Identifying molecules
Identifying molecules

An optical spectroscopy technique that uses a laser to detect molecules in the breath could help to diagnose diseases such as asthma or cancer. According to US researchers, they have improved a technique, known as cavity-enhanced direct optical frequency comb (OFC) spectroscopy, to be more sensitive and cover a larger spectral bandwidth. (Optics Express 16 2387)

"With our current system we can detect many tens of molecules with sensitivities near the 1 part per billion level," Michael Thorpe, a researcher from JILA, a joint venture between the National Institute of Standards and Technology and the University of Colorado, both US, told optics.org. "In the next 5-10 years I expect detection capability will extend further to the mid-infrared and the spectral bandwidth will increase to detect thousands of molecules simultaneously."

Why frequency combs?

OFC technology uses a modelocked laser to create broad spectral coverage. "Unlike single frequency laser systems, a frequency comb can detect many different molecules at the same time," commented Thorpe. "What's more, it is superior to mass spectrometer systems because it is better at distinguishing individual molecules, performs more rapid detections and is relatively inexpensive."

Whilst the idea of using frequency combs is not new, it has only recently been extended thanks to the availability of user friendly modelocked femtosecond fibre lasers. "These lasers can now be used to produce robust frequency combs capable of continuous operation without user intervention," commented Thorpe.

Thorpe's team uses a modelocked erbium-doped fibre laser that generates 100 fs pulses and covers a spectrum between 1.5-1.7 µm. By coupling these pulses into an optical enhancement cavity and using a virtually imaged phased array (VIPA) detector, a high spectral resolution of 800 MHz is achieved. "It is this unique combination of optical components that provides broad spectral coverage, high sensitivity and high resolution for analyzing complex gas samples," explained Thorpe.

Light detects breath molecules

The pulses of laser light were fired into an optical cavity, which contained the breath sample. The laser beam bounces back and forth within the cavity allowing the light to sample the entire volume. This increases the light-molecule interaction time, which in turn increases the sensitivity. By comparing the light coming out of the cavity with the light that went in, the JILA team could determine which frequencies of light were absorbed and by how much.

"Light transmitted from the cavity is dispersed into a two-dimensional pattern and imaged onto a camera by the VIPA spectrometer," explained Thorpe. "Computer databases and software compares the recorded spectrum against known molecular spectra to determine the quantities of the individual molecules contained in the gas sample."

Looking to the future

Apart from disease diagnosis via breath analysis, the approach could be useful for applications such as: monitoring of atmospheric greenhouse gases and analysing ice core samples for climate studies and detecting impurities in gases used to manufacture semiconductors.

The team expects clinical trials to be carried out in the next couple of years and plans to explore new laser systems, new types of optical cavities and new methods of detecting the transmitted light. "To reach its full potential, the device's spectral bandwidth and the number of molecules available for detection need to be increased by an order of magnitude," concluded Thorpe. "I'm fairly confident that the next generation system is just over the horizon."