Tunable lasers are necessary devices for modern-day telecommunications and sensing systems. Tunable lasers based on altering the refractive index of the tuning sections of distributed Bragg reflectors (DBR) by varying the injected electrical current show significantly more instantaneous random changes in the instantaneous lasing frequency (or excess FM-noise) compared to non-tunable distributed feedback lasers. We identifies shot noise and random carrier recombination as being the culprits of the excess FM-noise of DBR lasers, and demonstrated this by invoking detailed analytical and numerical analyses of the stochastic carrier fluctuations in the tuning sections, as well as a simplified quasi-static laser tuning model to convert the carrier fluctuations into random instantaneous fluctuations. We found that the spectral density of the FM-noise was mostly in the range 0.5 MHz–10 MHz, and this is in agreement with many independently published results. Our analytical treatment allowed us to conclude that it would be advantageous to reduce the refractive index dependence on the carriers in order to reduce the excess FM-noise while still maintaining the tuning functionality. The simplified numerical model allowed us to create a system simulator to help further develop signal processing techniques to counteract against these instantaneous laser frequency fluctuations.