Patent Description:
The fiber laser industry continues to increase laser performance metrics, such as average power, pulse energy and peak power. Pulse energy and peak power are associated with the storage and extraction of energy in the fiber while mitigating nonlinear processes that can have adverse impacts on the temporal and spectral content of the output pulse. Stimulated Raman Scattering (SRS) light is the result of one such nonlinear process associated with vibrations of the fiber media (e.g., glass). SRS is typically an undesired byproduct of fiber laser and/or fiber amplifier signal light passing through the optical fibers that these systems comprise.

Generation of SRS light can reduce power in an intended signal output wavelength. SRS generation can also destabilize laser emission resulting in undesired output power fluctuations. SRS generation may also have detrimental effects on the spatial profile of laser system emission. SRS may also be re-introduced in laser and amplifier systems by reflections from objects internal to, or external to, the laser system, such as optics used to manipulate the laser or amplifier output, or the workpiece to which the laser light output is applied. Such reflections can also destabilize the laser emission. Once generated, a laser and/or amplifier of a fiber system may amplify SRS light to the point of causing catastrophic damage to components internal to the system (e.g., a fiber laser, or fiber amplifier). The SRS light may also be detrimental to components external to the fiber system because the external components may not be specified for the wavelength of the SRS light. This mismatch in wavelength between what is delivered versus what is expected can lead to undesirable performance at the workpiece or may cause an eye safety concern for the external system in which the fiber system was integrated. As such, it may be desirable to suppress SRS generation within a fiber system, remove SRS light from a fiber system, and/or otherwise mitigate one or more of the undesirable effects of SRS light.

<CIT> discusses use of a slanted Fiber Bragg Grating (FBG) for simulated Raman scattering 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).

<CIT> describes a FBG taking deleterious light out of a fiber core without reflecting it into the fiber core. It also allows the unhindered transmission of useful light at a wavelength outside of the spectral band covered by the deleterious light.

<NPL> reports on the effects of spectral shaping of the output coupler fiber Bragg grating (OC-FBG) in a Yb-doped fiber laser on the laser emission spectrum for the purpose of inhabiting stimulated Raman scattering (SRS).

<CIT> describes a technique for eliminating feedback light in a highpower optical device.

Accordingly there is provided a fiber laser oscillator as detailed in claim <NUM>. Advantageous features are in the dependent claims. Also provided is a method of generating a light beam as detailed in claim <NUM>. Similarly, advantageous features are in the dependent claims thereto.

The material described herein is illustrated by way of example and not by way of limitation in the accompanying figures. For example, the dimensions of some elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference labels have been repeated among the figures to indicate corresponding or analogous elements. In the figures:.

One or more embodiments are described with reference to the enclosed figures. While specific configurations and arrangements are depicted and discussed in detail, it should be understood that this is done for illustrative purposes only. Persons skilled in the relevant art will recognize that other configurations and arrangements are possible without departing from the scope of the description. It will be apparent to those skilled in the relevant art that techniques and/or arrangements described herein may be employed in a variety of other systems and applications other than what is described in detail herein.

Reference is made in the following detailed description to the accompanying drawings, which form a part hereof and illustrate exemplary embodiments. Further, it is to be understood that other embodiments may be utilized and structural and/or logical changes may be made without departing from the scope of claimed subject matter. It should also be noted that directions and references, for example, up, down, top, bottom, and so on, may be used merely to facilitate the description of features in the drawings. Therefore, the following detailed description is not to be taken in a limiting sense and the scope of claimed subject matter is defined solely by the appended claims.

In the following description, numerous details are set forth. However, it will be apparent to one skilled in the art, that the present invention may be practiced without these specific details. In some instances, well-known methods and devices are shown in block diagram form, rather than in detail, to avoid obscuring the present invention. Reference throughout this specification to "an embodiment" or "one embodiment" means that a particular feature, structure, function, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrase "in an embodiment" or "in one embodiment" in various places throughout this specification are not necessarily referring to the same embodiment of the invention.

As used in the description of the invention and the appended claims, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term "and/or" as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items.

The terms "coupled" and "connected," along with their derivatives, may be used herein to describe functional or structural relationships between components. Rather, in particular embodiments, "connected" may be used to indicate that two or more elements are in direct physical, optical, or electrical contact with each other. "Coupled" may be used to indicated that two or more elements are in either direct or indirect (with other intervening elements between them) physical or electrical contact with each other, and/or that the two or more elements co-operate or interact with each other (e.g., as in a cause an effect relationship).

The terms "over," "under," "between," and "on" as used herein refer to a relative position of one component or material with respect to other components or materials where such physical relationships are noteworthy.

As used throughout this description, and in the claims, a list of items joined by the term "at least one of" or "one or more of" can mean any combination of the listed terms. For example, the phrase "at least one of A, B or C" can mean A; B; C; A and B; A and C; B and C; or A, B and C.

The term "luminance" is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. The term "numerical aperture" or "NA" of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. The term "optical intensity" is not an official (SI) unit, but is used to denote incident power per unit area on a surface or passing through a plane. The term "power density" refers to optical power per unit area, although this is also referred to as "optical intensity" and "fluence. " The term "radial beam position" refers to the position of a beam in a fiber measured with respect to the center of the fiber core in a direction perpendicular to the fiber axis. The term "radiance" is the radiation emitted per unit solid angle in a given direction by a unit area of an optical source (e.g., a laser). Radiance may be altered by changing the beam intensity distribution and/or beam divergence profile or distribution. The term "refractive-index profile" or "RIP" refers to the refractive index as a function of position along a line (1D) or in a plane (2D) perpendicular to the fiber axis. Many fibers are azimuthally symmetric, in which case the 1D RIP is identical for any azimuthal angle. The term "optical power" is energy per unit time, as is delivered by a laser beam, for example. The term "guided light" describes light confined to propagate within an optical waveguide. The term "cladding mode" is a guided propagation mode supported by a waveguide within one or more cladding layers of an optical fiber. The term "core mode" is a guided propagation mode supported by a waveguide within one or more core layers of an optical fiber.

Described herein are optical fiber devices, systems, and methods suitable for one or more of suppressing SRS generation within a fiber system, removing SRS light from a fiber system, and/or otherwise mitigating one or more undesirable effects of SRS within a fiber system. As described further below, a fiber laser oscillator that is to generate an optical beam may include a Raman reflecting output coupler <NUM> that strongly reflects a Raman component pumped within the resonant cavity, and partially reflects a signal component to sustain the oscillator and emit a signal that has a reduced Raman component. A Raman filtering output coupler may comprise a superstructure fiber grating, and such a grating may be chirped or otherwise designed to have a desired bandwidth.

<FIG> is a flow chart illustrating methods <NUM> for reducing Raman component power in resonant cavity emissions, in accordance with some embodiments. Methods <NUM> may be implemented with a fiber laser oscillator where signal power and/or fiber length of the resonant cavity is sufficient to generate a Raman component having significant power. Methods <NUM> may be implemented as a means of reducing energy of a Raman component of light coupled from a fiber laser oscillator, and for example, into another length of fiber outside of the resonant cavity.

Methods <NUM> begin at block <NUM> where a fiber laser oscillator comprising a length of doped fiber is energized, for example through any optical pumping technique. The fiber is to support at least one core propagation mode. Upon energizing, the oscillator generates a light beam having a signal component Is. The signal component Is may have any range of optical power per frequency or wavelength (W/nm) over a predetermined signal power spectrum. The signal power spectrum may be associated with a peak wavelength λs of some maximum optical power. The first signal spectrum may have any band characteristics, and may, for example, comprise a band known to be suitable for continuous wave (CW) and/or pulsed fiber laser systems (e.g., with a micrometer peak wavelength λs, such as <NUM>, <NUM>, <NUM>, <NUM>, etc.). In some exemplary embodiments, the signal component Is has a unimodal spectrum having a single peak power. The peak wavelength λs may be a center wavelength of the single-peaked spectrum, for example. Although the signal component Is may have any optical power, in some exemplary fiber laser embodiments the signal component Is power is at least 50W, advantageously at least 100W, and more advantageously at least 250W.

The light beam energized within the resonant cavity may further comprise a first Raman component Ir. The Raman component Ir may develop within the resonant cavity as a result of scattering phenomena associated with the fiber propagation media, for example. The Raman component Ir has some range of some power per frequency or wavelength (W/nm) over an "SRS" or "Raman" power spectrum comprising one or more Raman wavelengths. The Raman power spectrum may be associated with a peak wavelength λr of maximum optical power. The Raman component Ir spans wavelengths shifted longer (e.g., about <NUM>) from those of the signal component Is. The Raman component Ir may also have a broader band than signal component Is, for example as a result of noise. In some illustrative embodiments where the first signal component Is has a peak wavelength λs of <NUM>, the derivative Raman component Ir may have Raman peak wavelength λr around <NUM>. The power of the Raman peak wavelength λr may vary as a function of the signal power spectrum that stimulates the first Raman component Ir.

As described further below, the oscillator energized at block <NUM> is to further comprise a resonant cavity defined, in part, by a first optical reflector that strongly reflects one or more wavelengths within the Raman spectrum. In advantageous embodiments, at block <NUM> wavelengths within the signal spectrum are also reflected, but one or more Raman wavelengths are reflected more strongly than are one or more wavelengths within the signal spectrum. As such, at block <NUM>, a fraction of the signal spectrum that is output from the resonant cavity through the first reflector may be larger than a fraction of the Raman spectrum that is propagated through the first reflector. This signal selective reflector may therefore be further operated as a signal output coupler between the oscillator and another length of fiber.

A second reflector that strongly reflects the one or more wavelengths within the signal spectrum may further define the resonant cavity. The second reflector may be a "high reflector" that strongly reflects signal spectrum. The second reflector need not reflect Raman spectrum. One or more signal wavelengths may be reflected by this second reflector more strongly than one or more wavelengths within the Raman spectrum. As such, at block <NUM>, a fraction of Raman spectrum may be transmitted out of the resonant cavity through the second reflector. Any SRS energy allowed to exit one end of the resonant cavity may be dumped, for example into a suitable optical absorber and/or heatsink. Hence, in accordance with some embodiments herein, the relative strength of the first and second reflectors defining a laser oscillator may be tuned to be complementary between the Raman and signal spectrum so as to facilitate separation of these two spectra, with individual ones of the spectra transmitted primarily out of opposite ends of the fiber propagation media of a resonant cavity.

<FIG> is a schematic of a laser oscillator <NUM> including an output coupler for reducing Raman component power from a signal emission of the oscillator, in accordance with some embodiments. Oscillator <NUM> is to generate an optical beam by exciting a signal spectrum of light. Oscillator <NUM> comprises an optical cavity defined within a propagation fiber <NUM> by a first fiber grating <NUM> proximal to a first end of fiber <NUM>, and a second fiber grating <NUM> proximal to a second end of fiber <NUM>. Fiber <NUM> is suitable for supporting at least one guided core mode (i.e., fiber <NUM> may be single mode or multi-mode fiber). Within fiber <NUM> the signal component Is and the Raman component Ir may each propagate in a core guided mode lm<NUM>, for example. In some examples, the core guided mode is a linear polarized mode LP<NUM>, with one embodiment being the linearly polarized fundamental transverse mode of the optical fiber core, LP<NUM>. LP<NUM> has desirable characteristics in terms of beam shape, minimal beam expansion during propagation through free space (often referred to as "diffraction limited"), and optimum focus-ability. Hence, fundamental mode LP<NUM> propagation is often advantageous in the fiber laser industry. With sufficient core diameter Dcore,<NUM>, and/or NA contrast, fiber <NUM> may support the propagation of more than one transverse optical mode. For example, fiber <NUM> may comprise large mode area (LMA) fiber that is operable in an LMA regime, etc..

<FIG> are longitudinal and transverse cross-sectional views of fiber <NUM>, respectively, in accordance with some multi-clad fiber embodiments. Although a double clad fiber embodiment is illustrated, fiber <NUM> may have any number of cladding layers (e.g., single, triple, etc.) known to be suitable for optical fiber. In the example illustrated in <FIG>, fiber <NUM> has a central core <NUM>, and an inner cladding <NUM>, which is annular and encompasses core <NUM>. An annular outer cladding <NUM> surrounds inner cladding <NUM>. Core <NUM> and inner cladding <NUM> may have any suitable composition (e.g., glass of any of a variety of materials, such as, SiO<NUM>, SiO<NUM> doped with GeO<NUM>, germanosilicate, phosphorus pentoxide, phosphosilicate, Al<NUM>O<NUM>, aluminosilicate, or the like, or any combinations thereof). Outer cladding <NUM> may be a polymer or also a glass, for example. Although not depicted, one or more protective (non-optical) coatings may further surround outer cladding <NUM>.

Fiber <NUM> may have any suitable refractive index profile (RIP). As used herein, the "refractive-index profile" or "RIP" refers to the refractive index as a function of position along a line (e.g., x or y axis in <FIG>) or in a plane (e.g. x-y plane in <FIG>) perpendicular to the fiber axis (e.g., z-axis in <FIG>). In the example shown in <FIG>, the RIP is radially symmetric, in which case the RIP is identical for any azimuthal angle. Alternatively, for example as for birefringent fiber architectures, RIP may vary as a function of azimuthal angle. Core <NUM>, inner cladding <NUM>, and outer cladding <NUM> can each have any RIP, including, but not limited to, a step-index and graded-index. A "step-index fiber" has a RIP that is substantially flat (refractive index independent of position) within fiber core <NUM>. Inner cladding <NUM> may also have a substantially flat refractive index (RI) over DClad,<NUM>, with RI stepped at the interface between core <NUM> and inner cladding <NUM>. An example of one illustrative stepped RIP suitable for a fiber laser is shown in <FIG>. Alternatively, one or more of core <NUM> and inner cladding <NUM> may have a "graded-index" in which the RI varies (e.g., decreases) with increasing radial position (i.e., with increasing distance from the core and/or cladding axis).

Inner cladding <NUM> may have an area larger than that of the core <NUM>, and may also have a higher NA. Although core <NUM> and inner cladding <NUM> is illustrated as being concentric (i.e., a centered core), they need not be. One or more of core <NUM> and cladding <NUM> may also be a variety of shapes other than circular, such as, but not limited to annular, polygonal, arcuate, elliptical, or irregular. Core <NUM> and inner cladding <NUM> in the illustrated embodiments are co-axial, but may alternatively have axes offset with respect to one another. Although DClad,<NUM> and DCore,<NUM> are illustrated to be constants about a central fiber axis in the longitudinal direction (z-axis in <FIG>). The diameters DClad,<NUM> and DCore,<NUM> may instead vary over a longitudinal fiber length. In some exemplary embodiments, the core diameter DCore,<NUM> is in the range of <NUM>-<NUM> micron (µm) and the inner cladding diameter DClad,<NUM> is in the range of <NUM>-<NUM>, although other values for each are possible.

Returning to <FIG>, fiber <NUM> includes a doped fiber length <NUM>, which may include any suitable optically active gain media. In some embodiments, doped fiber length <NUM> comprises rare-earth ions such as Er<NUM>+ (erbium), Yb<NUM>+(ytterbium), Nd<NUM>+(neodymium), Tm<NUM>+(thulium), Ho<NUM>+(holmium), or the like, or any combination thereof. One or more cladding layers may surround the core of fiber <NUM> and/or of doped fiber length <NUM>. Laser oscillator <NUM> is optically coupled to a pump light source <NUM>, which may be a solid state diode laser, or lamp, for example. Where doped fiber length <NUM> comprises a multi-clad fiber, pump light source <NUM> may be coupled into a cladding layer in either a co-propagating or counter-propagating manner.

Raman reflective output coupler <NUM> is operable as a signal output coupler (OC) that is to transmit out of oscillator <NUM> a fraction of signal spectrum Is propagating in a core mode of fiber <NUM>. Raman reflective output coupler <NUM> is further operable as an SRS suppression grating that reflects Raman spectrum Ir propagating in a core (e.g., fundamental) mode of fiber <NUM> into a counter-propagating core (e.g., fundamental) mode of fiber <NUM>. Raman reflective output coupler <NUM> is sufficiently wavelength sensitive to discriminate one or more signal wavelengths from one or more Raman wavelengths. In the illustrated embodiment, Raman reflective output coupler <NUM> is a superstructure fiber grating (SS-FG) comprising a plurality of smaller fiber gratings placed in proximity to one another, as described further below. Raman reflective output coupler <NUM> may advantageously have a lower reflectivity at least at the peak signal wavelength λs than at the peak Raman wavelength λr. <FIG> is a graph illustrating percent transmission as a function of wavelength for a fiber grating that is suitable for reducing Raman component power in resonant cavity emissions, in accordance with some embodiments. Raman reflective output coupler <NUM> may display the transmission characteristics illustrated in <FIG>, for example having high (e.g., more than <NUM>%) transmission over a first wavelength band that includes peak signal wavelength λs, and a low (e.g., less than <NUM>%) transmission over a second wavelength band that includes peak Raman wavelength λr.

Fiber grating <NUM>, being proximal to an end of fiber <NUM> opposite Raman reflective output coupler <NUM>, is to also strongly reflect at least the signal component Is, and may therefore be operable as a "high reflector" having a higher reflectivity at the peak signal wavelength λs. In exemplary embodiments, fiber grating <NUM> may further have a reflectivity at the peak signal wavelength λs that is higher than its reflectivity at the peak Raman wavelength λr. In some such embodiments, reflectivity of fiber grating <NUM> at the peak Raman wavelength λr is lower than reflectivity of the Raman reflective output coupler <NUM> at the peak Raman wavelength λr. Fiber grating <NUM> may therefore be further operable to transmit out of oscillator <NUM> a fraction of Raman spectrum Ir propagating in a core mode of fiber <NUM>. Fiber grating <NUM> may also be sufficiently wavelength sensitive to discriminate one or more signal wavelengths from one or more Raman wavelengths, with a reflectivity complementary to that of Raman reflective output coupler <NUM>. Fiber grating <NUM> may have a wider or narrower bandwidth than Raman reflective output coupler <NUM>. In the illustrated embodiment, fiber grating <NUM> is a single fiber Bragg grating (FBG). Optionally however, fiber grating <NUM> may have a more complex architecture (e.g., including one or more of superstructure, chirp, or apodization).

<FIG> is an expanded cross-sectional view of a length of fiber that includes a superstructure fiber grating (SS-FG) <NUM>, in accordance with some embodiments. SS-FG <NUM> may be employed as Raman reflective output coupler <NUM> (<FIG>), for example. SS-FG <NUM> can produce multiple reflection peaks from a single grating writing process that does not rely on multiple phase masks, and may occupy considerably less fiber length than would multiple FBGs. SS-FG <NUM> is to interact with the core modes electric field and induce a modulation of amplitude or phase within a long periodic (LP) structure. Interaction can be direct, or evanescent. Structural modulations of the grating can therefore be located within the cladding, or even comprise external surface perturbations. However, in the example illustrated, SS-FG <NUM> comprises refractive index (RI) perturbations <NUM> within at least fiber core <NUM> over a superstructure grating length L1. In the illustrated example, SS-FG <NUM> is within a double-clad fiber, for example having one or more of the attributes described above for fiber <NUM>. RI perturbations <NUM> have a refractive index n<NUM> that is higher than a nominal core index n<NUM>. For embodiments where outer cladding <NUM> has an index m, and inner cladding <NUM> has an index n<NUM>, RI within mode SS-FG <NUM> may vary as n<NUM><n<NUM><n<NUM><n<NUM>. RI perturbations <NUM> may impact light guided within core <NUM> over a target range of wavelengths while light outside of the target band may be substantially unaffected by RI perturbations <NUM>. As shown, SS-FG <NUM> comprises a plurality of subgratings <NUM>, each having a short period. Subgratings <NUM> are adjacent to each other and separated by some spacing to have a long period.

<FIG> is graph of refractive index over a length of SS-FG <NUM>, in accordance with some embodiments. As shown, RI has an amplitude modulation in which the period P is defined as the sum of the length of one subgrating L2, and the length of one non-grating gap Lgap. SS-FG <NUM> has a duty cycle D that is the ratio of the subgrating length L2 to the superstructure period P. Each subgrating <NUM> may have a period Λ. The subgrating sections <NUM> define a broad reflection band and together the plurality of subgratings <NUM> define peaks within the broad reflection band. Various reflection peaks suitable for reflection of the Raman component Ir can be achieved by defining the period and duty cycle of SS-FG <NUM>. Subgrating parameters (e.g., period A, RI modulation amplitude) may be predetermined to specify the fraction of light reflected at each of the peak signal wavelength λs and the peak Raman wavelength λr. The subgrating period A and/or RI modulation amplitude may be controlled to reflect the signal component Is and the Raman component Ir by amounts appropriate for both signal output coupling and strong Raman reflection.

The subgrating period A may vary as a function of the Raman spectrum, but is generally to be less than half of the peak Raman wavelength λr, which is a sufficiently short period that wavelength within the Raman component Ir will satisfy a Bragg condition and be reflected back into a counter-propagating core mode. SS-FG <NUM> may therefore also be referred to as a superstructure fiber Bragg grating (SFBG or SS-FGB). In some further embodiments, the superstructure period P is significantly greater than half the peak Raman wavelength λr and may be ten, or more, times the half the peak Raman wavelength λr. In some specific examples where the peak Raman wavelength λr is <NUM>, subgrating period A is less than <NUM> (e.g., <NUM> if neff is assumed <NUM>), while superstructure period P may be between <NUM> and <NUM>.

<FIG> is a graph illustrating reflectivity of an exemplary SS-FG and power spectral distribution (PSD) for a fiber laser device employing an SS-FG, in accordance with some embodiments. The spectral functions illustrated may be generated from models of laser oscillator <NUM>, for example. Representative wavelengths suitable for high power fiber lasers are illustrated, but the information conveyed in <FIG> is applicable to a variety of other wavelengths that may be of interest in various applications. In <FIG> reflectivity as a function of wavelength is plotted to the dependent axis on the left, and PSD as a function of wavelength is plotted to the dependent axis on the right. The spectral distribution of reflectivity for the SS-FG demonstrates a primary reflection peak <NUM> that is centered at the peak Raman wavelength λr (e.g., ~<NUM>). This "Raman" reflection peak <NUM> has a strong grating reflectivity of ~<NUM> at the peak Raman wavelength λr, enabling the SS-FG to suppress Raman from laser oscillator emission. The Raman PSD spectrum <NUM> illustrated in dashed line is representative of a Raman power spectrum pumped by a signal PSD spectrum <NUM> having a peak signal wavelength λr (e.g., ~<NUM>) and what would be emitted from the oscillator absent the Raman reflection peak <NUM>. In contrast, in the presence of reflection peak <NUM>, the Raman power spectrum <NUM> exiting an oscillator through the SS-FG is of a significantly lower power and is notched into a multi-modal (e.g., double peaked) spectrum as a function of the bandwidth overlap between the SS-FG reflection and Raman PSD spectrums. The remaining Raman power spectrum reflected by SS-FG may be transmitted through the other reflector, for example where that reflector has narrow band reflectivity that is centered at the peak signal wavelength λs.

Secondary reflection peaks <NUM> at wavelengths outside of the Raman band periodically peak at a lower reflectivity values (e.g., ~<NUM>). The illustrated example shows how the SS-FG may be designed to have one of the secondary reflection peaks <NUM> centered at a predetermined peak signal wavelength λs (e.g., ~<NUM>), enabling the SS-FG to have sufficient reflection for oscillator operation and to serve the additional function of signal output coupler. Signal PSD spectrum <NUM> represents a signal that may be output from a resonant cavity through an SS-FG having a desired reflectivity at the peak signal wavelength λs.

The SS-FG responses illustrated in <FIG> are therefore well suited to laser oscillator <NUM> (<FIG>) where Raman reflective output coupler <NUM> complements fiber grating <NUM>, which has a suitable high reflector FBG architecture that is also centered at the peak signal wavelength λs. For example, in <FIG> the strong reflection peak <NUM> displayed by fiber grating <NUM> at the peak signal wavelength λs is shown in dashed line for comparison to the weaker reflection peak <NUM> displayed by the SS-FG. The strong reflection peak <NUM> displayed by the SS-FG at the peak Raman wavelength λr is further shown for comparison to the signal reflection peaks. In this example, fiber grating <NUM> is designed to have slightly more reflection bandwidth than the SS-FG, which may advantageously capture the entire signal bandwidth of the SS-FG OC.

In some other embodiments, a laser oscillator comprises an aperiodic (i.e., chirped) superstructure fiber grating. Relative to periodic superstructure embodiments, embodiments comprising chirp of either the refractive index amplitude modulation or period may offer a wider reflection bandwidth at each reflection peak, better countering the greater width of Raman spectrum for greater suppression of a Raman component from oscillator emission.

<FIG> is an expanded cross-sectional view of a length of fiber that includes a chirped SS-FG <NUM>, in accordance with some embodiments. A chirped SS-FG <NUM> may be employed as Raman reflective output coupler <NUM> (<FIG>). In the example illustrated, chirped SS-FG <NUM> again comprises refractive index (RI) perturbations <NUM> within at least fiber core <NUM> over a superstructure grating length L1. As noted above, structural modulations of the grating can be located within the cladding, or even comprise external surface perturbations in addition to, or in the alternative to, the core modulations illustrated. Within the double-clad fiber, RI perturbations <NUM> have a refractive index n<NUM> that is higher than a nominal core index n<NUM>, both of which are further illustrated in <FIG> for an exemplary period chirped SS-FG. For embodiments where outer cladding <NUM> has an index n<NUM>, and inner cladding <NUM> has an index n<NUM>, RI within mode chirped SS-FG <NUM> may vary as n<NUM><n<NUM><n<NUM><n<NUM>.

As further shown in 5D, RI perturbations <NUM> have a period that varies over subgrating length L2. The individual subgratings <NUM> may be identically chirped subgratings, or each sub-grating may have different chirp. In the illustrated embodiments, chirp is varied continuously over superstructure grating length L1. In one example, subgrating period A begins at a blue end of chirped SS-FG <NUM> with a minimum period Ai, and linearly increases across the entire grating length L1 to a maximum period Λi as if there were no gaps. In another example, grating index modulation is similarly varied across the entire grating length L1. In either example, super structure may then be created by periodically breaking up the chirp(s) with gaps, which in the illustrated example are uniform (equal) over grating length L1. Superstructure gratings with chirp varied over the entire grating length L1 will widen the reflection peaks (e.g., to generate the reflection spectra illustrated in <FIG> and described further below). Such superstructure gratings may also be fabricated with a single chirped phase mask, and a single amplitude mask to create gaps between subgratings <NUM>. In contrast, identical chirping of the individual sub-gratings <NUM> may widen the envelope of reflection peaks, but not necessarily widen the reflection peaks themselves. Fabrication of such a grating may be, for example, through point by point writing.

Each subgrating <NUM> is longitudinally asymmetric having a first "blue" end with and a second "red" end. Chirped SS-FG <NUM> is therefore asymmetric with the blue ends of the sub-gratings all facing a blue end of chirped SS-FG <NUM>. Grating periods Λ<NUM> and Λi may each vary based on the Raman spectrum to which chirped SS-FG <NUM> is tuned. In exemplary embodiments, the shortest grating period Ai is less than half of a center Raman wavelength. Hence, in some examples where the center Raman wavelength is around <NUM>, the shortest grating period Ai is <NUM>-<NUM>. The grating period may vary between the shortest and longest periods, for example by <NUM> or <NUM> of nm, according to any function (e.g., linear) over grating length L. The long superstructure grating period P may also be chirped, for example where gap length Lg is a function of z (not depicted). Chirping of the grating period P may, for example, narrow a reflection peak at a targeted Raman wavelength, and then wider peaks at the other side lobes. Alternatively, or additionally, the magnitude of index modulation may be chirped, for example where n4 and/or n3 are a function of z (not depicted).

<FIG> is a graph illustrating reflectivity of a chirped SS-FG OC and a high reflector of a fiber laser oscillator. The reflectivity responses illustrated in <FIG> are representative of fiber gratings that are suitable for use in fiber laser oscillator <NUM> (<FIG>), for example. In comparison to the corresponding reflectivity responses shown in <FIG>, the broader reflection bandwidth of the chirped SS-FG is evident in the strong reflection peak <NUM> displayed by chirped SS-FG OC at the peak Raman wavelength λr. The width of the strong reflection peak <NUM> is also greater than the width of strong reflection peak <NUM> displayed by fiber grating <NUM> at the peak signal wavelength λs. The weaker reflection peak <NUM> displayed by chirped SS-FG is also wider. In this example, fiber grating <NUM> is designed to have slightly less signal reflection bandwidth than the chirped SS-FG, which may be advantageous for defining the signal bandwidth of the oscillator.

For the SS-FG embodiments described above there is one reflection peak within the Raman band. In alternative embodiments, a SS-FG may display more than one such reflection peak within the Raman band. For such embodiments, the high reflectivity bandwidth is effectively broader, which offers the advantage of greater Raman suppression without a chirped architecture. <FIG> is a graph illustrating reflectivity of a Raman reflective output coupler and a high reflector of a fiber laser oscillator, in accordance with some such embodiments. The reflectivity responses illustrated in <FIG> are representative of fiber gratings that are suitable for use in fiber laser oscillator <NUM> (<FIG>), for example. Multiple strong reflection peaks <NUM> are closely spaced (e.g., by less than <NUM>) and span the Raman band. As shown, the strongest set of peaks are centered at the peak Raman wavelength λr with each reflection peak that is closer to the peak Raman wavelength λr being more reflective. The strong reflection peak <NUM> displayed by fiber grating <NUM> is further illustrated in dashed line for comparison. As shown, one lesser reflection peak <NUM> is at the peak signal wavelength λs to maintain OC functionality. In this example, fiber grating <NUM> is designed to have slightly greater signal reflection bandwidth than the SS-FG, which may be advantageous for capturing all the signal bandwidth of SS-FG.

The Raman reflective output couplers described above provide good SRS suppression and signal OC functionality even where the magnitude index of refraction modulation is limited (e.g., less than <NUM>-<NUM>). For materials systems where larger RI modulation is possible (e.g., <NUM>-<NUM>, or greater), a short uniform FBG becomes more capable of achieving acceptable SRS suppression and signal OC performance. For such embodiments, a single, short strong grating may provide broad reflectivity centered over the Raman wavelengths, and partial reflectivity for the signal OC within the side lobes of the reflection spectrum. As for a chirped SS-FG, a high reflector grating may be fabricated for narrow spectral bandwidth to complete the oscillator cavity. Apodization may also be employed to further tune the strength of the side lobes for the sake of emitting a desired fraction of the signal power. <FIG> is a graph illustrating reflectivity of an FBG, and of a high reflector in a fiber laser oscillator, in accordance with some alternative embodiments having large grating index contrast. The broad Raman reflection peak <NUM> is centered over the peak Raman wavelength λr. Raman reflection peak <NUM> is nearly <NUM> over a bandwidth exceeding <NUM> in this example where grating index contrast is ~<NUM>-<NUM>. One of the side lobes provides a reflection peak <NUM> centered over the peak signal wavelength λs. The high reflector reflection peak <NUM> is again shown in dashed line for comparison. For these embodiments, the high reflector may be fabricated with narrow bandwidth for to complete the oscillator cavity and define the signal spectrum that is emitted from the cavity.

The laser oscillator cavity architectures described above may be implemented within a variety of laser devices and systems according to a wide range of applications. As one example, <FIG> depicts a schematic of a master oscillator power amplifier (MOPA) system <NUM> having reduced Raman spectrum pumping. MOPA system <NUM> may be suitable for high power fiber laser applications, such as materials processing, etc. System <NUM> includes fiber laser oscillator <NUM> that is to generate an optical beam substantially as described above. Raman filtering OC <NUM> may be any Raman reflective grating suitable for coupling signal spectrum out of the resonant cavity, and may, for example have any of the attributes described above. Fiber laser oscillator <NUM> is optically coupled to a fiber power amplifier <NUM> through Raman filtering OC <NUM>. Fiber amplifier <NUM> is to increase the radiance of at least the signal spectrum excited by oscillator <NUM>. Fiber amplifier <NUM> includes a length of doped fiber length <NUM>, which may have any of the properties described above for doped fiber length <NUM>. For example, in some embodiments, doped fiber length <NUM> comprises rare-earth ions such as Er<NUM>+ (erbium), Yb<NUM>+(ytterbium), Nd<NUM>+(neodymium), Tm<NUM>+(thulium), Ho<NUM>+(holmium), or the like, or any combination thereof. Power amplifier may be pumped by any pump light source <NUM> (e.g., laser diode, lamp, etc.) as embodiments herein are not limited in this respect. In some embodiments, doped fiber <NUM> comprises a multi-mode fiber supporting multiple propagation modes within a fiber core. In some advantageous embodiments where doped fiber length <NUM> comprises single-mode fiber capable of supporting only one guided propagation mode within the fiber core, doped fiber <NUM> comprises a multi-mode fiber capable of supporting multiple propagation modes within the fiber core.

In accordance with the illustrated embodiments, fiber amplifier <NUM> is positioned between Raman filtering OC <NUM> and a delivery fiber <NUM>. Delivery fiber <NUM> is further coupled to a process head <NUM> where the optical beam propagating in delivery fiber <NUM> may be launched into free-space propagation.

Claim 1:
A fiber laser oscillator (<NUM>) to generate a light beam, the fiber laser oscillator comprising:
a length of optical fiber (<NUM>) comprising a core (<NUM>) and one or more cladding layers (<NUM>, <NUM>), where at least a portion (<NUM>) of the length of optical fiber is doped with a gain medium operable to excite at least a signal component of the light beam, the signal component associated with a first peak wavelength;
a first reflector (<NUM>) proximal to a first end of the length of optical fiber (<NUM>); and
an output coupler (<NUM>) separated from the first reflector by at least the length of optical fiber, wherein the output coupler has a lower reflectivity at the first peak wavelength than at a second peak wavelength associated with a Raman component of the light beam,
wherein:
the first reflector (<NUM>) has a higher reflectivity at the first peak wavelength than at the second peak wavelength;
the Raman component is to propagate in a fundamental core mode of the length of optical fiber; and
the output coupler (<NUM>) comprises a superstructure fiber grating, SS-FG (<NUM>), that is to reflect the Raman component into a counter-propagating core mode of the length of optical fiber so that Raman emission from the output coupler is reduced.