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The laser has revolutionized many areas of science and society, providing bright and versatile light sources that transform the ways we investigate science and enables trillions of dollars of commerce. In manufacturing, for example, robotic lasers have been programmable in a way that mechanical cutting tools had not been, with the same factory floor laser station capable of cutting, drilling, measuring, and in some cases welding and peening. Lasers are also becoming increasingly important for military owing to applications in communications, remote sensing, directed energy, and the production and diagnosis of materials in extreme environments.
One of the properties of lasers is the capability of generating a coherent monochromatic light beam, as opposed to incoherent, natural light, which is typically polychromatic, being emitted over a broad spectrum of frequencies. Soon after the advent of lasers, however, it was discovered that they can also produce broadband light in the form of trains of pulses, with duration ranging from the nanosecond down to the femtosecond temporal domain.
So, just how short is a femtosecond? One way to think of it, Kaertner says, is in terms of how far light can move in a given amount of time. Light travels about 300,000 kilometers (or 186,000 miles) in one second. That means it goes about 30 centimeters — about one foot — in one nanosecond. In one femtosecond, light travels just 300 nanometers — about the size of the biggest particle that can pass through a HEPA filter, and just slightly larger than the smallest bacteria. Another way of thinking about the length of a femtosecond is this: One femtosecond is to one second as one second is to about 32 million years.
A more precise but less common term is actually ultrashort pulse lasers; such lasers utilize ultrafast processes and emit light with very fast changes of optical power. Typical pulse repetition rates of ultrafast lasers are of the order of 100 MHz, but it is also possible to have only a few megahertz or many gigahertz.
Ultrafast pulsed lasers have seen exponential growth, with the number of filed patents increasing fivefold from about 100 to 500 per year. Several advanced niche applications have benefited from femtosecond laser processing, including in photonics, microelectronics, MEMS, and many other markets.
Picosecond and femtosecond pulsed lasers
The most widely used technique for the generation of short light pulses, in the picosecond and femtosecond range, is known as mode-locking, since the laser cavity modes are made to oscillate with some definite relation among their phases. There are mode-locked lasers emitting ultrashort pulses, i.e. light pulses with durations of femtoseconds or picoseconds: mostly below 100 ps, often even well below 100 fs.
The light emitted from a mode-locked laser thus consists of the coherent superposition of many sine waves of different frequencies, corresponding to the longitudinal modes, which interfere destructively except at the times when they are all in phase.
This interference pattern results in the emission of a train of ultrashort light pulses, spaced by the cavity roundtrip time, which is the time it takes for light to travel back and forth between the cavity mirrors. Each of these pulses has an ultrashort duration which, according to the Fourier transform theorem, is inversely proportional to the number of locked modes and therefore to the width of the frequency spectrum emitted by the laser.
The pulse duration dropped by three additional orders of magnitude, to the femtosecond regime, until the milestone result by Shank and coworkers, who generated in 1986 visible pulses with 6-fs duration by using a post-compression technique based on the nonlinear optical process of self-phase modulation in an optical fibre. This is a remarkable achievement because such pulses contain just a few cycles of oscillation of the carrier light wave (for visible light at λ = 600 nm the oscillation period is T = 2 fs) and are thus close to the ultimate limit of
pulse duration in the visible range.
In the following decades, important technical advances were introduced which greatly improved the reliability and the accessibility of femtosecond laser technology. Liquid gain media were replaced with solid-state ones, such as Ti:sapphire crystals and Yb:doped crystals and fibers, greatly increasing long-term stability and average power of the laser sources. Second- and third-order nonlinear optical effects, such as self-phase-modulation and optical parametric amplification, were used to further broaden the spectrum of the pulses and to tune their frequency, enabling to cover an extremely broad range from the mid-infrared to the ultraviolet.
Applications of femtosecond pulses
Exploiting this tunability, femtosecond pulses have become invaluable tools for physicists, chemists and biologists in order to investigate the ultrafast non-equilibrium processes occurring in atoms, molecules and solids, using a variety of spectroscopic techniques.
Ultrafast laser spectroscopy is a spectroscopic technique that uses ultrashort pulse lasers for the study of dynamics on extremely short time scales (attoseconds to nanoseconds). Dynamics on the as to fs time scale is in general too fast to be measured electronically. Most measurements are done by employing a sequence of ultrashort light pulses to initiate a process and record its dynamics. The width of the light pulses has to be on the same scale as the dynamics that is to be measured. Different methods are used to examine the dynamics of charge carriers, atoms, and molecules.
Femtosecond laser pulses can be used to shoot “slow-motion movies” of key photoinduced processes in bio-molecules, such a photosynthesis and vision, as well as to get insight into the non-equilibrium charge carrier dynamics underlying the operation of (opto)-electronic devices.
For medical uses, lasers have reduced the need for sterilization or anesthetics, and intense laser pulses can be delivered to internal tissues via optical fibers. In security, lasers can accelerate charged particles to relativistic energies, and beams of these particles can be used as probes of concealed materials in ways that complement the penetrating power and contrast available with X-rays. And in science, the high intensity and short duration laser pulses make possible new techniques for ultrafast imaging of a wide range of transiently evolving matter.
High-energy X-ray pulses with femtosecond duration could make it possible to obtain detailed images, and ultimately movies, of the dynamics of complex protein molecules, Kaertner says — something that can’t be done with existing techniques, and could be of great interest for biomedical research. But high-energy X-ray pulses that can probe these complex structures also destroy them in the process, so the pulse has to be so quick that the image can be obtained before the pieces fly apart. “If the pulse is short enough, all the X-rays diffract from the protein before it is destroyed,” Kaertner says. This is called diffraction before destruction. “It’s a hot field at the moment,” he adds.
Ultrafast lasers generate pulses shorter than about 1 ps, transferring energy to electrons before they interact with their environment. With high intensity, for example, an electron can receive energy from two photons before losing any of that energy in molecular vibrations or interactions with other electrons. That unique interaction introduces distinctive capabilities in microfabrication and direct manipulation of living cells.
Femtosecond lasers minimize heat transfer to ablate material without melting or re-solidification, leading to very clean microfabrication. In addition, when focused to a spot a few microns across, the beam can excite electrons with two low-energy photons in place of a single photon with twice the energy. This happens only at high intensity, limiting interaction with the target material to the focal region.
When a photopolymer is exposed to light of a specific wavelength, monomers join, creating a polymer. Two-photon polymerization (TPP) occurs when the material is exposed to femtosecond laser pulses of twice the polymerization wavelength. As this only happens near the focal point, TPP produces very fine features.
Professors Haibo Yu and Lianqing Liu of Northeastern University (Shenyang, China) and their colleagues used TPP to create self-assembled structures to mechanically trap individual cells. They created “micropillars” by exposing one location in a photopolymer to 240 fs pulses from a 1030 nm laser operating at 200 kHz. By varying laser power and moving the focus, they produced micropillars with diameters from about 2 to 7 µm and heights from 5 to 40 µm.
The intention was to create structures to hold individual cells without constant active intervention. Optical traps hold individual cells, but there is no easy way to retain multiple cells or hold them for a long time. This new capability now enables longitudinal studies of individual cells. So, ultrafast lasers micromachine structures useful for biological studies, but they can also directly modify the behavior of living cells.
Increasing the pulse energy
Mode-locked lasers produce pulses with relatively low energy, of the order of a few nanojoules (1 nJ = 10-9 J), which cannot be directly amplified due to damage occurring in the optical gain medium when the intensity exceeds a certain threshold. This problem was elegantly solved by the invention of the Chirped Pulse Amplification (CPA) technique. In CPA laser systems the ultrashort pulses are first temporally stretched, by sending them to an optical system in which their frequency components travel with different speeds and become temporally separated, resulting in a pulse-width lengthening by several orders of magnitude. The stretched pulses are then safely amplified without damaging the optical amplifier material and finally sent to a pulse compressor, which is an optical system where the relative delays of the different frequency components are reversed, thus restoring the original pulse duration.
The CPA technique, for which Donna Strickland and Gerard Mourou were honored with the Physics Nobel Prize in 2018, allowed to increase the energy of ultrashort pulses by over 10 orders magnitude, resulting in peak powers reaching the petawatt regime. Such incredibly high instantaneous powers, which exceed by more than two orders of magnitude the combined power of all the world electrical grids, enable completely new regimes of light-matter interaction, where electrons and ions can be accelerated by the laser light to relativistic speeds.
Besides their huge scientific impact, ultrashort light pulses are finding more and more real-world applications in materials processing, as they allow depositing energy in the irradiated volume in a very short time, avoiding heat diffusion and resulting in clean ablation without collateral damage. High intensity femtosecond lasers are used for drilling holes in metals, such as in the injectors of diesel engines or the stents used in vascular surgery.
Laser material processing is now a major component of the manufacturing process. Lasers accomplish tasks ranging from heating for hardening, melting for welding and cladding, and the removal of material for drilling and cutting. Typical intensities required for such tasks include heat treating at 103 – 104 W/cm2, welding and cladding at 105 – 106 W/cm2, and material removal 107 – 109 W/cm2 for drilling, cutting, and milling. High-intensity short pulse lasers, in particular, have unique capabilities for precision, mainly due to minimal thermal energy deposition in materials, resulting in negligible collateral damage beyond the desired interaction volume. This can yield high aspect ratio holes and precisely imprinted patterns unrealizable with long pulse or continuous wave (CW) lasers. Femtosecond laser processing for materials manufacturing is expanding as robust commercial lasers become available. Typical operating parameters for commercial lasers used for manufacturing include pulsewidths of 100-200 fs, peak.
They find also medical applications in refractive eye surgery, using the laser-assisted in situ keratomileusis (LASIK) procedure (where keratomileusis means surgical reshaping of the cornea) to correct myopia and astigmatism.
Beyond basic research, femtosecond lasers have many practical applications as well. The most common are in the micromachining of materials and in Lasik eye surgery — which was enabled by the development of robust femtosecond pulsed lasers. These extremely short pulses made it possible to deposit high energy to destroy material such as tissue on a tiny spatial scale, without having enough time for the energy to diffuse and damage surrounding tissue, Kaertner says. It is worth mentioning that in the last few years the European Commission funded the Extreme Light Infrastructure (ELI), a pan-European infrastructure completely devoted to the generation and application of ultra-intense laser sources.
Tapered double-clad fiber: The future of ultrafast high-power laser processing
Fiber, solid-state, and disk lasers are the most promising candidates for high-average-power generation. The outstanding characteristics of fiber lasers compared to solid-state and disk lasers include compactness, robustness, efficiency, ease of thermal management, and reliable beam quality. Significantly lower production and maintenance costs also make fiber-based approaches highly appealing for pico- and femtosecond high-repetition-rate kilowatt-level laser development.
In these lasers, properties of the active fiber itself are the limit to higher pulse energies. Today’s high-average-power fiber lasers generally use chirped-pulse amplification (CPA). However, in boost fiber-based amplifiers, even for highly stretched pulses the optical peak intensities can become very high, producing detrimental nonlinear pulse distortion or even destruction of the gain medium or other optical elements. Additionally, other nonlinear effects such as self-phase modulation, stimulated Raman scattering (SRS), mode instabilities, and poor output beam quality often arise in pulsed high-power systems limiting their performance.
The main approach to solving problems for pulsed signal amplification has been to enlarge the core diameter of the fiber. Special active fibers with large mode area were developed to increase the surface-to-active-volume ratio of active fibers and, hence, improve heat dissipation and elevate the threshold of nonlinear effects enabling power scaling. State-of-the-art high-power fiber-based technologies have already approached >1 kW in a single pulsed amplification channel2 and laid a cornerstone for future ultrashort multikilowatt-level fiber-based laser systems.
Traditionally, fiber diameter has been increased to enable higher pulse energies, but the beam quality of these large effective mode area (LMA) fibers is highly sensitive to any bending of the fiber. Now, tapered double-clad fiber (T-DCF) amplifiers offer the prospect of high power with excellent beam properties.
Developed in the European PULSE project, which includes Tampere University and Ampliconyx Oy (both in Tampere, Finland), T-DCF is a double-clad optical fiber formed using a fiber-drawing process that forms a taper along the length of the fiber. Both the core and cladding are tapered; the result of the changing fiber parameters is a chain of fiber amplifiers with ever-growing core diameter, combining features of conventional small-diameter, double-clad single-mode fibers with those of much larger-diameter, double-clad multimode fibers used for high-power amplification.
The fiber’s double-clad structure means that its core can be pumped with higher power than could be propagated only in the core. The absorption and conversion of pump light per unit length is higher in the tapered fiber compared to cylindrical fibers with similar levels of active ion doping. This is due to the improved clad mode mixing and the higher absorption at the thicker end of the taper due to the much thicker cladding. This also means that the rare-earth ion dopants are usefully concentrated at the wide end of a T-DCF, since the geometry defines their presence as directly proportional to the square of the diameter.
Project creates more powerful, versatile ultrafast laser pulse
Institute of Optics research sets record for shortest laser pulse for newly developed technology, work that has important applications in engineering and biomedicine. University of Rochester researchers are setting a new standard when it comes to producing ultrafast laser pulses over a broader range of wavelengths than traditional laser sources.
In work published in Physical Review Lettersin July 2020, William Renninger, an assistant professor of optics, along with members of his lab, describe a new device, called the “stretched-pulse soliton Kerr resonator,” that enhances the performance of ultrafast laser pulses. The work has important implications for a range of engineering and biomedical applications, including spectroscopy, frequency synthesis, distance ranging, pulse generation, and others. The device creates an ultrafast laser pulse—on the order of femtoseconds, or one quadrillionth of a second—that’s freed from the physical limits endemic to sources of laser light—what laser scientists call laser gain—and the limits of the sources’ wavelengths. “Simply put, this is the shortest pulse ever from a gain-free fiber source,” Renninger says.
Renninger and his team of graduate research and postdoctoral associates improved upon Kerr resonators, an exciting new alternative for generating femtosecond laser pulses that have been the subject of considerable research. The lab overcame a challenge to pulse duration in other versions of Kerr resonators by discovering a new soliton—a short burst or localized envelope of a wave—that maintains its shape while propagating at a constant velocity. The solitons generated in Renninger’s device differ from the solitons in other Kerr resonators, specifically in the shape and behavior of the stretching pulses they create. “It is stable in the sense it keeps repeating the same thing over and over, getting longer, then shorter, longer then shorter,” Renninger says.
The pulses “feature a broad spectral bandwidth and a compressed pulse duration of 210 femtoseconds, which is the shortest pulse duration observed to date from fiber Kerr resonators,” the researchers state in the paper. Lead author Xue Dong is a graduate research associate in the Renninger lab. In addition to Renninger, other coauthors are Qian Yang and Christopher Spiess, also graduate research associates in the lab, and Victor Bucklew, a former postdoctoral associate in the lab. The study was funded by in part by the University’s Technology Development Fund, a University Research Award, and by the National Institutes of Health.
Renninger, an expert in creating sources for femtosecond lasers, received his BS and PhD degrees in applied physics from Cornell University. Before joining the Institute of Optics, he was a postdoctoral associate and an associate research scientist in the Department of Applied Physics at Yale University. He recently received a National Science Foundation CAREER award, which includes funding to create open source access to information for designing and creating advanced lasers sources generating femtosecond pulses.
“There are now commercial products, but they’re very expensive. They are prohibitive for many research groups with limited budgets for equipment,” Renninger says. Much of the cost is for expertise, not components, so his group will use part of the CAREER funding to provide consulting for research groups at smaller universities in how to design and build femtosecond lasers for basic research. “The ultimate goal is to have a design guide published on our website for everybody,” Renninger says.
Combatting thermal lensing in high-power ultrafast laser systems, reported in June 2021
Ultrafast laser systems can be found in materials processing applications as well as medical lasers, semiconductor inspection, and nonlinear imaging and microscopy. These applications push ultrafast lasers toward higher and higher powers, although this technology is especially sensitive to thermal effects such as thermal lensing.
Heat can build up and cause gain media and intracavity optics to experience a refractive index gradient and even deform. These effects hinder the performance of ultrafast systems and can also make it impossible for them to mode-lock and produce pulses. Additionally, thermal lensing causes astigmatism in laser cavities. However, new highly dispersive intracavity mirror coatings minimize thermal effects. These advancements can create intracavity optics with negligible thermal effects, facilitating top-of-the-line ultrafast laser systems.
These types of systems also benefit external optics used outside of the laser cavity, such as those in high-energy ultrafast oscillators and amplifiers. External dispersive mirrors with thermal lensing-limiting technology offer a high degree of control over beam stability and pulse compression while minimizing detrimental thermal effects.
Thermal lensing
Thermal lensing limits the beam quality and power output of laser systems, particularly those operating at high powers. For ultrafast laser systems, these effects can ultimately lead to system failure by preventing the device from mode-locking to generate laser pulses. Ultrafast lasers operate in continuous-wave (CW) mode for cavity alignment. The lasers are then switched to a pulsed configuration for actual use, but thermal lensing may prevent mode-locking and pulse generation in this configuration.
Thermal lensing can misalign laser cavities, produce unwanted laser modes, cause beam pointing to drift, and cause astigmatism in the laser cavity. All of these effects contribute to unpredictable and diminished final system performance and could prevent ultrafast lasers from achieving higher powers. Thermally induced expansion as a result of heat buildup may also deform optical components, changing the radii of curvature of mirrors. This deformation shifts the focus position of those mirrors, which can potentially prevent the ultrafast system from mode-locking due to cavity misalignment.
The inability to produce pulsed beams renders ultrafast lasers useless. While there are not many available actions that can be done to manipulate the inherent thermal properties of gain media and optical component substrates, an opportunity exists for carefully choosing the proper dispersive mirror coatings to prevent thermal lensing.
Specialized highly dispersive mirrors
Advancements in dielectric coating design have recently enabled the development of highly dispersive ultrafast mirrors that maintain high reflectiveness and desired pulse compression while exhibiting negligible thermal effects. This combination of specifications is attained through careful manipulation of the various process parameters during coating deposition. While minimizing thermal stability is important, this cannot come at the cost of high reflectivity and a sufficiently negative group delay dispersion (GDD). Most optical media exhibit a positive GDD, so the negative GDD of dispersive mirrors is needed to compensate for this and compress pulses to the pulse duration demanded by a particular application.
Researchers have developed a novel thermal lensing-reducing coating. This new coating technology is beneficial for high-power, solid-state ultrafast lasers such as Yb:YAG, thulium, and holmium laser systems.
These novel dispersive mirror coatings were tested to determine the level of thermal lensing present. The temperature of intracavity dispersive mirrors inside a Yb:YAG thin-disk laser was measured using an infrared camera (FLIR SC305). The laser was tested in CW operation, as this is how the laser behaves before mode-locking can be achieved. A standard highly reflective mirror featuring a GDD of -3000 fs2, but not thermal lensing-reducing coatings, exhibited a rise in temperature of >50 K . This temperature change resulted in a deterioration of oscillator stability and a change in the emitted laser modes.
References and Resources also include:
https://www.sciencedaily.com/releases/2020/07/200724120149.htm
https://www.europhysicsnews.org/articles/epn/pdf/2019/02/epn2019502p11.pdf
