Combined Laser Beam Research Articles

Highly-efficient waveguide-type red–green–blue laser beam combiners for compact projection-type displays

A waveguide-type RGB laser beam combiner was fabricated and its optical properties were measured and analyzed through comparison with a simulation. The combiner has three different waveguides for RGB light inputs, three directional couplers, and a single output waveguide. The fabricated combiner shows an excellent multiplexing efficiency of as high as 96.4%. The size of the multiplexing region of the fabricated combiner is as small as 2.3 mm. Furthermore, by optimizing the combiner structure, it was found that the multiplexing efficiency hardly depends on the gap width of the directional coupler in the combiner. The polarization dependence of the multiplexing efficiency was also measured. The difference in multiplexing efficiency between the TE and TM modes is within 4% when using the optimized combiner structure, demonstrating the small polarization dependence of the multiplexing efficiency. Thus, the fabricated combiner exhibit’s high performance, as well as excellent fabrication reproducibility, leading to the realization of compact laser scanning image projectors.

Numerical analysis of non-collinear coherently combined laser beams interaction with micro-structured targets

In this work, we numerically investigate the laser beam intensification in a non-collinear coherently combined (NCCC) laser beams–micro-structured target interaction system, in the presence of pulse duration variation, down to few-cycle regime. A 2D numerical model based on finite-difference time domain (FDTD) method is performed in order to provide a spatio-temporal description of the electromagnetic field distribution in the vicinity of the laser–target interaction point in case of NCCC of multiple laser pulses. The results demonstrate that non-collinear coherent and spectral beam combination of three laser pulses in a micro-structured cone target allow a significant increase of the field intensity while a reduction of temporal duration of the combined pulse occurs. This numerical model offers an effective solution to increase the laser intensity in a micro-structured cone target by employing coherent and spectral beam combination of multiple laser sources and it can be used as an alternative method for field intensity enhancement in various laser–matter interaction experiments in ultra-intense regime.

Spectral linewidth narrowing of two broad-area blue laser diodes (445 nm) with a common external cavity.

Two watt-level broad-area laser diodes were simultaneously locked into a common external cavity made using a surface grating in a Littrow configuration. The spectral linewidth of the combined laser beam was narrowed down from over a nanometer to 10-15 pm (FWHM), and the output power was the sum of the power of the individually locked laser diodes. The spectrum of the combined laser beam can be tuned over a range of 2-3 nm by changing the tilt angle of the grating and varying the injection currents of each laser diode.

Research progress of stimulated Brillouin scattering pulse compression technique

Stimulated Brillouin scattering (SBS) as a third-order optical effect is widely used in laser beam combination, distributed fiber sensing, Brillouin lasers and other fields. In recent years, SBS pulse compression has also received special attention. Based on the energy transfer characteristics of the Brillouin amplification process, SBS pulse compression technology can compress nanosecond pulses to sub-nanosecond levels, and the peak power can be increased by 1-2 orders of magnitude. This paper systematically introduces the basic theory of SBS pulse compression, comprehensively discusses the influence of SBS compressor structure, gain medium, pump pulse and other factors on pulse compression characteristics, and looks forward to the development trends of SBS pulse compression. It provides a useful reference for the future study of SBS characteristics and a feasible scheme for the acquisition of high repetition frequency and high energy laser.

Transmission characters of wide-spectrum OAM beam in tunable atmospheric turbulence

In this paper, a wide-spectrum orbital angular momentum (OAM) based free space communication system is experimentally demonstrated. The wide-spectrum laser can be generated using narrow-linewidth pulses to pump nonlinear fiber and the OAM beam can be generated with spatial light modulator (SLM). In the experiment, a controllable atmospheric turbulence simulation device was used to ensure a unified test environment. The results show that with the combination of wide-spectrum laser and OAM beam, the received sensitivity can achieve a 4.23 dB gain compared with narrow-linewidth Gaussian beam-based communication system in weak atmospheric turbulence environment.

Half-cycle waveform control for producing a broad and intense harmonic spectral continuum and an isolated attosecond pulse

An effective method to control the half-cycle harmonic emission process through the combination of multiple laser beams is proposed. The control process can be separated into three steps. In the first step, by properly choosing the chirp and the chirp delay of the fundamental pulse the relevant negative and positive half-cycle waveform for extending the harmonic cutoff can be obtained. In the second step, using either an asymmetric inhomogeneous field or a half-cycle unipolar pulse, the harmonic cutoff from the negative or the positive half-cycle waveform can be further extended. In the third step, by properly adding an ultraviolet pulse, the harmonic yield can be enhanced due to ultraviolet resonance ionization. Finally, by superposing some harmonics on the harmonic spectra, a number of isolated attosecond pulses shorter than 70 as can be obtained.

Design and Realization of a Compact Efficient Beam Combiner, Based on Liquid Crystal Pancharatnam–Berry Phase Gratings

We demonstrate a laser beam combiner based on four photo-patterned Pancharatnam–Berry (PB) phase gratings, which is compact and has high diffraction efficiency for incident circularly polarized light. The nematic liquid crystal mixture E7 is used as anisotropic material, and the thickness of the layer is controlled by spacers. The beam combiner can bring two parallel laser beams closer to each other while remaining parallel. This work shows the potential to realize components based on flat optical LC devices.

Comparison between optimal configuration algorithms of fiber phased array

Optical fiber phased array can be used in high-power laser beam combination, lidar and other areas. The configuration of the optical fiber array is different from the microwave phased array, which has periodic problems that affect the energy intensity distribution of the main lobe. Starting from the physical model, in this paper we establish a theoretical model of optical phased array antenna array based on a set of concentric circular ring lattices, and propose a theory of the rapid synthesis of randomly configured interference field strengths through using analytical continuation method and Fourier transform method. The problem of sampling bandwidth and sampling number that should be paid attention to in the numerical simulation of discrete sampling are discussed, and the problem of quickly realizing the numerical simulation of multi-beam interference field is solved. Genetic algorithm and particle swarm algorithm for optimizing the configuration of optical phased array antennas are investigated with different populations. The convergence speeds and optimization efficiencies of the two algorithms are compared and analyzed. It is demonstrated that the peak side-lobe ratio PSR can be achieved to be better than 0.270 by the genetic algorithm optimized configuration array under the real fabricate parameter. The proposed method is expected to be used in the actual optical phased array antenna configuration to guide the optimal design of the antenna with low side lobes, and the proposed model is also expected to provide a certain reference value for the study of optimizing the non-differentiable objective function.

Modal decomposition of a fibre laser beam based on the push-broom stochastic parallel gradient descent algorithm

Compared with traditional methods, the stochastic parallel gradient descent (SPGD) algorithm used in modal decomposition has its own unique advantages of faster speed, wider application range, and low experimental effort, demonstrating many development prospects. However, the current SPGD algorithm only considers the modal weights of multimode optical fields and neglects the phase. More importantly, this algorithm is usually unsuitable for conducting modal decomposition of incoherent combined laser beams due to its serious ambiguities. In this paper, a novel modal decomposition algorithm, called the push-broom SPGD algorithm, is first proposed to achieve complete modal decomposition of optical fields emerging from multimode fibres. By iteratively evaluating the intensity profiles in different positions using the SPGD algorithm, this approach can obtain both amplitude and phase information with high accuracy. The proposed algorithm is preliminarily applied to the modal decomposition of an incoherent combined laser beam, and its feasibility is demonstrated via numerical simulations. By inserting a parallel plate, ambiguities can be solved and the influence of random noise is suppressed, demonstrating this algorithm’s potential.

Adjustable Ring Mode (ARM) laser welding of stainless steels

As a combination of the annular laser beam and gaussian laser beam, adjustable ring mode (ARM) laser is a promising technology for stabilizing laser-based welding process. Process status and fusion efficiency of laser-materials processing was investigated experimentally under different shape of laser modes, including ring laser mode, center laser spot alone, and ring/center dual spots, with welding speeds ranging from 5 to 100 mm/s. The process characteristics of the keyhole entrance, including the area, length, and width were monitored and quantified by a high-speed CCD camera with the selected image processing method. The process intensity of metallic vapor and plasma was further monitored by a spectrometer, and the relationship between plasma intensity and the keyhole entrance geometry was analyzed correspondingly. Additionally, individual effects of shaped laser beam on the geometric characteristics of the fusion zone across the weld cross-sections such as effective depth of penetration, and the area and width of the fusion zone were analyzed accordingly. Experimental results showed that the process variation of the keyhole entrance differ with different welding energy modes, and the main difference came from the variation of keyhole length. The plasma intensity had a positive relationship with the keyhole entrance in different shape of laser beam welding. The fusion efficiency of cross-section was quite different with different laser beams. This work provided a new investigation for researchers and engineers to have a better understanding of shaped laser beam on materials welding.

Multilayer dielectric coated Fabry Perot spectral beam combiner for high power, narrowband, widely tunable laser applications

Spectral beam combination of tunable laser beams demonstrated, using a multilayer dielectric coated single cavity Fabry Perot (FP) filter. A stack of 19 layers using TiO2 and SiO2 was optimally designed and fabricated using an electron beam evaporation technique. This filter was characterized for transmission spectrum, surface morphology and laser induced damage threshold. The sharp transmission spectrum facilitated the spectral combination of closer wavelengths (<10 nm) around 632.8 nm. This filter was utilized as an efficient spectral beam combiner as well as an amplified spontaneous emission (ASE) filter for the transmitting wavelength. This study finds direct application for tunable, multi-wavelength laser-based differential absorption lidar (DIAL) and atmospheric remote sensing applications. The application of this device was successfully demonstrated to spectrally combine high power, tunable copper vapor laser pumped dye laser beams. A mechanism to tune the FP transmission profile along with laser beam tunability was established.

Remote detection of heterogeneously spread alcohol vapors in the cabins of moving vehicles

This study concerns remote detection of alcohol vapors in cabins of moving vehicles. By means of a simplified description of propagation of two combined laser beams differently absorbed in alcohol vapors and in vehicle windowpanes, difficulties in detecting alcohol vapors in vehicle cabins using classical differential absorption method are demonstrated. Unreported in the literature, this method of remote detection of alcohol vapors in the air exhaled by a driver or a passenger in moving vehicles is proposed. The method uses two combined laser beams that illuminate different zones in the car cabin through the same windowpane. Based on the correlation between the spread of the transmission of these zones and the alcohol concentration in the air exhaled by a drunken person, it is possible to determine if the alcohol exists in the exhaled air. The effectiveness of the presented method is confirmed experimentally.

Two-beam coherent combining based on Ti:sapphire chirped-pulse amplification at the repetition of 1 Hz.

In this Letter, we report the demonstration of the coherent beam combining (CBC) of two-beam femtosecond laser pulses based on chirped-pulse amplification (CPA). To the best of our knowledge, this is the first experimental realization of coherent combination pulses from Ti:sapphire CPA systems. In this experiment, the combined laser beams operate at a repetition rate of 1Hz, and a reference laser is introduced to detect the phase error between the sub-beams. Finally, we realize a coherent combining efficiency of 90% which verified the feasibility for CPA CBC. Moreover, the influence of the amplification process on CBC is studied.

Fiber Positioner Based on Flexible Hinges Amplification Mechanism

Due to the nonlinear effect and the thermal effect, a single optical fiber has limitations in the output power. A beam combination of laser arrays based on adaptive optics can both improve the output power and ensure higher beam quality. This article puts forward one novel fiber positioner structure based on the flexible hinges amplification mechanism, which is the adaptive fiber optics collimator (AFOC), to correct tip-tilt aberration. The theoretical model was established, and the amplification ratios between the output displacement and device’s structure parameters were calculated. The first 6 orders of the mode shape of the vibration of amplification mechanism was obtained by using a modal analysis. Analysis results revealed an excellent performance for the flexible hinges amplification mechanism. The novel fiber positioner has good prospects for applications in laser beam combination systems for ideal tip-tilt control.

RGB laser speckles based 3D profilometry

Laser speckle is becoming popular for encoding objects for 3D profile reconstructions. However, the measurement accuracy of laser speckles based stereophotogrammetry is often not high due to the subjective speckles formed on the sensor plane of the camera. In this letter, we propose a method to enhance the measurement accuracy by encoding the surface of the object using red, green, and blue (RGB) laser speckles which are generated when a combined RGB laser beam passes through a ground glass diffuser. A 3CCD color camera is utilized to capture the object from two different perspectives. Interestingly, there is no crosstalk between different channels. Hence, the proposed system is practically equivalent to three synchronized monochrome vision systems having separate red, green, and blue laser speckle projections. Correspondences located by the RGB speckle image correlation are more accurate, because the structured information in the color image is three times that in the monochrome image. Experimental results validate the higher accuracy of the proposed method against the conventional method using monochrome laser speckles. Moreover, the motion robustness of the proposed method is verified on the measurement of the moving objects.

Artificial gauge fields with ultracold atoms.

Suitable combinations of laser beams can make neutral atoms behave like electrons in a magnetic field.

Incoherent Space Beam Combining of Fiber-Transmitted Semiconductor Lasers for Oil Well Laser Perforation

This paper demonstrates a 19 X 1 incoherent space beam combiner tailored for oil well laser perforation, which is used for an incoherent laser beam combining of 19 fiber-transmitted semiconductor lasers around a wavelength of 972 nm. The parameters of the combiner and its used optical lens are optimized, which is attributed to the theoretical analysis of the variation law between the radii, the spacing of collimating laser beams, and the spot overlap rate of the combined laser beam, respectively, and the simulation of the cross-sectional energy distribution of the beam combiner. A beam combining the power of 10.159 kW is achieved with an average beam combining efficiency of higher than 98.2%, a beam combining length of 300 mm, and a focal spot diameter of 21 mm. A laser perforation experiment is performed for a granite sample using a 10-kW space incoherent beam laser, and a perforation depth of 960 mm is obtained. This paper underscores the design philosophy of the incoherent space beam combiner with ultra-high laser power, a long beam combining length, a simple structure, and a high downhole transmission safety, and builds the foundation for applications of the incoherent space beam combined laser in the oil well perforation.

Demonstration of 2.5 J, 10 Hz, nanosecond laser beam combination system based on non-collinear Brillouin amplification.

We demonstrated a four-beam combined laser system based on Brillouin amplification with a nanosecond output of 2.5 J at a 10 Hz repetition rate. We used simulations and experiments to assess factors affecting the energy extraction efficiency of non-collinear Brillouin amplification. Our results indicate that higher efficiency can be achieved by adding pump beams to the Brillouin amplification process, which enhances optical field intensity and interaction strength. To the best of our knowledge, this is the first demonstration of a multi-beam combination system based on Brillouin amplification with an output of joule level energy, high peak power and nanosecond pulses in 10Hz repetitive operation.

Reaction kinetics and curing behavior of epoxies for use in a combined selective laser beam melting process of polymers

ABSTRACTSelective laser beam melting also known as selective laser sintering (SLS) of polymers is an additive manufacturing process, which enables the production of functional components. Additive manufacturing has its strength in areas where conventional manufacturing methods, such as injection molding, reach their limitations. Unfortunately, the SLS process is restricted regarding the materials that can be processed. This work clears the ground for multimaterial SLS parts, locally reinforced with thermosets. To introduce thermoset resins into the SLS process, the time–temperature curing of the resins have to be understood in order to assess the process behavior. The time–temperature behavior of the used resins is of main importance because of high temperature in the building chamber during laser sintering and long build time up to hours. For Polyamide 12, a standard material in SLS, the building chamber temperature is approximately 170 °C. Therefore, the used resin has to show a specific curing behavior at the building chamber temperature in SLS. Two epoxies, Araldite GY 764 and Araldite GY 793, are analyzed using isothermal and nonisothermal differential scanning calorimetry (DSC) in order to determine whether these resins can be used in SLS or not. Different approaches to model the curing behavior are applied, including the Kissinger method, the Ozawa–Flynn–Wall analysis, and the DiBenedetto equation. The curing behavior of the analyzed resins were finally compared to temperature profiles during SLS to estimate the curing behavior of these systems in SLS. At a medium building chamber temperature of 160 °C, both thermosets show a suitable curing behavior, because of a degree of cure that ranges between 0.15 and approximately 0.6 for 2 up to 10 min. Thus, the curing reaction starts after applying the resin on the powder bed surface and reaches a higher level after approximately 10 layers, assuming a layer time of 1 min. © 2018 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 46850.

Novel fabrication technique for phase-shifted fiber Bragg gratings using a variable-velocity scanning beam and a shielded phase mask.

A new method is proposed and demonstrated for fabricating phase-shifted fiber Bragg gratings (FBGs) using a variable-velocity scanning UV laser beam in combination with a shielded phase mask. The transmission properties of phase-shifted FBGs were analyzed based on coupled-mode theory and a transfer matrix method. The grating is divided into three parts to allow for easier analysis of FBG properties. These segments included a uniform FBG1 and FBG2 which were separated by a shielded section. A novel phase-shifted FBG was fabricated using this method, in which the refractive indices of FBG1 and FBG2 were different. Transmission properties of these phase-shifted FBGs were simulated numerically using MATLAB, and the experimental results and simulated results are in good agreement. In addition to the length and effective refractive index of the shielded section, the phase shift value of a phase-shifted FBG was also found to be dependent on the lengths and effective refractive indices of FBG1 and FBG2. Moreover, we predicted that changing the scanning velocity for fabricating FBG2 would adjust phase shift value, which exhibits a positive linear relationship with the scanning velocity. These results can provide guidelines for fabricating any phase shift value FBGs. This technique is simple, convenient, and may be developed further for use in fabricating novel tunable fiber filters or DFB fiber lasers.