the packaging of large spot-size optoelectronic devices

IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY—PART B, VOL. 20, NO. 4, NOVEMBER 1997 403

The Packaging of Large Spot-SizeOptoelectronic Devices

John V. Collins, Ian F. Lealman, Phil J. Fiddyment, Adrian R. Thurlow,Colin W. Ford, Dave C. Rogers, and Carole A. Jones

Abstract—The demand for more bandwidth to the home isstarting to drive optical fiber Telecommunication systems furtherinto the Access network. For fiber systems to reach directly intocustomer premises the cost of the optoelectronic recievers andtransmitters have to be significantly reduced. The major cost ofproducing these components is the active alignment step to couplean optical fiber to the semiconductor device. This paper detailshow by matching the output radiation pattern of the device tothe input radiation pattern of an optical fiber low-cost passivealignments processes can be utilized.

Index Terms—Fiber to the home, lasers, mode conversion,packaging, passive alignment.

I. INTRODUCTION

OVERCOMING the large difference between the outputradiation patterns of opto-electronic devices and the

acceptance angles of singlemode optical fibers has been thechallenge to packaging engineers for the past two decades.

Fig. 1 shows the output radiation pattern of a typical semi-conductor laser and the input acceptance angle of a single-mode optical fiber. Straightforward coupling of lasers to fibersdemonstrate 10 dB coupling, i.e only 10% of the laseremission is captured by the optical fiber. Traditionally anoptical element is interposed between the laser and fiberto increase the coupling. The simplest and a quite efficientelement is a lens polished on the fiber end. Fig. 2 showsthe coupling obtained using a lensed ended fiber. It can beseen that although the coupling is quite acceptable (60%)if the fiber is moved slightly from its optimum couplingposition then the coupling efficiency drops rapidly (Fig. 2).The process of aligning and fixing in place optical fibersto laser diodes to the very tight tolerances required hasdominated the cost of producing opto-electronic components.The packaging of optoelectronic devices can be considerablysimplified by increasing the spot size of their optical emissionto better match that of the optical fiber.

II. SPOT SIZE CONVERSION

Fig. 3 shows a schematic of the epitaxial layer structure ofa 1.55 m Fabry-Perot laser with an 8 well compressivelystrained active layer, [1]. The laser active area is tapered. Asthe optical mode encounters the tapered region it adiabatically

Manuscript received April 7, 1997; revised June 30, 1997.The authors are with BT Laboratories, Ipswich, Suffolk, U.K.Publisher Item Identifier S 1070-9894(97)07424-0.

Fig. 1. Laser to fiber coupling schematic.

Fig. 2. Coupling efficiency of laser to fiber.

expands into the underlying passive guide. The dimensionsof the passive guide were determined by the need to closelymatch the near field laser spot size to that of the cleaved fiber.A gaussian profile with a 9 m 1/e diameter was used tosimulate the electric field of the guided mode in the singlemode fiber. The laser spot size was then calculated, usinga variable grid finite difference program, for a wide rangeof guide dimensions. The width of the 1.1m wavelengthquaternary guide was calculated to be 7m, and its thickness0.07 m to give a close match to the fiber guided mode.Calculations of field overlap between the passive guide andtapered active layer showed that an active width of0.2

m was required at the end of the taper if the coupling lossbetween these two sections was to be kept below 1 dB [1].Calculation of the critical taper length for this structure andminimum active width [1], [2] yielded a minimum taper lengthof 440 m.

III. CHIP DIMENSIONAL CONTROL

If the opto-electronic device is going to be aligned ina passive manner, i.e. the device is not turned on during

1070–9894/97$10.00 1997 IEEE

404 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY—PART B, VOL. 20, NO. 4, NOVEMBER 1997

Fig. 3. Schematic of laser active region.

the alignment process, it is necessary to accurately definean alignment surface on the chip with respect to the lightemission area. In the laser production process the first stage ofphotolithography is used to define both the passive guide ofthe large spot size device [2] and to form channels at the edgesof the device to define scribe lanes. This allows the edges ofthe scribe lanes to be defined with an accuracy of 0.2 m,relative to the passive guide, determined by the undercut ofetchants used at later stages. In the final stages of processingthe buried scribe channels are exposed and the scribe lanesetched using 4:1 This forms a U shaped channelapproximately 20 m deep and 5 m wide, the top cornersof which are defined by the 1.1m quaternary of the passiveguide. The fabricated devices were cut into devices 1.1 mmlong by 300 m wide (defined by the scribe lanes). The lengthof the device was made up of 340m of untapered activeregion, a 460 m tapered region and 300m passive region.

IV. SILICON OPTICAL BENCH

Silicon micromachining techniques were used to fabricatethe optical bench on which the optical components werepassively aligned. The main features of the silicon opticalbench are a V-groove for fiber attachment and a silica stopagainst which the precision cleaved edge of the laser chip canbe aligned (Fig. 4).

The 15 m high silica stops were deposited by PlasmaEnhanced Chemical Vapor Deposition and etched by a com-bination of Reactive Ion Etching and wet chemical etching.Bond-pads were formed by e-beam evaporation of 50 nmtitanium, 200 nm platinum and 200 nm gold, and werepatterned using lift-off. Gold-tin (3 m) solder was thenthermally evaporated onto these metal pads, and also patternedusing lift-off. The fiber V-grooves were formed by anisotropicetching of (100) silicon wafers in ethylene diamine and py-rocatechol, using a silicon nitride mask. Grooves were etchedperpendicular to the fiber V-grooves (as shown in Fig. 4) toenable the adhesive used to hold the fiber in place to flowfreely underneath the fiber. A shallow saw-cut was made atthe end of the V-grooves to remove the sloping end-face, andallow the fiber to be brought close to the laser facet.

Fig. 4. Silicon optical bench.

V. PASSIVE ALIGNMENT

The lasers were bonded against the stop on the silicon benchon a conventional laser die bonder. A cleaved singlemodeoptical fiber was then laid in the V-groove on the optical benchand glued in place. A lid made from a piece of silicon withan etched V-groove was applied over the top of the fiber, toclamp the fiber firmly in place and provide rigidity. Couplingefficiencies as high as 55% have been obtained [3].

In order to compare the passive alignment performanceof the large spot lasers to conventional devices a wafer ofstandard lasers was fabricated with the precision cleavingfeature. These lasers were bonded onto silicon benches. Lensedfibers were required to give an acceptable coupling efficiency.As the fibers are aligned passively it is important that thelens on the fiber end is perfecly centred on the fiber core togive best coupling. Lensed fibers are fabricated by a numberof means, the most common being polishing, arc fusion oretching. Polished or fused lenses are not necessarily centeredon the fiber core which can lead to excess loss when couplingto a device. (Fig. 5). Etched lenses which rely on the differentetching properties of the fiber core to cladding in bufferedhydroflouric acid [4] are by their nature completely concentricand can therefore be considered for passive alignment. Fig. 6shows a schematic of an etched lensed fiber on a siliconoptical bench, Fig. 7 shows the coupling results from a batchof conventional lasers aligned to etched lensed fibers on siliconoptical benches. It can be seen that although there are somegood results the overall yield of well coupled devices is lowand would not be acceptable for a production process.

The yield is low because the very tight alignment tolerances(Fig. 8) cannot be met. Fig. 8 shows that the cumulative errorsfrom the device cleaving, the bench etching, device bondingand fiber fixing need to be less than 1micron to give goodcoupling compared to several micron (5 m) for the largespot laser (Fig. 2).

VI. NON-HERMETIC PACKAGING

Generally it has been necessary to hermetically packagelasers to achieve the reliability requirements for Telecom de-vices, however, it has been shown that nonhermetic packages

COLLINS et al.: PACKAGING OF LARGE SPOT-SIZE OPTOELECTRONIC DEVICES 405

Fig. 5. Coupling of polished lensed fibers with lens offset from center ofcore.

Fig. 6. Cross section of etched lensed fiber on optical bench.

Fig. 7. Coupling results of conventional lasers passively aligned on opticalbench.

can give the reliability required for local access applications[5]. The entire laser and the fiber end were covered in aprotective gel coating and then encapsulated in a standardpotting compound to demonstrate a potentially simple pack-aging process for lasers (see Fig. 9). Coating the laser andfiber end in the gel affects the laser performance. The laserthreshold increases slightly because the effective end face

Fig. 8. Typical laser to etched fiber coupling.

Fig. 9. Potted laser package.

mirror reflectivity is reduced. The coupling efficiency into thefiber rises as the laser light divergence angle is lower in thehigher refractive index gel than air and hence the couplinginto the fiber is further improved. Also, the refractive indexof the gel coating is very close to that of the fiber core whichsignificantly reduces the backreflection from the fiber endfaceinto the laser.

Coupling efficiencies are consistently achieved as high as65% with gel coated devices.

VII. PACKAGELESS LASER

Large spot lasers without the precision cleaving feature wereused to assess a very simple laser package, “the packagelesslaser” (Fig. 10).

Lasers were bonded to a heatsink and manipulated in frontof a fiber mounted in a specially designed ferrule (Fig. 10).When maximum coupling was achieved the assembly wasglued together. After assembly the device is covered in agel coating, coupling efficiences approaching 70%, have beenobtained [6].

Although this package requires active alignment, the assem-bly processes and equipment are quick and extremely simple.The tolerances to misalignment of the large spot laser (Fig. 2)means that these processes are suitable for automation.


Fig. 10. Schematic of “packageless laser.”

Fig. 11. Lifetest for device at constant output power 60�C.

VIII. L IFETEST RESULTS

Lifetest and thermal cycling tests have been carried out onthe nonhermetic silicon optical bench devices and the pack-ageless lasers. The thermal cycling tests have been graduallyincreased upto Bellcore specifications, (40 to 85 C) andno measureable changes in coupling have been seen after 98cycles.

Fig. 11 shows the lifetest results of a typical silicon opticalbench device running at 60C, 3 dBm constant fiber power.It shows that a stable output device can be obtained.

IX. A RRAY PACKAGING

The packaging of arrays of opto-electronic devices can alsobe simplified by increasing the spot size of each device in thearray.

One application is the need to address and route informationin a 100 Gb/s optical packet network [7]. The optical signalon a fiber needs to be split into eight paths, each path canthen be separately switched and assigned a specified delay, themodified signal is then recombined at the component output.Fig. 12 shows a schematic of the device.

The optical fiber enters the component on the left-hand side,it is coupled to a 1 8 splitter waveguide device fabricatedby flame hydrolysis on a silicon substrate. This splitter is thencoupled to an array of eight large spot size semiconductoramplifiers. The semiconductor amplifiers act as the switchingelements as well as amplifying the signal to overcome thecomponent losses. The amplifiers are coupled to anotherarray of flame hydrolysis waveguides, these waveguides haveprecision delays built into their optical path lengths and arerecombined onto a single optical fiber at the component output.

Fig. 12. Schematic of delay array device.

Fig. 13. Packaged delay array device.

Assembly of the component requires four optical alignmentprocesses. The most critical alignment steps are the alignmentsof the waveguide chips to the semiconductor laser amplifierswhere all six axes of movement require adjustment. Theamplifiers are turned on and their spontaneous emission is usedto monitor the best coupling through the silicon waveguidechips, each active alignment takes considerable time and effort.

With a conventional semiconductor amplifier array a lensarray would be required between either side of the amplifiersand the waveguides to give an acceptable coupling efficiency.In this case, the tolerances to misalignment would be less than1 m to obtain reasonable coupling across the whole array.Such a tight alignment tolerance and the number of piecepartsto be manipulated would make the assembly of such a devicealmost impossible.

Components have been assembled (Fig. 13), however evenwith the large spot amplifiers it is a difficult and time consum-ing process and is unlikely to be viable as a manufacturingmethod.

A passively aligned component based on an array of largespot-sized semiconductor amplifiers has been designed, asshown in Fig. 14. The amplifier array, which has angledfacets to reduce reflections inside the chip, is bonded againstalignment stops on an upstand on a silicon micromachinedcarrier. The waveguide splitter array and precision delaydevice are flipped onto the carrier and silicon oxide upstands

COLLINS et al.: PACKAGING OF LARGE SPOT-SIZE OPTOELECTRONIC DEVICES 407

Fig. 14. Micromachined delay array device.

on the splitter and delay device are aligned into groovesin the carrier. The input and output fibers are then finallyaligned to the waveguide devices. This passive assembly ofthis component will be considerably simpler and quicker thanthe actively aligned device and uses techniques already provenon simpler devices.

X. CONCLUSION

Lasers have been passively aligned to cleaved singlemodeoptical fibers on a silicon bench with coupling efficiencies ofover 50%. This is the highest known reported result.

Using the relaxed tolerances obtained from large spotsizelasers a very simple high performance laser package has alsobe produced.

The combination of semiconductor device developments,silicon micromachining and novel packaging techniques hasrealized complicated optoelectronic modules which will givethe technical performance and economic requirements neededfor future optical telecommunication networks.

ACKNOWLEDGMENT

The authors would like to acknowledge the support of thefollowing colleagues at BT Labs: R. G. Waller, L. J. Rivers, M.J. Harlow, S. D. Perrin, M. W. Nield, K. Cooper, R. A. Payne,B. M. Macdonald, R. Cecil, Y. Yeo, N. Lunt, and finally Dr.H. Ghafouri-Shiraz, Birmingham University, U.K..

REFERENCES

[1] I. F. Lealman, L. J. Rivers, M. J. Harlow, and S. D. Perrin, “InGaAsP/InPtapered active layer multiquantum well laser with 1.8 dB couplingloss to cleaved singlemode fiber,”Electron. Lett., vol. 30, no. 20, pp.1685–1687, 1994.

[2] I. F. Lealman, L. J. Rivers, M. J. Harlow, S. D. Perrin, and M. J. Robert-son, “1.56�m InGaAsP/InP tapered active layer multiquantum welllaser with improved coupling to cleaved singlemode fiber,”Electron.Lett., vol. 30, no. 11, pp. 857–859, 1994.

[3] J. V. Collins, I. F. Lealman, P. J. Fiddyment, C. A. Jones, R. G.Waller, L. J. Rivers, K. Cooper, S. D. Perrin, and M. W. Nield, “Passivealignment of a tapered laser with more than 50% coupling efficiency.”Electron Lett, vol. 31, no. 9, pp. 730–731, 1995.

[4] A. Kotsas, H. Ghafouri-Shiraz, and T. S. M. Maclean, “Microlensfabrication on single-mode fibers for efficient coupling from laserdiodes.”Optic. Quantum Electron., vol. 23 pp. 367–378, 1991.

[5] I. P. Hall “Non-hermetic encapsulation and assembly techniques foropto-electronic applications” inProc. 10th EMC, Copenhagen, Den-mark, May 1995.

[6] J. V. Collins, B. M. Macdonald, I. F. Lealman, and C. A. Jones, “Newtechnology developments make passive laser/fiber alignment a reality,”in Proc. Laser Diode Chip Packag. Technol. Conf. ’95, SPIE, 1995.

[7] D. C. Rogers, J. V. Collins, C. W. Ford, P. J. Fiddyment, J. Lucek, M.Shabeer, G. Sherlock, D. Cotter, K. Smith, J. D. Burton, C. M. Peed,A. E. Kelly, and P. McKee, “Demonstration of a programmable opticalpulse pattern generator for 100 Gbit/s metworks,”Electron. Lett., vol.31, pp. 2001–2002, 1995.

John V. Collins received the Higher National Certificate in electrical andelectronic engineering from Ipswich Civic College, U.K., the M.S. degree insemiconductor materials and devices from Thames Polytechnic, U.K., and iscurrently pursuing the Ph.D. degree at Birmingham University, Birmingham,U.K.

He joined BT Laboratories, Ipswich, U.K., in 1974, and has worked ona wide range of semiconductor related topics including GaAs encapsulation,Impatts and FET’s and semiconductor laser production, measurements, andreliability. For the past six years he has been investigating the packagingof opto-electronic devices. He is currently Team Leader of the low-costpackaging work, BT Labs, Suffolk, U.K.

Ian F. Lealman received the B.Sc. degree (1st class) in physics fromSouthampton University, U.K., in 1985, and is currently pursuing the Ph.D.degree at the University of Essex, U.K.

He joined BT, Ipswich, U.K. in 1985 to work on the development ofsemiconductor lasers. He was part of the team at BT that developed theBH FP laser that was successfully transferred to BT&D Technologies (nowHP), and subsequently spent six months on secondment at BT&D in 1989improving the manufacturing yield of the device. Since returning he hasworked on a number of projects including the development of high speedlasers and electroabsorption modulators. More recently he has been workingon the development of expanded mode devices for low cost optoelectronicsand of widely tunable lasers for WDM systems. He is Manager of the BTresearch project on advanced optoelectronic components.

Phil J. Fiddyment received the B.Sc. and M.Sc. degrees in physics fromThames Polytechnic, London, U.K., in 1968 and 1970, respectively.

He joined British Telecom Laboratories in 1964 where he worked on theresearch and development of silicon transistors, magnetic bubble memories,and semiconductor laser diodes. He retired from BT in 1996.

Adrian R. Thurlow joined BT Laboratories, Ipswich, U.K., as an Apprenticein 1976. On completing his apprenticeship he went on to work in the highreliability silicon devices area, concentrating primarily on ion implantation.In 1989, he joined the optical physics division developing methods utilizinglaser welding as a fiber to opto-electronic package fixing technique. He alsodeveloped single and multiple fiber assemblies designed to be used withthe new fixing method as well as developing further applications of laserwelding during the hermetic packaging of opto-electronic devices. He iscurrently exploring the potential for new, novel mobile terminals along withthe associated applications and wireless communications.

Colin W. Ford joined BT Laboratories, Ipswich, U.K., in 1969 (then GPORes. Dept.) developing packaging techniques and packages for submarinecable systems. This work was to continue through to 1976 when he progressedonto the development of multilayer circuit boards for the system X exchanges.During this period he established a prototype BTAB bonding system forSMART card applications and then in 1985 returned to device integrationand packaging. He was involved in the transfer of packaging technology toBT&D Laboratories (now Hewlett Packard, Ipswich) in the early 1990’s andhas since been responsible for developing high speed packages and the useof high power lasers as a packaging tool.


Dave C. Rogersreceived the B.A. and a D.Phil. degrees, both in physics,from Oxford University, Oxford, U.K., in 1986.

He is a Senior Professional Scientist in the Technology Research Unit, BTLaboratories, Ipswich, U.K. which he joined in 1986. He initially worked inthe Advanced Analytical Techniques Section on electrical and optical analysisof semiconductors and glasses. In 1991, he joined the Optical Glasses andFibers Group to work on semiconductor doped glasses and planar silicawaveguides for optical information processing applications. After spendingtwo years working on geographical information systems and business processmodeling, he recently joined the Advanced Optoelectronics Group, where he isnow working on integrated semiconductor optoelectronic devices for opticalinformation processing.

Carole A. Jones received the B.S. degree (first class honors) in appliedphysics and chemistry, and the Ph.D. degree in thin dielectric films, both fromthe University of Durham, Durham, U.K., in 1984 and 1987, respectively.

She spent one year as a Postdoctoral Research Assistant in the Departmentof Physics, Cambridge University, Cambridge, U.K., before joining BTLaboratories, Ipswich, U.K., in 1988. At BT she has worked on electro-opticpolymers and silicon micro-engineering, and is now studying the applicationof production management principles within BT.

the packaging of large spot-size optoelectronic devices

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