Optics letters最新文献

Metro-access networks are a type of optical network connecting metro hubs with various subnetworks, covering from rural to dense urban regions. In the long term, the metro-access network is expected to address hundreds of Tb/s aggregated traffic, which makes spectral efficient multiplexing techniques a must-have. Combining wavelength division multiplexing (WDM) and digital subcarrier multiplexing (DSCM) techniques is a possible successful industrial approach to cope with this challenge. However, the ever-growing demand for bandwidth/wavelength inevitably induces increased complexity and cost of the metro-access network architecture. In this Letter, we describe the design and experimental assessment of a semiconductor optical amplifier (SOA)-based optical add-drop multiplexer (OADM) with the coherent DSCM technique, which can dynamically add and drop traffic at any node within the network and enable lossless transmission at a relatively low cost due to photonic integration. The results show that the proposed architecture can support up to five nodes at a net data rate of 291 Gb/s/λ with negligible penalty from dropping and adding operations, indicating its great potential for long-term metro-access network adoption.

The referenced article [Opt. Lett.49, 6693 (2024)10.1364/OL.544099] has been retracted by the authors.

Nowadays, there is a surge of research reports on photovoltaic cells and photodetectors based on van der Waals (vdW) heterojunction. However, there are very few reports on the simultaneous realization of photoelectric conversion and storage in a single device. Herein, a simple method was utilized to successfully prepare a van der Waals heterojunction with a chargeable photoconductivity (CPC) effect between black phosphorus and quasi-two-dimensional electron gas. After a single optical illumination, the device generated photocarriers and stored them for up to 5 days, followed by a bias voltage applied in the dark to release a large photocurrent of 1.0 mA and a current ratio of up to 10⁸ before and after full optical charging. Firstly, experiments showed that the CPC effect critical temperature was approximately 100 K, which is 20 K higher than the highest temperature reported in the literature. Secondly, the CPC effect spectral range of the device increases from 405 nm-808 nm to 405 nm-1064 nm, including even general daylight, indicating that the device's responsivity and photon collection ability are more prominent. This discovery provides a new direction, to the best of our knowledge, for storage photoconductors and high-efficiency photoelectric storage devices.

Partial response equalization (PRE) is a spectral shaping technique that enhances robustness to the inter-symbol interference (ISI) due to bandwidth limitation in the high-speed intensity modulation and direct detection (IM/DD) systems. The simple 1/(1 + D) decoder can be used to decode the PR-equalized signal. However, it suffers from error propagation, leading to performance degradation. In this Letter, we propose a multiplication-free error corrector (MF-EC) to suppress the error propagation resulting from the 1/(1 + D) decoder of second-order PRE. The proposed method can effectively locate the beginning and end of burst errors with only additive operations. We experimentally demonstrate the performance of the proposed method in 256-Gb/s 4-ary pulse amplitude modulation (PAM-4) IM/DD systems. The results show that the PRE with MF-EC can effectively eliminate the error propagation of the 1/(1 + D) decoder, reducing the maximum length of burst errors from 15 to seven. Moreover, the proposed method outperforms the PRE with precoding and error-correlation based DFE (EC-DEF). As for the 500-m transmission scenario, the PRE with MF-EC exhibits similar performance to the PRE-MLSE decoder at the KP4-FEC threshold but can reduce the complexity.

Computational holographic displays typically rely on time-consuming iterative computer-generated holographic (CGH) algorithms and bulky physical filters to attain high-quality reconstruction images. This trade-off between inference speed and image quality becomes more pronounced when aiming to realize 3D holographic imagery. This work presents 3D-HoloNet, a deep neural network-empowered CGH algorithm for generating phase-only holograms (POHs) of 3D scenes, represented as RGB-D images, in real time. The proposed scheme incorporates a learned, camera-calibrated wave propagation model and a phase regularization prior into its optimization. This unique combination allows for accommodating practical, unfiltered holographic display setups that may be corrupted by various hardware imperfections. Results tested on an unfiltered holographic display reveal that the proposed 3D-HoloNet can achieve 30 fps at full HD for one color channel using a consumer-level GPU while maintaining image quality comparable to iterative methods across multiple focused distances.

A synaptic device is fundamental for memory and learning functions of neural networks. In this work, we demonstrated a GST (Ge2Sb2Te5)-based microring synapse with nonvolatile reconfigurable characteristics. The device shows bidirectional weight modulation during unidirectional crystallization or amorphization of GST, simulating long-term depression (LTD) and long-term potentiation (LTP). In addition, we adopted an anti-resonance pump scheme to reduce the fluctuations in pump energy coupled into the device to less than 12%. This scheme significantly enhances the programming precision of the microring synapse, achieving 24 resolvable states over 15 cycles. This work holds promise for laying the foundation for novel, to the best of our knowledge, photonic computing architectures and provides possibilities for the implementation of vision and adaptive optical neural networks that rely on bidirectional plasticity.

We develop compact microsphere self-interference lithography via a single laser beam incident into a self-assembled dual-layered microsphere array to achieve parallel fabrication of periodic units with nanopatterns (PUNs). Interference units with tens of millions are achieved through micron-thick dual-layered microsphere arrays. The periodic units with nanoholes (NHs), nanogrooves (NGs), and nanoslots (NSs) can be fabricated by simply varying incident laser polarization states. The minimum linewidth is 75 nm (∼λ/4.5), and the single-shot exposure area is up to 1 cm². An analytical model of polarization-dependent tri-beam interferences is developed to interpret the PUN formation. Au-coated PUNs demonstrate extraordinary performance for customized surface-enhanced Raman spectroscopy substrates, of which the polarization sensitivity can be regulated and the limit of detection is down to 3 × 10^-10 M. The present work opens up new opportunities for high-throughput laser parallel nanofabrication for various applications.

Flexible and compact optical switch is important for optical communications. In this Letter, a thermo-optical switch with wavelength division (de)multiplexing (WDM) function has been proposed and demonstrated experimentally. The switch is comprised of a 1 × 2 Bezier multimode interferometer (MMI) and an angled multimode interferometer (AMMI). The switch separates 1490 and 1550 nm wavelengths and realizes path reconfiguration simultaneously. The demonstrated device with a footprint of 13.63 × 0.385 mm² is built on a 2% refractive index difference silica platform. The excess loss is lower than 1.2 dB (the reference waveguide loss is larger than 5.6 dB). In the central wavelength, the cross talk is lower than -21.7 dB and the extinction ratio is larger than 21.7 dB. The 3 dB bandwidth is larger than 30.8 nm. The wavelength shift rates of central wavelength are ∼54 pm/nm in C band and ∼74.8 pm/nm in the S band with the width of the AMMI changed. The rise (10%-90%) and fall (90%-10%) times of the switch are 0.88 and 0.94 ms, respectively, with a maximum power consumption of 246.6  mW. The demonstrated switch combines WDM and router functions together. Scalability property makes it possible for a large port count number switch.

Germanate glass, owing to its high infrared transmittance, high refractive index, and excellent nonlinear optical properties, has become a key material in the field of photonics. Inducing micro-nanostructures on the surface of germanate glass using femtosecond lasers can impart new functionalities and applications to the material. In this study, self-assembled nanograting structures were successfully induced on the surface of germanate glass by femtosecond laser direct writing. The effects of laser parameters, including energy density, scanning speed, and polarization direction, on the grating morphology and periodicity were systematically investigated. We first found that curving nanogratings can be induced at a combination of high laser energy and low scanning speed. Straight nanogratings with steady periods can be obtained by reasonably increasing scanning speed. The nanograting period was found to change with varying polarization angles. By optimizing the process conditions, large-area, highly uniform nanograting arrays were successfully fabricated. Furthermore, by taking advantage of the rewritable characteristics of nanogratings, different micro-nanostructures with varying periods were produced by adjusting energy combinations in consecutive scans. These findings could extend the application of germanate glass in surface photonics and information technology.

Spot-size converters (SSCs) are key for efficient coupling of light between waveguides of different sizes. While adiabatic tapers are well suited for small size differences, they become impractically long for expansion factors around ×100, which are often required when coupling integrated waveguides and free-space beams. Evanescent couplers and Bragg deflectors can be used in this scenario, but their operation is inherently limited in bandwidth. Here, we propose a solution based on a parabolic dielectric interface that couples light from a 0.5μm wide waveguide to a 285μm wide waveguide, i.e., an expansion factor of ×570. We experimentally demonstrate an unprecedented bandwidth of more than 380 nm with insertion losses below 0.35 dB. We furthermore provide analytical expressions for the design of such parabolic spot-size converters for arbitrary expansion factors.