We investigate simplified fiber designs to mitigate thermal effects that limit the power scaling of large-mode area high-average power fiber lasers. We discuss the advantages of lowering the thermo-optical coefficient of the Yb active-doped core for different fiber designs. We demonstrate that mode shrinking can be fully suppressed not only in step index fibers, but also in more advanced ones such as large pitch fibers. Additionally, this approach will lead to a mitigation of transversal mode instabilities. We present first results with a step index fiber, where the influences of the co-dopants used to decrease the thermo-optical coefficient are discussed.

Nonlinearities are a persistent obstacle to power scaling in fiber lasers. Approaches to their management are often highly complex and difficult to implement. They can even introduce new problems that give rise to tradeoffs in system design. As a result, there is a continued need for complementary ways to eliminating them. This talk will focus on materials-based approaches to managing nonlinearities in high power fiber systems. The role of novel multicomponent glasses in scaling high power fiber systems will be envisaged.

Novel extra-large mode area active and passive fibers deigned to achieve up to 100 mJ and > 1 kW output power in industrial fiber laser systems will be presented. Three Yb-doped XLMA fiber sizes will be proposed, able to offer high extraction efficiency, MW peak power handling, manageable thermal load, excellent spliceability and stable beam profile. Fiber performance capabilities will be discussed with numerical simulation results. Active and passive fiber prototypes will be manufactured and tested. Key design aspects and performances including efficiency, beam profile, cladding absorption and photodarkening will be reported.

We present here the development of ultra-low NA large mode area neodymium doped alumino-phospho-silica fibers with different clad-to-core ratios for high power laser emission around 910nm. This ratio is determinant in the competition between the 3-level transition at 910nm and 4-level transition of neodymium at 1060nm. The study shows that the 30/130µm (core/cladding) fiber was the most efficient, with a record output power of 83W at 910nm, yielding a 47% slope efficiency and a good beam quality (M²~1.5). Parasitic power at 1060nm was kept lower than 1W and no sign of roll-off was observed at maximum pump power.

We present our first results on the coherent combination of four rod type Thulium doped fiber amplifiers. With this we achieved a pulse energy of 1.34 mJ and an average output power of 130 W at a repetition rate of 98 kHz with a compressed pulse duration of 123 fs. This system represents the highest output power mJ class laser system in the SWIR to date and proves the average power and pulse energy scaling capabilities of ultrafast Tm doped fiber amplifiers.

We report on far field energy distribution control using a coherent beam combining femtosecond digital laser employing 61 tiled channels. Each channel is considered as an individual pixel where amplitude and phase are controlled independently. Applying a phase difference between neighboring fibers or neighboring fiber-lines gives high agility for a far field energy distribution and paves the way for deeper exploration of phase patterns as a tool to further improve the efficiency achievable with tiled-aperture/filled-aperture setups and far field shaping on demand.

We demonstrate, to our knowledge, the first operation of a simultaneous spatial combining and CPSA system that provides 9mJ from two amplifier channels and coherently stacks 81 pulses to a single pulse and compresses the output to ~515fs duration. This demonstrates that CPSA with spatial coherent combining enables energy scaling of ultrashort pulses with fewer parallel channels. Future work will involve increasing energy per channel, average power per channel, the number of fiber channels and reducing the pulse duration.

A random distributed feedback Raman fiber laser (RRFL) based on graded-index (GRIN) fiber with brightness enhancement (BE) is demonstrated. Pumped by temporally stable amplified spontaneous emission source, the output power of 722 W at 1130 nm is achieved thanks to the effective suppression on higher-order Stokes generation. Besides, benefiting from the beam cleanup effect in GRIN fiber, the beam quality factor M2 of the signal laser at maximum output is 4.65 corresponding to the BE factor of 8.9. To the best of our knowledge, this is the record output power of RRFL based on GRIN fiber with BE.

To be cleared for public release

In this report, by comparing the RIN and beam pointing error of the signal laser at different output powers, the impact of the modal degeneration on both the spacial and temporal noise properties of high-power single-frequency fiber amplifiers will be demonstrated. The new finding reveal that obvious mode-related enhancement of the RIN could occur well below the conventional transverse mode instability (TMI) threshold of the fiber amplifier while the mode-related enhancement of the beam pointing error occurs near the TMI threshold. This work could provide a new insight for obtaining high-power, high spacial stability and low-noise single-frequency fiber lasers.

A novel phase modulation technique, calling multi-phase coded signal (MPCS) modulation, is demonstrated for stimulated Brillouin scattering (SBS) effect managing in high power narrow-linewidth polarization-maintained all-fiber amplifier. Combining with laser gain competition where two signal lasers operating at 1045 nm and 1064 nm are applied, the output power of 1064 nm laser could be boosted to 1023 W with FWHM linewidth of 4.6 GHz. The polarization extinction ratio (PER) is 13.3 dB and the output beam quality is measured to be M2=1.11. This work could provide a feasible method for linewidth controlling in high power narrow linewidth fiber amplifier.

We investigate the amplification of mid-infrared nanosecond laser pulses in a single-mode Er-doped fluoride fibre amplifier. The seed laser pulses in the wavelength range around 2790 nm are generated in a wavelength-tuneable, PPLN-based OPO, with sub-10 ns pulse durations and pulse energies of few 100 nJ at a repetition rate of 10 kHz. The single-stage fibre amplifier consists of a double-clad, single-mode Er-doped fluoride fibre with a doping concentration of 70000 ppm, pumped at 980 nm in counter-propagation to the laser signal. The sub-10 ns laser pulses are amplified to pulse energies above 35 µJ, resulting in gain values of more than 20 dB and peak powers above 5 kW. The gain bandwidth and wavelength range of the fibre amplifier are investigated at different gain values and wavelength tuning is demonstrated over the range from 2711 nm to 2818 nm.

In this work we present investigation of TMI in Yb-doped FA-LPF using spatially and temporally resolved imaging. We present ST-measurements that are clearly describing the system noises and that can be used to optimize amplifier systems in order to mitigate TMI. Comparative TMI measurements between the same FA-LPF in amplifier and laser configuration are also presented. These measurements show similar results with respect to optical efficiencies but large differences with respect to TMI.

A hundred-watt level spatial mode switchable all-fiber laser is demonstrated based on a master oscillator power amplifier scheme. The mode switchable seed laser is implemented by integrating a homemade acoustically induced fiber grating (AIFG) mode converter inside the cavity, and the output power was scaled to > 100 W in both LP01 and LP11 mode. Fast mode switching with millisecond scale switching time between LP01 and LP11 mode while operating in full power is realized through the AIFG driven by radio frequency modulation. This work could provide a good reference for realizing high-power agile mode-switchable fiber lasers for practical applications.

Record laser cooling of Yb-doped silica by 18.4 K was recently observed in vacuum. There is a strong indication pointing to even stronger laser-cooling of Yb-doped silica. In light of these results, a radiation-balanced fiber laser in which cooling from spontaneous emission offsets waste heat generation appears more feasible than before. We will discuss the possibility of a radiation-balanced fiber laser, the similarities and differences with the conventional fiber lasers, and performance issues.

We present the first all-polarization-maintaining Ytterbium-doped fiber laser oscillator mode-locked through a nonlinear optical loop mirror that works in a dispersion-managed dissipative soliton regime. Using a chirped fiber Bragg grating, we built a cavity of a net negative dispersion delivering ultrashort 2.3 nJ pulses at a repetition rate of 34.08 MHz. Chirped 3.96 ps pulses from the oscillator are recompressible down to 98 fs with a high temporal Strehl ratio.

Stable generation of ultrashort, high-energy pulses in the femtosecond range through passive harmonic mode locking is achieved in ultralong EDFA-based ring fiber oscillators using polarization-insensitive InN-based SESAMs and up to 25 km of standard communications fiber assisted by distributed Raman amplification. This demonstrates a new family of ultrafast fiber oscillators naturally operating at low repetition rates. The lasers can achieve fundamental mode locking for lengths below 10 km. By using TS-DFT the average laser spectrum is shown to correspond to that of individual pulses, confirming high-energy pulse generation without the need for external amplification and compression stages.

We report on our recent results for output scaling by a Thulium-doped fiber Mamyshev oscillator in the spectral 2 µm region. In order to further scale the output, a highly Thulium-doped double-clad fiber was implemented in the Mamyshev oscillator. A stable pulse train at a repetition rate of 19.7 MHz with an average power of 220 mW was observed. This corresponds to a pulse energy of 11 nJ at a pulse length of 2.4 ps. Further scaling of the output parameters was limited by soliton fission, which will be mitigated by introducing normal dispersive fiber sections in future experiments.

A gain-managed nonlinear (GMN) tapered fiber amplifier was experimentally demonstrated. The achieved single pulse energy was 707 nJ and the compressed pulse duration was 67 fs with 10 MW peak power at 1054 nm central wavelength.

We propose and demonstrate a wide-bandwidth unidirectional ring cavity wavelength-swept laser using a polygonal scanning wavelength filter and two semiconductor optical amplifiers (SOAs). The center wavelengths of the gain media of the two SOAs are 1020 nm and 1140 nm, respectively. At a scanning speed of 1.8 kHz of the WSL, a wide scanning band of about 223 nm with a 10-dB bandwidth from 959 nm to 1182 nm and an average optical power of 58.28 mW were obtained. It can be used as a broadband optical source for dynamic optical fiber sensor or optical coherence tomography.