Coherent Laser Beams Research Articles

Generation of partially coherent beams with controllable time-dependent coherence

A straight effective method to produce partially coherent beams with controllable time-dependent coherence is demonstrated. We theoretically deduce that a time-dependent partially coherent beam can be generated by imposing dynamic random phase on a coherent laser beam. The degree of coherence of the beam is determined by an amplitude control parameter of the dynamic random phase. We experimentally corroborate that after a completely coherent laser beam reflected from a spatial light modulator, loaded with a particular dynamic random phase, this beam is transformed into a partially coherent beam with time-dependent coherence.

Formation of surface nanolayers in chalcogenide crystals using coherent laser beams

We have shown a possibility to form laser modified surface nanolayers with thickness up to 60 nm in some ternary chalcogenide crystals (Ag3AsS3, Ag3SbS3, Tl3SbS3) The laser treatment was performed by two coherent laser beams split in a space. As the inducing lasers we have applied continuous wave (cw) HeCd laser at wavelength 441 nm and doubled frequency cw Nd: YAG laser at 532 nm. The spectral energies of these lasers were higher with respect to the energy gaps of the studied crystals. The optical anisotropy was appeared and defected by monitoring of birefringence at probing wavelength of cw HeNe laser at λ = 3390 nm. The changes of the laser stimulated near the surface layer morphology was monitored by TEM and AFM methods as well as by the reflected optical second harmonic generation at fundamental wavelength of microsecond CO2 laser generating at wavelength 10600 nm. This technique may open a new approach for the formation of the near the surface nanolayers in chalcogenides using external cw laser illumination.

Er/Pr-codoped borotellurite glasses as efficient laser operated nonlinear optical materials

We have discovered a rare opportunity to operate by the SHG at fundamental wavelength 10.6 μm of microsecond CO2 laser during simultaneous irradiation by 1064/532 nm bicolor laser beams of (60-x-y)TeO2-10B2O3-10BaO-10ZnO-10Na2O-(x)Er2O3-(y)Pr6O11 (x = 0.5, 1.0; y = 0); (x = 0; y = 0.1, 0.2, 0.3, 0.4, 0.5); (x = 0.5,1.0; y = 0.1, 0.2, 0.3, 0.4, 0.5) (mol%) glasses. The effect has shown a drastic sensitivity of the effect to the Er/Pr ratio. The effect exists only during the simultaneous illumination and applied dc-electric field at 5 kV/cm. Only completely reversible changes were observed. The effect is a consequence of photopolarization during the bicolor coherent laser beams and its efficiency is determined by the transfer of excitation between the Er and Pr ions.

Scintillation and bit error rate analysis of a phase-locked partially coherent flat-topped array laser beam in oceanic turbulence.

In this paper, the performance of underwater wireless optical communication (UWOC) links, which is made up of the partially coherent flat-topped (PCFT) array laser beam, has been investigated in detail. Providing high power, array laser beams are employed to increase the range of UWOC links. For characterization of the effects of oceanic turbulence on the propagation behavior of the considered beam, using the extended Huygens-Fresnel principle, an analytical expression for cross-spectral density matrix elements and a semi-analytical one for fourth-order statistical moment have been derived. Then, based on these expressions, the on-axis scintillation index of the mentioned beam propagating through weak oceanic turbulence has been calculated. Furthermore, in order to quantify the performance of the UWOC link, the average bit error rate (BER) has also been evaluated. The effects of some source factors and turbulent ocean parameters on the propagation behavior of the scintillation index and the BER have been studied in detail. The results of this investigation indicate that in comparison with the Gaussian array beam, when the source size of beamlets is larger than the first Fresnel zone, the PCFT array laser beam with the higher flatness order is found to have a lower scintillation index and hence lower BER. Specifically, in the sense of scintillation index reduction, using the PCFT array laser beams has a considerable benefit in comparison with the single PCFT or Gaussian laser beams and also Gaussian array beams. All the simulation results of this paper have been shown by graphs and they have been analyzed in detail.

Experimental study on off-axis scattering of flat top partially coherent laser beams when propagating under water in the presence of moving scatterers

Off-axis underwater scattering of spatially partially coherent Multi-Gaussian Schell-Model (MGSM) beams are compared with fully coherent Gaussian beams in both a stationary setting and in the presence of mechanically agitated scatterers in underwater environments. The analysis is carried out by comparing the mean intensities of scattered light, the normalized variance, and the scintillation index in various scenarios. Results indicate that fully coherent beams have increased off-axis scattered light variations in the presence of moving scatterers as compared with a spatially partially coherent MGSM beam. Additionally, in a stationary environment the coherent beam has less overall variations as expected due to the nature of constructing partially coherent MGSM beams. Metrics of normalized variance, scintillation index, and overall average intensity are discussed in the context of potential beam localization, reduced scattering, and off-axis detection.

Electronic structure and laser induced piezoelectricity of a new quaternary compound TlInGe3S8

New quaternary thallium indium germanium sulfide TlInGe3S8 was synthesized by co-melting high-purity elements at 1270 K. Its crystal structure was determined by X-ray single crystal method; it is a representative of its own structural type. TlInGe3S8 crystallizes in monoclinic space group Р21/a with lattice parameters a = 6.7245 (2) Å, b = 38.077 (1) Å, c = 6.7922 (2) Å, β = 90.616 (2)°, Z = 5. The as-synthesized TlInGe3S8 single crystal was probed with X-ray photoelectron spectroscopy (XPS). In particular, the XPS valence-band and core-level spectra were measured for pristine and Ar+ ion-irradiated surfaces of the TlInGe3S8 single crystal. The XPS measurements reveal partial sensitivity of the TlInGe3S8 single crystal surface with respect to Ar+ ion-irradiation. Comparison on a common energy scale of the X-ray emission S Kβ1,3 and the XPS valence-band spectrum allows for concluding that the main contributions of the S 3p states occur in the upper part of the valence band of TlInGe3S8, with also their substantial contributions in other valence band regions. Photoinduced piezoelectricity was explored versus the power of two coherent laser beams at 1540 nm Er:glass laser beams.

Fast Frequency Acquisition and Phase Locking of Nonplanar Ring Oscillators

Optical phase locking is a critical technique in space coherent optical communication and active coherent laser beam combining. In a typical optical phase locking loop based on nonplanar ring oscillators, the pull-in range is normally less than 1 MHz, limited by loop delay and frequency tuning bandwidth of the laser source. Phase locking cannot be achieved at large initial frequency differences. In this work, a fast laser frequency acquisition method is demonstrated. The frequency difference between the signal and local lasers was measured via frequency dividing and period counting, and the frequency control signal was generated by a frequency discrimination and control module, to reduce the frequency difference to the pull-in range of the loop. Under the coordinating function of the loop filter and the frequency discrimination and control module, phase locking under a large initial frequency difference was achieved. The frequency acquisition range reached 164 MHz, and the acquisition and locking time was measured to be 440 ms. Additionally, the acquisition time was shortened with the decrease in initial frequency difference.

UV-laser induced birefringence in olygoetheracrylate photopolymers

Laser induced birefringence was studied during coherent treatment by two coherent UV laser beams at wavelength 355nm. Principal role of the border separating photo-solidified and the surrounding non-solidified liquid-like fragments is demonstrated. The magnitudes of laser induced birefringence for probing 1150nm wavelength were varied within 10−4…10−2. It is demonstrated that one can operate by values of the birefringence versus polymer solidification time. The UV-induced birefringence achieves a saturation after about 45days. The reversible birefringence after switching off of the UV-solidification beams was equal to about 18% with respect to initially photo-stimulated. The discovered effect may be used for design and fabrication of electro-optical modulators operated by UV laser light. A principal physical mechanism is determined by an interaction between two coherent UV- nanosecond laser pulses at wavelengths of 355nm (third harmonic of Nd:YAG laser). The discovered effects allow to propose a new type of UV-operated photopolymer materials for optical recoding of information.

Coherent laser phase retrieval in the presence of measurement imperfections and incoherent light.

Phase retrieval is a powerful numerical method that can be used to determine the wavefront of laser beams based only on intensity measurements, without the use of expensive, low-resolution specialized wavefront sensors such as Shack-Hartmann sensors. However, phase retrieval techniques generally suffer from poor convergence and fidelity when the input measurements contain electronic or optical noise and/or an incoherent intensity contribution overlapped with the otherwise spatially coherent laser beam. Here, we present an implementation of a modified version of the standard multiple-plane Gerchberg-Saxton algorithm and demonstrate that it is highly successful at extracting the intensity profile and wavefront of the spatially coherent part of the light from various lasers, including tapered laser diodes, at a very high fidelity despite the presence of incoherent light and noise.

Cascaded multi-dithering technique using PZT modulators for high control bandwidth in coherent laser beam combining

A cascaded multi-dithering (CMD) technique using piezoelectric ceramic transducer (PZT) tubes as phase modulators is proposed as a tool for obtaining high control bandwidth in coherent laser beam combination. To prove its validity, eight coherent fiber beam elements were combined using the CMD technique with PZT tubes. As a result, residual phase error was recorded to be λ/54 at 100 Hz control bandwidth, which is comparable to that of a four laser beam combination in the previous experiment. To our knowledge, this is the first case to date of combining eight laser beam elements using PZT tubes as phase modulators and achieving such good results.

Surface texturing of electrospun fibres by photoembossing using pulsed laser interference holography and its effects on endothelial cell adhesion

Electrospun fibres are nowadays extensively studied for biomedical applications such as tissue engineering and drug delivery. Here, fibres for such applications are electrospun from photopolymer blends using a volatile solvent. The obtained nonwoven fibre mats were textured using a photoembossing technique that uses the interference of two coherent UV laser beams followed by a thermal development step. AFM measurements revealed that the patterned exposure using pulsed laser interference holography resulted in a surface texture on the fibres. The effect of temperature and UV dosage on fibre texturing was studied and was similar to results seen in photoembossed films. Fibronectin and cell adhesion was evaluated on textured fibres of 1 μm diameter with relief structures of 60 nm height and 2 μm pitch. Initial results did not seem to indicate improved cell adhesion for photoembossed fibres compared to their non-embossed counterparts.

Propagating of partially coherent laser beam in the near-resonant atomic gas

The characteristics of the light with various degrees of spatial coherence traveling in near-resonant atomic gas are investigated both experimentally and theoretically. The experimental results show that the coherence of partially coherent beams can get better after interaction with atoms under some certain conditions compared with that before interaction. The experimental results are explained theoretically by the method of spectroscopy absorption. Furthermore, partially coherent light has a better environmental adaptability than fully coherent light.

High-Q, Low-Threshold Monolithic Perovskite Thin-Film Vertical-Cavity Lasers.

A vertical-cavity surface-emitting perovskite laser is achieved using a microcavity configuration where CH3 NH3 PbI3 thin solid films are embedded within a custom GaN-based high-quality (Q-factor) resonator. This single-mode perovskite laser reaches a low threshold (≈7.6 µJ cm-2 ) at room temperature and emits spatially coherent Gaussian laser beams. The devices allow direct access to the study of perovskite gain dynamics and material robustness.

Analyzing the average intensity distribution and beam width evolution of phase-locked partially coherent radial flat-topped array laser beams in oceanic turbulence

In this research, an analytical expression for cross-spectral density matrix elements (and consequently, average intensity) of partially coherent flat-topped (PCFT) radial array laser beams in weak oceanic turbulence are derived based on the extended Huygens–Fresnel principle and the previously developed knowledge of the propagation of a partially coherent beam in atmosphere. Mean-squared beam width is calculated analytically using average intensity formula. The simulation is done by considering the effects of source parameters (such as the radius of the array setup’s circle and effective width of spectral degree of coherence) and turbulent ocean factors (such as the rate of dissipation of the turbulent kinetic energy per unit mass of fluid and relative strength of temperature-salinity fluctuations, Kolmogorov micro-scale, and the rate of dissipation of the mean squared temperature) in detail. It is found that when salinity fluctuations in the ocean dominate temperature fluctuations, the average intensity of the PCFT array beam becomes more broad and the array beam profile conversion process to a single wider Gaussian beam profile will occur at a faster rate. For the same turbulent conditions and the same initial beam width, the divergence of a flat-topped array beam is less than the Gaussian array beam. The simulation and calculation results are shown by graphs.

A theory for non-degenerate four-wave mixing in doped graphene

We present a theoretical study of the nonlinear optical (NLO) response of doped graphene to two coherent laser beams, of frequencies ω1 and ω2, resulting in the generation of radiation at frequency ωσ=2ω1−ω2. The two main ingredients of the developed theory are the interplay of interband and intraband electron motion, induced by the incident light waves, and the finite lifetime of excited electronic states, caused by electron scattering. Adopting a tight-binding approximation for the π-electronic band structure of graphene and the Genkin-Mednis formalism of the nonlinear conductivity theory of semiconductors, we calculate the third-order NLO susceptibility χ(3)(−ωσ;ω1,ω1,−ω2) responsible for the non-degenerate four-wave mixing process under consideration. Our calculations show the resonant enhancement of the |χ(3)| (up to a value of 2.8×10−7esu) when the frequencies ω1 and ω2 of the input beams are mat"ched to provide a resonance for the output photon energy ℏωσ with an effective optical gap of 2EF in the π-electronic band structure of doped graphene (EF is the Fermi energy of charge carriers in the graphene, tunable by an external gate voltage). The results obtained may be of practical interest for generating mid-infrared radiation from doped graphene pumped with two near-infrared laser beams.

Target reconstruction algorithm for four-beam sheared coherent imaging

Sheared-beam imaging, which is a nonconventional coherent laser imaging technique, can be used to better solve the problem of taking pictures with high resolution for remote targets through turbulent medium than conventional optical methods. In the previous research on this technique, a target was illuminated by three coherent laser beams that were laterally arranged at the transmitter plane into an L pattern. In order to obtain a high quality image, a series of time-varying scattered signals is collected to reconstruct speckled images of the same object. To overcome atmospheric turbulence, multiple sets of three-beam laser should be emitted, which increases data acquisition time. In this paper, aiming at the quasi real-time problem of conventional sheared beam imaging technique, we use four-beam laser with rectangular distribution instead of the traditional L type sheared three-beam laser to illuminate the target. According to this, we propose a target reconstruction algorithm for four-beam sheared coherent imaging to reconstruct four target images simultaneously in one measurement, which can acquire high quality images by reducing the amount of measurement and the speckle noise. Meanwhile, it can greatly reduce the amount of beam switching in multi-group emission and improve the imaging efficiency. Firstly, the principle of the four-beam sheared coherent imaging technique is deduced. Secondly, in the algorithm, the speckle amplitude and phase difference frames can be extracted accurately by searching for the accurate positions of the beat frequency components. Based on the speckle phase difference frames, four sets of wavefront phases can be demodulated by the least squares method, and wavefront amplitude can be obtained by algebraic operation of speckle amplitude. The reconstructed wavefront is used for inverse Fourier transform to yield a two-dimensional image. A series of speckled images is averaged to form an incoherent image. Finally, the validity of the proposed technique is verified by simulations. From the simulation results, the image quality of the proposed method is better than that of the traditional method in the same amount of measurement. Furthermore, on the premise of the same image quality, the data acquisition amount of the proposed method is 2-3 times as large as that of the traditional method. In other words, compared with that of the traditional method, the data acquisition time of the proposed method is reduced at least by half and the algorithm processing time is less. It can be concluded that the proposed imaging technique can not only improve the efficiency of target reconstruction, but also present a better way of imaging the remote moving targets.

A Comprehensive Study of Photorefractive Properties in Poly(ethylene glycol) Dimethacrylate- Ionic Liquid Composites.

A detailed investigation of the recording, as well as the readout of transmission gratings in composites of poly(ethylene glycol) dimethacrylate (PEGDMA) and ionic liquids is presented. Gratings with a period of about 5.8 micrometers were recorded using a two-wave mixing technique with a coherent laser beam of a 355-nm wavelength. A series of samples with grating thicknesses micrometers, each for two different exposure times, was prepared. The recording kinetics, as well as the post-exposure properties of the gratings were monitored by diffracting a low intensity probe beam at a wavelength of 633 nm for Bragg incidence. To obtain a complete characterization, two-beam coupling experiments were conducted to clarify the type and the strength of the recorded gratings. Finally, the diffraction efficiency was measured as a function of the readout angle at different post-exposure times. We found that, depending on the parameters, different grating types (pure phase and/or mixed) are generated, and at elevated thicknesses, strong light-induced scattering develops. The measured angular dependence of the diffraction efficiency can be fitted using a five-wave coupling theory assuming an attenuation of the gratings along the thickness. For grating thicknesses larger than 85 microns, light-induced scattering becomes increasingly important. The latter is an obstacle for recording thicker holograms, as it destroys the recording interference pattern with increasing sample depth. The obtained results are valuable in particular when considering PEGDMA-ionic liquid composites in the synthesis of advanced polymer composites for applications, such as biomaterials, conductive polymers and holographic storage materials.

Approach to atmospheric laser-propagation theory based on the extended Huygens-Fresnel principle and a self-consistency concept.

We propose a self-consistency condition based on the extended Huygens-Fresnel principle, which we apply to the propagation kernel of the mutual coherence function of a partially coherent laser beam propagating through a turbulent atmosphere. The assumption of statistical independence of turbulence in neighboring propagation segments leads to an integral equation in the propagation kernel. This integral equation is satisfied by a Gaussian function, with dependence on the transverse coordinates that is identical to the previous Gaussian formulation by Yura [Appl. Opt.11, 1399 (1972)APOPAI0003-693510.1364/AO.11.001399], but differs in the transverse coherence length's dependence on propagation distance, so that this established version violates our self-consistency principle. Our formulation has one free parameter, which in the context of Kolmogorov's theory is independent of turbulence strength and propagation distance. We determined its value by numerical fitting to the rigorous beam propagation theory of Yura and Hanson [J. Opt. Soc. Am. A6, 564 (1989)JOAOD60740-323210.1364/JOSAA.6.000564], demonstrating in addition a significant improvement over other Gaussian models.

Lateral optical binding between two colloidal particles.

An optical binding force between two nearby colloidal particles trapped by two coherent laser beams is measured by phase-sensitive detection. The binding force is long-range and spatially oscillatory. For identical linearly-polarized incident beams, the oscillation period is equal to the optical wavelength. For mutually perpendicular polarizations, a new force appears with half-wavelength periodicity, caused by double inter-particle scattering. This force is observable only with cross-polarized incident beams, for which the stronger single-scattering forces are forbidden by parity.

Laser Speckle Rheology for evaluating the viscoelastic properties of hydrogel scaffolds.

Natural and synthetic hydrogel scaffolds exhibit distinct viscoelastic properties at various length scales and deformation rates. Laser Speckle Rheology (LSR) offers a novel, non-contact optical approach for evaluating the frequency-dependent viscoelastic properties of hydrogels. In LSR, a coherent laser beam illuminates the specimen and a high-speed camera acquires the time-varying speckle images. Cross-correlation analysis of frames returns the speckle intensity autocorrelation function, g2(t), from which the frequency-dependent viscoelastic modulus, G*(ω), is deduced. Here, we establish the capability of LSR for evaluating the viscoelastic properties of hydrogels over a large range of moduli, using conventional mechanical rheometry and atomic force microscopy (AFM)-based indentation as reference-standards. Results demonstrate a strong correlation between |G*(ω)| values measured by LSR and mechanical rheometry (r = 0.95, p < 10−9), and z-test analysis reports that moduli values measured by the two methods are identical (p > 0.08) over a large range (47 Pa – 36 kPa). In addition, |G*(ω)| values measured by LSR correlate well with indentation moduli, E, reported by AFM (r = 0.92, p < 10−7). Further, spatially-resolved moduli measurements in micro-patterned substrates demonstrate that LSR combines the strengths of conventional rheology and micro-indentation in assessing hydrogel viscoelastic properties at multiple frequencies and small length-scales.