Coherent Beam Research Articles

Multilayer Kirkpatrick-Baez focusing mirrors with phase compensation for sub-20 nm focusing at the hard X-ray nanoprobe beamline of SSRF

The hard X-ray nanoprobe beamline is the first beamline to take advantage of the full coherent beam to attain the nanoscale focusing at the Shanghai Synchrotron Radiation Facility (SSRF). Here we introduce the beamline and specially go over the features of the multilayer Kirkpatrick-Baez focusing system and its supporting phase compensator system. The performance and stability of the phase compensator are also put to the test. By using the speckle scanning metrology, the wavefront of a focused beam was characterized and intensity distribution near the focus was reconstructed. The focusing performance was greatly enhanced by two phase compensations based on a global optimization technique, and a two-dimensional focal spot of 26 nm × 17 nm was achieved and maintained with good stability.

Optical switch based on tunable perfect photon absorption

We investigate the manipulation of optical field within quantum electrodynamics (QED) scheme. In our proposal, the cold three-level atoms interact with a two-sided cavity driven by two coherent beams with a relative phase. Coherent perfect absorption (CPA) appears when the beams are in phase. However, the perfect outputs occur when the beams are out of phase. On the basis of the phase modulation, an optical switch can be operated at a high efficiency. With a coherent control laser, the excited and metastable levels of the atoms are coupled. Electromagnetically induced transparency (EIT) is produced at two-photon resonance, which leads to four-band CPA. By virtue of the control laser, the switching efficiency can be maximized in a weaker probe field, which provides advantages over two-level atoms on weak-signal switching.

Twisted partially coherent solitons in strongly nonlocal nonlinear media

The analytical propagation formulae of twisted partially coherent beams in strongly nonlocal nonlinear media (SNNM) are derived. It is shown that a twisted partially coherent soliton (TPCS) can be formed in SNNM when the critical power arrives. The analytical expressions of the characteristic parameters of TPCSs are also derived, e.g., the critical power, soliton width, soliton coherence width, and soliton twist parameter. It is found that the characteristics of TPCSs can be manipulated by the twist parameter. In particular, a TPCS may convert to a soliton without twist when the critical coherence width arrives.

Characterization of Orbital Angular Momentum Beams by Polar Mapping and Fourier Transform

The recognition, decoding and tracking of vortex patterns is of increasing importance in many fields, ranging from the astronomical observations of distant galaxies to turbulence phenomena in liquids or gases. Currently, coherent light beams with orbital angular momentum (OAM) are of particular interest for optical communication, metrology, micro-machining or particle manipulation. One common task is to identify characteristic spiral patterns in pixelated intensity maps at real-world signal-to-noise ratios. A recently introduced combination of polar mapping and Fast Fourier Transform (FFT) was extended to novel sampling configurations and applied to the quantitative analysis of the spiral interference patterns of OAM beams. It is demonstrated that specific information on topological parameters in non-uniform arrays of OAM beams can be obtained from significantly distorted and noisy intensity maps by extracting one- or two-dimensional angular frequency spectra from single or concatenated circular cuts in either spatially fixed or scanning mode. The method also enables the evaluation of the quality of beam shaping and optical transmission. Results of proof-of-principle experiments are presented, resolution limits are discussed, and the potential for applications is addressed.

Synthesis of Robust Full Poincaré Polarization States via Spatial Coherence Engineering

The full Poincaré (FP) beam, encompassing all possible polarization states in its beam cross-section, has demonstrated advantages in various applications. However, conventional FP beams are typically considered as spatially fully coherent, rendering them sensitive to disturbances in the propagation path and susceptible to speckle effects. In this work, we propose an alternative approach to synthesize the optical beam with a FP polarization state through the spatial coherence engineering of a partially coherent beam. In this process, the FP polarization state is initially encoded into the spatial coherence structure of the beam source. We demonstrate that during the encoding process, the vector nature of the beam transitions from the FP polarization state to the spatial coherence structure of the source. However, during the propagation of the partially coherent beam, the vectorness reverts to the polarization state, resulting in the re-emergence of the encoded FP polarization in the output plane. We illustrate that the synthesized FP polarization state, achieved through spatial coherence engineering, is highly robust against obstructions in the propagation path. Furthermore, we examine the effect of the spatial coherence area of the beam on the quality of the recovered FP polarization state. The findings of this work can have valuable applications in optical trapping and optical imaging in complex environments.

Evolution properties of partially coherent electromagnetic elliptical vortex beam propagating through atmospheric turbulence

We obtained the analytical formulas of partially coherent electromagnetic elliptical vortex beam (PCEEVB) propagating through atmospheric turbulence with the help of the extended Huygens–Fresnel principle and explored the normalized average intensity distribution of PCEEVB in turbulence. It is worth noting that the PCEEVB appears new characteristics such as self-rotating, self-splitting, and self-healing in turbulence. It can be also found that the number of peaks caused by splitting of beams is relevant to the topological charge n. What’s more, reducing the ellipticity, magnifying the topological charge, and increasing the initial coherent length δ 0 can improve the anti-turbulence ability of PCEEVB in turbulence. The multi-phase screen simulation results are consistent with the theoretical results. Our results will have significant implications for laser communications, laser radar, and laser remote sensing.

Miniaturizing a coherent beam combining system into a compact laser diode module.

We present a laser module with dimensions of 76×43×15m m 3 that for the first time to our knowledge realizes a coherent beam combination in such a compact device, using two tapered amplifiers seeded by a distributed Bragg reflector ridge waveguide laser diode operating at 761nm in a single longitudinal mode. The generated combined optical power is up to 5W continuous wave with a combing efficiency of 85%. The phase of the system is controlled by the current in the ridge waveguide section of one of the tapered amplifiers. The phase-stabilization process is automated using a reverse hill-climbing algorithm and an ATmega328P microcontroller.

Coverage extended MMF-based indoor OWC using overfilled launch and diversity reception

Speckle patterns generated as coherent optical beams are reflected by scattering elements. Multimode fibers (MMFs) can modify the transverse intensity distribution of speckle patterns with macro perturbations, i.e., pressures, providing a simple and low-cost way to achieve equivalent beam-steering for indoor optical wireless communications (OWCs) with divergent optical beams. However, the received optical power (ROP) variance severely limits the mobility of user terminals. In this paper, the issue is alleviated by using the overfilled launch of MMFs and the diversity gain of multi-receivers. By adjusting the axial spatial coupling distance between the MMF and the single mode fiber (SMF) emitting coherent laser, the number of excited modes of MMF can be significantly increased at 1550 nm with negligible coupling and bending losses. In addition, the signal-to-noise ratio (SNR) enhancement obtained by applying two receivers is theoretically analyzed for the case when either thermal noise or shot noise is dominant. The experimental results demonstrate that the proposed scheme can efficiently compensate for the ROP inhomogeneity, and at the same time it can extend the achievable full steering angle up to 12° at a 1.5-m free-space distance for bit error rate (BER) values of less than 3.8 × 10−3.

Photonic integrated coherent beam combiner based on MMI and the SPGD algorithm.

Atmospheric turbulence severely degrades the optical wavefront of a propagating beam, which greatly reduces the coupling efficiency of free-space optical (FSO) receivers. Among the various methods to mitigate the effects, the use of a multi-channel receiver is more convenient and economical. After passing through the multi-channel receiver, multiple single-mode fibers (SMFs) are output and need to be combined. In this paper, we propose photonic integrated coherent beam combiners based on multimode interference (MMI) and the stochastic parallel gradient descent (SPGD) algorithm, which avoids detecting the light out of each channel and adding the data signal in the electrical domain. First, we propose a 4-channel coherent beam combiner based on a 4×1 MM, and about 21 iterations of the SPGD algorithm are required to enhance the combined optical power to a maximum of 96%. Furthermore, we demonstrate a combination of 16 beams using five 4×1 MMIs, which requires 140 iterations to enhance the combined power to 89%. This study offers theoretical insights to enhance the integration of FSO communication systems.

Flat-band Friedrich-Wintgen bound states in the continuum based on borophene metamaterials.

Many applications involve the phenomenon of a material absorbing electromagnetic radiation. By exploiting wave interference, the efficiency of absorption can be significantly enhanced. Here, we propose Friedrich-Wintgen bound states in the continuum (F-W BICs) based on borophene metamaterials to realize coherent perfect absorption with a dual-band absorption peak in commercially important communication bands. Metamaterials consist of borophene gratings and a borophene sheet that can simultaneously support a Fabry-Perot plasmon resonance and a guided plasmon mode. The formation and dynamic modulation of the F-W BIC can be achieved by adjusting the width or carrier density of the borophene grating, while the strong coupling leads to the anti-crossover behavior of the absorption spectrum. Due to the weak angular dispersion originating from the intrinsic flat-band characteristic of the deep sub-wavelength periodic structure, the proposed plasmonic system exhibits almost no change in wavelength and absorption at large incident angles (within 70 degrees). In addition, we employ the temporal coupled-mode theory including near- and far-field coupling to obtain strong critical coupling, successfully achieve coherent perfect absorption, and can realize the absorption switch by changing the phase difference between the two coherent beams. Our findings can offer theoretical support for absorber design and all-optical tuning.

Scattering of partially coherent electromagnetic beams by a sphere.

We examine the scattering of a partially coherent, partially polarized electromagnetic beam by a homogeneous sphere (Mie scattering). The degree of polarization and the Stokes parameters in the far zone are found to be strongly dependent on the state of coherence and polarization of the incident beam. In particular, we demonstrate the emergence of polarization singularities and show that partial spatial coherence gives rise to significant depolarization effects. In addition we explore the symmetry properties of the scattered field.

Bessel-Gauss coherently combined beams.

The application of a conical wavefront in the near field of the tiled aperture coherent beam combination (CBC) produces a segmented, similar to Bessel-Gauss irradiance pattern (BG-CBC), in the far field. The properties of such a structured optical field were numerically investigated. In contrast to that for the classical CBC, the power diffracted beyond the 'zero' BG-CBC diffraction order is more smoothly and homogenously distributed, not evidencing sharp maxima and dark regions typical for CBC. The novelty of the BG-CBC with segmented conical wavefront is a simple way of caustics elongation and redistribution of power density for the same CBC architecture.

Properties of a Partially Coherent Sine Beam in Non-Kolmogorov Turbulence

Properties of a Partially Coherent Sine Beam in Non-Kolmogorov Turbulence

Study on the effect of inter-channel phase error in coherent beam combining of laser

Study on the effect of inter-channel phase error in coherent beam combining of laser

Matrix formulation of the Gaussian expansion of coherent multiple beams in arbitrary dimensions.

Modeling the propagation of beams along laser beamlines is very challenging due to the multidimensional and multiscale configuration of the problem. Spatio-temporal couplings are particularly difficult to address with conventional numerical methods. Here we derive the Wigner function of a sum of Gaussian beams by calculating the multidimensional Fourier transform of the intercorrelation function of the fields. The matrix formulation allows for a simple propagation of the Wigner function in the framework of matrix optics. The relevancy of this approach is assessed by applying this model to one-dimensional and multidimensional configurations and by studying the influence of spatio-temporal couplings when considering propagation and dispersion by a diffraction grating.

Vortex harmonic generation in indium tin oxide thin film irradiated by a two-color field.

When a two-color Laguerre-Gaussian laser beam propagates through an indium tin oxide (ITO) material, the spatial distributions of odd- and even-order vortex harmonics carrying orbital angular momentum (OAM) are studied. The origin of vortex harmonics can be directly clarified by investigating their dependence on the incident laser field amplitude and frequency. In addition, it is shown that the spectral intensities of vortex harmonics are sensitive to the epsilon-near-zero nonlinear enhancing effects and the thickness of ITO materials. Thus the vortex harmonics can be conveniently tunable, which provides a wider potential application in optical communications based on high-order OAM coherent vortex beams.

Inorganic perovskite-based active multifunctional integrated photonic devices

The development of highly efficient active integrated photonic circuits is crucial for advancing information and computing science. Lead halide perovskite semiconductors, with their exceptional optoelectronic properties, offer a promising platform for such devices. In this study, active micro multifunctional photonic devices were fabricated on monocrystalline CsPbBr3 perovskite thin films using a top-down etching technique with focused ion beams. The etched microwire exhibited a high-quality micro laser that could serve as a light source for integrated devices, facilitating angle-dependent effective propagation between coupled perovskite-microwire waveguides. Employing this strategy, multiple perovskite-based active integrated photonic devices were realized for the first time. These devices included a micro beam splitter that coherently separated lasing signals, an X-coupler performing transfer matrix functions with two distinguishable light sources, and a Mach-Zehnder interferometer manipulating the splitting and coalescence of coherent light beams. These results provide a proof-of-concept for active integrated functionalized photonic devices based on perovskite semiconductors, representing a promising avenue for practical applications in integrated optical chips.

Change of polarization degree of light beams on propagation in curved space

Even in free space, which is commonly considered of as a flat space–time in most settings, the degree of polarization of a partially spatially coherent light beam changes as it travels. Similarly, the polarization degree would change when a partially spatially coherent light beam propagates in a curved space–time. The difference of the polarization degree between the curved space and flat space can reveal the essential structure of the curved space. In this work, we consider a simplest case of curved space known as Schwarzschild spacetime. We can simulate the Schwarzschild space–time as an optical material with an effective refractive index. The difference of the polarization degree of a light beam propagating in curved space and flat space can be achieved up to 5%, which is detectable in practical measurement. In addition, we have found that the partially spatially coherent light source is necessary for obtaining significant changes in polarization degree. Our results provide an alternative method to estimate the Schwarzschild radius of a massive object with the optical polarization degree measurement.

Vortex random lasing with tunable wavelength and orbital angular momentum

Random lasing with special structured light field has broad application prospects in various fields. However, the complex spatial modes of random lasing increase the difficulty of light field regulation and limit its practical application. Here, a vortex random lasing with dynamically tunable wavelength and orbital angular momentum is proposed based on the microfluidic channel. Different color random lasers are integrated into the same microfluidic channel for coarse control of the emission wavelengths from 462 to 685 nm by dynamically controlling the liquid flow. A special-shape cavity with a variable size of a gain region is further constructed to finely manipulate the emission wavelengths. Moreover, the vortex random lasing with tunable orbital angular momentum mode from −50 to 50 is realized. The results provide an outstanding strategy for generating the partially coherent vortex beams and may promote the practical applications of random lasers in the fields of sensing, imaging, and communication.

Multi-stage coherent beam combination of semiconductor optical amplifiers in ready-made fiber couplers

We report on multi-stage coherent beam combination (CBC) of continuous-wave (CW) outputs from semiconductor optical amplifiers (SOAs) in ready-made fiber couplers. The first CBC stage combines two 120-mW outputs from SOAs seeded by an extended-cavity diode laser (ECDL) at 1458 nm in a 2×2 50%:50% fiber coupler. Two beams generated by two such CBC setups are then combined in the second stage. By concatenating three stages we obtained an output power of 723 mW at 1458 nm from eight SOAs with a total combining efficiency of 75.3%. Stable power generation without interrupts nor degradation over three days was successfully implemented using a simple low-bandwidth servo system. An averaged single-stage combining efficiency of 89.5% deduced from seven CBC setups constituting the three-stage CBC is used to estimate scaling to further stages. As a practical application the output is used to second harmonic generation (SHG) in a nonlinear crystal to achieve an output power of 239 mW at 729 nm.