INTELLIGENT OPTICAL CROSS-CONNECT SUBSYSTEM ON A CHIP

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1 INTELLIGENT OPTICAL CROSS-CONNECT SUBSYSTEM ON A CHIP wwwenablencecom September, 200

2 Introduction Abstract: We report on an intelligent -channel subsystem on a chip that integrates switching functionality with power monitoring and power balancing The proposed subsystem on a chip includes 2 2 switches that form a strictly non-blocking switch matrix, optical power taps and integrated photodiodes for per-channel power monitoring, and variable optical attenuators for power level control The photocurrent signals from the photodiodes are used by a feedback electronic circuit to control the VOA s, thereby achieving closed-loop automatic power balancing on all the channels 2005 Optical Society of America OCIS codes: (30320) Integrated optics devices; (605470) Polymers; (300250) Optoelectronics; (30750) Components; (306750) Systems; ( ) Networks Large amounts of information traveling on multiple wavelengths around an optical network need to be switched at the network nodes Information arriving at a node is forwarded to its final destination via the best possible path, which is determined by such factors as distance, cost, and the reliability of specific routes The conventional way to switch the information is to convert the input fiber optical signal to an electrical signal, perform the switching in the electrical domain, then convert the electrical signal back to an optical signal that goes down the desired output fiber This optical-electrical-optical (O-E-O) conversion uses systems that are expensive, bulky, and are bit-rate/protocol dependent Optical cross-connects () allow to avoid the unnecessary O-E-O conversion, enabling O-O-O systems that use optical switching, which has significant advantages for carriers and service providers Optical switching involves lower capital expenditures (capex), as there is no need for a large amount of expensive high-speed electronics Furthermore, operational expenditures (opex) are decreased and reliability is increased because fewer network elements such as back-to-back terminals are required Reducing the complexity also makes for physically smaller switches Additionally, optical switches are relatively future-proof An electrical switch has electronics designed to detect incoming optical signals of specific bit rates and formats When the bit rate increases or when the format changes, the electronics need to be upgraded s route the optical signals directly, and are bit-rate/protocol transparent, so future upgrades of bit-rate or protocol can be accommodated without the need to upgrade the switch The technologies that can be used in s include planar lightwave circuits (PLC s), liquid crystals, holographic elements, semiconductor amplifiers, total internal reflection (eg, bubble switch), optomechanical switches (moving fiber), and moving-mirror-based micro-electro-mechanical systems (MEMS) Our proposed solution is based on a polymer-on-silicon PLC platform This integrated solution delivers low cost, small size, high optical performance, low electrical power consumption, high yield, high throughput, short cycle time, and faster time to market 2 Use in Networks N N s have a variety of uses at optical network nodes, including wavelength cross-connect switching for routing and wavelength shuffling for adding/dropping s used for wavelength cross-connect switching are particularly important in mesh networks The mesh topology offers increased network capacity, efficiency, and reliability through an increased number of connections and a higher level of redundancy This topology is highly desirable from an operational point of view, however its chief drawback is capex, because of the volume of hardware required Although the opex savings increase the return on investment and quickly outweigh the capex spent, the upfront capex investment has been a significant barrier to the deployment of mesh networks Our PLC-based solution addresses the capex concern because it takes advantage of integration, which inherently delivers high cost reduction for complex optical circuitry Figure shows a reconfigurable mesh network made up of interconnected islands of transparency As depicted in that figure, an is used at each node in the transparent sub-networks 2 Page

3 OEO Switch OEO Switch Fig A reconfigurable mesh network comprising two interconnected islands of transparency (solid and dashed) An is used at each node in the transparent sub-networks Reconfigurable ring networks also use N N switches, including in s and in reconfigurable optical add/drop multiplexers (ROADM s) Figure 2 shows the functions needed at nodes in optical networks with a ring topology ROADM DEMUX LONG HAUL RROAD OADM RO MET RO M ROAAD E DM M ACCESS D M DEMUX U X ROADM ROADM DEMUX Consumer FTTP SPLITTER Fig 2 Topology of a reconfigurable ring network, with the functions needed at nodes Figure 3 shows our PLC-based -channel ROADM solution for networks that require East/West separation In this two-module susbsystem, four strictly non-blocking s are used at the add and the drop ports to implement a full shuffle that achieves any wavelength from any port and any wavelength to any port flexibility DWDM Fiber Input from West MUX DEMUX DWDM Fiber 2 Output to West West Module t s x e W t s p a m o E o Dr Fr Add T x Add Drop To West From West DWDM Fiber Output to East Drop Add From East To East x x East Module MUX DEMUX x2 S witch Power Tap Photodiode VOA DWDM Fiber 2 Input from East Fig 3 Self-balancing -channel ROADM for a DWDM fiber pair in network architectures requiring East/West separation 3 Page

4 3 Intelligent Architecture Our -channel intelligent (i) subsystem on a chip integrates switching with power monitoring and power balancing The proposed subsystem on a chip (Fig 4) includes 2 2 switches that make up a strictly nonblocking switch matrix, optical power taps and integrated photodiodes for per-channel power monitoring, and variable optical attenuators (VOA s) for power level control The photocurrents generated by the photodiodes are used by a feedback electronic circuit to control the VOA s, thereby achieving closed-loop automatic power balancing on all the channels A pigtailed and packaged i chip (before lid sealing) is shown in Fig 5 INPUT INPUT 2 INPUT 3 INPUT 4 INPUT 5 INPUT 6 INPUT 7 INPUT OUTPUT OUTPUT 2 OUTPUT 3 OUTPUT 4 OUTPUT 5 OUTPUT 6 OUTPUT 7 OUTPUT x2 Switch VOA Power Tap Photodiode Fig 4 with power monitoring and automatic power balancing Fig 5 with power monitoring and automatic power balancing 4 Technology The proposed intelligent (i) subsystem on a chip is based on a polymer-on-silicon platform that allows hybrid integration of passive and active elements Waveguiding circuitry is built in an optical polymer, and it includes thermo-optic switches, VOA s, and power taps Out-of-plane coupling mirrors are formed by ablation of 45 slopes in the polymer waveguides with an Excimer laser, followed by metalization A self-aligning flip-chip process is used to mount photodetector arrays on top of the mirrors 4 Materials Organic polymers are a compelling choice as the base material for dynamic integrated optical components because, when designed, synthesized and processed properly, they offer high performance (the transparency of state-of-theart polymers is on par with that of silica, the birefringence is smaller than that of silica by two orders of magnitude), wide controllability of the refractive index contrast (the maximum n is an order of magnitude larger than that achievable in silica), tunability (the thermo-optic coefficient is more than an order of magnitude larger than that of silica), environmental stability, ease of hybridization, high yields, and low cost [] We developed state-of-the-art optical polymers that are highly transparent, with absorption loss values around or below 0 db/cm at all the key communication wavelengths (40, 30, and 550 nm) [2] In order to reduce absorption loss at telecom wavelengths such as the nm and nm windows, we synthesized halogenated polymers such as the one whose IR spectrum is shown in Figure 6, demonstrating high transmission at the telecom bands 4 Page

5 05 Loss (db/cm) Wavelength (nm) Fig 6 Near IR absorption spectrum of a DuPont halogenated acrylic polymer Figure 7a shows the insertion loss at 550 nm of a straight waveguide cut back to different lengths, exhibiting 0 db/cm propagation loss (slope) and 02 db fiber coupling loss for both sides of the chip (y intercept) Figure 7b depicts the insertion loss of a 3-cm-long straight waveguide tested between 500 and 570 nm wavelength, exhibiting low wavelength dependent loss (WDL) in this spectral window Insertion Loss (db) Chip Length (mm) 50 Wavelength (nm) Fig 7 Results of a cut-back experiment on a polymer optical waveguide, revealing 0 db/cm propagation loss and 02 db total chip-to-fiber coupling loss Insertion loss of a polymer waveguide between 500 and 570 nm, demonstrating low WDL Insertion Loss (db) The polarization dependent loss (PDL = loss TE loss TM ) varies with processing conditions The TE loss measured in planar waveguides can be higher than the TM loss when the vertical walls of the core have a higher degree of roughness than the horizontal boundaries, and it can be lower when the vertical evanescent tails overlap with an absorptive substrate or superstrate Our single-mode waveguides are well optimized by having minimal edge roughness and a well-confining material stack, and as a result have PDL values below 00 db/cm Our three-dimensionally cross-linked polymers undergo little molecular orientation during processing, and as a result have a birefringence (n TE n TM ) value that is extremely low (on the order of 0-6 ) A typical polarization mode dispersion (PMD) value in one of our devices is on the order of 00 ps The unique combination of large thermo-optic coefficient and low thermal conductivity makes polymers ideal materials for thermo-optic devices such as optical switches, variable attenuators, and tunable filters The thermo-optic effect is the change of refractive index, n, with temperature, T, and is commonly referred to as dn/dt For an amorphous polymer, the refractive index change is predominantly due to its density change Therefore, in order to increase the thermo-optic effect, we designed our polymers with high coefficient of thermal expansion (CTE) Our materials have a dn/dt value as high as / C, 40 times larger (in absolute value) than that of silica, and 3-5 times larger than that of common optical polymers such as polymethylmethacrylate (PMMA) and polycarbonate (PC) A proportionate decrease in power consumption results in thermo-optic switches and tunable devices In addition, these polymers have a T g well below the low end of telecom operating temperature specification (-40 C), and a large free volume The environmental stability of optical polymers is an important issue because most polymers do not have properties that are adequate for operation in communication environments A key characteristic for practical applications is the thermal stability of the optical properties since organic materials may be subject to yellowing upon thermal aging due to oxidation The presence of hydrogen in a polymer allows the formation of H-Halogen elimination products, which result in carbon double bonds, which are subject to oxidation Fortunately, the 5 Page

6 absorbing species from thermal decomposition are centered near the blue region of the spectrum, and the thermal stability is high at the datacom wavelength of 40 nm and even greater at the telecom wavelengths of 30 and 550 nm Heating is not caused only by the environment and by electrical heating electrodes on the chip, it can also be the result of high optical power propagating in the waveguides Figure shows the results of accelerated aging tests Our waveguides exhibit high resistance to heat from the environment (5000 hours at 75 C) and from guided 550 nm optical power (6000 hours at 5 W, a power level known to fuse silica fiber [3]) The resistance to humidity is also critical since optical absorption results from the overtone bands of the OH-stretch of water Our polymers stand up to 5 C 5% RH conditions Insertion Loss Variation (db) Hours, 75 C Hours, 5 W Optical Power Device -03 Device 2 Device 3-04 Device Time (hours) Time (hours) Fig Transmission of polymer waveguides held at 75 C for 5000 hours and 5 W optical power for 6000 hours Transmission Variation (db/cm) Telecom lifetime requirements of 20 years are easily met with these materials, and our polymeric components have passed Telcordia GR-209-CORE/GR-22-CORE qualification An example of a Telcordia test result is shown in Fig 9, where the variation in insertion loss for 4-channel variable optical attenuator (VOA) parts subjected to 00 cycles of -40 C to +5 C was within about ±0 db, with the pass criterion being ±05 db Similar distributions were obtained in all the Telcordia tests, hence our components passed the qualification testing with a large margin Temperature Cycling (-40 C to +5 C, 00 cycles) Parts, 44 Channels 9 7 Number of Channels nm 550 nm nm Change in Insertion Loss (db) 6 Page Fig 9 Results of a Telcordia test where 4-channel VOA parts were subjected to thermal cycling (-40 C to +5 C, 00 cycles)

7 42 Switches Thermo-optic N N switches can be interferometric switches based on directional couplers or Mach-Zehnder interferometers (MZIs), or they can be digital optical switches (DOS s) based on X junctions or Y junctions [4] The most widely used switch design is the Y-junction-based DOS (Y-DOS), because of its simplicity, and its insensitivity to applied electrical power, wavelength, polarization, ambient temperature, and dimensional variation The insensitivity to applied electrical power is what enables the digital behavior The building block in a Y-DOS is a small-angle (typically 0 ) 2 splitter with heaters on its arms [5], as shown in the schematic diagram of Fig 0a These splitters can be connected with bends and crossings to form M N switching matrices Each 2 splitter relies on adiabatic evolution of the mode profile in its two waveguides into the mode of the ON guide (the guide with the higher effective refractive index) when the OFF guide is heated to reduce its index, as shown in the computer simulation of Fig 0b Optical Signal OUT ~0 Heater Electrode (ON) Heater Electrode (OFF) Bonding Pad Optical Signal IN Channel Waveguide Fig 0 Schematic diagram of a 2 Y-DOS and computer simulation of such a device where the left arm is heated, allowing the light to exit the right arm The device is considered to have switched once it reaches the desired isolation value, which occurs at some level of electrical power dissipation in the electrodes, beyond which power level the device maintains the isolation, resulting in its well-known digital behavior (Fig ) A typical maximum time for restoration and restructuring of multiple circuit connections for SONET is 50 ms The measured response time for these thermo-optic switches is approximately 3 ms, a value that is adequate for system restoration 0 Optical Power (db) Output of ON Arm (under OFF heater) Digital Regime Extinction Per Stage = -30 db -40 Output of OFF Arm (under ON heater) Electrical Power (mw) Fig Operational characteristics of a Y-branch-based x2 digital optical switch DOS s in a 2 N configuration can be fabricated with 2 N - 2 s, and one electrode at each stage needs to be heated to perform the switching For instance a 4 switch can be built with three 2 s, as shown in Fig 2a, where the upper electrode in the first stage and the lower electrode in the second stage are powered to switch the light from port to port 3 As another example, a strictly non-blocking N N switch can be fabricated with 2N(N-) 2 s (Table ) [6] Fig 2b shows a 2 2 (or cross-bar) DOS built with four 2 s This switch is operated in the bar state by powering the four inner electrodes, while powering the four outer electrodes results in the cross state 7 Page

8 ' ' 2' 3' 4' 2 2' Fig 2 Schematic layouts of a 4 DOS and a 2 2 (cross-bar) DOS The dark electrodes are powered The 2 is shown switching from port to port 3 and the 2 2 is in the bar state TABLE The number of 2 switches needed in planar strictly non-blocking N N switches and the degree of maturity of the different switch sizes N Number of 2 s R&D Design R&D Fabrication Commercially Available , , , ,095,04 Figure 3 shows schematically the recursive tree design of the cross-bar switches we use in our technology This switching matrix consists of 2 2 switches providing a strictly non-blocking connectivity Page Fig 3 Architecture of an DOS-based switch matrix based on a recursive tree structure Each box represents a 2 switch The cross-bar switches exhibit a power dissipation of 40 mw per DOS, an insertion loss of 3 db, and an extinction of 45 db, mainly limited by the crosstalk at the crossings The small value of dn/dt and limited index contrast in silica PLC s has kept 6 6 switches from being implemented with DOS 2 switches Planar 6 6 switches implemented to date in silica have been based on MZIs [7] We implemented the first DOS-based 6 6 switch, using our high-index-contrast polymeric system [] Whereas the 6 6 switch shows the largest fabricated strictly non-blocking N N switch, significantly larger switching matrices have been designed and are in early development stages Using the highest n achievable in our polymer system, we designed the first planar optical cross-connect We developed multiple designs, with the most compact measuring cm 2, allowing 4 chips to fit on a standard 6-inch wafer [9]

9 43 Variable Optical Attenuators With the increasing complexity of WDM optical networks comes an increasing need for reliable, low cost VOA s that adjust the power level of optical signals with high accuracy and repeatability VOA s can be based on any switching principle including interferometry, modal transition, or mode confinement An interferometric approach involves using an MZI where heat can be applied to at least one of the arms to induce a phase shift between the two arms before they recombine, thereby controlling the level of optical power exiting the output guide Fig 4a shows a simulation of this device when power is applied to thermally induce a π phase difference between the optical signals in the two arms, causing the signals at recombination to form an asymmetric mode that radiates into the cladding, since it is not supported by the single-mode output waveguide, resulting in full attenuation Fig 4b shows the operational characteristics of an MZI VOA exhibiting very low power consumption of about 4 mw for 30 db attenuation [0] One performance specification that is typically difficult to achieve in VOA s is low PDL The PDL achieved in polymeric VOA s is under 02 db across the entire attenuation range, a value that is lower than that achieved in any other material system 0 Optical Output (db) Heater Power (mw) Fig 4 Computer simulation of a MZI VOA where heat is used to induce a π phase difference between the interferometer arms for full attenuation, and attenuation curve of such a VOA 44 Photodiode Arrays We have hybridly integrated photodiode arrays in our polymer platform for power monitoring, by flip-chip mounting array chips (Fig 5a) on top of out-of-plane mirrors (Fig 5b) at the ends of tap waveguides Various techniques have been proposed for the fabrication of out-of-plane mirrors in PLC s [] Our approach is based on ablating the polymer waveguide stack with an Excimer laser The ablation is followed by surface treatment for planarization, then by metalization for high reflectivity The mirror quality has been characterized by coupling light from a fiber into the input facet of a chip, and monitoring the tap-transmitted power by detection with the photodiodes The measured excess loss and PDL were 03 db and 0 db, respectively, a performance level that is adequate for our applications Fig 5 Photodiode arrays flip-chip mounted on top of out-of-plane mirrors fabricated by Excimer laser ablation The ability to tap optical power from a signal (typically 4-5%) and measure the power level with photodiodes enables quality of service (QoS) monitoring, and closed loop control of VOA s for accurate channel equalization In our integrated platform, the ease of building taps throughout a circuit and monitoring the power of multiple taps with photodiode arrays, provides added network reliability at a minimal cost 9 Page

10 5 Intelligent Performance The fiber-to-fiber insertion loss of the single-chip i of Fig 4, between 52 and 60 nm wavelength, is 4 db (including 5% tapped power) The PDL at minimum insertion loss from any port to any port is 02 db, the polarization mode dispersion (PMD) is 00 ps, and the chromatic dispersion (CD) is 0 ps/nm The switch isolation (or extinction) is 45 db, and the crosstalk from any port to any port is 50 db Table 2 lists all the key performance characteristics 6 Conclusion TABLE 2 Key performance characteristics of the single-chip -channel i Parameter Value Operating Wavelength Range nm Insertion Loss 4 db VOA Attenuation Range 5 db PDL at 0 db Attenuation 02 db PDL at 5 db Attenuation 04 db WDL 0 db Tap Ratio 5±05% PMD 00 ps CD 0 ps/nm Isolation (Exctinction) 45 db Crosstalk 50 db Return Loss 50 db Switch Response Time 3 ms PD Responsivity 00 A/W Power Consumption 4 W Optical Power Range per Channel -30 to +20 dbm Operating Temperature Range 0 70 C Storage Temperature Range C We presented an intelligent -channel subsystem on a chip that integrates switching functionality with power monitoring and power balancing The chip includes 2 2 switches that form a strictly non-blocking switch matrix, optical power taps and integrated photodiodes for per-channel power monitoring, and variable optical attenuators for power level control The technology is based on a hybridization-friendly polymer-on-silicon platform that has passed Telcordia GR-209-CORE/GR-22- CORE qualification 7 References [] L Eldada et al, Advances in polymer integrated optics, IEEE J Select Top Quant Electron 6, 54 (2000) [2] L Eldada, Advances in polymeric integrated optical componentry, Proc IPR, ITuH (200) [3] R Oron, Protecting the Optical Network, Fiberoptic Product News 9, 20 (2003) [4] N Keil et al, Integrated optical switching devices for telecommunications made on plastics, Proc Plastics in Telecom, (99) [5] L Eldada et al, Polymeric components for all-optical networks, Proc SPIE 3950, 7 (2000) [6] L Eldada, Polymer integrated optics: promise vs practicality, Proc SPIE 4642, (2002) [7] T Goh et al, Low-loss high-extinction-ratio silica-based strictly nonblocking 6 6 thermooptic matrix switch, Phot Technol Lett 0, 0 (99) [] FLW Rabbering et al, Polymeric 6x6 digital optical switch matrix, Proc ECOC 27, PD-7 (200) [9] J Fujita et al, Ultrahigh index contrast planar polymeric strictly non-blocking cross-connect switch matrix, Proc IPR 24 (2004) [0] NS Lagali et al, Ultra-Low Power and High Dynamic Range Variable Optical Attenuator Array, Proc ECOC 27, 430 (200) [] R Gerhardt et al, Hybrid integrated metro ring node subsystem on a chip, Photonics West, Proc SPIE 499 (2003)

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