HIGH POWER CLADDING PUMPED SINGLE MODE FIBER RAMAN LASER

Abstract

A Raman fiber laser source is configured with a feeding fiber delivering MM pump radiation to an inner cladding of double-clad MM Raman fiber laser. The MM pump beam radiation has a sufficient power to produce Raman scattering in the MM Raman fiber converting the pump radiation to a MM signal radiation at a Raman-shifted wavelength λram which is longer than a wavelength λpump of the pump radiation. The Raman laser source further has a pair of spaced reflectors defining therebetween a resonator for the signal radiation at a 1.sup.st Stokes wavelength and flanking at least part of the MM core of the Raman fiber which is provided with a central core region which is doped with impurities for enhancing Raman process. The reflectors and central core region are dimensioned to correspond to the fundamental mode of the MM signal radiation which is output from the Raman fiber with an M.sup.2 factor ≤1.1 and in a power range between a few kW and tens of kW.

Claims

1. A high power single mode (SM) Raman laser source comprising: an end pumped multi-clad Raman fiber having: an inner cladding, which receives multimode (MM) pump radiation propagating along a path at a wavelength λpump, and a MM core surrounded by the inner cladding and provided with a central core region, the pump radiation producing Raman scattering which induces a conversion of the pump radiation to a signal radiation at a Raman-shifted wavelength λram>than wavelength λpump, the central core region being dimensioned to confine substantially only a fundamental mode (FM) of the signal radiation and doped with impurities which enhance the Raman scattering; and spaced wavelength-selective reflectors defining therebetween a resonator for the FM at the wavelength λram which at least partially includes the central core region, the wavelength-selective reflectors being dimensioned to match the FM of the signal radiation, wherein the Raman fiber outputs the signal radiation in the FM with an M.sup.21.1 in a power range between a few kilowatts (kW) and tens of kWs.

2. The SM Raman laser source of claim I further comprising a MM feeding fiber located upstream from and delivering the pump radiation to an upstream end of the Raman fiber.

3. The SM Raman laser source of claim 1 or 2, wherein the MM feeding and Raman fibers are directly spliced to one another, the wavelength-selective reflectors being fiber Bragg gratings (FBG) formed in the central core region.

4. The SM Raman laser source of claim 1 or 2 further comprising a MM intermediary passive fiber spliced to opposing ends of respective feeding and Raman fibers and configured with a MM core which supports propagation of substantially only a FM of the intermediary fiber, wherein the FM of the intermediary and Raman fibers having respective mode field diameters match one another.

5. The SM Raman fiber laser of any of the above claims further comprising a SM output passive fiber spliced to a downstream end of the Raman fiber, the wavelength reflectors being respective fiber Bragg gratings written in cores of respective SM input and output passive fibers and optically aligned with the central core region.

6. The SM Raman laser source of claim 1 or 2 further comprising collimating and focusing lenses between spaced feeding and Raman fibers, and a slanted mirror between the lenses deflecting backreflected light off the path.

7. The SM Raman fiber laser of any of the above claims further comprising a plurality of fiber laser pumps having respective output fibers which are coupled together in a combiner so that outputs of respective fiber laser pumps constructively interfere with one another to produce the high power MM pump radiation at the pump wavelength λpump.

8. The SM Raman laser of any of the above claims, wherein the pump wavelength λpump is about 1070 nm and the signal wavelength tram is about 1120 nm.

9. The SM Raman fiber laser of any of the above claims, wherein the inner cladding and MM core of the Raman fiber are made from pure silica with respective refractive indices which match each other, an inner cladding region being doped with fluorine to provide a depression.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The above and other features and advantages will become more readily apparent from the subsequent specific description accompanied with the following drawings, in which:

[0015] FIG. 1 is a schematic view of the inventive all fiber Raman source in accordance with one structural modification.

[0016] FIG. 2 is the inventive Raman source utilizing free space communication between feeding and Raman fibers.

[0017] FIG. 3 illustrates the refractive index profile of the inventive Raman fiber.

[0018] FIGS. 4 and 5 illustrate respective field and intensity profiles of the fundamental mode in the exemplary inventive structure of FIGS. 1 and 2.

[0019] FIG. 6 illustrates the inventive Fiber laser source featuring a fiber Raman amplifier,

DETAILED DESCRIPTION

[0020] Reference will now be made in detail to embodiments of the invention. The features of the invention can be used individually or in combination with selected or all other inventive features in each of the disclosed configurations of the inventive Raman source. Wherever possible, same or similar reference numerals are used in the drawings and the description to refer to the same or like parts or steps. The drawings are in simplified form and are not to precise scale. The term “couple” and similar terms do not necessarily denote direct and immediate connections, but also include connections through intermediate elements or devices.

[0021] FIG. 1 illustrates a Raman fiber laser source 10 configured with multiple SM or MM laser pumps 2 which are preferably, but not necessarily based on fiber lasers each capable of outputting SM pump bright light having a power reaching a kilowatt (kW) level, For example, individual SM fiber laser pumps 2 each are operative to output 1 kW SM pump light. The SM fiber laser pumps may optionally have a master oscillator power fiber amplifier (MOPFA) configuration.

[0022] The output passive fibers 4 of respective laser pumps 2 are coupled together in a well-known manner by a fiber combiner having an output fiber 6 which is further referred to as the feeding fiber. The combination of multiple pump outputs results in a multi kW cumulative pump light that depends only on reasonable number of pumps 2 and their individual output powers. For example, a power range between 5 and 100 kW in a CW regime is readily accessible today. The beam quality of cumulative pump light is very good at these powers, but with forever increasing industrial demands, it could be further improved. The combiner coupling multiple output fibers to produce a low- or MM output is referred to as SM-LM or LM-/MM combiner depending on the modality of individual pumps and the M.sup.2 factor of the cumulative pump beam.

[0023] The feeding fiber 6 is configured with a cladding 8 surrounding a MM core 12 which guides the cumulative pump light towards a MM Raman fiber 14 which is butt spliced to the output end of feeding fiber 6. The MM fiber Raman laser (FRL) is configured to support the propagation of substantially only a single, FM through the beam cleanup effect of SRS in multimode fibers. High pump light powers require that feeding fiber 6 has MM core 12 with a relatively large dimeter, such as 50 to 100 μm which matches a waveguiding inner cladding 16 of double clad MM Raman fiber 14.

[0024] The diameters of MM Raman core 18 and cladding 16 are selected to ensure the effective absorption of the pump light over the shortest possible length of the MM Raman core. The latter outputs kW signal light in a fundamental mode at the desired signal light wavelength which is preferably the 1st Stokes wavelength.

[0025] To insure the near diffraction-limited signal light at the desired signal wavelength, such as 1.sup.st Stokes wave, Raman source 10 includes a combination of a central core region 30 in Raman fiber 14 and strong and weak FBGs 26 and 28, respectively, with respective pitches adjusted for the effective index of the FM. The spaced FBGs 26 and 28 are written directly in MM Raman core 18 to define a Raman resonant cavity for the desired 1.sup.st Stokes wavelength. The central core region 30 is incorporated within the resonant cavity and doped with impurities which are selected from boron, germanium, phosphorous or a combination of these. The central region 30 may occupy no more than 70% of the entire core area which is substantially the core region occupied by the FM, whereas HOMs tend to occupy the periphery of core area 18. Since central core region 30 is dimensioned to substantially match the mode field diameter of FM, it is amplified incomparably greater than the majority of HOMs which are thus reduced to the background noise at the output of Raman fiber 14. Accordingly, the above-disclosed structure ensures that the signal radiation emitted from Raman fiber 14 is in the FM.

[0026] Optionally, an intermediary MM passive fiber, not shown, can be spliced to the opposing ends of respective feeding and Raman fibers. Despite having a MM core, the intermediary fiber can be configured to support the propagation of only a FM with a MFD substantially matching that of the FM supported in Raman core 18. In this modification, strong FBG 26 may be written in the intermediary fiber. If a pure SM is required, the shown structure may have a SM output fiber 24 which is coupled to the output end of Raman fiber 14. The weak FBG 28 may be written in SM core 24 of output fiber 22. If both FBGs are formed onto respective intermediary and output passive fibers, central core region 30 of Raman fiber 14 may remain undoped.

[0027] FIG. 2 shows an alternative architecture of the disclosed Raman source 10. While the cumulative pump light from MM feeding fiber 6 propagates over free space, it is incident on a guiding optic assembly which includes spaced apart collimating and focusing lenses 25, 35 respectively. As a result, the pump light is focused on the cladding of SM passive fiber 20. Positioned between lenses 25 and 35 is a slanted reflector 45 preventing the propagation of backreflected Raman light toward upstream to SM laser pumps 2. The remaining structure is analogous to that of FIG. 1.

[0028] FIG. 3 illustrates an example of Raman fiber 14 of FIGS. 1 and 2 having 30 μm MM core 18 with central core region 30 being of about 20 μm. The tests conducted with this index profile of Raman fiber 14 show good results as can be seen in FIGS. 4 and 5 illustrating field and intensity profile for fundamental mode LPo. The configuration used in the tests include SM fiber laser pumps operated at 1070 nm, whereas SM Raman signal light was generated at 1120 nm. All of the above ranges of fibers shown in FIGS. 1 and 2 are exemplary and can be altered without however deviating from the scope of the invention. As shown, the refractive index profile of inner cladding 16 has a depressed portion due to the fluorine-doped inner region. Otherwise, the raised region of cladding 16 and Raman core 18 may be made of pure silica and thus have substantially the same refractive index.

[0029] FIG. 6 illustrates another aspect of the disclosure wherein fiber Raman source 50 is configured with a MM fiber Raman amplifier (FRA) 32. The realization of this requires a SM seed laser source 34 configured as a SM fiber laser or pigtailed SM diode laser. The seed 34 outputs a SM signal beam at the desired Raman wavelength λram, such as 1120 nm, which is guided in a SM fiber 36. The wavelength range of pump and therefore signal wavelengths is not limited to a 1-2 micron interval and extends well beyond it.

[0030] The seed 34 may include a SM DL or SM fiber laser. The SM fiber 36 may be directly spliced to a central fiber of a SM-MM combiner 48 or, as shown, to a strong FBG 38 which together with a weak FBG 46 are part of a central SM fiber laser pump 42. Here, like in the schematics of FIGS. 1 and 2, the SM-MM combiner is coupled to central and multiple SM fiber laser pumps 42 and 44, respectively, on the input end and to a feeding fiber 52 on the output end. The feeding fiber 52 is configured with a core supporting the SM signal beam, and a cladding guiding the cumulative pump beam at a wavelength λpump shorter than the desired Raman wavelength, for example, it may be 1070 nm. End-spliced to a double-clad FRA 32, the cumulative MM pump beam is coupled into the cladding of FRA, while signal beam into the MM core of the FRA.

[0031] Alternatively, element 42 of FIG. 6 may be configured as a seed laser outputting light at the first Stokes wavelength, such as 1120 nm. The rest of the configuration remains the same as disclosed above except for component 34.

[0032] As the cumulative pump beam continuously cross the MM core of ERA 32, its energy is transferred to a 1.sup.st Stoke wave. The suppression of high order Stokes waves is realized by the calculated length of the Raman fiber and the ratio between MM core and cladding diameters. The amplification of the fundamental mode at the expense of high order transverse modes is a result of matched MFDs of respective feeding and Raman fibers and their alignment. The MM Raman core may be provided with a central region doped with ions of standard Raman dopants and dimensioned to correspond to the MFD of the fundamental mode, like in the embodiments of FIGS. 1A, 1B and 2. If the residual, unabsorbed pump light still propagates through the cladding of the Raman fiber, a mode stripper may be arranged along the downstream stretch of a fiber train in any of embodiments of respective FIGS. 1, 2 and 6.

[0033] Although the present disclosure has been described in terms of the disclosed example, numerous modifications and/or additions to the above-disclosed embodiments would be readily apparent to one skilled in the laser arts without departing however from the scope and spirit of the following claims.