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A time-frequency-varying multicarrier generation scheme based on dynamic single-sideband recirculating frequency shifter is proposed and experimentally demonstrated. Through the use of two time-domain sinusoidal signals with stationary phase shift provided by a phase control module, multicarriers with flexible frequency intervals are obtained without carrier-to-noise ratio and flatness loss. We have experimentally attained multicarriers with 10-/12.5-/15-GHz carrier intervals in 3-nm bandwidth, and carriers with arbitrary intervals can be achieved when using the proposed scheme combined with a flexible wavelength selective switch. [ABSTRACT FROM AUTHOR]

A self-adaptive probabilistically shaped (PS) star-carrier-less amplitude/phase (CAP)-24 modulated passive optical network (PON) is proposed to adaptively adjust the probability distribution of the signal points. The proposed scheme is capable of accommodating different channels and transmission distances to obtain the optimal probability distribution. The employment of the simulated annealing algorithm in the constellation can significantly reduce the average signal power, as well as facilitating the flexible deployment and enhancement of the PON system. The experimental results show that the self-adaptive optimal signal transmission can be acquired when the probability ratio of signal points located in the three well-designed rings of the CAP-24 constellation is set as 0.724:0.175:0.101. Compared with the general PS-star-CAP-32 signal, the self-adaptive PSstar- CAP-24 signal has a gain of 1.5 dB at the 7% hard decision forward error correction (HD-FEC) limit. What is more, the proposed PS self-adaptive PS-star-CAP-24 can outperform the general PS-star-CAP-24 constellation by 0.2 dB at the 7% HD-FEC limit. [ABSTRACT FROM AUTHOR]

A rate-adaptive multilevel coded modulation (RA-MLC) scheme based on fixed code length and a corresponding decoding scheme is proposed. RA-MLC scheme combines the multilevel coded and modulation technology with the binary linear block code at the transmitter. Bits division, coding, optional interleaving, and modulation are carried out by the preset rule, then transmitted through standard single mode fiber span equal to 100 km. The receiver improves the accuracy of decoding by means of soft information passing through different layers, which enhances the performance. Simulations are carried out in an intensity modulation-direct detection optical communication system using MATLAB®. Results show that the RA-MLC scheme can achieve bit error rate of 1E-5 when optical signal-to-noise ratio is 20.7 dB. It also reduced the number of decoders by 72% and realized 22 rate adaptation without significantly increasing the computing time. The coding gain is increased by 7.3 dB at BER = 1E-3. [ABSTRACT FROM AUTHOR]

A photonic approach to generating square waveforms using a single dual-parallel Mach-Zehnder modulator driven by a single-tone radio frequency signal is proposed and demonstrated. The two sub MZM are set to be biased at the minimum transmission point and the maximum transmission point, respectively. By simply adjusting the modulation index to have a proper value, optical intensity with expression corresponding to the Fourier expansion of a square waveform is obtained, and square waveforms will be generated. Neither an optical filter nor an electrical component of frequency multiplier or hybrid is needed to guarantee the simplicity and the wide working frequency range. A proof-of-concept experiment is taken. Both 1- and 4-GHz square waveforms fitting well with the ideal ones are successfully generated. [ABSTRACT FROM AUTHOR]

We experimentally investigate the polarization insensitivity and cascadability of an all-optical wavelength converter for differential phase-shift keyed (DPSK) signals for the first time. The proposed wavelength converter is composed of a one-bit delay interferometer demodulation stage followed by a single semiconductor optical amplifier. The impact of input DPSK signal polarization fluctuation on receiver sensitivity for the converted signal is carried out. It is found that this scheme is almost insensitive to the state of polarization of the input DPSK signal. Furthermore, the cascadability of the converter is demonstrated in a two-path recirculating loop. Error-free transmission is achieved with 20 stage cascaded wavelength conversions over 2800 km, where the power penalty is <3.4 dB at bit error rate of 10-9. [ABSTRACT FROM AUTHOR]

Elastic software-defined optical networks greatly improve the flexibility of the optical switching network while it has brought challenges to the routing and spectrum assignment (RSA). A multilayer virtual topology model is proposed to solve RSA problems. Two RSA algorithms based on the virtual topology are proposed, which are the ant colony optimization (ACO) algorithm of minimum consecutiveness loss and the ACO algorithm of maximum spectrum consecutiveness. Due to the computing power of the control layer in the software-defined network, the routing algorithm avoids the frequent link-state information between routers. Based on the effect of the spectrum consecutiveness loss on the pheromone in the ACO, the path and spectrum of the minimal impact on the network are selected for the service request. The proposed algorithms have been compared with other algorithms. The results show that the proposed algorithms can reduce the blocking rate by at least 5% and perform better in spectrum efficiency. Moreover, the proposed algorithms can effectively decrease spectrum fragmentation and enhance available spectrum consecutiveness. [ABSTRACT FROM AUTHOR]

The wavelength-division multiplexing passive optical network (WDM-PON) is a potential technology to carry multiple services in an optical access network. However, it has the disadvantages of high cost and an immature technique for users. A software-defined WDM/time-division multiplexing PON was proposed to meet the requirements of high bandwidth, high performance, and multiple services. A reasonable and effective uplink dynamic bandwidth allocation algorithm was proposed. A controller with dynamic wavelength and slot assignment was introduced, and a different optical dynamic bandwidth management strategy was formulated flexibly for services of different priorities according to the network loading. The simulation compares the proposed algorithm with the interleaved polling with adaptive cycle time algorithm. The algorithm shows better performance in average delay, throughput, and bandwidth utilization. The results show that the delay is reduced to 62% and the throughput is improved by 35%. [ABSTRACT FROM AUTHOR]

An architecture for flattened and broad spectrum multicarriers is presented by generating 60 comb lines from pulsed laser driven by user-defined bit stream in cascade with three modulators. The proposed scheme is a cost-effective architecture for optical line terminal (OLT) in wavelength division multiplexed passive optical network (WDM-PON) system. The optical frequency comb generator consists of a pulsed laser in cascade with a phase modulator and two Mach-Zehnder modulators driven by an RF source incorporating no phase shifter, filter, or electrical amplifier. Optical frequency comb generation is deployed in the simulation environment at OLT in WDM-PON system supports 1.2-Tbps data rate. With 10-GHz frequency spacing, each frequency tone carries data signal of 20 Gbps-based differential quadrature phase shift keying (DQPSK) in downlink transmission. We adopt DQPSK-based modulation technique in the downlink transmission because it supports 2 bits per symbol, which increases the data rate in WDM-PON system. Furthermore, DQPSK format is tolerant to different types of dispersions and has a high spectral efficiency with less complex configurations. Part of the downlink power is utilized in the uplink transmission; the uplink transmission is based on intensity modulated on-off keying. Minimum power penalties have been observed with excellent eye diagrams and other transmission performances at specified bit error rates. [ABSTRACT FROM AUTHOR]

We propose a technique for the generation of optical frequency comb from a single source, which reduces the costs of optical access networks. Two Mach-Zehnder modulators are cascaded with one phase modulator driven by radiofrequency signals. With 10-GHz frequency spacing, the generated 40 optical multi-carriers have good tone-to-noise ratio with least excursions in their comb lines. The laser array at the optical line terminal of the conventional wavelength division multiplexed passive optical network (WDM-PON) system has been replaced with optical frequency comb generator (OFCG), which may result in cost-effective optical line terminal (OLT) supporting a large-capacity WDM-PON system. Of 40 carriers generated, each carrier carries 10 Gbps data based on differential phase-shift keying. Four hundred Gbps multiplexed data from all channels are successfully transmitted through a fiber span of 25 km with negligible power penalties. Part of the downlink signal is used in uplink transmission at optical network unit where intensity-modulated on-off keying is deployed for remodulation. Theoretical analysis of the proposed WDM-PON system based on OFCG are in good agreement with simulation results. The metrics considered for the analysis of the proposed OFCG in a WDM-PON system are power penalties of the full-duplex transmission, eye diagrams, and bit error rate. [ABSTRACT FROM AUTHOR]

We propose a theoretical mode division multiplexing (MDM) transmission model in a few mode fibers and numerically simulate the effects of mode coupling and modal dispersion on the transmitted signal. An adaptive signal processing with multiple-input multiple-output equalizers based on recursive least squares constant modulus algorithm (CMA) is proposed and demonstrated. The simulation results show that the mode signals are equalized sufficiently with a faster convergent speed than regular CMA. The equalization algorithm presented is more adaptive to complicated channels and applicable to the MDM system. [ABSTRACT FROM AUTHOR]

A hierarchical modulation with multilevels is proposed for an optical single-carrier frequency division multiplexing access (SC-FDMA) system. It can mitigate the nonlinearity by reducing the peak-to-average power ratio (PAPR) of the SC-FDM signal. According to different optical signal-to-noise ratio requirements, the adaptive bit allocation can be implemented on different levels during hierarchical modulation. In the experiment, the PAPR of the hierarchical-modulated SC-FDM signal outperforms the conventional SC-FDM signal by 0.7 dB. Signals with 4- and 6-bit hierarchical modulation are successfully demodulated by the optical network unit with power penalties less than 0.2 and 0.45 dB, respectively. [ABSTRACT FROM AUTHOR]

In an optical multiple input multiple output (MIMO) transmission system, mode coupling and mode dispersion have a significant impact on the signal recovery processes at the receiver. We describe a polarizationbased MIMO transmission by using mode division multiplexing and polarization division multiplexing over fewmode multicore fiber. We used three optical modes with x-y polarizations for 6 × 6 MIMO transmission over 75 km length of few-mode multicore fiber at 20 Gbps quadrature phase shift keying signals. Optical MIMO transmission is successfully recovered by using a recursive least squares constant modulus algorithm (RLSCMA). The simulation results show that RLSCMA enhanced the system performance of optical MIMO transmission system by employing sufficient adaptive equalization. [ABSTRACT FROM AUTHOR]

We propose an all-fiber electric field sensor using a U-bent single-mode fiber integrated with liquid crystal. The sensing mechanism is based on the interference between whispering-gallery modes and core mode in U-shaped bent fibers. We have experimentally investigated the influences of applied electric voltage as well as polarization state of the incident light on the transmission spectral characteristics of the proposed electric field sensor. The experimental results indicate that the transmission spectra are highly sensitive to the applied voltage and the highest sensitivity reaches 20.4221 nm/kV. The dip wavelengths and transmission loss at transmission dip exhibit a periodic variation behavior in response to the light polarization state rotation by 360 deg, which is in agreement with our theoretical analysis. [ABSTRACT FROM AUTHOR]

We present a large-core single-mode “windmill” single crystal sapphire optical fiber (SCSF) design, which exhibits single-mode operation by stripping off the higher-order modes (HOMs) while maintaining the fundamental mode. The “windmill” SCSF design was analyzed using the finite element analysis method, in which all the HOMs are leaky. The numerical simulation results show single-mode operation in the spectral range from 0.4 to 2 μm in the windmill SCSF, with an effective core diameter as large as 14 μm. Such fiber is expected to improve the performance of many of the current sapphire fiber optic sensor structures. [ABSTRACT FROM AUTHOR]

A type of single crystal sapphire optical fiber (SCSF) design is proposed to reduce the number of guided modes via a highly dispersive cladding with a periodic array of high- and low-index regions in the azimuthal direction. The structure retains a "core" region of pure single crystal (SC) sapphire in the center of the fiber and a "cladding" region of alternating layers of air and SC sapphire in the azimuthal direction that is uniform in the radial direction. The modal characteristics and confinement losses of the fundamental mode were analyzed via the finite element method by varying the effective core diameter and the dimensions of the "windmill"-shaped cladding. The simulation results showed that the number of guided modes was significantly reduced in the windmill fiber design, as the radial dimension of the air and SC sapphire cladding regions increase with corresponding decrease in the azimuthal dimension. It is anticipated that the windmill SCSF will readily improve the performance of current fiber optic sensors in the harsh environment and potentially enable those that were limited by the extremely large modal volume of unclad SCSF. [ABSTRACT FROM AUTHOR]

An acousto-optic fiber interferometer has been proposed and experimentally demonstrated by employing two MgF2 sandwiches to implement concatenated flexural acoustic wave modulation onto singlemode optical fibers. The transmission spectrum of the acoustic grating pair has been experimentally investigated. Experimental results indicate that interferometric spectral fringes possess a frequency sensitivity as large as -499.0 nm/MHz due to the Mach-Zehnder interference. Moreover, the applied radio frequency signal voltage for flexural wave generation has a great impact on the transmission spectral properties. The work presented would be of importance for the understanding of the acousto-optic interaction mechanism in concatenated acoustic fiber gratings and is helpful for the design of related acousto-optic fiber devices. [ABSTRACT FROM AUTHOR]

An acoustic birefringence suppression scheme for acoustic fiber gratings has been proposed and experimentally demonstrated. Experimental observation of the transmission characteristics of the single-mode-fiber-based (SMF-based) acoustic grating confirms that introduction of one segment of solid-core photonic crystal fiber (PCF) with air-hole array in the cladding region helps to eradicate resonance peak splitting induced by acoustic birefringence, which has been explained by our theoretical analysis from the acoustic wave modulation method and fiber cross-section structure. Moreover, experimental results indicate that the introduction of the solid-core PCF does not have a considerable impact on the frequency dependence of resonance wavelength for the SMF-based acoustic fiber grating, which ensures the performances of the PCF-assisted SMF-based acousto-optic tunable filter for practical applications. [ABSTRACT FROM AUTHOR]