Gigacycle-bandwidth coherent-light traveling-wave amplitude modulator

We have constructed an optical amplitude modulator having a bandwidth of approximately one Gc/s and requiring about a fifty volts drive to produce a 100 per cent amplitude modulation. The wide-bandwidth low drive power performance is obtained by using the traveling-wave approach previously used in the gigacycle bandwidth phase modulator. The electro-optic material is KDP cut in the same orientation used in the phase modulator and combined with other optical elements to produce retardation-amplitude modulation. Temperature variations through the KDP produce a deflection and decollimation of the output beam. Experiments have been conducted to determine the magnitude of these effects. Corrective measures have been devised. Comparatively good agreement between the computed and the measured bandwidth has been obtained.     

X-Band Ladder-Line Traveling-Wave Maser

An X-band ruby traveling-wave maser (TWM) has been constructed and tested in which the RF structure is a double-ridge ladder line and the signal is coupled into and out of the structure with coaxial lines. The pump power is propagated in a waveguide mode and the device is operated at and below liquid helium temperatures. A four-inch electromagnet was used. The TWM was operated at the push-pull point with a pump frequency of 24 Gc and a signal frequency of approximately 9.65 Gc; the magnetic field was 4.1 kilogauss. Electronic gains of 15 db have been obtained and bandwidths as high as 130 Mc were observed. Reduced structure losses and longer sections of ruby promise a G/sup 1/2/B of 1100 Mc (30 db over 35 Mc).     

A wide-band microwave light modulator

An experimental potassium dihydrogen phosphate (KDP) light modulator with a 10 percent (half-m) bandwidth at 6 GHz is described. When arranged for amplitude modulation, a modulation depth of 40 percent was obtained with 10 watts of input power at band center. With a different optical arrangement, phase modulation is also possible. The modulator uses a ring-plane traveling-wave circuit with cylindrical KDP crystals filling the space inside the rings. The crystals are used in the longitudinal mode, i.e., with the light beam along the optic axis, thus avoiding some of the thermal problems associated with transverse node operation. The circuit provides adequate cooling for the crystals and CW or pulsed operation at average RF levels of 10 watts is possible. Measurements indicate that the limitation in bandwidth is due mainly to the dispersion of the circuit and that with suitable modifications bandwidths of 20 percent are quite feasible.     

Attenuation Constant of Lunar Line and T-Septate Lunar Line

The attenuation constant /spl alpha/ of the lunar line and that of the T-septate lunar line were derived from the average power loss W/sub L/ and the average power transfer W/sub T/ in each line, that is the ratio, W/sub L//2W/sub T/. The average power loss and the average power transfer for the lunar line and for the T-septate lunar line were derived from their respective field functions. The theoretical attenuation constant of a typical lunar line is less than 0.7 db/ 100 ft for frequencies greater than 2000 Mc. The theoretical attenuation constant of a typical T-septate line is less than 0.9 db/ 100 f t for frequencies greater than 1000 Mc. Experimental measurements of the attenuation constant of a T-septate lunar line agree with the theoretical value. In the 200 to 2000 Mc frequency band, the lunar line and the T-septate lunar line offer a compact and light package without an appreciable sacrifice in peak power handling capacity or attenuation.     

The traveling-wave phototube part II: Experimental analysis

This paper reports a detailed experimental investigation of the traveling-wave phototube (TWP) as a broad-band demodulator of microwave-modulated light. As Part II of a theoretical and experimental analysis of the interaction between a density-modulated photoelectron beam and a slow-wave circuit, it examines the output power, bandwidth and voltage modewidth of the traveling-wave phototube as a function of photocurrent, beam voltage and interaction length. Three types of experiments are described: 1) direct demodulation of light, amplitude-modulated at 3 Gc by a cavity-type modulator, 2) measurement of the microwave shot noise from photoelectron beams initiated by light from both coherent and incoherent sources and 3) comparison of the previous two measurements with the microwave shot noise of a thermionic electron beam. Comparisons of the results of these experiments with the predictions of the theory of Part I are drawn, showing good agreement. In particular, a number of new effects which are predicted by the detailed analysis of Part I, and which had not been previously observed or predicted are reported.     

Broad-band multisection electrooptic modulators

A general solution has been derived for expressing the conversion of power from a base frequency to a modulation sideband using a multisection electrooptic modulator. The objective is to obtain broadband frequency modulation of a laser by the use of multiple collinear electrooptic crystals with microwave power levels well below the breakdown threshold. It is found that segmented structures lead to greater bandwidths, which increase with the number of modular sections. This is achieved by adjusting the phases of microwaves between sections to maximize the single-sideband conversion efficiency. It is shown that a 10.6-μm CO₂ laser modulator with six geometrically identical CdTe sections can potentially achieve a 3-dB bandwidth of nearly 6 GHz using a 10-kW traveling-wave tube operating at 16-GHz center frequency     

10 GHz bandwidth traveling-wave LiNbO3optical waveguide modulator

A broad-band traveling-wave electrooptic modulator of Ti-diffused LiNbO3optical waveguide was designed, constructed, and tested at 0.63 μm over a bandwidth of 10.5 GHz. Using a novel asymmetric electrode configuration as a microwave waveguide, practically smooth frequency response of the modulator was obtained. For 300 mW drive power, the phase modulation index was 1 rad at 2 GHz and 0.53 rad at 10 GHz, while the intensity modulation index was 64 and 34 percent, respectively.     

Microwave Variable Attenuators and Modulators Using PIN Diodes

The PIN diode is a double diffused junction with an intrinsic layer separating the P and N regions. At frequencies above 100 Mc, the diode ceases to be a rectifier because of carrier storage and transit time effects. Its shunt capacitance is quite small because of the separation of the P and N regions by the I layer. Conductivity of the I region can be varied by a dc bias current and the device becomes an electrically variable resistor which can be used for microwave attenuators and modulators up to frequencies as high as 20 Gc. The PIN junctions are mounted on posts which are inserted in a 50-ohm strip transmission line as shunt elements, and a number of these elements, spaced a quarter wavelength apart at midband, are used to form an attenuator. At the appropriate bias current, yielding 50-ohm junction resistances, the diode elements are reactively compensated by choice of post dimensions so that they are effectively pure resistances, yielding an image attenuation of 4.2 db per element. Many elements can be used to attain any desired total attenuation and higher impedance end elements can be used to improve the SWR. Bandwidths of 4 to 1 with low SWR in both ON and OFF conditions are achievable. Maximum attenuation of 60 db, insertion loss of 1 db, and SWR of 1.5 are typical for a 12-diode attenuator and powers of the order of watts can be handled with negligible harmonic generation. When used as a pulse modulator, rise times of the order of 10 nsec are achievable.     

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.     

Wide-band microwave light modulation

A method for obtaining extremely wide-band light modulation by traveling-wave interaction in electro-optic or magneto-optic materials is described. When the intrinsic velocity of microwaves in the material is appreciably less than that of light, the synchronism conditions require the light to be directed obliquely with respect to the wave vector of the microwave field. Structures are described which satisfy this condition, and in which the microwave field is largely concentrated in the electro-optic medium, resulting in very little dispersion and economical use of microwave power. The use of the linear electro-optic effect in potassium dihydrogen phosphate (KDP) in such applications is considered.     

Improvements in Light Modulators of the Traveling-Wave Type

Wide-band modulation of light by means of the electro-optic effect requires a traveling-wave type of interaction, with the modulation field traveling with the same phase velocity as the light in some suitably proportioned structure. If electro-optic material is lossy at the modulation frequencies, the modulating field is strongly attenuated with a resultant low-modulation efficiency. A scheme is analyzed here in which power is continuously fed into the light-carrying guide to make up for the attenuation as the wave progresses down the guide. By suitably tapering the coupling and the uncoupled propagation constant, the electric field can be maintained constant in the light-carrying guide and the "coupled" propagation constant in this guide can be maintained in synchronism with the light wave, thereby increasing the modulation efficiency.     

Theory of TEM Diode Switching

The theory of TEM diode switching is presented for purpose of understanding and designing TEM microwave diode switches. A few experimental results are reported for the purpose of supporting the theory and demonstrating the exceptional bandwidth possible. An analysis is given of the switching action of one and of two or more diodes as well as the biasing of the center conductor of a TEM transmission line over broad-frequency bandwidths without interacting the RF signal. The use of point-contact germanium, and gold-bonded germanium diodes for TEM switching is discussed. Some considerations of switching speed and maximum power-handling capacity are given. A coaxial transmission line switch has been constructed in which two gold-bonded diodes provide 26-db or greater isolation and insertion ranging from 1.6 db to less than 1 db from 40 Mc to 4000 Mc. The addition of a bias lead should increase the insertion loss 0.4 db or less over the 100-to-1 bandwidth, the maximum increase being at the upper and lower bounds.     

Analysis of a new microwave low-loss and velocity-matched III-Vtransmission line for traveling-wave electrooptic modulators

A new microwave transmission line with low loss and velocity matched to a lightwave for a III-V traveling-wave electrooptic modulator has been analyzed using the spectral domain technique. A bandwidth of >40 GHz and a half-wave voltage V_π of <6 V are predicted for the modulator     

LiNbO3traveling-wave light modulator/switch with an etched groove

A Ti dffused lithium niobate traveling-wave interferometric light modulator/switch with a groove excavated at the electrode gap has been fabricated and tested at microwave frequency. The groove suppresses the undesirable light coupling between the two parallel waveguides for the phase shifting section so that the drive voltage is decreased by reducing the separation of the parallel waveguides, or of the electrodes. In addition, the groove decreases the effective index for the modulating wave to reduce the velocity mismatch between light wave and microwave so that the bandwidth is broadened. The modulation experiment was carried out from dc to 15 GHz at 633 nm light wavelength. For the modulator with the electrodes 6 mm long and 10 μm apart, the half-wave or switching voltage was 3 V, the extinction ratio was 18 dB, the 3 dB bandwidth was 12 GHz and aP/Delta fof 1.5 mW/GHz was obtained.     

高效微带线聚合物电光调制器的研制

设计、制作并测试了1.55 m 光波长的微带线行波电极电光调制器。如果聚合物材料的电光系数33=30 pm/V,中心电极L 为20 mm,设计的调制器性能参数半波电压为6.70 V。用自主合成的发色团分子组成二阶非线性光学聚合物材料做为芯层制作的聚合物调制器,对调制器的各项性能参数进行了直流、低频和微波性能的测试,采用不同极化方法,在1.55 m 波长上测得低频半波电压分别为10.5 V(电晕极化)和4.9 V(接触极化),折算得芯层材料的电光系数分别为21 pm/V 和45 pm/V。测得消光比为24 dB。用矢量网络分析仪测试电极系统的微波性能,S 参数反映了此电极系统具有低的微波传输损耗和反射损耗。     

高速InP基半导体电光调制器行波电极结构研究

对PIN型和NIN型两种InP基Mach-Zehnder电光调制器的电极结构进行了数值仿真研究,从而确定出适于这两种电光调制器的行波电极结构。仿真结果表明,NIN型电光调制器可采用简单的单臂类微带电极,而PIN型电光调制器需采用周期容性负载电极,以达到良好的阻抗匹配特性和传输特性。进一步地,提出了将串联推挽式行波电极结构应用于PIN型电光调制器,可以简化制作工艺并获得良好的微波特性。     

Electro-optic diffraction grating for light beam modulation and diffraction

The interaction between a strong traveling microwave signal and an optical beam in an electro-optic material is described in the limit of very high microwave dielectric constant. The interaction produces effects analogous to those produced by a moving diffraction grating. When the optical beam is wider than the wavelength of the microwave signal, the first grating order is resolved from the zero-order or main beam. Under this condition two types of devices become possible: 1) a beam deflector which can position an optical maser beam on, for example, 10⁵distinct points with negligible crosstalk and with address times of order 10^-7s, 2) a baseband light intensity modulator which is founded on the fact that light deflected into the first-order beam by the microwave signal is removed from the main beam. The amount deflected into the first-order beam is proportional to the microwave power; the intensity modulation follows the microwave envelope. The power required for a given modulation depth is inversely proportional to the seven halfs power of the dielectric Constant. As an example, for a not unrealistic choice of dielectric constant of 10⁴, complete transfer from the zero-order to the first-order beam requires 5 watts of microwave power. The interaction length is of order one centimeter and the interaction bandwidth is essentially unlimited. As a baseband modulator the maximum instantaneous bandwidth is of order 10 percent of the subcarrier frequency. Experimental verification is provided in an earlier paper [1].     

Transmission line aspects of the design of broad-band electrooptic traveling-wave modulators

This paper proposes a two-layer traveling-wave type electrooptic modulator structure to maximize modulation bandwidth from transmission line aspects. This structure can be designed to satisfy the velocity matching and impedance matching condition simultaneously. A design method for this structure is discussed.     

Proposed method for measuring picosecond pulsewidths and pulse shapes in CW mode-locked lasers

A new method is proposed for measuring the widths, and possibly even the detailed shapes, of picosecond pulses in CW mode-locked lasers. The pulses are passed through an electro-optic modulator, which is biased for zero transmission with zero applied voltage, and which is driven at a harmonic of the pulse-repetition frequency. The variation in average light transmission through the modulator is monitored while the relative phase between the light pulses and the modulator drive is varied. The pulsewidth may be deduced from one such measurement made at a microwave light-modulation frequency sufficiently high that the pulsewidth is a finite fraction of a period at the modulation frequency. By making such measurements at a number of harmonics, the complete Fourier expansion (including both amplitudes and phases) of the picosecond pulse envelope can, at least in principle, be determined without ambiguity.     

X band travelling-wave light-intensity modulator using ZnS crystals

A travelling-wave light-intensity modulator is described which uses two natural crystals of ZnS. This prototype needs a peak microwave pulse of 2 kW for 45% modulation, and has an overall bandwidth of 15% at 9.75 GHz.