External cavity diode laser diffraction11/13/2023 ![]() The configuration of the ECDL is depicted in Fig. The ECDL produced a narrow Lorentzian fitted linewidth of 95 kHz, spectral purity of 2.9 MHz, and long-term frequency stability of 5.59 × 10 − 12, exhibiting an excellent performance. In this paper, we use a readily available broad bandwidth (∼ 4 nm) interference filter to achieve single mode operation in an ECDL. Nevertheless, the interference filters used in these designs have an extremely narrow bandwidth (∼ 0.3 nm) comparable to the intrinsic mode spacing of the diode laser that are not readily available at a broad range of wavelengths, resulting in higher cost, restraining the popularization of this method. Īn alternative approach is to simultaneously employ a narrowband interference filter, placed in the linear cavity as the wavelength discriminator, and a mirror located at the end of the cavity as the optical feedback component, and has been proved to have a greater alignment tolerance and wider tunability. However, these designs are sensitive to the ambient pressure and optical misalignment induced by the mechanical and thermal deformation. One common method to construct an ECDL is using a diffraction grating as the optical feedback and wavelength discrimination component in either the Littrow or Littman–Metcalf configurations. In general, narrow linewidths can be effectively achieved by external cavity diode lasers (ECDLs). Moreover, lasers with narrow linewidths have great potential in Faraday anomalous dispersion optical filters. Refer to LEW-17486-1.Narrow linewidths (< 1 MHz) are essential in a variety of laser applications, such as atom clocks, atomic physics, precise measurements, and coherent light communication. 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. For further information, access the Technical Support Package (TSP) free on-line at under the Physical Sciences category. 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. The length of the external cavity (including the mirror radius) determines the longitudinal mode spacing. The zeroth-order output of the diffraction grating is used as the laser output. 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. ![]() Thus, many wavelength components are simultaneously oscillating in the external cavity. In this configuration, each wavelength component diffracted from the grating is reflected from the mirror back to the point of diffraction. 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). 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. ![]() 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. A tunable filter - preferably, a monochromator or a spectrometer - is used to select a portion of the output spectrum. For example, the broadband ECDL is easily coupled into multiple-pass optical-path-lengthenhancement cells. Unlike light-emitting diodes, the ECDL offers the greater brightness, simpler fiber coupling, and superior spatial propagation properties of a laser. The broadband ECDL can, therefore, be likened to a light-emitting diode except that the spectrum incorporates the external- cavity mode structure. 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 Feedback Mirror Is Made Curved (in contradistinction to flat) to make it select a range of wavelengths(in contradistinction to a single wavelength). 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. 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. Glenn Research Center, Cleveland, OhioĪ broadband external-cavity diode laser (ECDL) has been invented for use in spectroscopic surveys preparatory to optical detection of gases. ![]()
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