Brightness preserving fiber beam combiner for reduced nonlinearities and intense radiation damage durability

Abstract

Method for adapting a fiber beam combiner to transmit at least 20 kW of optical power without noticeable bulk material damage mechanism effect and destructive nonlinearities, the method comprising: connecting an adiabatic beam combiner with a splice connection to an input facet of a graded index fiber which has a core doped with an index increasing material, further comprising the step(s) of: restricting the numerical aperture of the graded index fiber, and/or selecting the index increasing material with a Raman gain lower than that of GeO.sub.2 such as Al2O3 or Y2O3, and/or placing a shroud tube around the graded index fiber core, said shroud tube comprising a fluorine-doped silica tube.

Claims

1. A method for adapting a fiber beam combiner to transmit at least 20 kW of optical power without noticeable bulk material damage mechanism effect and destructive nonlinearities, the method comprising: connecting an adiabatic beam combiner with a splice connection to an input facet of a graded index fiber which has a core doped with an index increasing material, said graded index fiber having a numerical aperture, placing a shroud tube around the graded index fiber core, which adapts said beam combiner to transmit at least 20 kW of optical power without noticeable bulk material damage mechanism effect and destructive nonlinearities, and restricting the numerical aperture NA to be 0.15<NA<0.18.

2. The method according to claim 1, wherein the shroud tube comprises a fluorine-doped silica tube.

3. The method according to claim 1, further comprising selecting the index increasing material with a Raman gain lower than that achieved by a typical 20 mol % of GeO.sub.2 dopant level.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

(2) FIG. 1 is an illustration of a lower peak index graded index core fiber, combined with lowered-index shroud tube that encircles the core, in accordance with an embodiment of the invention.

(3) FIG. 2 is an illustration of a prior art refractive index profile of a graded index fiber.

(4) FIG. 3 is a flow chart of connecting an adiabatic beam combiner with a splice connection to an input facet of a graded index fiber, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

(5) In contrast to the prior art, the present invention relates to a large combiner whose waist width can confine a few-mode beam to a level that it is compressed to give the lowest BPP value, yet is enough wide such that it maintains intensity levels that are factors below the silica bulk damage. This dielectric/bulk damage threshold is mostly referred to as 5 w/m.sup.2 and 10 w/m.sup.2 for exit facet (silica-air boundary) and internal glass (i.e. material continua), respectively (Dawson et-al, Optics Express (2010)).

(6) Traditional GI (graded-index) fiber fabrication is done by gradually doping of materials into the core, that, as a side effect, have high nonlinearity gain, of which SRS (stimulated Raman scattering) becomes a major challenge. A very common material used for index increasing is GeO.sub.2. However, the most prominent characteristic of this addition is its Raman Gain, which is 8-9 times higher than of pure silica (g.sub.R(SiO.sub.2)1.05e13 m/w, with spectral gain peak at stokes wave shifted by v13 THz from the laser pump line). Using a gradually varying GeO.sub.2 concentration in a core with peak concentration corresponding to traditional acceptance angle values (NA=0.29), with MM beam, yields an overall SRS gain that is of factors higher (anywhere between that of the host and the dopant) than of silica.

(7) In order to battle this effect, embodiments of the present invention cover the following guidelines:

(8) Use lower peak NA values (like, but not restricted to 0.15<NA<0.25), which is translated to lower GeO.sub.2 concentration (% mole or % weight). For example: if a peak NA of 0.15 is used instead of the traditional 0.29 value, only about 5% (mol) of the GeO.sub.2 is used (instead of 20% in the prior art).

(9) Use index-increasing materials with lower Raman gain g.sub.R with respect to GeO.sub.2. Non-limiting examples of index-increasing materials instead of GeO.sub.2 are Al.sub.2O.sub.3 and Y.sub.2O.sub.3. Their refractive indices in the 1 m spectral region are n1.75 and n1.9 at =1070 nm, respectively. Both either broaden and reduce the SRS peak gain spectrum, and are therefore appropriate to use for this purpose. Typical overall SRS attenuation of 50%.

(10) Use an additional lower index shroud tube around the GI core; this leads to the same effective NA as if silica clad with standard GeO.sub.2 was used. For example: if a (e.g., 5% mol) fluorine-doped silica tube is placed around the core, an effective confinement of typically 0.25 NA is obtained within the silica core. In such case, doping GeO.sub.2 into the core to the required parabolic profile can be achieved with a much lower concentration, to the vicinity of 5% mol.

(11) Use larger exit facet of the combining element (and corresponding waist size) to reduce power densities, lead to reduces bulk damage likelihood and lower nonlinear scatterings. (An example is given in the following paragraph).

(12) Using larger GI core, in correspondence with the larger combiner s exit aperture (see former sections). This enables comfortable injection of the signal from the combiner into the delivery fiber, as well as accommodating the larger signal, compared to prior arts. For example, if a combiner is fabricated with a double-size output exit diameter with respect to standard GI fibers (50 m and 62.5 m are very common), and a roughly similar increase of beam spot size is also applied, power density is fourfold reduced. This enables roughly four times higher optical power delivery (bulk damage aspect) and fourth-exponential power higher SRS threshold.

(13) The present invention provides an efficient and high beam quality combiner device of several fiber lasers, with high nonlinearities threshold, high bulk damage threshold and lower concentration of dopant material.

(14) FIG. 1 is an illustration of a lower peak index graded index core fiber, combined with lowered-index shroud tube that encircles the core, in accordance with an embodiment of the invention. The overall effect is of high control on core's profile, effectively providing similar performance of higher refraction-index materials, while using a lower absolute dopants concentration. Also shown in FIG. 1 is the absence of a central refraction-index void, responsible for some of the brightness decrease previously observed with prior art GI fibers.