Slanted FBG for SRS suppression

An example apparatus includes an optical fiber including a core and cladding, the core being situated to propagate an optical beam along a propagation axis associated with the core, and at least one fiber Bragg grating (FBG) situated in the core of the optical fiber, the fiber Bragg grating including a plurality of periodically spaced grating portions situated with respect to the propagation axis so that light associated with Raman scattering is directed out of the core so as to reduce the generation of optical gain associated with stimulated Raman scattering (SRS).

FIELD

The field of the disclosed technology is generally fiber lasers and fiber amplifiers that can generate Stimulated Raman Scattering.

BACKGROUND

The maximum power of a CW or pulsed fiber source is ultimately limited by the onset of nonlinear processes in the optical fiber, such as Stimulated Raman Scattering (SRS), Stimulated Brillouin Scattering (SBS), self-phase modulation (SPM), four-wave mixing (4WM), etc. Such non-linear processes generally set a power ceiling for conventional continuous-wave lasers producing multiple kW of output power and pulsed laser sources in beams having high beam quality (e.g., single-mode, few-mode). As optical powers increase, Stimulated Raman scattering (SRS) is one of the most significant nonlinear processes establishing this ceiling. Suppression of SRS is thus desirable for power scaling of fiber lasers and amplifiers.

SUMMARY

In some examples of the disclosed technology, apparatuses can comprise an optical fiber that includes a core and cladding, the core being situated to propagate an optical beam along a propagation axis associated with the core; and at least one fiber Bragg grating (FBG) situated in the core of the optical fiber, the fiber Bragg grating including a plurality of periodically spaced grating portions situated with respect to the propagation axis so that light associated with Raman scattering is directed out of the core so as to reduce the generation of optical gain associated with stimulated Raman scattering (SRS).

In additional examples of the disclosed technology, apparatuses can comprise a gain fiber that includes an actively doped core defining a propagation axis and a pump cladding surrounding the actively doped core, the gain fiber being situated to generate a signal beam; one or more pump sources optically coupled to the gain fiber to provide pump light for generation of the signal beam; a high reflector fiber Bragg grating (FBG) optically coupled to an end of the gain fiber active core and situated to reflect the signal beam propagating in the active core of the gain fiber; a partial reflector FBG optically coupled to an opposite end of the gain fiber active core and situated to partially reflect the signal beam and to transmit an output beam; and at least one slanted stimulated Raman scattering (SRS) FBG situated in the gain fiber so as to direct light associated with Raman scattering out of the gain fiber so as to reduce the generation of optical gain associated with SRS in the gain fiber.

In further examples of the disclosed technology, apparatuses can comprise an optical fiber that includes a core and cladding, the core being situated to propagate an optical beam along a propagation axis associated with the core; and at least one spectrally selective component optically coupled to the core of the optical fiber, the spectrally selective component including at least one optical redirecting portion situated at a non-perpendicular angle with respect to the propagation axis so that light associated with Raman scattering is directed out of the core so as to reduce the generation of optical gain associated with stimulated Raman scattering (SRS).

DETAILED DESCRIPTION

In some examples, values, procedures, or apparatus' are referred to as “lowest”, “best”, “minimum,” or the like. It will be appreciated that such descriptions are intended to indicate that a selection among many used functional alternatives can be made, and such selections need not be better, smaller, or otherwise preferable to other selections. Examples may be described with reference to directions indicated as “above,” “below,” “upper,” “lower,” and the like. These terms are used for convenient description, but do not imply any particular spatial orientation unless the context clearly indicates a particular orientation.

As used herein, optical radiation refers to electromagnetic radiation at wavelengths of between about 100 nm and 10 μm, and typically between about 500 nm and 2 μm. Examples based on available laser diode sources and optical fibers generally are associated with wavelengths of between about 800 nm and 1700 nm. In some examples, propagating optical radiation is referred to as one or more beams having diameters, asymmetric fast and slow axes, beam cross-sectional areas, and beam divergences that can depend on beam wavelength and the optical systems used for beam shaping. For convenience, optical radiation is referred to as light in some examples, and need not be at visible wavelengths.

Representative embodiments are described with reference to optical fibers, but other types of optical waveguides can be used having square, rectangular, polygonal, oval, elliptical or other cross-sections. Optical fibers are typically formed of silica (glass) that is doped (or undoped) so as to provide predetermined refractive indices or refractive index differences. In some, examples, fibers or other waveguides are made of other materials such as fluorozirconates, fluoroaluminates, fluoride or phosphate glasses, chalcogenide glasses, or crystalline materials such as sapphire, depending on wavelengths of interest. Refractive indices of silica and fluoride glasses are typically about 1.5, but refractive indices of other materials such as chalcogenides can be 3 or more. In still other examples, optical fibers can be formed in part of plastics. In typical examples, a doped waveguide core such as a fiber core provides optical gain in response to pumping, and core and claddings are approximately concentric. In other examples, one or more of the core and claddings are decentered, and in some examples, core and cladding orientation and/or displacement vary along a waveguide length.

As used herein, numerical aperture (NA) refers to a largest angle of incidence with respect to a propagation axis defined by an optical waveguide for which propagating optical radiation is substantially confined. In optical fibers, fiber cores and fiber claddings can have associated NAs, typically defined by refractive index differences between a core and cladding layer, or adjacent cladding layers, respectively. While optical radiation propagating at such NAs is generally well confined, associated electromagnetic fields such as evanescent fields typically extend into an adjacent cladding layer. In some examples, a core NA is associated with a core/inner cladding refractive index, and a cladding NA is associated with an inner cladding/outer cladding refractive index difference. For an optical fiber having a core refractive index ncoreand a cladding index nclad, a fiber core NA is NA=√{square root over (ncore2−nclad2)}. For an optical fiber with an inner core and an outer core adjacent the inner core, a cladding NA is NA=√{square root over (ninner2−nouter2)}, wherein ninnerand nouterare refractive indices of the inner cladding and the outer cladding, respectively. Optical beams as discussed above can also be referred to as having a beam NA which is associated with a beam angular radius. While multi-core step index fibers are described below, gradient index designs can also be used.

In the examples disclosed herein, a waveguide core such as an optical fiber core is doped with a rare earth element such as Nd, Yb, Ho, Er, or other active dopants or combinations thereof. Such actively doped cores can provide optical gain in response to optical or other pumping. As disclosed below, waveguides having such active dopants can be used to form optical amplifiers, or, if provided with suitable optical feedback such as reflective layers, mirrors, Bragg gratings, or other feedback mechanisms, such waveguides can generate laser emissions. Optical pump radiation can be arranged to co-propagate and/or counter-propagate in the waveguide with respect to a propagation direction of an emitted laser beam or an amplified beam.

The term brightness is used herein to refer to optical beam power per unit area per solid angle. In some examples, optical beam power is provided with one or more laser diodes that produce beams whose solid angles are proportional to beam wavelength and beam area. Selection of beam area and beam solid angle can produce pump beams that couple selected pump beam powers into one or more core or cladding layers of double, triple, or other multi-clad optical fibers.

Fiber Bragg gratings are described herein as one or more Bragg grating that are “written” into optical fibers to produce a refractive index variation associated with a Bragg reflection. Fiber Bragg gratings can be written in optical fibers in a variety of ways to produce the corresponding refractive index variation, including during or after the fabrication of the optical fiber in which the FBG is to be written. The photosensitivity of an optical fiber generally allows the refractive index to be changed by an incident optical beam provided with suitable characteristics (wavelength, intensity, pulse duration, etc.) provided the fiber also has suitable composition characteristics. In typical examples, FBGs are written into lengths of active or passive optical fiber cores with pulsed lasers, such as excimer or femtosecond sources, situated to selectively irradiate the core in a spatially periodic pattern to produce a refractive index variation, or modulation, in the core corresponding to a desired reflectivity spectrum.

Stimulated Raman scattering (SRS) typically transfers light propagating in a fiber core from a signal wavelength to a longer wavelength, known as Stokes shifts, or shorter wavelengths, known as anti-Stokes shifts. As will be discussed further herein, introducing a wavelength-dependent distributed loss in a fiber allows SRS to be suppressed without causing loss of the signal beam, thereby preventing SRS buildup and allowing power scaling at the signal wavelength. Such a distributed loss can be provided by one or more fiber Bragg gratings (FBG) written at one or more locations along the fiber. The FBG is angled so as to reject light at the SRS wavelength from the fiber core. The rejected light can be directed out of the core and into the cladding or out of the cladding. Examples discussed herein can be implemented in fiber lasers or fiber amplifiers, including broadband continuous-wave systems, short-pulsed systems (e.g., less than 1 ns), quasi-continuous-wave systems, etc.

In some examples, FBGs can be written with the grating being tilted with respect to the fiber beam propagation axis. Design parameters for the FBGs include the grating angle, the length of the grating, the depth of the grating (e.g., the amplitude of the index modulation), whether the FBG is located along the entire fiber or at specific locations, and whether the FBG has a uniform period or is made non-uniform (e.g., to broaden its spectral bandwidth or otherwise influence its reflectivity). In some examples, such as with multimode fibers, multiple FBG patterns can be overwritten on the same location so as to address different modes, wavelength, polarizations, or other beam properties. FBGs written at different locations can also have different patterns. The one or more FBGs cause SRS light to be directed out of the fiber through conduction, dissipation, optical processes, or combinations thereof. In some examples, the FBG can be written along the entire length of the active optical fiber. In other examples, the FBG is written along only a portion of the active optical fiber. Various features of the examples herein can also be applied to other examples herein.

Referring toFIG. 1and corresponding exploded viewsFIGS. 1A-1D, a fiber laser100includes an active fiber102that includes a core104and cladding106surrounding the core104(as best seen inFIG. 1B). The core104is situated to propagate a signal beam108and the cladding106is situated to propagate a pump beam110. The core104of the active fiber102typically includes one or more active dopants that can be excited by the pump beam110as the pump beam110propagates through the core104to allow laser amplification of the signal beam108.

A passive fiber section112is fiber-spliced to one end of the active fiber102at a splice114. As best seen inFIG. 1A, the passive fiber section112includes passive core116and a cladding118surrounding the passive core116each having a similar diameter to the core104and cladding106of the active fiber102. In some examples, adjacent or adjoining fiber sections, such as the active fiber102and the passive fiber section112, can have cores and claddings of dissimilar diameter, geometry, or other fiber parameters.

The passive fiber section112also includes an HR FBG120written into the passive core116that is highly reflective at the wavelength (or range) of the signal beam108and that is transmissive at the wavelength (or range) of the pump beam110. A fiber-based pump combiner122is spliced to the passive fiber section112at a splice124and includes a plurality of pump sources126coupled to corresponding pump fiber inputs128. The pump sources126produce light at the wavelength of the pump beam110and are combined with the fiber-based pump combiner122for coupling into the cladding118of the passive fiber section112.

Another passive fiber section130is fiber-spliced to the other end of the active fiber102at a splice132. As best seen inFIG. 1C, the passive fiber section130includes a passive core134and cladding136that are generally aligned at the splice132with the core and cladding104,106of the active fiber102in order to receive the signal beam108with minimal loss. The passive fiber section130includes a PR FBG138written into the passive core134and that is partially reflective at the wavelength of the signal beam108so as to allow a portion of the signal beam108to propagate past the PR FBG138to form an output beam140.

The passive fiber section130also includes a SRS FBG142written into the passive core134with a selected refractive index pitch so that the SRS FBG142is highly reflective, or otherwise produces a scattering or optical redirecting effect, at a wavelength range of a first Stokes shift from the signal beam108that is associated with Raman scattering. During laser operation, the large power density of the signal beam108tends to surpass various nonlinear thresholds and to be associated with the generation of an SRS beam144that can cause various deleterious effects, including degradation of one or more components of the fiber laser100and ultimately failure of the fiber laser100. The SRS FBG142is situated in the passive core134to receive the SRS beam144as it is generated and to direct the SRS beam144out of the passive core134. In typical examples, the SRS FBG142includes one or more refractive index varied portions that are arranged at a non-perpendicular angle with respect to a propagation axis146of SRS beam144in the passive core134. In further examples, the SRS FBG142can be replaced with another spectrally selective component, such as a fused wavelength-division multiplexer type coupler or a micro-optic based spectral filter. Spectrally selective components can be situated in-line with the passive fiber section130and some examples can include controlled free-space propagation and in-spectral filtering of the free-space beam.

In the architecture of the fiber laser100shown inFIG. 1, a peak power density for the signal beam108typically occurs in proximity to the PR FBG138. Thus, situating the SRS FBG142adjacent to the PR FBG138towards the active fiber102allows the FBGs138,142to be written successively in the same passive fiber section130, thereby increasing manufacturing efficiency, and to increase attenuation of the SRS beam144as it is being generated in the fiber laser100. By inhibiting generation of the SRS beam144through selective attenuation in the oscillator, e.g., the core volume situated between the HR and PR FBGs120,138, of the fiber laser100, other components of the fiber laser100, such as the pump sources126, the fiber-based pump combiner122, and the active fiber102, can be protected from damage associated with SRS.

A delivery fiber148is spliced to the passive fiber section130at a splice150and provides a propagation path out of the fiber laser100for the output beam140. The SRS FBG142is also situated to receive a residual SRS beam152(shown inFIG. 1D) that is emitted from the delivery fiber148, reflected from a processing target (now shown), and coupled back into the delivery fiber148and passive core134. The SRS FBG142also directs the residual SRS beam152out of the passive core134to remove the residual SRS beam152from the fiber laser100. By removing the residual SRS beam152, additional SRS gain in the fiber laser100associated with the SRS beam144is reduced or eliminated, further protecting the fiber laser100and its components.

InFIG. 2a fiber laser200is shown in a master-oscillator power-amplifier architecture. The fiber laser200includes an active oscillator fiber202optically coupled with an optical splice204at a first end to a passive fiber section206and optically coupled with an optical splice208at a second end to another passive fiber section210. The active oscillator fiber202includes a core and cladding and generates a signal beam in the core. One or more fiber-coupled pump sources212is optically coupled to the passive fiber section206with an optical splice214. The fiber-coupled pump source212delivers a pump beam to the active oscillator fiber202so that the signal beam can be generated in the core of the active oscillator fiber202. The passive fiber section206includes an FBG216in the core of the passive fiber section206that is highly reflective at the wavelength of the signal beam so that the signal beam generated in the core of the active oscillator fiber202and propagating towards the FBG216is substantially reflected by the FBG216. The passive fiber section210includes an FBG218that is partially reflective at the wavelength of the signal beam to allow a portion of the signal beam to propagate past the FBG218. The passive fiber section210also includes an FBG220that includes refractive index variation features that generally form a non-perpendicular angle with respect to a propagation axis of the signal beam in the core of the passive fiber section206. The FBG220is situated to couple out an SRS beam that propagates in the core of the active oscillator fiber202and the passive fiber sections206,210as the SRS beam is being generated so as to reduce the destructive effects associated with Raman gain or other nonlinear effects.

A passive fiber section222is spliced at a splice224to an emitting end of the passive fiber section210and receives the portion of the signal beam allowed to propagate past the FBG218. The passive fiber section222includes an optical isolator225that is situated to extract amplified spontaneous emission (ASE) or other optical noise associated with the signal beam. The passive fiber section222is fiber spliced via splice226to a passive fiber input228of a pump combiner230that is all-glass. One or more fiber-coupled pump sources232are also coupled to the pump combiner230via fiber inputs234. A passive fiber section output236of the pump combiner230delivers the signal beam from the passive fiber section222and the pump beam from the pump source232to an active amplifier fiber238via an optical splice240. As the signal beam propagates in the core of the active amplifier fiber238, it increases in power to form an output beam242that is emitted from the fiber laser200. A passive fiber section244is optically coupled with an optical splice246to the active amplifier fiber238and includes an FBG248situated in the core of the passive fiber section244at a non-perpendicular angle with respect to the propagating signal beam. The FBG248couples out SRS light propagating in the core so as to inhibit the generation of SRS and to reduce the damaging effects associated with SRS. The passive fiber section244is also spliced at a splice250to a delivery fiber252that is situated to receive the output beam242with a reduced amount of SRS light.

FIG. 3shows an example of a fiber laser300that includes a plurality of SRS FBGs302written into a core of an active gain fiber304at selected positions along the length of the active gain fiber304. The fiber laser300can include components similar to fiber laser100, including an HR FBG306situated in a core of a passive fiber length308spliced to the active gain fiber304at a splice305, a PR FBG310situated in a core of a passive fiber length312spliced to the active gain fiber304at a splice307, a pump source314coupled to a pump fiber315that is coupled to the passive fiber length308with a splice316, and a delivery fiber318coupled to the passive fiber length312at a splice319. The HR and PR FBGs306,310form an oscillator with the gain medium provided by the core of the active gain fiber304. An output beam320is emitted from an end of a delivery fiber318that can be used for various material processes as well as for other lasers (such as for pumping another laser or combining into a more powerful beam).

As Raman scattering occurs that is associated with the amplification of a signal beam in the core of the active gain fiber, SRS can occur and increase in gain competing with the signal beam and transferring energy from the signal beam. The SRS FBGs302, which can generally be situated at a non-perpendicular angle with respect to the core of the active gain fiber304, are distributed along the length of active gain fiber304so as to introduce a distributed SRS loss that inhibits the generation of SRS along the length of the active gain fiber304. In general, the pitch of the refractive index variation associated with the SRS FBGs302situated in the active gain fiber304can be different from the pitch of an SRS FBG situated in a passive fiber section, due to dissimilarities in material composition between the active and passive core. The output beam320thus has a reduced SRS power content as compared with beams of conventional lasers.

InFIG. 4, another example of a fiber laser400is shown that is directed to suppression of SRS. The fiber laser400includes an active gain fiber402coupled to opposite passive fiber sections404,406, one or more pump sources408coupled to the passive fiber section404, and a delivery fiber410coupled to the passive fiber section406for receiving and delivering a high power output beam412. An FBG414is written into the core of the passive fiber section406at a chirped periodicity. For example, the period of the refractive index variation provided in the core by the FBG414varies along length of the core, such as with sequentially increasing distances. In some examples, the periodicity is varied to expand the reflectivity spectrum so as to extend over the wavelength (or range) associated with SRS of the output beam412. The FBG414is also angled with respect to the propagation path of the output beam412in order couple the SRS light out of the core of the fiber laser400.

FIG. 5shows an example of a fiber500with an active core that includes FBG portions502substantially distributed along the length of the fiber500. In various examples, the FBG portions502can be evenly spaced from one another as well with variable spacing. The FBG portions502introduce very low loss for pump light and signal light propagating in the core but introduce significant loss at Raman wavelengths associated with the signal light so that SRS is inhibited during active laser operation. Each FBG portion502can include one or more refractive index varied portions. The cumulative effect of the FBG portions502distributed along the fiber500acts to reduce the growth of SRS in a fiber laser in which the fiber500is situated. The FBG portions502can be similar to the plurality of FBGs302in the fiber laser300, though the FBG portions502generally include fewer refractive index varied portions than the FBGs302(e.g., a single dip and rise or rise and dip in refractive index may suffice) or the spacing between adjacent FBG portions502is closer, or both.

FIG. 6is a plot600depicting SRS gain in a fiber laser as a function of the output beam power of the fiber laser. Referring to line602, in a conventional fiber laser, SRS generally increases as the continuous-wave power of the fiber laser rises into the kW regime, leading to detrimental effects within the fiber laser and to the characteristics of the output beam of the fiber laser. Line604shows predicted SRS performance in accordance with examples of the disclosed technology. As the continuous-wave output beam power of the fiber laser increases and the threshold for SRS generation associated with the output beam is passed, SRS gain is maintained at a minimum with FBG-based loss components introduced into the fiber laser system inhibiting the generation of SRS.

FIG. 7shows a plot700of FBG reflectivity with respect to optical wavelength. In typical examples herein, the reflectivity characteristics of an FBG used to inhibit generation of SRS are selected to coincide with an SRS wavelength associated with an output beam. For example, a reflectivity spectrum702generally coincides with a center wavelength of a first Stokes shift from an output beam wavelength (e.g., 1080 nm) of a fiber laser. In additional examples, a reflectivity spectrum704is selected with a spectral offset706shifting the reflectivity spectrum704in relation to a desired reflectivity spectrum702. In different examples herein, SRS-related FBGs can be situated in proximity to heat loads of different magnitude resulting in different localized temperatures. For higher temperatures, one or more FBGs situated to introduce SRS loss can have a reflectivity spectrum that is offset so that as temperature of the FBG increases during normal fiber laser operation, the corresponding reflectivity spectrum shifts to coincide with an SRS wavelength. For example, spectral offsets can include between 0.01 and 0.1 nm, 0.1 nm and 1.0 nm, 1 nm and 10 nm, or greater. Separate SRS-related FBGs can be situated in a fiber laser and have different spectral offsets to correspond with the temperature associated with the region in which the FBGs are situated. Moreover, a plurality of SRS-related FBGs can have different offsets (or no offsets) to ensure that variation in temperature does not affect the ability to maintain SRS suppression.

In view of the many possible embodiments to which the principles of the disclosed technology may be applied, it should be recognized that the illustrated embodiments are only representative examples and should not be taken as limiting the scope of the disclosure. Alternatives specifically addressed in these sections are merely exemplary and do not constitute all possible alternatives to the embodiments described herein. For instance, various components of systems described herein may be combined in function and use. We therefore claim all that comes within the scope and spirit of the appended claims.