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This paper proposes a multiple-input multiple-output (MIMO) terrestrial-satellite laser communication uplink system based on optical spatial modulation with medium-based modulation (OSM-MBM) joint modulation. The Málaga(M) distribution is used to describe the combined effects of near-ground turbulent light intensity scintillation, beam wander and angle of arrival fluctuation. The closed expression of the bit error rate (BER) of the OSM-MBM joint modulation MIMO terrestrial-satellite laser communication uplink is derived. The simulation analyzes the influence of transmitting radius, receiving aperture, beam divergence, zenith angle and signal-to-noise ratio on the system error performance, and compares it with traditional modulation, media-based modulation and optical space modulation. Finally, the experiment is verified by Monte Carlo. This research provides a theoretical reference for the engineering realization of the modulation technology for the MIMO terrestrial-satellite laser communication uplink system. [ABSTRACT FROM AUTHOR]

Existing laser scanning approaches for highly reflective surfaces generally require multiple scans to cope with specular reflection. This paper proposes a dual-camera, line-laser scanning strategy to reconstruct them efficiently, which only needs a single scanning path. To bypass complicated point cloud processing algorithms for outlier elimination, a feasible image processing scheme was designed to delete the disturbed stripe regions. By merging two sets of point clouds computed from residual stripes on a common world frame, these removed noise patches can be compensated by the opposite camera. Therefore, the data fusion result can reconstruct the 3D geometry of targets completely. Experimental results demonstrate good performance with the ability to handle highly reflective surfaces. The scanning period is significantly shortened and scanning accuracy can meet the industrial requirement. [ABSTRACT FROM AUTHOR]

In this paper, the dynamics of quantum correlation quantified by local quantum uncertainty (LQU) and local quantum Fisher information (LQFI) for two uncoupled qubits driven by classical phase noisy laser have been investigated. The results show that, both LQU and LQFI exhibit similar behaviours, either display exponential decay or oscillatory decay with revivals depending on the ratios of classical phase noisy laser rate and system-environment coupling strength. Beside, our results also confirm the amount of quantum correlation measured by LQU is strictly bounded by LQFI in an open system. Finally, we propose a scheme to protect quantum correlation against classical phase noisy laser with the technique of weak measurement and reversal. [ABSTRACT FROM AUTHOR]

This paper reports an experimental study of the shot-to-shot intensity and spectral distribution fluctuations in the femtosecond coherent pulses from the storage ring (SR) based seeded free electron laser (FEL) at DELTA. A unique implementation of a gated ICCD camera coupled to a Czerny-Turner monochromator is utilized to reveal the details of the individual spectra and intensity of femtosecond pulses accompanied by a significant synchrotron background. This technique demonstrates the potential to study the dynamics of the electron bunches in an SR by scanning over a single electron bunch with seeded femtosecond laser pulses at a subharmonic repetition rate. To the best of our knowledge, this picture provides the first direct experimental evaluation of shot-to-shot intensity and spectral fluctuation in seeded SR-based FELs. [ABSTRACT FROM AUTHOR]

Hafnium diboride (HfB2) is used in hypervelocity re-entry vehicles (such as intercontinental ballistic missile (ICBM) or ICBM heat shields), nuclear reactors and aerodynamic leading edges because of its strong tensile strength and high thermal resistance. HfB2 is an ultra-high-temperature ceramic (UHTC) with a melting point of 3250°C. In this paper, we study the performance of HfB2 as a saturable absorber in Q-switched lasers. HfB2 has a non-saturable absorptance of 2.5 dB/µm, a saturable absorptance of 0.8 dB/µm and a saturation fluence of 12 µJ/cm2. In addition, the saturation lifetime is estimated as 989.3 ps, while the recovery lifetimes are estimated as 254.9 and 22.1 ps. As a saturable absorber in our Q-switched laser, we observed pulses with durations between 880 and 2000 ns. With the addition of an acousto-optic modulator, we have observed Q-switching mode-locking, with pulses as low as 250 ns. HfB2 can potentially work at very high power, since its damage fluence is no less than 361 mJ/cm2. [ABSTRACT FROM AUTHOR]

In this paper, we investigate a thermal coherent state defined with the Lie–Trotter product formula under the formalism of the thermo field dynamics. In the definition of our thermal coherent state, we treat the thermalizing operator and the displacement operator symmetrically. We examine its uncertainty relation and quasiprobability distributions. Although this thermal coherent state is equivalent to the conventional ones except for different parameterizations and a phase factor, it is convenient for describing an experimental setup of the optical parametric oscillator laser. [ABSTRACT FROM AUTHOR]

Underwater target detection and ranging in seawater are of interest in unmanned underwater vehicles. This study presents an underwater detection system that synchronously scans a collimated laser beam and a narrow field of view to circumferentially detect an underwater target. Hybrid methods of range-gated and variable step-size least mean squares (VSS-LMS) adaptive filter are proposed to suppress water backscattering. The range-gated receiver eliminates the backscattering of near-field water. The VSS-LMS filter extracts the target echo in the remaining backscattering and the constant fraction discriminator timing method is used to improve ranging accuracy. The optimal constant fraction is selected by analysing the jitter noise and slope of the target echo. The prototype of the underwater detection system is constructed and tested in coastal seawater, then the effectiveness of backscattering suppression and high-ranging accuracy is verified through experimental results and analysis discussed in this paper. [ABSTRACT FROM PUBLISHER]

This paper demonstrates the use of a tapered fibre in generating a highly stable and tunable dual-wavelength fibre laser. By unique arrangement of polarization controller, adjustable spacing range between 0.94 and 3.32 nm and side mode suppression ratio (SMSR) up to 50 dB were recorded. The results were achieved at laser pump power of 94.7 mW. The inter-modal interference is achieved through the use of a non-adiabatic tapered fibre, made by a systematic flame brushing technique. The tapered fibre suppresses the mode competition in the 3-m erbium-doped fibre (EDF) gain medium. Over 60 min, the laser exhibited very high stability with acceptable peak power and SMSR. The proposed EDF laser operates from 1556.71 to 1562.13 nm range. [ABSTRACT FROM PUBLISHER]

The contributions of two energetically highest molecular orbitals to the harmonic emission rate are analysed for a two-component laser field. For diatomic molecules exposed to the elliptically polarised field, the emission from the highest-occupied molecular orbital (HOMO) is dominant for various molecular orientations with respect to the laser field. However, the contribution of the lower molecular orbital (HOMO-1) can become significant or even dominant for some molecular orientations. We introduce the ratio of the coherent over the incoherent sum of the HOMO and HOMO-1 contributions as a quantitative measure of the significance of the particular molecular orbital. Also, the gaseous medium response is different for the left and right elliptically polarised light and the molecular characteristics are imprinted into this difference. Moreover, for the orthogonally polarised two-colour (OTC) laser field the relative contributions of HOMO and HOMO-1 depend to a great extent on the relative phase between the field components. The importance of the HOMO-1 can be assessed by the relative error which is made if the harmonic spectra are obtained only with the HOMO contribution. Finally, we investigate the interference of the contributions of two highest molecular orbitals. We show that, for the OTC field, the destructive interference depends linearly on the intensity of the field components. Also, the interference minima shift towards the higher energies with the increase of the component wavelength. [ABSTRACT FROM AUTHOR]

A method for flexibly generating dual-chirp microwave waveform (DCMW) is proposed and experimentally demonstrated. The scheme uses a monolithic integrated three-section laser, which consists of a front section laser, a phase section and a rear section laser. Under the appropriate control current, the integrated laser works in the period-one (P1) state, and the two beams of different frequencies generated by the integrated laser can produce stable beat frequency. After photoelectric conversion, the generated DCMW has the flexible tunability of the central frequency. Through theoretical analysis and experimental demonstration, the DCMW with time-bandwidth-product (TBWP) of 6 × 105 and chirp rate of ± 0.03 GHz/μs is obtained at different centre frequencies. In addition, the auto-ambiguity function of the generated DCMW is also studied and it is proved that the proposed scheme can improve the range-velocity resolution, so it has application prospects in microwave photonic radar systems. [ABSTRACT FROM AUTHOR]

In this paper, a novel scheme is proposed to obtain the single-mode laser using parity-time symmetry breaking. Four analogue micro-rings are optically coupled in the square configuration in a way that two micro-rings on the square diameter experience gain and the other two experience loss. The coupling coefficient for horizontal and vertical coupling between micro-rings is considered to be different. According to this new scheme, the symmetry of structure is destroyed, and thus the degenerate modes that can disrupt the stability of laser, disappeared. Due to the coupling of the resonators, four super-modes, which are coupled pairwise, are generated. The Eigen-frequency and parity-time symmetry breaking of the two super-modes depend on the difference between horizontal and vertical coupling coefficient; however, these parameters depend on the summation of the coupling coefficients for the other two. Results show that the laser output is single mode in a specific condition of coupling coefficient and gains. It is concluded that the threshold of pump parameter is improved in the new scheme, relative to the two coupled micro-rings. [ABSTRACT FROM PUBLISHER]

We report on a compact 808 nm diode-pumped passively Q-switched (PQS) Nd:YVO4 laser at 1064 nm with black phosphorus (BP)/ indium selenide (InSe) heterostructure as a saturable absorber (SA). When the transmission of the output coupler (OC) was 30%, the maximum continuous-wave (CW) output power of 10.3 W was obtained with a slope efficiency of 52.4% under the incident pump power of 22 W. The maximum average output power of 3.1 W was obtained at 9.29 W pump power, corresponding to the repetition rate of 598 kHz and the pulse width of 455 ns during pulse operation, further power scaling was limited by the damage threshold of the saturable absorber mirror (SAM). The results sufficiently validated that two-dimensional (2D) BP/InSe heterostructure could be used as an optical modulator for near-IR pulsed laser sources. [ABSTRACT FROM AUTHOR]

Angled surfaces are significant for obtaining complex 3D glass structures to be employed in precise optical components such as light guides. However, it is challenging to fabricate angled structures in glass with high precision. In this study, femtosecond laser irradiation followed by a chemical etching process is used to create a 45-degree angled surface. The roughness values of the angled top and bottom surfaces are compared and the parameters are optimized to achieve maximum surface quality. A 45-degree angled surface with <300 nm roughness and <±0.5° deviation is obtained in a given area of 1 × 1.5 mm2. Using this approach, various complex structures have been successfully fabricated with high quality and precision. Results indicate that this method can be adapted to the high-precision glass manufacturing industry. [ABSTRACT FROM AUTHOR]

In underwater turbulent medium, field correlations are found when the incidence is a laser beam array. Variations of the field correlations against the variations in the ring radius of laser array beam, number of beamlets composing the laser array, source size, underwater turbulence parameters, i.e. the ratio of temperature to salinity contributions to the refractive index spectrum, rate of dissipation of mean-squared temperature and rate of dissipation of kinetic energy per unit mass of fluid, are investigated. Field correlations of laser arrays are found to be larger than the field correlations of the single beams. The effect of underwater turbulence is to reduce the field correlation of laser arrays. [ABSTRACT FROM AUTHOR]

A multi-wavelength switchable random fibre laser based on a double Sagnac-loop filter is proposed. The threshold of laser is 27 mW and the slope efficiency of the total power output to pump power is about 0.04%. In the experiment, stable and switchable laser outputs of single-, dual-, tri- and quad-wavelength channels are obtained by adjusting the polarization controller at room temperature. The wavelength positions are tuneable within the range of 1556–1564 nm and intervals are switchable between 2.5, 5 and 7.5 nm. Under 300 mW pump power, average power intensity of −24dBm is achieved. Furthermore, the output of fibre laser is relatively stable with less than 0.2 nm wavelength drift and less than 1.5 dBm power fluctuation within one hour. The proposed random fibre laser has promising application prospects for distributed sensing systems and biomedical imaging. [ABSTRACT FROM AUTHOR]

We demonstrate significant temporal contrast improvement for a petawatt-class Ti:sapphire femtosecond laser by introducing a temporal pulse filter based on a self-diffraction process. A double chirped-pulse-amplification laser system was set up to stretch, amplify and recompress the generated first-order self-diffraction pulse. The maximum energy of 130 μJ has been achieved for one first-order self-diffraction seed pulse. By optimizing the initial chirp value, the spectral bandwidth was slightly broadened for the first-order self-diffraction pulse and the cleaned pulse did not exhibit visible spectral distortions in the air environment. The measurement results for the incident pulse and the recompressed first-order self-diffraction pulse showed that the temporal contrast was improved by seven orders of magnitude, i.e. from ∼105 to ∼1012 at −25 ps before the main pulse. Such a temporal contrast is beneficial for laser-matter interaction experiments with high focused intensity up to 1022 W/cm2. [ABSTRACT FROM AUTHOR]

N-type heavy doping and tensile strain can significantly improve the luminescence efficiency of Ge materials. The active region of the quantum dot(QD) structure can realize high luminous efficiency due to its 3D carrier confinement. In this study, we constructed a Si-based QD array laser by introducing a 0.25% biaxial tensile strain and 4 × 10 19 cm − 3 N type heavy doping to Ge. A band-lifting method was proposed to determine the optimum doping concentration and thus improve the luminescence efficiency of Ge. To obtain accurate characteristics of the laser, we constructed a calculation model of three band transitions and all of the k-space quantum transitions of Ge and other relevant modified models. Results showed that the laser power was 5.31 μ W at a voltage of 2.5 V, and the laser wavelength reached 1519.4 nm at room temperature. The proposed laser can be used as a light source compatible with the Si-based CMOS process. [ABSTRACT FROM AUTHOR]

We analyse a method of high repetition rate, efficient terahertz wave difference frequency generation (DFG) by using a self-Raman Nd-doped vanadate laser to pump 4'-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST). A semi-analytical model is introduced that combines the rate equations (for the self-Raman dual-wavelength laser) and the coupled-wave equations (for the DAST-based DFG). Calculations were performed on a diffusion-bonded Nd:YVO4-YVO4 laser with an acousto-optic Q-switch. Parameters (including transmittances at output mirror and crystal lengths) were optimized to obtain high output. Sub-milliwatt average power is predicted under current pump condition. [ABSTRACT FROM AUTHOR]

Spontaneous photon bursts are observed in the output collected from a mesoscale semiconductor-based laser near the lasing threshold. Their appearance is compared to predictions obtained from Laser Rate Equations and from a Stochastic Laser Simulator. While the latter is capable of predicting the observed large photon bursts, the photon numbers computed by the former produces a noisy trace well below the experimentally detectable limit. We explain the discrepancy between the two approaches on the basis of an incorrect accounting of the onset of stimulated emission by the Rate Equations, which instead are capable of complementing the physical description through topological considerations. [ABSTRACT FROM AUTHOR]

Azimuthal smoothing (AS) that enables the focal spot of a laser quad to rotate in a period of a few picoseconds can improve the illumination uniformity. However, the smoothing performance of azimuthal smoothing is worse than that of smoothing by spectral dispersion (SSD) due to the low rotation speed in the center area of the focal spot. A combined implementation of the schemes including smoothing by spectral dispersion, radial smoothing (RS) and azimuthal smoothing is discussed to achieve better and faster illumination uniformity. The theoretical models of the combined smoothing scheme have been built up to analyse the smoothing performance. The key parameters affecting the smoothing performance of the combined smoothing scheme are analysed and optimized, including the topological charge and the wavelength difference. The simulation results show that the illumination uniformity of the laser quad can be significantly improved in multi-directions by the combined smoothing scheme. [ABSTRACT FROM AUTHOR]

We propose a laser cooling mechanism based on velocity-selective double-dark resonance that leads to a temperature significantly lower than the one-photon recoil limit. This mechanism benefits from sharp and high-contrast spectra which are induced by the two electromagnetically induced transparency windows due to the interacting dark-state resonances. It is theoretically demonstrated that four-level atoms illuminated by two counter-propagating probe beams and two additional beams directed perpendicularly to the other two exhibit new cooling effects; for red detuned probe lasers, atoms can be subject to a strong viscous force with an extremely small diffusion, characteristic of heating caused by the stochastic nature of spontaneous emission processes. By quantum mechanical simulations, we then find that the lowest temperature in EIT windows approaches 1 nK for the case of mercury. However, the different broadening mechanisms can destroy the cooling, so that the lowest temperature can increase to the recoil temperature. [ABSTRACT FROM AUTHOR]

To control the polarization of attosecond pulse trains (APTs), the most widely used way is to adjust the intensity ratio between the two components in a bicircular laser field. Here we demonstrate another effective means to obtain highly elliptically polarized APTs. For this purpose, we propose a circular-elliptical laser field, which consists of a circularly polarized field with its coplanar counter-rotating elliptical harmonic pulse, in order to generate high-order harmonics by He atom. We show that the polarization of APTs depends evidently on the ellipticity ξ of harmonic laser field. Most importantly, nearly circular APTs can be obtained with a proper ξ for driving laser field. Our results contribute to the generation of high elliptically polarized APTs with s-state atom. [ABSTRACT FROM AUTHOR]

In order to realize a wavelength-tuneable fibre-laser output, a ring-cavity erbium-doped fibre laser based on an all-fibre Mach–Zehnder interferometer (MZI) is proposed and experimentally tested. The MZI consists of a single-mode fibre, two segments of coreless fibre, and a seven-core fibre. For the proposed fibre laser, the length of the gain medium is 4 m and the lasing threshold is 75 mW. By adjusting the loss of the laser cavity, switchable single-wavelength laser emission is realized across the range of 1527.6–1549.9 nm and the wavelength interval is less than 2.4 nm; the peak power difference of each lasing wavelength is less than 7.9 dB. Tuneable dual- and three-wavelength laser outputs were obtained by adjusting the polarization controller. The 3-dB linewidth was less than 0.57 nm. The single- and dual-wavelength laser output power fluctuations were less than 1.4 and 1.7 dB, respectively, when monitored over a period of 30 min. [ABSTRACT FROM AUTHOR]

We demonstrate the generation of mode-locked Thulium-Doped Fibre Laser by employing a newly developed saturable absorber (SA) based on copper (Cu) thin film. The SA was prepared by depositing nano-sized particles of Cu onto the surface of polyvinyl alcohol (PVA) film through the E-Beam evaporation process. A stable mode-locking pulse train operating at 1951 nm was successfully generated by introducing the Cu PVA SA into a laser cavity. The laser generated a pulse train at the fundamental frequency of 8.5 MHz with a calculated minimum pulse width of 14.8 ps. This demonstration proves that the Cu PVA based SA is suitable for generating mode-locked fibre laser at 2 µm region. [ABSTRACT FROM AUTHOR]

Based on the extended Huygens–Fresnel integral formula, the analytical expressions for partially coherent four-petal Gaussian beam propagating in oceanic turbulence are derived, and the influences of coherence length, beam orderNand the parameters of oceanic turbulence (the rate of dissipation of turbulent kinetic energy per unit mass of fluid, the rate of dissipation of mean square temperature and the relative strength of temperature and salinity fluctuations) on average intensity properties are investigated using numerical examples in detail. The results show that the beam with the higher beam orderNor coherence length will lose its initial four-petal profiles slower. It is also indicated that the beam will evolve into a Gauss-like beam more rapidly with increasing oceanic turbulence strength. The results have the potential application in underwater laser communication using a partially coherent four-petal Gaussian beam. [ABSTRACT FROM PUBLISHER]

We present a study of the interaction of coherent field effects on population of two bound atomic states of a dense atomic gas interacting in a cooperative regime. We provide a theoretical description of this phenomenon using density matrix equations. We discuss the implications and applications of the present studies to quantum amplification by superradiant emission of radiation action. A classical model is developed to gain physical insights into qaser physics. [ABSTRACT FROM PUBLISHER]

A dual-wavelength self-Q-switched operation of Nd:GYSGG laser with different output couplers is proposed and demonstrated. In self-Q-switched operation, two laser lines between 1056.86 nm and 1060.23 nm were found. Under the pump power of 4 W, the shortest pulse width of 2.02 μs was obtained with a maximum average output power of 565 mW and the optical conversion efficiency of 14.13%. The pulse repetition rate and single pulse energy were 50.2 kHz and 11.25 μJ, respectively. The system is very stable and suitable for generation of multi-wavelength pulses. [ABSTRACT FROM AUTHOR]

A novel all-optical flip-flop based on a chirped nonlinear distributed feedback laser structure is proposed. The flip-flop does not require a holding beam. The optical gain is provided by a current injection into an active layer. The nonlinear wave-guiding layer consists of a chirped phase shifted grating accompanied with a negative nonlinear refractive index coefficient that increases in magnitude along the wave-guide. In the ‘OFF’ state, the chirped grating does not provide the required optical feedback to start lasing. An optical pulse switches the device ‘ON’ by reducing the chirp due to the negative nonlinear refractive index coefficient. The reduced chirp grating provides enough feedback to sustain a laser mode. The device is switched ‘OFF’ by cross gain modulation. GPGPU computing allows for long simulation time of multiple SET–RESET operations. The ‘ON/OFF’ transitions delays are in nanoseconds time scale. [ABSTRACT FROM PUBLISHER]