Power scaling of resonantly cladding-pumped, Yb-free, Er-doped LMA fiber lasers

The results of the recent major efforts in power scaling of resonantly cladding-pumped Yb-free Er fiber lasers are presented. Commercially available Yb-free Er-doped large mode area Er60-20/125 (LMA) DC fibers were tested in two regimes: (i), as a booster amplifier in a single-frequency (SF) MOPA configuration and, (ii), in a Bragg grating (FBG) based narrowband fiber laser configuration. We obtained ～28.5 W of output power at 1590 nm, the highest power reported so far out of Yb-free Er-doped LMA fiber with resonant cladding pumping. The achieved optical-to-optical conversion slope efficiency of ～56.6% is also believed to be the highest efficiency ever reported from Yb-free Er-doped fiber laser with resonant cladding pumping.     

Third-order nonlinear optical response in transparent solids using ultrashort laser pulses

The third-order optical nonlinearity, χ ⁽³⁾, is measured in transparent glasses (BK7 and fused silica) and crystals (BaF₂ and quartz) using 36-fs, 800-nm laser pulses and the optical Kerr gate (OKE) technique; values are found to lie in the range 1.3–1.7×10^-14 esu, in accordance with theoretical estimates. We probe the purely electronic response to the incident ultrashort laser pulse in fused silica and BK7 glass. In BaF₂ and quartz, apart from the electronic response we also observe contribution from the nuclear response to the incident ultrashort pulses. We observe oscillatory modulations that persist for ～400 fs. The response of the media (glasses and crystals) to ultrashort pulses is also measured using two-beam self-diffraction; the diffraction efficiency in the first-order grating is measured to be in the range of 0.06–0.13 %. Third harmonic generation due to self-phase matching in the transient grating geometry is measured as a function of temporal delay between the two incident ultrashort pulses, yielding the autocorrelation signal.     

High efficiency silicon nitride grating coupler

In this paper, we have designed, fabricated and characterized silicon nitride grating couplers with high efficiency at 1490 nm. The devices are fabricated using deep UV photolithography with resolution requirement of ～500 nm. The grating coupler fabricated yields a peak coupling efficiency of ?5.1 dB. The 1-dB bandwidth of the grating coupler is 60 nm.     

Investigation of thermo-mechanical behaviour of diffractive optical elements for CO2 lasers

Phase diffractive optical elements are at present proposed for use with infrared high power lasers, for material surface treatment applications. However, the heating of the component exposed to several kilowatts per square centimetre can be a problem for practical implementation. The deformations due to thermal expansion of a gold binary diffraction grating under high power CO₂ laser exposure at 10.6 μm are estimated by a Finite Element Method. They are compared to the geometrical tolerances obtained by a rigorous electromagnetic Fourier modal method which is used to calculate the optical performances. Several exposure parameters (duration, average laser power) and grating parameters (period, line space ratio) are investigated. The laser exposure should be limited to a few milliseconds with a power density on the grating of 10⁴ W/cm², so that the amplitude of the deformations does not exceed the 75 nm tolerance on the grating depth. One is thus assured that the diffraction efficiency in the first order remains superior to 38.5%.     

非相干光纤激光组束中光栅衍射效率的研究

提高非相干光纤激光组束的组束功率,需要增大光栅对组束激光的衍射效率。通过理论分析和数值仿真,结果表明对中心激光入射角偏移及组束光角偏移的精确控制是提高光栅衍射效率的光健,高的衍射效率对应较大的光栅周期和较小的光栅厚度。对于中心激光入射波长1060nm,应选择光栅频率200~400mm-1,对应的光栅厚度2~4mm。     

Oblique incidence effect on steering efficiency of liquid crystal polarization gratings used for optical phased array beam steering amplification

A liquid crystal polarization grating (LCPG) is proposed that amplifies the steering angle of a liquid crystal optical phased array for non-mechanical beam steering, taking advantage of its high steering efficiency under normal incidence. However, oblique incidence may play an important role in the overall steering efficiency. The effect of oblique incidence on steering efficiency of a LCPG was analyzed by numerically solving the extended Jones matrix and considering propagation crosstalk. The results indicate that the outgoing laser beam is amplitude-modulated under the effect of oblique incidence and behaves as a sinusoidal-modulated amplitude grating, which diffracts certain energies to non-blazed orders. Over-oblique incidence may even eliminate the steering effect of the incident beam. The modulation depth of the induced amplitude grating was found to be proportional to the product of sinusoidal value of oblique incidence angle and the LC layer thickness, and inversely proportional to the periodic pitch length of the LCPG. Both in-plane incidence and out-of-plane incidence behave similarly to influence the steering efficiency. Finally, the overall steering efficiency for cascaded LCPGs was analyzed and a difference of up to 11 % steering efficiency can be induced between different LCPG configurations, even without considering the over-oblique incidence effect. Both the modulation depth and final steering efficiency can be optimized by varying the LC birefringence and layer thickness.     

Si homojunction structured near-infrared laser based on a phonon-assisted process

We fabricated several near-infrared Si laser devices (wavelength ～1300 nm) showing continuous-wave oscillation at room temperature by using a phonon-assisted process induced by dressed photons. Their optical resonators were formed of ridge waveguides with a width of 10 μm and a thickness of 2 μm, with two cleaved facets, and the resonator lengths were 250–1000 μm. The oscillation threshold currents of these Si lasers were 50–60 mA. From near-field and far-field images of the optical radiation pattern, we observed the high directivity which is characteristic of a laser beam. Typical values of the threshold current density for laser oscillation, the ratio of powers in the TE polarization and TM polarization during oscillation, the optical output power at a current of 60 mA, and the external differential quantum efficiency were 1.1–2.0 kA/cm², 8:1, 50 μW, and 1 %, respectively.     

Dye-concentration-dependent lasing behaviors and spectral characteristics of cholesteric liquid crystals

The laser behavior and spectral changes occurring in cholesteric liquid crystals with varying dye-doped concentrations were investigated when pumped at 532 nm. It was found that the long-wavelength band edge and the laser line exhibit a blue shift over 21 nm with increasing dye concentration. The circularly polarized fluorescence spectra were examined, and the location of the sense reversion of circular polarization was determined to coincide well with the discrete lasing lines. The blue shift can be ascribed to the decrease in average refractive index and pitch of the dye-doped cholesteric liquid crystals. The dependence of the slope efficiency and threshold energy on the dye concentration can be attributed to the shift in photonic stopband and the change in penetration depth of excitation. The temperature and incident angle of pumping beam also have a significant impact on the lasing properties. The optimal dye concentration is found to be 0.5 wt% at 30.5 °C with an incident angle of 10°. The laser emission located at 601.4 nm with slope efficiency of 4 % was achieved above the threshold energy of 14.3 μJ.     

高功率全光纤激光器特性

介绍了采用国产光纤光栅研制的全光纤激光器,单端泵浦获得468 W的连续激光输出。从理论上分析了光纤光栅的反射率与波长的关系,计算了输出谱宽值,与实验测得的数据相符。全光纤激光器的光-光转换效率达到70%,且随着功率的增加,光纤光栅的中心波长有向长波方向漂移的趋势。在最高输出功率下180 s之内输出功率波动在0.04%以内。     

Theory of a rotating-mode coaxial infrared grating laser

Analysis is presented for a novel interaction that makes possible a compact infrared grating laser configuration. The laser utilizes an annular axis encircling beam that is confined by an axial magnetic field and passes along a grating blazed axially on the center conductor of a coaxial resonator. A smooth cylindrical outer conductor completes the Fabry-Perot resonator to provide the necessary feedback for sustained oscillation. Linear analysis leads to an eigenvalue equation for cavity resonance frequencies, start-oscillation currents, and a gain-bandwidth relationship. The latter permits estimation of an upper bound on the ideal-beam power extraction efficiency. The nonlinear studies make use of analytical and numerical methods to gain a detailed understanding of electron motion in the presence of both axially focusing and radio frequency fields, and to determine the nonlinear efficiency. There is significant degradation of the interaction efficiency when the beam has finite annular thickness. Efficiency enhancement by means of a down-tapered axial field is demonstrated. A point design for lasing at ~160 μm in the far-infrared, utilizing a 100-kV beam with a pitch angle of 55° is presented. The start-oscillation current is a sensitive function of the resonator quality factor (or finesse); for a well-designed resonator, it is on the order of tens of Amperes     

Eye-safe Raman laser based on BaTeMo2O9 crystal

An eye-safe Raman laser is realized with BaTeMo₂O₉ (BTM) nonlinear crystal for the first time. By using a diode-end-pumped acousto-optically Q-switched Nd:YVO₄ laser as the pumping source, the BTM crystal converts the fundamental laser at 1,342 nm to first-Stokes laser at 1,531 nm successfully. With an incident power of 10.8 W and a pulse repetition rate of 25 kHz, the average output power at 1,531 nm is obtained to be 0.83 W, corresponding to a diode-to-Stokes conversion efficiency of 7.7 %. The pulse width is 11 ns, and the peak power is 3.0 kW.     

A ring ZnGeP2 optical parametric oscillator pumped by a Ho:LuAG laser

We have demonstrated that we believe to be the first ring ZnGeP₂ (ZGP) optical parametric oscillator (OPO) pumped by a pulsed Ho:LuAG laser. The maximum output power of the ring ZGP OPO laser was 5.51 W at 13.1 W incident Ho pump power, corresponding to a slope efficiency of 59.0 %. The ZGP OPO laser produced 14 ns mid-infrared pulses in the 3.72–4.01 and 4.37–4.75 μm spectral regions simultaneously. In addition, the continuous wave Ho:LuAG laser generated 26.5 W of linearly output at 2,094.4 nm at the absorbed Tm pump power of 49.9 W.     

Development of narrow linewidth, micro-integrated extended cavity diode lasers for quantum optics experiments in space

We present a micro-integrated extended cavity diode laser module for experiments on rubidium Bose–Einstein condensates and atom interferometry at 780.24 nm onboard a sounding rocket. The micro-integration concept is optimized for space application. The laser chip, micro-lenses, a volume holographic Bragg grating, micro-temperature sensors and a micro-thermoelectric cooler are integrated on an aluminium nitride ceramic micro-optical bench with a foot print of only 50 × 10 mm². Moveable parts are omitted to allow for a very compact and robust design. The laser module provides an output power of more than 120 mW at a short term (170 μs) linewidth of 54 kHz, both full-width-at-half-maximum. The laser can be coarsely tuned by 44 GHz with a continuous tuning range of 31 GHz. The micro-integration technology presented here can be transferred to other wavelengths.     

Room temperature efficient continuous wave and Q-switched operation of a Ho:YAP laser

We report the continuous wave and acousto-optically Q-switched operation of a Tm:YLF-pumped Ho:YAP laser at room temperature. Continuous wave output power of 6.8 W at 2118 nm was obtained under the incident pump power of 13.4 W, corresponding to a slope efficiency of 65.6% and a conversion efficiency of 50.7%. For the Q-switched mode, a maximum pulse energy of 1.28 mJ and a minimum pulse width of 31 ns at the repetition rate of 5 kHz were achieved, resulting in a peak power of 41.3 kW. In addition, the Ho:YAP laser was employed as a pumping source of ZGP optical parametric oscillator, the total average output power of which was 3.2 W at 4.08 and 4.41 μm with a slope efficiency of 69.5%, corresponding to the diode-to-mid-IR conversion efficiency of 9.0%.     

Design and realization of active infrared imaging system based on power-over-fiber technique

We present an active infrared monitoring system based on the power-over-fiber technique. The system realizes the following main functions: the power supply via optical fibers, the laser lighting, the image acquisition and processing. In the demonstration system, the high-power laser light (at the wavelength of 808 nm) in the base station is transmitted to the remote unit via a 200-m long multi-mode fiber, whose core diameter is 200 µm. The remote unit includes an optical beam splitter, a power manager module behind a photovoltaic power converter (PPC) to ensure a quasi-maximum power-supply, a camera, a microcontroller, and an optical transmitter. As the laser beam enters the remote unit, it is divided into three parts by an optical beam splitter. The first part is converted by the PPC to provide the required electrical power of the remote unit. Besides, to improve the power-supply ability of PPC, a maximum power point tracking technique is applied, and more than 77% of PPC’s maximum output power can be obtained. The other two parts of the laser beam pass through respective beam-shaping lenses and are used directly for the infrared laser lighting. Therefore, the active infrared monitoring is achieved without extra laser lighting sources. The collected image data are transmitted via another single-mode fiber to the base station for further data processing. Experiment result shows an active and unnoticed image monitoring in the dark environment.     

High-efficiency high-power QCW diode-side-pumped zigzag Nd:YAG ceramic slab laser

A high-efficiency high-power diode-side-pumped quasi-continuous wave (QCW) Nd:YAG ceramic slab laser using zigzag optical path was demonstrated. With an integrating sphere technique, the scattering and absorption coefficient of the ceramic slab were measured to be 0.0024 and 0.0016 cm^?1 at 1,064 nm, respectively. Under a pump power of 6.69 kW, an average output power of 2.44 kW at 1,064 nm with a repetition rate of 400 Hz was achieved, corresponding to an optical-to-optical efficiency of 36.5 %. As far as we know, this is the highest conversion efficiency reported for QCW side-pumped single slab Nd:YAG ceramic laser.     

UV beam-assisted efficient formation of surface relief grating on azobenzene polymers

An efficient method is demonstrated to improve the formation of a surface relief grating (SRG) with a high amplitude. The SRG is realized by a two-beam interference technique (λ=532 nm) in an azobenzene side-chain copolymer. An UV laser beam (λ=355 nm), called assisting beam, was used to accelerate the photoisomerization process from cis-form to trans-form, resulting in a rapid cis-form ? trans-form cycle. This UV beam-assisted method allowed to increase the diffraction efficiency of the formed SRG as well as its depth. Stable gratings with modulation depth as high as 410 nm were obtained at room temperature with moderate laser power.     

基准光栅重构傅里叶变换轮廓术

应用傅里叶轮廓变换术进行三维形貌测量中,为了获得待测物体的高度相位信息,通常需要采集两幅图像.因此当光学系统发牛变动时,必须重新采集基准光栅图像,不利于快速测量.提出一种从变形光栅图像中获取基准光栅图像信息的测量方法.首先在变形光栅图像中记录基准光栅信息,然后通过傅里叶分析提取基准光栅频率信息,通过图像分析获得基准光栅相位信息,最后重构出一幅完整的基准光栅图像,实现三维物体形貌测量.实验结果验证了该方法的可行的.     

Diode-pumped high-efficiency passively Q-switched Yb:CaYAlO<Subscript>4</Subscript> laser

We experimentally demonstrated a high-efficiency passively Q-switched Yb:CaYAlO₄ (Yb:CYA) laser based on a semiconductor-saturable absorber mirror (SESAM), and the output characteristics of the laser including the average output power, repetition frequency, pulse duration, pulse energy and pulse peak power were investigated by adopting output mirrors with different transmittances. When the transmittance of the output mirror was 5% and the pump power was set at about 7.76 W, a maximum average output power of as high as 2.480 W was achieved with a slope efficiency and light-to-light efficiency of the Q-switched Yb:CYA laser of up to 37.1% and 31.2%, respectively.     

Highly efficient continuous-wave composite Nd:YAG laser at 1,112 nm under diode pumping directly into the emitting level

We present an efficient, high-brightness laser at 1,112 nm by combining the direct pumping technique with an 885 nm laser diode and the composite crystal. Output power as high as 12.8 W at 1,112 nm is achieved under 22.2 W of absorbed pump power and it yields an optical-to-optical efficiency of 57.7 % and a slope efficiency of 64.0 % with respect to the absorbed pump power. To the best of our knowledge, both of these optical-to-optical and slope efficiencies with respect to the absorbed pump power are the highest values ever reported for 1,112 nm Nd:YAG lasers. Modeling of the temperature rise and stress induced in the laser crystals, with and without the undoped cap, and employing the pump at 808 and 885 nm are performed, respectively. Contributions of the composite crystal geometry and of the pump at 885 nm to lowering the threshold power, enhancing the optical-to-optical and the slope efficiencies with respect to the absorbed pump power are discussed, respectively.