Spectral Beam Combining System Research Articles

High-power spectral beam combining based on 890nm laser diode array

Spectral beam combining (SBC) in the fast axis (FA) direction based on the laser diode array (LDA) consisting of eighteen 400 μm broad-area laser diodes at ∼ 890 nm is demonstrated. For scaling higher output power, it is essential to take into account the catastrophic optical mirror damage (COMD) of the LDA. The gain characteristics have been studied based on the LDA front-facet reflectivity and the output coupler (OC) reflectivity. While the combination of the LDA with 0.5 % front-facet reflectivity and the OC with 3 % reflectivity is adopted, the SBC system delivers an 808 W maximum peak power with 89 % optical-optical conversion efficiency. The beam quality factor β is measured to be 2.3 in the FA direction. Moreover, an excellent SBC power stability is confirmed at full output power, indicating the power fluctuation of less than 0.8 % (rms) over 60 min.

Monolithic linear array fiber end cap for spectral beam combination

In this study, a fusion splicing system that facilitates the splicing of a monolithic linear array fiber end cap is constructed. The fusion splicing thermal field of the end cap irradiated by a CO2 laser is theoretically simulated using the finite element method. The experimental fusion splicing temperature agrees well with the simulation results. The splicing strength, misalignment, and high-power withstanding capacity of the fusion-spliced monolithic linear 3-fiber end cap are investigated. The use of the fabricated device in a dual-grating spectral beam combination (SBC) is also demonstrated with three-channel laser beams, achieving a combined beam with near-diffraction-limited beam quality M2=1.101. It shows that the development of the monolithic linear array fiber end cap is meaningful for achieving a compact high-brightness SBC system.

Dynamics and stability conditions of a spectrally combined beam based on a diode laser array

In the spectral beam combining (SBC) system based on a diode laser array (DLA), optical feedback is required to lock the wavelengths of the diode lasers, but excessive feedback may lead to instability. To analyze the dynamics and stability conditions of the SBC system based on DLA, a model for the round-trip propagation of the feedback beam in the external cavity is established, and the optical feedback is calculated using the diffraction integral method. On this basis, a stability condition (the feedback-factor is smaller than 0.07) is introduced to assess the potential stability of the SBC system. If the feedback-factor exceeds 0.07, the combined laser can be in chaos. Based on the stability condition, we analyzed the influence of external-cavity parameters on the external-cavity feedback. The results indicate that the optical feedback becomes increasingly strong with the increasing focal length of the transform lens and the reflectivity of the output coupler, while it decreases with the increasing reflectivity of the front facet of the diode emitter. This work paves the way toward the stabilized spectral beam combining system based on a diode laser array.

Theoretical analysis of beam quality degradation induced by laser spectral characteristics in spectral beam combining system

We establish a model based on the dispersion of the diffractive grating to investigate the influence of the spectral envelope and wavelength stability on the combined beam quality M 2 of the spectral beam combining (SBC) system. We obtain the analytic expressions of combined beam quality corresponding to the second-moment-spectral width of the beamlets, which can be used to forecast the combined beam quality for any integrable spectral envelope shape of beamlets in an SBC system. Additionally, we find that the center wavelength shift of beamlets has a significant impact on the combined beam quality. The second-moment-based width of the center wavelength shift distribution among all beamlets should be less than 0.5 times the spectral width. Here, the spectral width is the weighted root mean square of all beamlets’ spectral width, where the weight is the power of each single channel.

Analysis of crosstalk in spectral beam combining of diode laser bar

Spectral beam combining (SBC) of laser diodes has been proven to be an effective method to improve beam brightness. The crosstalk between different emitters in the SBC system will significantly affect the beam-combining efficiency and output beam properties without effective control. The interfering factors of the crosstalk beam intensity in semiconductor laser SBC have been theoretically analyzed, and experiments have been built to carry out the SBC of a semiconductor laser bar with a high filling factor of 80%. It is found that by adding an inverted Kepler telescope, reducing the focal length of the transmission lens or increasing the spacing of the grating and the output coupling mirror, the crosstalk can be suppressed. In an experiment of beam combining with five emitters, the ratio of the first-order crosstalk peak energy to that of the central beam spot is about 0.34 when the spacing of the grating and the output coupling mirror is 50 mm and the focal length of the transmission lens is 200 mm. The ratio decreases to 0.035 when the spacing of the grating and the output coupling mirror is 200 mm. With the addition of an inverted Kepler telescope with a magnification of 2, the ratio decreases to 0.085. In addition, the ratio decreases to 0.07 when the focal length of the transmission lens is 80 mm.

Numerical analyses of a spectral beam combining multiple Yb-doped fiber lasers for optimal beam quality and combining efficiency

Physical parameters of a spectral beam combining (SBC) system for multiple Yb-doped fiber lasers (YDFLs) were identified and numerically analyzed to obtain the optimal beam quality and the combining efficiency. We proposed an optimal range of the parameters that can be utilized in SBC systems. For a practical SBC system composed of a multi-layer dielectric grating and a transform mirror, we systematically varied input laser parameters such as the incident angle, beam diameter, laser linewidth, spectral spacing, number of beams, and their spatial separation. Characteristics of diffracted beams by the SBC system were numerically analyzed using a Fourier modal method (FMM). The beam quality M2 and the combining efficiency, η, were optimized by varying the laser beam parameters. We found that M2 and η were most affected by the angle of incidence and the laser linewidth, respectively. We presented the optimal parameters for three, five, and seven linear beam array SBCs along with a range of allowed parameters that could be used in the laser power scaling.

Simulation and experimental study of laser-induced thermal deformation of spectral beam combination grating.

The multilayer dielectric (MLD) grating is a critical device for combining multiple laser beams into a single beam in a spectral beam combining (SBC) system. We established a theoretical thermal deformation model of the laser-irradiated MLD grating. Thermal deformation on the surface of the grating is simulated according to a series of parameters including the laser irradiation time, laser power density, and substrate size. To verify the model, we exposed a 960 l/mm, 50×50×1.5 mm3 grating to a laser power density of 3.61 kW/cm2 and observed the temperature change. We used a Twyman-Green interferometer to measure the interference fringes on the grating surface. Based on the Fourier-transform method and a Zernike polynomial fitting method, the real-time grating surface profile is reconstructed. The results show that substrate thickness increase or area decrease can reduce thermal deformation, the average decreases are 18.3% and 19.9%, respectively. The discussion and analysis of the grating thermal deformation are potentially valuable for designing grating to decrease the thermal deformation and improve the combined beam quality of a SBC system.

Accurate temperature measurement of a spectral beam combination grating based on VO2 film.

In a spectral beam combination system, temperature increase of the multilayer dielectric grating (MDG) worsens the far-field beam quality of the output laser. To accurately monitor the surface temperature of the MDG, this study deposits VO2 phase-change film on the lowest layer of multilayer dielectric films in the MDG and tests the transmittance with a probe laser. Based on this measurement, the surface temperature of the MDG can be calculated. Additionally, the study analyzes the influence of VO2 film on the surface electric field and the -1 diffraction efficiency of the MDG and presents a specific example of using VO2 film to test high reflector temperature. The study concludes that VO2 film is a feasible method of measuring temperature and better than an infrared thermal imager.

Analysis of beam characteristic in the single and dual grating spectral beam combining of fiber laser array with pointing deviation

As a basis of evaluating output laser performance, maintaining excellent beam quality is of great importance for the construction of the spectral beam combining (SBC) system. The influence of pointing deviation on the beam characteristic is studied in SBC of different technical solutions theoretically. The incident light field of the fiber laser array with the pointing deviation is derived by the transformation of coordinates, and the change of the combined beam quality with normal distributed random perturbations is discussed by the principle of beam diffraction and the statistical analysis. When the pointing deviation of the laser array is only 0.5 mrad, the beam quality factor of the SBC system will increase to (2.80±0.68) for the single grating SBC and (4.10±1.29) for the dual grating SBC. It can be concluded that the pointing deviation has a significant effect on the beam characteristics of the SBC systems. Compared with the single grating SBC, the dual grating scheme is more sensitive to pointing deviation.

Spectral interval compressing structure in spectral beam combination system for diode laser arrays

An efficient spectral beam combining (SBC) system for diode lasers with compressed spectral interval is proposed and demonstrated experimentally. This structure consists of a transmission grating and a reflector paralleled to the grating, which can make the beam diffracted twice by only one transmission grating. The advantage of this configuration is that it compresses the spectral interval of the output beam into half compared to the traditional single-grating setup and improves the utilization rate of the grating element compared to the previous double grating system. The experimental results demonstrate that the spectral intervals of a 3-emitter diode laser array (DLA) and a 5-emitter DLA with traditional single-grating structure are 4.36 and 4.69 nm, respectively. In the modified system, the spectral intervals of the same DLA with 3 and 5 emitters are 2.15 and 2.35 nm, respectively, which shows that this method can compress the spectrum interval effectively and will be promising for a practical SBC system.

Modeling and Analysis of the Influence of an Edge Filter on the Combining Efficiency and Beam Quality of a 10-kW-Class Spectral Beam-Combining System

Filter-based spectral beam combining (FSBC) is a promising power-scaling concept for high-power, broad-linewidth fiber lasers, as it relaxes the requirements for linewidth control and also the sizes of the individual beams. As the combining element in the FSBC system, the steep-edge filter plays a major role in achievement of the combining efficiency and the beam quality. In this case, we combine the uncorrelated surface roughness model and the combining efficiency model, and we conduct a comprehensive analysis of the effects of surface roughness, thickness error, and incident angle on the filter’s optical properties and the combining efficiency, in order to determine the optimal configuration for the laser beam-combining system. The simulation results show a good agreement with the measured ones. Meanwhile, through the adoption of the angular spectrum theory, this paper has also conducted a preliminary analysis of the influence of the combining elements on the quality of the combined beam, and some theoretical instructions on the future design of the spectral beam-combining system are provided.

Beam quality analysis and optimization for 10 kW-level spectral beam combination system

Filter-based spectral beam combining provides a promising approach for power scaling and brightness enhancement, especially for broad-linewidth fiber lasers. In this paper, we have demonstrated the beam combining experiments of two high power broad-linewidth fiber lasers using an edge filter as the combining element and achieved 10.25 kW combined power with an efficiency of 97%. Due to the thermal lens effect, the combined beam quality shows obvious signs of deterioration, which occurs in all cases of high power spectral beam combination system, including grating-based and prism-based schemes. The surface deformation and the thermal behavior are monitored in real-time so as to research the mechanism of the combined beam quality deterioration during the combining process. The primary reason for the decline of beam quality could be attributed to the thermal induced change in the refractive index, which not only results in an inhomogeneous wave-front aberration but causes the different focal waists of the incident channels. To counterbalance these aberrations, an air-cooling device and a passively thermally compensated optical system based on CaF2 windows are integrated into the system. In this case, the thermal lensing effect is minimized and the beam quality deterioration factor (BQD) of the combined output beam has been improved 52% in horizontal direction and 101% in vertical direction, showing the great potential of the filter-based approach to achieve a combined beam with high power and excellent beam quality.

Theoretical analysis of beam quality degradation in spectral beam combining of fiber laser array with beam deviation

Aimed to maintain excellent beam quality, the degradation of M2 factor is theoretically studied in compact spectral beam combining (SBC) system of fiber laser array. Considering that the output beams from a laser array usually have axial translation and angular deflection, the correction of incident light field is built by the transformation of coordinates. Using the propagation model and the statistics, properties of the combined beam with perturbations of a single emitter or entire laser array are respectively discussed in detail. The degradation of M2 factor is 0.14(±0.075) when the axial translation of entire laser array satisfies normal distribution with standard deviation of 100 μm. While the angular deflection is introduced with standard deviation of 10/3 mrad, the degradation increases to 3.57(±1.28). Owing to non-overlap ensemble of the incident beam on the grating, the deflection angle will translate the far-field beam spot after the diffraction, which causes a dramatic degradation of the beam quality. Considering the axial translation and the angular deflection simultaneously, the overall effect on the beam quality is dominated by the angular deflection. For applicable requirements of M2 < 2, 3 mrad angular deflection is claimed with axial translation less than 300 μm in the compact SBC system. These analyses provide a valid basis for building the experimental system of SBC.

High-efficiency polarization-independent wideband multilayer dielectric reflective bullet-alike cross-section fused-silica beam combining grating.

A high-efficiency polarization-independent wideband multilayer dielectric reflective bullet-alike cross-section (combination of trapezoidal-rectangular grating profile) fused-silica beam combining grating (BCG) used in the -1st order for spectral beam combining (SBC) is designed and fabricated. Exact grating profile parameters are optimized by using the rigorous coupled-wave analysis and simulated annealing algorithm. As a comparison, traditional pure trapezoidal and pure rectangular gratings are also designed. Numerical results show that such a bullet-alike cross-section BCG exhibits wide bandwidth with the lowest maximum electric field enhancement in the grating material, which is greatly beneficial for the promotion of the power scaling level of the grating-based SBC system. A two-step dry-etching procedure is developed to fabricate such a grating. The averaged diffraction efficiency of greater than 91% was experimentally demonstrated.

基于双光栅的光纤激光光谱合成关键技术研究进展(特邀)

A spectral beam combination(SBC) system with dual Multi-Layer Dielectric(MLD) grating dispersion compensation configuration, which can combine multiple fiber lasers into a common aperture beam with high beam quality and relax the linewidth requirement of the individual fiber laser, has become gradually one of the most important technique route of fiber laser beam combining. The basic principle and the key technologies of the SBC system with dual-MLD-grating configuration were introduced and analyzed briefly. The recent progress of the main key technologies about high power beam combinable narrow linewidth fiber laser, high power high efficiency short wavelength narrow linewidth fiber laser, large dispersion high efficiency MLD grating and high integration dense beam combining were reviewed. The recent progress on the key technologies of SBC with dual-MLD-configuration in Institute of Applied Electronics, China Academy of Engineering Physics, were also presented. The potential of the SBC with dual-MLD-configuration was prospected.

Beam modulation due to thermal deformation of grating in a spectral beam combining system.

As the power of a spectral beam combining (SBC) system increases, the temperature of the multilayer dielectric grating (MDG) inevitably rises under the influence of high-power continuous-wave (CW) laser irradiation. Hence, thermal deformation of the MDG occurs, along with degeneration of the combined beam properties. In this study, we experimentally and theoretically investigate the influence of the MDG thermal deformation on the combined beam properties. An experimental setup is first proposed, in which beam quality M2, beam profile, and MDG wavefront deformation are investigated. The experimental results indicate that the beam quality clearly degrades and the MDG wavefront deformation becomes more significant with increasing pump-CW power density. On this basis, a calculation model for MDG thermal deformation in SBC systems is proposed. The results indicate that MDG wavefront deformation becomes more significant, combined beam profile becomes deformed, and beam quality of the combined beam degrades with increasing power density. Further, thermal expansion of the substrate is a crucial factor that induces MDG wavefront deformation and far-field intensity modulation.

Influence of thermal deformation of a multilayer dielectric grating on a spectrally combined beam.

In spectral beam combining (SBC) systems, temperature rise inevitably happens to the multilayer dielectric grating (MDG) due to irradiation of the high-power continuous-wave (CW) laser, resulting in thermal deformation of the MDG and degradation of the characteristics of the combined beam due to thermal deformation of the grating. The calculation model of thermal deformation of the MDG has been proposed, and the distributions of temperature field and thermal deformation of the grating have been analyzed. On this basis, a propagation model of a SBC system has further been built up. By utilizing the diffraction integral method and the principle of incoherent superposition, the intensity distribution and the beam quality of the combined beam with different power densities based on the MDG have been numerically calculated and analyzed in detail. The results show that both the maximum temperature rise and the thermal deformation of the MDG increase dramatically with the increasing power density of the incident laser beam. Side lobes appear in the intensity distribution of the combined beam due to thermal deformation of the MDG. Furthermore, the side lobes of the combined beam become increasingly obvious with increasing power density. Additionally, the beam quality of the combined beam degrades significantly with increasing power density of the incident laser beam.

Effect of crosstalk on combined beam characteristics in spectral beam combining systems

In a spectral beam combining (SBC) system, crosstalk always happens because stray lights are inevitable due to fabrication errors of optical components and ‘smile’ effect of laser arrays. Two kinds of crosstalk, including the crosstalk generated between two adjacent emitters of the laser array (ad-crosstalk) and that generated between two non-adjacent emitters (non-ad-crosstalk), have been analyzed. The equivalent light of the crosstalk model has been proposed, and the propagation model of the SBC system with the crosstalk has been built up. On this basis, influences of above two kinds of the crosstalk on the combined beam have been numerically simulated and discussed in detail. The results show that the wavelength composition of the combined beam varies evidently owing to the existence of the crosstalk. With the increasing of the crosstalk intensity, the beam quality of the combined beam degrades, and the side lobes of intensity distribution of the combined beam become more and more obvious. Furthermore, the influence of the non-ad-crosstalk on the beam quality is more serious than that of the ad-crosstalk.

고출력 광섬유 레이저 기술의 현황 및 전망

Over the past two decades, fiber-based lasers have made remarkable progress, now having reached power levels exceeding kilowatts and drawing a huge amount of attention from academy and industry as a replacement technology for bulk lasers. In this paper we review the significant factors that have led to the progress of fiber lasers, such as gain-fiber regimes based on ytterbium-doped silica, optical pumping schemes through the combination of laser diodes and double-clad fiber geometries, and tandem schemes for minimizing quantum defects. Furthermore, we discuss various power-limitation issues that are expected to incur with respect to the ultimate power scaling of fiber lasers, such as efficiency degradation, thermal hazard, and system-instability growth in fiber lasers, and various relevant methods to alleviate the aforementioned issues. This discussion includes fiber nonlinear effects, fiber damage, and modal-instability issues, which become more significant as the power level is scaled up. In addition, we also review beam-combining techniques, which are currently receiving a lot of attention as an alternative solution to the power-scaling limitation of high-power fiber lasers. In particular, we focus more on the discussion of the schematics of a spectral beam-combining system and their individual requirements. Finally, we discuss prospects for the future development of fiber laser technologies, for them to leap forward from where they are now, and to continue to advance in terms of their power scalability.

Beam properties in a spectral beam combining system based on trapezoidal multilayer dielectric gratings

Aimed at the application of multilayer dielectric gratings (MDGs) in spectral beam combining (SBC) technologies, a theoretical model of trapezoidal MDGs has been proposed and a beam propagation model of a SBC system based on trapezoidal MDGs has further been built up. The phase modulation introduced by the MDG involves optical path difference on the top surface of the relief, that in the relief and that of the multilayer dielectric films, which is affected by the relief groove depth, the relief duty cycle, base angles of the trapezoidal relief, the beam incidence angle, and so on. Using the diffraction integral method and the principle of incoherent superposition, the intensity distribution of the combined beam of the SBC system based on the MDG has been numerically calculated and analyzed. On this basis, the beam quality of the combined beam has been calculated by using the intensity second-order moment method, and the effect of both parameters of the MDG and the incidence angle of the central beam on the characteristics of the combined beam has been discussed in detail. The results show that the beam quality of the combined beam after passing through SBC is significantly better than that of the laser array before passing through SBC. Reasonable selection of the parameters of the trapezoidal MDG and the incidence angle of the central beam has no effect on the beam quality of the combined beam but influences the power of the combined beam. However, the fabrication errors of the trapezoidal MDG have significant effect on both the beam quality and the power of the combined beam.