Optics enhance CO₂ laser performance

08 Jul 2004

When it comes to directing high-power CO₂ laser light, special optics made from metals and semiconductors are needed. Paul Maclennan looks at the components available.

From Opto & Laser Europe July/August 2004

Over the past 30 years, metal processing using carbon dioxide (CO₂) lasers has become the preferred way of achieving fast and precise results in cutting and welding. This is due to an ever-increasing amount of available laser power (6kW and beyond), and improvements in the gas-flow dynamics and the optics that deliver the laser beam to the workpiece. Below is an outline of the optics commonly available for use at 10.6µm, the laser's wavelength of operation.

Cavity optics The CO₂ laser cavity is a unique environment where high optical durability is essential. Ultraviolet (UV) radiation from the plasma damages optics unless they are either uncoated or suitably treated with UV-resistant coatings, and replacing cavity optics is time-consuming and expensive. The minimum number of optics required in a cavity to generate a laser beam is two - a rear mirror and an output coupler. Some lasers employ fold mirrors, which are used in extended-length cavities.

The rear mirror is usually designed to reflect as much laser power back into the cavity as possible and is often called a maximum reflector. Such reflectors tend to be made from silicon (Si) and metals such as copper (Cu) and molybdenum (Mo). The latter two are natural reflectors at 10.6µm, although Cu is best used with a coating of gold to prevent tarnishing. Si (and Cu) can also have a coating applied to reflect 99.9% of the incident power at 10.6µm. Mo is less frequently used these days, but found some use in early pulsed-cavity environments.

In cases where it is necessary to monitor power fluctuations, the rear mirror can be manufactured to transmit a fraction of a percent of the power onto a monitor detector situated behind the mirror. In this case the optic is often called a leaky mirror. These mirrors can be made from germanium (Ge), gallium arsenide (GaAs) or zinc selenide (ZnSe). Quite often system designers opt for Ge or GaAs as they reflect well in the visible spectrum, which is useful for cavity alignment. Although these materials have relatively poor optical performance at high power, they can function as leaky mirrors because they are required to transmit so little energy.

The output coupler is designed to reflect part of the power back into the cavity; the rest is transmitted in the form of the output laser beam. Output couplers must be able to transmit 10.6µm (or 9.4, 9.6 or 10.2µm in some lasers). The most commonly used material - ZnSe - transmits 83% in its uncoated form and is mainly preferred for its low absorption. The components are usually finished with coatings to reflect between 50 and 70%. GaAs is sometimes preferred as an output coupler material because it has good thermal-conductivity properties and is relatively inexpensive compared with ZnSe.

The fold mirrors used to extend the length of the laser cavity are often made from Cu or Si. Cu is usually chosen in circumstances where very high power is involved, as it is possible to tap direct water-cooling channels into the rear surface of a Cu optic to extend its lifetime. In-cavity fold mirrors work best when they are coated with a highly reflective (99.9%, typically) UV-resistant coating such as SuperMax.

Outside the cavity After the beam exits the cavity it has to be brought to the target or workpiece. It is highly unlikely that the raw beam will be suitable for the intended task, so equipment involving mirrors and lenses in various forms is required to deliver the beam, ideally with minimum absorption of the laser power. Delivering a high-power CO₂ laser beam by flexible fibre is not possible because a suitable material is not yet available.

As the beam leaves the cavity it is usually linearly polarized and most applications require the beam to be moved in both the x and y axes. Although improved metal cutting can be achieved by aligning the beam's polarization to the direction of the cut, most laser users rely on a compromise and use a phase retarder to convert the beam from a linear polarization to a circular polarization that ensures equal cutting performance in any direction.

Reflective phase retarders can be produced either on Cu- or Si-based substrates. Typically these range in diameter from 25 to 100mm, depending on beam size. The recommended minimum clear aperture of any optic in the beam delivery system is 1.5 times the incident full-beam diameter (or twice the diameter with very-high-power systems). The state of circular polarization must be maintained to ensure uniform cut quality across the cutting bed. Zero phase-retardation optics, called turning mirrors, may be used to maintain the circular polarization.

Turning mirrors, available from most reputable optics manufacturers, have a virtually zero phase-shift characteristic. Gold-coated Cu (99% reflectivity) is one of the most commonly selected options. In systems where the external mirrors need to be cleaned frequently it is wise to select highly durable mirrors, such as SuperMax-coated Cu or Si types (these typically reflect 99.9% at 10.6µm).

Beam splitters can be added to a system to split the main beam into a number of sub-beams for use at several workstations simultaneously. These partial reflectors need to transmit 10.6µm and any injected visible beam, so ZnSe is the only option.

Focusing lenses. Transmitting lenses can only be used in systems up to levels of about 6kW unless they can be kept perfectly clean and well cooled. Beyond these levels optics should be reflective, including those at the focusing head. Transmitting lenses are usually made from ZnSe, although GaAs and Ge have some benefits due to relatively high refractive indices. ZnSe lenses are by far the most common in applications involving CO₂ laser systems. They usually range in diameter from 0.5 to 3 inch, the most popular being either 1.5 or 2 inch. They are able to withstand the power levels of most lasers currently available and, thanks to their commonality in the marketplace, they represent excellent value to the end-user. The bulk absorption of laser-grade ZnSe is as low as 0.05% per cm thickness. Surface finish and coatings together contribute about 0.12% to the overall component absorption. The laser-induced damage threshold of a good-quality coated ZnSe lens is 3000W per millimetre of beam diameter. This is sufficiently high for most modern-day applications, although this figure is reduced when back-spatter impacts on the lens.