Refractive index change in ytterbium-doped fibers induced by photodarkening and thermal bleaching

Progress and Summary of Photodarkening in Rare Earth Doped Fiber

In this paper, we summarize the research on photodarkening in optical fibers. The causes of photodarkening in fiber, the influence of photodarkening on fiber laser, the experimental device of photodarkening, and the mathematical model used to study the phenomenon of photodarkening are described in detail. At the end of the paper, we summarize the means and methods to suppress photodarkening.

On the measurement of fundamental mode bend loss in large-mode-area optical fibers

The measurement of fundamental mode bend loss is thoroughly studied in large-mode-area few-mode optical fibers. The influencing factors, including spectral properties of the light source, modal power content and cladding light, are experimentally investigated. Monte Carlo simulations are performed to help in understanding and illustrating the distribution of the variations. Practical guidelines and an example setup are provided for precise and accurate measurements.

Deep UV-induced near-infrared photodarkening of Er/Yb-doped and undoped phosphate fibers

Photodarkening (PD) of Er/Yb-doped and undoped phosphate fibers caused by pulsed 193 nm irradiation from an ArF excimer laser to form Bragg grating mirrors is investigated. Doped and undoped phosphate fibers exhibit the same level of significant UV-induced PD loss, which is associated with the formation of a color center band at 467 nm. The UV-induced absorption extends into the NIR and creates a loss on the order of 1 dB/cm across the C-band. Photo-bleaching by a high-power supercontinuum source, and thermal-bleaching processes are performed on photodarkened samples. Both bleaching processes are found effective to fully erase the UV-induced PD loss.

Slope efficiency over 30% single-frequency ytterbium-doped fiber laser based on Sagnac loop mirror filter

A high-slope-efficiency single-frequency (SF) ytterbium-doped fiber laser, based on a Sagnac loop mirror filter (LMF), was demonstrated. It combined a simple linear cavity with a Sagnac LMF that acted as a narrow-bandwidth filter to select the longitudinal modes. And we introduced a polarization controller to restrain the spatial hole burning effect in the linear cavity. The system could operate at a stable SF oscillating at 1064 nm with the obtained maximum output power of 32 mW. The slope efficiency was found to be primarily dependent on the reflectivity of the fiber Bragg grating. The slope efficiency of multi-longitudinal modes was higher than 45%, and the highest slope efficiency of the single longitudinal mode we achieved was 33.8%. The power stability and spectrum stability were <2% and <0.1%, respectively, and the signal-to-noise ratio measured was around 60 dB.

In situ FBG inscription during fiber laser operation

We demonstrate the inscription of a 266 nm UV femtosecond pulse-induced fiber Bragg grating (FBG) in an Yb-doped fiber during optical pumping at 976 nm and the initiation of lasing with increasing grating reflectivity. Output spectra show the emission of the pumped fiber changing from the broad-ranged amplified spontaneous emission in the nonlasing case to the narrow-range laser operation due to the enhancement of FBG reflectivity during inscription. The proposed technique enables the direct characterization and control of FBG performance in fiber lasers. After FBG fabrication, we investigate the spectral characteristics of the fiber laser for different laser powers and study the influence of a thermal treatment of the FBG.

Optical repetition rate stabilization of a mode-locked all-fiber laser

We designed an all-fiber mode-locked Erbium laser with optically stabilized repetition rate of 31.4 MHz. The stabilization was achieved by changing the refractive index of an Ytterbium-doped fiber in the resonator via optical pumping at a wavelength of 978 nm; and for long-term stability the local temperature of the fiber was additionally controlled with a thermo-electric element. The repetition rate was stabilized over 12 hours, and an Allan deviation of 2.5 × 10⁻¹² for an averaging time of 1 s could be achieved.

Photodarkening kinetics as a function of Yb concentration and the role of Al codoping

We investigated the photodarkening (PD) kinetics of two fiber series with variation of the Yb content for constant Al concentration or constant ratio of Al/Yb, respectively. The results show the outstanding importance of the absolute value of Al concentration also in the case of fibers with strongly reduced Yb content. An Al/Yb ratio of 5 to 6 is not sufficient to mitigate PD loss. Moreover, a model to describe PD loss and rate constant as functions of Yb concentration and excitation is suggested that links measurements of PD in single fibers of the same type (variation of Yb inversion) and in fiber series (constant Yb inversion).