MOPA Lasers: Making

Guest Contributor: Victor

The Master Oscillator Power Amplifier, or MOPA, for short—is a technological leap above the traditional Q-switch technology in Fibre Lasers. Where the traditional Q-switch used a fixed length frequency pulse, MOPA is able to utilise a variable length of pulse which brings with it many new opportunities and improved processes. Among these is a greater control over the effects on the materials under which the laser is being operated, as well as a more gentle execution in procedures; moreover, the most impressive feat is the imbuing of colour onto materials such as steel, titanium, and aluminium, the latter of which can now see deeper, darker greyscales. It should also not go unsaid that the internals of a MOPA laser are far more sustainable than traditional lasers, able to accrue countless more hours of use before requiring replacement.

In steel and titanium, true colour is capable of being produced through a process popularly referred to as laser annealing; wherein, the potency of the laser is reduced to simply heat the surface of the material without vaporising it, the localised heating forms a layer of oxide below the surface of the metal—in essence, a more efficient and precise form of anodisation. To better provide the information on how colour is made, the thin film interference phenomenon comes into play. When an observer views the material’s surface, light is reflected between a medium—in this case, the oxide layer into the substrate—and back as a colour relative to the relationship between the constructive and destructive interference of the two layers; the heat of the laser, therefore, chemically alters the refractive index of the medium respective to the temperature applied. For example, 220° C is recognised to produce a yellow hue, while 290° C will produce a blue hue.

On the topic of rust resistance in stainless steel, the presence of a thin layer of chromium oxide is assured as the process of passivation is present; the process creates a layer that is meant to weather the abuse of well… the weather, without getting worn or removed—it’s a layer that has a natural reproducibility much like the aluminium oxide layer that naturally protects aluminium, which makes it a prime candidate for objects like road signs. In laser engraving, however, one must be careful and understand the importance of properly tuned settings