Low Group Delay Research Articles

Hole-Assisted Graded-Index Four-LP-Mode Fiber With Low Differential Mode Group Delay Over C+L Band

In this paper, in order to achieve low differential mode group delay (DMGD) over the C+L band, we propose the hole-assisted graded-index few mode fiber (FMF) design supporting four nondegenerated linear polarized (LP) modes for a mode division multiplexing system. In order to determine the fiber core parameters, an optimization has been made between the enlargement of the effective area $({{A_{eff}}})$ and the reduction of macrobending loss (MBL). The ${A_{eff}}$ about 107 $\mu {m^2}$ for LP01 mode at the wavelength of 1550 nm and the MBL lower than 0.005 dB/100 turns for LP02 mode at the curvature radius of 30 mm and at the wavelength of 1550 nm are achieved simultaneously. The numerical simulation results prove that the hole-assisted structure can greatly reduce the absolute value of DMGD of the FMF. Particularly, the maximum DMGD less than |35| ps/km within the spectrum range from 1530 to 1630 nm is achieved. A tolerance analysis against manufacturing errors has been discussed, and we further propose a self-managed low DMGD transmission line solution with the controllable air-hole size.

Design of multiplier-less minimum-phase filters based on sharpening compensated comb filters

Minimum-phase (MP) filters have all zeros inside and/or unit circle. As a consequence, the group delay of an MP system is always less than that of non-minimum phase systems, having the equal magnitude responses. Minimum-phase (MP) filters find applications where it is necessary to have a low group delay, like in communications, speech processing, and predictive coding, among others. This paper presents a novel simple method for the direct design of low-pass minimum-phase (MP) filters. Method is based on design of two compensated combs, using a multiplier-less minimum-phase compensator, and sharpening technique. The first comb defines the stop band and pass band of the MP filter, while the second comb decreases side lobes of the first comb, thus increasing attenuation of the resulting MP filter. Knowing that all zeros of comb filter are on the unit circle, the compensated comb is also a MP filter. Similarly, under the special condition, the sharpening of multiplier-less compensated comb may also result in a MP multiplier-less filter. The benefit of the proposed method is illustrated in the provided design examples.

A Family of Square-Root Nyquist Filter With Low Group Delay and High Stopband Attenuation

A simple method for square-root Nyquist filter design with low group delay and high and fast decaying stopband attenuation is presented. The method is based on a frequency domain design procedure and can address various roll-off factors. The computation procedure is presented and justified analytically. Numerical results show that for a given group delay, the stopband attenuation performance of this proposed technique outperforms the most relevant existing techniques.

Design of compact bandpass filter for WiMAX and UWB application using asymmetric SIRs and DGS

This paper considers a novel compact tri-band microstrip bandpass filter, the design of which employs asymmetric SIRs and DGS for achieving the characteristics of low insertion loss, high selectivity, wider range of bandwidth, and low group delay for 2.5/3.4 GHz (WiMAX) and 4.14–5.32 GHz (UWB) bands. The novel filter design implies the intentional selection of impedance ratio R and the length of the microstrip of asymmetric SIRs; in addition, DGS is used to improve the coupling strength of the last band. The scattering parameters of the three passbands have the following values: insertion losses S21 are–0.26/–0.07/–0.05 dB, and return losses S11 are–11.29/–19.25/–22.64 dB, respectively. The response of the filter was simulated using Ansoft HFSS simulator.

Visible supercontinuum generation in a graded index multimode fiber pumped at 1064 nm

We observe efficient supercontinuum generation that extends into the visible spectral range by pumping a low differential mode group delay graded index multimode fiber in the normal dispersion regime. For a 28.5 m long fiber, the generated spectrum spans more than two octaves, starting from below 450 nm and extending beyond 2400 nm. The main nonlinear mechanisms contributing to the visible spectrum generation are attributed to multipath four-wave mixing processes and periodic spatio-temporal breathing dynamics. Moreover, by exploiting the highly multimodal nature of this system, we demonstrate versatile generation of visible spectral peaks in shorter fiber spans by altering the launching conditions. A nonlinearly induced mode cleanup was also observed at the pump wavelength. Our results could pave the way for high brightness, high power, and compact, multi-octave continuum sources.

Low Group Delay Dispersion Optical Coating for Broad Bandwidth High Reflection at 45° Incidence, P Polarization of Femtosecond Pulses with 900 nm Center Wavelength

We describe an optical coating design suitable for broad bandwidth high reflection (BBHR) at 45° angle of incidence (AOI), P polarization (Ppol) of femtosecond (fs) laser pulses whose wavelengths range from 800 to 1000 nm. Our design process is guided by quarter-wave HR coating properties. The design must afford low group delay dispersion (GDD) for reflected light over the broad, 200 nm bandwidth in order to minimize temporal broadening of the fs pulses due to dispersive alteration of relative phases between their frequency components. The design should also be favorable to high laser-induced damage threshold (LIDT). We base the coating on TiO2/SiO2 layer pairs produced by means of e-beam evaporation with ion-assisted deposition, and use OptiLayer Thin Film Software to explore designs starting with TiO2/SiO2 layers having thicknesses in a reverse chirped arrangement. This approach led to a design with R > 99% from 800 to 1000 nm and GDD < 20 fs2 from 843 to 949 nm (45° AOI, Ppol). The design’s GDD behaves in a smooth way, suitable for GDD compensation techniques, and its electric field intensities show promise for high LIDTs. Reflectivity and GDD measurements for the initial test coating indicate good performance of the BBHR design. Subsequent coating runs with improved process calibration produced two coatings whose HR bands satisfactorily meet the design goals. For the sake of completeness, we summarize our previously reported transmission spectra and LIDT test results with 800 ps, 8 ps and 675 fs pulses for these two coatings, and present a table of the LIDT results we have for all of our TiO2/SiO2 BBHR coatings, showing the trends with test laser pulse duration from the ns to sub-ps regimes.

Group Delay Management and Multiinput Multioutput Signal Processing in Mode-Division Multiplexing Systems

Multi-input multi-output (MIMO) digital signal processing (DSP) for mode-division multiplexing (MDM) may have high complexity, owing to a plurality of modes and a potentially long group delay (GD) spread in multimode fiber (MMF). This paper addresses the management of GD in MMF and its implications for the complexity and performance of MIMO DSP. First, we review the generalized Jones and Stokes representations for modeling propagation in MMF, and describe key GD properties derived using the two representations. Then, we describe three approaches for GD management: 1) optimized fiber design, 2) mode coupling, and 3) GD compensation. For approach 1), we explain design principles for minimizing the GD spread. We review experimental results to date, showing that fabrication nonidealities significantly increase the GD spread, and this approach alone may not achieve sufficiently low GD spread. For approach 2), we describe mechanisms for inducing intragroup and intergroup coupling. We describe mode scrambler designs based on photonic lanterns or long-period fiber gratings, both of which can ensure strong intergroup coupling. For approach 3), we review GD-compensated system design principles and show that GD compensation is only partially effective in the presence of random intragroup or intergroup coupling. Finally, we provide an overview of adaptive MIMO frequency-domain equalization algorithms. Considering tradeoffs between complexity, performance, and adaptation time, we show that the GD spread is a key factor determining the feasibility of MIMO DSP, and its feasibility requires judicious GD management.

Novel approach of Microstrip Tri-Band Bandpass Filter for GSM, WiMAX and UWB Application

In this paper, a novel approach of miniature tri-band microstrip bandpass filter is design for the application of GSM(1.8GHz),WiMAX(3.4GHz) and UWB(6.5-8.1GHz) using Asymmetric SIRs and DGS to achieved the basic characteristic of microstrip filter such as low insertion loss, high selectivity, wider range of bandwidth, low group delay. The novel filter is design intentionally selecting the impedance ration(R) and length of the microstrip of the asymmetric SIRs and DGS is used to improve the coupling strength of the desired band. The measurement entities of the novel filter for GSM (1.8GHz), WiMAX (3.4GHz), and UWB (6.5-8.1GHz) are insertion losses (S21) are -0.07dB/-0.21dB/-0.12dB,and return losses (S11) are -31dB/15dB/30dB respectively. The response of the filter was simulated using An soft HFSS Simulator.

10 Spatial mode transmission using low differential mode delay 6-LP fiber using all-fiber photonic lanterns.

To unlock the cost benefits of space division multiplexing transmission systems, higher spatial multiplicity is required. Here, we investigate a potential route to increasing the number of spatial mode channels within a single core few-mode fiber. Key for longer transmission distances and low computational complexity is the fabrication of fibers with low differential mode group delays. As such in this work, we combine wavelength and mode-division multiplexed transmission over a 4.45 km low-DMGD 6-LP-mode fiber by employing low-loss all-fiber 10-port photonic lanterns to couple light in and out of the fiber. Hence, a minimum DMGD of 0.2 ns (maximum 0.357 ns) is measured after 4.45 km. Instrumental to the multi-mode transmission system is the employed time-domain-SDM receiver, allowing 10 spatial mode channels (over both polarizations) to be captured using only 3 coherent receivers and real-time oscilloscopes in comparison with 10 for conventional methods. The spatial channels were unraveled using 20 × 20 multiple-input multiple-output digital signal processing. By employing a novel round-robin encoding technique, stable performance over a long measurement period demonstrates the feasibility of 10x increase in single-core multi-mode transmission.

Delay Spread Reduction in Mode-Division Multiplexing: Mode Coupling Versus Delay Compensation

Reduction of the group delay (GD) spread is crucial for minimizing signal processing complexity in mode-division multiplexing. Strong mode coupling and GD compensation (concatenating different fibers with opposing GD ordering) are two approaches for reducing the end-to-end GD spread. In this paper, we study the GD behavior in systems where mode coupling and GD compensation are both present. Using a propagation model in generalized Stokes space, we describe the evolution of the GD variance by coupled differential equations. By integration of these equations, we evaluate the GD variance in GD compensated systems with different mode coupling lengths and GD compensation lengths. When the mode coupling length is much longer than the GD compensation length, a low GD variance can be obtained as a result of GD compensation. By contrast, when the mode coupling length is much shorter than the GD compensation length, GD compensation becomes ineffective, but a low GD variance can be obtained as a result of strong mode coupling. The largest GD variance is obtained when the mode coupling length is comparable to the GD compensation length.

Analysis and design of miniaturized multisection crossover with open stubs

ABSTRACTA compact branch‐line crossover using open stubs structure is proposed. Even and odd mode analysis is adopted to derive the analytical formulas for a crossover with wide bandwidth and a compact size. The proposed crossover operating at 2.5 GHz has a fractional bandwidth of 23% (from 2.2 to 2.78 GHz) with high isolation, a high return loss, and low insertion loss. It also has a low group delay deviation of about 0.38 ns. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:2673–2677, 2015

Low-Differential-Mode-Group-Delay 9-LP-Mode Fiber

We report the optimization of a few-mode fiber, adapted to multiple-input multiple-output mode division multiplexing, that supports 9 LP modes (15 spatial modes). The fiber is fabricated using standard multimode processes, which allows for tight process tolerances, yielding low differential mode group delays (<155 ps/km) with low attenuations (<0.22 dB/km), large effective areas (≥95 μm2) and low bend losses.

11.25-ms-Group-Delay and Low-Complexity Algorithm Design of 18-Band Quasi-ANSI S1.11 1/3 Octave Digital Filterbank for Hearing Aids

This paper presents a novel algorithm and architecture design for 18-band quasi-class-2 ANSI S1.11 1/3 octave filterbank. The proposed design has several advantages such as lower group delay, lower computational complexity, and lower matching error. The technique we developed in this paper can be summarized as follows: 1) a simple low-pass filter (LPF) and discrete cosine transform (DCT) modulation are utilized to generate a uniform 9-band filterbank first, and then all elements of z <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> are replaced by all-pass filters to obtain a non-uniform filterbank; 2) a fast recursive structure and variable-length algorithm is further developed to efficiently accomplish DCT modulation. Thus, the spectrum of LPF can be easily spanned and flexibly extended to the location of the desired central frequency; 3) after employing the multi-rate algorithm, an 18-band non-uniform filterbank is generated from two 9-band sub filterbanks by following the proposed design steps and parameter determinations. Compared with the latest Liu 's quasi-class-2 ANSI S1.11 design, the proposed method-I (Proposed-I) totally has 72.8% reduction for multiplications per sample, 11.25-ms group delay, and 59 additions decreased per sample. Moreover, the maximum matching error of the proposed method-II (Proposed-II) is averagely equal to 1.79 dB much smaller than that of the latest Wei 's design. For the proposed variable-length DCT modulation, only 2 adders, 2 multipliers, 2 multiplexers, and 5 registers are required for hardware implementation after applying VLSI retiming scheme. Overall, the proposed filterbank design would be a new solution for future applications in the area of hearing aids.

High selectivity and common‐mode suppression wideband balanced bandpass filter using slotline resonator

Novel wideband balanced bandpass filter with high selectivity and common-mode (CM) suppression is constructed with slotline resonator. When the intersection points of the microstrip feedline and slotline resonator are close to the midpoint of the slotline resonator, the coupling between the fundamental mode of the slotline resonator and microstrip feedline can be maximised, whereas the second mode can be suppressed. Making use of this feature, the third-order wideband filter with low insertion loss, wide upper-stopband and flat group delay is designed by using the slotline resonator and two quarter-wavelength microstrip resonators paralleled to the slotline resonator with tapped-line coupling at their short-circuited ends. Based on the third-order single-ended filter structure, a novel balanced filter with a 1 dB fractional bandwidth up to 72% is designed with low insertion loss and enhanced CM suppression. To improve the selectivity, the fifth-order wideband balanced filter with wide upper-stopband is designed with parallel coupling structure between two slotline resonators. The corresponding CM suppression is better than 42 dB within the whole differential mode passband. The wideband balanced filter is designed and fabricated, and good agreement between measurements and simulations is obtained.

Measurements of differential group delay and chromatic dispersion for LP01 and LP11 modes of few-mode fibers with depressed claddings

The chromatic dispersion of the LP11 mode in a few-mode fiber with low differential group delay is evaluated experimentally. A differential group delay of less than 100 ps/km around 1550 nm can be evaluated with the modal interferometer method. The white-light interferometric method confirms that the LP11 mode is behind the LP01 mode in the 1300–1600 nm wavelength region. The difference between the chromatic dispersion values of the two modes is less than 0.5 ps/km/nm, which means that the optical pulse spreading characteristics of the LP11 mode are similar to those of the LP01 mode for fibers with depressed claddings.

All-Optical Continuously Tunable Flat-Passband Microwave Photonic Notch Filter

A new all-optical microwave photonic notch filter (MPNF) is presented. It is based on controlling the amplitude and phase of the optical carriers and RF phase modulation sidebands via a diffraction-based Fourier-domain optical processor. It has the ability of realizing a flat-passband, narrow-notch, and large-free-spectral-range amplitude response and a group delay response with very few ripples. The carrier and sideband optical phase controls enable a continuous notch frequency tuning operation to be realized without altering the response shape. The filter has a simple structure and a wide bandwidth as it only involves optical components. Experimental results demonstrate an MPNF that exhibits a flat passband with <; 1 dB ripples, a deep notch of > 35 dB, and low group delay ripples of <; ±25 ps, together with the continuous notch frequency tunability and excellent long-term stability and repeatability.

Time-division-multiplexed few-mode passive optical network.

We demonstrate the first few-mode-fiber based passive optical network, effectively utilizing mode multiplexing to eliminate combining loss for upstream traffic. Error-free performance has been achieved for 20-km low-crosstalk 3-mode transmission in a commercial GPON system carrying live Ethernet traffic. The alternative approach of low modal group delay is also analyzed with simulation results over 10 modes.

Minimum Phase Property of Chebyshev-Sharpened Cosine Filters

We prove that the Chebyshev sharpening technique, recently introduced in literature, provides filters with a Minimum Phase (MP) characteristic when it is applied to cosine filters. Additionally, we demonstrate that cascaded expanded Chebyshev-Sharpened Cosine Filters (CSCFs) are also MP filters, and we show that they achieve a lower group delay for similar magnitude characteristics in comparison with traditional cascaded expanded cosine filters. The importance of the characteristics of cascaded expanded CSCFs is also elaborated. The developed examples show improvements in the group delay ranged from 23% to 47% at the cost of a slight increase of usage of hardware resources. For an application of a low-delay decimation filter, the proposed scheme exhibits a 24% lower group delay, with 35% less computational complexity (estimated in Additions per Output Sample) and slightly less usage of hardware elements.

Compact planar UWB filter using cascaded resonators

This paper presents a planar ultra wide band filter employing chip inductors and complementary split ring resonators. Asymmetric interdigital coupled lines are used for achieving bandpass characteristics. Lower side transmission zeros are introduced using LC resonators consisting of chip inductors and circular patch capacitors. The filter has low insertion loss and flat group delay in its passband, steep roll off rate and good stopband attenuation. Measurement results for transmission and reflection characteristics of the fabricated filter are in good agreement with simulated response.

Wideband SiGe BiCMOS transimpedance amplifier for 20 Gb/s optical links

A novel bandwidth enhancement technique based on a modified regulated cascode (RGC) transimpedance amplifier (TIA) is proposed and realized. The post stage of the TIA adopts capacitive degeneration and π-type peaking network, maintaining the wideband and low group delay characteristics without increasing power dissipation. The TIA is implemented in a 0.18 µm SiGe BiCMOS technology and tested with an on-chip 300 fF capacitor to emulate a photodiode. The measured transimpedance gain is 61 dBΩ with a −3 dB bandwidth of 15 GHz. The chip consumes 32 mW power from a single 3.3 V supply and occupies the area of only 0.3 mm2.