Researchers at the University of Nebraska (Kearney, NE) have reported successful use of light-emitting diodes (LEDs) instead of laser sources to perform cavity-ring-down spectroscopy (CRDS). (In CRDS, a high-finesse optical cavity containing the gas or liquid specimen enables very sensitive measurements; see www.laserfocusworld.com/articles/219797). While LEDs do not provide the high spectroscopic resolution of laser sources, the low cost and broad bandwidth of LED sources could extend the use of CRDS to a much broader range of potential applications in the form of inexpensive, portable optical-sensing systems.
Traditionally, CRDS has been performed with pulsed-laser sources offering spectral resolution well under 1 nm. The bulk, expense, and relatively high electrical-power consumption of pulsed-laser sources have tended to confine such systems to the laboratory, however.
The development of cavity-ring-down spectroscopy systems based on continuous-wave (CW) lasers (switched on and off periodically by an acousto-optic modulator, for instance) offers the possibility of using relatively inexpensive and compact laser-diode sources; however, these only operate over relatively narrow spectral bandwidths that, while providing high spectral resolution, also limit the breadth of potential applications for systems in the field.
Handling a weak signal
Light-emitting diodes offer much broader bandwidth and are much less expensive than laser sources in terms of cost and power consumption, but the inefficiency of coupling the relatively diffuse light from an LED through the entrance mirror to a ring-down cavity yields an output signal too weak to be measured using the conventional waveform-sampling approach (photodiode or photomultiplier with oscilloscope). The Nebraska researchers got around this problem by measuring the signal cumulatively using a photomultiplier tube and photon-counting electronics (see figure). The LED source was a yellow-green LED, switched on and off electronically at frequencies ranging from 5 to 16.5 kHz. Each ring-down waveform took about two minutes to acquire; the spectral resolution at full-width half-maximum was ±12 nm.
“Light-emitting-diode CRDS offers certain practical advantages when high spectroscopic resolution is not needed,” said Jon Thompson, an assistant professor at the University of Nebraska. Potential applications, using currently available LEDs and ring-down mirrors in visible and near-IR spectral regions, include sensing of gases such as iodine, bromine, ozone, nitrogen dioxide, and the nitrate radical. An additional application of the LED-CRDS technique may be to monitor atmospheric particulates that affect air quality and visibility. “It would be exciting to extend the LED method into the UV region where it could be applied to more absorbers, but so far work with the LED technique has only been in the visible region,” Thompson said. “But there is no fundamental reason that the technique can’t be applied to other frequencies by using different LED wavelengths, provided an LED has been developed at the desired wavelength.”
Possible methods for improving the performance of LED-based systems include using brighter or multiple LED sources (see www.laserfocusworld.com/articles/254166). Another possibility might be to excite the resonator cavity from within, thereby avoiding coupling losses through the cavity’s entrance mirror. Significant improvements might eliminate the need for photon-counting electronics and enable the use of an oscilloscope or fast data-acquisition card, he said.
The researchers are also considering the merits of CW LED illumination, which would enable sensing through cavity-enhanced spectroscopy instead of CRDS. Some work has already been done in that field with very positive results, Thompson said.2 Cavity-enhanced spectroscopy measures integrated output power, which, like the ring-down time constant, depends on the contents of the cavity, but unlike the ring-down time constant, can be measured using a CW signal. Continuous-wave measurements, however, introduce concerns related to source/detector drift and stray light.
Making the LED CRDS technique available for widespread commercial use would require additional development to create an integrated system. The university has filed a provisional patent and hopes to interest a small instrument company in further developing the commercial aspects of the technique. In the meantime, the researchers are looking into making LED-based measurements in other spectral regions and developing broad-bandwidth sensing systems covering spectral regions up to 50 nm wide.
REFERENCE
1. J.E. Thompson and K. Myers, Measurement Science and Technology 18, 147 (January 2007).
2. S. M. Ball et al., Chem. Phys. Lett. 398, 68 (2004).