We have systematically investigated random lasing properties in weakly scattering systems composed of a macroporous silica disk immersed in a dye solution where the solvent is a mixture of two alcohols. Controlling the refractive index of the mixed solvent allows us to vary the scattering strength over a wide range. We have found two different scattering regimes where sharp spectral spikes with linewidth less than 1.0 nm, i.e., random laser with coherent feedback, appear in emission spectra. When the refractive index contrast between the solvent and the silica is very small, random lasing with coherent feedback is observed although the system appears nearly transparent. The coherent feedback vanishes when the refractive index contrast is increased up to a critical value, while further increase in the refractive index contrast results in the revival of the coherent feedback. We suggest that the existence of underlying microcavities plays an important role in the very weakly scattering regime (ballistic) while other mechanisms such as amplified extended modes may lead to the coherent feedback in lasing oscillation when the scattering strength increases.
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