Laser Alignment System for Tunable Diode Laser Sensors

Introduction
Tunable Diode Laser (TDL) instruments, such as the Unisearch Associates LasIR, operate by projecting a beam of laser light through a process gas to a photodiode detector. Target gas concentration along the beam path at the measurement location is then computed based on the output of the detector

Alignment of the beam between the launch optics and detector can sometimes be a challenge for users of TDL instruments. This is particularly true for situations where beam misalignments are caused by periodic movements of the launch optics and detector due to thermal transients or other phenomena associated with the process.

To address this issue, Bergmans Mechatronics LLC has developed the Laser Alignment System (LAS), which can be interfaced to TDL instruments for continuous and automatic beam alignment.

System Description
When the LAS is interfaced with a Unisearch LasIR instrument, the integrated system operates as follows. Laser light from the LasIR is transmitted along a fiber optic cable to a lens within the launch optics enclosure and is then projected into the process gas. To enable control of the horizontal and vertical direction of the beam, the lens is fixed to an adjustable mount which is actuated by a pair of computer-controlled stepper motors. The laser light passes though the process gas and, when the beam is aligned, enters the LasIR detector. The detector signal is then transmitted to the LasIR for measurement of the target gas concentration.

The LAS also receives the LasIR detector signal and independently evaluates the signal strength. An optimization algorithm in the LAS Processing Unit adjusts the two stepper motors in the launch optics enclosure with the objective of maximizing the detector signal strength. Through continuous maximization of the signal strength, the LAS is able to keep the beam aligned despite relative movements of the launch optics and detector.

Additional information about the LAS can be found in the following documents.

Schematic of Laser Alignment System Integrated with LasIR

Laser Alignment System Integrated with LasIR Ammonia Monitor