A computer simulation of the wide-band reflection-type gyrotron-TWA

A computer-aided design of the wide-band reflection-type gyrotron-TWA is presented in this paper. Using a set of equations for the self-consistent large-signal interaction the bandwidth characteristics of a wide-band reflection-type gyrotron-TWA with both tapered waveguide and tapered magnetic field could be estimated. Computer simulation shows that under defined conditions a 3 dB instantaneous bandwidth of nearly 25 % at a center frequency of 35 GHz with a 60 cm long rf waveguide could be obtained.     

Intra- and extra-cavity spectral broadening and continuum generation at 1.5 μm using compact low-energy femtosecond Cr:YAG laser

Spectral broadening mechanisms at 1.5 m in a Kerr-lens mode-locked Cr⁴⁺:YAG laser and in a tapered silica fiber have been analyzed. It is shown theoretically and experimentally that both approaches allow generation of coherent smooth spectra with up to 400-nm bandwidth, as required for optical coherence tomography and other applications. The necessary power levels of 200 pJ are accessible to the directly diode-pumped femtosecond Cr⁴⁺:YAG lasers. The spectral broadening in the tapered silica fiber at 1.5 m is different from that at 800 nm, because of the compensation of the Raman shift by the third-order dispersion at 1.5 m. The existence of the optimum taper-waist length is predicted and explained. PACS 42.65.Tg; 42.81.Dp     

VSWR reduction for large millimeter-wave cassegrain radiotelescopes

Multiple reflections in large radiotelescopes used for astronomical spectroscopy cause characteristic modulations of the observed spectrum (baseline ripple). For a given mechanism, the magnitude of the effect depends primarily on the reflection coefficient, which for the most important paths is proportional to . Although ripple is thus generally negligible at mm-wavelengths, there are some instances where it may still be significant, and a recent experiment at the15 m James Clerk Maxwell Telescope was severely affected. We describe a technique which has proved effective in reducing the ripple to an acceptable level.The purpose of this research note is to draw this technique to the attention of the astronomical community. We calculate the reflection coefficient for a typical instrument and show that it can be greatly reduced over a very broad bandwidth by means of a tapered absorber in the centre of the secondary mirror. The graphical analysis of the tapered absorber also demonstrates that the scattering cones and circular absorbers widely used in radioastronomy donot in fact reduce the reflection coefficient significantly.     

Microwave dielectric properties of Ba(Zn<Subscript>1/3</Subscript>Ta<Subscript>2 / 3</Subscript>)O<Subscript>3</Subscript> for application in high power waveguide window

Higher dielectric constant, low dielectric loss and good transmission characteristicshave been the goal for developing the ceramic waveguide window for high power windowapplications. The choice of materials having high k with low dielectric lossand reduced window size is key parameters to achieve maximum microwave transmissionwithout unleashing microwave dissipation. The microwave dielectric properties ofsynthesized Ba(Zn_1/3Ta_{2 /3})O₃ (BZT) ceramics have been studied for high power windowapplications. The structural studies are correlated with microwave dielectric propertiesof BZT. The maximum values of dielectric constant ?_r =30, Q ×f₀ = 102 THz and near zero temperaturecoefficient of resonance frequency were obtained for BZT ceramics sintered at thetemperature of 1550 °Cfor 4 h. The measured results are used to design a tapered transition from air filledwaveguide to narrow (reduced width and height) dielectric filled waveguide using Heckenslinear taper at a specific frequency. The simulation result shows that the lowerreflection loss is obtained for the tapered transition of the narrow BZT window ascompared to the standard waveguide BZT window. The return loss of –34 dB is obtained forS-bandwaveguide window with a bandwidth of 675 MHz. The return loss observed in the narrow BZTwindow is –46 dB with a bandwidth of 570 MHz at a center frequency of 3.63 GHz. Most ofthe disadvantages in conventional windows will be rectified using the design of the tapertransion employing narrow waveguide window in high power applications.     

Analysis and simulation on the performance of a gyro-penitron amplifier

A cylindrical waveguide gyro-peniotron amplifier is analysed theoretically in this paper. By a ballistic method and directly using the energy conservation in the beam-wave system, a set of interaction equations for the cylindrical waveguide gyro-peniotron amplifier is derived. Computer simulation shows that under defined conditions an interaction efficiency of 47%, an output power of 240kW at a gain of 22. 5 dB and a 3 dB-instantaneous bandwidth of 1. 9% for a central frequency of 35GHz, TE₀₃ mode gyro-peniotron amplifier could be achieved. The influence of the operation parameters on the amplifier performance is also presented.     

Three‐dimensional ultra‐broadband integrated tapered mode multiplexers

The demonstration of a three‐dimensional tapered mode‐selective coupler in a photonic chip is reported. This waveguide‐based, ultra‐broadband mode multiplexer was fabricated using the femtosecond laser direct‐write technique in a boro‐aluminosilicate glass chip. A three‐core coupler has been shown to enable the multiplexing of the LP₀₁, LP and LP spatial modes of a multimode waveguide, across an extremely wide bandwidth exceeding 400 nm, with low loss, high mode extinction ratios and negligible mode crosstalk. Linear cascades of such devices on a single photonic chip have the potential to become a definitive technology in the realization of broadband mode‐division multiplexing for increasing optical fiber capacity.     

High performance of 1.55 μm DFB integrated with vertically tapered self-aligned spot-size converter

A new type of self-aligned spotsize converter (SSC) integrated 1.55 m DFB lasers had been proposed in this article. The upper optical confinement layer and the butt-coupled tapered thickness waveguide were regrown simultaneously, which not only offered the separate optimization of the active region and the integrated SSC, but also reduced the difficulty of the butt-joint selective regrowth. The vertical and horizontal far field angles were 9° and 12° respectively, the 1- dB misalignment tolerance were both 3.6 and 3.4 m. The directed coupling efficiency to tapered single mode fiber was 48%.     

Optimization of the operational bandwidth in air-core photonic bandgap fibers for IR transmission

We determine the optimal inner glass ring thickness in infrared air-core photonic bandgap fibers (PBGFs) with 19-cell and 7-cell cores. For PBGFs with a 19-cell core, we find that an inner ring thickness of , where Λ is pitch, yields the widest operational bandwidth, which is nearly 6%. The operational bandwidth increases as the refractive index decreases from 2.8 to 2.0. For PBGFs with a 7-cell core, one needs to draw fiber with a ring thickness of less than to achieve a comparable operational bandwidth.     

Self-consistent simulation of cyclotron autoresonance maseramplifiers

A self-consistent, one-dimensional model of the cyclotron autoresonance maser (CARM) amplifier is developed, and numerical simulations based on this model are described. Detailed study results of the CARM gain and efficiency for a wide range of initial energy and velocity spreads are presented. The interaction efficiency is found to be substantially increased when the axial magnetic field is tapered. Efficiencies of greater than 41% are obtained for a 140-GHz CARM amplifier with a tapered axial magnetic field and a 700-kV 4.5-A electron beam with parallel velocity spreads of less than 1%. A discussion of the nonlinear bandwidth and interaction sensitivity to axial-field inhomogeneities is presented     

Effect of nonuniform mechanical stresses on the domain structure of iron borate

A magnetooptic method is used to study the effect of nonuniform radial mechanical stresses on the domain structure, magnetic susceptibility, and magnetic hysteresis loops of a FeBO₃ single crystal. When a magnetic field is applied in the basal plane of FeBO₃ along the stress vector, a system of tapered domains appears in the crystal during magnetization. These domains exist in a certain temperature-dependent field range H₀ ≤ H ≤ H _c . The appearance of a system of tapered domains is found to substantially affect the technical magnetization of a stressed crystal. The results obtained are discussed within the thermodynamic theory of a domain structure. A theoretical model used is shown to adequately describe the experimental temperature and field dependences of the ratio \({D \mathord{\left/ {\vphantom {D {\sqrt L }}} \right. \kern-\nulldelimiterspace} {\sqrt L }}\) (where D and L are the average width and length of a tapered domain, respectively). The calculated value of D is approximately 1.3 times smaller than the experimentally observed domain width.     

Determination of velocity distribution of metallic atoms in plasma devices with a rapid-frequency-scan dye laser

A rapid-frequency-scan dye laser was developed for the measurement of the velocity-distribution functions of impurity atoms sputtered due to the high-temperature plasma-surface interaction. In order to confirm the effectiveness of this rapid-scan technique, measurements of the Doppler profiles of Fe atoms sputtered by Ar ion-beam bombardment with energy 3 keV were performed. A scanning range of more than 40 pm at 300 nm was obtained with a spectral bandwidth of 1.5 2 pm and good linearity. Doppler profiles were determined in various scattering conditions, and excellent performance of this technique was demonstrated. The distortion error caused by line saturation was analytically discussed, and it is shown that the effect of this error is not so serious (less than 10%) even for highly saturated excitation.     

Simulation of femtosecond pulse propagation in sub-micron diameter tapered fibers

Ultrashort pulse propagation and supercontinuum generation in tapered and microstructured optical fibers is usually simulated using the corrected nonlinear Schrödinger equation. One of the underlying approximations is the use of a wavelength-independent effective area or, equivalently, of a constant nonlinear coefficient . In very thin waveguide structures with strong light confinement, including silica wires and sub-micron tapered fibers and some microstructured fibers, the validity of such an approximation comes into question. In this paper we present an improved model in which all modal properties are fully taken into account as functions of the wavelength. We use comparative numerical simulation to identify certain regimes in which an improved model is needed for quantitatively correct results. PACS 02.70.Hm; 02.60.Cb; 42.65.Wi; 42.65.Re; 42.81.Pp     

Solid-state tapered optical fibre devices

In this paper we report two novel evanescent-field devices based on tapered singlemode optical fibres. In both devices single-mode optical fibre is tapered to waist diameters of around 2 m such that transmission remains close to 100%. In the first device successive coating layers of sol-gel containing fluorescent material are baked on. The major application is in the field of sensing through changes in the absorption and fluorescence characteristics. In the second device, the taper is immersed in a solution of poly(vinyl acetate), also containing fluorescent material. This solidifies, creating a solid-state device. Applications are miniaturized amplifiers and lasers.     

W-Band Second-Harmonic Gyrotron Traveling Wave Amplifier with Distributed-Loss and Severed Structures

The second harmonic TE₀₂ gyrotron traveling wave amplifier (gyro-TWT) is a high-power, broadband, millimeter-wave amplifier with a low applied magnetic field. Mode-selective interaction circuits were applied to suppressing spurious oscillations. However, the mode-selective interaction circuit may perturb the operating mode in the gyro-TWT. A multi-stage gyro-TWT design with distributed-loss and severed structures is proposed to stabilize the amplification. This study presents a nonlinear analysis of typical oscillations, including absolute instability, gyrotron backward oscillation (gyro-BWO) and reflective oscillation. The lossy and severed sections of the multi-stage gyro-TWT seem to increase effectively the start-oscillation currents of the absolute instability, gyro-BWO, and reflection oscillation. The multi-stage gyro-TWT is predicted to yield a peak output power of 215 kW at 89.9 GHz with an efficiency of 14.3 %, a saturated gain of 60 dB and a bandwidth of 1.7 GHz for a 100 kV, 15 A electron beam with an axial velocity spread _z/ _z = 5%.     

Theoretical investigation of noncollinear phase-matched parametric four-photon amplification of ultrashort light pulses in isotropic media

The amplification of light signals (angular frequency _S in some isotropic media (D₂O, fused silica, and Schott type SF10 glasses) by noncollinear phase-matched parametric four-photon interaction ₁+_2S+₁ is studied theoretically. Computer simulations are carried out for fundamental and second-harmonic pump pulses of a mode-locked Nd: glass laser. Degenerate interaction (wavelength ₁=₂=1054nm or 527 nm) and nondegenerate interaction (₁=1054nm, ₂=527 nm are considered. Characteristic phase-matching parameters and gain parameters versus wavelength are determined. Limitations by spectral bandwidth, optical absorption, optical damage, self-phase modulation, self-focusing and stimulated Raman scattering are analysed.     

Frequency stability of an optically pumped cesium beam frequency standard

Results obtained in an experimental optically pumped cesium beam frequency standard in which a single semiconductor laser is used for the state selection and the atom detection are reported. The separation between the two interaction regions is equal to 21 cm. This gives a 500 Hz linewidth which is observed with a signal to noise ratio equal to 10,000 in a 1 Hz noise bandwidth. A quartz crystal oscillator is frequency controlled by the atomic transition. The measured short term frequency stability is given by _y()=2×10^–12–1/2 for 1 s<500 s. Prospects for improvement of this frequency stability are discussed.     

Detailed analysis of active mode-locking

We extend earlier work on the theory of active mode-locking in a laser with a very long gain recovery time and obtain approximate closed-form solutions. We show how the results can be reduced to the well-known Kuizenga and Siegman formulae in the limit of small modulation depth and large laser bandwidth.We also discuss the physical relevance of the cavity supermodes in determining the stability properties of the mode-locked laser. We show that when the modulation depth is too small or the bandwidth too large, different supermodes have similar energies and we argue that under these circumstances, the laser will not be able to sustain mode-locked operation.     

New results for near-millimeter wave isolators and phase shifters based on magnetoplasmons on GaAs substrates

Results are presented for planar insulated image guide structures modelling surface magnetoplasmon based non-reciprocal devices for the near-millimeter wave range. Sample results using GaAs substrates show acceptable performance for isolators over a bandwidth of 65 GHz in the 400 GHz range and differential phase shifters over a bandwidth of 30 GHz in the 550 GHz range with a 10% phase shift variation.This work was supported in part by the U.S. Navy, Office of Naval Research, under contract N-00014-80-C-0982.     

Quantum-limited FM-spectroscopy with a lead-salt diode laser

Ultrasensitive absorption spectroscopy of NO₂ was performed with a tunable lead-salt diode laser (TDL) using a single-tone high-frequency modulation (FM) technique. With a detection bandwidth of 200 kHz, an optical density of 2.7 × 10^–5 was detectable at SNR of 1. The detectable optical density could be further improved by reducing the detection bandwidth in agreement with the f relationship, reaching 2.5 × 10^–6 at a detection bandwidth of 1.56 kHz. Normalized to 1 Hz bandwidth, the demonstrated performance would then correspond to a detectable optical density of 5.9 × 10^–8. This detection limit agrees well with the calculated quantum limited performance based on the measured laser power, modulation index, noise figure of the electronic components, and other parameters of the apparatus. These measurements and calculations show that by implementation of the FM technique, the sensitivity of the present TDL absorption spectrometers (TDLAS) can be improved by at least a factor of 10 and possibly even of 100. Such a sensitivity improvement would greatly extend the applicability of TDLAS for trace gas analysis, especially in atmospheric monitoring.     

On the Possibility of a Considerable Frequency Shift for ``Whispering Gallery' Waves with the Use of Sound

An analysis of the propagation of whispering gallery waves in a dynamic cavity was carried out. The cavity was formed by a tapered quartz rod and the plane interface of two regions of the rod with different refractive indexes moving along its axis. The maximum frequency shift of light in such a cavity was shown to be determined by its Q factor and an attainable jump n. The possibility of using acoustic waves to realize the dynamic cavity was considered.