Leonard M. Johnson

An analytic and experimental comparison of direct and external modulation in analog fiber-optic links

Analytic lumped-element small-signal models of directly and externally modulated analog fiber-optic links are derived. Incremental modulation efficiency is defined and used to compare the performances of these modulation techniques. In experiments to optimize link RF-to-RF gain and noise figure, the measurements obtained agreed with calculations to within approximately=1 dB. The externally modulated link was operated with two different impedance matching circuits. With a low-pass match the bandwidth was 150 MHz, and the link transducer gain was 1 dB; with a bandpass match the bandwidth was 22 MHz, the link transducer gain was 11 dB, and the noise figure was 6 dB. The directly modulated link was operated with a low-pass match. In this case, the bandwidth was 1 GHz, the link transducer gain was -14 dB, and the noise figure was 33 dB. These experimental results were achieved with no amplification,. >

Serrodyne optical frequency translation with high sideband suppression

A Ti:NiNbO/sub 3/ serrodyne optical frequency shifter with 40-dB overall sideband suppression has been demonstrated. This performance can be achieved using either a laser diode or a broadband superluminescent diode as an optical source. The device utilizes a single-polarization waveguide design and a specialized sawtooth drive circuit. >

Advanced Fabrication Processes for Superconducting Very Large-Scale Integrated Circuits

We review the salient features of two advanced nodes of an 8-Nb-layer fully planarized process developed recently at MIT Lincoln Laboratory for fabricating single flux quantum (SFQ) digital circuits with very large-scale integration on 200-mm wafers: the SFQ4ee and SFQ5ee nodes, where “ee” denotes that the process is tuned for energy-efficient SFQ circuits. The former has eight superconducting layers with 0.5-μm minimum feature size and a 2-Ω/sq Mo layer for circuit resistors. The latter has nine superconducting layers: eight Nb wiring layers with the minimum feature size of 350 nm and a thin superconducting MoNx layer (Tc ~ 7.5 K) with high kinetic inductance (about 8 pH/sq) for forming compact inductors. A nonsuperconducting (Tc <; 2 K) MoNx layer with lower nitrogen content is used for 6-Ω/sq planar resistors for shunting and biasing of Josephson junctions (JJs). Another resistive layer is added to form interlayer sandwich-type resistors of milliohm range for releasing unwanted flux quanta from superconducting loops of logic cells. Both process nodes use Au/Pt/Ti contact metallization for chip packaging. The technology utilizes one layer of Nb/AlOx-Al/Nb JJs with critical current density Jc of 100 μA/μm2 and minimum diameter of 700 nm. Circuit patterns are defined by 248-nm photolithography and high-density plasma etching. All circuit layers are fully planarized using chemical mechanical planarization of SiO2 interlayer dielectric. The following results and topics are presented and discussed: the effect of surface topography under the JJs on the their properties and repeatability, Ic and Jc targeting, effect of hydrogen dissolved in Nb, MoNx properties for the resistor layer and for high-kinetic-inductance layer, and technology of milliohm-range resistors.

High-performance optical analog link using external modulator

An experimental fiber-optic analog link with a noise figure of only 6-dB, a 104-dB intermodulation-free dynamic range (measured using a 10-Hz noise bandwidth), and an RF-to-RF gain of 11 dB at 50 MHz is discussed. The link includes no electronic amplification. It uses a very sensitive bandpass impedance-matched Ti:LiNbO/sub 3/ interferometric modulator and an input optical power of 55 mW at 1.32 mu m.<<ETX>>

Fabrication Process and Properties of Fully-Planarized Deep-Submicron Nb/Al–

A fabrication process for Nb/Al-AlOx/Nb Josephson junctions (JJs) with sizes down to 200 nm has been developed on a 200-mm-wafer tool set typical for CMOS foundry. This process is the core of several nodes of a roadmap for fully-planarized fabrication processes for superconductor integrated circuits with 4, 8, and 10 niobium layers developed at MIT Lincoln Laboratory. The process utilizes 248 nm photolithography, anodization, high-density plasma etching, and chemical mechanical polishing (CMP) for planarization of SiO2 interlayer dielectric. JJ electric properties and statistics such as on-chip and wafer spreads of critical current, Ic, normal-state conductance, GN, and run-to-run reproducibility have been measured on 200-mm wafers over a broad range of JJ diameters from 200 nm to 1500 nm and critical current densities, Jc, from 10 kA/cm2 to 50 kA/cm2 where the JJs become self-shunted. Diffraction-limited photolithography of JJs is discussed. A relationship between JJ mask size, JJ size on wafer, and the minimum printable size for coherent and partially coherent illumination has been worked out. The GN and Ic spreads obtained have been found to be mainly caused by variations of the JJ areas and agree with the model accounting for an enhancement of mask errors near the diffraction-limited minimum printable size of JJs. Ic and GN spreads from 0.8% to 3% have been obtained for JJs with sizes from 1500 nm down to 500 nm. The spreads increase to about 8% for 200-nm JJs. Prospects for circuit densities > 106 JJ/cm2 and 193-nm photolithography for JJ definition are discussed.

\hbox{AlO}_{\rm x}\hbox{/Nb}

Inductance of superconducting thin-film inductors and structures with linewidth down to 250 nm has been experimentally evaluated. The inductors include various striplines and microstrips, their 90° bends and meanders, interlayer vias, etc., typically used in superconducting digital circuits. The circuits have been fabricated by a fully planarized process with 8 niobium layers, developed at MIT Lincoln Laboratory for very-large-scale superconducting integrated circuits. Excellent run-to-run reproducibility and inductance uniformity of better than 1% across 200-mm wafers have been found. It has been found that the inductance per unit length of stripline and microstrip line inductors continues to grow as the inductor linewidth is reduced deep into the submicron range to the widths comparable to the film thickness and magnetic field penetration depth. It is shown that the linewidth reduction does not lead to widening of the parameter spread due to diminishing sensitivity of the inductance to the linewidth and dielectric thickness. The experimental results were compared with numeric inductance extraction using commercial software and freeware, and a good agreement was found for 3-D inductance extractors. Methods of further miniaturization of circuit inductors for achieving circuit densities> 106 Josephson junctions per cm2 are discussed.

Josephson Junctions for VLSI Circuits

The polarization-mixing technique is used to reduce the cubic nonlinearity of an integrated-optical interferometric modulator at microwave frequencies. Two-tone intermodulation distortion is reduced by 14 dB at a constant modulation depth of 3% over the frequency range of 3.6-5.6 GHz. The polarization-mixing technique requires no high-speed electronics and can be extended to higher frequencies.<<ETX>>

Inductance of Circuit Structures for MIT LL Superconductor Electronics Fabrication Process With 8 Niobium Layers

Analog fiber-optic links using an integrated-optical intensity modulator have been demonstrated and analyzed. Experimental links operate at frequencies from 40 MHz to 22 GHz with electrical gain up to 11 dB, noise figure as low as 6 dB, and intermodulation-free dynamic range of up to 113 dB-Hz2/3. The gain, noise figure, and dynamic range are shown to have a simple dependence upon a few link design parameters. Other factors affecting link performance, such as stimulated Brillouin scattering, interferometric intensity noise, modulator linearization, and addition of an optical amplifier, are also discussed briefly.

Linearization of an interferometric modulator at microwave frequencies by polarization mixing

Th design and evaluation of high-sensitivity lumped-element, bandpass modulators based on the integrated-optical Mach-Zehnder interferometer are presented. The optimum impedance-matched modulator response is formulated. Then a passive resonant drive circuit that achieves this response is described. In contrast to the usual baseband case, the maximum response of the impedance-matched modulator is independent of modulator length. Experimental results are given for several modulators built in lithium niobate using this circuit. Undesirable acoustic resonances are observed, and a technique for their suppression is demonstrated. >

Optimization of externally modulated analog optical links

Analog fiber-optic links using an integrated-optical intensity modulator have been demonstrated. These links operate at frequencies from 50 MHz to 22 GHz with electrical gain u to 1 1 dB noise figure as low as 6 dB and intermodulation-free dynamic range of up to 1 13 dBHz2/3. The design factors determining optical link performance are discussed briefly and the effect of adding electronic amplifiers is analyzed. SUMMARY Analog optical links are attractive for many applications ranging from cable television distribution to radar-signal transmission at microwave frequencies. These systems often require a link with a low noise figure low electrical insertion loss and high dynamic range. We have demonstrated several intensitymodulated optical links that use a diode-pumped Nd:YAG laser and an integrated-optical modulator to achieve high performance by all these measures. 13 We will first discuss these experimental links and then show how their performance could be improved further by adding electronic amplifiers. Figure 1 illustrates the optical link used in our experiments with an electronic preamplifier and postamplifier added. (The experimental results reported here refer to the electrical ports of the optical link Figure 1. Block diagram of the optical link with an electronic preamplifier (Al) and postamplifier (A3) added. (Experimental results in the paper refer to the optical link alone without the amplifiers. ) 252 / SPIE Vol 1371 High-Frequency Analog Fiber Optic Systems(1990) OPTICAL LINK 161 884-1 INTEGRATED-OPTICAL PIN