Continuously tunable narrow-linewidth fiber lasers

Abstract: Narrow linewidth tunable fiber lasers have become an important tool in research and in-field applications thanks to their high beam quality, great spectral performance, compact structure and environmental robustness. Many methods have been introduced and developed throughout decades to fulfill the need for rapid wavelength adjustment of these fiber lasers. While maintaining a high-level spectral performance, the ease of manipulation and cost effectiveness of the tuning operation are considered beneficial for the lasers used in real world, and therefore, they are the main focus of the work presented in this thesis.                                        An accurate tuning method with narrow linewidth and compact configuration was achieved in the first work. A polarization independent semiconductor optical amplifier (SOA) was used as the gain medium in a unidirectional fiber ring cavity with a circulator connected to a 6-meter long chirped fiber Bragg grating (CFBG). The laser wavelength was chosen by setting the modulation frequency of the SOA the same as the harmonics of the fundamental repetition rate of the light reflected at a specific point on the CFBG. Careful management of the drive current and pulse width helped to generate laser light of narrow linewidth (less than 0.03 nm) and with low power variation (1.46 dB) over a tuning range of 40 nm.                                                      One example of the application of a tunable fiber source was discussed in the second work. An efficient Erbium Ytterbium fiber amplifier, which is seeded by a distributed feedback laser, was designed for continuous-wave differential absorption light detection and ranging (CW DIAL) of atmospheric CO2-concentration. It had a linewidth of 3 MHz, a tuning range of 2 nm over the CO2 absorption peaks at 1.572 μm and an output power of 1.3 W. Wavelength tuning is achieved by adjusting the drive current to the seed laser. Results from the initial CW DIAL testing demonstrate that this tunable fiber source meets the high demands for range resolved atmospheric CO2 monitoring.                                                                                  To conclude, two narrow linewidth tunable fiber lasers have been demonstrated based on different tuning mechanism. The rapid and accurate tuning operation with low output power variation is achieved in both works. The great spectral properties of these fiber sources make them powerful tools to be used in applications such as optical communication, remote sensing, spectroscopy, optical coherence tomography, and many more.