Design and implementation of a microcontroller-based closed-loop tuning-controller for a tunable dye laser

Abstract

This thesis presents the design, implementation and testing of a microcontroller-based closed-loop tuning-controller (CLTC) for a tunable dye laser. The specific goal is to precisely control and maintain the tuning of a tunable dye laser. This laser is used as a transmitter in a resonance lidar system that operates at 589nm and measures the mesospheric sodium layer. During operation, the operating frequency (or tuning) of the laser drifts due to environmental changes and the resonance signal (and measurement quality) is reduced. The controller uses the current signal from a sodium Hollow Cathode Lamp (HCL) to sense changes in the laser tuning. The CL TC incorporates both analog and digital circuitry. The amplifiers in the analog circuit have both sufficient slew rate (>5 V/us) and gain-bandwidth product (45x10⁶ Hz) to accurately capture the HCL signal. A high pass filter in the analog circuit removes the floating bias of the HCL and provides a stable signal to the digital circuit. The digital circuit is based on a Motorola 68HC12 microcontroller. The microcontroller monitors the signal from the HCL as it scans the laser wavelength and tunes the laser to the wavelength that maximizes the amplitude of the HCL signal. The CL TC can scan the laser at the resolution determined by the mechanical hardware of the laser system (0.14 pm), which is a factor of six times greater than resolution provided by the manufacturer software (1 pm). Our tests show that the CL TC can both tune the laser to and maintain the laser at the resonance frequency. The microprocessor software can be readily changed to accommodate changes in the optical sensor and the scanning methodology.