The first demonstration of laser action in ruby was made in 1960 by T. H. Maiman of Hughes Research Laboratories, USA. Many laboratories worldwide began the search for lasers using different materials, operating at different wavelengths. In the UK, academia, industry and the central laboratories took up the challenge from the earliest days to develop these systems for a broad range of applications. This historical review looks at the contribution the UK has made to the advancement of the technology, the development of systems and components and their exploitation over the last 60 years.

This dataset contains the raw data and analysis scripts for results presented in the paper. " Bright x-ray radiation from plasma bubbles in an evolving laser wakefield accelerator"

We are currently focusing on the improvement of contrast pedestal (CP) in the compressed laser pulse of PW Ti:Sapphire lasers. In our previous studies, we have identified the stretcher in our laser system as the source of CP. In order to underpin the true origins of CP, we have quantitatively characterised the surface quality of large optics used in the Gemini laser stretcher, where the laser beam is spatially dispersed and the spectral phase noise is induced by the optical surface roughness. We have measured the surface profiles of 2 different gold gratings, the new and old grating, and back mirror to a very high precision (~ a fraction of nm) by using ZYGO Dynafiz, with a spatial resolution of ~50µm over a width up to ~320mm, an unprecedented combination of very high spatial resolution with a very wide field of view. The surface roughness of the large curved mirror was determined experimentally. We have developed a simple physical model to deal with the influence of the surface roughness on the contrast pedestal. Based on the measured surface profiles and by taking the actual laser beam size into account, we are able to determine the spectral phase noise induced by the optical surface roughness in the stretcher. Consequently, we are able to accurately evaluate the impact of individual large optics in the stretcher and an overall impact of the stretcher on the contrast pedestal. The calculated contrast induced by both stretches with the new and old gratings are in an excellent agreement with the experimental results measured by the Sequoia scan. For the stretcher with the old grating, the grating is the dominant impact factor on the contrast. However, for the stretcher with the new gold grating of higher quality, the impact of the curved mirror on the contrast is comparable to that of grating. This implies that the influence of curved mirror on the contrast pedestal becomes more significant when the surface quality of grating is further improved. It is clearly observed that the impact of back mirror on the contrast is more than one order of magnitude lower than that of gratings and also much lower than that of curved mirror. In conclusion, we have demonstrated a novel method to evaluate the impact of large optics in the stretcher on the contrast pedestal by precisely quantitative characterization of optical surface quality. It is possible to accurately predict the contrast pedestal based on the stretcher configuration and precise characterisation of the optical surface in the stretcher prior to the construction of actual CPA high power laser system.

We describe the use of a genetic algorithm to apply active feedback to a laser wakefield accelerator at a higher power (10 TW) and a lower repetition rate (5 Hz) than previous work. The temporal shape of the drive laser pulse was adjusted automatically to optimize the properties of the electron beam. By changing the software configuration, different properties could be improved. This included the total accelerated charge per bunch, which was doubled, and the average electron energy, which was increased from 22 to 27 MeV. Using experimental measurements directly to provide feedback allows the system to work even when the underlying acceleration mechanisms are not fully understood, and, in fact, studying the optimized pulse shape might reveal new insights into the physical processes responsible. Our work suggests that this technique, which has already been applied with low-power lasers, can be extended to work with petawatt-class laser systems.

We report on the depletion and power amplification of the driving laser pulse in a strongly driven laser wakefield accelerator. Simultaneous measurement of the transmitted pulse energy and temporal shape indicate an increase in peak power from 187±11  TW to a maximum of 318±12  TW after 13 mm of propagation in a plasma density of 0.9×10^{18}  cm^{-3}. The power amplification is correlated with the injection and acceleration of electrons in the nonlinear wakefield. This process is modeled by including a localized redshift and subsequent group delay dispersion at the laser pulse front.

In this paper we review the design and development of a 100 J, 10 Hz nanosecond pulsed laser, codenamed DiPOLE100X, being built at the Central Laser Facility (CLF). This 1 kW average power diode-pumped solid-state laser (DPSSL) is based on a master oscillator power amplifier (MOPA) design, which includes two cryogenic gas cooled amplifier stages based on DiPOLE multi-slab ceramic Yb:YAG amplifier technology developed at the CLF. The laser will produce pulses between 2 and 15 ns in duration with precise, arbitrarily selectable shapes, at pulse repetition rates up to 10 Hz, allowing real-time shape optimization for compression experiments. Once completed, the laser will be delivered to the European X-ray Free Electron Laser (XFEL) facility in Germany as a UK-funded contribution in kind, where it will be used to study extreme states of matter at the High Energy Density (HED) instrument.

We describe how active feedback routines can be applied at a limited repetition rate (5 Hz) to optimize high-power (>10 TW) laser interactions with clustered gases. Optimization of x-ray production from an argon cluster jet, using a genetic algorithm, approximately doubled the measured energy through temporal modification of the 150 mJ driving laser pulse. This approach achieved an increased radiation yield through exploration of a multi-dimensional parameter space, without requiring detailed a priori knowledge of the complex cluster dynamics. The optimized laser pulses exhibited a slow rising edge to the intensity profile, which enhanced the laser energy coupling into the cluster medium, compared to the optimally compressed FWHM pulse (40 fs). Our work suggests that this technique can be more widely utilized for control of intense pulsed secondary radiation from petawatt-class laser systems.

The first two amplifiers of the Gemini laser system (pulse energy up to the Joule-level) operate at 10 Hz before being split into two alternating 5 Hz beams, which separately feed into TA2 and TA3. While the repetition rate of the Quantel pump lasers in the final Gemini amplifier limits TA3 operations to 0.05 Hz, TA2 can be operated at 5 Hz. Until now TA2 has reduced the repetition rate to a maximum of 1 Hz because of the practical challenges of increasing to the full capability. In this report we address these issues and describe the first experimental campaign conducted with the full 5 Hz repetition rate. Furthermore, this allowed us to employ active feedback routines [1] to directly optimize experimental parameters using controllable elements in the laser system.

We present an overview of the cryo-amplifier concept and design utilized in the DiPOLE100 laser system built for use at the HiLASE Center, which has been successfully tested operating at an average power of 1kW. Following this we describe the alterations made to the design in the second generation system being constructed for high energy density (HED) experiments in the HED beamline at the European XFEL. These changes are predominantly geometric in nature, however also include improved mount design and improved control over the temporal shape of the output pulse. Finally, we comment on future plans for development of the DiPOLE laser amplifier architecture.

We report a novel method to accurately evaluate the impact of stretcher gratings on the contrast pedestal of high power laser pulses by precise quantitative characterization of the grating surface. The results are in a good agreement with the experimental measurement. We have, for the first time, established a relation between the contrast pedestal and natural property of stretcher gratings.

Summary form only given. The nonlinear process of cross polarized wave (XPW) generation, which has been broadly used for temporal contrast enhancement and spectral broadening [1, 2], continue to attract interest in different application schemes. A feature of space-time focusing (STF), which occurs at the focus of spatially dispersed beam, is to deliver highly localized intensity at the focus without incurring high non-linear losses during long distance propagation. A number of nonlinear optical processes have been used to study and to utilize STF: two-photon fluorescence [3], harmonic generation [4], plasma formation [5] and refractive index modification in solids [6]. In this paper XPW generation in BaF 2 has been studied at the SST conditions of a compressed femtosecond pulse.

With recent progress in chirped pulse amplification (CPA) [1] technology, extreme laser power and intensity as high as ~10 W/cm has been made available by the new generation Petawatt (PW) class Ti:Sapphire amplifiers with an ultrashort pulse duration of around 30fs [2-7]. Ultra-high intensity lasers have proved to be a very powerful and efficient drive source to accelerate electrons to multi-GeV energies. Such lasers can also be used to accelerate protons to generate multi-MeV proton beams with high brightness and low emittance, paving the way towards compact sources for cancer therapy. There is also increasing interest in using ultra-intense lasers for high harmonic generation from solid density plasma to produce subfemtosecond coherent XUV/x-ray pulses with extreme brightness. The temporal contrast of the laser pulses plays a crucial role in these high field laser-matter interaction experiments [8-10]. Clean and high temporal quality laser pulses are essential to restrict any destructive preplasma dynamics, as excessive prepulse intensity can significantly modify, damage or even destroy the solid state targets due to formation of a preplasma prior to the arrival of the main pulse [11-13]. For these reasons, many workers have devoted significant effort to improving the contrast of CPA laser pulses.

We report on the successful demonstration of a 100 J-level, diode pumped solid state laser based on cryogenic gas cooled, multi-slab ceramic Yb:YAG amplifier technology. When operated at 175 K, the system delivered a pulse energy of 107 J at a 1 Hz repetition rate and 10 ns pulse duration, pumped by 506 J of diode energy at 940 nm, corresponding to an optical-to-optical efficiency of 21%. To the best of our knowledge, this represents the highest energy obtained from a nanosecond pulsed diode pumped solid state laser. This demonstration confirms the energy scalability of the diode pumped optical laser for experiments laser architecture.

We report the generation of 108 J pulses of duration 10 ns at 1 Hz with a conversion efficiency of 21% confirming the energy scalability of DiPOLE, a DPSSL based on cryogenically-cooled Yb:YAG amplifier technology.

Ultra-high intensity Ti:Sapphire lasers have proved to be a very powerful drive source to accelerate electrons and protons, producing ultrafast coherent X-ray pulses and high quality bright proton beams, where the temporal contrast of the driving laser pulses plays a crucial role in restricting any destructive preplasma dynamics prior to the main pulse. In our previous study, we have identified that the second grating in the stretcher is mainly responsible for the contrast pedestal (CP), as significant spectral phase noise was introduced by the grating surface roughness in a spatially dispersed beam [1]. We have also noted that for the same surface roughness level a transmission grating will generate ~4 times smaller spectral phase noise than a reflection grating, which should result in significant reduction in the CP. In order to experimentally explore the advantage of the transmission gratings, we have built up a transmission grating stretcher, based on a similar concept to that of the Gemini laser [Reference?] but on a smaller scale, and an identical gold grating stretcher with the same stretching factor. The stretched pulses were recompressed to a near transform-limited short pulse by a gold grating compressor, and the contrast of the compressed pulses was measured using a Sequoia as shown in Fig.1. Significant improvement in the CP by 1~2 orders of magnitude demonstrated the superior performance of the transmission grating over the reflective grating [2]. In order to investigate the effect of the grating surface roughness on the CP in detail, we measured the grating surface profiles using an interferometer. The measured peak-to-peak value of the spectral phase noise for the gold grating is ~0.56 radians with a standard deviation of 0.086, which is about 4 times larger than that of transmission grating. Based on the grating surface measurement, we have quantitatively analyzed the impact of the grating surface quality on the CP. The calculated temporal profiles of laser pulses are shown in Fig. 2, and are in relatively good agreement with the experimental results in Fig.1.

An ultra-short pulse reconstruction software, validated through a set of experimental measurements on the front-end of the Vulcan laser at the RAL Central Laser Facility is presented. The measurements were acquired in Target Area Petawatt of the Vulcan laser, both using a conventional autocorrelation technique and the GRENOUILLE technique in order to compare the results. The FWHM of the laser pulses considered came out to be comparable for the two techniques. In this experimental campaign for the first time a technique different from the autocorrelation one has been used for a PW class laser as Vulcan.

Space-time focusing of spatially-chirped laser pulses is used to generate a cross-polarized wave (XPW) in a single BaF2 crystal. We demonstrate 65 uJ output of XPW filter with improved spectral and temporal quality.

We propose, for the first time, a novel pulse stretcher for high power lasers using two transmission gratings and demonstrate its superiority over conventional, reflective grating stretchers in terms of pulse temporal quality. We show that, compared to a conventional stretcher with the same stretching factor, the transmission-grating based stretcher yields more than an order of magnitude improvement in the contrast pedestal.

Advances in X-ray imaging techniques have been driven by advances in novel X-ray sources. The latest fourth-generation X-ray sources can boast large photon fluxes at unprecedented brightness. However, the large size of these facilities means that these sources are not available for everyday applications. With advances in laser plasma acceleration, electron beams can now be generated at energies comparable to those used in light sources, but in university-sized laboratories. By making use of the strong transverse focusing of plasma accelerators, bright sources of betatron radiation have been produced. Here, we demonstrate phase-contrast imaging of a biological sample for the first time by radiation generated by GeV electron beams produced by a laser accelerator. The work was performed using a greater than 300 TW laser, which allowed the energy of the synchrotron source to be extended to the 10-100 keV range.

We propose, for the first time, a transmission grating stretcher for high power lasers and demonstrate its superiority over conventional, reflective gold grating stretchers in terms of pulse temporal quality. We show that, compared to a conventional stretcher with the same stretching factor, the transmission-grating based stretcher yields more than an order of magnitude improvement in the contrast pedestal. We have also quantitatively characterized the roughness of the grating surfaces and estimated its impact on the contrast pedestal.