Deterioration Of Beam Quality Research Articles

Mitigating transverse mode instability in fiber laser oscillator by employing direct pump modulation

The transverse mode instability (TMI) is a thermal-optical effect, inducing a sudden deterioration of beam quality and average power. This non-linear effect will greatly limit the power scaling of fiber lasers. We demonstrate a direct pump modulation to mitigate TMI in a 30 μm-core-diameter all-fiber laser oscillator. It only relies on function generator and pump drivers without the need of feedback control units. The groups of pump LDs and pump drivers and the phase difference between the channels of function generator have a great impact on the beam stability. Compared with the TMI threshold 180 W in continuous wave (CW) mode, the maximum average output power of a stabilized beam is increased up to 310 W via a multi-parameter-based direct pump modulation.

Commissioning the Muon Campus external beamline: first beam optics measurement

The Fermilab Mu2e experiment aims to observe charged lepton flavor violation through the direct conversion of a muon to an electron. This can be accomplished with the considerable challenge of manipulating and transporting large numbers of protons confined to a narrow phase space region without significant particle losses or deterioration of beam quality before reaching the production target. In this paper we report the first results from the commissioning the beamline upstream the Mu2e production target. We first discuss the evolution of the beam distribution in x-y plane after collecting a series of beam profiles along the line. Then, we discuss the beam evolution in phase-space by measuring the beam's Twiss parameters using two independent techniques. Finally, with the aid of numerical simulations we compare our measured data with design parameters and discuss similarities and differences observed.

Theoretical model and numerical study of effect of target reflected light on high-power fiber laser

<sec>In a high-power fiber laser system, the reflected light generated when the laser hits the target may be recoupled to the laser and amplified by the laser, thereby damaging the laser system. This situation is particularly serious for a high-power laser system, such as spectral beam combining a high power fiber laser, which lacks effective light-return protection. In order to solve the above problems, it is necessary to integrate various physical effects in the whole system link, evaluate and analyze the influence of reflected light on the operating state of the system, so as to optimize the optical path layout and system structure in the beginning of the design of fiber laser to avoid unnecessary losses. Based on the atmospheric transmission theory, fiber rate equation and medium heat conduction equation model, the effect of reflected light on high-power fiber laser is analyzed. In this paper is conducted the numerical simulation of coupling efficiency of reflected light of high-power fiber laser. It is found that under certain atmospheric conditions, the reflected power is related to the transmission distance, the offset angle of optical axis, divergence angle, and the offset of center position of the beam, and will affect the output power, beam quality factor, thermal effect and the signal-to-noise ratio of the stimulated Raman scattering spectrum of the fiber laser. The coupling efficiency of reflected light has a maximum value at a certain transmission distance. For example, the reflected light power up to 140 W is obtained when the transmission distance is 1500 m, which will largely affect the laser system. The reflected power is affected by the offset angle of optical axis far less than by the transmission distance when transmission increases from 500 m to 2000 m. For example, a change of less than 0.1 W can be obtained when offset angle increases from 0.11° to 0.44°. It is also shown that as the divergence angle changes from 0 to 15<i>''</i>, the coupling power decays nearly exponentially with the divergence angle. The coupling efficiency is close to 1% near 12<i>''</i>, which is almost negligible. The output beam quality of the laser system is also affected by the beam quality of reflected light. Deterioration of the beam quality of reflected light will lead to the deterioration of the laser output beam quality. As the reflected light power enters the fiber laser system and increases, the forward output power will decrease and the backward signal power will increase, and the Raman power will increase rapidly near the fiber output end. When the reflected light is present, the thermal effects caused by selecting the gain fiber with different pump absorption coefficients are very important. The stimulated Raman scattering (SRS) rejection ratio decreases with the increase of pump absorption coefficient. For example, the SRS rejection ratio decreases by 5 dB when pump absorption coefficient increases from 1.5 dB/m to 4.5 dB/m, resulting in a decrease in the signal-to-noise ratio of the laser, which will reduce the reliability of the fiber laser system.</sec><sec>In the design and test of spectral beam combining systems for high-power fiber lasers, the attention should be paid to the reflected light in the entire process, which includes the outer optical path and the internal optical path of the laser. The comprehensive constraints of multiple key indicators are analyzed, and the probability of system damage or reliability degradation due to reflected light is evaluated. The research results of this paper are of certain guiding significance in selecting suitable outer optical path layout and system parameters and also in optimizing the design of high energy fiber laser system.</sec>

Dynamic effects of temperature-dependent aberration factors on the beam quality of LD end-pumped lasers

Considering the temperature dependence of the thermal conductivity of the gain medium, the aberration factor of the thermal lens becomes a function of temperature. The influence of the aberration factor on critical spot size is analyzed, the actual value of critical spot size is recalculated, and the range of beam quality deterioration is re-pointed. An asymmetric flat cavity experiment is built, and a symmetrical flat cavity is used as the supporting experiment. The widened beam quality degradation range and the dynamic operating point of the fundamental mode beam are pointed out. Experimental results match the simulation.

Accurate thermal stress induced birefringence measurement of laser crystal with cavity ring-down technique

One of the main sources of beam quality deterioration encountered in high-power diode-pumped solid-state lasers is the thermal stress induced birefringence. Cavity ring-down (CRD) technique is applied to the accurate measurement of thermal stress induced birefringence of laser crystal under pump diode laser illumination. Measurements of the pump laser illumination induced temperature profile and thermal stress induced birefringence retardance distribution in the laser crystal are performed and compared to numerical calculations to determine the absorption and thermal diffusivity of the laser crystal and further the relevant stress-optical constant. The experimental results demonstrated that CRD is a highly sensitive and accurate technique to measure the thermal stress induced birefringence and relevant stress-optical constant of laser crystals, which is helpful for the evaluation and optimization of the performance of high-power laser systems.

Experimental Study on Transverse Mode Instability Characteristics of Few-Mode Fiber Laser Amplifier Under Different Bending Conditions

We studied the transverse mode instability (TMI) characteristics of few-mode fiber laser amplifier employing ytterbium-doped fiber (YDF) with core/cladding diameter of 30/400 μm under different bending diameters. The gain fiber is coiled in ellipse shape with minimum bending diameter changed from 9 cm, 10 cm, 11 cm to 12 cm in the bending part, and the experimental results show an anomaly characteristic of TMI. The TMI threshold improved from 772 W, 1149 W and 1458 W to 1641 W when the bending diameter of YDF increase from 9 cm, 10 cm, 11 cm to 12 cm, accompanied by the deterioration of beam quality from 1.60, 1.67, 1.87 to 1.95. This is mainly because tightly bending in few-mode fiber will promote the overlap between modes and lead to mode coupling, which may trigger TMI. Besides, the bending losses of HOMs decrease while bending diameter increasing, so that there is higher output efficiency and deteriorated beam quality when bending diameter increase. Although the phenomenon is not agreed with common TMI with near single-mode fiber laser, the result will be helpful for the design and power scaling of few-mode fiber lasers.

Comparison of spectral beam combining based on reflecting volume Bragg grating, with and without collimating lenses

AbstractOn the basis of the principle of spectral beam combining (SBC), a simple combining system using a reflecting volume Bragg grating as a combiner was designed in which two focusing lenses were used to control the incident angles of input beams, and two concave lenses were used to solve the problem of beam divergence caused by focusing lenses. The effects of the beam divergence angle (DA) on diffraction efficiency and beam quality were analyzed theoretically. The simulation results showed that a larger DA will reduce diffraction efficiency and lead to deterioration of beam quality. In the SBC experiment, the performances of the output beam with and without concave lenses were compared. The experimental results showed that, when the concave lenses were not inserted, the measured output power was 503 mW with a combining efficiency of 85.3% and the beam quality factor of 1.68. The experiment further showed that the combined power was 566 mW with a combining efficiency of 96% and a beam quality factor of 1.31 when concave lenses were used.

Mode Evolution of High Power Monolithic PM Fiber Amplifiers in the Presence of SRS Effect

In this work, mode evolution in the presence of stimulated Raman scattering (SRS) effect has been studied in a 2 kW polarization-maintained (PM) fiber amplifier. It proved that the SRS-induced mode distortion can be observed at 40 dB of signal to Raman noise ratio in a PM fiber amplifier, which is significantly lower than that in a non-PM system. By using mode decomposition (MD) method, the SRS-induced mode distortion has been analyzed quantitatively by decomposing the signal and the Raman light individually, and the signal mode evolution shows a dissimilar phenomenon compared with the SRS-induced transverse mode instability. The SRS effect in large mode area fiber lasers result in continuous excitation of signal high order modes (HOMs), which leads to the signal beam quality deterioration and the output power rollover. The power fraction of HOMs increases by 30% with the Raman power ratio only being 1.67% at the maximal output power, and the Raman Stokes light mainly exists in fundamental mode distribution, which can be attributed to the beam cleanup effect.

Theoretical and experimental research on crosstalk in spectral beam combining of a laser diode bar with a high fill factor.

Spectral beam combining is an important way to improve the brightness of semiconductor laser beams. For a spectral beam combining system, crosstalk between different emitters would cause the deterioration of beam quality and the reduction of beam combining efficiency, especially for the laser diode bar with a high fill factor. In this paper, an analysis model of the spectral beam combining system with crosstalk is established. The most important advantage of this model is that it can analyze the spectral distribution of the combined beam and also give a relatively good estimation for the beam quality parameters, such as beam size and far-field divergence angle. This model is verified by the experimental results. Furthermore, based on the theoretical model, a method for eliminating crosstalk is developed. By introducing a spatial filter inside the grating external cavity, the crosstalk between different emitters is blocked in the far field, and the beam quality is improved. In the experiment of beam combining of five emitters, after crosstalk is eliminated, the divergence angle of the combined laser beam is reduced from 10.09 to 4.73 mrad, the beam parameter product is reduced from 2.95 to 0.91 mm⋅mrad, and the power of the main lobe is improved from 1.77 to 2 W.

Active elements in the form of thin rods of square cross section for multichannel laser amplifiers

We study thermally induced phase distortions of laser radiation in active elements in the form of thin rods of square cross section. The absence of aberrations and beam quality deterioration is demonstrated for these elements, due to which they can be used along with rods having a round cross section, differing from the latter by much higher technological efficiency of fabrication and mounting in the cooling system. A concept of compact and reliable multichannel solid-state amplifier based on such active elements is proposed, which allows one to use a multichannel fibre laser as a master system.

Excellent performance of a cryogenic Nd:YAlO3 laser with low wavefront distortion based on zero thermal expansion.

High-power solid-state lasers with good beam quality are attracting great attention on account of their important applications in industry and military. However, the thermal effects generated in the laser host materials seriously limit power scaling and degrade the beam quality. Thermal lensing and thermally induced wavefront deformation are the main causes of the beam quality deterioration. Here we investigate the performance of a zero thermal expansion (ZTE) solid-state laser gain material. In a proof-of-principle experiment, an ${a}$-cut rod ${\rm Nd}\!:\!{{\rm YAlO}_3}$ (Nd:YAP) perovskite crystal is chosen to be the gain medium for ZTE around 180 K. The laser performance spanning the temperature range from 80 to 290 K is studied. The maximum output power and minimum threshold pump power were obtained at a temperature of 180 K. Moreover, the measured thermal focal power and peak-to-valley value of the wavefront distortion also reach a minimum at this temperature, an additional benefit from the crystal's ZTE coefficient. We envisage that these results will open a new route towards the development of high-power and high-beam-quality lasers through the use of ZTE gain materials.

Synchrobetatron resonance of crab crossing scheme with large crossing angle and finite bunch length

Crab crossing scheme is an essential collision scheme to achieve high luminosity for the future colliders with large crossing angles. However, when bunch length of one or both colliding beams is comparable with the wavelength of the crab cavity voltage, the nonlinear dependence of the crabbing kick may present a challenge to the beam dynamics of the colliding beams and impact the beam quality as well as the luminosity lifetime. In this paper, the results of nonlinear dynamics in the crab crossing scheme are presented, using both analytical and numerical studies. The result indicates that higher-order synchro-betatron resonances may be excited in the crab crossing scheme with large crossing angle, which causes the beam quality deterioration and luminosity degradation. The studies also reveal possible countermeasures to suppress the synchro-beta resonance, hence mitigate the degradation of beam quality and luminosity.

Experimental research on transmission performance on OAM beam and Gaussian beam in atmospheric turbulence channel

The transmission performance of orbital angular momentum(OAM) beam and Gaussian beam in atmospheric turbulent channel was studied and compared experimentally. The beam spreading, pointing error, power jitter and bit error rate of the two beams were measured respectively, the OAM beam loaded with modulated signals and the Gaussian beam were as carrier. The experimental results show that in the atmospheric turbulence channel, compared with Gaussian beam, beam spreading ratio of the OAM beams reduces by 10.5%, variance of power jitter falls 0.13, dispersion circle diameter of the pointing error decreases by 30.4%, and more focused on the distribution of the center of the beam axis. The lowest detection sensitivity of the OAM beam carrier system reaches −28.97 dBm, which improved 2.5 dB compared with that of Gaussian beam. The experimental results verify that OAM beam is less affected by turbulence than Gaussian beam in atmospheric turbulence channel, and the deterioration of beam quality of OAM beam is much less than Gaussian beam with the increase of turbulence intensity. The results of the experiment provide a reference for the development and application of free space laser communication in atmospheric turbulent channel.

Analysis of temperature-induced liquid crystal phase control beam quality deterioration

Analysis of temperature-induced liquid crystal phase control beam quality deterioration

Monte Carlo-based analysis of the photon beam fluence with air gap thickness between Linac head exit window and patient’s skin in radiotherapy treatments

Linear accelerators (Linac) are used in radiation therapy treatment and its technology improvement ensures high dosimetry quality that should be conserved for high radiotherapy efficiency. However, does the air gap between the exit window of Linac head and patient’s skin alters the physical properties of the photon beam? The objective of this study is to assess the physical properties changes of photon beam fluence according to air gap thickness under the Linac head. The air gap under the Linac head is the last material in the photon beam path; it induces alterations in the beam quality before reaching the patient’s skin. The Varian Clinac 2100 head and the air gap up to the phantom surface are modelled using Monte Carlo BEAMnrc code; the nominal beam energy is 6 MV. The BEAMDP code is used to extract the photon fluence. The photon beam fluence is affected by the air gap under Linac head and decreases by six times due to the photon beam attenuation with air gap thickness; in addition to increasing of beam contamination by scattered photons and electrons. Thus, the air gap induces the beam quality deterioration which is evaluated in terms of photon fluence with air gap thickness. To remove the particles contaminations and conserve integrally the photon beam quality, the number of the photon interactions with air atoms should be as low as possible under Linac head up to patient’s skin and ensure a higher quality of the radiotherapy treatment of deep tumour.

Experimental study on spherical aberration and beam quality factor of LD end-pumped laser

A method for judging the locations of dynamic stable zones boundaries and mode separations is presented by spherical aberration “transient variation” Experiments on spherical aberration in dynamic stable zones of an end-pumped laser are carried out. Based on the inversion of the wavefront information, the “beam quality deterioration zone” is investigated. The multiple relationships among the thermal focal length of the low-order modes are calculated. Based on the discussion of experimental results, the changing law of the sign of spherical aberration is studied. A theoretical model of the relation among the cavity configuration, spherical aberration and beam quality can be established in addition. The experimental results agree well with the theory.

Thermo-optic behaviour of intrinsically single-mode low-numerical aperture (NA) ytterbium-doped fibre

A simplified model based on the finite-element method is proposed to study the thermo-optic behavior in high-power fiber amplifiers. It is shown that thermally induced refractive index change results in a thermal lens effect and ultimately leads to multimode behavior even in an intrinsically single-mode fiber. Besides, experimental investigations were carried out to characterize the thermo-optic behavior of an intrinsically single-mode large mode area 20/400 ytterbium-doped fiber with core numerical aperture (NA) ~ 0.04 in an all-fiber amplifier. Output power up to 1.68 kW was achieved with a deterioration of beam quality from M2 < 1.1 to 1.7. It is explained that due to the uneven distribution of axial heat load, an intrinsically single-mode fiber gradually turns into a combination of a multi-mode fiber and a single-mode fiber with the growing heat load and then the leakage core modes trigger transverse mode instability. Moreover, a near diffraction-limited output power of 1.35 kW from an all-fiberized low-NA fiber amplifier is obtained.

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.

Four-wave mixing effect on high-power continuous-wave all-fiber lasers

A four-wave mixing effect on high-power continuous-wave fiber lasers has been demonstrated theoretically and experimentally. Detailed theoretical description of phase matching is presented and we found that the phase matching condition is satisfied at the frequency shift of 5.16 THz. While the intensity in fiber core region is more than about 394 MW/cm2, the four-wave mixing products of 1100 nm and 1060 nm were also observed in high-power all-fiber laser. The comparison shows that the experiment result is in good agreement with the simulation result. In addition, the beam quality deterioration for the laser is caused by the four-wave mixing effect and the mode instability. The [Formula: see text] factor measured at maximal intensity of 478 MW/cm2 is 2.80.

Thin-film filter wavelength-stabilized, grating combined, high-brightness kW-class direct diode laser

We report on kW-class dense wavelength beam combining of a laser diode module consisting of ten broad-area laser diode bars by using a novel multi-laser cavity approach based on a thin-film filter (TFF) as a dispersive optical element. The wavelength-stabilized output of the TFF cavity is beam combined upon a -1st order transmission grating. Hereby, a cylindrical telescope is used for linear dispersion-matching between the TFF and the combiner grating. On the basis of simulations of the resulting beam quality deterioration, we are able to optimize the cavity and the combiner setup for optimal beam quality preservation. We demonstrate a highly efficient direct diode laser with 1.1-kW output power and a symmetrical beam parameter product of about 6mm × mrad (95 % power content) in both beam axis.