Optics letters最新文献

Multi-singularity structured beams carrying SU(2) modes offer significant potential for expanding communication capacity by leveraging their three orthogonal mode degrees of freedom (DoFs): central orbital angular momentum (OAM), sub-beam OAM, and coherent-state phase. Despite considerable advancements in the recognition of SU(2) modes, challenges remain in the sorting of these modes for signal demultiplexing, including multi-mode conversion and mode separation. In this work, we propose a novel, to the best of our knowledge, SU(2) mode sorting strategy based on off-axis phase modulation, which converts SU(2) modes into quasi-Gaussian modes with customized diffraction angles by combining conjugate phase and gradient phase modulation, thus sorting coaxially transmitted SU(2) modes to distinct spatial positions. We demonstrate the simultaneous sorting of 18 SU(2) modes (including three central OAM, three sub-beam OAM, and two coherent-state phases) with minimal cross talk, achieving levels below -13.8 dB. As a proof of concept, we have developed an SU(2) mode multiplexing communication link that successfully transmits 100 Gbit/s quadrature phase-shift keying (QPSK) signals with bit error rates (BERs) below 10^-5. Our approach provides flexibility in the spatial reallocation of SU(2) modes and holds promise for advancing multidimensional multiplexing communication and high-efficiency shift-keying (SK) communication.

Quantum random number generator (QRNG) allows for the production of truly unpredictable random numbers, thanks to the inherent randomness available in quantum mechanics. However, its practical implementation is facing several challenges, including the practical security loophole, high-speed real-time randomness extraction, and large-scale production. In this work, we address these challenges with a chip-based, source-independent quantum random number generator achieving 20 Gbps real-time generation rate. It ensures the practical security through a source-independent security framework and the modeling of measurement devices. A bandwidth exceeding 2 GHz and a clearance reaching 10 dB is achieved by a silicon-based homodyne detector. Analog-to-digital conversion and randomness extraction are integrated on a single data collection and processing board, where the optimized parallel randomness extraction algorithm on a field programmable gate array achieves a throughput up to 28 Gbps. The results promise a high-speed and practically secure quantum random number generation on a chip, paving the way for its large-scale deployments and widespread applications.

Hyperspectral LiDAR (HSL) enables the simultaneous acquisition of the surface geometry and spectral signatures of remote natural targets, making it valuable for various applications such as material probing, automated point cloud segmentation, and vegetation health monitoring. We present a first proof-of-concept study of a broadband dual-comb HSL system based on a 1 GHz dual-comb supercontinuum (SC). The SC spans from 820 to 1300 nm, generated via coherent spectral broadening of a free-running single-cavity dual-comb oscillator at 1053 nm in a single nonlinear photonic crystal fiber. The HSL system achieves a sub-µm ranging precision on a non-cooperative target at an update rate of 670 Hz. The shot-noise limited electronic dual-comb interferograms furthermore encode the spectral information of the target reflection across the SC bandwidth. This allows the capture of precise 3D point clouds with spectral signatures, unlocking new possibilities for spectrum-based material classification.

We demonstrate a self-homodyne detection method to stabilize a continuous-wave 1550-nm laser to a 1-km optical fiber delay line, achieving a frequency instability of 6.3 × 10^-15 at a 16-ms averaging time. This result, limited by fiber thermal noise, is achieved without the need for a vacuum system, highlighting the potential of our approach for ultra-stable laser systems in non-laboratory environments. The system utilizes only a few passive fiber optic components and a single balanced photodetector, significantly simplifying the laser stabilization process while maintaining high performance. The entire optical setup is compactly packaged in a portable metal air-tight case.

We propose an iterative optimization approach for wavefront reconstruction in a rotational shearing interferometer based on Zernike polynomials. Two incorrect shearing amounts are optimized simultaneously, enabling the absolute reconstruction of wavefronts with high uniformity in the root mean square (RMS). Simulations under noisy conditions show that the reconstruction error, in terms of peak-to-valley (PV) and RMS, remains within the μλ range. The approach was experimentally validated by identifying asymmetric aberrations in an imperfectly collimated light source, showing strong agreement with the wavefront obtained from a Shack-Hartmann sensor.

The pumping mechanism based on the cross-phase modulation (XPM) effect can achieve a flat broadband supercontinuum (SC) output. In this paper, we demonstrate a novel, to the best of our knowledge, scheme for SC generation from a large mode area (LMA) erbium-ytterbium co-doped fiber (EYDF) amplifier based on this XPM effect by utilizing both the gain and dispersion characteristics of the EYDF, achieving a hundred-watt-level flat broadband SC output. The scheme consists of two pulsed lasers with a synchronized trigger signal and a LMA-EYDF amplifier. In order to enhance the XPM effect effectively, the central wavelengths of two pulsed lasers are selected as 1030 nm and 1535 nm according to the calculated group velocity curve of the passive fiber in the EYDF amplifier. The output powers of 1030 nm and 1535 nm pulses are scaled to 58 W and 131 W after the EYDF, where the dual-wavelength is first amplified and the output power is the highest level in the 1 µm/1.5 µm dual-wavelength lasers reported so far. After the dual-wavelength lasers undergo the nonlinear accumulation in the EYDF amplifier, the final SC with a spectrum spanning the 770 nm-2055 nm at -10 dB level excluding the residual pump peak is achieved and an output power of 104 W is obtained. This is the widest spectral bandwidth at -10 dB level and the highest output power in the reported SC generation from an EYDF amplifier. This work provides a common platform to generate a high-power flat broadband SC independent of fiber design, further promoting the development of LMA fiber-based SC sources.

A spiral phase filter can perform a radial Hilbert transform (RHT) and is useful in isotropic edge enhancement. For selective edge enhancement, the inclusion of anisotropy warrants the filter to be replaced. In this Letter, we introduce for the first time, to our knowledge, a novel and versatile filter that can be tuned between isotropic/anisotropic edge detection and contrast enhancement protocols. To achieve this, we use a lemon-star polarization dipole: a special kind of spin-orbit beam that is a superposition of spin and orbital angular momentum states of light. We devised a 4f imaging setup in microscope configuration to encode the object Fourier spectrum into inhomogeneous polarization distribution. The novelty and advantages of the proposed method lie in selecting the spatial frequency content through polarization transformations in the image reconstruction path, just before the detector, without altering the Fourier plane parameters. Considering a scalar-to-vector diffraction approach and invoking the polarization degree of freedom of light, the edge enhancement capabilities of a lemon-star polarization dipole and a monopole (star or lemon) are shown through experiment results.

To address the challenges posed by chromatic dispersion (CD)-induced power fading and nonlinear signal distortions in double-sideband (DSB) intensity modulation and direct detection (IM/DD) transmission systems, a combination of a Volterra feed-forward equalizer (VFFE) and a Volterra decision-feedback equalizer (VDFE) is widely employed. However, the conventional VFFE-VDFE exhibits significant computational complexity, particularly for longer memory lengths. In this Letter, a low-complexity cluster-assisting look-up table-based VDFE (CLUT-VDFE) is proposed to effectively reduce the computational complexity associated with compensating for CD and nonlinear distortions. By utilizing CLUTs, all multiplication operations required for the implementation of the VDFE are completely eliminated. To validate the effectiveness of the proposed CLUT-VDFE, experiments on a C-band 100-Gb/s PAM-4 transmission system over a 60-km standard single-mode fiber (SSMF) are conducted. The experimental results show that the CLUT-VDFE not only achieves comparable equalization performance to the conventional VDFE but also effectively eliminates multiplication operations and significantly saves 48.8% real-valued additions.

We report on an investigation of InGaAs/GaAs quantum well-dot superluminescent diodes (SLDs) based on what we believe to be a novel and simple design of stripe waveguides. The design employing a chip side facet as a component of the SLD structure allows effective suppression of optical feedback, thus increasing the optical power. The test SLDs under study emitting in 950-1150 nm spectral range show CW optical power as high as 150 mW in combination with broad emission spectra of 20 nm full width at half maximum (FWHM).

Ultra-stable optical cavities with adjustable zero-crossing temperatures feature low thermal expansion and low-temperature control power consumption. We develop a re-entrant cavity featuring flexible and nondestructive zero-crossing temperature tuning capabilities, with a tunable range of 49°C. Using the same ultra-low expansion glass (ULE) batch with a zero-crossing temperature of 16.0(4)°C, we experimentally demonstrate a re-entrant cavity with a higher zero-crossing temperature tuning to 24.7(4)°C, significantly increasing the operational range compared to traditional sandwich cavities. The ultra-stable laser system developed on this re-entrant cavity shows a thermal noise limited performance of 1.05(1) × 10^-15 at 0.2 s and a good long-term performance, making it suitable for portable applications such as space-borne laser sources.