Laser Beacon Research Articles

Analysis of speckle-beacon size impact on wavefront sensing and closed-loop control in deep turbulence conditions

The impact of the laser beacon size on the performance of an ideal phase-conjugation-type adaptive optics (AO) system is analyzed using wave-optics-based numerical simulation. The analysis includes wavefront aberration sensing and closed-loop control for laser beam propagation both in vacuum and in distributed (volume) atmospheric turbulence with the beacon beam scattering off a flat extended target with Lambertian surface roughness. For mitigation of the impact of target-induced speckle effects on the performance of AO systems, speckle-average wavefront sensing and control approaches are introduced and analyzed. The results demonstrate that proposed speckle-average phase conjugation control algorithms enable partial mitigation of turbulence-induced aberrations in presence of strong speckle modulations.

Modelling and experimental testing of an optical synchronisation beacon designed for high‐loss satellite quantum communication

AbstractLong‐distance free space quantum key distribution based on CubeSats can be used to establish global quantum secure communication networks, with potential commercial applications benefitting from the low cost of its design and launch. Detecting single‐photon level optical pulses sent from space requires highly accurate and robust timing systems to pick out signals from the noise. For such high‐loss applications, we envisage a low‐repetition (sub‐MHz) beacon laser emitting short (ns) high‐peak‐power pulses from which interpolated quantum signal arrival windows can be derived. We firstly study theoretically the effects of jitter on the efficiency of gating quantum signals including all important jitter sources, and then experimentally investigated it by changing the clock jitter, and the result shows that the greater jitter will reduce the gating rate of the signal. The experimental interpolation error is tested against loss under laboratory conditions giving results close to our model. We also found that the jitter introduced by the Doppler effect can be ignored with a repetition rate larger than 1 kHz. This model can be directly used for the performance analysis and optimisation of all quantum and non‐quantum systems using similar synchronisation schemes over terrestrial free space or fibre.

The Neutrino Mediterranean Observatory Laser Beacon: Design and Qualification

This paper encapsulates details of the NEMO laser beacon’s design, offering a profound contribution to the field of the time calibration of underwater neutrino telescopes. The mechanical design of the laser beacon, which operates at a depth of 3500 m, is presented, together with the design of the antibiofouling system employed to endure the operational pressure and optimize the operational range, enhancing its functionality and enabling time calibration among multiple towers. A noteworthy innovation central to this development lies in the battery system. This configuration enhances the device’s portability, a crucial aspect in underwater operations. The comprehensive design of the laser beacon, encompassing the container housing, the requisite battery system for operation, electronics, and an effective antibiofouling system, is described in this paper. Additionally, this paper presents the findings of the laser beacon’s qualification process.

Разработка обобщенного алгоритма расчета энергетических характеристик электронно-оптических компонентов лазерной навигационной системы, функционирующей в условиях космического вакуума

The article highlights the components of a laser navigation system and analyzes them. A mathematical model of an optical navigation system formed by a radiation source, a transmission medium (space vacuum) and a receiver are constructed. A technique for estimating the energy characteristics of the photosensitive matrix of the orbiter's receiver has been developed. Based on the constructed technique, an algorithm was developed for calculating the energy characteristics of the electron-optical components of the lunar beacon according to the given energy characteristics of the light-sensitive matrix and the optical system of the orbiter. Examples of practical calculations for lunar day and lunar night are given. The correctness of the results obtained in the examples of calculations was checked. The algorithm developed during the study can be used to select electro-optical components while solving the problem of creating a navigation system based on laser beacons. Object of research: laser navigation system. Subject of research: a generalized algorithm for calculating the energy characteristics of the electro-optical components of laser navigation system operating in space vacuum. Main results: a generalized algorithm for calculating the energy characteristics of the electron-optical components of laser navigation system and a mathematical model of a navigation system formed by a radiation source, a transmission medium (cosmic vacuum) and a receiver included in it.

Characteristics of a Gaussian focus embedded within spiral patterns in common-path interferometry with phase apertures

A phase-only method is proposed to transform an optical vortex field into desired spiral diffraction–interference patterns. Double-ring phase apertures are designed to produce a concentric high-order vortex beam and a zeroth-order vortex beam, and the diffracted intensity ratio of two beams is adjustable between 0 and 1. The coherent superposition of the two diffracted beams generates a brighter Airy spot (or Poisson spot) in the middle of the spiral pattern, where the singularity for typical vortex beam is located. Experiments employing circular, triangular, and rectangular phase apertures with topological charges from 3 to 16 demonstrate a stable, compact, and flexible apparatus for vortex beam conversion. By adjusting the parameters of the phase aperture, the proposed method can realize the optical Gaussian tweezer function and the optical vortex tweezer function simultaneously along the same axis or switch the experimental setup between the two functions. It also has potential applications in light communication through turbulent air by transmitting an orbital angular momentum-coded signal with a concentric beacon laser.

Wavelength- and Angle-Selective Photodetectors Enabled by Graphene Hot Electrons with Tamm Plasmon Polaritons

Recently, two-dimensional materials have attracted attention owing to their special optical characteristics and miniaturization, with low thickness as well as extremely high responsivity. Additionally, Tamm plasmon polariton (TPP) resonance can be observed by combining a metal film and a one-dimensional (1D) photonic crystal (PC), where an electric field confinement is located at the metal-1D PC interface. In this study, a graphene layer combined with a TPP is proposed as a wavelength- and angle-selective photodetector. The graphene layer is located where the strong field confinement occurs, and the photocurrent response is significantly enhanced with increasing absorption by over four times (from 62.5 μA⋅W-1 to 271 μA⋅W-1 and undetected state to 330 μA⋅W-1 in two different samples). Moreover, the graphene-TPP photodetector has wavelength and angle selectivity, which can be applied in LiDAR detecting, sun sensors, laser beacon tracking, and navigational instruments in the future.

Calibration and Verification of Pointing and Tracking System for Optical Communication Terminal

This paper presents the experimental verification of the pointing and tracking system of the flight model of an optical communication terminal for small satellites. The terminal, called LaserCube, fits in two CubeSat units and is conceived to operate onboard spacecraft starting from the 6 U (CubeSat units) form factor. The terminal is provided with a dual stage pointing system, composed of a coarse pointing mechanism used to orient the optical head and a fast-steering mirror for the fine pointing of the received and transmitted lasers. The coarse pointing mechanism is based on the parallel platform configuration and alone can provide rms pointing accuracy when operating in conjunction with a beacon laser sent by a remote terminal and used for feedback. The control algorithm operates the two pointing stages synergically, targeting an overall pointing accuracy of rms while rejecting the disturbances coming from the satellite bus. The pointing system of the LaserCube flight model has been calibrated and experimentally verified in laboratory conditions that replicate real operational scenarios for what concerns the beacon signal attenuation and the disturbances generated by the host satellite. A dedicated setup was used to directly measure the pointing error and asses the system performance.

Electro-Optical Sensors for Atmospheric Turbulence Strength Characterization with Embedded Edge AI Processing of Scintillation Patterns

This study introduces electro-optical (EO) sensors (TurbNet sensors) that utilize a remote laser beacon (either coherent or incoherent) and an optical receiver with CCD camera and embedded edge AI computer (Jetson Xavier Nx) for in situ evaluation of the path-averaged atmospheric turbulence refractive index structure parameter Cn2 at a high temporal rate. Evaluation of Cn2 values was performed using deep neural network (DNN)-based real-time processing of short-exposure laser-beacon light intensity scintillation patterns (images) captured by a TurbNet sensor optical receiver. Several pre-trained DNN models were loaded onto the AI computer and used for TurbNet sensor performance evaluation in a set of atmospheric propagation inference trials under diverse turbulence and meteorological conditions. DNN model training, validation, and testing were performed using datasets comprised of a large number of instances of scintillation frames and corresponding reference (“true”) Cn2 values that were measured side-by-side with a commercial scintillometer (BLS 2000). Generation of datasets and inference trials was performed at the University of Dayton’s (UD) 7-km atmospheric propagation test range. The results demonstrated a 70–90% correlation between Cn2 values obtained with the TurbNet sensors and those measured side-by-side with the scintillometer.

Internal phase control of coherent fiber laser array without ambiguous phase based on double wavelength detection.

High-power fiber lasers have been widely utilized in manufacturing, medical care, and many other fields. Due to mode instability, nonlinear effects, and so on, the output power of a monolithic fiber laser is limited. Coherent beam combining (CBC) of fiber lasers is a promising way to obtain higher output power. An all-fiber CBC structure with internal phase detection has a compact construction and potential for a larger fiber laser array. For the existing internal active phase control of an all-fiber structure, π phase ambiguity always occurs because of double passing the fiber path. Additional compensation is needed under this condition, and the compactness of the system will decrease. In this paper, internal phase control of an all-fiber structure based on double wavelength detection without π-ambiguity is proposed. By adding a beacon laser with a different wavelength, phase locking of a coherent fiber laser array can be achieved internally without π-ambiguity. A corresponding math model is established, and a phase matched condition is derived. The spectral width of the beacon laser is analyzed, and the result shows that it can reach tens of nanometers with a proper optical path difference. Simulations of seven, 19, and 37 beams are carried out, and the results show that the structure proposed in this paper has the ability to achieve phase control with good robustness. The control bandwidth in the simulation is better than 1 kHz. By properly designing elements, the structure is expected to achieve high-power CBC of an all-fiber structure experimentally.

Analysis of ground-based optical means of observing laser beacons on board near-Earth spacecraft

Analysis of ground-based optical means of observing laser beacons on board near-Earth spacecraft

Airborne quantum key distribution with boundary layer effects

With the substantial progress of terrestrial fiber-based quantum networks and satellite-based quantum nodes, airborne quantum key distribution (QKD) is now becoming a flexible bond between terrestrial fiber and satellite, which is an efficient solution to establish a mobile, on-demand, and real-time coverage quantum network. However, the random distributed boundary layer is always surrounded to the surface of the aircraft when the flight speed larger than 0.3 Ma, which would introduce random wavefront aberration, jitter and extra intensity attenuation to the transmitted photons. In this article, we propose a performance evaluation scheme of airborne QKD with boundary layer effects. The analyzed results about the photon deflection angle and wavefront aberration effects, show that the aero-optical effects caused by the boundary layer can not be ignored, which would heavily decrease the final secure key rate. In our proposed airborne QKD scenario, the boundary layer would introduce ∼3.5 dB loss to the transmitted photons and decrease ∼70.9% of the secure key rate. With tolerated quantum bit error rate set to 8%, the suggested quantum communication azimuth angle between the aircraft and the ground station is within 55∘. Furthermore, the optimal beacon laser module and adaptive optics module are suggested to be employed, to improve the performance of airborne QKD system. Our detailed airborne QKD performance evaluation study can be performed to the future airborne quantum communication designs.

The gateware calibration unit for the KM3NeT telescope

The KM3NeT collaboration is currently deploying the first detection units of a neutrino observatory in the Mediterranean Sea, which, once completed, will be equipped with thousands of so-called digital optical modules. In addition to the detection units KM3NeT has designed an independent calibration unit, housing a set of calibration instruments, including e.g. an acoustic beacon and a laser beacon. The calibration unit and the embedded software developed to operate it are presented here.

Tracking System for Fast Moving Nodes in Optical Mobile Communication and the Design Rules

Free space optical communication has been applied in many scenarios because of its security, low cost and high rates. In such scenarios, a tracking system is necessary to ensure an acceptable signal power. Free space optical links were considered unable to support fast moving nodes, such as unmanned aerial vehicles, cars or pedestrians, in optical mobile communication because existing tracking schemes fail to track the nodes accurately and rapidly. In this paper, we propose a novel tracking system exploiting multiple beacon lasers. Each beacon laser’s power is measured to estimate the orientation of the target. Unlike existing schemes which drive servo motors multiple times based on consecutive measurements and feedback, our scheme can directly estimate the next optimal targeting shift for the servo motors based on a single measurement, allowing the tracking system to converge much faster. Closed-form outage probability expression is derived for the optical mobile communication system with ideal tracking, where pointing error and moving statistics are considered. To maintain sufficient average power and reduce the outage probability, the recommended beam divergence of the main laser is expressed in closed form as a function of the target’s statistics of random shift, providing insights to the system design.

The concept of high-precision trajectory measurements using a coupled system of an onboard optical arc meter-interferometer and optical laser beacons

The concept of high-precision trajectory measurements using a coupled system of an onboard optical arc meter-interferometer and optical laser beacons

Multifield acquisition technology for a narrow beacon laser on the non-orbit determination platform.

The application of free space optical communications technology between unmanned airborne vehicles (UAVs) can significantly improve its communications rate and capacity. However, it will produce strong disturbance due to the change of external conditions when the UAV is flying, which will affect the acquisition probability of a light beacon. The wide beacon can generally increase the acquisition range and acquisition probability, but there are disadvantages of increased weight and energy consumption. We report on an advanced method to improve the performance of narrow beacon spatial acquisition under the condition of UAV drift. The acquisition probability model has been established to study narrow beacon spatial acquisition considering UAV drift. A multiscanning method is proposed to improve the acquisition probability instead of simply increasing the scanning area to overcome severe UAV drift, which is also proven by experimental validation. Moreover, through the control method of active disturbance rejection control (ADRC), the pointing accuracy of multifield fast scanning is improved under the condition of external interference. This technique is extremely useful to develop a smaller, lighter terminal for UAV optical communications in the future.

Efficient cascade lasing on over 17 wavelengths from two-photon excitation of the cesium [formula omitted]D states

A broad survey is taken to assess the agility of the cesium 62D states in the generation of cascade lasing from 455 to 3650 nm. The experiment utilized a heat pipe with 0 to 10 Torr of He, cesium densities of 1014 to 1017 atoms per cm3, and it is pumped by a pulsed dye laser with 1 to 30 mJ/pulse. Cascade lasing, generated by amplified spontaneous emission with no aid of a cavity, is observed on over 17 transitions—nearly every dipole allowed transition from energy levels below 62D. Many lasing lines possess 100 to 229 μJ of energy, slope efficiencies of 4 to 7%, low thresholds favorable for diode pumping, and together have combined optical conversion efficiencies as high as 10%±3%. This agile system could have applications in infrared counter measures, target illuminator lasers, beacon lasers, underwater communication, and other tactical laser applications.

High-power narrow-band mode-locked sodium laser via double-stage sum-frequency generation

A high-power, narrow-band, mode-locked sodium D2a laser with a high sum-frequency efficiency is demonstrated by mixing actively mode-locked Nd:YAG 1064 and 1319 nm lasers in two LiB3O5 (LBO) crystals. The mode-locked lasers at 1064 nm (23.02 W output) and 1319 nm (17.98 W output) are generated from two diode, side-pumped, Nd:YAG, master oscillator power amplifier laser systems. A macro-micro pulse sodium beacon laser with a 30.2 W output power, 0.3 GHz (0.3 pm) linewidth, and a beam quality given by M2 = 1.23 is obtained with a conversion efficiency of 73.6%. Compared with a single LBO crystal, the sum-frequency efficiency is enhanced by 14.5%. A laser coupled amplitude equation model, including the acousto-optic mode-locker, is applied for theoretical and numerical analyses of the sum-frequency conversion efficiency. The experimental results agree well with the theoretical calculations.

Simulations of ELT-GMCAO Performance for Deep Field Observations

The Global-Multi Conjugated Adaptive Optics (GMCAO) approach offers an alternative way to correct an adequate scientific Field of View (FoV) using only natural guide stars (NGSs) to extremely large ground-based telescopes. Thus, even in the absence of laser guide stars, a GMCAO-equipped ELT-like telescope can achieve optimal performance in terms of Strehl Ratio (SR), retrieving impressive results in studying star-poor fields, as in the cases of the deep field observations. The benefits and usability of GMCAO have been demonstrated by studying 6000 mock high redshift galaxies in the Chandra Deep Field South region. However, a systematic study simulating observations in several portions of the sky is mandatory to have a robust statistic of the GMCAO performance. Technical, tomographic and astrophysical parameters, discussed here, are given as inputs to GIUSTO, an IDL-based code that estimates the SR over the considered field, and the results are analyzed with statistical considerations. The best performance is obtained using stars that are relatively close to the Scientific FoV; therefore, the SR correlates with the mean off-axis position of NGSs, as expected, while their magnitude plays a secondary role. This study concludes that the SRs correlate linearly with the galactic latitude, as also expected. Because of the lack of natural guide stars needed for low-order aberration sensing, the GMCAO confirms as a promising technique to observe regions that can not be studied without the use of laser beacons. It represents a robust alternative way or a risk mitigation strategy for laser approaches on the ELTs.

High-repetition-rate 100 W level sodium beacon laser for a multi-conjugate adaptive optics system.

A 100 W level kilohertz repetition-rate microsecond (µs)-pulse all-solid-state sodium beacon laser at 589 nm is demonstrated for the first time, to the best of our knowledge, via combining two independent µs-pulsed lasers. Each beamlet is generated by the sum-frequency mixing of pulsed 1064 and 1319 nm lasers in a lithium triborate (LBO) crystal, which operate at 500 Hz pulse repetition frequency with 61 W $p$p-polarized and 53 W $s$s-polarized output, respectively. An incoherent sequence combining technology of polarized laser beams is employed to add the two beamlets. The average power of the combined beam is up to 107.5 W with a combining efficiency of 94.3%. The combined beam has a 1 kHz repetition rate with ${\sim}{120}\;\unicode{x00B5} {\rm s}$∼120µs pulse duration and beam quality ${M^2} = {1.41}$M2=1.41. The central wavelength with a linewidth of ${\sim}{0.3}\;{\rm GHz}$∼0.3GHz is locked to a sodium ${{\rm D}_{2a}}$D2a absorption line. To the best of our knowledge, this is a record-high power operating at kilohertz for µs-pulsed solid-state sodium beacon lasers.

Design and in-orbit test of a high accuracy pointing method in satellite-to-ground quantum communication.

To meet the requirement of high-accuracy pointing of quantum signals in satellite-to-ground quantum communication, this paper proposes a flexible satellite-based pointing method that changes the fine tracking point to solve the problem from point-ahead angle and ground beacon laser offset. This method does not require the use of a point-ahead mechanism and can detect the pointing angle in real time. Detailed algorithms and analysis are given. The method has been verified in orbit on the quantum science satellite Micius. The satellite-to-ground test results show that the quantum signal pointing accuracy is between 0.5∼1.0 µrad, which meets the efficiency requirements of satellite-to-ground quantum communication.