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Flexible slab laser gives industry an alternative

27 Mar 2008

A laser that produces high-quality light in a variety of beam shapes could be adapted to suit a range of applications. Marie Freebody speaks to Keming Du of EdgeWave to find out more about its slab laser technology and the areas that could benefit from the design.

Today's industrial applications tend to look to fibre, disk and diode-pumped solid-state as the laser of choice. But one German company pioneering a different approach is EdgeWave, which believes that slab lasers offer a feature set not to be found in any of its competitors.

According to Keming Du, chief executive officer and founder of EdgeWave, the most important advantage of its Innoslab diode-pumped slab design is the unique combination of features that it offers. "Innoslab provides short pulse length, high repetition rate, high pulse energy and scalability, while maintaining high beam quality," he told OLE. "We also have the ability to change the beam profile from a line to a square or circular shape."

Du began pioneering the Innoslab design back in 1996 during his time at the Fraunhofer Institute of Laser Technology (ILT) in Aachen. So convinced by the commercial potential of the design, he chose to form EdgeWave as a spin-off from ILT in June 2001. Since then the company has gone from strength to strength and has customers all over the world using slab lasers for applications ranging from solar-cell scribing to glass drilling.

What is a slab laser?
As its name suggests, a slab laser uses a plate with a rectangular cross-section as the gain medium. This shape allows slab lasers to avoid the optical distortion that is often associated with rod lasers.

"When a rod laser is pumped, energy is deposited within the crystal causing it to heat up. This heat is conducted in two dimensions from the middle of the crystal to the surface, causing deformation of the laser beam and limiting its quality," explained Du. "The crystal in a slab laser however, is a very thin plate of the order of 1 mm in height. This limits heat conduction to just one dimension so that you don't have the same level of stress or optical deformation."

EdgeWave advantages
While retaining all of the benefits of a traditional slab laser, Du pushes the design to another level. His key breakthrough uses partial pumping of just one layer of the slab crystal by an arrangement of stacked diode lasers. The pumped volume has a linear cross-section and only partially covers the cross-section of the slab crystal.

"The crystal has a cross-sectional area of 12 x 1 mm. Instead of pumping the entire crystal, we focus the pumping beam to 12 x 0.4 mm using a microlens array," explained Du. "By pumping a 0.4 mm flat layer in the middle of the crystal we reduce the optical distortion that occurs at the corners of the slab where the geometry of the crystal structure is broken."

Thanks to a short resonator length of just 70 mm, the end result is a high-quality beam and pulses with a duration of around 10 ns. By increasing the width of the crystal, the power and pulse energy can be increased without changing the beam profile or quality.

The crystal is typically Nd doped, however, unlike disk lasers any type of crystal can be used such as Nd:YLF, Nd:YAG or Nd:YVO4. Disk lasers require a crystal with a very high absorption coefficient and doping capacity. "For most applications, Yb:YAG is the only crystal that can be used in disk lasers as it can be doped to 20% maximizing absorption in the thin disk," explained Du. "In a slab laser we can use any type of crystal and absorption can be increased by increasing the length."

Du believes that his Innoslab concept was made possible thanks to the evolution of high-power diode lasers. "Lamp-pumping beams do not have enough energy or power to be concentrated into a line shape," commented Du. "A diode laser, however, can produce a high-quality line-shaped beam. Combining this with the partial pumping technique means that you can generate a high-quality laser beam."

Apart from high-power diode lasers for pumping, figure 1 shows the other components that are key to the Innoslab design. A planar waveguide and microlens array are used to homogenize and focus the pumping beam before it reaches the crystal. A dichroic-coated resonator mirror (M1) transmits the pumping light at 800 nm and reflects the laser emission at 1064 nm. Another resonator mirror (M2) is placed at the other end of the crystal.

Pulses are produced at a repetition rate of 50 kHz using a Pockels cell (electro-optic Q switcher) and a polarizer. "The Pockels cell contains a BBO crystal, which rotates the plane of the laser beam when a voltage is applied," explained Du. "The subsequent polarizer will then block the light, effectively closing the cavity. When the voltage is switched off, the cavity opens providing a short pulse of laser light."

A variety of wavelengths including the second and third harmonics can be generated in this way. "Our lasers can produce light at 1064, 532 and 355 nm with average powers of 600, 120 and 20 W respectively," commented Du. "We are currently working on increasing the power in the second harmonic (532 nm) from 120 to 300 W."

Beam shapes
Many lasers require additional effort and cost to produce different beam shapes. Innoslab lasers on the other hand can produce a variety of beam shapes without the need for diffractive optics or a microlens array. "We can get a square top hat down to 30 x 30 µm, which is difficult to obtain using a microlens array or diffractive optics," commented Du.

A circular (Gaussian) beam and a square (2D top hat) beam are produced using a cylindrical telescope to make the sides of the beam symmetrical. For a Gaussian beam an additional spatial filter is used to eliminate the side lobes. "The natural beam produced is a line-shaped beam that means that the intensity distribution along the line is homogeneous and across the line is Gaussian," explained Du. "This is used for a lot of applications such as for ablation processes and for solar-cell scribing."

Applications
The ability to tailor the beam shape means that Innoslab lasers can be adapted for use in applications such as drilling, structuring, cleaning, polishing, surface marking, sub-surface engraving, cutting and rapid prototyping.

One application that is only possible thanks to Innoslab's unique combination of properties is glass drilling. Glass is typically drilled from above using excimer or CO2 lasers. Innoslab, however, focuses the beam through the glass to a very small spot on the opposite surface.

"This allows the ablated material to escape freely without disturbing the path of the laser beam leading to effective drilling," commented Du. "The process requires high pulse energy and short pulse duration, as well as a high-quality beam for focusing to a small spot."

A line-shaped beam can be used to ablate a strip of material more efficiently than using a circular beam. Similarly a square-shaped beam can be used for solar-cell scribing. Conventionally, a Gaussian beam is used, but thermal damage can be caused at the edges of the beam where energy is not high enough to ablate.

"By using a square-shaped beam, you can use much less energy to get a very well-defined boundary," explained Du.

• This article originally appeared in the March 2008 issue of Optics & Laser Europe magazine.

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