Search

A block-based mode decomposition (BMD) algorithm is proposed in this paper, which reduces computational complexity and enhances noise resistance. The BMD uses randomly selected sample blocks of the beam images to restore mode coefficients instead of all pixels in the beam images. It allows for blocks of any shape, such as triangles. In noise-free simulations, compared to the spatially degenerated mode decomposition (SPMD) algorithm, the BMD algorithm requires only 1% of the multiplication operations, thereby significantly increasing the computational efficiency while maintaining a high mode decomposition accuracy. In simulations with noise, increasing the signal-to-noise ratio (SNR) reduces decomposition errors across all configurations. The amplitude error of BMD can outperform SPMD by 15 dB. The experimental results show that BMD has a better performance than SPMD.

The performance of mode division multiplexing (MDM) systems is limited by mode-dependent loss (MDL), which seriously deteriorates the unitarity of the transmission matrix, making the traditional detection algorithms ineffective. In order to mitigate the effect of MDL in MDM transmission and improve the efficiency of digital signal processing at the receiver, a novel coordinate descent (CD)-based box-constrained detection algorithm using adaptive moment estimate (CDbox-Adam) is proposed. In the process of each coordinate update for the CDbox detection algorithm, the adaptive moment estimation (Adam) method adaptively updates the step size for different parameters from the first moment estimate and the second moment estimate of the gradients. To simplify the process of each coordinate updating of the CDbox-Adam algorithm, the CDbox-Adamax detection algorithm is also proposed. The CDbox-Adamax detection algorithm generalizes the 2-norm update rule of weights to the infinite norm. At the same time, an early stop criterion is proposed to efficiently avoid unnecessary iterations. Simulation results show that the early stop criterion reduces the number of iterations of the proposed detection algorithms. For a 12-mode MDM system impaired by MDL, the average number of iterations of the proposed detection algorithms is less than the CDbox detection algorithm. Thus, the proposed detection algorithms present lower computational complexity than the CDbox detection algorithm. Furthermore, compared with the conventional minimum mean square error (MMSE) detection algorithm, the proposed detection algorithms, CDbox-Adam and CDbox-Adamax, gain signal to noise ratio (SNR) improvement of 2.3 dB and 1.6 dB at BER = $10^{-5}$ when MDL = 5 dB with QPSK and 16QAM, respectively. In the presence of MDL = 5 dB and 10 dB, when compared with the proposed detection algorithms with perfect CSI, the proposed detection algorithms with LS channel estimation requires 5.1 dB and 2.5 dB more of SNR to get the same BER performance, respectively. Moreover, the results suggest that at high SNR the proposed detection algorithms outperform the least mean squares (LMS) and recursive least squares (RLS) algorithms for modal dispersion (MD) and MDL-impaired MDM systems by using least squares (LS) channel estimation, which indicate their strong potential for MDM transmissions.

Parity-time (PT) symmetry has garnered significant attention in the field of photonics owing to its remarkable properties. High-performance lasers, which require simultaneous gain and loss, serve as an ideal platform for harnessing the exceptional properties of PT symmetry. In this study, we present and experimentally verify a polarimetric PT-symmetry fiber laser that uses a polarization-maintaining fiber Bragg grating (PM-FBG). This design allows for tunable, switchable dual-wavelength operation with narrow linewidth and orthogonal single-polarization characteristics. By converting two equal-length yet distinct polarization states into two spatial subspaces sharing a common wavelength space, we can achieve PT symmetry. Regulating the polarization controllers (PCs) enables interconversion between PT-symmetry phases. During the PT-symmetric phase (or PT exact phase), a dual-wavelength laser is realized with wavelength fluctuations below 0.098 nm and power fluctuations below 2.175 dB. At this stage, light propagates through both ordinary and extraordinary wavelength-spaces. During the PT-broken phase, one waveguide has a net gain and the other incurs a net loss, leading to a single longitudinal mode (SLM) laser output. The wavelength and power fluctuations of left-wavelength λ1 and right-wavelength λ2 remain below 0.002 nm and 0.770 dB over a 20-minute period, during single-wavelength operation. By adjusting the axial strain on the PM-FBG, the laser output can be tuned within the wavelength range of 1549.754 nm to 1551.690 nm, while maintaining a linewidth variation of less than 0.130 kHz. As the wavelength is adjusted, the azimuth sum of λ1 and λ2 consistently remains close to 90 degrees, accompanied by degrees of polarizations exceeding 99%.

This paper describes a novel compact fiber sensor with high sensitivity for the simultaneous measurement of underwater salinity and temperature by simulation. The structure is based on a highly birefringent, asymmetric, full-circular hole photonic crystal fiber (HB-A-FCH PCF), which achieves the birefringence value of 2.83 × 10-3 at the wavelength of 2.2 µm. In addition, with ethanol-filled specific air holes, the temperature sensitivity of HB-A-FCH PCF is remarkably improved. Therefore, considering the high birefringence property, we simulate a Michelson interferometer-based sensor setup with air holes as fluid microchannels and simultaneously measure salinity and temperature through the spectral responses of the x- and y-polarization state. Theoretical analysis shows the sensor can achieve the salinity and temperature sensitivity up to 5.1472 nm/% and −0.8987 nm/°C in x-polarized fundamental mode, respectively, whereas the salinity and temperature sensitivity up to 4.5386 nm/% and −0.8000 nm/°C in y-polarized fundamental mode, respectively.

In this paper, we propose and design a magnetic field and temperature sensor using a novel petaloid photonic crystal fiber filled with magnetic fluid. The PCF achieves a high birefringence of more than 1.43 × 10−2 at the wavelength of 1550 nm via the design of material parameters, air hole shape and the distribution of the photonic crystal fiber. Further, in order to significantly improve the sensitivity of the sensor, the magnetic-fluid-sensitive material is injected into the pores of the designed photonic crystal fiber. Finally, the sensor adopts a Mach–Zehnder interferometer structure combined with the ultra-high birefringence of the proposed petaloid photonic crystal fiber. Magnetic field and temperature can be simultaneously measured via observing the spectral response of the x-polarization state and y-polarization state. As indicated via simulation analysis, the sensor can realize sensitivities to magnetic fields and temperatures at −1.943 nm/mT and 0.0686 nm/°C in the x-polarization state and −1.421 nm/mT and 0.0914 nm/°C in the y-polarization state. The sensor can realize the measurement of multiple parameters including temperature and magnetic intensity and has the advantage of high sensitivity.

A CS2-filled Er-doped fiber, which can provide high nonlinearity and gain, low and flat all-normal dispersion profile, low loss and single-mode operation in the wavelength range of 1500-2500 nm, is proposed and analyzed by numerical simulation. When the 1900-nm 0.1-ps initial pulse with a peak power of 30 kW is injected into the active fiber, the generated supercontinuum (SC) spectrum has a double-hump structure in the 1550-nm window and covers about 800-nm wavelength range spanning from 1400 to 2200 nm at -20 dB level. Furthermore, the 1550-nm pulse can be obtained synchronously with the 1900-nm initial pulse when the amplified pulse with a temporal double-peak shape is passed through a band-pass filter with its center wavelength at 1550 nm.

Abstract This paper introduces a planarisation SiO2 layer into the configuration of ZnO‐silicon‐on‐insulator for constructing an acoustical resonant cavity to enhance the acoustic density on a silicon‐on‐insulator waveguide. The improved configuration with a planarisation SiO2 layer can increase the acousto‐optical interaction by about three times and 20% compared with acousto‐optical configuration of ZnO pads only in an interdigital transducer region and all over a silicon on insulator. Moreover, an introduction of a planarisation SiO2 layer can reduce acoustic reflection on optical waveguide and optical waveguide loss for an acousto‐optical device on silicon‐on‐insulator optical waveguides.

Compared with successive cancellation list (SCL) decoders, belief propagation (BP)-based decoders suffer performance loss in middle-and high-signal-to-noise ratio (SNR) regions. By analyzing the behavior of the incorrect decoding results of the bit-flipping BP decoder with a critical set of order ω (BFBP-CSω), we found that undetected errors mainly contribute to the error floor. Based on this observation, we proposed a belief propagation decoder with multiple bit-flipping sets (BFSs) and stopping criteria (BP-MF-MC) in this work. We use multiple stopping criteria to identify undetected errors and a small BFS to find an additional estimated codeword given by the bit-flipping BP (BFBP) function. For uncorrected errors, we use multiple BFSs to find estimated codewords with the BFBP function. Furthermore, we propose a method to dynamically generate a BFS based on the submatrix check. This method can remove unnecessary bit-flipping positions and increase the order of the critical set. Then, the best codeword is selected from all estimated codewords according to the maximum likelihood principle. Numerical results show that BP-MF-MC performs similarly to the cyclic redundancy check-aided SCL (CA-SCL) decoder with list size 16 and is slightly worse than CA-SCL with list size 32.

An analysis model for an optical waveguide microcantilever sensor is developed combining the optical and mechanical models. An improved optical waveguide microcantilever sensor with a buffer is provided and taken as an example to explore using the analysis model. A systematic and detailed discussion about the couplers for the input waveguide to optical waveguide cantilever and the optical waveguide cantilever to the output waveguide of the improved waveguide cantilever sensor is presented. The sensitivity of an improved optical waveguide cantilever sensor is evaluated by analyzing the input/output waveguide, buffer, microcantilever, and gap. An improved optical waveguide microcantilever sensor by adding a buffer shows a sensitivity of 5.7 × 10-4nm-1, which is improved by 51.3%, compared with a conventional optical waveguide microcantilever sensor. The design of an optical waveguide cantilever sensor is a trade-off of different design parameters. These will be helpful for the study of an optical waveguide cantilever sensor.

An image compression-encryption scheme utilizing the combination of fiber specklegram-based compressive sensing (CS) and double random phase encoding (DRPE) was demonstrated. The original image is compressed and encrypted by CS and then re-encrypted by DRPE. The measurement matrices for CS are constructed from multimode fiber specklegrams resulting from modes excitation and interference, which show better performance compared with those based on Gaussian random variables. Since fiber specklegrams have strong dependence on configuration, such measurement matrix can be constructed either by one specklegram or by specklegrams from different launched wavelengths. The double-encrypted image demonstrated good safety and reliability in several common attacks and presented high key sensitivity, which indicates good performance of the proposed scheme.

We report a dual-wavelength fiber laser sensor based on a uniform fiber Bragg grating (UFBG) and a Polyvinyl alcohol (PVA) film-coated long-period grating (LPG) as sensor probe for simultaneous detection of relative humidity (RH) and temperature. Two UFBGs and the LPG were inscribed in three single mode fibers utilizing phase/amplitude mask technology and mobile scanning technique via the 248 nm UV laser. In our experiment, the RH function based on the differential intensity measurement at dual-wavelength output from 30% to 85% is in a quadratic equation, and the RH sensitivity coefficient of 0.358 dB/% shows good linear relationship and stability from 55% to 85%. Meanwhile, the temperature sensitivity coefficients based on wavelength detection and differential intensity detection are 9.1 pm/°C and 0.21 dB/°C, respectively. The structure of dual-wavelength fiber laser sensor with high signal to noise ratio (SNR), narrow spectral width and good stability enables simultaneous measurement of the RH and temperature with high accuracy and good repeatability.

The three-core photonic crystal fiber (TC-PCF) is proposed to obtain large group-velocity dispersion (GVD) because of the coupling effects between the fundamental modes (FMs) of the central core and higher-order modes (HOMs) of the two side cores. The supermodes of the TC-PCF can provide concave anomalous- and normal-GVD profiles with large peak values at the maximum dispersion wavelengths (MDWs), which can be shifted in three wavelength windows of 1030 nm, 1550 nm and 1900 nm by properly tuning the refractive indexes of the cores or the air-hole diameters. Furthermore, the numerical results show that two segments of the TC-PCFs with large anomalous GVDs and opposite values of the third-order dispersion (TOD) can provide much higher efficiency of pulse compression than one segment of the TC-PCF. Additionally, the TC-PCFs with large normal GVDs can be used to stretch pulses due to their low nonlinearities and short fiber lengths.

A single longitudinal mode (SLM) dual-wavelength switchable erbium-doped fiber laser (DW-EDFL) based on polarization-maintaining fiber Bragg grating Fabry–Perot cavity (PMFBG-FP) cascaded multiple sub-ring cavities (MSCs) is proposed. A PMFBG-FP with a narrow-band transmission peak and MSCs was implemented as an optical filter to achieve stable dual-wavelength laser output and guaranteed SLM status. By stretching the PMFBG, a highly stable dual-wavelength tunable output could be achieved with a maximum tuning interval of 0.17 nm. The optical signal-to-noise-ratio (OSNR) at dual-wavelength lasing was higher than 57 dB, and the optimal wavelength and power fluctuations within 0.5 h were 0.01 nm and 0.79 dB, respectively. Meanwhile, the measured linewidths of each wavelength were 1.55 kHz and 1.65 kHz, respectively. The measured polarization states of the two laser wavelengths were linear and orthogonal, with a degree of polarization (DOP) of nearly 100%.

Establishing parity-time (PT) symmetry in non-spatial space is a promising way to simplify the PT-symmetry system structure. In this paper, an implementation of PT symmetry in optical polarization space is reported. By utilizing the polarization multiplexing technology, PT symmetry is formed in overlapping spatial parameter space. The degeneracy of eigenmodes with two distinct PT phases in polarization space is demonstrated. In the PT-symmetric phase, the eigenmodes have real eigenfrequencies that respect PT symmetry, exhibiting broken degeneracy (mode splitting); in the PT-broken phase, the eigenmodes are degenerate with a pair of complex conjugate eigenfrequencies. The sharp-pointed peak filter response in the PT-broken phase due to the strong field localization is characterized, which explains the mode-selection mechanism of PT-symmetry breaking. The polarization-space PT symmetry is applied in a 7-km single-loop optoelectronic oscillator, and a stable single-mode oscillation signal is generated with a phase noise of −138 dBc/Hz at 10 kHz and side-mode suppression ratio of 49 dB. The approach expands the parameter spaces to carry out PT symmetry and could promote the integration of the PT-symmetry photon system.

In this paper, a relative humidity (RH) and temperature sensing scheme with high sensitivity based on an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. A polyvinyl alcohol-Fiber Bragg grating-Fabry Perot (PVA-FBG-FP) filter, in which the PVA film acts as an RH-strain transducer, is employed in the OEO to implement a single passband microwave photonic filter (MPF). In the OEO loop, the oscillating frequency is determined by the center frequency of the PVA-FBG-FP filter and the laser source. When the external RH or temperature changes, the central wavelength of PVA-FBG-FP filter is shifted, leading to the change in the center frequency of the MPF. By simply monitoring the variation of oscillating frequency using an electrical spectrum analyzer or a digital signal processor, the measurements for the RH and temperature can be achieved. In the experiment, the sensitivities of the RH and temperature are measured to be as high as 508.3 MHz/%RH and 1.8118 GHz/°C, respectively.

We propose a novel technique for joint modulation format identification and frequency offset (FO) estimation. The linear frequency modulation signal which is obtained by a direct current signal and Fractional Fourier Transform (FrFT) is superimposed on the data symbols to mark and identify the modulation formats. Simulation results demonstrate our method can identify the DPBPSK, DPQPSK, DP8PSK, DP16QAM, DP32QAM and DP64QAM modulation formats correctly before chromatic dispersion compensation even the optical signal to noise ratio is extremely low. Additionally, due to the excellent time and frequency analysis properties of FrFT, the FO estimation range is greatly extended compared with the traditional Viterbi-Viterbi method.

An ultrafast high-sensitivity refractive index (RI) and temperature-sensing system based on an optoelectronic oscillator (OEO) is proposed and demonstrated in this paper. A Fabry-Perot fiber Bragg grating (FP-FBG), which combines a gap with two FBGs in a silica V-shaped slot and characterizes a narrow notch in the reflection spectrum, is incorporated in the OEO to implement a microwave photonic filter and perform oscillating frequency selection. A microwave signal is generated by the OEO, whose oscillating frequency is determined by the center frequency of the FP-FBG notch, which varies with the surrounding environments. The RI or the temperature change can be accordingly measured by monitoring the frequency shift of the microwave signal using an electrical spectrum analyzer or a digital signal processor. An experiment is performed to verify the proposal. An RI sensitivity of 413.8 MHz/0.001RIU and a temperature sensitivity of 2516 MHz/°C are successively demonstrated.

A photonic-assisted microwave mixer using a Sagnac-loop-based modulator and polarization-dependent modulation is proposed and experimentally demonstrated. In this scheme, a dual-parallel Mach-Zehnder modulator (DPMZM) is incorporated in a Sagnac loop, where light is single-sideband (SSB) modulated along one direction with no modulation along the opposite direction due to the traveling-wave velocity mismatch in DPMZM. The SSB-modulated light and unmodulated optical carrier (OC) with opposite propagation directions and orthogonal polarizations are combined via a polarization beam combiner, and thus, a partial orthogonal SSB-modulated signal is obtained, which is then injected into a Mach-Zehnder modulator (MZM) via a polarization controller to ensure the polarization of the unmodulated OC paralleling with the transverse electric (TE) principal axis of MZM. Thus, only the OC is modulated by the data, and the first-order sidebands pass through the MZM directly without modulation based on the polarization dependence of MZM. After a polarizer for polarization interference and a photodetector for optical-to-electrical conversion, a phase-stable microwave signal is obtained. By adjusting the bias point of MZM, clear eye diagrams, and waveforms of 1 Gb/s on-off keying (OOK), amplitude-shift keying (ASK), and binary-phase-shift keying (BPSK) signals at 8 GHz are experimentally achieved.

A novel technique to implement a multitap microwave photonic filter with positive and negative coefficients based on the inherent birefringence in a $\hbox{LiNbO}_{3}$ phase modulator is proposed and demonstrated. In the proposed filter, a microwave signal is applied to the phase modulator and an optical polarizer is connected at the output of the phase modulator to perform polarization interference and phase-modulation to intensity-modulation (PM–IM) conversion. Thanks to the inherent birefringence in the $\hbox{LiNbO}_{3}$ crystal, a $\pi$ phase shift is obtained by adjusting the wavelength spacing between two adjacent wavelengths, which leads to the generation of a positive coefficient and a negative coefficient. An equivalent experiment is performed. Four-tap and six-tap microwave photonic filters with positive and negative coefficients are experimentally demonstrated. The reconfigurability of the four-tap and six-tap microwave photonic filters is also investigated.