The system holds diamonds below a focused beam of ultraviolet light. The light has been focused so tightly that it burns a small microscopic pit onto the surface of the gem. The depth of this pit is extremely shallow (~1 or 2 microns).
The color of the pit can vary from nearly transparent to totally black. The LMS Cold Laser System creates characters by moving the diamond in X and Y axis while the laser is firing. This is much like holding a pencil still, while moving the paper below it to form characters. Marking on the girdle of a gem is the most popular use for the LMS Cold Laser System. The diamond can be marked over the entire diameter of the gem through a series of rotations.
The laser is kept in focus, like a camera, by moving the fixed lens, or objective, up and down. This motion is called Z-axis motion. In addition to moving the diamond, and the objective, the user has many other controls at his disposal. Laser energy can be varied, much like an artist varies the firmness by which he holds his brush.
Laser spot size can be adjusted to achieve the effect of selecting different brush widths. In addition, the laser can be spaced to produce separate dots, or one continuous line. In a very real sense, the LMS Cold Laser System is much like an artists pallet. The end result depends as much upon the skill of the user, as well as the sophistication of the technology.
Laser Interaction With Diamonds
Diamonds are transparent to wavelengths well beyond those visible by the human eye. The chart below details the typical wavelength of light that can be obtained from commercially available lasers. Diamonds are transparent to laser light from 0.200mm in the UV to well past 10mm in the Infra Red. This transparency makes it difficult for light to be absorbed onto the surface to form a legible mark. The LMS Cold Laser System solves this problem by emitting light in the deep UV (193nm or 0.193mm). At this wavelength one can mark a diamond, with little transmission of light into the body of the gem.
This lack of transmission is critical since it means that the laser can mark without the need for any other process or preparation. Other laser marking systems need to ‘ink’ or prepare the diamond before they begin marking. This is done to allow the light to be absorbed by the ink, and a mark formed by the heat generated by the process. The LMS Cold Laser System doesn’t need to generate this type of thermal stress to mark, and therefore presents less risk to the gem during the marking process.
Cracking, Thermal Ablation, and Other Destructive Issues
The 193nm Excimer laser ablates a mark on the diamond in a ‘cold’ process. This process does not appreciably affect the area around the mark. In the picture below one can see that the surrounding material shows little sign of thermal stress after exposure to laser energy.
With most other lasers the area of thermal stress is nearly as large as the mark itself. Having said this, the 193nm Excimer is not completely free from thermal effects. One must be careful in overlapping marks at the same point, near or on a fractured area. It is possible to initiate a crack, or enlarge a crack with excessive energy, and extended laser duration with some stones. In general, however, it takes quite a large amount of energy and a long dwell time for the 193nm Excimer to risk opening a crack in a diamond.