ОПТИКА И СПЕКТРОСКОПИЯ, 2013, том 115, № 3, с. 522-526
ЛАЗЕРЫ ^^^^^^^^^^^^
И ИХ ПРИМЕНЕНИЕ
УДК 621.373.826
COMPACT PASSIVELY Q-SWITCHED Nd:YAG LASER FOR 2D MICROMACHINING © 2013 г. H. Aman, M. Rafiq
Diode-Pumped Solid—State Laser Lab, National Institute of Lasers and Optronics, Islamabad, Pakistan
E-mail: haroon111@gmail.com Received December 15, 2012
We present a compact passively q-witched diode end pumped Nd:YAG laser at 1064 nm for 2D micromachin-ing. It consists of a 5.5 cm long plano-concave end pumped resonator carrying a Cr:YAG passive q-switch inside the cavity. With an optical conversion efficiency of 46% and 33% the laser emits 1.4 W in CW and 986 mW in q-switched mode at a current of 2.5 A. After using a 2 mm circular aperture the output in seen in TEM00 mode with a single pulse energy of 5 mJ. The laser produced circular holes of diameter 75 p.m in 25 p.m thick Tantalum foils. Actual results of 1D and 2D machining are shown along with the diffraction patters of the samples.
DOI: 10.7868/S0030403413090043
INTRODUCTION
Laser Micromachining is a technique to make holes (circular apertures), patterns, marks, and structures of very small dimensions (from nanometers upto few microns) on different materials using high power lasers. These can by used in micro-fabrication, optical imaging, diffraction, measurements and sensors, interference and research purposes. For micromachin-ing we need sufficient power of the laser to affect the target, tight focusing on the surface can create smaller structures. Different materials have different ability to absorb the incoming laser wavelengths. So combining the affect of laser wavelength, focusing of the lens and laser power we can perform micromachining in few microns easily. The stability of the system and the alignment of the components play very important role on the quality of the output. A vibrating system can cause overlapped patterns whereas the misalignment of the focusing lens on the target surface can create elongated holes. One important parameter is the incoming beam quality and its duration of the impact. The shorter the pulse duration of the laser the finer and smaller holes will be created whereas the shape of the drilled holes depends on the incoming beam quality. Hence q-switched lasers are more favorable as compared to the CW lasers. In case of end pumped Nd:YAG lasers usually have good beam quality, for this reason end pumped lasers are preferable over the side pumped lasers but these lasers usually provide low output energies as compared to the side pumped lasers, so we have to design a compact and efficient system that has a good beam quality and sufficient energy to impact the target surface. Now a days laser micromachining is utilized in a wide variety of applications and research areas such as surface damaging or modification of optical materials [1, 2] and semiconductor [3]
such as polymers [4], silicon [5—7], nonlinear crystals [8, 9], and metals [10]. Some applications in microengineering are precision drilling and high density meshes, micro photolithography, production of micro-channels, slots, high resolution patterning of electrodes or complex structures, microelectronics and micro sensors. Circular apertures of different diameters can be easily prepared or specific patterns like 2D grid can be fabricated which may be used as optical filters for experiments in optical image processing, laser microscopy, holography and optical amplification. To use the system efficiently the laser output should have high peak power which comes from either increased pulse energy or narrow pulse width. Several lasers such as q-switched Nd:YAG lasers, mode locked lasers or femtosecond lasers are being used in micromachining and micro-structuring on the surface of thin films. But practically in most of the experiments of optical imaging apertures of diameter larger than 100 ^m are useful as smaller apertures provide low output which result in degraded contrast of the images.
Many laser systems are capable to perform micro-machining such as CW laser [11], q-switched laser [12], picosecond laser [13], femtosecond laser [14, 15] and modelocked laser [16] but all of them usually require very high input pulse energy. In most of the cases available Nd:YAG lasers require very high pulse energy and high electrical inputs because of electro-optic q-switching with water cooling requirements which makes the system bulky where as the stability is also an issue for such kind of systems due to the water cooled systems which provide less accuracy of temperature control due to which these systems have less accuracy and repeatability of the output as compared to the conductively cooled systems which provide stable output during long time operations.
Output coupler Aperture Focusing lens
FAN TEC Laser diode
Cr:YAG
Nd:YAG
Foil attached with mount
Laser diode driver
Position controller
Translation stage
Fig. 1. Optical setup for 2D laser micromachining system.
In this paper we report an efficient passively q-switched Nd:YAG laser operating at 1064 nm wavelength and its application for two-dimensional laser micromachining.
EXPERIMENTAL SETUP
The experimental setup of the 2D laser micromachining system is shown in the Fig. 1. It includes a 5 W (808 nm) CW laser diode (emitter dimension = 200 x x 1 ^m) as a pumping source connected with a Peltier cooler (Thermo Electric Cooled TEC system) attached which maintains the temperature requirement in order to keep a stable output at 808 nm which varies 0.3 nm/°C so that it can be efficiently absorbed in the Nd:YAG crystal. In the peltier cooler heat is transported out due to the motion of charges. The laser diode driver (operated with AC/DC + 12V adaptor) supply the necessary input towards the thermoelectric controller which maintains the output requirement for both the laser diode and the TEC. The heat from the Peltier cooler is pushed away by a fan. A double convex lens with diameter 5 mm, focal length 5 mm is placed in front of diode to focus the input @808 nm at the Nd:YAG crystal (diameter 3 mm, length 8 mm, doping 1%, AR@808 nm, HR@1064 nm/HR@808 nm, AR@1064 nm) with a damage threshold of 20 W/cm2 a passive q-switch Cr4+:YAG (dia = 3 mm, length = = 2 mm, AR/AR@1064 nm) is placed inside the cavity to achieve pulses of duration 10 ns at a rep-rate of 197 Hz. A plano-concave output coupler (dia = = 10 mm, ROC = 200 mm, T = 10%@1064 nm) is used to form a plano-concave resonator of length 5.5 cm. The cavity is aligned to obtain maximum output in TEM00 mode. The overall length of the laser system is approximately 15 cm, with output beam diameter 1.5 mm. After successful operation the laser diode gives a maximum CW output of approximately 3 W at 808 nm whereas after the laser is aligned and optimized it pro-
vides 1.4 W and 986 mW in CW and q-switched modes at 1064 nm, with an optical efficiency of 46% and 33% respectively as in Fig. 2. Before using the laser for drilling its spatial beam profile is observed by scanning a photodiode across the laser beam, the output of photodiode can be stored and displayed in computer, a plot ofbeam profile in TEM00 mode is shown as Fig. 3. In q-switched mode the laser emits a maximum pulse energy of 5 mJ per pulse with a pulse width of 10 ns (peak power = 0.5 MW).
After the laser is fully operational a 2 mm circular aperture is placed in front of the laser output to get a collimated beam. To focus the incoming laser beam on the target a double convex lens with focus 1.2 cm and anti-reflection coated at 1064 nm is used whereas the target which is a thin metallic foil is fixed in front of a mount which is attached with a translation stage for
Output current, A
3000 r
■ Laser diode output at 808 nm
» CW laser output
* Q-switched laser output
2000 -
1000
0 -
2.0 2.5 Input current, A
Fig. 2. Output power of pump diode (squares), CW (circles) and Q-switched Nd:YAG laser (triangles) versus input current.
OOTHKA H CnEKTPOCKOnHa tom 115 № 3 2013
Intensity, a.u. 1.0
4 8
Distance, mm
Fig. 3. Beam profile of the q-switched Nd:YAG laser.
linear movement whereas a vertical knob is utilized to vary the vertical displacement. The focusing lens is also attached with a movable mount to control fine movement along the optical axis in this way the position of the target on the lens focus can be achieved and at that situation arcing can be seen easily. The translation stage is driven by a position controller which displays the counts corresponding to the movement on a screen and it can be precisely controlled upto few microns, in this way the position of target foil can be varied for 2D micromachining. Hence the system is used for making holes in different patterns and marks with different spacing, sizes from one dimension to two dimensions.
RESULTS AND DISCUSSION
We use our system to prepare single, double, triple and multiple circular apertures in the 25 ^m thick
0
0
Fig. 4. Result of a single laser drill in the Tantalum foil.
cps 15 -
10 -
5 -
10
15
20
Energy, keV
Fig. 5. EDS of the exposed portion of foil.
ОПТИКА И СПЕКТРОСКОПИЯ том 115 № 3 2013
5
(a)
(b)
(c)
Fig. 6. Results of linear micromachining (a) single slit (b) double slit (c) triple slit.
Tantalum foil by laser drilling from one to two dimensions during which we achieve a minimum of 75 ^m diameter circular holes the Scanning Electron Microscope (SEM) image of it is shown in Fig. 4 the corresponding Energy Dispersive Spectroscopy (EDS) graph is also shown in Fig. 5 which shows the relative concentration of Ta and other elements at the position of laser drill, as the experiment was carried out in air and the temperature on the laser focusing point at the
target foil may get sufficiently higher where oxidation can take place easily so that is why a small concentration of oxygen is present and cracking is seen clearly at the portion where oxidation takes place in Fig. 4. During linear drilling in
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