LASER MACHINING SYSTEMS AND METHODS FIELD OF INVENTION [0001] The present disclosure relates generally to laser machining, and more particularly, to laser machining systems and methods using laser beams having uniform beam profile and narrow beam divergence for forming zero and negative taper machined cuts. BACKGROUND OF INVENTION [0002] In laser machining, a laser beam generation device is used in conjunction with an optical system to direct the laser beam onto a workpiece that is to be machined. The impingement of the laser beam on the workpiece locally melts and/or vaporizes the workpiece material to produce or extend a hole or cut in the workpiece. The location of the laser beam impingement point on the workpiece may be controlled by moving one or both of the laser beam and the workpiece relative to one another to thereby control the geometry of the hole or cut. [0003] Laser machining of workpieces often produce edges along the cut features that exhibit taper. One approach to avoid tapered edges is to use a special trepanning head to rotate the laser beam at a spot location with a fixed tilt angle. The part remains stationary. Such an approach is only good for small hole drilling. Another approach is to rotate the part physically while keeping the laser beam at the same incident angle, which need high precision CNC stage (usually 4-5 axis) and sophisticated control software to achieve a zero taper cut. [0004] There is a need for laser machining systems and methods using laser beams having uniform beam profile and narrow beam divergence for forming zero and negative taper machined cuts. BRIEF DESCRIPTION OF INVENTION [0005] The present disclosure provides, in a first aspect, a method for laser machining a workpiece. The method includes directing, from an F-theta lens having a long focal length of greater than about 250 millimeters, a laser beam at a non-perpendicular beam tilt angle from an optical axis of the lens having a top-hat profile and a narrow beam divergence angle of between about 1 degree and about 3 degrees towards a workpiece disposed on a stage movable in at least an X-direction and a Y-direction, engaging the directed laser beam with the workpiece disposed in the usable field of view, moving the workpiece and the directed laser beam relative to each other, and removing portions of the workpiece with the directed laser beam to define a machined surface. [0006] The present disclosure provides, in a second aspect, a laser micromachining system for laser machining a workpiece. The laser micromachining system includes a laser, a converter, an F-theta lens a stage, a beam steering device, and a controller. The laser is operable to generate a laser beam having a Gaussian profile. The converter is operable to convert the laser beam having the Gaussian profile into the laser beam having the top-hat profile. The F-theta lens has a long focal length of greater than about 250 millimeters. The stage is operable to support and move the workpiece movable in at least an X-direction and a Y-direction. The beam steering device is operable for receiving the laser beam having the top-hat profile and directing the laser beam through the F-theta lens at an angle from an optical axis of the lens towards the workpiece on the stage. The laser beam is directable over a range of angles from the optical axis of the lens defining a usable field of view having a perimeter and having a narrow beam divergence angle of between about 1 degree and about 3 degrees. The controller is operable to control motion of said stage and/or said beam steering device to orientate the laser beam having the top-hat profile through the F-theta lens and onto the workpiece removing portions of the workpiece with the directed laser beam to define a machined surface. BRIEF DESCRIPTION OF DRAWINGS [0007] The foregoing and other features, aspects and advantages of this disclosure will become apparent from the following detailed description of the various aspects of the disclosure taken in conjunction with the accompanying drawings, wherein: [0008] FIG. 1 is a schematic diagram of a laser machining system in accordance with aspects of the present disclosure operable for machining a workpiece; [0009] FIG. 2 is are side elevational, cross-sectional views of a plurality of machined cuts made using the laser machining system of FIG. 1 employing a laser beam having a top-hat profile; [0010] FIG. 3 are side elevational, cross-sectional views of a plurality of machined cuts made using a conventional prior art laser machining system employing a laser beam having a Gaussian profile; [0011] FIG. 4 are side elevational, cross-sectional views of a plurality of machined cuts made using the laser machining system of FIG. 1 employing an F-theta lens with a long focal length; [0012] FIG. 5 are side elevational, cross-sectional view of a plurality of machined cuts made using a conventional prior art laser machining system a lens having a short focal length; [0013] FIGS. 6-8 are pictorial illustrations of a laser machining method in accordance with aspects of the present disclosure; and [0014] FIG. 9 is a flowchart of a method for laser machining a workpiece in accordance with aspects of the present disclosure. DETAILED DESCRIPTION [0015] Each embodiment presented below facilitates the explanation of certain aspects of the disclosure, and should not be interpreted as limiting the scope of the disclosure. Moreover, approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as "about," is not limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. When introducing elements of various embodiments, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the elements. The terms "comprising," "including," and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements. As used herein, the terms "may" and "may be" indicate a possibility of an occurrence within a set of circumstances; a possession of a specified property, characteristic or function; and/or qualify another verb by expressing one or more of an ability, capability, or possibility associated with the qualified verb. Accordingly, usage of "may" and "may be" indicates that a modified term is apparently appropriate, capable, or suitable for an indicated capacity, function, or usage, while taking into account that in some circumstances, the modified term may sometimes not be appropriate, capable, or suitable. Any examples of operating parameters are not exclusive of other parameters of the disclosed embodiments. Components, aspects, features, configurations, arrangements, uses and the like described, illustrated or otherwise disclosed herein with respect to any particular embodiment may similarly be applied to any other embodiment disclosed herein. [0016] The present disclosure addresses and enhances, inter alia, laser machining processes, and more specifically, laser machining systems employing a laser beam having less divergence and a more uniform profile so that less beam tilt angle is required for achieving zero or negative taper machined cuts. For example, the technique of the present disclosure may include converting a laser beam having a Gaussian beam profile into a laser beam having a top-hat beam profile, and passing the top-hat laser beam through an F-theta lens having a long focal length. As described in greater detail below, the directed top-hat profile laser beam from the F- theta lens has a generally narrow angle of divergence and high intensity. In addition, the directed top-hat profile laser beam from the F-theta lens is operable for forming zero taped cuts closer to the optical axis of the F-theta lens and spaced apart from the perimeter of the usable field of view of the F-theta lens. Line cuts and grooved slots may be readily implemented in combination with a two-dimensional stage for supporting and moving the workpiece to be machined. With such a technique for laser machining, systems employing the same may be less expensive compared to conventional laser machining requiring special trepanning heads or expensive precision CNC machines. Such a technique may also be operable with less power loss compared to conventional laser machining processes. Further, such a technique may be suitable for ceramic matrix composite tensile bar cutting and may be applied to other features such as seal slot as well. [0017] FIG. 1 is a schematic diagram of a laser machining system 10, in accordance with aspects of the present disclosure, operable for machining a workpiece having a zero taper or a controlled or angled taper. For example, laser micromachining system 10 may generally include a laser 20, a laser beam converter 30, a beam steering device 40, beam shaping optics 50, a movable stage 60 on which is supportable a workpiece 12, and a controller 70. [0018] The laser source may be any type of laser system that is capable of producing a laser beam of sufficient power, coherency, pulse width, pulse repetition time, and wavelength to be compatible with the performing the desired machining operations upon the selected workpiece. For example, the laser source may be a solid state, CO2, or fiber laser having a power of about 0.1 Watts to about 20,000 Watts. As those skilled in the art will appreciate, the optical components used in the laser system are operably compatible with the laser source so as to avoid damaging those components during operation. Laser 20 may emit a laser beam 22 or pulses having a Gaussian profile distribution or spatial properties over a cross section that are converted or reshaped in converter 30 to a laser beam 24 or laser pulses having a top hat profile distribution or spatial properties over a cross-section. In such an intensity profile of the top hat profile, the intensity of the beam is relatively constant across the cross section such as diameter of the laser beam, unlike the intensity profile of the Gaussian beam. Accordingly, the edges of the shaped beam have approximately the same intensity as the center of the beam, providing reduced intensity drop off at the beam's edge. The Gaussian to top-hat converters may be an optical device to convert one or two-dimensional single-mode Gaussian laser beam profiles to a flat top profile, while minimizing transmission loss. The converter may employ refractive elements, diffractive elements, optical fibers, other operable components, and combinations thereof. [0019] Beam steering device 40 may include a movable mirror 42 for receiving laser beam 24 having a top hat profile distribution and redirecting laser beam 24 towards beam shaping optics 50. For example, an x-y tilt mirror may be used to position or scan a laser spot on the workpiece for machining. The mirror in the beam steering device may be dynamically and reciprocally tiltable around a first axis which is perpendicular to the optical axis A of beam shaping optics 50. The mirror in the beam steering device may also be dynamically and reciprocally tiltable around a second axis which is perpendicular to the first axis. For example, the beam steering device may include two galvanometer-based scanners, arranged one each on the x-and y-axes, and include a galvanometer, a minor (or mirrors) and a servo driver board that controls the beam steering device. [0020] Beam shaping optics 50 may include an F-theta lens having a long focal length such as a focal length greater than about 250 millimeters, greater than 250 millimeters, between about 250 millimeters and about 420 millimeters, greater than about 420 millimeters, or greater than 420 millimeters. The combination of the beam steering device 40 and beam shaping device 50 results in a controllable directed laser beam 28 having a top hat profile towards workpiece 12 disposed on stage 60. The F-theta lenses is designed both to form an image on a flat plane and to provide a linear relationship between the scan length x and the scan or inclination angle θ, in accordance with the following so-called F-Theta condition: x = f * θ. Scan length x is simply equal to the incident scan angle θ multiplied by the focal length f, i.e. the position of the spot on the image plane is directly proportional to the scan angle. This eliminates the need for complicated electronic correction required with standard scanning lenses. [0021] The laser beam does not pass directly though the center of the focusing lens. Instead, the laser beam enters the lens at an angle θ relative to optical axis A of the lens. The lens bends the laser beam, causing the beam to reach the workpiece at a beam tilt angle α relative to the center axis of the lens. Beam tilt angle α depends on the lens geometry and the distance between the laser beam and the center axis of the lens. Varying the distance between the laser beam and the central axis will change beam tilt angle α. In one aspect, during laser machining the laser beam may remain at a constant distance from the optical axis of the lens, resulting in a constant beam tilt angle α. Directed laser beam 28 is directable on beam tilt angle α relative to central optical axis A of beam shaping optics 50 over a field of view FOV having a perimeter P. [0022] Stage 60 may include motion control devices (not shown) for holding workpiece 12 and moving the workpiece in relation to the laser beam axis along an X-direction and a Y-direction. For example, the stage may include an X-axis linear motor and a Y-axis linear motor. It will be appreciated that a suitable stage may be movable with up to six axes which include translation in three orthogonal axes (X, Y and Z) and rotation about the three axes. [0023] Controller 70 may be an operable computing unit including a processer or microprocessor, one or more input and output devices, and one or more memories. Controller 70 is operably connected to laser 20, beam steering device 40, beam shaping optics 50, and stage 60. Controller 70 is operable to direct the laser to emit the laser beam or pulses along the laser beam path and coordinate the beam steering optics and the stage to position the workpiece relative to the directed laser beam to cause the directed laser beam to engage the workpiece to effect the laser machining. The command signals from the controller are generally output based on programming instructions stored in memory, and the functions of each of the programming instructions are performed by the logic of the computing unit. The various components such as the beam steering device may include their own controllers that transmit data to and from the controller. Moreover, the controller could be incorporated into a computer, such as a personal computer. [0024] FIG. 2 illustrates laser machining employing the system of FIG. 1 in accordance with aspects of the present disclosure and in which a laser beam or pulses have a top hat intensity profile distribution for obtaining three cuts at different beam tilt angles α in workpiece 12 relative to vertical optical axis A of the beam shaping optics. FIG. 3 illustrates a prior art approach for laser machining employing a laser beam or pulses having a Gaussian intensity profile distribution for obtaining three cuts at different beam tilt angles α in workpiece 12 relative to vertical optical axis A of the beam shaping optics. As illustrated in the comparison between FIGS. 2 and 3, use of a top-hat beam profile reduces the taper angle β1 (e.g., between about 3 degrees and about than 5 degrees) of a machined cut or surface compared to a taper angle β2 of a cut or machined surface made using a laser beam having a Gaussian profile. In addition, less of a beam tilt angle α1 (e.g., between about 4 degrees and about 7 degrees) is needed to achieve zero taper cut using a laser beam having top hat profile compared to the beam tilt angle α2 when employing a laser beam having a Gaussian intensity distribution profile. Further, less beam tilt angle α and taper angle β allows machining a zero taper cut, e.g., at a smaller distance X1 from the optical axis A of the beam shaping optics within a larger region inside the perimeter P (FIG. 1) of the scan head field of view FOV (FIG. 1) compared to a distance X2 using a laser beam having a Gaussian intensity distribution profile. [0025] FIG. 4 illustrates laser machining employing the system of FIG. 1 in accordance with aspects of the present disclosure employing beam shaping optics 50 such as an F-theta lens having a long effective focal length EFL1 (e.g., greater than about 250 millimeters) for obtaining four cuts at different angles a relative to a vertical optical axis A of the beam shaping optics. FIG. 5 illustrates a prior art approach for laser machining employing a laser beam or pulses employing a beam shaping optics having a short effective focal length EFL2 for obtaining three cuts at different angles a relative to a vertical optical axis A of the beam shaping optics. As illustrated in the comparison between FIGS. 4 and 5, using an F-theta lens with a long focal length has a less beam divergence angle