single passband signal processing function within the optical link via shifting the dispersion induced RF fading of the modulated bipolar optical broadband source. The configuration can overcome the distortion in shifted dispersion induced RF fading caused by the non-uniformity in the group delay slope of dispersive delay lines. Therefore, a wider spectrum source can be used to improve the selectivity of the passband response. Experimental results verify the new proposed structure and demonstrate a shape-invariant tunable bandpass filter with a 3 dB bandwidth of 157 MHz over 0-20 GHz range. [ABSTRACT FROM PUBLISHER]

The dielectric properties of 0.4Ba0.6Sr0.4TiO3-0.6La(Mg0.5Ti0.5)O3 ceramics and their tunability under external optical fields are investigated at room temperature by means of terahertz time-domain spectroscopy. Application of the optical field leads to an appreciable tuning of the permittivity, which reaches up to 16%. Meanwhile, the dielectric loss changes about 21%. From the results, we find that the change of refractive index has a linear relationship on scale with the applied light power. These findings are attributed to the internal space charge field in the ceramic caused by the excited free carriers. [ABSTRACT FROM AUTHOR]

A low-power all-optical switching is presented based on the all-optical tunable Fano-like resonance in a two-dimensional nonlinear ferroelectric photonic crystal made of polycrystalline lithium niobate. An asymmetric Fano-like line shape is achieved in the transmission spectrum by using two cascaded and uncoupled photonic crystal microcavities. The physical mechanism underlying the all-optical switching is attributed to the dynamic shift of the Fano-like resonance peak caused by variations in the dispersion relations of the photonic crystal structure induced by pump light. A large switching efficiency of 61% is reached under excitation of a weak pump light with an intensity as low as 1 MW cm−2. [ABSTRACT FROM AUTHOR]

We present a high transmission, small bending radius, 180^\circ waveguide bend based on triangular lattice air holes two-dimensional (2-D) planar photonic crystal. The desired high transmission is achieved by altering the waveguide structure at the bending region. Drop hole defects are introduced at the bending region to guide the incoming electromagnetic wave. Simulation results based on the 2-D finite difference time domain method show that normalized transmission as high as 99.4% is achieved at 1550 nm optical wavelength and it is >94\% for nearly the entire optical C-band. In addition, the small bending radius enhances the suitability of our design to be used in ultra-compact photonic integrated circuits. [ABSTRACT FROM PUBLISHER]

Metasurfaces, a two-dimensional (2D) form of metamaterials constituted by planar meta-atoms, exhibit exotic abilities to tailor electromagnetic (EM) waves freely. Over the past decade, tremendous efforts have been made to develop various active materials and incorporate them into functional devices for practical applications, pushing the research of tunable metasurfaces to the forefront of nanophotonics. Those active materials include phase change materials (PCMs), semiconductors, transparent conducting oxides (TCOs), ferroelectrics, liquid crystals (LCs), atomically thin material, etc. , and enable intriguing performances such as fast switching speed, large modulation depth, ultracompactness, and significant contrast of optical properties under external stimuli. Integration of such materials offers substantial tunability to the conventional passive nanophotonic platforms. Tunable metasurfaces with multifunctionalities triggered by various external stimuli bring in rich degrees of freedom in terms of material choices and device designs to dynamically manipulate and control EM waves on demand. This field has recently flourished with the burgeoning development of physics and design methodologies, particularly those assisted by the emerging machine learning (ML) algorithms. This review outlines recent advances in tunable metasurfaces in terms of the active materials and tuning mechanisms, design methodologies, and practical applications. We conclude this review paper by providing future perspectives in this vibrant and fast-growing research field.

Crossing a barrier: Molecules with saturated ER2 units (E=C or Si, R=electron‐releasing group) inserted between two π‐conjugated segments have electronic and optical properties that resemble those of cross‐conjugated molecules (see figure). This cross‐hyperconjugation provides a deeper understanding of the conjugation phenomenon, and is an alternative to cross‐conjugation in the design of molecules for nano and materials applications. [ABSTRACT FROM AUTHOR]

Abstract: Although optical reading systems are important tools to transfer data from a paper form to electronic databases, the impact of system fine-tuning on the final error rate is not usually considered. At the end of a multi-step process involving paper form design training of operators, and fine-tuning procedure, the final rate of error can be reduced from 0.65% to 0.05%. Fine-tuning should be introduced as a standard procedure while using optical reading systems. [Copyright &y& Elsevier]

In this paper, the wavelength interrogation of optical sensors by novel thermo-optic tunable filters with multiple partially reflected slanted mirrors in silicon-on-insulator (SOI) technology is presented. The tunable filter is used both for tunable band selection of broadband light source interrogating the optical sensors, as well as for precise detection of interrogation optical wavelength. New interrogator design is numerically studied by 2-D finite-difference time-domain and finite-element method for a compact structure. The investigation demonstrates the performance of an interrogator on SOI, 1 cm long, having about 1 pm wavelength resolution and 1 ms scanning over 40 nm range at 1550 nm, with an operation power of about 16 mW. [ABSTRACT FROM PUBLISHER]

We propose and demonstrate a new modified Lyot filter to independently ensure the channel amplitude equalization and the bandwidth tuning functions. The device consists of hybrid liquid crystal interferometers. We show numerically and experimentally that for a modified four-stage Lyot filter, the two first interferometers ensure equalization function with a maximum extinction ratio, while the last interferometer allows full-width at half-maximum tuning function by using, in particular, the optical canceling approach. Such a device is highly desirable in optical telecommunications, especially where in the core network the optical filters must perform several other functions in addition to their basic amplitude filtering action. [ABSTRACT FROM PUBLISHER]

A continuously tunable microwave fractional Hilbert transformer (FHT) implemented based on a nonuniformly spaced photonic microwave delay-line filter is proposed and demonstrated. An FHT has a frequency response with a unity magnitude response and a phase response having a phase shift between 0 and π at the center frequency. A seven-tap photonic microwave delay-line filter with nonuniformly spaced taps is designed to provide such a frequency response. The advantage of using nonuniform spacing is that an equivalent negative coefficient can be achieved by introducing an additional time delay leading to a π phase shift, corresponding to a negative coefficient. An FHT operating at a center frequency around 8.165 GHz with a tunable order between 0.24 and 1 is implemented. A classical HT operating at a center frequency of 7.573 GHz with a bandwidth greater than 4.5 GHz is also implemented. The use of the classical HT to perform temporal Hilbert transform of a Gaussian-like electrical pulse is demonstrated. [ABSTRACT FROM PUBLISHER]

640 Gb/s all-optical ON-OFF keying signal regeneration based on nonlinearities in a periodically poled lithium niobate (PPLN) waveguide is demonstrated and characterized. The data are transferred from the distorted original optical time division multiplexing (OTDM) signal onto a high quality and locally generated 640 GHz clock by pump depletion simultaneously operating a wavelength conversion. The thresholding behavior and the saturation effect of pump depletion in the PPLN waveguide for low and high pump power values, respectively, allow for a noise compression on the wavelength-converted replica of the original signal. The use of a stable clock with a shorter pulsewidth also allows for a sort of sampling of the original OTDM frame in the bit time center, resulting in a pulse reshaping that eliminates all noise contributions on the pulse tails. This operation produces a reduction of the original time jitter of the signal. The sampling operation also results in a compression of the new OTDM frame pulses compensating for the residual chromatic dispersion effects. After having characterized the transfer function of the proposed regeneration scheme, we evaluate its effectiveness as a function of optical signal-to-noise ratio (OSNR), pulse broadening, and time jitter of the input signal. Regeneration performance is measured in terms of bit error rate (BER) and power penalty @ BER = 10-9. Error-free operation can be recovered and maintained over an input signal OSNR range of 8 dB. This results in a BER improvement up to several orders of magnitude, obtained when the input OSNR is reduced by 20 dB with respect to the baseline. The time jitter of input signals with original jitter values from 100 to 300 fs has been reduced of nearly one order of magnitude to values lower than 40 fs. We also demonstrate that a pulse broadening up to 20% of the original pulsewidth can be compensated. Finally, wavelength tunability of the regenerator input and output signal can be, in principle, guaranteed over the whole C-band and beyond. [ABSTRACT FROM PUBLISHER]

This study synthesized a europium (Eu3+) complex Eu(DBM)3Cl-MIP (DBM = dibenzoyl methane; Cl-MIP = 2-(2-chlorophenyl)-1-methyl-1H-imidazo[4,5-f][1,10]phenanthroline) dispersed in a benzyl methacrylate (BMA) monomer and treated with ultraviolet (UV) light for polymerization. Spectral results showed that the europium complex containing an antenna, Cl-MIP, which had higher triplet energy into the Eu3+ energy level, was an energetically enhanced europium emission. Typical stacking behaviors of π-π interactions between the ligands and the Eu3+-ion were analyzed using single crystal X-ray diffraction. Regarding the luminescence performance of this europium composite, the ligand/defect emission was suppressed by dispersion in a poly-BMA (PBMA) matrix. The underlying mechanism of the effective enhancement of the pure Eu3+ emission was attributed to the combined effects of structural modifications, defect emissions, and carrier charge transfer. Fluorescence spectra were compared to the composite of optimized Eu3+ emission where they were subsequently chelated to four metal ions via carboxylate groups on the BMA unit. The optical enhanced europium composite clearly demonstrated highly efficient optical responses and is, therefore a promising application as an optical detection material. [ABSTRACT FROM AUTHOR]

We present theoretical analysis of simultaneous photonic frequency downconversion, based on four-wave mixing (FWM), and optical subcarrier modulation in an electroabsorption modulator (EAM) for broadband millimeter wave (mm-wave) over fiber uplinks. A model of FWM generation in an EAM is established. The FWM efficiency with an EAM at dc bias with and without RF modulation is analyzed, including the impact of bias voltage, modulation voltage, and frequency detuning. The calculated FWM efficiency agrees well with the measured. The signal-to-noise ratio (SNR) of photonic frequency downconverted RF signal is also analyzed, considering the nonlinear distortion due to the EAM nonlinearities. Moreover, calculated error vector magnitude using theoretical SNR is compared with the measured for multiband orthogonal frequency division multiplexing ultrawideband mm-wave signals. [ABSTRACT FROM PUBLISHER]

A novel structure and principle of a tunable optical fiber filter is proposed, including a fiber Bragg grating loop mirror, an electromagnetic force based cantilever tuning unit, and a micro-displacement tuning unit. The electromagnetic force based cantilever tuning unit is used to change the pass band of the optical fiber filter, and the micro-displacement tuning unit is used to change the channel spacing of the comb filter within the pass band. The electromagnetic force is produced by an electromagnetic coil and permanent magnet element, and the micro-displacement driver includes a PZT piezoelectric ceramic and its driving circuit. Principles and structures are introduced. Theoretical and experimental results indicate the feasibility of the proposed idea. And some key problems about the cantilever and the PZT structure are discussed. [ABSTRACT FROM PUBLISHER]

Earlier investigations have demonstrated that a narrow bandwidth intracavity filter in a semiconductor optical amplifier (SOA) fiber ring laser results in an output spectrum comprising multiple longitudinal modes, whereas a large bandwidth filter generally leads to quasi-single-mode oscillation. In this paper, we report on a numerical model simulating the SOA fiber ring laser dynamics and compare its predictions with experimental results obtained with two different SOAs. We demonstrate that the observed experimental behavior results from the interplay between the complex transfer function of the intracavity optical filter, fast gain saturation and four-wave mixing (FWM) due to the linewidth enhancement factor. The dispersion introduced by the complex transfer function of the filter can lead to modulation instability, which plays a key role in the laser dynamics that can evolve into a single-longitudinal mode (SLM) operation. We also simulate the behavior of the laser emission when the wavelength of the optical filter is finely tuned. [ABSTRACT FROM PUBLISHER]

The article presents a study which discusses a wavelength add drop multiplexer (WADM) design for intelligent optical networks. It states that the proposed multiplexer with poled electro-optical polymer material and trapezoidal waveguide geometry enables WADM's wavelength tuning speed at the coupling length of 1.5 millimeter (mm) and resonance wavelength of 1550 nanometer (nm). It adds that the capabilities of the device make WADM as a vital element in intelligent optical network.

A linearity tunable fiber ring laser based on closed-loop piezoelectric ceramics (PZT) is proposed and successfully demonstrated. The fiber Bragg grating (FBG) of the ring cavity is fixed on the PZT, and the strain gauge attached on the PZT is monitoring the error, the PZT driving system is compensating the nonlinear hysteresis of the PZT real-timely. Experimental results show that the tuning curve of the fiber ring laser is linear with the tuning range nearly 0.9 nm. Furthermore, the wavelength fluctuations of the tunable fiber ring laser are kept within ±0.01 nm, and the power variation is kept within ±0.3 dB. [ABSTRACT FROM AUTHOR]

Tunable single-mode injection-locked optoelectronic oscillator (OEO) with low phase-noise is presented in this article. A master oscillator contributed by amplifier and tunable band-pass filter (TBPF) is embedded in the general single loop OEO. The oscillation frequency is determined by the center frequency of the TBPF. A portion of master oscillation signal is injected into the slave OEO loop, then it is locked by OEO signal after it passes through a long optical delay line with high Q value. With the RF isolator, the master oscillation signal with poor phase-noise is avoided directly mixing in the OEO signal. With the configuration a tunable range from 1.93 to 17.69 GHz with 68 dB suppression of the nearest-neighbor spur is realized. The single-sideband phase noise is below −112 dBc/Hz at an offset of 10 kHz within the whole tunable range. The frequency drift of the OEO system is < ± 2 kHz within 1 h. With low cost and simple system, tunable single-mode RF signal with low phase-noise and high frequency-stability is generated. [ABSTRACT FROM AUTHOR]

All-pass, wavelength-selective, slowlight optical buffers using cascaded micro-ring resonator filters are theoretically and experimentally studied in the context of optical packet switching router architectures. Two optical buffers fabricated in high-index contrast silica technology are compared: an 8-ring buffer with narrow resonances for wavelength selective delay and a 32-ring buffer for distortion-free delay at 10 and 40 Gb/s. Accurate measurements and configurations of the transmission of the buffers allow for precise simulations that accurately predict pattern dependent distortions, signal integrity and performance extrapolation to higher data rates. Continuously tunable delays are demonstrated up to 8 bits with minimal distortion. [ABSTRACT FROM PUBLISHER]

We propose a novel and compact all-optical device for tunable slow and fast light propagation in parallel coupled structures consisting of two dissimilar optical nanowires. Advancement/delay tunability is achieved by varying the spacing between two nanowires. We develop a simplified analytical model to describe the dependence of achievable time advancement/delay and pulse broadening on the diameter of optical nanowires and it is indicated that this model agrees well with numerical results. Simulation results show that achievable maximum group-index change is about 1.4 and a 10-mm-long device can advance/delay a 2-ps pulse by 15.9 ps with pulse broadening factor equaling to 2. [ABSTRACT FROM PUBLISHER]

All-optical tunable delay plays an important role for enabling high-throughput optical routing, such as buffer for contention resolution and timing synchronization. We proposed and demonstrated a widely tunable optical delay using optical Kerr-switch-based wavelength converter and pulse pre-chirping for compensating chromatic dispersion in fiber delay. With wideband operation provided by a Kerr-switch wavelength converter, a tunable delay range up to 8.56 ns was achieved. Meanwhile, pulse pre-chirping helped to alleviate group-velocity dispersion (GVD)-induced penalty, and provided simultaneous nonreturn-zero (NRZ)-to-RZ format conversion functionality. [ABSTRACT FROM PUBLISHER]

In this paper, we analyze in detail the characteristics of surface electromagnetic modes that can propagate along a periodically corrugated, perfectly conducting wire. We show how these modes, termed spoof surface plasmon polaritons, resemble surface plasmon polaritons supported by metallic wires at optical frequencies. The important point is that the dispersion relation of spoof surface plasmon polaritons is mainly controlled by the geometry of the corrugation. This fact allows the tuning of the properties of these modes via changes in corrugation geometry. Important applications lie in high-confinement waveguiding at far-infrared and microwave frequencies, including the possibility of efficient propagation to the tip of tapered structures for focusing. [ABSTRACT FROM PUBLISHER]

A tunable band-selection interleaver filter switch formed on a silicon-on-insulator (SOI) substrate for dense wavelength-division-multiplexing (DWDM) system was fabricated, and its fundamental performances were experimentally demonstrated. The device consists of monolithic integration of a Michelson interferometer (MI) structure with two arm-waveguides with different lengths, appropriately designed for ITU-grid separation, a phase-modulation electrode on one arm and tunable wideband Bragg gratings reflectors. An SOI rib waveguide structure with a medium mode size was adopted for low loss and easy fabrication. A bandwidth of the grating, formed by electron beam (EB) lithography and deep reactive-ion etching (Deep-RIE), was rather large of 4 nm at -10 dB transmission level, from which a large coupling coefficient of the Bragg grating of 105 cm-1 was evaluated. A large tuning range of the Bragg grating of 17 nm was obtained. An extinction ratio of the interleaver filter was about 18 dB, and the interleaving switching was also attained with an applied electric power of 50 mW and a switching speed of about 1 ms. [ABSTRACT FROM PUBLISHER]

We demonstrate a very simple implementation of a single-bandpass microwave photonic finite-impulse response filter (MPF) with discrete tuning of the bandpass frequency and reconfigurability of the bandpass shape. We also discuss the notion of wavelength sampling in an MPF. A theoretical analysis has been developed to exploit the properties of the proposed MPF. [ABSTRACT FROM PUBLISHER]

A novel technique to generate binary phase-coded, direct-sequence ultra-wideband (DS-UWB) signals in the optical domain is proposed and demonstrated. In the proposed system, the wavelengths from a laser array are modulated by a Gaussian pulse, which is sent to a multichannel optical frequency discriminator, to generate a UWB monocycle or doublet pulse sequence with a predetermined phase-code pattern. By tuning the wavelengths of the laser array, or by tuning the states of polarization of the wavelengths, the generated pulse shape and code pattern can be changed. The key device in the system is the multichannel dispersive chirped fiber Bragg grating (FBG), which functions, in combination with a dispersive fiber, as a multichannel frequency discriminator with a step-increased group-delay response, to ensure the generated UWB sequence to have uniform time spacing among the chips and to compensate for the fiber-induced chromatic dispersion. The proposed scheme is experimentally demonstrated. A multichannel chirped FBG is designed and fabricated. Binary phase-coded DS-UWB signals with different code patterns are experimentally generated. [ABSTRACT FROM PUBLISHER]

The materials and processes for fabrication of monolithically integrated microelectromechanical systems-based microspectrometers operating in the short-wavelength IR range is presented. Using low-temperature surface micromachining techniques, compatible with a range of IR sensor technologies, silicon-nitride-based tunable Fabry-Perot filter structures with distributed Bragg mirrors made of Ge/SiO/Ge layers have been monolithically integrated with HgCdTe photoconductors. The stress within and between the many layers of the structure has been eliminated or compensated by stress tuning of the deposition conditions. The demonstrated microspectrometers have a tuning range of 1.8-2.2 mum with relative peak transmission of 70% and full-width at half-maximum of 80plusmn10 nm. [ABSTRACT FROM PUBLISHER]

A novel 3-D hollow waveguide (HWG) is proposed and demonstrated to realize low propagation loss and polarization insensitivity for widely tunable and temperature insensitive photonic devices used in future dense wavelength division multiplexing networks. Low polarization dependence and low propagation loss, at the same time, are realizable by optimizing the shape of our 3-D structure. The modeling and experiment of the novel 3-D HWG is presented for low propagation loss and low polarization dependence. [ABSTRACT FROM PUBLISHER]

Tunable optical delay lines have many applications for high-performance optical switching and signal processing. Slow light has emerged as an enabling technology for achieving continuously tunable optical delays. Delay reconfigurability opens up a whole new field of nonlinear signal processing using slow light. In this paper, the authors review recent advances in slow-light-based optical signal processing, with a focus on the data fidelity after traversing the slow light elements. The concept of slow-light-induced data pattern dependence is introduced and is shown to be the main signal degrading effect. We then propose and experimentally demonstrate phase-preserving slow light by delaying 10 Gb/s differential phase-shift keying (DPSK) signals with reduced DPSK pattern dependence. Spectrally efficient slow light using advanced multilevel phase-modulated formats is further described. With this technique, doubled bit-rate signals can be transmitted through a bandwidth-limited slow light element. We finally show several novel slow-light-based signal processing modules. Unique features such as multichannel operation, variable bit-rate capability, and simultaneous multiple functions are highlighted. [ABSTRACT FROM PUBLISHER]

We experimentally demonstrate optically tunable delay line in a silicon microring resonator with a 20- m radius. The delay-tuning mechanism is based on the red shift of the resonance induced by thermal nonlinear effect. We investigate the delay performance of three modulation formats-non-return-to-zero (NRZ), return-to-zero (RZ), and differential phase-shift keying (DPSK) signals at different data rates. Tunable delay is achieved by controlling the power of the continuous-wave (CW) pump with very low tuning threshold, which could be used in microring-resonator-based slow-light structure. [ABSTRACT FROM PUBLISHER]

Continuously tunable optical delay lines based on parametric process in optical fibers are described theoretically and demonstrated experimentally. Performance limits are outlined and compared to alternative approaches capable of continually adjustable delay management. The current record of 105 ns tunable optical delay at 10 Gb/s is presented, with a delay-bandwidth product of 1055. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

A dual-shutter MEMS variable optical attenuator (VOA) is designed for advanced tuning functions such as linear attenuation relationship and simultaneous coarse and fine tunings. The mechanism behind is to take advantage of the additional shutter to render one more degree of freedom for attenuation adjustment. Although dual-shutter VOAs with asymmetric functionalities have been reported before, these intrinsic capabilities owing to asymmetry have not been extensively investigated. In experiment, the fabricated VOA device has demonstrated a linear tuning over a 20-dB range with respect to the driving voltage of one shutter, and it has also realized simultaneously coarse tuning (2.5 dB/V) and fine tuning (0.1 dB/V) by the two shutters. Ideally, the tuning can start from any available working point, linear to any controlling parameter, at any slope of linearity, and with any tuning resolution. Theoretical attenuation model has also been developed to provide a roadmap for the VOA design and choice of working point. An interesting finding is that over a certain range the linear attenuation can be obtained by moving a fixed aperture rather than by reducing the aperture size, which greatly relaxes the difficulty of shutter position control. The measured results match well with the theoretical data, implying the possibility of developing a look-up table to locate the shutter positions quickly. The dual-shutter VOA accomplishes these features without the need of high-precision control systems and therefore gives a structure-based rather than a control-system-based solution, clearly advantageous over the previously developed VOAs. [ABSTRACT FROM PUBLISHER]

In this paper, an optically controlled tunable delay scheme has been proposed using four-wave mixing (FWM) wavelength conversion in a 35-cm highly nonlinear bismuth-oxide fiber (Bi-NLF) together with group velocity dispersion (GVD) in a chirped fiber Bragg grating (CFBG). The Bi-NLF offers a very large nonlinearity and gives rise to significant FWM over a short fiber segment. With the use of a CFBG, a delay range over 185 ps has been experimentally demonstrated. To investigate the performance of the tunable delay, we have applied the scheme for variable delays of 10-Gb/s amplitude-shift keying (ASK) and differential phase-shift keying (DPSK) data signals. The bit error rate (BER) measurements show a power penalty of less than 3.5 dB for both amplitude- and phase-modulated data formats. To further increase the delay time, the CFBG has been replaced by a dispersion compensated fiber (DCF) that provides a wider bandwidth of operation. A variable delay up to 840 ps has been obtained using dual-pump FWM that offers a conversion bandwidth of about 40 nm. The large conversion range helps to minimize GVD-induced pulse distortion as a shorter DCF can be used for a given delay. The Bi-NLF provides an enhanced stimulated Brillouin scattering (SBS) threshold, a reduced latency, and an increased compactness of the approach for practical applications. [ABSTRACT FROM PUBLISHER]

This paper presents a clustering-based color segmentation method where the desired object is focused on. As classical methods suffer from a lack of robustness, salient colors appearing in the object are used to intuitively tune the algorithm. These salient colors are extracted according to a psychovisual scheme and a peak-finding step. Results on various test sequences, covering a representative set of outdoor real videos, show the improvement when compared to a simple implementation of the same K-means oriented segmentation algorithm with ad hoc parameter setting strategy and with the well-known mean-shift algorithm. [ABSTRACT FROM AUTHOR]

This paper presents the time analysis of the Bragg wavelength shift in fiber gratings based on the elastodynamics theory. The model is valuable for estimating an upper limit for the tuning speed when traction forces are applied to the free end of the grating. A comparison between the single-mode silica optical fiber and polymer optical fiber is performed, showing the differences in time response, tuning speed, and Bragg wavelength shift when the same longitudinal load is applied on both types of fibers. [ABSTRACT FROM PUBLISHER]

A continuously tunable photonic microwave bandpass filter with positive and negative coefficients implemented using an optical phase modulator and chirped fiber Bragg gratings (FBGs) is proposed and experimentally demonstrated. The positive and negative coefficients are generated through optical phase-modulation to intensity-modulation conversion by reflecting the phase-modulated optical carrier from linearly chirped FBGs (LCFBGs) with positive and negative dispersions. The tunability of the filter is realized by changing the wavelength of the optical carrier such that it is reflected at different physical locations in the LCFBGs. A two-tap microwave bandpass filter with a free spectral range tunable from 1.14 to 4.55 GHz is experimentally demonstrated. [ABSTRACT FROM PUBLISHER]

We demonstrate group velocity dispersion (GVD) and dispersion slope (DS) compensation devices based on enhanced thermal chirping of glued fiber Bragg gratings (FBGs). By gluing a uniform FBG onto a uniform Zn channel for the GVD compensator and a linearly tapered Zn channel for the DS compensator, the thermal sensitivity of the FBG is enhanced. The thermal chirping of the FBG is given by the thermal expansion of a Zn channel, the thermooptic effect, and the thermal expansion of the optical fiber. Controlling the temperature distributions of the uniform FBG performs the tuning of GVD and/or DS, as well as their center wavelengths. These compensation devices can tune their GVD or DS while maintaining a constant center wavelength and change their center wavelengths while preserving constant GVD or DS values. The GVD can be tuned from -123.39 to -57.29 ps/nm with the center wavelength at 1552.9 nm, while the center wavelength can be shifted 2.2 nm with a GVD value of about -105 ps/nm. On the other hand, the DS can be tuned from -8.88 to -24.44 ps/nm2 with the center wavelength at 1554.5 nm, while the center wavelength can be shifted 0.6 nm with a DS value of about -10.4 ps/nm2. [ABSTRACT FROM PUBLISHER]

A polarization-independent two-port guided-wave electrooptically tunable bandpass filter with programmable spectral output in Ti: LiNbO3 at the 1530-nm wavelength regime is presented. The design utilizes an asymmetric interferometer with interdigital electrode sets on its arms that allow the control of the polarization coupling coefficient on individual sets electronically for the simultaneous tuning and apodization of the spectral output. The operation of a device with 16 electrode sets is demonstrated, and good agreement with theoretical predictions is obtained. The number of selectable channels depends on the number of electrode sets. A 3-dB bandwidth of ~1 nm and a tuning range of 10 nm are measured. Nearest sidelobes ~20 dB below the main peak are realized. Fiber-to-fiber insertion loss is < 4.6 dB. The maximum voltage that is used for selecting channels at plusmn4.8 nm away from the center channel wavelength with apodization is 17.3 V. [ABSTRACT FROM PUBLISHER]

Slow light has been proposed as a potential solution to all-optically tunable delay line. However, a slow-light element may degrade data quality in an optical communication system while decreasing group velocity of optical pulses. In this paper, pattern dependence of signal distortion is identified as a main reason for data degradation, which is caused by narrow-band amplitude and phase responses of the slow-light elements. We define figure of merit involving pulse delay and data degradation to optimize slow-light devices. It is shown that the pattern dependence can be reduced by detuning slow-light devices away from the signal carrier frequency, which allows using narrow-band slow-light techniques to increase normalized delay up to 0.8, with Q improvement of 2 dB. [ABSTRACT FROM PUBLISHER]

We summarize our progress made on developing widely tunable monochromatic terahertz sources. They have been implemented based on difference-frequency generation (DFG) in GaSe, ZnGeP, and GaP crystals. Using a GaSe crystal, the output wavelength was tuned continuously in the range of 66.5 to 5664 m (from 150 to 1.77 cm with the peak power reaching 389 W. Such a high peak power corresponds to a conversion efficiency of 0.1% (a photon conversion efficiency of 19%). A further optimization on the terahertz beam parameter may result in higher output powers and conversion efficiencies. Our experimental results indicate that within the range of 100-250 mum, the output peak powers were higher than 100 W. On the other hand, based on DFG in a ZnGeP crystal, the output wavelength was generated to be tunable in the ranges of 83.1-1642 mum and 80.2-1416 mum for two phase-matching configurations. The output power reached 134 W. Using a GaP crystal, the output wavelength was tuned in the range of 71.1-2830 m, whereas the highest peak power reached 15.6 W. GaP offers an advantage for tuning the output wavelength compared with GaSe and ZnGeP since crystal rotation is no longer required. Instead, one just needs to tune the wavelength of one mixing beam within a narrow bandwidth of 15.3 nm. Based on power scaling, a shoe box-sized tunable terahertz source is feasible. We also review our recent results obtained following the investigation of backward DFG and feasibility studies on backward parametric oscillation. The terahertz radiations produced by DFG have pulse durations of about 5 ns, wide tuning ranges, and narrow linewidths, which are quite different from the broadband terahertz pulses. We also describe a few important applications that were realized by taking advantage of the wide tuning range and narrow linewidths of terahertz pulses such as chemical sensing and differentiation of isotopic variants by measuring the rotational spectra of gases and terahertz imaging. [ABSTRACT FROM PUBLISHER]

Efficient frequency conversion of high-repetition-rate femtosecond and picosecond pulses with wide tunability throughout the visible, blue, and UV is demonstrated in the nonlinear optical crystal BiB3O6 (BIBO). Using direct single-pass second harmonic generation of Kerr-lens-mode-locked Ti:sapphire laser, average output powers close to 1 W at conversion efficiencies in excess of 50% are demonstrated with broad spectral coverage across 370-450 nm in the near-UV and blue spectral range. Output pulse durations of 220 fs and 2.8 ps have been obtained at a pulse repetition rate of 76 MHz. By implementing a synchronously pumped femtosecond optical parametric oscillator (OPO) based on BIBO, the generated spectral range is further extended to cover the entire visible spectrum, across the green-yellow-orange-red regions. Average output powers of up to 270 mW in pulses of 120 fs at 76 MHz repetition rate have been generated across the 480-710 nm range. Internal frequency doubling of the visible OPO signal pulses in the crystal of beta-BaB2O4 has permitted high-repetition-rate femtosecond pulse generation with wide tunability across 250-350 nm in the UV at up to 100 mW of average power. The potential of BIBO for efficient frequency conversion of high-intensity pulses is also demonstrated through third harmonic generation of amplified microjoule picosecond pulses at 1064 nm. Output pulse energies of 216 muJ in 29 ps duration at 25 Hz repetition rate have been obtained at 355 nm with overall conversion efficiencies of up to 50%. [ABSTRACT FROM PUBLISHER]

In this paper, we report the performance-analysis results of our proposed high-speed and low-cost feedback-control method of a tunable-dispersion compensator (TDC) for adaptive dispersion compensation in all-optical dynamic-routing networks. In this method, we monitor the received waveform in the time domain and control a TDC repeatedly to reshape the waveform by means of the steepest descent method. Transmission experiments and simulations show that the proposed method can compensate for the dispersion quickly over a wide dispersion range. The compensation range is from -6000 to 6000 ps/nm in 10-Gb/s transmission. The compensation time is 1-2 s for dispersions within 1000 ps/nm. This method is applicable to the adaptive dispersion compensation in all-optical dynamic-routing networks [ABSTRACT FROM PUBLISHER]

We theoretically evaluate the performance of tunable channel-selective wavelength shift based on cascaded sum- and difference-frequency generation by the use of two pump lights in periodically poled lithium niobate waveguides. In double-pass configurations, the functions of wavelength add/drop and wavelength shift are easy to integrate in the same waveguide. Analysis shows that a longer waveguide more competently adapts narrower channel spacing in wavelength-division-multiplexed (WDM) systems. This wavelength shifter is flexible due to the almost separable operations of the two pumps: The channel is selected by setting the first pump, and the wavelength-shifting value is tuned by adjusting the second pump. This wavelength shifter has a very large dynamic region. For a 2.56-cm-long waveguide, the maximum dynamic region is as broad as 67 nm in a 0.4-nm channel-spacing WDM system. The dynamic region is mainly dominated by the limitation of multiple-channel crosstalk in a dense WDM system. However, it is dominated by the limitation of single-channel efficiency fluctuation in a coarse one [ABSTRACT FROM PUBLISHER]

We present the first monolithic widely tunable 40-Gb/s transceivers. The devices integrate sampled grating distributed Bragg reflector (SG-DBR) lasers, quantum-well electroabsorption modulators (EAM), low-confinement semiconductor optical amplifiers (SOA), and uni-traveling carrier (UTC) photodiodes for state-of-the-art light generation, modulation, amplification, and detection. A relatively simple high-flexibility fabrication scheme combining quantum-well intermixing (QWI) and blanket metal-organic chemical vapor deposition (MOCVD) regrowth was used to integrate components with performance rivaling optimized discrete devices. The SG-DBR/EAM transmitters demonstrate 30 nm of tuning, 39-GHz bandwidth, low-drive voltage, and low power penalty 40-Gb/s transmission through 2.3 km of fiber. The SOA/UTC photodetector receivers provide 23-28 dB of gain, saturation powers up to 18.6 dBm, and -20.2 dBm of chip-coupled sensitivity at 40 Gb/s. By connecting the transmitters and receivers off-chip, we demonstrate 40-Gb/s wavelength conversion [ABSTRACT FROM PUBLISHER]