New diffraction-limited laser systems with variable output parameters operating in visible spectral range on copper atomic transitions for advanced material micromachining

New considerably improved laser systems (LS) featuring beam propagation factor equal to unit, two different laser pulse durations and an average output power from several tens of Watts up to 50 W are reported. Micron-sized machining is also demonstrated.


Introduction
The primary advantage of metal vapor lasers over other laser classes is the possibility to oscillate in any spectral range on the transitions of various metal atoms and ions even in a single laser tube.Very soon that advantage was overcome by the solid state laser systems based on optical parametric oscillators [1], Raman-shifted alexandrite lasers [2,3], quantum cascade lasers [4], harmonic frequency [5], frequency mixing [4,6] and continuum spectrum generation.The highest average laser power of 550 W in the visible spectral range [7] has been produced by the copper vapor laser using a single gas-discharge tube.Applying several copper vapor power amplifiers (PAs) an average output power higher than 10 kW has been also achieved and used for pumping of a multiple-stage dye laser for isotope separation [7].Though the low-temperature versions of this perspective laser, namely copper halide vapor lasers [8] and copper HyBrID (Hydrogen Bromide In Discharge) laser [9], have undeniable priorities over it and have been well developed and applied in the industry for micromachining of different materials, several advantages of the copper vapor lasers, such as higher average laser power, lower pulse repetition frequency, higher peak pulsed power, lasing stability, etc., are still unsurpassed.
In this paper, beam quality of the laser radiation delivered by a small-bore Cu vapor master oscillator (MO1) operating with a flat-flat stable cavity and a negative branch unstable resonator is compared with the laser radiation of the CuBr vapor MO (MO2), which is similar to the one described in [10].Two Power Amplifiers (PAs) with CuBr lasant and a considerably increased discharge zone are also studied in two different optical schemes.Two the best of our knowledge, an average laser power of 50 W at a beam propagation factor (M 2 ) equal to unit is obtained for the first time.Micron-sized ablation and machining of 0.4-mm Si sample are also accomplished.

Experimental setup
New high-temperature gas-discharge tube plotted in figure 1 is developed and used as MO1.Some features of the laser tubes are presented in table 1.A BeO ceramic insert with an inside diameter of 6 mm and a length of 30 cm is coaxially sleeved in the basic tube made of fused quartz.Compact thermal ZrO2 powder insulation of the discharge zone is applied to ensure the high operating temperature of 1500 0 C required.Ceramic inserts made of Al2O3 and confining the active zone with an inside diameter of 4 mm and a length of 15 mm are coaxially sleeved in the BeO ceramic insert.The Cu pieces are placed between the Al2O3 ceramic inserts.The construction of the CuBr vapor laser tubes, MO and PA pulsed power supplies and their synchronization were described in details in [10].The average laser power is measured through a calorimetric power-energy meter (Scientech Inc.Vector S310).Achromatic lenses with focusing length of 6, 12, 20, 40 and 100 cm are used as objectives, while the lenses with focusing length of 2, 3 and 4 cm are aspheric, in order to overcome the spherical aberrations.For precise material microprocessing an X-Y table (Aerotech Inc.) with digital control is used.Irradiated samples are characterized through optical confocal scanning microscope Zeiss LSM900.

Results and discussion
Preliminary study on the laser beam quality shows that Cu vapor laser tube placed in a standard flat-flat stable cavity produces laser radiation with unacceptable quality for precise material microprocessing despite of its small-bore of 4 mm.The two studied LS, namely MO1-DPA1-PA2 (LS1) and MO2-PA1 (LS2), are depicted in figure 2   Table 3 summarizes the obtained laser parameters, such as pulse recurrence frequency prf, total average laser power at the atomic Cu 510.6-and 578.2-nm lines laser pulse duration τp (FWHM) and laser pulse energy Ep.Total average output power Pout is measured before the target switching on and off the corresponding power supplies.It is well known that the use of double-pass PA reduces the average output power in comparison to the single-pass PA.Unfortunately, the low power extraction for the MO1 cannot saturate a single-pass PA.For further increase of the average laser power laser system with three laser tubes is used.
Applying the experimental method described in detail in [10] and the data given in [11] and [12], M 2 is obtained for several laser beam diameters D defined by the PAs aperture and various focal distances f.Table 4 summarizes the results obtained, where d th = f.θ00 is the diameter of a TEM00 Gaussian beam.Average value of M 2 is M 2 av = M 2 1av + M 2 2av = 1.100.Using the aspheric lens with focusing distance of 100 cm, images of microcraters (a) and microchannels (b) drilled or cut in Si sample are presented in figure 4, respectively.
Laser spot diameters without the heat affected zones, which increase with the laser power increase, determined from the images given in figure 3 and 4 are 1 μm and 53 μm.These values correspond to beam divergence of about 50 μrad.Beam divergence for a 20-cm TEM00 Gaussian beam is 32.5 μrad.
Thus the value of M 2 is about 1.5.Assessment of M 2 from the crater diameter or channel width is quite imprecise method, due to its dependence on the laser power or laser pulse energy.
Microcutting with multi-contour trepanning and wobbling of 0.4-mm Si sample is also realised.

Conclusions
The highest beam quality achieved so far with laser systems oscillating on the metal self-terminating transitions is confirmed by precise measurement of the threshold average output power for volumetric optical breakdown at different focal distances and various laser beam diameters.Diffraction-limited laser radiation with record-high average output power of 50 W is also obtained.Precise ablation and micromachining of Si samples with crater diameter or trench width of 1 μm is accomplished applying aspheric 2-cm focusing lens.

Figure 1 .
Figure 1.Schematic diagram of Cu vapor laser tube.

Table 1 .
Features of the discharge tubes: da -active zone diameter; la -active zone length; Va -active zone volume.

Table 2 .
Characteristics of optical elements.

Table 3 .
Characteristics of laser output.

Table 4 .
Laser spot diameter determined through volumetric optical breakdown for 510.6-nm wavelength.