The use of computer generated holograms together with spatial light modulator (SLM) enable highly parallel laser micromachining. Usually SLM is used for splitting the original laser beam to desired number of beams with equal intensity. However, this technique also enables that the intensity of every beam can be controlled individually. Example of the hologram designing procedure for separation of the original beam to 400 beams with individually controlled intensity is presented. The proposed technique is demonstrated by femtosecond laser ablation of grayscale pictures so that grey scale of the pixel is addressed with corresponding beam intensity in the ablated picture.
© 2014 Optical Society of America
Computer generated holograms (CGH) have been around for decades and they are convenient way to create programmable diffractive optics. Together with the spatial light modulator (SLM) they can be used for various laser beam shaping tasks in micromachining [1–10]. Recently the power handling capacity of SLMs has increased so that they can be applied with relatively high peak and average power pulse femtosecond lasers. The use of the femtosecond laser enables generation of small spot sizes and ablation features. Ablation of the small features usually requires only a small fraction of laser power to be delivered to the ablation spot. When using only a single beam for the ablation of the small features this process is time consuming.
Micromachining using femtosecond laser with relatively high pulse power together with SLM the original beam can be divided up to hundreds, or even to thousands beams and still have the energy of the individual beam above the ablation threshold of the material. This division of the original beam to multiple beams enables the utilization of all laser power regardless of the machining task. Parallel micromachining together with SLM technology enables simultaneous controlling of various ablation parameters like position, size, shape and period. One important and overlooked possibility of this technique is individual control of intensity of each beam and this opens new possibilities for the laser micromachining.
The probable reason for omission of parallel processing in laser micromachining is that the hologram designing is perceived as complicated and time consuming. There is a lack of proper commercial hologram designing tools that would be suitable for micromachining purposes. Here we present designing method resulting relatively fast calculation of the holograms producing a matrix of beams with individually controlled intensity. The method is based on the Iterative Fourier Transform Algorithm (IFTA) [11–13] with camera compensation. Usually IFTA is used for the phase retrieval using constant target amplitude. Here IFTA is used so that the phases of the signal diffraction orders are kept constant at image plane between the iteration cycles and amplitude corrections are incorporated using camera feedback loop. This feedback enables one to compensate errors coming from environmental changes in laser and optical setup. Previously camera feedback has been used in similar fashion for improving the uniformity of CGH based on the multiplexed phase Fresnel lenses  and second harmonic optimization of CGH .
Technique is demonstrated for parallel micromachining where original laser beam is divided up to a 400 beams of varying intensity. The beams are used for copying of grey scale images to the silicon surface. Grey scale of the pixel in the original picture is addressed with corresponding beam intensity to ablate different size area. This parallel processing combining femtosecond laser and SLM enables ablation with feature sizes down to a few micrometers using very simple optical setup.
2. Setup and hologram designing
Experiments were done using Quantronics Integra-C femtosecond laser providing up to 3 mJ pulse energy and 120 fs long pulses at 790 nm central wavelength with 1 kHz repetition rate. Hamamatsu X10468 series LCOS-SLM (Liquid Crystal on Silicon Spatial Light Modulator) was used in the experiments. SLM has 600 x 792 pixels and the pixel size was 20 μm x 20 μm. The pulse energy was limited to 1.5 mJ due to the limited peak power handling of the SLM. The set-up used in the experiments, consisting computer controlled SLM, energy attenuator, imaging system and translation stage, is shown in Fig. 1. Lens F1 = 500 mm collimates beams from SLM and F2 = 80 mm focus beams on the sample surface. Lens F3 = 400 mm focuses beams to camera 1 which is used to measure the amplitudes for IFTA. Camera 2 with lens F4 = 200 mm is used to monitor ablation process. Matlab software by Mathworks was used to implement IFTA, capture the measured intensities from the camera and feed the holograms to the SLM.
The size of the ablated features can be controlled by hologram and scaled by changing the magnification of the imaging system. The hologram on SLM is a combination of holograms for the desired intensity distribution and 500 mm Fresnel lens. Fresnel lens remove the unwanted zero diffraction order from the image plane . In addition to the Fresnel lens, beam blocker was applied to the focus of the lens F1 in order to remove zero order. If zero order is not blocked, it can add a non uniform background to the designed intensity pattern.
IFTA is an efficient method for designing multilevel phase holograms with high diffraction efficiency and low speckle formation. Iterative design of the holograms was done using procedure shown in Fig. 2. Iteration starts by forming target signal amplitude which is then combined with a random initial phase to form complex electric field at image plane. This field is transferred to component plane using Fourier transforms. Amplitude of the field in the component plane is replaced with the Gaussian profile of the used laser. Next the amplitude at image plane is resolved either by transferring the field by Fourier transforms or by inserting phase to SLM and by taking an image using camera, which is real image of the amplitude at image plane. Amplitude is then compared to required target signal intensity and if result is acceptable iteration ends. If result is not acceptable then target signal amplitudes are adjusted by dividing original amplitude values with measured values. The phase of the signal obtained from the iteration cycle is replaced with the initial random phase. Iteration is continued until result is acceptable or stagnation of the iteration is reached. This stagnation of the iteration results from the fact that the selected random phase is not going to give acceptable result. In this case new starting phase must be selected and iteration is started from beginning.
The proposed implementation of IFTA is selected to minimize the number of iteration cycles and calculation time of the hologram. As a drawback of this implementation, more energy goes to the diffraction orders outside the signal window than when applying IFTA using phase freedom. However, this is not a problem as long as the intensity of these orders is kept under the ablation threshold of the processed material. The camera feedback loop also helps to align the setup and to correct amplitude errors caused by less than ideal laser beam profile.
The parallel micromachining using beams with different intensities is demonstrated experimentally by ablating grey scale images. Grey scale of the pixel is addressed with corresponding beam intensity in the ablated picture. Original laser beam is divided to the matrix of beams with each beam corresponds a pixel on the ablated picture. Higher beam intensity produces larger ablation area and results a darker pixel. In Fig. 3 this procedure is illustrated with hologram in Fig. 3(a) designed to produce matrix of beams, shown in Fig. 3(b), with rows of equal intensity while intensity is gradually changing in the columns and resulting ablated pattern in Fig. 3(c).
Note that the diameter of the ablated hole is not linearly related to the laser fluence and this must be taken into account when designing holograms. In Fig. 4 is shown measured diameter of ablated hole as a function of used fluence for silicon, measured from the test patterns shown in the Figs. 3(b) and 3(c). Into the data of diameter squared a linear fitting can be made when fluence is plotted with logarithmic scale. Using this linear fitting one can extrapolate the value for ablation threshold Fth = 0.125 J/cm2 of the silicon. The result correspond well to the values given in the literature [16,17]. Using this information it is possible to calculate how much energy the hologram must deliver to each position in order to get desired ablated spot size. Note that the variation in order of few percents can be detected in the intensities that are designed to be equal. However, this is only problematic when using fluence values close to the threshold value of the material. In this case a few percent variation on fluence results a much bigger variation to the hole size.
In Fig. 5 is shown this ablation process in larger scale by copying a portrait of the co-author Dr. Kaakkunen. The original picture is sampled to the desired amount of pixels and divided to the sub-images of 20 x 20 pixels. This is the amount of the beams that we can divide our original laser beam so that the energy of the individual beams is well above the ablation threshold of the silicon. The hologram is calculated for every sub-image and intensity patterns are ablated successively in order to form the whole portrait. Note that the energy level for the ablation of each hologram must be calculated and adjusted in order to join different sub-images. The portrait is ablated using 42 different holograms. Each hologram is designed so that the adjacent pixels are generated with different holograms in order to prevent interference of the adjacent beams at the sample surface. The size of the ablated picture is 0.95 x 1.4 mm. The maximum size of the simultaneously ablated image that can be obtained is restricted by the optical set-up and the pixel size of the SLM. Each hologram was illuminated with 40 pulses. The ablation process took 30 seconds where 7 seconds was used for laser illumination and rest of the time was consumed by the adjustment of laser energy levels on SLM between the different holograms. The energy is adjusted with computer controlled attenuator. The average calculation time for the each hologram was 25 seconds.
When ablating larger image areas than shown above the sample must be moved between the consecutive ablations. In Fig. 6 this ablation process with larger scale is demonstrated by copying a portrait of Mr. Kekkonen. Now the image is divided to 150 sub-images and thesample is moved between the successive ablations using translation stage. In Fig. 7 are shown close-up microscope images of the ablated patterns. Images show that the holograms can be designed so that the ablations can be joined to each other almost seamlessly. The diameter of the ablated spot area varies from 1 μm to 15 μm. The size of the portrait is 9.6 x 6.4 mm. The portrait is ablated using 200 pulses with each hologram. The ablation process took 10 minutes where 30 second was used for laser illumination and rest of the time was consumed by laser energy adjustment and translation table. Note, that the processing time can be greatly diminished, for example, by using galvanometer scanner instead of translation table for moving the illumination on the processing area. Average calculation time for each hologram was 3 seconds.
In this paper we have shown that beam matrix of varying intensities can be utilized for complex ablation task with high precision. SLM together with CGH is used for the generation of the beam matrix of varying intensities. A procedure based on the IFTA with camera compensation for the calculation of the holograms is presented. Varying amplitudes give us the control over the size and depth of the ablation feature and parallel ablation gives the increased processing speed compared to conventional single beam ablation. Technique is demonstrated for ablation of grey scale images to the silicon surface using parallel micromachining where original femtosecond laser beam is divided up to a 400 beams of varying intensities. Ablation feature sizes down to a few micrometers were produced using this technique. Various laser micromachining applications, for example engraving, drilling and marking, can benefit from this technique.
This work was funded by the Finnish Funding Agency for Technology and Innovation (TEKES).
References and links
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