High gain fiber-solid hybrid double-passing end-pumped Nd:YVO4 picosecond amplifier with high beam quality

We propose a fiber-solid hybrid system which consists of a semiconductor saturable absorber mirror(SESAM)modelocked fiber seed with a pulse width of 10.2 ps and a repetition rate of 18.9 MHz,a two-level fiber pre-amplifier and a double-passing end-pumped Nd:YVO4 amplifier.In the solid-state amplifier,to enhance the gain and the extraction efficiency,a specially designed structure in which the seed light passes through the gain medium four times and makes full use of population inversion is used as the double-passing amplifier.Besides,the beam filling factor(the ratio of the seed light diameter to the pump light diameter)and the thermal lens effect of the double-passing amplifier are considered and its optical-to-optical conversion efficiency is further improved.To preserve the beam quality of the double-passing amplifier,a new method of spherical-aberration self-compensation based on the principles of geometrical optics is used and discussed.Our system achieves a maximum average power of 9.5Wat the pump power of 28W,corresponding to an optical-to-optical efficiency of 27%.And the beam quality factor M^2 reaches 1.3 at the maximum output power.

Using Target Ablation for Ion Beam Quality Improvement

During the laser foil interaction, the output ion beam quality including the energy spread and beam divergence can be improved by the target ablation, due to the direct laser acceleration （DLA） electrons generated in tile ablation plasma. The acceleration field established at the target rear by these electrons, which is highly directional and triangle-envelope, is helpful for the beam quality. With the help of the target ablation, both the beam divergence and energy spread will be reduced. If the ablation is more sufficient, the impact of DLA-electron-caused field will be strengthened, and the beam quality will be better, confirmed by the particle-in-cell simulation.

Influence of spherical aberrations on fundamental mode beam quality under different laser resonators

Spherical aberrations of the thermal lens of the active media are severe when solid state lasers are strongly pumped. The fundamental mode profile deteriorates due to the aberrations. Self-consistent modes of a resonator with aberrations are calculated by using the Fox-Li diffraction iterative algorithm. Calculation results show that the aberration induced fundamental mode beam quality deterioration depends greatly on the resonator design. The tolerance of a flat-flat resonator to the aberration coefficient is about 30λ in the middle of stability, where λ is the wavelength of laser beam. But for a dynamically stable resonator, 2λ of spherical aberration will create diffraction loss of more than 40%, if inappropriate design criteria are used. A birefringence compensated laser resonator with two Nd：YAG rods is experimentally studied. The experimental data are in quite good agreement with simulation results.

Influence of mode competition on beam quality of fiber amplifier

Theoretical and experimental studies of the influence of the mode competition on the output beam quality of fiber amplifiers are presented. Rate equations and modal decomposition method are used in the theoretical model. In the experiment, the output beam-quality factor of a fiber amplifier, which is based on a Yb-doped double-clad large mode area fiber as a function of the seed beam quality and the pump power of the amplifier, is measured. The experimental results are consistent with the theoretical analysis.

Beam quality improvement of the high-energy KTA-OPO based on a confocal unstable cavity with Gaussian reflectivity mirror

A high-energy 100-Hz optical parametric oscillator(OPO)based on a confocal unstable resonator with a Gaussian reflectivity mirror was demonstrated.A KTA-based OPO with a good beam quality was obtained when the magnification factor was 1.5,corresponding to the maximum signal(1.53μm)energy of 56 m J and idler(3.47μm)energy of 20 m J,respectively.The beam quality factors(M^(2))were measured to be M^(2)_(x)=5.7,M^(2)_(y)=5.9 for signal and M^(2)_(x)=8.4,M^(2)_(y)=8.1 for idler accordingly.The experimental results indicated that the beam quality positively changed with the increase of magnification factors,accompanied by an acceptable loss of pulse energy.

Transverse mode instability mitigation in a high-power confined-doped fiber amplifier with good beam quality through seed laser control

In this work,a confined-doped fiber with the core/inner-cladding diameter of 40/250μm and a relative doping ratio of 0.75 is fabricated through a modified chemical vapor deposition method combined with the chelate gas deposition technique,and subsequently applied in a tandem-pumped fiber amplifier for high-power operation and transverse mode instability(TMI)mitigation.Notably,the impacts of the seed laser power and mode purity are preliminarily investigated through comparative experiments.It is found that the TMI threshold could be significantly affected by the seed laser mode purity.The possible mechanism behind this phenomenon is proposed and revealed through comprehensive comparative experiments and theoretical analysis.Finally,a maximum output power of 7.49 kW is obtained with the beam quality factor of approximately 1.83,which is the highest output power ever reported in a forward tandem-pumped confined-doped fiber amplifier.This work could provide a good reference and practical solution to improve the TMI threshold and realize high-power high-brightness fiber lasers.

Investigations on beam quality improvement of an NCPM-KTA-based high energy optical parametric oscillator using an unstable resonator with a Gaussian reflectivity mirror

Beam quality improvements by a large margin for signal and idler beams of a high energy 100 Hz KTiOAsO_(4)(KTA)non-critical phase matching(NCPM)optical parametric oscillator(OPO)were demonstrated using an unstable resonator configuration instead of a plane-parallel one.Theoretically,influences of cavity lengths and transmission of an output coupler on the OPO conversion efficiency for both were numerically simulated.For OPO based on an unstable resonator with a Gaussian reflectivity mirror,the maximum pulse energies at the signal(1.53μm)and idler(3.47μm)were about 75 mJ and 26 mJ,respectively.The corresponding beam quality factors of the signal were M_(x)^(2)=9.8 and M_(y)^(2)=9.9,and M_(x)^(2)=11.2 and M_(y)^(2)=11.5 for the idler.As a comparison,128 mJ of signal and 48 mJ of idler were obtained with the plane-parallel resonator,and the M~2 factors of the signal were M_(x)^(2)=39.8 and M_(y)^(2)=38.4,and M_(x)^(2)=32.1 and M_(y)^(2)=31.4 for the idler.Compared with a plane-parallel cavity,over eight times and three times brightness improvements were realized for the signal and idler light,respectively.

500-W high average power,high beam quality Nd:YAG solid-state laser with one focusing reflector

In order to explore a 56-J high energy, high frequency lamp-pumped pulsed Nd：YAG solid-state laser, the main factors influencing the higher laser output energy are analyzed, the relation between output power and input power and reflectivity is simulated theoretically, and the effective measures to improve the efficiencies of the laser are brought forward. As a result, pulse width is tunable between 0.1 and 10 ms, frequency between 1 and 1000 Hz. When the input electrical power is 12 kW, the laser can output maximum single pulse energy of 56 J and average power of 500 W with the beam quality of 16.5 mm.mrad, total electro-optic efficiency of 4.2%, and the in stability of ±2% output power. It is indicated that the parameters corresponds with the results of theoretical analysis and simulation.

A tunnel regenerated coupled multi-active-region large optical cavity laser with a high quality beam

A novel coupled multi-active-region large optical cavity structure cascaded by a tunnel junction is proposed to solve the problems of facet catastrophic optical damage （COD） and the large vertical divergence caused by the thin emitting area in conventional laser diodes. For a laser with three active regions, a slope efficiency as high as 1.49 W/A, a vertical divergence angle of 17.4~, and a threshold current density of 271 A/cm~ are achieved. By optimizing the structural parameters, the beam quMity is greatly improved, and the level of the COD power increases by more than two times compared with that of the conventional laser.

High power and beam quality continuous-wave Nd:Gd VO_4 laser in-band diode-pumped at 912 nm

We report on the performance of a continuous-wave Nd:Gd VO_4 laser in-band diode-pumped at 912 nm with high output power and excellent beam quality. The laser produced an output power of 19.8 W at 1063 nm with an optical efficiency of 59.3% and slope efficiency of 62.7%. The laser threshold was ~2.04 W of the absorbed pump power, and the laser output beam quality was ≤ 1.2 in the horizontal and vertical directions. The strength of thermal lensing at full output power(33.4 W of absorbed power) was measured to be an average of 8.6 diopters.It is shown that thermal lensing is reduced by a factor of 2 with respect to the Nd:YVO_4 lasers, thus opening a way for further output-power scaling.

Photoinjector beam quality improvementby shaping the wavefront of a drive laser with oblique incidence

To increase the quantum efficiency （QE） of a copper photocathode and reduce the thermal emit- tance of an electron beam, a drive laser with oblique incidence was adopted in a BNL type photocathode rf gun. The disadvantageous effects on the beam quality caused by oblique incidence were analyzed qualitatively. A simple way to solve the problems through wavefront shaping was introduced and the beam quality was improved.

The Degradation of Beam Quality in Large-Core Optical Fiber

The degradation of beam quality in large-core fiber is investigated experimentally. It is found that the output beam quality factor M2 is a compound tanh function of the fiber lengths and misalignment launch results that the beam quality degraded faster.

Propagation of the off-axis superposition of partially coherent beams through atmospheric turbulence

The propagation properties of the off-axis superposition of partially coherent beams through atmospheric turbulence and their beam quality in terms of the mean-squared beam width w（z） and the power in the bucket （PIB） are studied in detail, where the effects of partial coherence, off-axis beam superposition and atmospheric turbulence are considered. The analytical expressions for the intensity, the beam width and the PIB are derived, and illustrative examples are given numerically. It is shown that the maximum intensity/max and the PIB decrease and w（z） increases as the refraction index structure constant Cn^2 increases. Therefore, the turbulence results in a degradation of the beam quality. However, the resulting partially coherent beam with a smaller value of spatial correlation parameter γ and larger values of separate distance Xd and beam number M is less affected by the turbulence than that with a larger value of y and smaller values of Xd and M. The main results obtained in this paper are explained physically.

Mode stability analysis in the beam-wave interaction process for a three-gap Hughes-type coupled cavity chain

Based on space-charge wave theory, the formulae of the beam-wave coupling coefficient and the beam-loaded conductance are given for the beam-wave interaction in an N-gap Hughes-type coupled cavity chain. The ratio of the nonbeam-loaded quality factor of the coupled cavity chain to the beam quality factor is used to determine the stability of the beam-wave interaction. As an example, the stabilities of the beam-wave interaction in a three-gap Hughes-type coupled cavity chain are discussed with the formulae and the CST code for the operations of the 2π, π, and π/2 modes, respectively. The results show that stable operation of the 2π, π, and π/2 modes may all be realized in an extended-interaction klystron with the three-gap Hughes-type coupled cavity chain.

Performance of Diagnostic Neutral Beam for the HT-7 Tokamak

Diagnostic neutral beam （DNB）, combined with spectral diagnostics, is employed to measure the ion temperature in HT-7. The factors affecting the extracted beam are studied in the experiment for the high performance diagnostic neutral beam. A 6.5 A extracted hydrogen current at 43 keV of 100 ms was obtained after optimization. The extracted beam has a proton ratio as high as 40%, and can penetrate into the core plasma after neutralization to measure the ion temperature effectively.

A 115 ps,100 Hz high-beam-quality laser based on transient stimulated Brillouin scattering pulse compression

This work demonstrates the generation of short pulse duration and high-beam-quality laser pulses using transient stimulated Brillouin scattering at a high repetition rate.Thermal effects and optical breakdown are identified as the main factors that restrict energy reflectivity and beam quality under high repetition rates and transient situations.Through experimental analysis,the interaction length and focal point size are determined to be the key parameters in reducing the thermal effect by reducing the absorption of the laser pulse by the medium.The obtained results show that pulses with a duration of 175 ps and beam quality M^(2)of around 1.2 can be achieved with a maximum energy reflectivity of over 40%under an interaction length of 50 mm.Furthermore,at an interaction length of 90 mm,a pulse output with a minimum duration of 115 ps(0.5τQ)is achieved.

Diamond Raman laser:a promising high-beam-quality and low-thermal-effect laser

Stimulated Raman-scattering-based lasers provide an effective way to achieve wavelength conversion.However,thermally induced beam degradation is a notorious obstacle to power scaling and it also limits the applicable range where high output beam quality is needed.Considerable research efforts have been devoted to developing Raman materials,with diamond being a promising candidate to acquire wavelength-versatile,high-power,and high-quality output beam owing to its excellent thermal properties,high Raman gain coefficient,and wide transmission range.The diamond Raman resonator is usually designed as an external-cavity pumped structure,which can easily eliminate the negative thermal effects of intracavity laser crystals.Diamond Raman converters also provide an approach to improve the beam quality owing to the Raman cleanup effect.This review outlines the research status of diamond Raman lasers,including beam quality optimization,Raman conversion,thermal effects,and prospects for future development directions.

High power semiconductor laser array with single-mode emission

The semiconductor laser array with single-mode emission is presented in this paper.The 6-μm-wide ridge waveguides(RWGs)are fabricated to select the lateral mode.Thus the fundamental mode of laser array can be obtained by the RWGs.And the maximum output power of single-mode emission can reach 36 W at an injection current of 43 A,after that,a kink will appear.The slow axis(SA)far-field divergence angle of the unit is 13.65.The beam quality factor M;of the units determined by the second-order moment(SOM)method,is 1.2.This single-mode emission laser array can be used for laser processing.

Zernike-coefficient extraction via helical beam reconstruction for optimization(ZEHBRO) in the far field

The spatial distribution of beams with orbital angular momentum in the far field is known to be extremely sensitive to angular aberrations,such as astigmatism,coma and trefoil.This poses a challenge for conventional beam optimization strategies when a homogeneous ring intensity is required for an application.We developed a novel approach for estimating the Zernike coefficients of low-order angular aberrations in the near field based solely on the analysis of the ring deformations in the far field.A fast,iterative reconstruction of the focal ring recreates the deformations and provides insight into the wavefront deformations in the near field without relying on conventional phase retrieval approaches.The output of our algorithm can be used to optimize the focal ring,as demonstrated experimentally at the 100 TW beamline at the Extreme Light Infrastructure-Nuclear Physics facility.

Ultra-compact post-compressor on-shot wavefront measurement for beam correction at PHELIX

In order to reach the highest intensities,modern laser systems use adaptive optics to control their beam quality.Ideally,the focal spot is optimized after the compression stage of the system in order to avoid spatio-temporal couplings.This also requires a wavefront sensor after the compressor,which should be able to measure the wavefront on-shot.At PHELIX,we have developed an ultra-compact post-compressor beam diagnostic due to strict space constraints,measuring the wavefront over the full aperture of 28 cm.This system features all-reflective imaging beam transport and a high dynamic range in order to measure the wavefront in alignment mode as well as on shot.