Optical Phase Research Articles

Ultra‐Compact and Efficient Integrated Multichannel Mode Multiplexer in Silicon for Few‐Mode Fibers

AbstractSpace‐division multiplexing (SDM) is one of the key enabling technologies to increase the capacity of fiber communication systems. However, implementing SDM‐based systems using multimode fiber has been challenging with the need for compact, low‐cost, and scalable mode de/multiplexer (DE/MUX). Here a novel integrated mode MUX for few‐mode fibers (FMFs) is presented which can launch up to eight spatial and polarization channels. The new design is composed of a 2D multimode grating coupler (MMGC), highly compact spot size converters (SSCs), and adiabatic directional couplers (ADCs). Eight data lanes in FMFs can be selectively launched with integrated optical phase shifters. Experimental results reveal efficient chip‐to‐fiber coupling with peak efficiencies of −3.8, −5.5, −3.6, and −4.1 dB for LP01, LP11a, LP11b, and LP21b modes, respectively. Thanks to the use of an integrated subwavelength Mikaelian lens for mode‐independent field size conversion with loss ≤−0.25 dB, the total footprint of the MMGC and SSCs is only 35×35 µm2. The proposed design shows great potential for densely integrated photonic circuits in future SDM applications.

Attosecond electron microscopy by free-electron homodyne detection

Time-resolved electron microscopy aims to track nanoscale excitations and dynamic states of matter at a temporal resolution ultimately reaching the attosecond regime. Periodically time-varying fields in an illuminated specimen cause free-electron inelastic scattering, which enables the spectroscopic imaging of near-field intensities. However, access to the evolution of nanoscale fields and structures within the cycle of light requires sensitivity to the optical phase. Here we introduce free-electron homodyne detection as a universally applicable approach to electron microscopy of phase-resolved optical responses at high spatiotemporal resolution. In this scheme, a phase-controlled reference interaction serves as the local oscillator to extract arbitrary sample-induced modulations of a free-electron wavefunction. We demonstrate this principle through the phase-resolved imaging of plasmonic fields with few-nanometre spatial and sub-cycle temporal resolutions. Due to its sensitivity to both phase- and amplitude-modulated electron beams, free-electron homodyne detection measurements will be able to detect and amplify weak signals stemming from a wide variety of microscopic origins, including linear and nonlinear optical polarizations, atomic and molecular resonances, and attosecond-modulated structure factors.

Asymptotics for singular limits via phase functions

The asymptotic behavior of solutions as a small parameter tends to zero is determined for a variety of singular-limit PDEs. In some cases even existence for a time independent of the small parameter was not known previously. New examples for which uniform existence does not hold are also presented. Our methods include both an adaptation of geometric optics phase analysis to singular limits and an extension of that analysis in which the characteristic variety determinant condition is supplemented with a periodicity condition.

Image based automatic wind velocity profiler for adaptive optics

Abstract Adaptive optics (AO) systems correct optical phase aberrations of the incoming light generated by the atmosphere. To do so simultaneous estimators of the atmospheric turbulence parameters are required. For the family of wide-field AO systems (WFAO), this information must be stratified in altitude. Among these vectorized estimations, wind profiling in altitude is needed for the reduction of temporal errors in AO loops or for the estimation of turbulence coherence time. This paper proposes a turbulence wind profiler called image processing based peak tracking algorithm (IPTA). IPTA is an image-processing based approach that automatically and reliably estimates wind velocity for several turbulent layers along the line of sight. The estimation of each wind layer is achieved by tracking peaks produced in cross-correlation maps from pairs of wavefront sensors (WFSs) slopes using the technique known as SLODAR (slope detection and ranging). Results for simulated and on-sky WFS datasets demonstrate that IPTA outperforms one of the state of the art wind profiler methods (the profiler covariance parametrization of wind velocity (CAW)) in terms of accuracy and speed. Results also show that, in terms of execution time, our method scales better when the number of WFS lenslets is increased. Being an open source and reliable tool, we believe IPTA can be a useful wind profiler for the adaptive optics community.

Sensing analysis of self-mixing and Michelson interferometry with neural-network-based phase extraction

Self-mixing interference (SMI) and Michelson interference (MI) with the phase extraction method based on a neural network (NN) for sensing is analyzed in this work. Since MI and SMI signals are predominantly a cosine-like function of optical phase, the NN’s core task involves mapping the cosine function to phase values, which is solved by a tectonic characteristic function. Experimental results show that the movement information carried by MI and SMI signals can be accurately measured by the NN with a phase sensitivity of 10−3 rad, yielding outcomes that align with the simulations. Especially, algorithms are essential for extracting real-time motion direction of objects in MI. The simplicity of real-time motion direction extraction is the key advantage of SMI, while further utilization of variational mode decomposition is necessary to mitigate the impact of periodic optical frequency variation. This work offers a potentially valuable contribution to develop interferometry for engineering applications.

Photon Generation Scheme of 32‐Fold Millimeter‐Wave Signal Based on Mach‐Zehnder Modulator

AbstractThis paper proposes a 32‐tupling frequency millimeter‐wave (MMW) filter‐free system based on four Mach‐Zehnder Modulators (MZM) connected in parallel and cascaded with a simple radio‐fiber (RoF) link structure. The four MZMs are all at the maximum transmission point (MATP), and the radio frequency (RF) driving voltage phase difference between MZMs is π /2. The center carrier is suppressed by using an optical attenuator (OATT) and an optical phase shifter (OPS). Two parallel MZMs can generate ±8th order and ±12th order optical sidebands, and the ±4th order optical sidebands can be suppressed by adjusting the modulation index m of the MZM, using cascaded two dual‐parallel MZMS(DPMZM) and the phase difference of the RF signal source is π/4 to generate ±16th order optical sidebands. The theoretical analysis and simulation experiments are performed for the scheme proposed in this paper. The results show that the simulated and theoretical values of the optical sideband suppression ratio (OSSR) for ±16th order optical sideband signals are 60.02 and 59.96 dB, respectively, and the simulated and theoretical values of the RF sideband suppression ratio (RFSSR) for the 32‐tupling MMW signal are 56.34 and 53.94 dB, respectively.

Zero-Offset Frequency Locking of Lasers at Low Optical Powers With an Optical Phase Locked Loop

Zero-Offset Frequency Locking of Lasers at Low Optical Powers With an Optical Phase Locked Loop

A Variable Carrier Generation for Heterodyne LDV with an Optical Phase-locked Loop

A heterodyne laser Doppler vibrometer (LDV) requires a frequency shifter to generate a carrier for the vibration information in the detector signal. The carrier frequency should be carefully selected in order to obtain the intended measurement range with a demodulation bandwidth that avoids existing noise sources. The traditional frequency shift in heterodyne LDV is realized with an acoustooptical modulator, which can only generate a fixed carrier frequency. In this paper, a variable carrier generation method based on an optical phase locked loop (OPLL) is demonstrated. Our setup implements a feedback loop to control the phase of the second laser to synchronize a reference laser. In lock-in status the carrier for the vibration information associated with a local oscillator signal. In this paper, we obtain the laser diode parameters relevant to performance and on the design of the photodetector and loop filter. Finally, the performance of the lock-in OPLL, variable carrier as well as velocity measurement is reported and the reliability of the proposed method is evaluated.

Optical wave propagation phase for mKdV spherical electric flux density in sphere space

In this article, we illustrate surface flow alfa-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$alfa-$$\\end{document} microfluidical mKdV spherical electromotive phase in sphere space. Also, we obtain antiferromagnetic axially electrical alfa-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$alfa-$$\\end{document}microfluidical mKdV electric flux path circuit. We have electrical alfa-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$alfa-$$\\end{document}microfluidical geometrical mKdV free surface flow density in sphere space. Finally, we design antiferromagnetic wave propagation for alfa-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$alfa-$$\\end{document} microfluidical geometric mKdV surface flux density.

Phase to intensity modulation and fading compensation using a single ring resonator for radio-over-fiber links.

In this study, we present a simulation-based analysis of radio-over-fiber (ROF) transmission links incorporating both phase modulation (PM) and a single ring resonator (RR) as the modulation transformer (MT). This configuration offers cost-effectiveness, enhanced operational stability, facile reconfiguration, and heightened robustness. The optimization of the RR involves a comprehensive adjustment of the power coupler coupling coefficient (k) and the roundtrip optical phase shift (φ) to attain superior characteristics in terms of power output, bandwidth, dispersion, and nonlinearity, individually. The simulation encompasses the transmission of diverse data formats, including QPSK, 16QAM, and 16QAM-based OFDM, modulated by the PM-RR system. The results reveal a 0.25 dB improvement in nonlinearity tolerance, increased power, and superior fading mitigation compared to the conventional intensity modulation (IM) approach. Furthermore, through careful tuning of the phase response, the Q factor of the PM-RR system exhibits an enhancement exceeding 40% over a 100 km fiber length when compared to the Mach-Zehnder modulator (MZM) system.

Non-Invasive High-Resolution Imaging of In Vivo Human Myelinated Axons

This work aims to reveal the microscopic (2–3 micrometer resolution) appearance of human myelinated nerve fibers in vivo for the first time. We analyzed the myelinated retinal nerve fibers of a male patient without other neurological disorders in a non-invasive way using the transscleral optical phase imaging method with adaptive optics. We also analyzed the fellow eye with non-myelinated nerve fibers and compared the results with traditional ocular imaging methods such as optical coherence tomography. We documented the microscopic appearance of human myelin and myelinated axons in vivo. This method allowed us to obtain better details than through traditional ocular imaging methods. We hope these findings will be useful to the scientific community to evaluate neuro-retinal structures through new imaging techniques and more accurately document nerve anatomy and the pathophysiology of this disease.

Optimizing Wavelength for Enhanced Cycling Durability of Laser‐Induced Phase Change in Sb2S3 Films: A Survey on Optical Transmission Phase Shift

AbstractThe advancement of nonvolatile reconfigurable photonic elements has led to the development of Sb2S3 as a desirable phase change material for optical phase modulation in visible and near‐infrared wavebands. However, achieving long‐lasting cycling durability of the phase change remains a formidable challenge. Traditional characterization methods like transmittance or reflectance measurement fail to provide a quantitative assessment of the degree of phase change. To address this, the study focuses on enhancing the cycling durability of complete phase change in Sb2S3 films by manipulating the wavelength of a femtosecond laser for amorphization. To accurately measure optical transmission phase shifts during phase change experiments, a novel liquid‐crystal retardance matching technique based on the Mach–Zehnder interferometer is developed. This approach allows for a direct and quantitative assessment of the degree of phase change without relying on an optical model. The findings reveal that longer wavelengths and multi‐pulse irradiation with increasing pulse energy significantly enhance the cycling durability of the phase change. Additionally, the cost‐effective customization of nonvolatile photonic elements is demonstrated using laser direct writing, eliminating the need for lithography. It is anticipated that this work will drive advancements in Sb2S3‐based reconfigurable devices and contribute to the broader field of nonvolatile reconfigurable photonic elements.

Single-shot digital optical fluorescence phase conjugation through forward multiple-scattering samples.

Aberrations and multiple scattering in biological tissues critically distort light beams into highly complex speckle patterns. In this regard, digital optical phase conjugation (DOPC) is a promising technique enabling in-depth focusing. However, DOPC becomes challenging when using fluorescent guide stars for four main reasons: the low photon budget available, the large spectral bandwidth of the fluorescent signal, the Stokes shift between the emission and the excitation wavelength, and the absence of reference beam preventing holographic measurement. Here, we demonstrate the possibility to focus a laser beam through multiple-scattering samples by measuring speckle fields in a single acquisition step with a reference-free, high-resolution wavefront sensor. By taking advantage of the large spectral bandwidth of forward multiply scattering samples, digital fluorescence phase conjugation is achieved to focus a laser beam at the excitation wavelength while measuring the broadband speckle field arising from a micrometer-sized fluorescent bead.

Optical biosensor based on weak measurement for ultra-sensitive detection of calreticulin in human serum.

A novel real-time optical phase sensing method based on the Mach-Zehnder interference principle has been proposed for the detection of calreticulin (CRT) levels in human serum samples. In this approach, anti-CRT antibodies are utilized to capture CRT molecules in serum, leading to a phase shift in both the measuring and reference arms of the system. By employing the concept of weak amplification within the framework of weak measurements, it becomes feasible to continuously monitor the response of CRT in real-time, allowing for the precise determination of serum CRT content at the picomolar level. Our achievement may pave the way in establishing CRT as a diagnostic biomarker for a wide range of medical applications, including rheumatoid arthritis.

Chiral Lithography with Vortex Non‐diffracted Laser for Orbital Angular Momentum Detection

AbstractChiroptical response, demonstrating chiral interaction between optical vortex and chiral structure, plays an important role in variety of fields like optics and material science. However, the flexibility and efficiency of chiral structure fabrication are limited due to mask requirement and a point‐by‐point constructing strategy. In this paper, a novel chiral lithography method is proposed that utilizes optical vortex phase beam shaping to achieve chiral structure processing with high efficiency and flexibility. By programming topological charges of the vortex phase, chiral structures with adjustable appearance, rotation, and chirality can be produced using femtosecond laser single pulse exposure. Transmittance measurements of fabricated chiral structures array confirm a 66% helical dichroism that is predicted by simulation. Moreover, with the aid of convolutional neural networks (CNN), an accuracy of 98% in Orbital Angular Momentum (OAM) recognition can be achieved. This chiral lithography method provides an alternative for chiral structure fabrication and holds promise in the development of chiral optics, optical communications, and next‐generation optical devices.

Single-loop tunable PT-symmetric optoelectronic oscillator based on a phase modulator.

In this paper, a single-loop tunable parity-time (PT) symmetric optoelectronic oscillator (OEO) based on a dual-mode optical phase modulator (PM) and a microwave photonic filter (MPF) has been proposed, analyzed, and designed. By adjusting the polarization angle θ between the input linearly polarized light and the z axis of the PM, the two-mode splitting ratio of the PM can be controlled, which results in a PT-symmetric structure based on a single physical loop. By adjusting the pump light wavelength, the center frequency of the stimulated Brillouin scattering (SBS)-based MPF can be tuned, which enables a fine tunable output frequency. The results reveal that, by adjusting the pump light wavelength with a step size of 0.0001nm, a frequency tuning accuracy of 12.5MHz can be obtained. Simultaneously, the OEO output frequency can be tuned from 0.9 to 22GHz, while the side-mode suppress ratio (SMSR) is 53dB, and the phase noise is -133.8d B c/H z at a frequency offset of 10kHz.

Flexible and accurate system calibration method in microscopic fringe projection profilometry.

The three-dimensional (3D) measurement task of complex microstructures holds paramount significance in the domains of precision manufacturing and inspection. The calibration of the 3D system heavily determines the final reconstruction accuracy. The widely adopted system calibration method is phase-height mapping (PHM) and stereo vision (SV) based. The former can be applied directly to the calculation without considering the imaging model of the system, but it relies on highly precise and expensive translation stages or standard blocks. The latter's accuracy cannot be guaranteed because it is difficult to accurately calibrate the projector. In this paper, we establish an optically coupled microscopic fringe projection profilometry system that consists of a Scheimpflug pinhole projector and a super-low distortion bi-telecentric camera. We introduce a simplified 3D system calibration approach that combines phase modulation transfer and ray propagation. Our method enables the simultaneous calibration of the system, including the calibration of the projector, camera, and the phase to a 3D coordinates relationship, using only a 2D target. The calibrated projector's external parameters are used to obtain the target's complete poses, and then the direct mapping coefficients of the phase to the 3D coordinates can be obtained through the optical geometry structure and phase labels. Comparable experiments verify the feasibility of the proposed method.

Electrically tunable birefringence in nanophotonic waveguide with 2D electron gas in semiconductor heterojunction

An electrically tunable birefringence in a nanophotonic waveguide based on MgZnO-CdZnO semiconductor heterojunction is numerically proposed. Electrical control is realized with two-dimensional electron gas (2DEG) at the interface of ZnO based heterojunction and is used to control the polarization. The waveguide is engineered to enhance interaction of guided optical mode(s) with 2DEG providing us with a wide tuning in modal birefringence. We perform numerical analysis of phase and polarization characteristics of 2DEG based nanophotonic waveguide. Electrical tuning of 0.0112 in modal birefringence is reported by varying the applied voltage from 0 Vto −10 V. The proposed device also exhibits voltage variable optical phase modulation in both TE and TM polarizations. The proposed work can be utilized in applications where integrated photonic devices with variable polarization control are desirable.

Toward optimal performance of systems for digital optical phase conjugation

Toward optimal performance of systems for digital optical phase conjugation

All-optical high-order subharmonic mixer with a high signal-to-noise ratio and stable performance.

A photonics-based high-order subharmonic mixer, which enables a low-frequency LO source to be used for high-frequency RF signal frequency downconversion, is presented. It is based on an optically injected semiconductor laser, which is oscillated in the period-one state, sandwiched between two optical phase modulators. It has the advantages of a simple and compact structure, wide bandwidth, absence of electrical components, reconfigurable subharmonic mixing operation, stable output IF signal performance, high signal-to-noise ratio, infinite LO-to-RF port isolation, and high LO-to-IF port isolation. Furthermore, it is suitable for use in remote antenna applications. We set up a proof-of-concept experiment that demonstrates a reconfigurable second-, fourth-, sixth-, and eighth-order subharmonic mixing operation for different input RF signal frequencies and powers. The experimental results also demonstrate that the proposed structure exhibits a stable output IF signal performance, which overcomes the IF signal phase stability problem in the reported high-order subharmonic mixers.