High power CW mid-infrared laser

Abstract

A continuous wave (CW) laser with mid-IR output is disclosed. The ring is pumped at multiple locations and the laser produces a mid-IR output. Multiple pump gain sections of the ring provide a power scaled output. The gain sections can be positioned in a single resonant cavity, in multiple resonant cavities, and in a MOPA architecture with associated focusing, steering, and combining optics.

Description

[0001] This application is a divisional application of the Chinese invention patent application No. 201780016958.1 entitled “High-power CW mid-infrared laser” corresponding to PCT international application No. PCT / US2017 / 017428 with an international application date of February 10, 2017, after entering China.

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS

[0003] This application claims priority to U.S. Provisional Application Serial No. 62 / 294,747 filed on February 12, 2016, which is incorporated herein by reference. This application is related to application PCT / US2016 / 023778 filed on March 23, 2016, which was published on September 29, 2016 and has publication number WO2016154309A1, which is incorporated herein by reference. Technical Field

[0004] The present invention relates to continuous wave (CW) IR solid-state lasers, and more particularly to solid-state lasers having a rotating ring gain medium and improved output characteristics. Background Art

[0005] Mid-infrared (mid-IR) laser sources operating in the 2-10 m spectral range are in great demand for various defense-related applications, including free-space communications, countermeasures, remote sensing, as well as industrial process control and medical applications. Advances in optically pumped lasers based on Cr- and Fe-doped II-VI broadband semiconductors have enabled access to large output powers in the 1.9-5 μm spectral range.

[0006] Publication WO2016154309A1 (hereinafter referred to as '309) describes such a laser system that uses a conductively cooled rotating ring gain medium to scale power. As the ring rotates, the pump radiation point follows a path around the ring. The pumping is continuous, but as the ring rotates, the ring region is pumped intermittently as it moves in and out of the pump beam. This intermittent pumping of the medium produces a power benefit similar to intermittent quasi-CW (QCW) pumping, but with continuous high-power output. As a result, the laser exhibits pure CW output with a maximum output power of 27.5W at 2450nm and a maximum output power of 13.9W at 2940nm, with average slope efficiencies of 63.7% and 37.4%, respectively.

[0007] Power scaling of Cr2+:ZnS / Se lasers is of interest for a variety of applications in industrial, medical, material processing, and defense applications. For successful applications of these mid-IR lasers, it is particularly important to achieve output powers of tens or hundreds of watts near the 2940nm output wavelength, which is the wavelength at which most medical applications occur. Output wavelengths near the gain maximum of these materials (2.4μm) are also very attractive for material processing applications where high throughput is particularly important, such as the processing of polymers.

[0008] Increased power scaling and wavelength selectivity are needed to meet the demands of medical applications and materials processing within the mid-IR spectrum. Summary of the invention

[0009] The above articulated needs are met by the disclosed CW mid-IR laser configured with a rotating ring gain medium and pumped at multiple locations on the ring to increase pump power, distribute gain, and scale output power.

[0010] More specifically, in at least one embodiment, the present invention provides a continuous wave ("CW") mid-infrared laser comprising: a resonant cavity; a gain medium formed as a ring, and more preferably, formed as a ring defined by an inner circumferential surface, an outer circumferential surface, a first face, and a second face, the gain medium having a first non-fixed gain portion and a second non-fixed gain portion, the first non-fixed gain portion and the second non-fixed gain portion being positioned within the resonant cavity to provide mid-IR gain, and the second non-fixed gain portion between the first face and the second face being positioned to provide supplemental mid-IR gain; a first pump source; a second pump source; a rotatable mounting structure in thermal contact with the ring; and a motor coupled to the rotatable mounting structure to rotate the ring axis. The first non-fixed gain portion of the ring in transit through the resonant cavity receives pump light sufficient to emit a CW laser beam in the mid-IR spectrum, and the second non-fixed gain portion of the ring in transit receives pump light sufficient to provide optical gain and increase mid-IR output power.

[0011] In a preferred embodiment, the gain medium is a TM:II-VI, such as Cr:ZnS or Cr:Zn Se.

[0012] In other embodiments, the CW laser of the present invention will have one or more optical cavity elements for narrowing the spectral linewidth of the mid-IR output and for mid-IR output wavelength selection.

[0013] In other embodiments, the gain medium is a composite stacked ring gain element.

[0014] In other embodiments, the first non-fixed gain portion and the second non-fixed gain portion both provide gain in the resonant cavity.

[0015] In other embodiments, the first non-fixed gain section provides gain as a master oscillator and the second non-fixed gain section provides one-way amplification of the output of the master oscillator.

[0016] In a preferred embodiment, the resonant cavity comprises a cavity mirror proximate the first face of the ring and an output coupler proximate the first face of the ring.

[0017] In other embodiments, an optical element is included, disposed proximate to the second face of the ring, for redirecting the light path from the first gain section to the second gain section.

[0018] In other embodiments, optical elements are included for focusing the first pump source and the second pump source, respectively, to the first gain portion and the second gain portion of the ring.

[0019] In other embodiments, one or more lens elements are included for focusing mid-IR light.

[0020] In other embodiments, the first non-fixed gain portion provides gain in the first resonant cavity and the second non-fixed gain portion provides gain in the second resonant cavity.

[0021] In other embodiments, the first non-fixed gain section provides a first output, the second non-fixed gain section provides a second output, and a combiner combines the first output and the second output into a mid-IR output. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The above and other aspects, features and advantages of the present disclosure will become more apparent from the following drawings, in which:

[0023] Figure 1A A perspective view of the gain medium is provided.

[0024] Figure 1B A plan view of the gain medium ring is provided.

[0025] Figure 1C FIG. 1 is an exemplary optical schematic diagram of a 4-lens laser cavity embodiment of a CW mid-IR laser of the present invention.

[0026] Figure 2 A diagram of a 4-lens laser cavity embodiment of a CW-type mid-IR laser of the present invention is provided.

[0027] Figure 3 FIG. 1 is an exemplary optical schematic diagram of a master oscillator power amplifier (MOPA) for a CW medium IR laser of the present invention.

[0028] Figure 4 A diagram of a MOPA embodiment of the present invention for a CW type mid-IR laser is provided.

[0029] Figure 5 FIG. 1 is an exemplary optical schematic diagram of a dual-cavity embodiment of a CW mid-IR laser of the present invention.

[0030] Figure 6 A view is provided of an embodiment of a wavelength selector for a CW type mid-IR laser of the present invention.

[0031] Figure 7 Provided by Figure 2 Input-output curve produced by the laser.

[0032] Figure 8 Provides measurements by Figure 2 The output spectrum produced by the laser.

[0033] Fig. 9 Provided by Figure 4 Input-output curve generated by the master oscillator.

[0034] Fig.10 Provided by Figure 4 The master oscillator produces the measured output spectrum.

[0035] Fig.11 Provided by Figure 4 Input-output curve generated by the MOPA.

[0036] Fig.12 Provides calculated Figure 4 The gain curve of the power amplifier.

[0037] Fig.13 Provided by Figure 6 Input-output curve produced by a wavelength selective oscillator.

[0038] Fig.14 Provides measurements by Figure 6 Output spectrum produced by a wavelength selective oscillator.

[0039] Fig.15 Plots of thermal decay at different levels of total pumping are provided.

[0040] Fig.16 Various views of an annular cooling system of an embodiment of the present invention are provided.

[0041] Fig.17 Various views of an improved annular cooling system of an embodiment of the present invention are provided.

[0042] Fig.18 A view of a composite stacked ring gain element according to an embodiment of the present invention is provided. DETAILED DESCRIPTION

[0043] Reference will now be made in detail to embodiments of the invention. Wherever possible, the same or similar reference numerals are used in the drawings and the specification to indicate the same or similar components or steps. The drawings are in simplified form and are not drawn to exact scale. Unless expressly stated otherwise, the words and phrases in the specification and claims have their ordinary and customary meanings to those skilled in the art of diode lasers and fiber lasers. The word "coupled" and similar terms do not necessarily mean direct, proximate connection, but may also include mechanical and optical connections via free space or intermediate elements.

[0044] The present invention demonstrates a significant advance in power scaling of Cr:ZnS and Cr:ZnSe laser systems. Unprecedented sub-kW output powers are achieved in CW operation through a very simple and reliable laser design. Due to this unprecedented output power, the present invention provides a wide range of opportunities for chromium-based crystal laser ("CL") systems to be used in a variety of industrial applications in the field of material processing.

[0045] Table 1 below summarizes the results of several examples of the present invention.

[0046]

[0047]

[0048] Power scaling has been achieved using a flexible optical system architecture that provides multiple pump sources to the corresponding gain sections of the ring. Figure 1A , Figure 1B and Figure 1C , a rotating ring gain medium 10 is pumped from pump sources 11A, 11B at different locations on the gain ring. The pump light is transmitted through dichroic mirrors 12A, 12B and can be collimated by corresponding collimating lenses 13A, 13B to effectively form pump spots 14A, 14B on the gain medium at gain sections 15A, 15B. In at least one embodiment, the pump light is incident on a first side 16 of the gain medium. It should be understood that the pump light can be propagated to the ring in different ways, such as using mirrors in addition to or instead of lenses, propagating through focusing lenses before the dichroic mirrors, and alternating pumping scenarios consistent with end-pumped laser designs.

[0049] The ring is mounted on an annular base 17 which is rotated by a motor 18 to form a gain section track 19. As shown schematically, the locations of the pump points are arranged in a pattern that is evenly spaced in increments of about 180 degrees around the ring. However, different azimuthal orientations of the pump points relative to the axis of the ring may be used. For example, if the ring axis is not coplanar with the optical path, the azimuthal pump point spacing may be less than 180 degrees and the laser path spacing may be less than the full diameter of the pump point track around the ring. Radial alignment of the pump points may superimpose multiple pump points on a single track, but alternative alignments may be used.

[0050] In at least one embodiment, the gain sections can overlap at multiple locations along the redirected light path including one or more redirecting mirrors 21. The redirecting mirrors receive light from a first gain section of the ring and redirect the light propagation path so that the path intersects the ring a second time at a second gain section. Additional lenses 22 can be used between the ring and the steering mirrors for beam focusing, etc.

[0051] Depending on the position of the cavity mirror 23 and the output coupler 24, the laser resonator can be formed advantageously and flexibly in different ways. Figure 1C , the dichroic mirrors 12A, 12B steer the beam path to the cavity mirror forming one end of the resonator, and to the output coupler disposed on the same side of the ring forming the resonator output end. In this way, the gain medium intersects the optical path twice between the cavity mirror and the output coupler. Each gain section is pumped by a respective pump source 11A, 11B. This effectively connects the two active gain sections of the ring in series. The dual pump configuration may include optical elements such as a lens 22 to propagate the optical radiation, for example, to image a pump spot region from one gain section to another gain section, or to relay a beam waist from one gain section to another gain section. Each pump may be controlled to vary the laser output power and customize the laser resonator performance characteristics. The entire resonator may be modeled and optimized for output optical power, beam quality, and beam stability.

[0052] Figure 2A view of a generic dual-pump 4-lens laser cavity embodiment is shown in FIG, which is a general optical scheme for a high-power tunable CW Cr:ZnS / Se laser system based on a rotating gain element. This laser scheme represents a flexible and configurable CL system design and covers the widest selection of Cr laser output characteristics with minimal internal reconfiguration. Although polycrystalline gain materials are used due to ease of manufacturing, the invention is not limited to this. Not all configurations are shown. In the CL fixed wavelength laser system, the stepper motor 25 tuning mechanism 26 and the tuning element 27 (e.g., a diffraction grating) are replaced by a high reflectivity (HR) end mirror. The output 24 exits at element 28, an output coupler, or a volume Bragg grating (VBG). The pump collimator 29 can be a collimator TLM-100. The choice of pump configuration and gain element depends on the required output power and other parameters. The conversion from a high-power system (>15W) to a low-power system (<15W) can be achieved by replacing the rotating ring gain element with a pair of conventional planar Cr:ZnS / Se gain crystals. The approximate footprint of the laser head housing is shown with dashed lines (dimensions in mm). The identified gain element is in the form of a rotating ring, allowing the gain element to be mounted on a mandrel and providing surface area to juxtapose such a ring to thermal management structures and materials.

[0053] The advantages of the 4-lens laser design include a wide range of CW Cr:ZnS / Se laser systems, including: tunable lasers, fixed wavelength lasers, high-power and low-power lasers, and single-frequency lasers (not tested). Other advantages include multiple pump sources in a single cavity (226W total pump from dual TLM-100-1908 lasers has been tested), and flexible configurations of gain media, such as rotating ring Cr:ZnS and Cr:ZnSe elements, and dual gain elements made from a pair of conventional slab gain crystals (for low-power lasers). These advantages offset the complexity of the laser cavity.

[0054] In at least one embodiment, reference Figure 3 , a dichroic mirror 30 is used as a cavity mirror on a first side 34 of the gain loop, and an output coupler 31 is located in the optical path on the opposite side 35 of the loop to form a resonator so that the single pump point on the loop is within the resonator. Thus, the resonator is formed with the first gain section acting as a master oscillator. The output of the master oscillator is passed through the second gain section acting as a single pass amplifier, and the output 32 is extracted using a dichroic mirror 33.

[0055] The present invention provides a simple single-lens master oscillator and one-way power amplifier embodiment. Figure 4In , a view of a single lens single pump channel cavity design is shown. The master oscillator design shown in 40 represents a viable industrial laser model with a fixed output wavelength, where random polarization of the laser beam is acceptable. The use of rotating gain material at two locations simultaneously allows extremely efficient lasing.

[0056] The pump collimator 42 can be a collimator TLM-100-1098. The master oscillator pump can be focused by pump lens 13A and then enter the resonator at fixed input mirror 43. Lens 44 in the master oscillator is a cavity lens, and the resonator cavity output exits at element 45, which can be an OC or VBG. The master oscillator output is focused in the power amplifier 41 by focusing lens 46. Lens 13B acts as a pump / collimating lens, and the dichroic mirror 12 extracts the MOPA output 47.

[0057] In addition, instead of Figure 4 The (single resonator) MOPA system shown in Figure 1 can be used to construct a pair of master oscillators with polarization beam combining capability, which will double the total output power. Figure 4 The MOPA configuration of the CL laser platform is shown.

[0058] The advantages of the single-lens laser design are a very simple and efficient master oscillator design with a trivial alignment algorithm, low misalignment sensitivity and high reliability. In the case of a dual master oscillator system with polarization beam combining capability, the input mirror can be replaced with an appropriate HR / AR dichroic coating of the gain element. Independent beam shaping of the master oscillator laser mode and the pump light allows further efficiency improvements. With a single VBG grating as output coupler, ultra-narrow linewidths can be achieved. Widely tunable lasers are not feasible in such single-lens cavity designs.

[0059] In at least one embodiment, reference Figure 5 , resonators 50A and 50B are formed around different gain portions of the ring, respectively. Each resonator can provide a separate laser output at 51A and 51B. The resonators can be polarized so that the output from each resonator can be combined with a polarization combiner 52 to power scale the combined output 53. Multiple resonators can have different characteristics, including power, wavelength, and spectral width. Different resonator characteristics can be provided for non-polarized beam combining.

[0060] When embodiments of the present invention include a broadband gain medium, wavelength tuning can be achieved by using a tuning element as a cavity mirror, as is known in the art. For example, a volume Bragg grating (VBG) can be used as a tuning element in place of an output coupler, or a diffraction grating can be used in place of an end mirror. An adjustment mechanism can be used to vary the tuning element and select the resonant wavelength.

[0061] In at least one embodiment, reference Figure 6 , shows a view of a single lens laser cavity 60 for a CL-2940-25 master oscillator with a cavity lens 64 and a multi-mirror wavelength selector. In addition to a wavelength-specific high reflector 63 (HR) and an output coupler 62 (OC), a selector mirror 61 is used in the resonator cavity. For example, the selector mirror can be coated similarly to the HR to provide additional cavity losses to assist in wavelength selection. In conjunction with the HR and the selector mirror, the transmission range of the OC can be different from that of the HR and the selector mirror. For example, the HR and selector can have high reflectivity from 2940nm to 3200nm and high transmittance from 1567nm to 2700nm. The OC can have a reflectivity of 80% at 2940nm and high transmittance at 2000nm to 2800nm. This combination can be used to convert operation from 2760nm to 2940nm with a simple design, higher efficiency, and lower intracavity losses. Independent beam shaping of the master oscillator laser mode and the pump light allows for further efficiency improvements.

[0062] The selector mirror configuration can be used with a dual-pump parallel resonator architecture and polarization beam combining for power scaling. This can double the output power, for example, up to 60 W at a total pump power of 200 W is expected through improved thermal management of the rotating ring gain element.

[0063] It is expected that the future availability of VBGs in this spectral range can be applied to broaden narrow spectral line widths and improve wavelength control. In alternative systems, cutoff filters can be used as part of the bandwidth broadening strategy. Applications and capabilities in wavelength selection and tuning can correspond to different embodiments of dual-pump parallel resonator rotating ring lasers.

[0064] Figure 7 The input-output characteristics of a CL-2500-130-D laser system with two versions of output couplers are plotted: (1) 50% dielectric broadband OC; and (2) YA G etalon plate. The laser is pumped with two TLM-100-1908-WC fiber laser modules. The very linear dependence of the output power on the pump level indicates that the gain of the active elements is far from saturated and that higher pump power levels can be tolerated (with proper thermal management).

[0065] Figure 8 The measured output spectrum of a CL-2500-130-D laser system at maximum pump power level is shown. The laser cavity is non-dispersive and does not contain any spectrum control optics. This results in a broad linewidth of approximately 40 nm.

[0066] Fig. 9 The input-output characteristics of a CL-2500-70-D laser with 2 versions of output couplers and a CL-2300-60-D with wavelength set by a VBG grating are plotted. The laser is pumped with a single TL M-100-1908-WC fiber laser module. The laser cavity is Figure 4 The simplest possible single-lens cavity is shown in . The perfectly linear dependence of the output power on the pump level indicates that the gain of the active elements is far from saturated and that higher pump power levels can be tolerated (with proper thermal management).

[0067] Fig.10 The ultra-narrow output spectrum of the CL-2300-60-D laser system at maximum output power (~62W) obtained by using a VBG element as an output coupler is shown (Optigr ate, VBG 2300, 50% diffraction efficiency, 1nm bandwidth). The upper limit estimate of the laser linewidth of FWHM is <0.25nm, which is actually the device function of the Bristol 721 laser spectrum analyzer.

[0068] Fig.11 The performance of a MOPA configuration tested with a CL-2300-60-D master oscillator is plotted. Despite the fairly high output power as well as the absolute and slope efficiencies, the flip in the maximum pump indicates a relatively inefficient heat dissipation in a single-pass power amplifier compared to a 4-lens laser cavity. Excessive heating of the gain channel of the power amplifier also results in a distorted Gaussian beam profile (see Table 1 for details). Therefore, it seems more feasible to use a 4-lens optical cavity which is more conducive to a single-pass MOPA system.

[0069] Fig.12 The calculated gain of a single-pass power amplifier of a CL-2300-60-D laser system is illustrated. The maximum value of the amplifier channel pumping is set to 117 W, and the master oscillator power is variable. These results show that a rotating ring gain element can be used very successfully for amplification of low-power CLT laser systems (e.g., single-frequency tunable laser sources, where a rotating ring gain element configuration cannot be used directly due to frequency stability reasons; another example of a laser source that can benefit from a rotating ring amplifier is a mode-locked CLPF system). The figure also clearly shows that further power amplification of about 2 times the gain can be achieved by using a 4-lens cavity as a two-stage power amplifier.

[0070] Fig.13 The input-output characteristics of the CL-2940-25-D master oscillator based on a single-lens cavity with a multi-mirror wavelength selector are plotted. The power rollover is caused by inefficient cooling of the rotating ring gain.

[0071] Fig.14The measured output spectrum of the CL-2940-25-D master oscillator at maximum pump power level is shown. The laser cavity is typically non-dispersive and does not contain any spectrum narrowing optical elements. This results in a broad linewidth of approximately 20 nm (at FWHM).

[0072] The observed output power is at 2940nm output wavelength ( Fig.13 ) and in one-way MOPA ( Fig.11 ) indicates that the simple method of rotating ring cooling used in these prototypes may not be sufficient. To investigate the thermal performance of the CL-XXXX-XX-D laser system, the following measurements were performed:

[0073] The pump system was immediately turned on at 100% pump power level (total power maximum 226W) and the output power of the CL-2500-130-D laser system was recorded over time until a maximum drop of 10% was observed. The pump source was then turned off for a few minutes. The same measurements were then repeated at smaller levels of pump power (decreasing in 10% steps) until thermal flipping became negligible.

[0074] refer to Fig.15 , which shows the thermal decay of the CL-2500-130-D laser output power at different total pump levels. The percentage labels show the pump power level for each on / off cycle.

[0075] from Fig.15 It can be seen that when the maximum pump power is reduced, the thermal decay slows down and becomes smaller until it drops to negligible values ​​at 50% of the total pump. In the case of 2940nm output wavelength and single-pass amplifier, the thermal decay becomes more obvious under low laser efficiency conditions (more pump power is converted to heat). It is also noted that the beam quality does not degrade despite the output power drop. This observation further suggests that the main reason for the output power decay is thermal quenching due to the relatively inefficient cooling of the rotating ring. Modifications to the annular cooling device used to improve the testing are discussed below.

[0076] like Fig.16 The tested design of the rotating ring with mounting / cooling flange shown includes Crystal to heat sink contact area: 1296mm2 Heat sink to air contact area: 4213mm2 ZnSe layer thickness between heat sink and laser beam trace: 5.5mm

[0077] In the tested design, a rotating ring 70 is mounted between two aluminum flanges 71 coupled to a motor shaft 72. The ring has an outer diameter of 50 mm and its internal mounting hole is 20 mm. The ring rotates at a maximum speed of 5000 rpm. The flanges are cooled at room temperature using two compressed air jets 73, with an air pressure of about 65 psi at the supply nozzle.

[0078] From laser experiments it can be seen that this annular cooling method is sufficient to enable total pump power along the laser beam trace 74 to be as high as 100 W at the central wavelength (2300-2600 nm) and to drop to a maximum of 50 W at the tail of the Cr:ZnSe emission spectrum.

[0079] Disadvantages of the current approach include the relatively small contact area between the ring gain element and the heat sink (cooling flange). The estimated contact area is approximately 1296 mm 2 Also, the distance between the laser beam trace and the heat sink is relatively large (5.5 mm) (ZnSe thermal conductivity is much less than that of metal, so the extra thickness increases thermal resistance). The small number of cooling fins on the mounting / cooling flange results in a relatively small contact area between the heat sink and the cooling air (approximately 4213 mm 2 ), which may result in insufficient cooling air flow.

[0080] To alleviate thermal issues and increase the maximum acceptable pump power, Fig.17 The modified annular cooling system in Figure 1 shows a set of simple modifications to the cooling system. These modifications include a crystal-to-heat sink contact area of ​​2693 mm 2 ; The contact area between the radiator and the air is 29053mm 2 and the ZnSe layer thickness between the heat sink and the laser beam trace is 2.0 mm. In addition to the modified internal flange 75, an additional external heat sink 76 on the outer edge of the ring further improves heat dissipation. The expected cooling air flow jet will distribute the flow between all the blades of the rotating flange.

[0081] To further improve the heat dissipation of the gain element and achieve kW-level output power, more advanced thermal management is needed. One of the most promising approaches is to use a Cr:ZnSe ring 80 stacked between an outer flange 81 and an inner flange 82 made of a highly conductive material (e.g., copper or carbon fiber or a composite thereof). Fig.18 A conceptual design of a stacked ring gain element is shown.

[0082] Other aspects of laser system components, gain materials, ring characteristics, pumping, cooling, and ring rotation are found in the incorporated '309 reference, particularly in paragraphs

[029] through

[038] of the '309 specification and in the drawings of '309.

[0083] Various changes may be made to the disclosed structures without departing from the spirit and essential characteristics of the present invention.Therefore, all matters contained in the above description should be interpreted as illustrative only and in a limiting sense, the scope of the present disclosure is defined by the appended claims.

Claims

1. A continuous wave CW laser with mid-infrared output, comprising: Resonant cavity; a gain medium formed as a ring defined by an inner circumferential surface, an outer circumferential surface, a first face, and a second face, the gain medium having a first non-fixed gain portion and a second non-fixed gain portion, the first non-fixed gain portion being between the first face and the second face and positioned within the resonant cavity to provide mid-infrared gain, the second non-fixed gain portion being between the first face and the second face and positioned to provide mid-infrared gain; The first pump source; The second pump source; a rotatable mounting structure in thermal contact with the ring; as well as a motor coupled to the rotatable mounting structure to rotate the structure about an axis; wherein The resonant cavity includes a cavity mirror near a first face of the ring and an output coupler near the first face of the ring, and an optical element for redirecting an optical path from the first non-fixed gain portion to the second non-fixed gain portion, the optical element being disposed near a second face of the ring, Each of the first and second non-fixed gain portions of the ring receives light from a first pump source and a second pump source, The first non-fixed gain section and the second non-fixed gain section are each configured to provide gain in the resonant cavity; and The resonant cavity emits a mid-infrared output.

2. The CW laser according to claim 1, wherein the gain medium is selected from polycrystalline transition metal doped II-VI materials TM: II-VI.

3. The CW laser of claim 1, wherein the gain medium is characterized as Cr:ZnSe.

4. The CW laser of claim 1, wherein the gain medium is characterized as Cr:ZnS.

5. The CW laser according to claim 1, further comprising: At least one optical cavity element for mid-infrared output wavelength selection.

6. The CW laser according to claim 1, further comprising: At least one optical cavity element is used to narrow the spectral linewidth of the mid-infrared output.

7. The CW laser of claim 1, wherein the gain medium is a composite stacked ring gain element.

8. The CW laser according to claim 1, further comprising: An optical element for focusing a first pump source and a second pump source, respectively, to the first non-fixed gain portion and the second non-fixed gain portion of the ring.

9. The CW laser according to claim 1, further comprising: One or more lens elements for focusing mid-infrared light.

10. The CW laser of claim 1, wherein the resonant cavity is configured such that the first non-fixed gain portion and the second non-fixed gain portion form a cascade.

11. The CW laser of claim 1 , wherein the first pump source and the second pump source are positioned on the same side of the ring.

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