High-power picosecond/femtosecond lasers create new applications

Cold ablation, cold cutting and cold drilling with ultrashort laser pulses has been a vision for industrial applications for more than two decades. Early experiments in the past decade using titanium sapphire amplifiers to generate ultrafast lasers have demonstrated the potential of ultrashort laser pulses for precision machining. But how short can a pulse be to meet the requirements of precision machining? What happens when a laser pulse hits a material? What are the requirements for the time range of the pulse and the material?

Action principle, action time, energy density

Taking the absorption of laser pulses by metals as an example, it is basically an energy transfer process from the laser pulse to the electrons of the metal material. For a pulse with a duration of nanoseconds, a temperature equilibrium process will occur between the electrons and the crystal lattice in which they are located , and eventually begin to melt the material until it partially evaporates.

The shorter the pulse, the faster the energy is transferred to the electrons in this process. Under ideal conditions, if the pulse is short enough, there is not enough time for temperature equilibrium to occur between the electrons and the lattice. Next, the "hot electrons" (relative to the cold lattice) interact with the lattice in two ways: After a characteristic time, the heat from the electrons begins to diffuse to the surrounding lattice. This electron-phonon relaxation time is a property of matter and its typical value is 1-10ps. In roughly the same time range, but with a slight delay, a sudden energy transfer occurs between the hot electrons and the lattice, resulting in a phase explosion, i.e., the evaporation of the activated body.

From the above explanation, we can draw the following two basic conclusions:

1. The duration of the laser pulse must be short enough to prevent the temperature equilibrium process between the electrons and the lattice. For metals and most other materials, the pulse duration is required to be between 1 and 10 ps or even shorter.

2. Because there is a time delay between heat diffusion and ablation, there is always residual heat, even in the shortest pulse case.

Therefore, cold working must be defined as processing with minimal heat diffusion, which requires pulse durations between 1 and 10 ps or even shorter.

Although a short duration of picosecond/ femtosecond laser pulses is a necessary condition for cold processing, it is not enough to have sufficiently short pulses. If the hot electrons are "overheated" due to too high laser energy density, the heat diffusion effect will be more obvious and the entire process will be transformed into a thermal process. Generally speaking, the energy density of about 1J/cm2 is the optimal energy critical point for ablation processing with picosecond/femtosecond laser pulses without generating measurable thermal effects, that is, at this time, the optimal low thermal penetration depth is obtained.

Linear absorption and nonlinear absorption

However, achieving the optimal energy critical point is not easy. In addition to the factors mentioned above that determine the thermal impact, the optical penetration depth determines which part of the laser pulse is absorbed at what depth.

For mild ablation, light penetration depth should be in the 1 μm region or even shallower for three main reasons:

1. The depth of light penetration determines the depth of ablation. Ablation with too deep a depth will no longer be considered as gentle ablation, because it will lead to rough surfaces and edges, especially for hard and brittle materials, and microcracks will be generated.

2. If the light penetration depth is too great, the ablation process will become inefficient because most of the laser pulses may not be absorbed, resulting in a large waste of energy.

3. For selective ablation of substrate materials (such as thin film patterning on the insulator of thin film solar cells ), excessive penetration depth of light may cause damage to the substrate material.

The effect of linear absorption in femtosecond and picosecond pulses is often neglected because the peak pulse powers are so high that nonlinear absorption through multiphoton processes dominates linear absorption. This is misleading if the boundary conditions for pulse duration and energy density are met. To

illustrate this, Figure 1 shows the absorption curve of silicon for a pulse with an energy density of 1 J/cm2. For pulses of 6 ps and even longer, linear absorption absolutely dominates nonlinear absorption. This does not change even for pulses of 500 fs: nonlinear absorption is still too low to achieve the desired light penetration depth of 1 µm.

Figure 1 Absorption curve of silicon for laser pulses with an energy density of 1J/cm2

For pulses with a pulse duration of 6 ps (left), the linear absorption dominates over the nonlinear absorption. Even for pulses with a duration of 500 fs (right), the nonlinear absorption is so low that the desired optical penetration depth of 1 µm cannot be achieved.

Choose a UV wavelength so that the theoretical best performance is the same as in practice (e.g., for dicing silicon wafers ). For some purposes, in processing silicon wafers, a green wavelength may be sufficient.

Femtosecond and picosecond pulses with appropriate energy density and wavelength are suitable for material processing applications that require very low thermal effects. In addition, the technical methods for generating picosecond pulses can be greatly simplified for their duration. Direct diode pumping and amplification ( power scaling) without chirped pulse amplification (CPA) are essential for the success of ultrashort pulse technology in the industrial market. In fact, for a cost-effective application in industrial micromachining, the average output power must be increased to 50W or even higher.

Optical fiber and disc combination

Rod lasers (first lamp-pumped and later diode-pumped) were introduced in the 1970s . Rod lasers and diode-pumped disk lasers both made great strides in the 1990s, surpassing the beam quality limitations of high average power, making them the most reliable technology choice for kilowatt-class CW applications in industry.

Fiber laser technology and disk laser technology are superior to traditional rod laser technology because they use a larger heat dissipation surface than the laser active body, allowing the TEM00 to operate continuously at a power level of 500W or even higher. Under the same brightness , the small fiber core diameter makes the laser intensity in the fiber laser much higher than that of the disk laser.

However, when amplifying picosecond and femtosecond pulses, the high light intensity can lead to nonlinear effects such as self-phase modulation or Raman scattering, which requires the addition of complex chirped pulse amplification in ultrafast fiber amplifiers or limits the maximum achievable pulse energy to 6μJ or even lower. Using disk laser technology as an amplifier for picosecond pulses enables high peak powers (up to 100MW) and low light intensities without incurring nonlinear effects.

To achieve picosecond lasers with high pulse energy (up to 250 μJ) and high average power (up to 100 W), a master-oscillator power amplifier with the following unique configuration is required: a passively mode-locked fiber laser based on telecommunication components serves as a monolithically integrated, cost-effective, and reliable light source for low-power and low-pulse-energy picosecond pulse generation.

The output power of the fiber laser is amplified by 5 orders of magnitude using a disk laser, reaching over 100W in infrared and 60W in green , with a pulse frequency range of 200-800kHz, without the need for complex chirped pulse amplifiers. Even at these power levels, an excellent beam quality of M2 < 1.3 is achieved. In addition, the output beam quality remains at the above level for every optional parameter combination of the laser.

Power reaching the workpiece

The most important task of using ultrafast lasers to achieve fine processing is to manipulate the laser beam and convert the laser power into maximum production efficiency and quality. The entire processing process needs to fully consider the geometric characteristics of the workpiece and the requirements of processing accuracy to build the final processing system. The system will require a set of optical components such as scanners, F-Theta lenses , focusing elements, wave plates, perforated optical elements, and many other components.

The entire process also needs to consider linear or rotary processing. Neither the most advanced linear processing nor scanners have dynamically applied pulse frequencies exceeding 1MHz, although laser technology may be ready for development in this regard.

Injection nozzle drilling

Producing fuel-efficient, low-emission engines is a major challenge facing the automotive industry. The key to solving this problem is to achieve cleaner fuel combustion, which can be achieved by optimizing the fuel nozzle.

Drilling the fuel injector with a high-power picosecond laser produces very sharp edges, no burrs or melting in the hole, and a very smooth surface, which can achieve an optimized fuel spray. In addition, the taper of the fuel injector can be from positive, zero to negative, providing a certain degree of freedom for optimizing the injection process. The average power of 50W combined with the pulse energy of up to 250μJ can achieve high-speed, high-quality drilling (see Figure 2).

Figure 2: Drilling of the fuel injector nozzle using a high-power picosecond laser and spiral drilling optics.

Produces burr-free and smooth holes

Semiconductor Wafer Cutting

Currently, the most advanced method for separating computer chips from a silicon wafer is to cut them with a diamond saw. However, thin silicon wafers are very fragile, which is a major challenge for diamond saws. Because of the mechanical contact, the wafer must be sawed very carefully to avoid damaging the wafer or causing damage on the cut edge.

Picosecond lasers, combined with high-speed and precise linear processing, can be used as a non-contact tool to cut wafers faster than diamond saws. Another advantage of using picosecond pulsed lasers is that they can achieve higher cutting quality and negligible heat-affected zones, so that the cut edges will have higher strength, which is very important for the wafer to maintain good mechanical loading in the next processing step (see Figure 3).

Figure 3 Cutting semiconductor wafers with picosecond pulse lasers can not only increase production,

It also improves the cut quality and provides greater strength to the cut edge