Narrow Optical Filters Research Articles

Design and analysis of tunable optical filter based on silicon-on-insulator photonic crystal for visible light communication

In this paper, tunable narrow bandwidth band-stop and band-pass optical filters for visible light communication (VLC) are designed and numerically investigated using a photonic crystal (PC) structure. Designing a tunable filter for visible light communication is crucial for enhancing spectral utilization and maximizing communication efficiency. A rectangular defect with optimized sides length utilized in the center of the PC structure based on the desired filtering characteristics. To obtain a favorable narrow bandwidth filter for visible light wavelengths, this defect is filled with a specific liquid crystal. In addition, an electric field is applied to the proposed structure in order to change the refractive index of the liquid crystal and achieve tunability in the desired range. The functional wavelength ranges of band-pass and band-stop filters are 550–570 nm and 565–585 nm, respectively. Applying an electric field causes a shift in the central wavelengths of the filters. It is shown that a 1.5 nm central wavelength shift for a band-stop filter and a 0.9 nm central wavelength shift for a band-pass filter can be achieved by changing the voltage source up to 1.5 V. In addition, the impact that the variation in the length of the rectangular defects’ sides has on the filtering characteristics of the proposed device is studied.

Narrow bandpass filter using Octonacci photonic quasicrystal

Abstract In this paper, we propose structures for tunable narrow bandpass optical filters covering the visible range of 400–800 nm. The transmittance spectra of s-polarized (TE mode) light in one-dimensional photonic crystals are obtained by using a theoretical calculation based on the transfer matrix method. Here, photonic crystals and photonic quasicrystals composed of Si and SiO2 are organized according to some generations of the Octonacci sequence. The number of output channels and their central wavelengths are found to be dependent on the generation number and the angle of incidence. The results show an increase in the quality factor as the angle of incidence increased. Therefore, the proposed structures are believed to be suitable for efficient narrow bandpass filters operating in the visible range.

Inverse design of hybrid metal-dielectric guided mode resonance optical filters with a deep learning neural network and Fano function matching

In this work, we propose and implement a machine learning method of using a forward deep learning neural network and Fano function inverse matching to design and optimize hybrid metal-dielectric guided mode resonance narrow linewidth optical filters. First, a forward deep learning neural network is trained with a small design sample set generated with finite difference time domain physical simulations. The trained forward neural network is then used to generate a large sample set of three million designs. In inverse matching process, filter peak wavelength and spectral linewidth are two matching parameters first used for down selecting designs from the large sample set to a small sample set. Because of the asymmetric nature of the guided mode resonance filter spectral line-shape, Fano functions are used to match against the spectra in the small sample set to find the design with narrow filter linewidth. Optical transmission filters with linewidth between 6.8 nm and 8.7 nm are designed in visible spectrum.

Electromagnetically induced transparency, narrow absorption and transient response in a three-photon excitation process

The probe absorption characteristic of a four-level ladder-type system, in presence of two strong coherent radiation fields, is investigated analytically by following the density operator method. The parameters of 85Rb 5S1/2→5P3/2→5D5/2→63P3/2 transition are used in the simulation. In addition to the control field acting between the first excited state and the second excited state, if a switching field is applied to couple the second excited state to a Rydberg state, two-photon electromagnetically induced transparency (EIT) is converted into double EIT (DEIT) window having a strong central absorption peak. In a thermal vapor, the double EIT window is transformed into three transparency windows depending on the values of the control and switching Rabi frequencies. These multiple transparency windows vanish for certain combinations of the Rabi frequencies of the control and switching fields and the probe absorption is enhanced. It is also demonstrated how a very narrow optical band stop filter can be conceptualized by using three-photon excitation scheme. The transient optical response of the medium under three-photon transition is also investigated under both Doppler free and Doppler broadened conditions.

Inexpensive multispectral imaging device

Multi/hyperspectral imaging is an important technique to obtain color data and spectral monitoring. It is a precise remote sensing technique that includes two or more bands in the visible and invisible spectrum. This study demonstrates the design and construction of a low-cost, multispectral imaging system using a single linear array sensor and a varifocal lens coupled to individual spectral band-pass filters. The system allows the automated data acquisition of raw spectral data for seven spectral bands, each with different wavelengths. The raw data are visualized in real-time to generate images of spectral bands to obtain the desired spectral information on the computer graphical screen. Sensor arrays with spectral responsivity between 0.3 and 1.05 μm and motorized filter disks with a linear guide are used for acquiring data from specific wavelengths. Axis movement of the imaging device is achieved by a linear guide for spatial monitoring. Seven different bands are scanned and transferred to the computer via motorized band-pass filter sets with the imaging device. The multispectral imaging device has high accuracy with a 0.9936 linear correlation coefficient (r2) and averaged noise of the total system is below 1% (0.7%). Although the present configuration uses seven spectral bands including visible and narrow bandpass optical filters to obtain multispectral information from an object, the system is designed to have a replaceable and selectable motorized rotating filter, easily remounted, and adjusted up to forty spectral bands with 20 nm full width at half maximum (FWHM).

Micro-integrated high-power narrow-linewidth external-cavity tapered diode laser at 762 nm for daylight imaging

Solar radiation is a challenge for laser-based daylight imaging since it decreases the signal-to-noise ratio (SNR) of the imaging. Here we demonstrate a micro-integrated external-cavity tapered diode laser system for daylight imaging. It emits light at a wavelength of 762 nm, chosen because of its overlap with an oxygen absorption band in the solar radiation spectrum. The integrated laser system consists of a tapered diode amplifier as gain medium and an external volume Bragg grating for spectrum stabilization and narrowing, thus a narrow bandpass optical filter can be used to improve the SNR further. The laser system can be operated in both continuous wave (CW) mode and pulsed mode by modulating the injected current to the amplifier. In CW mode operation, 1.3 W of output power is obtained with an emission spectral linewidth of 4 pm, and a beam propagation factor in the slow axis, M2, of 1.7 (4σ). In pulsed mode operation with a trigger signal of a 50μs pulse width and a 10 kHz repetition rate, 2.0 W of peak output power is achieved with an emission spectral bandwidth of 0.2 nm, and an M2 in the slow axis of 1.9 (4σ). The modulation depth is almost 100%.

Ultra-narrow passband-tunable filter based on a high-Q silicon racetrack resonator

An ultra-narrow narrow passband-tunable optical filter employing a high-Q silicon racetrack resonator is proposed and experimentally demonstrated on a SOI platform. The high-Q silicon racetrack resonator is realized by utilizing the multimode waveguide racetrack, and the Q factor is measured as high as 8.1×105. The structure of the device is based on a thermally tunable Mach-Zehnder interferometer coupled racetrack. The tunability of the bandwidth is realized by tuning the coupling coefficient between the racetrack resonator and the input or output ports. Finally, the bandwidth of the filter can be tuned from 1.92 to 11.00 pm (240 MHz to 1.375 GHz), and the free spectral range is about 0.28 nm (35 GHz), with the footprint of 0.21mm2.

Demonstration of Faraday anomalous dispersion optical filter with reflection configuration

A narrow linewidth Faraday anomalous dispersion optical filter (FADOF) with reflection configuration is achieved for the first time based on the cesium (Cs) ground state 6S1/2 to the excited state 6P3/2 transition at 852 nm. Compared with the conventional FADOF with transmission configuration, reflection-type FADOF can greatly improve the transmittance of optical filter under the same experimental parameters, because it allows signal light to go and return through the atomic vapor cell. In our experiment, peak transmittance at Cs 6S1/2 F = 4–6P3/2 transition is 81% for the reflection-type FADOF, and while 54% for the transmission-type FADOF when the temperature of Cs vapor cell and the axial magnetic field are 60 °C and 19 G. The idea of this reflection-type FADOF design has the potential to be applied to the FADOF operating between two excited states to obtain higher transmittance.

Ultra simple low-power RF signal detection based on an optoelectronic feedback DFB laser.

An ultra simple and low cost method to detect low-power RF signal is proposed and experimentally demonstrated based on optoelectronic feedback DFB semiconductor laser. To our knowledge, according to public reports, this is the simplest photonics-assisted method which avoids using high sensitive optical modulators and narrow bandwidth optical filters. The RF signal, which matches the oscillation mode at the relaxation oscillation peak of the DFB laser, is amplified based on optoelectronic feedback. The RF signal from 1 to 4.5 GHz can be detected by adjusting the frequency of relaxation oscillation which is related to the laser bias current. The system provides a maximum gain of 15 dB for the low-power RF signal. The sensitivity of the system can reach up to as high as -97 dBm. Considering the real application of the detection system, the properties like dynamic range, resistance to large signals and performance for detecting modulated RF signal are also investigated.

Analysis of Dielectric Waveguide Grating and Fabry-Perot Modes in Elastic Grating in Optical Detection of Ultrasound.

In our previous work, we have demonstrated that dielectric elastic grating can support Fabry–Perot modes and provide embedded optical interferometry to measure ultrasonic pressure. The Fabry–Perot modes inside the grating provide an enhancement in sensitivity and figure of merit compared to thin film-based Fabry–Perot structures. Here, in this paper, we propose a theoretical framework to explain that the elastic grating also supports dielectric waveguide grating mode, in which optical grating parameters control the excitation of the two modes. The optical properties of the two modes, including coupling conditions and loss mechanisms, are discussed. The proposed grating has the grating period in micron scale, which is shorter than the wavelength of the incident ultrasound leading to an ultrasonic scattering. The gap regions in the grating allow the elastic grating thickness to be compressed by the incident ultrasound and coupled to a surface acoustic wave mode. The thickness compression can be measured using an embedded interferometer through one of the optical guided modes. The dielectric waveguide grating is a narrow bandpass optical filter enabling an ultrasensitive mode to sense changes in optical displacement. This enhancement in mechanical and optical properties gives rise to a broader detectable pressure range and figure of merit in ultrasonic detection; the detectable pressure range and figure of merit can be enhanced by 2.7 times and 23 times, respectively, compared to conventional Fabry–Perot structures.

A Fiber-Attached Coupler for Transmission Bandpass Whispering Gallery Mode Resonator

A fiber attached coupler for transmission bandpass whispering gallery mode (WGM) resonator is proposed and experimentally demonstrated. The coupler fabricated by femtosecond laser ablation consists of a pair of curved waveguides with a curvature radius of 50 μm, by which the WGM field is excited and collected through evanescent field coupling, respectively. As a proof of concept, a polymer microsphere functioning as the WGM cavity is embedded into a femtosecond laser ablated hole between the two curved waveguides. A passband with a full width of 0.15 nm at half maximum is observed when the microsphere is surrounded by air, verifying the feasibility of the proposed coupler for constructing a WGM cavity-based transmission narrow bandpass optical fiber filter. Benefitting from the relatively high thermo-optical coefficient, the passband of a packaged transmission bandpass WGM resonator can be thermo-optically tuned with a sensitivity of 446.6 pm/°C.

Dual dielectric cap gold nanoslits array optical resonance filter with large figure-of-merit.

In this work, we investigate a gold nanoslits array optical transmission filter with dual dielectric cap layers on top of the metal nanoslits. By integrating a low index of refraction dielectric layer between a high index of refraction dielectric cap layer and the gold nanoslits, a narrow spectral linewidth optical filter with a transmission peak far away from the Rayleigh anomaly wavelength is shown. Furthermore, we propose a figure-of-merit as the ratio of the spectral distance between a transmission peak and the Rayleigh anomaly over the spectral linewidth to characterize the performance of gold nanoslits optical filters. It is shown that dual dielectric cap gold nanoslits array optical filters have significantly larger figure-of-merits than that of traditional single dielectric cap gold nanoslits array optical filters.

A Faraday Anomalous Dispersion Optical Filter Based on Rubidium Hollow-Cathode Lamp

Using a hollow-cathode lamp (HCL) to build a Faraday anomalous dispersion optical filter (FADOF) is a new method to realize narrow linewidth optical filters. In contrast to other atomic optical filters based on saturated vapors, which work at a relatively high temperature to maintain the atomic density, the HCL device using sputtered particles can work at a much lower temperature. In this work, a rubidium HCL-based FADOF (HCL-FADOF) working at 780 nm is established and carefully tested. With 20 mm cathode length, the transmittance can reach 29% under 18 mA discharge current and 260 G magnetic field at room temperature, which is equivalent to the performance of a saturated vapor-based FADOF (VC-FADOF) at more than 60 ∘C. This work provides a direct comparison of the performance of the HCL-FADOF and the VC-FADOF, which is of great benefit to further studies of atomic filters at normal temperature.

Magnesium fluoride as low-refractive index material for near-ultraviolet filters applied to optical sensors

This article describes the fabrication of MgF2 and MgO thin-film-based optical filters and compares the optical transmission of the filters over UV. The MgF2 thin-films were deposited by use of an e-beam technique and their optical properties were characterised by ellipsometry. The effect of substrate temperature on the optical properties was studied. The MgF2 optimum refractive indices were obtained with a substrate temperature between 200 °C and 300 °C. Optical simulations were performed to compare the performance of MgF2 and MgO in the fabrication of near-UV narrow bandpass optical filters. While MgO-based optical filters result in a higher transmittance peak intensity, especially at 350 nm, the MgF2 optical filters are narrower, present lower values of FWHM, a mean value of 20 nm. This feature could be especially relevant for specific applications on fluorescent optical sensors. Finally, a Fabry-Perot based on a MgF2/TiO2 optical filter was deposited, using an e-beam technique for the MgF2 thin-films and RF-sputtering technique for the TiO2 thin-films. The MgF2/TiO2 optical filter peak transmittance is approximately 70% close to 400 nm, as expected. The results are discussed with focus on applications in fluorescent optical sensors for peaks at 350, 370, 380 and 400 nm, respectively.

Modified RGB Cameras for Infrared Remote-PPG.

Multi-wavelength cameras play an essential role in remote photoplethysmography (PPG). Whereas these are readily available for visible light, this is not the case for near infrared (NIR). We propose to modify existing RGB cameras to make them suited for NIR-PPG. In particular, we exploit the spectral leakage of the RGB channels in infrared in combination with a narrow dual-band optical filter. Such camera modification is simple, cost-effective, easy to implement, and it is shown to attain a pulse-rate extraction performance comparable to that of multiple narrow-band NIR cameras.

Using optically filtered high-speed imaging to characterise expansion tube operating conditions

Traditionally, Pitot rake test models have been used to take time- and spatially resolved impact pressure measurements in impulse facilities for flow characterisation. When expansion tubes are used for the study of hypervelocity planetary entry phenomena, generally the post-shock flow in the test section strongly radiates. This paper outlines a simple method which uses a high-speed camera in addition to a conventional Pitot rake to improve estimates of experimental test time and core flow size by imaging post-shock radiative emission over the probes in the Pitot rake. This method can also be used with specific narrow bandpass optical filters to examine when the emission from key species remains constant in the test flow in both time and space. The selection and suitability of various optical filters for a high-enthalpy Earth entry scenario are examined in this paper, as well as the effect of radiation from flow contamination. Experimentally, it was found that the radiative emission generally rises quite abruptly at the end of the test time and sometimes in situations where the pressure remains constant. It was also seen that different optical filters focusing on different spectral features can show their abrupt rise at different times, giving differing values for the end of the test time. Unfiltered measurements were found to be compromised by contaminant radiative emission which was seen to be randomly distributed in time and space towards the end of the test time.

J-PLUS: photometric calibration of large-area multi-filter surveys with stellar and white dwarf loci

Aims. We present the photometric calibration of the 12 optical passbands observed by the Javalambre Photometric Local Universe Survey (J-PLUS). Methods. The proposed calibration method has four steps: (i) definition of a high-quality set of calibration stars using Gaia information and available 3D dust maps; (ii) anchoring of the J-PLUS gri passbands to the Pan-STARRS photometric solution, accounting for the variation in the calibration with the position of the sources on the CCD; (iii) homogenization of the photometry in the other nine J-PLUS filters using the dust de-reddened instrumental stellar locus in (𝒳 − r) versus (g − i) colours, where 𝒳 is the filter to calibrate. The zero point variation along the CCD in these filters was estimated with the distance to the stellar locus. Finally, (iv) the absolute colour calibration was obtained with the white dwarf locus. We performed a joint Bayesian modelling of 11 J-PLUS colour–colour diagrams using the theoretical white dwarf locus as reference. This provides the needed offsets to transform instrumental magnitudes to calibrated magnitudes outside the atmosphere. Results. The uncertainty of the J-PLUS photometric calibration, estimated from duplicated objects observed in adjacent pointings and accounting for the absolute colour and flux calibration errors, are ∼19 mmag in u, J0378, and J0395; ∼11 mmag in J0410 and J0430; and ∼8 mmag in g, J0515, r, J0660, i, J0861, and z. Conclusions. We present an optimized calibration method for the large-area multi-filter J-PLUS project, reaching 1–2% accuracy within an area of 1022 square degrees without the need for long observing calibration campaigns or constant atmospheric monitoring. The proposed method will be adapted for the photometric calibration of J-PAS, that will observe several thousand square degrees with 56 narrow optical filters.

Complexity Analysis of the Kramers–Kronig Receiver

We propose a low-complexity digital implementation of the Kramers–Kronig receiver and analyze its performance and complexity. In simulations and experiments, we find that a relatively small number of filter taps is sufficient to achieve a reasonably high accuracy for the phase retrieval and for the reconstruction of the complex field. We show that the Kramers–Kronig receiver performance strongly depends on details of the system design. Unnecessarily broad optical filters decrease the reception quality, because additional noise makes the violation of the minimum-phase condition more likely. Narrow optical filters, however, impose high local oscillator laser stability and reduce the flexibility of this kind of receiver architecture. Further, we demonstrate Kramers–Kronig reception of 16 QAM signals at a net data rate of 267 Gbit/s after transmission over 300 km of standard single-mode fiber. We compare the performance with a conventional intradyne receiver. In a back-to-back setting, we increase the net data rate to 300 Gbit/s.

Tunable high repetition rate actively mode-locked fiber laser at 2 μm

We demonstrate a tunable actively mode-locked fiber laser at 2 μm band. A segment of 4 m Tm-Ho-co-doped fiber is used as gain medium. Active mode locking pulse is realized by using intensity modulation and the signal source is high frequency sinusoidal signal. A tunable narrow bandwidth optical filter is used to narrow laser linewidth, suppress noise and achieve wavelength tuning. Stable actively mode-locked pulses with up to 2.2 GHz repetition rate is obtained, corresponding to 649 order harmonic mode-locked pulse train. The pulse width is about 200 ps. The signal-to-noise ratio of RF spectrum is 68 dB. The optical tuning range is 1907 nm~1927 nm.

Cholesteric cored photonic fibre as a narrow optical filter

ABSTRACTWe consider an electromagnetic wave propagating in a cylindrical waveguide filled with a cholesteric liquid crystal whose helical axis is along the cylinder axis. Maxwell equations are established and analytically solved by imposing the appropriated boundary conditions for the Transverse Electric fields. We calculate the dispersion relation of the cholesteric fibre, from the transcendental equation given by the boundary conditions, for three components of the modes. This involves to reconcile the simultaneous restrictions imposed by the cholesteric band gap and the wavelength tuning of the cylindrical structure. We found that the cholesteric fibre can be used as an optical filter from the infrared to ultraviolet spectrum, where it is a narrow frequency filter from the visible to ultraviolet. The wavelength of light travelling into the fibre could be selected by choosing the appropriate parameter values of the cholesteric fibre and the mode.