Optical Gels Come of Age
BRENT D. JOHNSON
Optical gels were originally designed to improve return loss in mechanical splices of single-mode fiber, but they also have proved to be effective at index matching for a variety of optical components. One of the key advantages of an optical gel is that it is a fluid, which allows it to seep in and around wiring during encapsulation. Once cured, the material is very soft and elastomeric, permitting thermal expansion without migration of the material or change in refractive index.
Today, optical gels have been put to use in myriad photonic devices, including optical sensors, photodiodes, medical instruments and flat panel displays. Jon Dahm of Lightstream Photonics said his company has been using optical gels from Lightspan LLC for the past nine months in the manufacture of handheld light-curing devices for dentistry and other applications.
Lightstream uses the LS-3357 gel with a refractive index of 1.57 to encapsulate its blue LEDs. The gel index-matches the light from the LED to a waveguide, which directs the light onto composite filling material that hardens when excited by blue light. Dahm said that he prefers this gel because of its elasticity, easy processing and low absorption, and because it does not yellow over time.
David Stone of Lightspan stressed the importance of using an optical material that is index-matched to the wavelengths that will be used in the application. Typically, values for optical gels are based upon the Sodium D line at 589 nm. However, for a gallium-nitride emitter at 411 nm, the index of refraction for LS-3357 is 1.60 instead of 1.57, yielding a light extraction efficiency of 9.1 percent.
In a previous application, Dahm had used an epoxy adhesive, but it didn't provide the required low stiffness for the handheld instrument application, and it had residual absorption in the short Temperature is not an issue with the LS-3357 gel, Dahm said. He has cooked the gel at 120 °C for up to six hours with no change in the optical or structural qualities of the material.
Lightstream's upgraded handheld light-curing device generates 5 percent more power than the previous unit, which Dahm attributed to the superior light extraction of the high-refractive-index substrate. He said he expects to see more dramatic performance statistics over the long term because of the gel's nonyellowing properties.