Microwave photonic (MWP) radars based on radio-frequency orbital angular momentum(RF-OAM) beams have been extensively researched recently, for their potential to provide high-azimuth and high-range resolution for radar imaging at the same time. However, the inherent divergence effect of RF-OAM beams limits the effective detection distance, which has become the main technical bottleneck of the long-distance MWP OAM radar. In this paper, the mathematical model of the MWP-controlled circular antenna array (CAA) is constructed, and the basic transmission properties of generated RF-OAM beams are investigated. Then each factor in the model is extracted for numerical simulation to explore its influence on the generation of RF-OAM beams, and the Bessel function term is found to be the factor leading to beam-divergence. Within the Bessel function term, the role that each physical element plays is analyzed, which is just consistent with our experimental experience and previous research results. Finally, the quantization equation of the RF-OAM beam divergence angle is fitted based on the properties of the Bessel function.
Low phase noise microwave oscillator is a key device for many applications, including radar, remote sensing, communication and test instrumentation. Photonics based optoelectronic microwave oscillator can overcome the limitation of the phase noise of the electrical microwave oscillators. However, there are some scaling issues for the optoelectronic oscillator, including frequency control and spurs. In this paper, we propose an injection-locked dual-loop optoelectronic oscillator to attain a frequency stable 10 GHz microwave signal with low phase noise and low spurs, simultaneously. Experimentally, single-sideband (SSB) phase noise of -140 dBc/Hz at 10 kHz offset and spurs below -130 dBc are achieved.
The time-wavelength optical pulse interleaver is an important component of a wavelength-interleaved photonic analog-to- digital converter (ADC). The two important performance indexes of interleavers are power imbalance and delay error. In order to reduce the power imbalance, a runway-shaped and shallow ridge silicon waveguide optical delay line (ODL) array is adopted. The measured average loss of the ODL is only 0.68 dB/cm. By measuring the delay of the ODL array, we optimize the ODL’s length to reduce the interleaver’s delay error. A four-channel interleaver with low loss and small delay error was fabricated on the silicon-on-insulator (SOI) platform, and the power imbalance is 0.9 dB. The root mean square (RMS) delay error is 0.34% and the crosstalk is below -20.3 dB.
Weakly-coupled mode division multiplexing (MDM) over few-mode fiber (FMF) for short-reach transmission has attracted great interest, which can avoid multiple-input-multiple-output digital signal processing (MIMO-DSP) by greatly suppressing modal crosstalk. In this paper, step-index FMF supporting 4 linearity polarization (LP) modes for MIMO-free transmission is designed and fabricated for the first time. Modal crosstalk of the fiber is suppressed by increasing the mode effective refractive index differences. The same fabrication method as standard single-mode fiber is adopted so that it is practical and cost-effective. The mode multiplexer/demultiplexers (MUX/DEMUX) consists of cascaded mode-selective couplers (MSCs), which are designed and fabricated by tapering the proposed FMF with single-mode fiber (SMF). The mode MUX and DEMUX achieve very low modal crosstalk not only for the multiplexing/demultiplexing but also for the coupling to/from the FMF. Based on the fabricated FMF and mode MUX/DEMUX, we successfully demonstrate the first simultaneous 4-modes (LP01, LP11, LP21 & LP31) 10-km FMF transmission with 10-Gb/s intensity modulation and MIMO-free direct detection (IM/DD).
Mode division multiplexing (MDM) has been widely investigated to enhance the capacities of passive optical networks (PONs), and weakly coupled transmission schemes are highly preferred to reduce the cost and complexity. In this paper, we have proposed an evolution scheme of the cascaded mode division multiplexing PON (MDM-PON) and conventional Gigabit PON (GPON), which supports bidirectional 1310/1490-nm transmission. This scheme is enabled by wavelength-insensitive weakly coupled (WIWC) few-mode fiber (FMF) and optical components. The bidirectional MDM-GPON transmission scheme with 2.5 Gb/s on-off-keying (OOK) modulation and simple direct detection over 10-km FMF and 10-km single-mode fiber (SMF) are experimentally demonstrated. The proposed MDM-GPON evolution scheme can achieve maximum compatibility by maintaining both the optical distribution network (ODN) and the optical network units (ONUs).
An optical pulse generation with repetition-rate of 20 GHz based on a 10 GHz optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The combination of an optical comb generator (OFCG) and an OEO can generate an optical pulse train with the features of high repetition-rate, narrow pulse-width and low timing-jitter, simultaneously. By tuning the DC bias voltage applied on the OFCG, the modulated output light of the OFCG appears as a 20 GHz single optical pulse train which has a repetition-rate twice of the 10 GHz modulation frequency of the OFCG. It improves the repetition-rate of the optical pulse. A microwave frequency divider with dividing ratio of 2 is used to obtain an electrical signal with frequency equals to the 10 GHz modulation frequency. The output of the microwave frequency divider is fed back to drive the OFCG to form a closed OEO loop. Compared with the former ways, it eliminates the optical bandpass filter (OBPF), which improves the power efficiency of the OEO loop.
A 24 GHz microwave frequency divider with variable frequency dividing ratio based on a super-harmonic injectionlocked tunable optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. Due to the nonlinearity of the dual-output Mach-Zehnder intensity modulator (DOMZM) and the photodiode (PD) used in the OEO loop, it can generate intermodulation components between the 24 GHz injection RF signal and the harmonics of the free-running OEO. If the frequency of any one of the intermodulation components locates at the locking range of the free-running OEO, the phase of the OEO will be synchronized to the 24 GHz injection RF signal. It realizes a microwave frequency divider. By tuning the oscillation frequency of the OEO to the second, third, or fourth sub-harmonic of the 24 GHz input RF signal, a microwave frequency divider with variable frequency dividing ratio of 2, 3, or 4 is realized. The achievable frequency dividing ratio is limited by the nonlinearity of the DOMZM and the injection RF power. A good phase noise performance of the OEO lead to a microwave frequency divider with low phase noise. The transient response of the RF phase of the output of the OEO during the process of super-harmonic injection locking is also measured.
KEYWORDS: Switching, Optical fibers, Eye, Single mode fibers, Optical switching, Digital signal processing, Signal detection, Receivers, Lithium, Networks
We propose and experimentally demonstrate an all-fiber optical mode switching structure supporting independent switching, exchanging, adding, and dropping functionalities in which each mode can be switched individually. The mode switching structure consists of cascaded mode selective couplers (MSCs) capable of exciting and selecting specific higher order modes in few-mode fibers with high efficiency and one multiport optical switch routing the independent spatial modes to their destinations. The data carried on three different spatial modes can be switched, exchanged, added, and dropped through this all-fiber structure. For this experimental demonstration, optical on-off-keying (OOK) signals at 10-Gb/s carried on three spatial modes are successfully processed with open and clear eye diagrams. The mode switch exhibits power penalties of less than 3.1 dB after through operation, less than 2.7 dB after exchange operation, less than 2.8 dB after switching operation, and less than 1.6 dB after mode adding and dropping operations at the bit-error rate (BER) of 10−3, while all three channels carried on three spatial modes are simultaneously routed. The proposed structure, compatible with current optical switching networks based on single-mode fibers, can potentially be used to expand the switching scalability in advanced and flexible short-reach mode-division multiplexing-based networks.
A compact 1.6×10μm2 germanium pin waveguide photodetector was demonstrated on a Silicon-on-Insulator substrate. The dark current of the photodetector was measured to be 0.66μA at -1V bias voltage, which is much lower than recently reported. The photodetector exhibited a 3-dB bandwidth of 20GHz at the wavelength of 1.55μm. A clear open eye
diagram at 10Gb/s was also obtained.
Multi- and single-carrier (SC) coherent optical fiber communication with frequency domain equalization (FDE) is
discussed with emphasis on 100-Gb/s operation. 120-Gb/s coherent optical (CO-SCFDE) system over 317-km standard
single-mode fiber is demonstrated. Coherent optical orthogonal frequency-division-multiplexing (CO-OFDM) and
single-carrier frequency-division-multiplexing scheme (CO-SCFDM) are theoretically and experimentally compared.
Single-carrier Frequency Division Multiple Address (SC-FDMA) technique has been proposed for next generation
passive optical network (PON). In this paper, we demonstrate both downstream and upstream SC-FDMA transmissions
with source-free single wavelength transmission.
The wavelength-division multiplexing passive optical network (WDM-PON) is considered to be a promising method to
overlay or upgrade the existing optical access network. A centralized WDM-PON in which the downstream wavelength
is reused to modulate the upstream has been studied comprehensively, for same wavelength operation of both
downstream and upstream, colorless operation and source-free of optical network units (ONU) can be realized in such
systems. We propose a scheme of centralized WDM-PON with non-return-to-zero/continuous-wave (NRZ/CW)
polarization multiplexed downstream and remodulated upstream using a CW injection-locked Fabry-Perot (FP) laser.
This scheme can eliminate the crosstalk of downstream data on the upstream and get higher extinction ratio of received
downstream signal. Experiment with 22.2km transmission of 10-Gb/s downstream signal and 622-Mb/s upstream signal
was realized using a commercial FP laser, with power penalties of 0.2dB and 0dB respectively. 2.5-Gb/s upstream signal
using an injection locked FP laser was also investigated, and 61 modes within 25.4nm span were locked with side-mode
suppression ratio of above 27dB.
KEYWORDS: Modulation, Radio over Fiber, Single sideband modulation, Phase shift keying, Signal generators, Semiconductor lasers, Single mode fibers, Networks, Heterodyning, Modulators
The seamless integration of broadband optical and wireless access networks is considered to be a promising solution for
next generation access networks which will provide high capacity and flexibility with lower cost. In such access
networks millimeter wave (mm-wave) radio over fiber (RoF) system is a key enabling technology due to its its large
bandwidth and short reach in atmosphere. In this paper, we will discuss some key technologies based on injection locked
lasers for mm-wave RoF systems, including all optical generation of mm-wave signal, up- and down-conversion, single
sideband modulation, and transmission.
KEYWORDS: Single mode fibers, Digital signal processing, Critical dimension metrology, Dispersion, Phase shift keying, Four wave mixing, Phase shifts, Nonlinear optics, Modulation, Signal attenuation
Digital signal processing (DSP) based coherent detection attracts extensive studies recently. DSP technology simplifies
coherent detection and brings it more advantages and channel impairments can be compensated electrically. The lumped
dispersion compensation scheme makes the fiber link simple, flexible and potentially cost effective. However, such
pseudo-linear long haul transmission systems are greatly degraded by intra-channel nonlinearities at high bit rates. The
optical pulses are highly dispersed and overlap strongly during propagation, resulting serious intrachannel four-wave
mixing (IFWM). IFWM introduces pattern dependent nonlinear phase shifts to the signals. It has been pointed out that
IFWM induced phase noise are correlated from symbol to symbol. The autocorrelation function has been studied in
detail. Here we extend the result to fiber link with single mode fiber (SMF) for transmission only. We studied the
statistics of IFWM induced phase noise in Return to Zero-Differetnitally coded Quadrature Phase Shift Keying (RZDQPSK)
transmission systems with identical fiber spans and SMF fiber spans numerically. The results show that the
phase noise correlation of neighbouring bits is reduced for the latter one but the correlation length is increased with fiber
span number and chromatic dispersion value.
86 Gbit/s hybrid polarization multiplexed OOK/DPSK signal is generated and transmitted at the
same wavelength. Clear eye diagrams of demultiplexed OOK and DPSK signals are obtained after
transmission over a 106.5 km SSMF fiber span.
We demonstrate a 10.7Gb/s-line-rate L-band WDM loop transmission over 1890km standard single-mode fiber (SSMF) with 100km amplifier spacing as well as non-return-to-zero (NRZ) format. For the first time, dispersion compensating fiber (DCF) plus chirped fiber Bragg grating (CFBG) is employed for hybrid inline dispersion compensation. The power penalty of each channel is less than 3dB after three loop transmission. The experimental results show that high-performance-CFBGs can be successfully used in ultra-long haul (>1000km) WDM systems. We also point out that all-CFBG compensation scheme is not suitable for re-circulating loop transmissions.
This paper proposes a new method which uses the Cross Gain Modulation (XGM) in the Semiconductor Optical Amplifier (SOA) to realize optical interleaving. Different from the traditional optical wavelength interleaving, the proposed optical interleaving is the interleaving of codes. Three modules and theoretical analysis are presented, and an experiment is designed to validate these kinds of optical interleaving modules.
We demonstrate transmission of 36×10-Gbit/s WDM signals over 4134 km of standard single-mode fiber using all-Raman amplification and NRZ format. Wide-band fiber delay loop depolarizers are used in the Raman pump unit to reduce the depolarization dependent Raman gain.
A novel driving scheme utilizing the resonance effect of the micro-mirror is presented to reduce the drive voltage of the silicon-based MEMS (microelectromechanical systems) torsion-mirror optical switch. A mixed bias of DC (direct current) and AC (alternating current) is adopted to drive the torsion mirror. Both the numerical simulation and experiment results show that the driving voltage of the optical switch is reduced by nearly 50%, from more than 270V to 130V.
Power management is discussed based on a WDM optical test network, in which ring laser clamped EDFAs are used to compensate loss and only the optical attenuators is needed for power regulation. All channels at the output of each node are adjusted to a preset power level, so signal power fluctuations and differences between channels are not passed to the later nodes. The transmission performance is also studied and a simple expression is derived for evaluating the possible input power to the fiber and the possible transmission distance in optical networks. The accumulation of optical amplifier noise and the maximum input power for the stable gain of the lasing-clamped EDFA are the dominant factors limiting the network scale, and restrict the preset input power to the fiber.
A novel scheme of OADM(Optical Add/Drop Multiplexer) is proposed. It has the property of arbitrary add/drop channel compositions ( up to 4 wavelengths). In order to supress more of the homodyne crosstalk, we proposed a novel scheme of utilizing OLM(Optical Loop Mirror) so that we can utilizes the demultiplexer back-and-forth, thus utilizes it both as a demultiplexer and a multiplexer.The results show that we get a 5dB improve on the crosstalk depression, that means a 3 times larger network scale than normal scheme of demultiplexer+switch array+coupler. We found that the reflection on the input port of demultiplexer is the main cause of homodyne crosstalk.
Two types of four-channel tunable optical add-drop multiplexers(OADM) are demonstrated which are based on fiber Bragg gratings(FBGs),optical circulators and switches. Both of them can dynamically select the drop channel or just bypass all WDM channels. The crosstalk from the adjacent channel is lower than -23dB and the isolation between the drop and add port is higher than 40dB.
FP resonator has a very nice character which can be easily used as an OADM device. It can drop the selected channels and reflect the other channels which don't satisfy the resonance requirements. It can be sued as both channels drop and bypass device. In this paper, the design of good reflection spectrum is considered. The factors which affect the reflection spectrum of FP resonator are carefully studied. When using two FP resonators which have both nice reflection and transmission spectrum, it can be realized an OADM node without a circulator.
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