A secure scheme based on Rubik’s Cube (RC) transformation algorithm with low calculation complexity is proposed and experimentally demonstrated in the orthogonal frequency division multiplexing passive optical network system (OFDM-PONs). The RC transformation algorithm is adopted to generate encryption key stream for symbol substitution, and the chaos is utilized to control the RC transformation. An experiment with 7.64Gbps 16-quadratureamplitude- modulation (QAM) encrypted OFDM data are successfully transmitted over 25km standard signal mode fiber (SSMF). The experimental results indicate that the proposed scheme is an effective and promising method for physical-layer secure optical communication to meet the demands for low implementation complexity and high security performance.
A digital-controllable broadband analog optical chaos source is proposed. An analog-digital hybrid electro-optic feedback loop is introduced to enhance the robustness of the semiconductor laser based chaos system. The scheme could generate a broadband optical chaotic signal of high dynamical complexity. Robust digital synchronization strategies can be used for the proposed optical chaos sources. Compared with the existing schemes, the adoption of a semiconductor laser with self-feedback reduces the requirement on the processing speed of digital components. Meanwhile, the hybrid system structure could be simplified markedly. The proposed chaotic source has potential in long-haul secure fiber communication, chaotic radar, random number generation, and so on..
We have experimentally demonstrated a direct-detection (DD) 112-Gbit/s 16 quadrature amplitude modulation (QAM) transmission over single-span 140-km standard single mode fiber (SSMF) with Kramers-Kronig receiver and a sparse I/Q Volterra filter (VF). The sparse I/Q VF was proposed in our previous work and it is based on dual-input real-valued Volterra series and ℓ1-regularization method. In this paper it is used for compensating the nonlinear distortion in a short-reach DD optical 16-QAM signal transmission system. In back to back case, sparse I/Q VF represents the great compensation ability to the saturation effect of the electrical amplifiers and the nonlinear sinusoidal transfer function of I/Q modulator. It provides around 1-order magnitude improvement of BER when reducing 84% complexity from full I/Q VF. For fiber transmission case, sparse I/Q VF can mitigate the fiber nonlinearity effectively and it achieves single-span 140-km transmission at hard-decision forward error correction (HD-FEC) threshold of 3.8 ×10-3 with less than half complexity of full I/Q VF. Besides, optical signal noise ratio (OSNR) performance at 120 km is measured and sparse I/Q VF reduces the required OSNR at HD-FEC threshold by 1.3 dB. In a word, we investigate the performance of sparse I/Q VF in short-reach optical 16-QAM transmission system and sparse I/Q VF reveals its potential in the growing short-reach applications, such as data center inter-connection and metropolitan area network.
Smart functional fiber devices are indispensable for optical fiber based communication and sensing systems, in which the integration of fiber structure with smart semiconductor materials is essential. Among various choices, the colloidal quantum dot (CQD) is of great interests and its advantages includes: (a) the concentration of the photo-excited electrons can be relatively high due to the quantum confinement in the CQD, which means the refractive index of CQD could be altered with less power consumption; (b) CQD is a nanoscale dispersion solution thus the functional material can be deposited conformally on fiber, silicon chips or other complex structure surface; (c) CQD is a highly tunable material, and the chemical composition and operation wavelength can be specially designed for any wavelength window.
In this work, we developed a platform to deposit the CQD with fiber structures for smart sensing and communication applications. First, we construct optical fiber antennas for gas sensing, in which the CQD conformally coated on the fiber surface behave like olfactory receptors. The gas molecules adsorbed on the quantum dots change the local carrier concentration in quantum dot solids, which leads to a change in their refractive index. This interaction of gas molecules with quantum dots could be transformed into optical signals through the optical fiber antennae. Owing to the large surface area, highly tunable physical and chemical properties of colloidal quantum dots, combined with the versatile fiber microstructure, the optical fiber antennae with quantum dots offer a new degree of freedom to precise, real-time and large-scale gas monitoring. Second, by using the light-excited carriers in CQD, a light controlled fiber device has been achieved as a delayed interferometric comb filter. The transmission spectrum can be easily tailored by mW-scale optical power. This device was applied in a 50 km standard single mode fiber (SSMF) based communication system with 12.5 Gbs On-Off-Keying (OOK) direct modulation for chirp management and dispersion compensation to achieve the error-free transmission.
KEYWORDS: Digital signal processing, Channel projecting optics, Polarization, Signal processing, Optical amplifiers, Optical fibers, Signal detection, Orthogonal frequency division multiplexing, Spatial resolution, Single mode fibers
A novel Brillouin optical time-domain analysis (BOTDA) system is demonstrated using intensity-modulated optical orthogonal frequency division multiplexing probe signal and direct detection (IM-DD-OOFDM) for stimulated Brillouin spectrum (SBS) measurement without frequency sweep operation. The SBS is reconstructed by channel estimation algorithm and Brillouin frequency shift (BFS) is located by curve fitting. The IM-DD-OOFDM BOTDA is demonstrated experimentally with 25m spatial resolution over 2 km standard single mode fiber.
Employing the multiple spatially independent channels offered by multicore fiber (MCF), spatial-division multiplexed (SDM) hybrid Raman and Brillouin distributed sensor has been demonstrated. The interrogation of Raman and Brillouin scattering are performed in distinct cores of the MCF, allowing for flexible management of the input powers between each of them, which is actually incompatible in hybrid Raman/Brillouin distributed system using single mode fibers (SMFs) due to the limitation of nonlinear effects. The proposed SDM system employs shared devices to generate pump pulse, but separate interrogation fiber channels. The combination of Raman and Brillouin sensing provides the capability of discriminative measurement between temperature and strain.
KEYWORDS: Signal to noise ratio, Signal processing, Electronic filtering, Optical fibers, Raman scattering, Temperature sensors, Spatial resolution, Fiber optics sensors, Fusion splicing, Temperature metrology, Reflectometry
We demonstrate a distributed temperature sensor based on quasi-single mode (QSM) Raman scattering in few mode fibers (FMFs). The FMF allows much larger input pump power before the initiate of stimulated Raman scattering compared with the standard single mode fiber (SSMF) and mitigates the detrimental differential mode group delay (DMGD) existing in the conventional multimode fiber (MMF) based Raman distributed temperature sensor (RDTS). Distributed temperature sensing is realized using conventional RDTS hardware over 20km FMFs within 90s, with a spatial resolution of 3m. The temperature resolution is 2.3°C @10km and 6.7°C @20km, respectively.
In polarization optical time domain reflectometry (POTDR) system, the performance of polarimetric measurement is largely constrained by the low signal to noise ratio (SNR) due to the weak Rayleigh backscattering and the degradation of the degree of polarization (DOP) of signal light. It will be indispensable to improve the SNR without sacrificing the DOP of backscattered signal for a sufficient dynamic range. In this paper, a Simplex coded POTDR (sc-POTDR) system was proposed and demonstrated. The relationships between the signal’s DOP and coding length/bit width were studied. Both numerical simulations and experiments show that the signal’s DOP has nothing to do with the length of Simplex code and only reducing the bit width can suppress the temporal depolarization effect. Applying 511-bit Simplex codes, a coding gain of 10.125dB has been demonstrated.
KEYWORDS: Connectors, Polarization, Signal detection, Ferroelectric materials, Signal to noise ratio, Attenuators, Light sources, Signal attenuation, Mirrors, Fiber optics sensors
A novel scheme of quasi-distributed vibration disturbances detection system based on incomplete Polarization optical time domain reflectometry was proposed. The system was enhanced by employing Fresnel-reflection caused by FC/PC connector, which can improve the signal’s SNR significantly, while the temporal depolarization effect can be almost completely suppressed. Without performing any data averaging, the intrusion event can be detected and located precisely/instantaneously with good stability. Also the frequency components of vibration events applying on sensing fiber can be obtained with large dynamic range. It shows a very good potential in intrusion detection, vibration frequency measuring, etc.
Although the Master Oscillator Power-Amplifier (MOPA) based fiber laser has received much attention for laser marking process due to its large tunabilty of pulse duration (from 10ns to 1ms), repetition rate (100Hz to 500kHz), high peak power and extraordinary heat dissipating capability, the output pulse deformation due to the saturation effect of fiber amplifier is detrimental for many applications. We proposed and demonstrated that, by utilizing Genetic algorithm (GA) based optimization technique, the input pulse profile from the master oscillator (current-driven laser diode) could be conveniently optimized to achieve targeted output pulse shape according to real parameters' constraints. In this work, an Yb-doped high power fiber amplifier is considered and a 200ns square shaped pulse profile is the optimization target. Since the input pulse with longer leading edge and shorter trailing edge can compensate the saturation effect, linear, quadratic and cubic polynomial functions are used to describe the input pulse with limited number of unknowns(<5). Coefficients of the polynomial functions are the optimization objects. With reasonable cost and hardware limitations, the cubic input pulse with 4 coefficients is found to be the best as the output amplified pulse can achieve excellent flatness within the square shape. Considering the bandwidth constraint of practical electronics, we examined high-frequency component cut-off effect of input pulses and found that the optimized cubic input pulses with 300MHz bandwidth is still quite acceptable to satisfy the requirement for the amplified output pulse and it is feasible to establish such a pulse generator in real applications.
Recently, offset-QAM based coherent WDM (CoWDM) has been proposed to build up spectrally-efficient multi-carrier superchannels. Compared with Nyquist wavelength division multiplexing (N-WDM) and orthogonal frequency division multiplexing (OFDM), offset-QAM based CoWDM can relax the stringent transmitter-side requirements for spectrum shaping and achieve significant transmission performance improvement. In order to efficiently utilize the sampling rate of commercially available analog-to-digital converter (ADC) and decrease the receiver-side implementation complexity, multi-carrier group detection scheme is investigated in offset-QAM based CoWDM where multiple carriers are simultaneously detected within single coherent receiver, followed by carrier separation in the digital domain through the 4-point discrete Fourier transform (DFT) method at the baseband. Here, we demonstrate a transmission of five-carrier 100 Gb/s polarization-multiplexed offset-16QAM signal with 12.5 GHz channel spacing. Through 3-carrier group detection, the sampling rate per-carrier is reduced to 1.33 times symbol rate in terms of 50 GS/s ADC and there is only 0.35 dB required OSNR penalty at BER=10-3 compared with conventional single channel coherent detection. Meanwhile, good tolerance of coherent receiver analog bandwidth is secured and receiver bandwidth is reduced to 8 GHz. Moreover, 0.5 dB required OSNR penalty at BER=10-3 is obtained given 18 GHz ADC bandwidth. Besides, we find that side carriers suffer from severer performance degradation than the central carrier with limited ADC resolution and only 0.08 dB and 0.2 dB required OSNR penalty at BER=10-3 are secured with 6 bits ADC resolution for central carrier and side carriers, respectively.
Recently, few-mode fiber (FMF) based mode division multiplexing (MDM) transmission together with multi-input multi-output (MIMO) signal processing technique is ideal candidate to solve future single mode fiber (SMF) capacity crunch. Most existing mode division multiplexers/demultiplexers (MMUX/DEMMUX) have a specific mode orientation for high-order non-circular symmetric mode. Taking the phase plate based DEMMUX as example and converting LP11 mode to fundamental LP01 mode, we need optimize input mode orientation the same as the phase pattern of phase plate. In this submission, we propose and experimentally demonstrate a spatial mode rotator based on mechanically induced twisting and bending in a step-index FMF. We theoretically find that the mode coupling strength between vector modes with similar propagation constants is determined by the FMF bending and twisting. When the input LP11 mode cluster including TE01, HE21a, HE21b, and TM01 mode are properly perturbed, the output optical field is superposed as LP11 mode with a rotation. Therefore, the proposed spatial mode rotator is composed of three FMF coils with a radius of 16 mm, while the number of each coil is 2, 1, and 2, respectively. Consequently, we are able to rotate the LP11 mode with arbitrary angle within 360° range using the same conventional configuration of polarization controller (PC). The insertion loss of proposed spatial mode rotator is less than 0.82 dB, when the operation wavelength varies from 1540 nm to 1560nm. In particular, from the measured mode profile, there exists little crosstalk between LP01 mode and LP11 mode during mode rotation operation.
KEYWORDS: Digital signal processing, Receivers, Multiplexing, Signal detection, Analog electronics, Orthogonal frequency division multiplexing, Transmitters, Gaussian filters, Fiber optics, Polarization
To relax the limited sampling rate of an analog-to-digital converter (ADC) and to reduce the complexity of conventional fiber-optical superchannel coherent detection, we propose and demonstrate a joint digital signal processing (DSP) technique of Nyquist-wavelength division multiplexing superchannel with group detection. At the receiver side, three Nyquist-spaced channels with 12.5 Gbaud polarization multiplexing-quadrature phase shift keying signals are group detected with a sampling rate per channel of 1.33 times over the normal sampling rate. A modified carrier separation technique is then put forward to retrieve the high-frequency interference component of both the designated channel and its adjacent channels, which can subsequently be used to recover the designated channel with new constant modulus algorithm-based joint multiinput-multioutput equalizers. The results show that the proposed group detection and joint DSP algorithm can simultaneously improve the transmission performance and reduce the complexity of both the transmitter and receiver, regardless of bandwidth restrictions from the waveshaper, ADC module, and coherent receiver.
A fiber-optic methane gas sensor system is proposed and demonstrated with accurate gas concentration measurement
with a sensitivity of ~410ppm. We employ the polarization-maintaining photonic crystal fiber (PM-PCF) based Sagnac
loop filter to slice the spectrum of broadband light source so as to precisely match multiple absorption lines of the
methane gas. Meanwhile, a compact gas cell with multiple reflections is designed to enhance the interaction between
methane gas and the light beam. The proposed gas sensor is verified as low cost, temperature-insensitive, and high
sensitivity.
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