A method for analyzing a fluid flow, the method comprising: monitoring a first measurement region for density variations within the fluid, as the fluid passes the first measurement region, wherein monitoring the first measurement region for density variations within the fluid comprises monitoring the first measurement region for variations of the refractive index of the fluid, as the fluid passes the first measurement region; monitoring a second measurement region for density variations within the fluid, as the fluid passes the second measurement region, wherein monitoring the second measurement region for density variations within the fluid comprises monitoring the second measurement region for variations of the refractive index of the fluid, as the fluid passes the second measurement region; wherein monitoring the first measurement region for variations of the refractive index of the fluid is based on interferometry and monitoring the second measurement region for variations of the refractive index of the fluid is based on interferometry; characterized in that monitoring the first measurement region for variations of the refractive index of the fluid is based on measuring an intensity of interfering electromagnetic waves which travel along different paths through the first measurement region; monitoring the second measurement region for variations of the refractive index of the fluid is based on measuring an intensity of interfering electromagnetic waves which travel along different paths through the second measurement region; and determining whether a first density variation, which is detected by monitoring the first measurement region, and a second density variation, which is detected by monitoring the second measurement region, match comprises calculating a temporal correlation between the intensity of the interfering electromagnetic waves which travel along the different paths through the first measurement region and the intensity of the interfering electromagnetic waves which travel along the different paths through the second measurement region; wherein the first measurement region is delimited by a first optical path of a first beam and a second optical path of a second beam, wherein the first optical path differs from the second optical path; wherein the second measurement region is delimited by a third optical path of a third beam and a fourth optical path of a fourth beam wherein the third optical path differs from the fourth optical path; wherein the first and second beams (16a, 16b) are focused to different spots within the fluid flow; wherein the third and fourth beams (16c, 16d) are focused to different spots within the fluid flow; and wherein a distance between centers of the measurement regions (24a, 24b) is 0.01 m or less. monitoring a first measurement region for density variations within the fluid, as the fluid passes the first measurement region, wherein monitoring the first measurement region for density variations within the fluid comprises monitoring the first measurement region for variations of the refractive index of the fluid, as the fluid passes the first measurement region; monitoring a second measurement region for density variations within the fluid, as the fluid passes the second measurement region, wherein monitoring the second measurement region for density variations within the fluid comprises monitoring the second measurement region for variations of the refractive index of the fluid, as the fluid passes the second measurement region; wherein monitoring the first measurement region for variations of the refractive index of the fluid is based on interferometry and monitoring the second measurement region for variations of the refractive index of the fluid is based on interferometry; characterized in that monitoring the first measurement region for variations of the refractive index of the fluid is based on measuring an intensity of interfering electromagnetic waves which travel along different paths through the first measurement region; monitoring the second measurement region for variations of the refractive index of the fluid is based on measuring an intensity of interfering electromagnetic waves which travel along different paths through the second measurement region; and determining whether a first density variation, which is detected by monitoring the first measurement region, and a second density variation, which is detected by monitoring the second measurement region, match comprises calculating a temporal correlation between the intensity of the interfering electromagnetic waves which travel along the different paths through the first measurement region and the intensity of the interfering electromagnetic waves which travel along the different paths through the second measurement region; wherein the first measurement region is delimited by a first optical path of a first beam and a second optical path of a second beam, wherein the first optical path differs from the second optical path; wherein the second measurement region is delimited by a third optical path of a third beam and a fourth optical path of a fourth beam wherein the third optical path differs from the fourth optical path; wherein the first and second beams (16a, 16b) are focused to different spots within the fluid flow; wherein the third and fourth beams (16c, 16d) are focused to different spots within the fluid flow; and wherein a distance between centers of the measurement regions (24a, 24b) is 0.01 m or less.