Optical Bandpass Filter Research Articles

Enhancing signal quality in dense wavelength division multiplexed systems by eliminating crosstalk with cascaded optical bandpass filters

Non-orthogonal wavelength division multiplexing (WDM) channels are crucial in achieving significantly higher capacity and spectral efficiency than conventional schemes. Additionally, conventional schemes face challenges in electrical processing of wide-bandwidth signals. To suppress crosstalk in non-orthogonal WDM channels, the use of cascaded bandpass filters with suitable transmissivity and bandwidth is investigated for effective channel separation of large-capacity and multiplexed optical signals. This study employs the bandwidth ratio as a parameter, which is defined as the ratio of the channel spacing to the signal bandwidth, to quantify the extent of signal overlapping, and uses optical bandpass filters to filter non-orthogonal signals with a bandwidth ratio ranging from 1.0 to 0.75. The non-overlapping portion is set as the transmission bandwidth of the filter, and the crosstalk is maximally reduced with two-stage filters using Nyquist, super Gaussian, and Butterworth shaped filters. According to the analysis, the combination of Gaussian N = 1 and Gaussian N = 5 exhibits the lowest error vector magnitude (EVM) performance within a bandwidth ratio of 0.8 to 1.0. Simulations showed that a two-stage OBPF with a combination of Gaussian N = 1 and Gaussian N = 5 provides the best EVM performance at a bandwidth ratio between 0.85 to 1.0 within the combination of filter shapes and their orders we used. Additionally, the study includes an analytical investigation of the impact of amplified-spontaneous-emission noise associated with the Erbium-doped fiber amplifier (EDFA) and frequency resolution tolerance of the LCOS filter. The results demonstrate that cascaded EDFAs can be achieved down to OSNR of 10 dB at a minimum resolution of 4 GHz. Finally, the optimal filter shape is analyzed for different bandwidth ratios. The investigation shows that the optimal filter shape is independent of the function used and remains consistent across all bandwidth ratios. The tolerance of the filter is primarily affected by the −0.5 dB transmittance bandwidth between subchannels, which varies depending on the bandwidth ratio. Furthermore, a high tolerance is observed in the −20 dB transmittance bandwidth.

Towards a Multispectral Imaging System for Spatial Mapping of Chemical Composition in Fresh-Cut Pineapple (Ananas comosus).

With increasing public demand for ready-to-eat fresh-cut fruit, the postharvest industry requires the development and adaptation of monitoring technologies to provide customers with a product of consistent quality. The fresh-cut trade of pineapples (Ananas comosus) is on the rise, favored by the sensory quality of the product and mechanization of the cutting process. In this paper, a multispectral imaging-based approach is introduced to provide distribution maps of moisture content, soluble solids content, and carotenoids content in fresh-cut pineapple. A dataset containing hyperspectral images (380-1690 nm) and reference measurements in 10 regions of interest of 60 fruit (n = 600) was prepared. Ranking and uncorrelatedness (based on ReliefF algorithm) and subset selection (based on CfsSubset algorithm) approaches were applied to find the most informative wavelengths in which bandpass optical filters or light sources are commercially available. The correlation coefficient and error metrics obtained by cross-validated multilayer perceptron neural network models indicated that the superior selected wavelengths (495, 500, 505, 1215, 1240, and 1425 nm) resulted in prediction of moisture content with R = 0.56, MAPE = 1.92%, soluble solids content with R = 0.52, MAPE = 14.72%, and carotenoids content with R = 0.63, MAPE = 43.99%. Prediction of chemical composition in each pixel of the multispectral images using the calibration models yielded spatially distributed quantification of the fruit slice, spatially varying according to the maturation of single fruitlets in the whole pineapple. Calibration models provided reliable responses spatially throughout the surface of fresh-cut pineapple slices with a constant error. According to the approach to use commercially relevant wavelengths, calibration models could be applied in classifying fruit segments in the mechanized preparation of fresh-cut produce.

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.

Electro‐optical spiking neural networks using an enhanced optical axon with pulse amplitude modulation and automatic gain controller

AbstractVisible light communication can be leveraged to establish a wireless link between neurons in spiking networks even when neural areas are in relative motions. In electro‐optical spiking neural networks (SNN), parallel transmission is often achieved through wavelength division multiplexing (WDM). However, WDM can be prohibitive in certain applications due to the need for multiple narrow‐band transmitters and receivers with optical bandpass filters. Instead of WDM, an alternative approach of using non‐orthogonal multiple access is explored (NOMA) with a pulse amplitude modulation (PAM) scheme in optical axons to enable parallel neural paths in an SNN. To evaluate NOMA with PAM, the authors implement an electro‐optical SNN that controls the force of two anthropomorphic fingers actuated by the shape memory alloy‐based actuators. An optical reference channel is used to dynamically adjust the optical receiver's gain to improve the receiver's decoding performance. Experimental results demonstrate that the electro‐optical SNN can maintain control over the fingers and hold an object under varying channel conditions. Hence, the proposed system offers robustness against dynamic optical channels induced by the relative motion of neurons.

Demonstration of the First Outdoor 2-μm-Band Real-Time Video Transmission Free-Space Optical Communication System Using a Self-Designed Single-Frequency Fiber Laser

In the present paper, we demonstrate the first outdoor free-space optical communication (FSOC) system with real-time video transmission in a 2-μm-band with a state-of-the-art data rate performance of 10 Gbit/s and a communication distance of ∼177 m. The measured wide-open eye diagram and zero bit-error-rate also validated the high communication quality. The FSOC system is constructed with all homemade components, including a single-frequency (SF) thulium-holmium co-doped fiber laser (THDFL), a modulation module, a demodulation module, three thulium-doped fiber amplifiers, an optical bandpass filter, two optical-antennas, and a display system. As the most important component, the SF THDFL, enabled by a uniform fiber Bragg grating and a passive fiber ring-compound cavity, is self-designed and fabricated by our group, presenting high lasing stability centered at 2048.98 nm, an optical signal-to-noise ratio of >81 dB, a relative intensity noise of <-128.18 dB/Hz at >0.5 MHz and a linewidth of 5.290 kHz under a measurement integration time of 0.001 s. Our work is an important step forward for practical outdoor FSOC in a 2-μm-band, proving useful for engineers and scientists when our SF THDFL is implemented as a low-cost, high-performance laser source for the FSOC system.

Flexible optical fiber channel modeling based on a neural network module.

Optical fiber channel modeling, which is essential in optical transmission system simulations and designs, is usually based on the split-step Fourier method (SSFM), making the simulation quite time-consuming owing to the iteration steps. Here, we train a neural network module termed NNSpan to learn the transfer function of a single fiber (G652 or G655) span with a length of 80 km and successfully emulate long-haul optical transmission systems by cascading multiple NNSpans, which gives remarkable prediction accuracy, even over a transmission distance of 1000 km. Even when trained without erbium-doped fiber amplifier (EDFA) noise, NNSpan performs quite well when emulating the systems affected by EDFA noise. An optical bandpass filter can optionally be added after EDFA, making the simulation more flexible. Comparison with the SSFM shows that NNSpan has a distinct computational advantage, with the computation time reduced by a factor of 12. This method based on NNSpan could be a supplementary option for optical transmission system simulations, thus contributing to system designs as well.

Offset double-sideband signal field recovery at low CSPR using filter-assisted direct detection

The offset double-sideband (DSB) scheme has attracted attention because it can achieve field recovery at low carrier-to-signal power ratio (CSPR) and improve the spectral efficiency (SE) of self-coherent detection. In this paper, an offset DSB filter-assisted direct detection (FADD) scheme is proposed and studied for the first time, which uses an optical bandpass filter (OBPF) to filter out the optical carrier as the local oscillator for the receiver. As such, signal-signal beat interference (SSBI) can be eliminated. Since the offset DSB signal has a large frequency gap, the requirement for the bandwidth of the OBPF is not high. The performance of the proposed offset DSB FADD scheme is evaluated for 16-QAM signals. It is comprehensively compared with offset DSB carrier-assisted differential detection (CADD) scheme in terms of OSNR sensitivity, optimal CSPR, and receiver power sensitivity. In addition, the effect of frequency gap compression is significant as SSBI is completely eliminated from the system, thereby utilizing as many low-frequency resources as possible and improving SE. Finally, the proposed offset DSB FADD scheme is resilient to the wavelength offset up to several GHz between the transmitter laser and the center wavelength of the OBPF.

Investigations of the High-Power, Single-Frequency, Sub-Microseconds, 1548-nm Fiber Laser Based on the Optical Pulse Modulation

High-power, single-frequency lasers are important in the applications of coherent optical communication, Doppler wind lidar, and remote sensing. Here, a 10-kHz, 353-μJ, single-frequency, 574-ns, 1548-nm fiber laser system was demonstrated based on the optical pulse modulation. The 1548-nm, single-frequency seed laser with the linewidth of 1.25 kHz was modulated by two acousto-optic modulators (AOMs) into a 10-kHz pulsed laser. Two cascaded fiber amplifiers were spliced after the AOMs to scale up the pulse energy. The amplified spontaneous emission (ASE) was inhibited by the multistage amplification construction and the optical bandpass filters. A customized pulse profile was proposed to pre-compensate the temporal modulation and inhibit the stimulated Brillouin scattering (SBS) effect. The pulse duration of the amplified single-frequency pulse was 574 ns, with the peak power of 753 W. A numerical model was also built for investigating the dynamic of the interaction process inside the power amplifier. The simulated results indicate the maximum output power of the 10-kHz, customized single-frequency laser can be 3.984 W with a pulse duration of 381 ns.

Impact of ligands on the performance of band-stop and bandpass optical filter of cobalt sodium zinc borate glass

Impact of ligands on the performance of band-stop and bandpass optical filter of cobalt sodium zinc borate glass

Direct detection system based on a single photodiode receiving the independent quadruple-SSB signal.

We propose and verify a direct detection (DD) system based on a single photodiode (PD) receiving the independent quadruple-single-sideband (quadruple-SSB) signal. At the transmitter side, an I/Q modulator is utilized to modulate the independent quadruple-SSB signal, the signal is received via one PD without optical bandpass filters (OBPFs). Then, the independent quadruple-SSB signal is separated into four sideband signals by subsequent digital signal processing (DSP). In the scheme of back-to-back (BTB), 1-km and 5-km standard single-mode fiber (SSMF) transmission, the four sideband signals are extensively studied and analyzed. The simulation results show that the bit error rate (BER) of 1Gbaud, 2Gbaud and 4Gbaud independent quadruple-SSB signal can reach the 7% hard-decision forward error correction (HD-FEC) threshold of 3.8 × 10-3 when the received optical power (ROP) is -21, -20 and -17.2 dBm in 5-km SSMF transmission. Meanwhile, as the frequency interval gets wider, the crosstalk in the sideband signal reception can be mitigated and the BER decreases. This scheme for the first time demonstrates that the independent quadruple-SSB signal can further expand the system transmission capacity and enhance the spectrum efficiency. Our simplified independent quadruple-SSB signal direct detection system has a simple structure and high spectral efficiency, which will have a promising future in high-speed optical communication.

DEVELOPMENT OF A METHOD FOR CONTACTLESS TEMPERATURE MEASUREMENT IN 3 SPECTRAL RANGES

A 3-spectral method for non-contact temperature measurement has been developed. Three silicon photodiodes were used. The separation of the spectrum is done by optical bandpass filters and beamspliters. It is possible to work in three modes: three-spectral, two-spectral (spectral ratio) and one-spectral. In the first mode, the temperature of objects with an unknown and variable emissivity can be measured. The last two modes give good results respectively for gray bodies and for objects with known emissivity.

Modeling and optical characterization of γ-irradiated Cu2+Doped borate glasses as bandpass optical filters

Novel systems of alkali borate glasses with various CuO concentrations from 0.2 up to 1.5 mol% have been prepared by the conventional melt-quenching technique. Several optical, physical, and structural characteristics of the prepared glasses have been examined before and after incremental γ-irradiation doses. The intensity of the blue color of the glasses is controlled by CuO concentrations, with Cu+/Cu2+ in ionic equilibrium. UV/Vis absorption peaks centered at 740 nm can be correlated to a tetragonal twist of the octahedral structure due to 2B1g-2B2g electronic transition. Photoluminescence (PL) extinction spectra can be attributed to Cu+-Cu+ energy migration after Cu+→ Cu2+ energy transfer. Electron spin resonance (ESR) measurements show sharp signals at magnetic fields ranging from 3344 to 3326 Gauss (g = 2.08–2.09), corresponding to the divalent copper state (Cu2+) in octahedral units. FTIR spectra revealed stable structures towards irradiation. Either an increase in CuO content or irradiation led to greater UV absorption, lower PL intensity, lower bandgap (Eopt), lower density, larger molar volume, and slightly more intense ESR signals. Copper ions can be considered as modifiers since they increase the degree of disorder in the glassy network. UV/NIR band stop, FWHM, and center of peaks confirmed the high efficiency of the prepared Cu-doped glasses as colored optical bandpass filters in the green and near-blue regions for UV/IR light laser protection.

SOA-MZI Differential Transformation Approach Applied on Simultaneous Electro-Optical Mixing

We experimentally incubate a ground-breaking design, for the first time, of concurrent electro-optical semiconductor optical amplifier Mach–Zehnder interferometer mixing (SOA-MZI) based on a differential transformation methodology. Projecting the simultaneous electro-optical mixing system and improving its efficiency and quality achievement in optical and electrical features is a crucial task due to the characteristics of an optical pulse source (OPS) operating with a repetition rate of f= 58.5 GHz and a pulse width duration of 1 picosecond (ps). The resultant of the contemporaneous electro-optical mixing exhibits exceptional passive power stability, reaching 0.8% RMS over a two-hour period. Furthermore, when the optical bandpass filter is controlled at the data wavelength of 1540 nm, we achieve up to 30 dBm of the overall mean output power with an optical conversion gain of 46 dB and an exceptionally high optical signal-to-noise ratio reaching 80 dB. Using orthogonal frequency division multiplexing (OFDM) signals, each data subcarrier is modulated using 128 quadratic amplitude modulation (128-QAM) at carrier frequencies fk and simultaneously up-mixed to high aim frequencies nf±fk at the SOA-MZI output. Additionally, the resulting OFDM_128-QAM up-mixed signal is examined using the specifications for the error vector magnitudes (EVMs) and the electrical conversion gains (ECGs). The SOA-MZI mixing experiment can handle high frequencies up to 120 GHz. Positive ECGs are followed by a sharp reduction over the entire band of the aim frequencies. The highest frequency range achieved during the realistic investigation is shown at 2f+f4= 120 GHz, where the EVM reaches 8% with a symbol rate of 15 GSymb/s. Furthermore, the concurrent OFDM_128-QAM up-mixed signal achieves an absolute maximum bit rate of 80.4 Gbit/s. The investigation into the simultaneous electro-optical mixing regime is finally supported by unmatched characterization improvements.

Tunable Fano resonance analysis in plasmonic waveguide side-coupled multiple Taiji resonators

In this paper, we first propose and analyze tunable Fano resonance based on indirect interaction between two Taiji resonators which both side-couple with a MIM (metal–insulator–metal) waveguide. An S-shaped branch waveguide embedded in the Taiji ring can tailor the propagating modes and then varying the number of whispering-gallery (WG) modes in the Taiji resonator, leading to the convenient tuning of Fano resonance lineshape. By covering monolayer graphene on the waveguide, Fano resonance lineshape can be all-optically tuned with relatively low pump intensity because of the optical Kerr effect of graphene; by coating UV-glue on the proposed structure, temperature sensing with high sensitivity of −0.23 nm/°C is obtained because of large negative thermo-optic coefficient of UV-glue. Furthermore, up to 16 Taiji resonators are designed and coupled with a common MIM waveguide to observe and analyze Fano lineshape variation, finally realizing the optical band-stop filter with maximal bandwidth of 49.5 nm and optical band-pass filter with maximal bandwidth of 45.3 nm. This ultra-compact micro-structure of plasmonic waveguide coupling multiple Taiji resonators demonstrates good potential applications in the nonlinear optics, optical modulation, and high-sensitivity biochemical sensors.

Microwave Photonic PS-Pulse and 140 GHz RF Comb Generator

A microwave photonic RF comb generator is presented by combining an fs-pulse laser and a high-speed broadband photodetector module. The subsystem generates pulses with a FWHM of 5.8 ps and a flat RF comb up to 140 GHz. Furthermore, the capability of pulse distribution over fiber is investigated. The dispersion of the single mode fiber is managed by using an optical bandpass filter, because reducing the optical spectral width limits the pulse broadening. With this scheme, the electrical pulse shape remains constant up to 500 m of fiber transmission. For a demonstration with 1500 m of fiber, the optical bandpass filter dispersion management results in a tradeoff between minimum electrical pulse width and fiber length.

Design and performance of a repetition rate controllable and wavelength tunable L + U-band actively mode-locked erbium fiber laser

Mode-locked lasers draw great interest in the laser research field due to their diverse applications and easy achievement of ultra-short and high-intensity optical pulse trains. Actively mode-locked (AML) fiber lasers are more flexible compared to passively mode-locked fiber lasers owing to their ability to control repetition rates and pulse intervals electronically. The design and development of wavelength tunable AML fiber lasers operating beyond the C-band have attracted great attention from the research community. In this paper, a repetition rate controllable and wavelength tunable L+U-band AML erbium fiber laser (AML-EFL) based on a single standard 1480 nm pump laser and Mach–Zehnder modulator (MZM) as an intra-cavity intensity modulator is demonstrated using numerical simulation. The AML-EFL is created using a MZM driven by electrical Gaussian pulses and a tunable optical bandpass filter that is used to tune the laser cavity at any wavelength in the 1565–1645 nm range. The repetition rate of the AML-EFL is controlled from 500 kHz to 1 GHz. Trains of pulses with pulse widths of 103 ns and 50 ps and pulse intervals of 2 µs and 1 ns are successfully generated at repetition rates of 500 kHz and 1 GHz, respectively. A pulse energy of 626 nJ is obtained for a 500 kHz repetition rate. Signal-to-noise ratios of 30 and 32 dB are observed for trains of mode-locked pulses with repetition rates of 500 kHz and 1 GHz, respectively. In addition, slope efficiencies of around 66% for an L-band wavelength of 1582.1 nm and 30% for a U-band wavelength of 1633.6 nm are obtained considering the optimized parameters.

Structured cadmium sodium diborate glass optical bandpass filters doped with copper oxide: Impact of cadmium oxide

Glasses of cadmium sodium diborate doped with copper oxide were created. These glasses have optical filters in the ultraviolet and visible wavelength ranges. Cadmium oxide (CdO) is used to regulate the properties of glass and the range of optical filters. The 10.8 mol% CdO sample has a broad visible-near-infrared blocking region spanning 640–900 nm. Glass has triangular (BO3) and tetragonal (BO4) borate groups that are linked together by B–O–B bridges. When the CdO content increased from 10 to 13.3 mol%, the N4 ratio increased from 0.423 to 0.568, indicating the dominance of the BO4 units. As a result, the glass density (D) rose from 2.876 to 3.113 g/cm3, while the boron-boron (dB-B) separation fell from 3.731 to 3.603 Å. The optical transition (Eg) decreased slightly as CdO increased, from 3.107 eV to 3.021 eV, and the refractive index varied from 1.515 to 1.577 at 589 nm.

Integration of Raspberry Pi and antennas for multiplexing digital signals over a fiber optical communication system

A Fiber Optical Communication System (FOCS) is used to experimentally demonstrate the transmission of multi-channel information of two High Definition (HD) video signals and 18 digital TV channels. These signals are supplied using commercial Raspberry Pi boards and digital TV Antennas, respectively. Both digital signals and lightwaves of a couple of Distributed Feedback (DFB) lasers with wavelengths located at 1547.5 and 1552.5 nm interact with two Mach-Zehnder Intensity Modulators (MZ-IM). The resulting modulated optical signals from each modulator are combined by using an optical coupler. The combined optical field is amplified and transmitted through 30 km of single-mode optical fiber. To demultiplex the combined optical signals, a bandpass optical filter is used, then a PIN-photodetector is connected to recover HD video signals and the 18 digital TV channels. The measured electrical spectrum of the received signals shows that the transmission is performed without interference and the multi-channel information is successfully received in an HD display. The use of wavelengths with linewidths of 10 MHz and an optical fiber with a chromatic dispersion of 17 ps/nm-km allows for a maximum bit rate of approximately 11 Gbps to be achieved numerically. This result guarantees that HD video signals can be experimentally transmitted successfully with Bit Error Rate (BER) values between 10-6 and 10-15 when the optical power of lasers is tuned between 7 and 9 dBm.

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.

Simultaneous measurements of Ka-band microwave angle of arrival and Doppler frequency shift based on a silicon DPMZM.

We propose a system for the simultaneous measurements of Ka-band microwave angle of arrival (AOA) and Doppler frequency shift (DFS) based on a high-speed silicon dual-parallel Mach-Zehnder modulator (Si-DPMZM).. An echo signal drives a sub-MZM while the combination of a phase-delay echo signal and a transmitted signal drives the other sub-MZM. Two optical bandpass filters (OBPFs) are used to select the upper and lower sidebands of the Si-DPMZM output signal, detected by low-speed photodiodes, then generating two intermediate frequency (IF) signals. Thus, both AOA and DFS (with direction) can be obtained by comparing the powers, phases and frequencies of these IF signals. The estimation error of measured AOA is less than ±3° from 0 to ±90°. Meanwhile, the DFS at 30/40 GHz were measured with an estimated error of less than 9.8 × 10-10 Hz within ±1 MHz. In addition, the fluctuation of DFS measurement is less than 3 × 10-11 Hz within 120 minutes, indicating the high stability of the system.