The traveling-wave phototube part I: Theoretical analysis

The traveling-wave phototube (TWP) is a broad-band photodetector which is useful for demodulation of light that has been amplitude-modulated at a microwave frequency, and for optical heterodyning of coherent light signals with microwave difference frequencies. In either application the light beam(s) results in the emission of a current-modulated electron beam from a photocathode; interaction between this beam and a slow-wave circuit such as a helix is employed for broad-band detection of the microwave modulation. Part I of this paper presents a detailed analysis of the TWP beam-circuit interaction process, including the effects of loss, space charge and asynchronism. Computer results are given for both current modulation and velocity modulation excitations, since the latter can be important under certain operating conditions. Conclusions regarding the effects of beam current, circuit length and circuit loss are drawn which are important in the design, understanding and use of TWP's. The results of this analysis are also generally applicable to other cases where a slow-wave circuit is excited by an electron beam. In Part II of this paper, a detailed experimental analysis of the TWP will show good agreement with the predictions of this theoretical treatment.     

Microwave phototube design considerations

The general behavior of the traveling-wave microwave phototube (TWP) as a broadband light demodulator has been described in previous publications. This paper presents specific theoretical and experimental analyses. It is shown that at low average currents the microwave power output of a TWP is given byP = frac{1}{2}i^{2} R_{eq}whereiis the peak value of the microwave current modulation at the cathode (i.e., the ac current) and Reqis an "equivalent interaction resistance" given byR_{eq} = pi^{2}N^{2}Z_{c}whereNis the helix length in electronic wavelengths and Zcis the longitudinal beam-circuit interaction impedance. Typical values of Reqare from 10⁵to 10⁷ohms. The bandwidth is determined by the productf^{2}Z_{c}, which can vary less than 3 db over an octave. The major noise contributions are found to be shot noise and thermal noise. The power output and signal-to-noise calculations are verified by experiments on an S-band TWP at low average current levels. Preliminary analysis of the TWP at high average currents shows that even higher values of Reqshould be obtainable. However, in that case the frequency-dependent microwave interactions in the gun region may provide a serious bandwidth limitation.     

Measurement of the shot-noise reduction factor

A new method of measurement of the shot-noise reduction factor at microwave frequencies is described. In this method, the reduction factor is measured by using a traveling-wave phototube with oxide-coated cathode, on which an amplitude-modulated laser light is projected. The photodetected outputs, with and without the space-charge cloud near the cathode, are compared. The shot-noise reduction factor is calculated from the ratio of both outputs. Measuring frequencies (1320 and 2640 MHz), beam current density, cathode temperature, and beam-forming-electrode voltage are varied in this measurement. The shot-noise reduction factor tends to small values at lower frequency, and goes to unity at higher frequency than the plasma frequency at the potential minimum. The experimental results agree in general with the theoretical calculations done by Whinnery, Tien, and Moshman. The physical meaning of these results is interpreted as a plasma oscillation at the potential minimum. Also, the relation between the "Currie gun" and the shot-noise reduction is discussed.     

A transverse wave phototube for detection of microwave-frequency-modulated light

This paper describes the theoretical and experimental investigation of a novel means of detecting microwave-frequency-modulated light signals. The system consists of an optical dispersing element followed by a photocathode and transverse-field interaction circuit. The disperser causes the light beam to swing back and forth in accordance with the instantaneous optical frequency, resulting in the generation of a transversely modulated electron beam at the photocathode, which is then detected by the circuit. Results obtained with an experimental FM phototube system confirm the analyses of the optical and electron beam mechanisms for a small modulation index. Improved versions of the FM phototube are suggested, and it is shown that the approach may represent a superior means of directly detecting optical communications or reconnaissance signals as modulation techniques improve. In particular, the FM phototube can in principle utilize high-index modulation to increase the signal-to-shot noise ratio to a value only three dB below that of an ideal heterodyne detector, while inherently discriminating against background light.     

Gigacycle bandwidth coherent light traveling-wave phase modulator

A continuous duty coherent light phase modulator has been constructed which exhibits a bandwidth in the kilomega-cycles. A modulation index of unity has been obtained with a modulation peak power of about 12 watts. This power level is independent of the bandwidth of the modulation. This is contrasted with known microwave amplitude light modulators which require peak modulation power in the kilowatts and exhibit bandwidths of the order of 30 Mc. The extremely wide bandwidth operation with the low modulation power is obtained by applying the modulation voltage to the electro-optic material by means of a traveling-wave structure of the transmission line type. The direction of propagation of the light and of the modulation are colinear and the transmission line is designed so that the light progresses through the crystal in synchronism with the modulation on the line. The magnitude of the angular modulation was determined by illuminating the modulator with a ruby laser and examining the individual spectral components of the output with an S-band traveling-wave microwave phototube receiver. Similar to the behavior of conventional FM systems, a minimum in the carrier at a modulation index of 3.75 was observed as well as the appearance of the appropriate sidebands. These experiments were performed at 30 Mc and 1 Gc. It appears that the bandwidth of the modulator should be several octaves greater than 1 Gc, but this was not determined experimentally for equipment reasons. The attenuation of the 1 Gc modulation through the modulator and the balun at either end was 2 db and the attenuation of the light through the electro-optic crystals was about 6 db. A good portion of this later loss undoubtedly occurred at the interfaces between crystals.     

Conversion Loss and Noise of Microwave and Millimeterwave Mixers: Part 1--Theory

An analysis is presented for the conversion loss and noise of microwave and millimeter-wave mixers. The analysis includes the effects of nonlinear capacitance, arbitrary embedding impedances, nonideality of microwave diodes, and shot, thermal, and scattering noise generated in the diode. Correlation of down-converted components of the time-varying shot noise is shown to explain the "anomalous" noise observed in millimeter-wave mixers. Part 1 of the paper presents the theoretical basis for predicting mixer performance, while Part 2 compares theoretical and experimental results for mixers operating at 87 and 115 GHz.     

Effect of electron lenses on beam noise

Some types or electron guns used to form the beam of a low-noise beam type microwave amplifier (e.g. traveling-wave tube) impress sharp potential discontinuities or velocity jumps on this beam. These discontinuities produce strong electrostatic lenses. This paper presents a theory supported by experimental results, showing that the effect of such lenses is to increase the beam noise. Further, axial magnetic fields are shown to reduce this undesirable effect.     

Noise in cathodoluminescence light

A theory is given of the noise in cathodoluminescence light and the results are compared with experiments on Sylvania 5BNP16 and RCA5WP11 cathode-ray tubes. The measurements indicate that the noise in cathodoluminescence is mainly caused by the shot noise of the primary beam. The noise intensity is proportional to the magnitude of the voltage applied to the tube for voltages above 10 kV. The spectral distribution of the noise corresponds to the square of the frequency response of the cathode-ray tube screen as expected theoretically. The application to light amplifiers is discussed.     

Potential-minimum noise in the microwave diode

An analysis of the potential-minimum region in the microwave diode is presented which predicts the amount of noise convection current at the potential minimum under conditions present in guns of low-noise traveling-wave tubes and klystrons. Contrary to previous work by one of the authors, this analysis partially includes the effects of finite transit angle between the cathode and potential minimum. The results indicate that under certain conditions of space-charge limitation, the noise convection current may be larger than full shot noise, whereas under other conditions reduction by as much as a factor of four may be obtained. When the results of this work are applied to the noise-figure theory of traveling-wave tubes and klystrons, it is found that their minimum obtainable noise figure may be in the vicinity of 4 db rather than the previously predicted 6 db with reasonable cathode current densities. For very large current densities or low operating frequencies, the present theory indicates that there is no theoretical limit to the minimum noise figure of such devices, in agreement with the previous work.     

A proposed FM phototube for demodulating microwave-frequency-modulated light signals

A microwave phototube for demodulating frequency-modulated light signals is proposed. The demodulation is based upon the conversion of the frequency-modulated light into space-modulated light via an optical dispersing element. This space-modulated light is then incident on a photocathode where it is the source of transverse electron beam waves. A complete spectral analysis of the demodulation process is presented. It is shown that a quasi-steady-state viewpoint, i. e., that of an optical signal with slowly varying frequency is permissible only if the optical resolution is sufficiently low. Design parameters for a phototube based on the use of a Michelson echelon are presented. A related scheme employing a Fabry-Perot etalon is also discussed.     

Gyrotron-TWT Operating Characteristics

The gyrotron traveling-wave tube (gyro-TWT) is a new type of millimeter amplifier which employs the electron cyclotron maser instability as a basis for the electron-electromagnetic wave interaction. A C-band gyro-TWT, employing the fundarnental cyclotron resonance interaction with the circularly polarized TE/sub 11/ dominant waveguide mode, has been constructed and tested. Initial power measurements yielded an output power of 50 kW at 60-kV beam voltage with 16.6-percent efficiency and 6-percent bandwidth. These measurements were recorded with a flat magnetic field. Subsequent experimental testing yieIded, for a magnetic field increasing in magnitude towards the output portion of the tube, 128-kW and 65-kV beam voltage at 24-percent electronic efficiency. The maximum efficiency was 26 percent at 120.5-kW peak power, with an instantaneous bandwidth of 7.25 percent as measured in a high-beam power mode (65 kV, 7 A). In the low-beam power mode (40 kV, 4 A), the efficiency was 9.8 percent at 18.8-kW peak power at 9.3-percent instantaneous bandwidth. Additional experimental results of AM and PM modulation coefficients, spectral purity, phase Iinearity, and noise figure are presented.     

A solution to the problem of spontaneous deflection of the beam in TWT's with low helix voltage

The spontaneous deflection of the beam in low voltage, typically low noise, traveling-wave tubes has been studied and attributed to the accumulation of stray charges on the glazed helix rods. A means of measuring the tendency of the helix to show these charging effects will be described. A method of processing of the complete helix which prevents the accumulation of charges but does not seriously affect its cold loss or other microwave properties will be reviewed. Traveling-wave tubes that have been made with helices processed by this technique cannot be made to show charging effects or beam deflections. Six tubes have accumulated 22,000 tube hours with no failures.     

Carbon disulfide traveling-wave Kerr cells

Carbon disulfide has identical microwave and optical dielectric constants, as well as extremely low optical and microwave loss. These properties make it possible to construct long traveling-wave fight modulators at microwave frequencies using the Kerr electrooptic effect induced in CS2by an electric field propagating on a TEM transmission line. Several experiments with traveling-wave Kerr cells consisting of resonant strip transmission lines immersed in CS2are described. A decrease in the microwave power required for modulation by a factor of two, by cooling the modulators to a temperature of -55°C, is demonstrated. Simultaneous modulation of light at two microwave frequencies by excitation of two of the longitudinal modes of the strip line resonator is also described. Relatively high efficiency modulation with long devices of this type is also reported. In these experiments, the microwave power required for large depths of modulation is reduced by almost two orders of magnitude compared to previously reported CS2light modulators, and is within less than a factor of two of the calculated power for cells up to 44 cm in length. For longer cells, increasingly larger than predicted powers are required.     

Traveling-wave tube gain fluctuations with frequency

Periodic traveling-wave tube gain fluctuations with frequency are described in terms of fundamental tube parameters and readily measurable quantities using the simplified one-wave theory of Pierce. An analysis of the feedback existing in a traveling-wave amplifier due to mismatches in the discontinuity regions is presented. Thus, the form, periodicity, and magnitude of these fluctuations and their dependence upon the beam voltage are quantitatively determined. The microwave structure is analyzed from a network point of view and the circuit parameters are defined with mismatches in the lumped center attenuator and at the ends of the slow-wave transmission line. The gain fluctuations are related to the cold circuit parameters and standing wave measurements. Pertinent experimental data, indicating the correlation of gain fluctuations to vswr variations, are included, in addition to an elementary discussion of the distortion effects possible due to such periodic gain variations in a traveling-wave tube.     

Picosecond pulse response of broad-band guided-wave interferometric light modulators

Picosecond pulse operation of the guided-wave light modulator is reported in this paper. First, we analyze the time responses of phase and intensity modulators of the traveling-wave type, and the modulated output waveforms related to the modulating pulses are discussed. It is shown that there is an optimum interaction (or electrode) length of the modulator whereby the drive voltage is minimized for a desired output pulse width. The traveling-wave push-pull light intensity modulator of Ti-diffused LiNbO3waveguides was modulated by a pulse train of 1 GHz repetition frequency, and the modulated output was observed by the image tube streak camera modified for sinusoidal scan at the same frequency. The duration of the modulated light pulses was less than 45 ps, which shows good agreement with the predicted one.     

An experimental photomixer image tube

An experimental optical mixer for a superheterodyne receiver has been constructed for the detection of beats in the 2-4 Gc region. This phototube incorporates an image dissector with a traveling-wave tube helix structure. It can be used to search an image field by electronic scanning for optical beats. Construction details and test results are presented.     

A Traveling-Wave Electron Deflection System

Several types of traveling-wave electron deflection structures that can be used in microwave oscillographs are described and compared. An interaction structure consisting of a folded wire over a plane is considered in detail, both theoretically and experimentally. A general analysis of the interaction of electrons with sinusoidally varying transverse electric fields is presented and is applied to traveling-wave deflection systems. This analysis gives quantitative information about the interdependence of deflection and drift space lengths, beam velocities, frequencies and phase velocities along the structure. Limitations on the design and performance of traveling-wave deflection systems can be determined from this analysis.     

Analytical model of shot noise in double-barrier resonant-tunnelingdiodes

The shot noise in double-barrier diodes is analyzed using the stationary-state approach to resonant tunneling through the first quasi-bound level. Significant deviations from full shot noise are predicted. Significant shot noise suppression occurs in the entire positive differential resistance region below the current peak, and shot noise enhancement occurs in the negative differential resistance region above the peak. The physical basis for these effects is assumed to be the modulation of the double-barrier transmission probability by charge stored in the first quasi-bound level in the quantum well. The analysis confirms microwave noise measurements of high-speed double-barrier diodes     

Mixing and detection of coherent light in a bulk photoconductor

The use of a bulk photoconductor as a detector for microwave-modulated light or as a mixer to detect the beat frequency between coherent optical sources is studied. A model for the photoconductor as a parametric conductance is presented and used in the analysis of a dc-biased detector and an ac-biased mixer-detector arrangement. The results of experimental measurements on both dc- and ac-biased detectors are presented and found to be in good agreement with the theoretically predicted performance. The bulk photoconductor is compared with the traveling-wave phototube and the PIN photodiode in both signal power and SNR and is found to compare favorably with these devices if the semiconductor parameters can be reasonably optimized and if it is biased with fairly high fields. The desired semiconductor properties are that the lifetime should be on the order of the reciprocal of the angular frequency of modulation, the mobility should be high, and the dark conductance of the sample should be very small.     

Parametric Simulation and Optimization of Cold-test Properties for a 220 GHz Broadband Folded Waveguide Traveling-wave Tube

Characterized with full-metal structure, high output power and broad bandwidth, microfabricated folded waveguide is considered as a robust slow-wave structure for millimeter wave traveling-wave tubes. In this paper, cold-test (without considering the real electron beam) properties were studied and optimized by 3D simulation on slow-wave structure, for designing a 220 GHz folded waveguide traveling-wave tube. The parametric analysis on cold-test properties, i.e., phase velocity, beam-wave interaction impedance and cold circuit attenuation, were conducted in half-period circuit with high frequency structure simulator, assisted by analytical model and equivalent circuit model. Through detailed parametric analyses, interference between specified structural parameters is found on determining beam-wave interaction impedance. A discretized matrix optimization for interaction impedance was effectively carried out to overcome the interference. A range of structural parameters with optimized interaction impedance distributions were obtained. Based on the optimized results, a broadband folded waveguide with cold pass-band of about 80 GHz, flat phase velocity dispersion and fairly high interaction impedance was designed for a 220 GHz central frequency traveling-wave tube. A three-dB bandwidth of 20.5 GHz and a maximum gain of 21.2 dB were predicted by small signal analysis for a 28 mm-long lossy circuit.