Honghuan Lin

Combined numerical model of laser rate equation and Ginzburg–Landau equation for ytterbium-doped fiber amplifier

We propose a combined model of the laser rate equation and the Ginzburg–Landau equation for simulation of the amplification of the ultrashort pulse in an ytterbium-doped fiber amplifier. An iterative procedure is designed for the repetition rate pulse sequence. The combined numerical model is compared with the pure dynamic rate equation for the amplification of a nanosecond single frequency pulse where both models are applicable. Good agreement is achieved. The combined model is then applied for the amplification of high repetition rate chirped pulse amplification in ytterbium-doped single-mode fiber. The slope efficiency, the wavelength shift, and the output spectrum width at different repetition rates are numerically investigated.

5kW GTWave fiber amplifier directly pumped by commercial 976nm laser diodes.

With home-made fiber perform and special fiber drawing & coating technique, a new-type of (3 + 1) GTWave fiber theoretically designed for bi-directional pump method, was successfully fabricated and justified of integrating multi-kW pump energy from commercial 976nm laser diodes. This (3 + 1) GTWave fiber amplifier demonstrated uniform absorption of pump light and easy thermal management characteristics along the whole fiber length. This amplifier is capable of simultaneously aggregating 5.19kW pump power at 976nm and finally generating 5.07kW laser output at 1066.5nm with an optical-to-optical efficiency of 74.5%, the first publically-reported multi-kW GTWave fiber directly pumped with commercial 976nm laser diodes to the best of our knowledge. No power roll-over was found at 5kW level and further power scaling can be expected with more pump power. The results indicate that GTWave fiber is a competitive integrated fiber device to collect enough pump energy from low-cost commercial laser diodes for multi-kW fiber laser development.

Yb-Doped Aluminophosphosilicate Laser Fiber

By using modified chemical vapor deposition system combined with chelate precursor doping technique, we report on the fabrication and characterization of Yb-doped aluminophosphosilicate (Al2O3-P2O5-SiO2) laser fiber. Based on a master oscillator power amplifier laser setup and pumped directly by 976 nm laser diodes, 3.1 kW laser at ~1064 nm was achieved with a slope efficiency of 78.4%. Benefiting from codoped Al and P, the laser output power showed no evidence of roll-over. The linear fitting of the output power versus the pump power shows the potentiality for further power scaling. The results indicate that chelate precursor doping technique is a competitive method for rare-earth-ion-doped fiber preform fabrication, and aluminophosphosilicate host material has potentiality to develop multi-kW level laser fibers.

Tapered inner-cladding fiber design for uniform heat deposition in Ytterbium-doped fiber amplifiers

A method for designing double-clad fiber with tapered inner cladding and uniform core is proposed for linear pump power profile, i.e. uniform heat deposition, in the ytterbium-doped fiber amplifier. The analytical formula for the inner-cladding diameter profile along the fiber is given. The inner-cladding diameter near the pump injection port is determined purely by the diameter of the doped region, the number density of the doped ions, the absorption cross section at the pump wavelength and the length of the fiber. The simplified linearly varying inner-cladding diameter is proven to have a smoother heat deposition profile with lower maximum thermal load in both the co-pumping scheme and the counter-pumping scheme.

Tapered cladding diameter profile design for high-power tandem-pumped fiber lasers

The thermal effect has become the biggest limiting factor regarding the further power scaling of single mode fiber lasers, and it can lead to coating failure and transverse mode instability. A tapered cladding diameter profile design is proposed for the tandem-pumped fiber laser in this work, as it can smooth the temperature profile and reduce the maximum temperature rise within the fiber tremendously. The improvement in performance of the fiber design is verified by analytical and numerical results.

A fiber-based polarization–rotation filter utilized to suppress the FM-to-AM effect in a large-scale laser facility

The frequency modulation to amplitude modulation (FM-to-AM) effect is one of the few remaining scientific issues in a large-scale laser facility which could prevent fusion ignition. A fiber-based polarization?rotation filter which consists of a fiber polarizer, a section of polarization-maintaining fiber, a half-wave plate and a polarization beam splitter is proposed to suppress the FM-to-AM effect. Theoretical analysis and experimental demonstration in the preamplifier module of the SG-III laser facility prove the effectiveness of this filter.

Power balance on the SG-III prototype facility

With the methods of time-division multiplexing in Frontend and angle detuning in FOA, each beam pulse on SG-III prototype facility is controlled independently and so the systematic variations of power imbalance are eliminated entirely.

Coherent and incoherent combination of Gaussian beams employing lens array distributed on the spherical chamber

Coherent and incoherent combination of Gaussian beams employing a lens array distributed on the spherical chamber is theoretically analyzed. The output field of each source in the array is coupled through an individual optical system whose local optical axis coincides with the radial direction of the chamber. The resulting intensity profile near the origin is derived. The intensity profile and power in the bucket on the target for rectangular and hexagonal arrangement are numerically calculated. The influences of the center-to-center separation and the ring number of the focusing lens array are given. The synthetic intensity profile of incoherent combination changes little for a lens array scale much smaller than the chamber size. In contrast, the synthetic intensity profile of coherent combination shows an interference pattern with a sharp central peak and sidelobes.

Recent progress of the Integration Test Bed

The Integration Test Bed (ITB) is a large-aperture single-beam Nd:glass laser system, built to demonstrate the key technology and performance of the laser drivers. It uses two multipass slab amplifiers. There are four passes through the main amplifier and three passes through the booster amplifier. The output beam size is 360mm by 360mm, at the level of 1% of the top fluence. The designed output energy of ITB at 1053nm is 15kJ in a 5ns flat-in-time (FIT) pulse, the third harmonic conversion efficiency is higher than 70%. The first phase of the ITB has been completed in July 2013. A series of experiments demonstrated that laser performance meets or exceeds original design requirements. It has achieved maximum energies at 1053nm of 19.6kJ at 5ns and 21.5kJ at 10ns. Based on a pair of split third harmonic generation KDP crystals, the third harmonic conversion efficiency of about 70% and 3ω mean fluences as high as 8.4 J/cm2 have been obtained with 5ns FIT pulse.

Parameter space for the collective laser coupling in the laser fusion driver based on the concept of fiber amplification network.

Collective laser coupling of the fiber array in the inertial confinement fusion (ICF) laser driver based on the concept of fiber amplification network (FAN) is researched. The feasible parameter space is given for laser coupling of the fundamental, second and third harmonic waves by neglecting the influence of the frequency conversion on the beam quality under the assumption of beam quality factor conservation. Third harmonic laser coupling is preferred due to its lower output energy requirement from a single fiber amplifier. For coplanar fiber array, the energy requirement is around 0.4 J with an effective mode field diameter of around 500 μm while maintaining the fundamental mode operation which is more than one order of magnitude higher than what can be achieved with state-of-the-art technology. Novel waveguide structure needs to be developed to enlarge the fundamental mode size while mitigating the catastrophic self-focusing effect.