Cavity Configuration Research Articles

Research on Flow Characteristics of Upstream Cavity with Labyrinth Seals in Axial Compressor

The sealing between rotating components and stator components has become one of the main issues to be studied in the compressor. The mixing of stator root leakage flow and the mainstream can seriously affect the performance of the compressor. This article does a series of work on numerical calculation of plane diffuser cascade with the stator cavity and three-stage labyrinth seal. It also analyzes the details of the flow structure on upstream cavity. In this paper, the 3D streamlines distribution of the cascade corner region is studied, respectively for the no leakage case and the mixing mechanism of the secondary flow and mainstream with leakage. On this basis, the upstream cavity configuration is optimized and some results are obtained as fellows. The mainstream in the blade leading edge into the upstream cavity comes into being the secondary flow, which similar to leakage flow. It can affect the highest 80% leaves of the high range. Leakage flow is mainly influence on the performance of the blade root flow field and weakly of next to casing area. Compared to no leakage case, angular separation position ahead of time and range increased when there is leakage. Thus, added rib on both sides of the vessel wall can reduce the total pressure loss of the S3 section, and the relative position of ribbed effect significantly. The research shows that first layer of rib is better when set on the hub wall surface, the total pressure loss coefficient decreased by 3.49%.

Optical axis maladjustment sensitivity in a triangular ring resonator.

This paper estimates and measures the influences of mismatch and misalignment on continuous-wave cavity ring-down spectroscopy. We describe the theoretical differences in maladjustment effects on power transfer under different resonant cavity configurations and present a method for reducing the impact of maladjustment. Finally, we demonstrate a method for measuring the effect of maladjustment on power transfer when a Gaussian beam is matched to a triangular ring resonator.

Solid state ASE from an oligomer (HOTF) in polymethyl methacrylate

Solid state laser material based on oligomer 9,9,9′,9′,9″,9″-hexakis(octyl)-2,7′,2′,7″-trifluorene (HOTF) doped poly methyl methacrylate (PMMA) was fabricated. The absorption spectrum showed only one band at 355 nm with different concentration ratios (7–12 mM); thus the broad absorption band could be attributed to the α-phase formation. In addition, there was no new band detected at the end of the spectrum as the concentration increased. This indicates the absence of the β-phase formation for all concentrations used. On the other hand, HOTF exhibited two distinct emission bands at 420 and 470 nm for 7 mM concentration. When the concentration was increased to 9 mM, the intensity of the band 470 nm increased. Further increase the concentration to 12 mM, the intensity of the band at 420 nm totally vanished and there was only one band at 470 nm. Therefore, the band at 470 could be attributed to excimer state. However, the results revealed that there is a strong correlation between quantum yield of fluorescence and fluorescence life-time, absorption cross section, and emission cross section. Under pulsed laser excitation. The ASE spectrum of HOTF has been obtained using a transverse cavity configuration where the conjugated HOTF was pumped by the third-harmonic of Nd:YAG nanosecond pulsed laser (λex = 355 nm). We demonstrate that HOTF in the solid state could produce an ASE peak at 420 nm. The obtained results were compared with the HOTF and a conducting polymer poly (9,9-dioctylfluorene) (PFO)in the liquid state.

Peculiarities of ocular prosthetics in congenital anophthalmia and microphthalmia

Aim to determine optimal terms of the primary ocular prosthetics, to develop the most auspicious regimen of adaptation to the ocular prosthesis in children with congenital anophthalmia and microphthalmia.
 Material and methods. A total of 46 children aged from 1 month to 16 years with congenital defect were under observation. Among patients with congenital microphthalmia, only unpromising eyes were subject to ocular prosthetics. Examination methods in the laboratory included external examination of the orbit, palpebral fissure, and eyelids. The state of the cul-de-sac of eyelids, the configuration of the conjunctival cavity, the anterior segment of the abnormally small eyeball were assessed. Photography was performed to achieve a dynamic control of external prosthetics signs of, and to evaluate the face symmetry.
 Results. Best results were observed at early stepwise ocular prosthetics with consideration of features of the ocular prosthesis material, without prior surgery. Long-term cosmetic performance of children with congenital anophthalmia and microphthalmia directly depended on age at which the non-surgical treatment began, on the timely replacement of the ocular prosthesis, compliance to the regimen developed for the adaptation to the prosthesis.
 Conclusion. This study showed that the terms of primary ocular prosthetics are of crucial importance for the symmetrical development of soft tissues and facial skeleton. Prosthetics for patients with congenital anophthalmia should be started at the first month of life. The optimal term for primary prosthetics in congenital microphthalmia depends on the length of the antero-posterior axis at birth. If the axial length is less than 7.5 mm, prosthetics should be started at the first month of life, if the axis is longer than 10 mm no later than from the fourth month of life.

High-power broadly tunable grating-coupled external cavity laser in green region.

A broadly tunable grating-coupled external cavity laser system in the green region is demonstrated by employing a commercially available broad-area green laser diode. A simple Littrow-type external cavity configuration was adopted, and two gratings with different diffraction efficiencies were used as the light feedback elements. When a grating with the first-order diffraction efficiency of 50% and the zeroth-order diffraction efficiency of 31% was used, a tuning bandwidth of 11.0 nm with an output power near 400 mW was achieved. When a grating with the first-order diffraction efficiency of 10% and the zeroth-order diffraction efficiency of 78% was used, a tuning bandwidth of 5.0 nm with an output power near 750 mW was achieved. Meanwhile, the spectral linewidth was narrowed significantly down to 0.08-0.18 nm from the free-running linewidth of 2.81 nm due to the external mode selection effect. Such high-power broadly tunable green EC lasers are important for second harmonic generation.

Tunable near-infrared optical vortex parametric laser with versatile orbital angular momentum states.

We demonstrate a tunable vortex laser with versatile orbital angular momentum (OAM) states based on a singly resonant optical parametric oscillator formed of a noncritical phase-matching LiB3O5 crystal. The selective generation of a signal (idler) output with three OAMs, including an upconverted (negative) OAM, is achieved simply by appropriate shortening (or extending) of the cavity. The compact cavity configuration also allows for the generation of the signal (idler) output with various OAMs by simply tuning the signal wavelength. The vortex output is tuned within the wavelength region of 0.74 to 1.84μm with a maximum pulse energy of 2.16mJ from a pump energy of 9.3mJ.

Investigation of the Brillouin effect in highly nonlinear hafnium bismuth erbium doped fiber

AbstractThis paper demonstrates the generation of the Brillouin fiber laser (BFL) with 0.5 m long hafnium bismuth erbium doped fiber (HBEDF) as a hybrid gain medium. As the gain medium is pumped by a 980 nm laser diode at the power of 170 mW, a stable Stokes is observed at the output power of −2.96 dBm with Brillouin pump (BP) power of 3 dBm. The laser operates at wavelength shifted by 0.09 nm from the BP. To the best of the authors' knowledge, this is the first report of a compact BFL without employing any lengthy SMF, which features a simple cavity configuration.

Improving Measurement Sensitivity for a Displacement Sensor Based on Self-Mixing Effect

When a part of light emitted by a laser is back-reflected or back-scattered from an external target and re-enters the laser cavity, both the laser intensity and its wavelength can be modulated. This is so-called self-mixing effect (SME). An SME-based displacement sensor is featured with compact physical setup and interferometric measurement resolution. However, when a target has a surface with very low reflectivity, the sensing signal is very weak and noisy. The system may even lose its sensing capability. It is desired to develop a method for improving the sensing performance in this case. We propose a configuration of dual external cavities for the SME-based sensing system, where one of the cavities is used to provide prefeedback for improving the sensing sensitivity and the other is related to the target to be measured. First, a sensing model is derived for the proposed configuration and shows that the magnitude of the sensing signal can be enhanced by setting the prefeedback cavity with suitable location and high optical feedback strength. Then, experimental investigations are conducted to demonstrate the feasibility of the proposed approach. This paper provides an important guidance for designing an SME-based sensor with high sensitivity.

On the resonant hydroelastic behaviour of ice shelves

Rhythmic hydroelastic oscillations of ice shelves are a key mechanism believed to affect several phenomena observed in Polar Regions, such as the disintegration of ice shelves due to ocean wave impact or even the formation of localised distinctive atmospheric waves. The fundamental and lower hydroelastic modes of an ice-shelf/sub-ice-shelf cavity configuration can be studied by coupling shallow water theory and flexure dynamics of a slender, floating, cantilever beam. A crucial aspect of the analysis is the selection of appropriate boundary conditions at the grounding line of the ice shelf and at the freely floating end. The present study aims to determine appropriate and realistic homogeneous boundary conditions for eigenproblems of resonant ice-shelf vibrations. Through the formulation and solution of a wave impact Reflection-Transmission problem, frequencies that maximise specific norms of the ice-shelf response are identified. It is established that homogeneous conditions on the sub-ice-shelf cavity wave potential value, applied at the front of an ice-shelf, produce eigenfrequencies that in general match the norm maximisation frequencies. The methodology is employed for the prediction of characteristic periods of the Ross and Larsen C ice shelves.

Tunable 3 µm optical vortex parametric oscillator

We propose a tunable millijoule-level 3-µm vortex laser formed with a 1-µm ns vortex pulse-pumped quasi-phase matching MgO-doped periodically poled lithium niobate (MgO:PPLN) optical parametric oscillator with a singly resonant plane-parallel cavity configuration. The wavelength of the vortex output is tuned within the range of 3.360–3.677 µm merely by controlling the temperature of the MgO:PPLN crystal. The maximum vortex output energy of 2.14 mJ is obtained at the maximum pump energy of 21 mJ, and corresponds to an optical to optical conversion efficiency of 10.2% and a photon conversion efficiency of 33.5%. The handedness of the vortex output is controlled just by inverting the handedness of the pump vortex beam.

Sensitivity analysis and gradient-based optimisation of feed spacer shape in reverse osmosis membrane processes using discrete adjoint approach

Spacers play a significant role in enhancing water permeation in membrane desalination processes but also increase pressure drop and specific energy consumption for freshwater production. The complex coupling of nonlinear channel flow along with mass transfer of water and salts across the membrane makes it challenging to determine an optimal spacer design. Conventional methods employ a design loop based on manual design and re-evaluation, which is time-consuming and typically will only find an improved, but not an optimal design. This paper presents an alternative approach that employs the gradient-based adjoint method to explore an optimal design in complex shapes with many design parameters. The novel design methodology provides an effective tool for designers to pursue advanced spacer designs. To elucidate the design method with sensitivities, the spacer in cavity, submerged and zigzag configurations are first analysed with the computed gradients of pressure loss and flow permeation with respect to the displacement of the grid nodes on spacer surface. Then, an optimisation case of cylinder spacers in the zigzag configuration finds an optimal spacer that has 24% pressure loss reduction and only 0.43% permeation drop as compared to a standard cylinder spacer, indicating the new method's potential in finding high-performance spacer designs.

Flexible double-cladding ytterbium fibre based 9 W mode-locked laser with 102 fs compressible pulse duration

We report on a high power mode-locked femtosecond fiber laser oscillator with flexible double-cladding (DC) ytterbium-doped fiber (Yb-fiber) as gain medium and a fused fiber combiner as a pump coupling scheme, based on the nonlinear polarization evolution effect in a ring cavity configuration with a dual-stage birefringent plate as the spectral filter. Laser pulses with a pulse energy as high as 122 nJ and compressible pulse duration as short as 102 fs at 1041 nm center wavelength could been obtained. The experimentally recorded power fluctuations were better than 0.42% RMS and 0.40% RMS in 3 h under 5 W and 9 W output power, respectively. The measured beam quality factors M2 were 1.3 in both the horizontal and perpendicular directions. To the best of our knowledge, this is the highest output power generated from a flexible DC Yb-fiber-based ultrashort pulse mode-locking fiber laser oscillator pumped with a fusion spliced combiner.

Wavelength-spacing-controllable multi-wavelength fiber laser based on a Lyot-Sagnac filter.

A wavelength-spacing-controllable multi-wavelength polarization-maintaining erbium-doped fiber laser (MW-PM-EDFL) in a linear cavity configuration is presented based on a Lyot-Sagnac filter and a nonlinear optical loop mirror (NOLM). Using a Lyot-Sagnac filter as the wavelength selective filter makes the wavelength spacing controllable by adjusting the effective length of polarization-maintained fiber (PMF) segments. An NOLM inducing wavelength-dependent cavity loss can be used as an amplitude equalizer, which is employed to suppress the mode competition resulting from the homogeneous line broadening effect of the erbium-doped fiber in the cavity. Thus, a stable wavelength-spacing-controllable MW-PM-EDFL is realized. In this experiment, 20 stable lasing wavelengths are obtained with a pump power of 170mW at 980nm when the length of the PMF is 13m. When the length of the PMF is 8m, 10 stable lasing wavelengths are obtained at 185mW pump power. The power fluctuations and wavelength shifts are less than 2dB and 0.02nm in 1h at room temperature. The demonstrated multi-wavelength fiber laser has great potential for applications in optical communications and optical sensing systems.

Theoretical Study on Passively Mode-Locked Fiber Lasers with Saturable Absorber

ABSTRACTIn this paper, passively mode-locked erbium-doped fiber lasers based on saturable absorber (SA) have been theoretically studied. The energetics and pulse properties for different fiber laser cavity configurations have been investigated and the effects of each component (active fiber, passive fiber, and SA) in the laser cavity have been studied. This numerical study takes into account the temporal change in the saturable absorption (dq/dt).The presented simulations could be highly useful for understanding, optimizing, and improving passively mode-locked fiber lasers with SA.

A multitasking device based on electromagnetically induced transparency in optical cavities

Quantum memories and optical transistors are elementary building blocks for the implementation of many devices for quantum computers and quantum communication. The realization of experiments that can perform both capabilities in the same setup can open an avenue of possibilities in the development of such practical quantum technologies. We theoretically investigate the feasibility of implementing a quantum memory and an optical transistor in the same setup using a combination of electromagnetically induced transparency and cavity quantum electrodynamics (cavity-EIT) for single- and two-sided cavity configurations. This was accomplished by considering a single three-level atom in $$\Lambda $$ configuration coupled to a single electromagnetic mode of the cavity and a suitable temporal shape for the EIT control field. An optical transistor in cavity-EIT can be realized in a symmetric cavity with two output channels while a high efficient quantum memory must be performed in a single-sided one. From the master equation and input–output formalisms for the intracavity and outside fields, respectively, we obtain the upper bound of $$50\%$$ for the memory efficiency with perfect transistor action in two-sided cavities and values close to $$100\%$$ for the efficiency with a limited transistor effect for the single-sided setup in the high cooperativity regime. Thus, we showed that a dual device, which operates as a quantum memory and an optical transistor in the same setup of cavity-EIT, can be accomplished with some limitation in one of those capabilities.

Correlation matrix methods to assess the stirring performance of electromagnetic reverberation chambers

The use of correlation matrices to evaluate the number of uncorrelated stirrer positions of electromagnetic reverberation chambers has widespread applications in electromagnetic compatibility. We present a review of recent methods based on multivariate correlation functions that relates statistical inhomogeneities in space (frequency) to the reduction of uncorrelated cavity configurations. Full wave finite-difference time domain simulations of an actual reverberation chamber are performed through an in-house parallel code. The efficiency of this code allows for capturing extensive inhomogeneous/anisotropic reverberation fields at frequencies close to the lowest usable frequency (LUF) of the chamber. The concept of effective independent position is revised in light of random sampling and a model-driven relation with the probability distribution of correlation matrix entries is used to take into account spatial (frequency) inhomogeneities. Driven by extensive simulation data, an empirical probability density function is found for the correlation matrix elements to be non-central t-student distributed with asymmetry increasing towards low frequencies.

Semiconductor Laser in Distributed Optical Feedback Regime

In this work, we first analyse the behaviour of semiconductor laser in the presence of weak-to-moderate feedback from a single (lumped) and double external cavity setting. Analysis of laser diode in dynamic double cavity configuration shows that the dynamic double cavity does not disturb the stability of the laser device, but it introduces a considerable shift in the emission frequency that is directly related to the feedback strength and the relative phase difference between the phase of the two mirrors, and the phase of electric field inside the laser diode. Conditions for maximum and minimum impact of the laser emission wavelength have been derived. Addressing the issue of distributed optical feedback, optical feedback produced by more than two external mirrors has been also studied. To this aim we have first generalized the Lang and Kobayashi deterministic rate equations model to arbitrary number of external reflectors and provide the corresponding steady-state solution, which is done for the first time. To the best of our knowledge, this kind of study is carried out for the first time.

External cavity type-I quantum well cascade diode lasers with a tuning range of 440 nm near 3 μm.

A three-stage cascade GaSb-based diode laser heterostructure with an enhanced optical gain spectral bandwidth was designed and fabricated. The gain broadening was achieved by varying the thickness of the type-I quantum wells in different stages of the cascade active region from 10 to 14nm. The structures were processed into bent ridge gain chips with virtually eliminated feedback from the anti-reflection-coated angled facet. The external cavity devices based on a novel gain chip design demonstrated a record wide tuning range from 2.79 to 3.23μm in a Littrow cavity configuration at 20°C.

Route towards extreme optical pulsation in linear cavity ultrafast fibre lasers

Pathways towards the generation of extreme optical pulsation in a chaotic transition regime in a linear fibre laser cavity configuration are presented. In a thulium mode-locked fibre laser, extreme events that can be controllably induced by manipulating the cavity birefringence for pulse energies exceeding the single soliton pulse operating regime are studied in detail for the first time. While a solitonic pulsation structure at the fundamental repetition rate is maintained, additional energy is shed in a chaotic manner, leading to broader spectral generation and shorter pulse durations whose behaviour deviates significantly from a classical statistical distribution. These pulses display markedly different characteristics from any previously reported extreme events in fibre lasers associated with multiple solitons and pulse bunching, thus presenting a novel observation of extreme pulsation. Detailed noise studies indicate that significant enhancement of relaxation oscillations, modulation instability and the interplay with reabsorption mechanisms contribute in this transient chaotic regime. The extreme pulsation generated in a compact fibre laser without any additional nonlinear attractors can provide an attractive platform to accelerate the exploration of the underlying physics of the chaos observed in mode-locked laser systems and can lead to novel fibre laser cavity designs.

An FSS Based Correlation Reduction Technique for MIMO Antennas

This communication presents a new technique to reduce the correlation coefficient value between adjacent antenna elements in multiple-input-multiple-output (MIMO) configurations. Unlike the conventional works which utilize port isolation enhancement methods for this purpose, the proposed approach adopts decorrelation of the radiation patterns of individual elements. To achieve this, a phase-gradient frequency selective surface is used as a superstrate above the antenna elements in a low profile Fabry–Perot cavity configuration. The effectiveness of the method is demonstrated by designing the system for a two-element MIMO antenna at 5.25 GHz. The proposed technique results in more than 95% reduction in the correlation coefficient value, thus projecting a better MIMO performance.