2324 OPTICS LETTERS / Vol. 20, No. 22 / November 15, 1995

Transmission of 1-Gbitys data patterns through smart pixels byuse of picosecond laser pulses

Gabriela Livescu, Leo M. F. Chirovsky, Theodore Mullally, and Arza Ron*

AT&T Bell Laboratories, Murray Hill, New Jersey 07974

Received July 5, 1995

We demonstrate the transmission of 1-Gbitys data patterns through various GaAsyAlGaAs smart pixels byusing free-space optics and trains of mode-locked picosecond laser pulses spaced 1 ns apart as optical inputsand outputs. 1995 Optical Society of America

Arrays of fast smart pixels1 form the heart of high-speed, high-throughput photonic switching systems2

and spatial light modulators.3 Large-scale fabri-cation of such optoelectronic circuits, which useoptics for input and output, and electronics for amplifi-cation, memory, and logic functions, has challenged themonolithically integrated GaAs technology4; however,the emerging GaAs-on-Si complementary metal-oxidesemiconductor hybrid technology5 is very promisingfor overcoming the difficulties of integration. Forhigh throughput, it is necessary to have large arraysof smart pixels operated at very high data rates. In1993–1994, research at AT&T Bell Laboratories6,7

demonstrated an aggregate throughput of 2.5 Gbitsysin a five-stage system, with each stage consistingof 4 3 4 arrays of 2 3 1 routing nodes, operated at155 Mbitsys. However, newly emerging communica-tion services, such as transport and switching of video,television, and animated graphics, will certainly re-quire not only much larger arrays but also ever-higherdata rates, for throughputs in the range of terabitsper second. Therefore it is imperative to developcircuits andyor methods of operation that will allowthe increased speed of individual smart pixels. Ourresearch addresses this issue, describing a new methodof operation of the smart pixels in which short laserpulses are being used in an asynchronous method.

For our demonstration we used smart pixelsfabricated by the monolithic GaAsyAlGaAs field-effect-transistor (FET) self-electro-optic-effect device technol-ogy.8 The two circuits that we used are described byFig. 1. One of them [Fig. 1(a)] is a simple receiver–transmitter pair, in which the receiver stage consists ofdetectors only and the transmitter stage consists of oneFET and one modulator. We recently showed that thiscircuit has switching times as short as 200 ps.9,10 Theother circuit [Fig. 1(b)] is more sophisticated, alsoextensively characterized previously,11 showing ex-cellent high-speed capabilities: 650 Mbitsys forreceiver–transmitter pairs and 1 Gbitys for thereceivers alone.12 The receiver stage has two reverse-biased GaAsyAlGaAs quantum-well detector diodes,Set and Reset, onto which the two input beams areincident. Clamping diodes limit the input voltageswing on the gate of the first FET, and a bufferedFET inverter serves as an amplifier. The trans-mitter stage consists of one or two reverse-biased

0146-9592/95/222324-03$6.00/0

Read quantum-well modulator diodes, driven by aFET inverter. The output beam is ref lected off theRead diode, which is a ‘‘normally ON’’ modulator: itsref lectivity is high when the voltage drop on it is zero.

The essence of the asynchronous pulsed opera-tion13 of the simple circuit of Fig. 1(a) is describedby Fig. 2. The input pulses, Set and Reset, and theoutput pulses, Read, are staggered by a few hun-dred picoseconds from each other, in order to allowthe time necessary for the circuit to switch into thestate required by the input before it is read by theoutput.9,10 At the wavelengths used here (856 nm)the Read diode has a high ref lectivity (is ON) withoutvoltage on it (VRead ­ 0) and has a low ref lectivity(is OFF) when a voltage is applied (VRead ­ 8 V ). Ahigh Set or a ‘‘1’’ produces an increase in Vgate, whichmakes the FET conducting. As a result, the voltagedrop on the output Read diode Vread increases, and thequantum-well modulator becomes absorbing and its re-f lectivity decreases: it is being turned ‘‘OFF’’. Thus

Fig. 1. Circuit schematics of the two smart pixels used inthis research.

1995 Optical Society of America

November 15, 1995 / Vol. 20, No. 22 / OPTICS LETTERS 2325

Fig. 2. Asynchronous pulsed operation of smart pixelcircuit.

Fig. 3. Schematic description of the split-and-delay tech-nique used to produce a train of four pulses 1 ns apart, fromeach pulse generated by the mode-locked Ti:sapphire laser.BS, beam splitter; PBS, polarizing beam splitter.

a high Set pulse or a ‘‘1’’ input will produce a low or a‘‘0’’ output, and vice versa. Note that for this devicethe output is inverted. In the case of the circuit ofFig. 1(b) the output is not inverted.

The output of the device is read by the Read beam,which arrives later. In addition to being ref lected offthe Read diode and generating the output signal attime 1, the Read pulse also produces the photocurrentnecessary to discharge the Read diode and return it to

its original state. For the FET also to be brought backto its original state, ready for the next data-carryingSet pulse, a constant-intensity Reset pulse follows ev-ery Set pulse. As illustrated in Fig. 2, the Reset pulsemust arrive before the Read pulse, because the FETmust be insulating for the Read diode to be discharged.This will not, however, affect the accuracy of the out-put: the Read pulse first reads the state and thenchanges it.

Note that the data to be transmitted are encoded inone of the input beams only, with interleaved constant-amplitude pulses in the second one, whose role is toReset the circuit. The pulsed output can thus serveas input for a next stage, allowing for cascadableoperation.

The optical inputs and outputs are 1.5-ps pulses froma commercial mode-locked Ti:sapphire laser, whoserepetition rate is 82 MHz (12-ns separation betweenpulses). By means of a split-and-delay geometry,described in Fig. 3, a train of two and then four pulsesseparated by 1 ns is generated for every pulse emittedby the laser, simulating a 1-GHz rate. Each of thefour pulses is subsequently split into three beams: theSet, followed by the Reset a few hundred picosecondslater, and then by the Read. One can create aninput pattern in the Set beam by exploiting the factthat the different pulses have different polarizations(the smart pixels are polarization independent). Notethat the first two Set pulses have polarizations thatare orthogonal to the last two. Therefore, by usinga polarizer, one can allow either the first two Setpulses, generating the 1100 pattern, or the last two,generating the 0011 pattern. Without the polarizer,all four pulses arrive at the sample, which correspondsto the 1111 pattern. The 0000 pattern correspondsto the absence of the Set beam. There are constant-intensity Reset pulses interleaved with these data-carrying Set pulses; constant-intensity Read pulses areused for the optical output.

Fig. 4. Data pattern transmitted through the diode-clamped receiver–transmitter pair. The input patternsare indicated.

2326 OPTICS LETTERS / Vol. 20, No. 22 / November 15, 1995

We used this setup to operate both circuits inFig. 1. An example of the pattern transmittedby the diode-clamped receiver–transmitter pairis shown in Fig. 4. The input patterns are alsoindicated in the figure. The normalized outputshown here is the difference RON 2 ROFF ; the actualcontrast ratio was 2:1. The operating wavelengthwas 856 nm. The range of input pulse energiesfor which we could transmit the data pattern was200 fJypulse to 1.5 pJypulse, limited at the lowerend by the sensitivity of the detector and at theupper end by the saturation of the device. At a1-GHz repetition rate, this energy range corresponds toaverage optical powers of 0.2–1.5 mW. Simulationsindicate that hybrid GaAs-on-Si complementary metal-oxide semiconductor smart pixels, which will utilizetransimpedance amplif iers, will operate with muchsmaller energies.

In conclusion, we transmitted a 1-Gbitys data pat-tern through various smart pixels by using free-spaceoptics and picosecond laser pulses. We simulated the1-Gbitys data pattern with trains of picosecond mode-locked laser pulses spaced 1 ns apart.

We thank L. A. D’Asaro, S. Hui, and B. Tseng forfabricating the smart pixels and R. Leibenguth forgrowing the GaAsyAlGaAs structure.

*Permanent address, Department of Chemistry andSolid State Institute, Technion, Haifa, Israel.

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