1. Introduction

High power fiber lasers are among the most promising laser types for industrial materials processing applications. The advantages of fiber lasers include excellent beam quality and thermo-mechanical properties, high efficiency and long maintenance-free lifetime. The disadvantages are usually related to the complexity and high cost of the system.

Pump light can be coupled into a fiber by end pumping and/or by side pumping. End pumping [1,2] is straightforward and efficient and slope efficiencies in excess of 80 % have been obtained. On the other hand, end pumping has some obvious limitations in the maximum pump power since there are only two fiber ends available for pumping. Side pumping [3–7] is attractive, because there is no principal upper limit to the launchable pump power and because the fiber ends remain free. In practice, however, high pump power levels are usually reachable with side pumping only by using a great number of coupling structures, which increases the complexity of the system.

In this paper we introduce an axially symmetric fiber side pumping scheme that enables the efficient launch of kilowatt-level pump power into the fiber through a single coupling structure. The method is essentially based on an axially symmetric high-brightness and high-power diode laser light source that has been presented in Refs. 8 and 9. This source is capable of launching kilowatt-level pump light into a single side pumping coupling structure of suitable design. In this paper such axially symmetric coupling structures are presented and their operation together with the light source of Refs. 8 and 9 is investigated experimentally.

2. Axially symmetric pump source

A high power diode laser device utilizing stacked single emitters and axial symmetry [8] is a suitable light source for axially symmetric side pumping owing to its good beam quality and essentially circular focus.

Figure 1 schematically presents the profile cross sectional view of the axially symmetric diode laser device. In Fig. 1 the plane of fast axis divergence, perpendicular to the pn-junction plane, is in the plane of paper. Only two sectors are shown in Fig. 1. Additional sectors can be placed symmetrically around the axis. This way kilowatt-level output power can be generated, as explained in more detail in Ref. 8. The light source has a hollow optical axis, whereby the fiber can be threaded through the device in such a way that the coupling area of the axially symmetric coupling structure is at the right location with respect to the output beam of the pump source. In addition to good beam quality, the use of single emitters in the pump source facilitates cooling and provides high reliability.

 

Fig. 1. Profile cross sectional view of an axially symmetric multiwavelength diode laser source with kilowatt-level output power.

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Recently, it has been experimentally demonstrated [9] that the axially symmetric light source is capable of focusing kilowatt-level light power into a circular spot with 1/e ² diameter of the order of 200 µm using a focusing lens with 50 mm diameter and 100 mm focal length. Such high brightness combined with kilowatt-level output power makes this light source extremely attractive for axially symmetric side pumping of optical fibers.

3. Axially symmetric coupling structures

Axially symmetric coupling structures preferably have the same, typically circular, geometry as the fiber. In the following such coupling structures are presented and the conditions for successful coupling are considered. The structures can be manufactured by adding material to and/or by removing material from the fiber.

Figure 2 schematically shows the profile cross section of a fiber with a coupling structure created by adding material to the fiber. The fiber diameter is 2R, the height of the coupling structure is H and the length of the coupling structure (at height H) is L. The sides of the coupling structure form angles γ and θ to the normal of the fiber. The angle of incidence of the incoming pump ray is α and it hits the coupling structure at height h. The angle of refraction is β. The sign definitions of the angles α, γ, and θ are indicated in Fig. 2. The index of refraction of the coupling structure is n ₂ and the index of refraction of the surrounding medium is n ₁. According to the law of refraction n ₁sin α=n ₂sin β. The incident ray will be coupled into the fiber if the condition of total internal reflection is fulfilled, β+γ>0 and

Figure 3 schematically shows the profile cross section of a fiber with a coupling structure created by removing material from the fiber. For reasonable rays α+γ>0. The condition for successful coupling can now be written as

As can be seen, coupling does not depend on R in this case. An additional requirement for successful coupling in both above mentioned cases is that n ₂sin (β+γ)<NA, where NA is the numerical aperture of the fiber.

 

Fig. 2. Axially symmetric coupling structure created by adding material to the fiber.

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Fig. 3. Axially symmetric coupling structure created by removing material from the fiber.

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A coupling structure manufactured by adding material to the fiber provides a larger pump area as compared to a corresponding coupling structure manufactured by removing material from the fiber. Furthermore, such a coupling structure will not weaken the fiber mechanically. If both addition and removal of material are utilized, the pump area can be maximized.

It is worth mentioning that one coupling structure provides access to the fiber for two or more pump sources. If necessary, it is straightforward to scale the pump power to the desired level by increasing the number of coupling structures and pump sources. However, as mentioned before, even one coupling structure of the presented type is sufficient for launching extremely high pumping powers if a suitable pump source is available. The distance between two coupling structures can be chosen to be such that the pump power is suitably absorbed to the fiber before reaching the next coupling structure.

4. Experiment

The demonstration of the axially symmetric light source, presented in Ref. 9, was successful although the prototype still suffered from variations in the quality of the fast axis collimation of the laser emitters. These imperfections were due to deficiencies of the technology of microlens attachment. It was considered desirable to eliminate these imperfections for the demonstration of the present work, but the assembly of another prototype was not possible for reasons of finance and time. Therefore, the operation of the light source was simulated by the stepwise movement of one successfully collimated broad area edge emitting high power diode laser emitter with 150 µm stripe width. The diode emitted at 803 nm and it was manufactured by Coherent Finland, Inc. The fast axis of the diode laser was collimated with a microlens (Grintech GT-LFCL-100-024-50-C2), and the slow axis was collimated with a cylindrical lens of 100 mm focal length (Edmund Optics K46-023). The emitter was moved across a focusing lens of 50 mm diameter and 100 mm focal length (Edmund Optics K47-317) and the light was registered using a CCD array located 0.5 mm before the focal plane. This way the light field of the pump source was simulated and built up, step by step. The principle of this procedure is illustrated in Fig. 4.

 

Fig. 4. Experimental setup for studying the suitability of an axially symmetric pump source in the pumping of a fiber with an axially symmetric coupling structure.

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A single image was recorded on the CCD array for each of the six radial positions of the diode laser: ±24 mm, ±18 mm and ±12 mm. After that an average image was calculated from the individual images. This average image, representing the combined beam 0.5 mm before the focal plane of the focusing lens, is shown in Fig. 5. From the optical point of view, the procedure carried out in the experiment produces the same output beam as a corresponding axially symmetric diode laser device with two sectors and successful fast axis collimation. Therefore the experiment, although simple, demonstrates the potential of the proposed method in a proper and reliable way.

Two circles with Ø 80 µm and Ø 400 µm have been drawn into Fig. 5. Together they define a coupling area that can be created into a typical Ø 400 µm double clad fiber by removing 160 µm material from the surface of the fiber by e.g. laser ablation. The fraction of pump light falling inside the coupling area was calculated by subtracting the amount of light outside the Ø 400 µm circle and inside the Ø 80 µm circle from the total amount of light in Fig. 5. According to calculations over 90 % of the launched light fell inside the coupling area.

As was mentioned in Section 3, a coupling structure created by removing material from the fiber provides a smaller coupling area than a corresponding coupling structure created by adding material to the fiber. The experiment therefore corresponds to a situation with the most stringent requirements for the pump source.

Finally, the coupling efficiency of the focused diode laser light into a real fiber was studied experimentally. For this purpose, the CCD array was replaced by one end of a passive double clad large mode area fiber having Ø 390 µm outer cladding and >0.46 cladding NA (Liekki Passive-20/390DC). The other fiber end was placed in front of a light power meter. Both fiber ends were cleaved perpendicularly to the fiber axis and remained uncoated. The length of the fiber was approximately 0.5 m. The collimated diode laser was again moved across the focusing lens and the light power transmitted through the fiber was read with the power meter. The diode laser was operated in room temperature in pulsed mode with 0.1 % duty-cycle. The pulse power transmitted through the fiber as a function of the radial position of the diode laser with respect to the axis is presented in Fig. 6. The pulse power before fiber coupling was also measured. Unfortunately, the protection tube of the detector blocked part of the focused beam and, consequently, only a maximum reading of 1.9 W was obtained. The protection tube didn’t affect the measurement of fiber-coupled light, since the fiber end was placed inside the tube, almost into physical contact with the detector. The light power of the emitter used in the experiment is typically approximately 2 W [9], so it can be concluded that the coupling efficiency was high at radial distances from -22 mm to +22 mm.

 

Fig. 5. The experimental combined beam 0.5 mm before the focal plane of the focusing lens. Over 90 % of the launched pump light is between the two circles indicating the coupling area.

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Fig. 6. The pulse power transmitted through the fiber as a function of diode laser radial distance from the axis.

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According to Fig. 6 the transmitted light power decreases at the periphery of the focusing lens. This can be understood if some part of the peripheral beams hit the edges of the focusing lens and/or lens holder, thus experiencing significant aberrations and/or being blocked away. In addition, it was not possible to verify that the fiber end was exactly at the same position as the CCD array had been. Therefore a fraction of the focused peripheral beams may have fallen outside the fiber end. Nevertheless, the fiber coupling experiment verifies that as long as the light falls within the coupling area, it will be efficiently coupled to the fiber.

These experiments indicate that by utilizing the prototype design of Ref. 9 (5 W single emitters, 8 sectors, 3 wavelengths) together with successful fast axis collimation and a suitable axially symmetric coupling structure, kilowatt-level pump power can be efficiently coupled into a fiber, even at a low maximum NA of 0.24. In principle, such a low NA enables the use of robust all-glass fibers, having no low-index polymer claddings that are prone to damage at high power levels. In addition, large core fiber, optimized doping profile and pumping configuration together with active cooling can be utilized in addressing the potential thermal issues in power scaling to high power [10,11]. For example, at 1 kW pump power level the maximum temperature of the fiber can be reduced from ~300 °C to ~100 °C by proper fiber design and pump configuration [10]. It is also of importance to pay attention to the structure and cleanliness of the fiber facets and coupling structures. It has been estimated that power scaling beyond 10 kW in a single-fiber configuration is feasible with proper design [2].

In the experiments presented in this paper an axially symmetric pump source with two sectors was simulated, but the number of sectors can be increased, as explained in Refs. 8 and 9, to scale the pump power into kilowatt-range and to yield full coverage of the coupling area of a single coupling structure by pump light at high efficiency. Since the method can provide practically unlimited pump power without the need to e.g. splice the core of the active fiber, it provides new possibilities for studying the ultimate power limits of fiber lasers.

5. Conclusions

A fiber side pumping scheme based on axially symmetric coupling structures and pump sources has been proposed. The principle combines both the straightforward approach of end pumping and the scalability of side pumping. The conditions for successful coupling have been discussed and the potential for efficient coupling of kilowatt-level pump power into the fiber through a single coupling structure has been experimentally verified. In the experiments over 90 % of the launched light fell inside the coupling area and the light falling within the coupling area was efficiently coupled to the fiber. Since the proposed method can provide practically unlimited pump power without the need to splice the active fiber core, it opens new possibilities for studying the ultimate power limits of fiber lasers.