Dangpeng Xu

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.

Comprehensive Investigations on 1053 nm Random Distributed Feedback Fiber Laser

We design a 1053 nm Yb3+-doped random fiber laser (YRFL) based on the combination of Yb3+ -doped fiber (YDF) and single mode fiber (SMF), in which the YDF provides active gain, while SMF offers random distributed feedback. The numerical model is established based on the rate equation of Yb3+-doped laser, and it is modified and developed to calculate the power performance of YRFL. The numerical results show that the YDFs length and the pumping scheme could influence the laser power performance. Then, the forward and backward pumped YRFL both with 9.5 m YDF and 2 km SMF are experimentally demonstrated whose threshold powers are 1 and 1.1 W, respectively. The obtained optical slope efficiencies are 45.2% and 40.9% for these two cases. The experimental results agree with the calculated results well. This work gives a general study on active gain based random fiber lasers, which can enrich the operation wavelength range of random fiber laser and may broaden its application fields to high energy large-scale laser facilities.

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.

Research of beam smoothing technologies using CPP, SSD, and PS

Precise physical experiments place strict requirements on target illumination uniformity in Inertial Confinement Fusion. To obtain a smoother focal spot and suppress transverse SBS in large aperture optics, Multi-FM smoothing by spectral dispersion (SSD) was studied combined with continuous phase plate (CPP) and polarization smoothing (PS). New ways of PS are being developed to improve the laser irradiation uniformity and solve LPI problems in indirect-drive laser fusion. The near field and far field properties of beams using polarization smoothing were studied and compared, including birefringent wedge and polarization control array. As more parameters can be manipulated in a combined beam smoothing scheme, quad beam smoothing was also studies. Simulation results indicate through adjusting dispersion directions of one-dimensional (1-D) SSD beams in a quad, two-dimensional SSD can be obtained. Experiments have been done on SG-III laser facility using CPP and Multi-FM SSD. The research provides some theoretical and experimental basis for the application of CPP, SSD and PS on high-power laser facilities.

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.

Intermodal interference induced significant frequency modulation to amplitude modulation conversion in a broadband large-mode-area fiber laser

The conversion of the FM-to-AM effect induced by intermodal interference in the broadband large-mode-area (LMA) fiber laser was first investigated theoretically and experimentally. The numerical simulation results show that the spectrum transfer functions are different at different positions of the LMA fiber end face owing to the intermodal interference, so the output broadband pulses are different. We attain the similar results in the experiment when measuring the output pulse with the single mode fiber sampling oscilloscope. Whereas there is no amplitude modulation for the output pulse when measured by the bulk detector owing to the orthogonal characteristic of the eigenmodes.

Polarized millijoule fiber laser system with high beam quality and pulse shaping ability

The coherent amplification network (CAN) aims at developing a laser system based on the coherent combination of multiple laser beams, which are produced through a network of high beam quality optical fiber amplifiers. The scalability of the CAN laser facilitates the development of many novel applications, such as fiber-based acceleration, orbital debris removal and inertial confinement fusion energy. According to the requirements of CAN and the front end of high-power laser facilities, a millijoule polarized fiber laser system was studied in this paper. Using polarization maintaining Ytterbium-fiber laser system as the seed, and 10-μm core Yb-doped fiber amplifier as the first power amplifier and 40-μm core polarizing (PZ) photonic crystal fiber (PCF) as the second power amplifier, the all-fiber laser system outputs 1.06-mJ energy at 10 ns and diffraction limited mode quality. Using 85-μm rod-type PCF as the third power amplifiers, 2.5-mJ energy at 10-ns pulse width was obtained with better than 500:1 peak-to-foot pulse shaping ability and fundamental mode beam quality. The energy fluctuation of the system is 1.3% rms with 1-mJ output in one hour. When using phase-modulated pulse as the seed, the frequency modulation to amplitude modulation (FM-to-AM) conversion ratio of the system is better than 5%. This fiber laser system has the advantages of high beam quality, high beam shaping ability, good stability, small volume and free of maintenance, which can be used in many applications.

Progress on the suppression of FM-to-AM modulations in SG-III laser facility

FM-to-AM modulations are harmful to output characteristics of large-scale laser facility. In SG-III laser facility, some key techniques have been employed to suppress FM-to-AM modulations. Firstly, phase modulator was arranged at the end of fiber laser injection system to avoid GVD and PMD in the SM fiber system. Secondly, a fiber-based polarization rotated filter was proposed to suppress FM-to-AM modulations independently on each beam. Finally, less wave plates were employed and liquid crystal modulators were coated for decreasing weak etalon effect in preamplifier system. The results indicated that it can reduce the modulation depth less than 10% at a modulation frequency of 2.488GHz in SG-III laser facility.

Research of time fiducial and imaging VISAR laser for Shenguang-III laser facility

Time fiducial laser is an important tool for the precise measurement in high energy density physics experiments. The VISAR probe laser is also vital for shock wave diagnostics in ICF experiments. Here, time fiducial laser and VISAR light were generated from one source on SG-III laser facility. After generated from a 1064-nm DFB laser, the laser is modulated by an amplitude modulator driven by 10 GS/s arbitrary waveform generator. Using time division multiplexing technology, the ten-pulse time fiducial laser and the 20-ns VISAR pulse were split by a 1×2 multiplexer and then chosen by two acoustic optic modulators. Using the technique, cost of the system was reduced. The technologies adopted in the system also include pulse polarization stabilization, high precision fiber coupling and energy transmission. The time fiducial laser generated synchronized 12-beam 2ω and 4-beam 3ω laser, providing important reference marks for different detectors and making it convenient for the analysis of diagnostic data. After being amplified by fiber amplifiers and Nd:YAG rod amplifiers, the VISAR laser pulse was frequency-converted to 532-nm pulse by a thermally controlled LBO crystal with final output energy larger than 20 mJ. Finally, the green light was coupled into a 1-mm core diameter, multimode fused silica optical fiber and propagated to the imaging VISAR. The VISAR laser has been used in the VISAR diagnostic physics experiments. Shock wave loading and slowdown processes were measured. Function to measure velocity history of shock wave front movement in different kinds of materials was added to the SG-III laser facility.