Q-switched CO2 laser

Abstract

A CO2 laser configured to generate infrared electromagnetic radiation with an optical element including a frequency selective structure having a substantially periodic pattern of features. A frequency response of the optical element is configured to change upon receipt of a signal. A Q factor of the CO2 laser changes upon receipt of the signal. A laser marking system can incorporate the CO2 laser.

Description

[Technical Field]

[0001] Aspects and implementations of the present disclosure are generally directed to a tunable optical element including a frequency-selective structure having periodic patterned features. The frequency response of the tunable optical element is controllable and configured to change upon receiving a signal. The tunable optical element can be incorporated into various optical devices to at least partially control the optical device. For example, the tunable optical element can be incorporated as a mirror or output coupler of a laser (e.g., a carbon dioxide (CO2) laser). The tunable optical element can be used to Q-switch the laser upon receiving a signal. [Background technology]

[0002] Lasers are used in various industrial production lines to perform processes on workpieces, for example, by etching, engraving, machining, cutting, and / or welding. Laser marking systems can be used to imprint markings, such as images, identification numbers, expiration dates, and / or barcodes, onto the surface of various products. Marking of a material can be affected by ablation (i.e., removal of material), a change in color, or a change in the surface texture of the material. Marking by all of these processes occurs when the temperature of the material irradiated by the laser beam exceeds the threshold of the physical process being affected. The physical properties of the material determine the threshold temperature. Important material properties are the absorptivity, thermal diffusivity, and thermal conductivity at the wavelength of the laser irradiation. Therefore, for any material, the laser energy density (measured in J / mm) required to reach the threshold temperature that will affect the desired marking process can be calculated. 2There is a specific value for the laser energy density (which can be expressed as t). The laser energy density required to mark a particular material will at least partially determine the required laser type (e.g., CO2 laser) and beam parameters (e.g., spot size, power, etc.). Laser energy density can be understood as the peak power of the laser multiplied by the laser on-time (the time the laser beam is incident on the material) divided by the area of ​​the material illuminated by the laser beam (i.e., (P*t) / (spot size)). The optical design of the laser determines the spot size. Spot sizes can include diameters of about 50 μm or greater. Spot sizes can include diameters of about 700 μm or less.

[0003] The energy or power applied to a material by a laser over time is determined, at least in part, by the application (e.g., the material being marked, the time available to mark the material, the complexity of the mark, etc.). For example, typical paper-based packaging materials can be marked using low-power (e.g., 10-30 watts) continuous-wave lasers. As another example, metals generally require high-peak power (e.g., about 1 kW or more) lasers with short on-times (e.g., about 1 μs or less) to overcome the metal's relatively rapid heat dissipation and preserve the desired mark contrast. As a further example, some plastics, such as HDPE, require high peak power to reach the temperatures necessary for readable marking.

[0004] It can be difficult to produce a laser beam with the amount of power necessary to affect a desired change in a metal workpiece with a laser operating in a continuous wave (constant output) mode. Some lasers, utilizing a technique known as Q-switching, are capable of producing a pulsed output beam containing optical pulses with much higher peak power than those produced by the same laser operating in a continuous wave mode. While Q-switches are generally known, there remains a need for compact, relatively inexpensive Q-switches that can be manufactured on a large scale for lasers operating in specific wavelength ranges (e.g., infrared wavelengths). Summary of the Invention

[0005] According to a first aspect of the present disclosure, there is provided an optical element including a frequency selective structure having a substantially periodic pattern of features, wherein a frequency response of the optical element is configured to change upon receipt of a signal.

[0006] The optical element advantageously allows a user to control the frequency response of an optical system that includes the optical element. For example, the optical element can be used to Q-switch a laser by changing the frequency response to reflect, transmit, and / or absorb more or less electromagnetic radiation within a given frequency range. The optical element advantageously enables a compact Q-switching system having a lower form factor than known Q-switches. The optical element advantageously has improved energy efficiency compared to known Q-switches. The optical element is cheaper to manufacture than known Q-switches.

[0007] A feature of a substantially periodic pattern can have a periodicity that varies by less than about 10%. A feature of a substantially periodic pattern can be referred to as a periodic pattern feature.

[0008] The signal can be generated by a controller. The signal can include thermal, electrical, and / or optical energy. The signal can include an applied electromagnetic field. The signal can include electromagnetic radiation. The signal can include an applied voltage or bias.

[0009] The frequency selective structure can act as a bandpass or bandstop optical filter for a frequency range of incident electromagnetic radiation. The bandpass or bandstop behavior of the frequency selective structure, and / or the optical element as a whole, can change upon receiving a signal such that a frequency range of electromagnetic radiation that was previously passed is now rejected, or vice versa. In some embodiments, the frequency selective structure can include a frequency selective surface.

[0010] The reflectivity, transmittance and / or absorption of the optical element may vary over a range of frequencies of electromagnetic radiation upon receiving a signal.

[0011] The reflectivity, transmittance, and / or absorptance of an optical element can be changed on command to enable the optical element to function as a Q-switch for a laser. For example, the reflectivity of the optical element can be set to a first, relatively low value (e.g., less than about 50%), which can induce relatively large losses in the laser's oscillating cavity. Changing the reflectivity to a second, relatively high value (e.g., about 99%) for a short period based on the laser's pulse width (e.g., about 100 μs or less) can reduce the losses in the lasing cavity and allow lasing to occur, thereby Q-switching the laser. The second value can be an optimal value. The optimal value can depend on the type of laser used, the desired characteristics of the pulses generated by the laser, the application of the pulses generated by the laser, etc.

[0012] The range of frequencies of electromagnetic radiation can include infrared radiation.

[0013] Infrared lasers, such as CO2 lasers, have a variety of applications, including marking products on production lines. Some materials, such as some plastics, may require relatively high-power pulses of infrared radiation to mark them. Optical elements are used to Q-switch the infrared laser, which can advantageously enable marking of more materials. The optical elements can be configured to have a controllable frequency response to infrared radiation. The optical elements can be configured to have a controllable frequency response for wavelengths of about 8 μm or greater. The optical elements can be configured to have a controllable frequency response for wavelengths of about 15 μm or less. The optical elements can be configured to have a controllable frequency response for one or more wavelength bands centered around about 9.3 μm, about 9.6 μm, about 10.2 μm, and about 10.6 μm.

[0014] The optical elements of the present disclosure can be used to Q-switch other types of lasers, such as ultraviolet (UV) lasers and / or visible lasers.

[0015] The optical element may include a switching component operably coupled to the frequency selective structure.

[0016] The switching component can take the form of a layer on the optical element. The switching component can at least partially surround the periodic pattern features of the frequency selective structure. The frequency selective structure can be embedded in or otherwise surrounded by the switching layer. The switching component can be adhered to the frequency selective structure. For example, the frequency selective structure can be comprised of a periodic pattern features provided (e.g., etched or deposited) on the active switching layer. The periodic pattern features can include a metal. The switching component can include a semimetal (e.g., graphene). The switching component can include a semiconductor (e.g., GaAs) having a tunable value of charge carrier density. The switching component can be GaAs. The switching member can include a liquid crystal (e.g., E7, a liquid crystal provided by Merck KGaA, a company based in Darmstadt, Germany). The switching component can include a phase change material (e.g., vanadium dioxide).

[0017] The optical element can include a signal transmission component operably coupled to the switching component and configured to receive a signal and transmit a signal to the switching component.

[0018] The signal transmission component may include indium tin oxide, patterned metal such as gold, patterned silicon, gallium arsenide, and / or another semiconductor material. The optical element may include a deformable material configured to change the periodicity and / or geometry of features of the periodic pattern upon receiving a signal.

[0019] The deformable material may form part of an actuator configured to respond to a signal, which may generate or vary an electric and / or magnetic field configured to cause deformation of the deformable material.

[0020] The deformable material may be an elastic material, which advantageously reduces the energy required to reverse a change to the frequency response of the optical element, as the elastic material returns to its original state when the force acting on it is removed.

[0021] Deformable materials can be formed from a wide class of materials whose stress, strain, and / or elastic properties are used to change the shape of the material upon application of a stimulus via thermal, mechanical, and / or electrical means. Suitable deformable materials include piezoelectric materials, shape memory metals (e.g., Ni-Ti alloys), shape memory polymers, and rubber.

[0022] The geometry of the features of the periodic pattern can include the shape, size, and relative placement of the features of the periodic pattern.

[0023] The refractive index of the optical element can be configured to change upon receiving a signal.

[0024] The permittivity and / or permeability of the optical element may be configured to change upon receiving a signal.

[0025] The conductivity and resistivity of the optical element can be configured to change upon receiving a signal.

[0026] The periodic pattern of features can include an array of geometric features configured to at least partially determine the frequency response of the optical element.

[0027] The frequency response of the optical element can depend at least in part on the periodicity and / or geometric characteristics of the features of the periodic pattern. For example, the features can include slots, dipoles, crosses, rings, split rings, etc. Complementary features can have complementary effects on the frequency response of the optical element. For example, a slot can act as a passband filter by increasing the transmittance of a wavelength range of radiation, while a dipole can act as a stopband filter by decreasing the transmittance of a wavelength range of radiation. The frequency selective structure can include a grid array of slots, dipoles, crosses, rings, split rings, and / or any other geometric features.

[0028] The periodicity and / or geometry of the features of the periodic pattern can be selected depending at least on the wavelength of light incident on the optical element. For infrared electromagnetic radiation generated by a CO2 laser, the features can have a periodicity of, for example, about 1 μm or greater. For infrared electromagnetic radiation generated by a CO2 laser, the features can have a periodicity of, for example, about 10 μm or less. For infrared electromagnetic radiation generated by a CO2 laser, the features can have a width and / or length of, for example, about 0.3 μm or greater. For infrared electromagnetic radiation generated by a CO2 laser, the features can have a width and / or length of, for example, about 3 μm or less.

[0029] Some features, such as split ring resonators, can reduce the range of wavelengths of electromagnetic radiation that fall within the frequency response of the optical element, which can advantageously provide a highly targeted frequency response, i.e., a narrow passband or stopband that requires only a relatively low-energy signal to tune.

[0030] The features can be made from a material (e.g., metal) on the substrate using an additive process, and / or the complementary features can be made using a reductive process (i.e., removing material). For example, a slot and a dipole can be considered complementary features. The periodic pattern of features can be made by either method (i.e., additive and / or reductive processes). The features can be made using an imprinting technique, e.g., nanoimprinting, using a master mold.

[0031] The array can include tuning elements configured to at least partially determine the frequency response of the optical element.

[0032] The tuning element can include one or more geometric features (eg, bars) configured to change the geometry of the frequency selective structure to adjust the frequency response of the optical element.

[0033] The features of the periodic pattern can be configured to act as polarizers.

[0034] Frequency-selective structures can be polarization-sensitive or polarization-insensitive, depending on the geometry of the periodic surface features. This provides different ways to control lasers through polarization-dependent effects. Having a polarization-sensitive frequency-selective structure is particularly advantageous for certain applications (e.g., CO2 lasers) because the radiation generated by the laser is polarized.

[0035] The optical element can include a metamaterial.

[0036] Metamaterials can form part of frequency-selective structures. Metamaterials can include metals and dielectrics. Metamaterials can be plasmonic metamaterials, i.e., they can have plasmonic resonant properties. Metamaterials can be configured to respond to infrared wavelength electromagnetic radiation. By designing the physical geometry of the array of features that make up the metamaterial and simultaneously selecting materials surrounding these geometric features based on the desired switchable electromagnetic properties, metamaterial-based laser Q-switches can be created. Metamaterials can be configured, for example, as frequency-selective structures including graphene formed on a ZnSe base.

[0037] The optical element may include a semiconductor, the charge carrier density of which may be configured to change upon receipt of a signal.

[0038] Semiconductors can include Si, GaAs, Ge, InP, GaAlAs, and / or many others. The optical and electrical properties of semiconductors can be controlled by doping, which introduces impurities into the lattice structure of the material. Lithography and deposition techniques can create patterns and features on a substrate to perform various functions. By combining frequency-selective structures and switching components into a single device, monolithic Q-switches can be created. The advantage of using semiconductor-based designs is the ability to create low-loss, highly efficient, metal-free structures. Changing the charge carrier density of the semiconductor changes the refractive index of the frequency-selective structure, which can then change the transmittance, reflectance, and / or absorption of the optical element. The optical element can include a phase-change material configured to change its phase upon receiving a signal.

[0039] The term "phase change material" covers a wide range of compounds that undergo a physical change when exposed to an external stimulus. Phase change materials are used in a wide variety of applications, including random access memory devices, CDs and DVDs, and smart windows. Chalcogenide glasses used in random access memory devices change between amorphous and crystalline states. This transition can cause microscopic changes in resistivity in response to electrical stimuli. GeSbTe, a chalcogenide glass used in CDs, changes between amorphous and crystalline states using optical stimuli, changing the surface's reflectivity. Smart windows utilize the insulator-metal phase transition of VO3 to change reflectivity with temperature. Phase change materials can be configured to change between amorphous and crystalline states, or vice versa, upon receiving a signal. Phase change materials can also be configured to change between insulating and metallic states, or vice versa, upon receiving a signal. Changing the phase of the phase change material changes the conductivity and resistivity of the frequency selective structure, which can change the transmittance, reflectance and / or absorption of the optical element.

[0040] The phase change material can include VO2.

[0041] Vanadium dioxide (VO2) can be a preferred phase-change material for implementing a CO2 laser Q-switch. The insulator-to-metal phase transition of VO2 can be stimulated by thermal, electrical, or optical means. It has been shown that the insulator-to-metal phase transition can be stimulated by electrical or optical sources in the sub-microsecond time domain. Therefore, an optical element consisting of a frequency-selective structure and an active layer of VO2 would be sufficient to rapidly change the frequency response of the optical element and thereby Q-switch the laser.

[0042] The optical element may include a liquid crystal, the optical properties of which may be configured to change upon receiving a signal.

[0043] Liquid crystals are a family of materials that simultaneously exhibit the properties of a liquid and a crystal under appropriate conditions of temperature and / or concentration. The optical properties of liquid crystals vary depending on the liquid crystal material. A common property of liquid crystals is that their birefringence changes with the application of a potential difference. This signal can include applying a voltage to an optical element that constitutes the liquid crystal, thereby changing the birefringence behavior of the liquid crystal, thereby changing the polarization properties and / or refractive index of the liquid crystal. This can change the transmittance, reflectance, and / or absorption of the optical element. Changing the optical properties of the liquid crystal with the application of a bias can change the transmittance, reflectance, and / or absorption of a frequency-selective structure. Such optical elements can be used to Q-switch lasers, such as CO2 lasers. Liquid crystals can include E7.

[0044] The optical element may include graphene, and the dielectric constant of the graphene may be configured to change upon receiving a signal.

[0045] Graphene is a single atomic layer of carbon whose dielectric constant can be changed by the application of an electrical bias. An optical element for Q-switching a laser can consist of a frequency selective structure, a graphene layer, and a means for biasing the graphene (e.g., a metal contact). Alternatively, the graphene can be lithographically processed to incorporate periodic pattern features of the frequency selective structure. Biasing the graphene causes a change in the electrical dielectric constant of the optical element, which can change the transmittance, reflectance, and / or absorption of the optical element.

[0046] The frequency selective structure may include a plurality of frequency selective layers configured to at least partially determine the frequency response of the optical element.

[0047] Having multiple frequency selective layers advantageously allows for fine tuning of the frequency response of the optical element, thereby providing an optical element with a sharper frequency response and / or a multi-frequency response. For example, a first layer of the multiple frequency selective layers may be configured to have a desired interaction (e.g., reflect, absorb, or transmit) with radiation having a wavelength of about 10.2 μm, while another layer may be configured to have a desired interaction with radiation having a wavelength of about 10.6 μm.

[0048] According to a second aspect of the present disclosure, there is provided a laser including the optical element of the first aspect.

[0049] The laser can be suitable for material processing of a target. Material processing can include one or more of marking the target, engraving the target, cutting the target, etc. The laser can include a resonant cavity including two mirrors that define an optical path through a gain medium. One of the mirrors can be a substantially fully reflective mirror (e.g., a "rear mirror" or "fold mirror"), and the other mirror can be a partially reflective mirror (i.e., an "output coupler") for outputting pulses of radiation.

[0050] The optical element may be a reflector, for example a mirror. The reflector may be "highly reflective" (for example having a reflectivity of 95% or more, for example about 99%).

[0051] The laser can include a controller configured to provide a signal to the optics, for example to Q-switch the laser.

[0052] The Q factor of the laser can change as it receives a signal.

[0053] The optical element can include a first state in which the Q factor of the laser has a first value and a second state in which the Q factor of the laser has a different value. Laser Q-switching can be performed by providing a signal to the optical element to switch the optical element between the first and second states. The switching component can be configured to change the controllable optical property between the first and second states upon receiving the signal.

[0054] The bandpass or bandstop behavior of the optical element can be relatively narrow, such that small changes caused by the signal induce relatively large changes in the frequency response of the optical element.

[0055] The geometry of the frequency selective structure (e.g., feature periodicity) and the electromagnetic properties (e.g., refractive index, permittivity, permeability, conductivity, resistivity, etc.) of the surrounding material (e.g., switching component) can determine the frequency response of the optical element. For example, increasing the conductivity of the switching component can decrease the reflectivity of the optical element, making the optical resonant cavity of the laser comprising the optical element more lossy and potentially decreasing the Q-factor of the laser. The frequency characteristics of the frequency selective structure can be selected based on the operating wavelength range of the laser.

[0056] The optical element may form part of the rear mirror of the laser.

[0057] The periodic pattern features can be designed so that the frequency selective structure acts as a bandpass filter at the operating wavelength of the laser. In this case, losses within the resonant cavity are high and lasing is suppressed. When a signal changes the frequency response of the optical element, the frequency selective structure acts as a regular rear mirror with very low radiation losses, allowing stimulated emission and lasing to occur.

[0058] The optical element may form part of the output coupler of the laser.

[0059] The reflectivity of the output coupler can be chosen to optimize the output power of the laser while minimizing radiation losses in the lasing cavity. The optimal reflectivity of the output coupler is highly dependent on the laser design. However, once optimized, the reflectivity of the output coupler typically remains fixed. By incorporating an optical element into the laser as the output coupler, the reflectivity of the output coupler can be changed on command, allowing the laser to be Q-switched.

[0060] The optical element may form part of a fold mirror of a laser.

[0061] The signal can change the frequency response of the optical element over the operating range of the laser's wavelength from a highly reflective state to a highly transmissive or absorptive state, or vice versa. When in a highly reflective state, the resonant cavity can produce relatively low photon loss (i.e., a high Q factor). When highly transmissive or absorptive, the resonant cavity can produce relatively high photon loss (i.e., a low Q factor).

[0062] The optical element may form part of the passive optics of the laser.

[0063] When an optical element is incorporated into a laser as a passive optical component, the signal can change the frequency response of the optical element from a highly transmissive state (causing a low loss state in the resonant cavity, i.e., a high Q factor) to a partially transmissive or absorbing state (causing a high loss state in the resonant cavity, i.e., a low Q factor).

[0064] According to a third aspect of the present disclosure, there is provided a laser marking system for marking a target, comprising the laser of the second aspect.

[0065] According to a fourth aspect of the present disclosure, there is provided a method of marking a target with radiation, comprising using the laser marking system of the third aspect.

[0066] According to a fifth aspect of the present disclosure, there is provided a method of Q-switching a laser to produce pulses of radiation, comprising providing an optical element of the first aspect to a laser and using a controller to provide a signal to the optical element to Q-switch the laser.

[0067] According to a sixth aspect of the present disclosure, there is provided a CO2 laser configured to generate infrared electromagnetic radiation. The CO2 laser comprises an optical element including a frequency selective structure having a substantially periodic pattern of features. The frequency response of the optical element is configured to change upon receiving a signal. The Q factor of the CO2 laser changes in response to receiving the signal.

[0068] The CO2 laser can be configured to generate infrared electromagnetic radiation having an average power of about 10 W. The CO2 laser can be configured to generate infrared electromagnetic radiation having an average power of about 30 W. The CO2 laser can be configured to generate infrared electromagnetic radiation having an average power of about 50 W. The CO2 laser can be configured to generate infrared electromagnetic radiation having an average power of about 100 W.

[0069] The CO2 laser can include a laser resonant cavity containing a CO2 gain medium. The CO2 laser can include a radio frequency excitation source configured to excite the CO2 gain medium to produce infrared electromagnetic radiation.

[0070] The RF excitation source can be configured to provide RF power of about 150 W or more. The RF excitation source can be configured to provide RF power of about 1 kW or less. The RF excitation source can be configured to provide RF power at a frequency of about 80 MHz or more. The RF excitation source can be configured to provide RF power at a frequency of about 120 MHz or less. The RF excitation source can be configured to provide RF power at a frequency of about 100 MHz or more. The RF excitation source can be configured to provide RF power to the CO2 gain medium for a duration of about 0.1 μsec or more. The RF excitation source can be configured to provide RF power to the CO2 gain medium for a duration of about 1.0 μsec or less. The RF excitation source can be configured to continuously provide RF power to the CO2 gain medium. The pulse duration of the RF excitation source can be controlled by a user of the CO2 laser. The pulse duration of the RF excitation source can depend at least in part on one or more of the desired laser pulse energy for marking the product, the pulse repetition frequency or product rate, and the average power of the CO2 laser.

[0071] The CO2 laser can include a control system configured to provide a signal to the optical element to Q-switch the CO2 laser. The control system can be configured to provide a separate control signal to a radio frequency excitation source to excite the CO2 gain medium.

[0072] The optical element may form part of the rear mirror of a CO2 laser.

[0073] The CO2 laser may include a fold cavity having a fold mirror, and the optical element may form part of the fold mirror of the CO2 laser.

[0074] The optical element can include silicon or GaAs. These reflective substrates are compatible with the CO2 laser plasma present in CO2 lasers. The complex refractive index (n+jk) of silicon at a wavelength of approximately 10.6 μm can be approximately (3.4179+j0.0001223). The complex refractive index (n+jk) of GaAs at a wavelength of approximately 10.6 μm can be approximately (3.2646+j0.00029).

[0075] The optical element may form part of the output coupler of a CO2 laser.

[0076] The optical element may form part of the passive optical components located within the laser resonant cavity of a CO2 laser.

[0077] The optical element can include ZnSe, GaAs, Ge, or ZnS. These transparent substrates are compatible with the CO2 laser plasma present in a CO2 laser. The complex refractive index (n+jk) of ZnSe at a wavelength of about 10.6 μm can be about (2.4028). The complex refractive index (n+jk) of GaAs at a wavelength of about 10.6 μm can be about (3.2646+j0.00029). The complex refractive index (n+jk) of Ge at a wavelength of about 10.6 μm can be about (4.0038). The complex refractive index (n+jk) of ZnS at a wavelength of about 10.6 μm can be about (2.1925+j0.002).

[0078] The optical element can include graphene. The electrical permittivity of the graphene can be configured to change upon receiving a signal. The complex refractive index (n+jk) of the graphene at a wavelength of about 10.6 μm can change between about (4.45−j4.34) and about (14.43−j0.08) upon receiving a signal.

[0079] The optical element may include a phase change material configured to change phase upon receiving a signal.

[0080] The phase change material can include VO2, whose complex refractive index (n+jk) at a wavelength of about 10.6 μm can change between about (2.1+j0.16) in an insulating state and about (7.8+j5.8) in a metallic state upon receiving a signal.

[0081] The optical element can include a semiconductor. The charge carrier density of the semiconductor can be configured to change upon receiving a signal. The semiconductor can form part of a photoconductive device. The photoconductive device can be configured to receive an optical signal (e.g., a laser pulse) to Q-switch the CO2 laser.

[0082] The semiconductor may include at least one of GaAs, Si, or Ge.

[0083] The optical element may include a liquid crystal, the optical properties of which may be configured to change upon receiving a signal.

[0084] The signal can include a bias voltage. The bias voltage can be greater than 0 V. The bias voltage can be about 20 V or less. The bias voltage can be applied for about 1 ns or more. The bias voltage can be applied for about 100 μs or less. The bias voltage can be applied for about 150 ns or less. The duration for which the bias voltage is applied can be selected depending at least in part on the desired laser pulse energy to be generated by the CO2 laser. The desired laser pulse energy to be generated by the CO2 laser can depend on the application (e.g., medical or industrial) of the CO2 laser.

[0085] The signal can include a laser pulse. The laser pulse can be generated by a short-pulse laser diode. The short-pulse laser diode pulse can have a pulse width of 1 nsec or more. The short-pulse laser diode pulse can have a pulse width of 100 μsec or less.

[0086] The reflectivity, transmittance and / or absorptivity of the optical element may be changed over a range of frequencies of infrared electromagnetic radiation upon receipt of a signal.

[0087] The optical element may include a switching component operably coupled to the frequency selective structure.

[0088] The optical element may include a signal transmission component operably coupled to the switching component configured to receive a signal and transmit a signal to the switching component. The signal transmission component may include a layer of conductive material. The layer of conductive material may include at least one of gold, nickel, aluminum, and indium tin oxide (ITO). The optical element may include an insulating material. The insulating material may include one or more of aluminum oxide and hafnium oxide.

[0089] The optical element may include a deformable material configured to change the periodicity and / or geometry of features of the substantially periodic pattern upon receiving a signal.

[0090] The refractive index of the optical element can be configured to change upon receiving a signal.

[0091] The permittivity and / or permeability of the optical element may be configured to change upon receiving a signal.

[0092] The conductivity and resistivity of the optical element can be configured to change upon receiving a signal.

[0093] The substantially periodic pattern of features can include an array of geometric features configured to at least partially determine the frequency response of the optical element.

[0094] The array can include tuning elements configured to at least partially determine the frequency response of the optical element.

[0095] The features of the substantially periodic pattern can be configured to act as a polarizer. The optical element can include a metamaterial.

[0096] The frequency selective structure may include a plurality of frequency selective layers configured to at least partially determine the frequency response of the optical element.

[0097] The infrared electromagnetic radiation can be configured to mark the product.

[0098] The CO2 laser can be configured to produce short pulses of infrared electromagnetic radiation. CO2 lasers can be configured to produce short pulses (pulse width >0.1 nsec and <500 μsec) of infrared electromagnetic radiation.

[0099] The signal can be configured to control a bias level of a switching component of the optical element. The signal can be configured to control the timing between a laser output command of the CO2 laser and the initiation of Q-switching of the CO2 laser.

[0100] Infrared electromagnetic radiation can form part of industrial processes. CO2 lasers can form part of industrial systems. For example, CO2 lasers and the infrared electromagnetic radiation they generate can be used for laser marking and coding, engraving, drilling, cutting, perforating, welding (metal and plastic), surface treatment (laser peening, hardening, polishing, roughening, blackening), rust removal, paint removal, etc.

[0101] Infrared electromagnetic waves can be part of medical processes. CO2 lasers can be part of medical systems. For example, CO2 lasers and the infrared electromagnetic radiation they generate can be used as laser scalpels, in ENT and head and neck surgery, gynecological surgery, lesion and tumor removal, vascular surgery, oral soft tissue surgery, enamel removal, implant dentistry, tattoo removal, laser-induced nociceptive potentials for migraine treatment, burn scar treatment, skin resurfacing, birthmark removal, mole and viral wart removal, skin aging, facial scar removal, etc.

[0102] According to a seventh aspect of the present disclosure, there is provided a laser marking system for marking a target, comprising the CO2 laser of the sixth aspect.

[0103] According to an eighth aspect of the present disclosure, there is provided a method of Q-switching a CO2 laser to produce infrared electromagnetic radiation, comprising pumping a CO2 gain medium of the CO2 laser, the method including generating a signal that alters the frequency response of a frequency selective structure having a substantially periodic pattern of features on an optical element of the CO2 laser, thereby producing pulses of infrared electromagnetic radiation.

[0104] Generating the signal may include operating a controller to generate an electrical signal or an optical signal (eg, a laser pulse).

[0105] The infrared electromagnetic radiation can be configured to mark the product.

[0106] The method can include generating short-pulse infrared electromagnetic radiation.

[0107] CO2 lasers can be configured to produce short pulses (pulse width >0.1 nsec and <500 μsec) of infrared electromagnetic radiation.

[0108] According to a ninth aspect of the present disclosure, there is provided a method of marking a target with infrared electromagnetic radiation, comprising exciting a CO2 gain medium of a CO2 laser. The method comprises generating a signal to Q-switch the CO2 laser by varying the frequency response of a frequency selective structure having a substantially periodic pattern of features on an optical element of the CO2 laser, thereby generating pulses of infrared electromagnetic radiation. The method includes directing the pulses of infrared electromagnetic radiation toward the target.

[0109] Generating the signal may include operating a controller to generate an electrical signal or an optical signal (eg, a laser pulse).

[0110] The method can include generating short-pulse infrared electromagnetic radiation.

[0111] CO2 lasers can be configured to produce short pulses (pulse width >0.1 nsec and <500 μsec) of infrared electromagnetic radiation.

[0112] It will be understood that the exemplary signal durations (e.g., RF power signal, Q-switch signal (e.g., bias voltage signal), etc.) described above are for CO2 laser marking applications, and that durations other than those described above are possible depending on the CO2 laser application requirements.

[0113] It will be understood that while the exemplary materials described above are suitable for CO lasers, the use of other materials should not be excluded, for example, metal mirrors can be used as the base material for optical elements according to the present disclosure.

[0114] The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component illustrated in various figures is represented by a like numeral. For clarity, not every component is labeled in every figure. [Brief explanation of the drawings]

[0115] [Figure 1] FIG. 1 is a schematic plan view of an embodiment of a laser including a tunable optical element, according to an aspect of the present disclosure. [Figure 2] 1A and 1B illustrate a tunable optical element including a frequency-selective structure having periodic patterned features, according to one aspect of the present disclosure. [Figure 3] FIG. 1 illustrates a tunable optical element including a frequency-selective structure having a periodic pattern of features and a switching component operably coupled to the periodic pattern of features, according to one aspect of the present disclosure. [Figure 4] FIG. 4 is an exploded view of the adjustable optical element of FIG. 3. [Figure 5] 1 is an exploded view of an adjustable optical element comprising a frequency selective structure, a switching component, and a signal transmission component configured to receive a signal and transmit a signal to the switching component, according to one aspect of the present disclosure. FIG. [Figure 6A] 1 illustrates a tunable optical element comprising a frequency selective structure including an array of geometric features and a switching component including a phase change material including VO 2 on a ZnSe substrate, according to one embodiment of the present disclosure. [Figure 6B] 6B shows the optical transmission frequency response of the tunable optical element of FIG. 6A in the insulating and metallic phases after receiving a signal. [Figure 6C] 6B shows the optical reflection frequency response of the tunable optical element of FIG. 6A in the insulating and metallic phases after receiving a signal. [Figure 7A] FIG. 1 illustrates a tunable optical element comprising a frequency selective structure, a switching component comprising graphene, and a ZnSe substrate according to one embodiment of the present disclosure. [Figure 7B] FIG. 7B shows the optical transmission frequency response of the tunable optical element of FIG. 7A with different applied chemical potentials. [Figure 7C] 7B shows the optical reflection frequency response of the tunable optical element of FIG. 7A with different applied chemical potentials. [Figure 8A]1A and 1B show optical transmission frequency responses in biased and unbiased states of a tunable optical element with a frequency selective structure and a switching component including liquid crystal E7 on a ZnSe substrate according to one embodiment of the present disclosure. [Figure 8B] 8B illustrates the optical reflection frequency response of the tunable optical element of FIG. 8A in a biased and unbiased state. [Figure 9A] FIG. 1 illustrates the optical transmission frequency response for a range of charge carrier densities of a frequency selective structure, a tunable optical element with a switching component comprising a semiconductor including GaAs, according to one embodiment of the present disclosure. [Figure 9B] FIG. 9B illustrates the optical reflection frequency characteristics of the tunable optical element of FIG. 9A for a range of charge carrier densities. [Figure 10] 1 is a schematic perspective top view of a portion of a frequency selective structure including a periodic pattern of slots, according to one aspect of the present disclosure; FIG. [Figure 11] 1 is a schematic perspective top view of a portion of a frequency selective structure including a periodic pattern of crosses, according to one aspect of the present disclosure; [Figure 12] 1 is a schematic perspective view from above of a portion of a frequency selective structure including periodically patterned rings, according to one aspect of the present disclosure; [Figure 13] 1 is a schematic perspective top view of a portion of a frequency selective structure including a periodically patterned split ring, according to one aspect of the present disclosure; [Figure 14] FIG. 1 is a schematic perspective side view of a CO laser according to one embodiment of the present disclosure. [Figure 15] FIG. 15 is a schematic cross-sectional view of the output coupler of the CO laser of FIG. [Figure 16] 15 is a schematic cross-sectional view of the rear mirror of the CO2 laser of FIG. [Figure 17] FIG. 1 is a schematic perspective view of a CO laser including a folded cavity, according to one embodiment of the present disclosure. [Figure 18] 18 is a schematic cross-sectional view of two of the fold mirrors and output coupler of the CO2 laser of FIG. [Figure 19]18 is a schematic cross-sectional view of two of the fold mirrors and output coupler of the CO laser of FIG. 17 further comprising passive optical components, according to one embodiment of the present disclosure. [Figure 20] 1 is a schematic diagram of a laser marking system including a CO2 laser according to one embodiment of the present disclosure. [Figure 21] 1 is a flowchart of a method for Q-switching a CO2 laser to produce infrared electromagnetic radiation, according to one embodiment of the present disclosure. [Figure 22] FIG. 1 illustrates a method for marking a target with infrared electromagnetic radiation according to an aspect of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0116] The aspects and embodiments disclosed herein may be practiced or carried out in various ways and are not limited to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings.

[0117] Aspects and embodiments disclosed herein include optical elements including frequency-selective structures having a substantially periodic pattern of features. The substantially periodic pattern of features may be referred to as periodic pattern features. However, it will be understood that strict periodicity is not required. The periodicity of the pattern of features may vary by approximately 10% or less. Optical elements may be described as tunable optical elements because the frequency response of the optical element is controllable. That is, the effect of the optical element on incident electromagnetic radiation can be controlled or adjusted to have different effects on different wavelengths at different times. For example, the optical element may be adjusted to be highly transmissive to a first wavelength at a first time and highly reflective to the first wavelength at another time after receiving a signal. The frequency response of the tunable optical element is configured to change upon receiving a signal. Aspects and embodiments disclosed herein also include lasers with tunable optical elements for use in various implementations, such as laser marking systems.

[0118] Laser marking systems can be used in production lines to mark various types of targets or articles. For example, laser marking systems can be used to imprint bar codes, unique identification marks, expiration dates, or other information on articles passing through the production line. Visible and near-infrared Q-switched lasers are commonly used for marking in laser marking systems. The present invention advantageously enables the size and cost of such visible and near-infrared Q-switched lasers to be reduced. The present invention also advantageously enables the use of carbon dioxide (CO2) Q-switched lasers for laser marking applications. The optical elements of the present disclosure can be used to Q-switch other types of lasers, such as ultraviolet (UV) lasers and / or visible lasers. CO2 lasers typically generate infrared electromagnetic radiation beams in four primary wavelength bands centered at 9.3, 9.6, 10.2, and 10.6 micrometers (μm). Higher peak laser output powers may be desired to mark certain materials (e.g., metals) that exhibit higher marking thresholds. In various aspects and embodiments disclosed herein, a CO laser including tunable optics for Q-switching the laser can be utilized to generate relatively high peak laser power levels that may be desirable for marking workpieces including materials that exhibit relatively high thermal conductivity. The laser system embodiments disclosed herein are not limited to use in laser scanning or marking systems and can be utilized in any of a number of industrial or commercial embodiments. The tunable optics for Q-switching the laser can be utilized with any other laser technology or wavelength by appropriately scaling the periodic features and / or by the selection of materials used.

[0119] Generally, a laser includes two mirrors and a gain medium. The two mirrors are arranged to form a resonant cavity with the gain medium between them. The quality factor (Q factor) of a laser's resonant cavity is a measure of the laser's ability to sustain lasing. The Q factor of a laser is proportional to the ratio of the energy stored in the resonant cavity (i.e., the energy pumped into the gain medium) to the energy loss during one round trip of an electromagnetic wave through the resonant cavity (i.e., the loss of photons within the resonant cavity). A high Q factor is required to initiate and sustain lasing in a laser's resonant cavity. The lower the Q factor, the greater the photon loss within the resonant cavity, making it more difficult to initiate and sustain lasing action via stimulated emission of radiation. The energy stored in the resonant cavity is determined, at least in part, by the gain medium and the ability to efficiently pump energy into the gain medium using an external source, such as direct current (DC) or radio frequency (RF) power. Photon losses in the resonator can be determined, at least in part, by, for example, the reflectivity of the mirrors, diffraction losses, photon interactions with the resonant cavity walls, and absorption losses within the resonant cavity. For laser oscillation to occur and for the laser to have a high Q, the lifetime of a photon circulating within the resonant cavity must be much longer than the time it takes for the photon to travel the round-trip length of the resonant cavity.

[0120] Q-switching of a laser can be achieved by controlling a loss mechanism so that the Q-factor of the laser's resonant cavity can be switched from a high value to a low value, or vice versa. This allows for the generation of short laser pulses with high peak power, rather than continuous-wave output with low peak power. Q-switching can be achieved by placing a variable attenuator or other form of switchable (i.e., controllable) photon loss mechanism within the laser's resonant cavity. When the loss mechanism is in a loss state (i.e., when the loss mechanism causes photon loss within the resonant cavity), the Q-factor of the laser can be reduced to a level where oscillation cannot be initiated. The variable attenuator or loss mechanism can be referred to as a Q-switch. In some embodiments, a variable attenuator can be used as a Q-switch, but various mechanisms that can be used to control the Q-factor of a laser's resonant cavity or to control the generation or output of electromagnetic radiation from a laser's resonant cavity can be used as a Q-switch.

[0121] To generate high-power laser pulses using Q-switching, the Q-switch is set to a low Q value for the laser's resonant cavity so that energy is pumped into the laser's resonant cavity while preventing lasing. Energy introduced into the laser's gain medium causes atoms and / or molecules in the gain medium to transition from their ground state to an excited state, resulting in a population inversion in the gain medium. The energy stored in the resonant cavity increases with continued pumping of energy into the gain medium, but lasing does not begin due to the low Q value of the resonant cavity (i.e., photons in the resonant cavity are lost before significant stimulated emission from the gain medium occurs). After a certain time, the energy stored in the resonant cavity may reach a maximum level (a condition called "gain saturation"), which can be determined at least in part by factors such as the amount of energy pumped into the gain medium and the loss of photons due to spontaneous emission from the pumped gain medium. After the resonator is gain saturated (or in some embodiments before complete gain saturation is achieved), the state of the Q-switch can be changed or switched to increase the Q-factor of the resonant cavity, thereby allowing the onset of lasing (i.e., a significant amount of stimulated emission of radiation from the gain medium). The intensity of the laser light within the resonant cavity rises rapidly due to the large amount of energy stored in the gain medium. Radiation can be rapidly emitted from the resonant cavity, resulting in a short pulse (in some embodiments, on the order of nanoseconds or less) of high-intensity laser radiation. The short pulse of high-intensity laser radiation can have a peak power thousands of times or more greater than the power of laser radiation generated when the laser is operated in continuous-wave mode.

[0122] Q-switching is used in some solid-state and fiber laser technologies to generate short pulses (femtosecond to nanosecond duration) and high peak powers (megawatts to gigawatts) for various applications in materials processing. Mechanisms used to Q-switch such lasers include electro-optic crystals, acousto-optic crystals, and saturable absorbers.

[0123] Despite the proven benefits of using short pulse lasers for materials processing, there are no viable Q-switched CO2 laser products on the market. Existing technology solutions are costly, increase laser size to an unmanageable extent (e.g., lasers too large for production line use), and suffer from performance limitations (e.g., Q-switch inefficiency, poor absorption, Q-switch inefficiency, pulse frequency, lifetime).

[0124] Aspects and embodiments of the invention disclosed herein include tunable optical elements and methods for Q-switching lasers that are inexpensive, can be manufactured in compact sizes, and are actively driven while consuming minimal power compared to known Q-switches. Tunable optical element embodiments that can function as Q-switching devices in lasers disclosed herein are switchable between loss and low-loss states, e.g., between transmissive and reflective states, via an external control signal. Tunable optical element embodiments disclosed herein can form part of an output coupler, rear mirror, fold mirror, or passive optical component in a laser's resonant cavity. Tunable optical element embodiments include a frequency-selective structure having periodic patterned features. In some embodiments, the frequency-selective structure can include a frequency-selective surface. The frequency-selective structure can include a metamaterial (e.g., a metasurface) with plasmonic resonance properties. Tunable optical element embodiments can further include a switching component in communication with the periodic patterned features to change a controllable optical property (e.g., reflectance, transmittance, and / or absorbance property) of the tunable optical element. In this manner, the tunable optical element can be used to control photon loss in the resonant cavity of the laser and the Q-factor of the resonant cavity. Q-switching the resonant cavity can be achieved by providing a signal to a switching component to change the frequency response of the tunable optical element on command (e.g., changing the reflectivity, transmittance, and / or absorptivity of the tunable optical element over a frequency range of electromagnetic radiation). The tunable optical element can form part of a CO2 laser, and the frequency range of electromagnetic radiation can include infrared (e.g., wavelengths between about 8 μm and about 15 μm).

[0125] The present invention is applicable to any laser technology, and in particular to any CO laser technology. The optical elements of any DC-pumped laser, RF-pumped waveguide laser, slab laser, or free-space laser can be converted into tunable optical elements for Q-switching the laser. An example of a waveguide-configured laser including tunable optical elements that function at least in part as a Q-switching device is shown schematically in FIG. 1. Laser 100 includes a tube 105 that defines a gain region, a fully reflective rear mirror 110, and a partially reflective output coupler 115 that defines a resonant cavity of laser 100. The resonant cavity may include a fold mirror (not shown) to extend the length of the cavity. While element 105 is shown and described herein as a tube acting as a waveguide, it should be noted that in other embodiments, element 105 may include a conduit having another cross-section, such as an elliptical, square, or rectangular cross-section. In still other embodiments, the gain region may be constrained by a metal plate that also functions as an electrode, as in a slab laser configuration.

[0126] When operating in continuous wave mode, electromagnetic energy, e.g., radio frequency (RF) energy, is applied to the tube 105 to excite a gain medium (e.g., a gas mixture) within the tube 105. The RF energy can have a frequency of about 27 MHz or greater. The RF energy can have a frequency of about 120 MHz or less. The RF energy can have a frequency of about 95 MHz. The gas mixture can include, for example, He:N2:CO2. In some exemplary embodiments, the output coupler 115 can be configured to reflect photons exiting the tube 105 back into the tube 105, allowing the remaining photons to exit the laser 100 as a laser beam (not shown). The output coupler 115 can be configured to reflect about 45% or more of the incident photons. The output coupler 115 can be configured to reflect about 95% or less of the incident photons. The output coupler can be configured to reflect about 80% of the incident photons. A Q-switched laser can include an output coupler 115 with a reflectivity lower than that of a continuous wave laser.

[0127] In some embodiments, the rear mirror 110 may be a silicon mirror that may be coated with silver, gold, or any other highly reflective material (e.g., having a reflectivity of about 97% or greater at a selected operating wavelength). The rear mirror 110 may exhibit a reflectivity of about 99.8% for photons generated in the laser. A fold mirror, if used, may be formed from the same material as the rear mirror 110 and have substantially the same reflectivity as the rear mirror 110. The output coupler 115 may be formed from a material such as zinc selenide or other material and coated with a desired material selected to be partially reflective to photons of a particular wavelength generated by the laser 100, or to a particular wavelength of interest in the case of a laser that generates photons having multiple wavelengths. The waveguide body 105m may be formed from a dielectric material, such as aluminum oxide (Al2O3, alumina), and may be substantially pure, e.g., 99.9% or greater pure alumina, so as to be substantially free of impurities that may interfere with the purity of the gain medium within the laser 100. The gas mixture within laser 100 may include, for example, a mixture of 5% xenon, 57% helium, 19% nitrogen, and 19% CO2, although other gas mixtures known in the art may alternatively be used. The gas mixture may fill the entire interior volume 130 of laser 100, and tube 105 may be open at its ends so that the gas mixture circulates through the interior volume 130 of tube 105 and body 135 of laser 100. Tube 105 may have an outer diameter of about 5 mm and an inner diameter of about 3 mm in a waveguide configuration, or may be larger in a free-space configuration.

[0128] The laser 100 may further include a tunable optical element 140 with a frequency-selective structure having a periodic pattern of features that forms part of any of the optical components of the laser 100. Q-switching the laser 100 may involve varying the frequency response of the tunable optical element 140 over the operating bandwidth of the laser 100. Disclosed herein are four exemplary implementations that incorporate the tunable optical element 140, including the frequency-selective structure, as part of the laser's optical components for Q-switching the laser 100. A first exemplary implementation includes incorporating the tunable optical element 140 as part of the rear mirror 110. The rear mirror 110 is typically a highly reflective surface (e.g., having a reflectivity of about 97% or greater, e.g., about 99.5%) and contributes relatively little to photon loss within the resonant cavity of the laser 100. When the adjustable optical element 140 forms part of the rear mirror 110 (e.g., replacing a fixed highly reflective coating), the rear mirror 110 is switchable between a highly reflective state and a more transmissive and / or absorbing state for a wavelength range of electromagnetic radiation corresponding to the operating bandwidth of the laser. By altering the frequency response of the rear mirror 110 in this manner, it is possible to change the degree of photon loss in the resonant cavity, thereby Q-switching the laser 100 to produce intense pulses of radiation.

[0129] A second exemplary implementation includes incorporating a tunable optical element 140 as part of the output coupler 115 (e.g., replacing a fixed partially reflective coating). Typically, the reflectivity of the output coupler 115 is designed to achieve the desired output power of the laser 100 while simultaneously reducing photon loss in the resonant cavity. The optimal reflectivity of the output coupler 115 depends on the laser design. In known lasers, the reflectivity of the output coupler 115 remains fixed once optimized. By incorporating a tunable optical element 140 as part of the output coupler 115, the frequency response (e.g., reflectivity) of the output coupler 115 can be changed upon receiving a signal (i.e., on command). The reflectivity of the output coupler 115 can be set to an extremely low value, inducing significant photon loss in the resonant cavity. Changing the reflectivity of the output coupler 115 back to its optimal value for a short period of time reduces photon loss in the resonant cavity, allowing lasing to occur, thereby Q-switching the laser 100 to generate powerful radiation pulses.

[0130] A third exemplary implementation includes incorporating a tunable optical element as part of a fold mirror of a folded cavity laser (not shown). The tunable optical element can have a first state in which the frequency response of the fold mirror is highly reflective to reduce photon loss in the resonant cavity and enable lasing. The tunable optical element can have a second state in which the frequency response of the fold mirror is highly transmissive and / or highly absorptive, increasing photon loss in the resonant cavity and preventing lasing from occurring. By providing a signal that switches the tunable optical element between the first and second states, the laser can be Q-switched to generate intense radiation pulses.

[0131] A fourth implementation includes incorporating the adjustable optical element 140 as part of the passive optical components 120 in the radiation path of the laser 100. For example, the passive optical component can be located between the tube 105 and the output coupler 115. Other implementations can place the passive optical component 120 in front of the rear mirror 110, or a fold mirror (not shown), or any other suitable location in the laser beam path. The adjustable optical element 140 can have a first state in which the frequency response of the adjustable optical element 140 is highly transmissive to reduce photon loss in the resonant cavity and allow lasing to occur. The adjustable optical element 140 can have a second state in which the frequency response of the adjustable optical element 140 is highly absorptive to increase photon loss in the resonant cavity and prevent lasing from occurring. By providing a signal to switch the adjustable optical element 140 between the first and second states, the laser can be Q-switched to generate powerful radiation pulses.

[0132] Embodiments of the tunable optical element 140 that include a frequency selective structure can be considered to be capacitive or inductive. That is, the tunable optical element 140 and the frequency selective structure can be configured to operate as a bandpass or bandstop optical filter, respectively, for a range of wavelengths of incident electromagnetic radiation. The bandpass or bandstop operation of the frequency selective structure and the tunable optical element 140 as a whole can be changed upon receiving a signal such that a frequency range of electromagnetic radiation that was previously passed is now blocked, and vice versa.

[0133] The features of the periodic pattern of frequency selective structures can include an array of simple features, such as an array of slots and / or dipoles. Alternatively or additionally, the features of the periodic pattern of frequency selective structures can include an array of more complex features, such as an array of rings or split-ring resonators. For example, slots can function as band-pass optical filters by transmitting radiation of a range of wavelengths more than radiation of other wavelengths. The wavelengths passed by the slots can correspond to the geometric shape of the slots. As another example, dipoles can act as band-stop optical filters by absorbing a range of wavelengths of radiation more than other wavelengths of radiation. The wavelengths absorbed by the dipoles can correspond to the geometric shape of the dipoles.

[0134] Split-ring resonators can reduce the range of wavelengths of electromagnetic radiation that fall within the bandpass or bandstop function of the optical element, thereby advantageously providing a highly targeted frequency response over a narrow range of wavelengths. Relatively small changes in the shape and / or electromagnetic properties of the tunable optical element 140 (e.g., using relatively low-energy signals) can subsequently result in relatively large changes in the frequency response (e.g., reflectivity, transmittance, and / or absorption) of the tunable optical element. Any array of features may additionally include tuning elements configured to at least partially determine the frequency response of the tunable optical element 140. The tuning elements may include one or more geometric features (e.g., bars) configured to change the geometry of the frequency-selective structure to adjust the frequency response of the tunable optical element 140.

[0135] The periodic pattern features of the frequency selective structure of the optical element 140 can be configured to act as a polarizer. That is, the periodic pattern features can be polarization sensitive or polarization insensitive depending on the geometry of the feature pattern. The geometry of the frequency selective structure (e.g., the periodicity of the periodic pattern features) and the electromagnetic properties (e.g., refractive index, permittivity, permeability, conductivity, resistivity, etc.) of any combination of components (e.g., switching components) can at least partially determine the frequency response of the optical element 140.

[0136] The frequency selective structure can include multiple frequency selective layers configured to at least partially determine the frequency response of the optical element 140. Having multiple frequency selective layers advantageously allows for further fine tuning of the frequency response of the tunable optical element 140, thereby providing a tunable optical element with a sharper frequency response and / or multiple frequency responses. For example, a first layer of the multiple frequency selective layers can be configured to have a desired interaction (e.g., reflect, absorb, or transmit) with radiation having a wavelength of about 10.2 μm, while another layer can be configured to have a desired interaction with radiation having a wavelength of about 10.6 μm.

[0137] To operate as a Q-switch in the laser 100, the frequency response of the tunable optical element 140 and the manner in which the frequency response changes upon receiving a signal can be tailored to the operating wavelength of the laser. The frequency response can be tailored by designing the physical shape of the periodic pattern of features forming the frequency-selective structure and / or by selecting materials operably coupled to (e.g., contacting and / or surrounding) the periodic pattern of features based on desired switchable optical and electromagnetic properties. For example, the tunable optical element 140 can include a switching component operably coupled to the frequency-selective structure. The switching component can take the form of a layer on the tunable optical element. The switching component can at least partially surround the periodic pattern of features of the frequency-selective structure. The frequency-selective structure can be embedded in or otherwise surround the switching component. The switching component can be bonded to the frequency-selective structure. For example, the frequency-selective structure can include a periodic pattern of features provided (e.g., etched or deposited) on the active switching layer. The periodic pattern of features can include metal. The switching component may comprise a material whose electromagnetic properties are changeable, for example a phase change material such as vanadium dioxide.

[0138] The rear mirror 110, which includes an optical element 140 having a frequency-selective structure, can have a periodic pattern of features designed so that in one state of the switching component, the tunable optical element 140 acts as a bandpass optical filter at the operating wavelength of the laser 100. This induces a relatively high loss of photons in the resonant cavity, thereby suppressing lasing. When the switching component changes to the other state upon receiving a signal, the tunable optical element 140 acts as a highly reflective mirror, reducing the loss of photons in the resonant cavity and allowing lasing to occur. This switching can be induced by changing one or more physical properties of the frequency-selective structure and / or the switching component. Switching the tunable optical element 140 (i.e., changing the frequency response of the optical element) can be achieved by changing the charge carrier density in a portion of the tunable optical element (e.g., the switching component), such as in a photoconductor or semiconductor material. Other approaches include changing the phase of the material (e.g., from amorphous to crystalline, or from insulator to metal), using a deformable material to change the periodicity and / or geometry of features in a periodic pattern, changing the refractive index and / or permittivity and / or permeability of the material, etc. These changes can be achieved by applying electric and / or magnetic fields, injecting current, biasing with a voltage, or applying optical energy to the tunable optical element 140.

[0139] Possible implementations of tunable optical elements including frequency-selective structures capable of functioning as Q-switches in lasers are shown schematically in FIGS. 2-5. A Q-switch optical element 200 including a frequency-selective structure 205 is shown in FIG. 2. A Q-switch optical element 300 in which a switching component 305 is provided with periodic patterned features of the frequency-selective structure is shown in FIG. 3. An exploded view of a Q-switch optical element 400 including a frequency-selective structure 405 and a switching component 410 is shown in FIG. 4. An exploded view of a Q-switch optical element 500 including a frequency-selective structure 505, a switching component 510, and a signal transmission component 515 configured to receive and transmit signals to the switching component is shown in FIG. 5. The signal transmission component 515 can include, for example, indium tin oxide, patterned metal such as gold, patterned silicon, gallium arsenide, and / or another semiconductor material. The signal transmission component 515 can include multiple layers. Different layers of the signal transmission component 515 can include different materials. If an optical signal is used to control the frequency response of the optical element, signal transmission component 515 may not be necessary, and the optical switching energy can be applied separately from the Q-switched optical element.

[0140] The frequency selective structures, switching components, and signal transmission components of Figures 2-5 can be formed from a variety of different materials to achieve optical elements with the same or different frequency responses. The following text provides examples of possible implementations.

[0141] A first possible implementation is an optical element including a phase change material configured to change phase upon receiving a signal. Phase change materials can be incorporated into optical elements, for example, as switching components. The term "phase change material" covers a wide range of compounds that undergo a physical change when exposed to an external stimulus. Phase change materials are used in a wide variety of applications, including random access memory devices, compact discs (CDs) and digital versatile discs (DVDs), and smart windows. Chalcogenide glasses used in random access memory devices change between an amorphous and a crystalline state. During this transition, resistivity can change at a microscopic level in response to an electrical stimulus. Chalcogenide glasses (e.g., GeSbTe) used in compact discs use optical stimuli to change between amorphous and crystalline states, thereby changing the reflectivity of the surface. Smart windows utilize the insulator-to-metal phase transition of sesquioxides (e.g., VO) to change reflectivity with temperature.

[0142] The phase change material can be configured to change between an amorphous state and a crystalline state, or vice versa, upon receiving a signal. The phase change material can be configured to change between an insulating phase and a metallic phase, or vice versa, upon receiving a signal. Changing the phase of the phase change material changes the electromagnetic properties (e.g., refractive index and / or conductivity and resistivity) of the frequency selective structure, thereby changing the transmittance, reflectance, and / or absorption of the optical element. The phase change material can include vanadium dioxide (VO). VO is a preferred phase change material for implementing CO laser Q-switches. The insulator-to-metal phase transition of VO can be stimulated by thermal, electrical, or optical means. It has been shown that the insulator-to-metal phase transition can be stimulated by electrical or optical sources in the sub-microsecond time domain.

[0143] FIG. 6A schematically illustrates a tunable optical element 600 including a substrate component 605, a frequency-selective structure 610, and a switching component 620 including VO (e.g., a layer of VO). The optical element 600 can rapidly change its frequency characteristics upon receiving a signal. Thus, the optical element 600 can be used to Q-switch a laser (e.g., the laser of FIG. 1). The optical element 600 further includes a signal-transmission component 630 operably coupled to the switching component 620 and configured to receive and transmit the signal to the switching component 620. In the example of FIG. 6A, the signal-transmission component 630 includes indium tin oxide (ITO), the frequency-selective structure 610 includes an array of split-ring resonators 640, and the substrate 605 includes ZnSe.

[0144] 6B is a diagram illustrating the optical transmittance frequency characteristics of the optical element 600 of FIG. 6A in the insulating phase 650 before receiving a signal and in the metallic phase 655 after receiving a signal. The optical transmittance frequency response of the optical element 600 is shown for wavelengths of radiation between about 9.7 μm and about 11.6 μm (i.e., in the infrared spectrum of wavelengths). At about 10.6 μm, the transmittance of the optical element 600 in the insulating phase 650 is much greater than the transmittance of the optical element 600 in the metallic phase 655.

[0145] 6C is a diagram illustrating the optical reflectance frequency characteristics of the optical element 600 of FIG. 6A in the insulating phase 650 before receiving a signal and in the metallic phase 655 after receiving a signal. The optical reflectance frequency characteristics of the optical element 600 are shown for wavelengths of radiation between about 9.7 μm and about 11.6 μm (i.e., in the infrared spectrum of wavelengths). At about 10.6 μm, the reflectance of the optical element 600 in the insulating phase 650 is much lower than the reflectance of the optical element 600 in the metallic phase 655.

[0146] 6B and 6C show the significant change in frequency characteristics of optical element 600 due to the transition of VO from the insulating phase to the metallic phase upon receiving a signal. If optical element 600 forms part of the rear mirror of a CO laser configured to generate pulses of radiation having a wavelength of approximately 10.6 μm, optical element 600 can contribute to a laser with a high Q factor when VO is in the metallic phase by operating as a highly reflective rear mirror. A signal can be applied to change the phase of VO to the insulating phase, thereby reducing the reflectivity of the rear mirror for wavelengths of approximately 10.6 μm and thereby reducing the Q factor of the laser. This increases the number of photons lost within the resonant cavity, preventing laser oscillation while the gain medium is being pumped. Once the gain medium is sufficiently pumped, another signal can be applied to change VO back to the metallic phase, allowing lasing to occur, thereby generating powerful radiation pulses via Q-switching.

[0147] A second possible implementation is an optical element including graphene, where the electrical permittivity of the graphene is configured to change upon receiving a signal. Graphene is a monoatomic layer of carbon, and its electrical permittivity can be changed by applying an electrical bias. As shown in FIG. 7A , an optical element 700 for Q-switching a laser can include a support structure or substrate 705, a frequency selective structure 710, a switching component 720 including graphene, a means for biasing the graphene 735, an electrically insulating material 745, and a signal transmission component 730. In the example of FIG. 7A , the substrate 705 includes ZnSe, the switching component 720 includes a layer of graphene, and the means for biasing the graphene 735 includes two electrical contacts 735. The two electrical contacts 735 can be formed, for example, from at least one of gold, silver, nickel, and / or aluminum. The two electrical contacts 735 can be connected to a voltage source (not shown). The insulating material 745 can include, for example, at least one of SiO2, HfO2, and / or Al2O3. The signal transmission component 730 includes indium tin oxide (ITO). The signal transmission component 730 can operate in a manner similar to the gate of a transistor. In this manner, the electrical contact 735 and the signal transmission component 730 can operate in a manner similar to a transistor. The signal transmission component 730 is optional. In an alternative embodiment, the voltage signal can be switched directly and the single transmission component 730 can be omitted or grounded. The frequency selective structure 710 includes an array of split ring resonators 740. Alternatively or additionally, the graphene 720 can be lithographically processed to incorporate the periodic pattern of features 740 of the frequency selective structure 710. Biasing the graphene 720 can cause a change in the electrical permittivity of the optical element 700, thereby changing the transmittance, reflectance, and / or absorption of the optical element 700.

[0148] 7B and 7C show the variation of the transmission and reflection frequency characteristics, respectively, of the optical element 700 of FIG. 7A as a function of the applied bias Ef, acting as a signal. -4FIG. 7B shows how the transmission and reflection frequency responses of optical element 700 change at wavelengths between about 9.7 μm and about 11.6 μm for applied biases Ef of 0.1 eV (760), 0.1 eV (765), 0.2 eV (770), 0.3 eV (775), and 0.5 eV (780). FIG. 7B shows the transmission frequency response shift with varying bias. The peak transmission of about 100% is approximately 5*10 -4 At a bias of 0.1 eV (765), a peak transmission occurs at a wavelength of approximately 10.6 μm. At a bias of 0.1 eV (765), a peak transmission of approximately 100% occurs at a wavelength of approximately 10.9 μm. At a bias of 0.2 eV (770), a peak transmission of approximately 95% occurs at a wavelength of approximately 11.5 μm. A maximum transmission of approximately 40% is achievable across the wavelengths shown (approximately 11.6 μm) at a bias of 0.3 eV (775). A maximum transmission of approximately 5% is achievable across the wavelengths shown at a bias of 0.5 eV (780) (approximately 11.6 μm). An optical element 700 having such adjustable transmission frequency characteristics can be implemented, for example, as an output coupler for Q-switching a CO laser.

[0149] FIG. 7C shows how varying the bias of the graphene shifts the reflection null (i.e., the minimum reflectance) in the frequency response of the optical element 700, resulting in a change in reflectance at a CO wavelength of 10.6 μm. -4 A minimum reflectance of approximately 0% occurs at a wavelength of approximately 10.6 μm with a bias of 0.1 eV (765). A minimum reflectance of approximately 0% occurs at a wavelength of approximately 10.9 μm with a bias of 0.1 eV (765). A minimum reflectance of approximately 5% occurs at a wavelength of approximately 11.5 μm with a bias of 0.2 eV (770). A minimum reflectance of approximately 60% is achievable over the wavelengths shown (approximately 11.6 μm) with a bias of 0.3 eV (775). A minimum reflectance of approximately 95% is achievable over the wavelengths shown (at approximately 11.6 μm) with a bias of 0.5 eV (780). An optical element 700 having such a tunable reflectance frequency response can be implemented, for example, as a rear mirror for Q-switching a CO laser.

[0150] A third possible implementation is an optical element including a liquid crystal, where the optical properties of the liquid crystal are configured to change upon receiving a signal. Liquid crystals are a family of materials that simultaneously exhibit liquid and crystalline properties under appropriate conditions of temperature and / or concentration. The optical properties of liquid crystals vary depending on the liquid crystal material. A common property of liquid crystals is birefringence, i.e., polarization sensitivity, where the refractive index changes with the application of a potential difference (bias). Applying a voltage to an optical element including a liquid crystal can change the polarization properties and / or refractive index of the liquid crystal, thereby changing the transmittance, reflectance, and / or absorption of the optical element. The variation in the optical properties of the liquid crystal due to the applied bias can change the transmittance, reflectance, and / or absorption of a frequency-selective structure. Such an optical element including a liquid crystal, such as E7, can be used to Q-switch a laser, e.g., a CO2 laser.

[0151] Figure 8A shows the change in optical transmission frequency response due to birefringence of an optical element comprising a frequency-selective structure, an E7 liquid crystal switching member, and a supporting substrate comprising ZnSe. The optical transmission frequency response of the optical element is shown for emission wavelengths between approximately 9.7 μm and approximately 11.6 μm (i.e., within the infrared emission spectrum of CO₂ laser wavelengths). A bias voltage can be used to control the birefringence behavior of the liquid crystal and thereby the frequency response of the optical element. When no bias voltage is applied (790), a peak of approximately 100% transmission occurs at approximately 10.6 μm, the primary CO₂ laser emission wavelength. When a bias voltage is applied (792), the refractive index of the liquid crystal changes, shifting the approximately 100% transmission peak to approximately 10.9 μm and reducing the transmission of the optical element at approximately 10.6 μm to approximately 25%. Figure 8B shows the complementary optical reflection frequency response of the optical element of Figure 8A. When no bias voltage is applied (790), a 0% reflectance trough occurs at the primary CO laser emission wavelength of approximately 10.6 μm. When a bias voltage is applied (792), the refractive index of the liquid crystal changes, the 0% reflectance trough shifts to approximately 10.9 μm, and the reflectance of the optical element at approximately 10.6 μm increases to approximately 75%.

[0152] A laser output coupler including such a liquid crystal switching frequency-selective structure can be used to Q-switch a laser. For example, if the optical element forms part of the output coupler of a CO laser configured to generate pulses of radiation having a wavelength of approximately 10.6 μm, the optical element can contribute to a laser with a low Q when no bias voltage is applied (i.e., Figure 8A) by operating as a high-transmission output coupler. This increases the amount of photons lost in the resonant cavity, preventing lasing while the gain medium is being pumped. Once the gain medium is sufficiently pumped, another signal can be provided, applying a bias voltage to the liquid crystal (i.e., Figure 8B), thereby increasing the reflectivity of the output coupler and increasing the Q of the laser, generating intense pulsed radiation at 10.6 μm via Q-switching.

[0153] A fourth possible implementation is an optical element including a semiconductor, where the charge carrier density of the semiconductor is configured to change upon receiving a signal (e.g., optical radiation or other electromagnetic field). The semiconductor can include Si, GaAs, Ge, InP, GaAlAs, and / or many others. The optical and electrical properties of a semiconductor can be controlled by doping, which introduces impurities into the semiconductor's lattice structure. Lithography and deposition techniques can be used to create patterns and features on a substrate to perform various functions. For example, a monolithic Q-switched optical element can be fabricated by integrating frequency-selective structures and switching components into a single device. An advantage of semiconductor-based designs is the ability to create metal-free structures with low loss (i.e., low photon absorption at infrared wavelengths) and high efficiency. Modifying the charge carrier density of the semiconductor can change the refractive index of the optical element, thereby changing the transmittance, reflectance, and / or absorption of the optical element. For example, a semiconductor material including a p-n junction can exhibit a first optical passband when unbiased, such that free charge carriers exist within the bulk of the material. Reverse biasing a pn junction creates a depletion region with few free charge carriers, which can change the optical passband of the semiconductor.

[0154] FIG. 9A shows the optical transmission frequency response of an optical element including a frequency selective structure and a switching component including a semiconductor at three different values ​​of charge carrier density ((794) to (796)). FIG. 9B shows the optical reflection frequency response of the optical element of FIG. 9A at three different values ​​of charge carrier density ((794) to (796)). In the example of FIGS. 9A and 9B, the switching component includes GaAs. The optical transmission and reflection frequency responses of the optical element are shown for emission wavelengths from about 9.7 μm to about 11.6 μm (i.e., including the primary CO2 laser emission wavelength of about 10.6 μm). The first charge carrier density (794) is about 10 per cubic centimeter. 14The first charge carrier density (794) can represent the optical element without an applied electromagnetic field or any applied electromagnetic radiation (i.e., before receipt of a signal to switch the frequency response of the optical element). The second charge carrier density (795) can represent approximately 10 per cubic centimeter. 16 The second charge carrier density (795) can represent the optical element after application of the first electromagnetic field or first electromagnetic radiation (i.e., after receiving a first signal to switch the frequency response of the optical element). The third charge carrier density (796) can be approximately 10 per cubic centimeter. 18 The third charge carrier density (796) can represent the optical element after application of a second electromagnetic field or second electromagnetic radiation (i.e., after receiving a second signal to switch the frequency response of the optical element). The magnitude of the second electromagnetic field or second electromagnetic radiation can be greater than the magnitude of the first electromagnetic field or first electromagnetic radiation.

[0155] 9A and 9B, in a first (i.e., lowest) charge carrier density state (794), the transmittance of the optical element at the dominant wavelength of the CO laser at about 10.6 μm is at a maximum of about 93%, and the reflectance of the optical element is at a minimum of about 3%. When the charge carrier density increases to a second charge carrier density (795) after receiving a first signal, the transmittance of the optical element at about 10.6 μm decreases to about 22%, and the reflectance of the optical element increases to about 38% (i.e., the frequency response of the optical element changes). When the charge carrier density increases to a third (i.e., highest) charge carrier density state (796) after receiving a second signal, the transmittance of the optical element decreases to about 0%, and the reflectance of the optical element increases to about 96%. Thus, the frequency response of the optical element can be tuned by application of an electromagnetic field or electromagnetic radiation to tune the charge carrier state of the semiconductor switching component.

[0156] When the optical element forms part of the rear mirror of a CO laser configured to generate pulses of radiation having a wavelength of approximately 10.6 μm, the optical element can contribute to a laser with a low Q factor when no electromagnetic field or electromagnetic radiation is applied by operating as a high-transmission rear mirror. This increases the number of photons lost in the resonant cavity, preventing lasing while the gain medium is being pumped. It can also provide a signal to remove the reverse bias from the GaAs semiconductor. Once the gain medium is sufficiently pumped, an electromagnetic field or electromagnetic radiation can be applied to the GaAs semiconductor switching component to increase the reflectivity of the rear mirror, including the optical element, at a wavelength of approximately 10.6 μm, thereby increasing the Q factor of the laser and allowing lasing to occur. This results in the generation of powerful radiation pulses at 10.6 μm via Q-switching by the optical element.

[0157] The above-described possible implementations are just a few of the many ways to configure and switch Q-switched optical elements, including frequency selective structures.

[0158] The frequency-selective structure of an optical element can take many different forms. FIGS. 10-13 provide four different examples of a portion of a frequency-selective structure including an array of different geometric features that can form part of any of the previously described optical elements. FIG. 10 schematically depicts a top perspective view of a portion of a frequency-selective structure 800 including a periodic pattern of slots 810 according to one embodiment of the present disclosure. In the example of FIG. 10 , the periodic pattern includes a 10×10 grid array of slots 810 with a periodicity of about 3 μm, forming a frequency-selective structure 800 having a length of about 30 μm and a width of about 30 μm. FIG. 11 schematically depicts a top perspective view of a portion of a frequency-selective structure 820 including a periodic pattern of crosses 830 according to one embodiment of the present disclosure. In the example of FIG. 11 , the periodic pattern takes the form of a 10×10 grid array of crosses 830 with a periodicity of about 3 μm, forming a frequency-selective structure 820 having a length of about 30 μm and a width of about 30 μm. FIG. 12 schematically illustrates a top perspective view of a portion of a frequency-selective structure 840 including a periodic pattern of rings 850, according to one embodiment of the present disclosure. In the example of FIG. 12, the periodic pattern includes a 10×10 grid array of rings 850 with a periodicity of about 1.5 μm, forming a frequency-selective structure 840 with a length of about 15 μm and a width of about 15 μm. FIG. 13 schematically illustrates a top perspective view of a portion of a frequency-selective structure 860 including a periodic pattern of split rings 870, according to one embodiment of the present disclosure. In the example of FIG. 13, the periodic pattern includes a 10×10 grid array of split rings 870 with a periodicity of about 1.5 μm, forming a frequency-selective structure 860 with a length of about 15 μm and a width of about 15 μm.

[0159] The size of any of the frequency-selective structures (e.g., the number and / or size of individual array elements, as well as the periodicity of the array) can be selected based at least in part on the beam size of the laser beam to be incident on the frequency-selective structure. In any of FIGS. 10-13, the geometric features can be formed using additive or reductive manufacturing techniques, such as lithography, deposition, etching, nanoimprinting, etc. FIGS. 10-13 illustrate portions of frequency-selective structures that can form part of an optical element according to one embodiment of the present disclosure. In practice, the frequency-selective structures can be larger than those shown in FIGS. 10-13. For example, when used to Q-switch a laser, the array can be approximately 3 mm by 3 mm. That is, an optical element can incorporate approximately 10,000 of the arrays shown in FIGS. 10-13.

[0160] FIG. 14 schematically illustrates a side perspective view of a CO2 laser 900 according to one embodiment of the present disclosure. The CO2 laser 900 is configured to generate infrared electromagnetic radiation (not shown). The CO2 laser 900 includes an optical element (not shown) including a frequency-selective structure having a substantially periodic pattern of features. The frequency response of the optical element is configured to change upon receiving a signal, such that the Q-factor of the CO2 laser 900 changes upon receiving the signal. The optical element can be any of the optical elements described above and depicted in FIGS. 2-6A and 7A, including any of the frequency-selective structures depicted in FIGS. 10-13 described above.

[0161] The CO2 laser 900 comprises a laser resonant cavity 901 containing a CO2 gain medium (not visible in FIG. 14). The CO2 laser 900 comprises a radio frequency ("RF") excitation source 904 configured to excite the CO2 gain medium in the laser cavity 901 to generate infrared electromagnetic radiation. The RF excitation source 904 can be configured to provide RF power of approximately 150 W or more. The RF excitation source 904 can be configured to provide RF power of approximately 1 kW or less. The RF excitation source 904 can be configured to provide RF power at a frequency of approximately 80 MHz or more. The RF excitation source 904 can be configured to provide RF power at a frequency of approximately 120 MHz or less. The RF excitation source 904 can be configured to provide RF power at a frequency of approximately 100 MHz or more. The RF excitation source 904 can be configured to provide RF power to the CO2 gain medium for a duration of approximately 0.1 μsec or more. The RF excitation source 904 can be configured to provide RF power to the CO2 gain medium for a duration of approximately 1.0 μsec or less. The RF excitation source 904 can be configured to continuously provide RF power to the CO2 gain medium. The pulse duration of the RF excitation source 904 may be controlled by a user of the CO2 laser. The pulse duration of the RF excitation source 904 can depend at least in part on one or more of the desired laser energy (e.g., pulse energy), the average power of the CO2 laser 900, the pulse repetition frequency, and, in the case of a laser marking system use case, the overall product rate.

[0162] The CO2 laser 900 can be configured to generate infrared electromagnetic radiation having an average power of about 10 W. The CO2 laser 900 can be configured to generate infrared electromagnetic radiation having an average power of about 30 W. The CO2 laser 900 can be configured to generate infrared electromagnetic radiation having an average power of about 50 W. The CO2 laser 900 can be configured to generate infrared electromagnetic radiation having an average power of about 100 W. The CO2 laser 900 can be configured to generate short-pulsed infrared electromagnetic radiation. The CO2 laser 900 can be configured to generate short-pulsed (pulse width >0.1 nsec and <500 μsec) infrared electromagnetic radiation.

[0163] The CO2 laser 900 includes a control system 902 configured to provide a signal to the optical elements to Q-switch the CO2 laser 900. The signal can include a bias voltage. The bias voltage can be greater than 0 V. The bias voltage can be approximately 20 V or less. The bias voltage can be applied for approximately 1 ns or more. The bias voltage can be applied for approximately 150 ns or less. The duration for which the bias voltage is applied can be selected depending at least in part on the desired output energy (e.g., laser pulse energy) to be generated by the CO2 laser 900. Alternatively, the signal can include a laser pulse. The laser pulse can be generated by a short-pulse laser diode. The control system 902 is also configured to provide another control signal to an RF excitation source 904 to excite the CO2 gain medium within the laser cavity 901. The signal can be configured to control bias levels of switching components of the optical elements. The signal can be configured to control the timing between a laser output command of the CO2 laser 900 and the initiation of Q-switching of the CO2 laser 900.

[0164] The CO2 laser 900 includes a feedthrough 906 configured to transmit power from an RF excitation source 904 to a CO2 gain medium within a laser resonant cavity 901. The CO2 laser 900 includes a fluid input 908 for filling the laser cavity 901 with CO2 gas. The CO2 laser 900 includes an output coupler 910. The output coupler 910 can include a partially reflective mirror configured to output infrared electromagnetic radiation (e.g., pulses of infrared electromagnetic radiation) generated by the CO2 laser 900. The CO2 laser includes a rear mirror (not visible in FIG. 14) facing the output coupler 910 such that the CO2 gain medium is located between the output coupler 910 and the rear mirror. That is, the laser resonant cavity 901, including the CO2 gain medium, is formed between the rear mirror and the output coupler 901. Optical elements, including frequency-selective structures, can be positioned in several different locations in the CO2 laser 900. FIGS. 15-17 show different examples of where optical elements can be positioned in a CO2 laser.

[0165] FIG. 15 schematically illustrates a cross-sectional view of the output coupler of the CO laser of FIG. 14. Infrared electromagnetic radiation (not shown) generated by the CO laser propagates along a bore 912 within the laser cavity 901 and interacts with an output coupler 910. The bore may be formed, for example, of alumina. The output coupler 910 may be formed, for example, of ZnSe. The output coupler 910 reflects a portion of the infrared electromagnetic radiation back through the bore 912 toward a rear mirror (not shown). The output coupler 910 also transmits a portion of the infrared electromagnetic radiation out of the laser resonant cavity 901 as output laser light. In the example of FIG. 15, an optical element 914 including a frequency-selective structure having a substantially periodic pattern of features forms part of the output coupler 910 of the CO laser. As previously described, the frequency characteristics of the optical element 914 are configured to change upon receiving a signal such that the Q factor of the CO laser changes upon receiving the signal. The optical element 914 may include ZnSe, GaAs, Ge, or ZnS. These transparent substrates are compatible with the CO2 laser plasma present in the bore 912 of the laser cavity 901. The complex refractive index (n+jk) of ZnSe at a wavelength of about 10.6 μm can be approximately (2.4028). The complex refractive index (n+jk) of GaAs at a wavelength of about 10.6 μm can be approximately (3.2646+j0.00029). The complex refractive index (n+jk) of Ge at a wavelength of about 10.6 μm can be approximately (4.0038). The complex refractive index (n+jk) of ZnS at a wavelength of about 10.6 μm can be approximately (2.1925+j0.002).

[0166] FIG. 16 schematically illustrates a cross-sectional view of the rear mirror 916 of the CO laser of FIG. 14. Infrared electromagnetic radiation (not shown) generated by the CO laser propagates along a bore 912 within the laser cavity 901 and interacts with the rear mirror 916. The rear mirror 916 is configured to reflect substantially all of the infrared electromagnetic radiation back through the bore 912 toward an output coupler (not shown). The rear mirror 916 may be formed of, for example, silicon. In the example of FIG. 16, an optical element 918 including a frequency-selective structure having a substantially periodic pattern of features forms part of the rear mirror 916 of the CO laser. As previously described, the frequency characteristics of the optical element 918 are configured to change upon receiving a signal such that the Q factor of the CO laser changes upon receiving the signal. While in the example of FIG. 15, the optical element 918 forms part of an output coupler configured to at least partially transmit the infrared electromagnetic radiation, in the example of FIG. 16, the optical element 918 forms part of a rear mirror 916 configured to reflect the infrared electromagnetic radiation. Thus, the optical element 918 of FIG. 16 is formed of a different material than the optical element 916 of FIG. 15. It will be understood that the two implementations shown in FIGS. 15 and 16 are not to be used simultaneously. The optical element 918 of FIG. 16 may comprise, for example, silicon or GaAs. These reflective base materials are compatible with the CO laser plasma present in the bore 912 of the laser cavity 901. The complex refractive index (n+jk) of silicon at a wavelength of approximately 10.6 μm may be approximately (3.4179+j0.0001223). The complex refractive index (n+jk) of GaAs at a wavelength of approximately 10.6 μm may be approximately (3.2646+j0.00029).

[0167] FIG. 17 schematically illustrates a perspective view of a CO laser 920 including a fold cavity 922, according to one embodiment of the present disclosure. The CO laser 920 is configured to generate infrared electromagnetic radiation (not shown). The CO laser 920 includes an optical element (not shown) including a frequency-selective structure having a substantially periodic pattern of features. The frequency response of the optical element is configured to change upon receiving a signal, such that the Q factor of the CO laser 920 changes upon receiving the signal. The optical element may be any of the optical elements described above and depicted in FIGS. 2-6A and 7A, and may include any of the frequency-selective structures described above and depicted in FIGS. 10-13.

[0168] The fold laser resonant cavity 922 includes a CO2 gain medium (not visible in FIG. 17). The CO2 laser 920 includes a radio frequency ("RF") excitation source (not shown) configured to excite the CO2 gain medium within the fold laser cavity 922 to generate infrared electromagnetic radiation. The RF excitation source may be substantially the same as the RF excitation source 904 described above with reference to FIG. 14, details of which will not be repeated here to avoid unnecessary duplication.

[0169] The CO2 laser 920 can be configured to generate infrared electromagnetic radiation having an average power of about 10 W. The CO2 laser 920 can be configured to generate infrared electromagnetic radiation having an average power of about 30 W. The CO2 laser 920 can be configured to generate infrared electromagnetic radiation having an average power of about 50 W. The CO2 laser 920 can be configured to generate infrared electromagnetic radiation having an average power of about 100 W. The CO2 laser 900 can be configured to generate short-pulsed infrared electromagnetic radiation. The CO2 laser 900 can be configured to generate short-pulsed (pulse width >0.1 nsec and <500 μsec) infrared electromagnetic radiation.

[0170] The CO2 laser 920 includes a control system 924 configured to provide a signal to the optical elements to Q-switch the CO2 laser 920. The signal can include a bias voltage. The bias voltage can be greater than 0 V. The bias voltage can be about 20 V or less. The bias voltage can be applied for about 1 ns or more. The bias voltage can be applied for about 100 μs or less. The bias voltage can be applied for about 150 ns or less. The duration for which the bias voltage is applied can be selected depending at least in part on the desired output energy (e.g., laser pulse energy) to be generated by the CO2 laser 920. Alternatively, the signal can include a laser pulse. The laser pulse can be generated by a short-pulse laser diode. The control system 924 is also configured to provide another control signal to an RF excitation source to excite the CO2 gain medium within the fold laser cavity 922.

[0171] The CO2 laser 900 includes an output coupler 910. The output coupler 910 can be configured with a partially reflective mirror configured to output infrared electromagnetic radiation (e.g., pulses of infrared electromagnetic radiation) generated by the CO2 laser 920. The CO2 laser 920 includes multiple fold mirrors 926, 928. In the example of FIG. 17, the CO2 laser 920 includes four fold mirrors 926, 928, two of which are visible. The fold mirrors 926, 928 are configured to direct the infrared electromagnetic radiation along the parallel length of the fold cavity 922. The CO2 laser 920 includes a rear mirror (not visible in FIG. 17) facing one of the fold mirrors 926 such that a CO2 gain medium is located between the output coupler 910 and the rear mirror. That is, a fold laser resonant cavity 922 containing the CO2 gain medium is formed between the rear mirror and the output coupler 910. The optical elements, including the frequency selective structure, can be placed in several different locations in the CO laser 920, which includes the fold cavity 922. Figures 18 and 19 show different examples of where the optical elements can be placed in the CO laser 920.

[0172] FIG. 18 schematically illustrates a cross-sectional view of two of the fold mirrors 926, 928 and an output coupler 910 of the CO laser of FIG. 17. Infrared electromagnetic radiation (not shown) generated by the CO laser propagates along a bore 912 within the fold laser cavity 922 and interacts with the output coupler 910. The bore 912 may be formed, for example, of alumina. The output coupler 910 may be formed, for example, of ZnSe. The output coupler 910 reflects a portion of the infrared electromagnetic radiation back through the bore 912 toward a rear mirror (not shown). The output coupler 910 also transmits a portion of the infrared electromagnetic radiation out of the fold laser resonant cavity 922 as output laser light. In the example of FIG. 18, an optical element 914 including a frequency-selective structure having a substantially periodic pattern of features forms part of one of the fold mirrors 926 of the CO laser. As previously described, the frequency characteristics of the optical element 914 are configured to change upon receiving a signal, thereby changing the Q-factor of the CO2 laser upon receiving the signal. The optical element 914 may include silicon or GaAs. These reflective base materials are compatible with the CO2 laser plasma present in the bore 912 of the fold cavity 922. The complex refractive index (n+jk) of silicon at a wavelength of approximately 10.6 μm may be approximately (3.4179+j0.0001223). The complex refractive index (n+jk) of GaAs at a wavelength of approximately 10.6 μm may be approximately (3.2646+j0.00029).

[0173] FIG. 19 schematically illustrates a cross-sectional view of two of the fold mirrors 926, 928 and output coupler 910 of the CO laser of FIG. 17 , further including a passive optical component 930, according to one embodiment of the present disclosure. In the example of FIG. 19 , an optical element 914 including a frequency-selective structure having a substantially periodic pattern of features forms part of the passive optical component 930. As previously described, the frequency response of the optical element 914 is configured to change upon receiving a signal, thereby changing the Q-factor of the CO laser upon receiving the signal. The passive optical component 930 is disposed in the fold laser resonant cavity 922 of the CO laser. In the example of FIG. 19 , the passive optical component 930 is disposed between two of the fold mirrors 926, 928 of the CO laser. The passive optical component 930 may be disposed elsewhere within the fold cavity 922 of the CO laser. In the example of FIG. 19 , the passive optical component 930 includes a transmissive disk on which the optical element 914 is mounted. The passive optical components 930 can include, for example, ZnSe, GaAs, Ge, or ZnS. The optical elements can include, for example, ZnSe, GaAs, Ge, or ZnS. These transparent substrates are compatible with the CO2 laser plasma present in the bore 912 of the CO2 laser fold cavity 922. The complex refractive index (n+jk) of ZnSe at a wavelength of approximately 10.6 μm can be approximately (2.4028). The complex refractive index (n+jk) of GaAs at a wavelength of approximately 10.6 μm can be approximately (3.2646+j0.00029). The complex refractive index (n+jk) of Ge at a wavelength of approximately 10.6 μm can be approximately (4.0038). The complex refractive index (n+jk) of ZnS at a wavelength of approximately 10.6 μm can be approximately (2.1925+j0.002).

[0174] 15, the optical element 914 includes graphene. The dielectric constant of the graphene can be configured to change upon receiving a signal. The complex refractive index (n+jk) of the graphene at a wavelength of about 10.6 μm can change between about (4.45−j4.34) and about (14.43−j0.08) upon receiving a signal.

[0175] In the examples of Figures 15, 16, 18, and 19, the optical element includes graphene. The dielectric constant of the graphene can be configured to change upon receiving a signal used to Q-switch the CO2 laser. The complex refractive index (n + jk) of the graphene at a wavelength of about 10.6 μm can change between about (4.45 - j4.34) and about (14.43 - j0.08) upon receiving the signal. The optical element can be composed of different materials. For example, the optical element 914 can be composed of a phase change material configured to change its phase upon receiving a signal. The phase change material can include, for example, VO2. The complex refractive index (n + jk) of VO2 at a wavelength of about 10.6 μm can change between about (2.1 + j0.16) in an insulating state and about (7.8 + j5.8) in a metallic state upon receiving the signal. Alternatively, the optical element 914 can comprise a semiconductor. The charge carrier density of the semiconductor can be configured to change upon receiving the signal. The semiconductor may include, for example, GaAs, Si, or Ge. When the signal provided to the optical element to Q-switch the CO2 laser is a laser pulse, the semiconductor may form part of a photoconductive device configured to interact with the laser pulse.

[0176] As described above, the optical element may include a signal transmission component operably coupled to the switching component and configured to receive and transmit signals to the switching component. The signal transmission component may include a layer of conductive material. The layer of conductive material may include at least one of gold, nickel, aluminum, and indium tin oxide (ITO). The optical element may include an insulating material. The insulating material may include, for example, one or more of aluminum oxide and hafnium oxide. The insulating material may include other materials.

[0177] FIG. 20 schematically illustrates a laser marking system 940 including a CO laser 942 according to one embodiment of the present disclosure. The CO laser can be the same as any of the CO lasers described above and illustrated in FIGS. 14-19. This will not be described again here to avoid unnecessary duplication. The laser marking system 940 includes a power supply 944 configured to provide power to the CO laser 942. The laser marking system 940 includes a control system 946 configured to provide a signal to Q-switch the CO laser 942 and another control signal to excite a CO gain medium within the CO laser. In the example of FIG. 20, infrared electromagnetic radiation 946 generated by the CO laser 942 is configured to mark a product 952. The CO laser 942 can be configured to generate pulses of infrared electromagnetic radiation 946. The product 952 can be one of multiple products 950-954 moving along a production line 960 (e.g., a conveyor belt).

[0178] FIG. 21 shows a flowchart of a method for Q-switching a CO2 laser to generate infrared electromagnetic radiation according to one embodiment of the present disclosure. A first step 971 of the method includes pumping a CO2 gain medium of the CO2 laser. A second step 972 of the method includes generating a signal that alters the frequency response of a frequency-selective structure having a substantially periodic pattern of features on an optical element of the CO2 laser, thereby generating pulses of infrared electromagnetic radiation. Generating a signal can include operating a controller to generate an electrical or optical signal (e.g., a laser pulse). The infrared electromagnetic radiation generated by the method can be configured to mark a product. The method can include an optional step of generating short-pulse infrared electromagnetic radiation. The CO2 laser can be configured to generate short pulses of infrared electromagnetic radiation (pulse width >0.1 nsec and <500 μsec).

[0179] FIG. 22 illustrates a method for marking a target with infrared electromagnetic radiation according to one embodiment of the present disclosure. A first step 981 of the method includes pumping a CO2 gain medium of a CO2 laser. A second step 982 of the method includes generating a signal to Q-switch the CO2 laser by altering the frequency response of a frequency-selective structure having a substantially periodic pattern of features on an optical element of the CO2 laser, thereby generating pulses of infrared electromagnetic radiation. A third step 983 of the method includes directing the pulses of infrared electromagnetic radiation toward the target. Generating a signal can include operating a controller to generate an electrical or optical signal (e.g., a laser pulse). The method can include the optional step of generating short pulses of infrared electromagnetic radiation. The CO2 laser can be configured to generate short pulses (pulse width >0.1 nsec and <500 μsec) of infrared electromagnetic radiation.

[0180] It will be appreciated that the CO2 lasers described herein can be used in many different applications. CO2 lasers can be used in industrial applications, such as laser marking and coding, engraving, drilling, cutting, perforating, welding (metal and plastic), surface treatment (laser peening, hardening, polishing, roughening, blackening), rust removal, and paint removal. CO2 lasers can also be used in medical applications, such as laser scalpels, ENT and head and neck surgery, gynecological surgery, lesion and tumor removal, vascular surgery, oral soft tissue surgery, enamel resection, implant dentistry, tattoo removal, laser-induced nociceptive potentials for migraine treatment, burn treatment, skin surface preparation, birthmark removal, mole and viral wart removal, skin aging, and facial scar removal. The desired power delivered by a CO2 laser depends, at least in part, on the application. For example, for soft material applications, such as most surgical procedures, CO2 lasers can generate infrared electromagnetic radiation with a power in the range of 10-20 W. As another example, for some metal laser processing applications, a CO2 laser can generate infrared electromagnetic radiation with a power in the range of 40-50 W. As a further example, for some plastic laser processing applications, a CO2 laser can generate infrared electromagnetic radiation with a power of approximately 30 W.

[0181] Having thus described several aspects of at least one embodiment, it will be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure and to be within the spirit and scope of this disclosure. The operations of the methods disclosed herein may be performed in an order other than that illustrated, and one or more operations may be omitted, substituted, or added. One or more features of any one embodiment disclosed herein may be combined with or substituted for one or more features of any other disclosed embodiment. Accordingly, the foregoing description and drawings are by way of example only.

[0182] The phrases and terms used herein are for purposes of description and should not be considered limiting. As used herein, the term "plurality" refers to two or more items or components. As used herein, dimensions described as "substantially similar" should be considered to be within about 25% of each other. The terms "comprise," "include," "hold," "have," "comprise," and "accompany" are open-ended terms meaning "including, but not limited to," whether in the specification or claims. Thus, the use of such terms is meant to encompass the subsequently listed items, and equivalents thereof, as well as additional items. The use of ordinal terms such as "first," "second," and "third" in the claims to modify claim elements does not, in itself, imply a priority, precedence, or order of one claim element relative to others, or a chronological order in which method operations are performed, but is merely used as a marker to distinguish one named claim element from other elements having the same name (but for the use of ordinal terms).

Claims

1. CO configured to generate infrared electromagnetic radiation 2 a laser, an optical element including a frequency selective structure having a substantially periodic pattern of features, wherein a frequency response of the optical element is configured to change upon receipt of a signal; The CO 2 The Q value of the laser changes upon receiving the signal. 2 laser.

2. CO 2 a laser cavity including a gain medium; The CO 2 a radio frequency excitation source configured to excite the gain medium to generate infrared electromagnetic radiation; The CO of claim 1, 2 laser.

3. providing the signal to the optical element; 2 The laser is Q-switched, Another control signal is provided to the RF excitation source to control the CO 2 pumping the gain medium; 3. The CO according to claim 2, comprising a control system configured to: 2 laser.

4. The optical element is 2 A CO2 laser according to any one of claims 1 to 3, forming part of the rear mirror of a laser. 2 laser.

5. a fold cavity having a fold mirror, the optical element being 2 A CO2 laser according to any one of claims 1 to 3, forming part of the fold mirror of a laser. 2 laser.

6. 6. The CO according to claim 4 or 5, wherein the optical element comprises silicon or GaAs. 2 laser.

7. The optical element is 2 A CO2 laser according to any one of claims 1 to 3, forming part of the output coupler of a laser. 2 laser.

8. The optical element is 2 A CO2 laser according to any one of claims 1 to 3, forming part of a passive optical component located within the laser cavity of a laser. 2 laser.

9. 9. The CO according to claim 7 or 8, wherein the optical element comprises ZnSe, GaAs, Ge or ZnS. 2 laser.

10. 10. The CO optics of claim 1, wherein the optical element comprises graphene. 2 laser.

11. 10. The CO2 sensor of claim 1, wherein the optical element comprises a phase change material configured to change phase upon receiving the signal. 2 laser.

12. The phase change material is VO 2 12. The CO of claim 11, 2 laser.

13. 10. The CO 2 optical element according to claim 1, wherein the optical element comprises a semiconductor. 2 laser.

14. 14. The CO2 of claim 13, wherein the semiconductor comprises at least one of GaAs, Si, and Ge. 2 laser.

15. 10. The CO sensor of claim 1, wherein the optical element comprises a liquid crystal, and the optical properties of the liquid crystal are configured to change upon receiving the signal. 2 laser.

16. 16. The CO2 sensor of claim 10, wherein the signal comprises a bias voltage. 2 laser.

17. 16. The CO2 sensor of claim 10, wherein the signal comprises a laser pulse. 2 laser.

18. 18. The CO2 sensor of claim 1, wherein the reflectance, transmittance and / or absorptance of the optical element varies over a frequency range of infrared electromagnetic waves upon receiving the signal. 2 laser.

19. 19. The CO2 laser of claim 1, wherein the optical element comprises a switching component operatively coupled to the frequency selective structure. 2 laser.

20. 20. The CO of claim 19, wherein the optical element comprises a signal transmission component operably coupled to the switching component and configured to receive the signal and transmit the signal to the switching component. 2 laser.

21. 21. The CO2 sensor of claim 1, wherein the optical element comprises a deformable material configured to modify the periodicity and / or geometry of features of the substantially periodic pattern upon receiving the signal. 2 laser.

22. 22. The CO2 sensor of claim 1, wherein the refractive index of the optical element is configured to change upon receiving the signal. 2 laser.

23. 23. A CO2 sensor according to any one of claims 1 to 22, wherein the permittivity and / or permeability of the optical element is adapted to change upon receiving a signal. 2 laser.

24. 24. The CO sensor of claim 1, wherein the electrical conductivity and resistivity of the optical element is configured to change upon receiving the signal. 2 laser.

25. 25. The CO2 optical element of claim 1, wherein the substantially periodic pattern of features comprises an array of geometric features configured to at least partially determine a frequency response of the optical element. 2 laser.

26. 26. The CO of claim 25, wherein the array includes tuning elements configured to at least partially determine a frequency response of the optical element. 2 laser.

27. 27. The CO2 optical fiber of claim 1, wherein the substantially periodic pattern of features is configured to act as a polarizer. 2 laser.

28. 28. The CO 2 optical element of claim 1, wherein the optical element comprises a metamaterial. 2 laser.

29. 29. The CO2 optical element of claim 1, wherein the frequency selective structure comprises a plurality of frequency selective layers configured to at least partially determine the frequency response of the optical element. 2 laser.

30. 30. The CO2 sensor of any one of claims 1 to 29, wherein the infrared electromagnetic radiation is configured to mark a product. 2 laser.

31. The CO 2 31. The CO laser of any one of claims 1 to 30, wherein the laser is configured to generate short pulses of infrared electromagnetic radiation. 2 laser.

32. The signal is controlling bias levels of switching components of said optical element; The CO 2 The laser output command of the laser and the CO 2 Controlling the timing between the onset of Q-switching of the laser; The CO2 according to any one of claims 1 to 31, 2 laser.

33. 33. The CO2 radiation of any one of claims 1 to 32, wherein the infrared electromagnetic waves form part of an industrial process. 2 laser.

34. 34. The CO2 radiation therapy device of any one of claims 1 to 33, wherein the infrared electromagnetic waves form part of a medical process. 2 laser.

35. The CO according to any one of claims 1 to 33. 2 A laser marking system for marking a target, comprising a laser.

36. CO to generate infrared electromagnetic radiation 2 1. A method of Q-switching a laser, comprising: The CO 2 Laser CO 2 pumping a gain medium; The CO 2 generating a signal to modify the frequency response of a frequency selective structure having features in a substantially periodic pattern on an optical element of the laser, thereby generating pulses of infrared electromagnetic radiation; A method comprising:

37. 37. The method of claim 36, wherein the infrared electromagnetic radiation is configured to mark a product.

38. 38. A method according to claim 36 or claim 37, comprising generating short pulses of infrared electromagnetic radiation.

39. 1. A method for marking a target with infrared electromagnetic radiation, comprising: CO 2 Laser CO 2 pumping a gain medium; The CO 2 and varying the frequency response of a frequency selective structure having a substantially periodic pattern of features on an optical element of the laser, thereby generating a signal for the CO 2 generating a signal to Q-switch the laser, thereby generating the pulses of infrared electromagnetic radiation; directing the pulse of infrared electromagnetic radiation at the target; A method comprising:

40. 40. The method of claim 39, comprising generating short pulses of infrared electromagnetic radiation.

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