An apparatus and method for analyzing biological cells and other particles using an external laser cavity. Microfluidic channels contain and transport biological cells to be analyzed. A laser diode provides light for cell analysis. An external cavity is provided between one surface of the laser diode and a mirror opposite thereto. A microlens set focuses the light on only one cell as it passes through the external cavity. The presence of the cell in the external cavity gives a weak feedback toward the laser diode. The emission frequency and the output power of the laser are both functions of the length of the external cavity. Therefore, the variation of cavity length can be deduced from these parameters, where the variation is caused by changing the refractive index or size of the cell in the cavity.