DESC:
TECHNICAL FIELD
[0001] The present disclosure relates generally to the field of laser generation. In particular, the present disclosure relates to the generation of tuneable, high-power and scalable laser output.

BACKGROUND
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] Cascaded Raman fibre lasers enable high power fibre lasers at any arbitrary wavelength, which are otherwise not accessible with conventional rare-earth doped fibre lasers. This is due to the availability of Raman gain based on Stimulated Raman Scattering (SRS) at any arbitrary wavelength, within the transmission window of optical fibre.
[0004] In conventional systems, a cascaded Raman resonator, comprising a series of nested cavities at each intermediate stokes wavelength, is used for cascaded Raman conversion. The series of nested cavities in these systems are formed by a pair of wavelength selective Fibre Bragg Gratings (FBGs), which, in turn, decide both the output signal and the input pump wavelengths.
[0005] This limits the wavelength agility of such systems, as the maximum wavelength tuneability of 120nm with <7W of output power has been demonstrated with the help of either tuneable FBGs or tuneable intra-cavity filters.
[0006] Raman fibre lasers based on Random Distributed Feedback (RDFB) has been demonstrated to overcome this limitation, where, in these systems, both the Raman gain due to SRS and RDFB due to Rayleigh backscattering are available at any arbitrary wavelengths. These features enable them to be free from wavelength selective Fibre Bragg Gratings (FBGs) and hence wavelength agile.
[0007] However, the abovementioned system lacks power scaling, as the maximum output power demonstrated by the above system is ~10W. The first reason for this is the formation of supercontinuum at high power operations due to propagation of higher order stokes wavelengths in the anomalous dispersion regime of the passive fibre used for Raman gain. This reduces the spectral purity and hence the output in-band power.
[0008] The second reason for the lack of power scaling is, due of the broadband nature of the RDFB, cascaded Raman termination at the desired wavelength to enable power scaling is difficult at high power operations.
[0009] There is therefore a requirement in the art for a means for tuneability of laser output that can be scaled at any arbitrary wavelength.
[0010] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
[0011] In some embodiments, the numbers expressing quantities or dimensions of items, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
[0012] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[0013] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all groups used in the appended claims.

OBJECTS
[0014] A general object of the present disclosure is to provide a device for tuning of laser output wavelength.
[0015] Another object of the present disclosure is to provide a device with high power laser output.
[0016] Another object of the present disclosure is to provide a device with a capability to terminate cascading or laser amplification above a predetermined wavelength value.
[0017] Another object of the present disclosure is to provide a device for tuning of laser output which is scalable at any arbitrary wavelength.

SUMMARY
[0018] The present disclosure relates generally to the field of laser generation. In particular, the present disclosure relates to the generation of tuneable, high-power and scalable laser output.
[0019] In an aspect, the present disclosure provides a device for tuning laser output, the device comprising: an input laser source; one or more Raman fibres configured to receive laser beam from the input laser source, the one or more Raman fibres adapted to amplify wavelength of the received laser beam based on the number of Raman fibres, wherein, due to Raman scattering, the laser beam is scattered in forward and backward directions; a wavelength division multiplexer adapted to couple the input laser source to the one or more Raman fibres, the wavelength division multiplexer configured to isolate the input laser source from the laser beam scattered in the backward directing by diverting the laser beam scattered in the backward direction away from the input laser source; a two-way filter coupled to the wavelength division multiplexer and configured to receive the scattered laser beam from the wavelength division multiplexer, the filter configured with a predetermined wavelength cut-off value to enable filtering of wavelengths of the received scattered laser beam above the predetermined value; and a feedback coupler configured to receive the filtered scattered laser beam and feed it back to the two-way filter to allow further filtering of the returned scattered laser beam, wherein the returned scattered beam is fed back to the one or more Raman fibres through the wavelength division multiplexer to enable generation of laser beam with amplified power and an amplified wavelength within the predetermined value.
[0020] In an embodiment, the input laser source is a rare-earth doped fibre laser provided with a tuneable wavelength filter to enable generation the laser beam in at the corresponding tuneable wavelengths. In another embodiment, the fibre laser is coupled with a backward pump power amplifier to amplify the power output from the fibre laser.
[0021] In another embodiment, the Raman fibre is a passive optical fibre with a high index core, small effective area and high numerical aperture.
[0022] In another embodiment, the two-way filter is a bent Raman fibre, and wherein varying the length and bend of the Raman fibre allows for tuning of the cut-off wavelength value of the two-way filter.
[0023] In another embodiment, the input laser source is an ytterbium (Yb) doped fibre laser.
[0024] In an aspect, the present disclosure provides a system for tuning laser output, the system comprising: an input laser source; one or more Raman fibres configured to receive laser beam from the input laser source, the one or more Raman fibres adapted to amplify wavelength of the received laser beam based on the number of Raman fibres, wherein, due to Raman scattering, the laser beam is scattered in forward and backward directions; a wavelength division multiplexer adapted to couple the input laser source to the one or more Raman fibres, the wavelength division multiplexer configured to isolate the input laser source from the laser beam scattered in the backward directing by diverting the laser beam scattered in the backward direction away from the input laser source; a two-way filter coupled to the wavelength division multiplexer and configured to receive the scattered laser beam from the wavelength division multiplexer, the filter configured with a predetermined wavelength cut-off value to enable filtering of wavelengths of the received scattered laser beam above the predetermined value; and a feedback coupler configured to receive the filtered scattered laser beam and feed it back to the two-way filter to allow further filtering of the returned scattered laser beam, wherein the returned scattered beam is fed back to the one or more Raman fibres through the wavelength division multiplexer to enable generation of laser beam with amplified power and an amplified wavelength within the predetermined value.
[0025] In an embodiment, the input laser source is a rare-earth doped fibre laser provided with a tuneable wavelength filter to enable generation the laser beam in at the corresponding tuneable wavelengths.
[0026] In another embodiment, two-way filters with different cut-off wavelength values are placed on a rotating wheel mount to enable quick access to any suitable two-way filter.
[0027] In another embodiment, the input laser source is an ytterbium (Yb) doped fibre laser.
[0028] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF DRAWINGS
[0029] The accompanying drawings are included to provide a further understanding of the present disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[0030] FIG. 1 illustrates an exemplary schematic representation for a tuneable wavelength, cascaded Raman fibre laser, in accordance with an embodiment of the present disclosure.
[0031] FIG. 2 illustrates a schematic representation of a high-power tuneable input pump source.
[0032] FIG. 3 illustrates a schematic representation of a low-power tuneable wavelength seed laser source.
[0033] FIG. 4 illustrates a schematic representation of an exemplary backward pump power amplifier.
[0034] FIGs. 5A and 5B illustrate the output power and wavelength tuning characteristics respectively, of the pump laser source.
[0035] FIGs. 6A and 6B illustrate the transmission and spectral characteristics respectively, of a wavelength division multiplexer.
[0036] FIGs. 7A and 7B illustrate the transmission characteristics of different bulk thin film based short-pass filters for with cut-off wavelengths of 1400nm and 1450nm respectively.
[0037] FIG. 8A illustrates an exemplary experimental setup to demonstrate working principle for generating high-power, tuneable wavelengths Raman fibre laser, in accordance with an embodiment of the present disclosure.
[0038] FIG. 8B illustrates the output power of the laser as a function of wavelength.
[0039] FIG. 8C illustrates the wavelengths tuning characteristics of the laser.

DETAILED DESCRIPTION
[0040] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0041] If the specification states a component or feature “may”, “can”, “could”, or “might” be included or have a characteristic, that particular component or feature is not required to be included or have the characteristic.
[0042] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[0043] Exemplary embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. These exemplary embodiments are provided only for illustrative purposes and so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those of ordinary skill in the art. The invention disclosed may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Various modifications will be readily apparent to persons skilled in the art. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Moreover, all statements herein reciting embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future (i.e., any elements developed that perform the same function, regardless of structure). Also, the terminology and phraseology used is for the purpose of describing exemplary embodiments and should not be considered limiting. Thus, the present invention is to be accorded the widest scope encompassing numerous alternatives, modifications and equivalents consistent with the principles and features disclosed. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail so as not to unnecessarily obscure the present invention.
[0044] The use of any and all examples, or exemplary language (e.g., “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non – claimed element essential to the practice of the invention.
[0045] The present disclosure relates generally to the field of laser generation. In particular, the present disclosure relates to the generation of tuneable, high-power and scalable laser output.
[0046] In an aspect, a rare-earth doped fibre laser with tuneable wavelength (moderate tuning in the emission window of the rare-earth doping) is used as a pump source.
[0047] In another aspect, high power at a required wavelength (higher than emission wavelength of the rare-earth doped fibre laser) is obtained by wavelength conversion through a series of cascaded Raman shifts.
[0048] In another aspect, by a combination of emission wavelength of the rare-earth fibre laser and the number of Raman shifts, continuous, broadband tuning of output wavelength of laser is achieved. This requires broadband optical feedback in a Raman laser, achieved by using the method of Random Raman fibre lasers, which uses a half open cavity architecture with a broadband feedback in the backward direction.
[0049] The primary problem to implement the wide tunability in a power scalable fashion is that, at higher powers, it is impossible to terminate the cascaded Raman conversion to the required output wavelength. The output signal is always accompanied by spurious light in the adjacent Raman orders due to broadband Raman gain. This reduces the power scaling ability and efficacy of this laser.
[0050] In another aspect, a mechanism to terminate the cascade of the Raman conversion to the required wavelength band even at high powers in a Random Raman fibre laser configuration is achieved by utilising a short-pass filter in the feedback path in the backward direction. This feedback is essential for the efficient functioning of the laser. By tailoring the spectral properties of this feedback filter, in particular the short-pass filter profile with a sharp cut-off, termination of the cascaded Raman conversion can be achieved to the required wavelength band.
[0051] FIG. 1 illustrates an exemplary schematic representation for a tuneable wavelength, cascaded Raman fibre laser, in accordance with an embodiment of the present disclosure. The laser generator (100) comprises: an input pump source; a passive optical fibre; a wavelength division multiplier; a filter mechanism to filter backward propagating Raman signal; and a mechanism to feed back the filtered signal into the passive optical fibre.
[0052] In an embodiment, the input pump source is a high-power, tuneable Ytterbium (Yb) doped fibre laser (102).
[0053] In another embodiment, the passive optical fibre is also referred to as “Raman fibre” (104) and provides Raman gain and normal dispersion over its operating wavelength range.
[0054] In another embodiment, the wavelength division multiplexer (“WDM”) (106) is a fused fibre that couples the input pump source (102) into the Raman fibre (104) and spatially separates the Raman stokes signal generated inside the Raman fibre (104) from the input pump source (102).
[0055] In another embodiment, the filter mechanism is a short pass filter (108) with varying cut-off to filter backward propagating Raman stokes signal, either through a bulk thin-film based optical filters or the bend induced loss characteristics of a speciality Raman filter fibre.
[0056] In another embodiment, the mechanism to feed the filtered signal from the varying cut-off short pass filter (108) back into the Raman fibre (104) can be a feedback coupler (110).
[0057] In another embodiment, depending on the power value of the pump input source (102), Raman stokes signals are generated in both forward and backward directions at the Raman fibre (104). The Raman stokes signals travelling in the backward direction are coupled into the cross-port of the WDM (106), where they undergo filtering depending on the cut-off wavelength of the short pass filter (108) used.
[0058] In another embodiment, the feedback coupler (110) feeds the filtered signals back into the short pass filter (108), where it undergoes further filtering before entering the Raman fibre (104) through the WDM (106).
[0059] In another embodiment, the Raman stokes signals generated above the cut-off wavelength of the short pass filter (108) are suppressed, and consequently, the process of stimulated Raman scattering at these wavelengths are also suppressed. Thus, the Raman cascade at wavelengths below the cut-off of the short pass filter (108) is terminated.
[0060] In another embodiment, by adjusting the cut-off wavelength of the short pass filter (108), high power laser over a range is achieved over a wide bandwidth. In an embodiment, said range is from 8W at shorter wavelengths to 33W at longer wavelengths.
[0061] In an embodiment, the system for a tuneable wavelength Raman fibre laser comprises: a high-power, tuneable wavelength input pump source; a wavelength division multiplexer; a Raman gain medium; a short pass filter; and a feedback coupler.
[0062] FIG. 2 illustrates a schematic representation of a high-power tuneable input pump source. In an embodiment, the source (200) is based on a standard master oscillator and power amplifier (“MOPA”) configuration.
[0063] In another embodiment, said source (200) is an Yb-doped fibre laser and comprises: a continuously tuneable wavelength, low power seed laser as the master oscillator; a backward pump power amplifier; and an isolator.
[0064] In an embodiment, the isolator (204) is placed in between the seed laser (202) and the power amplifier (206) to isolate the seed laser from any unwanted back reflections and amplified spontaneous emission (“ASE”) noise from the power amplifier.
[0065] FIG. 3 illustrates a schematic representation of a low-power tuneable wavelength seed laser source (300). In an embodiment, said source comprises: a pump source; a pump combiner; a gain fibre; a pump dump; a coupler; an isolator; and a tuneable filter.
[0066] In an embodiment, the seed source (300) is in a standard unidirectional ring configuration with a uniform output over the entire wavelength tuning range. In another embodiment, the uniform output is ~12W.
[0067] In another embodiment, the gain medium is an Yb-doped double-clad fibre. In another embodiment, the gain medium is 13m long. The peak cladding absorption near 975nm is 1.65dB/m and at 13m length, 99% of pump absorption is ensured. In another embodiment, the core of the fibre is 5µm in diameter with a numerical aperture of 0.14, and the cladding is 125µm in diameter with a numerical aperture of 0.46.
[0068] In another embodiment, the pump source (302) is a fibre coupled pump laser diode operating at 976nm at an output power of 20W.
[0069] In another embodiment, a pump combiner (304) can be used when more than one pump source is used as input, to power combine the sources.
[0070] In another embodiment, the un-absorbed pump power is removed by stripping the polymer coating (outer cladding) at the end of the gain fibre (306) in a process referred to as pump dump (308).
[0071] In another embodiment, a manual thin film (fabry-perot) based fibre optic tuneable filter (314) is used to tune the wavelength from 1050nm to 1100nm. The tuning resolution of the filter is 0.02nm and the 3dB bandwidth is 1nm. The maximum insertion loss due to the filter is 3.8dB.
[0072] In another embodiment, a high-power inline isolator (312) operating at 1064nm is placed in the ring cavity to ensure unidirectional operation. In another embodiment, the isolator (312) provides an isolation greater than 28dB and has an insertion loss of less than 2.6dB.
[0073] In another embodiment, to extract the laser power from the ring cavity, a 90/10 fused coupler (310) is used. The coupler (310) provides an insertion loss of 0.71dB between the input and output (90% port). The output power measured from the coupler (310) at the 90% port is 12W at all wavelengths (1050nm to 1100nm).
[0074] In another embodiment, the overall efficiency of the seed laser source (300) is affected by the various insertion losses and also by additional splice losses. In another embodiment, the efficiency of the seed laser source (300) is 60% with respect to the input optical pump power of 20W at 976nm.
[0075] FIG. 4 illustrates a schematic representation of an exemplary backward pump power amplifier (400). In an embodiment, said amplifier comprises: a pump source; a pump combiner; a gain fibre; and an isolator.
[0076] In an embodiment, said amplifier uses an Yb-doped fibre as gain medium. The length of the gain medium is 20m and it has the same parameters as the gain medium used for the seed laser source (300).
[0077] In another embodiment, the pump source (402) is three fibre-coupled pump laser diodes operating at 976nm at a maximum output power of 50W.
[0078] In another embodiment, a (6+1) x1 pump combiner (404) (tapered fibre bundle) is used to power combine the three pump laser diodes. In another embodiment, the insertion loss of the combiner (404) is 0.65dB.
[0079] In another embodiment, the un-absorbed pump power is removed by stripping the polymer coating (outer cladding) at the end of the gain fibre (406) in a process referred to as pump dump (408).
[0080] In another embodiment, a high power in-line isolator (410) is placed before the tuneable wavelength seed laser in order to protect said seed laser from any unwanted back reflections and amplified spontaneous emission noise from the amplifier. In another embodiment, the isolator (410) provides an isolation greater than 20dB with an insertion loss of less than 1.2dB.
[0081] In an embodiment, the backward pumping configuration has the advantages of large range linewidth tuneability and higher efficiency. In an embodiment, efficiency of the power amplifier (400) is 81% and maximum output power at full power is ~130W.
[0082] FIGs. 5A and 5B illustrate the output power and wavelength tuning characteristics respectively, of the pump laser source.
[0083] FIGs. 6A and 6B illustrate the transmission and spectral characteristics respectively, of a wavelength division multiplexer.
[0084] In an embodiment, the WDM, as shown in FIG. 1 is a 1117/1480nm standard fused-fibre based, which is spliced to the high-power tuneable wavelength pump laser source.
[0085] In another embodiment, as illustrated in FIG. 6A, the WDM provides ~80% transmission, including the splice loss between the input and output of the high-power tuneable wavelength pump laser source. This corresponds to the measured output power of ~104W at all wavelengths.
[0086] In another embodiment, the output through port of WDM is connected to a passive optical fibre for Raman gain and random distributed feedback (“RDFB”). The WDM couples the input high-power tuneable wavelength pump laser source into the passive fibre while spatially separating the backward propagating Raman stokes signal from said input laser source. The spatial separation is achieved by coupling the backward propagating Raman stokes signal to the cross port at the input side of the WDM.
[0087] In another embodiment, as illustrated in FIG. 6B, a higher fraction of longer wavelengths backward propagating Raman stokes signals are coupled to the cross port of the WDM. The cross port at the input side of the WDM is connected to a low-pass filter with a tuneable cut-off wavelength, and a feedback coupler.
[0088] In an embodiment, the Raman gain medium referred to in FIG. 1 is provided by a passive optical fibre with a high index core, small effective area and a high numerical aperture of ~0.2. The fibre also provides the RDFB.
[0089] In another embodiment, the high refractive index and small effective area of the core enables a high peak Raman gain efficiency of 2.5(W•km)-1.
[0090] In another embodiment, the high numerical aperture provides large normal dispersion of approximately -80ps/nm.km over a wide wavelength operating range, which prohibits the generation of supercontinuum, by suppressing the modulational instability which, in turn, is necessary to kickstart the supercontinuum generation process.
[0091] FIGs. 7A and 7B illustrate the transmission characteristics of different bulk thin film based short-pass filters for with cut-off wavelengths of 1400nm and 1450nm respectively.
[0092] In an embodiment, the bend-loss characteristics of speciality Raman fibre are also utilised to implement the crucial sharp cut-off, short-pass filtering mechanism. The Raman fibre has a “W” shaped index profile, wherein the index variation is structures such that it provides large fundamental bend losses for wavelengths above a fixed cut-off wavelength, while maintaining very low losses for the wavelengths below the cut-off wavelength.
[0093] In another embodiment, the cut-off wavelength can be varied by varying the bend and the length of the bent fibre.
[0094] In another embodiment, FIGs. 7A and 7B illustrate transmission characteristics of two filters with cut-off wavelengths of 1400nm and 1450nm respectively. To achieve the cut-off wavelength a 5m length of Raman fibre is bent to a diameter of 1.2cm for 1400nm cut-off and to a diameter of 2.6cm for a 1450nm cut-off.
[0095] In another embodiment, the combined filters are packaged in a way to enable easy and rapid reconfiguration to vary the cut-off wavelength.
[0096] In an embodiment, the feedback coupler referred to in FIG. 1 is an 80/20 fused fibre-based tap coupler with output ends spliced together to provide feedback. This corresponds to a theoretical broadband reflection coefficient of 32% across the entire operating wavelength range, with insertion losses of 1.13dB and 7.02dB for the 80% and 20% ports respectively.
[0097] In another embodiment, along with the Raman gain medium, the tap coupler forms a half open cavity and enables preferentially seeded forward Raman scattering.
[0098] In another embodiment, the effect of point mirror is weak, in terms of threshold of the Raman scattering, and as such, a weak broadband point mirror is sufficient to enable preferential forward Raman scattering. The maximum power going in the backward direction (entering the feedback coupler) at steady state is ~4W. This is significantly low, compared to the forward output power.
[0099] FIG. 8A illustrates an exemplary experimental setup (800) to demonstrate working principle for generating high-power, tuneable wavelengths Raman fibre laser, in accordance with an embodiment of the present disclosure.
[0100] In an embodiment, the tuning range of the input pump source corresponds to at least one Raman shift from 1050nm to 1100nm. On coupling the source into a Raman fibre, Raman scattering occurs in forward and backward directions. The backward propagating Raman stokes signals are coupled to a WDM at the cross port on its input side.
[0101] In another embodiment, combined short-pass filters with different cut-of wavelengths, together with a feedback coupler, filter and couple only Raman stokes signals below the cut-off wavelength as feedback. In another embodiment, the bulk thin film filters are placed on a rotating wheel mount so that filters can be reconfigured quickly and easily.
[0102] In another embodiment, the cut-off wavelength of the short pass filters fixes the range of wavelengths across which feedback is provided. This, in turn, fixes the order of stokes shifts in the cascaded Raman process.
[0103] In another embodiment, the tuning wavelength of the high power Yb-doped fibre pump laser fixes the wavelength within each stokes order. In an exemplary implementation, an extra fibre is spliced at the output of the WDM, causing the coupled power into the Raman fibre to drop to ~90W due to splice loss.
[0104] In another embodiment, different output laser wavelengths can be achieved by simply tuning the Yb-doped laser wavelength and filtering position.
[0105] In another embodiment, lower powers at smaller wavelengths is brought about by a longer length of Raman fibre. This ensures higher power at higher order Raman shifts. By limiting the tuning range, that is, limiting the number of Raman orders used, higher net output power is obtained.
[0106] FIG. 8B illustrates the output power of the laser as a function of wavelength. In an embodiment, the performance of the system described in FIG. 8A is shown in comparison with a similar system, but without feedback filtering. A saturation in output power is seen in the system without feedback filtering (852) but not with the proposed system (854). This indicates that the current architecture can be power scaled, and its wavelength span extended. In another embodiment, a minimum of 8W at lower wavelengths and a maximum of 33W at longer wavelengths is observed.
[0107] FIG. 8C illustrates the wavelengths tuning characteristics of the laser. In an embodiment, for a few representative wavelengths, continuous tuneability is shown from 1118nm to 1575nm. Across most of the wavelength span, a high degree of wavelength conversion is achieved with >70% of light in the required wavelength. This is due to feedback filtering, without which, this fraction reduces to <40% at longer wavelengths.
[0108] Thus, the present invention provides a system for the generation of high-power, widely tuneable Raman fibre laser, scalable at any arbitrary wavelength. The issue of termination of Raman cascade is solved by using a short-pass cut-off filter to filter out backward propagating Raman stokes to wavelengths below the cut-off wavelength.
[0109] It should be apparent to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive patient matter, therefore, is not to be restricted except in the spirit of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “includes” and “including” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Where the specification claims refer to at least one of something selected from the group consisting of A, B, C ….and N, the text should be interpreted as requiring only one element from the group, not A plus N, or B plus N, etc. The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practised with modification within the spirit and scope of the appended claims.
[0110] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

ADVANTAGES
[0111] The present disclosure provides a device for tuning of laser output wavelength.
[0112] The present disclosure provides a device with high power laser output.
[0113] The present disclosure provides a device with a capability to terminate cascading or laser amplification above a predetermined wavelength value.
[0114] The present disclosure provides a device for tuning of laser output which is scalable at any arbitrary wavelength.

,CLAIMS:

1. A device for tuning laser output, said device comprising:
an input laser source;
one or more Raman fibres configured to receive laser beam from the input laser source, the one or more Raman fibres adapted to amplify wavelength of the received laser beam based on the number of Raman fibres, wherein, due to Raman scattering, the laser beam is scattered in forward and backward directions;
a wavelength division multiplexer adapted to couple the input laser source to the one or more Raman fibres, the wavelength division multiplexer configured to isolate the input laser source from the laser beam scattered in the backward directing by diverting the laser beam scattered in the backward direction away from the input laser source;
a two-way filter coupled to the wavelength division multiplexer and configured to receive the scattered laser beam from the wavelength division multiplexer, said filter configured with a predetermined wavelength cut-off value to enable filtering of wavelengths of the received scattered laser beam above the predetermined value; and
a feedback coupler configured to receive the filtered scattered laser beam and feed it back to the two-way filter to allow further filtering of the returned scattered laser beam,
wherein the returned scattered beam is fed back to the one or more Raman fibres through the wavelength division multiplexer to enable generation of laser beam with amplified power and an amplified wavelength within the predetermined value.

2. The device as claimed in claim 1, wherein the input laser source is a rare-earth doped fibre laser provided with a tuneable wavelength filter to enable generation the laser beam in at the corresponding tuneable wavelengths.

3. The device as claimed in claim 2, wherein the fibre laser is coupled with a backward pump power amplifier to amplify the power output from the fibre laser.

4. The device as claimed in claim 1, wherein the Raman fibre is a passive optical fibre with a high index core, small effective area and high numerical aperture.

5. The device as claimed in claim 1, wherein the two-way filter is a bent Raman fibre, and wherein varying the length and bend of the Raman fibre allows for tuning of the cut-off wavelength value of the two-way filter.

6. The device as claimed in claim 1, wherein the input laser source is an ytterbium (Yb) doped fibre laser.

7. A system for tuning laser output, said system comprising:
an input laser source;
one or more Raman fibres configured to receive laser beam from the input laser source, the one or more Raman fibres adapted to amplify wavelength of the received laser beam based on the number of Raman fibres, wherein, due to Raman scattering, the laser beam is scattered in forward and backward directions;
a wavelength division multiplexer adapted to couple the input laser source to the one or more Raman fibres, the wavelength division multiplexer configured to isolate the input laser source from the laser beam scattered in the backward directing by diverting the laser beam scattered in the backward direction away from the input laser source;
a two-way filter coupled to the wavelength division multiplexer and configured to receive the scattered laser beam from the wavelength division multiplexer, said filter configured with a predetermined wavelength cut-off value to enable filtering of wavelengths of the received scattered laser beam above the predetermined value; and
a feedback coupler configured to receive the filtered scattered laser beam and feed it back to the two-way filter to allow further filtering of the returned scattered laser beam,
wherein the returned scattered beam is fed back to the one or more Raman fibres through the wavelength division multiplexer to enable generation of laser beam with amplified power and an amplified wavelength within the predetermined value.
8. The system as claimed in claim 7, wherein the input laser source is a rare-earth doped fibre laser provided with a tuneable wavelength filter to enable generation the laser beam in at the corresponding tuneable wavelengths.

9. The system as claimed in claim 7, wherein two-way filters with different cut-off wavelength values are placed on a rotating wheel mount to enable quick access to any suitable two-way filter.

10. The system as claimed in claim 7, wherein the input laser source is an ytterbium (Yb) doped fibre laser.