Chirped pulse amplification
Chirped pulse amplification is a technique for amplifying an ultrashort laser pulse up to the petawatt level with the laser pulse being stretched out temporally and spectrally prior to amplification. CPA is the current state-of-the-art technique used by all of the highest-power lasers in the world, with the exception of the ≈500 TW National Ignition Facility; some examples of these lasers are the Vulcan laser at the Rutherford Appleton Laboratory's Central Laser Facility, the Diocles laser at the University of Nebraska–Lincoln, the GEKKO XII laser at the GEKKO XII facility in the Institute for Laser Engineering at Osaka University, the OMEGA EP laser at the University of Rochester's Lab for Laser Energetics and the now dismantled petawatt line on the former Nova laser at the Lawrence Livermore National Laboratory. Apart from these state-of-the-art research systems, a number of commercial manufacturers sell Ti:sapphire-based CPAs with peak powers of 10 to 100 gigawatts. CPA for lasers was introduced by Donna Strickland and Gérard Mourou at the University of Rochester in the mid-1980s, work for which they received the Nobel Prize in Physics in 2018.
Before the peak power of laser pulses was limited because a laser pulse at intensities of gigawatts per square centimeter causes serious damage to the gain medium through nonlinear processes such as self-focusing. For example, some of the most powerful compressed CPA laser beams in an unfocused large aperture can exceed intensities of 700 GW/cm2, which if allowed to propagate in air or the laser gain medium would self-focus and form a plasma or cause filament propagation, both of which would ruin the original beam's desirable qualities and could cause back-reflection damaging the laser's components. In order to keep the intensity of laser pulses below the threshold of the nonlinear effects, the laser systems had to be large and expensive, the peak power of laser pulses was limited to the high gigawatt level or terawatt level for large multi-beam facilities. In CPA, on the other hand, an ultrashort laser pulse is stretched out in time prior to introducing it to the gain medium using a pair of gratings that are arranged so that the low-frequency component of the laser pulse travels a shorter path than the high-frequency component does.
After going through the grating pair, the laser pulse becomes positively chirped, that is, the high-frequency component lags behind the low-frequency component, has longer pulse duration than the original by a factor of 1000 to 100000. The stretched pulse, whose intensity is sufficiently low compared with the intensity limit of gigawatts per square centimeter, is safely introduced to the gain medium and amplified by a factor of a million or more; the amplified laser pulse is recompressed back to the original pulse width through reversal of the process of stretching, achieving orders-of-magnitude higher peak power than laser systems could generate before the invention of CPA. In addition to the higher peak power, CPA makes it possible to miniaturize laser systems. A compact high-power laser, known as a tabletop terawatt laser, can be created based on the CPA technique. There are several ways to construct stretchers. However, a typical Ti:sapphire-based chirped-pulse amplifier requires that the pulses are stretched to several hundred picoseconds, which means that the different wavelength components must experience about 10 cm difference in path length.
The most practical way to achieve this is with grating-based compressors. Stretchers and compressors are characterized by their dispersion. With negative dispersion, light with higher frequencies takes less time to travel through the device than light with lower frequencies. With positive dispersion, it is the other way around. In a CPA, the dispersions of the stretcher and compressor must cancel out; because of practical considerations, the compressor is designed with negative dispersion, the stretcher is therefore designed with positive dispersion. In principle, the dispersion of an optical device is a function τ, where τ is the time delay experienced by a frequency component ω; each component in the whole chain from the seed laser to the output of the compressor contributes to the dispersion. It turns out to be hard to tune the dispersions of the stretcher and compressor such that the resulting pulses are shorter than about 100 femtoseconds. For this, additional dispersive elements may be needed.
Figure 1 shows the simplest grating configuration, where long-wavelength components travel a larger distance than the short-wavelength components. Only a single grating is used, with extra mirrors such that the beam hits the grating four times rather than two times as shown in the picture; this setup is used as a compressor, since it does not involve transmissive components that could lead to unwanted side-effects when dealing with high-intensity pulses. The dispersion can be tuned by changing the distance between the two gratings. Figure 2 show
Gaius Appuleius Diocles
Gaius Appuleius Diocles was a Roman charioteer. Gaius Appuleius Diocles was born in 104 A. D in Lamecum, the capital city of Lusitania, province of Emerita Augusta, his father owned the family was comparatively well off. Diocles is believed to have started racing at the age of 18 in Ilerda, This first notable victory outside his native land brought him international fame and encouraged him to go to Rome, and gained a reputation good enough to get himself called up to the ‘big leagues’ at the capital of the Roman empire. He became known as the Lamecus and henceforth brought fame and renown to his native ancient city of Lamecum. Within the city, a statue was erected on top the fountain in front of the garden known today as Jardim do Campo, located in the centre of town, he most raced four-horse chariots and in most of his races he came from behind to win. Diocles is notable for owning an rare ducenarius, a horse that had won at least 200 races. Records show that he won 1,462 out of the 4,257 four-horse races he competed in and was placed in an additional 1,438 races.
The ‘champion of charioteers’ is one of the best-documented ancient athletes, most because he was such a star at the famous Roman Circus Maximus. Being the best in the field seems to have allowed Diocles to perfect his showmanship. Many of his victories took the form of a ‘come from behind’ crossing of the finish line at the last possible moment; the crowds loved it. Any race with Diocles became the ‘featured event’ of the day; this helped Diocles make more money. His winnings totaled 35,863,120 sesterces, equivalent to 358,631.20 gold aureus or 2,600 kg of gold over $15 billion in today's dollars based on the cost of living and the median wage of the time, an amount which could provide a year's supply of grain to the entire city of Rome, or pay the Roman army at its height for a fifth of a year. Classics professor Peter Struck describes him as "the best paid athlete of all time". Wealth of today's sports stars is'no match for the fortunes of Rome's chariot racers', Murray Wardrop, Telegraph.co.uk, 13 Aug 2010 Charioteers and Racing Factions The Origins and Evolution of the Lusitano "Roman Life", Mary Johnston, 1957 30November 2016 KerrySullivan Gaious Appuleius Diocles
Diocletian, born Diocles, was a Roman emperor from 284 to 305. Born to a family of low status in Dalmatia, Diocletian rose through the ranks of the military to become Roman cavalry commander to the Emperor Carus. After the deaths of Carus and his son Numerian on campaign in Persia, Diocletian was proclaimed emperor; the title was claimed by Carus' surviving son, but Diocletian defeated him in the Battle of the Margus. Diocletian's reign marks the end of the Crisis of the Third Century, he appointed fellow officer Maximian as Augustus, co-emperor, in 286. Diocletian reigned in the Eastern Empire, Maximian reigned in the Western Empire. Diocletian delegated further on 1 March 293, appointing Galerius and Constantius as Caesars, junior co-emperors, under himself and Maximian respectively. Under this'tetrarchy', or "rule of four", each emperor would rule over a quarter-division of the empire. Diocletian purged it of all threats to his power, he defeated the Sarmatians and Carpi during several campaigns between 285 and 299, the Alamanni in 288, usurpers in Egypt between 297 and 298.
Galerius, aided by Diocletian, campaigned against Sassanid Persia, the empire's traditional enemy. In 299 he sacked Ctesiphon. Diocletian achieved a lasting and favourable peace. Diocletian separated and enlarged the empire's civil and military services and reorganized the empire's provincial divisions, establishing the largest and most bureaucratic government in the history of the empire, he established new administrative centres in Nicomedia, Mediolanum and Trevorum, closer to the empire's frontiers than the traditional capital at Rome. Building on third-century trends towards absolutism, he styled himself an autocrat, elevating himself above the empire's masses with imposing forms of court ceremonies and architecture. Bureaucratic and military growth, constant campaigning, construction projects increased the state's expenditures and necessitated a comprehensive tax reform. From at least 297 on, imperial taxation was standardized, made more equitable, levied at higher rates. Not all of Diocletian's plans were successful: the Edict on Maximum Prices, his attempt to curb inflation via price controls, was counterproductive and ignored.
Although effective while he ruled, Diocletian's tetrarchic system collapsed after his abdication under the competing dynastic claims of Maxentius and Constantine, sons of Maximian and Constantius respectively. The Diocletianic Persecution, the empire's last and bloodiest official persecution of Christianity, failed to eliminate Christianity in the empire. Despite these failures and challenges, Diocletian's reforms fundamentally changed the structure of Roman imperial government and helped stabilize the empire economically and militarily, enabling the empire to remain intact for another 150 years despite being near the brink of collapse in Diocletian's youth. Weakened by illness, Diocletian left the imperial office on 1 May 305, became the first Roman emperor to abdicate the position voluntarily, he lived out his retirement in his palace on the Dalmatian coast. His palace became the core of the modern-day city of Split in Croatia. Diocletian was born near Salona in Dalmatia, some time around 244.
His parents gave him the Greek name Diocles, or Diocles Valerius. The modern historian Timothy Barnes takes his official birthday, 22 December, as his actual birthdate. Other historians are not so certain, his parents were of low status. The first forty years of his life are obscure; the Byzantine chronicler Joannes Zonaras states that he was Dux Moesiae, a commander of forces on the lower Danube. The often-unreliable Historia Augusta states that he served in Gaul, but this account is not corroborated by other sources and is ignored by modern historians of the period; the first time Diocletian's whereabouts are established, in 282, the Emperor Carus made him commander of the Protectores domestici, the elite cavalry force directly attached to the Imperial household – a post that earned him the honour of a consulship in 283. As such, he took part in Carus' subsequent Persian campaign. Carus's death, amid a successful war with Persia and in mysterious circumstances – he was believed to have been struck by lightning or killed by Persian soldiers – left his sons Numerian and Carinus as the new Augusti.
Carinus made his way to Rome from his post in Gaul as imperial commissioner and arrived there by January 284, becoming legitimate Emperor in the West. Numerian lingered in the East; the Roman withdrawal from Persia was unopposed. The Sassanid king Bahram II could not field an army against them as he was still struggling to establish his authority. By March 284, Numerian had only reached Emesa in Syria. In Emesa he was still alive and in good health: he issued the only extant rescript in his name there, but after he left the city, his staff, including the prefect Aper, reported that he suffered from an inflammation of the eyes, he travelled in a closed coach from on. When the army reached Bithynia, some of the soldiers smelled an odor emanating from the coach, they opened its curtains and inside