Feedback Device Eliminates Mode Hops

A nonmechanical electronic feedback control that reportedly provides dramatically improved mode-hop-free continuous range tuning could make it easier to detect many gases in the atmosphere, from carbon dioxide emissions to bioterrorism agents, said the Montana State University researchers who have developed it.

The device grew out of research seeking ways to monitor carbon dioxide sequestered underground. Carbon sequestration has emerged as a possible way to reduce CO₂emissions into the atmosphere, but one challenge has been finding a way to monitor for leaks, the university said in a statement.

Every gas absorbs light at different wavelengths due to its molecular structure. Researchers can determine what a gas is by observing how laser light is absorbed and by knowing the wavelength at which absorption occurs. The concentration of the gas can be determined by how much absorption occurs. Diode lasers are the most common laser used for this kind of work, due to their relatively low cost, low power requirements and compactness. However, such analysis has suffered from gaps, known as "mode hops," in which tuneable diode lasers are unable to access many wavelengths. As a result, some gases become very difficult to detect.

The MSU device eliminates those gaps in the tuning spectrum, allowing researchers to provide smooth continuous tuning with diode lasers. Applying small currents to thermally expand a diode laser can extend tuning ranges by greater than a factor of 70. The researchers have demonstrated a continuous mode-hope free tuning range of greater than 65 GHz when applying the technique to an optical sensor measuring diatomic oxygen. Studies based on quantum well diode lasers have produced external cavity diode lasers with a tuning range of 50 to 100 nm.

The external cavity diode laser feedback system extends the tunable range for applications including laser spectroscopy, optical sensing and chemical detection, including differential lidar spectral hole burning, the university said.

For more information, visit: tto.montana.edu/technologies