el%3A19880578

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
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