Zhongwei Fan

Femtosecond off-axis digital holography for monitoring dynamic surface deformation

We present a femtosecond off-axis digital holography for investigating the dynamic reversible surface change of a metal film induced by femtosecond laser pulses with fluences near the ablation threshold. A reflection Michelson interferometer (RMI) and a transmission Mach–Zehnder interferometer (TMZI) are integrated in the same setup for recording digital holograms. The RMI is used to measure the laser-induced surface deformation of the metal film, while the TMZI is used to analyze the refraction index change of the metal film induced by the femtosecond laser pulses. Experimental results show that both surface modification and refraction index change of chromium metal film can be observed when femtosecond laser pulses are below and above the ablation threshold. Based on the experimental results, the physical processes of the metal induced by femtosecond laser pulses are given qualitatively.

High beam quality 5 J, 200 Hz Nd:YAG laser system

A high beam quality, all-solid-state Nd:YAG laser system of high-repetition frequency has been built for Thomson scattering diagnosis. A 1.7 times diffraction limited output beam at a pulse energy of 5 J at 1064 nm is achieved for the first time with a pulse duration of 6.6 ns (FWHM) and a repetition rate of 200 Hz; the output energy stability is 4.9% peak-to-valley over 6000 shots. A novel pulsed laser system (Supplementary Fig. S1) with high average power and high beam quality has recently been built by Dr Zhong-Wei Fans group at the Academy of Opto-Electronics, Chinese Academy of Sciences. Both the laser diode side-pumped rod and slab crystals are integrated into the amplifier (AMP) system. A 1.7 times diffraction-limited output beam at a pulse energy of 5 J at 1064 nm is achieved for the first time with a pulse duration of 6.6 ns (FWHM) and a repetition rate of 200 Hz; the output energy stability is 4.9% peak-to-valley over 6000 shots. The test results are shown in Figure 1a and 1b. The laser system is constructed in a master oscillator power amplifier (MOPA) configuration, as shown in Figure 1c, with four components: a single-frequency seed laser, pre-amplifier unit, beam control unit, and post-amplifier unit. The pre-amplifier consists of a three-stage, side-pumped rod amplifier. The dimensions of the rod crystals are φ3 mm× 67 mm with a Nd3+ concentration of 0.8% for AMP1 and AMP2, and φ6.35 mm× 140 mm with a Nd3+ concentration of 0.6% for AMP3 and AMP4. The techniques of the stimulated Brillouin scattering phase-conjugate mirror (SBSPCM) and adaptive optics are implemented in the beam control unit to correct the wavefront distortion dynamically. The postamplifier unit is composed of a three-stage, large slab amplifier. The dimensions of the slab crystals are 138 mm (L) × 35 mm (W)× 7 mm (D) with a Nd3+ concentration of 0.6% for AMP5, AMP6 and AMP7. The single-frequency seed laser produces an output power of 8.58 μJ with a pulse duration of 33.9 ns (FWHM) at a 200-Hz repetition rate. The root-mean-square (RMS) fluctuation in pulse energy is smaller than 1% and the beam quality is better than 1.12 times diffraction limited. The seed pulses first pass through the pre-amplifier and then through the control unit, at which the beam shaping is applied. The pulse energy is amplified to 300 mJ. The pulse duration is 30.5 ns and the beam quality is better than 1.4 times diffraction-limited. After passing through the post-amplifier, the pulse energy reaches 5 J with 3.2 times diffraction limited beam quality. An adaptive optics system is applied for wavefront correction, and the beam quality is improved to 1.7 times diffraction limited. The core parts of the laser system include the single-frequency laser source, slab amplifier module with high-energy storage efficiency, and the phase-conjugated, stimulated Brillouin scattering mirror. The single-frequency source is an active Q-switching laser utilizing acousto-optic modulation. Single-longitudinalmode operation is achieved by applying a Fabry-Pérot (FP) etalon. The length of the resonant cavity can be accurately controlled using piezoelectric ceramics, while sampling precision can be improved using a smoothing algorithm. To compensate for the influence of the environment on the length of the resonant cavity, the fuzzy proportional-integral-derivative (PID) control algorithm is implemented. With the above setup, single-frequency pulses of high stability can be achieved at the nanosecond (ns) level. The single slab amplifier can store as much as 2.5 J, with a small signal gain 45, depolarization loss o2%, pumping homogeneity better than 90%, and single-pass wavefront distortion better than 0.15 λ (RMS) when fully loaded. In the SBS-PCM, FC-770 is chosen as the SBS medium. Attributed to the specific cleaning and fine purification during the preparation processes, the load capacity of the SBS-PCM is thus improved. When pumping energy reaches 1.1 J (220 W, 200 Hz), optical breakdown is prevented and a phase-conjugate reflectivity higher than 98% is achieved.

Picosecond laser system with 30-W average power via cavity dumping and amplifying

We present a picosecond laser system with high energy by technologies of cavity dumping and amplifying. Firstly, pulses with 10 ps and ∼520 nJ were obtained by cavity-dumped mode-locked laser at 10 kHz repetition rate. Secondly those pulses were seeded into a side-pumped regenerative amplifier (RA). Then pulses output from the regenerative amplifier were amplified by two four-pass post amplifiers. From the laser system pulses with an average power of 30 W corresponding to 3 mJ pulse energy were achieved with the pulse-width of 25.4 ps at repetition rate of 10 kHz.

Further localization of optical field for flower-like silver particles under laser radiation

We find the surface optical field of submicron silver particles can be further localized by their surface roughening, which leads to a greatly enhanced local field. The flower-like silver particles were prepared and the extinction spectra were measured in our work. A high enhancement factor of 1.3 × 107 was obtained by surface-enhanced Raman spectrum. Besides that, the correlation between the surface plasmon resonance mode and surface local field was also studied by theoretical analysis. The results show the plasmon resonance modes of flower-like silver particles include dipole and multipole plasmon resonances. For the dipole plasmon resonance at long wavelength, the local field near polar regions will be further localized as a result of the rough surface. In contrast, the interaction of local field between surface structures will cause hotspots for the multipole plasmon resonance in the short-wave region. Based on these factors, we demonstrate that the rough-surface-induced field superposition results in the redistribution and enhancement of the optical field around the particles.

Beam shaping and compensation for high-gain Nd:glass amplification

Beam shaping and compensation are reported for a high Nd:glass amplification system based on the liquid crystal spatial light modulation technique. As a typical application, a signal Gaussian beam of 1 nJ, with a pulse width of 3 ns, was amplified to 5 J with a flat-top intensity distribution. A spot modulation of 1.41 on the dimension of 50 mm × 50 mm was obtained. The minor beam distortion was further eliminated by applying a closed loop control. Most parameters including pulse energy, frequency, spot intensity distribution, etc., can be measured and calculated simultaneously via this proposed system. Moreover, the structure of image transfer was confirmed to be capable of maintaining high-quality image transmission in the amplification process.

Erbium-Doped Fiber Lasers Operated in a Strong Normal Dispersion Regime at Low Repetition Rate

We report on a passively mode-locked erbium-doped fiber laser with a repetition rate as low as 1.2 MHz in a strong normal dispersion regime. The laser was mode locked via spectral filtering. It generated a pulse with large and linear chirp that can be dechirped to 430 fs outside the cavity.

Dual-core antiresonant hollow core fibers

In this work, dual-core antiresonant hollow core fibers (AR-HCFs) are numerically demonstrated, based on our knowledge, for the first time. Two fiber structures are proposed. One is a composite of two single-core nested nodeless AR-HCFs, exhibiting low confinement loss and a circular mode profile in each core. The other has a relatively simple structure, with a whole elliptical outer jacket, presenting a uniform and wide transmission band. The modal couplings of the dual-core AR-HCFs rely on a unique mechanism that transfers power through the air. The core separation and the gap between the two cores influence the modal coupling strength. With proper designs, both of the dual-core fibers can have low phase birefringence and short modal coupling lengths of several centimeters.

Two-beam combined 3.36 J, 100 Hz diode-pumped high beam quality Nd:YAG laser system

In this paper, we develop a diode-pumped all-solid-state high-energy and high beam quality Nd:YAG laser system. A master oscillator power amplifier structure is used to provide a high pulse energy laser output with a high repetition rate. In order to decrease the amplifier working current so as to reduce the impact of the thermal effect on the beam quality, a beam splitting-amplifying-combining scheme is adopted. The energy extraction efficiency of the laser system is 50.68%. We achieve 3.36 J pulse energy at a 100 Hz repetition rate with a pulse duration of 7.1 ns, a far-field beam spot 1.71 times the diffraction limit, and 1.07% energy stability (RMS).

High efficiency and good beam quality of electro-optic, cavity-dumped and double-end pumped Nd:YLF laser

In this paper, we describe a Nd:YLF laser based on high-speed RTP electro-optical cavity dumping technique. Two home-made 150 W fiber pump modules are used from both sides to pump Nd:YLF crystal. Coupling systems are the key elements in end-pumped solid-state lasers, the aberrations of which greatly affect the efficiency of the lasers. In order to get high efficient and good quality laser output, the optical software ZEMAX is used to design a four-piece coupling system. When the pumped energy is 32 mJ at the repetition rate of 1 Hz, the output energy is 6.5 mJ with 2.5 ns pulse width. When the pumped energy is 13.1 W at the repetition rate of 200 Hz, the output energy is 2.2 W with small M2 factor where Mx2 is 1.04, and My2 is 1.05, and the light-light conversion efficiency is up to 16.8%.

Microjoule level femtosecond optical pulses with double-cladding fiber-based nonlinear chirped-pulse amplification

An all-fiber high-energy nonlinear chirped-pulse amplification system with two-stage double-cladding fiber amplifiers is presented in this paper. It generates pulses with the energy up to 10 μJ at 20 kHz repetition rate and total accumulated nonlinear phase shift as large as 12.8π. The pulses can be compressed to 137 fs with the compensation of mismatched third-order dispersion by self-phase modulation in the fiber amplifiers.