**4. Microhole drilling for processing drawing dies using ultrafast Yb:KGW laser**

Laser micromachining techniques are currently used due to the broad applications across the manufacturing sectors. Among the major applications, laser microhole drilling and cutting have much attention. For microhole drilling, the conventional fabrication method, lithography, which requires advanced facilities and numerous multiple steps, is limited in material type and geometry. Currently used drilling and cutting with nanosecond (ns) or longer pulsed laser are always accompanied with contamination to the surrounding material, melt zone, and recast layer. Although the geometrical precision could be improved by using ns laser techni‐ ques, the quality and precision achievable are still limited due to the subsequently uncontrolled deposition of the melt. Due to the high energy input and thermally induced stress, drilling and cutting using picosecond pulsed laser still have disadvantages, e.g. cracks and heat-affected zone in the surrounding area. Not only the accuracy but also the reliability of the process is affected. For ultrafast lasers, energy deposition occurs on a timescale that is short compared to atomic relaxation processes. Ultrafast lasers suppress thermal diffusion and thus reduce heat-affected zone.

The shape of the output spectrum for equal pump power in both arms of pumping is shown in **Figure 13(a)**. The gain narrowing effect is noticeably well—FWHM spectral width is ~1.5 times narrower compared with the spectrum of master oscillator pulses. Compression

This gain narrowing effect can be suppressed, for example, by making the pump power of Yb:KYW crystals not to be equal [9]. We changed the pump power launched on the Np-cut and Ng-cut crystals to the ratio of 3:2 [15]. The experimental measurements showed that the spectral width became broader and its shape was modified considerably. In this case, the spectral width was measured to be 11 nm and the pulse length was measured to be 210 fs for assuming

 profile. This method has a drawback that the restriction of pumping power on one crystal results in the restriction of total output power in expense of pulse width. For example the

Another way to suppress the spectrum narrowing is to use preliminary spectrum shaping [11] as it was discussed earlier. The example of output spectrum for extra-cavity spectrum shaping by filter Lyot is shown in **Figure 13(b)**. The optical spectrum showed a characteristic "bell" shape with a spectral FWHM bandwidth of 11 nm. Such a bandwidth provides smooth output pulse with width of intensity autocorrelation trace 305 fs that gives the pulse length of τFWHM = 182 fs for sech2 pulse profile as shown in **Figure 13(d)**. This pulse length is close to the pulse length of 160 fs defined by aberrations in the stretcher-compressor module. To measure the ultrashort pulse width, we used a PulseCheck autocorrelator (APE GmbH). The inset of

Inserting a spectrum shaper inside the cavity of regenerative amplifier, we obtained approx‐ imately the same spectral width but less output power of about 20%. It is connected with accumulated effect of intra-cavity losses by Lyot filter inside the cavity. Thus combination of Lyot filter outside the cavity as a spectral shaper and identical pump power for two slabs in the dual-slab regenerative amplifier provides optimal condition of output power and pulse

Beam quality *M*<sup>2</sup> of output beam was below 1.2 at output power <12 W that allows the beam focusing to small spot size of 5–10 μm. High average output power, with more than tens of

**4. Microhole drilling for processing drawing dies using ultrafast Yb:KGW**

Laser micromachining techniques are currently used due to the broad applications across the manufacturing sectors. Among the major applications, laser microhole drilling and cutting have much attention. For microhole drilling, the conventional fabrication method, lithography, which requires advanced facilities and numerous multiple steps, is limited in material type and geometry. Currently used drilling and cutting with nanosecond (ns) or longer pulsed laser are always accompanied with contamination to the surrounding material, melt zone, and

**Figure 13(d)** shows that there is no noticeable peak beyond the range of 1.5 ps.

μJ, and beam quality are important for industrial microprocessing applications.

provides 265 fs output pulses under this condition as shown in **Figure 13(c)**.

output power dropped 37% in our experimental conditions.

sech2

48 High Energy and Short Pulse Lasers

length.

**laser**

When processing transparent materials with ultrafast laser, the high intensity of focal volume induces multiphoton or tunnel ionization, and then subsequent electron heating or avalanche ionization, which gives rise to the efficient absorption of light. This phenomenon is observed by the nonlinear nature of the ultrafast laser interaction with the transparent materials. This generates the unique capability of transparent material processing using ultrafast laser.

**Figure 14.** Scanning electron microscope (SEM) views of the structure on bearing land in drawing dies (a) using femto‐ second (fs) laser and (b) using nanosecond (ns) laser.

Its promising application is drawing dies-hole drilling. Wire drawing is a deformation and metalworking process used to reduce the cross-section of a wire by pulling the wire through a drawing die. For drawing very fine wire, a single crystal diamond die is used. The drawing die consists of three zones: cone-shape entrance, bearing land, and back relief. The die bearing determines the size of the wire. As demand for microwire increases across the manufacturing sectors, large scale machines are currently used to produce wire of microlevel, but it is not economical in an industry. Micromachining with ultrafast laser, which made the hole size small, has been reported. Smooth surfaces, also, are generally preferred for precision machin‐ ing. **Figure 14** shows the SEM view of the bearing surface of drilled hole in dies. The compar‐ ison with fs and ns laser drilling results shows the advantage of fs laser drilling. **Figure 14(a)** shows a ripple for which spacing is generally <200 nm. The orientations of the ripple structures are parallel to each other. Similar ripple structure has been observed in various materials for fs laser drilling. Uneven and rough structures are shown in **Figure 14(b)**. It is clear that the material removal during the dies-hole drilling is accomplished by the formation of melt. Compared with two methods, micromachining with ultrafast laser creates much cleaner and smoother hole.

Ultrafast laser micromachining is an emerging technology for high-precision and cold-ablation material processing. For its advantages and potential uses, suitable ultrafast laser and laser operating parameters such as wavelength, repetition rate, average power, pulse duration, spot size, beam quality, and sample moving speed must be selected to achieve desired high-quality micromachining. In the near future, ultrafast laser micromachining will be used in various sectors including sub-micron material processing, surface structuring, photonics devices, biomedical devices, microfluidics, displays, and solar applications.
