This PDF file contains the front matter associated with SPIE Proceedings Volume 10811, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Laser wakefield accelerators (LWFA) hold great potential to produce high-quality high-energy electron beams (e beams), and wiggling of these LWFA e beams either in the Conventional period magnetic field structure (undulator radiations), strong focusing laser wakefield (betatron radiation), or intense laser fields (Compton scattering) can emit high-energy x-ray photons. By experimentally generating the high-quality LWFA e beams with a good stability and repeatability, we have recently produced tunable quasi-monochromatic ultrahigh brilliance MeV γ-ray via the self-synchronized all-optical Compton scattering scheme and realized a scheme to enhance betatron radiation by manipulating transverse oscillation of electrons in a deflected wakefield with a tilted shock front. The concurrent generation of high-quality e beams and bright x-rays in a compact LWFA may provide practical applications in ultrafast pump-probe study and x-ray radiology fields.

Single or double femtosecond Bessel laser beams are employed to generate multiple annular beams with different central wavelengths. A set of annular beams with bandwidths of several tens of nanometers and different peak wavelengths discretely distributed in the visible and infrared wavelength range are generated. No supercontinuum emission is observed. The propagation directions of these annular beams are wavelength-dependent and different from the propagation direction of the pump femtosecond Bessel beams, so four-wave mixing and secondary parametric process are considered to be the generation mechanism of these annular beams, which is further confirmed by numerical simulations. The energy conversion efficiency of single generated annular beam is in the order of 10-6. It is expected that specific resonant structures of the third-order nonlinear optical susceptibility of the sample used in experiments can enhance the energy conversion efficiency at certain wavelengths. These simultaneously generated colorful annular beams are actually composed of light pulses with nearly the same durations as the pump femtosecond pulses, which can find important applications where multi-wavelength ultrashort light pulses are needed, such as optical telecommunications, sensing, and pump-probe measurements. It is considered that stimulated four-wave mixing and corresponding cascaded parametric process are the main generation mechanism.

Low noise laser sources with narrow linewidth and low intensity noise are key tools in a broad range of applications such as optical sense, microwave photonics, coherent optical communications and so on. The report gives an introduction about our research progress of the narrow-linewidth single frequency DFB fiber lasers. An internally developed DFB fiber laser is developed. The noise characteristic of laser output is studied in detail. The frequency noise in low Fourier frequency is reduced via using the low noise pump and intracavity optical negative feedback. Furthermore, an injecting lock scheme is used to reduce the intensity noise. A fiber laser output with low frequency noise and low intensity noise has been demonstrated.

Detailed experimental investigations are presented on suppressing mode instability and stimulated Raman scattering by varying the pumping power distribution in a large mode area all-fiber amplifier with fiber core diameter of 20μm using bi-directional configuration. Results reveal that compared to employing co-directional pumping scheme, the fiber amplifier employing counter-directional pumping scheme can enhance the MI threshold power from 1250W to 1447W. Optimizing the pumping power distribution can further strength the mitigation of mode instability, such as the ratio of 57% and 66%, the threshold power is 2176W (highest output power) and 2150W respectively. For the ratio of 49%, which means almost identical scale of co-pumped light and counter-pumped light, the threshold power is 1934W. On the other hand, raising the proportion of backward pumping power can also mitigate the stimulated Raman scattering. 66% of backward pumping power can acquire 2150W output power and 20dB signal-to-noise ratio of the Raman peak, which indicates that optimizing the pumping power ratio can suppress mode instability and stimulated Raman scattering simultaneously.

In this paper, we demonstrate 47 GHz linear polarized fiber amplifier injected by a simple fiber oscillator laser seed source with narrow linewidth and near diffraction-limited beam quality. Output powers of 419 W, 778 W, and 1107 W are achieved with 3dB linewidth of 26.5 GHz, 41 GHz, and 47 GHz, respectively. The M2 is 1.25 in the x-direction and 1.23 in the y-direction at the maximum laser power, respectively. The measured PER is large than 96% during the power scaling process. However, The SRS is observed when the laser power is 1107, the SNR is about 47dB, which means that the SRS effect has become a serious limitation for further power scaling of such PM-amplifier seeded by the fiber oscillator laser seed source.

We have demonstrated here, to the best of our knowledge, for the first time the suppression of stimulated Raman scattering (SRS) in a monolithic fiber laser oscillator using chirped and tilted fiber Bragg gratings (CTFBGs). We designed and inscribed CTFBGs in large-mode-area (LMA) fibers according to the operating wavelength of the fiber laser oscillator. A maximum suppression ratio nearly 19 dB or 23 dB is achieved CTFBG insert before the OC grating or after the HR grating. By reducing the insertion loss and improving the transmission spectrum of the CTFBG, a promotion in laser efficiency could be achieved. This work provides a novel idea for SRS suppression in a high-power all-fiber oscillator system, which is very useful for the output power increasing of fiber oscillators.

Based on a method for coupling of guided light from tapered single-mode fibers to hollow-core fibers, we report midinfrared emission around 3.1-3.2 μm from acetylene-filled hollow-core fiber gas laser system. The laser operating at both CW and pulsed regime is investigated. The maximum pulse average power of ~0.125 W (~250 nJ pulse energy) at both P(11) and P(13) pump transitions is obtained with a 2 m length of fiber. And the maximum CW power is 185 mW by P(15) pump transition at 1.1 mbar gas pressure with a power conversion efficiency of ~14% when the fiber is 10 m.

High power density as the critical performance of laser diode pumps significantly affects both efficiency and power of a solid state laser. In this report, we designed a new packaging structure that two laser bars bonded on the top and bottom of a MCC, respectively, to achieve higher power density at the same bias current or the same power density at a reduced bias current with respective to one laser bar on a MCC. We achieve 1KW output power at a lower bias current 450A with 2.3W/A slope efficiency from a dual-bar MCC at a duty cycle of 8% (200 μs/400 Hz). Other performances like spectral width broadening, wavelength shift and reliability about 1KW quasi-CW high power laser diodes and 5KW for one vertical stack with five dual-bar micro-channel coolers (MCCs) also are discussed. The reliability of dual-bar MCC packaging structure is also studied by life-time testing, and the output peak power of all devices degraded less than 5% after working for 1353 hours.

A low-cost, compact intracavity deformable mirror (DM) consisting of a mirror unit, a heater unit, a cooler unit and a base unit is proposed to compensate the thermal distortion of a linear resonator passively Q-switched (PQS) laser in this paper. The thermal distortion of the PQS laser is measured using the active deflectometry method. Simulation results indicate that the surface shape of the DM (DMSS) matches well with the measured thermal distortion at the given pumping current. Experiment results verify that the PQS laser with the designed DM could achieve high output power and good beam quality at high pumping currents, as the DM prominently compensates the thermal distortion in the laser.

The semiconductor laser diode has the advantage of low cost, high efficiency, and compactness, but the beam divergence is too large to directly use. The phase-locked laser array is an efficient way to control the lateral lasing mode, which can help to achieve narrow farfield.. Though the lasing mode of phase-locked laser array can be an in-phase mode via Ywaveguide, integrated with phase shifter and external cavity, it still has a large side lobe in the farfiled. We demonstrated an on-chip phase and amplitude manipulation method to suppress the side-lobe in the farfield. The intensity of the sidelobes decrease from 0.307 to 0.109 and the integral energy of the main lobe increase from 52.5% to 60.5%

Planar laser induced fluorescence (PLIF) diagnosis technology has been widely applied in flow field study and combustion diagnosis. Due to the complexity of the experimental environment of practical PLIF applications, The PLIF diagnostic system needs a good environment adaptation. In this paper, we reported a high energy Nd:YAG MOPA laser with repetition rate of 500Hz, which was applied in PLIF diagnostic system. A diode-laser side-pumped Nd:YAG module, which was pumped from five directions and optimized for better gain distribution, was employed to build a EO Q-switched Nd:YAG oscillator. A stable structure design of oscillator resonant cavity was used to improve the environmental adaptability of Nd:YAG laser. In our environmental adaptation experimentation, the laser oscillator has an energy fluctuation of <5% in the temperature range from 5°C to 45°C. In order to scale the pulse energy to meet the PLIF system requirements, we employed three 2500W diode-laser five-direction-side-pumped Nd:YAG modules as laser amplifiers to build MOPA system. Finally, the laser pulse energy of ~18mJ was amplified to 68mJ at 1064nm. Using KTP crystal as a frequency doubling crystal, we obtained a pulse energy of 35.6mJ at 532nm. The developed Nd:YAG laser has been used in our 500Hz-PLIF diagnostic system successfully.

The recent development of high harmonic spectroscopy (HHS) has opened new pathways to probe the structure and dynamics of molecules. Here we report our recent experiments about the measurement of molecular structure and dynamics with HHS. It is shown that the molecular orbitals can be reconstructed with the high harmonic spectra. Moreover, we will demonstrate how to probe the molecule rotation with femtosecond or attosecond resolution.

The relationship between the longitudinal-mode structure (LMS) of the laser and the relative intensity noise (RIN) properties was investigated in this paper after an all-solid-state continuous-wave (CW) single-frequency 1064 nm laser with output power of 50.3 W was achieved. The LMS of the laser was manipulated by controlling the temperature of the deliberately introduced nonlinear lithium triborate (LBO) crystal in the resonator. When the laser worked with single-longitudinal-mode (SLM) operation, the stable RIN spectrum was observed and measured. Moreover, with the decrease of the nonlinear conversion efficiency (NCE), the frequency and amplitude of the resonant-relaxation oscillation (RRO) peak regularly shifted to the higher frequencies and increased, respectively. However, with further decrease of the NCE, the laser began to work with the multi-longitudinalmode (MLM) or mode-hopping operation and the unstable RIN spectra of the laser were both observed not only at low frequencies but also at high frequencies. Once the NCE was moved away, the MLM or mode-hopping can only enhance the fluctuation of the laser RIN spectrum below the frequency of RRO. The experimental results definitely revealed that the key to achieve a stable high power laser with low intensity noise was to realize single-frequency operation of the laser with free MLM and mode-hopping.

We report here a novel two-stage approach Raman cascade laser source operating at 2.8 μm. In the first stage, a pulsed pump laser at 1064.6 nm is coupled into a methane-filled hollow-core fiber and efficiently transfer to 1st Stokes wave at 1543.9 nm. The quantum efficiency is ~ 87%, which is achieved in 2 bar methane pressure and 2 m fiber length. In the second stage, the 1st Stokes wave is coupled into another methane-filled hollow-core fiber as the pump source, 2nd Stokes wave at 2.8 μm is obtained with a high quantum efficiency of ~75%, which is achieved in 11 bar methane and 2.2 m fiber length. The record total quantum efficiency (from the 1064 nm pump laser to Stokes wave at 2.8 μm) of ~ 65% is achieved, which is 1.6 times the previous reported value. This kind of gas filled hollow-core cascade Raman laser source provides a potential method to obtain high efficiency mid-infrared laser sources with low threshold in various applications.

We reported an active multipass stretcher which can deliver chirped pulse with high energy, adjustable duration and high beam quality. The stretcher system is based on a Martinez stretcher and a regenerative amplifier. The stretched pulse duration can be adjusted without changing the layout. With 1 ns stretched pulse output per roundtrip, chirped pulse with more than 10 ns can be obtained after several round-trips. A laser amplifier is introduced into the cavity to compensate the energy loss caused by the diffraction efficiency of gratings, and several millijoules pulse energy can be obtained after stretching and amplification. Benefit with the advantage of the regenerative structure, the stretched pulses have an excellent beam quality with M2 of 1.1. This novel stretcher is significant for high peak power laser systems.

A SESAM mode-locked Yb:YAG thin disk laser was designed and analyzed based on the conjugated dual parabolic mirrors multi-pass pumping scheme. In the experiment, 3.6W mode-locked pulses were obtained at the repetition rate of 38.3MHz with a 5% transmission output coupler at 1030 nm. And the phenomenon of double pulses and chaotic-QML were observed in the high pumping power. Moreover, it was found that the pumping power range of the CW modelocking was very narrow in our experiment.

We have demonstrated here, to the best of our knowledge, for the first time the suppression of stimulated Raman scattering (SRS) in a monolithic fiber laser oscillator using chirped and tilted fiber Bragg gratings (CTFBGs). We designed and inscribed CTFBGs in large-mode-area (LMA) fibers according to the operating wavelength of the fiber laser oscillator. A maximum suppression ratio nearly 19 dB or 23 dB is achieved CTFBG insert before the OC grating or after the HR grating. By reducing the insertion loss and improving the transmission spectrum of the CTFBG, a promotion in laser efficiency could be achieved. This work provides a novel idea for SRS suppression in a high-power all-fiber oscillator system, which is very useful for the output power increasing of fiber oscillators.

Nanoseconds duration square pulses generated by passively mode-locked fiber lasers have lots of applications in scientific research, mechanical processing and optical communication. We present square pulses generation in an Ybdoped figure-of-eight fiber laser by using nonlinear optical loop mirror (NOLM) mode locking technique. The generation of square pulses in the passively mode-locked fiber laser is based on dissipative soliton resonance (DSR) theory. The length of the Yb-doped fiber (YDF) in the cavity is 1 m. The ytterbium ion absorption of the YDF is 250 dB/ m at 975 nm and it is pumped by a 980 nm laser diode. A ~500 m single mode fiber (SMF 1060-XP) with a dispersion of -3.3 ps/ (nm· km) is used to increase the cavity length and nonlinear effects. The total length of the cavity is about 502 m. The stable output square pulses start to appear when the pump power reaches 147.46 mW, which is the threshold of modelocking. The fundamental repetition rate of the pulse train is 399.88 kHz, corresponding to the cavity length of 502 m. The output pulse duration broadens from 56.4 ns to 218.4 ns with the pump power increasing from the mode-locked threshold to 481.59 mW, while the peak power maintains a constant value. The maximum output energy of a single pulse is 8.48 nJ at the pump power of 481.59 mW. Both of the pulse duration and the output energy increase linearly with the pump power because of the DSR. Future work is needed to increase the stability and width of the square pulse.

In a longitudinally excited CO₂ laser that had a 45 cm-long discharge tube and a fast discharge circuit, we investigated control of the laser pulse waveform by adjusting the gas medium. In a longitudinal discharge at a low pressure of 10 kPa or less, the laser pulse waveform dramatically changed with the gas pressure and the mixing ratio due to the changing discharge formation time, which varied from 57 ns to 262 μs. A low-pressure (<2 kPa) CO₂-rich gas produced a tail-free short laser pulse with a width of about 100 ns. A high-pressure (<2 kPa) CO₂-rich gas and a low-pressure (<6 kPa) N₂- rich gas produced a short laser pulse containing a spike pulse with a width of about 100 ns and a pulse tail with a length of 10−200 μs. A high-pressure (<6 kPa) N₂-rich gas produced a long laser pulse with a pulse width of 10−20 μs. Therefore, the laser pulse waveform of the longitudinally excited CO₂ laser could be controlled.

Coherent beam combining of multiple laser beams is an attractive approach for scaling up the output power from fiber laser sources while maintaining their beam quality. A key challenge in such a beam combining approach is the need for active phase synchronization between the different amplifier arms. Stochastic parallel gradient descent (SPGD) algorithm is a popular method for synchronizing the phase across a large number of sources as it has a relatively simple hardware requirement and is scalable. In this work, we demonstrate a fiber-optic Mach-Zehnder Interferometer (MZI) with stabilized output using the SPGD algorithm implemented in a commercial controller board. We investigate the implementation of the SPGD algorithm and study key performance factors such as convergence time and output signal noise.

Based on the heat conduction theory, a theoretical model of HgCdTe detector irradiated by 10.6μm CW laser is constructed. The thermal effect of the detector is analyzed by finite element method when the laser spot velocity is 0m/s, 4mm/s and 10mm/s respectively with the peak power density of 50KW/cm² . The results show that, the temperature of irradiation point rises rapidly when v=0 mm/s, but the rising speed will be slower and slower and finally reaches the equilibrium temperature under the combined effect of laser irradiation and heat conduction. In the case of relative motion, the position of the peak temperature gradually shifts along the velocity direction. The peak temperature gradually decreases with the increase of the moving speed, and the width of the temperature peak gradually widens with the increasing speed. Relative motion should be considered when studying laser radiation and laser protection.

The work presents for the first time a comparative study of mode-locked figure-8 laser, in which two independently pumped active media are located either in the same or in different cavity loops. It is shown that the NALM2 configuration (both active media in the same cavity loop) delivers both higher average and peak radiation power. Flexibility of NALM/NALM2 technologies is further demonstrated for implementation of algorithmic electronically driven control over radiation mode-locking regimes. Also discussed are the results of experimental testing of electronic methods relying on NALM/NALM2 technologies for setting desired generation regimes.

Thulium-doped fiber laser is one of the most promising high power mid-infrared sources which attracts lots of attention recently. However, there is no comprehensive theoretical model which can be used for precise simulation of the performance of the pulsed Thulium-doped fiber laser. A combined theoretical model is proposed in this work by integrating the laser rate equation and Ginzburg-Landau equation into the iteration process. Good agreement between the experiments and simulations is achieved in a Thulium-doped fiber amplifier employing counter-pumping scheme.

An all-reflective transient-grating based self-referenced spectral interferometry (TG-SRSI) device is proposed. Except for a thin transparent Kerr medium used for self-referenced pulse generation, no transmitted material used in the device, which enables few-cycle pulses characterization with center wavelength from ultraviolet to near infrared. An 800 nm/8.1 fs and an 1800 nm/14.3fs pulse are characterized successfully using this device which proves its ability

Intense broadband optical-vortex pulses are expected to introduce new phenomena in nonlinear optical physics. We experimentally demonstrate the acquirement of about 500μJ, 220nm bandwidth (650nm – 870nm), and 16fs vortex ultrashort pulses by using a hollow fiber compressor. The Gaussian beam from a Ti:sapphire laser system at 800nm/50fs is transformed into vortex beam by using a spiral phase plate (SPP) firstly. Then, the narrowband optical vortex beam is spectrally broadened based on SPM in a 50-cm long hollow core fiber (HCF) filled with Ar at 0.5 bar, which is dispersion compensated using chirped mirrors. The total pulse energy transform efficiency is up to 56%. It is also found that orbital angular momentum (OAM) can be transferred to the newly generated spectral components by SPM effect..

This paper designs an automatic laser arc hybrid welding system. The system has the function of automatic control, which is convenient for setting and adjusting parameters, and the welding attitude control is stable and accurate. To a certain extent, it solves the problems of high requirements and poor adaptability of laser welding system in practical application, and improves the engineering adaptability of laser-arc hybrid welding.