Symmetric bifurcations and phenomena of polarization symmetry breaking in a single-mode gas laser

An analysis is carried out of the optical cavities whose natural modes have the form of Gaussian beams with complex astigmatism. In the case of coincident eigenvalues of the cavity ray matrix, the wave beams were found to be determined not only by the geometrical optics characteristics of a cavity, but also by additional cavity-independent parameters. A detailed analysis is given for the fundamental mode of such cavities. Concrete cavities possessing these properties are noted.     

Modes of optical cavities containing selecting components

A model of the field mode structure in optical cavities containing selecting components is suggested. The cavity also contains field-attenuating or field-enhancing media where elements of the 4×4 symplectic detour matrix of the cavity become complex. The model makes it possible to determine the stability of the cavity, non-Hermiticity of higher modes, and the complex astigmatism of the eigenmode field. Conditions for unilateral (unidirectional) and bilateral (bidirectional) stability are formulated. A cavity showing unilateral stability at the first transverse mode is described.     

针剂中异物的光电检测方法研究

针剂在实际生产过程中容易混入异物颗粒,异物颗粒影响人体健康,因此针剂在出厂前必须进行异物检测.本文提出了一种基于光阻法的针剂异物的检测方法,介绍了检测原理及其信号处理方法,单瓶静态实验表明,该方法与人工检测方法比较,符合率为96.30%.针对检测区域和测试时间过小所造成的漏检可能,提出了双光路检测方法,并通过实验论证了其可行性.     

Modelling the interior sound field of a railway vehicle using statistical energy analysis

The sound field in train compartments, treated as a series of connected air cavities, is modelled using statistical energy analysis, SEA. For the case under study, with five cavities in series and the source in the second cavity, a closed-form solution is obtained. An adjusted SEA model is used to predict the rate of spatial decay within a cavity. The SEA model is validated using results from a ray tracing method and from scale model measurements. For the octave bands 500–4000 Hz, good agreement is shown between the results from measurements, the ray tracing and the SEA model, for the two saloons closest to the source cavity (a vestibule). The SEA model was shown to slightly underestimate the rate of spatial decay within a cavity. It is concluded that a reasonable cause is the additional diffusion due to the seating.     

Complex ABCD transformations for optical beams with complex astigmatism

The method of the ray 4 × 4 ABCD matrices is extended to the three-dimensional optical systems of first order possessing complex astigmatism and containing selective elements with loss and field gain. These systems are found to have complex matrix elements. Properties of such matrices are studied, and a number of examples are considered that are of practical interest. It is shown that in the systems under consideration, like in the systems without loss and gain, such matrices retain the symplecticity property, on which the use of the method applied to specific models of optical-wave propagation is based.     

Dispersive optical bistability in a Fabry-Perot cavity

This is a numerical analysis of the steady-state Maxwell-Bloch equations in a Fabry-Perot cavity with standing waves, atomic detuning and cavity mistuning; the atoms are homogeneously broadened. In the Mean-Field Approximation (MFA) limit, the resulting output-input characteristics agree exactly with the analytical predictions for a ring cavity. This leads to the derivation of a state equation for the Fabry-Perot cavity in the MFA limit identical to that for a ring cavity. Agreement with the MFA within 4% are obtained when αL < 3, the mirror reflectivity is above 90%, and the cavity mistuning below 4% of its free spectral range. Simulations beyond the scope of the MFA are also made and the case of purely dispersive optical bistability is also considered.     

Compact diode-side-pumped Nd:YVO<Subscript>4</Subscript> laser in grazing-incidence configuration

The analysis of compact CW diode-side-pumped grazing-incidence-geometry Nd:YVO₄ laser designs is presented. An output power of 5 W (λ=1064 nm) was produced at 17 W of diode pump (conversion efficiency of 30%) in single transverse TEM₀₀ mode operation at high laser beam quality (M_x ²≈1.05 and M_y ²≈1.01). The resonator geometry was analyzed by applying generalized 4×4 matrix modeling of the spatial mode size, including the impact on the laser operation of cavity astigmatism and a thermal lens in the laser slab. The simplicity and compactness of the laser cavities allow their use for technological applications. Received: 31 July 2002 / Published online: 22 January 2003 RID="*" ID="*"Corresponding author. Fax: +44-20/7594-7744, E-mail: m.damzen@ic.ac.uk     

Effect of cavity geometry on the performance of a gyrotron

The effect of interaction cavity geometry on the gyrotron performance in terms of cold cavity parameters (Q value and axial electric field profile), Ohmic wall loss, start oscillation current and interaction efficiency are analyzed in detail in this article. The measurement of Q value and axial electric field profile is also performed for the cavities with different geometries by using non-destructive and perturbation techniques, respectively. Scattering matrix code is used for the computation of Q value and axial electric field profile and results are compared with the experimental data. A Particle-in-Cell code and a specific beam–wave interaction computation code based on generalized non-linear theory are used in the efficiency calculations. For all numerical and experimental analyses, the case of 42 GHz, 200 kW gyrotron is considered here.     

Control of boundary layer on a flat plate by means of cavity flow

This paper describes the control of boundary layer on a flat plate by means of cavity flow. In this study, classifying the shapes of cavities into circular arc, rectangle and triangle makes the discussion, and the depths of the cavities are changed systematically. It is made clear, by numerical calculations and experiments, what states of flow are shown in the internal parts of cavities and what kinds of influence are exerted to the boundary layers in the upstream and downstream flows. As a result, the following facts are made clear. By taking up properly the depth ratios (k/c) of cavities complying with the individual cavity shapes, the boundary layer thickness in the flow upstream and downstream of the cavities can be controlled. Meanwhile with any of the cavities, the existence of the minimum boundary layer thickness (δ/δ _m ) _min min is seen at a depth ratio.     

Measuring of ultrahigh unloaded Q factor using excitation of a superconducting cavity by the electron beam

A precision method for measuring of ultrahigh unloaded Q factors of superconducting cavities is proposed which is based on the excitation of oscillation in the cavity by electron beam. In this measuring, the cavity is not connected to any external microwave circuit; its unloaded Q factor is determined from the loss of electron-beam power, which can be measured with high accuracy.     

Modelling the excitation field of an optical resonator

Assuming the paraxial approximation, we derive efficient recursive expressions for the projection coefficients of a Gaussian beam over the Gauss--Hermite transverse electro-magnetic (TEM) modes of an optical cavity. While previous studies considered cavities with cylindrical symmetry, our derivation accounts for “simple” astigmatism and ellipticity, which allows to deal with more realistic optical systems. The resulting expansion of the Gaussian beam over the cavity TEM modes provides accurate simulation of the excitation field distribution inside the cavity, in transmission, and in reflection. In particular, this requires including counter-propagating TEM modes, usually neglected in textbooks. As an illustrative application to a complex case, we simulate reentrant cavity configurations where Herriott spots are obtained at cavity output. We show that the case of an astigmatic cavity is also easily modelled. To our knowledge, such relevant applications are usually treated under the simplified geometrical optics approximation, or using heavier numerical methods.     

Possibility of suppressing quantum light fluctuations when excess photon fluctuations occur inside a cavity

Using the optical excitation of a high-Q cavity as an example, it is shown that when light is observed at the output of this cavity, effective suppression of the photocurrent shot noise below the quantum limit is in general independent of the parameters of the stationary state of the field oscillator (in particular, it is independent of the rms photon fluctuations) inside the cavity and can occur not only at any allowed negative value but even at a positive value of the Mandel parameter. It was assumed in solving the problem that the cavity is optically excited by superimposing the radiation of a sub-Poisson laser and a laser with excess photon noise. A formal solution was obtained in terms of the kinetic equation for the density matrix of the actual fields (inside the laser cavities and the empty cavity), which is derived here on the basis of the Heisenberg-Langevin quantum equations, taking into account directed propagation of the field from the laser cavities inside the empty cavity. The resulting kinetic equation can also be used to solve other physical problems, since it is applicable to optical systems that contain, in principle, an arbitrary number of coupled cavities and interference mixers. Zh. éksp. Teor. Fiz. 111, 1579–1600 (May 1997)     

Optical-axis perturbation in nonplanar ring resonators

Several findings on the optical-axis perturbation of nonplanar ring resonators have been obtained identical by utilizing the augmented 5 × 5 matrix formulation and augmented 6 × 6 matrix formulation, respectively. It has been found out that in the whole region of 0 < L/R < 2, the longer the mirror radius, the higher the sensitivity of optical-axis decentration, while the total cavity length is L and the radius of the curvature mirrors is R. The sensitivity of optical-axis tilt in the region of 0 < L/R < 2 has been carried out too. The optical-axis decentration and optical-axis tilt inside the region of L/R > 2 have been discussed. The differences of the optical-axis perturbation between planar and nonplanar ring resonators have also been analyzed. These interesting findings are important to the cavity design of nonplanar ring resonators.     

Remote entanglement for quantum networks

Quantum networks are distributed many-body quantum systems with tailored topology and controlled information exchange. We present two schemes to generate remote entanglement, in atomic external degrees of freedom and between cavities. In the first scheme, we entangle two atoms with their cavities in momentum space through Bragg diffraction. Thereafter, in order to trace out the cavities, we let resonantly interact an auxiliary atom with each cavity. In the last, we perform quantum measurement on two auxiliary atoms and get remote entangled state in atomic external degrees of freedom. In the second scheme, we have a three cavities system. The other two cavities, A and B, are entangled with indistinguishable modes of cavity, C. Performing quantum measurement on third cavity, C, we disentangle it from the system and the cavities, A and B, become entangled.     

Generalised ABCD matrix treatment for laser resonators and beam propagation

A generalised ABCD matrix treatment for laser resonators and beam propagation is developed for computer programming. In this treatment, imaginary parts are introduced into the matrixes for all optical elements, and the beam quality factor M² and the index of the medium are also taken into account. When the imaginary parts of the complex matrix are zero and the beam quality factor M² and medium index are unity, the stated method is transformed back into the more classic ABCD matrix format in which the fundamental-mode Gaussian beam transmits through real elements in the vacuum. Based on this method, laser resonator software is realised by the VB programme language. The software can be used to analyse and design stable/unstable standing/travelling cavities, phase-conjugate cavities and beam transformations.     

Exponential decrease of thermodynamic functions of photon gas in cubic and spherical cavities with low adiabatic invariants

The Planck radiation spectrum of ideal cubic and spherical cavities with low adiabatic invariants, γ = TV ^1/3, is discrete and strongly dependent on the cavity geometry and temperature. This behavior is a consequence of the random distribution of the state weights in the cubic cavity and of the random overlapping of successive multiplet components, in the case of a spherical cavity. The total energy density of cavities with low adiabatic invariant, γ (obtained by summing up the exact contributions of the eigenvalues and their weights) no longer obeys the Stefan-Boltzmann law. The new law includes a corrective factor depending on γ, which imposes an exponential decrease of the total energy density to zero, when γ → 0. A similar behavior is demonstrated for specific heat and for all other thermodynamic functions of photon gas in cubic and spherical cavities with low adiabatic invariants. This special quantum regime, defined by the limits of principal quantum numbers or by adiabatic invariants, is shown to be similar for cubic and spherical cavities.     

Complex ray representation of the astigmatic Gaussian beam propagation

The method of Gaussian beam ray-equivalent modelling, first proposed by Arnaud, is generalized to the case of general astigmatism. It has been shown that a generally astigmatic Gaussian beam can be properly represented by two complex rays, or equally by four real rays, which are treated by the well-known propagation equation and ray tracing method in geometric optics, and from which the beam parameters are easily obtained. Illustrative numerical examples are given. The equivalence between the complex-ray treatment and the generalized ABCD law is also shown.     

Design of ring cavities for c.w. passively mode-locked dye lasers

A simple approach for the analysis and design of ring cavities for c.w. passively mode-locked dye lasers is suggested. The approach is based on the representation of the intracavity beam as a series of two matched beams, each considered to be formed in a linear cavity configuration. Useful expressions are obtained for the cavity parameters in terms of the intracavity beam properties of interest for two different beam configurations. The expressions can be used for the correct design of ring resonators.     

The effect of external cavity reflectivity on the dynamic behaviors of quantum cascade laser

The effect of external reflectivity on the dynamic behaviors of quantum cascade laser coupled to external cavity is theoretically investigated. Our model is based on three-level rate equations including the dependence of the loss on external cavity parameters. We find in particular that the effective reflectivity, photon lifetime and threshold current are strongly influenced by the external cavity length and external reflectivity. The optimum external cavity length (L_ext) is about 4?cm. We also show that the external cavity reflectivity influences significantly the dynamics of the electron number in the upper and lower laser levels, population inversion, and photon numbers in the Fabry Perot and in the external cavities. In addition, the external cavity reflectivity dependences of turn-on delay time (t_th) is also studied and discussed.     

Structural, electronic, vibrational, and elastic properties of SWCNTs doped with B and N: an ab initio study

A microscopic model is developed in order to analyse the effects of dissipations on single-photon transport in a coupled cavity array where one of the cavities is coupled to a three-level atom and both cavities and the three-level atom are coupled to an external environment. By employing the quasi-boson approach, the single-photon transmission and reflection amplitudes are found exactly for the Ξ-type, V-type and Λ-type three-level atoms. We focus on the dissipation properties in the case of the Λ-type system. Comparing the dissipative case with the nodissipative one, it can be found that the dissipations of the cavities and the Λ-type three-level atom significantly affect the transmission amplitude of single-photon transport. Whether the atom is in tune with the resonant frequency of the cavity or not, incomplete reflection is mostly caused by atom dissipation near the middle dip of the single-photon transport spectrum, while reduced transmission appears to be mainly controlled by cavity dissipation. Dissipations broaden the line width of the single photon transport spectrum.