The influence of pump beam spatial modulation on angular divergence and temporal characteristics of subnanosecond gain-switched Ti: Sapphire laser was investigated. It was shown, that the change of Ti: Sapphire refraction index by pump light may result in considerable change of laser characteristics. Spatial modulation with frequencies > 15-20 mm^-1 induce scattering in laser crystal and leads to the increase of angular divergence in lasers with flat resonator mirrors and large Fresnel number. At the same time induced scattering produces coupling of light within laser aperture, which can result in synchronous generation of all active volume and reduced laser pulse duration.

To obtain seed pulses of large energy, high contrast and broad spectral bandwidth for the high power laser system, a new cross-polarized wave (XPW) generator based on the gadolinium gallium garnet (GGG) crystal is designed. For the injected fundamental wave (FW) pulses exceeding tens of millijoule, the output XPW characteristics of conversion efficiency, spectral broadening and contrast improvement are theoretically analyzed. We also investigated the influence of spatial distribution characteristics of the injected chirped FW pulses on the output XPW. With incident FW pulses of 8 order super gaussian spatial profile for instance, conversion efficiency of ~73%, spectral broadening ratio (SBR) of ~2.7 (from 40nm to 108nm) and contrast boost by >7 orders of magnitude can be obtained. The impact of the FW spatial modulation on XPW was sensitive to the modulation depth, which is significant for spatial quality control of the laser beam in high power laser systems.

Optical properties of the fog haze of ammonium sulphate particles including sphere, ellipsoid and cluster consisting of 10 spheres are calculated by using Discrete dipole approximation method (DDA); The change of the efficiency factor with these particles of size parameters and the variation of the ratio of scattering field intensity and the incident field intensity with the scattering angle are obtained and analyzed. The calculation these parameters laid a solid foundation for the following processing of the scattering characteristics of complex particles and the light transmission in the medium of fog haze.

Q-switched fiber lasers emitting at around 3 μm have been widely applied in various scientific and industrial fields, such as molecular spectroscopy, laser surgery, material processing, remote sensing, and mid-infrared (mid-IR) supercontinuum source generation. Au nanocages (Au-NCs) have attracted much attention recently due to their outstanding saturable absorption properties including broadband absorption, ultrafast optical response (a few picoseconds), and large third-order optical nonlinearity coefficient that caused by local surface plasmon resonance (LSPR). We propose and demonstrate a tunable Er³⁺ doped ZBLAN fiber laser using Au-NCs as a Q-switcher for the first time. Nonlinear absorption of the Au-NCs was measured by a home-made Ho³⁺/Pr³⁺ co-doped mode-locked fiber laser at 2850 nm. The measured modulation depth, saturation intensity and non-saturation loss are 10.73%, 0.11 MW/cm² and 3.26%, respectively. The central wavelength of the Q-switched pulses could be tuned across 54.1 nm (from 2753.0 to 2807.1 nm). The Q-switched fiber laser delivers a maximum average power of 253.7 mW with corresponding pulse energy of 4.06 μJ and pulse width of 1.30 μs at repetition rate of 62.5 kHz. Our work shows the Au-NCs are promising saturable absorbers (SAs) for 3 μm mid-infrared (mid-IR) pulse generation.

The design of annular doping region located in the cladding can reduce signal overlap with the doped region in order to reduce saturation and minimize gain compression, which has important applications in EDFAs. Here, we present the design and power scaling characterization of a cladding-pumped amplifier with ytterbium dopant located in an annular region near the ultra low NA core in the cladding, which is found to be a promising way to achieve multi-kilowatt single mode fiber lasers. The ultra low NA ensures that the fiber amplifiers operate in single mode state, which results to that the fiber amplifiers are free of the limitation of the transverse mode instability, and that the mode field of the signal laser extends into the cladding to extract gain amplification. The annular ytterbium-doped region located in the cladding can overcome the contradiction between high doping concentration and ultra-low NA design, which can simultaneously obtain high pump absorption with ultra low NA. The size of annular ytterbium-doped region under different core NA has been studied for various core sizes, which shows that the optimal size of annular ytterbium-doped region is related to the core NA and the core size. Detail analysis of high power amplification of cladding-ring-up-doped ultra low NA single mode fiber amplifier has been presented, which includes various nonlinear effects and thermal effects. It shows that, due to the specific design, the single mode characterization of the fiber is less influenced by the detrimental thermo-optic effect, which means that the cladding-annular-doped ultra-low NA fiber has high mode instability threshold than the ultra-low NA fiber with the core being fully uniformly doped. The cladding-pumped fiber amplifiers based on cladding-annular-doped ultra low NA fiber has the capability to achieve >10kW single mode fiber lasers.

In this paper, based on a linear polarized, white noise signal (WNS) phase modulated all fiber amplifier, the self-pulsing characteristics of polarization maintaining (PM) amplifier at the different phase modulation parameters with the same linewidth are analyzed experimentally. It is demonstrated that the self-pulsing threshold is closely related to the details of the modulation spectrum affected by the frequency and the signal power of the WNS, and it is near impossible to calculate by the linewidth. Besides, by suppressing the self-pulsing effect, we obtain an output of 2009 W with a linewidth of 22 GHz. The polarization extinction ratio (PER) is larger than 15 dB, and the M² is lower than 1.2.

We report on the first demonstration of pulsed regime of Raman laser based on a multimode graded-index fiber directly pumped by a CW multimode laser diode. Proof-of-principle experiments have been performed with a 3.7-km multimode graded-index fiber with 62.5-μm core pumped by 976-nm high-power laser diode and cavity formed by bulk mirror and fiber Bragg grating with intra-cavity acousto-optic modulator providing Q-switching or mode locking. At 27.2-kHz repetition rate corresponding to the laser cavity round-trip frequency (i.e. in mode-locking regime), stable nanosecond pulses with peak power of ~300 W have been observed both at the 1st (1018 nm) and 2nd (1064 nm) Stokes orders. At that, the beam quality of generated pulses is greatly improved as compared to that for the pump diode (M²>20) reaching M²=2 for the 2nd-order Stokes wave.

The peak power of pump pulse is a key factor in the generation of supercontinuum source. Observably, as the peak power of the pump pulse increases, the spectral range of the supercontinuum becomes wider. In order to study the blue shift limit of PCF fiber at different peak powers, in our experiment, the change in peak power is achieved by introducing a different length of chirped fiber after the oscillator to vary the pulse width. The pump source is a self-made laser with pulse duration, operating wavelength and repetition rate of 12 ps, 1064 nm and 68 MHz, respectively, which are injected into the photonic crystal fiber after three stages of amplification. Finally, a supercontinuum with an average power of 358 W in the spectral range of 466 nm to 2400 nm was achieved. Experiments have shown that the introduction of large positive chirp has a significant effect on the supercontinuum of the 10 W class, but for a supercontinuum with a sufficiently high average power (over 100 W level supercontinuum spectrum). after the peak power threshold is exceeded, further blue shift of the spectrum cannot be achieved by increasing the peak power, but the high peak power helps to improve the spectral flatness of the supercontinuum. The four-wave mixing, dispersive wave generation, radiation trapping with the soliton play much important role in the blue-shift of SC spectrum, but the short-wave edge is limited by the group velocity matching condition, which is determined by the dispersion characteristics of the PCF, not only peak power of the pump pulse. In order to further extend the short-wave spectrum, other methods are required, for example, changing the structural characteristic of the PCF, etc.

Broadband low-coherence light has been applied in many fields such as optical imaging, atmospheric optical communication and laser inertial confinement fusion(ICF). It’s meaningful to investigate the temporal shaping technology of broadband low-coherence light. We propose and experimentally demonstrate a scheme of achieving all-optical arbitrary temporal shaping of broadband low-coherence light based on saturable absorption effect. Compared to traditional temporal shaping schemes such as electro-optic modulation(EOM) shaping or acousto-optic modulation(AOM) shaping, the modulation on the shaped pulse profile is much smaller, reduced from 14% (by acousto-optic modulator) to 3%(close to the ASE noise). Furthermore, we explore the evolution of shaped pulse contrast (intensity ratio between front edge and end edge) with the pump energy and contrast changing. The contrast of shaped pulse can be adjusted from 1:1.32 to 1:1.02 when pump contrast is 1:2, and from 1:2.25 to 1:1.90 when pump contrast is 1:3. And the maximum contrast of shaped pulse can reach 1:17 within the measurement accuracy. By changing the energy and contrast of pump, the contrast of the shaped pulse can be controlled flexibly. It’s useful to achieve all-optical arbitrary temporal shaping and pre-compensation during pulse amplification.

In this paper, we present a theoretical model to describe the passively-Q output characteristics of quasi-three level Ho³⁺-doped Fluorotellurite fiber lasers. According to the model, we have studied the factors impacting on the output characteristics of the laser through numerical simulation method. The calculating program of the theoretical model is written using the Matlab language. We obtain the passively-Q output laser with the pulse repetition rate of 13.1 kHz, pulse width of 28.63ns, peak power of 25W, and pulse energy of 0.34 μJ at the pump power of 0.1W. When the pump power increases, the pulse width of the laser decreases, the pulse repetition rate linearly increases, both the pulse energy and the peak power also increases. The pulse width of the laser linearly increases and the pulse energy increases when the length L does. When the output coupler transmission T increases, both the pulse width and the peak power of the laser decrease. The pulse energy of the laser firstly increases and then decreases when the output coupler transmission T does. We qualitatively analyze what causes the change laws of the outputlaser characteristics, such as the pulse width and the pulse energy.

An active range detection system with high power laser source requires a high precision of pulse timing. Although the trigger time stamp can be obtained electrically on the circuit board level, it is crucial to detect the actual transmitting laser pulse signal to monitor the malfunction of the laser. To detect the signal, inserting a reflector in the optical path is suggested so that the photodiode can detect the signal directly. To insert the reflector and pulse detection photodiode in the optical path of the transmitting laser, it is required to confirm the obscuration ratio of the laser module by the shadow of the reflector. The shape of the reflector was set to be rectangular due to the manufacturability although the elliptical reflector shows a better performance in the aspect of the vignette. As a result, it is confirmed that the obscuration is tolerable enough to cover the requirement of 0.8 of the nominal irradiances of the signal of each pixel when the reflector angle is set to be 47 ° and insertion depth is between 0.9 mm and 2 mm.

In this work, deuterium loaded Yb-doped fiber has been proposed to mitigate mode instability in laser oscillator. Experimental results reveal that mode instability threshold power rises from ~459W to ~533W and ~622W at the condition of pristine fiber and fiber loaded with deuterium for 2 weeks and 4 weeks respectively. Mode instability threshold power is raised by more than 16% and 35% after 2 and 4 weeks deuterium loading compared to pristine fiber respectively, and laser slope efficiency is not affected by deuterium loading. The experimental results indicate that deuterium loading is effective in mode instability mitigation and showing potentials in further power scaling of high power fiber lasers.

Beam stabilization is critical in an adaptive optics beam clean-up system to improve the power concentration in the desired area during a period of time. In these systems, the average wavefront slope from a Shack-Hartmann wavefront sensor is widely employed as the feedback of beam stabilization. However, when the adaptive optics system is applied to improve the beam quality of a high-power solid-state slab laser, the “M” shaped aberration at the edge of the slab and the fluctuating intensity distribution of the beam would induce some errors when using the average slope to measure the direction of the beam. In this paper, we present the numerical analysis of beam direction detection errors in solid-state slab laser beam clean-up systems using the average slop. At first, we calculated the direction detection errors with the aberations composed of the first 64 Legendre polynomials using the average slope. Then we measured the wavefront of a solid-state slab laser with a Shack-Hartmann wavefront sensor, and evaluated the influence of the “M” shaped aberration and fluctuating intensity distributions both in the time and frequency domains. It is clear that these factors bring in significant detection errors. Finally, we proposed a method by removing some edge sub-apertures when calculating the average slope.

In high-power laser systems for inertial confinement fusion, hot images may be intense enough to damage expensive optical components. Basing on split-step fast-Fourier-transform algorithm, the intensity and location of hot images of a spectrally dispersed sinusoidal phase modulated laser beam is numerically investigated. It is found that, the time-average intensity of hot image firstly decreases monotonically with the modulation depth and eventually reaches a certain minimum, then increases slowly and reaches a saturated value; in addition, with the modulation frequency and the grating dispersion coefficient increasing, the time-average intensity of hot image decreases generally; while the location of hot image always does not change. Finally, the influence of the distance from the obscuration to the nonlinear optical element on the suppression effect of spectral dispersion of sinusoidal phase modulated light on formation of hot images is discussed. The result is helpful to appropriately choose parameters of the spectrally dispersed sinusoidal phase modulated light to minimize the threat of optical damage of hot image and improve the performance of high-power laser system.

A 589 nm yellow laser is developed for dermatological applications by single-pass frequency doubling of a linearly-polarized narrow linewidth 1178 nm Raman fiber laser in a periodically poled MgO-doped near-stoichiometric LiTaO₃ crystal. Up to 6.4 W continuous-wave 589 nm laser is obtained with a conversion efficiency of 14%. The wavelength flexible cascaded Raman fiber laser combined with the single-pass frequency doubling device has advantages of small volume, low cost and easy operation. This device is very suitable for use in the medical field. By optimization, the expected power can be further improved.

Output beam characteristics of a LD-pumped multimode graded-index fiber Raman laser with different cavity configurations are studied. It has been shown that specially designed 976-nm FBGs inscribed by femtosecond pulses enable selection of an individual transverse mode: fundamental LP₀₁ mode is generated in case of FBG localized in the center of graded-index fiber core, whereas next-order LP₁₁ mode is generated in case of FBG shifted by ~8 um from the center. Corresponding beam shapes and spectra are observed in the laser output. At that, output power at the same pumping is sufficiently higher in case of LP₁₁ FBG. Optimization of output characteristics is also performed resulting in generation of ~50 W power at 976 nm with beam quality parameter M²≈2.

This PDF file contains the front matter associated with SPIE Proceedings Volume 11181, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.