Travelling through a multiple optical band-pass filters the optical signal experiences spectral narrowing due to pass-band narrowing of filtering devices and instability of central frequency of light sources due to temperature changes in dense wavelength division multiplexing (DWDM) systems. This could be the main factor of degradation in long haul transmission systems. The pass-band narrowing of optical band-pass filter depends on the shape of the amplitude square transfer function. The fastest pass-band narrowing at several cascaded optical band-pass filters is observed for the Lorenzian filter, whereas the slowest – for flat-top filters (Raised-Cosine) [1]. From other point of view, narrowing of pass-band in band-pass filters could be used for realization of novel applications like all-optical modulation format conversion. In this paper we propose a linear phase-to-intensity modulation format conversion with a Raised-Cosine band-pass filter. The main advantage of this approach is to use simplified receiver configuration which could decrease the complexity of the overall system. Also could give advance because of better transmission properties and resistance to nonlinear optical effects for phase modulation format.