Patent Application
20210057864
The present disclosure relates generally to lasers and, more specifically, to waveguide gas lasers.
Lasers (e.g., gas lasers) operate by pumping energy into an active medium (e.g., a gas such as carbon dioxide (CO2)) in order to extract some of that energy as useful laser light for engraving, cutting, or other applications. In CO2 lasers (e.g., lasers in which the active medium is CO2), the pumping comprises an electrical discharge across a resonator in which the active medium is present. Typically, the greater the pumping energy, the more laser light can be generated until a saturation point, beyond which laser output remains constant or even decreases. For lasers of all types, a significant parameter is the consistency of the output power over time. The consistency is typically measured as a percentage variation in power over a specified interval, such as 30 minutes. CO2 lasers are often operated without closed loop feedback for power. As such, CO2 lasers may experience an intrinsic power variation, subject to both internal heating and environmental factors. Other measures of the performance of a laser is “rise time” and “fall time,” which refer to how quickly the laser light emitted by a resonator turns on and off in response to electrical pumping of the laser gas (e.g., the active medium, also referred to as the gain medium). While there are some contributions from circuitry, the limiting factors for the rise time and fall time are intrinsic to the laser gas and the design of the resonator.
In radio frequency (RF) driven CO2 lasers, mechanisms such as gas ionization rate and discrete gain impose lower limits on rise and fall time. Another significant measurement for the performance of a laser is wall plug efficiency. Wall plug efficiency is a measure of how much laser power is generated for a given amount of power consumed by the apparatus that produces the laser. For example, a 100 Watt laser that draws 1,000 Watts of electricity from a facility power outlet would be 10% efficient. Much of the power injected in to a gain medium becomes waste heat as part of the laser light forming process. However, the efficiency can be improved by optimization of the gain medium (e.g., the gas mixture), more aggressive cooling techniques, and/or reduced resonator optical losses.
Another important measure of the performance of a laser is the degree to which polarization of the light output of a laser can be controlled. The light output from lasers can be 100% polarized, 0% polarized (i.e., unpolarized), or between 0-100% polarized (i.e., partially polarized). Control of the polarization is significant in materials processing applications, such as laser engraving, marking, or cutting. The degree of polarization (DOP) and state of polarization (SOP) of laser light are determined by internal resonator properties, such as polarization-dependent reflectivity from mirror and waveguide surfaces. In addition to the performance aspects of lasers discussed above, another significant parameter is reliability, as the ratios of different gases within the gain medium may change over time. Furthermore, the surface of a waveguide within the resonator may degrade over time, causing losses due to absorption and/or scattering of the laser light within/from the waveguide surface.
In one embodiment, a laser is provided. The laser includes a ceramic core defining an airtight enclosure that contains a laser gas. The airtight enclosure defines a waveguide slab laser cavity. An interior surface of the waveguide slab laser cavity is coated with a layer of metal. The laser also includes a plurality of mirrors forming a resonator in the waveguide slab laser cavity. The laser additionally includes a plurality of electrodes positioned outside the airtight dielectric enclosure such that the laser gas contained in the waveguide slab laser cavity is excited when an excitation signal is applied to the plurality of electrodes.
The metal, in some embodiments, may be gold. The metal may additionally or alternatively include silver, copper, nickel, and/or platinum. The layer may have a thickness that ranges from less than a micrometer to more than a hundred micrometers. In some embodiments, the layer of metal is discontinuous. For example, the layer may be formed from a set of stripes of the metal across the interior surface of the waveguide. In other embodiments, the layer may be formed from sputtering deposits of the metal on the interior surface. The layer may have a greater optical reflectivity than ceramic. Additionally or alternatively, the layer may be smoother than ceramic. In some embodiments, the layer may be bonded to the interior surface of the waveguide with a glass binder. The layer, in some embodiments, may include gold nanoparticles.
In another embodiment, a laser is provided. The laser includes a core formed from a first material and at least partially defines a waveguide slab laser cavity. An interior surface of the waveguide slab laser cavity is coated with a layer of a second material that is different than the first material. The laser also includes a plurality of mirrors forming a resonator in the waveguide slab laser cavity. Further, the laser includes a plurality of electrodes positioned such that laser gas contained in the waveguide slab laser cavity is excited when an excitation signal is applied to the plurality of electrodes.
The first material used in the core, in some embodiments, may include metal or glass. In some embodiments, the first material includes ceramic. Further, the second material may include gold. The second material, used in the layer, may have a greater optical reflectivity than the first material used in the core.
In yet another embodiment, a laser is provided. The laser includes a core formed from a first material and at least partially defines a waveguide slab laser cavity. An interior surface of the waveguide slab laser cavity is coated with a layer of a second material that is different than the first material. In some embodiments, the interior surface is coated with multiple layers that are configured to operate as multiple layers of dielectric laser mirrors that utilize optical interference to increase a power output of a laser. In some embodiments, the first material may include ceramic, while the second material may include gold.
For the purposes of promoting an understanding of the principles of the invention, reference will now be made to an illustrative embodiment shown in the attached drawings and specific language will be used to describe the same.
One illustrative embodiment of a laser 10, according to the present disclosure, is shown in the diagrams of
In terms of reliability, the enhanced waveguide surface may decrease the rate at which the gas components (e.g., the mixture of gases within the gain medium) wear out (e.g., disassociate into different gasses and/or otherwise change from a predefined ratio). That is, in typical gas lasers, a portion of the CO2 population can disassociate into CO and O. Hence that portion of the population can no longer contribute to laser action. Long term, in such lasers, the CO and O can be permanently segregated by adsorption, oxidation, or other pathways. The enhanced waveguide surface disclosed herein can favor recombination of CO and O into CO2 to increase the instantaneous population of CO2. Furthermore, the increased recombination inhibits those other pathways for CO and O removal from the active mixture by maintaining their preferred state as CO2 molecules. Additionally, while typical CO2 lasers may wear out due to degradation of the waveguide surface (e.g., from oxidation, adsorption, photobleaching, color center formation, or other processes occurring in the discharge volume), thereby reducing the net optical gain and output power (e.g., from absorption and/or scattering), the enhanced waveguide surface may inhibit degradation since the surface can be made non-reactive. As such, the enhanced waveguide surface serves to retain the desired population of the initial gas molecules.
The laser 10 generally includes a body 12 formed primarily from ceramic and a coating of gold or other material, as described in more detail herein. The body 12 defines a slab laser cavity 14, a number of mirrors 16, 18, 20 forming a resonator 22 in the slab laser cavity 14, and a number of electrodes 24, 26 positioned outside the body 12 (and, hence, outside the slab laser cavity 14). The laser 10 may have a Fresnel number of less than about 0.3. The Fresnel number of a laser may generally be given by the formula: NF=a2/(λL), where a is half of the resonator aperture, λ is the wavelength of the laser beam, and L is the resonator length. A Fresnel number less than about 0.3 may generally define a true waveguide laser cavity. A Fresnel number greater than about 3 may generally define a free-space laser cavity in which there may be little or no interaction between the beam and the walls of the laser cavity. Fresnel numbers between about 0.3 and about 1.5 describe a quasi-free-space laser cavity of varying degrees of interaction between the beam and the walls of the laser cavity.
The body 12 of the laser 10 is illustratively embodied as being formed of two mirror image components: a left-side component 12a and a right-side component 12b. It will be appreciated that, in other embodiments, the body 12 may be formed as an integral component or may be formed from three or more components. The body 12 and/or components 12a, 12b may also be non-symmetrical in some embodiments. In the illustrative embodiment, the body 12 comprises Alumina (Al2O3) with a purity between 95% and 99.9%. In other embodiments, the ceramic body 12 may be formed of Beryllium Oxide (BeO), Aluminium Nitride (AlN), certain types of glass and/or glass ceramics, or any material with similar electrical mechanical and thermal properties, or combinations thereof. It is also contemplated that the body 12 may be formed of other suitable materials (including non-ceramics). However, as described in more detail herein, the interior surfaces are coated with gold or other materials to provide enhanced performance and reliability.
In the illustrative embodiment, the interior of the body 12 defines a slab laser cavity 14 having a generally rectangular cross-section. The slab laser cavity 14 is partially defined by two opposing walls 32 and by two other opposing walls 34 of the body 12 (see
As can be seen in
The illustrative embodiment of laser 10 also includes a number of mirrors 16, 18, 20. Two mirrors 16, 20 are supported by a mirror mount 28, which is positioned at the front end of the slab laser cavity 14. The other mirror 18 is supported by a mirror mount 30, which is positioned at the rear end of the slab laser cavity 14. In the illustrative embodiment, the mirror mounts 28, 30 are sealed to the body 12 to form an airtight, or vacuum-sealed, enclosure. Specifically, the mirror mounts 28, 30 are coupled to the body 12 using an epoxy. It will be appreciated that an airtight seal may also be provided by various other methods, including, but not limited to, brazing, welding, glass fritting, etcetera, and that the components 12a, 12b of the body 12 may be affixed to one another using similar methods. It is also contemplated that one or more of the mirrors 16, 18, 20 may be affixed directly to the body 12, without the use of a mirror mount. It will also be appreciated that the mirror 20 may be a window without a reflective surface in other embodiments (e.g., in embodiments that utilize an unstable resonator). It is further contemplated that, in some embodiments, the mirrors may be placed inside a larger airtight enclosure containing the laser gas (which enclosure might be formed any number of materials, such as ceramic, glass, and/or metal).
In the illustrative embodiment, the airtight enclosure formed by the body 12 and the mirror mounts 28, 30 is filled with a laser gas. The laser gas may be any mixture of gases (multiple elements and/or multiple molecules) sufficient to produce a gain medium in the slab laser cavity 14 when excited. The primarily Alumina body 12 of the illustrative embodiment allows coupling of RF power without the need for an RF feed-through. In other embodiments, the body 12 does not itself form an airtight enclosure but, instead, is positioned inside a larger airtight enclosure containing the laser gas (which enclosure might be formed any number of materials, such as ceramic, glass, and/or metal).
In the axis of slab thickness (i.e., the dimension parallel to the walls 34 of the ceramic body 12), the spacing between the walls 32 is close enough to allow the resonating photons to be guided by the walls 32 of the slab laser cavity 14. In the unstable axis, the beam is coupled out of the slab laser cavity 14 across the edge of the mirror 16.
As described above, the illustrative laser 10 includes a number of electrodes 24, 26 positioned outside the slab laser cavity 14 and adjacent to the walls 32 of the body 12. The laser gas disposed in the slab laser cavity 14 will be excited when an excitation signal is applied to the electrodes 24, 26. In the illustrative embodiment, the electrodes 24, 26 are each received in an elongated slot 48 formed in an exterior surface of the body 12. When an RF excitation signal is applied to the electrodes 24, 26, the laser gas in the slab laser cavity 14 is excited and, through capacitive coupling, produces a plasma gain medium. While the electrodes 24, 26 are shown as elongated electrodes in the illustrative embodiment, it is contemplated that the electrodes 24, 26 may have different geometrical forms, may be segmented, and may be flat or at an angle to the walls 32 of the body 12. It is also contemplated that, in some embodiments, one or more of the electrodes 24, 26 may be positioned adjacent and, thus, at least partially define the slab laser cavity 14.
The use of electrodes 24, 26 that are external to the body 12, in the illustrative embodiment, may provide several advantages. For instance, RF energy is coupled into the slab laser cavity 14 through the walls 32 of the body 12, eliminating the need for feedthroughs and allowing heat to be coupled away more efficiently from the plasma. In other embodiments, electrodes of different geometrical forms may be used to excite only certain portions of the laser gas in the slab laser cavity 14. In other embodiments, single segments of multiple segment electrodes may be excited individually. Furthermore, the use of external electrodes 24, 26 allows for a continuous waveguide up to the surface of the mirrors 16, 18 without exciting the portion of laser gas directly adjacent to the mirrors 16, 18. It is well known that a gap between the end of the waveguide and the mirrors may introduce losses into the laser; the larger the gap, the higher the losses. On the other hand, a distance must be maintained between the excited plasma and the mirrors to avoid damage to mirror surfaces. Using external electrodes 24, 26 protects the mirrors 16, 18 from the plasma but still allows the body 12 to guide the laser radiation to the mirrors 16, 18 without any interruption in the waveguide surfaces. As noted above, however, the use of electrodes that are external to the body 12 is not required in all embodiments.
Referring now to
The enhanced waveguide surface may implement a catalytic effect, an oxidation prevention effect, an adsorption prevention effect, a surface charge control effect, an optical loss reduction effect, an optical polarization discrimination effect, a diffraction effect, a photonic bandgap effect, an optical interference effect, or a combination of two of more of the aforementioned effects. In the illustrative embodiment, catalysis and optical loss reduction provide enhanced laser performance. The presence of gold inside the CO2 laser 10 provides a catalytic enhancement of the CO2 population. Disassociation and recombination of CO2 molecules is a localized phenomenon within the laser discharge volume (e.g., between the electrodes 24, 26) for a sealed gas laser. The presence of a catalyst at a remote or adjacent surface aids in the recombination of CO and O into CO2 when those species happen to traverse near to the catalytic event. It is also possible that the catalytic effect may be enhanced by ultraviolet radiation that is a byproduct of the discharge.
In the illustrative embodiment, the presence of gold on the most immediate surfaces of the waveguide resonator 22 surrounding the discharge volume increases the catalytic effect. The gold, in the illustrative embodiment, is bonded to the interior ceramic surfaces of the waveguide resonator 22 using a glass binder. The porosity of the ceramic enables adhesion and long-term durability of the gold coating that was not previously possible with earlier laser designs.
Optical loss reduction is provided by the finite improvement of optical reflectivity of the waveguide surfaces within the spectral range of operation for the laser 10. While bare Alumina ceramic offers adequate reflectivity in standard ceramic core lasers, the gold coated surface improves the reflectivity, thereby reducing the distributed loss that is accumulated along the full length of the waveguide resonator 22. Moreover, the gold coating may also smooth the waveguide surface on a scale relevant to surface scattering effects (e.g., another loss mechanism in waveguide channels). The bare ceramic surface may retain defects from fabrication steps (e.g., grinding) or material inclusions that serve as scattering centers for the propagating guided mode.
In any laser resonator, performance is constrained by the balance of gain and loss provided by the active amplification medium (e.g., gain medium) and the constituent optical components. The negative impact of resonator losses can have a compounding effect when lost light becomes a source of heat in the resonator. In some embodiments, the finite reflectivity difference between bare Alumina and gold may have such a compounding effect. That is, the lower reflectivity of Alumina may cause some light to be attenuated as the guided mode propagates the waveguide resonator 22, and a portion of that light may be absorbed by the Alumina, producing heat. Above a certain temperature, the laser emission provided by the energy transitions in the CO2 population may become disfavored. Hence, any incremental heat in the waveguide resonator 22 may limit output power.
While the coating (e.g., the layers 70, 72) is described above as a gold coating, in other embodiments, the surface of the waveguide resonator 22 may be coated with films of other metals, such as silver, copper, nickel, and/or platinum. Additionally or alternatively, the enhanced surface (e.g., the layers 7072) of the waveguide resonator 22 may include ceramic glazes or glassy surfaces, engineered for minimal waveguide loss and/or catalytic effect. Additionally or alternatively, the enhanced waveguide surface (e.g., the layer 70, 72) may include wavelength-scale or subwavelength structures engineered for minimal waveguide loss and/or catalytic effect. The enhanced waveguide surface may include a layered structure analogous to multi-layer dielectric laser mirrors that utilize optical interference to enable improved performance. In some embodiments, the enhanced waveguide surface may include an embedded or partially embedded structure to integrate with the body 12. Although the illustrative embodiment utilizes a ceramic core, where the enhanced waveguide surface is attached to the ceramic body 12, in some embodiments, the enhanced waveguide surface may be used with a metal body or a glass body vessel. In such embodiments, the laser may include a waveguide formed of a first material (e.g., a ceramic insert) that is coated with layer of a second material (e.g., gold) that is different than the first material, in order to improve one or more properties of the laser.
Referring generally now to
As mentioned above, the laser 10 may also include one or more thermal dissipation or thermal transfer devices 56 to prevent excess temperatures in the laser 10. For instance, water-cooled plates or water-cooled electrodes might be used in some embodiments. In the illustrative embodiment, the body 12 is generally sandwiched between two heat sinks 56. Each heat sink 56 is thermally coupled to one of the electrodes 24, 26 to pull heat from that electrode and, indirectly, from the slab laser cavity 14 in the body 12. Between the heat sink 56 and the respective electrode 24, 26, a thin material 60 is used to electrically insulate the electrode 24, 26 from the heat sink 56 and, at the same time, thermally conduct heat to the heat sink 56. In the illustrative embodiment, the thin material 60 is comprises a thick paste that is loaded with Alumina and cures to a soft rubber-like material. It is also contemplated that a thin ceramic strip could be used for the thin material 60. The thin material 60 may also be placed between the electrodes 24, 26 and the body 12 to fill any air gaps that might exist and promote thermal conduction. In any case, the material 60 will have a high dielectric constant (e.g., in the range of 8 or 9). The high dielectric constant of the material 60 causes the capacitance between one of the electrodes 24, 26 and the heatsink 56 to be much higher than the capacitance between the electrodes 24, 26.
To avoid uncontrolled tuning of the resonance beyond the best frequency match to the RFPA 50, the capacitance of the laser 10 should be kept constant. Since the capacitance between each electrode 24, 26 and heat sinks 56 pair is very sensitive to the spacing, the laser 10 is designed to maintain the heat sink 56 at a predetermined distance from the respective electrode 24, 26 irrespective of the temperature of the body 12 (over the range of operating temperatures for the laser 10). In the illustrative embodiment, the spacing between the two heat sinks 56 is maintained by spacers 62 that are made of a material that closely matches the coefficient of thermal expansion of both the body 12 and the electrodes 24, 26. These spacers 62 also support the body 12 and maintain the close spacing between the each electrode 24, 26 and the adjacent heat sink 56. The spacers 62 may be formed of several possible materials, including, but not limited to, Titanium or 7075-T6 Aluminum, depending upon the particular body 12 and electrodes 24, 26 being used. In other embodiments, the corners of the body 12 may be drilled through (in a way that avoids the gas reservoirs 36, 38) to allow threaded inserts to be installed. In these embodiments, the body 12 itself becomes the spacer 62 as it expands and contracts with changing temperature.
While the invention has been illustrated and described in detail in the foregoing drawings and description, the same is to be considered as illustrative and not restrictive in character, it being understood that only illustrative embodiments thereof have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.
This application claims the benefit of U.S. Provisional Patent Application No. 62/888,741, filed Aug. 19, 2019, the entire disclosure of which is incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| 62888741 | Aug 2019 | US |
