Abstract
The intensity noise and frequency noise of diode lasers and external cavity diode lasers (ECDLs) need to be carefully characterised to ensure they meet required specifications in many telecommunications and spectroscopic applications. Previously, we demonstrated that a relatively long cavity (~30-50 cm), Littrow grating configuration, external cavity diode laser (ECDL), has a measured frequency noise spectrum [ 1 ] like that predicted for an ECDL with a plane high reflectance external mirror [2], Here we report on how the grating dispersion can be used to control the bandwidth of this frequency noise. Figure 1 shows the calculated frequency noise spectrum [2] for an ECDL with a high reflectance plane mirror (8, the feedback factor, is 1011). The cavity length is 50 cm and the longitudinal mode spacing is 300 MHz. Also shown on this figure are the normalised grating response functions for a 1200 lines/mm grating used in a Littrow configuration at 850 nm, where the number of lines of the grating illuminated increases from 10,000 for the flattest curve to 50,000 lines for the most dispersive response. In laser systems in general, to suppress adjacent laser modes requires a change in grating reflectance of the order of 1 to a few percent. In contrast, ECDL’s with a high reflectance (>90%) plane mirror as the external reflector operate single frequency without the need of a dispersive element [3]. The grating dispersion is necessary, however, to suppress the frequency noise peaks at multiples of the longitudinal mode spacing frequency.
© 1996 Optical Society of America
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