Zhihua Huang

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.

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.

Passive energy jitter reduction in the cascaded third harmonic generation process

In free electron laser (FEL) systems with ultraviolet (UV) laser driven injectors, a highly stable UV source generated through cascaded third harmonic generation (THG) from an infrared (IR) source is a key element in guaranteeing the acceptable current jitter at the undulator. In this letter, the negative slope of the THG efficiency for high intensity ultrashort IR pulses is revealed to be a passive stabilization mechanism for energy jitter reduction in UV. A reduction of 2.5 times the energy jitter in UV is demonstrated in the experiment and simulations show that the energy jitter in UV can be reduced by more than one order of magnitude if the energy jitter in IR is less than 3%, with proper design of the THG efficiency curve, fulfilling the challenging requirement for UV laser stability in a broad scope of applications such as the photoinjector of x-ray FELs.

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.

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.

Demonstration of resonant backward Raman amplification in high-density gas-jet plasma

Backward Raman amplification was observed in a 0.7 mm-long high-density gas jet plasma. The 800 nm 30 fs seed pulse was amplified by a factor ~28, with an output energy of 2.8 mJ. The output spectra showed that the waveband around 800 nm was significantly amplified. The experimental result demonstrated that the resonant Raman amplification can be realized in high-density plasma against strong plasma instability.

Generation of high average power supercontinuum involve visible spectrum

A high average-power all-fiber supercontinuum laser source is constructed. By integrating series techniques together, the output average power achieves 65W with the spectrum range covering two octaves from 540nm to 2200nm. To our knowledge, there has been never reported similar supercontinuum source with such high average power, broadband spectrum and picosecond pulse width.

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.