Ultrashort pulsed lasers, such as femtosecond lasers, are increasingly becoming easy-to-use plug-and-play devices suitable for a wide range of industrial and biomedical applications.Fifteen years ago, these lasers were large mainframes that required daily cleaning of the optics, regular maintenance of the cooling water, and constant optimization of the laser parameters.
Today, solid-state and fiber-optic platforms using proven photonic crystal fiber amplification technology and chirped-pulse amplification architectures have produced compact, reliable and lower-cost femtosecond lasers.
In today's ultrashort sources, the laser cavity is gas-tight, and even larger disk or plate amplification cavities are enclosed for more effective isolation from the environment. This means that modern ultrafast lasers no longer need to be manually calibrated in the field and are less affected by changes in temperature or humidity.
"Decades ago, if you sneezed hard next to an ultrafast laser, it could misalign or lose mode-locking," said Heather George, TRUMPF product manager, "The advent of passively mode-locked seed lasers, rather than active mode-locked seed lasers, has made industrial ultrafast lasers possible."
Complete femtosecond laser sources are now available at a variety of price points, as well as flexible pulse durations, pulse energies, average powers and beam parameters.
Many of these systems go further by integrating features such as automatic pre-compensation for unwanted group velocity dispersion or integrated acousto-optic modulators, which allow control of average power, energy per pulse and repetition rate.
"Aside from minor and slow performance degradation associated with natural component aging, the complete laser does not require any periodic adjustments or re-optimization by the user, and it should be remotely diagnostic and offer remote service intervention to maximize uptime." said Marco Arrigoni, director of marketing at Coherent Corp.
While user-friendly ultrafast systems are becoming increasingly available, a better understanding of the parameters of these lasers can help improve their throughput, quality, and application efficiency. Fiber-based ultrafast lasers can operate for years with little maintenance and are relatively affordable. Typical femtosecond fiber lasers with output powers of less than 10 W, repetition frequencies between 80 and 100 MHz, and pulse energies between 10 and 20 nJ cost about $50,000, about half the price of earlier products.
However, the cost increases as the average power/pulse energy increases. Current ultrafast lasers have average powers between 10 and 200 W, pulse widths of less than 300 fs, pulse energies between 0.1 and 2 mJ, and burst energies of 8 mJ. These lasers are priced in the $80,000 to $100,000 range.
Bernhard Wolfring, product manager for ultrafast lasers at TOPTICA Photonics AG, says that the design solution must take into account the cost versus the physical characteristics of the laser tool required. "The minimum power required does not allow costs to be reduced below a certain level," he says. "On the other hand, the maximum power requirement helps to avoid oversizing the system in terms of cost and functionality. The result is usually an optimal balance between design and cost, and design and parameters, for specialized laser systems."
Application One: Materials Processing
Femtosecond lasers continue to be used in a wide range of materials processing applications, such as cutting foils for flat panel displays, micromachining medical scaffolds and wafer scribing.
Femtosecond pulses result in better quality in micromachining applications compared to pico- or nanosecond pulses, in part because femtosecond pulses minimize the impact of thermal defects, such as heat-affected zones (HAZ) or debris around the processing area. It is worth noting that there is a bottom line to these advantages: for most materials, pulses shorter than 350 fs do not improve machining efficiency and may require more expensive optics. In addition, the pulse width itself is only part of the problem.
Hui Imam, director of strategic marketing for ultrafast lasers at NKT Photonics, says: "We think pulse width can be a bit misleading. The important parameter is peak power, which is the amount of energy delivered in a short femtosecond. The higher the peak power for a given short femtosecond pulse, the more material is ablated with less thermal impact."
Reduced heat treatment is critical for temperature- or mechanically-sensitive materials such as Nitinol, polymers, drug-injected materials, or thin dielectrics.
The pulse energy and average power of ultrafast fiber lasers are limited by the fiber damage threshold. Amplification structures such as plate and disk amplifiers can produce higher pulse energies and average powers. But they also come with a larger footprint, higher cost and more stringent cooling requirements.
Average power and repetition rate determine the maximum pulse energy that can be achieved with a single laser pulse. For most material processing applications, the optimum pulse energy depends on the so-called ablation threshold.
The threshold varies from material to material, but once the pulse energy exceeds the material's ablation threshold, the process becomes saturated. In essence, the plasma created during the ablation process absorbs subsequent pulses, which increases heat and reduces processing efficiency.
"For ablation of most materials, the typical pulse energy when using femtosecond pulses is between 0.02 and 0.2 mJ." George said.
The extremely high power density of femtosecond laser pulses also induces two- or multi-photon absorption in the material, resulting in three-dimensional structures with fine resolution beyond the limits of optical diffraction. In contrast to traditional micro/nano fabrication techniques, femtosecond laser processing offers both nanoscale feature size and three-dimensional architectural capabilities.
The amount of light energy transmitted per unit area (called laser fluence) determines the ablation rate efficiency (mm3/min/W). For most materials, the optimal peak fluence value that combines the highest processing quality with the most efficient use of light energy is approximately 1 J/cm2.
Jim Bovatsek, Senior Applications Engineering Manager at MKS Spectra-Physics, says, "Fluxes below the peak result in a sharp drop in efficiency, while higher fluxes result in a gradual decrease in efficiency." This allows for higher throughput by running at higher repetition rates, which results in higher average power.
At some point, however, either the auxiliary motion/scanning equipment doesn't move fast enough, or the material's ability to dissipate residual heat energy is insufficient, or both, and the result is a less-than-ideal heat-affected zone.
For cutting materials such as nitinol, a laser repetition frequency and pulse energy of 100 kHz and ~80 µJ, respectively, appear to be an upper limit before a heat-affected zone (HAZ) begins to form, while a pulse frequency of greater than 2 MHz with a power of greater than 100 W can be used to cut polymer films such as polyethylene terephthalate and polyimide, Bovatsek said.
Application 2: Marking medical parts
Femtosecond lasers can be the ideal technical solution for marking reusable medical devices if the production volume compensates for the price of the laser.
There is a growing application for marking medical devices with permanent 2D barcodes in black or dark colors, which can be used to register these tools and keep track of when they are cleaned.
Typically, marking these items requires the use of lower cost nanosecond pulsed lasers. These markers are chemically treated to be corrosion resistant. However, femtosecond lasers produce an indelible mark that does not corrode and oxidize over time, so additional chemical treatment steps may not be necessary.
Processing metals with picosecond or femtosecond lasers produces small periodic structures at the nanoscale that show up as high-contrast black markings," says George at Tonson. These black markings are independent of viewing angle and show black contrast at any viewing angle."
Research continues into whether femtosecond pulses can achieve better marker quality than picosecond pulses. However, higher repetition rates allow for faster scanning and thus shorter cycle times. Therefore, medical marking applications must consider the trade-off between speed and quality.
In black marking, low pulse energies (<0.05 mJ) and high repetition rates (1 MHz) are used," says Daniel Huerta-Murillo, laser applications engineer at Trafotek. Higher pulse energies result in structured materials, while insufficient pulse energy produces low-contrast markings."
Application 3: Welding and cutting
Processing brittle materials such as glass is another emerging industrial market for femtosecond lasers.
According to Antonio Castelo, photonics technology manager at the European Photonics Industry Consortium (EPIC), glass processing requires an accurate combination of wavelength and pulse energy. Failure to use the correct parameters often results in additional polishing steps being added at the end of the process.
Castelo says, "Some processes used for glass and polymer materials may require different wavelengths in the near- and mid-infrared, and a full range of solutions is now available in 2- and 3-micron."
Cutting and welding of glass or clear brittle plastics requires a special optic to achieve a certain pulse profile known as a Bessel beam. This beam creates a series of linear foci on the material being processed, similar to a thin laser knife that can modify the material in a single pass.
The maximum thickness that can be processed is limited by the pulse energy. The thicker the material, the higher the pulse energy required.
For glass welding, the pulse energy ranges from 0.01 mJ to 0.04 mJ, depending on the type of material, and for glass cutting, a pulse energy of 0.1 mJ to 2 mJ can be used, depending on the thickness of the sample to be processed," says ThruPoint's Huerta-Murillo. Laser cutting of glass plates up to 12 mm thick has already been realized in the Thomson Application Laboratory."
Pulse duration is another important factor in glass processing. For example, cutting clear glass can be done with picosecond pulses. But for welding clear glass, femtosecond pulses are more useful because they achieve higher peak power, which causes the glass to melt in a specific localized area.
Application 4: Multiphoton microscopy
Femtosecond pulses are particularly well suited for inducing multiphoton applications that are valuable for biological and scientific imaging applications such as nonlinear microscopy, two-photon optogenetics, and three-photon imaging.
The primary end users in these markets are biologists and neuroscientists with limited optical backgrounds. Adding functionality to simplify laser tools is particularly attractive to these end users.
It always comes back to the added value for the user, most often in terms of ease of use and usability," says Arrigoni of Coherent. Users in core imaging labs, who are frequently rotating and inexperienced, may benefit from a full suite of turnkey performance and readily accept two times the price of a turnkey laser system."
About 80 percent of the scientific applications for ultrafast lasers are realized with turnkey lasers, which have an average power of 10 W, pulse frequencies between 1 kHz and 10 MHz, pulse widths that are adjustable between 20 fs and 200 fs, and tunable wavelengths between 200 nm and 1,000 nm.
For femtosecond lasers targeting multiphoton phenomena in life sciences applications, the pulse energy or peak power is usually the most important factor, Wolfring said. To get good results, these parameters must be kept within a certain range. If the power is too low, the efficiency of the two-photon process may not be sufficient to produce microscopic images with good contrast. If the parameters are too high, the microscope image may show burnt tissue samples.
In general, multiphoton imaging applications require lasers to output tens to hundreds of nanojoules of energy between 1 and 100 MHz pulse frequencies to support fast image scanning and avoid damage to biological samples.
In principle, the shorter the pulse, the higher the nonlinear effects, but maintaining a short pulse width while propagating through the optical system is an important factor; parameters such as dispersion and compensation effects become important.
How to balance the ideal with reality?
From spectroscopy to photonics computing, ultrafast lasers are still finding new areas of application.
Customers in these fields demand flexibility and tunability from their lasers, but the lifetimes of ultrafast sources are not yet matched by the size of the market. according to Florian Emaury, CEO of Menhir Photonics, the trade-offs for delivering products to these emerging markets are a challenge worth meeting.
The design of ultrafast laser systems for these markets must balance customer needs with reasonable deliverables in terms of reliability and manufacturability. Building a robust turnkey system requires iterative steps - starting with determining the minimum specifications needed for the desired application.
Emaury says that customer requirements are always reasonable for what they need, but customers seldom consider what they want in terms of reliability and repeatability for the system they need. Considering the cost of ownership of the laser over many years is key.
Future progress?
Ultrafast lasers are becoming more compact, in addition to being easier to use and more competitively priced. Fiber-based systems allow for more flexible beam delivery, which allows for easy integration into narrow production lines, microscope systems or medical environments.
Smaller footprints also allow ultrafast lasers to be installed in smaller devices, such as photonic computing architectures, where the precision of ultrashort pulses allows photonic microprocessors to perform calculations faster with less energy.
Emaury says, "We see this as a market with very high demand, and we plan to produce hundreds of thousands, if not millions, of lasers per year." Of course, for now these lasers will be smaller than the size of a cell phone, but they will be the core component of any high-end computer."
Lasers specialized for photonic computing will need to provide gigahertz-level repetition rates and precisely time each pulse within a 10fs window.
While this and other markets may require complex parameters not yet possible with today's lasers, their recent advances are fueling new developments.
As ultrafast lasers evolve, more robust and versatile performance in terms of repetition rate, wavelength, and pulse duration will inevitably lead to a big break in the expansion of their applications. But there seems to be some sort of mutual give-and-take between how the technology drives the market.
"The development of fiber lasers in recent years has shown that new laser wavelengths and new power levels, which can be achieved through new concepts such as chirped pulse amplification, provide an important technological push for fiber lasers to enter the current key markets." Wolfring said. Translated with www.DeepL.com/Translator (free version)
