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This work is concerned with the analysis of the signals which can be recovered in a phase-sensitive detector after either transmission through or reflection from a Fabry–Perot interferometer of a phase-modulated light. The frequency responses are obtained and compared, the effects of amplitude modulation which result in offsets and line shape distortions are analyzed, the conditions to reduce the in-loop frequency noise are examined taking into account the electronic noise, when the source is locked to the cavity by means of a high gain control system.

We present an analysis of the Fabry–Perot response to a phase‐modulated light in the reflection mode, by considering the general problem of the lock‐in detection at the pth harmonics of the rf modulating frequency. Suitable frequency modulation conditions for servo‐locking purposes are obtained and the values of modulation index which maximize the sensitivity for the first, third, and fifth harmonics are found. Moreover, we investigate the effects of the residual amplitude modulation introduced by the electro‐optic frequency modulator, the presence of laser amplitude and frequency noise, and the dependence of the achievable closed‐loop frequency fluctuation spectrum on the modulation index and detection noise.

The study of the evolution of a two-level atom crossing an electromagnetic field with a sinusoidally shaped amplitude is of interest in frequency metrology because a few Cs beam standards share this type of realization. The Magnus expansion method helps to find a solution to this problem giving approximate analytical results, which are made easy to handle by a series expansion in the detuning between atom resonance and radiation frequency. Numerical checks show that rather simple expressions provide an accuracy better than 10−3 in the matrix elements of the evolution operator for useful ranges of power levels and frequency differences. Two examples concerned with Cs beam standards are discussed.

We report on the absolute frequency stabilization of Yb-Er:glass lasers against different acetylene absorption lines in the 1.5 /spl mu/m spectral region. An external frequency modulation technique was adopted to independently stabilize two diode-pumped erbium microlasers against the P(15) rovibrational line at 1534.099 nm. A long-term frequency stability below 10/sup -9/ (200 kHz) was measured on both the closed-loop error signal and on the beat note frequency. From the beat signal analysis, a 1 ms laser linewidth of 50 kHz was observed. The Allan standard deviation of the beat frequency was below 10/sup -10/ for integration times between 10 ms and 1 s and as low as 4/spl times/10/sup -11/ around 0.1 s.

An External-Cavity Laser Diode was frequency locked on molecular absorption line R(3) of hydrogen iodide at 1.541 μm using the wavelength modulation technique. This molecular reference is very important in many fields like HD-WDW telecommunication system and interferometry. Preliminary measurements of the closed-loop error signal provided a relative frequency stability of 9.5 times 10-10 in a 160 Hz acquisition bandwidth.

Frequency-modulation spectroscopy of the CH3D molecule is performed to assess a novel optical frequency standard at 1.54- m wavelength. Numerical integration of the demodulated first derivative signals provides the Voigt absorption profiles for the R(6,3) transition at 1543.78 nm at different gas pressures. Deconvolution with the Gaussian (Doppler) line provides the corresponding Lorentzian (collisional) line as a function of the gas pressure. Measurements of the collisional broadening contribution, at three different gas pressures, provide a collisional broadening coefficient of 37 MHz/kPa for the R(6,3) line.

An analysis of the transition probability for an atomic beam excited by resonant microwave radiation in a TE/sub 01n/ cavity fed in the center is presented. Results of the numerical integration of the Schrodinger equation and an analytical approximation obtained by means of a Magnus expansion are given. The agreement is satisfactory up to the n = 7 case, which is the highest-order examined. No asymmetry was assumed in the center feed, mainly because its effect can be shown to be very small for a cylindrical cavity, since the TE/sub 01n/ mode in that kind of structure has very low losses on the side walls. The analytical and numerical results support the idea that a Ramsey cavity is not always the only solution for guiding microwaves in a beam standard. >

Summary form only given. In this paper we summarize some results of a rather general analysis of the relevant properties of the lock-in detectable signal. when phase (frequency) modulated light impinges on a photodetector after reflection off a Fabry Perot cavity (Pound-Drever-Hall topology). In particular we will consider the problem of the non-ideal behavior of the electro-optic phase modulator device.

The first results of optical pumping of the /sup 3/P/sub 1/ level in the excited metastable triplet of Mg are reported. A nearly total depletion of the above level has been achieved in a metastable atomic beam through resonant interaction with the /sup 3/S/sub 1/ (3s4s) level. For this purpose, a cavity-stabilized coumarin 6 ring dye laser has been used. The outlook for an optically pumped Mg frequency standard is also discussed on the basis of the results reported. >

We will describe a system of three heterodyne receivers. Its mixers are based on SIS tunnel junctions which allow a very sensible downconversion of the detectable signal from 94 GHz, 225 GHz and 345 GHz to 1.5 GHz. This will allow the detection of molecular rotational transition lines from diffuse molecular clouds. The current status of a 94 GHz receiver, prototype of MASTER, will also be described. Technical design and cryogenic problems solution will be shown and we will focus our attention on the optical coupling technique based on Gaussian beam analysis

Summary form only given. Frequency modulated laser light, when synchronously detected after reflection from a Fabry-Perot cavity, give rise to well known discrimination curves, if the source is tuned across the resonance. In lineshape calculations, the laser field is normally assumed to be monochromatic. Several different models describe the laser phase/frequency noise, namely phase diffusion model (PDM), FM or phase modulation from a band-limited Gaussian noise (GNM), and a generalized phase jump model (PJM). Our work deals with the analysis of the dependence of the static frequency response on the parameters pertinent to each model. Moreover, in the PSD case, approximate expressions for the intensity spectrum are given.

The frequency of the RI(32) CO2 line has been measured with respect to a 3.39 μm He22Ne laser stabilized to the F2 (2) P(7) transition in thev 3 band of CH4. The value obtained isv=29477160862±12 kHz, in good agreement with the only value reported in the literature. Using the available data of the RI(32)-RI(30) (difference) frequency, the following value can be derived for the RI(30) line: ν=29442483320±18 kHz.

In the first part of this paper the characteristics of frequency deviation and modulation bandwidth required for phase-locking optically pumped far infrared lasers to reference synthesized signals are examined. Then different fast modulation techniques suitable for this purpose are discussed pointing out their particular features. Experimental results concerning laser frequency modulation by Stark effect, injection locking and electrically variable reactances (e.g Schottky diodes, plasma cell) are reported and compared.

The absorptions of OsO4 in coincidence with CO2 laser emissions have very attractive metrological characteristics, as has been demonstrated by several high-resolution frequency measurements. We evaluate in this work the possibilities of realizing a frequency standard at 30 THz based on an OsO4 molecular beam with a Ramsey-type interaction technique which could allow an accuracy level of 10-14 or better with a high short-term stability. The possible interaction and detection techniques are discussed in order to evaluate the observed line-width and the signal-to-noise ratio, taking into account the molecular flux achievable with a multichannel source. The main uncertainty sources are also discussed.

The influence of microwave phase noise on rf‐induced constant voltage steps generated by the ac Josephson effect is studied. Different circuit models for the junction excitation are considered: dc and rf voltage generators, dc and rf current generators, dc current and rf voltage generators. The analysis has been carried out assuming a power and band limited phase noise modulation in the first two cases, while in the last one a white frequency modulation spectrum is required for a treatment based on the Fokker‐Planck equation. Some experimental results are reported, in order to display the basic phenomenological behavior.

The absorptions of RuO4 coincident with12C16O2 and14N216O laser lines have been observed with sub-Doppler resolution and their frequency distances from the laser line-centers have been measured. These preliminary results suggest a possible extension of the absolute reference frequency grid up to the 10.9 μm region, based on transitions of hyperfine-structure free heavy molecules.

An analysis of abrupt-junction millimetric varactor doublers using Schottky diodes is performed and numerically implemented to evaluate conditions of maximum output power. This power level, the efficiency, and the circuit parameters have been derived as a function of the geometrical and physical parameters of the junction. Physical phenomena which allow the application of the model up to the plasma resonance frequency in epilayer are taken into account. Comparison of available experimental data with the theory developed is repotted.

The phase lock of backward-wave-oscillators operating in the submillimeter range requires the reconsideration of some theoretical aspects in order to take into account the high operation frequency and the high flicker noise level of these sources. Experimental data referring to the frequency noise spectrum of a 600-GHz carcinotron are reported together with some guidelines for properly designing the control phase-locked loop. The results achieved in a practical realization are presented and compared to the theoretical considerations developed. The experimental results demonstrate the possibility of achieving a satisfactory signal-to-noise ratio and narrow linewidth in the phase stabilization of a 600-GHz carcinotron. >

Great advances have been made in the recent years in the field of phase-coherent frequency synthesis into the infrared region of the electromagnetic spectrum. Most important are the improvements achieved in the phase noise of the reference oscillators, in the modulation and control capabilities of important intermediate oscillators such as optically pumped FIR lasers, and in general in the understanding of the problems to be dealt with, both of noise level and of multiplication efficiency. In this paper a short review is given of the progress occurred in the field in the last years, and a projection is attempted for the ultimate possibilities of the techniques presented. The importance of reaching the 30 THz region with a coherent signal is discussed in view of a coherent frequency scale from VHP to the infrared.

In high resolution atomic or molecular beam spectroscopy, where the Ramsey interrogation method is used, one of uncertainty sources in determining the resonance frequency accurately is the phase-shift of the electromagnetic radiation in the cavity. This phenomenon, which depends on losses and asymmetries, is analyzed in a general way for a transverse atomic beam dimension much larger than the transition wavelength, a case which occurs in Mg or Ca beam frequency standards and the effects of different misalignments in collimated or divergent beams are examined. Numerical evaluations have been performed in the special case of an experimental Mg beam frequency standard. Divergence plays an important role in determining the cavity phase shift frequency error which is reduced about 100 times with respect to the case of a collimated beam.

Experimental results of harmonic generation in the submillimetric wavelength region using carcinotrons as fundamental sources are reported. The multiplier is a commercial cross-waveguide structure equipped with a Schottky barrier diode operating as a non-linear resistance. Power levels useful for many spectroscopic applications can be obtained with these wideband tunable sources.

A nonlinear analysis of varactor frequency multipliers has been performed and numerically implemented to obtain the quantities that must be specified to define the quality of a multiplier used to generate high spectral purity signals for metrological purposes. The theoretical model has proved adequate to predict the existence of complicated hysteresis phenomena, confirmed by experimental investigation. The practical cases examined refer to abrupt-junction doublers and triplers operating in self-bias conditions. The circuit parameters have been determined for maximum efficiency at a given generator available power. Then, some effects of input power, frequency, and tuning variations have been investigated, and calculated curves including hysteresis cycles are shown. Beside the optimization of the multiplier operation, the equations given are sufficiently general to yield the information requested on multipliers of this kind, where the power handling capabilities of the varactor are not fully exploited but the high spectral purity of the input signals must be preserved. As an example, one of the most important features of the frequency multiplier, i.e., the AM-PM conversion, was determined in a practical case for varying input power levels.

Some experimental results of video detection in the submillimeter range with Schottky-barrier diodes are reported in the first part of this note and different mounts are compared. The dependence of video responsivity on the frequency for the devices examined seems to obey more likely an ƒ −4 law than the theoretically expected ƒ −2 . The second part concerns harmonic multiplication with Schottky diodes; experimental results are reported for 100 GHz multiplication up to the 6th harmonic.

High precision frequency measurements of the3P1-3P0 transitions have been performed for the three natural abundant Mg-isotopes in a metastable atomic beam. A second order hyperfine theory for two-electron atoms allows the determination of the centre-of-gravity for25Mg and then the evaluation of the isotopic shift displacements. The high resolution frequency measurements (10−8) show a noticeable departure of the total shift from a linear dependence with respect to the atomic mass.

In this paper the problem of phase stabilization of submillimetric backward wave oscillators (BWO's) is considered. Experimental results are given about the phase-lock of a 600 GHz BWO to a quartz crystal reference, via a frequency multiplier chain of high phase spectral purity. The spectral analysis of the locked BWO is performed both on the IF and on the submillimetric output signal.

We describe the realization of a source of Mg atoms in the metastable triplet 3s3p3P and we report a theoretical model describing the dynamic process of production of metastable atoms via electron impact excitation. Experimental results concerning atomic flux and velocity distribution are reported and compared with the theoretical model; the efficiency of production of metastable atoms was 40%, in good agreement with the computed value, and the velocity distribution showed a dependence from the discharge current close to the theoretical prediction.

In order to evaluate the spectral purity degradation produced by varactor doublers, the transfer properties of amplitude and phase fluctuations have been determined with reference to a theoretical analysis developed in a previous work. Quasi-stationary conditions have been assumed and the input and output tuning circuits have been approximated with a linear relation between frequency and reactance. The effect of the bias circuit has been taken into account both for fixed-bias and self-bias conditions. Then the additive-noise contribution of the doubler circuit elements have been obtained. Numerical examples and curves are given for all the evaluated quantities.

The interaction between a metastable atomic beam and a standing-wave radiation field is analyzed taking into account the effects of the natural decay between the interrogation zones and in the detection region; moreover the problem is concerned with atomic beams with cross section much wider than the radiation wavelength (multiwavelength cross section). The analysis is carried out in the density matrix formalism and numerical results obtained refer to the new frequency standards based on fine structure transitions within the Mg and Ca metastable triplets.

Advances towards the realization of an absolute frequency standard in the submillimetre region are reported. The 3P1 ↔ 3P0, Δmj = 0 fine-structure transition in the metastable triplet of 24Mg has been observed in an atomic beam with a Ramsey-type interrogation technique. Its frequency, measured with an accuracy of ± 3 × 10-11 with respect to a Cs standard, turned out to be ν0 = (601 277 157 860 ± 20) Hz.

Parametric oscillators with a superconducting output cavity are of great interest as sources of very high frequency stability. In this paper, a careful analysis is performed for a parametric oscillator model based on an abrupt junction varactor with fixed-bias voltage and ideal shunt mode operation. Steady-state operation, transfer functions for amplitude and phase fluctuations, and additive noise contributions are examined. An overall estimation of system performance as regards to the phase instabilities shows that although the foreseen characteristics are Iess outstanding than expected from a previously reported analysis, this device compares favorably with the most important available sources based on atomic transitions or on superconducting cavity-stabilized oscillators.