High-speed two-dimensional laser scanner based on Bragg gratings stored in photothermorefractive glass

A high-speed free-space wavelength-multiplexed optical scanner with high-speed wavelength selection coupled with narrowband volume Bragg gratings stored in photothermorefractive (PTR) glass is reported. The proposed scanner with no moving parts has a modular design with a wide angular scan range, accurate beam pointing, low scanner insertion loss, and two-dimensional beam scan capabilities. We present a complete analysis and design procedure for storing multiple tilted Bragg-grating structures in a single PTR glass volume (for normal incidence) in an optimal fashion. Because the scanner design is modular, many PTR glass volumes (each having multiple tilted Bragg-grating structures) can be stacked together, providing an efficient throughput with operations in both the visible and the infrared (IR) regions. A proof-of-concept experimental study is conducted with four Bragg gratings in independent PTR glass plates, and both visible and IR region scanner operations are demonstrated.     

High-efficiency bragg gratings in photothermorefractive glass

Photosensitive silicate glasses doped with silver, cerium, fluorine, and bromine were fabricated at the Center for Research and Education in Optics and Lasers. Bragg diffractive gratings were recorded in the volume of these glasses with a photothermorefractive process (exposure to UV radiation of a He-Cd laser at 325 nm is followed by thermal development at 520 degrees C). Absolute diffraction efficiency of as much as 93% was observed for 1-mm-thick gratings with spatial frequencies up to 2500 mm(-1). No decreasing of diffraction efficiency was detected at low spatial frequencies. Original glasses were transparent (absorption coefficient less than 1 cm(-1)) from 350 to 4100 nm. Induced losses in exposed and developed glass decreased from 0.3 to 0.03 cm(-1) between 400 and 700 nm, respectively, and did not exceed 0.01-0.02 cm(-1) in the IR region from 700 to 2500 nm. Additional losses caused by parasitic structures recorded in the photosensitive medium were studied.     

Origin of crystallization-induced refractive index changes in photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a multi-component silicate that undergoes localized refractive index decrease after UV-exposure and thermal treatment for partial crystallization. Based on this refractive index change, high efficiency volume Bragg gratings have been developed in PTR glass and have been successfully used for laser beam control. However, despite the fact that this type of glass has been widely studied and used over the last 20 years, the origin of the refractive index change upon crystallization is poorly understood. In this paper, we introduce three possible mechanisms (the precipitation of nano-sized NaF crystals and the associated local chemical changes of the glass matrix, the volumetric changes due to relaxation, and the local residual stresses) for the refractive index decrement in PTR glass and estimate the partial refractive index change due to each mechanism. Refractive index measurements are compared with high temperature XRD experiments and a general approach for the simulation of the refractive index change in PTR glass is proposed. We show that among the studied variables the residual stresses surrounding the crystals are the main responsible for the local refractive index decrement in this glass.     

Temperature and intensity dependence of Yb-fiber laser light absorption in water

Absorption of CW Yb-fiber laser light of 1.07 μm wavelength in water has been measured at different water temperatures and laser intensities. The absorption coefficient was estimated to be 0.135 cm(-1) at 25 °C water temperature, and this was found to decrease with temperature at a rate of 5.7 × 10(-4) cm(-1) °C(-1). The absorption coefficient increased significantly when the laser beam was focused in water, and the increase depended on the distance of the focal point from the water surface. This has been attributed to the absorption and scattering losses of laser radiation in a cavity formed in water by the focused beam at laser intensities in the megawatts per square centimeter and higher range.     

Analysis of fiber bragg gratings by a side-diffraction interference technique

A new nondestructive, noncontact, and sensitive technique for fiber Bragg grating geometry and index-fault location measurements is presented. Two plane-wave probe laser beams are incident upon the grating from the side at angles that satisfy the Bragg-reflection condition. An interference pattern is formed behind the fiber between the first-order diffracted beam (from one probe beam) and the zero-order transmitted beam (from the second probe beam). The axial grating index modulation and the grating period are functions of the fringe visibility and the fringe period, respectively. The method is sensitive and is applicable even in the case of relatively weak gratings. Unchirped and chirped Bragg gratings have been studied with the proposed technique. We demonstrate accuracies of 1 x 10(-4) for measurement of the index modulation and 0.01 nm for measurement of the period. As well as for the analysis of most already-fabricated gratings, this technique is useful for in situ analysis of a long fiber Bragg grating as such a grating is translated along its axis during the fabrication process.     

Injection-seeded pulsed alexandrite laser for differential absorption lidar application

We describe a Q-switched alexandrite laser injection seeded with a cw single-mode titanium-sapphire laser. The reported experimental results show that this system meets the frequency stabilization required for differential absorption lidar measurement of humidity, pressure, and temperature. The emission of the cw titanium-sapphire master oscillator is locked to an atmospheric absorption line by means of a servoloop with derivative spectroscopy. The spectral position is stabilized within ±3.5 × 10(-4) cm(-1) (10 MHz) of the peak of the line over 1 hr. The alexandrite laser emits pulses of 30 mJ in 500 ns, with a spectral linewidth of ≈ 3.3 × 10(-3) cm(-1) (100 MHz). The position of the centroid of the emitted spectrum has a standard deviation of 6 × 10(-4) cm(-1) (18 MHz) and is held within ±1.3 × 10(-3) cm(-1) (40 MHz) of the peak of the absorption line over 1 h.     

A developed optical-feedback cavity ring-down spectrometer and its application

A developed spectrometer based on optical-feedback cavity ring-down spectroscopy (OF-CRDS) has been demonstrated with a distributed feedback laser diode and a V-shaped glass ceramic cavity. The laser is coupled to the V-shaped cavity, which creates an absorption path length greater than 2.8 km, and resonance between the laser frequency and the cavity modes is realized by modulating the cavity length instead of tuning the laser wavelength to obtain a higher resolution. A noise-equivalent absorption coefficient of ~2.6 × 10(-8) cm(-1)Hz(-1/2) (1σ) is determined with spectral resolution of ~0.003 cm(-1) and spectral range of 1.2 cm(-1). As an application example, the absorption spectrum measurement of water vapor in the spectral range of 6590.3~6591.5 cm(-1) is demonstrated with this spectrometer.     

Investigation of stimulated Brillouin scattering media perfluoro-compound and perfluoropolyether with a low absorption coefficient and high power-load ability

The correlations between stimulated Brillouin scattering (SBS) characteristics of perfluoro-compound (PFC) and perfluoropolyether (PFPE) and their chemical structure are analyzed in detail and a series of new PFC and PFPE are reported. In the Nd:YAG laser system, the absorption coefficient, optical breakdown threshold (OBT), SBS threshold, and Brillouin frequency shift of new media such as FC-87, FC-43, HT-55, and DET are measured. Parameters such as gain coefficient, Brillouin linewidth, and phonon lifetime are calculated. The results demonstrate their good SBS properties: the absorption coefficients are below 10(-3)cm(-1) and OBTs are above 100 GW/cm(2). These media also exhibit a series of unique physicochemical properties, i.e., high heat-resistance, high oxidation stability, good chemical inertness, and insulation properties. The discovery of new media not only diversifies SBS medium, but also improves the performance of the SBS system, thereby laying a good foundation for the application of a SBS phase conjugation mirror in a high-power laser system.     

Modeling the reflection of a laser beam by a deformed highly reflective volume bragg grating

A model is presented for analyzing volume Bragg gratings having large diffractive strength that may become distorted upon the passage of high-power laser light. One result of this analysis is that very little distortion in a volume Bragg grating can greatly reduce the reflectivity. This is a critical issue in the design of systems in which these devices are used to combine high-power laser beams.     

Experimental method for high-accuracy reflectivity-spectrum measurements

An experimental method for accurate measurements of the reflectivity spectrum of mirrors is presented. It combines the noise reduction obtained with multiple beam reflections on two identical mirrors; high-beam quality, owing to the use of single-mode optical fibers; and high immunity against intensity variations of the beam. This method is demonstrated for characterizing a 30-period GaAs/Al(0.65)Ga(0.35)As distributed Bragg reflector designed for long-wavelength vertical-cavity surface-emitting lasers. Its peak reflectivity is found to be 99.43 ? 0.04% at 1.562 mum, and an optical absorption coefficient of alpha = 36 ? 6 cm(-1) is derived. The peak internal reflectivity of this distributed Bragg reflector used as the top mirror in a wafer-fused vertical-cavity surface-emitting laser is calculated to be 98.87 ? 0.12%, and the transmission is 0.28%.     

Intracavity absorption spectroscopy with a tunable multimode traveling-wave ring Ti:sapphire laser

A tunable multimode unidirectional traveling-wave Ti:sapphire laser was developed to measure in situ the atmospheric absorption spectra using intracavity absorption spectroscopy. The effective absorption path length was 2100 km. O2 and H2O vapor lines in atmosphere with absorption coefficients of 10(-6)-10(-8) cm(-1) were measured with uncertainties <5%, and the absorption coefficients were in agreement with those estimated from the HITRAN database. By tuning the wavelength, a weak absorption line with an absorption coefficient of 10(-9) cm(-1) was measured with a sensitivity of 2×10(-10) cm(-1). The sensitivity was limited by the residual parasitic variation that appeared in the spectrum.     

Bragg gratings printed upon thin glass films by excimer laser irradiation and selective chemical etching

Photon-induced property changes of sputter-deposited GeO(2)-SiO(2) thin glass films were investigated. Irradiation with ArF laser pulses induced the changes in refractive index of -10% and volume of +30% in the film without ablation. A Bragg grating with a positive sinusoid wave pattern was printed upon the film by irradiation with ArF excimer laser pulses through a phase mask. The irradiated area could be quickly etched by a HF solution. The ratio of etching rate of irradiated area to unirradiated area was higher than 30. A Bragg grating with a surface relief pattern was successfully formed on the film by irradiation with excimer laser pulses followed by chemical etching. Diffraction efficiency of the gratings increased by 25 times with the etching.     

Stress measurements in glass by use of double thermal gratings

We developed a nondestructive and noncontact method for measuring stress at the midplane of tempered glass plates that uses Bragg scattering from a pair of thermal gratings. These gratings are formed by 1064-nm beams from a seeded Nd:YAG laser, and we measure the polarization state of light from a 532-nm beam that scatters from both thermal gratings. The change in polarization of the doubly scattered light with separation between the two gratings allows measurement of the in-plane stress. A model of the Bragg scattering efficiency, experimental investigations of the scattered beams, and stress measurements are reported.     

基于狭缝整形的低损耗飞秒光纤光栅制备

为了解决飞秒激光逐点法制备的光纤布拉格光栅（Fiber Bragg Gratings, FBG）损耗较高的问题,利用逐面法完成飞秒光纤光栅制备的实验研究。运用高斯光束传播的基本理论,通过狭缝整形技术对聚焦的飞秒激光能量分布进行等高宽整形,突破在光纤横截面内诱导产生圆形折射率调制的难点,最终制备得到低损耗的飞秒光纤光栅。开展不同狭缝宽度制备FBG的光谱特性对比实验,结果表明：利用光斑直径为5.0 mm的飞秒激光光束刻写FBG时,采用宽度为1.7 mm的狭缝制备得到的FBG插入损耗降低至0.15 dB,短波损耗降低至0.5 dB,验证了基于狭缝整形的低损耗飞秒光纤光栅制备方法的有效性。针对狭缝法制备的FBG反射率分散问题,提出控制折射率匹配液填充量以及调整飞秒激光能量的方法,并优化光束聚焦流程,成功降低FBG反射率的分散度。本研究对推动飞秒光纤光栅在大容量、高链路损耗等环境中的实际应用具有重要意义。     

Fluorescence of Ta(2)O(5) thin films doped by kilo-electron-volt Er implantation: application to microcavities

Luminescent layers are prepared by the implantation of kilo-electron-volt Er ions into tantalum pentoxide (Ta(2)O(5)) thin films made by ion plating. The implantation fluences range from 3.3 × 10(14) to 2 × 10(15) ions/cm(2), and the energies range from 190 to 380 keV. Refractive index, extinction coefficient, and losses on guided propagation are investigated. We show that these Er-implanted layers present an absorption as low as that of the nonimplanted films. When optically pumped with an Ar(+) laser (λ = 0.488 μm) beam, implanted films show peaked fluorescence spectra centered near 1.53 and 0.532 μm. We show that the fluorescence intensity is correlated with the intensity of the pump beam in the region where Er ions are implanted. Radiation patterns of Er ions located inside a single layer or inside a Ta(2)O(5)/SiO(2) dielectric stack made by ion plating are also investigated. We show that, in any case, spontaneous emission of Er ions can be spatially controlled.     

Binary gratings for CO(2) laser-beam diagnostics

Properties of binary gratings for beam diagnostics of high-power CO(2) lasers are investigated. The layout uses a modal theory that includes absorption of metallic surfaces by an impedance boundary condition. Radiation-resistant gratings were fabricated in copper with photolithography combined with galvanic technology. In this way gratings have been realized in which the first-order diffraction efficiencies are polarization independent. Use of a second grating to reduce elliptical beam distortion is demonstrated. Finally it is shown that Rayleigh anomalies for binary diagnostic gratings are accompanied by high absorption.     

Auger recombination in III-nitride nanowires and its effect on nanowire light-emitting diode characteristics

We have measured the Auger recombination coefficients in defect-free InGaN nanowires (NW) and InGaN/GaN dot-in-nanowire (DNW) samples grown on (001) silicon by plasma-assisted molecular beam epitaxy. The nanowires have a density of ～1 × 10(11) cm(-2) and exhibit photoluminescence emission peak at λ ～ 500 nm. The Auger coefficients as a function of excitation power have been derived from excitation dependent and time-resolved photoluminescence measurements over a wide range of optical excitation power density. The values of C(0), defined as the Auger coefficient at low excitation, are 6.1 × 10(-32) and 4.1 × 10(-33) cm(6)·s(-1) in the NW and DNW samples, respectively, which are in reasonably good agreement with theoretical predictions for InGaN alloy semiconductors. Light-emitting diodes made with the NW and DNW samples exhibit no efficiency droop up to an injection current density of 400 A/cm(2).     

Improving the holographic sensitivity of dichromated gelatin in the blue-green part of the spectrum by sensitization with xanthene dyes

The holographic properties of dichromated gelatin (DCG) sensitized with various xanthene dyes were studied, and results are reported. The sensitivity of dyed DCG in the green part of the spectrum compared with that of pure DCG (215 mJ/cm(2)) was significantly improved by addition of Rhodamine 6G (140 mJ/cm(2)) or Erythrosin B (90 mJ/cm(2)). Diffraction gratings were recorded with a He-Ne laser at 543.5 nm. The maximum diffraction efficiency was found to be 32% at normal incidence of the readout beam; it was 80% at the Bragg angle.     

IR transparency of the glass of ZnCl2-KBr-PbBr2 system

IR transparency and some properties of halide glass of composition 48ZnCl₂-48KBr-4PbBr₂ (mol%) have been investigated as the basic study on the development of IR glass fiber for CO₂ gas laser. The glass transition temperature Tg, crystallization temperature T_c, softning temperature T_s, and linear thermal expansion coefficient α of the glass were found to be 45–46°C, 100°C, 54°C, and 570×10^?7 / °C, respectively. The refractive index of the glass for He-Ne laser emission (632.8 nm) was about 1.63. The amount of impurity which decreases the transmissibility for CO₂ laser beam due to the absorption in the wavelength region 10–11 μm could be reduced by preparing the glass from a batch containing NH₄ Cl under reactive atmosphere of CCl₄ or CBr₄ in a glove box filled with He gas of dew point ?62～ ?45°C. The minimum value of absorption loss of the glass for CO₂ gas laser measured by laser calorimetry was about 20 dB/m.     

High-efficiency dielectric reflection gratings: design, fabrication, and analysis

We report on reflection gratings produced entirely of dielectric materials. This gives the opportunity to enhance the laser damage threshold over that occurring in conventional metal gratings used for chirped-pulse-amplification, high-power lasers. The design of the system combines a dielectric mirror and a well-defined corrugated top layer to obtain optimum results. The rules that have to be considered for the design optimization are described. We optimized the parameters of a dielectric grating with a binary structure and theoretically obtained 100% reflectivity for the -1 order in the Littrow mounting for a 45 degrees angle of incidence. Subsequently we fabricated gratings by structuring a low-refractive-index top layer of a multilayer stack with electron-beam lithography. The multilayer system was fabricated by conventional sputtering techniques onto a flat fused-silica substrate. The parameters of the device were measured and controlled by light scatterometer equipment. We measured 97% diffraction efficiency in the -1 order and damage thresholds of 4.4 and 0.18 J/cm(2) with 5-ns and 1-ps laser pulses, respectively, at a wavelength of 532 nm in working conditions.