Finding faults with photonics

Quality control is critical to the cost and integrity of a manufactured product. It is also key to customer satisfaction in terms of product lifetime, warranty costs, and (where applicable) safety. One aspect of ensuring quality typically involves product inspection and testing, which can range from routine destructive analysis of representative product samples to (preferably) automated methods that are fast, noncontact, and nondestructive.

While vision-based systems can often provide such pass/fail testing capability in real time, if the final quality of the product is based on quantitative content specifications-­such as with pharmaceuticals-then more sophisticated approaches are required. Multispectral imaging is one such approach, and with the help of a liquid-­crystal tunable filter it is now being applied to nondestructive ­assessment of fruit texture (see p. 99). In another ­example, terahertz-based time-domain imaging or time-domain ­molecular spectroscopy is being investigated for use in nondestructive inspection and identification. The generation of arbitrary terahertz waves for these and related applications is a crucial aspect of whether terahertz systems can be broadly applied (see p. 67).

At the component level, the sensors in these and similar systems are -typically tailored to the application involved. Parameters such as speed, -wavelength, noise, and sensitivity must all be taken into account. -Devices for sensing in the ultraviolet, for instance, present a completely different set of -design challenges from those that operate in the infrared (see p. 79). And sensing at low light levels places special demands on the sensitivity of the -detector; among the solutions for these applications are single-photon -avalanche photodiodes (see p. 93).

As for safety, product inspection of any type is usually only part of the ­story-a car is, after all, only as safe as its driver. Within the laser community safety has become an increasingly important issue over the years, gaining prominence most recently thanks to the fools who direct laser pointers at airplanes in flight. Senior editor Hassaun Jones-Bey takes a closer look with a historical perspective at safety issues in our “40 Years: Then and Now” series on p. 52.