We propose an approach for recognizing the pose and surface material of diverse objects, leveraging diffuse reflection principles and data fusion. Through theoretical analysis and the derivation of factors influencing diffuse reflection on objects, the method concentrates on and exploits surface information. To validate the feasibility of our theoretical research, the depth and active infrared intensity data obtained from a single time-of-flight camera are initially combined. Subsequently, these data undergo processing using feature extraction and lightweight machine-learning techniques. In addition, an optimization method is introduced to enhance the fitting of intensity. The experimental results not only visually showcase the effectiveness of our proposed method in accurately detecting the positions and surface materials of targets with varying sizes and spatial locations but also reveal that the vast majority of the sample data can achieve a recognition accuracy of 94.8% or higher.
KEYWORDS: Eye, Education and training, Picosecond phenomena, Radio over Fiber, Optical sensing, Affine motion model, Amplitude modulation, Data modeling, Machine learning, RGB color model
A designed visual geometry group (VGG)-based convolutional neural network (CNN) model with small computational cost and high accuracy is utilized to monitor pulse amplitude modulation-based intensity modulation and direct detection channel performance using eye diagram measurements. Experimental results show that the proposed technique can achieve a high accuracy in jointly monitoring modulation format, probabilistic shaping, roll-off factor, baud rate, optical signal-to-noise ratio, and chromatic dispersion. The designed VGG-based CNN model outperforms the other four traditional machine-learning methods in different scenarios. Furthermore, the multitask learning model combined with MobileNet CNN is designed to improve the flexibility of the network. Compared with the designed VGG-based CNN, the MobileNet-based MTL does not need to train all the classes, and it can simultaneously monitor single parameter or multiple parameters without sacrificing accuracy, indicating great potential in various monitoring scenarios.
A pivotal issue of the conventional optical fiber communications networks is to meet the explosively increasing requirement in data traffic. In order to meet this ever-increasing demand, there have been a lot of research and industrial development efforts to utilize the photon in various dimensions such as wavelength division multiplexing (WDM), space division multiplexing (SDM), mode division multiplexing (MDM) and so on. Fueled by emerging bandwidth-hungry applications, orbital angular momentum (OAM) modes and their multiplexing have recently gained much attention due to its special doughnut-shaped intensity distribution, as well as its unique helical phase wavefront with the theoretically infinite topological value. The OAM modes with different topological charge values are orthogonal to each other, which has provided a new degree of freedom in MDM. In this paper, we propose and design a Ge-doped air-core ring fiber, which can support numerous OAM modes. By varying the mole fraction of GeO2 and adjusting the structure parameter, including the air-core radius and the GeO2-doped ring width, we study the influence of the different fiber parameters on the total supported OAM mode number. The hollow silica fiber with a 50-μm air core and a 1.5-μm thickness of Gedoped ring is designed in simulation to support fiber eigenmodes up to HE112,1 and EH107,1. This provides 436 OAM modes at 1550 nm while maintaining radially single mode condition. Moreover, it can support more than 400 OAM modes from 1260 nm to 1625 nm, covering O, E, S, C, and L bands.
Light-carrying orbital angular momentum (OAM) has recently drawn extensive attention from researchers due to its unique field distribution. As a result of the intrinsic orthogonality among OAM modes with different topological charge values, they can be used as a modal basis in the mode-division multiplexing (MDM) optical communications systems. For fiber-based optical systems, chromatic dispersion induces temporal optical pulse broadening, which seriously limits the rate of information transmission. Consequently, dispersion compensation fiber is promising for mitigating the chromatic dispersion of the complex beam in the optical fiber. We propose and design a novel germanium-doped silica ring fiber composed of two high refractive index ring regions that can support high order OAM modes with large negative dispersion. We numerically investigate the high-order OAM modes guiding property in the proposed fiber by using a full-vector finite-element mode (FEM). Since Ge-doped silica has similar physical properties to silica, they can be easily combined with a tunable mole fraction of GeO2. Through varying the mole fraction of GeO2 and optimizing the structure parameter, we obtain a large negative dispersion of up to -99,685 ps/(nm·km) for OAM11,1 mode at the wavelength of 1614.2 nm. Furthermore, we engineer the chromatic dispersion of some other OAM modes and investigate the effects of fiber parameters on the dispersion, which indicates that the fiber we design is able to support all the OAMℓ,1 modes (∣l∣≤11) with highly negative dispersion. The designed fiber with tailorable negative dispersion can be applied to compensate for positive dispersion in the OAM-based optical systems.
Spatial-division multiplexing (SDM) techniques have been purposed to increase the capacity of optical fiber transmission links by utilizing multicore fibers or few-mode fibers (FMF). The most challenging impairments of SDMbased long-haul optical links mainly include modal dispersion and mode-dependent loss (MDL), whereas MDL arises from inline component imperfections, and breaks modal orthogonality thus degrading the capacity of multiple-inputmultiple- output (MIMO) receivers. To reduce MDL, optical approaches include mode scramblers and specialty fiber designs, yet these methods were burdened with high cost, yet cannot completely remove the accumulated MDL in the link. Besides, space-time trellis codes (STTC) were purposed to lessen MDL, but suffered from high complexity. In this work, we investigated the performance of space-time block-coding (STBC) scheme to mitigate MDL in SDM-based optical communication by exploiting space and delay diversity, whereas weight matrices of frequency-domain equalization (FDE) were updated heuristically using decision-directed recursive-least-squares (RLS) algorithm for convergence and channel estimation. The STBC was evaluated in a six-mode multiplexed system over 30-km FMF via 6×6 MIMO FDE, with modal gain offset 3 dB, core refractive index 1.49, numerical aperture 0.5. Results show that optical-signal-to-noise ratio (OSNR) tolerance can be improved via STBC by approximately 3.1, 4.9, 7.8 dB for QPSK, 16- and 64-QAM with respective bit-error-rates (BER) and minimum-mean-square-error (MMSE). Besides, we also evaluate the complexity optimization of STBC decoding scheme with zero-forcing decision feedback (ZFDF) equalizer by shortening the coding slot length, which is robust to frequency-selective fading channels, and can be scaled up for SDM systems with more dynamic channels.
For next-generation long-haul communication systems, space-division multiplexing (SDM) is suggested as a promising technique for providing orders of magnitude increase in transmission capacity. Optical amplifiers for multimode fibers are the crucial components to realize SDM systems, while few-mode fiber (FMF) has the advantage of strong intensity overlap between different modes that allows sharing of single pump across multiple signal modes. FMF-based erbiumdoped fiber amplifiers (FM-EDFA) have been intensively studied. Since SDM is exceedingly reliant on DSP technique and thus has more stringent requirement on noise performance, FMF-based distributed Raman amplifier (FM-DRA) benefits from relatively lower noise figure (NF) compared with FM-EDFA. Yet much less work has been done in this area. To implement FM-DRAs in SDM systems, their performance should be carefully optimized. In this paper, modal gain and saturation characteristics of intermodal Raman amplification in FMFs have been fully investigated by tailoring the refractive-index profiles and doping levels of different FMF designs. For 50-km-long FMF with -3 dBm signal at 1550 nm and a 20-25 dBm pump at 1455 nm, the optimized modal-equalization of Raman gain and NF are provided depending on various fiber cross-section and pumping configuration with respect to modes/wavelengths. The pumpsignal modal-overlap integrals for each of the four mode-groups with normalized frequency V=5 have been exploited, resulting in a mean gain of 10 dB within 1 dB of equalization for 16 gain coefficients. Our results show it should be possible to design FMF with larger intermodal nonlinearity and better dispersion characteristics to achieve desirable Raman gain.
KEYWORDS: Digital signal processing, Telecommunications, Multiplexing, Optical fibers, Optical communications, Signal processing, Space division multiplexing, Receivers, Fourier transforms, Dispersion, Polarization, Digital filtering, Refractive index, Systems modeling
Mode-division multiplexing (MDM) transmission systems utilizing few-mode fibers (FMF) have been intensively explored to sustain continuous traffic growth. The key challenges of MDM systems are inter-modal crosstalk due to random mode coupling (RMC), and largely-accumulated differential mode group delay (DMGD), whilst hinders mode-demultiplexer implementation. The adaptive multi-input multi-output (MIMO) frequency-domain equalization (FDE) can dynamically compensate DMGD using digital signal processing (DSP) algorithms. The frequency-domain least-mean squares (FD-LMS) algorithm has been universally adopted for high-speed MDM communications, mainly for its relatively low computational complexity. However, longer training sequence is appended for FD-LMS to achieve faster convergence, which incurs prohibitively higher system overhead and reduces overall throughput. In this paper, we propose a fast-convergent single-stage adaptive frequency-domain recursive least-squares (FD-RLS) algorithm with reduced complexity for DMGD compensation at MDM coherent receivers. The performance and complexity comparison of FD-RLS, with signal-PSD-dependent FD-LMS method and conventional FD-LMS approach, are performed in a 3000 km six-mode transmission system with 65 ps/km DMGD. We explore the convergence speed of three adaptive algorithms, including the normalized mean-square-error (NMSE) per fast Fourier transform (FFT) block at 14–30 dB OSNR. The fast convergence of FD-RLS is exploited at the expense of slightly-increased necessary tap numbers for MIMO equalizers, and it can partially save the overhead of training sequence. Furthermore, we demonstrate adaptive FD-RLS can also be used for chromatic dispersion (CD) compensation without increasing the filter tap length, thus prominently reducing the DSP implementation complexity for MDM systems.
KEYWORDS: Digital signal processing, Filtering (signal processing), Digital filtering, Finite impulse response filters, Optical filters, Data communications, Modulation, Signal generators, Wavelength division multiplexing, Transmitters, Spectral resolution, Analog electronics
To support the ever-increasing demand for high-speed optical communications, Nyquist spectral shaping serves as a promising technique to improve spectral efficiency (SE) by generating near-rectangular spectra with negligible crosstalk and inter-symbol interference in wavelength-division-multiplexed (WDM) systems. Compared with specially-designed optical methods, DSP-based electrical filters are more flexible as they can generate different filter shapes and modulation formats. However, such transmitter-side pre-filtering approach is sensitive to the limited taps of finite-impulse-response (FIR) filter, for the complexity of the required DSP and digital-to-analog converter (DAC) is limited by the cost and power consumption of optical transponder. In this paper, we investigate the performance and complexity of transmitter-side FIR-based DSP with polarization-division-multiplexing (PDM) high-order quadrature-amplitude-modulation (QAM) formats. Our results show that Nyquist 64-QAM, 16-QAM and QPSK WDM signals can be sufficiently generated by digital FIR filters with 57, 37, and 17 taps respectively. Then we explore the effects of the required spectral pre-emphasis, bandwidth and resolution on the performance of Nyquist-WDM systems. To obtain negligible OSNR penalty with a roll-off factor of 0.1, two-channel-interleaved DAC requires a Gaussian electrical filter with the bandwidth of 0.4–0.6 times of the symbol rate for PDM-64QAM, 0.35–0.65 times for PDM-16QAM, and 0.3–0.8 times for PDM-QPSK, with required DAC resolutions as 8, 7, 6 bits correspondingly. As a tradeoff, PDM-64QAM can be a promising candidate for SE improvement in next-generation optical metro networks.
KEYWORDS: Interference (communication), Computer simulations, Fiber optic communications, Adaptive optics, Signal to noise ratio, Optical communications, Telecommunications, Digital signal processing, Lithium, Signal processing
Few-mode fiber (FMF) transmission system has become an emerging technology to overcome next capacity crunch. To compensate the large accumulated differential mode group delay (DMGD) and random mode coupling in FMF transmission systems, frequency domain least mean square (FD-LMS) algorithm has been proposed and proven as the most hardware efficient approach. Except for the hardware complexity, convergence speed is another major consideration of adaptive FD-LMS algorithm, especially in FMF system with large accumulated DMGD. We propose two algorithms to improve the convergence speed of the adaptive FD-LMS in FMF transmission systems. One is signal power spectrum density (PSD) dependent adaptive algorithm, which adopts variable step size that is the reciprocal of the power level in each frequency bin. The other is noise power directed adaptive FD-LMS algorithm, which adopts the step size of each frequency bin rendering the posterior errors that will converge to background noise in additive white Gaussian noise channel. Our simulation results show that, in a 3000 km FMF transmission system with 35-ps/km DMGD and optical signal to noise ratio (OSNR) of 14 dB, the noise power directed algorithm and PSD dependent algorithm can improve the convergence speed by 54% and 35% over conventional adaptive FD-LMS algorithm with negligible increase in hardware complexity. We also proposed a complexity reduced single-stage adaptive equalizer for compensating both chromatic dispersion (CD) and DMGD simultaneously. The single-stage method can save 6% complex multiplication over conventional two-stage equalization method in a 1000 km FMF transmission system with 76-ps/km DMGD.
We proposed an ultra-wide range chromatic dispersion (CD) measurement method using coherent detection and digital
signal processing. We measured the average phase at frequency ± f as a function of frequency f, and estimated the total
accumulated CD with polynomial fitting algorithm. The method is compatible with modern coherent optical networks
and capable of measuring CD of at least ± 16000 ps/nm with an accuracy of better than ± 50 ps/nm.
We demonstrated the QPSK signal generated by a single phase-modulator (PM-QPSK) is more tolerant to nonlinearity
than that generated by the Mach Zehnder Interferometer (MZI-QPSK). With simulation of WDM system at 28
Gsymbol/s per polarization the performance of PM-QPSK and MZI-QPSK are compared when dispersion management
is used. After 800 km's transmission in single-polarization transmission system, MZI-QPSK outperforms PM-QPSK
because of its better tolerance to ISI. After 400 km's transmission in polarization multiplexing system, PM-QPSK
outperforms MZI-QPSK by comparing the symbol error ratio with optimized launch power, which is because PM-QPSK
has better nonlinearity tolerance in dispersion managed system.
We demonstrate the generation of an 160-GHz optical pulse train using a 40-GHz driven phase modulator followed by two stages of delayed interferometers. The simulation shows that the pulse width of the generated chirp-free 160-GHz carrier-suppressed return-to-zero optical pulse train is 3.13 ps
Based on the equivalent resistance and finite element method, the current distribution in active region is analyzed with current injection from ring electrode. To improve the injection efficiency and reduce the resistance of p-type DBR, we design reverse mesa structure and achieved low threshold current InGaAs vertical cavity surface emitting laser operation. In situ thickness monitoring and controlling in MBE growth is also studied.
The structures of InGaAs/GaAs strained QW vertical cavity suurface-emitting lasers with low threshold current have
been grown on tilted substrate by a modificatoy MBE system. The VCSEL structure contains 23.5 pairs n-type DBR , a
strained InGaAs/GaAs 3QW active region ,and 20.5 pairs p-type DBR. The emission wavelength of InxGaixAs IGaAs
QW lasers as a function of indium contents (x) and QW width has been studied theoretically and experimentally. The
experimental results compared with theoretical calcuulation were in good agreement. The device measurements showed
that room temperature CW operation of VCSEL has been achieved with the threshold current as low as 0.7 mA at 9430
A wavelength, and quantum efficiency above 12%.
Temperature controlling is important for optoelectronic device's stable and reliable working, especially in poor circumstance or for long time. This article represents an active temperature control system used to control the temperature of a tunable narrow-linewidth external cavity semiconductor laser. A double deck cooling structure controlled by an intelligent driver is reported and the feature of the system is investigated.
A compact programmable tuning external cavity laser has been developed using a microprocessor as the controller in which a stepper motor is employed to execute the motions of the grating for wavelength continuous tuning and a dual stage Peltier cooling system as precise temperature controller. A tuning range of 77 nm and 1GHz/microstep tuning resolution are achieved. This device can be widely applied to spectroscopy, spectral measurements, metrology, remote sensing, and so on.
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