Tunable Wavelength Filter Research Articles

Heterogeneous quantum dot/silicon photonics-based wavelength-tunable laser diode with a 44 nm wavelength-tuning range

A heterogeneous wavelength-tunable laser diode combining quantum dot and silicon photonics technologies is proposed. A compact wavelength-tunable filter with two ring resonators was carefully designed and fabricated using silicon photonics technology. The tunable laser combining the wavelength-tunable filter and an optical amplifier, which includes InAs quantum dots, achieved a 44.0 nm wavelength-tuning range at around 1250 nm. The broadband optical gain of the quantum dot optical amplifier was effectively used by the optimized wavelength-tunable filter. This heterogeneous wavelength-tunable laser diode could become a breakthrough technology for high-capacity data transmission systems.

Transmission type tunable wavelength filters based on polymer waveguide Bragg reflectors

In WDM communication systems, a compact low-cost tunable wavelength filter is highly demanded. Polymeric Bragg reflector devices are suitable for this purpose because the large thermo-optic effect of the polymer enables widely tunable wavelength filters with simple device structure. To direct the filtered signal in the forward direction rather than the backward direction, a waveguide mirror device is integrated. A compact package was then achieved by attaching a fiber-connecting receptacle and a high-speed PD on each side of the chip. The tunable filter exhibited a tuning range of 14nm, a 3-dB bandwidth of 0.45nm, and a 15-dB bandwidth of 1.54nm. The device exhibited a low polarization dependence of 0.08nm, which is the first demonstration in polymeric tunable filters.

Vertically-Aligned Polymer Stabilized Liquid Crystals (VA-PSLC) with a Curing Voltage for Fast-Response Wavelength-Tuning Applications

We present experimental results obtained for a Vertically-Aligned Polymer Stabilized Liquid Crystal (VA-PSLC) with a curing voltage. The curing voltage was found to help reduce the otherwise strong scattering effect of the VA-PSLC. This liquid crystal mode was placed inside a Fabry-Perot cavity to achieve a wavelength tunable filter. Wavelength tuning range was found to decrease as curing voltage increased, which is consistent with what we expected since molecules had a larger pretilt angle when the curing voltage was higher. Shortening of response time (>10X) was found as polymer concentration increased since the polymer effect helped improve the response speed. These filters can have potential applications in wavelength tuning applications (e.g. WDM) in telecommunication systems where high speed is desirable. This new liquid crystal mode of VA-PSLC with a curing voltage can also find applications where both fast response time and pure phase modulation are desirable.

Narrow-band wavelength tunable filter based on asymmetric double layer metallic grating.

In this paper, the optical properties of asymmetric double layer metallic gratings are presented theoretically. The asymmetric structure is achieved by two main factors: one corresponding to moving alternatively metal nanowires of the top layer metallic grating, the other corresponding to possessing different thickness of the top and down layer metallic gratings. Our proposed structure shows one remarkable narrow-band transmission dip at normal incidence, which is distinct different from that of symmetric structure. The results are further confirmed by using different numerical computation methods, and explained by the analytical model of Fano-like resonance. We find that, only when the thickness of the down layer metallic grating has certain fixed value, transmission dip can be transformed from two to only one dip even if the existence of symmetry breaking. However, the wavelength position of the dip can be easily controlled by adjusting the thickness of the top layer metallic grating without the need to modify the structure period, and the width of metal nanowire. Moreover, the influence of other structure parameters on the dip is also investigated. Surprisingly, in order to keep the appearance of one dip in the transmission spectrum of designed structure, there is a good linear approximation between the refractive index of waveguide layer and the thickness of down layer metallic grating, and the relation of waveguide layer thickness and the thickness of down layer metallic grating satisfy secondary polynomial fitting. This work can be used to develop subwavelength metallic-grating-based and narrow-band tunable wavelength filters.

Ultra-compact wavelength-tunable quantum-dot laser with silicon-photonics double ring filter

Compact, wavelength-tunable light sources are desired for the enhancement of information communication technology and bio-imaging applications. We propose a compact, wavelength-tunable laser diode with a wide wavelength-tunable range around 1230 nm consisting of a quantum-dot optical amplifier and a silicon photonic tunable filter. High-quality InAs quantum dots grown with the sandwiched sub-nano separator technique were used as the optical gain medium. The wavelength-tunable filter was constructed with ring resonators fabricated using silicon photonics. The single-mode laser oscillation was demonstrated with a 28.5-nm wavelength-tunable range.

Narrow-spectral-linewidth silicon photonic wavelength-tunable laser with highly asymmetric Mach–Zehnder interferometer

We propose a narrow-spectral-linewidth silicon photonic wavelength-tunable laser with a novel external wavelength-tunable filter, which consists of two silicon ring resonators with different circumferences and a highly asymmetric Mach-Zehnder interferometer (MZI), the two optical paths of which have significantly different lengths. Calculations and experimental results indicated that the gain difference between longitudinal modes was increased by the highly asymmetric MZI. Consequently, a narrow spectral linewidth of 12kHz and a stable single-mode oscillation were obtained.

Compact silicon photonic wavelength-tunable laser diode with ultra-wide wavelength tuning range

We present a wavelength-tunable laser diode with a 99-nm-wide wavelength tuning range. It has a compact wavelength-tunable filter with high wavelength selectivity fabricated using silicon photonics technology. The silicon photonic wavelength-tunable filter with wide wavelength tuning range was realized using two ring resonators and an asymmetric Mach-Zehnder interferometer. The wavelength-tunable laser diode fabricated by butt-joining a silicon photonic filter and semiconductor optical amplifier shows stable single-mode operation over a wide wavelength range.

Wavelength-Tunable Filters Utilizing Arrayed Waveguide Gratings for Colorless/Directionless/Contentionless Optical Signal Drop in ROADMs

The wavelength-tunable filter is a key device for creating colorless, directionless, and contentionless reconfigurable optical add-drop multiplexers. Although arrayed-waveguide gratings (AWGs) can realize compact and cost-effective tunable filters, crosstalk in wavelength-division-multiplexing systems increases as the frequency grid spacing decreases. To resolve this problem, we propose a tunable-filter architecture that can eliminate the crosstalk by utilizing two specific (commeasurable) cyclic AWGs in conjunction with multiple switches based on symmetric Mach-Zehnder interferometers and an asymmetric Mach-Zehnder interferometer (AMZI) filter. To verify the technical feasibility of the proposed architecture, we fabricate a monolithic prototype using planar-lightwave-circuit technology. We confirm, for the first time, its excellent performance by measuring insertion loss, polarization-dependent loss, and bit error ratios (BERs). Alternative tunable-filter configurations employing multiple AMZI-based filters are newly investigated for future applications.

Design of tunable asymmetric directional coupler filter using periodically segmented Ti:LiNbO3 waveguides

A tunable wavelength filter is designed using Ti:LiNbO3 asymmetric directional coupler. One arm of the directional coupler is made of continuous waveguide while the other arm is periodic segmented. This segmentation reduces the lithographic steps and hence the fabrication cost required for such asymmetric coupler based filter. The propagation constant of periodical segmented and continuous waveguides are analysed using Effective-Index-based Matrix Method. The tunability is achieved using electro-optic property of LiNbO3.

Franz-Keldysh Effect in GeSn Detectors

The fundament for silicon based photonics is Ge grown on Si and SOI. A simple photonic integrated circuit (PIC) consist of an integrated light source, a waveguide, a modulator and a detector. Due to the fact that Ge is an indirect semiconductors, the light source is the main challenge for PICs. One concept is to add Sn into the Ge matrix. Theoretical calculations predict that Ge1-xSnx becomes a direct semiconductor for a Sn content of x = 10 % and therefore is an interesting material mixture for photonic devices integrated on Si. Recent publications have shown that detectors using GeSn as active material excess 40 GHz bandwidth. For PICs, also the modulation of the light is needed. Beside Mach-Zehnder modulators, the modulator can be realized using the Franz-Keldysh (FK) effect. Due to the FK effect, the absorption in the material can be changed by varying the applied electric field.In this paper we report the FK effect in GeSn PIN diodes with Sn concentrations up to 4.2 %. In order to analyze the absorption of GeSn, vertical PIN diodes with different Sn content were produced. First, the epitaxial layers were grown with a solid source molecular beam epitaxy system [1] on B-doped Si (100) substrates with a high specific resistance ρ > 1000 Ωcm. Fig. 1(a) shows the vertical layer sequence. The layer stack consists of a vertical PIN structure. Starting with a 400 nm B-doped Ge contact layer (1020 cm-3), the active layer with 300 nm intrinsic Ge1-xSnx and a 10 nm Ge spacer follow. The top contact consist of 100 nm Sb-doped Ge (1020 cm-3) and 100 nm Sb-doped Si (1020 cm-3). The three investigated samples have a Sn content of x = 0 %, 2 % and 4.2 %, respectively. The GeSn PIN heterojunction photodiodes are produced in a quasi-planar technology. The device processing by double mesa etching, passivation and metallization is similar to earlier reported GeSn pin photodiodes [2]. A schematic high frequency device structure of the GeSn pin photodetector with the typical ground signal ground (GSG) probe configuration with a pitch of 100 µm is shown in Fig 1(b).The electrical and optical characterization of the diodes is done with on-wafer measurement of the current-voltage characteristics. As a light source, a super-continuum laser with a tunable acousto-optical wavelength filter is used. The light couples into the diode through a lensed glass fiber. From measuring the dark current and photo current, the responsivity of the GeSn detectors were determined by dividing the photo current by the optical power.Fig. 2 shows the responsivity of the GeSn PIN diodes with Sn concentrations of x = 0 %, 2 % and 4.2 %. With increasing Sn content, the bandgap shifts to higher wavelengths and therefore, the responsivity increases for higher wavelengths. A voltage dependence of the optical responsivity is only shown near the direct bandgap Eg,Γ. This is due to the FK effect.From the measurement of the responsivity and reflectivity, the internal quantum efficiency can be calculated and further the absorption is determined. The electro-optical modulation from the FK effect was clearly observed in GeSn material. The maximum absorption ratio Δα/α0 for the 2 % Sn sample at room temperature was 1.5 at a wavelength of 1806 nm and a voltage swing of 3 V.

Optofluidic tunable plasmonic filter based on liquid-crystal microcavity structures

A theoretical demonstration of Optofluidic tunable wavelength filter in the near-infrared regime has been presented. By incorporating nematic liquid crystal (LC) into the slot resonator, a nanoscale LC optical filter is proposed and numerically investigated. The finite difference time domain method is used to simulate the optical characteristics of the plasmonic nanostructure. Two approaches are considered to tune the filter’s resonant wavelength, a fine tuning based on optofluidic control LC length and a coarse tuning based on LC birefringence. Both fine tuning and coarse tuning are successful for manipulating the filter’s resonant wavelength. This plasmonic structure permits the high-precision control of transmit of the input surface plasmon polaritons. The filters may have important potential application in highly integrated optical circuits.

Bragg reflector waveguide array with ferroelectric liquid crystal cladding for optical tunable wavelength filters

A tunable wavelength filter array is proposed, which consists of a 1 × 6 multimode interferometer silicon waveguide splitter and Bragg reflector waveguides with ferroelectric liquid crystal (FLC) cladding. The transmission characteristics of the proposed device were reported, which were theoretically and experimentally obtained. Different operating wavelengths were obtained by using different waveguide widths with the same grating period. The stop bandwidth we obtained with a sampled grating was narrower than that in conventional uniform grating structures. Bistable tuning operation with a 0.8 nm shift in wavelength characteristics, which had six different Bragg wavelengths, was demonstrated. The distribution of the optical axis in the FLC cladding layer was also measured using a system for birefringence measurements.

Spectral radiance source based on supercontinuum laser and wavelength tunable bandpass filter: the spectrally tunable absolute irradiance and radiance source

A new spectrally tunable source for calibration of radiometric detectors in radiance, irradiance, or power mode has been developed and characterized. It is termed the spectrally tunable absolute irradiance and radiance source (STAIRS). It consists of a supercontinuum laser, wavelength tunable bandpass filter, power stabilization feedback control scheme, and output coupling optics. It has the advantages of relative portability and a collimated beam (low étendue), and is an alternative to conventional sources such as tungsten lamps, blackbodies, or tunable lasers. The supercontinuum laser is a commercial Fianium SC400-6-02, which has a wavelength range between 400 and 2500 nm and a total power of 6 W. The wavelength tunable bandpass filter, a PhotonEtc laser line tunable filter (LLTF), is tunable between 400 and 1000 nm and has a bandwidth of 1 or 2 nm depending on the wavelength selected. The collimated laser beam from the LLTF filter is converted to an appropriate spatial and angular distribution for the application considered (i.e., for radiance, irradiance, or power mode calibration of a radiometric sensor) with the output coupling optics, for example, an integrating sphere, and the spectral radiance/irradiance/power of the source is measured using a calibration optical sensor. A power stabilization feedback control scheme has been incorporated that stabilizes the source to better than 0.01% for averaging times longer than 100 s. The out-of-band transmission of the LLTF filter is estimated to be < -65 dB (0.00003%), and is sufficiently low for many end-user applications, for example the spectral radiance calibration of earth observation imaging radiometers and the stray light characterization of array spectrometers (the end-user optical sensor). We have made initial measurements of two end-user instruments with the STAIRS source, an array spectrometer and ocean color radiometer.

Tunable filter based on cavity electro-optic modulation for DWDM applications

In this paper the authors have demonstrated the feasibility and the realization of a wavelength tunable filter with an electro-optic modulation effect realized on diffused Ti:LiNbO3 waveguide. The two reflectors of the cavity consist in two Bragg׳s mirrors of identical period Λ=1.81μm and whose total number N=140. Both mirrors have been engraved using the Focused Ion Beam technique up to the depths l1=l2=2.7μm separated by a distance of lc=500μm. We obtained thanks to electro-optic modulation effect a tunable shift whose value is 0.67nm in a free spectral range FSR=1.59nm for a voltage variation up to 60V. This designed and realized probe will find many DWDM (Dense Wavelength Division Multiplexing) applications.

Latching operation in a tunable wavelength filter using Si sampled grating waveguide with ferroelectric liquid crystal cladding.

A tunable wavelength filter fabricated with a latching function is proposed. The proposed tunable wavelength filter consists of a silicon sampled grating waveguide and ferro-electric liquid crystal (FLC) cladding. The sampled grating waveguide in a silicon-on-insulator (SOI) wafer achieved narrower stop bands than that with the conventional uniform grating structure. Enhanced wavelength shift was also obtained due to the increased effect in FLC by using a thinner silicon core. Bistable switching operation with the fabricated device, which was latching without state-sustaining power, was successfully demonstrated. Its switching and latching characteristics are also reported.

The Tunable Optofluidics Waveguide Design Based on the Novel Dual Side-Coupled Cavities Plasmonic Structure

We present a tunable wavelength filter in plasmonic metaldielectricmetal (MIM) side-coupled-cavity waveguide with optofluidics pump system proposed to realize tunable mechanism. The peak wavelength can shift by manipulating the length of liquid column and the effective refractive index. The finite difference time domain method is used in the numerically simulated experiment and the resonant wavelengths from 1000 to around 1800nm had been analyzed. The results reveal that the resonant wavelengths are proportional to the liquid volume length and refractive index of liquid in the cavity. This waveguide filter can be used in integrated optical circuits.

Tunable electro-optic wavelength filter based on lossy-guided mode resonances

In this work an optical fiber tunable filter based on lossy guided-mode resonances (LGMR) is proposed. It consists of a multilayer structure deposited onto the surface of a plastic cladding removed multimode fiber. The first layer is used to generate the LGMR and to work as the first electrode as well; the second one to tune the filter and the outer layer forms the other electrode. The fabricated filter has demonstrated a good sensitivity to the applied voltage showing a change of the LGMR wavelength of 0.4 nm/V. Among other applications, this filter is intended to be used as electro-optic wavelength filter or modulator.

A tunable notch filter using microelectromechanical microring with gap-variable busline coupler

A microelectromechanical tunable notch filter using silicon-photonic freestanding waveguides is proposed, and the basic characteristics are experimentally investigated. The proposed filter is composed of a wavelength-tunable silicon microring resonator and a busline switch. The tunable microring consists of freestanding single-mode waveguides and air-gap directional waveguide couplers. The optical path length of the microring is varied physically by a displacement of electrostatic comb-drive actuator. The busline switch consists of a gap-variable waveguide coupling mechanism, which enables coupling the tunable microring with the busline by another electrostatic comb-drive actuator. During the wavelength tuning of microring, the busline can be disconnected from the microring. Therefore, the proposed device operates as a hitless wavelength-selective switch if they are connected in series. The waveguides are 320 nm in width and 340 nm in thickness. The resonant wavelength shift of the microring is 9.96 nm at the voltage of 26 V with the actuator displacement of 1.0 μm. The coupling to busline is adjusted from the switch-off state at the gap of 600 nm to the switch-on state corresponding to the critical coupling condition at the gap of 383 nm. The whole size of the wavelength-tunable filter with hitless mechanism is about 150 μm by 80 μm. Due to the capacitive operation of the comb-drive actuators, the power consumption is negligibly small.

Application of finite impulse response network concept in wavelength-tunable electro-optic filter

We present a model of wavelength-tunable electro-optic filter on x-cut, y-propagation Ti-diffused lithium niobate. The filter is composed of two polarization beam splitters and a polarization converter which is controlled by an N-stage cascade of alternating coupled-mode and phase-shifted electrodes. The concept of finite impulse response digital filtering is used to calculate the voltage matrices and in the z field, which control the mode coupling and phase shift respectively. By changing the input voltages, a wavelength-tunable bandpass filter or a non-periodic multi-wavelength selector can be obtained. For the bandpass filter, the rectangular degree of passband can be improved by increasing the number of cascading stages . If is equal to 24, the side mode suppression ratio (SMSR) could reach 25 dB. And the passband can be tuned by simply changing the voltage matrix . For the multi-wavelength selector, the design process can be fulfilled by the discrete Fourier transform method. The bandwidth of each selected wavelength increases but the number of selected wavelengths decreases when the number of cascading stages decreases. If is equal to 24, a bandwidth of 0.29 nm with a SMSR of 12 dB is achieved within a 8 nm range centered at 1550 nm.

Tunable filters for multispectral imaging of aeronomical features

Multispectral imaging of optical emissions in the Earth’s upper atmosphere unravels vital information about dynamic phenomena in the Earth-space environment. Wavelength tunable filters allow us to accomplish this without using filter wheels or multiple imaging setups, but with identifiable caveats and trade-offs. We evaluate one such filter, a liquid crystal Fabry–Perot etalon, as a potential candidate for the next generation of imagers for aeronomy. The tunability of such a filter can be exploited in imaging features such as the 6300–6364Å oxygen emission doublet, or studying the rotational temperature of N2+ in the 4200–4300Å range, observations which typically require multiple instruments. We further discuss the use of this filter in an optical instrument, called the Liquid Crystal Hyperspectral Imager (LiCHI), which will be developed to make simultaneous measurements in various wavelength ranges.