Spatial Mode Selection Research Articles

High-brightness 300 W fiber laser at ~950 nm.

Fiber lasers operating in the 900 nm wavelength band have garnered significant interest due to their extensive array of applications. However, their development is still hindered by various factors, with the output power currently being restricted to the hundred-watt level. Despite numerous proposed solutions, achieving higher power outputs in this wavelength band remains a challenge. In this Letter, we have pioneered a new approach that harnesses stimulated Raman scattering within a fiber nonlinear gain framework. We have engineered a high-brightness laser diode source by utilizing techniques such as spectral beam combining and polarization beam combining, which are then employed directly for pumping. Through meticulous optimization of the fiber laser system and by incorporating brightness enhancement strategies such as multimode fiber beam cleaning and resonant cavity spatial mode selection, we have achieved a record-breaking output power of 315 W with an outstanding beam quality factor of M2 = 2.45 for high-brightness fiber lasers at ∼950 nm.

Non-Hermitian kagome photonic crystal with a totally topological spatial mode selection.

Recently, the study of non-Hermitian topological edge and corner states in sonic crystals (SCs) and photonic crystals (PCs) has drawn much attention. In this paper, we propose a Wannier-type higher-order topological insulator (HOTI) model based on the kagome PC containing dimer units and study its non-Hermitian topological corner states. When balanced gain and loss are introduced into the dimer units with a proper parity-time symmetric setting, the system will show asymmetric Wannier bands and can support two Hermitian corner states and two pairs of complex-conjugate or pseudo complex-conjugate non-Hermitian corner states. These topological corner states are solely confined at three corners of the triangular supercell constructed by the trivial and non-trivial kagome PCs, corresponding to a topological spatial mode selection effect. As compared to the non-Hermitian quadrupole-type HOTIs, the non-Hermitian Wannier-type HOTIs can realize totally topological spatial mode selection by using much lower coefficients of gain and loss. Our results pave the way for the development of novel non-Hermitian photonic topological devices based on Wannier-type HOTIs.

Excitation of surface plasmon polaritons by diffraction-free and vector beams.

Surface plasmon polaritons (SPPs) are traditionally excited by plane waves within the Rayleigh range of a focused transverse-magnetic (TM) Gaussian beam. Here we investigate and confirm the coupling between SPPs and two-dimensional Gaussian and Bessel-Gauss wave packets, as well as one-dimensional light sheets and space-time wave packets. We encode the incoming wavefronts with spatially varying states of polarization; then we couple the respective TM components of radial and azimuthal vector beam profiles to confirm polarization-correlation and spatial-mode selectivity. Our results do not require material optimization or multi-dimensional confinement via periodically corrugated metal surfaces to achieve coupling at a greater extent, hereby outlining a pivotal, yet commonly overlooked, path towards the development of long-range biosensors and all-optical integrated plasmonic circuits.

Study on the Spatial Structure and City Form Construction of River Valley-Type Cities in the Context of Artificial Intelligence—A Case Study of Northwest China

This paper uses Northwest China as an example to present an in-depth study and analysis of river valley cities’ spatial structure and city form in the context of artificial intelligence. The paper focuses on alleviating the “core-edge” spatial structure locking effect, achieving the “industry-space” scale effect and cooperation, solving the problem of endogenous dynamics to promote regional and urban development, and promoting various main functional areas based on the derivation discussion and spatial growth model selection. The study focuses on the exploration of paths to alleviate the “core-edge” spatial structure’s locking effect, realize the scale effect and cooperation ability of “industry-space,” solve the problem of endogenous power to promote regional and urban development, and promote the new urbanization path of different main functional areas based on the discussion and spatial growth mode selection. We measure the evolution characteristics of industrial and spatial elements and the level of intersystem synergistic development to address the spatial growth and industrial transformation of the northwest region urban agglomeration in the special development period, discover the defects of the northwest China urban agglomeration’s spatial growth, and select the appropriate spatial growth model in the foreseeable spatial and temporal scope. The new city spatial form in the northwest region, which strengthens regional characteristics, is the spatial form combined with the new city plan form in two dimensions. This paper investigates the relationship between regional characteristics and urban spatial carriers by developing spatial planning design ideas and methods that reflect the regional characteristics of new cities in the northwest region, in order to improve the spatial quality of new cities in the northwest region and their sense of belonging to the city, provide a new spatial expansion idea for small cities in river valleys in the Qinling Mountains of southern Shaanxi, and enrich the spatial quality of new cities in the northwest region.

The generation of femtosecond optical vortex beams with megawatt powers directly from a fiber based Mamyshev oscillator.

Numerous approaches have been developed to generate optical vortex beams carrying orbital angular momentum(OAM) over the past decades, but the direct intracavity generation of such beams with practical output powers in the femtosecond regime still remains a challenge. Here we propose and experimentally demonstrate the efficient generation of high-peak-power femtosecond optical vortex pulses from a Mamyshev oscillator (MO) based on few-mode polarization-maintaining (PM) ytterbium-doped fibers (YDFs). By employing an appropriate intracavity transverse spatial mode selection technique, ultrafast pulses carrying OAM with selectable topological charge of l=±1 are successfully generated with an average output power of∼5.72W at ∼24.35MHz repetition rate, corresponding to a single pulse energy of ∼235nJ.The chirped pulses can be compressed to ∼76fs outside the cavity, leading to a pulse peak power of ∼2.2MW. To the best of our knowledge, this isby far the highest pulse energy and peak power for optical vortex pulses ever generated directly from a fiber oscillator. This unprecedented level of performance should be of great interest for a variety of applications including materials processing and imaging.

Analysis and experimental demonstration of spatial mode selection in a coherently combined fiber laser.

Self-phasing due to spatial mode selection in a two-element passively coupled fiber laser is studied. We find that the addition of a second supermode in a coupled resonator results in a 90% increase in the average output power and nearly π/2 radians of passive phase adjustment versus applied phase errors between the gain elements. These results require a phase of zero (modulo 2π) between the beams in the external cavity. These findings are supported by an eigenmode analysis of the resonator and show that beam recycling is a useful resonator design feature but must be appropriately implemented to obtain beneficial results.

Demonstration of an electric field conjugation algorithm for improved starlight rejection through a single mode optical fiber

Linking a coronagraph instrument to a spectrograph via a single mode optical fiber is a pathway towards detailed characterization of exoplanet atmospheres with current and future ground- and space-based telescopes. However, given the extreme brightness ratio and small angular separation between planets and their host stars, the planet signal-to-noise ratio will likely be limited by the unwanted coupling of starlight into the fiber. To address this issue, we utilize a wavefront control loop and a deformable mirror to systematically reject starlight from the fiber by measuring what is transmitted through the fiber. The wavefront control algorithm is based on the formalism of electric field conjugation (EFC), which in our case accounts for the spatial mode selectivity of the fiber. This is achieved by using a control output that is the overlap integral of the electric field with the fundamental mode of a single mode fiber. This quantity can be estimated by pair-wise image plane probes injected using a deformable mirror. We present simulation and laboratory results that demonstrate our approach offers a significant improvement in starlight suppression through the fiber relative to a conventional EFC controller. With our experimental setup, which provides an initial normalized intensity of $3\times10^{-4}$ in the fiber at an angular separation of $4\lambda/D$, we obtain a final normalized intensity of $3\times 10^{-6}$ in monochromatic light at $\lambda=635$~nm through the fiber (100x suppression factor) and $2\times 10^{-5}$ in $\Delta\lambda/\lambda=8%$ broadband light about $\lambda=625$~nm (10x suppression factor). The fiber-based approach improves the sensitivity of spectral measurements at high contrast and may serve as an integral part of future space-based exoplanet imaging missions as well as ground-based instruments.

Mode-selective amplification in a large mode area Yb-doped fiber using a photonic lantern.

We demonstrate selective spatial mode amplification in a few mode, double-clad Yb-doped large mode area (LMA) fiber, utilizing an all-fiber photonic lantern. Amplification to multi-watt output power is achieved while preserving high spatial mode selectivity. We observe gain values of over 12 dB for all modes: LP01, LP11a, and LP11b, when amplified individually. Additionally, we investigate the simultaneous amplification of LP01+LP11a and LP11a+LP11b, and the resultant mode competition. The proposed architecture allows for the reconfigurable excitation of spatial modes in the LMA fiber amplifiers, and represents a promising method that could enable dynamic spatial mode control in high power fiber lasers.

Learning-Based Adaptive Transmission for Limited Feedback Multiuser MIMO-OFDM

Performing link adaptation in a multiantenna and multiuser system is challenging because of the coupling between precoding, user selection, spatial mode selection and use of limited feedback about the channel. The problem is exacerbated by the difficulty of selecting the proper modulation and coding scheme when using orthogonal frequency division multiplexing (OFDM). This paper presents a data-driven approach to link adaptation for multiuser multiple input mulitple output (MIMO) OFDM systems. A machine learning classifier is used to select the modulation and coding scheme, taking as input the SNR values in the different subcarriers and spatial streams. A new approximation is developed to estimate the unknown interuser interference due to the use of limited feedback. This approximation allows to obtain SNR information at the transmitter with a minimum communication overhead. A greedy algorithm is used to perform spatial mode and user selection with affordable complexity, without resorting to an exhaustive search. The proposed adaptation is studied in the context of the IEEE 802.11ac standard, and is shown to schedule users and adjust the transmission parameters to the channel conditions as well as to the rate of the feedback channel.

Schmidt-like coherent mode decomposition and spatial intensity correlations of thermal light

We experimentally study the properties of coherent mode decomposition for the intensity correlation function of quasi-thermal light. We use the technique of spatial mode selection developed for studying the transverse entanglement of photon pairs, and show that it can be extended to characterize classical spatial correlations. Our results demonstrate the existence of a unique, for a given thermal source, basis of coherent modes, correlated in a way much resembling the Schmidt modes of spatially entangled photons.

Analysis of spatial emission structures in vertical-cavity surface-emitting lasers with feedback from a volume Bragg grating

We investigate the spatial and spectral properties of broad-area vertical-cavity surface-emitting lasers (VCSEL) with frequency-selective feedback by a volume Bragg grating (VBG). We demonstrate wavelength locking similar to the case of edge-emitters but the spatial mode selection is different from the latter. On-axis spatial solitons obtained at threshold give way to off-axis extended lasing states beyond threshold. The investigations focus on a self-imaging external cavity. It is analyzed how deviations from the self-imaging condition affect the pattern formation and a certain robustness of the phenomena is demonstrated.

Spatial Mode Selection by the Phase Modulation of Subwavelength Plasmonic Grating

The extraordinary transmission of the subwavelength gold grating has been investigated by the rigorous coupled-wave analysis and verified by the metal–insulator–metal plasmonic waveguide method. The physical mechanisms of the extraordinary transmission are characterized as the excitation of the surface plasmon polariton modes. The subwavelength grating integrated with the distributed Bragg reflector is proposed to modulate the phase to realize spatial mode selection, which is prospected to be applied for transverse mode selection in the vertical cavity surface-emitting laser.

Efficient Sum Rate Maximization and Resource Allocation in Block-Diagonalized Space-Division Multiplexing

For space-division multiplexing (SDM) via block diagonalization on multiuser multiple-input multiple-output (MIMO) wireless downlink, it is shown that receive antenna selection (RAS) is necessary for maximizing the achievable sum rate. This is true even when all receive antennas are equipped with radio frequency (RF) chains and RAS reduces the upper bound on the broadcast sum capacity, and when the orthogonalized channels use optimal processing. Similarly, spatial-mode selection (SMS) is necessary for sum rate maximization when receive-weight matrices are used for spatial-mode allocation. RAS/SMS may release transmission resources that can fully be utilized via additional user scheduling to yield further sum rate gains. Optimal user selection for sum rate maximization is subsumed within an exhaustive RAS/SMS process for multiantenna terminals, and both selection processes become identical for single-antenna terminals. RAS/SMS thus helps reduce the performance gap from the optimal sum capacity even for small user pool sizes. A block antenna/mode selection approach is introduced to help overcome the drawbacks of existing algorithms. Since RAS/SMS involves antenna/mode ranking, a systematic method for resource allocation with sum rate loss minimization is inherently provided. This way, a streamlined process that combines user selection, RAS/SMS, and resource allocation is developed for sum rate maximization of block-diagonalized SDM.

Non-tissue-like features in the time-of-flight distributions of plastic tissue phantoms

We measure high-temporal-resolution time-of-flight distributions of picosecond laser pulses in the visible and near-infrared, scattered in the forward direction by solid and liquid phantoms, and compare them to those obtained by using ex vivo tissues. We demonstrate that time-of-flight distributions from solid phantoms made of Delrin, Nylon, and Teflon are modulated by ripples that are absent in the biological samples and disappear when the temporal and/or angular resolution of the measuring apparatus is decreased. This behavior prevents the use of such materials as tissue phantoms when spatial mode and time selection are required, such as in imaging methods exploiting early arriving photons.

Spectral and spatial single mode emission from a photonic crystal distributed feedback laser

The authors report on the fabrication and characterization of photonic crystal distributed feedback lasers, which are a generalization of distributed feedback and angled distributed feedback lasers. Spectral and spatial mode selection is provided by a two-dimensional, rectangular lattice of etched air holes that is tilted relatively to the cleaved facets. InGaAs∕AlGaAs laser layers emitting around 1μm were used for the fabrication of the devices. The laser emits at a single wavelength with a side mode suppression ratio of 50dB. The emission is concentrated in a narrow far field, indicating that a coherent mode is formed over the entire width of the device.

Homodyne measurement of the average photon number

We describe a scheme for measurement of the mean photon flux at an arbitrary optical sideband frequency using homodyne detection. Experimental implementation of the technique requires an acousto-optic modulator in addition to the homodyne detector, and does not require phase locking. The technique exhibits polarization and frequency and spatial mode selectivity, as well as much improved speed, resolution, and dynamic range when compared to linear photodetectors and avalanche photodiodes, with potential application to quantum-state tomography and information encoding using an optical frequency basis. Experimental data also support a quantum-mechanical description of vacuum noise.

Spatial and spectral mode selection of heralded single photons from pulsed parametric down-conversion

We describe an experiment in which photon pairs from a pulsed parametric down-conversion (PDC) source were coupled into single-mode fibers with heralding efficiencies as high as 70%. Heralding efficiency or mode preparation efficiency is defined as the probability of finding a photon in a fiber in a definite state, given the detection of its twin. Heralding efficiencies were obtained for a range of down-conversion beam-size configurations. Analysis of spatial and spectral mode selection, and their mutual correlation, provides a practical guide for engineering PDC-produced single photons in a definite mode and spectral emission band. The spectrum of the heralded photons were measured for each beam configuration, to determine the interplay between transverse momentum and spectral entanglement on the preparation efficiency.

Three-mirror resonator with aspheric feedback mirror for laser spatial mode selection and mode shaping

The spatial-mode characteristics are studied for a three-mirror resonator consisting of a diffractive aspheric third mirror aligned collinearly with a standing-wave two-mirror resonator. Simulations predict that enhanced spatial-mode selection and mode shaping can be achieved by this three-mirror cavity. A three-mirror Nd:YVO/sub 4/ solid-state laser with an optimized aspheric third mirror has been designed and implemented to oscillate in a designed fundamental Gaussian mode. The modal discrimination was measured to be 1.43 and the propagation factor M/sup 2/ was 1.02.

Spatial-Mode Selection for the Joint Transmit and Receive MMSE Design

To approach the potential MIMO capacity while optimizing the system bit error rate (BER) performance, the joint transmit and receive minimum mean squared error (MMSE) design has been proposed. It is the optimal linear scheme for spatial multiplexing MIMO systems, assuming a fixed number of spatial streams p as well as a fixed modulation and coding across these spatial streams. However, state-of-the-art designs arbitrarily choose and fix the value of the number of spatial streams p, which may lead to an inefficient power allocation strategy and a poor BER performance. We have previously proposed to relax the constraint of fixed number of streams p and to optimize this value under the constraints of fixed average total transmit power and fixed spectral efficiency, which we referred to as spatial-mode selection. Our previous selection criterion was the minimization of the system sum MMSE. In the present contribution, we introduce a new and better spatial-mode selection criterion that targets the minimization of the system BER. We also provide a detailed performance analysis, over flat-fading channels, that confirms that our proposed spatial-mode selection significantely outperforms state-of-the-art joint Tx/Rx MMSE designs for both uncoded and coded systems, thanks to its better exploitation of the MIMO spatial diversity and more efficient power allocation.

Properties of a phase-conjugate etalon mirror and its application to laser resonator spatial-mode control.

The concept of a phase-conjugate etalon mirror consisting of one flat and one aspheric surface is introduced. This new element can be used as an end mirror of a conventional resonator to promote spatial-mode selection and mode shaping. A phase-conjugate etalon designed for the fundamental Gaussian mode is experimentally implemented and tested with a single-mode He-Ne laser.