Extended Cavity Research Articles

Design and experimental analysis of W-band extended interaction klystron amplifier

Beam loading is an important parameter in extended interaction klystron, which can be used to analyze the influence of beam on resonant frequency and ohm loss <i>Q</i>, and study the match condition between input cavity and external power source, etc. Based on the kinematical theory, law of induce current, principle of charge conservation under the small signal condition, and one-dimensional (1D) mode, the transit time effect of electron in <inline-formula><tex-math id="M1">\begin{document}${\text{π}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20182194_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20182194_M1.png"/></alternatives></inline-formula> mode standing wave electric treld in a multiple-gap resonator is analyzed, and the expressions of electron load conductance and electron load susceptance in the multiple-gap resonator are presented. The results show that the electron load conductance of extended interaction cavity can change in a bigger extension than that of traditional single gap cavity, which means that the loaded <i>Q</i> of extended interaction cavity can be adjusted in a bigger extension to realize a desired <i>Q</i>. And the results also show that the electron load susceptance of extended interaction cavity can change in a bigger extension than that of traditional single gap cavity, which means that the loaded frequency of extended interaction cavity can also be easily adjusted to a desired value. The influence of gap number on the power exchange between beam and microwave is also investigated, which shows that the maximum power exchange between beam and microwave electric field increases with the number of resonator gaps increasing, and so does the efficiency of klystron. A W band extended interaction klystron amplifoer is designed by the above theory analysis and three-dimensional (3D) PIC code. The simulation results show that when beam voltage is 20.8 kV, current is 0.28 A, input power is 30 mW at a frequency of 94.77 GHz, the extended interaction klystron can produce 443 W output power. The responding electron efficiency is 7.6%, the gain is 41.7 dB, and the 3 dB bandwidth exceeds 150 MHz. The extended interaction klystron is machined and tested, and the experimental results show that the maximum output power of 175 W is obtained with a beam of 300 mA, a voltage of 20.8 kV, and an input microwave power of 30 mW at a frequency of 95.37 GHz in a magnetic field of 0.62 T. The responding electron efficiency is 2.8%, the gain is 34.6 dB, the 3-dB bandwidth exceeds 90 MHz. This study is meaningful for designing and developing greater power extended interaction klystrons.

Design of a circular dual‐loop antenna for a GPS array element using an extended cavity structure

AbstractThis article proposes a circular dual‐loop antenna for a very small Global Positioning System array with an extended cavity structure. The antenna element consists of lower and upper circular loops printed on a high‐dielectric ceramic substrate and an extended cavity structure above the ground plane to improve the isolation characteristic. To demonstrate the suitability of the proposed antenna, antenna characteristics are measured in a full‐anechoic chamber, and its performances such as bore‐sight gain, mutual coupling, and near‐field as a function of extended cavity height are analyzed. The results confirm that the proposed antenna structure can minimize the gain degradation by improving the isolation characteristic and is therefore suitable for use in very small arrays.

Dual-laser frequency-stabilized cavity ring-down spectroscopy for water vapor density measurements

Absolute measurements of water vapor densities have been carried out using a new concept of frequency-stabilized cavity ring-down spectroscopy. Our scheme is based on the use of a pair of phase-locked extended cavity diode lasers, emitting at 1.39 m, one of them being locked to a self-referenced optical frequency comb synthesizer. An intrinsically stable high-finesse optical cavity that tracks the laser frequency scans has been used. High quality, comb-calibrated, absorption spectra have been recorded in a certified H2O/N2 gas mixture at different gas pressures, in coincidence with the 2 transition of the H2 18O + band. Water vapor mole fractions have been determined with a statistical uncertainty of 0.6. Systematic deviations have been identified and carefully quantified, thus leading to an overall uncertainty of 0.8.

Nonlinear structure-extended cavity interaction simulation using a new version of harmonic balance method

This study addresses the nonlinear structure-extended cavity interaction simulation using a new version of the multilevel residue harmonic balance method. This method has only been adopted once to solve a nonlinear beam problem. This is the first study to use this method to solve a nonlinear structural acoustic problem. This study has two focuses: 1) the new version of the multilevel residue harmonic balance method can generate the higher-level nonlinear solutions ignored in the previous version and 2) the effect of the extended cavity, which has not been considered in previous studies, is examined. The cavity length of a panel-cavity system is sometimes longer than the panel length. However, many studies have adopted a model in which the cavity length is equal to the panel length. The effects of excitation magnitude, cavity depth, damping and number of structural modes on sound and vibration responses are investigated for various panel cases. In the simulations, the present harmonic balance solutions agree reasonably well with those obtained from the classical harmonic balance method. There are two important findings. First, the nonlinearity of a structural acoustic system highly depends on the cavity size. If the cavity size is smaller, the nonlinearity is higher. A large cavity volume implies a low stiffness or small acoustic pressure transmitted from the source panel to the nonlinear panel. In other words, the additional volume in an extended cavity affects the nonlinearity, sound and vibration responses of a structural acoustic system. Second, if an acoustic resonance couples with a structural resonance, nonlinearity is amplified and thus the insertion loss is adversely affected.

5-nJ Femtosecond Ti3+:sapphire laser pumped with a single 1 W green diode

We report a Kerr-lens mode-locked, extended-cavity femtosecond Ti3+:sapphire laser directly pumped at 520 nm with a 1 W AlInGaN green diode. To obtain energy scaling, the short x-cavity was extended with a q-preserving multi-pass cavity to reduce the pulse repetition rate to 5.78 MHz. With 880 mW of incident pump power, we obtained as high as 90 mW of continuous-wave output power from the short cavity by using a 3% output coupler. In the Kerr-lens mode-locked regime, the extended cavity produced nearly transform-limited 95 fs pulses at 776 nm. The resulting energy and peak power of the pulses were 5.1 nJ and 53 kW, respectively. To our knowledge, this represents the highest pulse energy directly obtained to date from a mode-locked, single-diode-pumped Ti3+:sapphire laser.

Extended interaction oversized coaxial relativistic klystron amplifier with gigawatt-level output at Ka band

In this paper, an extended interaction oversized coaxial relativistic klystron amplifier (EIOC-RKA) with Gigawatt-level output at Ka band is proposed. We introduce the oversized coaxial and multi-gap resonant cavities to increase the power capacity and investigate a non-uniform extended interaction output cavity to improve the electronic efficiency of the EIOC-RKA. We develop a high order mode gap in the input and output cavities to easily design and fabricate the input and output couplers. Meanwhile, we design the EIOC-RKA by using the particle-in-cell simulation. In the simulations, we use an electron beam with a current of 6 kA and a voltage of 525 kV, which is focused by a low focusing magnetic flux intensity of 0.5 T. The simulation results demonstrate that the saturated output power is 1.17 GW, the electronic efficiency is 37.1%, and the saturated gain is 57 dB at 30 GHz. The self-oscillation is suppressed by adopting the absorbing materials. The proposed EIOC-RKA has plenty of advantages such as large power capacity, high electronic efficiency, low focusing magnetic, high gain, and simple structure.

70 femtosecond Kerr-lens mode-locked multipass-cavity Alexandrite laser.

We report, to the best of our knowledge, the shortest femtosecond pulses generated from a Kerr-lens mode-locked (KLM) Alexandrite laser operating near 750nm. The Alexandrite gain medium was pumped with a continuous-wave (cw), 532nm laser, and the performance of both the short and extended resonators was investigated. The use of an extended cavity eliminated the multi-wavelength spectral instabilities observed during the cw operation of the short cavity. Furthermore, since the repetition rate of the Alexandrite laser was reduced from 107 to 5.6MHz, the resulting increase in the intracavity pulse energy provided enhanced Kerr nonlinearity and eliminated the Q-switching instabilities during mode-locked operation. The KLM MPC Alexandrite laser produced nearly transform-limited, 70fs pulses at a pulse repetition rate of 5.6MHz with only 1W of pump power. The time-bandwidth product was further measured to be 0.331.

2-GHz carbon nanotube mode-locked Yb:YAG channel waveguide laser.

We demonstrate GHz-repetition rate mode-locked operation of a femtosecond-laser-inscribed Yb:YAG channel waveguide laser using single-walled carbon nanotube saturable absorber mirror (SWCNT-SAM). A 6.3-mm-long, type II Yb:YAG waveguide laser with an extended cavity configuration delivers mode-locked picosecond (ps) pulses at GHz repetition rates. The dispersion of the laser cavity is compensated by the combination of a multi-functional output coupler and the Gires-Tournois interferometer (GTI) effect arising from an air-gap between the facet of the waveguide and the output coupler. The incident beam fluence on the SWNCNT-SAM is controlled by adjusting two intracavity lenses to avoid optical damage on the polymer nanocomposite matrix containing the SWCNTs. The average output power of our mode-locked waveguide laser is 322 mW at a pump power of 3.2 W. Nearly Fourier-limited, stable 2-ps-short pulses are generated at a repetition rate of 2.08 GHz.

Differential Probe Fed Liquid Crystal-Based Frequency Tunable Circular Ring Patch Antenna

A differential probe-fed liquid crystal (LC)-based frequency tunable circular-ring patch antenna is presented. Besides, cavity model is extended to analyze the LC-based antenna for the differential operation. According to the cavity model, the permittivity and loss tangent of the LC are extracted from the measured differential reflection coefficients. Acceptable agreements among the measurement, simulation, and calculation for differential impedances, reflection coefficients, and radiation patterns are obtained and also validate the extraction method and model. More importantly, the extended cavity model can provide a deep physical insight into the LC-based antenna.

Frequency Measurement of Tellurium Lines Near Calcium

We report measurement of frequencies of tellurium lines with respect to atomic calcium employing a 423 nm commercial violet laser diode placed in an extended cavity. Doppler-free saturated absorption spectroscopy of molecular tellurium is performed on three lines, which are lower and higher in energy to the calcium 4s2 1S0–4s4p 1P1 transition. The measured frequency difference of 717(13) MHz between the strong absorption line, #1508 in the tellurium atlas and the calcium transition is in agreement within 10% of the values available from literature. Wavenumbers of two new tellurium lines are derived from the measurement. In addition, spectroscopy and laser cooling and trapping of francium (n = 7) on the ns 2S1/2–(n + 1)p 2P3/2 transition and spectroscopy of Te2 lines near francium could be performed employing the 423 nm diode laser.

Frequency modulation of an extended cavity diode laser for 2D raman cooling in a cesium clock

We used frequency modulation of an extended cavity master diode laser in an optical set-up for an atomic clock experiment at NIST. We modulated at low frequencies from 3.2GHz to 4.8GHz at different powers in order to generate sidebands at those frequencies to inject in laser beams which trap and cool cesium atoms in an atomic clock.

Nonlinear beam self-cleaning in a coupled cavity composite laser based on multimode fiber.

We study a coupled cavity laser configuration where a passively Q-switched Nd:YAG microchip laser is combined with an extended cavity, including a doped multimode fiber. For appropriate coupling levels with the extended cavity, we observed that beam self-cleaning was induced in the multimode fiber thanks to nonlinear modal coupling, leading to a quasi-single mode laser output. In the regime of beam self-cleaning, laser pulse duration was reduced from 525 to 225 ps. We also observed a Q-switched mode-locked operation, where spatial self-cleaning was accompanied by far-detuned nonlinear frequency conversion in the active multimode fiber.

1 GHz InP on-chip monolithic extended cavity colliding-pulse mode-locked laser.

A record low-repetition rate from an on-chip monolithic InP extended cavity colliding-pulse mode-locked laser is experimentally reported. The device, fabricated in generic InP-based active-passive integration technology, makes use of integrated mirrors to enable its use as a building block within a photonic integrated circuit. This structure allows us to generate an electrical frequency comb with mode spacing of 1GHz, determined by the 40.5mm long resonator. Passive and hybrid mode-locking regime conditions are experimentally demonstrated. In the passive regime, an electrical beat tone at the fundamental repetition rate with an electrical linewidth (LW) of 398kHz and a signal-to-noise ratio (SNR) >30 dB is measured. In the hybrid regime, the optical comb is locked to a continuous wave signal source, improving the LW of the generated signal and the SNR>40 dB.

JOKARUS - design of a compact optical iodine frequency reference for a sounding rocket mission

We present the design of a compact absolute optical frequency reference for space applications based on hyperfine transitions in molecular iodine with a targeted fractional frequency instability of better than 3 × 10−14 after 1 s. It is based on a micro-integrated extended cavity diode laser with integrated optical amplifier, fiber pigtailed second harmonic generation wave-guide modules, and a quasi-monolithic spectroscopy setup with operating electronics. The instrument described here is scheduled for launch end of 2017 aboard the TEXUS 54 sounding rocket as an important qualification step towards space application of iodine frequency references and related technologies. The payload will operate autonomously and its optical frequency will be compared to an optical frequency comb during its space flight.

Light–matter interactions in multi-element resonators

We investigate light–matter interactions in multi-element optical resonators and provide a roadmap for the identification of structural resonances and the description of the interaction of single extended cavity modes with quantum emitters or mechanical resonators. Using a first principle approach based on the transfer matrix formalism we analyze, both numerically and analytically, the static and dynamical properties of three- and four-mirror cavities. We investigate in particular conditions under which the confinement of the field in specific subcavities allows for enhanced light–matter interactions in the context of cavity quantum electrodynamics and cavity optomechanics.

Study of extra wide coherent optical combs generated by a QW-based integrated passively mode-locked ring laser.

We present an investigation of an InP quantum-well-based integrated extended cavity passively mode-locked laser which shows extra broad frequency comb generation. The ring laser was characterized in frequency and time domains for a range of the current levels injected in the semiconductor optical amplifier. The study showed an increase of the bandwidth to over 40nm at the -20 dB level. The coherence between the longitudinal modes in the wide comb is demonstrated by the characterization of a spectrally filtered signal in time and RF domains. The relative time delay across the optical comb was measured.

Microresonator-stabilized extended-cavity diode laser for supercavity frequency stabilization.

We demonstrate a simple, compact, and cost-effective laser noise reduction method for stabilizing an extended-cavity diode laser to a 3×105 finesse mirror Fabry-Perot (F-P) cavity, corresponding to a resonance linewidth of 10kHz, by using a crystalline MgF2 whispering gallery mode microresonator. The laser linewidth is reduced to sub-kilohertz such that a stable Pound-Drever-Hall error signal is built up. The wavelength of the pre-stabilized laser is tunable within a large bandwidth covering the high-reflection mirror coating of an F-P supercavity.

Numerical and experimental investigation of the means for reducing the aeroacoustic loads in an extended rectangular cavity at subsonic and transonic freestream velocities

The results of a comprehensive investigation including numerical calculations and experiments with models in a wind tunnel and a vehicle under flight conditions aiming to find the ways of reducing the pressure fluctuation levels in an extended cavity at subsonic and transonic freestream velocities are presented. It is shown that the reduction of these loads can be achieved using the means which have demonstrated their effectiveness for cavities with the open-type flows, for example, a permeable deflector and the bevelling of the rear wall but only in the case of a given combination of their geometric parameters. The mechanisms of the action of these devices on the flow, thanks to which the intensity of the wave disturbances generated by the rear wall is reduced, the instability wave growth in the mixing layer behind the deflector is limited, and the fluctuation level in the cavity decreases, are investigated. The results of numerical investigations of the flow in a cavity with a permeable deflector are apparently among the first.

Autonomous frequency stabilization of two extended-cavity diode lasers at the potassium wavelength on a sounding rocket

We have developed, assembled, and flight-proven a stable, compact, and autonomous extended cavity diode laser (ECDL) system designed for atomic physics experiments in space. To that end, two micro-integrated ECDLs at 766.7 nm were frequency stabilized during a sounding rocket flight by means of frequency modulation spectroscopy (FMS) of 39^K and offset locking techniques based on the beat note of the two ECDLs. The frequency stabilization as well as additional hard- and software to test hot redundancy mechanisms were implemented as part of a state-machine, which controlled the experiment completely autonomously throughout the entire flight mission.

Use of Quasiregular Resonator Cavities with Short Phase Correctors in Gyrotrons Operated at Higher Cyclotron Harmonics

We discuss the possibilities of using quasiregular resonator cavities with short irregularities, which ensure correction of the wave phase, in low-power gyrotrons operated at higher cyclotronfrequency harmonics. The use of such phase correctors can help with solving two problems, namely, increasing the selectivity of excitation of a higher cyclotron harmonic and decreasing the diffraction Q-factor of the gyrotron wave excited in an extended cavity.