The behavior of a semiconductor laser subject to filtered optical feedback is studied in dependence on the width of the filter. Of special interest are pure frequency oscillations where the laser intensity is practically constant. We show that frequency oscillations are stable in a large region of intermediate values of the filter width, where the dispersion of the filter is able to compensate for the well-known phase-amplitude coupling of the semiconductor laser. Our stability diagram covers the entire range from a very narrow filter, when the system behaves like a laser with monochromatic optical injection, to a very broad filter, when the laser effectively receives conventional (i.e., unfiltered) optical feedback. © 2007 Optical Society of America OCIS codes: 140.5960, 190.3100, 000.4430. We consider a semiconductor laser with filtered optical feedback (FOF), where a part of the laser light is spectrally filtered and reinjected into the laser after the round-trip time of the feedback loop. In an experimental setup spectral filtering can be realized, for example, by a Fabry-Perot interferometer where optical isolators prevent unwanted reflections; see Fig. 1. The filter itself is characterized by the detuning ⌬ between the laser frequency and the filter center frequency, and by the filter width ⌳. These two parameters offer additional control over the feedback light, which may be used to influence the dynamics of the laser; see also, for example, [1-4] for other optical feedback schemes. The dynamics of the FOF laser has been considered in a number of experimental and theoretical studies; see, for example, [5][6][7][8]. Their focus has been on the influence of the detuning, the feedback strength, and the external round-trip time. By contrast, studies of the influence of the filter width ⌳ have focused so far on the two limiting cases of an extremely narrow filter and of an extremely broad filter. Namely, the narrow-filter limit reduces to a laser with optically injected light at the filter frequency, while in the broad-filter limit spectral filtering is lost so that the system reduces to a laser with conventional optical feedback [9-11]; in both limits spectral filtering can be neglected. However, in a real system where the feedback light is subject to spectral filtering, intermediate filter widths are of interest.In this paper we study how the behavior of the FOF laser is influenced by the filter width ⌳ over several orders of magnitude, ranging from zero up to 4 GHz. This is motivated by recent experimental measurements in [12], where the vital influence of ⌳ in an intermediate range was revealed by changing the distance between the two mirrors of the FabryPerot interferometer. Of special interest are frequency oscillations (FOs) of the system, which are characterized by an absence of oscillations of the power of the laser. In this respect FOs are very different from the well-known relaxation oscillations (ROs) that are a typical feature of semiconductor lasers. In fact, in light of the strong amplitude-...