Abstract
In various embodiments, cold-start times and performance of wavelength-beam-combining laser resonators are improved via adjustment of the operating wavelengths and/or temperature of beam emitters within the resonators.
Claims
1. A method of operating a wavelength-beam-combining (WBC) resonator, wherein the WBC resonator comprises an emitter having (i) a gain bandwidth defining a range of operating wavelengths at which a gain of the emitter exceeds a predetermined effective gain level, and (ii) a nominal operating wavelength (a) falling within the gain bandwidth at an operating temperature and (b) falling outside of the gain bandwidth at a startup temperature lower than the operating temperature, the method comprising: providing the emitter having a temperature equal to the startup temperature; applying heat to the emitter to increase the temperature thereof; and thereafter, operating the emitter to emit a beam at the nominal operating wavelength, whereby the temperature of the emitter increases to the operating temperature during operation.
2. The method of claim 1, wherein (i) operating the emitter comprises applying to the emitter a current greater than a lasing threshold current of the emitter, and (ii) applying heat to the emitter comprises applying to the emitter a simmer current less than the lasing threshold current.
3. The method of claim 1, wherein applying heat to the emitter comprises locally heating the emitter via a heat source external to the emitter.
4. The method of claim 3, wherein the heat source comprises at least one of a resistive heater, an infrared heater, or a thermoelectric heater.
5. The method of claim 1, wherein the nominal operating wavelength of the emitter is a wavelength of visible light or ultraviolet light.
6. The method of claim 1, wherein the nominal operating wavelength of the emitter is a wavelength of blue light.
7. The method of claim 1, wherein the startup temperature is approximately equal to a temperature of an ambient environment in which the WBC resonator is disposed.
8. The method of claim 1, wherein (i) the WBC resonator comprises a cooling system utilizing a fluid coolant, and (ii) the startup temperature is approximately equal to a temperature of the fluid coolant.
9. The method of claim 1, wherein the WBC resonator comprises: a plurality of additional emitters each having a nominal operating wavelength different from the nominal operating wavelength of the emitter; a dispersive element configured to receive beams emitted by the emitter and the plurality of additional emitters and combine the beams into a multi-wavelength beam; and disposed optically downstream of the dispersive element, a partially reflective output coupler configured to (i) receive the multi-wavelength beam, (ii) transmit a first portion of the multi-wavelength beam from the WBC resonator as an output beam, and (iii) reflect a second portion of the multi-wavelength beam back toward the dispersive element.
10. The method of claim 1, further comprising: combining, within the WBC resonator, the beam emitted by the emitter with beams emitted by a plurality of additional emitters, to thereby form a multi-wavelength beam; transmitting a first portion of the multi-wavelength beam from the WBC resonator as an output beam; and propagating a second portion of the multi-wavelength beam back to the emitter and the plurality of additional emitters to stabilize the beams emitted by the emitter and by the plurality of additional emitters.
11. The method of claim 10, further comprising applying heat to the plurality of additional emitters to increase a temperature thereof, and, thereafter, operating the plurality of additional emitters to emit beams therefrom.
12. The method of claim 10, further comprising processing a workpiece with the output beam.
13. The method of claim 12, wherein processing the workpiece comprises at least one of cutting, welding, etching, annealing, drilling, soldering, or brazing.
14. The method of claim 12, wherein processing the workpiece comprises physically altering at least a portion of a surface of the workpiece.
15. A method of operating a wavelength-beam-combining (WBC) resonator, wherein (A) the WBC resonator comprises an emitter having (i) a gain bandwidth defining a range of operating wavelengths at which a gain of the emitter exceeds a predetermined effective gain level, and (ii) a nominal operating wavelength (a) falling within the gain bandwidth at an operating temperature and (b) falling outside of the gain bandwidth at a startup temperature lower than the operating temperature, and (B) the emitter is operable at a nominal drive current greater than a lasing threshold current to produce a beam having the nominal operating wavelength, the method comprising: initiating operation of the emitter, at the startup temperature, by applying to the emitter an overdrive current greater than the nominal drive current; and when a temperature of the emitter increases to the operating temperature, decreasing the applied current to the nominal drive current.
16. The method of claim 15, wherein the applied current is decreased gradually from the overdrive current to the nominal drive current as the temperature of the emitter increases to the operating temperature.
17. The method of claim 15, further comprising, before initiating operation of the emitter, applying heat to the emitter to increase the temperature thereof.
18. The method of claim 17, wherein applying heat to the emitter comprises applying to the emitter a simmer current less than the lasing threshold current.
19. The method of claim 17, wherein applying heat to the emitter comprises locally heating the emitter via a heat source external to the emitter.
20. The method of claim 19, wherein the heat source comprises at least one of a resistive heater, an infrared heater, or a thermoelectric heater.
21. The method of claim 15, wherein the nominal operating wavelength of the emitter is a wavelength of visible light or ultraviolet light.
22. The method of claim 15, wherein the nominal operating wavelength of the emitter is a wavelength of blue light.
23. The method of claim 15, wherein the WBC resonator comprises: a plurality of additional emitters each having a nominal operating wavelength different from the nominal operating wavelength of the emitter; a dispersive element configured to receive beams emitted by the emitter and the plurality of additional emitters and combine the beams into a multi-wavelength beam; and disposed optically downstream of the dispersive element, a partially reflective output coupler configured to (i) receive the multi-wavelength beam, (ii) transmit a first portion of the multi-wavelength beam from the WBC resonator as an output beam, and (iii) reflect a second portion of the multi-wavelength beam back toward the dispersive element.
24. The method of claim 15, further comprising: combining, within the WBC resonator, the beam emitted by the emitter with beams emitted by a plurality of additional emitters, to thereby form a multi-wavelength beam; transmitting a first portion of the multi-wavelength beam from the WBC resonator as an output beam; and propagating a second portion of the multi-wavelength beam back to the emitter and the plurality of additional emitters to stabilize the beams emitted by the emitter and by the plurality of additional emitters.
25. The method of claim 24, further comprising processing a workpiece with the output beam.
26. The method of claim 25, wherein processing the workpiece comprises at least one of cutting, welding, etching, annealing, drilling, soldering, or brazing.
27. The method of claim 25, wherein processing the workpiece comprises physically altering at least a portion of a surface of the workpiece.
28.-89. (canceled)
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which:
[0037] FIG. 1A is a graph of exemplary cold and hot gain curves, and their overlap, for an emitter having a finite positive working range in accordance with embodiments of the present invention;
[0038] FIG. 1B is a graph of exemplary cold and hot gain curves, and their overlap, for an emitter having zero working range in accordance with embodiments of the present invention;
[0039] FIG. 1C is a graph of exemplary cold and hot gain curves, with no meaningful overlap, for an emitter in accordance with embodiments of the present invention;
[0040] FIGS. 2A-2C schematically depict techniques for improving the cold-start performance of emitters exhibiting the behavior depicted in FIG. 1B via application of simmer current (FIG. 2A), application of overdrive current (FIG. 2B), or both (FIG. 2C), in accordance with embodiments of the present invention;
[0041] FIG. 3 schematically depicts a technique for improving the cold-start performance of emitters in which the emitter operating wavelength is actively changed during operation, in accordance with embodiments of the present invention;
[0042] FIG. 4 is a schematic diagram of a wavelength beam combining (WBC) resonator in accordance with embodiments of the present invention;
[0043] FIG. 5 is a graph of simulated wavelength and position shifts of a resonator output beam as a function of the rotation angle of a folding mirror in accordance with embodiments of the present invention;
[0044] FIG. 6A schematically depicts the effect of folding mirror rotation on beam position in accordance with embodiments of the present invention;
[0045] FIG. 6B schematically depicts the reduction of beam shift of an output beam via movement of the folding mirror rotation axis in accordance with embodiments of the present invention; and
[0046] FIGS. 7A-7C are graphs schematically depicting the relationship between resonator wavelength and emitter wavelength from cold start in accordance with various embodiments of the present invention.
DETAILED DESCRIPTION
[0047] FIG. 1A is a graph of exemplary cold and hot gain curves, and their overlap, for a diode emitter. In FIG. 1A and later figures, G.sub.L refers to the gain curve at cold status, i.e., low temperature (e.g., the temperature at startup), while G.sub.H refers to the gain curve at hot status, i.e., high temperature (e.g., during sustained operation). B refers to the gain bandwidth, which is the width of the gain curve at the effective gain level (EGL) of the emitter, which is typically at 90% gain or higher. S refers to the shift in wavelength () of the gain curve experienced in the transition from cold status to hot status.
[0048] In the example of FIG. 1A, the gain bandwidth S is larger than the wavelength shift S, resulting in a finite positive working range W, which is equal to the difference between B and S. An emitter locked to a wavelength within the range W, for example at the depicted wavelength .sub.0, will have a fast rising time because it will generate power at cold status at a level comparable to that generated at hot status. Example emitters exhibiting such behavior include at least some semiconductor laser emitters emitting at near-infrared or longer wavelengths. Techniques disclosed in the '807 patent may be successfully applied to such emitters to increase the width of the range W and therefore improve laser performance.
[0049] In the example of FIG. 1B, the gain bandwidth B is narrower than the wavelength shift S, resulting in zero working range above the effective gain level EGL. However, the cold and hot gain curves still do overlap at gain levels lower than EGL but at meaningful gain levels, represented by the shaded area in FIG. 1B. In the example of FIG. 1B, an emitter locked to wavelength .sub.0, which is near the optimized point of the hot gain curve, will not produce sufficient power at cold status. Therefore, a laser system incorporating such emitters will rise more slowly at cold start and require more time to reach sustained stable operation.
[0050] In the example of FIG. 1C, the gain bandwidth B is substantially narrower than the wavelength shift S, resulting in no meaningful overlap of the cold and hot gain curves. Emitters exhibiting such behavior include various diode lasers emitting at visible (e.g., blue, blue-violet, violet) wavelengths and/or ultraviolet wavelengths. In such cases, at emitter locked at a hot wavelength .sub.0 will be fully wavelength-unlocked at cold start, and therefore may produce little or no power at cold start, resulting in a much slower rise time to sustained stable operation. For simplicity, it is assumed that emitter drive currents may be raised instantaneously from zero to a preset operating current. As such, cold status refers to a low temperature of the emitter (typically the ambient room temperature or the temperature of cooling fluid utilized in the laser system), rather than low current levels.
[0051] FIGS. 2A-2C schematically depict techniques for improving the cold-start performance of emitters exhibiting the behavior depicted in FIG. 1B via application of simmer current (FIG. 2A), application of overdrive current (FIG. 2B), or both (FIG. 2C). By applying simmer current, an emitter may be preheated and thus be cold started from a higher temperature. In FIG. 2A, the dashed curve G represents the gain curve of such a preheated emitter. As shown, with a preheated emitter, the resulting wavelength shift S in the transition to hot status may be smaller than the gain bandwidth B, resulting in a finite positive working range W (equal to the difference of B and S). In various embodiments, the applied simmer current is limited to a level below the laser threshold current of the emitter; thus, in various embodiments the amount of resulting heat applied to the emitter may be limited. In various embodiments, instead of or in addition to applying a simmer current to the emitter, a local heater or heat source (e.g., an infrared heater, a resistive heater, and/or a thermoelectric cooler/heater may be utilized to heat one or more emitters in the laser system. The local heat source may apply heat to the emitter(s) at (and/or before) cold start and then be gradually or immediately switched off once cold start has been initiated. In various embodiments, the local heat source may be abruptly turned off once the emitter has achieved hot status and the concomitant elevated operating temperature. In various embodiments, the heat applied by the local heat source may be gradually decreased as the operating temperature of the emitter increases due to the operating current utilized thereby; in such embodiments, the local heat source may be turned off once the emitter has reached hot status and its operating temperature.
[0052] FIG. 2B schematically depicts an embodiment of the invention in which overdrive (or overshoot) current is applied to the emitter to effectively increase the gain level at cold start. As shown, the emitter gain curve G is shifted higher, resulting in a wider gain bandwidth B at EGL. For simplicity, assuming that the overdrive current is decreased linearly back to the nominal operating current during the transition from cold to hot operation, the resulting working range W may be calculated by W=(B+B)/2S. Since B is less than S, embodiments of the invention apply a sufficient overdrive current such that (B+B)/2 is greater than S.
[0053] FIG. 2C schematically depicts embodiments in which both simmer current (and/or local heating) and overdrive current are applied to the emitter to achieve faster cold-start performance. Again assuming a linear decrease of the overdrive current back to the nominal operating current during the transition from cold to hot operation, the resulting working range W may be calculated by W=(B+B)/2S. In various embodiments, because diode emitters may become less efficient (and thus run at hotter temperatures) over their working lifetimes, the working range W achieved utilizing the above methods may be increased to compensate.
[0054] FIG. 3 schematically depicts embodiments of the invention in which the emitter operating (i.e., locked) wavelength is actively changed during operation, a technique which may be applied to emitters exhibiting any of the behaviors depicted in FIGS. 1A-1C. However, such embodiments may be particularly applicable to emitters exhibiting the behavior depicted in FIG. 1C (e.g., emitters configured to emit visible (e.g., blue, blue-violet, violet) or ultraviolet wavelengths). As shown in FIG. 3, the emitter operating wavelength is changed (e.g., increased) from .sub.0 at cold status (i.e., at and/or before startup), to .sub.0 at an intermediate status where the temperature of the emitter is between the low temperature at cold status and the high temperature at hot status, and finally to .sub.0 at hot status (i.e., where the temperature of the emitter has stabilized at its higher operating temperature). In such embodiments, the impact of the gain curve shift at cold start is effectively eliminated, and the resulting working range W is equal to the gain bandwidth B. Such embodiments of the invention are additionally advantageous because the operating wavelength may be continually set at or near the peak of the gain curve at each temperature, resulting in high power efficiency of the laser system.
[0055] FIG. 4 schematically depicts a system and technique for adjusting the emitter operating wavelength in a WBC resonator in accordance with the embodiments depicted in FIG. 3. FIG. 4 schematically depicts various components of a WBC resonator 400 that, in the depicted embodiment, combines the beams emitted by nine different multi-beam emitters, i.e., emitters from which multiple beams are emitted from a single package, such as diode bars. Embodiments of the invention may be utilized with fewer or more than nine emitters. In accordance with embodiments of the invention, each emitter may emit a single beam, or, each of the emitters may emit multiple beams. The emitters in FIG. 4 are depicted as each emitting a single beam for clarity and convenience of illustration. The view of FIG. 4 is along the WBC dimension, i.e., the dimension in which the beams from the bars are combined. The exemplary resonator 400 features nine diode bars 405, and each diode bar 405 includes, consists essentially of, or consists of an array (e.g., one-dimensional array) of emitters along the WBC dimension. Each emitter of a diode bar 405 may emit a non-symmetrical beam having a larger divergence in one direction (known as the fast axis, here oriented vertically relative to the WBC dimension) and a smaller divergence in the perpendicular direction (known as the slow axis, here along the WBC dimension).
[0056] In various embodiments, each of the diode bars 405 is associated with (e.g., attached or otherwise optically coupled to) a fast-axis collimator (FAC)/optical twister microlens assembly that collimates the fast axis of the emitted beams while rotating the fast and slow axes of the beams by 90, such that the slow axis of each emitted beam is perpendicular to the WBC dimension downstream of the microlens assembly. The microlens assembly also converges the chief rays of the emitters from each diode bar 405 toward a dispersive element 410. Suitable microlens assemblies are described in U.S. Pat. No. 8,553,327, filed on Mar. 7, 2011, and U.S. Pat. No. 9,746,679, filed on Jun. 8, 2015, the entire disclosure of each of which is hereby incorporated by reference herein.
[0057] As shown in FIG. 4, resonator 400 also features a set of SAC lenses (or slow-axis collimators) 415, one SAC lens 415 associated with, and receiving beams from, one of the diode bars 405. Each of the SAC lenses 415 collimates the slow axes of the beams emitted from a single diode bar 405. After collimation in the slow axis by the SAC lenses 415, the beams propagate to a set of interleaving mirrors 420, which redirect the beams toward the dispersive element 410. The arrangement of the interleaving mirrors 420 enables the free space between the diode bars 405 to be reduced or minimized, and also reduces or minimizes the overall wavelength locking bandwidth. Upstream of the dispersive element 410 (which may include, consist essentially of, or consist of, for example, a diffraction grating such as the transmissive diffraction grating depicted in FIG. 4), a lens 425 may optionally be utilized to collimate the sub-beams (i.e., emitted rays other than the chief rays) from the diode bars 405. In various embodiments, the lens 425 is disposed at an optical distance away from the diode bars 405 that is substantially equal to the focal length of the lens 425. Note that, in various embodiments, the overlap of the chief rays at the dispersive element 410 is primarily due to the redirection of the interleaving mirrors 420, rather than the focusing power of the lens 425.
[0058] Also depicted in FIG. 4 are lenses 430, 435, which form an optical telescope for mitigation of optical cross-talk, as disclosed in U.S. Pat. No. 9,256,073, filed on Mar. 15, 2013, and U.S. Pat. No. 9,268,142, filed on Jun. 23, 2015, the entire disclosure of which is hereby incorporated by reference herein. Resonator 400 may also include one or more folding mirrors 440 for redirection of the beams such that the resonator 400 may fit within a smaller physical footprint. The dispersive element 410 combines the beams from the diode bars 405 into a single, multi-wavelength beam, which propagates to a partially reflective output coupler 445. The coupler 445 transmits a portion of the beam as the output beam of resonator 400 while reflecting another portion of the beam back to the dispersive element 410 and thence to the diode bars 405 as feedback to stabilize the emission wavelengths of each of the beams.
[0059] In accordance with embodiments of the invention, the resonator locking wavelengths of the emitters 405 may be altered via adjustment of the folding angle of the folding mirror 440. As shown in FIG. 4, one or more actuators 450 may be utilized to tune the locking wavelengths of the emitters by altering the mirror folding angle, i.e., the angle at which the folding mirror 440 intercepts and redirects the beams toward the output coupler 445. In various embodiments, the angle and position of the output coupler 445 remain unchanged, and therefore the pointing of the output beam remains unchanged even as the folding mirror 440 (and the resulting operating/locking emitter wavelengths) are adjusted. However, the resonator output beam may be shifted in position at the output coupler 445 in the WBC dimension. In order to reduce or minimize this output beam position shift, the folding mirror 440 may be positioned as closely as possible to the dispersive element 410, either upstream or downstream thereof. For example, the distance between the folding mirror 440 and the dispersive element 410 may be less than 300 mm, less than 200 mm, less than 100 mm, or less than 75 mm. In various embodiments, the distance between the folding mirror 440 and the dispersive element may be at least 20 mm, at least 30 mm, at least 40 mm, or at least 50 mm. In various embodiments, in order to accommodate the output beam position shift on the output coupler 445, the output coupler 445 may be sufficiently large, at least in the WBC dimension. For example, the output coupler 445 may have a size greater than the expected output beam position shift by at least a factor of 50, at least a factor of 20, or at least a factor of 10. In such embodiments, any possible distortion or edge-effect-related to the output coupler 445 will not affect the beam, despite the position shift. In various embodiments, the output coupler 445 may have a size (e.g., diameter) of at least 8 mm, at least 10 mm, at least 12 mm, at least 14 mm, at least 16 mm, at least 18 mm, or at least 20 mm. The size of the output coupler 445 may be, in various embodiments, at most 50 mm, at most 40 mm, or at most 30 mm.
[0060] In various embodiments, the one or more actuators 450 may be responsive to, and thus controlled by, a controller (or control system) 455. The controller 455 may be provided as either software, hardware, or some combination thereof. For example, the system may be implemented on one or more conventional server-class computers, such as a PC having a CPU board containing one or more processors such as the Pentium or Celeron family of processors manufactured by Intel Corporation of Santa Clara, Calif., the 680x0 and POWER PC family of processors manufactured by Motorola Corporation of Schaumburg, Ill., and/or the ATHLON line of processors manufactured by Advanced Micro Devices, Inc., of Sunnyvale, Calif. The processor may also include a main memory unit for storing programs and/or data relating to the methods described herein. The memory may include random access memory (RAM), read only memory (ROM), and/or FLASH memory residing on commonly available hardware such as one or more application specific integrated circuits (ASIC), field programmable gate arrays (FPGA), electrically erasable programmable read-only memories (EEPROM), programmable read-only memories (PROM), programmable logic devices (PLD), or read-only memory devices (ROM). In some embodiments, the programs may be provided using external RAM and/or ROM such as optical disks, magnetic disks, as well as other commonly used storage devices. For embodiments in which the functions are provided as one or more software programs, the programs may be written in any of a number of high level languages such as PYTHON, FORTRAN, PASCAL, JAVA, C, C++, C #, BASIC, various scripting languages, and/or HTML. Additionally, the software may be implemented in an assembly language directed to the microprocessor resident on a target computer; for example, the software may be implemented in Intel 80x86 assembly language if it is configured to run on an IBM PC or PC clone. The software may be embodied on an article of manufacture including, but not limited to, a floppy disk, a jump drive, a hard disk, an optical disk, a magnetic tape, a PROM, an EPROM, EEPROM, field-programmable gate array, or CD-ROM.
[0061] In various embodiments, the controller 455 may also be utilized to control the flow of power (e.g., current) to the emitters 405 in order to, for example, apply simmer current and/or overdrive current thereto, as described above. The controller 455 may also be utilized to control local heaters (not shown in FIG. 4) utilized to apply heat to one or more of the emitters 405 (e.g., at or before cold start). In various embodiments, each emitter 405 may be associated with a separate local heater, or one local heater may be shared by two or more (or even all) of the emitters 405.
[0062] FIG. 5 is a graph of simulated wavelength and position shifts of the resonator output beam for an example resonator similar to resonator 400 as a function of the rotation angle of the folding mirror. In the example of FIG. 5, the resonator features a transmissive diffraction grating having a line density of 3.5/m, a resonator center wavelength of about 418 nm, a folding mirror located about 85 mm downstream of the grating, and a telescopic lens pair (i.e., equivalent to lenses 430, 435 in FIG. 4) having a focal length ratio of about 18. As shown in FIG. 5, as the rotation angle of the folding mirror is increased, both the locking wavelength and the position of the beam on the output coupler (in the WBC dimension) shift accordingly. In this manner, the operating wavelength may be adjusted during emitter operation to fall within the gain bandwidth of the emitter, even as it changes as a function of operating temperature, over the entire temperature range from cold status to hot status.
[0063] FIG. 6A schematically depicts the effect of folding mirror rotation on beam position. In FIG. 6A, beam 600 represents the chief ray of the center emitter in a WBC resonator propagating to a diffraction grating 605, where line 610 represents the normal to the grating 605. The resulting output (from the grating) beam 615 propagates to the output coupler (not shown) after being redirected by a folding mirror 620 (like folding mirror 440 of FIG. 4). For simplicity, the mirror 620 is depicted as arranged such that the output beam (or resonator beam) 615 is parallel to the incoming center chief ray 600; however, embodiments of the invention may be utilized to redirect output beams at other trajectories, as long as the output coupler is positioned to intercept the output beam accordingly. The output beam is typically normal to the feedback surface, i.e., the output coupler. As shown in FIG. 6A, rotating the mirror 620 by an angle alters the resonator beam propagation downstream of the grating 605 by an angle 2, and both the wavelength and the position of the resonator beam will be altered, as indicated by the line 615a in FIG. 6A. The wavelength shift is, in various embodiments, approximately equal to 2cos()/p, where p is the line density of the grating 605. This equation generally applies to embodiments in which there are no optics having optical power (i.e., lens power) in the WBC dimension disposed between the grating 605 and the output coupler. The wavelength shift in the example of FIG. 5, which is based on a resonator similar to that of FIG. 4, is about 25% smaller than the value that would be calculated from the above equation because of the presence of the telescope lens pair 430, 435, both of which have lens power in the WBC dimension.
[0064] In various embodiments, the beam shift S at the output coupler may be approximately equal to 2S/F, where S is the separation distance between the mirror 620 and the grating 605, and F is the beam size shrinkage factor in the WBC dimension caused by the telescopic lens pair (if present). The position shifts depicted in FIG. 5 result from a beam size shrinkage factor F of 18, which is equal to the focal length ratio of the lens pair.
[0065] In various embodiments of the invention, the position shift of the output beam at the output coupler may be reduced or minimized by adjusting the rotation axis of the folding mirror. FIG. 6B schematically depicts the reduction of beam shift of the output beam 615 via movement of the mirror rotation axis 625 a distance D away from the position at which the beam strikes the mirror 620. In various embodiments, the distance D is approximately equal to 2Scos()/sin(2). In this manner, as shown in FIG. 6B, the position shift of the output beam relative to the output coupler may be kept substantially constant, even as its operating wavelength changes due to rotation of the folding mirror 620.
[0066] In various embodiments of the invention, the resonator locking wavelength may also be adjusted by decentering one or more lenses in the WBC dimension. Such lenses include, but are not limited to, for example, lenses 425, 430, 435 in resonator 400 depicted in FIG. 4. Thus, in various embodiments of the invention, one or more lenses in a laser resonator, such as lenses 425, 430, 435, are configured to be decentered (i.e., translated) at least in the WBC dimension of the resonator. For example, the lenses may be coupled to one or more actuators configured to translate the lenses, and the one or more actuators may be responsive to the controller (e.g., as detailed above with respect to FIG. 4). The controller may be configured to decenter one or more of the lenses and translate the lenses during operation (and concomitant heating) of the emitters such that the lenses are centered in the WBC dimension when the emitters have reached their nominal operating temperatures. The induced wavelength shift will be proportional to d/f, where d is the lens decentering distance and f is the focal length of the lens. In various embodiments, lens decentering may not be preferred due to it requiring relatively larger adjustments than the mirror rotation adjustment of FIGS. 6A and 6B. Lens decentering may also induce larger beam position shifts relative to the output coupler, which may thus be more challenging to compensate for.
[0067] FIGS. 7A-7C schematically depict the relationship between resonator wavelength RW and emitter wavelength EW at cold start in accordance with various embodiments of the present invention. Specifically, FIG. 7A depicts the relationship between the optimized resonator locking wavelength RW and the emitter working wavelength EW at various times during the startup period of a WBC resonator without any adjustment of the working wavelength. For FIG. 7A, it is assumed, as is typical, that the WBC laser system is optimized at the emitter wavelength when in hot status, denoted in FIG. 7A as .sub.H. It is also assumed, for simplicity, that the driving current is applied to the emitters at time to instantly. The emitter wavelength represented by the EW curve may represent the peak wavelength, the central wavelength, or any other wavelength within the emitter effective bandwidth (i.e., B in FIGS. 1A-1C). It is also assumed that the emitter junction temperature will quickly rise to an intermediate level within the first fraction of a second (or even less than a millisecond) and then relatively slowly rise to the final temperature representative of operation at hot status. The EW curve, starting from the cold status wavelength XL and ending at the hot status wavelength .sub.H, is assumed to follow the same trend as the rise in emitter temperature. Depending on the thermal constant of the entire emitter, the entire duration t=t.sub.2t.sub.0 of emitter temperature rise or, equivalently, emitter gain wavelength shift, may take more than a second, more than 2 seconds, more than 5 seconds, or longer. In various embodiments, this duration may be less than 60 seconds, less than 30 seconds, or less than 10 seconds, for example.
[0068] If the emitter bandwidth is very narrow, for example in the case depicted in FIG. 1C, the WBC laser will exhibit a very slow cold start, because it will not produce resonator power above an effective level until, at time t.sub.1 in FIG. 7A, the emitter has attained a sufficiently high temperature so that the difference of emitter wavelength .sub.E, and the preset resonator locking wavelength .sub.H becomes smaller than the emitter effective bandwidth B, i.e., until (.sub.H.sub.E)<B. Application of simmer current and/or overdrive current, as depicted in FIGS. 2A-2C, will effectively move the EW curve closer to the RW curve at an earlier time, thereby reducing the laser startup time t. However, as mentioned above, such techniques may be insufficient in embodiments in which emitters have very narrow gain bandwidths, e.g., various visible-light (e.g., blue, blue-violet, or violet) and/or ultraviolet-light emitters. Thus, instead of or in addition to moving the EW curve via application of simmer current and/or overdrive current, embodiments of the invention effectively lower the RW curve by altering the resonator locking wavelength as a function of time during startup from cold start. Various such embodiments are schematically depicted in FIGS. 7B and 7C.
[0069] FIG. 7B schematically depicts an embodiment in which the WBC resonator is initially optimized at the emitter hot status wavelength .sub.H, and in which the resonator wavelength may be adjusted as described above in relation to FIGS. 4, 6A, and 6B. As shown, the adjustment of the resonator wavelength RW will not alter the behavior of the emitter wavelength EW during the time period t, which will follow the same curve as in FIG. 7A. In the embodiment of FIG. 7B, the actuator 414 is activated at time t.sub.0 to start rotating the folding mirror 440 and is calibrated so that the resonator wavelength is quickly shifted down from .sub.H to .sub.R, which is an intermediate wavelength approaching the emitter wavelength .sub.E. After the time t.sub.1, the resonator wavelength is adjusted to follow the EW curve until the hot status wavelength is attained. In embodiments in which the difference between .sub.R and .sub.E is smaller than the emitter effective bandwidth B, the laser rising time will be about t, which is shorter than the nominal rise time t in FIG. 7A.
[0070] In such embodiments, the laser rise time is limited by, at least in part, the response time of the actuator rotating the folding mirror and the required maximum rotation. In an exemplary embodiment, the wavelength shift rate is about 0.1 nm/degree, and the emitter junction temperature may rise over 70; therefore, the full wavelength shift at cold start will be around 7 nm, which corresponds to a 1.2 rotation of the mirror 440 of FIG. 4 in the embodiment of FIG. 5. Assuming, in an exemplary embodiment, that the emitter will complete a 30% shift of the total range during the actuator response period (e.g., in a time period in the sub-millisecond range), then the required wavelength adjustment will be about 5 nm, or about 4.5 nm if considering about 1 nm full bandwidth at 90% for the emitter, which corresponds to a minimum mirror tilt of about 0.8 in the embodiment of FIG. 4. Further assuming the actuator contact point on the mirror 440 is 10 mm away from the mirror rotating axis (for example the axis 625 shown in FIG. 6B), then the minimum actuator displacement will be 140 m. Utilizing as an example a Thorlabs off-the-shelf piezo actuator (P#PK2F SF1), which has a 220 m free stroke with 1 kHz no-load or about 330 Hz loaded resonant frequency, the minimum response time of the actuator for 140 m displacement is estimated to be 640 s.
[0071] FIG. 7C schematically depicts an embodiment in which the laser rise time from cold start is further minimized. In the embodiment of FIG. 7C, the resonator wavelength is adjusted to conform to the emitter cold wavelength at the initiation of the cold start. Such embodiments may be accomplished via two different techniques. First, the resonator wavelength may be initially optimized (i.e., with no mirror rotation) at the wavelength corresponding to the emitter hot status. Then, before cold start at time to, the actuator is preset so that the resonator locking wavelength is pre-shifted to the emitter cold status wavelength. The actuator is also calibrated to follow the emitter wavelength curve EW by gradually decreasing the rotation angle until the hot status wavelength is achieved at time t.sub.2. Alternatively, the resonator wavelength may be optimized at the wavelength corresponding to the emitter cold status, and the actuator is calibrated to follow the emitter wavelength curve EW by gradually increasing the mirror rotation angle until the hot status wavelength is achieved at time t.sub.2.
[0072] In contrast with the embodiment depicted in FIG. 7B, no abrupt change in resonator wavelength is required in the embodiment of FIG. 7C, and the rise time primarily depends on the drive current rise time rather than being limited by the actuator response time. The rise time of drive current for high-power diodes may be on the order of a few tens of microseconds or less. This greatly relaxed requirement on the actuator response time enables less-responsive means of adjusting resonator wavelength to be utilized, such as stepper motors and/or local heaters.
[0073] In various embodiments, the power of the WBC resonator may be further stabilized utilizing a feedback loop incorporated with the one or more actuators (via the controller) or other wavelength-adjustment means. For example, the resonator output power may be detected and utilized as a feedback signal to adjust the resonator locking wavelength to maximize output power. Such embodiments, as well as all embodiments of the invention detailed herein, may be utilized at times other than startup of the laser system from cold start. For example, the resonator wavelength may be advantageously adjusted to increase resonator power at later stages of laser emitter lifetime when the emitters become less efficient (i.e., operate at higher temperatures for the same driving current). In addition, cold start, as utilized herein, is not limited to the very initial startup of laser operation. Rather, cold start may also include the initiation of one or more (or even each) pulse when the laser system is being operated in pulsed mode, particularly when operating at short-duration pulses, when the emitters may always be operating near or at their cold status.
[0074] In various embodiments, the calibration of the wavelength adjustment (e.g., to follow the emitter wavelength curves in FIGS. 7B and 7C) may be accomplished via laboratory trials measuring startup time from cold start as a function of, e.g., mirror rotation. In addition or instead, the controller may be programmed to match the trend of wavelength shift predicted by thermal models of emitter temperature over time. Lookup tables and/or models may be generated to predict the initial emitter temperature status (e.g., cold, hot, or an intermediate temperature) of the emitter at each cold start based on operating modes and settings of the laser system (e.g., current level, pulse rate and duration, flow rate of cooling fluid, etc.) A feedback loop based on emitter temperature (measured by, e.g., thermistors or other temperature sensors) may also be incorporated into embodiments of the invention. Such calibration, feedback, and programming may be accomplished by those of skill in the art without undue experimentation.
[0075] After the optimized cold start of laser systems in accordance with embodiments of the present invention, the output beams of the laser systems may be propagated to a delivery optical fiber (which may be coupled to a laser delivery head) and/or utilized to process a workpiece. In various embodiments, a laser head contains one or more optical elements utilized to focus the output beam onto a workpiece for processing thereof. For example, laser heads in accordance with embodiments of the invention may include one or more collimators (i.e., collimating lenses) and/or focusing optics (e.g., one or more focusing lenses). A laser head may not include a collimator if the beam(s) entering the laser head are already collimated. Laser heads in accordance with various embodiments may also include one or more protective window, a focus-adjustment mechanism (manual or automatic, e.g., one or more dials and/or switches and/or selection buttons). Laser heads may also include one or more monitoring systems for, e.g., laser power, target material temperature and/or reflectivity, plasma spectrum, etc. A laser head may also include optical elements for beam shaping and/or adjustment of beam quality (e.g., variable BPP) and may also include control systems for polarization of the beam and/or the trajectory of the focusing spot. In various embodiments, the laser head may include one or more optical elements (e.g., lenses) and a lens manipulation system for selection and/or positioning thereof for, e.g., alteration of beam shape and/or BPP of the output beam, as detailed in U.S. patent application Ser. No. 15/188,076, filed on Jun. 21, 2016, the entire disclosure of which is incorporated by reference herein. Exemplary processes include cutting, piercing, welding, brazing, annealing, etc. The output beam may be translated relative to the workpiece (e.g., via translation of the beam and/or the workpiece) to traverse a processing path on or across at least a portion of the workpiece.
[0076] In embodiments an optical delivery fiber, the optical fiber may have many different internal configurations and geometries. For example, the optical fiber may include, consist essentially of, or consist of a central core region and an annular core region separated by an inner cladding layer. One or more outer cladding layers may be disposed around the annular core region. Embodiments of the invention may incorporate optical fibers having configurations described in U.S. patent application Ser. No. 15/479,745, filed on Apr. 5, 2017, and U.S. patent application Ser. No. 16/675,655, filed on Nov. 6, 2019, the entire disclosure of each of which is incorporated by reference herein.
[0077] In various embodiments, the controller may control the motion of the laser head or output beam relative to the workpiece via control of, e.g., one or more actuators. The controller may also operate a conventional positioning system configured to cause relative movement between the output laser beam and the workpiece being processed. For example, the positioning system may be any controllable optical, mechanical or opto-mechanical system for directing the beam through a processing path along a two- or three-dimensional workpiece. During processing, the controller may operate the positioning system and the laser system so that the laser beam traverses a processing path along the workpiece. The processing path may be provided by a user and stored in an onboard or remote memory, which may also store parameters relating to the type of processing (cutting, welding, etc.) and the beam parameters necessary to carry out that processing. The stored values may include, for example, beam wavelengths, beam shapes, beam polarizations, etc., suitable for various processes of the material (e.g., piercing, cutting, welding, etc.), the type of processing, and/or the geometry of the processing path.
[0078] As is well understood in the plotting and scanning art, the requisite relative motion between the output beam and the workpiece may be produced by optical deflection of the beam using a movable mirror, physical movement of the laser using a gantry, lead-screw or other arrangement, and/or a mechanical arrangement for moving the workpiece rather than (or in addition to) the beam. The controller may, in some embodiments, receive feedback regarding the position and/or processing efficacy of the beam relative to the workpiece from a feedback unit, which will be connected to suitable monitoring sensors.
[0079] In addition, the laser system may incorporate one or more systems for detecting the thickness of the workpiece and/or heights of features thereon. For example, the laser system may incorporate systems (or components thereof) for interferometric depth measurement of the workpiece, as detailed in U.S. patent application Ser. No. 14/676,070, filed on Apr. 1, 2015, the entire disclosure of which is incorporated by reference herein. Such depth or thickness information may be utilized by the controller to control the output beam to optimize the processing (e.g., cutting, piercing, or welding) of the workpiece, e.g., in accordance with records in the database corresponding to the type of material being processed.
[0080] The terms and expressions employed herein are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.
