Abstract:
A microfilter design system for use with a laser drilling system producing multiple sub-beams for parallel drilling operations includes an optical intensity detector illuminated by the multiple sub-beams of the laser drilling system. An analysis module operates the optical intensity detector to produce intensity measurement data for each of the multiple sub-beams. A memory operable with a data processing system stores the intensity measurement data for analysis.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application Serial No. 60/398,400 which was filed on Jul. 25, 2002 and is incorporated by reference herein. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention generally relates to laser drilling and particularly relates to microfilter design for laser drilling systems producing multiple sub-beams for parallel drilling operations.  
         BACKGROUND OF THE INVENTION  
         [0003]    Material ablation by pulsed light sources has been studied since the invention of the laser. Etching of polymers by ultraviolet (UV) excimer laser radiation in the early 1980s led to further investigations and developments in micromachining approaches using lasers—spurred by the remarkably small features that can be drilled, milled, and replicated through the use of lasers. A recent article entitled “Precise drilling with short pulsed lasers” (X. Chen and F. Tomoo, High Power Lasers in Manufacturing, Proceedings of the SPIE Vol. 3888, 2000) outlines a number of key considerations in micromachining. Other recent patents of interest include the following:  
           [0004]    U.S. Pat. No. 6,252,714, “Diffractive homogenizer with compensation for spatial coherence,” describes a diffractive homogenizer for receiving a beam of laser energy and producing a desired illumination pattern in a target plane. The homogenizer is made up of a plurality of diffractive sub-elements, each of which contributes to all or a portion of the desired image. By combining the contributions of many sub-elements to form the final image, a homogenizing effect is realized. In preferred embodiments, the sub-elements are designed to compensate for the finite spatial coherence of the incident laser beam and to control the numerical aperture distribution of the transmitted light. Each sub-element is composed of a large number of discrete pixels, each of which alters the phase of radiation passing therethrough by a selected amount. The pixel arrangement is chosen; using computer modeling and optimization techniques, such that the interference pattern created by the collective pixels in a sub-element makes up the desired image (or a portion thereof). A technique is also provided for reducing the intensity of the image formed by a selected sub-element, which may be located in a laser “hot spot”, by randomizing a selected percentage of the pixels located in that sub-element. This diffractive homogenizer is useful in various laser ablation and annealing, and other laser material processing applications.  
           [0005]    U.S. Pat. No. 6,243,209, “Method and apparatus for providing rectangular shape array of light beams,” describes a linear array of equal intensity optical beams transformed into a rectangular array of equal intensity optical beams, while the intensity of each beam is kept nearly constant. The transformation is performed using an optical element that has two coatings on the front surface and a reflective coating on the opposing back surface. The front surface is partially coated with a reflective coating and partially coated with an anti-reflective coating. The beams are incident upon the front surface, with some of the beams incident on each of the two different coatings on the front surface. The beams incident on the front surface are specularly reflected. The remaining beams are transmitted through the optical element to the back surface, reflected from the back surface, and transmitted back up through the optical element and exit from the front surface. The exiting beams are thus shifted laterally and transversely to define the desired rectangular array. The index of refraction, thickness of the optical element, and the incident angle of the beam are selected to achieve the desired arrangement of beams.  
           [0006]    U.S. Pat. No. 6,236,509, “Diffractive optical system with synthetic opening and laser cutting device incorporating this system,” describes an optical device for focusing a light beam. The device includes a Fourier diffractive element that can separate an incident beam into n beams along n directions that are symmetric about an optical axis. The device also includes a diffractive element including a Fresnel lenses capable of refocusing the n beams onto the optical axis. The device may be used with lasers and laser cutting devices.  
           [0007]    U.S. Pat. No. 6,025,938, “Beam homogenizer,” describes a beam homogenizer that minimizes undesired intensity variations at the output plane caused by sharp breaks between facets in previous embodiments. The homogenizer includes a hologram made up of irregularly patterned diffractive fringes. An input beam illuminates at least part of the hologram. The hologram transmits a portion of the input beam onto an output plane. In doing so, the energy of the input beam is spatially redistributed at the output plane into a homogenized output beam having a pre-selected spatial energy distribution at the output plane. Thus, the illuminated portion of the output plane has a shape predetermined by the designer of the homogenizer.  
           [0008]    U.S. Pat. No. 5,566,024, “Beam separation control and beam splitting by single blazed binary diffraction optical element,” describes two sets of two single blazed binary diffractive optical elements that form a beam separation control apparatus for expanding two closely spaced parallel beams into two wider spaced parallel beams or for contracting two wider spaced parallel beams into two closely spaced parallel beams. Four sets of two single blazed binary diffractive optical elements form a beam separation control apparatus for separating two closely spaced parallel beams into two wider spaced parallel beams for possible modulation or other optical effect, then returning the two beams to be closely spaced and parallel. A set of two adjacent and opposite single blazed binary diffractive optical elements can form a beam splitting apparatus or a beam combining apparatus.  
           [0009]    Ultrafast lasers generate intense laser pulses with durations from roughly 10 −11  seconds (10 picoseconds) to 10 −14  seconds (10 femtoseconds). Short pulse lasers generate intense laser pulses with durations from roughly 10 −10  seconds (100 picoseconds) to 10 −11  seconds (10 picoseconds). Along with a wide variety of potential applications for ultrafast and short pulse lasers in medicine, chemistry, and communications, short pulse lasers are also useful in milling or drilling holes in a wide range of materials. In this regard, hole sizes in the sub-micron range are readily drilled by these lasers. High aspect ratio holes are also drilled in hard materials; applications in this regard include cooling channels in turbine blades, nozzles in ink-jet printers, and via holes in printed circuit boards.  
           [0010]    Parallel processing of laser-milled holes is a key technique for increasing throughput in laser micromachining. Beamsplitting devices (beamsplitters) such as diffractive optical elements (DOEs) are used in laser micromachining to divide a single beam into multiple beams and thereby achieve parallel machining. However, such use of beamsplitters introduces technical challenges in hole geometry requirements and in the ability to produce consistent results. Such challenges need to be overcome in order to maintain consistency and repeatability in laser milling.  
           [0011]    Inkjet nozzle design, construction, and operation are all important factors in providing high quality inkjet print resolution. Inkjet nozzle designs, which typically include specific patterns of many ink jet holes, which in turn are also specific defined geometries, provide the templates for nozzle holes drilled in a thin foil or polymer to a particular shape. Each nozzle hole includes an input section, a shaped section and an exit hole section, and each exit hole section is preferably cut with a high degree of precision respective to the design pattern. In a particular nozzle inconsistency in nozzle hole shape leads to inconsistent expulsion of inks among the individual holes in an inkjet nozzle, which negatively affects print resolution. Therefore, imperfections in the shape of the inkjet nozzle holes respective to the design pattern negatively impact print quality.  
           [0012]    When a DOE is used to produce multiple sub-beams for parallel machining, generally there is variation in beam strengths among the sub-beams, i.e., some sub-beams are more intense than the average sub-beam strength and some are weaker than the average sub-beam strength. The variation is caused by the design and/or fabrication imperfections of the DOE. The beam strength variation among the sub-beams leads to size variations among the machined geometries. Stronger sub-beams tend to machine larger sizes. If the beam strength variation is too large that the machined geometries exceed the product specification, and thus means must be found to reduce the beam strength variation among the sub-beams of the DOE.  
           [0013]    Microfilters are used in equalizing sub-beam intensities to enable a parallel process laser drilling system to drill consistent workpiece geometries. One important application for such a use is in inkjet nozzle hole manufacture. However, the respective microfilter is also subject to factors derived from manufacturing errors and design limitations. In this regard, microfilters do not, as delivered, predictably sufficiently equalize the intensities of sub-beams in parallel process laser drilling systems because the microfilters are designed with inaccurate sub-beam intensity data. This data is inaccurate insofar as it is theoretical as based on design inputs of the beamsplitter, rather than being based on empirical measurements of actual sub-beam intensities. Current technology does not provide a way to empirically measure the intensities of sub-beams to a level of accuracy acceptable for use in designing a microfilter for use in precision parallel laser drilling.  
           [0014]    What is needed is a way to improve accuracy of measuring relative beam intensities in parallel process laser drilling system so that the design input parameters for microfilter design will be more accurate and so that microfilter designs and the resultant microfilters will improve to provide sufficiently balanced and homogeneous parallel subbeams for consistent hole manufacture. The present invention provides a solution to this need.  
         SUMMARY OF THE INVENTION  
         [0015]    According to the present invention, a microfilter design system for use with a laser drilling system producing multiple sub-beams for parallel drilling operations includes an optical intensity detector illuminated by the multiple sub-beams of the laser drilling system. An analysis module operates the optical intensity detector to produce intensity measurement data for each of the multiple sub-beams. A memory operable with a data processing system stores the intensity measurement data for analysis.  
           [0016]    The present invention provides a method for providing sub-beam impingement intensity control from a set of sub-beams generated from a parallel process laser system and impinged upon a target, where at least two of the sub-beams having an impingement separation at the target of less than about 260 microns by measuring the impingement intensity of each sub-beam to generate a sub-beam intensity measurement and attenuating the intensity of each sub-beam in response to the measurement.  
           [0017]    In preferred form, the invention uses, in the measuring step, a scanning diode and blocking plate with an aperture positioned to pass the sub-beam whose intensity is being measured to a target point, while at the same time blocking all adjacent sub-beams from passing to impinge upon the target point being measured.  
           [0018]    As should be readily appreciated, the invention also provides a laser cutting apparatus, such as used in manufacturing an inkjet nozzle, which uses a microfilter derived from the above steps.  
           [0019]    A number of advantages are provided with the invention. By providing a way to improve the accuracy of measuring relative beam intensities in a parallel process laser drilling system, further derived benefits of improved microfilter design parameters and improved mirofilter design are readily realized. A solution approach is also achieved for compensating for fabrication errors and minor defects in diffractive optical elements. An approach is also derived for compensating for final intensity variations between sub-beams emitted from a diffractive optical element. Print resolution in inkjet printers is also realized when the inkjet nozzles of the printer are manufactured with the benefit of the invention.  
           [0020]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0022]    [0022]FIG. 1 presents a schematic of a laser drilling system;  
         [0023]    [0023]FIG. 2 shows an intensity measuring system used in designing a microfilter;  
         [0024]    [0024]FIG. 3 shows a method of designing a microfilter using the intensity measuring system of FIG. 2;  
         [0025]    [0025]FIG. 4 shows a sample data plot of sub-beam data used in designing a microfilter in accordance with the present invention;  
         [0026]    [0026]FIG. 5 provides a perspective view showing major constituent components of an ink-jet printer; and  
         [0027]    [0027]FIG. 6 provides a schematic, cross-sectional view of an ink-jet head. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0029]    In overview, one embodiment of the present invention provides a method of designing a microfilter to be used in a parallel process laser drilling system, including the steps of providing a parallel process laser drilling system that generates a plurality of sub-beams in a pre-defined pattern, measuring the strength of each individual sub-beam for subsequent analysis, analyzing the intensity data in order to determine appropriate design parameters for a customized microfilter, and designing and fabricating the customized microfilter according to these design parameters.  
         [0030]    In another embodiment, a continuously scanning photodiode designed to empirically measure the intensities of all the sub-beams generated by a beamsplitter is employed in the measuring step. The photodiode scans the strength of the sub-beams generated by the beamsplitter at a constant rate of speed around the pattern of sub-beams, and a computer defines the pattern, stores the intensity data, and analyses the intensity data by normalizing the data against the intensity of the laser beam measured by a stationary photodiode.  
         [0031]    In yet another embodiment, each sub-beam is measured through an aperture in front of an intensity detector, and the intensities of a statistically significant sample of laser beam pulses are measured by a scanning photodiode through all sub-beams in a workpiece drilling pattern.  
         [0032]    In a further embodiment, a customized microfilter article is specifically designed from the measurements to equalize strength of its sub-beams within a parallel process laser drilling system.  
         [0033]    In another embodiment, an inkjet nozzle article is produced by a laser drilling system having the customized microfilter.  
         [0034]    Turning now to specific details in the preferred embodiments, FIG. 1 shows a simplified schematic of a laser drilling system  100 , including a laser  105 , a beam  107 , a shutter  110 , an attenuator  115 , a beam expander  120 , a spinning half-wave plate  125 , a first mirror  108 , a second mirror  117 , a third mirror  121 , a fourth mirror  122 , a piezo electric transducer (PZT) scan mirror  130 , a diffractive optical element (DOE)  135 , a plurality of sub-beams  137 , a scan lens  140 , a microfilter  145 , an image transfer lens  150 , and a workpiece  155 , arranged as shown. All elements of laser drilling system  100  are conventional in laser micromachining.DOE  135  is a highly efficient beamsplitter and beam array pattern generator that allows laser drilling system  100  to drill parallel holes in workpiece  155 .  
         [0035]    The pattern of sub-beams  137  output by DOE  135  is pre-determined by the specifications of the holes to be drilled in workpiece  155 . In an alternate contemplated embodiment pursuant to anticipated improvements in beam quality of excimer lasers, an excimer laser with a kinoform is used in place of DOE  135 . In one example, DOE  135  splits the single incident laser beam  107  from laser  105  into 152 beams in the form of 4 rows with 38 beams in each row. (The excimer/kinoform information is from Holmér and H{dot over (a)}rd&#39;s  1995  paper “Laser-machining experiment with an excimer laser and a kinoform” in Applied Optics which is hereby incorporated by reference herein).  
         [0036]    Scan lens  140  determines the spot size of sub-beams  137  upon workpiece  155 . The beam size that enters scan lens  140  must be less than or equal to the pupil size of scan lens  140 . Telecentricity is required to keep the incident angle between sub-beams  137  and workpiece  155  perpendicular, which is necessary to drill parallel holes in workpiece  155 . In the present invention, scan lens  140  is an f-theta telecentric (scan) lens. In alternate embodiments where the axes of the holes do not need to be parallel to each other, a non-telecentric scan lens is used.  
         [0037]    Microfilter  145  equalizes the uniformity of sub-beams  137  emitted from laser  105  and through DOE  135 . Microfilter  145  consists of dielectric coatings on a glass substrate, and is custom designed and fabricated according to the intensity patterns of sub-beams  137  of DOE  135 . In one example, microfilter  145  consists of two transmission values, 100% and 98%, in a pattern of 152 individual filters of 4 rows with 38 filters in each row that corresponds to the example given to DOE  135  above. In this example, each of the individual filters is circular in shape with a diameter of 250 microns.  
         [0038]    In operation, laser  105  emits beam  107  along the optical path identified in FIG. 1 above. Beam  107  propagates along the optical path, where it is incident upon first mirror  108 . First mirror  108  redirects beam  107  along the optical path, where it is incident upon shutter  110 . Shutter  110  opens and closes to selectively illuminate the material of workpiece  155 . Beam  107  exits shutter  110  and propagates along the optical path to attenuator  115 . Attenuator  115  filters the energy of laser  105  in order to precisely control ablation parameters. Beam  107  exits attenuator  115  and propagates along the optical path, where it is incident upon second mirror  117 . Second mirror  117  redirects beam  107  along the optical path, where it is incident upon beam expander  120 .  
         [0039]    Beam expander  120  increases the size of beam  107  to match the pupil size of scan lens  140 . Beam  107  exits beam expander  120  and propagates along the optical path, where it is incident upon third mirror  121 . Third mirror  121  redirects beam  107  along the optical path, where it is incident upon fourth mirror  122 . Fourth mirror  122  redirects beam  107  along the optical path, where it is incident upon spinning half-wave plate  125 . Spinning half-wave plate  125  changes the polarization of beam  107 . Upon exiting spinning half-wave plate  125 , beam  107  propagates along the optical path, where it is incident upon PZT scan mirror  130 . PZT scan mirror  130  moves in a pre-defined pattern using a drilling algorithm (which executes on computer such as computer  255 —see FIG. 2) to drill the holes in workpiece  155 . PZT scan mirror  130  redirects beam  107  along the optical path, where it is incident upon DOE  135 . DOE  135  splits beam  107  into a plurality of sub-beams  137 , which allow parallel drilling of workpiece  155 . Sub-beams  137  exit DOE  135  and propagate along the optical path, where they are incident upon scan lens  140 . Scan lens  140  determines the spot size of sub-beams  137  upon workpiece  155 . Sub-beams  137  exit scan lens  140  and propagate along the optical path, where they are incident upon microfilter  145 . Microfilter  145  equalizes the intensities of sub-beams  137 . Sub-beams  137  exit microfilter  145  and propagate along the optical path, where they are incident upon image transfer lens  150 . Image transfer lens  150  re-images the focal spots of sub-beams  137  onto workpiece  155 . Sub-beams  137  ablate workpiece  155  in a pattern according to the pre-defined milling algorithm.  
         [0040]    Beamsplitters such as DOE  135  generate sub-beams  137  that exhibit variable intensity distributions unacceptable for performing precision parallel process laser drilling. As will be further described, the present invention provides a way to compensate for these intensity variations in beamsplitters (e.g., DOE  135 ) by deriving a microfilter from empirically measured sub-beam intensities emitted from a specific beamsplitter.  
         [0041]    [0041]FIG. 2 shows an intensity measuring system  200  for designing microfilter  145 , including: beam  107 , DOE  135 , sub-beams  137 , scan lens  140 , aperture  240  positioned in front of a scanning photodiode  250 , first gated integrator  253 , computer  255 , pick-off mirror  260 , reference beam  265 , stationary photodiode  270 , and second gated integrator  273 .  
         [0042]    Intensity measuring system  200  is used to design a customized microfilter (e.g., microfilter  145 ) to equalize the intensities of sub-beams  137  from a beamsplitter (e.g., DOE  135 ) within a specific parallel process laser drilling system. The resultant, customized microfilter  145  is designed to work within a specific laser drilling system to drill a specific pattern and workpiece geometry in a specific workpiece  155 .  
         [0043]    Beam  107 , DOE  135 , sub-beams  137 , and scan lens  140  function as described above with respect to laser drilling system  100  in FIG. 1. Aperture  240  is a circular hole that allows sub-beams  137  to reach and be measured by scanning photodiode  250 . Aperture  240  ensures that sub-beams  137  are incident upon the target area of scanning photodiode  250  such that scanning photodiode  250  measures beam strength individually. Accurate measurement of individual sub-beams  137  is not possible when more than one sub-beam  137  is incident upon scanning photodiode  250  simultaneously. To avoid this problem of more than one sub-beam  137  being incident upon scanning photodiode  250  at one time, the size of aperture  240  is defined by the spacing between the holes in the targeted workpiece geometry and the beam diameter at focus, such that only one sub-beam  137  is incident upon scanning photodiode  250  at a given time but the aperture  240  is large enough to transmit the whole sub-beam  137 . In one example, with an 8×38-hole target pattern in an inkjet nozzle foil, the holes in the workpiece pattern are 250 microns apart and aperture  240  is circular with a diameter of 100-microns to minimize the possibility of more than one sub-beam being measured by scanning photodiode  250  at once.  
         [0044]    Scanning photodiode  250  is a low-noise, high dynamic range photodiode, such as those manufactured by Thorlabs, Inc. The opto-electronic response of photodiode  250  needs to be frequency-adequate. In one example, silicon photodiodes only respond to light of 185-1100 nm. Backward bias and low impedance are used to obtain high speed measurments. Scanning photodiode  250  is placed beyond the focal plane where sub-beams  137  diverge slightly, such that the strength is lower than in the focal plane. The signal-to-noise ratio is maximized by using the largest possible photodiode under full illumination with a response time that meets the particular requirements of the sub-beams  137  under measurement. The response time sets an accompanying limit on noise reduction when time-gating technique is used in the measurement. In one example where measuring laser  105  pulses at a repetition rate of 1 kHz, a millisecond is the allowable time for scanning photodiode  250  to measure and log the strength of a single sub-beam  137  pulse before the subsequent pulse is incident upon scanning photodiode  250 . In this same example, scanning photodiode  250  is a 1-mm square photodiode with a response time of less than 1 nanosecond into an impedance of 50 ohm.  
         [0045]    Scanning photodiode  250  linearly responds to the strength of sub-beams  137  and sends an electrical pulse to first gated integrator  253 . A log file is stored on computer  255  with all the measured pulses of sub-beam  137  for intensity analysis to be performed after all sub-beams  137  have been measured. In one example measuring an 8×38 pattern, scanning photodiode  250  takes approximately two hours to complete measuring all of sub-beams  137  in the pattern. In this example, it takes two hours to scan through the pattern, measuring a statistically significant sample of pulses from all sub-beams  137  in the pattern. In the same example, scanning photodiode  250  and aperture  240  move through the pattern one row at a time and one sub-beam after another sequentially in each row. The scanning speed is chosen such that a sufficient amount of data is collected from each sub-beam  137 . The moving path is predetermined according to the pattern geometry of sub-beams  137 .  
         [0046]    First gated integrator  253  and second gated integrator  273  integrate the pulse with a rolling average for a fixed number of pulses to determine the strength of sub-beams  137  and reference beam  265  respectively. First gated integrator  253  sends an analog signal with strength proportional to the strength of sub-beam  137  to computer  255 . Second gated integrator  273  sends an analog signal with strength proportional to the strength of reference beam  265  to computer  255 . First gated integrator  253  and second gated integrator  273  transfer pulsed input signal to a quasi-continuous output that is captured by computer  255 . In alternate embodiments, first gated integrator  253  and second gated integrator  273  are not required when a quasi-continuous laser is used for the measurement.  
         [0047]    Computer  255  is a computer with a connection to scanning photodiode  250 . Computer  255  contains an analog-to-digital converter to convert the level of analog signal from both gated integrator  253  and  273  to digital data. Computer  255  defines the scanning path of scanning photodiode  250 , stores intensity data of all scanned sub-beams  137  for subsequent analysis, and provides a means for subsequent analysis.  
         [0048]    Pick-off mirror  260  reflects a portion of beam  107 , creating reference beam  265 . One side of pick-off mirror  260  is coated with anti-reflection coating to avoid beam interference.  
         [0049]    Reference beam  265  is reflected from pick-off mirror  260  and is incident upon stationary photodiode  270 . Stationary photodiode  270  measures the beam strength of reference beam  265  and sends the intensity data to second gated integrator  273 . The physical specification of stationary photodiode  270  is the same as that of the scanning photodiode  250 . However, stationary photodiode  270  only measures the relative temporal variation; therefore, spatial mapping of reference beam  265  and the active area of stationary photodiode  270  are not required. The beam strength of reference beam  265  is used as a reference to mathematically compensate for any beam strength fluctuations occurring during the time scanning photodiode  250  measures all sub-beams  137 .  
         [0050]    In operation, beam  107  propagates along the optical path of a parallel process laser drilling system and is incident upon pick-off mirror  260 . A portion of beam  107  is reflected by pick-off mirror  260  as reference beam  265  towards stationary photodiode  270 . Stationary photodiode  270  measures the strength of reference beam  265  for use in compensating for strength variation in beam  107  over time. Data captured by stationary photodiode  270  is sent to computer  255  through gated integrator  273 . The remaining portion of beam  107  not reflected by pick-off mirror  260  continues along the optical path until it is incident upon DOE  135 . DOE  135  splits beam  107  into a plurality of sub-beams  137 , which allow parallel drilling of workpiece  155 . Sub-beams  137  exit DOE  135  and propagate along the optical path, where they are incident upon scan lens  140 . Scan lens  140  determines the spot size of sub-beams  137  upon workpiece  155 . Sub-beams  137  exit scan lens  140  and propagate along the optical path, where they pass through aperture  240  and are incident upon scanning photodiode  250 . Scanning photodiode  250 , mounted with aperture  240 , moves according to a pre-defined pattern stored on computer  255  in order to sequentially scan all sub-beams  137 . Scanning photodiode  250  measures the strength of a sample of pulses from each sub-beam  13  and sends the data to first gated integrator  253 . First gated integrator  253  sends the data to computer  255 . The intensity data are stored on computer  255  for subsequent analysis and use in designing microfilter  145 .  
         [0051]    [0051]FIG. 3 shows a method  300  of designing microfilter  145  using intensity measuring system  200  to provide the requisite accurate empirical data, including the steps below.  
         [0052]    Steps  310  through  360  provide a detailed description of how intensity distribution information of sub-beams  137  is determined for the design and manufacture of microfilter  145 .  
         [0053]    In Step  310 , determining required intensity equalization, a laser system operator or technician determines the intensity equalization of sub-beams  137  for producing a workpiece  155  that meets specifications. The relationship between the variation in the strength of sub-beams  137  and the variation in the desired workpiece geometry is experimentally established prior to the start of method  300 . Based on this relationship, the laser system operator or technician determines an acceptable tolerance in the variation of the intensities of sub-beams  137 , based on pre-determined experimental measurements.  
         [0054]    In Step  320 , determining pattern size and shape, the laser system operator or technician determines the pattern of sub-beams  137  to match the requirements of the desired product. In one example, an inkjet nozzle with 8 rows of 38 holes is to be drilled, requiring an 8×38 pattern to be determined. In alternate embodiments, other patterns may be used, as determined by product specifications.  
         [0055]    In Step  330 , focusing sub-beams through aperture, the assembly of aperture  240  and scanning photodiode  250  is placed in front of the sub-beam  137  such that aperture  240  is in the focal plane of sub-beams  137 . This focuses sub-beams  137  through aperture  240  in front of scanning photodiode  250 .  
         [0056]    In Step  335 , starting scanning photodiode, scanning photodiode  250  measures the strength of all sub-beams  137  by following a pre-determined scanning path.  
         [0057]    In Step  340 , measuring significant sample of sub-beam pulses, scanning photodiode  250  moves through the pre-determined scanning path at a constant rate of speed to capture a statistically significant sample of light pulses from a sub-beam  137  to ensure that the measurement uncertainty fits within the tolerances defined in step  310 . Stationary photodiode  260  also measures the strength of reference beam  265  simultaneously. In one example of using laser drilling system  100 , scanning photodiode  250  and stationary photodiode  270  capture about 2500 pulses of one sub-beam  137  as the scanning photodiode  250  moves across that sub-beam  137 . Widely understood techniques of gating and background subtraction are used with first gated integrator  253  and second gated integrator  273  to measure and produce log files of the intensities of sub-beams  137  and reference beam  265 , respectively. In the above example, a gating technique is used to selectively measure the output of photodiodes  250  and  270  when short pulses are present. Gating eliminates measurements taken when sub-beam  137  pulses were not incident upon scanning photodiode  250 , preventing these measurements (containing only noise) from use in the design of microfilter  145 . A rolling average over 30 pulses is also used to reduce the effect of noise on the laser pulses within the gated windows. This averaging technique also reduces measurement uncertainty (or standard deviation) statistically. Log files containing sub-beam identifiers and intensity data (along with the reference beam data) are stored on a computer (not shown) to be analyzed later in method  300 .  
         [0058]    In Step  350 , decision respective to all sub-beams in pattern having been measured, it is determined whether scanning photodiode  250  has measured the strength of all sub-beams  137 . If so, method  300  proceeds to step  355 ; if not, method  300  returns to step  340 .  
         [0059]    In Step  355 , stopping scanning photodiode, the assembly of scanning photodiode  250  and aperture  240  stops scanning, having reached the end of the pre-determined scanning path, and having measured the strength of all sub-beams  137 .  
         [0060]    In Step  360 , logging and analyzing sub-beam intensity data, the laser system operator or technician analyzes the strength of sub-beams  137  to produce input parameters used to design a customized microfilter  145 . Analysis of the intensities of sub-beams  137  is done by averaging the selected 500-pulse section around the center of the “plateau” area, subtracting the average “background” measurement (the intensity reading during the time between spots or “dark orders”) and normalizing the result respective to the strength of reference beam  265 . The strength of reference beam  265  is measured by stationary photodiode  270  at the same point in time as the intensity measurements of sub-beams  137  to mathematically remove any effect of strength variation in beam  105  during the time needed to complete method  300  as shown in FIG. 4.  
         [0061]    [0061]FIG. 4 shows a sample data plot of sub-beam data. In this sample, curve  402  is the strength measurement of sub-beam  137 , curve  404  is the strength of reference beam  265 , and curve  406  shows the normalized result. The two plateaus represent two of the 152 sub-beams measured. The resulting reduction in noise is clearly illustrated by the differential between the curves.  
                               TABLE 1                               Average   Reference               Beam   Bacground   Beam       Point   Intensity (B)   (D)   Intensity (R)   (B-D)/R                   . . .   . . .   . . .   . . .   . . .       1963   1.843   0.007   1.950   0.9415       1964   1.804   0.007   1.918   0.9369       . . .   . . .   . . .   . . .   . . .                  
 
         [0062]    Table 1 shows an example of the intensity analysis of sub-beams  137  taken from a picosecond laser drilling system (e.g. laser drilling system  100 ). Table 1 includes the intensity measurements from scanning photodiode  250  and the background level, the corresponding reference beam strength from stationary photodiode  270 , and the normalized result of the actual strength per each data point. The last column constitutes curve  406  in FIG. 4. In the above example, a rolling-average routine is used to transfer plateau shape of transmission curve to peak shape curve. A peak-finder routine thus is applied to find the center of each peak that is also the center of the “plateau” in curve  406  of FIG. 4. The 500-pulse average around the plateau center further statistically reduces the uncertainty associated with the measurement by about another factor of  22 . Each plateau is assigned a sub-beam ID and the beam strength is recorded as shown in Table 2.  
                                         TABLE 2                                       Average               “Bright Order”           Sub-beam ID   Intensity                                        1   0.9435           2   0.9872                      
 
         [0063]    In Step  370 , designing microfilter, the laser system operator or technician uses the empirically measured sub-beam  137  intensity data resulting from step  360  to design a customized microfilter  145 . The concepts of microfilter design are not discussed in detail here but should be apparent to those of skill in this art. In one example, each sub-beam  137  is matched with an individual filter with a transmission factor of the reciprocal of intensity within customized microfilter  145  to equalize the strength of sub-beams  137  before they are incident upon workpiece  155 .  
         [0064]    In Step  380 , fabricating microfilter, a manufacturer fabricates microfilter  145  via conventional methods. The design established in step  370  provides the manufacturing specifications for producing a microfilter with the proper apertures and coatings to meet the design needs.  
         [0065]    In summary, a high-level design process includes the following four steps. The first step is providing a parallel-process laser drilling system that generates a plurality of sub-beams  137  in a pre-defined pattern. The second step is measuring the strength of each individual sub-beam  137  for subsequent analysis. The third step is analyzing the intensity data of each sub-beam  137  in order to determine appropriate design parameters for a customized microfilter. The fourth step is designing and fabricating customized microfilter  145  according to these design parameters.  
         [0066]    The system and method of the present invention are used to make a customized microfilter article that is specifically designed to equalize strength of sub-beams within a parallel process laser drilling system.  
         [0067]    The system and method of the present invention are also used to produce an inkjet nozzle article with improved workpiece geometry, uniformity, and repeatability via use of a customized microfilter article designed to equalize strength of sub-beams within a parallel process laser drilling system. In this regard, a nozzle plate of an ink-jet head may be constructed with the laser drilling system of the present invention as further detailed in FIGS. 5 and 6.  
         [0068]    As shown in FIG. 5, an ink-jet printer  540  has an ink-jet head  541  capable of recording on a recording medium  542  via a pressure generator. Ink droplets emitted from ink-jet head  541  are deposited on the recording medium  542 , such as a sheet of copy paper, so that recording can be performed on the recording medium  542 .  
         [0069]    The ink-jet head  541  is mounted on a carriage  544  capable of reciprocating movement along a carriage shaft  543 . More specifically, the ink-jet head  541  is structured such that it can reciprocate in a primary scanning direction X in parallel with the carriage shaft  543 . The recording medium  542  is timely conveyed by rollers  545  in a secondary scanning direction Y. The ink-jet head  541  and the recording medium  542  are relatively moved by the rollers  545 .  
         [0070]    Turning now to FIG. 6, further details in in-jet head  541  are shown. Pressure generator  604  is preferably a piezoelectric system, a thermal system, and/or equivalent system. In this embodiment, the pressure generator  604  corresponds to a piezoelectric system which comprises an upper electrode  601 , a piezoelectric element  602 , and an under electrode  603 .  
         [0071]    A nozzle plate  614  (an instance of workpiece  155 ) comprises a nozzle substrate  612  and a water repellent layer  613 . The nozzle substrate  612  is made of metal, resin and/or equivalent material. The water repellant layer is made of fluororesin or silicone resin. In this embodiment, the nozzle substrate  612  is made of stainless steel and has a thickness of 50 um, and the water repellent layer is made of a fluororesin and has a thickness of 0.1 um.  
         [0072]    The ink-jet ink is filled in an ink supplying passage  609 , a pressure chamber  605 , an ink passage  611 , a nozzle  610 . Ink droplets  620  are ejected from nozzle  610  as pressure generator  604  pushes on pressure chamber element  606 .  
         [0073]    As a result f the present invention, very good nozzles are formed without flash and foreign matter (carbon etc) in the nozzle plate. Further, the accuracy of the nozzle outlet diameter is 20 um±1.5 um (a preferred predefined acceptable threshold value for tolerance between the perimeter and the excision edge of the 20 um diameter nozzle outlet).  
         [0074]    From the foregoing it will be understood that the present invention provides a system and method for cutting a workpiece with a laser cutting tool with a high degree of precision in the quality of the conformance of the dimensions of the removed portion to the dimensions of the design used in the cutting operation with special value in using a laser to mill exit holes in inkjet nozzles. While the invention has been described in its presently preferred form, it will be understood that the invention is capable of certain modification without departing from the spirit of the invention as set forth in the appended claims.