Comb Filter Research Articles

Widely tunable 2 μm continuous-wave and mode-locked fiber laser.

We propose and experimentally demonstrate a widely tunable mode-locked thulium-doped fiber laser. The laser can operate at both continuous-wave and mode-locked states at different pump power levels. A classic nonlinear polarization rotation structure is employed to obtain the passive mode-locked laser. A birefringence Lyot filter as a fiber comb filter is used to expand the tuning range. Thanks to the filtering component, the tuning range of the continuous-wave laser can reach 127nm (1823-1950nm). The tuning ranges of the single-wavelength and dual-wavelength mode-locked lasers are 94nm (1835-1929nm) and 87nm (1831-1918nm), respectively, with a 3.085MHz repetition rate and 75ps pulse width.

The Application of EMD Denoising in Sampled Grating Comb Filter Design

In order to design sampled grating comb filter, the effect of sampled fiber grating’s parameters on reflective spectrum was analyzed. Sampled grating comb filter can be applied in the intensive optical communication system. Sampled grating reflective spectrum was simulated by changing the parameters. Four kinds of sampled grating optical comb filter were designed. In order to optimize the function of sampled grating comb filter, a filtering analysis and denoising method by using EMD were proposed. This method divides intrinsic mode function into two modes by EMD. One of mode was that signal components mode play dominant role, the other mode was that ınoise component mode plays dominant role. The mode of noise component play dominant role was removed. It can denoise reflective spectrum through restructuring signal. The smooth reflective spectrum and delay spectrum can be got after denoising. So that the peak value of sampled grating can be judged accurately.

Stable and uniform multiwavelength erbium-doped fiber laser based on a microfiber knot resonator with a Sagnac loop reflector

In this paper, we proposed and demonstrated a stable and uniform multiwavelength erbium-doped fiber laser (EDFL) by using of the filer action of microfiber knot resonator (MKR) with a Sagnac loop reflector. By embedding the resonator to the ring cavity of the EDFL by means of a circulator, a multiwavelength EDFL is created. The number of the lasing lines with different wavelengths within 3-dB bandwidth is up to 42, the linewidth of each lasing line is about 0.05 nm and the side-mode suppression ratio is about 45 dB. The results demonstrate that the MKR with a Sagnac loop reflector can be employed as a high performance comb filter to realize a stable and uniform multiwavelength fiber laser.

Stripline bandpass filters with alternating coupling coefficients

This article shows the possibility in principle to design stripline comb filters with alternating signs of the coupling coefficients. The new feature of the stepped-impedance resonators of the quarterwave type was established. They provide the opposite sings of the electromagnetic coupling with adjacent resonators. Such resonators are asymmetric relative to the vertical axes, which is drawn through their middle part. The stripline passband filters with alternating signs of coupling coefficients have attenuation poles that are located to the left and to the right of the passband. Those poles improve the selectivity of such filters. The articles provide the measured characteristics of the stripline comb filter of the forth order with alternating coupling, which has the central frequency of f0 = 1835 MHz and the passband width of 90 MHz. The filter is constructed using a dielectric material with relative permittivity of er = 92 and has the dimensions of 7.4×4.2×2 mm.

Tunable dual-wavelength single-longitudinal-mode fiber laser based on spectral narrowing effect in a nonlinear semiconductor optical amplifier

A tunable dual-wavelength single-longitudinal-mode (SLM) fiber laser based on a nonlinear semiconductor optical amplifier (NL-SOA) and an optical comb filter is proposed and demonstrated successfully. The SLM operation principle is based on the spectral narrowing effect resulting from the inverse four-wave mixing (FWM) in a NL-SOA. By inserting the NL-SOA inside the fiber laser cavity, SLM lasing can be possibly achieved when stable laser oscillation is established after many roundtrips through the NL-SOA. Further, tunable dual-wavelength SLM output can be obtained by incorporating a tunable optical filter (TOF) and a comb filter inside the laser cavity to perform dual-wavelength selection function. As a result, tunable dual-wavelength SLM oscillation with a wavelength spacing of 0.4nm and a wavelength tuning range of about 56nm has been achieved. Each wavelength has a linewidth less than 8kHz and a side-mode suppression ratio (SMSR) larger than 40dB.

Continuously tunable polarization-independent zeroth-order fiber comb filter based on polarization-diversity loop structure

By selecting some optimal wave retarder combination (WRC) groups, we propose and experimentally implement a continuously tunable polarization-independent zeroth-order fiber comb filter based on a polarization-diversity loop structure. The selected WRC groups contain a set of two quarter-wave retarders (QWRs), a set of a QWR and a half-wave retarder (HWR), and a set of an HWR and a QWR. The filter was formed using a polarization beam splitter (PBS), one of the three selected WRC groups, and high birefringence fiber (HBF). One end of HBF was butt-coupled to the PBS so that its slow axis should be oriented at 45° for the horizontal axis of the PBS, and the other end was connected to the WRC group. Three kinds of comb filters were fabricated with the three selected WRC groups. Through theoretical analysis on light polarization conditions for continuous spectral tuning and filter transmittances, eight special azimuth angle sets of two wave retarders, which gave the transmittance function eight different phase shifts of 0 to –7π/4 with a –π/4 step, were found for each WRC group. Theoretical prediction was verified by experimental demonstration. It was also confirmed that the filter could be continuously tuned by the appropriate control of wave retarders.

Switchable triple-wavelength thulium-doped fibre laser based on a Mach–Zehnder interferometer and fibre ring filter

A switchable triple-wavelength thulium-doped fibre laser based on an all-fibre Mach–Zehnder interferometer and fibre ring filter with a polarization-maintaining fibre is proposed and experimentally demonstrated. In the proposed fibre laser, a Mach–Zehnder interferometer comprising two 1 × 2 optical couplers is inserted into the optical cavity to produce the comb filter effect. The fibre ring filter comprises two optical couplers with a 3:7 splitting ratio and a 2-m-long polarization-maintaining fibre to improve lasing stability. Single-wavelength lasing can be tuned continuously from 1864.4 to 1884.5 nm, and five different modes of dual-wavelength and switchable triple-wavelength lasers can be realized by changing the polarization state. The signal-to-noise ratios of all lasers are more than 33 dB. The maximum power fluctuations and wavelength variations are less than 1.5 dB and 0.3 nm at room temperature, respectively, and the 3 dB bandwidth is less than 0.2 nm. The results demonstrate that stable and switchable single- or dual-wavelength lasers can be generated using the designed fibre laser.

Tunable and stable multi-wavelength erbium-doped fiber laser based on a double Sagnac comb filter with polarization-maintaining fibers

In this paper, a tunable and stable multi-wavelength erbium-doped fiber laser that employs a double polarization-maintaining Sagnac loop comb filter is proposed and experimentally demonstrated. The two nested Sagnac loops are used as the comb filter, which was fabricated using two 3-dB couplers, two polarization controllers (PC), and two sections of polarization-maintaining fibers (PMFs) with lengths of 1.2m and 1.8m, respectively. In the proposed laser, a stable and tunable single-, double-, and triple-wavelength lasing was obtained experimentally in the spectral range of 1528–1564nm, the side mode suppression ratio (SMSR) was higher than 38dB, and the wavelength shifts and power fluctuations were less than 0.04nm and 4.7dB, respectively, for a duration of 10min at a room temperature of 26°C. Furthermore, the laser can emit quadruple-wavelengths through adjustment of the two PCs in the experiment.

Multi-fault diagnosis of gearbox based on resonance-based signal sparse decomposition and comb filter

Abstract Fault diagnosis of gearbox is very important for the security and reliability of the equipment. In actual working conditions, multiple faults usually occur in a gearbox. However, the multi-fault diagnosis in gearboxes is a challengeable problem because the signal measured from the gearbox with multiple faults is complex and non-stationary. Particularly, the weaker fault feature signal is generally submerged by the stronger one and background noise. In order to avoid missed diagnosis and misdiagnosis of multi-faults in a gearbox, a novel method called resonance-based signal sparse decomposition (RSSD) with comb filter (CF), namely the RSSD-CF method, is proposed in this paper. The RSSD-CF method is based on the RSSD method which can nonlinearly decompose the vibrational signal of the gearbox with multiple faults into the high resonance component and the low resonance component. To obtain good separation results, the stepwise optimization strategy is applied to the adaptive selection of the optimal decomposition parameters in the RSSD method. In RSSD-CF method, the collected signal is firstly separated into the high and the low resonance components through using the RSSD method with the optimal decomposition parameters. And then, both of the high and the low resonance components are demodulated with the Hilbert transform and the fault information can be found in Hilbert envelop spectra. Finally, CF is constructed to extract the weaker fault feature signal from resonance components and exclude the interference components. The effectiveness of the RSSD-CF method is evaluated by using two experimental cases in this paper. The results confirm the advantage of the proposed method over the traditional RSSD method and the wavelet decomposition for multi-fault diagnosis in gearboxes.

Two-pitch tracking in co-channel speech using modified group delay functions

Modified group delay functions are beginning to gain significance in the literature for formant estimation, speaker recognition and speech recognition. In particular, group delay functions have the property that they possess higher resolution compared to that of the magnitude spectrum. In this paper, modified group delay functions are used for the estimation and tracking of two pitches in concurrent speech. The power spectrum of the speech signal is first flattened to annihilate the system characteristics, while retaining the source characteristics. Group delay analysis of the flattened spectrum is performed and the predominant pitch is computed. Next, a comb filter is designed to remove the predominant pitch and its harmonics from the group delay spectrum. The residual spectrum is again subjected to group delay analysis and the next candidate pitch is again estimated using modified group delay processing. The first and second pass pitch trajectories are corrected using post processing. The performance of the proposed algorithm was evaluated on two datasets using two metrics; pitch accuracy and standard deviation of fine pitch error. Our results show that phase based processing holds promise in the context of multipitch estimation.

Design and FPGA implementation of multiplierless comb filter

SummaryThe main objective of this paper is to design and implement minimum multiplier, low latency structures of a comb filter. Multipliers are the most area and power consuming elements; therefore, it is desirable to realize a filter with minimum number of multipliers. In this paper, design of comb filters based on lattice wave digital filters (LWDF) structure is proposed to minimize the number of multipliers. The fundamental processing unit employed in LWDF requires only one multiplier. These lattice wave digital comb filters (LWDCFs) are realized using Richards' and transformed first‐order and second‐order all‐pass sections. The resulting structural realizations of LWDCFs exhibit properties such as low coefficient sensitivity, high dynamic range, high overflow level, and low round‐off noise. Multiplier coefficients of the proposed structures are implemented with canonic signed digit code (CSDC) technique using shift and add operations leading to multiplierless implementation. This contributes in reduction of number of addition levels which reduces the latency of the critical loop. A field programmable gate array (FPGA) platform is used for evaluation and testing of the proposed LWDCFs to acquire advantages of the parallelism, low cost, and low power consumption. The implementation of the proposed LWDCFs is accomplished on Xilinx Spartan‐6 and Virtex‐6 FPGA devices. By means of examples, it is shown that the implementations of the proposed LWDCFs attain high maximum sampling frequency, reduced hardware, and low power dissipation compared with the existing comb filter structures. Copyright © 2017 John Wiley & Sons, Ltd.

Simplified tunable fiber comb filter based on polarization-diversity loop configuration

A simple tunable fiber comb filter composed of two quarter-wave plates and a section of polarization-maintaining fiber is proposed. The states of polarization are investigated through theoretical analysis, and we present the means for continuous wavelength tunability using analytic expression. The optimal angle combination of each birefringent device could be deduced so that an arbitrary wavelength could be a channel center of the comb filter without transmission loss, which is verified experimentally.

Multi-pitch estimation based on multi-scale product analysis, improved comb filter and dynamic programming

There are many multi-pitch estimation methods, but most of them can't perform perfectly for intrusion pitch detection. For this reason, a new multi-pitch detection approach is proposed. This method consists on the autocorrelation function of the Multi-scale product calculation of the mixture signal, its filtered version by a rectangular improved comb filter and the dynamic programming of the residual signal spectral density. First, we analyze the composite speech. Then, we apply the autocorrelation on the multi-scale product (AMP). We find the first pitch which represents the dominant one. Then, we apply the rectangular comb filter which has adaptive amplitude to remove the resulting signal from the original one. We operate AMP on the residue to obtain a pitch estimation of the intrusion. To improve the residue pitch estimation, we apply the dynamic programming to the spectral density of the residual signal to get optimum pitches corresponding also to intrusion signal. After that, we compare the two resulting pitch residue series to choose the most appropriate. Finally, this method is evaluated using the Cooke database and is compared to other well-known techniques. Experimental results confirm the strength and the performance of the proposed approach.

Thin stripline bandpass filters for the centimeter band

This article considers the design of thin stripline (1 mm) bandpass filters in the centimeter band, which contain dielectrics with different relative permittivity er. This includes the choice of the resonators, the dielectric material, the preliminary assessment of resonators’ unloaded quality factors and taking into account the features of the frequency response curve of the filter. It was found, that the passband width of the thin comb filters with λ/4 resonators and array-type filters with λ/2 resonators cannot exceed 6% for any values of er and the length of resonators of at least 2 mm. The array-type filters with resonators of half-wave type and with alternating signs of the coupling factors between resonators were proposed. Under certain additional conditions, the attenuation poles appear in such filters, resulting in improved selectivity. The results of computer modelling of the frequency response curve of thin filters of cm band are presented. They are compared to the frequency response of other filters. The data obtained from computer modelling showed good correspondence with the experimental data.

Two high-order superfine comb filters based on HiBi Fiber Ring

Two high-order superfine comb filters based on HiBi Fiber Ring

Multiple comb filters and autocorrelation of the multi-scale product for multi-pitch estimation

This paper presents a new method that estimates the fundamental frequency in the case of a real noisy environment when many persons speak at the same time and considers the case of two speakers. It essentially gives an accurate estimation of the pitch characterizing the second speaker. The first pitch is determining by detecting the Autocorrelation of the Multi-scale Product (AMP) of the mixture signal. Then a multiple-comb filters is applied to eliminate the dominant signal. After subtracting the resulting signal from the mixture, we obtain the residual signal. Next, we reapply the AMP to the obtained signal to estimate the second pitch. We get a matrix of the second pitch candidates. We classify its elements into three groups. After, we calculate the mean of each column of the appropriate selected group. Finally, the intrusion pitches of each frame are obtained. Experiments are performed using Cooke database. The results show the robustness and effectiveness of the proposed approach.

采用可调Mach-Zehnder滤波的波长可切换掺铒光纤激光器

In order to realize a highly stable and tunable laser output, an Er3+-doped fiber laser based on Mach-Zehnder filter structure and grating array made by femtosecond laser was proposed, and an optical delay line(ODL) was set into one interference arm to tune the interference interval flexibly. In the designed configuration, one 976 nm laser diode (LD) was used for pumping; an Er3+-doped fiber (EDF) of 6 m length was selected as the gain medium; The grating array will selete the femtosecond light reflection loop cavity specific wavelength oscillation form; the M-Z structure, which was used as comb filter, was adopted to select the wavelength reflected by the grating array; Tuning laser output can be realized by adjusting the ODL. In the experiment, when the pump power was 100 mW, a stable and tunable single, dual and triple wavelength output laser was observed. The single wavelength tunable interval was 2 nm within 1 570-1 596 nm, and the side-mode suppression ratios (SMSRs) of all were more than 50 dB.

Low-rate DoS attack flows filtering based on frequency spectral analysis

In frequency domain, the power spectrum of Low-rate denial of service (LDoS) attacks is totally spread into the spectrum of normal traffic. It is a challenging task to detect and filter LDoS attack flows from the normal traffic. Based on the analysis of LDoS attack flows and legitimate TCP traffic in time and frequency domains, the periodicity of the TCP traffic and LDoS attack flows is explored to facilitate the research of network traffic processing. Hence, an approach of LDoS attack flow filtering based on frequency spectrum analysis is proposed. In this approach, the TCP traffic and LDoS attack flows are transformed from the time domain into the frequency domain. Then the round-trip time (RTT) is estimated by using frequency domain search method. Analysis of amplitude spectrum shows that TCP traffic energy is mainly concentrated on the points of n/RTT. Therefore, a comb filter using infinite impulse response (IIR) filter is designed to filter out the LDoS attack flows in frequency domain, while most legitimate TCP traffic energy at the points of n/RTT are pass through. Experimental results show that the maximum pass rate for legitimate TCP traffic reaches 92.55%, while the maximum filtration rate of LDoS attack flows reaches 81.36%. The proposed approach can effectively filter the LDoS attack flows while less impact on the legitimate TCP traffic.

Tunable fiber comb filter based on simple waveplate combination and polarization-diversified loop

By incorporating a simple waveplate combination (WPC) set composed of two waveplates, we propose a wavelength-tunable fiber comb filter based on a polarization-diversified loop (PDL). The simple WPC set includes three kinds of waveplate groups such as two quarter-wave plates (QWPs), a set of a QWP and a half-wave plate (HWP), and a set of an HWP and a QWP. The PDL is implemented by making a Sagnac birefringence loop comprised of a four-port polarization beam splitter (PBS), two waveplates, and polarization-maintaining fiber (PMF). In the PDL, one end of PMF is connected to one port of the PBS with its slow axis π/4 (45°) oriented with respect to the horizontal axis of the PBS, and the other end of PMF is concatenated with the waveplates. First, we investigated light polarization conditions required to continuously tune the absolute wavelength location of the proposed filter in terms of input and output states of polarization (SOPs) of a birefringence element, or PMF. Then, three analytic transmittances of the filter were derived for the three WPC sets with arbitrary orientation angles of waveplates through Jones matrix formulation. And eight specific orientation angle sets of two waveplates, which caused phase shifts increasing linearly from 0° to 315° by a step of 45° in a sinusoidal transmittance function, were found for each WPC set. In particular, it has been theoretically proved that an orientation angle set of two waveplates, which can induce an arbitrary phase shift in the sinusoidal transmittance function, always exists for each WPC set. This implies that the comb spectrum of the proposed filter can be continuously tuned within one channel bandwidth by the proper control of the waveplate orientation angles. Finally, the input SOPs of PMF and the wavelength-dependent evolution of its output SOP were examined on the Poincare sphere at the eight specific waveplate angle sets. The relationship between the wavelength tuning and the SOP evolution was also discussed.

Dose-efficient ultrahigh-resolution scan mode using a photon counting detector computed tomography system.

An ultrahigh-resolution (UHR) data collection mode was enabled on a whole-body, research photon counting detector (PCD) computed tomography system. In this mode, 64 rows of [Formula: see text] detector pixels were used, which corresponded to a pixel size of [Formula: see text] at the isocenter. Spatial resolution and image noise were quantitatively assessed for the UHR PCD scan mode, as well as for a commercially available UHR scan mode that uses an energy-integrating detector (EID) and a set of comb filters to decrease the effective detector size. Images of an anthropomorphic lung phantom, cadaveric swine lung, swine heart specimen, and cadaveric human temporal bone were qualitatively assessed. Nearly equivalent spatial resolution was demonstrated by the modulation transfer function measurements: 15.3 and [Formula: see text] spatial frequencies were achieved at 10% and 2% modulation, respectively, for the PCD system and 14.2 and [Formula: see text] for the EID system. Noise was 29% lower in the PCD UHR images compared to the EID UHR images, representing a potential dose savings of 50% for equivalent image noise. PCD UHR images from the anthropomorphic phantom and cadaveric specimens showed clear delineation of small structures.