Abstract: An intra-cavity frequency-doubling laser device includes a first mirror and a second mirror defining a resonance cavity therein, a gain media to produce a first lasing light in response to an excitation energy received from outside of the resonance cavity, a non-linear optical material to generate a second lasing light in response to the first lasing light. The second lasing light and the first lasing light have different frequencies. The first mirror is reflective to the first lasing light and the second lasing light. The second mirror is reflective to the first lasing light and at least partially transmissive to the second lasing light. A birefringent optical material in the resonance cavity can rotate the polarization direction of at least one of the first lasing light and the second lasing light. An optical axis of the birefringent optical material and an optical axis of the non-linear optical material have an angle between 30 and 60 degrees.