Ultrafast Lasers: Trends in femtosecond amplifiers—Ti:sapphire vs. ytterbium

This from Laser Focus World as a Juxtaposition.

Feb. 18, 2020

Ti:sapphire and ytterbium femtosecond amplifier technologies—one mature, the other quite dynamic—currently provide complementary performance, so the optimum choice is really application-specific.

Joseph Henrich , Steve Butcher , Marco Arrigoni

Amplified femtosecond laser pulses enable many diverse applications because their high peak power (electric field) and very short pulses produce highly nonlinear processes and exquisite temporal resolution. For many years, titanium sapphire (Ti:sapphire) was the unanimous gain material of choice for ultrafast oscillator/amplifier systems. Recently, ytterbium (Yb) doped crystals, and particularly fibers, have been used in a growing range of femtosecond amplifiers with quite different (that is, complementary) performance characteristics in terms of pulse energy and average power. This article gives an overview of the current state of both technologies and their applications, showing how the scaling flexibility of Yb is now beginning to close the performance gap between the two technologies and impact the traditional domains of Ti:sapphire technology.

Ti:sapphire amplifiers

The high gain of Ti:sapphire crystals results in amplifiers that are unrivaled at delivering the highest pulse energies and shortest pulse durations at the lowest price per millijoule. By using two stages of amplification—typically a regenerative amplifier followed by a single-pass amplifier—it is possible to reach >13 mJ at 1 kHz with a commercial amplifier such as the Legend Elite HE+ series from Coherent, without resorting to cryogenic cooling. Indeed, the limiting design factor in kilohertz Ti:sapphire amplifiers is heat extraction from the gain crystal and the relatively short lifetime of the upper laser level. This means that these millijoule/pulse amplifiers need to be thermoelectrically (TE) or water-cooled and operate best at average power levels in the 7–15 W range and 1–10 kHz repetition rates. Combining these high pulse energies with pulse durations as short as 25 fs results in a peak power of hundreds of gigawatts.

Ti:sapphire is now a mature amplifier technology, so new models are usually characterized by incremental improvements in output specifications like power or carrier envelope phase (CEP) stability, with continuing effort to increase reliability, environmental stability, and maintenance intervals, especially in the case of so-called one-box versions. Although Ti:sapphire is tunable in the 700–1080 nm range, amplifiers are typically designed for optimized operation near the 800 nm peak of the tuning curve and broad tunability is achieved by pumping one or more tunable optical parametric amplifiers (OPAs).

Applications using Ti:sapphire amplifiers

The unique combination of high pulse energy, short pulse width, and high peak power from Ti:sapphire amplifiers has enabled diverse applications in physics, chemistry, biology, and material sciences. One of the most sophisticated applications is attosecond physics, where high harmonic generation (HHG) is used to create ultrabroadband pulses at extreme-ultraviolet (XUV) wavelengths that can be compressed to produce isolated attosecond-scale pulses when the optical carrier is locked to the pulse envelope (CEP stabilization).

At the other end of the electromagnetic spectrum, Ti:sapphire amplifiers are well suited to generating terahertz pulses. These can be used, for example, to interrogate semiconductor materials. In integrated circuits, transient electric fields can reach tens of megavolts per centimeter. Solid-state physicists want to know how fundamental charge transport mechanisms vary at fields of this magnitude and higher. Typical breakdown fields for many semiconductor materials are around 1 MV/cm—therefore, failure (burning) will rapidly occur if higher static fields are applied to test these materials. One solution that enables even higher fields to be safely applied is to use subpicosecond terahertz pulses.

In the laboratory of professor Rupert Huber at the University of Regensburg (Regensburg, Germany), a high-stability Ti:sapphire amplifier has been used to pump two tunable OPAs with a terahertz wavenumber difference in their outputs to create terahertz pulses with inherent CEP stability. These are used to probe the behavior (including Bloch oscillations) of electrons in gallium selenide samples under the influence of resultant transient fields approaching 100 MV/cm. By electro-optical “stroboscopic” gating of the signal from the sample with an 8 fs probe pulse at the terahertz detector, the data yields important information about Bloch oscillations as well as coherent and interfering conductive mechanisms only revealed at these high fields and short time intervals.

Another area where Ti:sapphire amplifiers are increasingly used is 2D spectroscopy, where the optical signal (emission, harmonic conversion, etc.) from a sample is recorded as a function of the wavenumber of an ultrabroadband pulse from an OPA, providing a unique combination of structural and dynamic data (see Fig. 1). Most 2D spectroscopy measurements are made in the time domain and converted to the frequency domain using Fourier-transform (FT) algorithms. Instead of using light at one frequency, ultrafast pulses of broadband light are used so that all frequencies are recorded simultaneously.

The operational simplicity and stability of one-box Ti:sapphire amplifiers such as the Coherent Astrella are proving ideal for these type of experiments that are relatively complex and require data acquisition times measured in hours and days. For example, in the laboratory of Graham Fleming (University of California, Berkeley), scientists are using 2D spectroscopy to probe the fundamental physics in perovskite films that might be used in next-generation solar cells. In the laboratory of Wei Xiong (University of California, San Diego), researchers are using a unique type of 2D spectroscopy to study a CO2 reduction catalyst expected to be important for artificial photosynthesis.

Ytterbium amplifiers and applications

While Ti:sapphire amplifiers are a mature technology, Yb is more than 15 years younger and therefore more dynamic in terms of performance improvements. Unlike Ti:sapphire, Yb can also be used as a dopant in gain fibers that enable the thermal load from the optical pumping to be spread over a longer path with much larger surface area/volume. Even when used as a dopant in bulk material, this reduced thermal sensitivity for the lasing properties of Yb enables higher pumping average power compared to Ti:sapphire, and does not require cryogenic cooling.

In addition, the much better quantum defect (980 nm pumping/1040 nm lasing for Yb vs. 532 nm pumping/800 nm lasing for Ti:sapphire) means that less energy is wasted as heat. Finally, pump power from diodes at 980 nm is less expensive than from a diode-pumped laser at 532 nm. Consequently, Yb can be scaled to much higher average powers with a lower cost per watt, compared to Ti:sapphire amplifiers. In fact, Yb amplifiers can deliver tens of watts from the footprint the size of a desktop computer.

Despite advances in average power, typical Yb amplifiers are limited to pulse outputs of a few millijoules in the femtosecond regime and cannot reach the 10 mJ-class pulse outputs offered by Ti:sapphire amplifiers. Yb fiber systems face a limitation due to peak power inside very small fiber cores, while Yb bulk systems typically face a tradeoff between achievable energy and pulse duration.

The gain bandwidth in Yb is not as broad as in Ti:sapphire, so its pulses are naturally longer. Therefore, recompression after chirped-pulse amplification (CPA) in bulk (or natural dispersion in fibers) results in pulse widths around 250 to 300 fs. While this is short enough for many applications, it does not match the temporal resolution (and spectral bandwidth) of Ti:sapphire amplifiers used for pump-probe, 2D spectroscopy, and similar time-resolved experiments. There are, however, several ways to overcome this limitation.

Like Ti:sapphire amplifiers, Yb systems require an OPA to enable wavelength tuning. By using a hybrid design, the OPA greatly reduces the resulting pulse width while maintaining a useful tuning range. Such an OPA includes a noncollinear stage to generate pulse widths as short as 40 to 50 fs, followed by a high-power collinear stage which delivers very broad wavelength tuning.

The compact architecture of Yb amplifiers lends itself to additional improvements in the overall amplified tunable system. For example, the White Dwarf optical parametric chirped-pulse amplifier (OPCPA) from Class 5 Photonics (Hamburg, Germany) incorporates a Coherent Monaco Yb-fiber amplifier and the OPCPA together in a single, compact box. With this approach, the OPCPA extends the performance of Yb-based systems into the ultrashort (less than 9 fs) pulse regime as well as the broadly tunable regime with approximately 50 fs pulse duration, providing highly customizable performance in a single box.

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Carlos Castaneda – two versions

on Wikipedia

Carlos Castañeda (December 25, 1925 – April 27, 1998) was an American writer. Starting with The Teachings of Don Juan in 1968, Castaneda wrote a series of books that purport to describe training in shamanism that he received under the tutelage of a Yaqui “Man of Knowledge” named don Juan Matus.

Castaneda’s first three books—The Teachings of Don Juan: A Yaqui Way of Knowledge, A Separate Reality, and Journey to Ixtlan—were written while he was an anthropology student at the University of California, Los Angeles (UCLA). He wrote that these books were ethnographic accounts describing his apprenticeship with a traditional “Man of Knowledge” identified as don Juan Matus, allegedly a Yaqui Indian from northern Mexico. The veracity of these books was doubted from their original publication, and they are now widely considered to be fictional. Castaneda was awarded his bachelor’s and doctoral degrees based on the work described in these books.

Early life

according to his birth record, Carlos Castañeda was born Carlos César Salvador Arana, on December 25, 1925, in Cajamarca, Peru, son of César Arana and Susana Castañeda, both of them single. Later Castaneda would say he was born in São Paulo, Brazil in 1931 and that Castaneda was a surname he adopted later. Immigration records confirm the birth record’s date and place of birth. Castaneda moved to the United States in the early 1950s and became a naturalized citizen on June 21, 1957.

Castaneda married Margaret Runyan in Mexico in 1960, according to Runyan’s memoirs. Castaneda is listed as the father on the birth certificate of Runyan’s son C.J. Castaneda even though the biological father was a different man.

In an interview Margaret said they were married from 1960 to 1973, however Castaneda obscured whether their marriage even happened, and his death certificate even stated he had never been married.

Career

Castaneda’s first three books—The Teachings of Don Juan: A Yaqui Way of Knowledge, A Separate Reality, and Journey to Ixtlan—were written while he was an anthropology student at the University of California, Los Angeles (UCLA). He wrote that these books were ethnographic accounts describing his apprenticeship with a traditional “Man of Knowledge” identified as don Juan Matus, allegedly a Yaqui Indian from northern Mexico. The veracity of these books was doubted from their original publication, and they are now widely considered to be fictional. Castaneda was awarded his bachelor’s and doctoral degrees based on the work described in these books.

In 1974 his fourth book, Tales of Power, was published and chronicled the end of his apprenticeship under the tutelage of Matus. Castaneda continued to be popular with the reading public with subsequent publications that unfolded further aspects of his training with don Juan.

Castaneda wrote that don Juan recognized him as the new nagual, or leader of a party of seers of his lineage. Matus also used the term nagual to signify that part of perception which is in the realm of the unknown yet still reachable by man, implying that, for his own party of seers, Matus was a connection to that unknown. Castaneda often referred to this unknown realm as “nonordinary reality.”

While Castaneda was a well-known cultural figure, he rarely appeared in public forums. He was the subject of a cover article in the March 5, 1973 issue of Time which described him as “an enigma wrapped in a mystery wrapped in a tortilla”. There was controversy when it was revealed that Castaneda may have used a surrogate for his cover portrait. Correspondent Sandra Burton, apparently unaware of Castaneda’s principle of freedom from personal history, confronted him about discrepancies in his account of his life. Castaneda responded: “To ask me to verify my life by giving you my statistics … is like using science to validate sorcery.” Following that interview, Castaneda completely retired from public view.

Death

Castaneda died on April 27, 1998 in Los Angeles due to complications from hepatocellular cancer. There was no public service; Castaneda was cremated and the ashes were sent to Mexico. His death was unknown to the outside world until nearly two months later, on 19 June 1998, when an obituary entitled “A Hushed Death for Mystic Author Carlos Castaneda” by staff writer J. R. Moehringer appeared in the Los Angeles Times.

Four months after Castaneda’s death, C. J. Castaneda, also known as Adrian Vashon, whose birth certificate shows Carlos Castaneda as his father, challenged the authenticity of Castaneda’s will in probate court. The challenge was ultimately unsuccessful. Carlos’ death certificate states metabolic encephalopathy for 72 hours prior to his death, yet the will was purportedly signed 48 hours before Castaneda’s death.

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From Castaneda.com

Tensegrity® was developed by American author, anthropologist and shaman, Carlos Castaneda and his cohorts, Carol Tiggs, Taisha Abelar and Florinda Donner-Grau.

Castaneda, starting with The Teachings of Don Juan, wrote a series of books that describe his training with his shaman-teacher, don Juan Matus, who taught him everything he knew about energy, how to cultivate it within ourselves and then use it in our everyday lives.

Those 12 books have sold more than 28 million copies in 17 languages and have become the source for helping millions of people manifest the life of their dreams, by bringing the once closely held knowledge and wisdom of the Toltec shamans to anyone who desires an extraordinary life.