The wavelength tuning ranges of a grating external-cavity laser diode (ECLD) have been studied by the equivalent cavity method. The maximum tuning range (MTR) and the continuous tuning range (CTR), which are related to the maximum and the minimum threshold carrier densities, are deduced from the threshold condition. We define a ratio of the CTR to the MTR. This ratio is only determined by the reflectivities of the external and internal facets of the ECLD. The analysis shows that there is an appropriate combination of the external and internal-cavity reflectivities to obtain a given CTR in the design of an ECLD.
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