Generation of a single-longitudinal mode (SLM) in bidirectional ring lasers has direct impact on the laser linewidth and dynamic range of operation, when used in rotation sensing applications. Besides, operating at a specific wavelength helps in optimizing the performance of the system components. In this work, we report a novel method for generating SLM in bidirectional SOA-fiber ring laser using mechanically tunable Fabry-Perot filter with 1-nm bandwidth. The method is based on gain starvation by tuning the central wavelength of the filter in the blue edge of the gain-wavelength response. By adjusting the SOA driving current, the oscillation condition is satisfied mainly for single mode and bidirectional operation can be achieved simultaneously. The SLM operation is verified by monitoring the beating signal between the modes on an RF spectrum analyzer. Using an SOA with a small-signal gain of 20 dB at 300 mA pumping current and a gain bandwidth of 100 nm centered around 1490 nm; the central wavelength of the ring laser could be tuned from 1440 nm to 1480 nm with a side-mode suppression ratio of 25 dB.
The temperature dependence of the beating frequencies in multi-longitudinal mode hybrid semiconductor-fiber based ring lasers is studied theoretically and experimentally. The variation of the beating frequency with temperature is found to be smaller for larger cavity length and lower beating order. Measured frequency variation as low as -0.24 Hz/°C is obtained for cavity length of 2.7 km. The stability of the frequency is evaluated using the Allan variance technique. The measurement is carried out for different beating frequency orders. The lowest order beating frequency has about 20x better long-term frequency stability than the beating frequency of the 100th order.
Fiber lasers are gaining wide attention nowadays due to their high stability, high reliability, low cost and compactness. Frequency modulation of the laser system has many applications such as wavelength tuning, active mode locking, generation of optical frequency combs and fiber sensors in general. In this work, we report frequency modulation of fiber ring laser system using transmission-type corner cube in-plane MEMS phase modulator fabricated by DRIE technology on an SOI substrate. The fiber-coupled MEMS-based phase modulator is inserted in a multilongitudinal mode fiber ring laser, which has a free spectral range of 345 kHz. By varying the applied voltage on the MEMS device, a wide range of the frequency modulation index can be achieved.