2022, 10(5) Column
Research Articles Integrated Optics Optical Devices Surface Optics and Plasmonics Optoelectronics Silicon Photonics Optical and Photonic Materials Instrumentation and Measurements Fiber Optics and Optical Communications Nanophotonics and Photonic Crystals Imaging Systems, Microscopy, and Displays Lasers and Laser Optics Errata
Photonics Research 第10卷 第5期
Achieving full grating-lobe-free field of view with low-complexity co-prime photonic beamforming transceivers
Integrated photonic active beamforming can significantly reduce the size and cost of coherent imagers for LiDAR and medical imaging applications. In current architectures, the complexity of photonic and electronic circuitry linearly increases with the desired imaging resolution. We propose a novel photonic transceiver architecture based on co-prime sampling techniques that breaks this trade-off and achieves the full (radiating-element-limited) field of view (FOV) for a 2D aperture with a single-frequency laser. Using only order-of-
radiating elements, this architecture achieves beamwidth and sidelobe level (SLL) performance equivalent to a transceiver with order-of- elements with half-wavelength spacing. Furthermore, we incorporate a pulse amplitude modulation (PAM) row–column drive methodology to reduce the number of required electrical drivers for this architecture from order of to order of . A silicon photonics implementation of this architecture using two 64-element apertures, one for transmitting and one for receiving, requires only 34 PAM electrical drivers and achieves a transceiver SLL of with 1026 total resolvable spots, and 0.6° beamwidth within a FOV.
Integrated scanning spectrometer with a tunable micro-ring resonator and an arrayed waveguide grating
Integrated spectrometers with both wide optical bandwidths and high spectral resolutions are required in applications such as spectral domain optical coherence tomography (SD-OCT). Here we propose a compact integrated scanning spectrometer by using a tunable micro-ring resonator (MRR) integrated with a single arrayed waveguide grating for operation in the 1265–1335-nm range. The spectral resolution of the spectrometer is determined by the quality factor of the MRR, and the optical bandwidth is defined by the free spectral range of the arrayed waveguide grating. The spectrometer is integrated with on-chip germanium photodetectors, which enable direct electrical readout. A 70-nm optical bandwidth and a 0.2-nm channel spacing enabled by scanning the MRR across one free spectral range are demonstrated, which offer a total of 350 wavelength channels with 31-kHz wavelength scanning speed. The integrated spectrometer is applied to measure different spectra and the interference signals from an SD-OCT system, which shows its great potential for future applications in sensing and imaging systems.
Machine-learning-empowered multispectral metafilm with reduced radar cross section, low infrared emissivity, and visible transparency
For camouflage applications, the performance requirements for metamaterials in different electromagnetic spectra are usually contradictory, which makes it difficult to develop satisfactory design schemes with multispectral compatibility. Fortunately, empowered by machine learning, metamaterial design is no longer limited to directly solving Maxwell’s equations. The design schemes and experiences of metamaterials can be analyzed, summarized, and learned by computers, which will significantly improve the design efficiency for the sake of practical engineering applications. Here, we resort to the machine learning to solve the multispectral compatibility problem of metamaterials and demonstrate the design of a new metafilm with multiple mechanisms that can realize small microwave scattering, low infrared emissivity, and visible transparency simultaneously using a multilayer backpropagation neural network. The rapid evolution of structural design is realized by establishing a mapping between spectral curves and structural parameters. By training the network with different materials, the designed network is more adaptable. Through simulations and experimental verifications, the designed architecture has good accuracy and robustness. This paper provides a facile method for fast designs of multispectral metafilms that can find wide applications in satellite solar panels, aircraft windows, and others.
Continuous-wave operation of 405 nm distributed Bragg reflector laser diodes based on GaN using 10th-order surface gratings
Single longitudinal mode continuous-wave operation of GaN-based distributed Bragg reflector (DBR) laser diodes with 10th-order surface gratings is demonstrated. The DBR consists of periodic V-shaped grooves on a 2 μm wide ridge waveguide fabricated by using electron-beam lithography and plasma etching. The effect of different lengths of the DBR section and the gain section on the device performance has been studied. Periodic mode hops to the adjacent longitudinal Fabry–Perot resonator mode at shorter wavelength have been observed when increasing the operation current. Between the mode hops, single longitudinal mode emission at around 405 nm is achieved with a full width at half-maximum of 0.03 nm. A linear redshift of the emission wavelength with increasing temperature of 0.019 nm/K was derived.
Dual-focusing effect with a cylindrical vector-light characteristic (i.e., radial and azimuthal polarizations) is theoretically proposed and numerically demonstrated by spin-decoupled phase control with all-silicon metalens. Attributed to the polarization dependence, the pair of focusing cylindrical vector beams can be interchanged by orthogonally switching the polarization of incident light. We demonstrate the unique contributions of focused radial and azimuthal vector beams to longitudinal and transverse optical forces on glass spheres, respectively, by calculations based on the Maxwell stress tensor. This paper presents the use of all-silicon metalens with highly-compact vector beams, promising for applications such as multidimensional optical trapping.
Demonstration of electrically injected vertical-cavity surface-emitting lasers with post-supported high-contrast gratings
We experimentally demonstrate for the first time to our knowledge electrically injected vertical-cavity surface-emitting lasers (VCSELs) with post-supported high-contrast gratings (HCGs) at 940 nm. The HCG-VCSELs have two posts to support the air-suspended HCGs, which are realized by simple fabrication without critical point drying. The HCG-VCSEL achieves a threshold current of about 0.65 mA and a side-mode suppression ratio of 43.6 dB under continuous-wave operation at 25°C. Theoretically the HCG-VCSEL with a
-cavity for the transverse magnetic polarization has a smaller effective mode length of . Thus, the relaxation resonance frequency can be increased by 16% compared with that of the conventional VCSEL. The modulation speed of 100 Gbit/s for the HCG-VCSEL is expected in the on–off keying modulation format. Our easy design of HCG-VCSELs has great potential for applications in optical interconnects, sensing, illumination, and so on.
A solid-state active beamformer based on the annular-ring diffraction pattern is demonstrated in an integrated photonic platform. Such a circularly symmetric annular-ring aperture achieves a radiating element limited field of view. Furthermore, it is demonstrated that a multi-annular-ring aperture with a fixed linear density of elements maintains the beam efficiency for larger apertures while reducing the beamwidth and side-lobe level. A 255-element multi-annular-ring optical phased array with active beamforming is implemented in a standard photonics process. A total of 510 phase and amplitude modulators enable beamforming and beam steering using this aperture. A row–column drive methodology reduces the required electrical drivers by more than a factor of 5.
Broadband NIR-emitting Te cluster-doped glass for smart light source towards night-vision and NIR spectroscopy applications
Broadband near-infrared (NIR)-emitting materials are crucial components of the next generation of smart NIR light sources based on blue light-emitting diodes (LEDs). Here, we report a Te cluster-doped borate glass, which exhibits ultra-broadband emission around 980 nm with a full-width at half-maximum (FWHM) of 306 nm under blue light excitation. We propose adjustments of glass chemistry and processing condition as a means for topo-chemical tailoring of the NIR photoemission characteristics in such materials. Through implementing strongly reducing conditions during glass melting, Te clusters with broad NIR photoluminescence can be generated and stabilized once the melt is vitrified to the glassy state. Tunability of the NIR emission peak over the wavelength range of 904 to 1026 nm is possible in this way, allowing for fine adjustments of spectral properties relative to the stretching vibrations of common chemical bonds, for example, in water, proteins, and fats. This potentially enables high sensitivity in NIR spectroscopy. We further demonstrate potential application of glass-converted LEDs in night vision.
Conventional photodetection converts light into electrical signals only in a single electromagnetic waveband. Multiband detection technology is highly desirable because it can handle multispectral information discrimination, identification, and processing. Current epitaxial solid-state multiband detection technologies are mainly within the IR wave range. Here, we report epitaxial indium antimonide on gallium arsenide for IR and millimeter/terahertz wave multiband photodetection. The photoresponse originates from interband transition in optoelectrical semiconductors for IR wave, and surface plasmon polaritons induced nonequilibrium electrons for a millimeter/terahertz wave. The detector shows a strong response for an IR wave with a cutoff wavelength of 6.85 μm and a blackbody detectivity of
Jones at room temperature. For a millimeter/terahertz wave, the detector demonstrates broadband detection from 0.032 THz (9.4 mm) to 0.330 THz (0.9 mm); that is, from to the and bands, with a noise equivalent power of at 0.270 THz (1.1 mm) at room temperature. The detection performance is an order of magnitude better while decreasing the temperature to 170 K, the thermoelectric cooling level. Such detectors, capable of large scale and low cost, are promising for advanced uncooled multiband detection and imaging systems.
Measurement of sub-fm/Hz1/2 displacement spectral densities in ultrahigh-Q single-crystal microcavities with hertz-level lasers
Tracing a resonance frequency of a high quality factor (Q) optical cavity facilitates subpicometer displacement measurements of the optical cavity via Pound–Drever–Hall (PDH) locking scheme, tightly synchronizing a laser frequency to the optical cavity. Here we present observations of subfemtometer displacements on a ultrahigh-Q single-crystal
whispering-gallery-mode microcavity by frequency synchronization between a 1 Hz cavity-stabilized laser and a resonance of the cavity using PDH laser-cavity locking. We characterize not only the displacement spectral density of the microcavity with a sensitivity of over the Fourier offset frequency ranging from 15 mHz to 100 kHz but also a 1.77 nm displacement fluctuation of the microcavity over 4500 s. Such measurement capability not only supports the analysis of integrated thermodynamical and technical cavity noise but allows for minute displacement measurements using laser-cavity locking for ultraprecise positioning, metrology, and sensing.
Fringe projection profilometry (FPP) is widely used in optical three-dimensional (3D) measurements because of its high stability. In FPP, fringe distortion is an inevitable and highly complex systematic error that significantly reduces the 3D measurement accuracy. At this point, the existing causes of fringe distortion represented by gamma distortion, high-order harmonics, and image saturation have been effectively analyzed and compensated to restore high-quality fringe images. In this paper, we innovatively reveal a concealed cause of fringe distortion, i.e., intensity diffusion across pixels, which is induced by photocarrier diffusion between photodiodes. To the best of our knowledge, intensity diffusion has not been studied in the field of fringe restoration. Based on the motion of photocarrier diffusion, we theoretically analyze the mechanism of how the intensity diffusion affects FPP. Subsequently, an intensity diffusion model is established for quantifying the diffused intensity in each pixel, and an intensity diffusion correction algorithm is presented to remove the diffused intensity from the fringe images and correct the fringe distortion. Experiments demonstrate the impact of intensity diffusion on FPP, and the 3D measurement results prove the effectiveness of the proposed methods on improving the 3D measurement accuracy by correcting the fringe distortion.
Topological physics exploits concepts from geometry and topology to implement systems capable of guiding waves in an unprecedented fashion. These ideas have led to the development of photonic topological insulators, which are optical systems whose eigenspectral topology allows the creation of light states that propagate along the edge of the system without any coupling into the bulk or backscattering even in the presence of disorder. Indeed, topological protection is a fully coherent effect, and it is not clear to what extent topological effects endure when the wavefronts become partially coherent. Here, we study the interplay of topological protection and the degree of spatial coherence of classical light propagating in disordered photonic topological insulators. Our results reveal the existence of a well-defined spectral window in which partially coherent light is topologically protected. This opens up the design space to a wider selection of light sources, possibly yielding smaller, cheaper, and more robust devices based on the topological transport of light.
Understanding inter-site mutual mode interaction in coupled physical systems is essential to comprehend large compound systems, as this local interaction determines the successive multiple inter-site energy transfer efficiencies. In the present study, we demonstrate that only the non-Hermitian coupling can correctly account for the light transfer between two coupled optical cavities. We also reveal that the non-Hermitian coupling effect becomes crucial as the system dimension decreases. Our results provide important insight for handling general-coupled devices in the subwavelength regime.
We created an all-fiber solution for fast, continuous, and controllable tuning of Fano-like resonance. By embedding a graphene-coated fiber Bragg grating into one arm of a Mach–Zehnder interferometer, the narrow Bragg resonance interacts with a broad interference spectrum, forming a sharp asymmetric Fano-like resonance line shape. With the application of an electrical voltage over the graphene layer, the generated Joule heating shifts the Bragg resonance and consequently tunes the asymmetric Fano-like resonance line shape to a symmetric dip or electromagnetically induced transparency-like peak. Further, by exploiting two modulated states with reversed Fano-like resonance line shapes, an optical switch can operate with an extinction ratio of 9 dB. The well-engineered Fano-like resonance in an all-fiber structure opens up new horizons for applications of fiber gratings in optical signal processing, slow-light lasing, and fiber sensing.
Synchronized rotation of unit cells in a periodic structure provides a novel design perspective for manipulation of band topology. We then design a two-dimensional version of higher-order topological insulator (HOTI) by such rotation in a triangular photonic lattice with
symmetry. This HOTI supports the hallmark zero-dimensional corner states and, simultaneously, the one-dimensional edge states. We also find that our photonic corner states carry chiral orbital angular momenta locked by valleys, whose wave functions are featured by the phase vortex (singularity) positioned at the maximal Wyckoff points. Moreover, when excited by a fired source with various frequencies, the valley topological states of both one-dimensional edges and zero-dimensional corners emerge simultaneously. Extendable to higher or synthetic dimensions, our paper provides access to a chiral vortex platform for HOTI realizations in the terahertz photonic system.
Improving signal-to-background ratio by orders of magnitude in high-speed volumetric imaging in vivo by robust Fourier light field microscopy
Fourier light field microscopy (FLFM) shows great potential in high-speed volumetric imaging of biodynamics. However, due to the inherent disadvantage of wide-field illumination, it suffers from intense background, arising from out of the depth-of-field signal and tissue scattered noise. The background will not only deteriorate the image contrast, making quantitative measurement difficult, but also introduce artifacts, especially in functional imaging of the neuronal network activity in vivo. Here, we propose the robust Fourier light field microscopy (RFLFM), which suppresses the background in FLFM by introducing structured illumination and computational reconstruction based on HiLo. The superior performance of RFLFM is verified by volumetric imaging of biological dynamics in larval zebrafish and mouse in vivo, at a volumetric imaging rate up to 33.3 Hz. The statistical results show that the fluorescence background can be significantly depressed, with the signal-to-background ratio improved by orders of magnitude and the whole image contrast improved by as much as
times. Moreover, we stress that, in functional imaging of neuronal network activity in turbid brain tissues, our system can avoid artifacts resulting from background fluctuations, while conventional light field microscopy fails. As a simple but powerful tool, we anticipate our technique to be widely adopted in robust, high-contrast, high-speed volumetric imaging.
This work compares the four-wave mixing (FWM) effect in epitaxial quantum dot (QD) lasers grown on silicon with quantum well (QW) lasers. A comparison of theory and experiment results shows that the measured FWM coefficient is in good agreement with theoretical predictions. The gain in signal power is higher for p-doped QD lasers than for undoped lasers, despite the same FWM coefficient. Owing to the near-zero linewidth enhancement factor, QD lasers exhibit FWM coefficients and conversion efficiency that are more than one order of magnitude higher than those of QW lasers. Thus, this leads to self-mode locking in QD lasers. These findings are useful for developing on-chip sources for photonic integrated circuits on silicon.
The combination of grating-based frequency-selective optical feedback mechanisms, such as distributed feedback (DFB) or distributed Bragg reflector (DBR) structures, with quantum dot (QD) gain materials is a main approach towards ultrahigh-performance semiconductor lasers for many key novel applications, as either stand-alone sources or on-chip sources in photonic integrated circuits. However, the fabrication of conventional buried Bragg grating structures on GaAs, GaAs/Si, GaSb, and other material platforms has been met with major material regrowth difficulties. We report a novel and universal approach of introducing laterally coupled dielectric Bragg gratings to semiconductor lasers that allows highly controllable, reliable, and strong coupling between the grating and the optical mode. We implement such a grating structure in a low-loss amorphous silicon material alongside GaAs lasers with InAs/GaAs QD gain layers. The resulting DFB laser arrays emit at pre-designed 0.8 THz local area network wavelength division multiplexing frequency intervals in the 1300 nm band with record performance parameters, including sidemode suppression ratios as high as 52.7 dB, continuous-wave output power of 26.6 mW (room temperature) and 6 mW (at 55°C), and ultralow relative intensity noise (RIN) of
(2.5–20 GHz). The devices are also capable of isolator-free operating under very high external reflection levels of up to while maintaining high spectral purity and ultralow RIN qualities. These results validate the novel laterally coupled dielectric grating as a technologically superior and potentially cost-effective approach for fabricating DFB and DBR lasers free of their semiconductor material constraints, which are thus universally applicable across different material platforms and wavelength bands.
Dissipative solitons relying on the double balance between nonlinear and linear effects as well as cavity loss and gain have attracted increasing attention in recent years, since they give rise to novel operating states of various dissipative nonlinear systems. An optoelectronic oscillator (OEO) is a dissipative nonlinear microwave photonic system with a high quality factor that has been widely investigated for generating ultra-low noise single-frequency microwave signals. Here, we report a novel operating state of an OEO related to dissipative solitons, i.e., spontaneous frequency hopping related to the formation of dissipative microwave photonic solitons. In this operating state, dissipative microwave photonic solitons occur due to the double balance between nonlinear gain saturation and linear filtering as well as cavity loss and gain in the OEO cavity, creating spontaneous frequency-hopping microwave signals. The generation of wideband tunable frequency-hopping microwave signals with a fast frequency-hopping speed up to tens of nanoseconds is observed in the experiment, together with the corresponding soliton sequences. This work reveals a novel mechanism between the interaction of nonlinear and linear effects in an OEO cavity, extends the suitability and potential applications of solitons, and paves the way for a new class of soliton microwave photonic systems for the generation, processing, and control of microwave and RF signals.
The monolithic integration of soliton microcomb devices with active photonic components and high-frequency electronics is highly desirable for practical applications. Among many materials, silicon nitride (
) waveguide layers prepared by low-pressure chemical vapor deposition (LPCVD) have been the main platform for on-chip optical frequency comb generation. However, the high temperatures involved in LPCVD render it incompatible as a back-end process with complementary metal oxide semiconductor (CMOS) or active III-V compound semiconductor fabrication flows. We report the generation of coherent soliton frequency combs in micro-ring resonators fabricated in deuterated silicon nitride ( ) waveguides with a loss of 0.09 dB/cm. Deposited at 270°C by an inductance-coupled plasma chemical vapor deposition (ICP-CVD) process, the material preparation and fabrication flow are fully CMOS-compatible. These results enable the integration of silicon-nitride-based optical combs and photonic integrated circuits (PICs) on prefabricated CMOS and/or III-V substrates, therefore marking a major step forward in photonic technologies.
Polarization microwave-induced thermoacoustic imaging for quantitative characterization of deep biological tissue microstructures
Polarization optical imaging can be used to characterize anisotropy in biological tissue microstructures and has been demonstrated to be a powerful tool for clinical diagnosis. However, the approach is limited by an inability to image targets deeper than
due to strong optical scattering in biological tissues. As such, we propose a novel polarization microwave-induced thermoacoustic imaging (P-MTAI) technique to noninvasively detect variations in deep tissue by exploiting the thermoacoustic signals induced by four pulsed microwaves of varying polarization orientations. The proposed P-MTAI method overcomes the penetration limits of conventional polarization optical imaging and provides submillimeter resolution over depths of several centimeters. As part of the paper, the structural characteristics of tissues were quantified using a new parameter, the degree of microwave absorption anisotropy. P-MTAI was also applied to the noninvasive detection of morphological changes in cardiomyocytes as they transitioned from ordered to disordered states, providing a potential indication of myocardial infarction.
This erratum corrects Fig. 5 in
. 9, 1924 ( 2021) PRHEIZ 2327-9125 10.1364/PRJ.432292
A quantum dot (QD) mode-locked laser as an active comb generator takes advantage of its small footprint, low power consumption, large optical bandwidth, and high-temperature stability, which is an ideal multi-wavelength source for applications such as datacom, optical interconnects, and LIDAR. In this work, we report a fourth-order colliding pulse mode-locked laser (CPML) based on InAs/GaAs QD gain structure, which can generate ultra-stable optical frequency combs in the O-band with 100 GHz spacing at operation temperature up to 100°C. A record-high flat-top optical comb is achieved with 3 dB optical bandwidth of 11.5 nm (20 comb lines) at 25°C. The average optical linewidth of comb lines is measured as 440 kHz. Single-channel non-return-to-zero modulation rates of 70 Gbit/s and four-level pulse amplitude modulation of 40 GBaud/s are also demonstrated. To further extend the comb bandwidth, an array of QD-CPMLs driven at separate temperatures is proposed to achieve 36 nm optical bandwidth (containing 60 comb lines with 100 GHz mode spacing), capable of a total transmission capacity of 4.8 Tbit/s. The demonstrated results show the feasibility of using the QD-CPML as a desirable broadband comb source to build future large-bandwidth and power-efficient optical interconnects.
Structured illumination microscopy (SIM) is an advanced microscope system that provides superresolution capability with excellent imaging speed, which has become a practical tool for live-cell imaging. However, the bulky size is blocking the application of SIM in wider study fields and scenarios. Here, we developed a miniaturized SIM (Mini SIM) system that provided periodic illumination using a diffractive optical element (DOE) for the first time. This optimized phase-only DOE generated the two-dimensional sinusoidal illumination by optical Fourier transform with an illuminating objective lens, which substantially simplified and miniaturized the illumination system. We built up a Mini SIM prototype and demonstrated lateral superresolution imaging of fluorescence beads and A549 cell slides. The proposed Mini SIM greatly simplifies the experimental setup and may lead to important applications in bio-imaging.
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