Glass phosphor broadens LEDs for OCT

Japanese researchers plan to commercialize a low-cost LED lighting system that they say doubles the depth resolution of conventional OCT systems.

A team from Nagoya University has combined an infrared wideband glass phosphor with an LED for what it believes to be the first time. According to the group, the LED-glass phosphor unit provides wideband light suitable for high-resolution optical coherence tomography (OCT), while maintaining the benefits of LEDs (Applied Physics Express 2 032102).

"LEDs are high power, small, cheap and easy to incorporate into an OCT system, however, the full-width at half-maximum (FWHM) is not sufficient for high-resolution imaging," Shingo Fuchi, a researcher at Nagoya University, told optics.org. "We used a glass phosphor to broaden the FWHM of the LED for low-cost high-resolution OCT imaging."

Conventional OCT systems employ super luminescent diodes (SLEDs) or LEDs that emit in the near-infrared with spectral widths of around 50 nm. While such light sources are inexpensive, the resulting OCT depth resolution is insufficient for some medical applications.
 
By combining a near-infrared emitting phosphor with a 590 nm LED, the Nagoya group boosts the FWHM to 98 nm. The resulting light source emits an output power of 1mW at a peak wavelength of 1014 nm.

Although not directly measured, the depth resolution of the OCT system can be inferred from the coherence length of the lighting unit. Coherence length is the term used to define the distance between two sources which are able to interfere.

"We tested the coherence length of the lighting unit using a Michelson interferometer," commented Fuchi. "We achieved a coherence length of 4.6 µm, which should improve the depth resolution of the OCT to twice that of conventional SLEDs or LEDs."

The next step in the Japanese study is to install the light source in an OCT system and generate OCT images.