High Performance Optical Systems Research Articles

Electrically pumped laser transmitter integrated on thin-film lithium niobate

Integrated thin-film lithium niobate (TFLN) photonics has emerged as a promising platform for the realization of high-performance chip-scale optical systems. Of particular importance are TFLN electro-optic modulators featuring high-linearity, low driving voltage and low propagation loss. However, a fully integrated system requires integration of high power, low noise, and narrow linewidth lasers on TFLN chips. Here we achieve this goal, and demonstrate integrated high-power lasers on TFLN platform with up to 60 mW of optical power in the waveguides. We use this platform to realize a high-power transmitter consisting of an electrically pumped laser integrated with a 50 GHz modulator.

Sparse bi-step raster path for suppressing the mid-spatial-frequency error by fluid jet polishing.

The periodic ripple errors (mid-spatial-frequency (MSF) error) produced by computer-controlled sub-aperture polishing severely limit the improvement of high-performance optical systems. At the same time, the fluid jet polishing (FJP) method is non-destructive and non-contact, but it is still hard to widely use it due to the defect of small spot-size and low efficiency. In this paper, we found that FJP has a significant advantage in removing the residual periodic ripples in sub-aperture polishing. The mathematical model developed by complex spectrum optimization verifies the existence of the sparse "bi-step raster path" (BSRP), which can achieve efficient periodic ripple error removal by suppressing the first two-order peaks of the error spectrum. In the experiments, it was observed that the MSF error has been significantly reduced after BSRP processing while the surface form and surface roughness have not been deteriorated, which demonstrates the validity of the proposed method. The proposal of the BSRP provides a new approach for the application of FJP and the suppression of the MSF error.

Comprehensive Design Method of a High-Frequency-Response Fast Tool Servo System Based on a Full-Frequency Error Control Algorithm.

With the development of optoelectronic information technology, high-performance optical systems require an increasingly higher surface accuracy of optical mirrors. The fast tool servo (FTS) based on the piezoelectric actuator is widely used in the compensation machining of high-precision optical mirrors. However, with the low natural frequency of mechanical structures, hysteresis of the piezoelectric actuators, and phase delay of the control systems, conventional FTS systems face problems such as a low working frequency and a large tracking error. This study presents a method for the design of a high-performance FTS system. First, a flexure hinge servo turret with a high natural frequency was designed through multi-objective optimization and finite element simulations. Subsequently, a composite control algorithm was proposed, targeting the problems of hysteresis and phase delay. The modified Prandtl–Ishlinskii inverse hysteresis model was used to overcome the hysteresis effect and a zero-phase error tracker was designed to reduce the phase error. The experimental results reveal that the tracking error of the designed FTS system was <10% in the full frequency range (0–1000 Hz).

Tunable Flat-Top Filtering Response in Cascaded Resonant Waveguide Gratings

The applicability of resonant waveguide grating (RWG)-based structures as filters to control the spectral response in an optical communication system is investigated. A new physical model for the structure is established on the basis of the Fabry-Pérot (FP) etalon model and coupled leaky mode theory (CLMT). It is found that the flat-top spectral response of the filter is achieved by the combined effect of the guided-mode resonance of an RWG and its Fabry-Pérot resonance (FPR). The bandwidth-tunable spectral response of the filter can be varied according to the change in the eigenvalues of the RWG by changing the structural parameters such as strip width of the grating and incident angle. The flat-top and bandwidth-tunable RWG-based resonant filter is a promising application for high-performance optical communication systems.

Enabling precision coordinate metrology for universal optical testing and alignment applications

Optical designs for the next generation space science instruments call for unconventional, aspheric, and freeform (FF), prescriptions with tight tolerances. These advanced surfaces enable superior-performance, compact, and lower cost systems but are more challenging to characterize and, hence, to fabricate and integrate. A method was developed to characterize a wide range of optical surfaces, without requiring custom-made correctors, and to align them to each other for a high-performance optical system. A precision coordinate measuring machine, equipped with a non-contact, chromatic confocal probe, was used to measure numerous optics including large convex conics, high-sloped aspherics, several FF surfaces, and grazing-incidence x-ray optics. The resulting data were successfully reduced using custom-developed, advanced surface fitting analysis tool, to determine the optic’s alignment relative to the global and local coordinate systems, surface departure from design, and the as-built optical prescription. This information guided the modeling and the alignment of the corresponding as-built optical systems, including a flight system composed of a three-mirror anastigmat.

Cross-point effect of high-speed cross scratch on hard brittle materials

In modern day high-performance space optical systems, precision surface grinding of quartz glass freeform surface has been widely utilized in the initial stage of component machining. Investigations regarding material removal mechanisms present an essential link to ensuring both machining accuracy and quality. Currently, removal mechanisms of quartz glass investigations are mainly focused on straight line scratch and parallel scratch experiments. The material removal mechanism of high-speed cross scratch on machined surface is rarely considered. In order to investigate the removal mechanism of the high-speed cross scratch material, a new scratch test method based on trochoid feed was proposed. The cross-point effect of quartz glass was investigated through high-speed scratch experiment with variable cutting depth. The experimental results show that the cross-point area formed by high-speed cross-scratches is more damaged and enters brittle removal earlier than other areas. In the area of brittle removal, the damage distribution of high-speed cross scratches at the crossover point is approximately symmetrical. The cutting forces at the inlet and outlet of the intersection area are different, resulting in a different fracture morphology. The reflection fracture of stress wave caused by impact load is the key factor leading to the cross-point effect of the intersection area.

Kinematic mirror mount design for ultra-precision manufacturing, metrology, and system level integration for high performance visible spectrum imaging systems

High-performance freeform optical systems, designed for broad spectral imaging from the visible to the far infrared, place new demands on optical design, precision manufacturing, and precision metrology. To meet the tolerances on figure, roughness, and relative positioning in such systems requires the ability to perform metrology and manufacturing corrections on freeform optics in a continuous feedback loop. This feedback loop requires a common interface for machining and manufacturing platforms. This paper describes the design, analysis, and testing of such an interface suitable for use with single point diamond turning and deterministic micro-grinding. The interface utilizes a torsionally preloaded, robust, kinematic mount capable of supporting manufacturing process loads while maintaining the position repeatability in five degrees of freedom required for the measurement and correction of optical figure. Results from a prototype system demonstrate absolute in-plane position uncertainty less than 200nm and 50nm, respectively, and axial position uncertainty is 40nm absolute and 10nm relative. The absolute and relative angular positioning uncertainties less than 1µrad and 0.25µrad respectively. The results exceed the requirements for many optical systems. The mount is also suitable for use in opto-mechanical assembly, so that the same platform can be used for manufacturing, metrology, final assembly, testing, and service.

Avalanche Carrier Multiplication in Multilayer Black Phosphorus and Avalanche Photodetector.

A highly sensitive avalanche photodetector (APD) is fabricated by utilizing the avalanche multiplication mechanism in black phosphorus (BP), where a strong avalanche multiplication of electron-hole pairs is observed. Owing to the small bandgap (0.33 eV) of the multilayer BP, the carrier multiplication occurs at a significantly lower electric field than those of other 2D semiconductor materials. In order to further enhance the quantum efficiency and increase the signal-to-noise (S/N) ratio, Au nanoparticles (NPs) are integrated on the BP surface, which improves the light absorption by plasmonic effects. The BP-Au-NPs structure effectively reduces both dark current (≈10 times lower) and onset of avalanche electric field, leading to higher carrier multiplication, photogain, quantum efficiency, and S/N ratio. For the BP-Au-NPs APD, it is obtained that the external quantum efficiency (EQE) is 382 and the responsivity is 160 A W-1 at an electric field of 5 kV cm-1 (Vd ≈ 3.5 V, note that for the BP APD, EQE = 4.77 and responsivity = 2 A W-1 obtained at the same electric field). The significantly increased performance of the BP APD is promising for low-power-consumption, high-sensitivity, and low-noise photodevice applications, which can enable high-performance optical communication and imaging systems.

Wide-Angle Broadband Antireflection Coatings Prepared by Atomic Layer Deposition.

A novel broadband antireflective coating with ultra-low residual reflectance for light incidence angles from 0° up to 60° is presented. The system consists of an interference multilayer coating made by atomic layer deposition (ALD) combined with a low- n nanoporous SiO2 top-layer obtained by wet-chemical etching of an atomically mixed SiO2/Al2O3 ALD composite. The average residual reflectance measured at normal incidence for double-sided coated B270 glass substrates is only 0.5% in a broad spectral range from 400 to 1100 nm. The average reflectance of the substrate considering both front and rear sides decreased in the visible spectral range of 420-680 nm from 9.9 and 15.8 to 0.4 and 1.8% at an oblique angle of incidence (AOI) of 45° and 60°, respectively, by applying the hybrid ALD antireflection coatings. The measured average transmittance reaches 99.5% at AOI 6° in the 400-950 nm spectral range. Measurements three weeks after preparation show only a small reduction of the average transmittance to 99.3% in this spectral range spanning 550 nm. Ten months later, the average transmittance is still 99.0%, whereby the sample handling might have also affected the performance. The hybrid ALD system shows excellent conformal AR performance on a strongly curved B270 aspheric lens with a diameter of 50 mm and a height of 25 mm. The presented process is a promising route toward omnidirectional AR coatings on complex 3D optics, which are increasingly important for consumer and high-performance optical systems.

Measurement and correction of two-sided freeform optical elements with combined tactile-optical metrology equipment

Freeform optical elements are state of the art for several years to fabricate very high performance optical systems with the necessity of, e.g., strong folding in mirror system or correction of typical asymmetric aberrations in mirror systems as well as lens systems. For freeform mirror systems, in particular for metal mirrors, the metrology is well understood and iterative process chains are well established. For transmission elements with a freeform surface on both sides, manufacturing, metrology, and correction for both sides in a parallel manner is quite difficult. The article presents a method to measure such an optical element and correct it with a well-defined correction step to have both sides in a well-defined position to each other.

Seamless Convergence of Fiber and Wireless Systems for 5G and Beyond Networks

We propose different fronthaul systems for facilitating future mobile networks based on the seamless convergence of fiber-optic and wireless systems in the millimeter-wave (mmWave) bands. First, a flexible and high-performance wireless fronthaul system is proposed through an encapsulation of radio signals onto a converged fiber–mmWave system. A simultaneous transmission of three radio signals over the system is successfully demonstrated. Second, a high-performance optical self-heterodyne system is proposed and demonstrated for the generation and transmission of radio access signals in high-frequency bands. Third, a high-spectral-efficiency optical fronthaul system for the simultaneous transmission of multiple radio signals in different frequency bands is proposed using a subcarrier-multiplexing intermediate-frequency-over-fiber system. Satisfactory performance is experimentally confirmed for the transmission of three different radio signals in the microwave and low- and high-mmWave bands. The proposed systems can overcome the challenges and bottlenecks of the current mobile fronthaul systems and can be useful in different usage scenarios of 5G and beyond networks.

Fabrication of InP-based monolithically integrated laser transmitters

InP-based photonic integrated circuits (PICs) have aroused great interest in recent years to meet the needs of future high-capacity and high-performance optical systems. With the advantages of small size, low power consumption, low cost, high reliability, InP-based PICs are promising solutions to replace the multiple discrete devices used in various systems. In this paper, we will review the design, fabrication, key integration technology and performance of several kinds of InP-based monolithically integrated transmitters developed in our group in recent years. Particular attention will be paid to the electro-absorption modulated laser (EML), multi-wavelength distributed feedback (DFB) laser arrays, widely tunable distributed reflector (DBR) lasers and their arrays, integrated amplified feedback lasers (AFL), and few-mode transmitters.

Machining position-attitude optimisation in ion beam figuring of high-precision off-axis aspheric surfaces

High-precision off-axis aspheric components are widely applied in various high-performance optical systems. However, the complicated manufacturing features of off-axis aspheric make it difficult to achieve high-precision surface for traditional optical manufacturing methods. Ion beam figuring (IBF) is a non-contact and highly deterministic method for the final precision optical figuring. In this paper, two different machining position-attitude models are discussed, which have great influence on the machining difficulty and figuring accuracy of IBF. The child mirror coordinate position-attitude model (CCPAM), which is based on the robustness of IBF removal function to small disturbance of target distance and incidence angle, can effectively decrease the sag height and incidence angle, thereby reduce the beam errors and machining difficulty. A linear three-axis IBF system is successfully applied for the figuring of a large-aperture parabolic off-axis aspheric mirror based on the CCPAM and its best-fit sphere. With two iterations of 113 min, surface error is down to 7.658 nm RMS (effective aperture 310 mm × 310 mm). The figuring results indicate IBF can realise the rapid fabrication of high-precision off-axis aspheric based on the machining position-attitude optimisation.

Metalenses: Versatile multifunctional photonic components.

Recent progress in metasurface designs fueled by advanced-fabrication techniques has led to the realization of ultrathin, lightweight, and flat lenses (metalenses) with unprecedented functionalities. Owing to straightforward fabrication, generally requiring a single-step lithography, and the possibility of vertical integration, these planar lenses can potentially replace or complement their conventional refractive and diffractive counterparts, leading to further miniaturization of high-performance optical devices and systems. Here we provide a brief overview of the evolution of metalenses, with an emphasis on the visible and near-infrared spectrum, and summarize their important features: diffraction-limited focusing, high-quality imaging, and multifunctionalities. We discuss impending challenges, including aberration correction, and also examine current issues and solutions. We conclude by providing an outlook of this technology platform and identifying promising directions for future research.

High performance optical systems using MIM based plasmonic structures

In this paper, the design of high performance optical systems based on MIM based plasmonic structures is proposed. Using a design methodology based on optimization functions, two novel components are proposed: demultiplexer and gas sensor. Further optimization of these plasmonic systems, high performance optical demultiplexer is maintained with a narrow line-width down to 4 nm with acceptable power level around 30%. On the other side, highly sensitive optical sensor is proposed with sensitivity up to 2000 nm and a limit of detection down to 10−4.

Focus on microscopy, imaging, and nanotechnology

Focus on microscopy, imaging, and nanotechnology

Ultralinear heterogeneously integrated ring-assisted Mach–Zehnder interferometer modulator on silicon

A linear modulator is indispensable for radio frequency photonics or analog photonic link applications where high dynamic range is required. There is also great interest to integrate the modulator with other photonic components, to create a photonic integrated circuit for these applications, with particular focus on silicon photonics integration in order to take advantage of complementary metal–oxide–semiconductor compatible foundries for high-volume, low-cost devices. However, all silicon modulators, including the highest performing Mach–Zehnder interferometer (MZI) type, have poor linearity, partially due to the inherent nonlinearity of the MZI transfer characteristic, but mostly due to the nonlinearity of silicon’s electro-optic phase shift response. In this work, we demonstrate ultralinear ring-assisted MZI (RAMZI) modulators, incorporating heterogeneously integrated III–V multiple quantum wells on silicon phase modulation sections to eliminate the nonlinear silicon phase modulation response. The heterogeneously integrated III–V/Si RAMZI modulators achieve record-high spurious free dynamic range (SFDR) for silicon-based modulators, as high as 117.5 dB·Hz2/3 at 10 GHz for a weakly coupled ring design, and 117 dB·Hz2/3 for a strongly coupled ring design with higher output power. This is a higher SFDR than typically obtained with commercial lithium niobate modulators. This approach advances integrated modulator designs on silicon for applications in compact and high-performance analog optical systems.

The Role of High Performance Computing and Communication for Real-Time Biofeedback in Sport

This paper studies the main technological challenges of real-time biofeedback in sport. We identified communication and processing as two main possible obstacles for high performance real-time biofeedback systems. We give special attention to the role of high performance computing with some details on possible usage of DataFlow computing paradigm. Motion tracking systems, in connection with the biomechanical biofeedback, help in accelerating motor learning. Requirements about various parameters important in real-time biofeedback applications are discussed. Inertial sensor tracking system accuracy is tested in comparison with a high performance optical tracking system. Special focus is given on feedback loop delays. Real-time sensor signal acquisitions and real-time processing challenges, in connection with biomechanical biofeedback, are presented. Despite the fact that local processing requires less energy consumption than remote processing, many other limitations, most often the insufficient local processing power, can lead to distributed system as the only possible option. A multiuser signal processing in football match is recognised as an example for high performance application that needs high-speed communication and high performance remote computing. DataFlow computing is found as a good choice for real-time biofeedback systems with large data streams.

Structure optimization and fabricating capability analysis of an ion-beam machine for a subnanometer optical surface.

Ultraprecise and ultrasmooth surfaces become critical requirements for some high-performance optical systems. Ion-beam figuring (IBF) is a good and highly deterministic method for the final precision optical figuring. However, the uniform convergences of all spatial frequency surface errors are strongly dependent on the dynamic performance and ion-beam stability of the IBF machine. In this paper, only the dynamic performance is discussed, which is limited by the acceleration and velocity of the motion system. So we discuss these problems and their influences on figuring optical surfaces in detail. The structure optimization principle is based on the fabricating capability of ultraprecise surface errors in all spatial frequency ranges. With this requirement, the structure optimization of a quick-response platform is performed to improve its dynamic performance. Manufacturing experiments on a fused silica spherical concave surface (Φ135.7 mm, radius of curvature 340.5 mm) are accomplished, and the IBF machine can effectively correct the figure errors and improve the surface quality simultaneously. The IBF process realizes the uniform convergence of surface errors in all spatial frequency ranges, which is reduced down to 0.368 nm RMS, 0.204 nm RMS, and 0.087 nm RMS, respectively. The final results indicate that the performance of the new designed IBF machine meets the requirements well for the fabrication of a subnanometer optical surface.

Polar-grids based source-target mapping construction method for designing freeform illumination system for a lighting target with arbitrary shape.

The freeform optical system plays a key role in illumination engineering, and several methods have been reported to manage the design of such system. In this paper, an approach to generate the polar-grids based flux transportation mapping for an arbitrarily-shaped target is proposed based on the conventional variable separation method. The source emitting grid is divided along the azimuth angle and the zenith angle respectively under the spherical coordinate system. Then, the target grid is achieved by solving the flux integral equations in polar coordinates using separation of variables method. When establishing the target grid along the polar radius, a strategy based on uniformly scaling down the external contour of the target is introduced. According to the mapping, a smooth freeform surface is then generated using the geometric construction method according to Snell's law. Finally, an iterative feedback process is adopted to compensate the deterioration of the target distribution caused by surface construction errors and the extension of a real source. Based on this method, a series of freeform lenses are designed for a 1 × 1 mm(2) LED source to generate uniform, Gaussian and multiple-rings illumination distributions within different target regions. High-performance optical systems with the light utilization efficiency η over 0.8 and the relative standard deviation (RSD) of the simulated illumination distribution less than 0.1 are obtained simultaneously for all the cases.