High-power fiber combiners
High power fiber lasers are used in various industrial processes: material processing, marking of parts and products, cutting and welding. Due to its high efficiency and excellent qualities of its outgoing beam in combination with the compact design, fiber lasers have gained popularity.
Currently, there are two types of active fiber – GTWave and DoubleClad. The structure of the GTWave fiber has an optical fiber with an active core and optical fibers intended for the propagation of pump radiation. The scheme of such a fiber is illustrated in Fig. 1.
It is known  that by using GTWave fiber, it was possible to achieve the radiation power 5,2 kW. The main disadvantage of such fibers is the difficulty in input the pump radiation. The fiber has a small number of input points and limits the input pumping power due to the high probability of ignition.
The most popular and demanding today are fibers like DoubleClad. DoubleClad fiber has two clads: the first provides the propagation of useful radiation in the core, the second – the propagation of pump radiation. The cross-section of a double-clad fiber is shown in Fig. 2.
If we consider the scheme of a high- power laser (Fig. 3), then it can be seen that the main elements of the laser are the fiber-optic radiation pump combiners, mode field adapters, cladding power strippers and fiber end caps.
When designing fiber-optic components, it is necessary to take into account a lot of details that ensure reliable operation of the elements in the conditions of high radiation power. For example, losses in components that are normally for telecommunications applications can lead to the destruction of a high-power laser system due to a strong overheating. As a rule, the most vulnerable element of a high-power fiber laser is the pump combiner, which provides for the input of pump radiation into the fiber.
This article deals with a one of the most important types of components used in fiber lasers – fiber combiners, used as the element base of fiber lasers and amplifiers. Fiber combiners can be divided into several types:
• pump combiners of N Ч 1 structure. Devices of this type combine multiple pump radiation sources into one fiber (Fig. 4).
• pump combiners with a central signal channel of the (N + 1) Ч 1 structure. These devices combine several sources of pump radiation into one fiber, and additionally have a channel in the center into which a signal is fed for further amplification (Fig. 5).
• combiners of powerful signals of the SN Ч 1 structure. They are designed to combine several high-power fiber laser sources into a single fiber with a total power of 10 kW or more. The structure is similar to the Nx1pump combiners, the only difference being that each fiber is a signal one (Fig. 6).
OF FIBER FUSION IN THE COMBINERS
In the pump combiners of the N Ч 1 and (N + 1) Ч 1 multiple multimode fibers are used, which propagate the pump radiation. In the manufacture of such combiners, the tapering technology is used, when the fibers are drawn out with a decrease in their diameter and further are spliced to the corresponding output fiber. Tapering is characterized by a tapering coefficient (TR). It is defined as the ratio of the effective diameter of the input fiber to the diameter of the outlet one. Under the effective diameter of the input fiber, a fiber diameter is adopted which gives an equivalent cross-sectional area equal to the sum of the areas of the input fibers.
To avoid energy losses during the transmission of a light signal in the forward direction, it is necessary to preserve the brightness of the beam. Harmonization of the brightness of the input beam (in) and output (out) fibers with the corresponding numerical apertures NAin and NAout is carried out from the calculation that the tapering coefficient (TR) satisfies the following condition:
TR · NAin ≤ NAout. (1)
In practice, to reduce losses when welding fibers of different types, geometric and diffusion tapers are used (Fig. 7). Geometric tapers those produced by drawing the fiber with large geometrical dimensions, the diameter of the core and cladding are changed linearly. The fiber drawn to the required core diameter is cleaved, after which splicing is performed. Diffusion tapers are produced when the fiber is heated, resulting in diffusion of dopants from the core to the clad. Diffusion leads to an increase in the diameter of the core and to a decrease in the difference in the refractive index between the materials of the core and the clad, which in turn leads to an increase in the diameter of the mode field. The diffusion rate depends on the temperature, as well as on the chemical composition of the core matrix containing the elements Ge, Al, F, rare earth elements Yb, Er, Tm, etc.
To minimize losses and preserve the quality of the beam, it is important to take into account the effect of the transient characteristics both types of tapers. In certain taper length, the condition of adiabatic expansion is fulfilled, and the fundamental mode propagates without significant losses due to excitation of higher modes. Therefore, for fiber combiners, the frequency of the mode transition must be adiabatic, and the mode field at the output must coincide with the field of the mode of the output fiber.
A complete quantitative description of the propagation of the optical field through the transition region requires the solution of Maxwell’s equations with the corresponding boundary conditions based on the actual waveguide property of the fiber and the optical characteristics of the transition region. When the radiation propagates in the core of a weak waveguide, the parameters of the field in the fiber are determined by the Helmholtz equation . The interaction between the fields in the transition region can be described by the theory of coupled modes. Evolution of a field is a superposition of the eigen modes [5, 6]. In numerical terms, the Helmholtz equation can be solved by a method of propagation of the finite beam using commercially available software, e. g., BeamProp Software  based on the optical properties of the fiber with the corresponding boundary conditions. When the fiber undergoes heat treatment, the dopant in the core diffuses into the clad, which leads to a change in the optical properties of the fiber. The dopant diffusion moves of the alloying impurities in the fiber can be calculated from the Fick law  taking into account the distribution of the dopant in the fiber before diffusion.
When the two fibers are connected to each other by splicing joint, there is a loss in transmission at this point (transmission loss), which can appear because of the inconsistency of the mode field of the fibers to be welded and whether arise in the transition region (transition loss). The loss in the transition region occurs when the mode conversion is too fast or, by definition, is not adiabatic. The loss and error modes can be estimated based on the overlap integral of the amplitudes of the waveguide mode fields using the following equation:
where ϕA (r, θ) and ϕB (r, θ) are the normalized amplitudes of the mode fields for two fibers A and B.
When negotiating the mode fields, is important to consider that just a coincidence of the mode field diameters (MFD) does not mean the presence of zero-loss. Furthermore, the forms of the mode field must coincide, and the mode transition must be adiabatic, i. e. run with minimal loss. Ultimately, a thorough analysis of the distribution of the profile of the refractive index along the length of the fiber connection determines the optimal mode coupling.
Let us examine in more detail the method of matching the mode and brightness fields, using the example of pump-combiners by Lightcomm
When thermal expansion except of the core except for its diameter and the index of refraction, the number of mode propagating in the fiber also changes. And this means that, after diffusion, a single-mode fiber can turn into a multimode one. But this statement does not apply to the fused biconical tapering method (FBT). In this case, the refractive index and, accordingly, the numerical aperture remain unchanged, and therefore the number of propagating modes remains unchanged, where only the diameter of the core changes (Fig. 8).
For fibers with a stepped refractive index distribution profile mode can be determined analytically. Fig. 9 shows changes of the diameter a of the radiation mode field (λ = 1064 nm), depending on the size of the core for different fixed values of the numerical aperture.
For each value of the numerical aperture there is a definite value of the diameter of the core, where the diameter of the mode field is minimum. Any further reduction of the core size is actually the increase in the mode field diameter. For example, if the fiber diameter of 400 µm with a core of 20 µm and a numerical aperture of 0,06 is dragged over in the fiber of diameter 125 µm (TR = 3,2), then the core diameter will decrease to 6.25 µm. A mode field diameter thus will remain approximately the same, i. e. 18 µm (Fig. 10).
It is important to note that if two fibers have the same value of the mode field diameter, this does not necessarily mean that these two fibers have the perfect coordination of events. They can have different mode profiles and therefore make a significant loss specified in the equation (2).
It is necessary to carefully control the tapering, so that it occurs adiabatically. The general rule of thumb is that in order not to excite higher-order modes, the taper pitch should be sufficiently flat, and its parameters (r is the radius of the core of the fiber to be narrowed, β1 and β2 transmission constants of the main mode and higher order modes in the core of the fiber) shall correspond to the inequality condition (3).
This means that to ensure the transition of modes without losses, a certain minimum length of the taper is required. Large taper length provides low loss, but their manufacturing may require complicated and cumbersome equipment. Therefore, it is required to optimize the lengths of taper. To estimate the required length of the taper for different fibers, use the BeamProp software  to calculate the losses. In the simulation the tapered fiber has two sections: a taper and a straight line. During the calculation, the length of the taper section was changed for a given tapering coefficient (TR), and the length of the straight section in all cases was 5 mm. The results of the calculation for both cases (fiber No. 1: 20 m / 0.06 NA TR3 and fiber No. 2: 10 µm / 0.08 NA with TR2) are shown in Fig. 8. Their analysis showed that the minimum length of constriction should be about 12 mm.
Furthermore, care must be taken when conducting tapering. Otherwise, you may experience losses due to defects, macro- or micro-bending, and this will lead to worsening quality of the output beam and to loss increase.
The purpose of the output signal combiner is different from the purpose of the pump combiner. Its task is to combine several fiber laser outputs (fiber lasers). They can be combined either incoherently, as the union of several output fibers into one fiber, or coherently, by matching the phase of each channel. In this paper, the questions of coherent addition of radiation will not be considered.
In general, the method of creating combiners is similar for all types and includes the following steps: combining in a bundle, tapering, stripping and splicing.
One of the most convenient ways of combining fibers in a bundle is the use of quartz capillaries. The joining process begins with the packing of the desired number of fibers into a capillary with an internal diameter (ID) of not less than the outer diameter of the fiber bundle. In the future, this part of the capillary with the fiber in the clad will be the structural element of the combiner. Fig. 12 illustrates this concept using the example of a 7 : 1 combiner. In the next step, the capillary is drawn to obtain a straight portion of the inner diameter equal to the outer diameter of the fiber bundle cleaned from the polymeric clad. Further, the fibers themselves are cleaned of the clad, wiped and loaded into a previously prepared capillary. If it is necessary to use a signal fiber, then at this stage it shall be installed strictly in the center of the bundle. Fibers in the capillary in a place without a clad are dragged to the required value of TR with the required length of the taper (adiabatic taper). Often, for ease of cleavage, a small straight line is added after the taper. The discarded fiber bundle is welded to the corresponding output fiber with possible modulation matching. The lower part of Fig. 12 shows photographs of fiber combiner ends 7 : 1 and 19 : 1 created on installation Vitran GPX 3400.
The losses in the pump combiners should be minimal (this leads to minimum heating at the combiner). In this case, the heating temperature depends on the correct distribution of the numerical aperture in the tappering zone. If the fiber or fiber bundle narrows so that the law of total internal reflection is violated, then a sharp increase in losses occurs. Therefore, it is very important to correctly calculate the distribution of numerical aperture when the fiber is tapered. In this case, it is necessary to minimize the tapering coefficient, and also to reduce the area of "dark fiber". The "dark fiber" is any cross-sectional area through which radiation does not pass: a fiber clad or a capillary tube. To reduce these losses, dark fiber zones are etched with hydrofluoric acid (HF).
An important role is played by installations where tapering and fiber splicing is performed. There are installations with arc, heat and laser splicing. It is hard to say which of the are better ones, but laser apparatuses are much more popular (Fig. 13).
In conclusion we would like to emphasize the main points of design and production of high-power fiber combiners. High power pump combiners of "Lightcomm" are developed on the basis of the fused biconical tappering technology (FBT).
When manufacturing high-power fiber pumping combiners with a signal channel, the mode field of the input and output signal fibers is determined initially, and then the distribution of the fundamental mode radiation power along the transition length is calculated. In this case, the overlap integral of the mode fields of two fibers (formula 2) should tend to unity. Thus, the insertion losses are minimized.
The diameter of the spot of the mode field is chosen depending on the diameter of the fiber core in accordance with the saddle curve (see Fig. 9).
Based on the diameter comparison of the mode field between input and output, the corresponding fiber core size is calculated.
The brightness matching is calculated depending on the ratio of the core diameter of the input fiber (a), the number of input fibers (N), the output diameter (b) and the numerical aperture of the input and output fibers (NAa, NAb). After calculating the necessary parameters, the fibers undergo high-temperature processing.
The final stage in the manufacture of high-power pump combinators is the production of a casing with efficient heat dissipation (Fig. 1–5). Depending on the power and structure of the combiner, the packages differ in overall dimensions, because the higher the heating temperature, the greater the length of the thermal dissipation. Therefore, advanced production technology allows to work with a power of about 10 kW, which is an absolute record. Every month, Lightcomm produces more than 7,000 units. There are different variations of combiners structures such as N Ч 1, (N + 1) Ч 1, there are versions of polarization-maintaining amount united fibers 61 can reach.
Are currently the most popular structures 7 Ч 1, 19 Ч 1, (6 + 1) Ч 1 and (18 + 1) Ч 1 (see. Fig. 12), as these are the numbers of the fibers help to achieve maximum combining density and, consequently, increasing pump efficiency. Such structures as 37 Ч 1, (36 + 1) Ч 1 and (60 + 1) Ч 1 are rarely used, because of not very high economic efficiency (the set output power level, the cost of the combiner and all the laser diodes is measured in the number of dollars per watt), mainly in the military industry or research institutes. Structures (1 + 1) Ч 1 and (2 + 1) Ч 1 are used in low-power pulsed laser systems, e. g. MORA lasers or Q-switched lasers.
Table. 1 shows the main types of pump structures combiners (N + 1) Ч 1. Table. 2 shows the characteristics of the most commonly used pump combiner (6 + 1) Ч 1.
Fiber optic systems are developing rapidly, and more and more industries are applying them as technological solutions. Proper selection of components will not only ensure a long service life, but also the stability of the system. As is well known leader in the production of fiber-optic lasers and systems based on them is IPG Photonics. It should be noted that to achieve success in the field of production of lasers and laser systems can be by providing their own element base and unify solutions. Thus, Lightcomm provides required in optical elements needs to create fiber lasers to 10 kW in single-mode and less powerful in few-mode regime. The use of special fiber signal combiners opens up the possibility of achieving in excess of 10 kW power for industrial applications. Success in creating pumping modules companies BWT and II VI Laser Enterprise covers the need for the pump sources. Also, in China, there are manufacturers of fiber lasers themselves and the actual and systems, co-operation which has opened up new opportunities for consumers.