Laser Physics

The study of the reduction of an optical fiber by chemical etching has been suggested to determine the concentrations of sucrose in water and their refractive indices by evanescent waves using a coherent infrared source. The cladding of a single-mode optical fiber was removed at a rate of ~3.27 µm min⁻¹ using hydrofluoric acid until it reached a diameter of 7.3 µm, similar to the core of the fiber. This fiber was used to characterize sucrose solutions at different amounts employing a continuous wave infrared laser source at 1550 nm. The sucrose was dissolved in water to evaluate the quantitative sensor response based on the transmission relationship. The experimental results showed that the refractive indices obtained by the evanescent absorbance were in the range of 1.31–1.44 for concentrations of sucrose between 0% (water) to 65%. Additionally, it was determined that for concentrations higher than 65% of sucrose, the refractive index of the solution is similar to the core of the fiber, and therefore the total internal reflection was not possible. The results obtained in this work suggest that the etched optical fiber can be used as a refractive index sensor, which may play an important role in chemical applications.

With ever increasing world population, the demands on food safety and security are also expected to substantially increase over the next few decades. As agronomic practices, agricultural mechanization and plant breeding technologies have already been extensively exploited, novel techniques need to be explored and implemented to enhance crop production. To this end, the emerging area of laser-based technologies has shown potential to bring about another revolution in enhancing quantity, quality, and safety of foods. This paper presents an exhaustive review of the use of five non-invasive non-destructive laser-based techniques in agriculture, namely laser biostimulation, light detection and ranging, laser land levelling, laser-induced fluorescence spectroscopy, and Raman spectroscopy. Herein we provide the advantages, status quo and challenges of each of these techniques and conclude with recommendations for future work. A comprehensive review of literature reveals the untapped potential of laser applications in agriculture that has the potential to unleash the next agricultural revolution.

The emission of electrons from hot plasmas generated in the interaction of ultra-short (and ultra-high intensity) laser pulses with matter is often characterized by the so-called 'hot electron temperature'. In this article it is shown that this number is not unambiguous. The reason is the following: to assign a temperature to an electron spectrum, it is necessary to describe the spectrum with a distribution function. However, different types of distribution functions are in use, e.g. the Boltzmann or Maxwell distribution, leading to different electron temperatures in spite of providing nearly the same form of the electron spectrum. For this reason, the main characteristics of all these distribution functions are presented in this article and compared. Depending on the distribution function used, the value of the hot electron temperature varies by up to 30% and in extreme cases by more than a factor of four. This fact should always be kept in mind when comparing values of hot electron temperatures. In addition, the reasons for using equilibrium distributions to describe the characteristics of laser-produced electrons—although probably no thermodynamic equilibrium is prevailing—are discussed.

Quantum super-resolution involves resolving two sources below the Rayleigh limit using quantum optics. Such a technique would allow high-precision inter-satellite positioning and tracking on communication and navigation constellations. Due to the size, weight and power constraints typical of low-earth-orbit (LEO) satellites, a simple solution is often preferred. Here, we show that a balanced homodyne detection (BHD) setup using a shaped single-mode local oscillator can achieve super-resolution despite typical photonic losses. We further analyze the impact of a fluctuating and fixed centroid misalignment due to satellite pointing issues, and find that fixed misalignment is comparatively more detrimental to the performance of a BHD setup. Thus, our study provides a practical assessment of BHD to achieve super-resolution on a modern LEO satellite platform. Finally, we discuss how our analysis can be extended to stellar sources for astronomical applications.

Great concern regarding energy resources and environmental polution has increased interest in the study of alternative sources of energy. Biodiesels as an alternative fuel provide a suitable diesel oil substitute for internal combustion engines. The Raman spectra of pure biodiesels of soybean oil, olive oil, coconut oil, animal fats, and petroleum diesel are optically characterized for quality and biofuel as an alternative fuel. The most significant spectral differences are observed in the frequency range around 1457 cm⁻¹ for pure petroleum diesel, 1427 for fats biodiesel, 1670 cm⁻¹ for pure soybean oil, 1461 cm⁻¹ for soybean oil based biodiesel, 1670 cm⁻¹ for pure olive oil, 1666 cm⁻¹ for olive oil based biodiesel, 1461 cm⁻¹ for pure coconut oil, and 1460 cm⁻¹ for coconut oil based biodiesel, which is used for the analysis of the phase composition of oils. A diode pump solid-state laser with a 532 nm wavelength is used as an illuminating light. It is demonstrated that the peak positions and relative intensities of the vibrations of the oils can be used to identify the biodiesel quality for being used as biofuel.

We demonstrate a figure-of-9 all-fiber thulium-doped laser (TDFL) that generates 560 fs long pulses at 1948 nm wavelength. In order to achieve self-starting passive mode-locking, we utilize an in-fiber Faraday rotator to induce a nonreciprocal phase shift. To the best of our knowledge, this is the first all-fiber TDFL that combines an artificial saturable absorber (SA) with a chirped fiber Bragg grating (CFBG) as a wavelength-selective reflector. This cavity design is an excellent candidate to pump nonlinear processes such as supercontinuum and frequency comb generation since it does not require any SA material that degrades over time for mode-locking and could be made wavelength-tuneable via the CFBG.

The mode field intensity, spot size, central peak intensity evolution and adiabaticity are calculated for different points along the transition of an optical fibre taper that adiabatically tapers from the standard 125 nm down to 1 µm and then to 440 nm diameter for low loss operation at 1550 nm wavelength. The first section of the taper is evaluated using a weak guidance approximation. The second section is treated as a three-index layer structure (double-clad) and evaluated with eigenvalue equations for three refractive indices. The third and thinnest section of the taper is studied using an exact mode eigenvalue equation. The results show that the fundamental mode for the third section has a discontinuity at the fibre edge with a peak intensity larger than the intensity at the centre of the fibre. Since the guiding by the core disappears in the first section of the taper, the mode field does not simply reduce monotonously along the taper with the outer diameter of the fibre. By this novel approach, and for the first time, to the best of our knowledge, the taper shape that complies with the adiabaticity criterion, the mode intensity profile and the spot size (first Petermann definition) of the fundamental mode evolution, along their position on the taper are determined.

We demonstrated a 577 nm yellow laser source with a proper combination of Raman medium and frequency doubling medium, pumped externally. Cryogenic Yb:YAG oscillator acts as an external pump source, and it generates 1.2 mJ energy at 1 KHz emitting around 1029 nm. Using a Raman resonator with Ba(NO₃)₂ as Raman medium, stimulated Raman scattering around 1153.4 was generated. Finally, 577 nm yellow laser was generated using the frequency-doubling medium LBO. A maximum average output power of 65 mW with a pulse duration of 18 ns at 1 KHz was achieved.

Many different fields benefit from the usage of light sources emitting in the mid-infrared wavelength range (2–10 µm). A rising need for precise and fast sources in the mid infrared (mid-IR) is reflected in the development of a high-power, picosecond mid-IR source capable of generation at high repetition rates. In this work, we present the optimization of an optical parametric generator, pumped by a 3 W portion of total power of the Yb:YAG thin-disk laser (1.3 ps, 90 kHz, 90 W) by comparing a single-pass and double-pass arrangement output parameters in terms of output power dependences on input power, efficiency, beam profiles, stability, and spectra. The output tunability of both arrangements spanned from 1459 nm to 2891 nm, with the upper limit being influenced by the limited transmission of the dichroic components used in the setup above 2700 nm. It was shown that the double-pass arrangement increases the output power, from 17 mW in the single-pass arrangement to 193 mW in the double-pass arrangement at 1459 nm, resulting in over ten-fold output power increase.

We presented a comprehensive study of optimized loading of ultracold Cs atoms in a magnetic levitated crossed dipole trap. Moreover, we analyzed the optimized experimental parameters of the dipole laser for the opening time and the power intensity prior to switching on the magnetic field which formed the magnetically levitated dipole trap. The number of atoms as the function of sweep time from distinct laser power intensity to a fixed one is measured. And beyond that, the variation of atoms with the dipole laser power intensity per beam was studied experimentally ranging from 0 W cm⁻² to a fixed one for loading the dipole trap, and the number of atoms can be increased up to a maximum of ∼1.67 × 10⁶. Our research demonstrates that the 'pre-loading process' plays a crucial role in enhancing the loading efficiency of an optical dipole trap.

The radiation properties of the cross collision between a single electron and an intense laser pulse are researched by numerical simulation methods. Under the condition of tightly-focused laser, the electron trajectories, spatiotemporal distribution and spectrum are compared with that under non-tightly focused lasers. The results show that the torsion effect on the electron during the oscillation process is more notable after the tightly focused laser interacts with electron. The radiation it generates is asymmetric in space, and its time distribution is nearly unimodal and can be regarded as a single attosecond pulse. In frequency domain, the spectrum appears to be a supercontinuum. With the increase of beam waist radius, the symmetry of the spatial distribution enhances and time distribution also exhibits a three-peak structure that is symmetrical about the main peak. Furthermore, the spectrum changes from a supercontinuum to a multimodal distribution. The analysis turns out that tightly focused laser is more realistic compared to non-tightly focused laser or even plane wave, which benefits the design of high-quality x-rays in practical application.

The emission of electrons from hot plasmas generated in the interaction of ultra-short (and ultra-high intensity) laser pulses with matter is often characterized by the so-called 'hot electron temperature'. In this article it is shown that this number is not unambiguous. The reason is the following: to assign a temperature to an electron spectrum, it is necessary to describe the spectrum with a distribution function. However, different types of distribution functions are in use, e.g. the Boltzmann or Maxwell distribution, leading to different electron temperatures in spite of providing nearly the same form of the electron spectrum. For this reason, the main characteristics of all these distribution functions are presented in this article and compared. Depending on the distribution function used, the value of the hot electron temperature varies by up to 30% and in extreme cases by more than a factor of four. This fact should always be kept in mind when comparing values of hot electron temperatures. In addition, the reasons for using equilibrium distributions to describe the characteristics of laser-produced electrons—although probably no thermodynamic equilibrium is prevailing—are discussed.

In a fixed spectral range, single- and half-cycle electromagnetic pulses have the shortest duration. Half-cycle pulses are promising tools for ultrafast control of quantum systems. Previously, the possibility of using a sequence of single- and half-cycle attosecond pulses to generate and ultrafast control light-induced population difference gratings has been demonstrated. However, such studies have been carried out using different approximations, such as the sudden perturbation theory and the two-level model for the resonant medium. In this paper, based on the numerical solution of constitutive equations for elements of the density matrix and wave equation it is shown that it is possible to generate and control population gratings in a three-level medium without using the approximation of sudden perturbations used in previous studies. It is shown that taking into account the additional level of the medium does not lead to a violation of the effect of generating such gratings. This extends the applicability of previous results.

Optical solitons can find important applications in optical fiber communication systems. Here, we simulate extra-cavity modulation of a chirped Gaussian bisoliton in a 1 μm wavelength band. Several different soliton parameters are varied (including the amplitude ratio and time delay of orthogonal components, the projection angle, phase difference, pulse chirps and propagation distances), to effectively change the optical spectra and pulse shapes of the initial input chirped Gaussian bisoliton. For example, when the two branches in the optical fiber modulation system have the same or different fiber lengths, the modulated chirped Gaussian bisoliton will show obviously different properties in the time domain for orthogonally polarized components, while the corresponding optical spectra have no obvious differences. The simulation results reveal the effects of extra-cavity modulation of the chirped Gaussian bisoliton, which further explores the field of soliton shaping out of a fiber laser cavity.

The combustion of fossil fuels is primarily responsible for disrupting the carbon cycle equilibrium by releasing greenhouse gases (GHGs). Therefore, detecting GHG emissions from fossil fuels is extremely important. In this study, utilizing laser-induced breakdown spectroscopy (LIBS), a new method for real-time in-situ detection of carbon fluctuations during combustion has been developed. The combustion of fossil fuels is emulated through the controlled burning of candles within a confined area, and the elemental content of the surrounding air during this process is analyzed. Fluctuations in the intensity of CN spectral lines were tracked to reveal changes in carbon concentration. The backpropagation neural network (BPNN) is used to identify and verify local air with different carbon concentrations, and the predictions are accurate. In conclusion, the integration of BPNN and LIBS for the purpose of identifying variations in carbon content during combustion provides an effective method for environmental management.

The review presents the methods of generation of nonlinear coherent excitations in strongly nonequilibrium Bose-condensed systems of trapped atoms and their properties. Non-ground-state Bose–Einstein condensates are represented by nonlinear coherent modes. The principal difference of nonlinear coherent modes from linear collective excitations is emphasized. Methods of generating nonlinear modes and the properties of the latter are described. Matter-wave interferometry with coherent modes is discussed, including such effects as interference patterns, internal Josephson current, Rabi oscillations, Ramsey fringes, harmonic generation, and parametric conversion. Dynamic transition between mode-locked and mode-unlocked regimes is shown to be analogous to a phase transition. Atomic squeezing and entanglement in a lattice of condensed atomic clouds with coherent modes are considered. Nonequilibrium states of trapped Bose-condensed systems, starting from weakly nonequilibrium state, vortex state, vortex turbulence, droplet or grain turbulence, and wave turbulence, are classified by means of effective Fresnel and Mach numbers. The inverse Kibble–Zurek scenario is described. A method for the formation of directed beams from atom lasers is reported.

The extracellular matrix (ECM) is a three-dimensional multicomponent, and a structural meshwork constituted of many specialized macromolecules. Such macromolecules provide an essential scaffold to tissue cells and chemical signals involved in cell proliferation, survival, migration, and differentiation, which are crucial to tissue morphogenesis, homeostasis, and functions. Photobiomodulation (PBM) is based on non-ionizing radiations in the visible and infrared spectrum, emitted from low-power lasers, light-emitting diodes, and broadband light sources. PBM has been used for improving tissue repair, and successful results have been reported from experimental studies. In this review, studies were accessed by PubMed, and their findings on PBM-induced effects on the ECM were summarized. The results showed that low-power violet-red lights and near-infrared radiation modulate gene expression, cell proliferation, adhesion and differentiation, factors and enzymes, and structural constituents in the ECM. These results showed a dependence on radiation wavelength, fluence, irradiance, exposure time, emission mode, and cellular and tissue conditions. Such results suggest that the irradiation parameters, biological tissue type, and conditions should be considered for an effective therapeutic protocol aiming at tissue repair based on PBM-induced extracellular matrix remodeling.

With ever increasing world population, the demands on food safety and security are also expected to substantially increase over the next few decades. As agronomic practices, agricultural mechanization and plant breeding technologies have already been extensively exploited, novel techniques need to be explored and implemented to enhance crop production. To this end, the emerging area of laser-based technologies has shown potential to bring about another revolution in enhancing quantity, quality, and safety of foods. This paper presents an exhaustive review of the use of five non-invasive non-destructive laser-based techniques in agriculture, namely laser biostimulation, light detection and ranging, laser land levelling, laser-induced fluorescence spectroscopy, and Raman spectroscopy. Herein we provide the advantages, status quo and challenges of each of these techniques and conclude with recommendations for future work. A comprehensive review of literature reveals the untapped potential of laser applications in agriculture that has the potential to unleash the next agricultural revolution.

The tunable wavelength emission of an erbium doped fiber laser using a Fabry–Perot interference filter based on a fiber micro-ball lens (MBL) with a spherical shape is experimentally demonstrated. The filter is formed at the tip of a single-mode fiber by controlled electric arc discharge. The filter consists of a fiber MBL with a radius of 152.7 µm and a flat-convex mirror. A tunable single laser emission range of 1556.85–1569.72 nm is obtained when the mirror moves perpendicular to the fiber. Dual-wavelength laser emission with a separation of ∼12.9 nm corresponding to the free spectral range of interference modulation is obtained within the single laser wavelength tuning limits. The laser line exhibits full width at maximum half of 0.1 nm. The stability of the laser emission is also discussed. The use of a reliable tunable spectral filter for dual-wavelength emission and single tuning is demonstrated in the fiber laser's design. The proposed spectral filter configuration can be useful in different research areas, such as the coherent development of light sources, optical communications, and optical instrumentation.

The influence of linear and nonlinear asymmetric defects on light beam propagation in a one-dimensional photonic lattice has been numerically analysed. Defects are located in a uniform or composite lattice and can be linear or nonlinear in both cases. The results obtained for the uniform lattice were compared with those obtained for the composite lattice. The asymmetric defect width was varied. It was found that the width of asymmetric defects plays a significant role in light beam propagation. A comparison with the corresponding symmetric defects was also performed. Various types of strongly localized defect modes were found at the defect position as well as in the cavities between the asymmetric defects or an asymmetric defect and the interface. In addition to localized modes, we found reflection and transmission of light.

The emission of electrons from hot plasmas generated in the interaction of ultra-short (and ultra-high intensity) laser pulses with matter is often characterized by the so-called 'hot electron temperature'. In this article it is shown that this number is not unambiguous. The reason is the following: to assign a temperature to an electron spectrum, it is necessary to describe the spectrum with a distribution function. However, different types of distribution functions are in use, e.g. the Boltzmann or Maxwell distribution, leading to different electron temperatures in spite of providing nearly the same form of the electron spectrum. For this reason, the main characteristics of all these distribution functions are presented in this article and compared. Depending on the distribution function used, the value of the hot electron temperature varies by up to 30% and in extreme cases by more than a factor of four. This fact should always be kept in mind when comparing values of hot electron temperatures. In addition, the reasons for using equilibrium distributions to describe the characteristics of laser-produced electrons—although probably no thermodynamic equilibrium is prevailing—are discussed.

Quantum super-resolution involves resolving two sources below the Rayleigh limit using quantum optics. Such a technique would allow high-precision inter-satellite positioning and tracking on communication and navigation constellations. Due to the size, weight and power constraints typical of low-earth-orbit (LEO) satellites, a simple solution is often preferred. Here, we show that a balanced homodyne detection (BHD) setup using a shaped single-mode local oscillator can achieve super-resolution despite typical photonic losses. We further analyze the impact of a fluctuating and fixed centroid misalignment due to satellite pointing issues, and find that fixed misalignment is comparatively more detrimental to the performance of a BHD setup. Thus, our study provides a practical assessment of BHD to achieve super-resolution on a modern LEO satellite platform. Finally, we discuss how our analysis can be extended to stellar sources for astronomical applications.

We demonstrate a figure-of-9 all-fiber thulium-doped laser (TDFL) that generates 560 fs long pulses at 1948 nm wavelength. In order to achieve self-starting passive mode-locking, we utilize an in-fiber Faraday rotator to induce a nonreciprocal phase shift. To the best of our knowledge, this is the first all-fiber TDFL that combines an artificial saturable absorber (SA) with a chirped fiber Bragg grating (CFBG) as a wavelength-selective reflector. This cavity design is an excellent candidate to pump nonlinear processes such as supercontinuum and frequency comb generation since it does not require any SA material that degrades over time for mode-locking and could be made wavelength-tuneable via the CFBG.

We demonstrated a 577 nm yellow laser source with a proper combination of Raman medium and frequency doubling medium, pumped externally. Cryogenic Yb:YAG oscillator acts as an external pump source, and it generates 1.2 mJ energy at 1 KHz emitting around 1029 nm. Using a Raman resonator with Ba(NO₃)₂ as Raman medium, stimulated Raman scattering around 1153.4 was generated. Finally, 577 nm yellow laser was generated using the frequency-doubling medium LBO. A maximum average output power of 65 mW with a pulse duration of 18 ns at 1 KHz was achieved.

We presented a comprehensive study of optimized loading of ultracold Cs atoms in a magnetic levitated crossed dipole trap. Moreover, we analyzed the optimized experimental parameters of the dipole laser for the opening time and the power intensity prior to switching on the magnetic field which formed the magnetically levitated dipole trap. The number of atoms as the function of sweep time from distinct laser power intensity to a fixed one is measured. And beyond that, the variation of atoms with the dipole laser power intensity per beam was studied experimentally ranging from 0 W cm⁻² to a fixed one for loading the dipole trap, and the number of atoms can be increased up to a maximum of ∼1.67 × 10⁶. Our research demonstrates that the 'pre-loading process' plays a crucial role in enhancing the loading efficiency of an optical dipole trap.

We proposed a photonic crystal fiber (PCF) with ellipse air holes and high refractive index ring-core, which can stably support 82 orbital angular momentum states from the wavelength of 1.25–1.65 μm. The modes' average confinement loss keeps as low as 10⁻⁹ dB m⁻¹. In addition, the effective refractive index separation (Δn_eff) between HE_m+1,1 and EH_m−1,1 (m = 2–21) is up to 10⁻³, which makes the mode degeneracy to LP mode can be almost neglected. The dispersion curves of low order eigenmodes are low and flat, especially at the wavelength of 1.4 μm the minimum absolute value of chromatic dispersion for HE_1,1 mode is as low as 0.246 ps (nm·km)⁻¹. Furthermore, we also analyze the effect of ellipticity change on the cladding air holes, which instructively show the favorable fault tolerance of confinement loss and dispersion. The proposed PCF structure will be a potential candidate for high-capacity optical fiber communications.

Many different fields benefit from the usage of light sources emitting in the mid-infrared wavelength range (2–10 µm). A rising need for precise and fast sources in the mid infrared (mid-IR) is reflected in the development of a high-power, picosecond mid-IR source capable of generation at high repetition rates. In this work, we present the optimization of an optical parametric generator, pumped by a 3 W portion of total power of the Yb:YAG thin-disk laser (1.3 ps, 90 kHz, 90 W) by comparing a single-pass and double-pass arrangement output parameters in terms of output power dependences on input power, efficiency, beam profiles, stability, and spectra. The output tunability of both arrangements spanned from 1459 nm to 2891 nm, with the upper limit being influenced by the limited transmission of the dichroic components used in the setup above 2700 nm. It was shown that the double-pass arrangement increases the output power, from 17 mW in the single-pass arrangement to 193 mW in the double-pass arrangement at 1459 nm, resulting in over ten-fold output power increase.

We propose a new kind of compound optical vortex (COV) generator in this paper. The device consists of an inner spiral phase plate (SPP) and an outer annular spiral phase plate. There is an opaque band between two SPPs. Under the flat-top beam irradiation, concentric multi-ring COV rings with different topological charges in different radial radii can be generated. The theoretical analysis lays a theoretical foundation for the design of the COV generator, and the simulation results prove the effectiveness of the design. The unique characteristics of COV are discussed and some of its potential application scenarios are presented. This work provides a design method for generating COV using compound SPPs, and the advanced COV structure can help to expand the scope of utilization of vortex beam in optical tweezers, optical communication and other fields.

In this paper, we report on a direct bonding hybrid silicon evanescent laser with sampled Bragg grating structure based on the reconstruction equivalent chirp (REC) technique for the first time. By the design of the REC technique, the hybrid silicon evanescent laser in the +1st order channel is emitted. The optical mode is evanescently coupled between the III and V waveguide and silicon waveguide. A laser with 24 mA threshold current and 0.3 mW output power from silicon waveguide at 50 mA under the continuous wave operation is obtained.

The mode field intensity, spot size, central peak intensity evolution and adiabaticity are calculated for different points along the transition of an optical fibre taper that adiabatically tapers from the standard 125 nm down to 1 µm and then to 440 nm diameter for low loss operation at 1550 nm wavelength. The first section of the taper is evaluated using a weak guidance approximation. The second section is treated as a three-index layer structure (double-clad) and evaluated with eigenvalue equations for three refractive indices. The third and thinnest section of the taper is studied using an exact mode eigenvalue equation. The results show that the fundamental mode for the third section has a discontinuity at the fibre edge with a peak intensity larger than the intensity at the centre of the fibre. Since the guiding by the core disappears in the first section of the taper, the mode field does not simply reduce monotonously along the taper with the outer diameter of the fibre. By this novel approach, and for the first time, to the best of our knowledge, the taper shape that complies with the adiabaticity criterion, the mode intensity profile and the spot size (first Petermann definition) of the fundamental mode evolution, along their position on the taper are determined.