Ultrafast laser technology continues to reach new heights

Ultra short pulse 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, the volume of these lasers was still very large, requiring daily cleaning of optical components, regular maintenance of cooling water, and continuous optimization of laser parameters.

Nowadays, solid-state and fiber platforms using mature photonic crystal fiber amplification technology and chirped pulse amplification architecture have produced compact, reliable, and low-cost femtosecond lasers.

In today's ultra short light sources, the laser cavity is enclosed, even larger disk or plate amplification cavities are enclosed for more effective isolation from the environment. This means that modern ultrafast lasers no longer require manual calibration on site and are less affected by temperature or humidity changes.

Decades ago, if you sneezed hard next to an ultrafast laser, it could be inaccurate or lose its mode locking function, "said Heather George, product manager of Tongkuai." The emergence of passive mode locking seed lasers instead of active mode locking seed lasers has made industrial ultrafast lasers possible. "The entire femtosecond laser source now offers various price points, as well as flexible pulse duration, pulse energy Average power and beam parameters.

A femtosecond fiber laser laboratory device for generating broadband frequency combs in the visible spectral range

Many of these systems have further integrated functions such as automatic pre compensation for unwanted group velocity dispersion or integrated acoustooptic modulators, allowing for control of average power, energy per pulse, and repetition rate. Marco Arrigoni, Marketing Director of Coherent, said, "Except for small and slow performance degradation related to natural component aging, turnkey lasers should not require users to make any regular adjustments or re optimizations. They should have remote diagnostic capabilities and provide remote service intervention to maximize normal operating time.

Despite the increasing number of user-friendly ultra fast systems, a better understanding of the parameters of these lasers can help improve throughput, quality, and application efficiency. Fiber based ultra fast lasers require almost no maintenance, can last for many years, and are relatively affordable. A typical femtosecond fiber laser has an output power of less than 10W, a repetition frequency between 80MHz and 100MHz, and a pulse energy between 10nJ-20nJ. The price is approximately $50000, which is about half of the price of early products.

However, the cost will also increase with the increase of average power/pulse energy. At present, the average power of ultrafast lasers is between 10W-200W, with a pulse width of less than 300fs, a pulse energy of 0.1mJ-2mJ, and a burst energy of 8mJ. The prices of these lasers range from $80000 to $100000.

Bernhard Wolfring, Product Manager for Ultrafast Lasers at TOPTICA Photonics AG, said that design decisions must consider cost and the physical characteristics of the required laser tools. The minimum power requirement cannot reduce costs below a certain level. On the other hand, the maximum power requirement helps to avoid the system being too expensive and capable. The result is usually to achieve the optimal balance between design and cost, as well as design and parameters, for specialized laser systems.

material processing 

Femtosecond lasers continue to be widely used in material processing applications, such as flat panel display foil cutting, medical stent microfabrication, and wafer scribing. Compared to picosecond or nanosecond pulses, femtosecond pulses can achieve better quality in micro machining applications, partly because femtosecond pulses have the least impact on thermal defects, such as fragments around the heat affected zone or machining area. It is worth noting that these advantages also have limitations: for most materials, pulses shorter than 350fs do not improve processing efficiency and may require more expensive optical devices. 
Furthermore, the pulse width itself is only part of the problem.

Hui Imam, Strategic Marketing Director of Ultra Fast Lasers at NKT Photonics, said: "The pulse width may be a bit misleading, and the more important parameter is the peak power, which is the energy transmitted in a short femtosecond time. The higher the peak power of a specific short femtosecond pulse, the more material is ablated, and the smaller the thermal effect.

Reducing the sensitivity of heat treatment to temperature or mechanical sensitive materials such as nickel titanate, polymers, drug injection materials, or thin dielectrics is crucial. The pulse energy and average power of an ultrafast fiber laser are limited by the fiber damage threshold. Amplification structures such as plate and disc amplifiers can generate higher pulse energy and average power. But they have a larger footprint, higher costs, and stricter requirements for cooling.

The average power and repetition rate determine the maximum pulse energy that a single laser pulse can reach. For most material processing applications, the optimal pulse energy depends on the ablation threshold. Different materials have different thresholds, but once the pulse energy exceeds the material's ablation threshold, the processing process will become saturated. Essentially, the plasma generated during the ablation process absorbs subsequent pulses, thereby increasing heat and reducing processing efficiency.
For the ablation of most materials, the typical pulse energy when using femtosecond pulses is between 0.02mJ-0.2mJ. The extremely high power density of femtosecond laser pulses can also induce two-photon or multiphoton absorption in the material, resulting in a three-dimensional structure with fine resolution that exceeds the optical diffraction limit. Compared with traditional micro/nano manufacturing technologies, femtosecond laser processing has both nanoscale feature size and three-dimensional architecture capabilities.

The light energy transmitted per unit area (referred to as laser flux) determines the ablation efficiency (mm3/min/W). For most materials, the optimal peak flux value that balances the highest processing quality and the most effective utilization of light energy is approximately 1J/cm2. Jim Bovatsek, Senior Application Engineering Manager at MKS Spectra Physics, said: Fluxes below peak lead to a sharp decrease in efficiency, while higher fluxes lead to a gradual decrease in efficiency. This way, throughput can be improved by running at a higher repetition rate, resulting in higher average power.

However, at certain times, either the movement speed of auxiliary motion/scanning equipment is not fast enough, the material's ability to dissipate residual heat energy is insufficient, or both are combined, resulting in the formation of an unsatisfactory heat affected zone. Bovatsek said that for cutting materials such as nickel titanium, the laser repetition frequency and pulse energy of 100kHz and~80 µ J, respectively, seem to be an upper limit before the formation of a heat affected zone. However, when the pulse frequency is greater than 2MHz and the power is greater than 100W, it can be used for cutting polymer films such as polyethylene terephthalate and polyimide.

Ultra short pulse lasers are increasingly used for black marking of stainless steel medical devices, and laser energy can form indelible markings that are not easily corroded and oxidized.

Marking medical components
Femtosecond lasers are an ideal technical solution for marking reusable medical devices, provided that the production volume can compensate for the price of the laser. Marking black or dark permanent two-dimensional barcodes on medical devices is an increasingly widespread application.

Usually, marking these items requires the use of low-cost nanosecond pulse lasers. These markings have been chemically treated and have corrosion resistance. However, femtosecond lasers can produce indelible markings that are less prone to corrosion and oxidation over time, so additional chemical treatment steps may not be required. Processing metal with picosecond or femtosecond lasers can form small periodic structures at the nanoscale, displayed as high contrast black markings that are not affected by the viewing angle and can display black contrast at any viewing angle.

The research on whether femtosecond pulses can achieve better labeling quality than picosecond pulses is still ongoing. But higher repetition rates can accelerate scanning speed, thereby shortening cycle time. Therefore, medical marking applications must consider the trade-off between speed and quality.

Daniel Huerta Murillo, an application engineer at Tongkuai Laser, said that in black marking, low pulse energy (<0.05 mJ) and high repetition rate (1MHz) are used. Higher pulse energy can lead to material structuring, while insufficient pulse energy can result in low contrast labeling.

Editor | Ringier

Source: Yangtze River Delta Laser Alliance