For many telecommunication applications, low polarization-dependent loss (PDL) operation is mandatory. On the silicon-on-insulator photonic wire platform, this can be achieved using polarization diversity configurations. However, as we show here, when using two-dimensional grating couplers for near vertical fiber input and output, the low PDL bandwidth is limited. We propose and demonstrate the use of a phase shifter in one of the arms of the polarization diversity circuit to effectively reduce PDL (0.15-dB PDL is shown) and increase the low PDL bandwidth

Halir, Robert

Roelkens, Günther

Vermeulen, Diedrik

English

Robert  Halir

Diedrik Vermeulen

Günther Roelkens

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Reducing Polarization-Dependent Loss of Silicon-on-Insulator Fiber to Chip Grating Couplers

Ghent University Academic Bibliography

1Reducing Polarization Dependent Loss ofSilicon-on-Insulator Fiber to Chip Grating CouplersRobert Halir, Diedrik Vermeulen, and Günther RoelkensAbstract—For many telecommunication applications, low Po-larization Dependent Loss (PDL) operation is mandatory. On thesilicon-on-insulator photonic wire platform this can be achievedusing polarization diversity configurations. However, as we showhere, when using two dimensional grating couplers for nearvertical fiber input and output, the low PDL bandwidth is limited.We propose and demonstrate the use of a pi phase shifter in oneof the arms of the polarization diversity circuit to effectivelyreduce PDL (0.15 dB PDL is shown) and increase the low PDLbandwidth.I. INTRODUCTIONS ILICON-on-insulator photonic wire technology allows fora substantial scaling in the size of photonic integratedcircuits due to the high refractive index contrast that isavailable. Silicon waveguides with dimensions on the orderof 500 nm by 200 nm allow for single mode propagation andtight optical confinement. The high refractive index contrastmakes the waveguide structures very polarization dependenthowever, which is an issue in many practical applications.Therefore polarization diversity schemes are being proposedto tackle this problem. In one approach, a polarization splitterand rotator is being integrated on the chip, such that thetwo arms of the polarization diversity circuit are processingthe same polarization state. This resulted in relatively lowPDL (about 1 dB) [1], [2]. In [3], it was proposed to usea two-dimensional grating structure, which at the same timefulfills the role of optical fiber interface, polarization splitterand rotator, all on a 100µm2 area. Taking into account thefull bandwidth of the grating coupler spectrum, still quitesubstantial polarization dependent loss can be observed in theexperiment (1 to 2 dB PDL in the fiber to fiber coupling 1 dBbandwidth). In this paper we show that, by introducing a piphase shifter in one arm of the polarization diversity circuit,a substantial polarization dependent loss (PDL) reduction canbe obtained.II. CAUSES AND REDUCTION OF PDLIn a polarization diversity scheme like the one shown inFig. 1, light is coupled in and out of the circuit with fibersThis work was carried out in the framework of the Dutch Smartmix-Memphis project. Robert Halir acknowledges the Spanish Ministerio deEducación for the FPU scholarship with reference AP-2006-03355. D. Ver-meulen thanks the Institute for the Promotion of Innovation by Science andTechnology in Flanders (IWT) for a grant. G. Roelkens acknowledges theFund for Scientific Research Vlaanderen (FWO) for a postdoctoral grant.Robert Halir is with the Departamento de Ingeniería de Comunicaciones,ETSI Telecomunicación, Universidad de Málaga, 29010 Málaga, Spain (e-mail: robert.halir@ic.uma.es).Diedrik Vermeulen and Günther Roelkens are with the Departmentof Information Technology (INTEC), Ghent University - Interuniver-sity Microelectronics Center (IMEC), B-9000Gent, Belgium (e-mail: gun-ther.roelkens@intec.ugent.be)(website: http://photonics.intec.ugent.be)1450 1500 1550 1600 165000.050.10.150.20.25Wavelength(nm)CouplingefficiencyP1,inP2,inFig. 1. Schematic illustration of a polarization diversity circuit with 2Dgratings. The inset shows the simulated coupling efficiency of a 2D gratingfor polarizations P1,in and P2,in.positioned over the two dimensional grating couplers [3], [4].These couplers convert any polarization state in the fiber toTE polarized light in both arms of the polarization diversitycircuit. In order to avoid strong back reflections from thegratings, the fibers have to be slightly tilted, typically at ∼ 10◦with respect to the vertical. In both arms the light is thendirected to the integrated photonic device that performs thedesired functionality and is finally coupled into the output fiberusing an identical 2D grating coupler. In such a polarizationdiversity configuration there are three main sources of PDL.First, if the input and output fibers are not positioned alongthe diagonal of the 2D grating couplers, symmetry is lost andsome polarization states will couple more efficiently to andfrom the integrated waveguides than others. This contributionto PDL can be minimized with proper fiber alignment, asdetailed in section III. Second, due to fluctuations duringfabrication the devices in either arm may not behave iden-tically, thus introducing a different amount of loss. Trimmingor tuning of the devices may be used to cancel this effect. Insome device configurations the same device can be used forboth arms of the polarization diversity circuit, by propagationin opposite directions [5]. Finally, the grating couplers them-selves also introduce PDL. This is best understood consideringthe two linear polarizations at 0◦ and 90◦, P1,in and P2,in,shown in Fig. 1. Due to the near vertical coupling, P1,in isslightly tilted out of the grating plane, while P2,in lies inthe same plane as the grating coupler. Consequently P1,inand P2,in will not exhibit the same coupling efficiency. Thedifferent coupling efficiencies are shown in the inset of Fig. 1.They were obtained with 3D Finite Difference Time Domainsimulations of a 19×19 array of circular holes with a diameterof 400 nm and a 605 nm pitch. The outgoing waveguides form2an 83.8◦ angle to account for the effect of the tilted fiber (see[4]). Since the coupling efficiency of both polarizations onlycoincides at a specific wavelength, the grating coupler willgenerally introduce PDL. The reduction of this type of PDLis the main contribution of this paper and will be discussed inmore detail in the following.From symmetry considerations it is clear that if polarizationstate P1,in is aligned along the grating bisection line, it willcouple symmetrically to the upper and lower arm of thepolarization diversity circuit. This is schematically illustratedwith the solid arrows on the left side of Fig. 1. On the otherhand, polarization state P2,in will couple antisymmetrically,i.e. with a 180o phase shift between the upper and the lowerarm, as shown with the dashed arrows on the left side of Fig.1. If the optical signals in both arms experience the same phaseshift when propagating through the photonic device, the outputgrating will restore the original input polarization state. Thatmeans that when light with either polarization P1,in or P2,inis coupled in, it will experience the same coupling efficiencycurve (see inset of Fig. 1) twice, thus increasing PDL.Here we propose to introduce a pi phase shift in one ofthe arms of the polarization diversity circuit. As illustrated inon the right side of Fig. 1, this will cause the polarizationstates P1,in and P2,in to interchange their orientation at theoutput grating coupler. Consequently, either of these stateswill experience both coupling efficiency curves (one at theinput, and the other one at the output), so that the overallinput to output coupling is identical for both, and PDL van-ishes. Naturally, complete PDL compensation requires almostidentical input and output coupling efficiencies. This can beachieved because the grating couplers are closely together andare defined by state-of-the-art 193 nm DUV lithography, sothat they are virtually identical, and highly symmetric fiberalignment is possible using one dimensional grating couplersas polarization references (see section III).III. DEVICE AND MEASUREMENT SETUPTo demonstrate the PDL reduction concept we use a polar-ization diversity circuit as shown in Fig. 1, with the genericdevice substituted by a ring resonator (see inset of Fig. 2).The final purpose of the circuit is to act as tunable wavelengthrouter for FTTH applications. As such, in this prototype it hasto deliver two wavelengths to the home user, one with thedownstream data (λdown,1,2), and a continuous wave (λcw,1,2)that can be modulated at the user end. Both of these wave-lengths must be routed with low PDL. Furthermore, wave-lengths intended for other users should be suppressed, but PDLis less critical at these wavelengths. The component operateswith the downstream data and continuous wave wavelengthsat two consecutive transmission peaks of the ring resonators,while wavelengths of other users are located at transmissionminima. The recorded transmission spectrum is plotted in2. An insertion loss of 21 dB is obtained, which is due tothe limited efficiency of the grating couplers (approximately7 dB loss per coupler) and the 1 × 4 splitter implementedin the circuit. The efficiency of the grating couplers can beimproved however by using a silicon overlay, as shown in [6].Fig. 2. Measured spectral response of the wavelength router with all heatersturned off. The dashed envelope indicates the overall efficiency of the two 2Dgrating couplers. A schematic of the device is shown in the inset.Fig. 3. Microscope image of the wavelength router with waveguide heaters(phase shifters) and ring heaters.This, however, does not change the concept of PDL reduction.The free spectral range of the rings is ∼ 4.5 nm, while theirextinction ratio will still be optimized.The device was fabricated on a SOI wafer with a siliconthickness of 220nm using deep UV lithography and a twoetch depth process. A 70 nm etch depth was used for thegrating couplers, whereas the photonic wires were completelyetched. After cladding the device with DVS-BCB, titaniumheaters were defined on top of both ring resonators to allowfor wavelength tuning as well as matching the devices inboth arms. The pi phase shifter was also implemented with aheater on top of one of the arms of the polarization diversitycircuit. A microscope image of a wavelength router capableof addressing four users is shown in Fig. 3.To reduce fiber-misalignment induced PDL, the fibers werefirst aligned to a reference waveguide with one dimensionalgrating couplers. The high polarization dependence of thesecouplers was used to adjust the input polarization to the P2,instate shown in Fig. 1, using a polarization controller. Thefibers were then aligned for maximum transmission on the2D gratings. Since the fiber coupling efficiency is relativelyposition tolerant (typically ∼ 2µm for a 1 dB variation),this effectively minimizes PDL due to asymmetric powercoupling. However, a slight misalignment, that does notproduce significant coupling asymmetry, may still result in anunwanted phase shift between both arms. Current was suppliedindependently to the pi phase shifter and the ring heaters. ForPDL characterization a tunable laser source and the Fiberpro“PL 2000” PDL meter1 were employed. The latter employs thepolarization scanning method to determine PDL and allows forsimultaneous insertion loss measurements.1Available: http://www.fiberpro.com/products/ofc_pl2000.asp3IV. RESULTSFor device characterization, the transmission spectrum ofthe device was first recorded with the fixed P2,in polarizationstate (see Fig. 2). Without changing the fiber alignment, PDLwas then measured as a function of heater power at thetransmission peaks using the PDL meter.Initially, the ring heaters were switched off, and the powersupplied to the waveguide heater was varied. At a wavelengthof λ = 1556 nm this yields the dashed curve in Fig. 4(a). Asexpected, PDL decreases as the phase shift increases, reachinga minimum value of 0.5 dB at 14mW of heating power. Asmore power is applied, the phase shift exceeds the optimumvalue so that PDL increases again. The fact that the deviceexhibits a higher PDL with 30mW of power than withoutheating, can be attributed to imperfect fabrication or slightfiber misalignment that yield an initial non-zero phase shiftbetween the arms. Only marginal variations in insertion lossof ∼ 0.2 dB were observed while heating the device.The ring heaters are used to compensate for both theinitial fabrication differences between the rings and differentialthermal cross-talk from the waveguide arm heater. A trial anderror approach was used to determine which of the two ringheaters cancels the combination of these two effects, and thusfurther reduces PDL. When the waveguide heater was switchedoff, and power was supplied only to the previously chosenring heater (with the other ring heater off), PDL improvementswere negligible. This is shown by the dashed curve in Fig. 4(b)and indicates that the initial fabrication differences between therings were low. However, when the waveguide heater is fixed at17.8mW, different thermal crosstalk is induced in the rings, sothat they must be matched again with the ring heaters. This canbe seen in the solid curve in Fig. 4(b), where the waveguideheater is fixed at 17.8mW and PDL reaches a minimum of0.15 dB at 6mW ring heater power. Conversely, as shownin the solid curve in 4(a), at this power level for the ringheater the minimum PDL is achieved at 17.8mW waveguideheating power. The fact that the waveguide heating powersthat yield minimum PDL are slightly different depending onthe power supplied to the ring heaters, can again be explainedby thermal crosstalk from the ring heater to the polarizationdiversity waveguide arm. The 0.15 dB of remaining PDL areattributed to minor asymmetries in the fiber alignment.The optimum waveguide and ring heater power levels werefound to be generally slightly different for different transmis-sion peaks. However, choosing adequate levels, it is possibleto achieve low PDL at several consecutive peaks, inside the1 dB fiber to fiber coupling bandwidth. This is shown in tableI, from where it is clear that PDL well below 0.5 dB for thedownstream wavelength as well as continuous wave locatedat the next peak is readily achieved at λ = 1551.5 nm andλ = 1556 nm. Also notice PDL improvements of up to 1.4 dB(at λ = 1560.5 nm) are achieved when the heaters are used.By readjusting the heaters, low PDL can be obtained in adifferent wavelength range.V. CONCLUSIONSWe have shown experimentally that by using a pi phaseshift in one of the arms of a polarization diversity circuit0 10 20 300.150.511.52Waveguide heater power (mW)PDL (dB)(a) Ring heater fixed  Ring: OffRing: 6mW0 2.5 5 7.5 100.150.511.52Ring heater power (mW)(b) Waveguide heater fixed  Wg: OffWg: 17.8mWFig. 4. Measured PDL at λ = 1556 nm as a function of waveguide andring heater powers.TABLE IMEASURED PDL AT TRANSMISSION PEAKS WITH HEATERS ON AND OFF.WAVEGUIDE HEATER POWER ∼ 20mW, RING HEATER POWER ∼ 4mW.Wavelength(nm) 1542.5 1547.0 1551.5 1556.0 1560.5 1565.0PDL (dB)heaters on 0.8 0.4 0.4 0.35 0.6 1PDL (dB)heaters off 1.25 1.1 1.2 1.4 2.0 2.2the PDL induced by the near vertical grating couplers canbe reduced from more than 1 dB to only 0.15 dB, at a fixedwavelength. This requires careful alignment of the input andoutput fibers and exact matching of the devices in each arm.In the wavelength router used to demonstrate this concept,it was also possible to achieve PDL below 0.5 dB at severalconsecutive transmission peaks. This technique should enablelow PDL operation in a variety of polarization diversityconfigurations.REFERENCES[1] T. Barwicz, M. Watts, M. Popovic, P. Rakich, L. Socci, F. Kartner,E. Ippen, and H. Smith, “Polarization-transparent microphotonic devicesin the strong confinement limit,” Nat. Photonics, vol. 1, no. 1, pp. 57–60,2007.[2] H. Fukuda, K. Yamada, T. Tsuchizawa, T. Watanabe, H. Shinojima, andS. Itabashi, “Silicon photonic circuit with polarization diversity,” Opt.Express, vol. 16, pp. 4872–4880, 2008.[3] D. Taillaert, H. Chong, P. Borel, L. Frandsen, R. De La Rue, and R. Baets,“A compact two-dimensional grating coupler used as a polarizationsplitter,” IEEE Photonics Technol. Lett., vol. 15, no. 9, pp. 1249–1251,2003.[4] F. Van Laere, T. Stomeo, D. Taillaert, G. Roelkens, D. Van Thourhout,T. Krauss, and R. Baets, “Efficient Polarization Diversity Grating Cou-plers in Bonded InP-Membrane,” IEEE Photonics Technol. Lett., vol. 20,no. 4, 2008.[5] W. Bogaerts, D. Taillaert, P. Dumon, D. Van Thourhout, R. Baets, andE. Pluk, “A polarization-diversity wavelength duplexer circuit in silicon-on-insulator photonic wires,” Opt. Express, vol. 15, no. 4, pp. 1567–1578,2007.[6] G. Roelkens, D. Vermeulen, D. Van Thourhout, R. Baets, S. Brision,P. Lyan, P. Gautier, and J. Fédéli, “High efficiency diffractive gratingcouplers for interfacing a single mode optical fiber with a nanophotonicsilicon-on-insulator waveguide circuit,” Appl. Phys. Lett, vol. 92, pp.131 101–131 103, 2008.

Reducing polarization-dependent loss of silicon-on-insulator fiber to chip grating couplers

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Reducing polarization-dependent loss of silicon-on-insulator fiber to chip grating couplers

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