Technique for continuous tuning of optical fiber lasers

For the first time to the authors' knowledge, we have demonstrated how thermally controlled overcoupled fused fiber couplers and fiber loop mirrors based on these couplers can be used as broadband tuning elements in a fiber laser cavity. No bulk optical elements play any role in this technique. Temperature tuning the coupler results in a shift in the coupling ratio or in the effective output coupler transmission. For a fixed pump source, and for a given laser cavity, this shift causes the lasing wavelength to shift. We have continuously tuned an Er silica fiber laser in this manner over the range of 1527-1570 nm in a ring configuration, and, using a fiber loop mirror with these couplers in a linear Tm silica fiber laser cavity, we have achieved more than 50-nm broadband tuning over the range of 1850-1910 nm. The tuning range and the sensitivity to temperature depend on the degree of overcoupling of the loop mirror coupler.     

Modeling and analysis of an extrinsic Fabry-Perot interferometer cavity

A schematic representation of optical feedback between two resonator mirrors undergoing a phase shift each round trip as a function of the separation of the mirrors is studied. A transfer function modeling of the extrinsic Fabry-Perot interferometer (EFPI) is presented. Nyquist analysis has been used to forecast the operational stability and possibility of interference in an EFPI. The analysis with two perfectly parallel surfaces of the cavity shows efficient interference. The performance when there is some tilt between the two mirrors in the cavity is also studied and is presented. In this case some restricted interference is found.     

Organic microcavity light-emitting diodes with metal mirrors: dependence of the emission wavelength on the viewing angle

Organic microcavity light-emitting diodes typically exhibit a blueshift of the emitting wavelength with increasing viewing angle. We have modeled the shift of the resonance wavelength for several metal mirrors. Eight metals (Al, Ag, Cr, Ti, Au, Ni, Pt, and Cu) have been considered top or bottom mirrors, depending on their work functions. The model fully take into account the dependence of thephase change that occurs on reflection on angle and wavelength for both s and p polarization, as well as on dispersion in the organic layers. Different contribution to the emission wavelength shift are discussed. The influence of the thickness of the bottom mirror and of the choice and thickness of the organic materials inside the cavity has been investigated. Based on the results obtained, guidelines for a choice of materials to reduce blueshift are given.     

Nonstationary doppler effect and frequency-phase methods of investigation and control

A general theory of the nonstationary Doppler effect, which originates during wave passage through a medium whose properties vary in time, is elucidated. Relations are obtained for the frequency shift and phase difference, and numerical estimates are made in an example of ultrasonic oscillations. The advantages of methods of investigation and control based on the effect under consideration are shown.     

Enhanced gain and narrow linewidth of an optical cavity by the Doppler effect in a four-level atomic system

Abstract

A scheme for high gain and narrow linewidth of an optical cavity with a four-level atomic system is proposed by the Doppler effect via active Raman gain (ARG) process. Atomic motion leads to Doppler frequency shift which induces constructive interference for the linear susceptibility. The enhanced normal dispersion greatly narrows the cavity linewidth, and the amplified gain gives rise to a high cavity transmission. Simulation results show that the cavity linewidth based on ARG is about one order of magnitude narrower than that based on electromagnetically-induced transparency under the same conditions, and the cavity transmission intensity could be enhanced by nearly 30 times.     

Detection of nitrogen dioxide by cavity attenuated phase shift spectroscopy

We present initial results obtained from an optical absorption sensor for the monitoring of ambient atmospheric nitrogen dioxide concentrations (0-200 ppb). This sensor utilizes cavity attenuated phase shift spectroscopy, a technology related to cavity ringdown spectroscopy. A modulated broadband incoherent light source (a 430-nm LED) is coupled to an optically resonant cavity formed by two high-reflectivity mirrors. The presence of NO(2) in the cell causes a phase shift in the signal received by a photodetector that is proportional to the NO(2) concentration. The sensor, which employed a 0.5-m cell, was shown to have a sensitivity of 0.3 ppb in the photon (shot) noise limit. Improvements in the optical coupling of the LED to the resonant cavity would allow the sensor to reach this limit with integration times of 10 s or less (corresponding to a noise equivalent absorption coefficient of <1 x 10(-8) cm(-1) Hz(-1/2)). Over a 2-day-long period of ambient atmospheric monitoring, a comparison of the sensor with an extremely accurate and precise tunable diode laser-based absorption spectrometer showed that the CAPS-based instrument was able to reliably and quantitatively measure both large and small fluctuations in the ambient nitrogen dioxide concentration.     

Optical-axis perturbation in triaxial ring resonator

We report several findings on the optical-axis perturbation of monolithic triaxial ring resonator. A criterion, C, which represents the mismatching error of the monolithic triaxial ring resonator, has been found out and it cannot be decreased by modifying the angles of the terminal surfaces or the terminal mirrors of the resonator. When C not equal 0, an optimization method to share the mismatching error C in some specific directions equally and simultaneously has been proposed. The interesting findings are important to cavity design, cavity improvement, and alignment of the monolithic triaxial ring resonator.     

Miniaturized cavity ring-down detection in a liquid flow cell

A novel method for applying cavity ring-down spectroscopy in the liquid phase, compatible with LC analyses, is presented. The core of the setup is a home-built cavity ring-down flow cell (cell volume 12 microL) that is constructed using a silicon rubber spacer, which is clamped leak-tight between two high-reflectivity mirrors. The mirrors are in direct contact with the liquid flow, which provides for a small path length and short ring-down times. Inside the cavity there are no windows, reflection losses, or Brewster angles to be considered. Due to the small size of the presented cavity geometry, the setup can be implemented in conventional-size LC apparatuses. With a flow injection setup, a detection limit of 2.5 nM was obtained for Crystal Violet in ethanol, and the linear dynamic range of the system is at least 2 orders of magnitude. The method has the potential to become a powerful alternative for commercial LC UV/visible absorbance detectors.     

In-Situ Sizing of Powders and Crystals using Phase Doppler Technique

ABSTRACT

ABSTRACT Phase Doppler technique measures the velocity of single particles based on the frequency of oscillations in the light scattered by the particles. The phase shift between two oscillatory signals, collected by two spatially separated detectors, is used as a statistical measure of panicle size. It is shown experimentally that the second and fourth moments of phase shift distribution increase linearly with the second and fourth power of aerodynamic size D of irregular particles, provided that the aspect ratio is small. This allows one to determine the mean and standard deviation of D² in real time without taking a sample. The above result is based on combined measurements using the phase Doppler technique and the aerodynamic particle sizer, and has been consistent for a variety of particles, including alumina, com starch, quartz, and dust. Measurements were also taken to examine the growth and dynamics of crystals, while cooling down a solution of lysine monohydrochloride in a mixture of methanol and water. These data will also be presented and discussed.     

Behavior of a Light Solid in a Rotating Horizontal Cylinder with Liquid Under Vibration

Dynamics of a cylindrical body in a rotating cavity is experimentally studied under transversal translational vibrations of the cavity rotation axis. Experiments are run at high rotation rate, when under the action of centrifugal force the body shifts to the rotation axis (the centrifuged state). In the absence of vibrations, the lagging rotation of the body is observed, due to the body radial shift from the axis of rotation caused by gravity. The body average rotation regime depends on the cavity rotation rate. The vibrations lead to the excitation of different regimes of body differential rotation (leading or lagging) associated with the excitation of its inertial oscillations. The dependence of the differential speed of the body rotation on the vibration frequency is investigated. The body dynamics has a complex character depending on the dimensionless vibration frequency. The analysis of body oscillation trajectory revealed that the body oscillatory motion consists of several modes, which contribute to the averaged dynamics of the body and the flows in the cavity.     

Diagnostic techniques and UV-induced degradation of the mirrors used in the Orsay storage ring free-electron laser

Due to its low gain, the Orsay storage ring free-electron laser necessitates the use of high reflectivity mirrors. Three techniques for measuring the mirror losses are presented, based on cavity decay time measurements using either an external laser, the synchrotron radiation stored in the cavity, or the free-electron laser itself. The high signal-to-noise ratio allowed the detection of loss variations as low as 10(-7)/sec(1/2). From these diagnostics three distinct processes of UV-induced degradation of TiO2/SiO2 dielectric mirrors were identified. One was a surface absorption of the upper SiO2-air interface; it was not affected by annealing. The other two corresponded to a volume absorption of the layers which completely recovered after annealing.     

Laser on YLiF<Subscript>4</Subscript>:Tm<Superscript>3+</Superscript> crystal with resonant reflector

The possibility of using silicon crystal resonant reflectors as cavity mirrors in mid-IR lasers is considered. Generation in a YLiF₄:Tm³⁺ laser with cavity mirrors based on silicon plates has been obtained and studied. The parameters of lasing in the schemes with silicon reflectors and dielectric mirrors are compared.     

Control of superluminal transit through a heterogeneous medium

Abstract

We consider pulse propagation through a two component composite medium (metal inclusions in a dielectric host) with or without cavity mirrors. We show that a very thin slab of such a medium, under conditions of localized plasmon resonance, can lead to significant superluminality with detectable levels of the transmitted pulse. A cavity containing the heterogeneous medium is shown to lead to subluminal-to-superluminal transmission depending on the volume fraction of the metal inclusions. The predictions of the phase time calculations are verified by explicit calculations of the transmitted pulse shapes. We also demonstrate the independence of phase time on system width and the volume fraction under specific conditions.     

Development and Applications of Continuous-Wave Cavity Ring-Down Spectroscopy

Continuous-wave cavity ring-down spectroscopy (CW-CRDS) (using continuous-wave lasers) is now in widespread use for the sensitive detection of a range of different trace-gas species, including water vapor as a very important trace contaminant in many gases. It has also now been applied to monitor trace water vapor in a range of matrix gases, including those that are corrosive and have the potential for spectral interference with the target water-vapor species. The developments that have been carried out to achieve this will be discussed, and some of the applications, covering single sensors and multi-head sensors, will be presented. One limitation of the current sensor technology is that it uses mirrors that are highly reflective over a very restricted spectral range, and this limits a given sensor to the measurement of one or two gaseous species. Measurements of other species require the mirrors to be changed, as it is not currently practical to obtain mirrors with the required high reflectivity that also cover a large spectral range. The development of a new type of ring-down cavity that uses uncoated reflective optics, and which can be used from the ultraviolet to the infrared spectral regions, is presented. Examples of industrial and scientific applications are also presented.     

Microwave Properties of Yarns and Textiles Using a Resonant Microwave Cavity

Modified cavity perturbation methods for permittivity measurements of yarn and textile are described. A cylindrical cavity which is split into equal parts is designed to operate in the TE011 mode. The dielectric yarn is placed across the midplane of the cavity and a textile sample is placed in the cavity on a low-loss dielectric sheet in annular form. The shift in the resonant frequency and change in Q-factor are measured and the complex permittivity is calculated.     

State of polarization of a stochastic electromagnetic beam in an optical resonator

On the basis of the unified theory of coherence and polarization, we investigate the behavior of the state of polarization of a stochastic electromagnetic beam in a Gaussian cavity. Formulations both in terms of Stokes parameters and in terms of polarization ellipse are given. We show that the state of polarization stabilizes, except in the case of a lossless cavity, after several passages between the mirrors, exhibiting monotonic or oscillatory behavior depending on the parameters of the resonator. We also find that an initially (spatially) uniformly polarized beam remains nonuniformly polarized even for a large number of passages between the mirrors of the cavity.     

基于量子级联激光器的中远红外高反射率测量

利用光腔衰荡技术开展了中远红外波段的高反镜反射率测量研究.以中心波长9.7μm附近的脉冲量子级联激光器为光源,构建了高反射率测量实验装置.利用该装置对不同腔长下腔镜的反射率进行了测量,最终测定其反射率为99.9464%,测量重复性误差优于0.0014%.实验表明该测量装置重复性精度高,可用于中远红外高反镜反射率的精确测量.     

Vibration transducer using an optical cavity comprising birefringent mirrors

We have developed an optical-cavity system comprising birefringent mirrors for a vibration transducer. To obtain a dispersive-shape signal necessary for reading out the information of mirror vibration, we measure the polarization change of the cavity transmission light caused by the natural birefringence appearing on interferential mirrors; this effect is enhanced by the cavity resonance. Since there are no additional polarization-changing elements inside the cavity, we can achieve the high finesse that is indispensable for a highly sensitive vibration measurement. The principle and an experimental demonstration of the system are reported here.     

Generalized Doppler shift and time dilation by considering velocity and acceleration

Generalized Doppler shift for light is derived by considering both velocity and acceleration, and is used to calculate time dilation ratio. A circular mirror used to measure time is transformed to the elliptical mirror in a reference frame when it is in motion. The special relativistic Doppler shift is calculated in any propagation direction of light for a constant velocity of motion. Furthermore, the generalized Doppler shift is derived by considering both the velocity and the acceleration effect. The ratio of the frequency shifts are shown as a function of velocity, acceleration, and detection angle.     

Study of the reflectivity of ceramic materials for laser-cavity mirrors

High power lasers for industrial applications necessitate easy handling and ability to be subjected to fast movements. Thus, small volume and weight for these devices without prejudicing their accuracy of beam-fire and their integrity during the time is required. Solid state crystal Nd: YAG lasers were consequently considered. An important aspect under high power irradiation, is the system of mirrors of the cavity that must reflect back towards the laser active crystal source a sufficient radiant energy to activate laser pumping. At the same time the material of these mirrors must not degrade in short times (in the sense of industrial applications) and be sufficiently able to dissipate the produced heat. Ceramics are obvious candidates in particular alumina ceramic. After the test concerning the reflectivity of a wide range of white materials, it was realized that alumina ceramic is up to now the better and about which current technology allows easy manufacture of these mirrors. Certain rules must be followed to prepare white alumina mirrors able to maintain for sufficiently long times their reflectivity such as sintering at suitably high temperature in reducing atmosphere using very pure corundum-like spheroidal grains, correct granulometry of the utilized powders and treatment of the surface with a specific porcelain-like glaze before firing.