Increasing Cavity Length Research Articles

Optimal parameters of gyrotrons with weak electron-wave interaction

In low-power gyrotrons with weak electron-wave interaction, there is a problem of determining the optimal length of the operating cavity, which is found as a result of a tradeoff between the enhancement of the electron efficiency and the increase in the Ohmic loss share with increasing cavity length. In fact, this is the problem of an optimal ratio between the diffraction and Ohmic Q-factors of the operating gyrotron mode, which determines the share of the radiated rf power lost in the cavity wall. In this paper, this problem is studied on the basis of a universal set of equations, which are appropriate for a wide class of electron oscillators with low efficiencies of the electron-wave interaction.

Mode transition and oscillation suppression in supersonic cavity flow

Supersonic flows past two-dimensional cavities with/without control are investigated by the direct numerical simulation (DNS). For an uncontrolled cavity, as the thickness of the boundary layer declines, transition of the dominant mode from the steady mode to the Rossiter II mode and then to the Rossiter III mode is observed due to the change of vortex-corner interactions. Meanwhile, a low frequency mode appears. However, the wake mode observed in a subsonic cavity flow is absent in the current simulation. The oscillation frequencies obtained from a global dynamic mode decomposition (DMD) approach are consistent with the local power spectral density (PSD) analysis. The dominant mode transition is clearly shown by the dynamic modes obtained from the DMD. A passive control technique of substituting the cavity trailing edge with a quarter-circle is studied. As the effective cavity length increases, the dominant mode transition from the Rossiter II mode to the Rossiter III mode occurs. With the control, the pressure oscillations are reduced significantly. The interaction of the shear layer and the recirculation zone is greatly weakened, combined with weaker shear layer instability, responsible for the suppression of pressure oscillations. Moreover, active control using steady subsonic mass injection upstream of a cavity leading edge can stabilize the flow.

Numerical Investigation on Frequency Jump of Flow Over a Cavity Using Large Eddy Simulation

Large eddy simulation (LES) approach was used to investigate jumps of primary frequency of shear layer flow over a cavity. Comparisons between computational results and experimental data show that LES is an appropriate approach to accurately capturing the critical values of velocity and cavity length of a frequency jump, as well as characteristics of the separated shear layer. The drive force of the self-sustained oscillation of impinging shear layer is fluid injection and reinjection. Flow patterns in the shear layer and cavity before and after the frequency jump demonstrate that the frequency jump is associated with vortex–corner interaction. Before frequency jump, a mature vortex structure is observed in shear layer. The vortex is clipped by impinging corner at approximately half of its size, which induces strong vortex–corner interaction. After frequency jump, successive vortices almost escape from impinging corner without the generation of a mature vortex, thereby indicating weaker vortex–corner interaction. Two wave peaks are observed in the shear layer after the frequency jump because of: (1) vortex–corner interaction and (2) centrifugal instability in cavity. Pressure fluctuations inside the cavity are well regulated with respect to time. Peak values of correlation coefficients close to zero time lags indicate the existence of standing waves inside the cavity. Transitions from a linear to a nonlinear process occurs at the same position (i.e., x/H = 0.7) for both velocity and cavity length variations. Slopes of linear region are solely the function of cavity length, thereby showing increased steepness with increased cavity length.

Burst-mode pulse generation from a bismuth-doped germanosilicate fiber laser through self Q-switched mode-locking

A bismuth-doped germanosilicate fiber laser was experimentally demonstrated to engage in self-Q-switched mode-locking at a wavelength of 1470 nm. Mode-locked pulse bursts were generated from a ring cavity without using a saturable absorber when the cavity length was properly adjusted. At a cavity length of 65 m, repetition rates of ~31.5 kHz and ~3.088 MHz and temporal widths of ~4.7 μs and ~1.2 ns were measured for the Q-switching envelopes and mode-locked pulses under the Q-switching envelopes, respectively. The laser operating regime was observed to change from Q-switching to Q-switched mode-locking as cavity length increases by adding a segment of anomalously dispersive fiber. The output pulse characteristics such as pulse width and repetition rate of the Q-switching envelopes and mode-locked pulses were also investigated with respect to the cavity length.

Numerical analysis of AlGaAs/GaAs multi-quantum well superluminescent diodes

In this paper, we have investigated numerically AlGaAs/GaAs multi-quantum well superluminescent diodes. In these devices the dependence of optical gain, output power on the cavity length and the density states have been analyzed. It is observed that the optical gain and its FWHM bandwidth increase with the increasing density state. Furthermore, the output power increases with the increasing cavity length, whereas the FWHM bandwidth decreases.

Experimental Study on Dynamic Characteristics of Self-Excited Oscillation Pulsed Jet

The dynamic characteristics of the self-excited oscillation pulsed jet is tested by using pulsed jet impact variable cross-section beam in submerged conditions and make comparison with continuous jet, analyzing the effects of operating parameters and nozzle structure parameters on the dynamic characteristics of the pulsed jet. The result show that the oscillating frequency of jet decreases with the increase of cavity length while increases with the increase of the pump pressure. The energy of pulsed jet has obvious change with different cavity length.

Mode stability in photonic-crystal surface-emitting lasers with large κ1DL

We study mode stability in photonic-crystal surface-emitting lasers (PCSELs) with large coupling-coefficient-length product κ1DL(>6). We observe that mode competition occurs at high current levels above threshold. Our combined experimental and theoretical study provides the first evidence of the mode competition originating from the high-order band-edge modes. The decreased threshold margin between these competing high-order modes and the main lasing mode with increasing cavity length as well as the spatial hole burning effect may deteriorate the single-mode stability. Our finding is essential for designing single-mode high-power PCSELs for which the strategy to suppress the high-order modes must be considered.

Transverse modes of a laser using volume Bragg grating as the cavity mirror

A simulation and experiment were performed to demonstrate that a laser using volume Bragg grating as one of the cavity mirrors can achieve lasing even if the laser cavity length exceeds the traditional stable cavity condition. The laser transverse mode changes from a Gaussian beam into a ring-shaped mode as the laser cavity length increases from stable to unstable cavity conditions. At the same time, the effective modal reflectivity is reduced as the cavity length increases.

Hysteresis of mode transitions with varying cavity length of bottle-shaped thermoacoustic prime movers

Transition regions to higher resonant modes of a bottle-shaped thermoacoustic prime mover (neck: 5.39 cm long, 1.91 cm ID; variable cavity with a sliding piston: up to 38 cm long, 4.76 ID) were studied. The neck and cavity regions behave as coupled resonators. A variable cavity with a sliding piston was constructed to study the nature of the device as the cavity length is varied. The dominant mode of operation depends on the length of the cavity, favoring successively higher modes as the cavity length increases, occurring roughly where the higher mode overlaps with the fundamental frequency of the neck region. As the cavity length is increased, the transition of the dominant frequency from the fundamental to the first overtone occurs. However, when the length is then shortened, transition back to the fundamental does occur at the same piston position, revealing hysteresis. Three transitions to higher modes were observed. The hysteresis was studied as a function of input power (12.0–16.5 W) and stack volume filling factor (3.0–4.9%). Preliminary results indicate that the transition region occurs shallower in the cavity and the hysteresis widens as the input power is increased. Decreasing the stack mass causes an increase of the hysteresis width, but has no strong effect on the hysteresis depth.

Effects of Cavity Length on Optical Characteristics of Deep Violet InGaN DQW Lasers

The performance characteristics and their dependence to cavity length of deep violet InGaN DQW lasers emitting at 390 nm have been investigated using the Integrated System Engineering Technical Computer Aided Design (ISE TCAD) software. The focus of simulation was on the study of optical properties which were carried out with varying cavity length from 200µm to 600µm. The simulation results indicated that the cavity length strongly affects the optical properties of the violet InGaN DQW laser. They showed that the parameters related to the output power such as optical intensity increases by increasing cavity length due to increase of applied current to the laser system. The results also indicated that the parameters such as optical material gain, stimulated and radiative recombination which are related to quantum efficiencies and laser performance decrease by increasing cavity length. It was shown that the laser structure with the longer cavity length has the lower optical loss.

Cylindrical Surfaces Enable Wavelength-Selective Extinction and Sub-0.2 nm Linewidth in 250 $\mu\hbox{m}$-Gap Silicon Fabry–Pérot Cavities

In this paper, we propose two different designs of micromachined Fabry-Perot optical cavities, with first motivation of improving the quality factor (Q -factor) and in the same time allowing increased cavity length L. Our approach consists of providing a solution to the main loss mechanism in conventional FP cavities related to the expansion of the Gaussian light beam after multiple reflections inside the cavity. The first design is based on all-silicon cylindrical Bragg mirrors, which provide 1-D confinement of light. In addition to wavelength selectivity, the first design also demonstrates its potential for a new class of applications, including wavelength selective extinction through mode-selective excitation, where the fiber-to-cavity distance is used as the control parameter. The second design is based on cylindrical Bragg mirrors combined with a fiber rod lens to provide a complete solution for 2-D confinement of light. This approach outperforms the first design in terms of Q-factor, of nearly 9000 for around 250 μm-long cavity, which suggests its potential use for biochemical sensing and analysis as well as cavity enhancement applications requiring high Q.L values.

Travelling wave analysis on high-speed performance of Q-modulated distributed feedback laser

The structure of a Q-modulated distributed feedback laser is designed and simulated. A large reflectivity modulation of the rear reflector is achieved by using an anti-resonant cavity formed by two deep trenches with the one between the modulator and phase section filled by a high index dielectric material. The travelling wave model is presented to analyze the high speed performance of the laser. Due to the effect of the wave propagation in the structure, the modulation extinction ratio decreases with increasing cavity length. It is shown that 40 Gb/s RZ signal modulation can be achieved with an extinction ratio of 7 dB and 10 dB, respectively, for a cavity length of 500 μm and 300 μm.

Broad-range self-sweeping of a narrow-line self-pulsing Yb-doped fiber laser

The effect of broad-range (16 nm) self-sweeping of a narrow-line (less than 1 pm) Yb-doped fiber laser has been demonstrated experimentally. It is found that the effect arises from the self-sustained relaxation oscillations. As a result, the sweeping rate increases as square root of the laser power and decreases with increasing cavity length. Based on these results we propose a model describing dynamics of the laser frequency. The model takes into account the effects of gain saturation at the laser transition and spatial hole burning in the self-pulsing regime.

Study on the Frequency Characteristic of Self-Excited Oscillation Pulsed Water Jet

The self-excited oscillation pulsed water jet (SEOP Jet ) has been used extensively in oil and gas well drilling, mineral mining, cutting and industrial cleaning. In order to improve the valid stand-off distance of water jet pressure and its range of action, the model of oscillation frequency of jet was given on the basis of hydroacoustics and fluid dynamics for better understanding the principle of increasing jet pressure amplitude. The SD150test system was used to analyze the effects of dynamic parameters on the frequency, the effects of pump pressure and the cavity length on the frequency were studied in detail. The test results showed that the peak value of the SEOP Jet is larger by about 20% than that of continuous jet. The oscillation frequency increased while increasing pump pressure and decreased while increasing cavity length. There was an optimum cavity length corresponding to the pressure peak value of the SEOP Jet. Experimental result is in good agreement with that of theoretical analysis.

Study of Solitary-Wave-Induced Fluid Motions and Vortices in a Cavity Using a Two-Dimensional Viscous Flow Model

This study presents a combined numerical and experimental investigation of the free-surface variation and induced fluid motion for a solitary wave propagating past a submerged cavity (or trench). The formation of vortices and the trajectories of fluid particles showing the transport of fluid content within the cavity zone are examined. A two-dimensional viscous flow is simulated by solving the stream function and vorticity equations using the finite-analytic method. Equations of free-surface boundary conditions are discretized by a two-step finite-difference scheme. To obtain more detailed motions in a cavity, a transient boundary-fitted grid system with locally refined grids is adopted. Experimental measurements of the free-surface elevations and the visual observations of the vortex motion were carried out to compare to the numerical solutions. The simulated free-surface elevations and fluid particle motion at various times are found to agree reasonably well with measurements and recorded observations. The formation and subsequent growth of a pair of recirculating vortices around the front corner of the cavity are clearly simulated by the present model. The effects of cavity size and incident-wave height on the flow patterns and the transport displacement of the fluid particles along the vertical and horizontal directions are analyzed. The results indicate that the greater the incident-wave height, the larger the values of the horizontal and vertical transporting distances. With an increase of cavity length, the strength of induced up-rolling vortices and the amount of downstream transporting fluid particles increases. However, the depth of the cavity has an insignificant influence on the height of the up-rolling vortices.

Influence of Thermodynamic Effect on Blade Load in a Cavitating Inducer

Distribution of the blade load is one of the design parameters for a cavitating inducer. For experimental investigation of the thermodynamic effect on the blade load, we conducted experiments in both cold water and liquid nitrogen. The thermodynamic effect on cavitation notably appears in this cryogenic fluid although it can be disregarded in cold water. In these experiments, the pressure rise along the blade tip was measured. In water, the pressure increased almost linearly from the leading edge to the trailing edge at higher cavitation number. After that, with a decrease of cavitation number, pressure rise occurred only near the trailing edge. On the other hand, in liquid nitrogen, the pressure distribution was similar to that in water at a higher cavitation number, even if the cavitation number as a cavitation parameter decreased. Because the cavitation growth is suppressed by the thermodynamic effect, the distribution of the blade load does not change even at lower cavitation number. By contrast, the pressure distribution in liquid nitrogen has the same tendency as that in water if the cavity length at the blade tip is taken as a cavitation indication. From these results, it was found that the shift of the blade load to the trailing edge depended on the increase of cavity length, and that the distribution of blade load was indicated only by the cavity length independent of the thermodynamic effect.

Device Parameter Analyses of Solid-State Organic Laser Made by Self-Written Active Waveguide Technique

In this paper, we propose an evaluation method for device parameters of solid-state organic lasers. Using a self-written active (SWA) waveguide technique, a Fabry-Perot (FP)-type organic laser with a channel waveguide structure was fabricated, in which the cavity length and the feedback efficiency can be easily controlled. The lasing threshold of the fabricated organic laser showed a monotonic decrease with increased cavity length. From the experimental result, the optical net-gain coefficient and the loss coefficient could be quantitatively evaluated. The evaluation method described here would be an available means to estimate the performance of the laser device with organic compounds.

Casimir pressure on a thin metal slab

We consider the vacuum-field pressure on boundaries of a metal slab in the middle of a cavity with perfectly reflecting mirrors adopting the plasma model for the metal and paying special attention to the surface plasmon polariton contribution to the pressure. We demonstrate that, with increasing cavity length, the pressure on a thin (d << λp) slab in this system decreases from the Casimir pressure FC = −π2hc/240d4 at zero slab-mirror distances to the non-retarded force per unit area Fnr = 1.19(d/λp)FC in the case of an isolated slab. In the first case the pressure is entirely due to the photonic modes propagating through the metal whereas in the second case it is entirely due to the (nonretarded) surface plasmon modes supported by the free-standing thin slab. In either case the pressure decreases with the slab thickness. These considerations indicate that the vacuum-field pressure on a thin metal layer (and its modal structure) can be in a symmetric cavity significantly influenced when changing the cavity length.

Size Effect on ESD Threshold and Degradation Behavior of InP Buried Heterostructure Semiconductor Lasers

Optoelectronic components such as laser diodes and light-emitting diodes are vulnerable to electrostatic dis- charge (ESD) and electrical overstress (EOS). In this paper, we extensively study the size effect on ESD performance of buried heterostructure (BH) distributed feedback (DFB) InGaAsP/InP lasers. We show that the ESD threshold and degra- dation behavior of BH lasers are correlated with the cavity length and contact width. The ESD threshold increases linearly with increasing cavity length and contact width. For the ESD degradation behavior, the occurrence frequency of hard deg- radation, a behavior characterized with a sudden jump in threshold current during the ESD voltage ramp, decreases with increasing cavity length. We also show that the dielectric layer is influential in ESD performance. The physical mecha- nisms of the ESD behavior will also be discussed.

Enhancement in the gain of quantum dot laser by increasing overlap integral between electron and hole wave-functions

We report the influences of quantum dot (QD) shape on the lasing characteristics such as threshold current, slope efficiency, and wavelength stability. The average heights of the shape-engineered InAs/InAlGaAs QDs (SEQDs) and the conventionally-grown Stranski-Krastanov InAs QDs (CQD) were 10 ± 0.5 and 2.5 ± 0.5 nm, respectively. For the cavity length of 0.5 mm, the threshold current of the laser diodes (LDs) with the InAs/InAlGaAs SEQDs as an active layer (SEQD-LD) was decreased by 3.6 times smaller than that of the LDs with the InAs CQDs (CQD-LD). Also the slope efficiency for the SEQD-LD was increased to 0.15 from 0.087 W/A of the CQD-LD. While the lasing wavelength of the CQD-LD was red-shifted by 0.032 μm with increasing cavity length from 0.5 to 0.75 mm, the lasing emission of the SEQD-LD was red-shifted only by 0.012 μm with increasing cavity length from 0.5 to 1.5 mm. From these results, the gain of the QDLDs was enhanced by increasing the height of the QDs due to the enhancement in the confinement of the carrier wave-functions.