HIGH POWER METAL CLAD MODE ABSORBER
BACKGROUND OF THE DISCLOSURE Field of Disclosure
[001 ] The disclosure relates to high power fiber laser systems. In -particular, the disclosure relates to a clad mode stripper/absorber operative to trap and remove undesirable cladding-guided light in passive fibers.
Prior Ait
[002] High power single mode fiber laser systems ("HPSMFLS") are typically configured with one or more amplifying cascades each including an active double clad fiber which is either side- or end pumped by multimode (MM) pump light. The pump light propagating along a light-guide waveguide cladding is often not fully absorbed along the length of the active fiber and eventually coupled into a cladding of SM passive fibers. In addition, power losses at splices are also responsible for multimode light which propagates in a waveguidmg cladding under a polymeric protective sheath. Besides, baekreflecting light from the surface to he laser treated may also be coupled in the cladding. Even, at moderate pump light absorption rates varying between 10-12. dB the clad-guided light can easily reach 300-600 W at the final amplifying stage. The clad guided light is undesirable for the following reasons.
[003] Typically, the SM passive fiber of HPSMFLS is surrounded by a flexible polymeric protective sheath which has a lower refractive index than that of the cladding region. At fiber bends or spliced connections, the total reflection of the cladding region may be disturbed leading to the escape of clad-guided light into the protecti ve sheath. As a consequence, the protective sheath may be overheated and destroyed. Furthermore, the MM radiation earned In the cladding may damage the end regions of the fiber which is typically a passive, SM delivery fiber or SM passive fiber coupled between two gain
blocks, one of which, for example, is pumped in a direction counter to the signal propagation. Finally, if the dad-guided MM radiation reaches the end of the delivery fiber, the beam quality of the transmitted light signal may be worsened, which cars adversely affect other optical components and the processing quality.
[004] Devices configured to remove clad light and convert the light energy into the heat energy are known as cladding mode absorbers ("CMA"). Typically, a CMA is provided along a length of fiber stripped from the protective sheath, which covers the cladding, and configured as a light-guide polymer compound with a refractive index higher than that one of the cladding, The polymer compound absorbers typically allow decoupling cladding light with a power up to about 100 to about 400 W,
[005] Some of structural limitations of the known CMAs include a low resistance to mechanical stresses caused by non-uniform thermo-dependent expansion/contraction of fiber and CMA. Typically, mechanical stresses lead to micro bending losses in single mode ("SM") large mode area ("LMA") fibers and excitation of high order modes ("HOMs").
[006] Still a further limitation is associated with a relatively low thermal conductivity of polymer compound leading to its high temperatures. A 1 10JC temperature is generally considered critical, and higher temperatures lead to the destruction the CMA. This critical temperature can be even lower during abrupt temperature changes occurring upon energizing a fiber laser system.
[007] A need therefore exists for a HPSMFLS configured with a CMA which is characterized by high temperature- arid mechanical stress -resistant structure.
[008] A further need exists for a method of manufacturing the improved CMA.
SUMMARY OF THE DISCLOSURE
[009] These needs are satisfied by the disclosed metal light dad energy absorber ("MLCA") configured to absorb light while withstanding elevated temperatures and pro vide a stress-resistant structure.
[010] One aspect of the disclosure relates to a MLCA which, in contrast to the known polymer -compound-based absorbers, includes a filling of liquid metals and their alloys provided along a stripped region of protective coating. The filling is selected to absorb light and have a low melting temperature; high thermo-conductivity, desired density and good adhesion to the exposed cladding (quartz). As will be disclosed below, the disclosed absorber can withstand considerably higher input powers than known silicon- based absorbers.
[01 1] A farther aspect of the disclosure relates to improving the stress resistance of the disclosed MLCA. In particular the fiber with the absorber is further placed in a heat sink configured as a sleeve surrounding the filling and adhered thereto. Due to high thermo- conductive characteristics, the absorber operates as effective heat evacuating media between quartz and metal heatsink.
BRIEF DESCRIPTION OF THE DRAWINGS
[012] FIG. 1 is a diagrammatic view of a high power single mode fiber laser system ("HPSMFLS") provided with the disclosed metal light clad absorber ("MLCA")
[013] FIG. 1A is a .diagrammatic view of a. HPSMFLS including a master oscillator/power amplifier configuration ("MOPA") with the disclosed metal absorber provided on a passive fiber coupled between the components of the illustrated system,
[014] FIG, 2 is a diagrammatic view of a SM passive output fiber provided with the disclosed MLCBC.
[015] FIG, 3 is a diagrammatic view of a modified SM output fiber provided with the disclosed MLCEC.
[0.16] FIG. 4 is an exploded view of the disclosed MLCA mounted in a heat sink,
[017] FIG. 5 is a graph, illustrating dependence of maximal temperature of the sleeve of the MLCEC froin dispersed power.
SPECIFIC DESCRIPTION
[018] Reference will now be made in detail to the disclosed energy absorber, high power fiber laser system incorporating the absorber and a method for manufacturing the latter. 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 far from precise scale. For purposes of convemence and clarity only, directional terms .may be used with respect to the plane of the drawing sheets and not be construed to limit the scope. Unless specifically noted, it is intended that the words and phrases in. the specification and. claims be given the ordinaiy and accustomed meaning to those of ordinary skill in the fiber laser arts,
[019] FIG. 1 illustrates a diagrammatic view of a typical gain block 10 which is either alone or in combination with similarly configured gain blocks is operative to emit an output substantially SM beam reaching kW levels. The output beam is then delivered to the destination point by a delivery single mode ("SM") passive fiber 18. The latter is provided with a clad mode absorber 20 made from liquid metals, i.e., metals with a low- temperature melting point, or a combination of liquid metals and their alloy. The absorber 20, thus, is operative to trap and absorb unwanted modes guided along the clad of fiber 18,
[020] FIG, 1 A illustrates two gain blocks 10 - components of MOP A, which as known to one of ordinary skill in the art, includes a master oscillator MO and a power amplifier
PA. in this schematics, absorber 20, in addition to or alternatively to the delivery fiber, may be provided on a passive fiber 21 optically coupling the MO and PA, as shown. The fiber 21 may receive an unabsorbed pump signal from a pump 15 which, for instance, is coupled into the PA in a direction counter to a forward propagating direction of light signal,
[021 ] The gain block 10, for example, power amplifier PA, includes an active fiber 12, i.e. the liber doped with ions of one or more rare earth elements, such as ytterbium, erbium, thulium and etc, and single SM input and output passive fibers 14, 1.6, respectively. The active fiber 12 is configured with one or more claddings and a multimode core which, if desired, is configured to support substantially a fundamental mode at the desired wavelength, A multimode ("MM") pump light emitted by a pump unit 15, is coupled into a waveguide cladding, and as it propagates therealong, is gradually absorbed by the core. "Not all pump light is absorbed, some of it remains in the cladding. The unabsorbed pump and other differently originated and unwanted rnodes are guided further through the cladding of passive output fiber 16 and propagate further along the clad of delivery fiber 21 , The existence of the clad light is highly undesirable for a variety reasons discussed previously and should be minimized and desirably totally eliminated. Note that while the above discusses single mode systems and fibers, the disclosed absorber may be used in conjunction with multimode fibers.
[022] Referring to FIG. 2, fiber 18 or 21, besides core 22 and cladding 24, is further configured with a protective polymeric protective film (stripped and thus not shown) with a refractive coefficient lower than that one of the cladding. To create conditions for absorbing clad propagating modes, the film is stripped along a desired axial region of fiber 18, 2.1 exposing a stretch of cladding 24 which may be immersed into absorber 20 made of a liquid-base metal and/or its alloys 30. The metal, in contrast to polymer compound, has excellent reflective characteristics. The clad light and particularly those modes that propagate along the border between the metal coating and quartz periodically impinge upon the former. A large portion of light incident on metal 30 is absorbed; the
rest continues its propagation and is eventually greatly reduced or completely eliminated. The metal absorber 20 can withstand high mechanical stresses, higher temperatures and, although the absorption is still limited, it is distributed better than in traditional polymer compounds. Selecting the optimal composition of absorber 20 based on the known parameters of the entire fiber system and the desired location provide the removal of clad light with a maximum possible light power that can heat the composition at a temperature lower than a threshold temperature known to damage absorber 20 and other optical components of the system.
[023] While it is possible to simply apply and adhere liquid metal to quartz, surface stresses may still be considerabie because of imperfect adhesion of the liquid metal to the quartz. Hence, the surface of quartz is textured so that it is not smooth but has microscopic formations which improve a bond between quartz/clad and metal absorber. Also, the latter is selected to have a coefficient of -thermal expansion close to that one of silicon, which also reduces mechanical stresses.
[024] Liquid metals and their respective alloys exist in a liquid phase at room or about room temperatures and may include, among others. Gallium ("Ga") and its alloys, such as Galinstan. The latter is a composition of 68% Ga (Gallium), 21 ,5% In (Indium) and 10% Tin Sn and has a freezing point of about -1 " after which this material tends to expand, Galinstan, like other liquid metals and alloys, has a high degree of thermal conductivity which is superior to polymer compound and thus has excellent heat conducting and dissipating characteristics, The tests show that energy absorber made of this alloy can safely dissipate higher than 600 watts of optical power of the clad-guided light. The composition of alloy may be altered to withstand higher powers reaching a kW level, in particular, the mass fraction of the alloys" components is controllably adjusted so that MM light is removed from the cladding with a maximum possible light power heating the alloy at temperatures lower than a threshold temperature known to damage it.
[025] Referring to FIG, 3, to reduce a high power densit of clad-guided MM light and somewhat alleviate thermal loads on absorber 20, preferably, passive fiber 18, 21 may have a double bottleneck-shaped cross-section, which is also referred to as a twin bottle ("TB") fiber. The TB geometry is disclosed in detail in co-pending applications 12/559,284 and US 32/630,545 assigned commonly with the instant application and fully incorporated herein by reference. The configuration of TB fiber 18 or 21, thus, includes spaced apart relatively uniform, small -diameter end regions 28, relatively uniform large- diameter central region 34 and transition regions 32 between respective end and central regions which allow adiabatic expansion of guided light. Based on the foregoing considerations, absorber 20 is preferably, but not necessarily, provided along central region 34 wher the power density is relatively low, The diameter of absorber 20 ma be the same as the oute diameter of polymeric sheath or film 31 or greater than the fatter. TB fiber 18, 21 may further have another absorber 36 made of polymer compound which is located downstream from absorber 20. The central region 34 of fiber 18 may be configured with about a 300-micron diameter.
[026] Referring now to FIG, 4, high light powers may require a heat sink made from a ductile material, such as copper, wolfram alloys with copper and others which are configured with good thermal conductivity and minimal thermal expansion preventing fiber stresses. The preferred housing material does not react with the metal alloy. The sink includes a first part 38 and a second part 40 detaehably coupled to one another by fasteners 42, only one of which is shown, ana configured to enclose a length of passive fiber 18, 21. The length, of the latter is placed in a groove 44 filled with absorber 20 and provided in part 38. The opposite ends of the fiber's stretch within groove 44 are coupled to part 38 by an adhesive, such as silicone Gel.. The latter is also applied to absorber 20 and any other stripper provided on fiber 18, which may be also located in groove 44, so as render the structure hennetical. To prevent overheating of housing part 40, which covers the entire unit, and even further improve the air tightness of the unit, a foil 46, which may be made, for example, from Al, is packed between the fiber length and part
40. Alternatively, Silicone Gel, UV glues or epoxy may fully cover absorber 20 without the use of pari 40. These materials are flexible and efficiently compensate for thermal expansion of the metal,
[027] FIG. 5 illustrates the temperature of Die heat sink or housing enclosing the length of fiber 18 with the disclosed absorber from the dispersed power. Even with powers reaching 800 W, the absorber preserves its structural integrity.
[028] The disclosed absorber is particularly advantageous in the context of high power fiber laser systems. Although the above has mostly referred to the stripping capability of absorber 20 of forward propagating light, it also serves as a filter of backreflected light. Also, while the above disclosed configuration is described in the context of SM systems, the disclosed absorber can be efficiently used in MM systems as well. For example, it may be provided on a MM delivery fiber so as to minimize backrefiection. Furthermore, the disclosed configuration may be used in both continuous wave .and pulsed laser configurations,
[029] Variety of changes of the disclosed structure may be made without departing from the spirit and essential characteristics thereof. Thus, it is intended that all matter contained in the above description should be interpreted as illustrative only and in a limiting sense, the scope of the disclosure being defined by the appended claims.