MULTICORE FIBER LASER WITH INTEGRATED HIGH-BRIGHTNESS SIGNAL COMBINER

In some implementations, an optical system includes a multicore input fiber comprising multiple cores that are each configured to support an independent singlemode laser; a delivery fiber comprising a single core configured to support multiple modes; and a signal combiner, coupled to the multicore input fiber and coupled to the delivery fiber. In some implementations, the signal combiner is configured to receive multiple independent singlemode laser inputs from the multicore input fiber and to combine the multiple independent singlemode laser inputs into a multimode output that is provided to the delivery fiber.

TECHNICAL FIELD

The present disclosure relates generally to a high-power fiber laser architecture and to a laser architecture that includes a multicore fiber supporting several independent singlemode lasers with an integrated high-brightness signal combiner.

BACKGROUND

Laser power scaling includes techniques to increase an output power from a laser without changing the geometry, shape, or principle of operation of the laser. Power scalability, which is generally considered an important advantage in laser design, usually requires a more powerful pump source, stronger cooling, an increase in size, and/or a reduction in background loss in a laser resonator and/or a gain medium. For example, one approach to achieve power scalability is to use a master oscillator power amplifier (MOPA) architecture. For example, in a MOPA system, the master oscillator produces a highly coherent beam, and an optical amplifier is used to increase the power of the beam while preserving the main properties of the beam.

SUMMARY

In some implementations, an optical system includes a pump laser source; a multicore fiber laser comprising: an oscillator comprising an input side coupled to the pump laser source and an output side, wherein the oscillator comprises: an active fiber comprising multiple singlemode active fiber cores to convert pump light generated by the pump laser source into signal light; multiple first reflectors, respectively associated with the multiple singlemode active fiber cores, that are each configured to operate as a high reflector (HR) on the input side of the oscillator; and multiple second reflectors, respectively associated with the multiple singlemode active fiber cores, that are each configured to operate as an output coupler (OC) on the output side of the oscillator; and a power amplifier coupled to the output side of the oscillator, wherein the power amplifier comprises multiple cores that are matched to the multiple singlemode active fiber cores of the oscillator; a multimode delivery fiber; and a signal combiner, integrated with the multicore fiber laser, configured to receive multiple singlemode laser inputs from the multicore fiber laser and to combine the multiple singlemode laser inputs into a multimode output that is provided to the multimode delivery fiber.

In some implementations, an optical system includes a multicore input fiber comprising multiple cores that are each configured to support an independent singlemode laser; a delivery fiber comprising a single core configured to support multiple modes; and a signal combiner, coupled to the multicore input fiber and to the delivery fiber, wherein the signal combiner is configured to receive multiple independent singlemode laser inputs from the multicore input fiber and to combine the multiple independent singlemode laser inputs into a multimode output that is provided to the delivery fiber.

In some implementations, a method for operating an optical system includes receiving, by a signal combiner, multiple independent singlemode laser inputs from a multicore input fiber that comprises multiple cores that are each configured to support an independent singlemode laser, of the multiple independent singlemode laser inputs; combining, by the signal combiner, the multiple independent singlemode laser inputs into a multimode output; and providing, by the signal combiner, the multimode output to a delivery fiber comprising a single core configured to support multiple modes.

DETAILED DESCRIPTION

FIG.1Ais a diagram illustrating an example100A of a master oscillator power amplifier (MOPA) laser architecture, andFIG.1Bis a diagram illustrating an example100B of power scaling the MOPA laser architecture using an external signal combiner.

As described herein, laser power scaling generally refers to techniques that may be used to increase an output power from a laser without changing the geometry, shape, or principle of operation of the laser. Power scalability, which is considered an important advantage in laser design, usually requires a more powerful pump source, stronger cooling, an increase in size, and/or a reduction in background loss in a laser resonator and/or a gain medium. For example, one approach to achieve power scalability in a laser architecture is to use a MOPA architecture, where the master oscillator produces a highly coherent beam, and an optical power amplifier is used to increase the power of the beam while preserving the main properties of the beam. For example, in a MOPA architecture, the output from a low-power, single-frequency laser oscillator may be injected unidirectionally into an optical amplifier with greater output power capacity. A special case is a master oscillator fiber amplifier (MOFA), where the power amplifier is a fiber device. In other cases, a MOPA may include a solid-state bulk laser and a bulk amplifier, or a tunable external cavity diode laser and a semiconductor optical amplifier.

For example, referring toFIG.1A, example100A depicts a MOPA laser architecture that includes a multi-kilowatt (kW) pump source110comprising a set of laser diodes112, a combiner114, and a set of output fibers coupling the set of laser diodes112and the combiner114. As further shown, the MOPA laser architecture may include a master oscillator120configured to produce a highly coherent beam, where the master oscillator120may comprise a first reflector122(e.g., a first fiber Bragg grating (FBG)), an active fiber124, and a second reflector126(e.g., a second FBG). For example, the first reflector122may be used as a high reflector (HR) to reflect a high percentage of light emitted from the active fiber124between the combiner114and an input end of the active fiber124, and the second reflector126may be used as an output coupler (OC) at an output end of the active fiber124. The MOPA laser architecture may further include a power amplifier140, and a passive fiber130coupling the output end of the oscillator120to the power amplifier140(e.g., via the second reflector126configured as the OC at the output end of the active fiber124), as well as various other components in the optical chain (e.g., filters for undesired wavelengths or unabsorbed pump) or the like.

Although a MOPA configuration may be more complex than a laser that can directly produce the required output power, a MOPA configuration may achieve a required performance more easily (e.g., in terms of linewidth, wavelength tuning range, beam quality, or pulse duration) in cases where the required output power is high. In addition, a MOPA configuration may be used to modulate a low-power seed laser or may use an optical modulator between the seed laser (e.g., the oscillator120) and the power amplifier140rather than modulating a high-power device directly, may use an existing laser and an existing amplifier (or amplifier chain) and thereby obviate a need to develop a new laser with a higher output power, and/or may use an amplifier that has lower optical intensities compared with the intracavity intensities in a laser.

However, power scaling a MOPA laser architecture to higher and higher powers is challenging. For example, the oscillator120in a MOPA laser architecture should be maintained as near to singlemode as possible for stability, which is challenging because converting pump light to signal light in the oscillator120is limited by stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), or other nonlinear effects that pose serious hurdles to power scaling a singlemode laser. In particular, SRS is a nonlinear optical effect where energy from an optical beam is converted to a longer wavelength via vibrational and/or rotational modes or phonons being excited in the molecules of a glass medium. While this process may be useful for certain applications (e.g., to turn an optical fiber into a Raman amplifier or a tunable Raman laser), SRS is undesirable for multi-kW continuous wave (CW) industrial fiber lasers or quasi-CW kW fiber lasers used in the cutting and welding industries. For example, in industrial applications, SRS may transfer energy from one wavelength to another wavelength and/or limit the power that can propagate without unwanted loss and/or heating, which may negatively impact the industrial processes and/or cause damage to equipment. As power levels for industrial kW fiber lasers continue to increase, SRS, SBS, and other nonlinear effects become more problematic, and a need arises for techniques to suppress the nonlinear effects.

In some cases, power scaling in a MOPA laser architecture may be achieved by increasing a core diameter and/or a numerical aperture (NA) of the laser. However, increasing the core diameter and/or NA of the laser sacrifices brightness as a tradeoff for the increase in power. In other cases, power scaling in a MOPA laser architecture may be achieved by combining multiple lasers with an external signal combiner. For example, referring toFIG.1B, example100B depicts a MOPA architecture that includes multiple singlemode lasers with an external signal combiner150, where singlemode outputs from n singlemode lasers (where n is greater than one) are provided to an external signal combiner150, which then combines the n singlemode outputs to provide a multimode output via a delivery cable160. For example, the singlemode outputs from n singlemode lasers may be provided to the external signal combiner150via n separate fibers, which may then be twisted, tapered, fused, and spliced to the delivery cable160. Alternatively, the external signal combiner150may include a glass enclosure (e.g., a capillary tube), in which case the n separate fibers are input to the external signal combiner150, tapered within a taper region of the glass enclosure, and spliced to the delivery cable160. However, although an external signal combiner150can be used in this way to combine multiple lasers and scale power, the external signal combiner150increases cost and sacrifices brightness.

Some implementations described herein relate to a laser architecture that includes a multicore fiber that may support multiple independent singlemode lasers and a high-brightness signal combiner, which may be integrated with the multicore fiber to improve power scaling performance in a MOPA laser architecture or other monolithic fiber laser. For example, as described above, the master oscillator in a MOPA laser architecture is most stable when operating in a regime that is singlemode or near singlemode. Otherwise, transverse modal instabilities can arise when oscillator dimensions are not well-controlled. Furthermore, FBGs and/or other devices that are used as HR and/or OC reflectors are generally easier to write and/or measure when the reflector devices are singlemode or near singlemode. Accordingly, integrating a multicore fiber that supports several independent singlemode lasers with a high-brightness signal combiner may be used to generate a multimode output in a manner that may increase signal power, optimize brightness, minimize cost, and/or leverage an existing laser architecture.

FIG.2Ais a diagram illustrating examples200-1,200-2, and200-3of a multicore laser architecture that includes an integrated high-brightness signal combiner. In example200-1, the multicore laser architecture is an end-pumped MOPA architecture that includes a multicore oscillator220coupled to a multicore power amplifier fiber240, which is integrated with a signal combiner250that provides a multimode output via a delivery fiber260. Additionally, or alternatively, in example200-2, the multicore laser architecture is a MOPA architecture configured with a bi-directional pump, including a first pump laser source210-1that comprises a first set of diodes212-1and a first combiner214-1provided at an input end of the multicore oscillator220(e.g., to provide first pump light in a light propagation direction) and a second pump laser source210-2that comprises a second set of diodes212-2and a second combiner214-2provided at an output end of the multicore power amplifier fiber240(e.g., to provide second pump light in a direction opposite from the light propagation direction). As further shown inFIG.2A, in example200-3, the multicore laser architecture is an end-pumped multi-stage amplifier. In this case, in addition to the diodes212that are configured to provide pump light, the pump laser source210includes a seed diode216that may provide signal light and a pump-signal combiner218that may combine the pump light provided by the diodes212with the signal light provided by the seed diode216to generate input light to a multicore laser architecture that includes a multicore pre-amplifier fiber240-1and a second stage multicore power amplifier fiber240-2integrated with a high-brightness signal combiner250. As shown by example200-3, using the external seed diode216in a PSC architecture may eliminate a need for the first reflector222used as the HR at the input end of the active fiber and/or the second reflector226used as the OC at the output end of the active fiber.

Accordingly, as described herein, the multicore laser architectures shown inFIG.2Amay each include a multi-kilowatt (kW) pump source210comprising a plurality of laser diodes212and a combiner214or a pump-signal combiner218. In some implementations, the multi-kW pump source210may define a pump laser source configured to generate input light to the multicore laser architecture. As further shown, the multicore laser architecture may include a plurality of HR reflectors (e.g., a plurality of first FBGs)222provided at an input end of a multicore active fiber224(e.g., at an interface between the combiner214/218and the multicore active fiber224), or a passive multicore fiber matched to the multicore active fiber224. Each of the plurality of HR reflectors222is associated with one core of the multicore active fiber224. Further, a plurality of OC reflectors226(e.g., a plurality of second FBGs) may be provided at an output end of the multicore active fiber224(e.g., at an interface between the output end of the multicore active fiber224and a passive fiber coupling into the multicore power amplifier fiber240). Each of the plurality of OC reflectors226may be associated with one core of the multicore active fiber224. Further, an HR reflector222associated with one core of the multicore active fiber224is associated with the corresponding OC reflector226of the same core of the multicore active fiber224. Further, the multicore laser architecture may include a multicore oscillator220or a multicore pre-amplifier fiber240-1that may include the plurality of HR reflectors222, the plurality of OC reflectors226, and the multicore active fiber224.

In examples200-1and200-2, each oscillator of the multicore oscillators220may include one of the plurality of HR reflectors222, one core of the multicore active fiber224, and one of the plurality of OC reflectors226. The input light may be converted into signal light by the multicore oscillator220, and the signal light may then be amplified to a higher power level by the power amplifier fiber240. For example, in some implementations, the multicore active fiber224may include multiple cores, with reflectors222/226written into the different cores of the multicore oscillator220. In some implementations, the periods of the reflectors222/226may be different from one another, where varying the periods of the reflectors222/226allows different wavelengths to oscillate in each oscillator (e.g., in each core of the multicore oscillator active fiber224). Alternatively, in some implementations, the periods of the reflectors222/226may match one another to allow a specific wavelength to oscillate in each oscillator, or one of the reflectors222/226may be a narrow grating that overlaps with a broader grating. Alternatively, in some implementations, a single grating may be written across the entire fiber using a femtosecond laser or the like. In a configuration where a single grating is written across the entire fiber, the entire fiber may be exposed at once to write the same grating across all cores. In any case, by providing the multicore oscillator220with multiple independent cores, pump-to-signal conversion may be effectively multiplied without significantly increasing SRS or other nonlinear effects. For example, a single core in the oscillator may generally generate a given amount of power (e.g., based on conversion of pump power in that core), whereby doubling, tripling, quadrupling, or otherwise multiplying the number of independently operating cores in the multicore oscillator220may effectively multiply the pump-to-signal conversion that occurs within the multicore oscillator220. Furthermore, in example200-3, the multicore pre-amplifier fiber240-1and/or the multicore power amplifier fiber240-2may comprise a multicore fiber having similar properties as the multicore oscillator220described herein.

In some implementations, in order to maximize stability, a multicore fiber included in a multicore laser architecture may be configured to operate in a singlemode regime. For example, as described herein, the multicore fiber may be configured to be singlemode (e.g., designed to reflect only a singlemode of light), near singlemode (e.g., within a threshold of singlemode), a single transverse mode and a single polarization mode, a single transverse mode but not a single polarization mode, or the like. In any case, by operating the multicore fiber in a singlemode regime, the multicore laser architecture may avoid transverse modal instabilities that could otherwise arise if the parameters of the multicore active fiber or any other signal-carrying fiber within the multicore fiber were not well-controlled. Furthermore, fabricating multiple independent singlemode or near singlemode cores within one active fiber may simplify techniques used to write and/or measure the FBGs or other reflectors configured to operate as the HR reflector and/or the OC reflector. Accordingly, as described herein, the multicore laser architectures shown inFIG.2Ainclude one or more multicore fibers with multiple independent singlemode or near singlemode cores that may be fabricated within one fiber, which increases pump-to-signal conversion, reduces SRS gain, reduces photo darkening, and maintains stability coming out of the multicore fiber. In this way, a signal power from the multicore fiber may be increased, which reduces inversion and/or heating in the subsequent stage(s) that include the power amplifier(s) fiber240. Additionally, or alternatively, more than one mode may be carried in one or more cores of the multicore fiber. For example, in some implementations, the FBGs or other reflectors configured to operate as the HR reflector and/or the OC reflector may be used to achieve singlemode lasing in a multimode fiber, because the higher order modes would have different resonance wavelengths (e.g., starting with a slightly multimoded fiber, the FBG(s) could be used to give near singlemode performance, which may ease manufacturing tolerances).

In some implementations, the independent active fiber cores that are included within the multicore oscillator220can have different HR reflectors222and/or OC reflectors226that are fabricated to reflect different wavelengths prior to launching into the multicore power amplifier fiber240(e.g., each core of the active fiber may have a pair of FBGs222/226or other devices that are fabricated for a specific wavelength and used as the HR reflector222and the OC reflector226for a corresponding core, whereby each oscillator may function as an independent laser with different wavelength(s) within the multicore active fiber224). Additionally, or alternatively, rather than fabricating both the HR reflector222and the OC reflector226for a specific wavelength, only one reflector (e.g., the HR reflector222) may be fabricated for each wavelength while the other reflector (e.g., the OC reflector226) may be a wide-bandwidth grating. In this way, undesirable coherence effects may be suppressed when transitioning to the stages associated with the multicore power amplifier fiber240, which may be addressed by having a passive fiber between the multicore oscillator220and the multicore power amplifier fiber240in examples200-1,200-2or between the multicore pre-amplifier fiber240-1and the second stage multicore power amplifier in example200-3. In some implementations, brightness between the multicore fiber220/240-1and the multicore power amplifier fiber240/240-2can be increased by adding a mode-matched passive fiber (e.g., a quarter-pitch graded index fiber or an equivalent step index fiber). The multicore fiber can also be used with different pump wavelengths within the same pump combiner to enable more efficient conversion within the oscillator cores and later amplifier stage(s).

Referring toFIG.2B, in a first configuration, a multicore oscillator220-1that may be used in a multicore laser architecture with an integrated high-brightness signal combiner comprises a multicore active fiber224that has a plurality of gratings (e.g., HR gratings222and OC gratings226) fabricated directly into the independent active cores of the multicore active fiber224. For clarity, only one of the plurality of HR gratings222and only one of the plurality of OC226gratings are shown inFIG.2B, although in some implementations a plurality of gratings may be written into a respective plurality of cores. In this case, the single active fiber224includes multiple doped cores that, in cooperation with the associated FBGs222/226, act as independent oscillators and are separated from each other to satisfy a threshold level of crosstalk. For example, in some implementations, the multiple doped cores may be separated from each other to avoid or suppress crosstalk between the cores or to minimize a level of crosstalk between the cores. Alternatively, in some implementations, the separation between the cores may be reduced to enable a threshold level of crosstalk between the cores (e.g., in applications where crosstalk is desired, such as coherent beam combining, in which case the separation between the cores may be controlled to achieve a fixed phase relationship between the cores).

In the first configuration of the multicore oscillator220-1, one or more FBGs used as the HR reflector222at the input end of the multicore oscillator220-1and/or one or more FBGs used as the OC reflector226at the output end of the multicore oscillator220-1may be written directly into each core on both sides of the active fiber224with a femtosecond (FS) laser or other means. For example, in some implementations, a first FBG on the input end of the active fiber224may be configured to operate as the HR reflector222(e.g., with a reflectivity around 99%) and a second FBG on the output side of the active fiber224may be configured to operate as the OC reflector226(e.g., with a reflectivity around 10-20%). Alternatively,FIG.2Bdepicts a second configuration of the multicore oscillator220-2, which includes matched active and passive fibers. As described herein, matched active and passive fibers may generally have the same number of cores, the same relative positioning of cores within the fibers, and/or similar mode sizes and numerical apertures (NAs). Furthermore, as described herein, matched active and passive fibers may be oriented so that respective cores in the fibers align when the fibers are spliced together or otherwise coupled. In the case of the second configuration for the multicore oscillator220-2, matched multicore passive fibers with the HR reflector222and the OC reflector226(e.g., respective FBGs) written in each core may be spliced to both ends of a matched multicore active fiber224. For example, reference numbers228-1and228-2depict respective splice points where the matched multicore passive fibers are spliced to both ends of the matched multicore active fiber224. Furthermore, similar to the first configuration for the multicore oscillator220-1, a first FBG on the input end of the active fiber224acts as the HR reflector222and a second FBG on the output end of the active fiber224acts as the OC reflector226.

As indicated above,FIGS.2A-2Bare provided as examples. Other examples may differ from what is described with regard toFIGS.2A-2B. The number and arrangement of devices shown inFIGS.2A-2Bare provided as examples. In practice, there may be additional devices, fewer devices, different devices, or differently arranged devices than those shown inFIGS.2A-2B. Furthermore, two or more devices shown inFIGS.2A-2Bmay be implemented within a single device, or a single device shown inFIGS.2A-2Bmay be implemented as multiple, distributed devices. Additionally, or alternatively, a set of devices (e.g., one or more devices) shown inFIGS.2A-2Bmay perform one or more functions described as being performed by another set of devices shown inFIGS.2A-2B.

FIGS.3A-3Bare diagrams illustrating example cross-sections of a multicore active fiber that may be used in a multicore laser architecture that includes an integrated high-brightness signal combiner.FIGS.3A-3Bare diagrams illustrating example cross-sections300-1through300-6of a multicore active fiber that may be used in a multicore laser architecture with an integrated high-brightness signal combiner (e.g., in the multicore laser architectures shown inFIG.2A). For example, as shown inFIGS.3A-3B, the multicore active fiber may generally include an inner cladding320, an outer cladding330surrounding the inner cladding320, and multiple singlemode fiber cores310that are embedded in the inner cladding and used to convert pump light from a pump laser source into signal light that is launched into a power amplifier and/or to transmit combined pump and signal light into a power amplifier.

Referring toFIG.3A, cross-sections300-1,300-2, and300-3depict example configurations where the multicore active fiber includes two, three, or four identical cores310(e.g., the cores310have a uniform doping and a uniform core size) with a uniform separation from each other to avoid crosstalk between the cores310and/or to satisfy a threshold level of crosstalk between the cores310. However, the multicore active fiber may generally include N cores, where N is an integer greater than one (1).

Additionally, or alternatively, referring toFIG.3B, cross-sections300-4,300-5, and300-6depict example configurations where the multicore active fiber includes multiple cores310associated with different parameters. For example, as shown by cross-section300-4, the multicore active fiber may include multiple cores310with different core separations (e.g., a first core310may be separated from a second core310by a first distance and separated from a third core310by a second distance that is different from the first distance). Additionally, or alternatively, as shown by cross-section300-5, the multicore active fiber may include multiple cores310with different core sizes (e.g., different core diameters). Additionally, or alternatively, as shown by cross-section300-6, the multicore active fiber may include multiple cores310with a different doping in each core310(shown inFIG.3Bby variations in the fill patterns of the different cores310). In other examples (not explicitly illustrated), the multiple cores310may be twisted around a center axis of the active fiber, which is equivalent to bending of the cores310and may ensure that more singlemode signal is output by the multicore oscillator. In some implementations, the period of the twisting may be adjusted, which causes the bending diameter to change. Furthermore, in some implementations, twisting the cores310may result in a more uniform pump absorption across the different cores310.

As indicated above,FIGS.3A-3Bare provided as examples. Other examples may differ from what is described with regard toFIGS.3A-3B. The number and arrangement of devices shown inFIGS.3A-3Bare provided as examples. In practice, there may be additional devices, fewer devices, different devices, or differently arranged devices than those shown inFIGS.3A-3B. Furthermore, two or more devices shown inFIGS.3A-3Bmay be implemented within a single device, or a single device shown inFIGS.3A-3Bmay be implemented as multiple, distributed devices. Additionally, or alternatively, a set of devices (e.g., one or more devices) shown inFIGS.3A-3Bmay perform one or more functions described as being performed by another set of devices shown inFIGS.3A-3B.

FIGS.4A-4Care diagrams illustrating an example of a MOPA laser architecture400and example interfaces to couple a multicore oscillator420and a power amplifier440in the MOPA laser architecture. For example, as shown inFIG.4A, the MOPA laser architecture400may include a multi-kW pump410that comprises a plurality of laser diodes412and a combiner414, which may collectively define a pump laser source configured to generate input light to the MOPA laser architecture400. As further shown, the MOPA laser architecture may include a plurality of HR reflectors422(e.g., a plurality of first FBGs) provided at an input end of the multicore oscillator420(e.g., at an interface between the combiner414and an active fiber424), with one HR reflector422provided at the input end of each core of the active fiber424. As further shown, the MOPA laser architecture400may include a plurality of OC reflectors426(e.g., a plurality of second FBGs) at an output end of the multicore oscillator420(e.g., at an interface between the output end of the active fiber424and a passive or active output fiber430coupling into the power amplifier440), with one OC reflector426provided at the output end of each core of the active fiber424.

In some implementations, the output from the multicore oscillator420is spliced to a multicore power amplifier440that is matched to the multicore oscillator420for a final amplification stage. In general, as described herein, individual cores in the multicore oscillator420and the multicore power amplifier440each function as independent singlemode lasers. Furthermore, in some implementations, twisting may be applied to the multicore fiber used for the multicore oscillator420and/or the multicore power amplifier440to help with uniform pump absorption across the various cores. In some implementations, the multicore fiber used for the multicore power amplifier440may include multiple symmetric cores, multiple concentric cores, multiple offset cores, or other suitable core configurations that are matched to the output from the multicore oscillator220.

For example, referring toFIG.4A, reference number450-1depicts an example cross-section of the output end of the multicore oscillator420, which includes two active cores452that are embedded in a fused silica inner cladding454surrounded by a fluorine (F)-doped outer cladding456. As further shown, reference number460-1depicts an example cross-section of the input end of the multicore power amplifier440that is spliced to or integrated with the output of the multicore oscillator420. As shown, the multicore power amplifier440includes two active cores462matched to the two active cores452of the multicore oscillator420, a fused silica inner cladding464that surrounds the two active cores462and is matched to the fused silica inner cladding454of the multicore oscillator420, and an F-doped outer cladding466that surrounds the fused silica inner cladding464and is matched to the F-doped outer cladding456of the multicore oscillator420.

In another example, referring toFIG.4B, reference number450-2depicts an example cross-section in which the multicore oscillator420includes a central active core452-1and an offset active core452-2in a fused silica inner cladding454surrounded by an F-doped outer cladding456. As shown by reference number460-2, the cross-section of the multicore oscillator420is matched to a dual concentric core output fiber (e.g., a passive or active output fiber) that includes an inner core462-1matched to the central core452-1of the multicore oscillator420, an outer core462-2matched to the offset core452-2of the multicore oscillator420, a fused silica inner cladding464surrounding the dual concentric cores462-1,462-2, and an F-doped outer cladding466surrounding the fused silica inner cladding464.

In another example, referring toFIG.4C, reference number450-3depicts an example cross-section in which the multicore oscillator420has the same configuration as shown inFIG.4B. As shown by reference number460-3, the cross-section of the multicore oscillator420is matched to a dual offset core output fiber (e.g., a passive or active output fiber) that includes a central core462-1matched to the central core452-1of the multicore oscillator420, an offset core462-2matched to the offset core452-2of the multicore oscillator420, a fused silica inner cladding464surrounding the dual offset cores462-1,462-2, and an F-doped outer cladding466surrounding the fused silica inner cladding.

In other examples (not explicitly illustrated), the multicore power amplifier fiber440may have a confined doping, which is similar to a tapered core and may better confine the mode after the signal from the multicore oscillator420is launched into the multicore power amplifier fiber440. Additionally, or alternatively, the active fiber424of the multicore oscillator420may include a single center offset core452that may be twisted, where the mode in the single center offset core can be well-managed by controlling a period of the twisting.

As indicated above,FIGS.4A-4Care provided as examples. Other examples may differ from what is described with regard toFIGS.4A-4C. The number and arrangement of devices shown inFIGS.4A-4Care provided as examples. In practice, there may be additional devices, fewer devices, different devices, or differently arranged devices than those shown inFIGS.4A-4C. Furthermore, two or more devices shown inFIGS.4A-4Cmay be implemented within a single device, or a single device shown inFIGS.4A-4Cmay be implemented as multiple, distributed devices. Additionally, or alternatively, a set of devices (e.g., one or more devices) shown inFIGS.4A-4Cmay perform one or more functions described as being performed by another set of devices shown inFIGS.4A-4C.

FIGS.5A-5Care diagrams illustrating examples500-1,500-2, and500-3of a multicore laser architecture with an integrated high-brightness signal combiner. For example, as shown inFIGS.5A-5C, examples500-1,500-2, and500-3each include a MOPA laser architecture that comprises a multi-KW pump510, which includes a plurality of laser diodes512and a combiner514that may define a pump laser source configured to generate input light to the MOPA laser architecture. Alternatively, in some implementations, the multi-kW pump510may include a seed diode (not shown) and a pump-signal combiner514that may generate the input light to the multicore laser architecture. In some implementations, as further shown, the multicore laser architecture may include a multicore oscillator520that comprises a plurality of HR reflectors522(e.g., a plurality of first FBGs) provided at an input end of the multicore oscillator520(e.g., at an interface between the combiner514and an active fiber524of the multicore oscillator520) and a plurality of OC reflectors526(e.g., a plurality of second FBGs) at an output end of the multicore oscillator520(e.g., at an interface between the output end of the active fiber524and a passive or active output fiber530) and a multicore power amplifier540. As further shown in

FIGS.5A-5C, an output from the multicore power amplifier540may be provided to an integrated signal combiner550that combines singlemode inputs received from the various cores of the multicore power amplifier540into a multimode output that is then provided via a delivery fiber560.

For example, referring toFIG.5A, example500-1depicts a multicore laser architecture that may be integrated with a signal combiner550with a passive taper. For example, as shown inFIG.5A, the output from a multicore laser with symmetric cores may be tapered adiabatically with a roughly 3× taper ratio, and then spliced to a multimode delivery fiber (or delivery cable)560. For example, reference number552depicts a cross-section of the integrated signal combiner550at an input interface where the singlemode inputs are received from the multicore laser architecture, reference number554depicts a cross-section of the integrated signal combiner550at a splice point570where the signal combiner550is spliced to the multimode delivery fiber560, and reference number562depicts a cross-section of the multimode delivery fiber560. Accordingly, in example500-1, modes from the individual cores of the multicore laser architecture may expand adiabatically into the cladding as the fiber is tapered. In some implementations, the taper ratio may be configured to be sufficiently large to ensure that modes emerge fully out of the cores in order to optimize brightness. Otherwise, in cases where the modes from the individual cores are not well-controlled, the modes may couple into several modes in the multimode delivery fiber560, which may result in decreased brightness.

Furthermore, in some implementations, a surface treatment may be applied to the integrated signal combiner550, to cause the integrated signal combiner550to have a hydrophobic surface coating. For example, in some implementations, a hexamethyldisilazane (HMDS) (H3C)3Si chemical treatment layer may be applied to the surface of the signal combiner550, which may result in a changed chemistry of the surface of the signal combiner550. In this case, hydroxyl (OH) groups (silanol terminations) on a surface of the signal combiner550may be reacted with (methyl groups of) the HMDS to form a monolayer protective coating (e.g., an HMDS layer) on the signal combiner550. In other words, rather than a silica-based optical fiber (or other type of optical fiber or optical component) with a surface layer of oxygen molecules, each having a hydrogen molecule (e.g., silanol groups), the signal combiner550includes a surface layer of oxygen molecules, each having an HMDS group. The exposed HMDS groups form a hydrophobic surface, thereby preventing or reducing atmospheric water molecule based deposition surface contaminants on the signal combiner550and/or microcrack propagation via hydrolysis reaction. In this way, the use of an HMDS treatment (or another type of treatment) can reduce a need to provide a recoating or housing for the signal combiner550, thereby reducing manufacturing complexity and/or enabling further miniaturization.

Additionally, or alternatively, as shown inFIG.5B, and by example500-2, the signal combiner550may comprise a quarter-pitch graded index (GI) fiber. For example, as shown inFIG.5B, the output from a multicore laser architecture may be spliced to a matched piece of graded index fiber with a quarter-pitch length at a first splice point570-1, and the graded index fiber may then be spliced to the multimode delivery fiber (or delivery cable)560at a second splice point570-2. For example, inFIG.5B, reference number552depicts a cross-section of the graded index fiber at the first splice point570-1where the singlemode inputs are received from the multicore laser architecture, reference number554depicts a cross-section of the graded index fiber at a midpoint of the quarter-pitch length of the graded index fiber, and reference number556depicts a cross section of the graded index fiber at the second splice point570-2. In some implementations, relative to example500-1inFIG.5A, an additional splice570in the fiber assembly could potentially introduce more loss. Furthermore, in a similar manner as described above with reference toFIG.5A, the modes from the individual cores may couple into several modes in the multimode delivery fiber560in cases where the modes from the individual cores are not well-controlled, which may result in decreased brightness.

Additionally, or alternatively, referring toFIG.5C, example500-3depicts a multicore laser architecture that may be integrated with a signal combiner550with an active taper. For example, inFIG.5C, the multicore laser architecture and the multimode delivery fiber560are well-matched, in that the number of cores in the multicore laser architecture matches the number of supported modes in the multimode delivery fiber560, and each core in the multicore in the multicore laser architecture has a core size and an NA that matches a corresponding mode in the multimode delivery fiber560at an interface between the signal combiner550and the multimode delivery fiber560. For example, in a multicore fiber with four (4) cores, the4cores are matched to a delivery fiber560that supports 4 modes (e.g., LP01, LP11X, LP11Y, and LP02 modes). In some implementations, the core sizes and the NAs in the multicore laser architecture are fixed in such a way that when a precise taper is applied to the multicore fiber to form the integrated signal combiner550, each mode from the multicore fiber matches an exact mode in the multimode delivery fiber560. For example, in some implementations, a first tapered core may directly match an LP01 mode in the multimode delivery fiber560, a second tapered core may directly match an LP11X mode in the multimode delivery fiber560, a third tapered core may directly match an LP11Y mode in the multimode delivery fiber560, and a fourth tapered core may directly match an LP02 mode in the multimode delivery fiber560. Furthermore, in a similar manner as described above with reference toFIG.5A, a surface treatment may be applied to the integrated signal combiner550, to cause the integrated signal combiner550to have a hydrophobic surface coating to prevent or reduce atmospheric water molecule based deposition surface contaminants on the signal combiner550and/or microcrack propagation via hydrolysis reaction.

As indicated above,FIGS.5A-5Care provided as examples. Other examples may differ from what is described with regard toFIGS.5A-5C. The number and arrangement of devices shown inFIGS.5A-5Care provided as examples. In practice, there may be additional devices, fewer devices, different devices, or differently arranged devices than those shown inFIGS.5A-5C. Furthermore, two or more devices shown inFIGS.5A-5Cmay be implemented within a single device, or a single device shown inFIGS.5A-5Cmay be implemented as multiple, distributed devices. Additionally, or alternatively, a set of devices (e.g., one or more devices) shown inFIGS.5A-5Cmay perform one or more functions described as being performed by another set of devices shown inFIGS.5A-5C.

FIG.6is a flowchart of an example process600for operating an optical system that includes a multicore laser architecture with an integrated high-brightness signal combiner. In some implementations, one or more process blocks ofFIG.6are performed by a signal combiner (e.g., signal combiner250, signal combiner550, or the like).

As shown inFIG.6, process600may include receiving multiple independent singlemode laser inputs from a multicore fiber laser that comprises multiple cores that are each configured to support an independent singlemode laser, of the multiple independent singlemode laser inputs (block610). For example, the signal combiner250, the signal combiner550, or the like may receive multiple independent singlemode laser inputs from a multicore fiber laser200-1,200-2, and/or200-3, a MOPA laser architecture400, or the like that comprises multiple cores that are each configured to support an independent singlemode laser, of the multiple independent singlemode laser inputs, as described above.

As further shown inFIG.6, process600may include combining the multiple independent singlemode laser inputs into a multimode output (block620). For example, the signal combiner250, the signal combiner550, or the like may combine the multiple independent singlemode laser inputs into a multimode output, as described above.

As further shown inFIG.6, process600may include providing the multimode output to a delivery fiber comprising a single core configured to support multiple modes (block630). For example, the signal combiner250, the signal combiner550, or the like may provide the multimode output to a delivery fiber260,560, or the like comprising a single core configured to support multiple modes, as described above.

In a first implementation, the signal combiner comprises multiple symmetric cores, to receive the multiple independent singlemode laser inputs from the multicore fiber laser, that taper adiabatically to a splice point with the delivery fiber.

In a second implementation, alone or in combination with the first implementation, the signal combiner comprises a graded index fiber with a quarter-pitch length that is spliced to the multicore fiber laser at a first splice point and spliced to the delivery fiber at a second splice point.

In a third implementation, alone or in combination with one or more of the first and second implementations, a quantity of the multiple independent singlemode laser inputs received at the signal combiner from the multicore fiber laser equals a quantity of the multiple modes supported in the delivery fiber.

In a fourth implementation, alone or in combination with one or more of the first through third implementations, the multiple cores of the multicore fiber laser have respective core sizes and numerical apertures that match corresponding modes in the delivery fiber at a splice point between the signal combiner and the delivery fiber.

In a fifth implementation, alone or in combination with one or more of the first through fourth implementations, the multicore fiber laser is an end-pumped MOPA laser with a pump laser source and a combiner coupled to an input end of a multicore oscillator and a multicore power amplifier.

In a sixth implementation, alone or in combination with one or more of the first through fifth implementations, the multicore fiber laser is a MOPA laser with a bi-directional pump that comprises a first pump laser source and a first combiner coupled to an input end of a multicore oscillator and a multicore power amplifier, and a second pump laser source and a second combiner coupled to an output end of the multicore oscillator and the multicore power amplifier, wherein the first pump laser source and the second pump laser source are configured to generate pump light that propagates in opposite directions.

In a seventh implementation, alone or in combination with one or more of the first through sixth implementations, the multicore fiber laser is an end-pumped multi-state amplifier that comprises a pump laser source, a seed laser source, and a combiner coupled to an input end of a multicore pre-amplifier and a multicore power amplifier.

AlthoughFIG.6shows example blocks of process600, in some implementations, process600includes additional blocks, fewer blocks, different blocks, or differently arranged blocks than those depicted inFIG.6. Additionally, or alternatively, two or more of the blocks of process600may be performed in parallel.