The ability to amplify optical signals is of paramount importance in photonic integrated circuits (PICs). Recently, lithium niobate on insulator (LNOI) has attracted increasing interest as an emerging PIC platform. However, the shortage of efficient active devices on the LNOI platform limits the development of optical amplification. Here, we report an efficient waveguide amplifier based on erbium and ytterbium co-doped LNOI by using electron beam lithography and an inductively coupled plasma reactive ion etching process. We have demonstrated that signal amplification emerges at a low pump power of 0.1 mW, and the net internal gain in the communication band is 16.52 dB/cm under pumping of a 974 nm continuous laser. Benefiting from the efficient pumping facilitated by energy transfer between ytterbium and erbium ions, an internal conversion efficiency of 10% has been achieved, which is currently the most efficient waveguide amplifier under unidirectional pumping reported on the LNOI platform, to our knowledge. This work proposes an efficient active device for LNOI integrated optical systems that may become an important fundamental component of future lithium niobate photonic integration platforms.
2023, 11(10): 1733
Based on the one-dimensional periodic and Fibonacci-like waveguide arrays, we experimentally investigate localized quantum walks (QWs), both in the linear and nonlinear regimes. Unlike the ballistic transport behavior in conventional random QWs, localization of QWs is obtained in the Fibonacci-like waveguide arrays both theoretically and experimentally. Moreover, we verify the enhancement of the localization through nonlinearity-induced effect. Our work provides a valid way to study localization enhancement in QWs, which might broaden the understanding of nonlinearity-induced behaviors in quasi-periodic systems.Fibonacci-like waveguide arrays nonlinearity-induced effect localization enhancement
Chinese Optics Letters
2023, 21(10): 101301
Long-distance light detection and ranging (LiDAR) applications require an aperture size in the order of 30 mm to project 200–300 m. To generate such collimated Gaussian beams from the surface of a chip, this work presents a novel waveguide antenna concept, which we call an “optical leaky fin antenna,” consisting of a tapered waveguide with a narrow vertical “fin” on top. The proposed structure (operating around ) overcomes fundamental fabrication challenges encountered in weak apodized gratings, the conventional method to create an off-chip wide Gaussian beam from a waveguide chip. We explore the design space of the antenna by scanning the relevant cross section parameters in a mode solver, and their sensitivity is examined. We also investigate the dispersion of the emission pattern and angle with the wavelength. The simulated design space is then used to construct and simulate an optical antenna to emit a collimated target intensity profile. Results show inherent robustness to crucial design parameters and indicate good scalability of the design. Possibilities and challenges to fabricate this device concept are also discussed. This novel antenna concept illustrates the possibility to integrate long optical antennas required for long-range solid-state LiDAR systems on a high-index contrast platform with a scalable fabrication method.
2023, 11(9): 1570
Integrated optomechanical crystal (OMC) cavities provide a vital device prototype for highly efficient microwave to optical conversion in quantum information processing. In this work, we propose a novel heterogeneous OMC cavity consisting of a thin-film lithium niobate (TFLN) slab and chalcogenide (ChG) photonic crystal nanobeam coupled by a wavelength-scale mechanical waveguide. The optomechanical coupling rate of the heterogeneous OMC cavity is optimized up to 340 kHz at 1.1197 GHz. Combined with phononic band and power decomposition, 17.38% energy from the loaded RF power is converted into dominant fundamental horizontal shear mode (SH0) in the narrow LN mechanical waveguide. Based on this fraction, as a result, 3.51% power relative to the loaded RF energy is scattered into the fundamental longitudinal mode (L0) facing the TFLN-ChG heterogeneous waveguide. The acoustic breathing mode of the heterogeneous OMC is successfully excited under the driving of the propagating L0 mode in the heterogeneous waveguide, demonstrating the great potentials of the heterogeneous piezo-optomechanical transducer in high-performance photon–phonon interaction fields.
2023, 11(9): 1509
We propose and demonstrate a cryogenic thermo-optic (TO) modulator in x-cut thin-film lithium niobate (TFLN) with an NbN superconducting heater. Compared to a conventional metal heating electrode, a fast and energy-efficient modulation is obtained by placing an NbN superconducting heating electrode above the TFLN waveguide. The transition of the NbN superconducting electrode between superconducting and normal states turns the heating and cooling processes from continuous to discontinuous change. Thus, the energy consumption during the modulation process is reduced proportionally. The rise/fall time of the proposed device is 22 µs/15 µs, which has been the fastest response time reported in TFLN thermo-optic modulators so far. The presented TO modulator can easily be used at cryogenic temperatures and has great potential for applications in cryogenic optoelectronics.cryogenic modulator lithium niobate superconductance
Chinese Optics Letters
2023, 21(8): 081301
We propose an integrated W-band transmitter enabled by an integrated dual-mode distributed feedback (DFB) laser and cascaded silicon photonic microring modulators for next-generation wireless communication. 10 Gb/s error-free intensity modulation and direct detection W-band transmission are achieved in experiments by using the dual-mode DFB laser and two free-running lasers. Moreover, we conduct an experiment of dual-carrier modulation based on cascaded microring modulators, achieving 3 dB signal-to-noise ratio improvement and better signaling integrity for wireless communication. The proposed photonic integrated W-band transmitter will be a viable solution for a high-speed and low-power wireless communication system.
2023, 11(8): 1431
Based on the 90 nm silicon photonics commercial foundry, sidewall-doped germanium–silicon photodetectors (PDs) are designed and fabricated. The large designed overlap between the optical field and electric field achieves high responsivity while retaining high-speed performance. Even including the loss due to optical fiber coupling, the PD demonstrates an external responsivity greater than 0.55 A/W for transverse magnetic (TM) polarization and 0.65 A/W for transverse electric (TE) polarization at 1530 nm. A flat responsivity spectrum of is achieved up to 1580 nm for both polarizations. Their internal responsivities can exceed 1 A/W in the optical communication bands. Furthermore, with the aid of a 200 mm wafer-level test and analysis, the overall PDs of 26 reticles have a 3 dB optoelectrical bandwidth and a dark current at a bias voltage. Finally, the eye diagram performances under TE and TM polarizations, various modulation formats, and different input wavelengths are comprehensively investigated. The clear open electrical eye diagrams up to 120, 130, 140, and 150 Gbit/s nonreturn-to-zero are experimentally attained at a photocurrent of 1 mA. To the best of our knowledge, this is the first time that single-lane direct detection of record-high-speed 200, 224, 256, and 290 Gbit/s four-level pulse amplitude modulation (PAM) and 300, 336, 384, and 408 Gbit/s eight-level PAM optical signals has been experimentally achieved.
2023, 11(6): 961
Interface states are widely applied in waveguide devices. However, previous studies failed to achieve photonic and phononic interface states independent of each other in the same crystal structure depending on the behavior of the crystal structure, i.e., photonic or phononic crystals, making the function of interface states single. In this study, straight-line and circular photonic and phononic interface states were realized independently in sunflower-type crystals. In addition, with a defect and a metal barrier, interface states could remain almost undamaged. The results have the potential to achieve multi-function devices and reduce the cost of engineering applications.interface states photonic and phononic crystals sunflower-type crystals
Chinese Optics Letters
2023, 21(6): 061301
Ever-growing deep-learning technologies are making revolutionary changes for modern life. However, conventional computing architectures are designed to process sequential and digital programs but are burdened with performing massive parallel and adaptive deep-learning applications. Photonic integrated circuits provide an efficient approach to mitigate bandwidth limitations and the power-wall brought on by its electronic counterparts, showing great potential in ultrafast and energy-free high-performance computation. Here, we propose an optical computing architecture enabled by on-chip diffraction to implement convolutional acceleration, termed “optical convolution unit” (OCU). We demonstrate that any real-valued convolution kernels can be exploited by the OCU with a prominent computational throughput boosting via the concept of structral reparameterization. With the OCU as the fundamental unit, we build an optical convolutional neural network (oCNN) to implement two popular deep learning tasks: classification and regression. For classification, Fashion Modified National Institute of Standards and Technology (Fashion-MNIST) and Canadian Institute for Advanced Research (CIFAR-4) data sets are tested with accuracies of 91.63% and 86.25%, respectively. For regression, we build an optical denoising convolutional neural network to handle Gaussian noise in gray-scale images with noise level , 15, and 20, resulting in clean images with an average peak signal-to-noise ratio (PSNR) of 31.70, 29.39, and 27.72 dB, respectively. The proposed OCU presents remarkable performance of low energy consumption and high information density due to its fully passive nature and compact footprint, providing a parallel while lightweight solution for future compute-in-memory architecture to handle high dimensional tensors in deep learning.
2023, 11(6): 1125