Innovative freeform design methodology used for high efficiency coherent beam combination design study

The influence of turbulence on the coherent and incoherent combinations of off-axis Gaussian beams is studied in detail. The analytical propagation equation of the resulting beam in turbulence for the incoherent combination is derived. The mean-squared beam width, the power in the bucket and the β parameter are taken as the characteristic parameters of beam quality to compare the results of coherent and incoherent combinations of off-axis Gaussian beams propagating through atmospheric turbulence, and the reasonable physical explanations are also given. It is shown that, on one hand, both for the incoherent and coherent combinations the turbulence results in a beam spreading and a degradation of the maximum intensity, and the larger the individual beam numbers is, the less sensitive is the resulting beam to the effect of turbulence. On the other hand, the resulting beam for the incoherent combination is less affected by turbulence than that for the coherent combination.

Persistence and learning effects in design innovation: Evidence from panel data

Coherent beam combination laser system using SBS-PCM for high repetition rate solid-state lasers

Coherent beam combination of two thulium-doped fiber laser beams with the multi-ditheringtechnique

In order to obtain focal spot with high power and spot size comparable to wavelength scale, a novel approach to achieving the coherent combination of beam array by tightly focusing is proposed. The physical model of coherent beam combination of beam array via tightly focusing is built up by the use of the vector diffraction integral. Therefore the influences of beam configuration, polarization state, beam width, beam interval and numerical aperture of the tight focusing system on the characteristics of the combined beam are discussed in detail. The results indicate that the coherent combination effect of beam array with linear and circular polarization via tight focusing is the first best, and that with the radial polarization is the second best but that with the azimuthal polarization is the worst. The beam array of linear and circular polarization with rectangle configuration can be tightly focused onto center point, and the beam array with hexagon is also focused onto center point but with lower efficiency. In addition, by enlarging the beam width and the beam interval to a certain extent, the combination efficiency can be increased. By optimizing the beam configuration, beam width and interval, and selecting rational numerical aperture of the tightly focusing geometry, the focal spot with high energy concentration can be obtained with high beam quality and combination efficiency.

<sec>Ultrafast fiber laser sources with mJ-level pulse energy and kilo-watt average power are of particular importance for various science fields such as attosecond lasers. Currently, several large-scale facilities for attosecond lasers, including ELI-ALPS in Europe, SECUF in China, NeXUS in America and ALFA in Japan are under construction. High-performance femtosecond driven lasers are crucial for attosecond lasers and various ultrafast laser facilities. Fiber lasers have a large surface-to-volume ratio, which enables efficient cooling and is suitable for high average power amplification. However, due to small mode area of optical fibers, detrimental nonlinear optical effects such as self-phase modulation, four-wave mixing, and stimulated Raman scattering limit the peak power of pulse to hundreds of MW, corresponding to pulse energy of hundreds of μJ for femtosecond pulses in large mode area rod-type fibers. In addition, the average power of fiber lasers is limited by transverse mode instability, which reduces the stability and quality of beams above a certain threshold. In rod-type fibers, the threshold is about 250 W. Neither average power nor pulse energy emitted by single fiber meets the requirement for attosecond laser generation.</sec><sec>The average power and pulse energy can be further scaled by coherent beam combination, which involves splitting pulses caused by an frontend laser and recombining them after amplification. It is essential for coherent beam combination to maintain the coherence of pulse replicas, which usually involves high speed photodiode detectors, piezo-driven mirrors, and other electronics forming a feedback system to actively control the phase of all replicas. We present a high-energy high-power ultrafast fiber laser system by using filled-aperture coherent combination of four ytterbium-doped rod-type fiber amplifiers. The phase control is achieved by using stochastic parallel gradient descent method. The frontend includes a passively mode-locked Yb-fiber oscillator, a stretcher, a pulse picker, and three fiber pre-amplifiers, which delivers 1 MHz stretched pulses centered at 1032 nm with 700 ps duration and 20 W average power. The pulse is split into four replicas by polarization beam-splitter and half-wave plate pairs, and the replicas pass through delay lines formed by piezo-driven mirrors before amplification. The pulse replicas are equally split and amplified to ensure the same accumulated nonlinear phase, and are combined by thin film polarizer and half-wave plate pairs. A small portion of the combined pulse is split and collected by a photodiode detector after being filtered spectrally and spatially, serving as a signal for controlling phase. The combined pulse is compressed by a compressor using a double-pass diffraction grating pair consisting of two 1739 l/mm gratings.</sec><sec>At a repetition rate of 1 MHz, our four-channel Yb-fiber coherent beam combination system generates a combined average power value of 753 W and a combination efficiency of 87%. By utilizing an adjustable pulse stretcher and compressor, a 0.67 mJ, 242 fs near transform-limited pulse can be generated with a compressing efficiency of 89%. The compressed pulse is centered at 1032 nm, and the spectrum width is 8.8-nm. In the 30 min measurement, the root-mean-square of average power is less than 1% , while the residual phase error is less than <i>λ</i>/23, indicating excellent stability on different time scales. The beam quality factor of the 0.67 mJ compressed pulses is 1.17×1.11. At 500 kHz, we obtain pulses of 1.07 mJ and 247 fs with average power of 534 W, exhibiting similar efficiency, long-term stability, and beam quality. The residual phase error decreases below <i>λ</i>/29, indicating better short-term stability. Further scaling power and energy can be achieved by increasing the number of channels. By adding the delay stabilization system and pointing stabilization system, which are currently under development, an eight-channel CBC system can be used to generate 1 kW, 2 mJ pulses.</sec><sec>In this work, we implement a four-channel coherent beam combining system based on the SPGD method, and obtain compressed pulses of 673 W, 673 µJ, and 242 fs at 1 MHz and 534 W, 1.07 mJ, and 247 fs at 500 kHz. Both power and energy can be further improved by increasing the channel number, and adding the delay stabilization system and pointing stabilization system which are under construction. By adding coherent pulse stacking amplification technology, the coherent beam combining system ought to generate pulse energy as high as 100 mJ, which constitutes the energy source for applications such as laser wake-field acceleration.</sec>

In this Letter, a first (to our knowledge) demonstration of phase conjugate self-organized coherent beam combination is reported. A demonstration involving combination of two self-adaptive gain grating holographic laser resonators is presented. Output powers of up to 27 W, with a combination efficiency of 94%, are demonstrated. This technique coherently combines individual phase conjugate laser modules, which do not have predefined spectral or spatial modes. This removes the problem of finding shared resonator modes, which should allow coherent beam combination of much larger laser arrays than standard self-organized coherent beam combination.

We demonstrate the coherent beam combination of a hexagonal distributed high power fiber amplifier array. Six polarization-maintained fiber amplifiers are tiled into a hexagonal array by use of a novel beam combiner. The phase control signal for fiber amplifiers is generated by running a stochastic parallel gradient descent algorithm on a digital signal processor. The coherent beam combination with a total output power of 73.5 W is demonstrated. The contrast of the coherent combined beam profile is as high as 81% when the system is closed-loop controlled.

Michelson cavity technique provides an effective solution for coherent beam combination. In this paper, coherent beam combination of two fiber lasers by using an all-fiber Michelson cavity technique is demonstrated, and 11.75 W coherent output power with a coherent combination efficiency of 95.76% is obtained. The influences of the relation of coherent combination efficiency to laser cavity length difference, and the pump symmetry on coherent combination efficiency are studied experimentally. Experimental results show that the coherent combination efficiency is highly sensitive to the influence of laser cavity length difference, that the difference in combinination efficiency, induced by laser cavity length difference, can be as large as 18%, and that there is an optimal cavity length difference. The maximal influence of pump symmetry on coherent combination efficiency is within 2%, which can be neglected.

The development of high average power lasers is an area of major research and commercial effort. Moving to high average powers has been thwarted by thermal induced refractive index changes which limit the maximum attainable power at high beam quality. Coherent beam combination (CBC) has been an area of significant research effort as it offers the ability to run multiple low power, high quality, lasers which are then combined to produce one high quality, high brightness source. We demonstrate for the first time a novel system which employs both phase conjugation and self-organisation in order to coherently combine two self starting adaptive laser oscillators with high combination efficiencies. The advantage is that combination is performed passively without the requirement for active adjustment of phase shifting elements with computer control algorithms.

The single frequency dithering technique for coherent beam combination with one single modulation frequency and without a reference beam is presented for the first time to our knowledge. Coherent beam combination of four fiber amplifiers is successfully demonstrated by using the single-dithering technique. When the phase control system is in the closed loop, the fringe contrast of far-field intensity pattern is improved by more than 85% from 8% in open loop, and the residual phase error is less than lambda/20. Experimental results reveal that the single-dithering technique has great potential in scaling to a large number of beamlets with a simple phase control system and low cost.

Coherent beam combination has emerged as a promising strategy for overcoming the power limitations of individual fibre lasers. This approach relies on maintaining precise phase difference between the constituent beamlets, which are typically established using phase retrieval algorithms. However, phase locking is often studied under the assumption that the power levels of the beamlets remain stable, an idealisation that does not hold always in practical applications. Over the operational lifetime of fibre lasers, power degradation inevitably occurs, introducing additional challenges to phase retrieval. To address this, we propose a deep learning algorithm for single-step simultaneous phase and amplitude identification, directly from a single camera observation of the intensity distribution of the combined beam. By leveraging its ability to detect and interpret subtle variations in intensity interference patterns, the deep learning approach can accurately disentangle phase and power contributions, even in the presence of significant power fluctuations. Using a spatial light modulator, we systematically investigate the impact of power-level fluctuations on phase retrieval within a simulated coherent beam combination system. Furthermore, we explore the scalability of this deep learning approach by evaluating its ability to achieve the required phase and amplitude precision as the number of beamlets increases.

The coherent beam combination of kW fiber lasers with a filling aperture has been in research. An experiment is set up to achieve the coherent beam combination. The coherent beam combination of two fiber lasers with a filling aperture is realized, and the phase bandwidth of the beam combination is measured and analyzed. The laser spots before and after the combination are detected respectively. The phase noise of high power laser is measured and the factors affecting the phase noise are analyzed.

Jeanne Sacerdote, known as Madame Jenny, was one of many French women fashion designers in the first part of the twentieth century. After training at Pacquin and Bechoff-David, well-established French fashion houses, she opened House of Jenny in 1908. The French government in 1926 awarded the Chevalier, Legion of Honor to Jenny for her economic service. Her business weathered the First World War and the worst part of the world wide economic depression in the 1930s. It closed in 1940. Costume historians have mostly overlooked Jenny's success. This research defined Jenny's contributions to the French and American fashion business. The analysis used about 1000 images of Jenny's work and the work of the seven contemporaries; Chanel, Drecoll, Lanvin, Molyneux, Patou, Premet and Vionnet, to determine if she was innovative. The study of these contemporary dress designers indicated that she produced some innovative designs especially in the first twenty years. The research evaluated fashion reports, advertisements, and extant Jenny garments. Women's Wear Daily 's collection reports indicate that Jenny was a favorite with American manufacturers because her styles were easy to copy. The fashion reports and ads from American periodicals imply a high acceptance of her work by buyers and magazine editors. The study of the design and construction of Jenny garments evaluated the aesthetics of Jenny's work. Jenny's designs were wearable, but included details that made them unique. Her style was most like Chanel and Patou in its simplicity, but Jenny was creating uncluttered styles years before the House of Chanel was famous for them. The findings suggest that she was a very good businesswoman, making money when few others Paris dressmakers were doing so. Many scholars have overlooked Jenny.

Analysis of the multi-mismatch effect on coherent polarization beam combination system by using a multivariate-model