Standing-wave Cavity Research Articles

Equidistant optimization of elliptical superconducting rf standing wave cavities

Equidistant optimization of elliptical superconducting rf standing wave cavities

High output power, single mode, and TEM00 operation of a multiple gain chip VECSEL using a twisted-mode configuration.

A vertical external cavity surface emitting laser (VECSEL) has been developed for a sodium guide star application. Stable single frequency operation with 21 W of output power near 1178 nm with multiple gain elements while lasing in the TEM00 mode has been achieved. Higher output power results in multimode lasing. For the sodium guide star application, the 1178 nm can be frequency doubled to 589 nm. The power scaling approach used involves using multiple gain mirrors in a folded standing wave cavity. This is the first demonstration of a high power single frequency VECSEL using a twisted-mode configuration and multiple gain mirrors located at the cavity folds.

Onset of Visible Capillary Waves from High-Frequency Acoustic Excitation.

Remarkably, the interface of a fluid droplet will produce visible capillary waves when exposed to acoustic waves. For example, a small (∼1 μL) sessile droplet will oscillate at a low ∼102 Hz frequency when weakly driven by acoustic waves at ∼106 Hz frequency and beyond. We measured such a droplet's interfacial response to 6.6 MHz ultrasound to gain insight into the energy transfer mechanism that spans these vastly different time scales, using high-speed microscopic digital transmission holography, a unique method to capture three-dimensional surface dynamics at nanometer space and microsecond time resolutions. We show that low-frequency capillary waves are driven into existence via a feedback mechanism between the acoustic radiation pressure and the evolving shape of the fluid interface. The acoustic pressure is distributed in the standing wave cavity of the droplet, and as the shape of the fluid interface changes in response to the distributed pressure present on the interface, the standing wave field also changes shape, feeding back to produce changes in the acoustic radiation pressure distribution in the cavity. A physical model explicitly based upon this proposed mechanism is provided, and simulations using it were verified against direct observations of both the microscale droplet interface dynamics from holography and internal pressure distributions using microparticle image velocimetry. The pressure-interface feedback model accurately predicts the vibration amplitude threshold at which capillary waves appear, the subsequent amplitude and frequency of the capillary waves, and the distribution of the standing wave pressure field within the sessile droplet responsible for the capillary waves.

Secondary Raman and Brillouin mode suppression in two- and three-mirror-cavity diamond Raman lasers.

We report an investigation into secondary mode suppression in single longitudinal mode (SLM) 1240 nm diamond Raman lasers. For a three-mirror V-shape standing-wave cavity incorporating an intra-cavity LBO crystal to suppress secondary modes, we achieved stable SLM output with a maximum output power of 11.7 W and a slope efficiency 34.9%. We quantify the level of χ(2) coupling necessary to suppress secondary modes including those generated by stimulated Brillouin scattering (SBS). It is found that SBS-generated modes often coincide with higher-order spatial modes in the beam profile and can be suppressed using an intracavity aperture. Using numerical calculations, it is shown that the probability for such higher-order spatial modes is higher for an apertureless V-cavity than in two-mirror cavities due its contrasting longitudinal mode-structure.

Beam loading compensation of standing wave linac with off-crest acceleration

In E-Driven positron source of ILC, the generated positron is captured by a standing wave cavity. Because the deceleration capture method is employed, the positron is off-crest over the linac. Because the beam-loading is expected to be more than 1A in a multi-bunch format, the compensation is essential to obtain uniform intensity over the pulse. A conventional method for the compensation controlling the timing doesn’t work because RF and Beam induced field are in different phase. In this manuscript, we discuss the compensation with the off-crest acceleration case. A simple phase modulation on the input RF is a solution.

An angle-tuned polarization-independent multi-narrowband perfect absorber

We propose and numerically investigate an angle-tuned polarization-independent multi-narrowband perfect absorber, which comprises a simple gold nanocube array on SiO2/oxidized aluminum mirror layers. The oxidized aluminum mirror is able to support the surface plasmon polariton mode on the SiO2/Al2O3 interface. The multi-narrowband absorption contributes to the simultaneously excited quasi-metal–insulator–metal guided modes and standing wave cavity mode in the thick insulator layer by the incident light normal to the structure in the wavelength range 1400–2400 nm. Moreover, the absorption can be actively modulated by adjusting the incidence angle, and it is polarization independent. The perfect absorber is also suitable for sensing, with the figure of merit reaching 102 RIU−1 within the refractive index range from 1.3 to 1.36.

Nanosecond pulsed single longitudinal mode diamond Raman laser in the 1.6 µm spectral region.

We demonstrate the first nanosecond pulsed single longitudinal mode (SLM) intracavity-pumped diamond Raman laser, to the best of our knowledge. The eye-safe coherent source at 1634 nm, which was converted from the actively Q-switched 1342 nm Nd:YVO4 laser, yielded 4.35 W of multimode average output power with a pulse duration of 6 ns and peak power of 29 kW. By exploiting the spatial hole burning free gain mechanism in the Raman media, stable SLM operation was observed at low output power (0.46 W) for the free-running case. Furthermore, by incorporating an etalon in the fundamental standing-wave cavity, the spectral linewidth of the fundamental field was suppressed substantially below the diamond Raman gain linewidth and slightly less than the free spectral range of the mm-scale Raman resonator. Thereby, a much higher SLM output power of 1.63 W was obtained with a pulse duration of ∼9 ns and a spectral linewidth of ∼77 MHz.

Two-cavity dual-frequency Nd:YAG laser based on the principle of longitudinal mode selection by Fabry–Perot etalon

To obtain the dual-frequency laser output with large and tunable frequency difference, a design scheme of dual-frequency Nd:YAG laser with two standing-wave cavities sharing the common gain medium has been proposed, which is based on the principles of polarization splitting and single longitudinal mode selection of intracavity Fabry–Perot etalon. With each of the cavities containing a piece of Fabry–Perot etalon, the p- and s-polarized components of the laser at 1064 nm will be forced to oscillate simultaneously in single longitudinal mode in the linear and right-angle cavities, respectively. As a result, the orthogonally and linearly polarized dual-frequency laser at 1064 nm can be obtained. The principle of single longitudinal mode selection by use of the Fabry–Perot etalon has been analyzed, and the Fabry–Perot etalons have been designed. An experimental system of the two-cavity dual-frequency Nd:YAG laser at 1064 nm has been established, and the characteristics of single longitudinal mode oscillation of the two cavities have been investigated experimentally. The orthogonally and linearly polarized dual-frequency laser output at 1064 nm has been obtained; the main characteristics of the oscillating threshold and output power, the polarization state as well as the laser beam quality have been tested experimentally. The frequency difference of the dual-frequency laser has been tuned in turn to 16, 24, 37, and 76 GHz, by slightly adjusting the tilt angles of the intracavity Fabry–Perot etalons. Such a two-cavity dual-frequency Nd:YAG laser will be widely used in the synthetic-wave absolute-distance interferometry and other fields.

Design and characterization of a broadband multi-node bulk acoustic wave device for acoustofluidic applications

The classic approaches to generate the Acoustic Radiation Force (ARF) in a standing wave cavity are based on the use of a specific height for the cavity and a narrow-band acoustic frequency to reach the resonance. We present a robust multi-node cavity working over a broadband range of frequency. This approach allows large acoustophoresis manipulations. The design of a transparent cavity and the use of a broadband ultrasonic transducer allowed the characterization of the acoustic energy and the comparison with a simple 1-D model. The acoustic properties of the system were estimated through the effects of the acoustic radiation forces on particles suspension. From the particle velocities induced by the ARF, measured with a Particle Image Velocimetry (PIV) approach, we deduced the acoustic energy over a large frequency range. The automation of the setup allowed the acquisition of a large amount of data to make parametric studies. The results show a wide continuous operating range for the acoustic radiation forces. Furthermore, the occurrence of the resonance peaks allows an increase by a factor ten of the force magnitude. This broadband effect also enables an accurate control of the node positions by tuning the proper acoustic frequency.

Self-organization of human stem cells into spheroids in a multinode acoustic levitation

We present an approach based on acoustic levitation to grow the cells without wall interactions in order to promote 3D cell architecture (spheroïd, organoïd). The 3D microenvironment is indeed closer to In Vivo physiological behavior. The suspended cells are moved toward the acoustic pressure nodes where they are trapped and maintained in acoustic levitation in perfectly straight monolayers reaching very quickly the classical confluency of the cells in 2D culture. Interestingly, by maintaining the monolayers of MSCs in culture over a 24-h period, the MSCs spontaneously self-organized from cell sheets to cell spheroids with a characteristic time of about 10 h. This approach of 3D cell culture is based on the use of the acoustic wave coupled with microfluidics. We designed a standing wave cavity to generate a large acoustic radiation force (ARF) with an optical access, which allows the characterization of the self-organization dynamics. This 3D cell culture method has been validated on MSCs over 24h experiments. The MSCs viability has been checked. Moreover, they show a higher differentiation capacity compared to standard 2D culture conditions. These results open the path to long-time cell culture in acoustic levitation of cell sheets or spheroids for any type of cells.

Preparation of continuously tunable orthogonal squeezed light filed corresponding to cesium D<sub>1</sub> line

The non-classical light resonance on the cesium D<sub>1</sub> (894.6 nm) line has important applications in solid-state quantum information networks due to its unique advantages. The cesium D<sub>1</sub> line has a simplified hyperfine structure and can be used to realize a light-atom interface. In our previous work, we demonstrated 2.8-dB quadrature squeezed vacuum light at cesium D<sub>1</sub> line in an optical parametric oscillator(OPO) with a periodically poled KTP(PPKTP) crystal. However, the squeezing level is relatively low, and the tunability that has practical significance for squeezed light has not been further investigated. Theoretically, the increase of the transmittance of output mirror and the decrease of the intra-cavity loss of the OPO can improve the squeezing level. Here, we use super-polished and optimal coating cavity mirrors to improve the nonlinear process in OPO. We prepare 447.3 nm blue light from 894.6 nm fundamental light by a second harmonic generation cavity (SHG). The SHG is a two-mirror standing-wave cavity with a PPKTP crystal as the nonlinear medium. The power of generated blue laser is 32 mW when the incident infrared power is 120 mW. Using the blue light to pump an OPO, we achieve quadrature squeezed vacuum light at cesium D<sub>1</sub> line. The OPO is a two-mirror standing-wave cavity with a PPKTP crystal. The threshold of OPO is reduced to 28 mW. The squeezing level of generated quadrature squeezed vacuum light is increased to 3.3 dB when the pump power is 15 mW. Taking into account the overall detection efficiency, the actual squeezing reaches 5.5 dB. We inject a weak signal beam into the OPO cavity to act as an optical parametric amplifier (OPA), and test the tunability of squeezzed light. The blue light and the squeezed light are tuned by using a low-frequency triangular wave signal to scan the Ti: sapphire laser. Gradually increasing the amplitude of the scanning triangle wave signal, the generated bright squeezed light can be continuously tuned over a range around 80 MHz without losing the stability of the whole system. The generated squeezed light offers the possibility for the efficient coupling between the non-classical source and solid medium in the process of quantum interface.

Quantum Enhanced Cavity QED Interferometer with Partially Delocalized Atoms in Lattices.

We propose a quantum enhanced interferometric protocol for gravimetry and force sensing using cold atoms in an optical lattice supported by a standing-wave cavity. By loading the atoms in partially delocalized Wannier-Stark states, it is possible to cancel the undesirable inhomogeneities arising from the mismatch between the lattice and cavity fields and to generate spin squeezed states via a uniform one-axis twisting model. The quantum enhanced sensitivity of the states is combined with the subsequent application of a compound pulse sequence that allows us to separate atoms by several lattice sites. This, together with the capability to load small atomic clouds in the lattice at micrometric distances from a surface, make our setup ideal for sensing short-range forces. We show that for arrays of 10^{4} atoms, our protocol can reduce the required averaging time by a factor of 10 compared to unentangled lattice-based interferometers after accounting for primary sources of decoherence.

Rectangular dark pulses in all-normal dispersion fiber oscillator

We report the generation of rectangular dark pulses (RDPs) in ytterbium (Yb) doped fiber laser using a nearly 50:50 coupler based nonlinear optical loop mirror (NOLM) in a simple linear standing wave cavity. Width of the dark pulse decreases with increase in the pump power whereas depth of the rectangular pulse profile remains constant. The nonlinear reflectance characteristics of the NOLM used in the cavity has been experimentally investigated to understand its role in formation of RDPs. Numerical simulations were performed to show the evolution of dark pulses in a fiber oscillator in all normal dispersion configuration. Both experimental and numerical simulations indicate that reverse saturable absorption supports formation of dark pulses. With increase in the saturation energy, width of RDP was found to decrease in line with the experimental observations. Further, an ansatz is proposed to model RDPs. The ansatz is used in cubic quntic Ginzburg-Landau equation to establish relationship between pulse parameters and cavity parameters. Results from analytical formalism are in accordance with the experimental measurements and numerical simulations. Detailed theoretical analysis reveals that experimentally observed RDPs do not belong to domain wall type.

Development of a seven-cell S-band standing-wave RF-deflecting cavity for Tsinghua Thomson scattering X-ray source

A 2856-MHz, π-mode, seven-cell standing-wave deflecting cavity was designed and fabricated for bunch length measurement in Tsinghua Thomson scattering X-ray source (TTX) facility. This cavity was installed in the TTX and provided a deflecting voltage of 4.2 MV with an input power of 2.5 MW. Bunch length diagnoses of electron beams with energies up to 39 MeV have been performed. In this article, the RF design of the cavity using HFSS, fabrication, and RF test processes are reviewed. High-power operation with accelerated beams and calibration of the deflecting voltage are also presented.

Many-body phases of a planar Bose-Einstein condensate with cavity-induced spin-orbit coupling

We explore the many-body phases of a two-dimensional Bose-Einstein condensate with cavity-mediated dynamic spin-orbit coupling. By the help of two transverse non-interfering, counterpropagating pump lasers and a single standing-wave cavity mode, two degenerate Zeeman sub-levels of the quantum gas are Raman coupled in a double-$\Lambda$-configuration. Beyond a critical pump strength the cavity mode is populated via coherent superradiant Raman scattering from the two pump lasers, leading to the appearance of a dynamical spin-orbit coupling for the atoms. We identify three quantum phases with distinct atomic and photonic properties: the normal ``homogeneous'' phase, the superradiant ``spin-helix'' phase, and the superradiant ``supersolid spin-density-wave'' phase. The latter exhibits an emergent periodic atomic density distribution with an orthorhombic centered rectangular-lattice structure due to the interplay between the coherent photon scattering into the resonator and the collision-induced momentum coupling. The transverse lattice spacing of the emergent crystal is set by the dynamic spin-orbit coupling.

Optical cavity enhancement experiment of Duan-Lukin-Cirac-Zoller writing excitation process in atomic ensemble

The Duan-Lukin-Cirac-Zoller (DLCZ) process in the atomic ensemble is an important means to generate quantum correlation and entanglement between photons and atoms (quantum interface). When a write pulse acts on atoms, the DLCZ quantum memory process will be generated, which has been extensively studied. In the process a spontaneous Raman scattering (SRS) of a Stokes photon is generated, and a spin-wave excitation stored in the atomic ensemble is created at the same time. The higher probability of the generation of Stokes photons will cause more noise and reduce entanglement. On the contrary, the low generation probability of Stokes photons affects the success probability of entanglement distribution on a quantum repeater. How to increase generation probability of Stokes photons without causing more noise is an urgent problem to be resolved. In this work, a <sup>87</sup><i>Rb</i> atomic ensemble is placed in a standing wave cavity which resonates with the Stokes photon. This cavity has a trip length of 0.6 m and a free spectral range (FSR) of 256 MHz. The optical loss of all the optical elements in this cavity is 9%, of which 4% loss originates from the other optical elements and 5% loss from the vacuum chamber of the magneto-optical trap (MOT). The fineness of the cavity with the cold atoms is measured to be ~19.1. By calculating the total probability of Stokes photon emission out of the cavity, we derive the enhancement factor of this standing wave cavity when the cavity loss is <i>l</i>. When this cavity is locked with PDH frequency locking technique, we observe that the production probability of the Stokes photons is 8.7 times higher than that without cavity due to the optical cavity enhancement effect. Under this condition, the relationship between the generation probability of Stokes photons and the power of write beam is studied. The write excitation probability changes linearly with the power of write beam. This work provides an experimental solution to reducing the noise caused by time multimode operation in DLCZ scheme.

Research progress of quartz-enhanced photoacoustic spectroscopy based gas sensing

Laser spectroscopy based techniques have the advantages of high sensitivities, high selectivities, non-invasiveness and in situ, real-time observations. They are widely used in numerous fields, such as environmental monitoring, life science, medical diagnostics, manned space flight, and planetary exploration. Owing to the merits of low cost, compact volume and strong environment adaptability, quartz-enhanced photoacoustic spectroscopy (QEPAS) based sensing is an important laser spectroscopy-based method of detecting the trace gas, which was invented in 2002. Detection sensitivity is a key parameter for gas sensors because it determines their real applications. In this paper, focusing on the detection sensitivity, the common methods for QEPAS are summarized. High power laser including amplified diode laser by erbium doped fiber amplifier (EDFA), and quantum cascade laser are used to improve the excitation intensity of acoustic wave. The absorption line of gas molecules located at the fundamental bands of mid-infrared region is adopted to increase the laser absorption strength. Micro-resonator is employed to enhance the generated acoustic pressure by forming a standing wave cavity. Quartz tuning forks (QTFs) with low resonant frequency are used to increase the accumulation time of acoustic energy in itself. Multi-pass strategy is utilized to amplify the action length between laser beam and target gas in the prongs of QTF. The advantages and disadvantages of the above methods are discussed respectively. For the issues in real applications, the all-fiber strucure in near-infared region and mid-infrared region and miniaturization using three-dimensional(3D) printing technique for QEPAS sensor are summarized. A QEPAS technique based multi-gas sensor is used to quantify the concentration of carbon monoxide (CO), carbon dioxide (CO<sub>2</sub>), hydrogen cyanide (HCN), and hydrogen chloride (HCl) for post-fire cleanup aboard spacecraft, which is taken for example for the real application.Finally, the methods of further improving the sensitivity of QEPAS sensor are proposed.

Reflectionless dual standing-wave microcavity resonator units for photonic integrated circuits

We propose a novel photonic circuit element configuration that emulates the through-port response of a bus coupled traveling-wave resonator using two standing-wave resonant cavities. In this "reflectionless resonator unit", the two constituent cavities, here photonic crystal (PhC) nanobeams, exhibit opposite mode symmetries and may otherwise belong to a single design family. They are coupled evanescently to the bus waveguide without mutual coupling. We show theoretically, and verify using FDTD simulations, that reflection is eliminated when the two cavities are wavelength aligned. This occurs due to symmetry-induced destructive interference at the bus coupling region in the proposed photonic circuit topology. The transmission is equivalent to that of a bus-coupled traveling-wave (e.g. microring) resonator for all coupling conditions. We experimentally demonstrate an implementation fabricated in a new 45 nm silicon-on-insulator complementary metal-oxide semiconductor (SOI CMOS) electronic-photonic process. Both PhC nanobeam cavities have a full-width half-maximum (FWHM) mode length of 4.28 μm and measured intrinsic Q's in excess of 200,000. When the resonances are tuned to degeneracy and coalesce, transmission dips of the over-coupled PhC nanobeam cavities of -16 dB and -17 dB nearly disappear showing a remaining single dip of -4.2 dB, while reflection peaks are simultaneously reduced by 10 dB, demonstrating the quasi-traveling-wave behavior. This photonic circuit topology paves the way for realizing low-energy active devices such as modulators and detectors that can be cascaded to form wavelength-division multiplexed links with smaller power consumption and footprint than traveling wave, ring resonator based implementations.

Dual-wavelength rectangular pulses in a mode-locked ytterbium-doped fiber laser

We demonstrate dual wavelength rectangular pulses in an ytterbium-doped fiber laser in a simple, all-fiber standing wave cavity configuration. The laser cavity consists of a nonlinear optical loop mirror as a saturable absorber at one end and two fiber Bragg gratings at the other end at wavelengths of 1025 and 1055 nm. There are three distinct regimes of operation in the cavity with an increase in the pump power, one where only 1025 nm wavelength pulses appear, then both 1025 and 1055 nm pulses are supported in the cavity after that, only 1055 nm wavelength pulses are present in the cavity. In the intermediate pumping regime, two sets of pulses are supported in the cavity. These pulses move in the cavity at different speeds and on the cross-over there is no interaction in the pulses. The mode-locked pulses at the 1025 nm wavelength are polarized, whereas pulses corresponding to 1055 nm are nearly unpolarized. Pulses corresponding to both the wavelength bands exhibit a dissipative soliton resonance signature. The laser generates 60 mW total average output power at the maximum pump power of 260 mW at a ∼1.8 MHz repetition rate. The presented cavity mechanism can be utilized to generate various dual- or multi-wavelength mode-locked pulses.

Masked states of an atom coupled to a standing-wave cavity mode

The form of the eigenstates of an atom coupled to a cavity mode displaying a three dimensional periodic profile are obtained. It is shown that the quantized motion leads to degenerate states where the atomic degrees of freedom are masked, that is, upon detection of one component of this composite system the others remain in an entangled state. When the system is extended to include drive and dissipation it is found to undergo a dissipative quantum phase transition at a critical drive amplitude. Unlike other phase transitions reported in the literature, the degeneracy prepares the system in a superposition of incompatible states upon detection of the electromagnetic field. Probing the field hints at an order above the transition point that, due to state masking, allows for atomic coherence to survive at long times.