Integrated Optoelectronics for Optical Interconnections and Optical Processing

SFtdhana, Vol. 17, Parts 3 & 4, September 1992, pp. 431-449. © Printed in India.

Integrated optoe!ectronics for optical interconnections and optical processing

OSAMU WADA Fujitsu Laboratories Ltd., 10-1 Morinosato-Wakamiya, Atsugi 243-01, Japan

Abstract. Integrated optoelectronics using III-V compound semiconductor technology has so far shown exciting advances for application in optical telecommunication systems. New applications of this technology are in optical interconnections and signal processing systems. The technology is expected to be very effective in solving the wiring limit in data transmission within electronic systems, using the advantages of optical techniques such as high data transmission rate and high parallelism, and thus improve the performance of overall systems. Optical interconnection devices currently being developed aim both at multiplexing vast amounts of data and exhibiting flexible interconnection functions using the advantageous characteristics of light. Future research is expected to explore new techniques such as that for multiplexing and processing data in the wavelength divison as well as for integrating functional devices in two- dimensions. Synergetic collaboration among materials and processing, design and fabrication, and packaging areas is extremely important and this will lead to practical optical interconnections and signal processing systems.

Keywords. Integrated optoelectronics; optoelectronic integrated circuits (OEIC); photonic integrated circuits (pie); integrated optical functional devices; optical interconnections; multiplexing optical interconnection; flexible optical interconnection; optical signal processing; optical bistability; wavelength-division multiplexing (WDM); two-dimensionally integrated devices.

1. Introduction

Since the successful advances in discrete semiconductor optoelectronic devices such as lasers and photodiodes in the early 1970's, they have become the key to optical fibre transmission systems. This is due to the optical technique possessing advantages such as large bandwidth, long transmission distance and freedom from electromagnetic noise, cross-talk, impedance-mismatching and grounding problems, and the components being compact and lightweight. Optical fibre transmission systems are thus in wide use all over the world. Development in this field is currently directed both towards ultra-fast transmission systems operating at multi-gigabit per second (Gbps) 431 432 Osamu Wada

Inter-subsystem

.//,". •//,./ Board-lo-//1 Chip-to-/ ^' Holographic" chipJ /^/' '~ minor plate Trns ~ / Trns L~,,/,/'~-/t~ecv/ /,)f-7~__. / Figure 1. Schematic illust- ration showing optical interconnections implemented at different levels within electronic digital systems. rates and broadband integrated-services digital networks (ISDN) aiming at fibres-to- the-home applications. Application of optoelectronic devices is further expected to develop in new fields such as optical interconnections (Goodman et a11984) and signal processing in digital systems for solving interconnection problems limiting system speeds and performances. Building up this technology is essential for developing advanced systems such as tera-FLOPS (floating point operations per second) computers over the next century. Figure 1 illustrates a variety of forms of implementation of optical interconnections in electronic digital computing systems (Wada & Yamakoshi 1990, pp. 28-36). Inter-subsystem optical interconnection has already been applied in linking multi-processor units or a processor unit to peripherals such as storage units. Miniaturisation of optical components is strictly required for further application of optical interconnections into the board-to-board, chip-to-chip and even intra-chip levels. Discrete optoelectronic devices, however, cannot be utilized without being interfaced with other components such as electronic integrated circuits and guided-wave elements in practical systems. Integrated optoelectronics which organizes all necessary devices into an integrated form can realize compact, reliable components exhibiting performance and functions required for systems. The first experiment demonstrating the integration was reported in 1978-79 by the California Institute of Technology group (Yariv 1984), in which they integrated A1GaAs/GaAs double heterostructure (DH) lasers and GaAs metal-semiconductor field-effect transistors (MESFET) on a GaAs substrate. Such integration of optoelectronic devices with electronic circuits is referred to as optoelectronic integrated circuits (OEIC), which has become a widely accepted term since the research project organized by Japanese Mm for 1980-1987 (Hayashi 1986; Wada et al 1986; Matsueda 1987). During this decade, many advances were made in different areas of integration technology (Dagenais et al 1990). Not only have OEE transmitters and receivers been developed for telecommunications on both GaAs- and InP-based (Suzuki et al 1987) material systems, but researches on other styles of integrations have been initiated for generating novel optical signal processing functions for possible applications in future optical telecommunication and data processing systems. The purpose of this paper is to describe recent advances in integrated optoelectronics Integrated optoelectronics for optical interconnections 433 with a focus on the applications in optical interconnections and optical signal processing. Categories, advantages and possible applications of integrated optoelectronics are first explained. The current technological status of integration is shown by illustrating a couple of examples developed for optical telecommunications application. Then, requirements of integrated optoelectronics towards the application to optical interconnections and signal processing are discussed and technological trends are projected by reviewing proposed devices.

2. Categories, advantages and applications of integrated optoelectronics

We define integrated optoelectronics as a term encompassing all integrated devices and circuits that incorporate semiconductor optoelectronic elements. Figure 2 shows the three major categories of integrated optoelectronics referring to three basic functions: optoelectronic conversion, electronic signal processing and optical signal processing (Wada 1991, pp. 598-607). Lasers and photodetectors perform conversion of electrical signals to optical ones and vice versa. The most widely recognised definition of OEIC is in the combination of optoelectronic elements with electronic circuits. In an OEIC, either input or output is electronic and the other is optical as indicated in the figure. The integration of lasers and photodetectors with other optical signal processing circuits using waveguide devices and micro-optics elements provides various optical signal processing functions. This type of integration performed on a single semiconductor substrate has developed more recently than OEIC and is referred to as photonic integrated circuits (PIc) (Koch & Koren 1991). The term integrated optics has often been used for the integration of guided-wave devices on dielectric materials such as lithium niobate (Hunsperger 1982). Optical signals are provided at both interfaces and a variety of functions are generated by the use of the wave property of light. As the third category, novel optical functions such as optical switching and storage can be

Optoelectronic conversion r- .... ~ I Lasers and <~ }O/E/O } ~::> / photodetectors ...... I° Optical functional ~-~1 devices II • [ r------"-m oo,,os/i"VIL./1 E 0 Photonic ICs z • Electronic signal I /V__I/ E!ectronic processing <~ 0/0 n~ I/ .. circ?its "0~ 0 ,J/'Waveguide optics and micro-optics Opticalsignal processing Figure 2. Three major categories (shaded areas) ofintegrated optoelectronics with respect to three basic functions and devices (axis). The construction of integrated circuit and the signal form are indicated by box and arrows. (O: optical, E: electronic). 434 Osamu Wada

FunctionaLity

/~ Signalprocessing : Photonic switching Optical computing f ' Image processing , Intra-chip optical ' interconnection I I I I I I ,1 ,, Performance

' ,~,o~', Figure 3. Various areas of application of integrated / / o- optoelectronics shown with UcbS~r~brerloop respect to three different network divisions of advantages ex- Monufocturability Opticalinterconnection pected for integration.

generated by incorporating optoelectronic conversion and processing functions within an integrated device structure. This is achieved by designing the device structure to allow efficient interaction between electron and photon systems to create a novel signal processing function. Such an integrated optical functional device has basically optical signals at both interfaces, and is expected to be the key to future fully optical data processing systems. The advantages of integration can be represented best by OEIC (Wada 1990). It affects three major divisions, namely, performance, function and manufacture of components, as illustrated in figure 3. The integration of different devices on a common substrate can minimise parasitic reactions which are inevitably introduced by electrical wiring between discrete devices. This improves the compactness as well as the speed and noise characteristics of optoelectronic devices and is important for ultra-fast and optical coherent telecommunication systems for which large bandwidth light sources and receivers are needed. Integration reduces the number of components necessary for realising certain system functions. This simplifies the assembly, improves the reliability and lowers the cost of the overall system. Such improvements in manufacture can broaden the application of optoelectronic components into local area networks (LAN), subscriber loops and optical interconnections, where low-cost components are required in large volumes (Leheny 1989). There is another direction of the advantage of integration to exploit new functions. A variety of optical signal processing functions such as optical logic operation, wavelength conversion and multiplexing and light beam deflection can be realised by integrated functional devices, and also a capability of dealing with massive data can be achieved by arraying these integrated devices and circuits either one- or two-dimensionally. Such features open up the possibility of practical implementation of photonic switching, optical computing and image processing systems. In the same way, optical interconnections require functional devices having connection controllability particularly when they are incorporated at the chip level as lhe technology develops. Integrated optoelectronics for optical interconnections 435

DFB laser

AR~.M°dulat°r.@1

nP JS burying ~ctive layer Figure 4. Structure of ultra-fast, integrated light \ layer source consisting of a DFB laser and an absorption Absorption waveguide modulator butt-joint coupled on an InP layer substrate (after Soda et al 1990).

3. Integrated optoelectronics for telecommunications

Since the first fabrication of an OEIC transmitter, most efforts at developing integrated optoelectronics have been devoted to applications in telecommunication systems. In order to illustrate the current status of integration technology, we review in the following a couple of examples developed for both ultra-fast transmission and optical coherent systems. Figure 4 shows the structure of an ultra-fast light source consisting of a distributed feedback (DFB) laser and a waveguide absorption modulator monolithically integrated on an n÷-InP substrate (Soda et al 1990). The advantage of integration is in the minimisation of the wavelength chirping under ultra-fast modulation, which cannot be avoided when a laser is directly modulated by laser injection current. Such photonic integration provides excellent optical coupling between the constituent elements without any tedious alignment procedure. This device has very high performance, exhibiting very low chirp, less than 0.01 nm at 1.55 pm under a bit rate of 10 Gbps with a fibre-coupled output power of several mW and - 10 dB extinction with only 3 to 5 V bias. Photonic integration has also been used for fabricating a wavelength-division multiplexing (WDM) source which contains four distributed Bragg reflector (DaR) lasers combined through an optical amplifier to a Single output port. Four-channel WDM transmission with a wavelength separation of 2-5 nm has been demonstrated at 2 Gbps (Koch & Koren 1991). Figure 5 shows the structure of an optoelectronic integrated ultra-fast receiver consisting of an Si bipolar amplifier circuit and an InGaAs/InP avalanche photodiode

APD I~/inGaAsP Microtens hv \ n'-n;-!nP bP An~L-n InP+ - InP s,.\ \ ~ t ~AuZnAu I ~-~ ~ ~AuSn bump n+ InP sub , . ~~ /L SiN SI-Blpolar Amp. Figure 5. Structure of flip-chip i',~", ~ ~. ---~ ~ ...... integrated long-wavelength re- / p contact n contact ~ 1 ceiver combining a GalnAs/InP AuSnbump APD and a Si bipolar transistor Si Substrate amplifier (after Hamano et al 1991). 436 Osamu Wada

-15 iii I i i i iiiii I i i i i i1[11' '

E GalnAs PIN-PD - 20 NRZ, BER: Io Hetero-ept HEMT0 / / /+ / o -25 GoAs MESFET,,,,,, //f (3.

{3 U dFET: Xy/F~:S:'I

=e~ -30 o ~i, / / Besthybrid -- // PIN/FET > -35 g Flip-Chlp ._._ / /\ GoAs MESFET"~x/ / Best hybrid / / GoInAsAPD -40 ,,,,I , , I IIIll[ I I I tllll[ Figure 6. Plot of receiver sensitivity 0.1 I I0 versus bit rate for reported long- Bit ra~e (Gb/s) wavelength OEIC receivers.

(APD) flip-chip bonded on to the Si chip (Hamano et a11991). This receiver showed an excellent sensitivity of - 23 dBm with a bit error rate of 10-11 under a transmission bit rate of 10 Gbps. Such improvement of sensitivity has been realised by the reduction of noise induced by parasitic reaction at the device interface. A number of reports have been published on monolithic OEIC receivers integrating PIN photodiodes with amplifiers using not only GaAs-based and InP-based materials systems but also nonlattice-matched heteroepitaxial systems such as GaAs on InP. OEIC technology has also been utilised for fabricating ultra-fast transmitters. Recent demonstration of an InP-based DFB laser/heterojunction bipolar transistor (HBT) driver OEIC has achieved 10 Gbps modulation (Lo et al 1990). To show the current status of OEIC performance, we summarise in figure 6 the receiver sensitivity versus bit rate relationship for recent long-wavelength PIN photodiode-based OEIC receivers. Early OEICs suffered from degraded performance, but this problem has nearly been solved by the recent introduction of low-capacitance photodiodes and high-speed transistors such as high electron mobility transistors (HEMT) (Nobuhara et al 1988; Hayashi et al 1991), HaT (Chandrasekhar-et al 1990), heterojunction FET (Lee et al 1990) and heteroepitaxial GaAs MESFET (Suzuki et al 1987a). Flip-chip integrated receivers with PIN photodiodes (Sussman et a11985; Wada et a11991) and APD (Hamano et at 1991) have already been shown to be comparable to or even better than the best hybrid circuits. Integration can provide advanced optical signal processing function which usually needs tedious assembly. Figure 7 shows the structure and circuit diagram of a monolithic coherent optical receiver chip in which a waveguide directional coupler and a balanced pair of lateral structure PIN photodiodes are integrated using both surfaces of a semi-insulating (sI) InP substrate (Deri et al 1990). In this circuit, a large core size waveguide with GaInAsP/InP diluted multi-quantum well (MQW) structure was used to obtain an excellent coupling efficiency (0.2 dB) between the input fibre and the circuit. The signal and local oscillator powers are mixed in the coupler and fed into the detector through the transparent substrate to produce a beat frequency output. Heterodyne detection with a sufficient signal to noise ratio (> 20 dB) at 1.8 GHz was demonstrated for this circuit (Yasuoka et al 1991). Such photonic integration has also been applied to incorporate even a local oscillator laser within a chip, producing a compact coherent receiver (Takeuchi et al 1989; Koch & Koren 1991). Integrated optoelectronics for optical interconnections 437 . --Twin Signal -- ~ ~ ,~photodiodes uooo, ---l--f °u'p°' oscillator I 3-dBcoupler ~, [

Diluted-MQW Signal wave__Lot. OSC.

Mirror ~ J/l

Top view ~ SI-InP substrate

Figure 7. Equivalent circuit and schematic diagrams showing top and bottom surfaces of coherent receiver incorporating Botto~~ a directional coupler and twin-PIN photodiodes optically coupled through a transparent InP substrate.

As described above, integration techniques in different categories have been used to improve performance, function and manufacture of optoelectronic components. Although the examples shown above are mostly of InP-based circuits, which reflect the current trend of using single mode fibres, there have been a number of successful demonstrations on GaAs-based circuits including OEIC transmitters and receivers. Thus integrated optoelectronics is expected to be used in a variety of areas of telecommunication applications; one area is that of low-cost, easy-to-use components for widespread applications to LAN and subscriber loops, and the other is that of advanced components with high performance and intelligent functions for large data capacity transmission systems. Further development of the fabrication technology promises the implementation of integrated optoelectronics in practical telecommunication systems.

4. Integrated optoelectronics for optical interconnections and signal processing

As the performance and speed of electronic digital systems increase, the overall performance begins to be limited not only by the speed of the logic devices but also by the interconnection speed within the system. This difficulty is expected to be overcome by the use of optical interconnections between processors, boards, chips and gates instead of conventional electrical wiring (Goodman et al 1984). Not being limited in connections, the application of optical techniques extensively in signal processing can provide the means to handle massive amounts of data. In this direction, optical interconnections can be most effectively used when implemented with flexible connection functions such as reconfigurability and reprogrammability. Thus optical interconnections can be taken up as the basic technique to be applied for improving the overall performance of digital systems. In what follows, we explain the advantages and requirements of optical interconnections and discuss the role of integrated optoelectronics in developing optical interconnection devices. 438 Osamu Wada

Fully optical system Control signal

Optoelectronic system Control signal __j~l Figure 8. Schematicdiagram of optical interconnections in fully optical and optoelectronic systems. Parts affecting three key factors, data amount, interconnection function and transmission I:' 'd '1 I I I \ distance, are indicated by marks A, F and F '- L " F L, respectively.

4.1 Three key factors and requirements of optical interconnections

Figure 8 shows schematic diagrams of optical interconnections in both fully optical and optoelectronic systems. In a fully optical system, both input and output signals are optical and interconnection function is controlled with the aid of an interconnection device with external optical control signals. Optical signals are transmitted through media appropriate for the transmission distance. The first application of optical interconnections will be the introduction into existing electronic digital systems such as computers. Such a system would be optoelectronic, where signals are converted and the necessary function is performed by interconnection devices at both input and output interfaces with electrical control signals. For both fully optical and optoelectronic systems, there are three key factors for characterising optical interconnections, namely, the amount of data, the interconnection distance and the interconnection function. The data amount is determined by the transmission data rate and the data multiplexing technique. Data multiplexing can be performed not only in the time division (TDM)and in the space division (SDM) but also in wavelength (WDM) or optical frequency division (FDM), which is characteristic of the optical method. The interconnection distance varies from kilometres to microns depending on the interconnection level. The interconnection functions include both fixed point-to-point connections and more flexible connection with reconfigurability. Figure 9 shows areas of electrical and optical interconnections in the distance versus data rate relationship. Also shown are representative areas of present and feasible optical telecommunication systems. In the currently used advanced digital computers, for example, electrical interconnections are used at data rates below 100 Mbps over a distance of several tens of metres by various wiring with coaxial cables, for example. As the distance increases, electrical interconnections suffer from a frequency dependent loss induced by the skin effect (Tsang 1990). Low loss coaxial cables are used in particular systems such as cable-aided television (CATV)distribution systems to avoid this loss problem. However, these cables have large diameters and cannot be applied in the limited space within digital systems. Light can be confined in a small size waveguide and collimated in free space without any problem of cross-talk, providing high parallelism (Haugen et al 1986). Assuming a couple of microns sizes of the waveguide and interconnection device, a data channel density in the range of 104-10S/mm 2 should Integrated optoelectronics for optical interconnections 439

^ ~ r- LD modulation 10 3 km . uptical amp. ""~1 limit t~ntie~Optic9l lrunkl \ ~ [interconnectiom, ~ t lineJ \~ ~..iSingle- mode fiber ~ I km ~~----Groded - index ~~ ~ fiber ffl i:5 ~Ele'ctrical "11~ Im interconnections I~ Figure 9. Limits for representative transmission media shown in transmission 'N'~'4'~'~ I Superconducting distance versus bit rate relationship. Also ~"~"e i~ wires shown are areas in which optical I mm * i ~ I [~1 i i t p telecommunications and optical and IM IG tT electrical interconnections are effectively Data rate (bps) used. be achievable. The use of free space transmission provides higher density. Also longer transmission distances can be provided by optical interconnections up to the level where optical telecommunication systems are operating because of the large bandwidth in optical transmission media such as fibres. The data rate is restricted by the dispersion characteristics of the transmission media. The development of superconducting wires can extend the data rate of electrical interconnections in the future, provided practical techniques for implementation are developed. The maximum data rate of optical interconnections is practically limited by the light source modulation speed. Modulation of about several tens of gigabits per second is already possible in present lasers. Thus optical interconnections can provide extremely large data transmission capacity within limited time and space. An appropriate transmission medium Can be selected from fibres, waveguides and free space, depending on the distance needed, as shown in figure 10. To take full advantage of using light, optical interconnections are required most in high data rate applications as indicated by shaded areas in figure 10.

103 km Optical interconnections

G) I km 0 r" 2 (/t Electrical ! .... C3 Im

Figure 10. Areas of optical interconnections suitable for different optical transmission media shown in distance I mm I I I [ I I I i versus bit rate plane. Optical interconnec- I'M IG IT P tions are capable of higher bit rate regions Data rate (bps) (shaded areas) than electrical ones. 440 Osamu Wada

For achieving necessary signal processing functions using electrical interconnections, devices have to be wired in a complicated way with variations of distance and fan-out. As wiring density increases, problems of transmission speed, signal skew between channels, noise and cross-talk become more severe. High fan-out requirements constrain the impedance matching and grounding design. These problems may be avoided by increasing the power to drive heavier interconnection loads, which produces even worse problems of size and heat removal. Optical interconnections can solve most of these problems by their advantages of no cross-talk, no RE delay and no impedance mismatching. Optical signals will enable logic processing unlimited by transmission as the development of optical functional devices proceeds. Combining these functional devices with free space parallel interconnections will provide efficient functions for processing massive data such as that of images, leading to the application to optical computers and processing systems.

4.2 Optical interconnection devices

Figure 11 is a map to categorise interconnection devices using three key factors of interconnection. The interconnection distance ranges over kilometres for intercomputer, down to microns for intra-chip, gate-to-gate interconnections. The dimension used for multiplexing data includes time, space and other parameters characteristic of light, most typically the wavelength. The interconnection function can be, most simply, fixed point-to-point connections, but can be more flexible with

Interconnectil distance lg

ical Inter- itrol Computer I

Module

Board Chip ible cal Gate ~ection

f Reconfigurable Dynam~ Interconnection Fixed L__ Flexible --J function Figure 11. Map of optical interconnection devices with reference to three key interconnection factors (axes). Representative devices and circuits so far proposed are included. Two major groups (shaded areas) aim at multiplexing data in various dimensions and also generating flexible interconnection functions. Integrated optoelectronics for optical interconnections 441 reconfigurability and dynamic switching. For controlling the function, electrical control is easily used in optoelectronic systems and optical control is needed for fully optical systems. We note that there are two important categories of optical interconnection devicesas indicated by shaded areas in figure 11. One is multiplexing optical interconnections in which very large amounts of data are multiplexed by using various techniques such as TDM, SDM and WDM. Another category is flexible optical interconnections in which arbitrary connections among many signal channels are realised. One of the most promising techniques for generating flexibility is in the use of two-dimensionally integrated devices for high parallelism.

4.3 Multiplexing optical interconnection devices

For connecting subsystems or processors within a computer system, fibre optical links incorporating array lasers and array photodetectors are used. Twelve-channel, InP- based light emitting diode (LED) and PIN photodiode arrays have been used with the hybrid assembly technique for such links operating at 14 Mbps (Kaede et al 1990) to 200 Mbps (Ohta & Swarz 1991) over distances of kilometre range. OEIC techniques are applied for fabricating more compact, high-performance links. Figure 12 shows the structure and circuit diagram of a multi-channel OEIC transmitter array fabricated on Si-GaAs substrate (Wada et al 1989). A1GaAs/GaAs quantum well (QM) lasers and GaAs MESFET were fabricated using molecular-beam epitaxy. Lasers showed fairly uniform threshold current around 17 mA over eight channels. The overall OEIC array showed high-speed modulation up to 2 Gbps. Figure 13 shows the structure and circuit diagram of a GaAs-based OEE receiver array (Wada et al 1986). Planar metal-semiconductor-metal (MSM) photodiode involving a pair of Schottky barriers on GaAs was employed for simplifying the overall integrated structure as shown in figure 13. Uniform receiver sensitivity of 105 V/W +/- 10V/W and high- speed operation at 1.5Gbps were demonstrated with sufficiently low cross-talk (- 37 dB at 0.6 GHz) in an eight-channel array. As an application of these transmitter and receiver pairs, a 4 x 4 optical switch was fabricated using four-channel OEIC arrays and a GaAs electrical switch, the structure of which is shown in figure 14 (Iwama et al

Monitor photodiode Bias line "3 ,/\ A c ~ (~/GaAsFET//7 I AIGaAs/GoAs',,.ZA driver '/ / / / )

I S -Go , ,,,b. V vs _L Structure Circuit Figure 12. Structure and circuit diagram of multi-channel GaAs-based OEIC transmitter array containing Qw lasers, monitor photodiodes and GaAs MESFET driver circuits. 442 Osamu Wada

MSM-PD AI FET Au/AuGe

m m~/ r ~-~--~n-GOAs --Undoped GoAs SI-GoAs substrate

Structure

VD2

MSM-PD

OUT

Figure 13. Structure and circuit diagram of OEIC receiver incorporating Circuit GaAs MSM photodiodes and GaAs Ve vs MESFET amplifiers.

1988). This switch module showed full optical switch operation at 560MHz with cross-talk less than -20 dB. Similar GaAs-based OE[¢ techniques were used in multi-channel optical interconnection for computer links transmitting 1 Gbps signals over a kilometer. An OEIC receiver chip used had four MSM photodiode/preamplifier channels and deserialising and clock recovery circuits consisting of 8000 GaAs MESFET (Crow 1989, p. 83, 1991; Ewen et al 1991). Two-dimensional laser arrays have been developed using surface emitting laser diode (SELD) structures. Various efforts for lowering the threshold current such as introducing Qw laser structures and multi-layer, high-reflectivity mirrors have made cw operation possible at room temperature (Iga et al 1988). Such two-dimensional

Tapered end LD fiber array Coupling ~I J~'9 oo o ,,onc

~lC transmitter

4 x4 ' GaA, switch 0.83 ,m

Slanted end t" OEIC receiver <20 dB fibre array ~ P = 3.7 W PD Figure 14. Schematicdiagram of four-channel optical switch composed of four- channel GaAs-based OEIC receiver and transmitter arrays and a 4 x 4 GaAs-Ic switch. Integrated optoelectronics for optical interconnections 443

~~ Transmission ~__~ medium Figure 15. Block diagram of optical interconnection system exhibiting wavelength division multiplexed Tunable lasers Tunable detector,, switchingfunction. integration is extremely important for parallel interconnections between boards, for example, as has been suggested in figure 1. Computer-generated (CG) hologram mirrors can be used for the free space coupling of two-dimensional optical signals at the chip- to-chip level (Feldman et al 1988). Beam deflection lasers were fabricated using the current-induced refractive index change in an InP-based laser with a second-order grating structure (Yamashita et al 1990, pp. 125-128) and in a GaAs-based laser with twin-stripe waveguide structure (Mukai et a11985). Such lasers can provide capability of steering the beam effectively in three dimensions. Basic logic functions were demonstrated for twin-stripe lasers (Itoh et al 1991). Further studies particularly on suitable system architectures are required for fully utilising such beam steering functions. Wavelength tunable lasers have been developed for coherent optical telecommunication applications. The same concept should be useful for realising highly multiplexed data transmission in optical interconnections. In order to construct WDM optical interconnection systems, an example of which is depicted in figure 15, both tunable emitters and selective wavelength receivers must be developed. Figure 16 shows an example of filter characteristics obtained in a GaInAsP/InP DFB laser-based filter together with its schematic device structure (Tanaka et al 1991). The device operation principle is in the refractive index change in a tuning layer induced by the current injection (It). A very wide tuning range over 4.2nm was achieved around 1-55 #m with a very narrow FWHM of 0.02 nm and a fairly large isolation of 11 dB by the control of the tuning current over 3 mA. This indicates a possibility of multiplexing more than a hundred data channels in this wavelength range. Figure 17 shows a GaInAsP/InP wavelength conversion laser fabricated by integratiiag a two-section bistable laser and a DBR wavelength tuner (Kondo et al 1990). Optical bistability is generated within the two-section laser part in which the unconnected part of the cavity acts as a saturable absorber. The third contact is used for

mA ~4.2 nm~ Ia:O.981m ~'~ JO ~ Active layer [~Ia 10.02nr~ " i\\ 1 ! ~" J . ~ ~,Tl/J//'2~////////////////////////////////////.:J 1 "~ 5 I I dB ,, L,ght

~ l Tuning'/layer/,L /I I'+I°lli= "~ O, I~ '.~ - " / 'I, '~ _5l ~ ~l ~./4-shifted 8 I- od~/ I ~ corrugation / [t:3mA I YiIt =0 -io Figure 16. Wavelength de- pendence of transmission char- ~' -15 acteristics of GaInAsP/InP 1.55 1.552 1.554 1.556 1.558 1.560 laser-based wavelength filter Wavelength (tJm) device (after Tanaka et al 1991). 444 Osamu Wada

Tuni~ DBR section Electrode ~/'Phase shifter section Contact / ~ layer Activation 2 / n-,nP ~-~ Section Figure 17. Structure of wave- InGaAsP antimeltback length conversion laser integrating a bistable gain section, a InGaAsP active" -- • ~ "Corrugation phase shifter section and a DBR n-lnP / tuning section on an InP n.lnGaAsP / substrate (after Kondo et al guide 1990). matching the phase between the bistable and DBR parts. This integrated device exhibits a wide wavelength tunability over 3-5 nm around 1.54 #m. In additionto this, a variety of optical logic functions are possible on the basis of saturable absorber and gain quenching. By adjusting the active laser current (!a) with respect to the light-current hysteresis loop, the operation mode can be selected as is indicated in figure 18 (Nobuhara et al 1989; Wada & Yamakoshi 1990, pp. 28-36). Optical logic operations including flip-flop and several other functions are demonstrated at an optical input data rate of 1 Gbps. Integrated optical functional devices provide the capability to manipulate not only the intensity of light but also the wavelength, opening up the possibility of processing massive data.

4.4 Flexible optical interconnection devices

Flexibility of connection is a prerequisite for the wide use of optical interconnections in digital signal processing. Waveguide switches made either of dielectric materials such as LiNbO3 or III-V compound Semiconductors and liquid crystal light valves (LCLV) have been studied for flexible interconnections. More recent investigations have focused on the development of optical switching devices using III-V semiconductor heterostructures, as indicated in figure 11. Figure 19 shows a few examples of optical bistable switches based on III-V semiconductor materials (Wada & Yamakoshi 1990, pp. 28-36). Semiconductor etalons can be used in fully optical systems but tight wavelength control is essential because of the use of optical cavity structures. In order to make fully optical signal processing practical, materials with high nonlinearity have to be developed further. The use of III-V semiconductors must be useful in this area, too, as has been proposed by A1GaAs/GaAs MQW photo~efractive devices exhibiting subnanosecond response (Ralph et al 1989). Bistable lasers are characterised by their possessing optical gain, which is very useful for realising cascadability in multi-stage processing circuits. Operating speed is high because the device operates in high-injection condition, as has Figure 18. Diagrams showing Lo%,~ Memory Lout~ Inverter input light-current-output light .¢///A CNOT~ relations for various bias ~ NOR/NAND current settings in wavelength conversion lasers. Note that a --~ Li n ...... -~ LIn variety of optical logic functions are achieved (after Nobuhara ~ et al 1989). Integrated optoelectronics for optical interconnections 445

Type Effect Structure Feature Optical power All optical Etalon dependent --~ Bulkor i""~ refractive index MQW Tight wavelength LMi RRORA control Field dependent CR limited SEED absorption and response speed external feedback Integrable

VSTEP Phototransistor Wide wavelength DOES action and range LED/HPT external feedback Integrable

Optical gain Bistable Saturable laser absorption Wavelength conversion

Figure 19. Different types of devices for generating optical bistability. already been shown by an example in the previous subsection. The remaining problem of these devices is fairly large DC power consumption: efforts for lowering the threshold current of lasers are expected to solve this. Various integrated optical functional devices such as self-electrooptic devices (SEED) (Miller et al 1985), vertical-to-surface transmission electrophotonic (VSTEP) devices (Kasahara et al 1988), integrated LED/heterostructure phototransistors (HPT) (Matsuda et al 1990), double heterostructure optoelectronic switches (DOES) (Taylor et al 1986) and a few other similar devices have been proposed. Since the optical bistability they exhibit is due to external feedback loops, they usually have a tradeoff between the input power necessary for switching and the switching speed. Recent experiments with these devices have shown nanosecond speed with microwatt optical power corresponding to a switching energy below 1 pJ. The important advantages of these integrated functional devices include suitability for two-dimensional integration and controllability of interconnection function by external signals. Two-dimensional arrays of the above mentioned devices have been performed up to a level of 2 to 3 kbit. Based on these advantages, they must play an extremely important role in parallel flexible interconnections. A few basic switching and computing demonstrations, including a SEED-based system using the symbolic substitution architecture (Streibel et al 1989), have been reported (Athale 1990). It is noted from this discussion that two-dimensional integration of optical functional devices together with the supply of their control signals is indispensable for developing flexible optical interconnections and signal processing systems. This is clearly seen in a recent proposal for vertical inter-wafer optical interconnection in a Si- based multiwafer memory system. Figure 20 shows the structure of an optically coupled common memory (OCCM), which features fast free-space optical data transfer in the vertical direction combined with conventional electrical interconnections in the horizontal direction, providing intelligent memory for real-time parallel processor systems (Koyanagi et al 1990). Heteroepitaxial LED and photodiodes are integrated with the host Si memory cells in this system. The increase of data transfer speed is 446 Osamu Wada

Y-Z/ M,mor, ,+, , cou0,o0

Figure 20. Schematic diagram showing multi-layer structure ~\jCPU. i_I.J__7// v / Z~/ Memory,-, for three dimensional optically coupled common memory. Vertical inter-layer data transfer is carried out through 1 optical interconnections (after ~ Memory Koyanagi et al 1990). estimated to exceed 128 Gbps for four-layer design. Future development of fabrication technology, such as low-temperature heteroepitaxy will enable experimental verification.

5. Future technology

Integrated optoelectronics is useful for optical telecommunication systems and will become increasingly important for optical interconnections and signal processing systems. Future development is expected to cover a wide range of technological areas. Figure 21 illustrates three major technological areas to support the development of optical interconnection devices. As already mentioned, integrated optoelectronics will be a prerequisite. However, design of integrated optoelectronic circuits and devices depends on the requirements of application systems. In particular, for optical interconnections and signal processing applications, design must be determined through collaboration with architects to determine the most effective way of using optical techniques in particular target systems. Materials and device processing technologies are crucial for developing practic~il integrated circuits and devices. Integrating different types of elements is desired in many cases. Non-lattice-matched heteroepitaxy is important in combining materials suitable for electronics and optoelectronics. III-V semiconductor growth on Si

Figure 21. Basic technological areas for developing practical optical interconnection devices. Integrated optoelectronics for optical interconnections 447 substrates, for example, is an attractive technique for wafer and chip level interconnections. Processing techniques must be improved for uniformity over a large wafer area and for controllability and reproducibility in defining fine geometries. Dry processing techniques with the aid of electron and ion beams will be useful in this regard. An in-situ process to combine crystal growth and device processing has been proposed (Takamori et al 1987) and such challenges are very important for reliable device production. Requirements in the packaging of integrated optoelectronics include compactness, low-loss optical coupling, high-speed electrical interfacing and efficient heat removal. A great deal of effort is expected to be devoted to packaging technology, since it plays a more important role in device implementation, than to discrete devices. Multi-channel optical coupling is essential for two-dimensional array devices and this remains to be developed for free space coupling. Development of self-alignment techniques is necessary for practical assembly.

6. Conclusion

We have described recent advances in integrated optoelectronics for optical telecommunication systems and have also shown their applicability to optical interconnections and signal processing systems. Optical interconnections should be able to solve the wiring limit which conventional electronic digital processing systems face, provided practical devices are developed. Towards this goal, the importance of integrated optoelectronics technology is now being recognised widely in the research community and various challenges are being tackled. Further efforts with synergetic collaboration among different technological areas such as material and processing, design and packaging technologies hold the promise 6f realisation of practical optical interconnection devices.

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