查看更多>>摘要：? 2021 Elsevier B.V.We propose a novel Erbium-doped fiber laser sensor in a sigma-cavity configuration (EDFσL) for temperature sensing. In this configuration, the sigma-branch of the EDFσL, or the double-pass cascaded-chirped long-period fiber grating (C-CLPG), consists of an optical circulator, a C-CLPG, and a Faraday rotator mirror (FRM), and works as a reflective sensor device as well as a wavelength selection element with a comb-like spectrum, namely a channeled spectrum. The double-pass C-CLPG has the advantage of not only a flexible sensor arrangement, especially for remote sensing, but also the compensation effect for the polarization fluctuations in the sigma-branch caused by the measurand. In the experiment, it is shown that the FRM reflection scheme can stabilize the output polarization state of the double-pass C-CLPG during the temperature change around the C-CLPG, and it is found that the EDFσL enables a more stable and precise measurement than the previous simple EDF ring laser using the C-CLPG. Furthermore, it is also demonstrated that the oscillation is stabilized by the polarization compensation effect of the double-pass C-CLPG even if the polarization state is intentionally changed by applying the mechanical deformation near the C-CLPG.
查看更多>>摘要：? 2021 Elsevier B.V.An ultra-broadband and compact E00/E10 polymer mode converter composed of a symmetric Y-branch 3-dB coupler, a phase shifter and a 2 × 1 multimode interferometer is theoretically designed and experimentally demonstrated. Within the wavelength range from 1470 to 1690 nm, calculated results show that the mode crosstalk is less than –22.6 dB and insertion loss is less than 5 dB. Moreover, the calculated mode crosstalk and insertion loss are less than –29.8 dB and 3.6 dB in C-band (1530–1565 nm), respectively. To verify the proposed design, the waveguide mode converter was fabricated and polymer materials EPOcore and EPOclad are adopted as the core and cladding materials, respectively. The measured mode crosstalk varies from –26.2 dB to –15.4 dB and the insertion loss varies from 4.8 dB to 9.2 dB in the wavelength range from 1500 to 1630 nm. Benefit from the advantages of broadband and compactness, this proposed mode converter has potentials in future complex hybrid multiplexing applications.
查看更多>>摘要：? 2021 Elsevier B.V.In this work, we demonstrated an ultrasensitive parallel double Fabry–Perot interferometers (FPIs) sensor based on Vernier effect and Type II fiber Bragg grating (FBG) for simultaneous measurement of temperature and strain in the temperature range of 26 °C–1000 °C. Two FPIs are manufactured from hollow silica tube (HST), single mode fiber (SMF) and multimode fiber (MMF). The superposition of both waves from the FPIs produces an interference spectrum with a large envelope that has a temperature sensitivity of 6.98 pm/°C and a strain sensitivity of 127.32 pm/μ? at 800 °C. This strain sensitivity is ～7.6 times higher than that of a single FPI structure (without Vernier Effect). In addition, an FBG is inscribed in one of the FPIs using a femtosecond laser to provide additional temperature reading (temperature sensitivity ～15.14 pm/°C) and to serve as thermal compensation for the measurement. The FBG is encapsulated in glass capillary tube so that it is unaffected by the applied strain. The proposed sensor is robust and highly sensitive, and it is suitable for the applications under extreme temperature condition.
查看更多>>摘要：? 2021 Elsevier B.V.Security is of crucial importance to visible light communication (VLC) systems. In this paper, we propose and experimentally demonstrate a secure non-orthogonal multiple access (NOMA) VLC system with orthogonal frequency division multiplexing (OFDM) modulation, where the peak-to-average power ratio (PAPR) is also significantly reduced. Based on 2D chaotic map, the public key serves a two-fold purpose: to provide physical-layer security against eavesdropper, and to generate a set of phase-rotated candidate signals in the chaotic selected mapping (SLM) scheme where the signal with lowest PAPR is selected for transmission. The private keys further enhance the security and guarantee the privacy among the legitimate users via chaotic data scrambling. The experiment based on a direct current-biased optical OFDM (DCO-OFDM) VLC system with two legitimate users and one eavesdropper successfully verified the joint security enhancement and PAPR mitigation performance of the proposed scheme for NOMA VLC application.
查看更多>>摘要：? 2021 Elsevier B.V.Orthogonal frequency-division multiplexing-based passive optical network (OFDM-PON) with low-cost direct detection is the current research focuses of access networks. It can realize multi-dimensional bandwidth allocation based on subcarriers and time slots. In general, the inverse fast Fourier transform (IFFT) with Hermitian symmetric (HS) mapped symbols is applied to generate the real-valued baseband signal for intensity modulation. However, only half of the subcarriers can be employed to carry user data, while the other half are modulated by the corresponding conjugate data. Therefore, both IFFT and FFT are not fully utilized in HS-OFDM systems. Under the same bandwidth granularity (data subcarriers) and data transmission rate, the non-HS OFDM (NHS-OFDM) can reduce IFFT/FFT size by half. The hardware implementation and power consumption will be significantly reduced compared to the HS-OFDM. In this paper, we design and implement the baseband HS/NHS-OFDM transceivers based on field programmable gate array chips and data converters. On-chip resource usage and power consumption are analyzed and compared. It shows that the NHS-OFDM transmitter (receiver) can save up to 58% (58%) multipliers, 46.8% (41.7%) registers, and 48% (42.9%) look-up tables. About 11.9% (19.6%) of on-chip power consumption is reduced for the transmitter (receiver). In addition, the two real-time baseband transceivers are comparatively investigated in a directly-modulated laser-based direct detection transmission system. The bit error rate (BER) performance after 20 km standard single-mode fiber transmission is measured with both offline and real-time digital signal processing (DSP) algorithms. It exhibits that the NHS-OFDM system's power penalty is negligible at the BER of 3.8e?3, compared to the HS-OFDM. Moreover, the real-time receivers have a similar BER performance to corresponding offline ones.
查看更多>>摘要：? 2021 Elsevier B.V.The power distribution, high-order polynomial, bicubic spline interpolation, and geometric construction methods were used to reduce the edge thickness and weight of negative blended lenticular lenses. Four lenses with the same optical parameters were designed, simulated, machined, and evaluated using the four methods. Spherical and cylindrical power maps were generated and compared. The methods reduced the edge thickness by 78.73% for power distribution method, 77.40% for high-order polynomial method, 78.12% for bicubic spline interpolation method and 79.86% for geometric construction method for a 35 mm radius compared to that with the spherical surface. Geometric construction, power distribution, high-order polynomial, and bicubic spline interpolation reduced the lens weight by 43.86%, 35.98%, 33.06%, and 29.76%, respectively. The center optical area for the power distribution and geometric construction methods were larger than those with the other methods.
查看更多>>摘要：? 2021 Elsevier B.V.In this article we study saturation effects in the fluorescence spectra at the D2 transition in 87Rb excited by two counter-propagating laser beams of the same frequency. The effects appear as dips in the Doppler broadened fluorescence emission lines at the atomic transitions and in all but one of the cross-over resonances. The 5SF=1→5P3/2F=0, 1 cross-over presents a strong dependence in the laser intensity. For low intensities it appears as a weak dip, but at higher laser intensities an increase in fluorescence is clearly present. A rate equation calculation which includes the velocity profile is used to interpret the experimental data. For the 5SF=2→5P3/2F=1, 2 and 3 manifold, a five level model that takes into account the hyperfine structure of both initial and final states is in good agreement with the experiment. For the 5SF=1→5P3/2F=0, 1 and 2 manifold, it is necessary to include the magnetic sublevel structure of all hyperfine levels involved. The calculation also shows that the onset of this magnetic pumping effect occurs near the 5S→5P3/2 saturation intensity. In both cases the comparison between theory and experiment allowed a direct determination of the ratio of the laser intensity to the atomic saturation intensity.
查看更多>>摘要：? 2021 Elsevier B.V.Using a 90-m-long few-mode dispersion-shifted fiber, we experimentally demonstrate the high-gain and large-bandwidth parametric amplification for both the LP01 and the LP11 modes via intramodal four-wave mixing (FWM) processes. We find that the gain for intramodal FWM in the higher-order spatial mode LP11 exhibits novel characteristics due to the coupling between polarization and spatial modes. Moreover, for the 1535 nm signal input propagating along the LP01 and LP11 modes, the gain-equalized amplification can be realized through the two intramodal FWM processes. Our investigation has potential application in developing low-noise amplifiers for mode-division multiplexing communication systems.
查看更多>>摘要：? 2021 Elsevier B.V.The tuning nonlinearity of a laser is the main source that will deteriorate the spatial resolution in a frequency scanning interferometry (FSI) system. In the present study, a nonlinear error correction method based on a fiber resonator is proposed. Equal-spacing frequency sampling is achieved by resampling the raw signal at the position of the peaks of the fiber resonator signal. The effectiveness of the method is evaluated by checking the spectrum of a 2-m Michelson or Mach–Zehnder interferometer (MZI) and its residual nonlinearity in the FSI scheme. The distributed strain measurement is also performed, which further verified the usefulness of the method in the tuning nonlinearity of the laser in FSI system.
查看更多>>摘要：? 2021 Elsevier B.V.In this work, we present a numerical investigation of the pulse dynamics in an all-normal dispersion fiber laser mode-locked by a physical saturable absorber. We erected a theoretical model including the reverse saturable absorption (RSA) and spectral filtering effects. Utilizing the double-parameter scanning method, the pulse state distribution is mapped in the two-dimensional parameter space. The simulation results emphasize that the RSA greatly affects the transitions of pulse states and pulse properties. Moreover, we explore the multipulse region in detail and then demonstrate two completely different multipulse evolution dynamics. Based on this, we offer an analysis of dissipative soliton resonance multipulse generating in the physical saturable absorber-based ultrafast fiber laser.
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