Adaptive unsaturated technique for OFDM multimode fiber communication system

A novel adaptive unsaturated technique is proposed for Orthogonal Frequency Division Multiplexing (OFDM) multimode fiber communication. The core idea of this technique is that bits originally allocated to poor subcarriers are now transmitted by new extra subcarriers rather than the original good subcarriers used in adaptive modulation OFDM. It can reduce the system bit error rate (BER) which is mainly caused by some OFDM subcarriers located at the deep nulls in the high frequency region of multimode fiber. The simulation results indicate that adaptive unsaturated technique is more effective in reducing the BER of system than adaptive modulation and equal bit allocation. Moreover, adaptive unsaturated technique does not need complex bit allocation algorithm and each subcarrier has the same modulation format, so it is simple and practical.     

Application of Turbo codes in optical OFDM multimode fiber communication system

In this paper, Turbo Code is proposed in optical OFDM multimode fiber communication system in order to decrease the bit error rate (BER) of the system, which is mainly affected by the deep nulls of the magnitude response of multimode fiber in the high frequency region (above 3 dB). A simulation system in SIMULINK is established. Based on the system, the BER of the system with Turbo Code is compare to the systems with another two typical coding schemes including convolutional code (CC) and serially concatenated code (SCC) which uses a concatenation of convolutional and RS codes when transmitting 10 Gbps data over various length multimode fibers. Different transmitting rate is also considered. The results show that Turbo coded system has a lower BER than the other two systems and the Turbo coded system can transmit 10 Gbps data to the distance of 300 m with BER below 1e−6.     

A novel bits loading algorithm for adaptive modulation OFDM multimode fiber communication system

A novel bits loading algorithm is proposed for adaptively modulated optical OFDM (AMOOFDM) multimode fiber communication system to decrease the system bit error rate (BER) caused by some OFDM subcarriers located at the deep nulls in the high frequency region of multimode fiber. The simulation results show that the performance of the new algorithm is approximate to the optimal algorithm-Greedy algorithm, but its computational complexity is considerably lower, thus the new algorithm is easy to implement. Comparing to fixed bits loading, adaptive bits loading can reduce the system BER effectively.     

Allocation Algorithm for Optical OFDM Multimode Fiber Communication System

Abstract

A fast bits allocation algorithm is proposed for adaptively modulated optical orthogonal frequency division multiplexing multimode fiber communication system to decrease the system bit error rate caused by some orthogonal frequency division multiplexing subcarriers being located at the deep nulls in the high-frequency region of multimode fiber. Based on a lookup table scheme, the new algorithm dramatically decreases the computational complexity of the Greedy algorithm. The simulation results show that the new algorithm has the same allocation result as the Greedy algorithm, but the computational complexity is considerably lower. In addition, adaptive bits allocation can effectively reduce the system bit error rate.     

High-gain composite microstrip patch antenna with the near-zero-refractive-index metamaterial

A high-gain rectangular microstrip patch antenna which is covered by a single layer metamaterial (MTM) superstrate with the near zero refractive index is proposed. The refraction of the metamaterial at frequency 3.51 GHz–3.57 GHz is very close to zero. The metamaterial with the near zero refractive index is placed 42 mm above an ordinary rectangular microstrip patch antenna. The effectively zero refractive index behavior of metamaterial superstrate can gather the wave emitted from the microstrip patch antenna and collimate it toward the normal direction of the antenna. The finite element method (FEM) and the finite difference time domain (FDTD) method are used to study the characteristics of this antenna. The results of the two methods indicate that the realized gain of the proposed antenna is increased by more than 6 dB, and the antenna has a flatness high gain in the predicted frequency band, where the proposed MTM is designed to have a near zero index of refraction. Therefore, the high-gain antenna is effectively enhanced based on the near-zero-refractive-index metamaterial.     

Reduction of frequency fading and imperfect frequency response with pre-emphasis technique in OFDM-ROF systems

In this paper, a novel technique is proposed and experimentally demonstrated to reduce the effect of frequency fading (FF) and imperfect frequency response in direct-detection (DD) optical orthogonal frequency division multiplexing-radio-over-fiber (OFD-MROF) systems. To overcome FF effect in the optical fiber and imperfect frequency response in the optical and electrical devices at the high frequency, we pre-emphasize the power of the millimeter wave (mm-wave) OFDM sub-carriers appropriately in the center station. Experimental result of the proposed system shows the received sensitivity has been improved about 2 dB at the BER of 1 × 10^− 4 after 50 km SSMF transmission for 2.5 Gb/s OFDM signal carried on 60 GHz optical mm-wave compared to the original system without pre-emphasis technique.     

The improvement of Nyquist pulse shaping for all-optical OFDM system in multi-users network

The all-optical orthogonal frequency division multiplexing has a better spectral efficiency and a lower response requirement of modulators for high capacity transmission. In the system, the optical filter will degrade the performance of subcarriers which are far away from the center carrier. We proposed an improvement method of all-optical OFDM scheme using Nyquist pulse shape in the pulse source generator. Comparing a Nyquist shape pulse with a Gauss pulse in a 4 × 100 Gb/s DP-QPSK all-optical sampling OFDM system, the side lobe of transmitted spectrum can be effective suppressed, and the optical power will be more focused on the effective frequency band. By coherent receiver, the results show that the Nyquist pulse shaping can improve the OSNR and transmission performance of subcarriers which deviate mostly from the center frequency of optical filter. This improvement is of great benefit for multi-users system.     

Cost-effective and dispersion-tolerant cascade modulation for millimeter-wave-over-fiber applications

In millimeter-wave-over-fiber (MWoF) feeder systems, the received millimeter-wave signals at the remote antennas (RAs) can suffer from signal fading by chromatic dispersion of optical fiber. This can be substantially mitigated by Mach-Zehnder modulator (MZM) based photonic up-conversion technique. In this technique, the data signals at intermediate frequency (IF) are frequency up-converted to millimeter-wave frequency by an MZM biased at its transmission null point. However, this scheme requires a costly, high-speed MZM, which will hinder the widespread of this technique for cost-sensitive MWoF applications. Hence, we propose and demonstrate a cost-effective way of reducing the cost of MWoF optical transmitters based on photonic up-conversion technique. We employ a dual wavelength source composed of a directly modulated laser and a polarimetric filter. This source is used to generate a millimeter-wave tone signal and to frequency up-convert the IF data signals to millimeter-wave frequency. The dual wavelength source is also shared with numerous RAs for further cost reduction. Our experimental demonstration performed with 30 Msymbol/s 16-quadrature amplitude modulation signals shows that we can transmit the 20 GHz millimeter-wave signals over 25 km standard single-mode fiber without any transmission penalty.     

Enhanced performance for 10 Gb/s long reach RSOA based WDM-PON by using power pre-emphasized OFDM signal

An extended reach 10 Gb/s wavelength division multiplexing passive optical networks (WDM-PONs) system based on reflective semiconductor optical amplifier (RSOA) is proposed by using power pre-emphasized orthogonal frequency division multiplexing (OFDM) signal. Experimental results show that the proposed technique can effectively enhance the system performance against the limited bandwidth and chirp induced fading effect from direct modulation of RSOA. The receiver sensitivity is improved by 5 dB at the limit of BER for forward error correction (FEC) code over the 60 km and 85 km fiber transmission without any dispersion compensation module.     

Gaussian wavelet based dynamic filtering (GWDF) method for medical ultrasound systems

In this paper, a novel dynamic filtering method using Gaussian wavelet filters is proposed to remove noise from ultrasound echo signal. In the proposed method, a mother wavelet is first selected with its central frequency (CF) and frequency bandwidth (FB) equal to those of the transmitted signal. The actual frequency of the received signal at a given depth is estimated through the autocorrelation technique. Then the mother wavelet is dilated using the ratio between the transmitted central frequency and the actual frequency as the scale factor. The generated daughter wavelet is finally used as the dynamic filter at this depth. Frequency-demodulated Gaussian wavelet is chosen in this paper because its power spectrum is well-matched with that of the transmitted ultrasound signal. The proposed method is evaluated by simulations using Field II program. Experiments are also conducted out on a standard ultrasound phantom using a 192-element transducer with the center frequency of 5 MHz. The phantom contains five point targets, five circular high scattering regions with diameters of 2, 3, 4, 5, 6 mm respectively, and five cysts with diameters of 6, 5, 4, 3, 2 mm respectively. Both simulation and experimental results show that optimal signal-to-noise ratio (SNR) can be obtained and useful information can be extracted along the depth direction irrespective of the diagnostic objects.     

Performance of directly modulated OFDM data over multimode fiber link

The performance of orthogonal-frequency-division-multiplexing (OFDM) technology in the multimode fiber (MMF) optical communication system is investigated numerically. The mode dispersion in MMF is analyzed by utilizing the optical angular spectrum analysis method. Simulation results in terms of bit-error-rate (BER) and signal-to-noise ratio (SNR) show that the transmission distance can reach up to 2.5-km at 10^− 3 BER for 40-Gb/s data. Results of different modulation formats further indicate that better performance can be achieved by adopting multi-level modulation format.     

Acceleration target detection based on LFM radar

In radar systems, the echo signal caused by an accelerated target can be similarly considered as linear frequency modulation (LFM) signal. In high signal-to-noise ratio (SNR), discrete polynomial-phase transform (DPT) algorithm can be used to detect the echo signal, as it has low computation complexity and high real-time performance. However, in low SNR, the DPT algorithm has a large mean square error of the rate of frequency modulation and a low detection probability. In order to detect LFM signal in low SNR, this paper proposes a detection method, segment discrete polynomial-phase transform (SDPT), which means, at first, dividing the whole echo pulses into several segments with same duration in time domain, and then, using coherent accumulation method of DFT to segments, at last, processing this signal with DPT in intra-segment. In the case of a large number of segments, the SDPT can improve the output SNR. In addition, in a certain SNR, to the target signal with big sampling interval, large acceleration and less segments, this paper proposes an algorithm to detect the LFM signal generated from the combination of an improved DPT (IDPT) and fractional Fourier transform (FRFT). The output SNR of this algorithm is connected with the length of time delay. In the simulation, when the length of the time delay is 0.2 N, the output SNR is 2.5 dB more than that which results from directly using DPT. Finally, the detection performance and algorithm complexity of the proposed algorithm were analyzed, and the simulated and measured data verify the effectiveness of the algorithm.     

Modeling of the write and read back performances of hexagonal Ba-ferrite particulate media for high density tape recording

In this study, the signal-to-noise ratio (SNR) performances of longitudinally, randomly, and perpendicularly oriented particles, based on hexagonal barium ferrite (h-BaFe) platelets with an average volume of 2400 nm³ have been studied as a function of the recording head to media distance by numerical micromagnetic simulations. The distances from the write head to media and from the read head to media were varied independently. For a fixed read distance and varied writing distances, the SNR was decreasing in larger write distance. An optimum write distance of 40 and 50 nm was found for the longitudinally oriented media and the perpendicularly oriented media, respectively. The optimum write distance for longitudinally oriented media, 40 nm, resulted in the local minimum SNR for the perpendicularly oriented media. In most write distances the perpendicularly oriented media show the outstanding best performance, but near the write distance of 40 nm the longitudinally oriented media work as good as the perpendicularly oriented media. In a fixed write distance with various read distances, the SNR was almost constant in each media whereas the average signal amplitude was exponentially decayed in larger read head to media distance. The best SNR was found in the perpendicularly oriented media at write head to media distance d_write=20 nm and read head to media distance d_read=40 nm. The best SNR value is 11.9 and 24.4 dB in time domain and frequency domain, respectively.     

High precision line profile measurements on C acetylene using a near infrared frequency comb spectrometer

Line profiles of a rovibrational transition of ¹³C acetylene have been measured for various pressures in the near infrared region. In order to accomplish high precision in frequency, we have employed a diode-laser, the frequency of which is locked to an optical comb. By tuning the comb frequency we have achieved a continuous frequency tuning over 2 GHz for the measurement of Doppler broadened line profiles spread over 2 GHz. In addition we have stabilized the incident power of the laser on the sample cell by adjusting the gain of a fiber amplifier via a feed-back loop. Observed profile data have been analyzed by the generalized Voigt function to determine the Gaussian width precisely: at the present we realized a precision of 10^-3. The zero pressure line center position was determined with a precision of 10^-9.     

Tunable dual-wavelength terahertz wave power splitter

We demonstrate, for the first time, a tunable dual-wavelength terahertz wave power splitter based on the multimode interference effect and self-imaging principle in 1 × 2 × 2 photonic crystal waveguides. Both plane wave expansion method and finite-difference time-domain method are used to calculate and analyze the characteristics of the proposed device. The simulation results demonstrate that the power splitter not only split the input power into output1 and output2 branches with equal power at frequency of 1.09 THz, but also split the power into output3 and output4 branches symmetrically at frequency of 1.20 THz. Furthermore, for the frequency of 1.09 THz, the input terahertz wave power can be split into output1 and output2 branches with an arbitrary ratio by tuning the refractive index of the tuning rods.     

Magnetostriction trend of non-oriented 6.5% Si–Fe

Non-oriented electrical steel is produced in strip form typically 0.35–1.0 mm thick and containing 0–3 wt% silicon. It is well-known that non-oriented electrical steel is not quite isotropic but has small anisotropy. In the last decade, NKK produced 0.1 mm thick, non-oriented steel 6.5% Si which has applications such as in high-frequency transformer due to its high electrical resistivity, low core losses, near zero magnetostriction, and high permeability. The magnetostriction of 6.5% silicon steel samples with dimensions 280 mm×30 mm×0.1 mm was measured when magnetised sinusoidally between 0.5 and 1.0 T at frequencies between 0.5 and 6 kHz. Test samples were clamped at one end and the peak-to-peak displacement of the free end was measured with the aid of the single-point laser vibrometer. The average peak–peak magnetostriction was 0.2–0.25 με apart from a sharp rise to 1.2 με at 2 kHz magnetising frequency. This agrees well with the predicted value of 2 kHz for l=0.28 m, d=7430 kg/m³ and E=166 GPa. This shows that although the 6.5% silicon steel is often thought of as having near zero magnetostriction, care is needed to avoid lamination lengths corresponding to resonance points which could induce higher noise in laminated cores.     

Generation of radio signals using a novel Mach-Zehnder modulator with four arms

We have proposed and demonstrated a novel Mach-Zehnder modulation technique which employs a 1 × 4 multimode interference MMI coupler and four optical phase-modulator waveguides to generate optical single sideband (SSB) signals in radio-over-fiber (ROF) transmission link. It is shown that when the RF (radio frequency) modulation index is large, the optical SSB signal generated by conventional modulation scheme contains a significant part of undesired higher order harmonics, accordingly, much distortion in the RF signal was detected at the base station (BS). However, the main undesired higher order harmonics can be suppressed using our proposed modulation scheme and the performance of the transmission links were largely improved in single-channel and dense wavelength-division multiplexing (DWDM) cases.     

High-power multi-mode laser to single-mode pump conversion

A novel pump-sharing module for multi-channel amplifiers based on integration of high-power multimode semiconductor lasers and planar waveguide circuits is proposed. The feedback-free modules are designed for optimum coupling, throughput and power uniformity on silicon using BPM method and fabricated via the sol-gel process of photo patternable precursors. Modal properties of the multimode lasers are considered by studying spectrum and spatial distribution of laser output power at different temperatures. Employing an 1500 mW 980 nm multimode laser (TE polarized), via lens and Butt coupling up to 1250 mW of laser output is coupled into the multimode channel waveguides, which are tapered and branched to four single mode waveguides. The measured pump power from splitter outputs, randomly polarized, exceeds 600 mW. At constant temperatures ranging from 22 °C to 30 °C, the pump-sharing modules exhibit good stability with a power uniformity of <0.5 dB over hours.     

60 GHz Radio over Fiber Technology for Wireless Access by employing Narrow-Angle PSK Modulation

This paper investigates two key techniques used in 60 GHz Radio over Fiber (RoF) technology for wireless access, namely, the generation of 60 GHz signals and the distribution of 60 GHz local oscillator (LO). In the proposed model, a 60 GHz PSK signal is generated by a heterodyne of two subcarriers with narrow-angle PSK (NA-PSK) modulation, whose phase shift is equal to one half that of a normal PSK signal. Then we use a 60 GHz mixer to frequency mix two PSK signals in two different bands, 60 GHz and baseband. By doing this, the modulation information can be fully eliminated, resulting in a 60 GHz LO. In the wireless terminal, coherent demodulation is realized by a self-mixing of the transmitted 60 GHz PSK signal and LO. Thus no millimeter-wave (mm-wave) band oscillator is needed in the wireless terminal.