  We experimentally investigate generation of molecular nitrogen-ion lasers
with two femtosecond laser pulses at different wavelengths. The first pulse
serves as the pump which ionizes the nitrogen molecules and excites the
molecular ions to excited electronic states. The second pulse serves as the
probe which leads to stimulated emission from the excited molecular ions. We
observe that changing the angle between the polarization directions of the two
pulses gives rise to elliptically polarized molecular nitrogen-ion laser
fields, which is interpreted as a result of strong birefringence of the gain
medium near the wavelengths of the molecular nitrogen-ion laser.
