Higher-order Spatial Modes Research Articles

A Multiplexing Optical Temperature Sensing System for Induction Motors Using Few-Mode Fiber Spatial Mode Diversity

Induction motors are widely applied in motor drive systems. Effective temperature monitoring is one of the keys to ensuring the reliability and optimal performance of the motors. Therefore, this paper introduces a multiplexed optical temperature sensing system for induction motors based on few-mode fiber (FMF) spatial mode diversity. By using the spatial mode dimension of FMF, fiber Bragg grating (FBG) carried by different spatial modes of optical paths is embedded in different positions of the motor to realize multipoint synchronous multiplexing temperature monitoring. The paper establishes and demonstrates a photonic lantern-based mode division sensing system for motor temperature monitoring. As a proof of concept, the system demonstrates experiments in multiplexed temperature sensing for motor stators using the fundamental mode LP01 and high-order spatial modes LP11, LP21, and LP02. The FBG sensitivity carried by the above mode is 0.0107 nm/°C, 0.0106 nm/°C, 0.0097 nm/°C, and 0.0116 nm/°C, respectively. The dynamic temperature changes in the stator at different positions of the motor under speeds of 1k rpm, 1.5k rpm, 2k rpm with no load, 3 kg load, and 5 kg load, as well as at three specific speed–load combinations of 1.5k rpm_3 kg, 1k rpm_0kg, 2k rpm_5 kg and so on are measured, and the measured results of different spatial modes are compared and analyzed. The findings indicate that different spatial modes can accurately reflect temperature variations at various positions in motor stator winding.

Dead Zone Fault Detection Optimization Method for Few-Mode Fiber Links Based on Unexcited Coupled Higher-Order Modes

Dead Zone Fault Detection Optimization Method for Few-Mode Fiber Links Based on Unexcited Coupled Higher-Order Modes

Reduction of relative intensity noise in a diamond Raman laser

The relative intensity noise (RIN) characteristics of a continuous-wave diamond Raman laser are investigated for the first time. The results reveal the parasitic stimulated Brillouin scattering (SBS) that usually occurred with higher-order spatial modes in the diamond Raman resonator is a pivotal factor impacting the Raman longitudinal modes and deteriorating the RIN level. The diamond Raman laser automatically switches to single-longitudinal-mode operation and the RIN level is significantly decreased in the frequency range of 200 Hz to 1 MHz after the parasitic SBS is effectively suppressed through inserting a spatial aperture or a χ(2) nonlinear crystal into the cavity. Due to the introduction of additional nonlinear loss to the high intensity Raman fluctuations and the non-lasing spontaneous Raman modes, the χ(2) nonlinear crystal enables better performance in the RIN-level reduction compared to the spatial aperture which can only achieve SBS inhibition. The RIN reduction routes are well suited for various crystalline Raman media to achieve high power and low intensity noise laser at different wavelengths.

Experimental Demonstrations of Alignment and Mode Matching in Optical Cavities with Higher-Order Hermite-Gauss Modes.

Higher-order spatial laser modes have recently been investigated as candidates for reducing test-mass thermal noise in ground-based gravitational-wave detectors such as advanced LIGO. In particular, higher-order Hermite-Gauss (HG) modes have gained attention within the community for their more robust behaviors against random test-mass surface deformations and stronger sensing and control capacities. Inthis Letter we offer experimental investigations on various aspects of HG mode interferometry. We have generated purified HG modes up to the twelfth order HG_{6,6} mode, with a power conversion efficiency of 38.8% and 27.7% for the HG_{3,3} and HG_{6,6} modes respectively. We demonstrate for the first time the misalignment and mode mismatch-induced power coupling loss measurements for HG modes up to the HG_{6,6}. We report an excellent agreement with the extended numerical power loss factors that in the "small power loss" region converge to 2n+1 or n^{2}+n+1 for a misaligned or mode mismatched HG_{n,n} mode. We also demonstrate the wavefront sensing (WFS) signal measurement for HG modes up to the HG_{6,6}. The measurement result is accurately in accordance with theoretical WFS gain β_{n,n-1}sqrt[n]+β_{n,n+1}sqrt[n+1] for an HG_{n,n} mode, with β_{n,n-1} being the beat coefficient of the adjacent HG_{n,n} and HG_{n-1,n} modes on a split photodetector.

High-order-mode Brillouin random fiber laser based on distributed Rayleigh scattering in all-few mode fiber half-open cavity

A high-order-mode (HOM) Brillouin random fiber laser (BRFL) based on distributed Rayleigh scattering in an all-few-mode fiber random cavity is proposed and experimentally demonstrated. Compared to traditional BRFLs oscillating in the single-mode-fiber cavity, the few-mode-fiber random cavity allows the high-order spatial mode Stokes beams to be activated and amplified via intra-mode stimulated Brillouin scattering for sufficient gain. Besides random feedback of distributed Rayleigh scattering along few-mode fibers, a few-mode fiber Bragg grating (FM-FBG) is employed to provide the one-end transverse mode-dependent reflection, which facilitates a high-purity oscillating random laser mode switchable between LP01 and LP11 mode by readily tunning the working wavelength of the Brillouin pump. Consequently, the all few-mode fiber-based BRFL emits a 600-Hz narrow-linewidth LP11 mode with a high mode purity of ∼91.3 %, which paves the way for spatial multiplexing coherent communication, high-resolution imaging, highly sensitive sensing, etc.

Effects of higher-order spatial modes on the squeezing level in optical parametric amplification with waveguides

We investigate a limiting factor of waveguide optical parametric amplifiers used for generating broadband squeezed light. We found that spatially non-degenerate optical parametric amplification (OPA) between the fundamental modes and the higher-order modes generates noise, and that this effect can be avoided by detecting the squeezed light with the optimal spatial mode. The analysis is conducted assuming the waveguides with different dimensions, offering valuable insight into the waveguide design for squeezing.

Effective soliton order approach for scaling of pulse self-compression in hollow-core fibers

Optical solitons can undergo extreme temporal and spectral reshaping at high pulse intensities. These dynamics can be utilized to extract few-cycle pulses in the negative dispersion region in the case of higher-order spatial modes in hollow fibers. One can achieve self-compression up to few-cycle duration in hollow capillary fibers without any external optical post-compression schemes using this technique. These intense few-cycle pulses are an essential tool for revealing the ultrafast dynamics in the femtosecond to attosecond timescale, where the tunability and scalability of few-cycle pulses are a necessity. Utilizing the soliton dynamics, we can scale the pulses up to multi-millijoule energies as well as terawatts of peak power. Scaling relations are relations that can be used to produce propagation dynamics that are invariant and essentially identical for multiple sets of input conditions. But, for the same input soliton order, the scaling relations derived under different dispersion conditions, such as different gas pressure, result in somewhat different scaling laws. This leads to an ambiguity in the compression factor and compression length for any particular soliton order N. It is thus necessary to find an accurate soliton order which can scale the self-compression dynamics over different dispersion conditions. We numerically simulate soliton self-compression in an argon gas-filled HCF across a wide range of dispersion conditions and present a set of efficient scaling laws for soliton dynamics. We introduce an effective soliton order Neff for explaining the behavior of the dynamics in systems with high third-order dispersion (TOD) such as the HCF. The proficiency of the new scaling laws over the earlier scaling laws in literature is elucidated in this work. Thus, an inclusive set of scaling laws for generating high-energy few-cycle pulses are defined which are critical for generating single and trains of attosecond pulses, as well as electron and ion acceleration strategies in intense laser pulses.

High-power laser beam in higher-order Hermite–Gaussian modes

The sensitivities of current gravitational-wave detectors are limited around signal frequencies of 100 Hz by mirror thermal noise. One proposed option to reduce this thermal noise is to operate the detectors in a higher-order spatial laser mode. This operation would require a high-power laser input beam in such a spatial mode. Here, we discuss the generation of the Hermite–Gaussian modes HG2,2, HG3,3, and HG4,4 using one water-cooled spatial light modulator (SLM) at a continuous-wave optical input power of up to 85 W. We report unprecedented conversion efficiencies for a single SLM of about 43%, 42%, and 41%, respectively, and demonstrate that the SLM operation is robust against the high laser power. This is an important step toward the implementation of higher-order laser modes in future gravitational-wave detectors.

Secondary Raman and Brillouin mode suppression in two- and three-mirror-cavity diamond Raman lasers.

We report an investigation into secondary mode suppression in single longitudinal mode (SLM) 1240 nm diamond Raman lasers. For a three-mirror V-shape standing-wave cavity incorporating an intra-cavity LBO crystal to suppress secondary modes, we achieved stable SLM output with a maximum output power of 11.7 W and a slope efficiency 34.9%. We quantify the level of χ(2) coupling necessary to suppress secondary modes including those generated by stimulated Brillouin scattering (SBS). It is found that SBS-generated modes often coincide with higher-order spatial modes in the beam profile and can be suppressed using an intracavity aperture. Using numerical calculations, it is shown that the probability for such higher-order spatial modes is higher for an apertureless V-cavity than in two-mirror cavities due its contrasting longitudinal mode-structure.

Turbulence-resilient differential-phase-shift-keying free-space optical communications using automatic multi-mode optoelectronic mixing

Free-space-optical (FSO) communications can benefit from power-efficient modulation formats and detection methods. In general, differential-phase-shift-keying (DPSK) tends to be more optical-power efficient when detecting both output arms in the receiver as compared to intensity modulation. Unfortunately, turbulence can induce power coupling from a Gaussian mode to higher-order spatial modes, which can produce high power loss in the differential receiver. Here we demonstrate a 2.25-Gbit/s turbulence-resilient DPSK FSO link under emulated turbulence effects with a strength of 2ω0/r0∼5.5. The turbulence-induced modal coupling loss is automatically compensated by optoelectronic (O/E) mixing of many higher-order spatial modes. The experimental results, based on 200 random turbulence realization measurements, indicate that compared with a single-mode coherent receiver, the O/E multi-mode mixing can reduce the average turbulence-induced mixing loss by ∼14.6 dB.

Graded-azimuthal-index fiber as a control element for radial-index and orbital-angular-momentum modes of Laguerre-Gaussian beams

We propose an azimuthally varying graded-index fiber that can efficiently create and control a definite set of radial-index and orbital-angular-momentum (OAM) modes of Laguerre-Gaussian beams and their superpositions. A rigorous coupled-wave analysis formalism is developed, which provides a ready analysis for the azimuthal-refractive-index variation in the Laguerre-Gaussian basis. In particular, we present effective control in the generation and coupling of the various OAM modes, including radial-index modes of the Laguerre-Gaussian beam within the fiber, wherein the propagation length of the fiber and the wavelength act as the control parameters. We further present the possibility of switching every alternate channel in a wavelength division multiplexing protocol to a higher-order spatial mode within the optical fiber to substantially reduce interchannel crosstalk. The graded azimuthal index fiber can be utilized in high-speed fiber-optic communications as a versatile control element for OAM modes and the radial-index mode of the Laguerre-Gaussian beam transformation.

Fault detection sensitivity analysis and optimization of the few-mode fiber link under a dynamic spatial mode crosstalk cumulative effect.

Due to the cumulative effect of dynamic spatial mode crosstalk, it is difficult to obtain the amplitude distribution of backscattering with high purity. Hence, the fault detection sensitivity (FDS) of the few-mode fiber (FMF) link is deteriorated, and the fault location accuracy is limited. This work investigates the characteristics and optimization method of FDS for the FMF link under the dynamic spatial mode crosstalk cumulative effect. We establish a mathematical model and analyze the influence of dynamic spatial mode crosstalk on the FDS of LP01, LP11a, and LP11b modes. The crosstalk mentioned above is caused by different splice misalignments and rotation angles during FMF fusion splicing. The results show that the dynamic spatial mode crosstalk has a great influence on the detection sensitivity of the high-order spatial mode. To solve this problem, the FDS optimization method is proposed based on high-order mode waveform reconstruction. The coupling efficiency matrix at the fusion splice point is estimated by the Rayleigh backscattering waveform of each mode, and the Rayleigh backscattering waveform of high-order spatial modes is reconstructed. The simulation experiment is carried out based on the above theoretical model. The results show that the proposed method can effectively eliminate the influence of the cascading dynamic crosstalk accumulation on the fault loss of the high-order spatial mode, and then optimize the FDS of the FMF link. This study offers new opportunities to develop FMF fault detection devices with high detection sensitivity performance.

High order modes of intense second harmonic light produced from a plasma aperture

Because of their ability to sustain extremely high-amplitude electromagnetic fields and transient density and field profiles, plasma optical components are being developed to amplify, compress, and condition high-power laser pulses. We recently demonstrated the potential to use a relativistic plasma aperture—produced during the interaction of a high-power laser pulse with an ultrathin foil target—to tailor the spatiotemporal properties of the intense fundamental and second-harmonic light generated [Duff et al., Sci. Rep. 10, 105 (2020)]. Herein, we explore numerically the interaction of an intense laser pulse with a preformed aperture target to generate second-harmonic laser light with higher-order spatial modes. The maximum generation efficiency is found for an aperture diameter close to the full width at half maximum of the laser focus and for a micrometer-scale target thickness. The spatial mode generated is shown to depend strongly on the polarization of the drive laser pulse, which enables changing between a linearly polarized TEM01 mode and a circularly polarized Laguerre–Gaussian LG01 mode. This demonstrates the use of a plasma aperture to generate intense higher-frequency light with selectable spatial mode structure.

Laboratory demonstration of real-time focal plane wavefront control of residual atmospheric speckles

Current and future high contrast imaging instruments aim to detect exoplanets at closer orbital separations, lower masses, and/or older ages than their predecessors. However, continually evolving speckles in the coronagraphic science image limit contrasts of state-of-the-art ground-based exoplanet imaging instruments. For ground-based adaptive optics (AO) instruments, it remains challenging for most speckle suppression techniques to attenuate both the dynamic atmospheric as well as quasistatic instrumental speckles on-sky. We have proposed a focal plane wavefront sensing and control algorithm to address this challenge, called the fast atmospheric selfcoherent camera (SCC) technique (FAST), which in theory enables the SCC to operate down to millisecond timescales even when only a few photons are detected per speckle. Here, we present the first experimental results of FAST on the Santa Cruz Extreme AO Laboratory (SEAL) testbed. In particular, we illustrate the benefit of “second stage” AO-based focal plane wavefront control, demonstrating up to 5 × contrast improvement with FAST closed-loop compensation of evolving residual atmospheric turbulence—both for low and high order spatial modes—down to 20-ms timescales.

Reverse-Taper Mid-Infrared Quantum Cascade Lasers for Coherent Power Scaling

We present a reverse-taper quantum cascade laser (QCL) emitting at 4.6 μm, a novel-geometry device that can scale the output power while maintaining good beam quality. Buried-ridge waveguides with tapered and straight regions were formed by ICP etching and HVPE regrowth – the tapered region scales the output power, while the emitting facet is located at the narrow-end taper section, which provides mode filtering by suppressing high-order spatial modes. Beam profiles were observed under quasi-continuous-wave (QCW)/CW operation and beam quality (M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) measurements along with beam-stability measurements were performed – a small degree of collimated-beam centroid movement (<0.46 mrad, peak-to-peak) was observed, along with M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> values close to 1 up to ∼1 W QCW power. Devices of shorter cavity lengths were also investigated, indicating that the output power scales with the core-region volume but results in a small increase in angular deviation.

Metamaterial-enabled arbitrary on-chip spatial mode manipulation

On-chip spatial mode operation, represented as mode-division multiplexing (MDM), can support high-capacity data communications and promise superior performance in various systems and numerous applications from optical sensing to nonlinear and quantum optics. However, the scalability of state-of-the-art mode manipulation techniques is significantly hindered not only by the particular mode-order-oriented design strategy but also by the inherent limitations of possibly achievable mode orders. Recently, metamaterials capable of providing subwavelength-scale control of optical wavefronts have emerged as an attractive alternative to manipulate guided modes with compact footprints and broadband functionalities. Herein, we propose a universal yet efficient design framework based on the topological metamaterial building block (BB), enabling the excitation of arbitrary high-order spatial modes in silicon waveguides. By simply programming the layout of multiple fully etched dielectric metamaterial perturbations with predefined mathematical formulas, arbitrary high-order mode conversion and mode exchange can be simultaneously realized with uniform and competitive performance. The extraordinary scalability of the metamaterial BB frame is experimentally benchmarked by a record high-order mode operator up to the twentieth. As a proof of conceptual application, an 8-mode MDM data transmission of 28-GBaud 16-QAM optical signals is also verified with an aggregate data rate of 813 Gb/s (7% FEC). This user-friendly metamaterial BB concept marks a quintessential breakthrough for comprehensive manipulation of spatial light on-chip by breaking the long-standing shackles on the scalability, which may open up fascinating opportunities for complex photonic functionalities previously inaccessible.

Observation of Squeezed States of Light in Higher-Order Hermite-Gaussian Modes with a Quantum Noise Reduction of up to 10dB.

Mirror thermal noise will be a main limitation for the sensitivities of the next-generation ground-based gravitational-wave detectors (Einstein Telescope and Cosmic Explorer) at signal frequencies around 100Hz. Using a higher-order spatial laser mode instead of the fundamental mode is one proposed method to further mitigate mirror thermal noise. In the current detectors, quantum noise is successfully reduced by the injection of squeezed vacuum states. The operation in a higher-order mode would then require the efficient generation of squeezed vacuum states in this mode to maintain a high quantum noise reduction. In our setup, we generate continuous-wave squeezed states at a wavelength of 1064nm in the fundamental and three higher-order Hermite-Gaussian modes up to a mode order of 6 using a type-I optical parametric amplifier. We present a significant milestone with a quantum noise reduction of up to 10dB at a measurement frequency of 4MHz in the higher-order modes and pave the way for their usage in future gravitational-wave detectors as well as in other quantum noise limited experiments.

Monolithic elliptical dome Fabry–Perot microcavities exhibiting large birefringence

We report on the experimental and theoretical characterization of elliptically shaped Fabry–Perot microcavities fabricated through a controlled thin-film buckling process. Due to the highly astigmatic nature of the buckled mirrors, the cavity modes are well described as elliptical Hermite–Gaussian beams. In addition to lifting the typical degeneracy of higher-order transverse spatial modes, the cavities exhibit large polarization-mode splitting greater than 25 GHz in the 1550 nm wavelength range. This large, controllable, and highly predictable birefringence makes these cavities of interest for emerging applications in cavity quantum optics that rely on non-degenerate polarization modes.

Intermodal synchronization effects in multimode fibers with noninstantaneous nonlinearity

The synchronization of spatial modes in the formation of multimode solitons can be considered as the transverse analog of mode locking. In this process, the inherent Kerr nonlinearity in the fiber core binds different spatial fiber modes together. However, compared to their temporal counterparts, the binding mechanism in multimode solitons is rather weak and susceptible to perturbation. In order to mitigate this effect, the propagation in multimode fibers with noninstantaneous Kerr media (NKM) is theoretically investigated. Our study focuses on the dynamics of intermodal energy transfer for the two cases of weak and strong walk-off. Both the moment method and numerical simulations of a multimode expansion of the nonlinear Schr\"odinger equation are employed. Scenarios of pure, hybrid, and varying NKM are investigated. For pure NKM, it is found that for weak walk-off, higher order spatial modes are accelerated in the time domain when their energy is transferred to lower order modes, that is, an effect that has previously been discussed as self-cleaning. In the hybrid case, when an additional instantaneous Kerr effect is present, this results in an enhancement of intermodal nonlinear coupling, leading to prominent oscillating evolutions of the derivatives of energy and pulse center. For varying NKM, the nonlinear refractive index and the proportion of noninstantaneous Kerr nonlinearity may both vary with pulse width. In this case, energy transfer and temporal shift are essentially determined by the magnitude of nonlinear response time of NKM. Significant temporal self-splitting at the trailing edge is observed for the lowest order mode provided only that the response time is large enough and irrespective of strong or weak walk-off.

Fabrication of low-loss quasi-single-mode PPLN waveguide and its application to a modularized broadband high-level squeezer

A continuous-wave (CW) broadband high-level optical quadrature squeezer is essential for high-speed large-scale fault-tolerant quantum computing on a time-domain-multiplexed continuous-variable optical cluster state. CW THz-bandwidth squeezed light can be obtained with a waveguide optical parametric amplifier (OPA); however, the squeezing level has been insufficient for applications of fault-tolerant quantum computation because of degradation of the squeezing level due to their optical losses caused by the structural perturbation and pump-induced phenomena. Here, by using mechanical polishing processes, we fabricated a low-loss quasi-single-mode periodically poled LiNbO3 (PPLN) waveguide, which shows 7% optical propagation loss with a waveguide length of 45 mm. Using the waveguide, we assembled a low-loss fiber-pigtailed OPA module with a total insertion loss of 21%. Thanks to its directly bonded core on a LiTaO3 substrate, the waveguide does not show pump-induced optical loss even under a condition of hundreds of milliwatts pumping. Furthermore, the quasi-single-mode structure prohibits excitation of higher-order spatial modes and enables us to obtain larger squeezing level. Even with including optical coupling loss of the modularization, we observe 6.3-dB squeezed light from the DC component up to a 6.0-THz sideband in a fully fiber-closed optical system. By excluding the losses due to imperfections of the modularization and detection, the squeezing level at the output of the PPLN waveguide is estimated to be over 10 dB. Our waveguide squeezer is a promising quantum light source for high-speed large-scale fault-tolerant quantum computing.