Low - Cost Optical Transceiver for PON Using Micro …

1 11 Fujikura Technical Review, 20071. Introduction Fiber-to-the-home (FTTH) system is becomingwidely used now. Passive Optical Network (PON)system will be commonly selected because of its cost-effectiveness. In the PON system, every subscriber isrequired to install an Optical Network Unit (ONU) athis/her home. Therefore, cost reduction of the ONUis essential. A major portion of the ONU cost comesfrom an Optical Transceiver , and, therefore, the costreduction of the Transceiver is most have developed a novel Transceiver and anOptical Sub-Assembly (OSA), the key element of atransceiver, for Gigabit Ethernet-PON1)(GE-PON),which operates at 1 Gbit/s data rate Using Ethernettechnology. A newly developed compact and low-costBidirectional Optical Sub-Assembly (BOSA)2), calledmicro-BOSA, can make the Transceiver a small formfactor. The design of the Transceiver was also opti-mized for the Micro -BOSA, reducing its total cost.

2 The development was a joint work betweenFujikura Ltd. and Oki Electric Industry Co., Ltd. Themembers of Oki Electric Industry took charge of theoptical design and internal structure of the Micro -BOSA3)under their Silicon microlens technologies4).Optoelectronic Circuits & Systems R & D Center ofFujikura developed packaging and assembling ofmicro-BOSA and designed the Optical Outline of the PON systemFigure 1 shows the outline of the PON system thatis one of the FTTH systems. One Optical fiber from acentral office is divided by an Optical splitter on theway to subscribers. Multiple subscribers can there-Low - Cost Optical Transceiver for PON Using Micro -BOSAK oichiroMasuko1, KenjiNishide2, SusumuNakaya2, TeijiroOri2, OsamuKikuchi3, RyouSekikawa4and DaisukeShimura4A novel Micro -Bidirectional Optical Sub-Assembly (BOSA) has been developed in which opti-cal transmitting and receiving functions are incorporated in a single TO-CAN package.

3 A newcompact and cost-effective Passive Optical Network (PON) Optical Network Unit (ONU) trans-ceiver Using the Micro -BOSA has also been developed. Low-cost feature is achieved with a pas-sive alignment technology and a simplified integrated structure of the Micro -BOSA. Both elec-trical and Optical characteristics are sufficient to comply with Gigabit Ethernet (GE)-PONONU Transceiver specification. In this paper, the structure and characteristics of the transceiverare reported. Drop cableClosure with splittersAerial cableFeeder cable Central officeGE-PON OLTGE-PON OLTGE-PON PON Applied Electronics Technology Department of Optics and ElectronicsLaboratory2 Optoelectronic Circuits & Systems R & D Center 3 Optical Module R & D Department of Optoelectronic Circuits & Systems R &D Center4 Oki Electric Industry Co., driverBidirectional Optical Sub-Assembly(BOSA)DataBurstsignalDataSig naldetectRx circuit boardPD TO-CANO ptical fiber1,310nm1,490nm1,550nmPost amplifierPre-amplifierPDLDmPDWDMC onventional Transceiver for PON ONUTx circuit boardLD Block diagram of an ONU Optical share an Optical Line Terminal (OLT) in the cen-tral office.

4 Figure 2 shows the block diagram of a con-ventional GE-PON ONU Transceiver . In this system,upstream signal is carried by a 1,310 nm wavelengthlight, and downstream signal is carried by a 1,490 nmwavelength light, so that a bidirectional communica-tion is realized by a single Optical fiber. Burst trans-mitting function to operate laser diode (LD) only dur-ing a requested period is also required for the ONUsbecause they share one OLT. The function to shutoff a 1,550 nm signal is alsorequired because a video service on 1,550 nm isplanned in the Structure of the proposed Transceiver andthe Conventional transceiverConventional Transceiver generally uses a cubicoptical module shown as BOSA in Fig. 3. In this typeof module, a TO-CAN type LD and photodiode (PD)are attached to a cubic body. A Wavelength DivisionMultiplexing (WDM) filter is fixed inside the body soas to make a 45 angle to the LD and PD Optical the module itself along with the circuit board ofthe Optical Transceiver becomes complicated.

5 A pigtailfiber with an SC connector is usually attached to theBOSA. It results in high material and assemblingcost. Proposed Optical transceiverFigure 4 shows the appearance of the proposedoptical Transceiver . The Transceiver is designed tocomply with the Small Form Factor (SFF)Multisource Agreement (MSA) standard. An SC-typeoptical connector is mated to one end of the transceiv-er. Upstream signal is carried by a 1,310 nm wave-length light, and downstream signal is carried by a1,490 nm wavelength light because the Transceiver isdesigned for GE-PON ONU. The transmission datarate is 5 shows the internal structure of the prod-uct. A cylindrical object on the left side is the Micro -BOSA and on the right side is a circuit board for theIC with an LD driver and a post amplifier for signalreceiving. Outer shape and internal structure of the Micro -BOSAF igure 6 shows a schematic diagram of the Micro -BOSA chip.

6 A silicon Optical bench (SiOB) with opti-cal devices, called Micro -BOSA chip, is fixed in themicro-BOSA. An LD, a PD, Si microlens, a WDM fil-ter, a transimpedance amplifier (TIA), and capacitorsare mounted on the SiOB with passive alignmenttechnologies. Upstream signal from an LD is collimat-ed by an Si microlens, goes through a WDM filter, iscondensed by a ball lens and coupled into an opticalfiber. Downstream signal from the Optical fiber is col-limated by the ball lens, reflected by the WDM filter,condensed and bent a little downward by an Si12Tx circuit boardRx circuit boardBOSAO ptical Example of a conventional Outer shape of the proposed Optical Internal structure of the proposed Optical microlensPre-amplifierWDM filterUp-stream1,310nmDown-stream1,490nm Ball lensSi The Micro -BOSA , reflected by a mirror on SiOB and coupledinto a 7 shows the details of the Si microlens thatis a kind of diffractive )The lenses can be fabri-cated in wafer scale based on the conventional Si LSIfabrication technology so that the form can be con-trolled very precisely.

7 An LD coupling efficiency is designed to be 30% forthis Micro -BOSA, but a coupling efficiency higherthan 50% can be achieved. The Si microlens can beplaced precisely on a V-groove fabricated by orienta-tion-dependent etching on the SiOB because theouter shape of Si microlens is also precise. The outershape is fabricated Using MEMS 8 shows a cut model of the Micro -BOSA chip is attached to the ledge of astem. Electric pads and pins are connected by goldwires and hermetically sealed by a cap with a balllens. Finally, an SC receptacle is aligned and attachedby YAG welding. The outer shape just looks like aTransmitter OSA (TOSA) or a Receiver OSA (ROSA),but in this case, Optical transmitting and receivingfunctions are incorporated in one OSA. In this Micro -BOSA, it is important to suppress across talk because an LD and a PD are packaged in asingle TO-CAN package.

8 A TO-CAN cap with a balllens is designed whose inside surface is finished witha low-reflection coat. We also put a resin on the stemto absorb LD emission from a rear facet. With thesefeatures, an Optical cross talk is suppressed efficient-ly. To shutoff 1,550 nm or longer wavelength, anotheroptical filter is attached onto an SC receptacle facetnear the ball lens. So this Micro -BOSA can be used inthe system with video delivery. Optical designOptical system of the Micro -BOSA is designed inconsideration of the wavelength dependence of an Simicrolens. In the Optical system, the focal length getsshorter as the wavelength gets longer. On the con-trary, the wavelength of an FP-LD gets longer andefficiency gets lower as the temperature gets higher. Considering these characteristics, we align theoptical devices to achieve the highest coupling effi-ciency at a high temperature, which means a longerwavelength and a reduced operation current at a 9 shows examples of LD coupling effi-ciencies.

9 Dashed lines represent the efficiencies ofthe samples optimized at room temperature. Solidlines represent the ones optimized at high tempera-ture. Dotted line shows assumed characteristics with-out the wavelength dependence of the Si a low temperature, LD can be driven easily with ahigh current to get a high power, so we choose theoptical design optimized at high temperature as thesolid lines. 4. Characteristics of fabricated Characteristics of the Micro -BOSAT able 1 shows Optical and electrical characteristicsof the fabricated Micro -BOSA at room are good enough and the internal cross talk isalso suppressed efficiently. Characteristics of a transceiverAn Optical Transceiver with the developed Micro -BOSA was designed and fabricated. Figure 10 shows13 Fujikura Technical Review, 2007 StemPinsCap with ball lensSC A cut model of the proposed ( C)slope efficiency (W/A) Temperature dependence of slope An Si s bit error rate (BER) characteristics underPRBS 27 1 and 25 C.

10 An LD is operated at dBmoutput power, Gbit/s data rate on the conditionthat the transmitter (Tx) is on. Table 2 shows theirtemperature receiver sensitivity at BER of 10 12was dBm when the transmitter was disabled, but dBm when the transmitter was operated. Figure 11 shows the Optical waveform at of transmitter. Mask margin is 50%. Figure 12shows a burst Optical output waveform of the trans-mitter. The rise time is 5 ns with enough ConclusionA new Micro -BOSA where Optical transmitting andreceiving functions are incorporated in a single TOcoaxial OSA has been developed. A SFF transceiverfor PON ONU with the Micro -BOSA has also beendeveloped. Highly simplified and integrated structureand the low-cost feature of the Micro -BOSA were real-ized by passive alignment technologies. The charac-teristics of the Transceiver with the Micro -BOSA weregood enough for GE-PON ONU.