DC Motorized Actuator With Low Cost Feedback
The actuator required to automatically drive each of these stages also required customization. A DC motor was chosen for several reasons. Piezo actuators do not provide the requisite travel range and stepper motors require constant power in order to hold position, but it was decided to minimize the power budget for the laser. Siskiyou manufactures a range of DC motors with motion controlled by an industry standard 12-volt pulsing. However, DPSS Lasers wanted to operate these motors using only 6 volts to enable slower motion (i.e., higher resolution) and to further minimize power budget. As already noted, however, the stages were also equipped with heavy return springs.
Fortunately, there is a large overhead built into the design of these DC motors and it turned out that there is no problem operating them at 6 volts. But to ensure there are no issues in the field, Siskiyou tests these extensively with voltages down in the 1 to 3 volt range, while they are attached to the stages, using a precision laser interferometer and a retroreflection.
Just as important, DPSS Lasers needed to configure these motorized actuators for closed-loop operation. Many users of DC motors accomplish this by using some type of encoder to measure position. However, they did not want the added size and cost of using encoders in their compact laser heads. Again, a dialog between the two companies resulted in a design where Siskiyou assembles the motors with a compact circuit board incorporating a linear potentiometer all supplied by DPSS Lasers. This is located inside the motor assembly so that the spring-loaded potentiometer pin rides against the “key indicator.” This is a low-profile metal tongue that protrudes from the side of the actuator in many commercial actuators. This provides the (proprietary) feedback resolution needed to fully exploit the slow scanning and hence maximize the lifetime of the harmonic crystals.
Qualified Materials – Pristine Cavity
Another overarching theme in this project is the use of materials that are UV laser compatible. It is well-established that a common cause of early failure in UV lasers is failure of optical surfaces, many of which carry dielectric coatings. The problem arises because any dust or chemical contamination tends to ride along a laser beam and eventually reaches an optical surface. This leads to absorption of laser light at the surface, resulting in damage that further increases absorption, etc. In lasers intended for industrial applications, it is simply not practical to allow cleaning of the optics by the end user, who rightly wants to use the laser as a 24/7 tool, not a delicate scientific instrument. So instead, UV laser manufacturers take steps to eliminate the possibility of any cavity contamination or outgassing. This typically involves sealing the cavity, ensuring there are no mechanisms to generate contamination inside, and some type of cleaner inside the laser head to actively remove contamination.
The stages and actuators thus had to be made only with materials qualified by DPSS Lasers. This meant using the same materials that Siskiyou uses in vacuum compatible optomechanics, i.e., no organics and using crytox as the only lubricant. In addition, it required no anodizing. Normally the majority of stages, mounts and other optomechanics are black anodized to minimize scattered light issues. But, it turns out that when black anodized (oxidized) aluminum is exposed to small amounts of scattered UV laser light inside a sealed cavity, it eventually creates traces of dust. As shown in Figure 2, this is the reason why the stages are neither anodized nor powder coated.
In recent years, there has been a growing demand for ultraviolet lasers in applications such as micromachining and marking. This is because these lasers can deliver high spatial resolution and a minimized heat affected zone (HAZ). Improved laser reliability and lower cost of ownership have been keys to this market growth. While many might think of UV DPSS lasers as a mature technology, this work shows that they can be enhanced through the use of innovative designs and the clever use of components.