Storme, Marcel

Van Orshoven, Paul

Ghent University Academic Bibliography

Hybrid silicon lasers for optical interconnect(Invited Paper)Y. De Koninck, S. Keyvaninia, S. Stankovic, D. Van Thourhout, G. Roelkens and R. BaetsDepartment of Information Technology, Photonics Research Group, Ghent University - IMECCenter for Nano- and Biophotonics (NB-Photonics)Sint-Pietersnieuwstraat 41 B-9000 Ghent, Belgiumemail: yannick.dekoninck@intec.ugent.beAbstract—Hybrid III-V/silicon lasers for short reach opticalinterconnect applications need to have a modest power dissipationand preferably also a small footprint. Furthermore they shouldbe free of mode hops even under varying temperature conditions.In this paper a number of designs will be discussed that addressthis set of requirements.Index Terms—Silicon photonics, heterogeneous integration,hybrid lasers, optical interconnectsI. INTRODUCTIONAs the number of microprocessor cores that are integratedon a single die increases rapidly, the amount of data thathas to be delivered to and extracted from the chip scalesaccordingly. If this trend continues in the coming decade,electrical interconnects will no longer be able to deliverthe required bandwidth in a cost- and power-effective wayand optical interconnects will be needed both for on- andoff-chip communication in future generation integratedcircuits [1]. To offer sufficient advantage over electricalwiring, the power budget for such short-range, high-speedcommunication systems needs to be a few 100 fJ / bit for off-chip interconnects and even lower for on-chip communication.Only a fraction of the available power can be allocated to thetransmitter circuit, consisting of the laser and/or modulatorand its drive circuitry. To reach such ambitious goals, itis obvious that an efficient, low-threshold laser source isnecessary. Furthermore, wavelength division multiplexingis put forward to increase the effective bandwidth of thesystem, but this also puts stringent requirements on thespectral properties of the laser source: single mode operationis essential and instabilities such as temperature variationinduced mode-hops are not allowed. Mode hops should alsobe avoided in single channel systems so as to avoid bit errors[2].To solve this problem, a number of solutions have beenproposed. Externally generated laser light can be fed intothe circuit using a fiber or a laser source can be flip-chippeddirectly onto the chip. However, the highest degree ofefficiency and the smallest footprint can potentially bereached by heterogeneously integrating the laser sourcesonto the photonic/electronic IC [3]. In this paper we willreview a number of designs for such lasers that are suitablefor optical interconnects. We will focus on approachesthat have a wavelength selection mechanism defined ina) evanescent DFBb) tapered mode conversionc) resonant mirrorIII-VIII-VIII-VSOISOISOIFig. 1. Schematic overview of the operation of the evanescent DFB laser (a),III-V-confined laser with adiabatic mode-conversion (b) and resonant mirrorlaser (c)the silicon layer to avoid multi-mode operation or mode-hopping. Although we only discuss this type of lasers inthe context of optical interconnect, an array of single-modelasers, each operating at a well defined wavelength can alsobe used as a tunable light source in spectroscopic applications.Perhaps the most straight-forward approach is the siliconevanescent laser. The laser consists of an optical waveguidein the silicon layer and a wide III-V mesa bonded on top.The laser mode is mainly confined to the silicon waveguideand only its evanescent tail overlaps with the quantum wellsin the III-V layer. DFB-lasers are fabricated by etchingperiodic corrugations in the silicon wire to obtain distributedfeedback. Given the excellent spectral properties found inDFB lasers and the high-resolution fabrication technologyavailable for silicon photonics, evanescent DFB lasers caneasily be used in a wavelength division multiplexing schemeto boost the effective bandwidth through a single outputchannel. This approach was demonstrated for both direct [4]and adhesive bonding [5]. Due to the evanescent nature of thegain mechanism, the modal gain is limited, typically leadingto a lasing threshold for evanescent DFB lasers of about 20mA and a slope efficiency of 0.067 W/A, with a maximumoutput power of about 5mW at 100 mA driving current [4].Assuming typical values for the required optical power forerror-free operation, this leads to an energy consumption ofthe order of 1pJ/bit, which is far above the goals definedabove. Splitting the power into different spatially separatedchannels, each individually modulated, can alleviate thisissue, but still, reaching the 100fj/bit goal is very ambitious.II. RESONANT MIRROR LASERSIn order to reduce the power consumption of the laser,the overlap of the optical field with the III-V multiplequantum well region should be optimized and the laser cavityvolume should be minimized. However, still the wavelengthselective feedback is preferably implemented in the siliconwaveguide layer, given the fact that high accuracy patterningis more easily achieved in silicon than in III-V layers. Theserequirements can be realized by defining a waveguide in theIII-V layer which improves the confinement in the activeregion. In order to couple the light to the silicon waveguidelayer, typically an intra-cavity spot-size converter is usedto squeeze the mode out of the III-V mesa into the siliconwaveguide. In the silicon waveguide, wavelength selectivefeedback can be realized by using a combination of Bragggratings and ring resonators, which can be thermo-opticallytuned [6]. However, low-loss and low-back reflection tapersare difficult to realize in such a device geometry, which againimpacts the threshold current and slope efficiency of thedevice.Recently, we proposed a novel approach for realizinglow-power consumption, single mode lasers that are robustagainst mode hops based on resonant mirrors [7]. Still thelaser mode is confined to the III-V wire but instead ofadiabatically tapering all the light from the III-V wire tothe silicon layer towards the wavelength selective feedbackstructure, the III-V waveguide is now evanescently coupledto two silicon cavities, one at each facet of the III-V wire.The silicon cavities are identical and the coupling to the III-Vwaveguide provides high, narrow banded reflectivity backinto that III-V waveguide as described in [7]. Figure 2 showsa typical reflection and transmission spectrum of a singlesilicon cavity coupled to a III-V waveguide.The reflectivity of this mirror is mainly determined by thequality of the silicon cavity. One can prove that if the lossesin the cavity are sufficiently small, this mirror will reflect near100% of the incident light back into the III-V waveguide.This is similar to an add-drop filter, which is always criticallycoupled if the coupling section is symmetric and the lossesin the ring are sufficiently low.Typically, the silicon cavities are 20-30 µm long, and becausethey provide such high reflectivity, the laser gain section canalso be short, on the order of a few 10’s of micrometers.This means that the device length can be in the range of50-100µm, which is significantly shorter than the approachespresented earlier. The threshold current should decreaseproportionally, making this type of laser a viable candidateFig. 2. Typical simulated characteristic of a resonant mirror. This mirror isabout 30µm long and reflects 95% of the incident light (FWHM 8nm).as a light source in future optical interconnects.The reflection bandwidth of the resonant mirror is determinedby the coupling between the III-V wire and the silicon cavity.Decreasing the coupling will narrow the reflection band-widthand vice-versa. This way the reflection bandwidth can beengineered to reflect only one longitudinal laser mode. If thecavity is sufficiently short it can be designed to oscillate at afrequency aligned to the mirror reflection peak and to do soover a substantial temperature range.III. CONCLUSIONSHybrid silicon lasers based on bonded III-V layers on siliconwere discussed with respect to the challenges and trade-offsin their design and fabrication. Focus is on compact designsthat combine good light confinement in the gain layer withgood spectral control provided by grating structures patternedin silicon.IV. ACKNOWLEDGEMENTSThe authors acknowledge partial support from IWT throughthe SBO-Glucosens project as well as from ERC through theInSpectra project. Yannick De Koninck thanks the researchfoundation Flanders (FWO) for a research grant.REFERENCES[1] D. A. B. Miller, “Device Requirements for Optical Interconnects toSilicon Chips,” Proceedings of the IEEE, vol. 97, no. 7, 2009.[2] T. Heumier and J. Carlsten, “Detecting mode hopping in semiconductorlasers by monitoring intensity noise,” IEEE Journal of Quantum Elec-tronics, vol. 29, no. 11, pp. 2756–2761, 1993.[3] G. Roelkens, L. Liu, D. Liang, R. Jones, A. Fang, B. Koch, and J. Bowers,“III-V/silicon photonics for on-chip and inter-chip optical interconnects,”Laser & Photonics Reviews, vol. 4, no. 6, pp. 751–779, Nov. 2010.[4] A. W. Fang, E. Lively, H. Kuo, D. Liang, and J. E. Bowers, “A distributedfeedback silicon evanescent laser,” Optics Express, vol. 16, no. 7, pp.4413–4419, Mar. 2008.[5] S. Stankovic, G. Roelkens, and D. Van Thourhout, “In preparation.”[6] G.-H. Duan, C. Jany, A. Le Liepvre, J. Provost, D. Make, F. Lelarge,M. Lamponi, F. Poingt, J. Fedeli, S. Messaoudene, D. Bordel, S. Brision,S. Keyvaninia, G. Roelkens, and D. Van Thourhout, “10 Gb/s integratedtunable hybrid III-V/Si laser and silicon MZ modulator,” in ECOC, 2012.[7] Y. De Koninck, G. Roelkens, and R. Baets, “Cavity Enhanced ReflectorBased Hybrid Silicon Laser,” in IEEE Photonics Society Annual Meeting2010, 2010.

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Amice curiae, quo vadis? Het openbaar ministerie in privaatrechtelijke, administratieve en sociale zaken

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