Wavelength-Agile External-Cavity Diode Laser

John H. Glenn Research Center, Cleveland, Ohio

A prototype wavelength-agile external-cavity diode laser (ECDL) was demonstrated in the first phase of a continuing effort to develop noninvasive laser-based instruments that would measure absolute concentrations of trace gases. As envisioned, these instruments would offer a combination of high sensitivity, versatility, large bandwidth, long-term stability, accuracy, and reliability - attributes that would make them attractive for use in research on combustion.

The measurement principle to be implemented in the developmental instruments is that of wavelength-modulation spectroscopy (WMS). ECDLs are useful for detecting trace gases by WMS because their wavelength-tuning ranges are greater than those of diode lasers alone and, consequently, it is generally practical to design and operate an ECDL such that an absorption spectral feature characteristic of a molecular species that one seeks to detect lies within the nominal tuning range of the ECDL. In prior ECDLs, wavelength modulation is effected by using piezoelectric actuators to translate laser optics. This mechanical-translation approach limits achievable modulation frequencies to a few kilohertz, whereas frequencies of tens or even hundreds of kilohertz are needed to enable the use of high-sensitivity detection techniques in WMS. In addition, prior commercially available ECDLs include complex and expensive optical components.

A Littman-Metcalf Resonator is used as an external cavity resonator in the wavelength-agile ECDL.

The present prototype ECDL (see figure) is simple and inexpensive, relative to prior commercial ECDLs. Its design combines the stability of an external-cavity laser with the wavelength agility of a diode laser. The design allows for wavelength modulation of the ECDL by modulation of the diode gain element injection current. The external cavity is of a type known in the art as a Littman-Metcalf resonator, in which the zeroth-order output from a diffraction grating is used as the laser output and the first-order-diffracted light is retroreflected by a cavity feedback mirror, which establishes one end of the resonator. The other end of the resonator is the output surface of a Fabry-Perot resonator that constitutes the diode-laser gain element. Wavelength selectivity is achieved by choice of the angle of the diffracted return beam, as determined by position of the feedback mirror.

The diode laser in the prototype wavelength-agile ECDL is of a room-temperature ultraviolet type that has recently become commercially available. However, the design principle is just as well adaptable to other diode lasers. The feasibility of the developmental instruments has been demonstrated by using the prototype wavelength-agile ECDL as the source of light in a prototype WMS spectrometer for measuring concentrations of CH radicals in laboratory flames.

This work was done by Jeffrey S. Pilgrim and Daniel B. Oh of Southwest Sciences, Inc., for Glenn Research Center.

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