Tunable Spectral Filter Research Articles

Tunable spectral and spatial filters for the mid-infrared based on hyperbolic metamaterials.

In this paper we present the possibility of shaping the reflectivity characteristics of tunable hyperbolic metamaterials (THMMs). Using the example of voltage-sensitive graphene-based structures, we demonstrate the existence of spectral and spatial functionalities of edge and narrowband filters, controlled dynamically over a 3-5μm spectral range, that are important for both civilian and military applications. We also demonstrate that the adoption of apodization techniques in the THMM design leads to a reduction in the sidelobe's parasitic effect in edge filters, as well as providing the means to reshape the overall reflectivity characteristics, which not only unveiled the tunable angle aperture functionality but also significantly increased the potential for tailoring optical properties of THMM nanostructures in general.

Soft mold NanoImprint Lithography: a versatile tool for sub-wavelength grating applications

Due to its independency to the substrate used, Soft mold NanoImprint Lithography (S-NIL) is a technique of great interest in particular for the fabrication of optical devices. We demonstrate a mature pathway for the fabrication of optical filters from the conception to the optical characterization. Those filters can be fabricated on large surfaces (up to 6″ diameter wafers) with high conformity on various substrates. Quality of the transfer will be discussed throughout the process and optical performances compared to those obtained with classical techniques. In this paper we fabricated tunable spectral filters with a grating periodicity down to 260 nm and imprint surfaces up to 6″. Physical conformity of the gratings will be discussed in terms of long-range stitching obtained on 6″ Si hard mold, dimensional shrinkage during thermal NanoImprint on Zeonor® soft mold and conformity towards patterned hard mold throughout the process.

Suppression of amplitude modulation induced by polarization mode dispersion using a multi-degree-of-freedom fiber filter

Polarization mode dispersion (PMD) in fibers for high-power lasers can induce significant frequency modulation to amplitude modulation (FM-to-AM) conversion. However, existing techniques are not sufficiently flexible to achieve efficient compensation for such FM-to-AM conversion. By analyzing the nonuniform transmission spectrum caused by PMD, we found that the large-scale envelope of the transmission spectrum has more serious impacts on the amount of AM. In order to suppress the PMD-induced FM-to-AM conversion, we propose a novel tunable spectral filter with multiple degrees of freedom based on a half-wave plate, a nematic liquid crystal, and an axis-rotated polarization-maintaining fiber. Peak wavelength, free spectral range (FSR), and modulation depth of the filter are decoupled and can be controlled independently, which is verified through both simulations and experiments. The filter is utilized to compensate for the PMD-induced FM-to-AM conversion in the front end of a high-power laser facility. The results indicate that, for a pulse with phase-modulation frequency of 22.82 GHz, the FM-to-AM conversion could be reduced from 18% to 3.2% within a short time and maintained below 6.5% for 3 h. The proposed filter is also promising for other applications that require flexible spectral control such as high-speed channel selection in optical communication networks.

Wavelength tunable, linearly polarized mode-locked fiber laser using chirped fiber Bragg grating

We demonstrate a wavelength-tunable, linearly polarized mode-locked fiber laser using carbon nanotube and chirped fiber Bragg grating. The chirped fiber Bragg grating is mounted on a plastic cantilever and serves as a tunable spectral filter. Linearly polarized mode-locked pulses are obtained with typical pulse duration of ~7.89 ps and repetition rate of 8 MHz, respectively. The high polarization extinction ratio is maintained over 30 dB for a long period of time. The central wavelength of the mode-locked laser is tuned continuously by adjusting the cantilever, while maintaining the polarization state.

Experimental characterization towards an in-fibre integrated silicon slab based all-optical modulator

BackgroundDue to the industrially increasing interest in optics communication the field of all-optical modulation is becoming more and more popular. However, no in-fibre integrated miniaturized devices which provide current solutions were suggested as all-optical modulators with sufficient modulation depth and with low power consumption.MethodsIn this paper, an all-optical silicon modulator with improved modulation depth is presented. The modulator will be developed as an in-fibre device for optics communication applications. The modulator is a silicon slab, 50 and 470 μm thick which is coated on both sides to get an improved Fabry-Perot resonator for laser beam at wavelength of 1550 nm. The modulator operation is based on the plasma dispersion effect which is induced by a pulsed visible laser beam acting as the pump for the creation of free charge carriers.ResultsWe have proved the coating withstands high energy fluency of the laser pulse. The different recombination and heat processes are examined. Our silicon based Fabry-Perot resonator increases the intrinsic c-Si finesse to 30. The improved finesse is shown to have significant effect on the modulation depth up to 12 dB.ConclusionsTo the best of our knowledge this is the first experimental feasibility study of an all-optical modulator with sufficient modulation depth and with low power consumption which is also an in-fibre integrated miniaturized device based upon Si slab. Extensive experimental and numerical studies presented in this paper examine the influence of thermal effects and the sample parameters on the response time and on the modulation depth and the power consumption of the in-fibre integrated device. Apart from being a modulator, the proposed device can also act as tunable spectral filter.

Tunable filters for precise spectral gain control in ultra-short-pulse laser systems.

We present tunable spectral filters (TSFs) as a variable and precisely adjustable method to control the spectral gain of short-pulse laser systems. The TSFs provide a small residual spectral phase and a high damage threshold, and generate no pre- or post-pulses. The method is demonstrated for two different laser materials and can be applied as an intracavity compensation in regenerative amplifiers as well as a method for pre-compensation in high-energy multipass amplifiers. With this method, a full width at half-maximum bandwidth of 23.9 nm could be demonstrated in a diode-pumped, 50 J Yb:CaF2 amplifier.

Rapidly tunable optical parametric oscillator based on aperiodic quasi-phase matching.

A new optical parametric oscillator (OPO) architecture with high tuning speed capability is demonstrated. This device exploits the versatility offered by aperiodic quasi-phase matching (QPM) to provide a broad parametric gain spectrum without changing the temperature, angle, or position of the nonlinear crystal. Rapid tuning is then straightforwardly achieved using a fast intracavity spectral filter. This concept is demonstrated here for a picosecond synchronously pumped OPO containing an aperiodically poled MgO-doped LiNbO3 crystal and a rapidly tunable spectral filter based on a diffraction grating. Tuning over 160 nm around 3.86 μm is achieved at fixed temperature and a fast tuning over 30 nm in 40 μs is demonstrated. Different configurations are tested and compared. The cavity length detuning is analyzed and discussed. This device is successfully used to detect N2O by absorption. This approach could be generalized to other spectral ranges (e.g., visible) and temporal regimes (e.g., continuous-wave or nanosecond).

Hybrid Plasmonic Modulators and Filters Based on Electromagnetically Induced Transparency

A hybrid plasmonic modulator based on microdisk resonators enhanced with electro-optic polymers is presented. Modulation is achieved by dynamically controlling an electromagnetically induced transparency window using a push–pull driving configuration. The proposed device combines small footprint, high modulation depths, low insertion losses, and low power consumption. By properly selecting the voltage values and the microdisk radii, modulation at different wavelengths and tunable spectral filtering are achieved.

Tunable spectral filters based on metallic nanowire gratings

Via surface plasmon enhanced light transmission through bilayer metallic nanowire gratings (BMNGs), three pitches of BMNGs with red, green and blue colors are respectively designed and fabricated. The color gamut can reach 89% NTSC in CIE 1931 UCS with optimized design. Furthermore, by utilizing the character of surface plasmon resonance (SPR), which is sensitive to ambient refractive index, the brightness can be tuned to dark by changing the ambient materials. The tunability was demonstrated on a 520 nm gratings in both simulation and experiment, and the obtained contrast ratio is over 120:1. These characteristics indicated a tunable spectral filter based on surface plasmon resonance, promising for a bistable full-color-capability electronic paper display.

Multifunctional tunable multiwavelength erbium-doped fiber laser based on tunable comb filter and intensity-dependent loss modulation

A multiwavelength erbium-doped fiber laser based on tunable comb spectral filter and intensity-dependent loss modulation is proposed and experimentally demonstrated. The laser allows fine and multifunctional tunable operations of channel-spacing, peak-location, spectral-range, and wavelength-number. More specifically, channel-spacing switch from 0.4nm to 0.2nm and peak-location adjustment within half of free spectrum range are obtained via controlling the tunable comb filter. The wavelength-number and the spectral-range of the lasing lines can be accurately controlled by intensity-dependent loss modulation in the laser cavity, enabled by a power-symmetric nonlinear optical loop mirror. In addition, fine control over the wavelength-number at fixed spectral-range is realized by simply adjusting the pump power. More important, the tunable operation process for every type of specific parameter is individual, without influences for other output parameters. Such features of this fiber laser make it useful and convenient for the practical application.

Tunable terahertz spectral filter based on temperature controlled subwavelength InSb grating

In this paper, we theoretically investigate a tunable terahertz- (THz-) frequency filter based on sub-wavelength indium antimonide (InSb) grating by utilizing the dielectric property of InSb. Strong electric field localization on the surface of the sub-wavelength InSb grating at a certain resonant frequency is presented. This phenomenon can be attributed to the effect that arises from localized surface plasmon resonance (LSPR) excited on the surface of InSb grating by the THz wave. The resonant frequency can be tuned from 0.425THz to 1.364THz by thermally controlling the permittivity of the InSb grating. This shift can be attributed to the change in the permittivity of InSb with the number of thermally excited carriers. Furthermore, the resonance frequencies vary in a linear fashion with respect to temperature with the range of 225–325K. Therefore, the desired resonance frequency can be selected and tuned in a tuning range of 900GHz conveniently in the THz region.

Tunable supercontinuum light vector vortex beam generator using a q-plate

Spatially coherent multicolored optical vector vortex beams were created using a tunable liquid crystal q-plate and a supercontinuum light source. The feasibility of the q-plate as a tunable spectral filter (switch) was demonstrated, and the polarization topology of the resulting vector vortex beam was mapped. Potential applications include multiplexing for broadband high-speed optical communication, ultradense data networking, and super-resolution microscopy.

Potential-controlled filtering in quantum star graphs

We study the scattering in a quantum star graph with a Fülöp–Tsutsui coupling in its vertex and with external potentials on the lines. We find certain special couplings for which the probability of the transmission between two given lines of the graph is strongly influenced by the potential applied on another line. On the basis of this phenomenon we design a tunable quantum band-pass spectral filter. The transmission from the input to the output line is governed by a potential added on the controlling line. The strength of the potential directly determines the passband position, which allows to control the filter in a macroscopic manner. Generalization of this concept to quantum devices with multiple controlling lines proves possible. It enables the construction of spectral filters with more controllable parameters or with more operation modes. In particular, we design a band-pass filter with independently adjustable multiple passbands. We also address the problem of the physical realization of Fülöp–Tsutsui couplings and demonstrate that the couplings needed for the construction of the proposed quantum devices can be approximated by simple graphs carrying only δ potentials.

Dual-core optofluidic chip for independent particle detection and tunable spectral filtering

We present the first integration of fluidically tunable filters with a separate particle detection channel on a single planar, optofluidic chip. Two optically connected, but fluidically isolated liquid-core antiresonant reflecting optical waveguide (ARROW) segments serve as analyte and spectral filter sections, respectively. Ultrasensitive detection of fluorescent nanobeads with high signal-to-noise ratio provided by a fluidically tuned excitation notch filter is demonstrated. In addition, reconfigurable filter response is demonstrated using both core index tuning and bulk liquid tuning. Notch filters with 43 dB rejection ratio and a record 90 nm tuning range are implemented by using different mixtures of ethylene glycol and water in the filter section. Moreover, absorber dyes and liquids with pH-dependent transmission in the filter channel provide additional spectral control independent of the waveguide response. Using both core index and pH control, independent filter tuning at multiple wavelengths is demonstrated for the first time. This extensive on-chip control over spectral filtering as one of the fundamental components of optical particle detection techniques offers significant advantages in terms of compactness, cost, and simplicity, and opens new opportunities for waveguide-based optofluidic analysis systems.

Tunable dual-wavelength fiber laser based on a polarization-maintaining fiber Bragg grating and a Hi-Bi fiber optical loop mirror

We demonstrate experimentally the operation of a linear cavity dual-wavelength fiber laser using a polarization maintaining fiber Bragg grating (PM-FBG) as an end mirror that defines two closely spaced laser emission lines. The PM-FBG is also used to tune the laser wavelengths. The total tuning range is ∼8 nm. The laser operates in a stable dual-wavelength mode for an appropriate adjustment of the cavity losses for the generated wavelengths. The high birefringence (Hi-Bi) fiber optical loop mirror (FOLM) is used as a tunable spectral filter to adjust the losses. The FOLM adjustment was performed by the temperature control of the Hi-Bi fiber.

Polarization-independent tunable spectral slicing filter in Ti:LiNbO_3

A two-port polarization-independent tunable spectral slicing filter at the 1530 nm wavelength regime is presented. The design utilizes an asymmetric interferometer with a sparse index grating along its arms. The sparse grating makes it possible to select equally spaced frequency channels from an incident WDM signal and to place nulls between them to coincide with the signal comb frequency. The number of selected channels and nulls between them depends on the number of coupling regions used in the sparse grating. The free spectral range depends on the spacing between the coupling regions. The Z-transform method is used to synthesize the filter and determine the spectral response. The operation of a device with six coupling regions is demonstrated, and good agreement with theoretical predictions is obtained. A 3 dB bandwidth of ∼1 nm and thermal tuning over a range of ∼13 nm are measured.

The IMaX polarimeter for the solar telescope SUNRISE of the NASA long duration balloon program

On June 8th 2009 the SUNRISE mission was successfully launched. This mission consisted of a 1m aperture solar telescope on board of a stratospheric balloon within the Long Duration Balloon NASA program. The flight followed the foreseen circumpolar trajectory over the Artic and the duration was 5 days and 17 hours. One of the two postfocal instruments onboard was IMaX, the Imaging Magnetograph eXperiment. This instrument is a solar magnetograph which is a diffraction limited imager capable to resolve 100 km on the solar surface, and simultaneously a high sensitivity polarimeter ( ) and a high resolution spectrograph (bandwidth etalon as tunable spectral filter are remarkable. Currently the data obtained is being analyzed and the preliminary results show unprecedented information about the solar dynamics.

Wideband tunable Gaussian-shaped spectral filters based on dispersion engineering

Optical liquids can be used to engineer the dispersion characteristics of fibers by serving as the core or cladding to attain fundamental-mode cutoff effect. The short-/long-pass fiber filters are so made and concatenated to achieve widely thermo-optic tunable Gaussian-shaped spectral filters. The proposed wideband tunable Gaussian-shaped spectral filter provides a potential technique in application to high resolution bio-imaging.

Tunable Bandpass Filter With Solid-Core Photonic Bandgap Fiber and Bragg Fiber

A tunable spectral bandpass filter which includes two fibers with different bend loss edges-a solid-core photonic bandgap fiber with short wavelength bend loss edge and a Bragg fiber with long wavelength bend loss edge-is presented in this letter. The central wavelength and the bandwidth can be widely tuned around 1100 nm by controlling the bend diameters of the two fibers respectively. It can be a low-loss in-line fiber device due to its all solid structure and standard splice connection.

Biomedical Optical Applications of Liquid Crystal Devices

Liquid crystals exhibit large electro-optic effects which make them useful for a variety of applications as fast, compact, and tunable spectral filters, phase modulators, polarization controllers, and optical shutters. They were largely developed for liquid crystal displays and in the last decade for optical telecommunications, however their application in the field of optical imaging just started to emerge. These devices can be miniaturized thus have a great potential useful in miniature optical imaging systems for biomedical applications. Using a collection of tunable phase retarders one can perform: 1. Stokes parameters imaging for skin and eye polarimetric imaging; 2. Tunable filtering to be used for hyperspectral imaging, fluorescence microscopy, and frequency domain optical coherence tomography; 3. Adaptive optical imaging and eye aberrations correction; 4. Phase shift interferometric imaging; 5. Variable frequency structured illumination microscopy. Basic optics of liquid crystals devices is reviewed and some novel designs are presented in more detail when combined to imaging systems for a number of applications in biomedical imaging and sensing.