Patent Application
20160327802
Powerful lasers are used for cutting, drilling, welding, marking, engraving of materials, etc. In particular, radio frequency (RF)-excited gas lasers produce laser energy when a gas medium within the laser is excited by the application of RF energy between a pair of electrodes. An example of a gas laser is a carbon dioxide (CO2) laser.
The performance parameters of a laser, particularly an RF-excited gas laser, may generally be characterized by the laser power, power stability, and beam mode quality. Each of these performance parameters may be affected by one or more conditions within the laser itself. For instance, changing conditions of the gas within the electrodes of an RF-excited gas laser may affect the uniformity of the gas discharge within the electrodes. This then affects the M2 (pronounced “M-squared”) parameter, which is defined as the ratio of a beam parameter product (BPP) of an actual beam to that of an ideal Gaussian beam at the same wavelength (e.g., a “beam quality factor”). See, e.g., The Physics and Technology of Laser Resonators, Jackson and Hall eds. Other performance metrics include ellipticity (or asymmetry) and astigmatism. See standards ISO11145,ISO11146-1, and ISO11146-2.
The M2 factor performance metric characterizes how well, i.e., how tightly, a laser beam can be focused. A “perfect” beam, i.e., a beam with diffraction limited performance, is defined by M2=1.0. The term “diffraction limited” refers to a beam whose optical properties are limited only by the unavoidable physical phenomena of diffraction—i.e., the beam does not possesses any of the common aberrations that may negatively affect the properties of optical beams, such as, spherical aberration, coma, etc. The presence of these common aberrations (which may, e.g., be induced by the lenses and mirrors of the optical system) will put a lower limit on focused the spot size that that is larger than the diffraction limited spot size. On the other hand, a diffraction limited optical beam can be focused to the smallest theoretical spot size for a given size and wavelength of beam.
The ellipticity of a laser beam is defined as the ratio of spot widths along the major and minor axes of the beam at a particular location along the beam. The astigmatism of a laser beam is defined as the difference in the beam waist locations (locations of smallest spot size) of a laser beam. In other words, if one envisions travelling along an astigmatic beam, one may first see the beam size reach a minimum in the x-direction (with the y-direction still unfocussed) and then, travelling still further, the x-direction will expand while the beam size in the y-direction reaches a minimum. The distance along the beam between the x- and y-positions of minimum beam size characterizes the astigmatism of the beam. If there is ellipticity and/or astigmatism in a laser beam the symmetry of the beam will vary along the beam path.
For laser beams with elliptical and/or astigmatic outputs, the performance of the laser beam when used in materials processing applications will be detrimentally affected because the size and/or shape of the focused laser beam on the material being processed will be sub-optimal, e.g., the laser spot on the processed material may be larger than desired. Even a laser having perfect beam quality, i.e., M2=1.0, will produce poor results if the beam is elliptical when focused onto the part being processed. This is because optimal processing requires the highest possible optical power density to be incident on the surface of the part being processed. In an elliptical and/or astigmatic beam, the focused beam power density will be reduced from its theoretical maximum that would result from a perfectly circular beam.
In some cases, an elliptical beam may be corrected by the use of one or more cylindrical lenses on the output of the laser, but this may exacerbate the astigmatism of the beam. In addition, ellipticity may not be controlled consistently from laser tube to laser tube because the alignment and/or production build tolerances of the output of each laser may be slightly different. Accordingly, different focal length lenses may be required for each build. Even for the case of a single laser that may operate at different input powers and operating frequencies, changes inevitably occur in the output laser mode thereby requiring different beam conditioning arrangements to be used for each laser operating condition.
This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.
Illustrative embodiments of the present disclosure are directed to a laser that includes a first electrode and a second electrode separated by a gap region having a gap thickness between a first electrode inner surface and the second electrode inner surface. A discharge region is disposed within a central portion of the gap region with a lasing medium disposed within the discharge region. The discharge region is further disposed within an optical cavity. The laser further includes an output port for allowing an output beam of the laser to exit the optical cavity. The waveguide aperture includes an input port section having the input port that receives the output beam, an elongated waveguide body having internal bore formed therethrough and an output port that receives the waveguided beam and outputs a conditioned output beam of the laser. The transverse width of the internal bore has a transverse size that is small enough to cause the output beam to be waveguided by the internal bore.
Illustrative embodiments of the present disclosure are directed to a beam conditioning apparatus for conditioning an output beam of a laser. The beam conditioning apparatus includes an optical coupling element that couples the output beam of the laser into an input port of a waveguide aperture. The waveguide aperture includes an input port section comprising the input port that receives the focused output beam, an elongated waveguide body formed from an opaque material and having internal bore formed therethrough, and an output port that receives the waveguided beam and outputs a conditioned output beam of the laser. An inner surface of the internal bore forms a waveguide for the focused output beam and thereby generates a waveguided beam.
Illustrative embodiments of the present disclosure are directed to a waveguide aperture beam conditioner. The waveguide aperture beam conditioner comprises an input port section comprising a input port that receives an aberrated laser beam, an elongated waveguide body framed from an opaque material and having internal bore formed therethrough, and an output port that receives the waveguided beam and outputs a conditioned output laser beam. The inner surface of the internal bore forms a waveguide for the focused output beam and thereby generates a waveguided beam.
Illustrative embodiments of the present disclosure are directed to a method for conditioning an output beam of a laser. The method includes coupling the output beam of the laser into an input port of a waveguide aperture, waveguiding, by an inner surface of the internal bore, the focused output beam thereby generating a waveguided beam, and outputting, by an output port, the waveguided beam thereby generating a conditioned output beam of the laser. The waveguide aperture includes an input port section comprising the input port that receives the focused output beam and an elongated waveguide body formed from an opaque material and having internal bore formed therethrough.
Other aspects of the invention will be apparent from the following description and the appended claims.
Specific embodiments of waveguide conditioning for a high powered laser will now be described in detail with reference to the accompanying figures. Like elements in the various figures (also referred to as FIGs.) are denoted by like reference numerals for consistency.
In the following detailed description of embodiments, numerous specific details are set forth in order to provide a more thorough understanding of the laser tube with baffles. However, it will be apparent to one of ordinary skill in the art that these embodiments may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid unnecessarily complicating the description.
In general, one or more embodiments of the present disclosure are directed to a waveguide aperture beam conditioner and a laser system and method for reducing or eliminating both ellipticity, astigmatism and improving beam quality in a laser using the waveguide aperture beam conditioner. The disclosed examples refer particularly to hybrid waveguide unstable CO2 resonators but could in general be applied to any asymmetric laser beam without departing from the scope of the present disclosure.
In accordance with one or more embodiments, the high powered laser may be a Radio Frequency (RF)-excited gas discharge laser (e.g., a slab laser). The laser includes a housing containing a laser gas, where a pair of elongated, planar electrodes are disposed within the housing and spaced apart to define a narrow gap corresponding to a discharge region. A laser resonator is defined by placing mirrors at the ends of the electrodes. The electrodes may form a waveguide, or light guide, in one axis of the resonator and confine the lasing mode of the resonator in an axis perpendicular to the plane of the electrodes (the waveguide axis). The minors define the lasing mode in an axis parallel to the plane of the electrodes. This type of minor arrangement operates in the long axis of the slab discharge region as what is referred to as an unstable resonator (or unstable resonant cavity).
In accordance with one or more embodiments, the laser may be operated in a pulsed mode, particularly for drilling, cutting, etc. The pulse-repetition frequency (PRF) and the pulse duty-cycle may be selected according to the operation to be performed and according to the material on which the operation will be performed (e.g., PRF may typically range from less than 1 kilohertz (kHz) to over 100 kHz). As noted above, laser performance (e.g., output beam shape, pointing stability, discharge stability, etc.) can be affected at certain frequencies due to acoustic resonances, which may be caused by, among other things, perturbations in the gas discharge volume due to localized pressure variations in the gas.
After entering beam conditioning system 107, the elliptical/astigmatic output beam 109 is coupled into waveguide aperture 111. In accordance with one or more embodiments, the coupling may be accomplished using any appropriate optical coupling system, e.g., by using a focusing element 113 (shows as a minor, but may also be implemented as a lens) to focus the output beam 109 to the appropriate size for coupling into the hollow central waveguiding region of inner bore 111a of waveguide aperture 111, shown here in cross-section. Furthermore, in accordance with one or more embodiments, the input end of the inner bore 111a may be appropriately shaped so as to allow the direct coupling of the elliptical/astigmatic output beam 109 without the need for and additional coupling optics.
In accordance with one or more embodiments, the waveguide aperture 111 is long enough so that the focused beam 109 becomes guided by the walls of the inner bore 111a of the waveguide aperture 111. The waveguiding of the beam 109 thus forces the properties of the guided beam to be determined by the waveguide rather than the incoming laser beam 109 (which, as described above, may be slightly elliptical or astigmatic). In accordance with one or more embodiments, the length of the waveguide aperture 111 that will cause waveguiding of the beam 109, and thereby created a waveguided beam 109a, depends on the wavelength of the beam. In general, the length L of the waveguide aperture is large enough to cause the Fresnel Number NF of the waveguide to be less than approximately 1, or L is chosen such that NF<approximately 1, where NF=a2/Lλ.
As it exits the output port of the waveguide aperture 111, the now diverging conditioned output beam 109b may be incident onto collimating optical element 115 (shows as a mirror, but may also be implemented as a lens) what serves to collimate the now conditioned output beam 109b. Because the laser beam 109 was guided by the inner bore 111a of waveguide aperture 111, the shape of the conditioned output beam 109b at the exit of the aperture, e.g., as illustrated by intensity distribution 213 and cross section 221 shown in
As described above, in accordance with one or more embodiments of the invention, the beam quality may be improved by passing the output beam 201 through a waveguide aperture 205 as shown in
In accordance with one or more embodiments, the size of the focused beam relative to the size of the waveguide aperture falls within a range of about 0.4a<ω<0.7a where a is the half width of the inner bore 211 of the waveguide aperture 205 and ω is the beam waist at the focal plane of the focusing system 207. As shown by the intensity distribution 209, the cross section 201b of the focused output beam 201 may have side lobes in a manner similar to the original output beam 201 and thus, the beam waist ω may refer to the size of the central lobe only, excluding the side lobes.
As already described above in reference to
In accordance with one or more embodiments, the central bore 211 of waveguide aperture 205 may function as a multi-mode waveguide. For example, in accordance with one or more embodiments the EH11 and EH22 modes of the waveguide may be excited. However, as the EH11 and EH22 modes propagate down the length of the guide, the intensity profiles of the guided beam alternate between being “donut” shaped (when the EH11 and EH22 modes are 180 degrees out of phase) and Gaussian shaped (when the EH11 and EH22 modes are in phase). Accordingly, the length of the waveguide aperture is such that the EH11 and the EH22 modes for this diameter of aperture are in-phase at the output port, thereby insuring that the output mode is nearly Gaussian. For example, for a wavelength of 10.6 microns, a waveguide aperture having a circular bore diameter of 0.040 inches and a length of 3.0 inches will result in nearly Gaussian output.
As shown in
In accordance with one or more embodiments, the appropriate length for the waveguide aperture may be determined experimentally, by measuring the output mode shape for a number of different lengths and then choosing a length that produces the most Gaussian output mode, or other shapes such as donut or quasi flat-topped beam shapes by judiciously coupling/combining lower and higher order modes. In addition, the waveguiding behavior of the waveguide aperture may be modelled and the appropriate length determined from the numerical output of the model. For example, the complex amplitude of the EH1n mode of a guide having a hollow circular aperture may be given by
where J0 is the zero order Bessel Function of the first kind, u1n is the nth root of the Bessel function, given by the equation J0(un1)=0 , γ1n is the complex propagation constant of the EH1n mode, and a is the radius of the hollow circular core. The complex propagation constant is related to the phase coefficient β1n of the EH1n mode and the attenuation coefficient α1n of the EH1n mode by way of the relation
γ1n=β1n+iα1n (2)
The phase coefficient β1n is further expressed by
where λ is the wavelength, a is the guide radius, and ν is related to the complex refractive index of the guide wall. Accordingly, under the condition that the EH11 and EH12 modes are excited with complex amplitudes A11 and A12, the resulting field intensity from these two modes as a function of the position along the guide (i.e., along the z-direction) of the guide may be expressed as
Thus, under the assumptions given above, the optical intensity I(r,z) of the guided beam at every point z along the length of the guide may be computed. Of course, the above “closed form” equations are provided herein merely as an example and other models may be employed, including those that account for more than two propagating modes. Likewise, any physics-based numerical modelling solution available through commercial software packages may be employed without departing from the scope of the present disclosure.
In accordance with one or more embodiments, the shape of the central bore at the output of the waveguide aperture may have an extreme aspect ratio such that, when used in combination with an appropriately chosen focusing lens, a line focus output beam may be created. Employing a waveguide aperture having this type of architecture allows for the creation of a highly elongated output beam from a nearly symmetrical input beam without the additional spherical aberration the would result from the use of only cylindrical lenses for the beam shaping. Thus, in embodiments such as this, the function of the waveguide aperture may be to create a highly elongated beam from a less elongated, or even spherical beam. For example, in one or more embodiments, the range of aspect ratios could be from 5:1 to 20:1. Those skilled in the art will appreciate that one or more embodiments are not limited to those values. Also, those skilled in the art will appreciate that for aspect ratios over around 10:1, it is difficult to attain using standard lenses and mirrors without introducing spherical aberrations.
In accordance with one or more embodiments, the waveguide aperture may be formed from one, or more than one, section. For example,
In accordance with one or more embodiments, the waveguide aperture may be made of more than one section, e.g., as shown in
In the case of low power laser beams the waveguide aperture may be made of copper or aluminum and the high loss nature of these metals can add to further improve the beam quality of the laser by increasing the losses of any higher order mode components, compared to the fundamental mode, that may enter the waveguide without being filtered off by the aperture. For high power lasers, the losses of the main mode will be high enough that cooling of the waveguide aperture may be required.
In accordance with one or more embodiments, a waveguide aperture made of Al2O3 or BeO may have lower loss than copper or aluminum for the linearly polarized mode typically produced by a CO2 hybrid unstable-waveguide or hybrid unstable-free space type resonator. In other embodiments, a composite waveguide aperture, like that shown in
In accordance with one or more embodiments, the inter-electrode gap 606 is at least partially filled with a laser gain medium (not shown). Furthermore, in embodiments that employ a gas discharge as the laser gain medium, the inter-electrode gap 606 may also serves as a gas discharge region. In accordance with one or more embodiments, the discharge region is defined to be the space between the inner surfaces 603aand 605a of the elongated electrodes 603 and 605, respectively. In accordance with one or more embodiments, the inner surfaces 603a and 605a serve as two elongated resonator walls that bound the discharge region in a transverse direction, and, in some embodiments, may also serve as waveguiding surfaces for the intra-cavity laser beam in this transverse direction (y-direction). While the example shown in
The slab laser 601 shown in
In accordance with one or more embodiments, the electrodes 603 and 605 may have lengths of up to 1 meter, widths of up to 0.5 meters, and inter-electrode gaps on the order of 0.5-6.0 mm. However, other embodiments may use dimensions outside this range without departing from the scope of the present disclosure. In accordance with one or more embodiments, when radio frequency (commonly referred to as “RF”) power is applied to the gas lasing medium via elongated electrodes 603 and 605, a gas discharge forms within the inter-electrode gap 606. The laser energy then builds up within one or more modes, including a fundamental mode, of the optical resonator, eventually forming an intra-cavity laser beam (not shown) that travels back and forth between the output coupling mirror 611 and front cavity mirror 607 via rear folding mirror 609. Some fraction of the intra-cavity laser beam is transmitted through the output coupling minor 611 and forms output laser beam 615.
In the illustrative embodiment shown in
As alluded to above, in accordance with one or more embodiments, the inter-electrode gap region (or inner cavity region) may be filled with a gas lasing medium. For example, the gas lasing medium may be a mixture of one part carbon dioxide (CO2), one part nitrogen (N2), and three parts helium (He), with the addition of 5% xenon (Xe). The gas pressure may be maintained in a range of approximately 30-150 Torr, e.g., 90 Torr. However, other embodiments may use higher pressures without departing from the scope of the present disclosure. Other embodiments of the invention may use other types of gas lasers, examples of which are listed in Table 1.
Other gas mixtures may be used as well. For instance, some embodiments may use the following gas mixtures, or their isotopes, including portions of neon (Ne), carbon monoxide (CO), hydrogen (H2), water (H2O), krypton (Kr), argon (Ar), fluorine (F), deuterium, or oxygen (O2) and other gases, examples of which are listed in Table 1 above, at various other gas pressures, e.g., 30-120 Torr, e.g., 50 Torr; however, it will be appreciated that other gaseous lasing media may also be employed. For instance, one example of a lasing medium includes one or more of the following vapors: copper, gold, strontium, barium, a halide compound of copper, a halide compound of gold, a halide compound of strontium, a halide compound of barium, and other vapors, examples of which are identified but not limited to those shown in Table 1 above.
Returning to
The excitation energy supplied by the power supply 617 in the embodiment shown in
In accordance with one or more embodiments, the inner surfaces 603a and 605a of the first and second elongated planar electrodes 603 and 605, respectively, are positioned sufficiently close to each other so that the inter-electrode gap acts as a waveguide along the y-axis for the laser radiation. Accordingly, when acting as waveguide surfaces, the inner surfaces 603a and 605a also act as optical resonator surfaces in the transverse direction (y-direction). In accordance with one or more embodiments, waveguiding occurs when πN<<1, where N=D2/(4λL) is the Fresnel number of the guide and D is the width of the gap between the electrodes, L is the length of the optical cavity, and λ is the wavelength of the laser radiation. For a wavelength of about 10.6 microns, which is a common wavelength produced by a CO2 laser, the waveguiding criterion is satisfied if the inter-electrode gap is less than 2 mm for a guide length of 40 cm. However, in other embodiments, the inter-electrode gap is large enough to allow for free space propagation, e.g., Gaussian beam propagation, of the laser beam in the y-direction. Accordingly, in this free space configuration, these surfaces serve to define the thickness of the gas discharge region without acting as a waveguide for the laser radiation. Other embodiments may use an inter-electrode gap that is between the waveguiding criterion and complete free space propagation.
In accordance with one or more embodiments, the distance D4 may be smaller than the e−2 intensity width of the fundamental free space mode in the region of the resonator walls (i.e., smaller than the so-called “free space requirement”) such that the modes in the narrow axis (i.e., y-axis) are waveguide in nature. For example, in accordance with one or more embodiments, D4 may range between 1 mm and 4 mm, although smaller and larger values of D4 may be used without departing from the scope of the present disclosure. One or more embodiments may employ a so-called hybrid resonator configuration where waveguide propagation occurs in the y-direction and free space propagation occurs in the x-direction. In accordance with one or more embodiments, the distance D4 may be chosen to allow free space propagation (not shown) of the intra-cavity laser beam 211 in the narrow axis of the slab structure (the y-axis, i.e., the axis that is parallel to the gap).
In embodiments that employ a hybrid configuration the output laser transverse beam profile 700 (the cross section of the output beam in the x-y plane) may be elliptical and/or astigmatic, e.g., the size of the beam in the y-direction may be larger than the size of the beam in the x-direction (ellipticity) and/or the position of the waist of the beam in the y-direction is different from the position of the waist in the x-direction (astigmatism). To improve the properties of the output beam profile, waveguide beam conditioning may be employed in accordance with one or more embodiments of the invention as described in more detail above in reference to
Returning to
In accordance with one or more embodiments, for a properly chosen gap width D4, the intensity profile of the output beam in the waveguide direction can be made Gaussian-like, as in the free space direction. For example, the fundamental mode in the waveguide direction in rectangular symmetry may be approximated by a Gaussian beam with a waist of w2≈0.7a, where 2a is equal to the gap width D4. Thus, in accordance with one or more embodiments, for a given free space waist w1 occurring at the end of the waveguide, D4 may be chosen to satisfy w2≈0.7(D4/2). However, other widths may be used without departing from the scope of the present disclosure. In accordance with one or more embodiments, the waveguide beam conditioning systems and methods may be used to improve the beam quality of any of the lasers manufactured by Synrad, Inc. In other embodiments, the waveguide beam conditioning systems may be employed to condition the output beams of CO2 lasers that employ all-ceramic laser tubes, such as those manufactured by Iradion Laser, Inc. The examples of laser architectures discussed above are meant merely as examples and the waveguide beam conditioning systems and methods disclosed herein may be employed with any type of laser architecture without departing from the scope of the present disclosure.
As discussed above, the single-body waveguide aperture and multi-body waveguide aperture may include a single-piece or multi-piece body that is formed from any generally opaque material, e.g., the waveguide aperture may be formed from Al2O3, BeO, copper, aluminum or any other material that is opaque at the wavelength of interest, e.g., at 9-10 μm. Thus, the operation of the waveguide aperture may be contrasted with the operation of a traditional fiber optic that has a largely transparent core and cladding material. In a traditional fiber optic, the spatial extent of the transverse mode is large enough that it extends well into the transparent cladding material. To the contrary, in accordance with one or more embodiments of the waveguide aperture disclosed herein, the light will not substantially extend into the waveguide body due to the opacity of the material. For example, in the case of a copper or aluminum body, the opacity of the material is driven largely by the conductivity of the material with the optical field extending only a negligible amount into the copper itself, due primarily to skin depth effects. Thus, in accordance with one or more embodiments, the inner surface of the bore provides for a highly-multi-mode wave-guiding effect without the use of a traditional transparent cladding. Such a device differs in behavior from a typical optical fiber formed from a transparent material in that the optical loss of the waveguide aperture decreases as the diameter of the inner bore increases as shown in
In Step 1003, the focused output beam is waveguided by an inner surface of the internal bore, thereby generating a waveguided beam. During propagation of the waveguided beam, the various aberrations of the input beam are largely corrected because the waveguided beam takes the physical characteristics of the waveguide aperture modes, both with respect to phase and shape of the spatial mode. In Step, 1005, the waveguided beam is output by an output port, thereby generating a conditioned output beam of the laser.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.
