Yukun Wang

Diode-end-pumped passively mode-locked ceramic Nd:YAG Laser with a semiconductor saturable mirror

We report on a diode-pumped CW passively mode locked ceramic Nd:YAG laser with SESAM (semiconductor saturable absorber mirror), wavelength 1064nm. At a pump power of 7.6w, the pulse width was estimated to be ~8.3ps with repetition rate ~130MHz and the average output power was 1.59w. To our knowledge, this was the first demonstration that ceramic Nd:YAG was used for diode pumped CW passively mode locking.

Diode-pumped continuous wave and passively Q-switched Tm, Mg: LiTaO3 lasers

We have demonstrated the continuous wave and passively Q-switched Tm, Mg: LiTaO3 lasers for the first time. In continuous wave (CW) regime, a maximum CW output power of 1.03 W at 1952 nm was obtained, giving a slope efficiency of 9.5% and a beam quality M2 = 2.2. In passive Q-switching regime, a single walled carbon nanotube (SWCNT) was employed as saturable absorber (SA). The Tm,Mg:LiTaO3 laser has yielded a pulse of 560 ns under repetition rate of 34.2 kHz at 1926 nm, corresponding to a single pulse energy of 10.1 μJ. The results indicate a promising potential of nonlinear crystals in the applications for laser host materials.

Direct laser writing of pyramidal plasmonic structures with apertures and asymmetric gratings towards efficient subwavelength light focusing

Efficient confining of photons into subwavelength scale is of great importance in both fundamental researches and engineering applications, of which one major challenge lies in the lack of effective and reliable on-chip nanofabrication techniques. Here we demonstrate the efficient subwavelength light focusing with carefully engineered pyramidal structures fabricated by direct laser writing and surface metallization. The important effects of the geometry and symmetry are investigated. Apertures with various sizes are flexibly introduced at the apex of the pyramids, the focusing spot size and center-to-sidelobe ratio of which could be improved a factor of ~4 and ~3, respectively, compared with the conical counterparts of identical size. Moreover, two pairs of asymmetric through-nanogratings are conceptually introduced onto the top end of the pyramids, showing significantly improved focusing characteristics. The studies provide a novel methodology for the design and realization of 3D plasmonic focusing with low-noise background and high energy transfer.

Time delay compensation method for tip–tilt control in adaptive optics system

The time delay engendered by wavefront sampling and data processing inevitability exists in almost all the wavefront sensor (WFS) based adaptive optics (AO) systems. Also, when WFS is used for tip-tilt aberration detection, the time delay significantly reduces the tip-tilt correction performance of the AO system. In this paper, we focus on researching time delay in a tip-tilt (TT) control system and introduce a predicted signal compensation method (PSCM) to compensate the time delay by modifying the WFS detected signals. Based on a precise model of a TT dynamic control system, the detection delay of TT corrections included in a WFS detected signal can be compensated. Experiments are conducted in the lab: the pure integrator (I), proportional and integral (PI) wavefront TT controllers, and these controllers with PSCM are compared to test the efficiency of the PSCM for TT corrections. For the PI controller, the rejection bandwidth increases from 52 to 62xa0Hz by using PCSM; meanwhile, the open-loop phase margin increases from 45 to 60xa0deg. In addition, astronomical observation results are also given based on the PI wavefront TT controller. The PSCM improves the Strehl ratio by a factor of 1.3. The new method is proven to improve the AO system closed-loop performance not only for increasing the closed-loop rejection bandwidth but also in favor of the error attenuation at low frequency. Furthermore, the method does not introduce more noise to the system.

Wavefront detection method of a single-sensor based adaptive optics system

In adaptive optics system (AOS) for optical telescopes, the reported wavefront sensing strategy consists of two parts: a specific sensor for tip-tilt (TT) detection and another wavefront sensor for other distortions detection. Thus, a part of incident light has to be used for TT detection, which decreases the light energy used by wavefront sensor and eventually reduces the precision of wavefront correction. In this paper, a single Shack-Hartmann wavefront sensor based wavefront measurement method is presented for both large amplitude TT and other distortions measurement. Experiments were performed for testing the presented wavefront method and validating the wavefront detection and correction ability of the single-sensor based AOS. With adaptive correction, the root-mean-square of residual TT was less than 0.2 λ, and a clear image was obtained in the lab. Equipped on a 1.23-meter optical telescope, the binary stars with angle distance of 0.6″ were clearly resolved using the AOS. This wavefront measurement method removes the separate TT sensor, which not only simplifies the AOS but also saves light energy for subsequent wavefront sensing and imaging, and eventually improves the detection and imaging capability of the AOS.

Performance analysis of an adaptive optics system for free-space optics communication through atmospheric turbulence

The performance of free-space optics communication (FSOC) is greatly degraded by atmospheric turbulence. Adaptive optics (AO) is an effective method for attenuating the influence. In this paper, the influence of the spatial and temporal characteristics of turbulence on the performance of AO in a FSOC system is investigated. Based on the Greenwood frequency (GF) and the ratio of receiver aperture diameter to atmospheric coherent length (D/r0), the relationship between FSOC performance (CE) and AO parameters (corrected Zernike modes number and bandwidth) is derived for the first time. Then, simulations and experiments are conducted to analyze the influence of AO parameters on FSOC performance under different GF and D/r0. The simulation and experimental results show that, for common turbulence conditions, the number of corrected Zernike modes can be fixed at 35 and the bandwidth of the AO system should be larger than the GF. Measurements of the bit error rate (BER) for moderate turbulence conditions (D/r0u2009=u200910, fGu2009=u200960u2009Hz) show that when the bandwidth is two times that of GF, the average BER is decreased by two orders of magnitude compared with fG/f3dBu2009=u20091. These results and conclusions can provide important guidance in the design of an AO system for FSOC.

Hysteresis compensation of the piezoelectric ceramic actuators–based tip/tilt mirror with a neural network method in adaptive optics

Abstract. The intrinsic hysteresis nonlinearity of the piezo-actuators can severely degrade the positioning accuracy of a tip-tilt mirror (TTM) in an adaptive optics system. This paper focuses on compensating this hysteresis nonlinearity by feed-forward linearization with an inverse hysteresis model. This inverse hysteresis model is based on the classical Presiach model, and the neural network (NN) is used to describe the hysteresis loop. In order to apply it in the real-time adaptive correction, an analytical nonlinear function derived from the NN is introduced to compute the inverse hysteresis model output instead of the time-consuming NN simulation process. Experimental results show that the proposed method effectively linearized the TTM behavior with the static hysteresis nonlinearity of TTM reducing from 15.6% to 1.4%. In addition, the tip-tilt tracking experiments using the integrator with and without hysteresis compensation are conducted. The wavefront tip-tilt aberration rejection ability of the TTM control system is significantly improved with the −3u2009u2009dB error rejection bandwidth increasing from 46 to 62 Hz.

A high precision phase reconstruction algorithm for multi-laser guide stars adaptive optics*

Adaptive optics (AO) systems are widespread and considered as an essential part of any large aperture telescope for obtaining a high resolution imaging at present. To enlarge the imaging field of view (FOV), multi-laser guide stars (LGSs) are currently being investigated and used for the large aperture optical telescopes. LGS measurement is necessary and pivotal to obtain the cumulative phase distortion along a target in the multi-LGSs AO system. We propose a high precision phase reconstruction algorithm to estimate the phase for a target with an uncertain turbulence profile based on the interpolation. By comparing with the conventional average method, the proposed method reduces the root mean square (RMS) error from 130 nm to 85 nm with a 30% reduction for narrow FOV. We confirm that such phase reconstruction algorithm is validated for both narrow field AO and wide field AO.

Low repetition rate subnanosecond pulse laser generation from a diode pumped Nd:Lu3Al5O12 laser with electro-optic modulator and transmission semiconductor saturable absorber

By using the dual-loss modulated technology, i.e., adopting the electro-optic (EO) modulator and transmission semiconductor saturable absorber (transmission SSA) simultaneously, a diode-pumped doubly Q-switched and mode-locked (QML) Nd:Lu3Al5O12 (Nd:LuAG) laser at 1.06xa0μm has been realized for the first time. In comparison to the singly passively QML Nd:LuAG laser with transmission SSA, the doubly QML laser can generate more stable pulses with shorter pulse widths and higher peak powers. It can also be observed that the pulse duration of the Q-switched envelope decreases with increasing pump power. When the pump power exceeds 6.52xa0W for the first time, there is only one mode-locked pulse underneath a Q-switched envelope for this doubly QML Nd:LuAG laser. As a result, the subnanosecond pulse laser with 1xa0kHz repetition rate of EO and high stability can be generated. The shortest pulse duration generated is about 718xa0ps and the highest peak power reaches as high as 502xa0kW. The experimental results show that Nd:LuAG is an excellent alternative crystal for diode-pumped QML pulsed laser generation.

Responses of subshrub sagebrush rangeland in North-Western China to changes in livestock numbers and rainfall over 10 years (2004–2013)

Abstract Seriphidium transiliense, or subshrub sagebrush, is one of the most important forage resources for livestock in many arid lands, but such resources have been threatened by overgrazing for a long time. A 2013 vegetation survey of an original sagebrush rangeland (2400u2009ha) in the Tianshan Mountains revealed that nearly half of the area once under sagebrush has been replaced by annual species. This is due to the continual and rapid increase in livestock numbers over the past 30u2009years. Three smaller sites with different grazing scenarios: no grazing (Site N), a decline (Site 1), and an increase (Site 2) in livestock numbers, were selected to monitor recent changes (from 2004 to 2013) in vegetation biomass and the degree of dominance by sagebrush. Supplanting happened at Site 2 but little occurred at Site 1. There were significant differences between Site 2 and Site 1 in standing biomass in Spring and Summer, but no significant differences in Summer between Site 1 and Site N. The supplanting of sagebrush marks the completion of a dynamic process – in which rainfall plays a crucial role – of damage to sagebrush from heavy grazing in drought years and its rehabilitation by light grazing in some good years. Plant biomass loss was greatest when the land was first subjected to heavy grazing. Sagebrush tends to exist as a sole-dominant species rather than a companion species in sagebrush communities. This suggests that sagebrush continued to survive in pockets where grazing pressure had declined.