Introduction

Fibre lasers are a very convenient source of highly focused light for industrial applications, as they not only offer efficient operation, but are also compact, lightweight and resistant to shock. Currently, most designs of high-power fibre lasers are based on rare-earth-doped optical fibre that features a double layer of glass cladding and is end-pumped by a laser diode. Such lasers offer extremely efficient operation^1,2, but the small surface area of the fibre-end limits the maximum output power that can be achieved to around 1 kW.

Although the use of higher-power pump diodes or multiple fibre-coupled diodes can help increase the output power of the fibre laser³, these approaches are not ideal as they are costly to implement.

To overcome these issues, Hamamatsu Photonics K. K. Laser Group has recently developed the Fiber Disk Laser. This unique design is so named because the optical fibre is coiled into the shape of a flat disk. As explained later in this article, this special geometry brings several performance benefits. The result is a source that is capable of delivering high-power output while retaining the attractive qualities of lower-power, conventional fibre lasers.

Fiber disk laser design

A schematic of the Fiber Disk Laser can be seen in Fig. 1. The coiled fibre is pumped by several laser diodes. The light enters the fibre from the periphery and is gradually absorbed by the core as it passes through the disk. The laser output exits from the end of the fibre.

Figure 1: Schematic of the Fiber Disk Laser.

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In contrast to conventional designs in which the pump light is delivered through the ends or sides of a longitudinal fibre, the coiled-disk-pumping scheme offers a much larger area for introducing pump light. As a consequence, the design is compatible with wide-area, high-power laser diode bars, which consist of a linear array of single emitters. This compatibility means that the pump power can be significantly increased at low cost, which opens the door to cost-effective and convenient scaling of fibre laser output power. Furthermore, flat heat sinks can be attached to the top and bottom surfaces of the fibre disk for convenient and effective cooling⁴.

Ideally, all the pump light that enters the disk would contribute to pumping the rare-earth ions of the fibre core, but in reality, some of the incoming light is lost due to scattering processes within the fibre. These losses increase in proportion to the length of the fibre and reduce the laser extraction efficiency. When determining the optimum fibre length of the coil, the scattering that occurs as the pump light is transmitted through the disk must be taken into account, as well as other background losses (absorption and dispersion of the glass) and the absorption coefficient at the pumping wavelength of the fibre. Hamamatsu Photonics configures the fibre in a series of concentric rings that are in contact with each other and do not have any gaps. To cool the disk effectively, a soft, extremely thin, resin layer is placed between the fibre disk and a cooling plate. Figure 2 shows a photograph of the prototype laser head, which fits into a standard 19-inch rack and is approximately 100 mm tall.

Figure 2: The Fiber Disk Laser unit.

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Laser performance

Hamamatsu Photonics are currently investigating the performance of a Fiber Disk Laser that is doped with the rare-earth ytterbium (Yb³⁺). This laser combines high-power output with high quantum efficiency and slope efficiency. For the pumping of the Fiber Disk Laser, twelve water-cooled laser diode bars per module are used, each offering a maximum output of about 70 W. Figure 3 shows the input/output characteristics measured in laser tests. The emission wavelength is approximately 1,100 nm, with a continuous wave output power of 580 W from the fibre end and a slope efficiency of around 69%. The laser's beam parameter product is under 10 mm mrad, which provides laser beam quality and power density that are not attainable with conventional solid and gas lasers.

Figure 3: Input/output characteristics of the Yb³⁺-doped Fiber Disk Laser.

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Having subjected the fibre laser to cutting tests as illustrated in Fig. 4, we have demonstrated that it is suitable for use in the high-grade machining of materials. Figure 5 shows a steel plate with a thickness of 0.5 mm after cutting with the Fiber Disk Laser; a cut width of approximately 120 m was achieved, and this shows promise for cutting and welding applications.

Figure 4: Cutting test using a Fiber Disk Laser.

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Figure 5: Steel plate sample after cutting with the Fiber Disk Laser.

The cut width is about 120 m.

Hamamatsu Photonics

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Other potential uses of the laser include three-dimensional forming and surface treatment of materials. Two disks can be combined to form a Fiber Disk Laser in either a cascade or parallel configuration, producing a source that is capable of delivering a laser power in excess of 1 kW from a compact laser head.

Future work will involve studies of the long-term stability of the laser, tests of its machining performance (including its welding and cutting abilities) and an investigation into ways to boost the power output, through multistage configurations, for example.

Note

This article was submitted to Nature Photonics by a commercial organization and has not been peer-reviewed. Nature Photonics takes no responsibility for the accuracy or otherwise of the information provided.