Fourier domain mode locked (FDML) fiber lasers are broadband wavelength-swept ring systems with record sweep speeds. Lasing is achieved by synchronizing the roundtrip time of the optical field in the fiber delay cavity with the sweep period of a tunable Fabry-Pérot (FP) bandpass filter. Since their invention in 2006, FDML lasers have dramatically enhanced the capabilities of optical coherence tomography (OCT) and various sensing applications. However, the physical coherence limits, such as the maximum achievable coherence length, are yet unknown. An important breakthrough in reaching this limit is a recently experimentally demonstrated highly coherent operation mode over a bandwidth of more than 100 nm , referred to as the sweet spot. The sweet spot operation mode is characterized by nearly shot-noise limited fluctuations in the intensity trace of the laser with significantly enhanced coherence properties, whereas in conventional FDML laser systems the intensity trace is distorted by high frequency noise which negatively affects the coherence length. This ultra-low noise operating regime was generated by an almost perfect compensation of the fiber dispersion with a manually fine tuned chirped fiber Bragg grating and a highly synchronized sweep rate of the FP filter with an accuracy in the range of mHz. Polarization effects were controlled with a polarization maintaining semiconductor optical amplifier (SOA) gain medium and a polarization controller.
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