Patent Description:
At high optical power levels of single mode fiber laser systems, based on a double clad fiber, which includes a core, inner cladding and outer protective polymer and is operative to emit an output beam of the order of tens of kilowatts in substantially a single mode operation, the task of managing stray light propagating along the inner cladding becomes crucial. Even a small fraction of stray light in the cladding can heat a protective polymer to damaging temperatures and cause a catastrophic failure of the fiber.

High thermal stresses generated by a rapid temperature rise during the initial, switch-on operational phase of a high power fiber laser system, when the system's output increases from <NUM> Watt to the desired high power, are particularly damaging to double-clad fibers which are used in kW-level fiber laser systems. Typically, a time period associated with the switch-on phase does not exceed about <NUM> milliseconds and is characterized by a localized decoupling of clad modes at the upstream end of the fiber. Accordingly, the cladding modes need to be safely removed which is typically realized by a clad mode stripper ("CMS") the known configurations of which are briefly discussed hereinbelow.

One of numerous CMS configurations includes a small strip of high index silicone placed over the inner low index cladding along an upstream end of a fiber which is stripped from the protective polymer. However, because the coupling is localized and the length of the coated high index silicone is short, the latter is often overheated and decomposed.

United States Patent Application Publication No. <CIT> teaches a system for generating an optical beam from a fiber laser, including a fiber gain medium having a core and a cladding, the core being configured to convert radiation from a pump beam into an output beam, the cladding having a mode propagating section and a mode stripping section bounded on a near end and a distal end by the mode propagating section, the mode stripping section of the cladding being configured to scatter excess pump radiation received from the mode propagating section in a substantially outwardly radial direction. The system also includes a rigid support member into which the fiber gain medium is placed, the rigid support member completely encompassing the mode stripping section of the cladding and joined to the fiber at the mode propagating section of the cladding.

Another cladding mode stripper for stripping cladding modes of light from an optical fiber is disclosed in US Patent No. <CIT>.

Furthermore, Japanese Patent Application Publication No. <CIT> teaches a clad mode removing method for double-clad fibers.

Further relevant prior art is disclosed, for example, in <CIT> and <CIT>.

A need therefore exists for a device minimizing thermal stress during the switch on phase of a high power fiber laser system.

According to a first aspect, the present invention relates to a device for gradually removing cladding modes according to claim <NUM>. According to a second aspect, the present invention relates to a clad light absorber assembly according to claim <NUM>. According to a second aspect, the present invention relates to a single mode, SM, high-power fiber laser system according to claim <NUM>. Further aspects of the invention are set forth in the dependent claims, the drawings, and the following description of embodiments. The disclosed CMS satisfies this need by generating a low temporal thermal gradient dT/dt, wherein T is a temperature of the CMS's material at a desired output power level, and t is the time necessary for the output power to reach the desired level. The low temporal thermal gradient reduces the possibility of damaging the most vulnerable upstream part of a fiber during the switch-on phase of a high-power fiber system's operation during which a light power surges from zero to the desired kW level.

The disclosed CMS is configured with a layer of silicone covering the upstream end of a passive SM fiber coupled to the output of the amplifying fiber of a high-power SM fiber laser system. The fiber has a double-clad configuration in which the upstream portion of a protective polymer, covering the upstream part of the cladding, is replaced by a mixture which includes a host material and a plurality of diffusers embedded in the host material so as to define an upstream decoupling zone and at least one downstream decoupling zone.

At the start of the switch-on phase, a front of unabsorbed pump light is coupled into the cladding and, as it propagates along the very upstream of the CMS, the guided light is incident on diffusers along the upstream decoupling zone. The diffusers omnidirectionally scatter the incident light with a fraction of scattered light remaining in the host material or decoupling therefrom. In either case the material is heated which affects its refraction index.

The refractive index is a critical parameter that determines the operational properties of a photonic device, and knowing its dependence on temperature is crucial for designing photonic devices. In silicone, as temperature rises the refractive index lowers. Accordingly, scattered light heats the upstream zone of the CMS to a temperature which is insufficient to destroy the host material, but sufficient to lower its refractive index along this upstream zone of the CMS below that of quartz. As recognized by one of ordinary skill in the art, at his point, the cladding remains a waveguiding structure.

Accordingly, the un-scattered light and unabsorbed pump light following the front of the initially coupled unabsorbed pump light continue to propagate along the cladding and impinge upon diffusers of the downstream zone. As a consequence, the downstream zone of the CMS is heated by a further fraction of scattered decoupled light and reduces its refractive index below that of the cladding. Such a sequential decoupling of clad light may continue along the entire length of the CMS while the power increases to its desired level. By the time the latter is reached, the cladding light is removed by the CMS which remains structurally sound.

The host material of the CMS includes silicone with a refractive index which is substantially equal to or slightly lower than that of quartz/cladding at room temperature. Alternatively, silicone may have a slightly higher refractive index than that of the cladding at room temperature.

The diffusers may be selected from a variety of silicone-based materials which when embedded in silicone host material do not bond therewith. Accordingly, diffusers form scattering centers which interact with cladding light incident thereon.

One configuration of the disclosed CMS includes metal oxides such as Al<NUM>O<NUM> embedded in silicone. The distribution of these particles may have a uniform profile along the entire length of the CMS, step-wise profile with the CMS's downstream region having a higher concentration of diffusers than that of the upstream end, or gradually increasing profile towards the downstream region.

In alternative configuration of the disclosed CMS, silicone host material is mixed with diffusers also made from silicone but with a refractive index which is lower than that of the host material. Silicone based diffusers lower the effective refractive index of the mixture to be at most equal to that of quartz - material of cladding. The silicone-based diffusers do not bond with silicone host material and form centers of haziness which function as scattering centers in the host material.

The above and other features and advantages of the disclosed CMS will become more readily apparent from the following specific description in conjunction with the attached drawings, in which:.

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. The drawings are in simplified form and are far from precise scale. Unless specifically noted, it is intended that the words and phrases in the specification and claims be given the ordinary and accustomed meaning to those of ordinary skill in the fiber laser arts.

<FIG> illustrates a high power fiber laser system <NUM> operative to emit a system beam substantially in a fundamental mode. The system <NUM> is composed of one or more gain blocks <NUM> pumped by a plurality of pump sources. The latter are combined in a tapered fused bundle pump and signal combiner <NUM> so as to launch pump light into the cladding and signal into the core of a double clad gain fiber of gain block <NUM>. Pump light which is not absorbed by a gain medium continues its propagation along a path through a cladding of double clad fiber <NUM> and may detrimentally affect the integrity of a protective coating <NUM> covering the cladding. Hence the removal of cladding light is important and typically realized by a clad mode absorber or stripper ("CMS") <NUM> installed along the upstream of output fiber <NUM>. The coating <NUM> is particularly vulnerable during a switch-on operation phase of system <NUM> during which light power increases from <NUM> to the desired kW level within, as a rule, about one hundred milliseconds.

Referring to <FIG>, CMS <NUM> is configured with a housing <NUM> receiving a portion of fiber <NUM> which is provided with an absorber as disclosed below. The fiber <NUM> extends through a side wall of housing <NUM> and gradually lowers to its bottom <NUM>. In the following text, a gradually lowering or slacked portion of CMS <NUM> is further referred to as an upstream zone <NUM>, whereas the portion in contact with bottom <NUM> is referred to as a downstream zone <NUM> of CMS <NUM>. The housing <NUM> is further filled with a putting or host material and typically is closed by a lid <NUM>.

The fiber <NUM> is placed in a groove <NUM> (<FIG>) provided on the bottom of housing <NUM> so that downstream zone <NUM> is in continuous contact with the bottom of groove <NUM>. It has been noticed that even a small gap between downstream zone <NUM> and groove's bottom substantially reduces absorption of clad light by CMS <NUM>. However, placing entire fiber <NUM> in contact with the bottom of groove <NUM> substantially increases optical power density of the decoupled light during a switch-on phase which leads to a high failure rate of absorber <NUM>. It is believed that when the entire length of fiber <NUM> is in contact with the housing, the decoupled light, incident on the housing, generates high heat density which causes a putting material or silicone to burn. This problem can be alleviated by lifting at least upstream zone <NUM> above the bottom of the groove. A maximum distance between the very upstream of fiber <NUM> and the bottom preferably varies between about <NUM> and about <NUM>. At about <NUM> distance, the failure rate dropped to practically zero. Optionally, the very downstream end CMS <NUM> may be elevated as well.

Referring to <FIG>, disclosed CMS <NUM> is coated along a portion of cladding <NUM> after the removal of protective coating <NUM> of output double clad fiber <NUM> of block <NUM> of <FIG>. In accordance with the disclosure, CMS <NUM> is composed of host material having a coefficient of refraction only slightly above than or the same as or slightly lower than that of cladding <NUM> which is made of fused quartz having a refractive index of about <NUM> at the desired wavelength. Preferably, the refractive index of putting material varies between about <NUM> and <NUM>.

In accordance with one aspect of the disclosure, CMS <NUM> includes a mixture of host material <NUM>, such as silicone configured with a relatively high refractive index at room temperature, and silicone-based diffusers <NUM> with a refractive index lower than that of the host material. The embedded silicone-based diffusers <NUM> not only form centers of scattering, but also they lower effective index nab of the mixture to at most equal to, but preferably lower than refractive index ncl of quartz.

As unabsorbed pump light Lp is coupled into cladding <NUM>, a portion thereof may decouple into upstream zone <NUM> of CMS <NUM> and encounter diffusers <NUM> which scatter light incident thereupon within host material <NUM>. Some of the scattered light eventually leaves upstream decoupling zone <NUM> which gradually heats reducing the effective index even lower. As the index of CMS <NUM> along its upstream zone <NUM> falls even further below that of cladding <NUM>, the unscattered light continues its propagation along cladding <NUM> gradually losing its power as subsequent portions of light keep colliding with diffusers <NUM> along at least one or more downstream decoupling zones <NUM> (<FIG>). However, in contrast to upstream zone <NUM>, downstream zone <NUM> is already preheated and does not experience high thermal loads or high temporal gradient despite a greater amount of light decoupling from the cladding <NUM> and therefore high temperatures.

The cladding <NUM> is configured with ncl of about <NUM>. The host material <NUM> of CMS <NUM> at the desired wavelength preferably varies between about <NUM> and about <NUM>. The diffusers <NUM> may be mixtures of silicone and dimethyl group (dimethylsiloxane) and have a refractive index of about <NUM>. Alternatively, diffusers <NUM> may include fluorinated silicone with a refractive index lower than about <NUM>, or phenylated silicone with a refractive index higher than <NUM>. Typically, diffusers <NUM> are selected to reduce the effective refractive of the mixture to that of or lower than quartz. Preferably, the refractive index of diffusers <NUM> varies between about <NUM> and about <NUM>. The weight percentage of embedded diffusers may vary from about <NUM>% to about <NUM>% of total weight of CMS <NUM>, which leaves silicone host material <NUM> ranging between about <NUM> % and <NUM>%.

According to a further aspect of the disclosure, removal of cladding light is realized by diffusers <NUM> made of particles of aluminum oxide Al<NUM>O<NUM> which are embedded in host material <NUM>. As disclosed above, CMS <NUM> is configured with upstream and downstream decoupling zones <NUM>, <NUM> with respective relatively low concentration of Al<NUM>O<NUM> or similar materials particles or diffusers and relatively high concentration thereof. The upstream zone <NUM> may have between about <NUM>% and about <NUM>% diffuser weight concentration. The weight concentration of the diffusers in the downstream region may vary between about <NUM>% and <NUM>%. By way of example, upstream region <NUM> is composed of about <NUM>% diffuser weight concentration, whereas downstream region <NUM> is doped with about <NUM>% concentration. The refractive index of the host material in combination with different concentration of the diffusers along the length of CMS <NUM> minimizes the undesired consequences of the switch-on phase, as discussed hereinbelow.

<FIG> illustrate the importance of the upstream region with a low concentration of diffusers during the switch-on phase of the system's operation. The maximum desired output Pmax in a kW power level laser system is reached in about <NUM> milliseconds which typically corresponds to the end of the switch-on phase te, as shown in <FIG>. As the output power Po reaches its maximum, the power of decoupled light Pdc also at its maximum often resulting in unacceptably high temperatures at a location x of the CMS's length, which is very close to its upstream end as shown by a phantom line Pdcpa of <FIG>.

To reduce the decoupled power at the end of the switch-on phase, some of the decoupled light power should be dissipated prior to the peak of decoupling. This is attained by upstream region <NUM> of CMS <NUM> along which a portion of scattered clad light decouples from cladding <NUM>. Accordingly, by the end of the switch-on phase, when the output power of the system is at its highest, the clad light reaches downstream region <NUM> where it decouples at somewhat reduced power Pdci as shown in <FIG>.

A thermal stress corresponds to a thermal gradient dT/dx, where x is a location of decoupling. The thermally induced stress corresponds to a dPdci/dte at the end of the switch-on phase. Increasing the time of induced emission front reduces a temperature gradient at the time of decoupling with the maximum decoupled power Pdci being lower than a stress threshold on the fiber. The solid line Tin of <FIG> clearly shows that a stress on the fiber configured in accordance with the disclosure is lower than a threshold. In contrast, as indicated by dash-line Tpa representing the known prior art, the critical threshold is considerably exceeded. As a consequence, the disclosed structure is much more resistant to the detrimental effects of the decoupling during a switch-on phase than the known prior art systems.

<FIG> illustrates theoretically modeled curves <NUM> and <NUM> representing the CMS which is configured with only the host material and CMS of of <FIG> and <FIG>, respectively. During a switch-on phase, a maximum temperature in CMS <NUM> of the present disclosure is reached at t2-t1 later than the maximum temperature of the CMS made only from the host material. This temporal difference allows a portion of high numerical aperture cladding light incident on diffusers to scatter along upstream region <NUM> and pre-warm downstream region <NUM>. As shown, a thermal stress on the disclosed CMS represented by a dash line <NUM> never reaches a fracture threshold <NUM>. In contrast, a CMS configured according to the known prior art and represented by curve <NUM> exceeds a thermal fiber fracture threshold.

<FIG> clearly illustrates advantages of the disclosed structure which includes diffusers <NUM> made of aluminum oxide. The <NUM>% Al<NUM>O<NUM> upstream region of the inventive CMS remains slightly heated during a switch-on phase. Even the <NUM>% Al<NUM>O<NUM> downstream region of the disclosed CMS does not exhibit a detrimentally high temperature. A hot spot <NUM> with the highest temperature is generated downstream from the border between upstream and downstream decoupling zones. At the moment of initial, most intense decoupling, the host material of the disclosed CMS already waveguides the cladding light and thus does not significantly contribute to the temperature increase. Substantially only Al<NUM>O<NUM> diffusers interact with light along the CMS-cladding interface and scatter it so that some light incident on the diffusers remains in the host material. However, the light intensity within the material is such that a thermal stress does not damage the fiber. The maximum temperatures detected during multiple experiments do not exceed about <NUM>.

Referring to <FIG>, disclosed CMS <NUM> is also effective to distribute induced emission along a large portion of downstream region <NUM> during the laser system's operation following a switch-on phase. Both <FIG> illustrate hot spot <NUM> at the end of CMS <NUM> during a stable operation of laser system with respective highest temperatures not exceeding about <NUM>° C at <NUM> W of input power and about <NUM> at about <NUM> W of input power, respectively. With the combination of the disclosed host material and diffuser concentration, CMS <NUM> is operative to decouple substantially the entire cladding light including that during the switch-on phase. Although technologically difficult, even better results were obtained in the disclosed CMS not with a step-wise profile of diffuser concentration, but with a gradually increasing percentage of diffusers distributed along the entire length of the CMS.

By way of example, excellent results have been shown by a CMS <NUM> having a length of about <NUM>. The <NUM> long upstream region contains a mixture of silicone with a refractive <NUM> and <NUM>% of Al<NUM>O<NUM> diffuser concretion. The downstream region of CMS <NUM> has the same host material and <NUM>% of the diffusers. In order to protect the entrance area from sharp temperature rise, about <NUM> of entrance area was potted with silicone PS145 having a coefficient of refraction of about <NUM> doped with a <NUM>% of Al<NUM>O<NUM>. The structure was placed in a curved aluminum housing to facilitate decoupling of small numerical aperture modes. More than hundred CMSs configured in accordance with the disclosure were tested and none failed during the switch-on phase. The concentration of diffusers can be controllably altered subject to the limitation of the present disclosure.

Claim 1:
A device for gradually removing cladding modes (<NUM>) during a switch-on operational phase of a high-power single mode fiber laser system (<NUM>), comprising:
a double-clad fiber (<NUM>) including a core and a cladding (<NUM>) surrounding the core;
a light absorber being in direct contact with a portion of the cladding (<NUM>) and including a mixture of host material (<NUM>), wherein:
a plurality of light diffusers (<NUM>) is embedded in the host material (<NUM>) to provide the mixture with an effective refractive index equal to or smaller than a refractive index of the cladding (<NUM>),
the diffusers (<NUM>) are configured to scatter clad light incident thereupon along an entire length of the absorber so that an upstream zone of the absorber heats at a temperature lower than that of at least one downstream zone,
along the entire length of the absorber, the diffusers (<NUM>) are embedded in the host material (<NUM>) in step-wise manner with the upstream zone (<NUM>) provided with a lowest concentration of the diffusers or with gradually increasing concentration of the diffusers towards the downstream zone (<NUM>), and
the clad light is substantially removed from the cladding at an end of the switch-on phase.