WO2013126924A1

WO2013126924A1 - Fiber beam combiner

Info

Publication number: WO2013126924A1
Application number: PCT/US2013/027708
Authority: WO
Inventors: Yoav Sintov; Yariv Shamir
Original assignee: Soreq Nuclear Research Center; Klein, David
Priority date: 2012-02-26
Filing date: 2013-02-26
Publication date: 2013-08-29

Abstract

A fiber beam combiner assembly includes a high quality adiabatic beam combiner (10) which is connected with a splice connection (14) to an input facet of a graded index (GRIN) fiber (12).

Description

FIBER BEAM COMBINER

FIELD OF THE INVENTION

The present invention relates to a novel fiber beam combining device, which preserves the beam parameter product, i.e. beam-quality, of a multiple input beams, launched into its input fibers.

BACKGROUND OF THE INVENTION

Many commercially available fiber-based tapered fused bundles (TFB) power combiners are designed to be seeded with high multimode (MM) input beams, being combined into a larger MM output fiber. The performance of such combiners is such that the output signal, although capable of delivering high power levels, has a poor spatial beam parameter product (BPP). Thus, it markedly diverges and is not suited for directed radiation applications. (See, for example, US Patents 7046875 "Optical coupler comprising multimode fibers and methods of making the same" and 6434302 "Optical couplers for multi mode fibers".)

In contrast, strictly or nearly strictly single mode TFB combiners are capable of preserving the lowest obtainable BPP, in the case of incoherent side by side combining, by keeping adiabatic mode propagation.

The challenge that follows the beam combiner device is to further guide the signal in a way that will not diminish its brightness and will minimize possible power loss.

A common method is to make a splice connection between the end of the TFB combiner and another MM step index delivery fiber. In case that the output fiber is a multi mode fiber, the result is poor because of the large number of propagating modes that have large angular divergence in free space. In the event the output fiber supports a low number of modes, the device becomes difficult to fabricate, since the low NA (numerical aperture) of the output fiber makes the process extremely sensitive to mutual positioning. In both cases, the output signal has uniform low quality spatial properties.

SUMMARY OF THE INVENTION

In contrast to the prior art, the present invention uses a graded index fiber (GRIN fiber) as an output delivery fiber following the TFB combiner and has a unique feature of preserving the BQ (beam quality) of the input signals. Such fiber is usually (but not always) made as a parabolic index profile, which significantly helps in reconstructing the input signal. Moreover, even if this GRIN fiber is in the deep MM regime, this feature is preserved. Thus, one can benefit from the reduced sensitivity of the MM fiber to misalignments, as well as from its BQ preserving feature. In terms of power efficiency transfer, the large number of modes supported by this fiber make it very efficient, and as long as the incident signal Mode Field Diameter (MFD) is kept within about 70% of the core's diameter, almost all of the power is transferred.

The present invention relates to a fiber beam combining device which preserves the beam quality of a combined plurality of fiber sources. It is based on the unique feature of GRIN fibers to analytically match the input signal phases at evenly spaced periods. In contrast to common fiber combiners, related to standard step index MM fibers, making a splice connection of a beam combiner with a GRIN fiber yields the beneficial BQ preserving capability. The present invention provides an efficient and high beam quality preserving combiner device of several fiber lasers.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 is an illustration of a typical refractive index profile of a graded index fiber;

Figs. 2A and 2B are illustrations of two cases where the combined signal is transferred into a delivery fiber, wherein Fig. 2A shows a step index MM fiber (prior art) and Fig. 2B shows a graded index MM fiber, in accordance with an embodiment of the invention; and

Fig. 3 is a microscope image of the high quality splice connection between the end of the TFB combiner emerges on the left, and an output MM graded index fiber on the left side, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Reference is now made to Fig. 1, which illustrates a typical refractive index profile of a graded index fiber.

The following conditions are required to obtain and preserve a high quality brightness beam combining device (referring to Fig. 2B): 1. An input signal traverses a high quality adiabatic beam combiner 10 and then is introduced at the input facet of a graded index (GRIN) fiber 12.

2. A high quality splice connection 14 is performed between the two devices 10 and 12, so that the signal is not significantly distorted.

3. The output fiber is a GRIN fiber 12, preferably a parabolic index type. This fiber supports multiple numbers of modes, but has a unique feature of signal reconstruction; therefore the BQ is almost not affected while passing between both parts of the device.

The BQ is reconstructed analytically at certain periods along the output GRIN fiber. This occurs since the mode propagation constants within a GRIN fiber are equally spaced, making a L=2 / fi period length.

In between these points, BQ does not diminish significantly, so the whole device is not sensitive to ambient perturbations.

Fig. 3 illustrates a microscope image of the high quality splice connection between the end of a TFB combiner emerges on the left, and an output MM graded index fiber on the left side.

Claims

CLAIMS What is claimed is:

1. A fiber beam combiner assembly characterised by:

a high quality adiabatic beam combiner (10) which is connected with a splice connection (14) to an input facet of a graded index (GRIN) fiber (12).

2. The assembly according to claim 1, wherein said splice connection (14) is such that a signal from said combiner (10) to said GRIN fiber (12) is not significantly distorted.

3. The assembly according to claim 1, wherein said GRIN fiber (12) is a parabolic index type fiber.