Optics letters最新文献

The orbital angular momentum (OAM) of beams provides an additional degree of freedom and has been applied in various scientific and technological fields. Accurate and quantitative measurement of intensity distributions across different OAM modes, referred to as the OAM spectrum of a beam, is crucial. Here, we propose a straightforward and efficient experimental setup for measuring the OAM spectrum of a randomly fluctuating beam. By employing a modal decomposition analyzer, a randomly fluctuating light field can be decomposed into an incoherent superposition of a series of modes, followed by a coordinate transformation to calculate the OAM spectrum. This method is suitable for measuring the OAM spectrum of partially coherent beams and superposition of vortex beams. The experimental results are in good agreement with the theoretical predictions. Precise measurement of the OAM spectrum is critical for various applications in optical communications, quantum optics, and digital imaging.

In this Letter, an integrated deceptive sensing and secure communication scheme based on random subcarrier (RSC) orthogonal frequency-division multiplexing (OFDM) in a photonics-assisted millimeter-wave (MMW) system is proposed. Based on chaotic encryption on bit and constellation level, the RSC-OFDM signal is used to further disturb the transmitting signal and achieve sensing deception. Moreover, to make full use of sensing function to achieve collaborative security, sensing-aided dynamic parameter (DP) encryption is designed, which uses sensing information as the random seed to get encryption DPs and sequences. In the experiment, the RSC-OFDM signal integrated deceptive sensing and secure communication are successfully transmitted over a 1.2 m wireless channel in a 100 GHz MMW system. The results show that the proposed scheme can effectively improve the security of the system while ensuring bit error rate performance. The data rate can be up to 7.69 G bit/s and the scheme can achieve centimeter-level resolution and millimeter-level accuracy. Meanwhile, the scheme can successfully achieve distance deception between 0.25 and 8.03 m for 1.2 m target.

The Shack-Hartmann wavefront sensor (SHWS) is known for its high accuracy and robust wavefront sensing capabilities. However, conventional compact SHWS confronts limitations in measuring field-of-view to meet emerging applications' increasing demands. Here, we propose a high-density lens transfer function retrieval (HDLTR)-based SHWS to expand its field-of-view. In HDLTR-SHWS, an additional lens is introduced into the measurement system, which converges input wavefront with large aperture onto detectable aperture of sensor. A densely sampling set of phase delays is first employed to retrieve the transfer function of the lens and to isolate lens distortion, which is used to accurately demodulate convergent wavefronts and reconstruct incident wavefronts. We also utilize a global spot matching method to reconstruct the converged wavefront with a large dynamic range. Our experimental results demonstrate that the HDLTR-SHWS expands the field-of-view of SHWS by a factor of 24.9 and achieves an accuracy of less than λ/80.

Exceptional points (EPs) have been the subject of wide concern because of their unique physical properties and have produced many related applications. However, up to now, most non-Hermitian metasurfaces related to EPs focus on realizing a single function. It remains a challenge to integrate multiple functions into a single non-Hermitian metasurface while making it dynamically adjustable. In this work, we investigated how to combine the Pancharatnam-Berry (PB) phase and exceptional topological (ET) phase and used the polarization decoupling property of EPs to design and realize the integration of multiple functions on a metasurface with the assistance of vanadium dioxide (VO₂). As an example in applications, the phase-only metalens and dual-channel holograms can be realized and switched by changing states of the VO₂ and incident light polarization. This work will open a prospect for researching a multifunctional non-Hermitian metasurface and the design of switchable multifunctional nano-photonic devices.

In this study, we present an all-optical image reconstruction technique leveraging a diffractive deep neural network (D2NN) within a ring-core fiber (RCF) architecture. Orbital angular momentum (OAM) modes are employed to facilitate imaging transmission. We experimentally validate the efficacy of our approach for complex field diffractive image reconstruction through a multimode fiber (MMF) and RCF at a 1550 nm operating wavelength. The experimental results demonstrate the superior performance of the proposed method in mitigating RCF scattering-to-restoration transformation issues, significantly outperforming traditional MMF-based imaging correction techniques.

Frequency modulation of narrow-linewidth lasers can cause coherent backscattering in cladding-pumped fiber amplifiers. This detrimental effect can be observed in Tm-based fiber amplifiers and can be an additional limitation for power scaling applications. We investigate such instabilities in Tm- and Tm/Ho-doped fiber amplifiers for a wide range of design parameters (active fiber length, pumping scheme, dopant type) and operation regimes (laser frequency tuning rate, amplifier gain). For each amplifier configuration, the backward-propagating (BP) signal is found to peak at a specific laser frequency tuning rate, with an amplitude and a frequency that increase with increasing amplifier gain and fiber length.

Spectral scanning, which utilizes the dispersive effect of light, is a simple and robust method for solid-state beam steering in light detection and ranging (LiDAR) applications. Powered by a tunable laser source, optical frequency-domain reflectometry (OFDR) is a high-precision measurement scheme that is inherently compatible with spectral scanning. Here, we propose a spectral-scanning LiDAR based on OFDR technology and demonstrate that, by connecting the measured spectral reflectivity and group delay of the targets with the dispersion equation, their cloud point data can be obtained. Moreover, compared to the spectral-scanning LiDAR based on the frequency-modulated continuous-wave (FMCW) ranging method, our proposed LiDAR scheme offers a more than tenfold improvement in range resolution with a large number of angular pixels. This enhancement enables high-resolution 3D imaging along both the angular and range axes.

The long fiber length required for the amplification of bismuth-doped fiber (BDF) has hindered its practical application. In this paper, we propose and demonstrate a feasible method to improve the active absorption of bismuth active centers (BACs) by optimizing the drawing conditions, achieving a high gain with a short fiber length. The bismuth-doped phosphosilicate fiber (BPSF) preform was fabricated by the modified chemical vapor deposition (MCVD) process and drawn into fiber under nine different conditions. The results indicate that the active absorption of BACs increases as the drawing temperature increases and the drawing speed decreases within these drawing parameters. Meanwhile, the corresponding gain per unit length is improved. Furthermore, a maximum gain of 31.6 dB at 1350 nm with the >20 dB gain wavelength range of 1311-1401 nm was achieved in a double-pass double-pump configuration, using only 45 m BPSF. Meanwhile, the -3 dB bandwidth was 1328-1370 nm. The gain per unit length is 0.7 dB/m, which, to the best of our knowledge, is the highest gain per unit length reported for the BPSF.

The short-wave infrared range is highly significant for spectroscopic sensing and upcoming optical communication applications. Integrating active and passive photonic components is essential to achieve compact optical solutions. In this paper, we show, for the first time to our knowledge, a wavelength-selective detection system based on the heterogeneous integration of two grating-coupled InGaAs photodetectors operating at the 2µm wave band, with a wavelength selectivity provided by a dual-channel Mach-Zehnder interferometer fabricated using a silicon-on-insulator (SOI) wafer. A full system responsivity of 0.1 A/W is measured together with >9.5 dB rejection ratio at two wavelengths. To our knowledge, we achieve the lowest measured dark current density (7.6 × 10^-4 A/cm² at -2 V) with micro-transfer printed integrated detectors.

We propose a multidimensional (MD) quadrature phase shift keying (QPSK) symbol modulation and demodulation scheme based on random sequence extraction, which overcomes the limitations of traditional QPSK signals in performing probabilistic shaping. Unlike probabilistic shaping (PS) and geometric shaping (GS), our approach does not require modifications to the existing digital signal processing (DSP) architecture; it only necessitates the addition of sequence modulation and demodulation modules. Additionally, the source entropy (SE) can be flexibly adjusted. An experimental verification was conducted in a 50 m, 150 GHz photonics-aided wireless communication system. The results show that, under the same net data rate, the random sequence extraction QPSK scheme can achieve a maximum optical power gain of 0.7 dB. These findings indicate that the random sequence extraction QPSK (SE-QPSK) scheme can effectively enhance system performance, providing a promising outlook for future wireless communication.