The tunable external cavity diode laser is an important new tool used to evaluate and test the latest fiber-optic components and building blocks of modern telecommunications systems. A key factor enabling operation of this type of laser is the ability to angle-tune the wavelength-dependent key component (a diffraction grating) in the cavity with extreme precision and repeatability. When Hewlett Packard wanted to develop a laser with enhanced wavelength performance, their design required a linear actuator with 100-nm resolution over a travel range of several millimeters. This brief tells how a custom microstepper system was developed to meet the needs of this demanding application.
The diffraction grating is rotated by pushing on the end of a spring-loaded grating arm, which is several centimeters in length. The target was to tune the laser output with a wavelength resolution of 100 MHz, which translated into a positional resolution of 100 nm or better. Piezoelectric actuators were rejected for this application because they do not provide the necessary range of motion (several mm). DC-motorized actuators did not offer the necessary holding stability — the actuator is never truly stationary but is always oscillating around the target encoder position. Furthermore, they require the use of an encoder, which would add cost and complexity to the laser. On the other hand, a microstepping system offered the potential for high resolution over a long travel range, as well as excellent repeatability without an encoder.
The starting point for the OEM design was a micropositioner called the Nanomover®. This was developed by Applied Precision several years ago, initially for integration into test instrumentation for the semiconductor industry, and later as a stand-alone micropositioner. The Nanomover uses a conventional stepper motor in a microstepping mode, with a minimum microstep size of 0.036°. This fine rotary motion is converted into 25 mm of linear motion by a precision drive screw and a proprietary low-backlash linkage. The minimum step size is 50 nm. Software reduces the 500-nm lost motion (mechanical backlash) to deliver a repeatability specification of ±100 nm. Because power-down does not compromise this long-term repeatability, most applications do not need an encoder.
Modifying this basic design to meet the special requirements of this application required customizing the three major components of this positioning system: mechanics, electronics, and software.
Two significant mechanical issues were the operating temperature and the grating arm interface. A stepper motor draws current even when stationary and thus generates heat. Thermal drifts must be avoided in the laser cavity as these could compromise cavity/wavelength stability and wavelength calibration. For this reason, Hewlett Packard heats the entire laser cavity and stabilizes it at 55 °C. Our life tests showed that the original lubricant would lose its lubricity at these temperatures. A custom synthetic oil was formulated to eliminate this problem.
The interface between the positioner tip and pivot arm also required special consideration. As shown schematically in Figure 2, the positioner contacts a tungsten carbide tooling ball mounted in the grating arm. As with many motorized devices, the tip of this positioner rotates as it translates. This is not an issue in most applications. In this case, however, the tooling ball gradually translates across the surface of the rotating positioner tip as the grating arm moves. This required that the face of the positioner tip be truly perpendicular to the direction of travel. In addition, a testing protocol was designed which would not create even minute scratches or surface blemishes in the tip face, which could introduce errors into the motion-to-wavelength transfer function. During testing, the tip is protected with a small tungsten carbide plate, which has been lapped flat. This rotates with the tip, so that any wear scratches are produced on it instead of the tip.
The major electronic issues were related to voltage and systems integration. Since size was an issue, the design team initially integrated both the control and drive electronics on a single board, conforming to the Hewlett Packard template. These electronics had been redesigned to operate using the ±19V available in the laser. To further reduce cost, Applied Precision now simply licenses a surface-mount implementation of this circuit to Hewlett Packard for direct in-house automated fabrication.
Finally, the control electronics are based on a Z8 processor. This control system was originally designed to run in an ISA bus environment executing commands from a standard library set. To work in the Hewlett Packard laser meant designing a new register-level interface to work with their HPIB bus. Obviously, an entirely new command set had to be created to support this environment.
To summarize, as with any OEM motion control application, this work involved meeting two equally important goals: first, to provide the necessary performance, and second, to configure the system in such a way that it could be seamlessly integrated into the final product.