Low-Divergence Laser Hits the Spot

Brent D. Johnson

Temperature measurement in industrial applications can be challenging, particularly in harsh environments, where line-of-sight, high ambient temperatures or electromagnetic interference from nearby machinery limits direct readings. One solution is to install noncontact thermometers equipped with fiber optic sensing heads and cables in locations that are inaccessible to integrated temperature sensors.

Even for remote systems, it is still necessary to locate the precise spot where temperature is measured. To solve this problem, Raytek Corp. added a laser source that would send a beam back down the fiber into the remote sensing head and project a visible spot on the measurement location.

Unlike laser-sighted sensors that identify the target with a single laser dot, Raytek's system guides the beam through the same optical fiber that is carrying the infrared temperature signal from the target. It also uses the same infrared optics to focus the laser light and illuminate the target with a red glow, indicating the exact size of the measurement spot.

In the process of selecting a suitable laser source for this application, Steven King, Raytek's senior staff scientist, found the performance of standard laser diodes to be unacceptable, primarily because of the large angular divergence of the beams they emit.

"It was simply not possible to couple enough laser power into our fibers with standard laser diodes," King said. That is, extra focusing optics would be required -- an option that Raytek could not pursue because of space constraints.

He needed a self-contained laser diode module that could produce a highly focused circular beam without adding lenses. He opted for the CircuLaser manufactured by Blue Sky Research, which integrates its µLens technology with a standard laser diode to generate a 5-mW, 635-nm, low-divergence, diffraction-limited beam. The module mounts the µLens directly in front of the diode's emitting edge, capturing nearly 100 percent of the diode's power, circularizing the beam and correcting any aberrations.

The device demonstrated more than 80 percent efficiency coupling visible light into a single-mode optical fiber, up to twice that achieved by external macro-optics with standard laser diodes.