Stable injection-locking and well defined switching were obtained with these photocurrent levels. Switching with an AND gate was attained with the current source set to 15pA and its voltage limited to between 0 and 4V. Good contrast, 5,5dB, between the ‘0’and ‘1’ levels was obtained with no significant impairment in the reshaped output signal. Next, the decision SEED was operated bistably by setting the current-source voltage limits to 3.5 and 11V. The SEED oscillator bias was adjusted slightly to obtain a more symmetric waveform. The inverted output signal had good level contrast, but a 30ps wide triangular pulse appeared as an artefact in the low output state. This pulse resulted from the slow risetime of the clock pulse, causing a delay before the SEED switched to the low state. This feature could be removed by speeding up the clock transitions with a second bistable SEED positioned after the clock oscillator. Even with this extraneous pulse, the energy ratio between the regenerated Is and Os was better than 2 : 1. Optical gain through the bistable regenerator was achieved with a signal power of 0.87pW injected into both the clock recovery and decision SEEDs. The output optical power of the decision SEED was 3.28 pW, which after correcting for the output extinction ratio, corresponded to a 2dB gain in the peak/peak signal power through the regenerator. High absorption losses through the SEEDs ( o11 dB each) and optical coupling losses between stages ( z8 dB total) caused the low observed gain. However, the decision circuit easily distinguished between Os and Is when the injected clock power was 15dB greater than the signal power. Without optical amplifiers, this switching gain, minus the loss through the decision SEED, is the maximum expected gain in this type of regenerator. Summary: An all-optical regenerator based on AIGaAs-GaAs multiple-quantum-well SEEDs has been demonstrated. We believe that this is the first all-optical regenerator capable of signal detection, clock recovery and data retiming. Regeneration of 5kbit/s RTZ signals with 2dB gain was demonstrated, although the inherent speed limitations in the proposed configuration suggest that the circuit could operate at much higher rates5 The regenerator’s simplicity is suited to monolithic integration, which together with higher optical pump powers, might permit such high-speed operation in future optical circuits. Acknowledgments: We are grateful to A. C. Gossard, J. H. English, E. Carr, R. Trambarulo and J. Centanni for their contributions to the experiment. C. R. GILES T. LI T. H. WOOD C. A. BURRUS A T & T Bell Laboratories Crawford Hill Laboratory Holmdel, N J 07733, U S A 27th May 1988 D. A. B. MILLER A T & T Bell Laboratories Holmdel, NJ 07733, U S A 5GHz JOSEPHSON A/D CONVERTOR Indexing terms: Analogueldigital conversion, Convertors, Josephson junctions, Superconducting devices A four-bit A/D convertor using NbIAIOJNb Josephson junctions was fabricated. The circuit was designed to preserve the matching of characteristic impedances at all nodes. Four-bit A/D conversion was confirmed at a low frequency. The A/D convertor correctly operated at up to 5GHz for two-bit conversion. Introduction: Several types of A/D convertor using Josephson junctions have been proposed.’-5 Josephson A/D convertors are generally characterised by a high-speed conversion rate and a small number of comparators. Only N comparators are necessary to compose a flash-type N-bit A/D convertor. The Josephson A/D convertor proposed by Hamilton e t al.’ has the simplest construction, i.e., one comparator consists of one superconducting quantum interference device (SQUID). We designed and fabricated this type of four-bit A/D convertor. Design: Fig. 1 shows the equivalent circuit of the SQUID designed as a comparator. The designed critical current density is 3000 A/cm2. 1, L2 whhc 0 0 I L3 L3 Fig. 1 Equivalent circuit of SQUID designed as comparator I , = 0.21 mA, LI = 0.25pH, U = 0.8pH, L3 = 1.4pH, R, = 2.4R Fig. 2 shows the equivalent circuit of the designed four-bit A/D convertor. In designing the circuit, it is important to match the characteristic impedance for superconducting wiring at every node. This is vital to operating the circuit at a frequency over 1GHz. In measurements with a high clock frequency, 50 R coaxial cables connect measuring instruments with an A/D convertor chip immersed in liquid helium. Therefore, it is desirable to design the impedance of the superconducting strip lines in the chip to be 50R.This results in a 0.5 pm-wide strip line. However, 0.5 pm-wide Nb lines are hard to fabricate, so we designed the line to be 4 R for the power supply. Line impedance at 1/0 terminals was converted z =4n References 1 bias current MILLER, D. A. A,, GOSSARD, B., CHEMLA, D. S., DAMEN, T. C., WOOD, T. H., BURRUS, C. A. c., and WIEGMANN, w.: ‘The quantum well self- electro-optic effect device: optoelectronic bistability and oscillation, and self-linearized modulation’, IEEE J . Quantum Electron., 1985, QE-21, pp. 1462-1476 2 H., BURRUS, C. A., TUCKER, R. S., B.,CHEMLA, D. s., DAMEN, T. c., GOSSARD, WOOD, T. WEINER, J. S., MILLER, D. A. A. c., and WIEGMANN, w.: ‘1OOps waveguide multiple quantum well (MQW) optical modulator with 10 : 1 on/off ratio’, Electron. Lett., 1985,21, pp. 693694 3 MILLER, D. MANN, w., n 1 A. B., CHEMLA, D. S., DAMEN, T. C., GOSSARD, A. C., WIEGWOOD, T. H., and BURRUS, c. A,: ‘Bandedge electro- absorption in quantum well structures: the quantum confined stark effect’, Phys. Rev. Lett., 1984,53, pp. 2173-2177 K.: ‘Injection locking of microwave solid-state oscil4 KUROKAWA, lators’, Proc. IEEE, 1973,61, pp. 1386-1410 5 SCHMITT-RINK, s., CHEMLA, D. s., and MILLER, D. A. B.: ‘Theory of transient excitonic optical nonlinearities in semiconductor quantum-well structures’,Phys. Rev. B., 1985,32, pp. 6601-6609 850 SQUl analogue signal Fig. 2 Equivalent circuit of designed four-bit AID convertor Circled SQUID represents SQUID shown in Fig. 1 ELECTRONICS LETTERS 7th July 1988 Vol. 24 No. 14 by adding resistors to match the 50R cable. We designed all nodes to preserve impedance matching in all directions except for the analogue signal input terminal and the digital output terminals, where impedance matching was maintained in one direction only to avoid lowering the signal level. We used 3 pm-diameter Nb/AIOx/Nb Josephson junctions6 Nb, Mo, SiO,,’ and SiO,* were used for the superconducting wiring, resistors, insulators, and the protecting layer of the resistors, respectively. Results: Four-bit operation at a low clock frequency is shown in Fig. 3. Bias current is applied as a 0.2MHz square pulse. remaining between the 50 R cable and the terminals in the A/D convertor chip. This causes the apparently blunt waveforms. Conclusion: We fabricated a four-bit Josephson AID convertor, using Nb/AIOx/Nb Josephson junctions, and confirmed four-bit operation at 0.2 MHz and two-bit operation at 5GHz. The conversion rate of 5GHz is not the maximum conversion rate of the A/D convertor but the limit of the frequency that the package could respond to. S. OHARA T. IMAMURA S. HASUO Fujitsu Laboratories Ltd. 10-1 Morinosato- Wakamiya, Atsugi 243-01, Japan 23rd M a y 1988 References analogue signal ZAPPE, H. H.: ‘Ultrasensitive analog-to-digital convertor using Josephson junctions’, IBM Tech. Disc. Bull., 1975, 17, p. 3053 HAMILTON, c. A,, and LLOYD, F. L.: ‘Design limitations of superconducting A/D converters’, IEEE Trans., 1981, MAG-17, pp. 3414- bias current 3419 and LLOYD, F. L.: ‘Operation of a superconducting analog to digital convertor at short conversion time’, IEEE Trans., 1983, MAG-19, pp. 1186-1189 KAUTZ, R. L., bit 4 ( M S B ) PETERSON, D. A,, KO, H., JEWETT, R. E., NAKAJIMA, K., SPARGO, J. W., and VAN DUZER, T. : ‘A high-speed analog-to-digital converter using bit 3 - m 100 v s Fig. 3 Four digital outputs of AID convertor with analogue signal and bias current Since a SQUID is a latching device, the output of each bit returns to zero when bias current becomes zero to reset the SQUIDS. The triangular analogue signal is correctly converted to four-bit digital signals. Gray code is used for output coding. The A/D convertor chip was attached to a conventional chip carrier. Therefore, there is a parasitic inductance between the 50R cable and the chip. At low frequencies such as 0.2 MHz, however, the influence of this inductance is negligible. Next, we examined operation at a higher clock frequency. To remove the parasitic inductances between the cables and the chip, we used another chip having 50R superconducting strip lines, to which we connected the 50R coaxial cables. The superconducting strip lines and the A/D convertor chip were linked with very short A1 wires. Fig. 4 shows four-bit A/D conversion at 5GHz. Bias current was applied as a sinusoidal wave with DC offset. The ramp analogue signal was converted to two-bit digital Gray code. The value of 5 GHz is the fastest conversion rate reported so far. As mentioned above, digital outputs must be reset when bias current becomes zero, but the waveforms in Fig. 4 do not return to zero. This is due to an excess inductance L, /6861pi Fig. 4 Result of twto-bit operation at 5 GHz conversion rate 7th July 1988 Vol. 24 SINGLE MODE FUSED COUPLERS INSENSITIVE TO EXTERNAL REFRACTIVE INDEX Indexing terms: Optical fibres, Optical connectors and couplers, Multiplexing, Optical waveguide components Wavelength multiplexers produced using the fused taper technique have been fabricated to be insensitive to variations in the external refractive index, allowing them to be potted in silicone for environmental protection. The technique exploits two opposing dependencies by fabrication at a particular coupler cross-section. Introduction: The propagating field within the fused region of a coupler extends evanescently outside the silica fibre, hence the refractive index of the surrounding material influences the degree of coupling. The couplers are usually encapsulated in silicone resin to provide environmental and mechanical protection.’ As a result, because the refractive index of the silicone varies with temperature, the coupling ratio of the device has a temperature dependence. For a 3 dB (50/50split) coupler this effect is negligible over a wide temperature range (+ 125°C to - 5S°C), but for devices with a longer interaction length, such as 1300nm/1540 nm wavelength multiplexers, the resulting effect can be so deleterious that the devices must be left air clad for most applications. Such devices are considered less durable than encapsulated devices. This letter reports the fabrication of external refractive index insensitive devices, based on control of the fused crosssection geometry. Ins ELECTRONICS LETTERS Josephson self-gating-AND comparators’, IEEE Trans., 1985, MAG-21, pp. 2 W 2 0 3 NAKANISHI, T., and YOSHIKIYO, H.: ‘A new comparator for use in ultra-high speed Josephson AID converter’. Extended Abstracts of ISEC ’87, 1987, pp. 329-332 MOROHASHI, s., and HASUO, s. : ‘Experimental investigations and analysis for high-quality Nb/AI-AIOJNb Josephson junctions’, J . Appl. Phys., 1987,61, pp. 4835-4849 HOKO, H., IMAMURA, T., OHARA, s., and HASUO, s.: ‘Application of sputtered SiO, insulator to NbIAIJNb Josephson junctions’, J. Appl. Phys., 1987,62, pp. 3432-3435 SHIBAYAMA, H., HASUO, s., and YAMAOKA, T.: ‘Formulation of low defect density SiO, films for Josephson integrated circuits’, Appl. Phys. Lett., 1985,47, pp. 429430 No. 14 851
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