Heterodyne detection of the induced phase shift using a twin-laser phase-modulation technique is a reliable measurement of electron density. For long pulse duration experiments, the frequency and power stability of source laser becomes a critical requirement to obtain an adequate fringe resolution. Further, the difference in optical path lengths between reference and probing branch and the long propagation distance are the factors that can deteriorate fringe resolution and visibility due to distortion of the wavefront besides the loss of intensity. The long propagation distance is due to interferometer design requirement that the source laser must be installed outside the tokamak hall where SST-1 is situated, to avoid the effect of mechanical vibrations and stray magnetic field and the large path difference is to solve the interface problems with other diagnostics on the same port. The requirements imposed on FIR laser for the required resolution are (1) the frequency stability (2) the intermediate frequency stability and (3) the power stability.