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Broadband External-Cavity Diode Laser

This relatively simple, inexpensive device is suitable for use in survey spectroscopy.

A broadband external-cavity diode laser (ECDL) has been invented for use in spectroscopic surveys preparatory to optical detection of gases. Heretofore, commercially available ECDLs have been designed, in conjunction with sophisticated tuning assemblies, for narrow- band (and, typically, single-frequency) operation, as needed for high sensitivity and high spectral resolution in some gas-detection applications. However, for preparatory spectroscopic surveys, high sensitivity and narrow-band operation are not needed; in such cases, the present broadband ECDL offers a simpler, less-expensive, more-compact alternative to a commercial narrowband ECDL.

alt To be precise, the output of the tunable, broadband ECDL consists of many narrow spectral peaks spaced at narrow wavelength intervals that, taken together, span a broad wavelength band. The broadband ECDL can, therefore, be likened to a light-emitting diode except that the spectrum incorporates the external- cavity mode structure. Unlike light-emitting diodes, the ECDL offers the greater brightness, simpler fiber coupling, and superior spatial propagation properties of a laser. For example, the broadband ECDL is easily coupled into multiple-pass optical-path-lengthenhancement cells. A tunable filter — preferably, a monochromator or a spectrometer — is used to select a portion of the output spectrum.

The optical configuration of the broadband ECDL (see figure) is based on, but differs from, a standard configuration known in the art as that of the Littman-Metcalf design. Whereas heretofore, a flat feedback mirror would be used to select a single laser output wavelength, in the present case, a curved feedback mirror is used to select multiple wavelengths. Preferably, the feedback mirror is cylindrical or spherical and is positioned with its center of curvature at the point of diffraction (the intersection of the laser beam with the diffraction grating).

In this configuration, each wavelength component diffracted from the grating is reflected from the mirror back to the point of diffraction. Thus, many wavelength components are simultaneously oscillating in the external cavity. The wavelength range is determined by the range of angles intercepted by the mirror; hence, the wavelength range can be adjusted by moving the mirror to a different position on the diffraction circle. The zeroth-order output of the diffraction grating is used as the laser output.

The length of the external cavity (including the mirror radius) determines the longitudinal mode spacing. It is preferable to make this spacing smaller than the wavelength resolution of tunable filter, so that for the purpose of filtering, the ECDL spectrum can be regarded as continuous.

This work was done by Jeffrey S. Pilgrim of Southwest Sciences, Inc. for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17486-1.

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