Abstract: In this letter, we propose and experimentally demonstrate a color filter based on an annular aperture geometry working in the visible range. The device is built by configuring an array of annular apertures in a gold film suitable for transmission measurement. We show effective fine tuning of resonance peaks through precise geometric control of the aperture dimensions. Selective transmission through annular apertures of various sizes leads to continuous color tuning of transmitted electromagnetic waves. This may find potential for application in high-definition displays, optical filters, ultrafast switching, and bio-sensing.

Abstract: We demonstrate reconfigurable and tunable flat-top microwave photonic filters based on an optical comb source and a dispersive medium. Complex taps allowing flexible and tunable filter characteristics are implemented by programming the amplitude and phase of individual comb lines using an optical line-by-line pulse shaper. First, we implement a flat top filter by applying positive and negative weights across the comb lines, then tune the filter center frequency by adding a phase ramp onto the tap weights.

Abstract: Spatial division multiplexing has been proposed as an option for further capacity increase of transmission fibers. Application of this concept is attractive only, if cost and energy efficient implementations can be found. In this work, optical amplification and optical filter based signal processing concepts are investigated. Deployment of multi mode fibers as the waveguide type for erbium doped fiber amplifiers potentially offers cost and energy efficiency advantages compared to using multi core fibers in preamplifier as well as booster stages. Additional advantages can be gained from optimization of the amplifier module design. Together with transponder design optimizations, they can increase the attractiveness of inverse spatial multiplexing, which is proposed as an intermediate step. Signal processing based on adaptive passive optical filters offers an alternative approach for the separation of channels at the receiver which have experienced mode coupling along the link. With this optical filter based approach, fiber capacity can potentially be increased faster and more energy efficiently than with solutions relying solely on electronic signal processing.

Abstract: We present a theoretical and experimental study of the ultra-narrow bandwidth Faraday anomalous dispersion optical filter (FADOF) operating at the rubidium D1 line (795 nm). This atomic line gives better performance than other lines for the main FADOF figures of merit, e.g. simultaneously 71% transmission, 445 MHz bandwidth and 1.2 GHz equivalent noise bandwidth.

Abstract: The invention relates to an optical coherence microscopy system for fast, phase resolved imaging by means of optical coherence microscopy with decoupled illumination and detection apertures, producing a dark-field effect with an enhanced optical contrast. The setup uses a light source with an appropriate temporal coherence, an interferometer and an array detector combined with a spectrometer. The dark-field effect is produced by optical filter means in the illumination and detection paths, positioned in conjugated planes of the sample microscope objective. These optical means comprise for example refractive or diffractive elements, amplitude or phase masks, or programmable spatial light modulators. The object is scanned via a scanning unit allowing a point scan of the object.

Abstract: We demonstrate that a narrowband spectral filter with a ripples-free isolated transmission peak and wide acceptance angle can be constructed exploiting polarization properties of a metal film patterned on the subwavelength scale. Its transmission band can be engineered to be anywhere from the visible to microwaves.

Abstract: A microwave bandpass differentiator implemented based on a finite impulse response (FIR) photonic microwave delay-line filter with nonuniformly-spaced taps is proposed and experimentally demonstrated. To implement a microwave bandpass differentiator, the coefficients of the photonic microwave delay-line filter should have both positive and negative coefficients. In the proposed approach, the negative coefficients are equivalently achieved by introducing an additional time delay to each of the taps, leading to a π phase shift to the tap. Compared with a uniformly-spaced photonic microwave delay-line filter with true negative coefficients, the proposed differentiator features a greatly simplified implementation. A microwave bandpass differentiator based on a six-tap nonuniformly-spaced photonic microwave delay-line filter is designed, simulated, and experimentally demonstrated. The reconfigurability of the microwave bandpass differentiator is experimentally investigated. The employment of the differentiator to perform differentiation of a bandpass microwave signal is also experimentally demonstrated.

Abstract: We demonstrate an add/drop filter based on coupled vertical gratings on silicon. The concept is extended to implement a 1 by 4 wavelength division multiplexer with 3nm bandwidth, 1dB insertion loss and 16dB crosstalk suppression.

Abstract: We present the first hybrid-integrated optical phase-lock loop (OPLL) for use in high spectral purity photonic terahertz sources. We have achieved the necessary short loop delay to lock a 1-MHz linewidth slave laser by hybrid integration of the slave laser and photodetector on a silicon motherboard with silica optical waveguides and combining this with a custom-designed low-delay electronic loop filter circuit. The laser and photodetectors are InP-based and are flip chip bonded to silicon daughter boards, which are in turn attached to the motherboard. Delay between the slave laser and photodiode was approximately 50 ps. The heterodyne between slave and master sources has a linewidth of less than 1 kHz and achieved phase noise less than -80 dBc/Hz at an offset of 10 kHz. The slave laser can be offset from the master source by 2-7 GHz, using a microwave oscillator. This integrated OPLL circuit was used with an optical comb source and an injection-locked laser comb filter to generate high spectral purity signals at frequencies up to 300 GHz with linewidths <;1 kHz and powers of about -20 dBm, while the two integrated lasers could deliver a tunable heterodyne signal at frequencies up to 1.8 THz.

Abstract: A novel scheme to generate coherent 112 optical sub-carriers with subcarrier spacing of 25GHz is proposed and experimentally demonstrated. By using these coherent optical subcarriers, we have successfully generated 10-Tbit/s (112×100Gbit/s) single channel optical OFDM polarization multiplexing QPSK signal with the bandwidth of 2.8THz and the optical signal noise ratio greater than 20dB (0.1nm bandwidth for each subchannel). We have transmitted the 11.2-Tbit/s optical OFDM signal over 640-km SMF-28 with an amplifier span of 80km and EDFA-only.

Abstract: A tunable microwave photonic filter with complex coefficients is presented. The complex coefficient is generated using an all-optical radio-frequency (RF) phase shifter, which contains an optical differentiator to realize phase modulation to intensity modulation conversion. The phase of the RF signal can be tuned continuously by simply controlling the optical power through changing variable optical attenuators. A two-tap tunable filter with one complex coefficient is experimentally demonstrated and a continuous tuning range of a half free spectral range is realized without changing the shape of the frequency response.

Abstract: We experimentally and numerically demonstrate a planar metamaterial consisting of two asymmetrically positioned π-structures in a single unit that exhibits plasmonic analogue of electromagnetically induced transparency (EIT). Through the coupling of the constituent nanorod elements, the proposed structure enables fine spectral tuning of the EIT-like behavior and controlling the location of near field enhancement. Originated from the asymmetric cascaded π-structures, we introduce a more compact system which possesses the EIT-like characteristics and as well as much smaller mode volumes. Due to these properties, the proposed metamaterials can be utilized for a wide range of applications including bio-chemical sensors, optical filters and modulators and enhancement of non-linear processes.

Abstract: A small-aperture wavelength filter consisting of a guided-mode resonance grating and a cavity resonator integrated in a SiO 2 -based waveguide was designed. The operation principle was experimentally confirmed at around 1540-nm wavelength for the first time.

Abstract: An all-optical approach to realizing optical single-sideband (OSSB) modulation based on optical Hilbert transform implemented using a fiber Bragg grating (FBG) is proposed and demonstrated. In the proposed approach, an optical double-sideband (ODSB) signal is divided equally into two channels with the output at one channel being the in-phase (I) component and that at the other being the quadrature phase (Q) component. An FBG-based optical Hilbert transformer (HT) is incorporated in the Q channel, making the two sidebands out of phase at the output of the HT. The combination of the two optical signals from the two channels leads to the cancellation of one sideband, thus an OSSB signal is generated. An experiment is performed. An OSSB signal with a frequency from 6 to 15 GHz and a sideband suppression ratio (SSR) as large as 20 dB is generated. The transmission of the OSSB signal over a single-mode fiber of 45.6 km is also studied.

Abstract: A novel scheme to generate coherent 21 optical subcarriers with fixed frequency of 25 GHz is proposed and experimentally demonstrated. Using these optical subcarriers, we have successfully generated 1.96 Tb/s (21 × 100 Gb/s) polarization-multiplexing optical orthogonal frequency multiplexed (PM-OFDM) polarization multiplexed quadrature phase-shift keying (QPSK) optical superchannel signal with optical signal-noise ratio of each subchannel larger than 30 dB (0.1-nm bandwidth). We have transmitted this 1.96-Tb/s optical OFDM superchannel over 3200-km SMF-28 with amplification span of 80 km with erbium-doped fiber amplifier-only.

Abstract: An all-optical approach to implementing short-time Fourier transform (STFT) of a high-speed and broadband electrical signal is proposed and demonstrated for the first time to our knowledge. The STFT is implemented based on a temporal pulse shaping system incorporating an array of cascaded linearly chirped fiber Bragg gratings (LCFBGs). An electrical signal to be analyzed is applied to a Mach-Zehnder modulator (MZM) to modulate the optical spectrum of a time-stretched optical pulse from a mode-locked laser (MLL). Each individual LCFBG in the LCFBG array functions as a bandpass filter to filter a specific range of the spectrum, which is equivalent to applying a window function to the corresponding section of the temporal signal, and at the same time, as a dispersive element to implement real-time Fourier transform. A theoretical analysis is performed which is verified by a numerical simulation and a proof-of-concept experiment. The key feature of this technique is that STFT can be implemented in real time which can find applications in analyzing an electrical signal with a bandwidth up to several hundreds of gigahertz.

Abstract: A continuously tunable microwave fractional Hilbert transformer (FHT) implemented based on a photonic microwave delay-line filter is proposed and demonstrated. The photonic microwave delay-line filter with negative coefficients is realized based on polarization-modulation using a polarization modulator (PolM) and polarization-modulation to intensity-modulation conversion in an optical polarizer. The tunability of the fractional order is achieved by tuning the coefficient of the zeroth tap. An FHT with a tunable order from 0.3 to 1 is demonstrated. The accuracy of the FHT is evaluated; a phase deviation less than 5° within the passband is achieved.

Abstract: A broadband tunable, single-longitudinal-mode (SLM) ytterbium fiber laser based on a passive multiple-ring cavity (MRC) technique is proposed and demonstrated experimentally for the first time to our knowledge. Two different short ring cavities are inserted into the main ring cavity and serve as wideband mode filters to ensure SLM oscillation. With 1-m ytterbium-doped fiber as the gain medium, the SLM operation is achieved with over 60-nm wavelength tuning range at 100-mW pump power. The laser is very stable with output power of 6 dBm and an optical signal-to-noise ratio of higher than 53 dB in all the 60-nm tuning range.

Abstract: A fully tunable microwave photonic phase shifter involving a single semiconductor optical amplifier (SOA) is proposed and demonstrated. 360° microwave phase shift has been achieved by tuning the carrier wavelength and the optical input power injected in an SOA while properly profiting from the dispersion feature of a conveniently designed notch filter. It is shown that the optical filter can be advantageously employed to switch between positive and negative microwave phase shifts. Numerical calculations corroborate the experimental results showing an excellent agreement.

Abstract: A novel approach to interrogating in real time a linearly chirped fiber Bragg grating (LCFBG) sensor based on spectral-shaping and wavelength-to-time (SS-WTT) mapping with improved interrogation resolution and signal-to-noise (SNR) ratio is proposed and experimentally demonstrated. The proposed system consists of a mode-locked laser source, an optical interferometer incorporating an LCFBG, and a dispersive element. The optical interferometer has a spectral response with an increasing free spectral range (FSR). The incorporation of the LCFBG in the interferometer would encode the sensing information in the spectral response as a change in the FSR. After SS-WTT mapping, a linearly chirped microwave waveform is obtained. The correlation of the linearly chirped microwave waveform with a chirped reference waveform would provide a sharp correlation peak with its position indicating the wavelength shift of the LCFBG. A theoretical analysis is carried out, which is validated by a numerical simulation and an experiment. The experimental results show that the proposed system can provide an interrogation resolution as high as 0.25 μe at a speed of 48.6 MHz.

Abstract: We report a flat spectral Faraday anomalous dispersion optical filter (FS-FADOF) for sodium lidar. The physical and technical considerations for obtaining a FS-FADOF with a 3.5 GHz flat spectral transmission function are presented. It was found that the effective transmission of this filter was much higher (>94%) and more uniform than that of the ultranarrowband FADOF, and therefore were less sensitive to laser-frequency drift. Thus, the FS-FADOF can improve lidar efficiency and precision.

Abstract: An approach for the multiple-frequency millimeter-wave (mm-wave) signals generation is proposed and demonstrated, specifically, which can be applied to a radio-over-fiber (RoF) system with multiple base stations (BSs). In this scheme, optical double sideband (ODSB) modulation is achieved using a Mach-Zehnder modulator (MZM) to generate the two-sideband signals. New frequencies of the optical signals are obtained by using four-wave mixing (FWM) in a semiconductor optical amplifier (SOA). At the BSs, two different frequencies are achieved using a comb optical filter (COF), and which then input a photodiode (PD) to generate the mm-wave signals with the frequencies of 20, 40 or 60 GHz for different BSs, by mixing of these frequencies components. Experimental results verify that the proposed multiple-frequency mm-wave signals generation scheme for a RoF system with multiple base stations can work properly.

Abstract: In this paper, a novel nanometeric plasmonic narrow-band filter consisting of a metal-insulator-metal waveguide and a cavity embedded in the metal is proposed and numerically demonstrated by using the finite difference time domain method. The physical mechanism of the filter is elucidated. The results reveal that a narrow-band spectrum notch-filtering function can be realized with the proposed structure. The selection wavelength of the notch is proportional to the length of the cavity and inversely quasi-proportional to its width. The simple plasmonic filter structure has an ultracompact size with a length of a few hundred nanometers. Our results may open a way to construct nanoscale high-density photonic integration circuits based on plasmonic effects.

Abstract: An optical filter with tunable wavelength and bandwidth has been demonstrated using two cholesteric liquid crystals configured in a reflection mode, in which the incident light is first reflected by one cholesteric liquid crystal and then by the other one. The central wavelength can be tuned from 527 nm to 574 nm and the bandwidth can be changed from 10 nm to 80 nm. It has potential applications in many fields, especially in optical communications and multispectral and hyperspectral imaging systems.

Abstract: The apparatuses disclosed herein include a discharge lamp configured to emit ultraviolet and visible light, an optical filter configured to attenuate visible light, and a power circuit configured to operate the discharge lamp. Some embodiments of the apparatuses are configured such that ultraviolet light emitted from the discharge lamp and passed through the optical filter encircles an exterior surface of the apparatus. Some cases of the apparatuses include a sensor system configured to monitor a first parameter associated with the operation of the discharge lamp and a second parameter associated with the transmittance of the optical filter. Some of the apparatuses include a means for moving the optical filter while the apparatus is in operation. In some cases, the apparatuses may be configured such that the optical filter may be arranged in and out of alignment with the discharge lamp. In some embodiments, the optical filter may be a multifaceted.

Abstract: High-fidelity color image acquisition with a multispectral camera utilizes optical filters to separate the visible electromagnetic spectrum into several passbands. This is often realized with a computer-controlled filter wheel, where each position is equipped with an optical bandpass filter. For each filter wheel position, a grayscale image is acquired and the passbands are finally combined to a multispectral image. However, the different optical properties and non-coplanar alignment of the filters cause image aberrations since the optical path is slightly different for each filter wheel position. As in a normal camera system, the lens causes additional wavelength-dependent image distortions called chromatic aberrations. When transforming the multispectral image with these aberrations into an RGB image, color fringes appear, and the image exhibits a pincushion or barrel distortion. In this paper, we address both the distortions caused by the lens and by the filters. Based on a physical model of the bandpass filters, we show that the aberrations caused by the filters can be modeled by displaced image planes. The lens distortions are modeled by an extended pinhole camera model, which results in a remaining mean calibration error of only 0.07 pixels. Using an absolute calibration target, we then geometrically calibrate each passband and compensate for both lens and filter distortions simultaneously. We show that both types of aberrations can be compensated and present detailed results on the remaining calibration errors.