This PDF file contains the front matter associated with SPIE Proceedings Volume 12974, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

The surface of the gold film grating appeared to different degrees of carbon burning phenomenon under high energy laser irradiation, which resulted in the degradation of the grating performance. Thus, in this study, the main components and relative contents of organic contaminants in the wall and air at different positions in the chirped pulse amplification system were detected by gas chromatography and mass spectrometry. The organic molecules were volatilized from potential sources such as components and pump oil or dust produced by stray light irradiation of carbon-based materials. The contaminant C₁₂H₃₈O₅Si₆ was found at multiple sampling sites, indicating that the hydrocarbon molecules in the contaminant formed a chemical bond with the molecular structure of silicon and oxygen on the surface of the optical component. Compared to physical adsorption, this chemical bond adsorption is stronger and more difficult to remove. The effect of long-term vacuum organic contamination on the diffraction efficiency of the gold grating was not significant enough. On the contrary, organic residual contaminants were formed in the laser-irradiated area of the surface of the gold grating, and the diffraction efficiency was significantly reduced to two-thirds of the undamaged area. Many small organic molecules, particles and water molecules were deposited in the grooves on the surface of the gold grating, and carbonization occurs under the action of ultra-short pulse laser. A stress pit appeared in the center area of laser irradiation, causing damage to the surface of the grating.

Wavefront sensing is a technique for measuring wavefront aberrations in optical beams, playing a crucial role in various optical systems used for astronomical observations, laser communications, and inertial confinement fusion. As optical systems are being employed in increasingly extreme scenarios, wavefront aberrations have become more pronounced, often exhibiting high temporal variations, which demands more elevated rates of wavefront sensing. This paper introduces a novel wavefront sensing method based on cascaded phase modulation layers capable of achieving ultra-high wavefront sensing speed while maintaining satisfactory accuracy.

We propose a high-power, high-repetition-rate nanosecond slab laser amplifier that can operate at room temperature. The laser seed source uses a self-developed nanosecond laser with 500 W and 1064 nm. Based on the principle of multi-angle amplification of Nd:YAG slab structure, the seed source is injected into the Nd:YAG slab crystal at an incident angle of 10.2° and 20.6° after passing through the isolation system, the collimation system and the beam shaping system in turn. The rectangular output spot greatly improves the cleaning efficiency. The seed source operates in pulse mode, and the pump source of the amplifier operates in quasi-continuous mode. The pump uses a single side pump source with a wavelength of 808 nm and a bar number of 60 × 8. When the pulse width, repetition frequency and current of the pump are set to 200 μs, 400 Hz and 120 A, respectively, the output power of the seed source through the amplifier is 521 W and the magnification is 6 times. The output power of the seed source secondary through the amplifier is 872 W, and the magnification is as high as 19 times. When the pump pulse width, repetition rate and current are 400 μs, 400 Hz and 120 A, respectively, the output power of the seed source passing through the slab crystal twice is as high as 1747 W. The nanosecond slab laser amplifier with rectangular output spot designed by us can be applied to the field of high-efficiency laser cleaning.

Being simple to install and with excellent performance, gas-puff loads play an important role in the experimental study of Z pinch. The gas load in Z-pinch experiment is generated by Laval nozzle, which behaves as a supersonic transient gas flow with density between 10¹⁶ and 10¹⁷cm^-3. Studying the initial density distribution of the load gas flow is helpful to predict the nozzle performance, optimize the nozzle size structure, improve the implosion dynamics process, and finally achieve the purpose of improving the X-ray radiation yield.

We present numerical investigations to evaluate the performance of the second-harmonic generation (SHG) for a 100 J, 1 Hz Nd:glass laser operating at 1053 nm under different phase-matching conditions in a large-aperture DKDP crystal. Steady state temperature maps for different crystal length, as well as temperature bandwidth analysis under different phase-matching conditions are presented. Studies show that the SHG efficiency drops due to thermal dephasing for the type-I and type-II configurations are 14.6 % and 3.6 %, respectively. Since the type-I SHG implementation is more straightforward and cost-effective in the laser facility, we focused on how to improve its performance in the present work. Besides some existing routines, we proposed here a new approach to mitigate the thermal dephasing issue, by pre-detuning the phase-matching angle for the DKDP crystal. The results show that performance of type-I SHG can be greatly improved by this new approach, with the efficiency drop being reduced from 14.6 % to 3.9 %. We believe that our findings would be beneficial to the construction of ultra-short, ultra-intense laser drivers with reliability, cost-effectiveness and fewer number of large optics.

We demonstrated a CW mode-locked all solid-state Tm: CaGdAlO4(Tm: CGA) laser using SESAM as a mode-locked starting element. The laser cavity adopts an X-type four-mirror cavity structure, and the pump source is an 790nm LD. The minimum mode locking threshold of Tm: CaGdAlO₄ solid state laser was measured to be 7.54W, and when the pump power reached 14.79W, the corresponding maximum output power was 114mW at 1889nm, the repetition rate was 105.2 MHz, and the pulse width was 242.3ps. The experimental results show that Tm: CaGdAlO₄ crystal can obtain a stable 2μm picosecond pulse, which is very suitable as a gain material for 2μm ultrashort pulse lasers.

Spectral beam combining technique is one of major approaches in power scaling of fiber lasers. In order to maximize beam-combining performance, maintain a high beam quality in the combining process, the absorption-induced wavefront distortion of the resulting combined beam must be considered. By reducing the film absorption, the dichroic filters will be promising competitors to dielectric gratings as crucial spectral beam combining elements. In this paper, two different structures: less-cavities (LC) structure and more-cavities (MC) structure of dichroic filter layers were carefully designed and fabricated by ion-beam sputtering deposition with high-precision layer thickness monitoring method. A photothermal scanning system based on laser-induced surface thermal lensing (STL) effect was used for analyzing the 1064nm wavelength absorption of dichroic filter layers in passband and stopband. Dichroic filter with reduced in-bandpass absorption was found in MC structure by relaxing the electric field strength in thin films. This approach is highly expected to have great potential for fabricating promising spectral beam combining dichroic filters with less thermal effect and higher laser-induced damage threshold (LIDT).

Large mode field reverse polarization maintaining optical fiber coupler (LMFRPM-OFC) is the key optical fiber component of high power fiber laser, which plays an important role in the fields of high power polarized laser, etc. The polarization temperature dependence of LMFRPM-OFC means that reverse extinction ratio of polarized laser transmitted through the optical fiber coupler is affected by the temperature change. The temperature change will cause the birefringence and polarization state of the optical fiber coupler to change, thus affecting the stable transmission of polarized laser. Therefore, it is of great significance to study the polarization temperature characteristics of optical fiber couplers. In this paper, the polarization state of optical fiber coupler samples is tested. Test results: In the process of temperature change, the polarization state of the optical fiber coupler to change, and the change trend is opposite to the direction of temperature change. This phenomenon is due to the change of the structure in the stress zone in the LMFRPM-OFC caused by the temperature change, which affects the polarization state of the output high-power polarized lasers. In this paper, the polarization temperature dependence of LMFRPM-OFC is studied through active or passive cooling. The polarization temperature dependence of LMFRPM-OFC is effectively reduced through experiments of active cooling and passive cooling, and the reliability of polarization output of LMFRPM-OFC and high-power polarized lasers is improved.

The interactions of laser below moderate intensity with highly magnetized over-dense plasma are investigated analytically, which shows the capability of propagation into and probing of the over-dense plasma by right hand circularly polarized (RHCP) laser in the whistler mode, or when the axial magnetic field is above a threshold. The process of collisional heating is also studied, showing the dependence of heating depth and temperature on conditions of plasma, laser and magnetic field. A novel idea of direct laser ignition is brought up for the first time, where kJ/ns RHCP laser pulse will be used to heat up and ignite the compressed fuel directly with the assistance of strong axial magnetic field, different from other ignition schemes where laser should be first converted to shock wave, electron or ion beams.

The vortex beam can improve the resolution of the interferometry by converting the continuous phase shift into the rotation angle of the interference image. This research focuses on the high-resolution recognition of rotating interference images in the vortex beam interferometry. The present paper explores deep-learning technology by establishing a residual convolutional neural network to recognize the rotation angles of interference profiles. After well trained, the proposed network model is able to achieve an image rotation recognition resolution of 9.19 milli-radians, and the corresponding displacement measurement resolution is 0.92 nm. Due to the competitive resolution, the proposed method shows great potential in precise measurements.

High power laser transmission in optical system is a key link for laser coping with electro-optical imaging system. When laser goes through the entrance pupil of optical system, diffraction effect inevitably comes into being due to the limitation of the entrance pupil. The energy distribution on the photodetector caused by diffraction effect can have a serious impact on the target detection and recognition. The correction method of diffraction effect among the different optical systems has been proposed based on Fraunhofer diffraction theory in this paper. The simulations results have indicated that diffraction effect simulated by the oscillation envelope approximation have a good agreement with the results simulated by Bessel function on energy distribution profile and changing trend. The difference is the lack of fine modulation of sidelobes, but the modulation is lower than the sidelobes by 3 orders of magnitude. The agreement has proved that the correction method is practicable, which can effectively solve the effect estimation among the different types and F number of optical systems.

With the development of machine vision, remote sensing, and wearable displays, the applications of optical imaging systems are becoming more and more complex, and the requirements of intelligence, miniaturization and low cost is becoming higher. By breaking the constraint of rotational symmetry or simple aspherical application, and get more degree of surface shape freedom, freeform surfaces have brought new solutions to the optical imaging system. Meanwhile, traditional optical surface testing method is difficult to meet high-efficiency and high-precision at the same time since the non-rational symmetric characteristics. With the aid of modern photolithography, complicated computer generated hologram (CGH) can be produced for the null testing of optical freeform surfaces. Furthermore, CGH can provide references for system alignment while testing. In this paper, a CGH method of measuring freeform surface is presented, and the measurement uncertainty is analyzed. The result shows that for measuring the wavefront of a freeform surface with over 6-inch clear aperture, the measurement accuracy is less than λ/10.

An active spectrum tailoring scheme is proposed and demonstrated in this letter to enhance the energy distribution further towards the long wavelength side. By cascading Dy-doped fluoride fiber to the Er-doped ZBLAN fiber, the residual spectral peak at ~2.8 μm was eliminated, and a spectrally-flat, 3.5-W MIR-SC with power ratio beyond 3 μm up to 91.2% was achieved.

Mid-infrared supercontinuum has been widely used in biomedicine, spectroscopy and other fields. With the development of fluoride fiber, the output power of mid-infrared supercontinuum has gradually increased. In this paper, the supercontinuum generation system of InF3 fiber pumped by a supercontinuum laser with a spectral range of 1.9-2.6 μm is presented. By optimizing the cooling device, the supercontinuum laser with an output power of 26.2 W and an output spectral range of 1.95-4.5 μm is realized. To our knowledge, this is the first report of mid-infrared supercontinuum output with an average power exceeding 20 W in InF₃ fiber.

The mid-infrared fiber lasers operating in the 3-5 μm have attracted great interest due to wide applications. However, the soft glass fiber-based rare-earth-doped laser in the mid-infrared band has limitations in power enhancement and wavelength expansion. Gas-filled hollow-core fiber lasers provide a new way to realize mid-infrared output. Here, we demonstrate an optically pumped CO-filled hollow-core fiber laser. The pump source was a homemade Tm fiber laser operating at 2.33 μm. By tuning the wavelength of the pump source, mid-IR emission from 4.64 to 4.82 μm has been achieved.

In order to accurately analyze and evaluate the output beam quality of large-aperture laser amplifiers, we established a new method of simulating thermal effects which can analyze the distributed thermal effects in the direction of optical transmission. In this method, gain medium would be divided to several segments. Thermal effects and beam propagation in every segment are calculated and cascaded to the next one in the direction of optical transmission until the end of gain medium rod. To achieve propagation of aperture-level distance in these segments, a close-range propagation algorithm based on angular spectrum is proposed and its correctness is verified by comparing with theoretical results obtained by Fresnel diffraction. Though this new method, simulations of the thermal effects in simplified amplifiers with two gain medium rods that compensate each other at different apertures were carried out. The results show that residuals of depolarization loss increase with aperture. When the diameter is up to 35mm, the maximum depolarization loss is 98.8%. This new method of simulating thermal effects lays a solid foundation for the further research on large-aperture highrepetition- rate laser systems.

In inertial confinement fusion, precise alignment between the laser and the target is crucial for the success of laser fusion experiments. The shroud, which protects the target, retracts seconds before laser shooting, a process that inadvertently induces target drift, thus compromising alignment with the laser. We established an experimental framework, employing a laser interferometer of picometric precision, to monitor the target’s displacement during the shroud’s retraction. To counteract the target drift from the shroud’s motion, we implemented a feedforward compensation strategy grounded in Kalman filter theory, which ensures the target remains within the acceptable alignment error margins relative to the target chamber center post-shroud retraction. This approach bolsters the cryogenic target system’s stability, thereby improving the likelihood of successful laser fusion ignition.

Fused silica optics, as key components in high - power laser systems, are prone to produce laser damage with different sizes under laser irradiation. For large - size laser damage, CO₂ laser can be used for repairing process. For small - size laser damage, magnetorheological finishing (MRF) technique is appropriate. This work focused on the MRF repairing process of small - size laser damage on fused silica optical surface, the optimization of MRF processing parameters and the absorption characteristics of optics are investigated. First, the MRF processing parameters were determined by experimental method. Under the conditions of a wheel speed 200 rpm, flow rate 110 L/min, current 7.5 A, press depth 0.3 mm and polishing abrasive CeO₂, the volumetric removal efficiency of removal function was relatively high, up to about 1.108 mm³/min. Appling the optimized parameters in small - size laser damage repairing process, no "comet tail" defects occurred on the optical surface. After MRF repairing process, the surface roughness was restored by immersed CCOS (computer control optical surfacing) technique. For immersed CCOS process, it contained two stage: coarse polishing and fine polishing. With 120 min of coarse polishing and 40 min of fine polishing, the roughness Ra decreased from the initial 6.179 nm to 0.7 nm, which was basically restored to the initial state. Finally, the photo - thermal absorption of the optics before and after repairing process was detected on a weak absorption testing platform. The results showed that the surface absorption decreased from the initial 425.38 ppm to 125.92 ppm, and the laser energy absorption of the repaired fused silica optics was significantly reduced, which verified the validity of the combined process of MRF and immersed CCOS process. In a conclusion, the research results in this paper can provide important technical support for the rapid repairing of small - size laser damage on fused silica optical surfaces, which was conducive to improving the service life of high - power laser systems.

A comprehensive finite element physics model is constructed to analyze the thermal effects of a thin disk medium. Based on this physical model, the temperature and stress distribution inside the medium are obtained. According to the corresponding theory, the internal optical path difference (OPD) and thermal focal length of the medium can be calculated when subjected to thermal load. According to the numerical model of the thermal effect, which has been verified by experiments conducted on commercial thin disk purchased from Dausinger + Giesen GmbH (DG), the study investigated the influence of various factors on the thermal effect of the thin disk medium. Additionally, a thin disk medium based on photoadhesion technology has been developed. The thermal focal length of the thin disk medium has been compensated by adjusting the surface shape in advance to match the desired focal length, which is significantly higher than that of the uncompensated medium. Below a pump density of 1.27 kW/cm², the thermal focal length of the self-developed thin disk medium is comparable to that of the DG's thin disk medium.

Due to the rapid performance of taking away the generated heat of a diode pumped alkali laser (DPAL) system, a flowing diode pumped alkali laser (FDPAL) is thought to be helpful to mitigate the thermal effects and improve the power scaling ability of a DPAL system during the power scaling period. In general, a relatively perfect theoretical model for a FDPAL needs to take the laser kinetic, heat transfer, and computational fluid dynamics (CFD) into account at the same time. Until now, the commercial finite element method (FEM) soft can only simulate the fluid and thermal distributions in an alkali vapor cell for a FDPAL. The laser kinetic can only be effectively calculated by a coding soft. Therefore, the multi-physics coupling problem needs to be firstly tackled at the beginning of a design for a FDPAL system. In the paper, a loop iteration based co-simulation method is utilized to solve the multi-physics coupling problem during the simulation of a FDPAL. The temperature and fluid corresponded parameters of a FDPAL are obtained by a FEM soft. The laser kinetic corresponded parameters of a FDPAL are got by a coding soft. By constructing a java language based server, the calculated data of such two kinds of soft can be shared. Then, a main iteration based procedure with preset initial values is coded to control the running behavior and communication of the two kinds of soft. After several or several ten times loop iteration, the laser power, temperature distribution, and velocity distribution of a FDPAL can be theoretically investigated. It has been demonstrated that the co-simulation based calculating results show a good agreement with the experiment results.

Due to the thermal deposition, the switching ratio of the amplitude OASLM will be reduced, so it is difficult to play a better effect in the high average power laser system. In this paper, through the analysis of laser-induced temperature rise model and liquid crystal layer voltage model and experiments, we show that with the same aperture and same switching ratio, the tolerable optical power density will decrease with the larger irradiation spot area. When the spot diameter decreases from 8 mm to 3 mm without heat dissipation treatment, the temperature of the OASLM emitting surface decreases from 39.6 °C to 33 °C, and the laser tolerated power density improves from 32.8 W/ cm² to 120 W/ cm², but the total tolerated power decreases from 16.5 W to 8.5 W. Therefore, in order to improve the total power tolerance of the optical addressable spatial light modulator, it is still an important technical way to improve the clear aperture of the optical addressable spatial light modulator.

The short-wave infrared band correlated light source, especially near the 2.3 μm band, has a wide application prospect in the fields of optical metrology of combustion process and non-invasive blood glucose measurement. Especially, the 2.3 μm narrow linewidth source has unique advantages in the monitoring of polluted gas and as the pumping source of gas laser. Tm doped fiber laser is an effective method to realize narrow linewidth output in 2.3 μm band. At present, however, the output power of the 2.3 μm band narrow linewidth Tm fiber laser is only in the order of one hundred milliwatts. Here, we demonstrated the first watts-level single-frequency Tm³⁺-doped fiber laser operating at 2.3 μm.

This study examines the implications of cubic nonlinearity on second-harmonic generation (SHG) within high-intensity optical fields, elucidating how cubic nonlinearity inherent to the medium perturbs the ideal phase interrelation among the interacting waves, culminating in diminished energy conversion efficiency and inducing spectrum modulation. A methodology is advanced for identifying the optimal wave vector mismatch that maximizes SHG efficiency while circumventing second-harmonic (SH) pulse modulation. Through simulations of the SHG mechanism, conducted under varying intensities (1-5 TW/cm²) and contrasting durations (10 ns and 50 fs), the research underscores the efficacy of this approach. Notably, an optimal wave vector mismatch scenario facilitated the attainment of an 86% energy conversion efficiency, utilizing a 0.5 mm thick KDP crystal, with the procedure peaking at an intensity of 5 TW/cm² and a pulse duration of 48 fs.

Polarization smoothing (PS) can effectively improve the uniformity of the focal spot by changing the polarization distribution of the beam. In this paper, a technical scheme for achieving multidimensional PS by cascading birefringent wedge (BW) has been proposed. A near-field and far-field diffraction transmission model with multiple PS and a continuous phase plate (CPP) was first constructed, and parameters such as the wedge angle and placement position of BW are optimized through theoretical analysis to increase the number of sub-focal spots on the target plane and change the distribution direction. Then one-dimensional (1-D) power spectral density (PSD) and root mean square (RMS) were used to quantify the difference in the focal spot uniformity in different directions, the results show that the cascaded BW not only ensures the smoothing effect of a single BW, but also has a smoothing performance in a specific frequency band at specific wedge angles.

A diode pumped alkali laser (DPAL) provides a significant potential for construction of high-powered lasers in the future. To realize the scaling of a DPAL, heat management and flow field inside a vapor cell should be investigated. In this paper, a new kind of gas-flowing DPAL with a disc-type vapor cell was proposed. The gain medium of cesium and the buffer gas of ethane were filled in the vapor cell with the total pressure is about 1 atmosphere. The influence of the rotate speed of a cross-flow fan on the internal gas velocity, temperature, and output features of the laser was systematically studied. The corresponding experiment was carried out, and the output laser at 894.3 nm with power of 321 W was obtained.

Negative ion source photo-neutralization has extremely high neutralizing efficiency (>80%) and wall-plug efficiency (>60%) theoretically, making it one of the most promising technologies for neutral beam injection (NBI) in magnetic confinement fusion in the future. Due to the small absorption cross-section of laser on plasma (mainly H^-), the neutralization process requires a MW level laser flux; The increase in thickness of the laser target increases the volume of the neutralizer and the instability of the entire system; high power laser causes thermo-elastic deformation of the reflector and increases cavity loss. Based on the transmission characteristics and system layout of the NBI negative ion source, this paper designs a simplified resonant cavity containing multiple sets of mirrors to minimize the projection area of the entire optical path. Based on the diffraction theory of wave optics and the paraxial optical transmission matrix, the stability and laser transmission properties of this optical path are theoretically analyzed. Finally, the ray trajectory of the optical path is simulated using finite element software, and the deposited light power on the reflecting surface is calculated. The above results indicate the feasibility of the simplified resonant cavity and provide a theoretical basis for small-scale experiments of photo-neutralization.

Charge Coupled Device(CCD) in vacuum environment could be easily overheated, and the energy is not equally distributed on the surface. So the thermal design of the CCD plays a significant role in the procedure of the whole detection[1]. If a proper method is used for the cooling of the CCD, the temperature distribution would be more equally on the surface and the testing result of the CCD would be more accurate. In this paper, a thermal control system for CCD camera in vacuum was designed. CCD will be placed in a cavity filling with high-specific heat capacity gas, so that the energy generated from CCD could be reserved in the gas through thermal convection and heat radiation. In order to reduce the gas leakage, multilayer cavity was designed to maintain the gas pressure of the cavity. The result shows that the optimized multilayer cavity structure for cooling system of CCD can reduce overheated of CCD significantly with CFD (Computational fluid dynamics) method.

We adopt a flat-concave resonant cavity structure, and the laser propagates along the Zigzag optical path in the slab. In order to effectively reduce the thermal lens effect in the thickness direction of the slab crystal and reduce the local thermal effect at the pump region, we propose a module structure under the mixed cooling mode. The large surface under the slab is welded in the microchannel cooling heat sink, and the large surface on the module is cooled by strong convection, so that the crystal surface is evenly cooled, and the thermal distortion in the thickness direction of the slab crystal is suppressed. On the other hand, We use the fast axis collimated semiconductor laser stack as the pump source, and the pump light is coupled to the slab to achieve a uniform distribution of the pump intensity, which is through the optical waveguide and the aspheric lens. When the pump power is 9500W, the average output power is 4.352kW, and the corresponding optical-to-optical conversion efficiency is 45.8%. The repetition rate is 400Hz, the laser pulse width is 180μs, and the single pulse energy is 10.88J. The laser beam βx is 3.4 times of the diffraction limit, and βy is 1.5 times of the diffraction limit.

In this study, we compare and illustrate how weak end-facet reflections affect high-power fiber oscillators and fiber amplifiers' stimulated Raman scattering (SRS) threshold. The simulation results reveal that the enhancement of weak end-facet reflections could decrease the SRS threshold of high-power fiber lasers significantly, especially for fiber amplifiers employing phase-modulated single-frequency lasers as seed lasers. Further comparisons point out that weakening reflections at either the input or output end-facet provides an effective approach to improve the SRS threshold of high-power fiber lasers. In addition, weak end-facet reflections could even invalidate the strategy of suppressing the SRS effect through filtering out the Raman component in seed laser. The theoretical approach and findings may serve as a useful guide for designing high-power fiber laser systems with SRS limitations.