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Progress towards low-cost modules for SNIT

W. Rehm, H.P. Mayer Alcatel Corporate Research Center

Lorenzsir. 10, 70435 Stuttgart, Germany phone +49 71 1 8'11 45301, etnail: wrehm@rcs.sel.de

Introduction Optical communication requires large volumes of optoelectronic (OE) modules. Due to the required

mechanical precision (usually < 1 pm) and long time protection against environmental impacts modules are rathet- expensive. In addition, modules have to be integrated within system electronics. High volunie automatic soldering, however, is incompatible with long fiber pigtails. Therefore, a package integrated connector is an essential feature for low-cost applications.

Several technical approaches targeting individual aspects of the subject are widely reported in the lit- erature. Interesting is an approach [ l ] relying on C D laser technology, however, conimunication applica- tions have strongly different requirements for petformance, reliability and operation environment. Ap- proaches with beam deflections between OE chip arid optical fiber [2] are also reported. Common for all these solutions is a submount (ussembly platforrri), niostly made from silicon, that carries the active devices

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Fig. 1 Submount with stand-offs and lateral stops for laser chip

but ulso some mechanical [3] or optical feutures [4] for uctive or passive uliynmerit cluring assembly. All these subrnount up- procrches - despite of their iridividuul tecliriological differences -

transfer part of the required precisiori into well known batch proc- esses u r d reduce therewith overcill module fabrication cost.

Additioricrl iniportunt cost element of optoelectronic components is the outer package since this riot only defines fabrication cost but also the integratioti technology into the elecironics. Package solu- tions with ceramics [5] or plastics [6] reduce fubrication cost sigtiifi- cantly. Of concern, however, is U potential lorig term degradation due to moisture penctration into the puckage. A very important cost-reducing feature for the end user i s a package integrated fiber connector. Therewith, the module can be incorporated into elec- tronics bv fullv automated Drocesses atid standard Dick and dace

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with batch processes for soldering can be used sirice there is no fiber pigtail. Examples for ;hose solitions can be found, e.g., in [7].

Package Our modules have u common submount hosting the OE chips and a short piece of fiber. Fig. 1 shows

details of this arrangement for un edge ernittirig laser that is flip-chipped to the submount. 3-dimensional features on the submount surface urovide mechanical references. Corresponding features of the O'E chip establish the link to its internal structures. Precisely etched V-grooves host U shoti piece of optical fiber. Self-alignment principles ure used throughout subniourit fabricatiori to control dimensional uccuracy of the various features with respect to each othet-.

The subniounts ore fabricated frotn bare silicon wafers using thin film technologies. The rnosking process directly links the position of the lateral stops to the fiber V-groovc (self-ulignrrient) although these features are fabricated within individuul fubrica- tion steps under dedicated technological parameters.

During the fully passive assembly the pre-aligned (< 10 pni accuracy teq'd) OE chip is driven to its final position by surface

Fig.2 Package concept

0-7803-5947-W00/$10.00@2000 IEEE 714

Fig. 3 SEM of a structurecl submourli

tcrisiori forces of the liquid solder. l l i e ussernbly sequelice is computible with batch reflow of huririreds nf iridividuul sutmouiits at a time.

Central elenient of tho plustic puckage is u MT type opticul coiinechr. The submount with the OE chips (siit)niodule) is ~ j l u e d to ihc MT feri.uk, ct-eaiing CI very coinpwt devici! tliut oiily needs to he protected unci coririeded to tlie outside dectroriics. Fig. 2 shows the principles of the modules.

OE chips have been described iii more detail in [8, 91. A sub- module is presented iii Fig. 3. Typical dimensions ore :-1 x 5 t71n12.

Final aligrirnent precision is determined by the excess loss with respect to petiect alignment iricludincj the accurmy of chip ulign- . - Inent but also piece -port tabi-icotion- With perfec; ulignment the

coupling loss of the tapered laser- to (I cleuved SMF is uround 4.5 dE with un excess loss fi-om ussernt)ly of < 1 dB (fyyp.), corresponding lo an offset of less tlian 1.4 pm.

The pi.efabricated housing is made frorli standard illaterials as used for electronic purposes. The sub- module with its fibet coririector attached is clipped into place. Protection fr-orn erivirorimental influerices is achieved by ctncapsulution with epoxy rnaterinls. The device withstatids staridurd surfuce riiouritirig.

Modules Despite of the low-cost consiructiori very good performance data

could be verified. Althuugh t i o interrial Peltiei- c.ooling is provided tlie modules cite ccrpable of 2.5 rnW liglrt emission into CI SMF (@ 85 “C urnbierit). Modules, solclatecl to U test bonrd (Fig. 4) under stcrndurc SM1 cycling, prove the excellent siubility of the plastics rrrid tlie gluing. No degiuclatiorl of output ~ O W C I could t i t i detectecl ctveii rifler i.epeaieci solder cycles.

Long term reliabiliiy has beeri tcstcd. Tlie output power lias beeri kept constant to provide light emissioii of 1.25 rnW. Device terripcru- ture was rnuiniained a i 80 O C for the erititc test. The required drive current was found io be stable within i- 3 % of iis uveruge value. N o degradotioti wus observed.

Conclusions We have presented ci new concept lor low cost optoelectronic modules. Our results demonstrate the

capability of 1asei.s with a plastic package, that are compatible with autornutic surface inouiiting. They curl operate even at high temperatui-es withoui Peltier cooling.

Acknowledgements

~ i ~ , 4 L~~~~ module after starl. darcl SMT ussembly

The authors would like to thank all their- colleagues supporting it ie uctivity presented.

References 1. 2. 3. 4. 5. 6. 7 . 8 9

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