Yb-doped Large Mode Area Fiber Research Articles

Ultra-low NA step-index large mode area Yb-doped fiber with a germanium doped cladding for high power pulse amplification.

High concentration rare earth doped, large mode area (LMA) step-index fibers, which feature a very high cladding absorption per unit length at the pump wavelength, high efficiency, and excellent beam quality, are ideal for high power pulsed fiber lasers/amplifiers where large effective mode areas and short device lengths are crucial in order to reduce detrimental nonlinear effects associated with high peak power operation. In this Letter, we realize low numerical aperture (NA) high absorption fibers, simply by employing a germanium (Ge)-doped cladding rather than a pure silica cladding to offset the high refractive index associated with using a high concentration of ytterbium (Yb) in the core. This approach allows us to separate the two inter-linked fiber design parameters of pump absorption and NA in a step-index fiber. Using a conventional modified chemical vapor deposition process combined with solution doping, a low NA (0.04), LMA (475µm2) silica fiber is fabricated with a cladding absorption value of >20dB/m, which is the highest value among LMA step-index fibers with NA<0.06 so far reported to the best of our knowledge. The fabricated Yb-doped fiber was tested in a high-power picosecond amplifier system and enabled the generation of 190 ps laser pulses with a 101 µJ pulse energy and 0.5 MW peak power at an average power of 150 W.

Yb-doped large mode area fiber for beam quality improvement using local adiabatic tapers with reduced dopant diffusion.

A newly designed all-solid step-index Yb-doped aluminosilicate large mode area fiber for achieving high peak power at near diffraction limited beam quality with local adiabatic tapering is presented. The 45µm diameter fiber core and pump cladding consist of active/passively doped aluminosilicate glass produced by powder sinter technology (REPUSIL). A deliberate combination of innovative cladding and core materials was aspired to achieve low processing temperature reducing dopant diffusion during fiber fabrication, tapering and splicing. By developing a short adiabatic taper, robust seed coupling is achieved by using this Yb-doped LMA fiber as final stage of a nanosecond fiber Master Oscillator Power Amplifier (MOPA) system while maintaining near diffraction limited beam quality by preferential excitation of the fundamental mode. After application of a fiber-based endcap, the peak power could be scaled up to 375 kW with high beam quality and a measured M2 value of 1.3~1.7.

Long-term mode shape degradation in large mode area Yb-doped pulsed fiber amplifiers

Experimental observation of long-term output mode shape degradation in pulsed Yb-doped fiber amplifiers is reported for the first time. The process occurs in large mode area Yb-doped fibers and is caused by the formation of the long period grating responsible for power transfer from the fundamental mode to the first high-order mode. The linkage between the process and photodarkening was revealed.

Mode-selective amplification in a large mode area Yb-doped fiber using a photonic lantern.

We demonstrate selective spatial mode amplification in a few mode, double-clad Yb-doped large mode area (LMA) fiber, utilizing an all-fiber photonic lantern. Amplification to multi-watt output power is achieved while preserving high spatial mode selectivity. We observe gain values of over 12 dB for all modes: LP01, LP11a, and LP11b, when amplified individually. Additionally, we investigate the simultaneous amplification of LP01+LP11a and LP11a+LP11b, and the resultant mode competition. The proposed architecture allows for the reconfigurable excitation of spatial modes in the LMA fiber amplifiers, and represents a promising method that could enable dynamic spatial mode control in high power fiber lasers.

300 W all-fiber amplifier with distributed side-coupled pump configuration

The angle-polished side-pump coupler is fabricated by fused splicing angle-polished multimode fibers and 20/400 μm double-clad Yb-doped large mode area fiber together. The coupling efficiency of side-pumped coupler for 975 nm pumping light can reach up to 97%, and the leakage ratio of 1080 nm signal light is less than 2%. We analyze the performance of a single side-pumped coupler and also the influence of the distribution of cascade stage couplers on the efficiency of fiber amplifier. Using self-developed side-pumped couplers, we build a distributed side-pumped, ytterbium-doped double-clad all-fiber master oscillator amplifier, and an continuous and single mode laser of 1080 nm wavelength with a 303 W maximum output power is obtained. Finally, increasing the number of the side couplers to improve pump power, the output power of the amplifier becomes higher.

Single-Mode Propagation in Yb-Doped Large Mode Area Fibers With Reduced Cladding Symmetry

Single-Mode Propagation in Yb-Doped Large Mode Area Fibers With Reduced Cladding Symmetry

Multi-stage ytterbium fiber-amplifier seeded by a gain-switched laser diode

We demonstrated all-fiber amplification of 11 ps pulses from a gain-switched laser diode at 1064 nm. The diode was driven at a repetition rate of 40 MHz and delivered 13 µW of fiber-coupled average output power. For the low output pulse energy of 325 fJ we have designed a multi-stage core pumped pre-amplifier in order to keep the contribution of undesired amplified spontaneous emission as low as possible. By using a novel time-domain approach for determining the power spectral density ratio (PSD) of signal to noise, we identified the optimal working point for our pre-amplifier. After the pre-amplifier we reduced the 40 MHz repetition rate to 1 MHz using a fiber coupled pulse-picker. The final amplification was done with a cladding pumped Yb-doped large mode area fiber and a subsequent Yb-doped rod-type fiber. With our setup we reached a total gain of 73 dB, resulting in pulse energies of &gt;5.6 µJ and peak powers of &gt;0.5 MW. The average PSD-ratio of signal to noise we determined to be 18/1 at the output of the final amplification stage.

Build up and decay of mode instability in a high power fiber amplifier

State-of-the-art high power Yb-doped large mode area fibers have been developed to a performance level able to reach the so-called mode instability threshold. In this contribution we will discuss the experimental results regarding the temporal evolution (build up and decay) of this effect to come closer to a comprehensive understanding of its driving mechanisms. Our investigations prove that the relevant time scale for build up and decay of mode instability is in the millisecond range and thus deliver experimental evidence of underlying thermal effects. To the best of our knowledge these are the first systematic, time resolved investigations on that topic.

A 12 mJ 11 ns spectrally narrow fiber amplifier with a pulsed pump

We report on the fiber-based amplification of a flash lamp pumpedQ-switched Nd:YAG laser. At a repetition rate of 10 Hz and spectrum widthof 0.1 nm, an output single pulse energy of up to 12 mJ is generated in a95 µm core Yb-doped large mode area fiber with a seed pulse energy of 0.75 mJ and pulseduration of 11 ns. No stimulated Brillouin scattering, stimulated Raman scattering orsaturation are observed. Furthermore, higher pulse energies will be achieved with higherpump powers.

High-power all-fiber passively Q-switched laser using a doped fiber as a saturable absorber: numerical simulations

We report a design for a power-scalable all-fiber passively Q-switched laser that uses a large mode area Yb-doped fiber as a gain medium adiabatically tapered to an unpumped single-mode Yb-doped fiber, which serves as a saturable absorber. Through the use of a comprehensive numerical simulator, we demonstrate a passively Q-switched 1030 nm pulsed laser with 14 ns pulse duration and 0.5 mJ pulse energy operating at 200 kHz repetition rate. The proposed configuration has a potential for orders of magnitude of improvement in both the pulse energies and durations compared to the previously reported result. The key mechanism for this improvement relates to the ratio of the core areas between the pumped inverted large mode area gain fiber and the unpumped doped single-mode fiber.