Laser Cavity Mirror Research Articles

Efficient 2075-nm laser emission from Ho3+-doped fluorotellurite glass in a compact all-fiber structure.

In this Letter, we report an Ho3+-doped fluorotellurite glass all-fiber laser at 2075 nm. The gain fiber is pumped in-band with a 1976-nm fiber laser and connected by fusion splicing. A high-quality fusion splicing point with a loss of < 0.1 dB was obtained by finely adjusting the splicing power and offset. In addition, by optimizing the writing parameters, a third-order fiber Bragg grating (FBG) with a reflectivity of 98% was achieved at 2075 nm using the femtosecond laser direct-writing method. Using the FBG as the laser cavity mirror and a relatively short 28-cm-long home-made Ho3+-doped fluorotellurite fiber as the laser medium, a laser with a maximum unsaturated output power of 7.33 W was obtained, and the corresponding slope efficiency was as high as 93.4%. The first, to the best of our knowledge, demonstration of the fluorotellurite glass all-fiber ∼2.1-µm laser presented in this work may pave the way for a high-power 2.1-µm fiber laser with a compact structure.

Comment on “A study of the longitudinal laser modes of a semiconductor laser using optical coherence tomography” [Am. J. Phys. 75, 569–571 (2006)

Although the longitudinal mode spacing and longitudinal mode widths of a typical laser diode can, in principle, be determined from an optical coherence tomography signal, the values presented by Poddar et al. [Am. J. Phys. 75, 569–571 (2006)] do not agree with the Fourier transform theory. Also, the mode spacing is inconsistent with the peak separation in the interference signal. Moreover, the laser cavity mirror reflectivity calculated from the laser spectrum is incorrect because it ignores the gain and the loss within the laser cavity. This Comment aims to help readers who may struggle to understand that paper or to reproduce its results.

Fabrication of a laser cavity mirror in a large mode area fiber by an ultrashort pulse laser.

We present herein a method to fabricate a higher-order fiber grating (HOFG) for use as a fiber-cavity mirror in a fiber laser. The HOFG was fabricated by irradiating the Yb-doped large core of a double-clad fiber by a femtosecond pulsed laser. The HOFG served as a laser cavity mirror with a reflectance of 13.2% and yielded a laser line with a spectral full width at half-maximum of 0.56nm.

Hydrophilic bismuth sulfur nanoflower superstructures with an improved photothermal efficiency for ablation of cancer cells

Nanomaterials with intense near-infrared (NIR) absorption exhibit effective photon-to-thermal energy transfer capabilities and can generate heat to ablate cancer cells, thus playing a pivotal role in photothermal cancer therapeutics. Herein, hydrophilic flower-like bismuth sulfur (Bi2S3) superstructures with uniform size and improved NIR absorption were controllably synthesized via a facile solvothermal procedure assisted by polyvinylpyrrolidone (PVP), which could adjust the product morphology. Induced by an 808-nm laser, the as-prepared Bi2S3 nanoflowers exhibited much higher photothermal conversion efficiency (64.3%) than that of Bi2S3 nanobelts (36.5%) prepared in the absence of PVP. This can be attributed not only to the Bi2S3 nanoflower superstructures assembled by 3-dimensional crumpled-paper-like nanosheets serving as many laser-cavity mirrors with improved reflectivity and absorption of NIR light but also to the amorphous structures with a lower band gap. Thus, to achieve the same temperature increase, the concentration or laser power density could be greatly reduced when using Bi2S3 nanoflowers compared to when using Bi2S3 nanobelts, which makes them more favorable for use in therapy due to decreased toxicity. Furthermore, these Bi2S3 nanoflowers effectively achieved photothermal ablation of cancer cells in vitro and in vivo. These results not only supported the Bi2S3 nanoflowers as a promising photothermal agent for cancer therapy but also paved an approach to exploit new agents with improved photothermal efficiency.

Type I and II Bragg gratings made with infrared femtosecond radiation in high and low alumina content aluminosilicate optical fibers

Aluminosilicate fibers have very low Brillouin scattering gain coefficients, making them interesting fibers for nonlinear optical applications. We manufactured Bragg gratings in high (30 mol.%) and low (4 mol.%) alumina content optical fiber using 800 nm femtosecond pulse duration radiation and a phase mask. Grating spectral characteristics and thermal behavior are presented. Index modulations >10−3 were generated for fundamental pitched Bragg gratings, and >10−4 for higher-order gratings. Gratings were annealed at temperatures up to 900°C. Type II gratings written in fibers with lower alumina content showed better thermal stability than gratings written in fibers with higher alumina content. Bragg gratings in these fibers would be well suited as laser cavity mirrors in high-energy laser systems, as well as in telecommunication and sensor systems where Brillouin scattering restricts power scaling.

Self-assembled WO3−x hierarchical nanostructures for photothermal therapy with a 915 nm laser rather than the common 980 nm laser

Photothermal therapy (PTT) is limited by unsuitable photothermal agents and near-infrared (NIR) light. Herein, self-assembled PEGylated WO3-x hierarchical nanostructures, which could serve as excellent laser-cavity mirrors, were successfully prepared via a simple one-pot solvothermal route. The as-prepared WO3-x hierarchical nanostructures displayed strong near-infrared absorption. The absorption of pure water at 980 nm is 30 times higher than that at 915 nm, and the temperature of water only increased by 3.4 °C under the irradiation of a 915 nm laser with a power density of 1.0 W cm(-2) for 10 min, while the temperature of water increased as much as 15.1 °C for the 980 nm laser. With continuous excitation by 915 nm light, the photothermal conversion efficiency of these WO3-x hierarchical nanostructures was evaluated to be 28.1%. Thus, the WO3-x hierarchical nanostructures could serve as excellent laser-cavity mirrors of a 915 nm laser. The PTT study on cancer cells in vivo demonstrated that the WO3-x hierarchical nanostructures can generate enough heat for efficient photothermal therapy of cancer cells under the irradiation of a 915 nm laser with a power density of 1.2 W cm(-2) over a short period (5-10 min).

Thermal limitations of volume Bragg gratings used in lasers for spectral control

In this paper we present experiments and calculations of the property changes of a highly reflecting volume Bragg grating (VBG) when it is used as a laser cavity mirror. A small absorption of the reflected laser beam resulted in a laser output power roll-off, increased coupling through the VBG, and a change of the spectrum from a single to a double peak at high power. The simulations revealed that an inhomogeneous temperature distribution deformed the grating such that the diffraction efficiency was reduced and the light penetrated deeper into the VBG, which accelerated the deteriorating effects. We extrapolated the power limit found in our investigations for various beam radii and absorption coefficients.

Simultaneous phase and shape control of monodisperse NaLuF4:Yb, Er microcrystals and greatly enhanced upconversion luminescence from their superstructures

Simultaneous phase- and shape-controlled NaLuF4 microcrystals co-doped with Yb3+/Er3+ have been synthesized via a facile hydrothermal method by only changing the precursor pH value. The UC emission efficiency of these obtained microparticles showed a strong phase- and shape-dependency. Under irradiation of 980nm, the luminescence intensity of hexagonal NaLuF4 microcrystals presents an enhancement by at least 5.8 times relative to their cubic counterpart. Specially, an up to 168-fold enhancement of emission intensity from the NaLuF4 superstructures, compared with the microplates with identical hexagonal phase, was observed. Such improvement is likely to be dominated by the laser-cavity mirrors effect from the microcavities on their surface of the NaLuF4 superstructures. These results will permit a promising step to harness the upconversion phosphors in solid state lasers.

МНОГОСЛОЙНОЕ ЗЕРКАЛО ТОМСОНОВСКОЙ ДИАГНОСТИКИ В ЭКСПЕРИМЕНТЕ НА ТОКАМАКЕ Т-10

The study of changes in the properties of the laser cavity mirrors for Thomson scattering diagnostic experiment on T-10 tokamak is carried out. Fullfilled measurement provides the technology of mirrors validation and rejection to ensure the reliability and quality of the diagnostic. It is detected the erosion of enlightened input window for the laser radiation, which significantly affect the efficiency of diagnostic. The reasons for the erosion are unclear. Influence of carbon films on the characteristics of mirrors has not been detected.

Dual-wavelength distributed Bragg reflector semiconductor laser based on a composite resonant cavity

We report a monolithic integrated dual-wavelength laser diode based on a distributed Bragg reflector (DBR) composite resonant cavity. The device consists of three sections, a DBR grating section, a passive phase section, and an active gain section. The gain section facet is cleaved to work as a laser cavity mirror. The other laser mirror is the DBR grating, which also functions as a wavelength filter and can control the number of wavelengths involved in the laser action. The reflection bandwidth of the DBR grating is fabricated to have an appropriate value to make the device work at the dual-wavelength lasing state. We adopt the quantum well intermixing (QWI) technique to provide low-absorption loss grating and passive phase section in the fabrication process. By tuning the injection currents on the DBR and the gain sections, the device can generate 0.596 nm-spaced dual-wavelength lasing at room temperature.

Reflection spectrum analysis of stimulated Brillouin scattering dynamic grating

The reflection spectrum of Brillouin dynamic grating is calculated by a numerical method called piecewise approach. A sawtooth frequency modulation is proposed to be used in Brillouin dynamic grating localization along a fiber. It is shown that the reflection spectrum has a flat band where its linewidth depends on the amplitude of frequency modulation. The maximum reflection of dynamic grating can be adjusted by the amplitude of modulation and the power of pump waves, which generates the grating. This grating can be useful as a fiber laser cavity mirror.

Polarization sensitive near-complete reflection from photonic crystal slab in centered rectangular lattice

In this study, reflective properties have been systematically calculated for two-dimensional photonic crystal slabs in centered rectangular lattices with elliptical patterns for transverse electric polarization along perpendicular x- and y-directions. The slab structures can be geometrically optimized for a near-complete reflection over a broad wavelength range. The reflection from the photonic crystal slabs is polarization selective, allowing a complete reflection for one polarization and complete transmission for the other polarization, which could be useful for a laser cavity mirror without using Brewster windows. The dependence of the reflection on the slab thickness and the structural parameters demonstrates the effect of guided-mode resonances on the reflection.

Efficient 1.06 μm laser operation in an unprocessed Nd3+:BaGd2(MoO4)4 cleavage microchip

A 1.2 mm thick microchip cleaved from a 1.2 at.% Nd3+: BaGd2(MoO4)4 crystal was used as the laser gain medium directly. The laser cavity mirrors were coated on the microchip without cutting and polishing. In the 1.06 μm quasi-CW laser operation, the microchip was pumped by a Ti:sapphire laser with a duty cycle of 10%, the threshold is 10 mW, and the slope efficiency is 50%. In the CW laser operation pumped by a diode laser, the threshold and slope efficiency are 140 mW and 33%, respectively. The results demonstrate that the unprocessed surfaces of the cleaved microchip have good enough flatness and parallelism for laser operation.

All-fiber actively Q-switched fiber laser tuned by a pair of temperature controlled fiber Bragg gratings

We report an all-fiber actively Q-switched erbium-doped fiber laser, where the linear laser cavity mirrors are composed of two fiber Bragg gratings (FBGs). The laser oscillation wavelength could be tuned by this pair of temperature controlled FBGs. The Q-switching is achieved by an all-fiber phase modulation device. Using this system, we could obtain stable Q-switched laser pulses output, which could be optimized by tuning the reflection wavelengths of the two FBGs to be adjacent to each other. Instead of being modulated by the FBG filter in high-speed oscillation, this fiber laser system is operating in the Q-switched regime using an all-fiber phase modulator, producing a more stable laser output spectrum.

Room-temperature excitonic lasing from ZnO single nanobelts

Excitonic lasing from ZnO single nanobelts was observed at room temperature, which was due to exciton–exciton scattering processes appearing under intense light excitation. Morphologies of the nanobelts are rectangular shapes, so that crystalline facets of the nanobelts acted as laser-cavity mirrors. Therefore, mode spacings corresponding to cavity lengths of the respective nanobelts were observed in luminescence spectra. Lasing from ZnO single nanobelts at room temperature and their lasing properties are reported.

High-speed continuous tuneable fibre and waveguide lasers with controllable Bragg grating

The method for fast controllable tuning of radiation wavelength of fibre and waveguide lasers is proposed. Bragg grating written on waveguide electrooptic modulator is used both as a wavelength selective element and as laser cavity mirror. By applying control voltage to grating electrodes it is possible to shift grating reflectivity spectrum and, thus, to tune laser generation region. Further, frequency is continuously tuned within the grating reflectivity bandwidth by means of intracavity waveguide electrooptic cell. Spectral parameters of laser radiation as well as efficiency of generation and switching time have been calculated. The region of continuous tuning of such a laser for control voltage up to 5 V is about 0.2 nm and switching time is less than 0.7 ns.

Fabrication and performance of InGaAlAs MQW distributed Bragg reflector lasers with grooved surface gratings formed by reactive beam etching

Grooved surface grating distributed Bragg reflector lasers were fabricated by reactive beam etching after one-step growth of an InGaAlAs multiquantum well active layer and an InP cladding layer. A first-order diffraction grating with a 240 nm period was formed by utilising grooves etched into the cladding layer. The groove depths range between 550 to 900 nm. Single longitudinal mode operation was achieved for devices at all groove depths. DBR lasing with output power of up to 10 mA was obtained. The dependence of the lasing wavelength on device temperature is about 0.1 nm/°C, which indicates the same good stability as standard DBR/DFB lasers. Lasing performance (i.e. injection current against output light power and emission spectra) is discussed in terms of the groove depth of the grating. The relationship between the lasing wavelength shift and the groove depth was examined. The coupling coefficient of the Bragg reflectors are estimated to be about 80 – 180 cm-1 by using the relation between the threshold current density and the reflectivity of the laser cavity mirror.

Performance Comparison of Room-Temperature CO and CO2 Lasers, Used with the Same Apparatus.

Output performance and beam characteristics have beencompared between room-temperature, rf-dischargeexcited CO and CO2 lasers, used with the same apparatus. When switching the operating laser type, cavity mirrors and gas mixtures were replaced. Under optimized conditions, output powers of 340W and 380W were obtained with the CO and the CO2 lasers, respectively. With the CO laser, a much higher slope efficiency (28%) was achieved (slope efficiency with the CO2 laser was 20%), suggesting that a much higher operating efficiency at higher inputs was obtainable for the CO laser. It was also confirmed, experimentally and theoreticall, that for the higher-ordered Gaussian mode beams, the divergence angles of the CO and CO2 lasers were comparable for the given laser cavities with the same diameter and length. For the fundamental Gaussian mode, however, a lower divergence angle, and hence, a more highly focusable beam, can be obtained with the CO laser than with the CO2 laser.

Pulse duration limitations in a diode-pumped femtosecond Kerr-lens mode-locked Cr:LiSAF laser

We describe the results of experimental investigations into the minimisation of the pulse duration from a diodepumped Kerr-lens mode-locked Cr:LiSAF laser. Specifically, the effects on performance of changing the intracavity dispersion characteristics and the laser cavity mirrors are investigated and described. Pulse durations of less than 20 fs have been recorded. PACS: 42.55R; 42.60F In recent years there have been exciting developments in femtosecond laser technology, exploiting the wide bandwidth of solid-state laser materials such as Ti:sapphire and Cr:LiSAF. There have been significant improvements in both the convenience and performance of femtosecond pulse lasers, and this has encouraged a rapid expansion in the application of such sources, e.g. in time-resolved spectroscopic studies, biological imaging, and nonlinear optics. One aspect of femtosecond laser technology that has attracted special attention is the development of all-solid-state diode-pumped femtosecond solid-state lasers (DPFSSLs). These systems bring well-known advantages such as efficiency, compactness, reliability, and stability, all of which will help bring about the fullest exploitation of the potential of ultrashort pulse lasers. A number of alternative approaches to DPFSSLs have been demonstrated. The most attractive option, at least in terms of convenience and simplicity, is the direct-diode pumping of a suitable broadband gain material. Systems to date have typically used the material Cr3+:LiSrAlF6 (Cr:LiSAF) [or its sister material Cr3+:SrGaF6 (Cr:LiSGAF)] pumped with visible 670-nm laser diodes. Recently, the technique of Kerr-lens modelocking (KLM) (also known as self-mode locking) has been optimised for use in these diode-pumped lasers [1, 2] and proven to be both reliable and flexible. Semi-conductor saturable absorbers have been used as an alternative mode∗Current address: Microlase Optical Systems Ltd., 141 St. James Road, Glasgow, G4 ONG, UK locking technique, and several designs have been used successfully, see e.g [3–5]. Although a wide range of DPFSSLs have been reported, most of the progress in reducing the minimum pulse duration obtained from femtosecond lasers has occurred in systems which are not all-solid-state. Work in this field has been dominated by KLM Ti:sapphire laser systems, pumped with an argon ion laser (or frequency doubled flashlamp pumped Nd:YAG laser). Recently, there has been considerable progress in this field, for example [6, 7], and pulses as short as 7.5 fs have been produced [7]. This dramatic progress has been important in enabling new applications and studies to be made in previously inaccessible regimes. Ti:sapphire is used because it has a very wide emission bandwidth, and because its excellent thermal and spectroscopic properties allow high output powers. However, the absorption regions of Ti:sapphire are not compatible with direct diode pumping and this may act as a barrier to the full exploitation of the very short pulses which have been produced in these systems. In this paper we report on investigations into minimising the pulse duration from a diode-pumped Cr:LiSAF laser. Cr:LiSAF clearly has the required bandwidth, at 1900 cm−1 to support much shorter pulse durations than the 30 fs or longer typically seen in systems reported to date [2, 8–11], and there are many applications which would benefit from obtaining pulse durations closer to the sub-10-fs regime, in a convenient all-solid-state laser source. 1 Experimental background 1.1 The diode-pumped KLM Cr:LiSAF laser The Cr:LiSAF laser used for the experiments described in this paper is similar in design to the first reported KLM diodepumped Cr:LiSAF laser [1]. In a higher-power version of this laser we have previously reported routine operation with output powers of up to 42 mW at pulse durations down to 34 fs [2]. Also, in a similar laser we have reported on the generation of pulses as short as 18 fs in duration [12], al-

Characteristics of the application of retroreflectors in submillimetre laser cavities

An experimental investigation was made of the characteristic features of the use of trihedral corner-cube reflectors as laser cavity mirrors. It was found that tilting or rotation of the reflectors about their axis did not affect the output power or the polarisation of the output radiation.