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Abstract
Temporal and spatial shaping of laser beams is common in laser micromachining applications to improve quality and throughput. However, dynamic beam shaping (DBS) of ultrashort, high-power pulses at rates of hundreds of kHz has been challenging. Achieving this allows for full synchronization of the beam shape with high repetition rates, high-power lasers with zero delay time. Such speeds must manipulate the beam shape at a rate that matches the nanosecond to microsecond process dynamics present in laser ablation. In this work, we present a novel design capable of alternating spatial and temporal beam shapes at repetition rates up to 330 kHz for conventional spatial profiles and temporal shaping at nanosecond timescales. Our method utilizes a unique multi-aperture diffractive optical element combined with two acousto-optical deflectors. These high damage threshold elements allow the proposed method to be easily adapted for high power ultrashort lasers at various wavelengths. Moreover, due to the combination of the elements mentioned, no realignment or mechanical movements are required, allowing for high consistency of quality for high throughput applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Laser beam shaping has found uses in many applications advancing modern technologies and improving efficiency. Several fields include precision micromachining and micro-welding, super-resolution microscopy techniques such as STED, optical trapping, microlithography, micro-optics, various medical applications, optical data processing, and laboratory research [1–4]. Laser beam-shaping is an enabling process for increasing the stability, quality, and speed of laser-based processes [5,6]. Many processes that could benefit greatly from beam-shaping still do not utilize beam-shaping methods due to the time delays required to change the beam shape in real-time processing. Various temporal and spatial beam shapes are already a common practice in laser micromachining in microelectronic applications [5]. The capability to preform high quality beam shaping is needed for the delicate processing of advanced substrates where typically only several pulses remove a very thin layer. The increased usage of complex substrates with various thin stacked layers can also help greatly from fast beam shape changes to tailor optimal ablation conditions for the desired application. There are several common methods used for laser beam-shaping, each resulting in different forms of manipulating the intensity and phase distribution across the beam. Some examples are diffractive optical elements (DOE’s) [7–9], acousto-optic deflectors/modulators (AOD/Ms) [10–14], digital micromirror devices (DMDs) [15–18], and spatial light modulators (SLMs) [19–21]. Each type of device has unique characteristics that help define the optimal applications for each method. Phase elements can create any beam shape capable of being accurately described by a phase mask (within the limits of fabrication resolution). This is usually done by replicating the phase calculated by a Gerchberg–Saxton algorithm applied to the desired image. After which, the design is imposed into an optical element to manipulate the phase of the incident beam. Typical elements that adopt this method are SLMs and DOEs. DMDs find common use for beam shaping of two types, either by controlling a pixel’s on/off state by shifting the MEMS mirror or by controlling the uniform orientation of the whole array to correct for any aberrations in the resulting image/beam. AOD/Ms are commonly found as modulators for either controlling the power or the deflection angle of the incident beam by volume Bragg diffraction. Previously, AOD’s have been used to overcome limits set by mechanical mirror scanning in laser-matter interaction (LMI) applications, allowing order of magnitude faster scans and pulse to pulse positioning [22]. In this study, we focus on two of these methods, DOE’s for their many degrees of freedom in beam-shaping output along with the capabilities of using high-power laser pulses, and AOD’s for their fast switching capabilities. Table 1 presents the important characteristics for each type of beam-shaping method as can be used as a guide for choosing the proper method for an optical system. Our goal for having high-speed beam-shaping capability can be defined as a system that does not require any additional delay times when changing beam shapes. The only device that is capable of these switching speeds is AOD’s, as they are even used internally in laser architecture and therefore present an inherently limiting factor in the system reaching switching speeds in the hundreds of kHz. While AOD’s can be used for rapid beam-shaping as shown in [12], they have many limitations in producing unique spatial beam intensity distributions, specifically non-symmetrical shapes. Alternatively, DOE’s are devices with the highest resolution and most flexibility in beam-shaping but are passive devices. From this comparison, one can see that a shaping concept that can harness the advantages of both elements is of interest to many laser applications especially those that require high damage thresholds such as ultrafast high-power lasers. One concept that uses the combined advantage of AOM/D’s for power modulation and a multi-region DOE for beam-splitting is described in [23]. This shows a very promising method for the challenge of increasing throughput in ablation rates for uniform surface scanning applications by maintaining optical fluence value to maximize the specific ablation rates of the substrate [24]. While this concept shows the powerful combination we describe in this study, the use and placement of a multi-region DOE with AOD’s as deflectors creates a unique case of temporal and spatial beam-shapes presenting a solution to high-speed dynamic beam-shaping.
Table 1. Comparison of beam-shaping methods
Utilizing the design freedom with DOEs allows for several unique applications. While DOEs can be found in a variety of devices for producing Tophat or Bessel-like intensity distributions generated by an axicon, they can also be used as diffractive prisms with extremely high efficiency when tailored to a specific wavelength [25]. By combining these two functionalities on a single surface, fewer optical elements and higher overall system efficiency are achieved, increasing the viability for use with high power laser applications when both shaping, and deflection are required.
In cases where more than one beam shape is required for the process, the DOE must be mechanically removed from the optical path, resulting in increased calibration issues for misalignment after each move, see [26] for a detailed analysis of sensitivity to misalignments. This delay also presents a hindrance for high-throughput applications where mechanical steppers add valuable time delays (10s of milliseconds).
By developing a design that combines the fast switching times of an AOD with the high-quality beam shaping of a DOE we can select specific beam shapes at high speeds that do not reduce the process throughput, up to potential laser pulse repetition rates of 330 kHz. This results in high-speed spatial beam-shaping as well as a unique form of temporal beam-shaping when controlling the phase and pulsewidths of the input beam and RF signals. An additional benefit in this proposed method is that by not having any mechanical movements of the DOE or other optical elements, the result is perfect alignment accuracy each switching cycle. Further physical phenomena can also be explored as the interaction time of different pulse shapes can be tailored to optimize laser-matter interactions such as incubation effects.
2. Methods
The experimental setup (patented [27]) consists of 2 AODs and a unique multi-aperture DOE with dual functionality, see Fig. 1. The beam enters the system at the input of the first AOD which directs the first order deflection to the desired optical path (zero-order is dumped). The current design consists of three optical paths, each aligned to a specific region on the DOE for unique beam shapes. ${f_1}$ was chosen as a diffractive axicon, ${f_2}$ was left un-patterned for Gaussian beam to propagate, and ${f_3}$ was chosen as a Tophat beam shaper pattern. In addition to imposing the diffractive beam shaping pattern, the DOE also acts as a diffractive prism to direct the beam back to the second AOD window from each path. The DOE was designed to deflect the side beams at an angle of ${\pm} {1.5^0}$, this is so that with a distance of 22.9mm from each AOD, the deflection angle of the AODs would result in RF frequencies of ${\pm} 15{\; }MHz$, well within a higher diffraction efficiency. The path delay time for each of the side axis amounts to ∼2ps which is negligible for this design but can be utilized for temporal beam-shaping effects for interference patterning in the time domain (elaborated in section 4 on temporal shaping). This DOE was custom designed and developed to provide these unique characteristics.
Fig. 1. Illustration of the concept, where AOD1 defects the input beam, the DOE manipulates the phase for each angle separately and defects the beam to AOD2 which returns the beam to realign with the optical axis. ${O_{(0 )}}$ is the AOD zero order beam.
The laser used in the setup was a 532nm microchip laser by Teem Photonics with a pulsewidth of 750 ps and a maximum repetition rate of 4 kHz. This choice of wavelengths requires TeO2 to be used for the AODs resulting in a shear wave velocity of 660 m/s. Gooch & Housego AODs were used with a center frequency of 87 MHz and effective bandwidth of 55 MHz with a large aperture of 8.5mm to allow a beam of 3mm (1/e2) to propagate without affecting the beam-shaping qualities, as it is necessary to have ${\times} 2$ the aperture of the beam size for efficient DOE beam-shaping.
In addition to the elements described here, $\lambda /2$ waveplates are required for rotating the polarization before each AOD for optimal Bragg efficiency where a combined optimal point is required to be measured for all frequencies. Focusing optics for Fourier transform of the beam and a beam camera were used after the second AOD for imaging of the spatial profile. To design the optical system Zemax OpticStudio was used in which the AODs were simulated as apertures with diffractive gratings and separate orders for each frequency configuration. The DOE was inserted as a Blackbox provided by the manufacturer. The setup allows for the multifrequency operation where multiple paths can be combined resulting in multiple beam shapes with controlled intensity for each one. To utilize this, a digital complex signal is generated in MATLAB using an iterative algorithm to optimize the phase of each frequency to produce a maximum envelope within the AOD crystal. While this element has a maximum of three input frequencies, it can be scaled accordingly. The signal is then sent with a field-programmable gate array (FPGA) to a 16bit digital to analog converter (DAC) and passes amplification before entering each AOD transducer. The clocking time of the system is ∼1ns allowing for complex real-time multifrequency manipulation.
To determine the switching speed of beam-shaping we only need to consider the time it takes for the RF signal to pass through the interaction region with the beam, this is simply the spot size of the beam over the acoustic velocity multiplied by 3 to include the rise time and decay time. Due to our beam requirements, we had a 3mm beam at the AODs resulting in a time window of 4.5µsec. This should be multiplied by 2 to allow for rising and fall times resulting in a maximum switching rate of 9µsec ∼110 KHz. This can be further optimized by added beam expanding optics to the system to allow for beam-shaping with an input of 1mm at AODs resulting in a 3µsec switching rate for 330 kHz. To achieve much higher switching rates alternative AOD’s can be used such as quartz (fused silica) with acoustic velocities ∼${\times} 10$ that of the shear wave of TeO2 [28]. This range is several orders of magnitude higher than mechanical switching components, common for use of DOEs, and closely approaches optimal laser pulse repetition rates. See Fig. 2 for the full apparatus image.
Fig. 2. Optical setup of elements with two AODs and custom DOE with imaging optics for beam-shaping measurements. Elements are as follows: (a) Teem Photonics Laser source, (b) beam collimating optics, (c) $\lambda /2$ waveplate, (d) AOD1, (e) Multi-region diffractive optical element (DOE), (f) AOD2, (g) focus lens, (h) imaging beam camera.
3. Results
3.1 Energy efficiency
The first important characterization is the energy efficiency of the system. For many high-power laser processes where beam-shaping is critical, such as via formation, the power efficiency is a determining factor for methods of choice. As we are only operating with three frequencies on the AOD we can adjust the incident angle to assure a maximized Bragg efficiency. This is done by rotating the AOD to adjust the incident angle. Locating the Bragg angle is done by measuring intensity as a function of input RF power at each angle step for each of the three frequencies. The optimal position will be the overlap angle of all three peak powers, alternatively, a non-optimal point can be chosen that provides higher power for a single beam shape while being less efficient for others if the application requires it. In Fig. 3 this is shown by a calibration measurement where the optimal angle chosen provided above 80% efficiency for all beams within the AOD spec. The alignment for the second AOD is done similarly to the first. In the case where several beam shapes are desired to be used simultaneously, a slightly different efficiency would be obtained, and the phase of each frequency is needed to be adjusted to return to the optimal efficiency. The RF power can then be adjusted to control the desired intensity for each beam shape superimposed on each other. Additional methods for increasing the bandwidth efficiency can also be applied such as described in [29].
Fig. 3. Measurement of first-order output power from first AOD for each frequency (titled) used for its intentional beam shape as noted in titles. The vertical axis is for degree shifts, while the horizontal axis plots against the normalized RF power, one being the maximum input for the AOD (2.5 W). Colormap indicated the output power intensity in mW as measured.
Alignment of the DOE is also critical for beam-shaping quality and efficiency. As with any beam shaper, it is sensitive to any decentering and angular misalignments. Additionally, due to the multi-aperture design and the prism effect this DOE requires a high level of precision of the rotational angle of the element and distance from the AODs to achieve the optimally designed angle of incidence. A slight offset will result in non-uniform beam shapes as well as not realigning properly with the optical axis after the second AOD. The fabrication method used for this DOE was a 16-level lithography process with a grating spacing of 21.7um resulting in a simulated efficiency of 99%.
The efficiency of the DOE element as measured in the system resulted in 97% for center region Gaussian (no shaping area), 83% for square Tophat shape, and 77% for diffractive axicon ring shape. The losses to higher diffraction orders due to the angle of incidence on the diffractive prism were visible. Unlike for typical beam-shaping alignment when it is critical to assure proper beam collimation at the entrance point, this is difficult to measure for the deflected beams after propagating through the first AOD. While the DOE was designed to specific incident angles to match the AOD deflection angle, it would be more feasible to maintain direct entrance angles along the optical axis to assure collimation and add a separate relay lens before and after the DOE for deflection purposes. This additional relay may extend the optical path and may further be modified to increase the deflection angle to allow for a narrower AOD bandwidth. Combining these solutions can regain optical efficiency to maintain 85% from each AOD with near 99% efficiency of the beam shaper for a total system efficiency of 71% rather than the 55% demonstrated here.
While the total efficiency has several abating factors, it is important to mention the very high laser-induced damage threshold (LIDT) of the beam-shaping elements used as compared to other shaping methods, see Table 1. This high LIDT coupled with fast switching speeds presents a very promising method for beam-shaping for high throughput and high-power applications.
3.2 Spatial beam shapes
The resulting beam shapes are shown in Fig. 4 imaged with a beam camera and microscope. The resulting spot sizes were 97µm Gaussian, 220µm Tophat, 200µm Ring respectively. This was achieved with a focusing lens of 400mm where the Gaussian achieves the diffraction limit of the system. The maximum rate of the laser used in the setup was 4 kHz, testing for power and pointing stability was maintained within 0.2% while no optical issues were seen over a run of 5 minutes of beam switching every consecutive pulse. Some irregularities are seen in the beam shapes of Fig. 4, this is due to the quality of the input beam as well as having an ellipticity measured at 0.855. The high quality of DOE beam shapes is confirmed to not be disfigured after interacting with the AODs. Generating RF pulses at the maximum rate for the system disregarding the limitation of the laser is shown in Fig. 5. Each consecutive pulse profile is the result of a change in RF frequency controlling the deflection angles. If desired power modulation can also be achieved at this rate resulting in temporal shaping of the pulse energy.
Fig. 4. Resulting beam shapes at same focus point from left to right ${f_1}$-Ring, ${f_2}$-Gaussian, $\; {f_3}$-Tophat. Each beam shape is the result of steering the beam by different AOD RF frequencies through a different region on the multi-aperture DOE. Each image has a subfigure on the bottom left showing an optimal beam profile image.
Fig. 5. RF pulse train generation output to AODs at the maximum switching rate for the system. Spatial beam profiles are an example of a potential switching rate per pulse profile of 330 kHz. An annotation of RF frequencies is shown for each pulse.
3.3 Spatial and temporal combined beam-shaping
The designed apparatus can additionally adjust the temporal intensity profile of the recombined sub-beams by varying the respective amplitudes and phases of the frequency components in the RF drive signals. The mechanism of control depends on the relation between the duration of the laser pulse and the optical path length difference between the sub-beams. The optical path difference is accumulated due to the distance between diffractive regions on the DOE that each sub-beam passes through. In the constructed apparatus, with a 4mm spacing, this is on the order of several ps. If the pulsewidths are shorter than this time delay, then by applying a multi-frequency signal to the AODs, the recombined output will form a closely spaced pulse train, where each pulse has propagated through a different DOE region without overgoing temporal interference after realigning to the optical path at AOD2. As each region can have additional diffraction patterning for spatial beam-shaping each consecutive pulse within the pulse train can have a different spatial profile. For high power applications, similar pulse trains have shown promising value for controlling the plasma intensities for ablation applications resulting in a substantial improvement in quality and ablation efficiency [30,31].
In the case where the pulsewidths are longer than the optical path difference, they will undergo interference when recombining at the second AOD. To control this interaction the respective amplitudes and phases of the frequency components can be adjusted so that the extended output pulse or pulse burst has a desired temporal intensity profile. Figure 6 illustrates the concept of temporal beam-shaping by splitting a beam with a multi-frequency RF signal in AOD1, given by $F + \delta {f_i}$, and recombining the signal considering temporal interference at AOD2. The function of the DOE in this configuration is to behave as a diffractive prism that maintains collimation only diverting the respective beams at the required angle (negative of the incident angle; ${\theta _{out,i}} ={-} {\theta _{in,i}}$).
Fig. 6. Schematic illustration of the interference process of two coherent beams deflecting from AOD1 and recombining at AOD2 after passing through different diffractive prism regions on the DOE.
The pulse intensity profile at the exit AOD2 can then be described as follows:
For the temporal shaping to be effective, $\delta f$, the resulting summation shift in optical frequency and the laser pulse duration $\mathrm{\Delta }\tau $ should satisfy the overlap relation:
For example, using a pulse duration of 50ns and $\delta f = 20MHz$, the overlap value is $\gamma = 1$ while for 10ns the value $\gamma \; $is $0.2$, see Fig. 7 for temporal shapes several examples. In this case, a variety of temporal beam patterns can be generated by using multiple beams at different deflection angles and varying the phase of each RF signal. The multi-region DOE in the apparatus was designed for this temporal shaping with additional 4 diffractive prismatic regions, each with a different deflection angle. The pulse-width of the laser used is constant ∼800ps, resulting in a small overlap factor, while still too long for the mentioned pulse bursts. Alternatively, $\delta f$ can be returned to zero by aligning the AOD transducers with opposite orientations, thus eliminating the frequency shift and resolving coherence. The resulting temporal profile of the recombined sub-beams will behave as defined in Eq. (1).
Fig. 7. Graphs of simulated temporal shaping, full timescale represent the equivalent input pulse-width, the title denotes the gamma factor and average resulting pulsewidth of temporally shaped pulses.
Within each temporal profile, as shown in Fig. 7, a spatial profile is dynamically changing due to spatial and temporal interference. Simulating these interactions between 2 pulses, a Gaussian profile, and the resulting axicon profile from the DOE element is shown in Fig. 8. The pulses have equal phase shift and a $df$ of 40 MHz, initiating with a 50ns pulsewidth, replicating the leftmost instance in Fig. 7. Important to note that while this would be near impossible to measure, the laser-matter interactions that may result from this combined temporal and spatial beam-shaping may prove critical for applications such as high-resolution microscopy and laser material modification.
Fig. 8. Spatial shapes due to spatial and temporal interference between a 3mm Gaussian pulse and 5mm axicon, both initiating with a 50ns pulsewidth (see Visualization 1). Titles present the equivalent time as shown in Fig. 7 left.
4. Conclusion
In conclusion, a proposed concept for allowing high-quality dynamic beam-shaping has been established. An apparatus was developed capable of pulse to pulse beam-shaping, matching the switching speed of high laser repetition rates of 330 kHz, thus eliminating the requirement of mechanical movements. The system presents a method that has the potential to vastly increase the adoption of high-quality beam shaping for laser-matter interaction applications where the throughput/alignment requirements are high and therefore have not yet accepted beam-shaping as a viable solution. The additional capability of temporal shaping with the proposed setup presents a novel method of producing nanosecond pulse trains from much longer pulses as well as creating pulsetrains for ultrashort pulses with the combined control of spatial profiles. The methods presented in this paper have the potential of greatly advancing beam-shaping in laser micromachining and microscopy.
Funding
Israel Innovation Authority (65500).
Acknowledgments
The authors would like to acknowledge Dr. Ofer Fogel, Dr. Roman Vander, and Mr. Boris Kling for their professional discussions and advice.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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