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ULTRA-HIGH BW OPTICAL COMMUNICATION USING OXYGEN PLASMA ASSISTED InP BASED HYBRID Si LASER

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PRATTAY BIN ABDUL WAHAB1, RAQIB AHMED ASIF Department of Electrical Engineering & Computer Science

North South University, Dhaka, Bangladesh

prattay@gmail.com1

MASUDUL HAIDER IMTIAZ*, MST.RUMANA AKTAR SUMI, KAZI RIZWANA MEHZABEEN, SUSMITA GHOSE Department of Applied Physics, Electronics & Communication Engineering,

University of Dhaka, Bangladesh

masudul4145@gmail.com*

Abstract:

This paper demonstrates the development of a hybrid integration platform to build major electrically driven photonic active devices: silicon evanescent DFB lasers, silicon evanescent amplifiers, silicon evanescent waveguide photo-detectors or pre-amplified photo-detectors on a single silicon platform consisting of III-V gain layers with passive waveguide for board to board and for chip to chip optical communication for tera scale computing. The structural description proposed here may push forward the manufacturing of photonic ICs to make high speed communicational imaginings to reality and to start a new era.

Keywords: DFB – SEL; DFB-HSL; Mach- Zehnder; Modulator; MZM.

1. Introduction

Moore’s Law [1] describes a long-term trend in the history of computing hardware, in which the number of transistors that can be placed inexpensively on an integrated circuit has doubled approximately every two years. The relentless push of Moore’s Law has allowed data rates to soar, internet traffic to swell, and wired and wireless technology to cover continents. Now-a-days Silicon is used to manufacture electronics integrated circuits at high volume by using CMOS fabrication process. In using silicon to manufacture optoelectronic integrated circuit, a new process is not necessary to develop. Silicon photonics offers the solution by which optoelectronic integrated circuits can be manufactured in state of the art technology at high volume and also at low cost using the same CMOS fabrication process. In order to siliconizephotonics, we need a light source which is cheap; to split, route and direct light in the silicon chip, an optical modulator to encode data on continuous light wave, a photo detector to convert optical signals to electrical signals, high volume of production with minimum cost and necessary intelligence for photonic control.

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Fig. 1. Concept of a future integrated terabit silicon optical Tx containing 25 hybrid silicon lasers multiplexed into single output fiber. (Courtesy Intel

the whole process expensive, time consuming and mass production limited. Associated coupling loss is not a prime concern for low temperature wafer bonding technology using oxygen plasma.

High expense of photonic devices and absurd assembly of photonic components onto single silicon chip led photonic industry to stay far behind than electronic industry. The objective of this paper was to provide a schematic solution to integrate the individual photonic components like silicon evanescent laser, amplifier, modulator, photo-detector or pre amplified photo-detector together to form an optoelectronic device, which can bring optical interconnects to chip to chip and hence, tera scale computing to reality. It is based on the hybrid waveguide structure consisting of III-V gain layers and a silicon passive waveguide. Through the mode overlap to the III-V gain region, the optical mode in the hybrid waveguide experiences optical gain or absorption lead to build efficient electrically driven photonic active devices on the silicon photonics platform. The amount of mode overlap in III-V gain layers and silicon waveguide layer can be engineered by manipulating silicon waveguide dimensions as well as the thickness and refractive index of each III-V layer. Very recently, the Intel researchers have developed Si APD with built-in amplification section [3]. Using this Si APD, the evanescent preamplifier and photo detector are eliminated in this paper and presented a model of an Optical De-Mux using hybrid silicon laser as transmitter and Silicon APD as receiver. Fig.1 shows what a proposed terabit integrated optical transceiver could look like. It consists of a row of small, compact hybrid silicon lasers, each generating laser light at a different wavelength (color) and directed into a row of high-speed silicon modulators that encode data onto each of the different laser wavelengths. An optical multiplexer would combine these individual data streams together into one output fiber, in the process all of these signals can be simultaneously sent down a fiber without interfering with each other. Twenty five hybrid silicon lasers integrated with twenty five silicon modulators (each running at 40Gbps) results terabit per second of optical data transmitting from a single integrated chip [4].

2. Description

2.1. Unavailability of silicon in optical communication

Although a good guider of light, but, silicon is an inefficient light emitter because of a fundamental limitation called indirect bandgap that prevents its atoms from emitting photons in large numbers when an electrical charge is applied. Instead, silicon emits heat as a form of lattice vibration. Having lack of strong electro-optic effect, it is not very good at modulating a laser beam. And, it is quite poor at photo-detection (converting photons into electrons) that is normally carried out at infrared wavelengths. To overcome the limitations of silicon, many approaches have been aimed like overcoming the indirect band structure by using spatial confinement of the electron, introduction of rare earth impurities as optically active dopants, use of Raman scattering to achieve optical gain.

2.2. Hybrid Silicon Laser

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Figure 2. (a) Cross-section schematic of a hybrid silicon laser. (b) When voltage is applied to the contacts, current flows, and the electrons (-) and holes (+) recombine in the center and generate light. (Courtesy Intel)

patterned silicon photonic chip (alignment of InP-based material is not required at all) [5]. Fig. 2(a) below is a cross-section of hybrid silicon laser, showing the InP-based gain material (orange) that generates the laser light bonded on top of a silicon waveguide (gray). When a voltage is applied to electrical contacts shown in yellow in Fig. 2(b), electrons flow from the negative contacts toward the positive contact, encounter holes in the semiconductor lattice, recombine and emit photons. Generated light is guided by the silicon waveguide through the glue layer.

This bonding technique can be performed at the wafer or die level, depending on the application, and could provide a solution for large-scale optical integration onto a silicon platform. Lasers will no longer have to be attached and aligned separately before, so, dozens maybe even hundreds of lasers can be created with one bonding step. We can have different wavelengths for every laser by just modifying the silicon waveguide properties which was done previously by altering the doping percentages and base materials. The optical mode can obtain electrically pumped gain from the III–V (like InP and GaAs etc) while being guided by the underlying silicon waveguide region [4].

2.3. Distributed Feedback Hybrid Silicon Laser (DFB-HSL)

Distributed feedback lasers are attractive for optical communications since they have a single longitudinal mode output and their short cavity lengths allow for low threshold currents while still producing mille watt regime output power. Its output wavelength is determined by grating pitch. Grating based lasers are more dependent on grating periodicity [6].

2.4. Mach- Zehnder Modulator (MZM)

Mach- Zehnder modulators mainly created from Lithium Niobate (LiNbO3) are suited for use in metro, long-haul (LH) and ultra long-long-haul (ULH) optical transport applications. However the silicon photonics offers also small MZM fabricated from silicon. The incoming optical signal is split equally, sent down two different optical paths. After a few centimeters, the two paths recombine, causing optical waves to interfere with each other (schematically shown in Fig.3). If the phase shift between the two waves is 0°, the interference is constructive and the light intensity at the output is high (on state); if the phase shift is 180°, the interference is destructive, light intensity is zero (off state) [7]. Phase shift and output intensity are controlled by changing the delay through one or both of optical paths by means of electro-optic effect, causes refractive index to change in presence of an electric field [8].

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Fig.4. Transmitting section of the integrated photonic devices

2.5. Silicon Evanescent Amplifier

Optical amplifiers are the key components in realizing the high level of photonic integration as they compensate optical losses from the individual photonic elements. If a high speed modulator of 40Gbps is used, then we need an amplifier which also can amplify data rate of 40Gbps too. Similar thing can be used in the photo detector for pre–amplification. In the case of pre–amplification, dynamic characteristics and power penalty are important for high speed detection. An electrically pumped hybrid amplifier was fabricated and tested in [9] exhibits 13dB on – chip gain and only 0.5 dB penalty for data rates up to 40Gbps. In order to increase the gain of the amplifier, the optical confinement factor in the III – V region needs to be increase [10]. Unlike Laser, having lack of feedback system, light is amplified only by stimulated emission, may cause noise. To minimize optical

feedback, the sample is diced at a 7˚ with the normal to the polished facet plane, then an antireflection coating of Ta2O5 is applied to each facet [11].

3. Integrated Photonic components on a single Silicon Platform

The active photonic components used for different purposes like producing, modulating, amplifying or detecting light, have been manufactured on a common silicon platform with the same cross-sectional features but individually. Fig. 4 shows the Tx section of the device which we have visioned that has the potential to bring optical communication inside the computers. Ten fabricated hybrid silicon lasers aligned together are connected to the hybrid silicon based Mach- Zehnder modulator by a silicon waveguide is shown in Fig 5. Instead of electro absorption modulator [12] based silicon evanescent structure (speed 10Gbps), we chose Mach– Zehnder because of its high speed above 40Gbps [13]. Only advantage of using electro absorption modulator is that it could minimize the space of the integrated photonic components on a single silicon platform. It requires only one mesa structure whereas it requires two for Mach– Zehnder which is not a big deal. In the hybrid silicon technology, by changing the structure of the mesa or III–V, different active components can be achieved, but not for the case of laser. The ten lasers shown in Fig.7a will emit light of same wavelength. However, because of the novelty of the device just by individually changing the structure of the silicon waveguide ten different wavelengths can be produced. If ten hybrid silicon lasers were integrated with ten Mach– Zehnder modulator (each running at 40 Gbps) the result would be 400 Gbps of optical data transmitting from a single integrated silicon chip [14].

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Fig.6. Continuous wave lasing spectrum for various current levels.(Fang, A)

3.1.Distributed Feedback Silicon Evanscent Laser (DFB-SEL)

Fig.5 shows the layout of DFB – SEL, consists of a 200 µm long gain region. 80 µm long tapers are formed by linearly narrowing the III–V mesa region above the silicon waveguide. The spontaneous emission spectrum can be used to extract device parameters, such as effective indices, group index, amount of phase shift in the center of DFB and k value from the DFB [15]. The light-current (L-I) characteristics of the DFB-SEL measured on chip by collecting light out of both sides of the laser with integrated silicon evanescent photo-detectors shows that at 10 °C, the lasing threshold is 25 mA with a maximum output power of 5.4 mW. Fig. 6 below shows the spectral shift under CW operation for various current levels. It can be seen that the laser stays single mode throughout the various current injection levels. Using DFB – SEL, ten hybrid silicon lasers can emit ten different wavelengths [15]. Fig. 7b shows ten fabricated DFB–SEL connected with ten Mach–Zehnder modulator via silicon waveguide.

3.2.Integrated Amplifier with silicon evanescent photo-detector

Silicon evanescent amplifier can be easily integrated with the silicon waveguide after the optical signals were modulated. Recently hybrid silicon based amplifier attained the capability for high speed amplification up to 40Gbps.The light detector is one of the prime active photonic devices in the integrated circuits. However the hybrid silicon based photo detectors can detect the light wave up to the 5Gbps. So it is the photo detector which needs more research on how to upgrade the detection speed. Very recently monolithically Ge/Si with a gain bandwidth of 340 GHz has come. It can detect light wave up to 10 Gbps. More research is being done on this particular Ge/Si photo detector. However the DFB–SEL laser produces wavelength in the range of 1500 nm, but the Ge/Si photo detectors can only response to the wavelengths in the range of 1300 nm. Even the researches are optimistic that by mixing III–V material with germanium can absorbed the light in the range of 1500 nm and avalanche process can take place inside the doped silicon. Fig.9 shows the complete transmitting section of integrated photonic components on single chip.

3.3.Complete transceiver platform for integrating devices

In the receiving section for photo detection we have used hybrid waveguide silicon evanescent photo detector. The receiving signals are weak so it needs to be amplified before being detected by photo detector. In this design signal get amplified and also get detected at the same time. In the hybrid waveguide detector, there is a 50 – 50 chance of the light wave to get amplified or get absorbed. And the performance of the device is not up to the mark. However there is another type of photo-detector named pre–amplified photo detector. In a pre– amplified photo detector the light wave gets amplified first and then it get absorbed. It is the best choice for integrated photonic components to have pre–amplified photo detector.

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Fig. 7. (a) Ten DFB – SEL laser with Mach – Zehnder Modulator (b) final Transmitting section of the integrated active photonic components on a common Silicon platform

4. Optical mode confinement factor

Optical confinement is the most important factor on determining the individual photonic component performance [16].The confinement factor in the III–V mesa and on its corresponding silicon waveguide needs to be varied from device to device. If the width of the mesa is wide then the optical confinement in the mesa region will increase, although the confinement factor in the silicon waveguide is always greater than the III–V mesa. For laser where optical gain is important, the III–V mesa width is increased, similar thing for the amplifier. But for modulator and photo detector thin width is okay because all it need to detect and to modulate. The height and the width of the silicon waveguide determine the optical confinement in the silicon region. In the integrated photonic component above fundamental optical mode is considered, so a fixed height and width is taken into account on the single silicon substrate. Series of picture of cross section of the optical confinement by using specific software BeamPROPTM are shown in the Fig.9. The optical mode confinement factor must be significantly greater because the optical mode in the III–V mesa remains in the mesa but it is the optical mode in the silicon waveguide which guide the light into other photonic components.

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silicon waveguide height and the slab height is fixed at 0.7 μm and 0.1 μm respectively. (Courtesy to Hyundai Park)

Fig. 9. (2) Simulated (top) and measured (bottom) optical mode profiles with different waveguide widths. The waveguide height is 0.76 μm with a slab height of 0.1 μm. The calculated silicon and the quantum well confinement factors are specified at the bottom. (Courtesy to

Hyundai Park)

5. Acknowledgement

The authors thank Dr. Hyundai Park(university of California, Santa Barbara, Dept. of Electrical and Computer Engineering, Santa Barbara, USA) for proving help and support he gave being 8000 miles away. To him the authors will always be in debt. The authors also appreciate discussions with Dr. Abdul Awal, Professor, Department of EECS, North South University, Bangladesh.

6. Conclusion

The device presented in this work is based on the hybrid waveguide structure consisting of III-V gain layers and a silicon passive waveguide. A low temperature oxygen plasma assisted wafer bonding process was used for device fabrication. A low temperature anneal step was important to minimize the material degradation from the large thermal mismatch between silicon and InP as well as to achieve a good scalability for wafer level bonding process. It is this wafer bonding which enable the possibility for having optical computing to a reality. This silicon evanescent device platform provides unique advantages over other hybrid integration techniques in terms of integration capability with silicon passive devices (high coupling efficiency, and low reflection) and manufacturability to fabricate many devices with a single bonding step. Future work, including studies on device performance and reliability, will expedite the developments of silicon-based optoelectronic circuits integrated with on-chip silicon evanescent photonic active devices overcoming the indirect band gap characteristic of silicon.

7. References

[1] Neil Savage, Samuel K.Moore, “Linking with light.”, IEEE Spectrum, vol 39. no. 8, pp. 32-36, August 2002. [Online]. Available: http://www.usc.edu/dept/engineering/eleceng/Adv_Network_Tech/Html/publications/IEEESpectrum.8.8.02.pdf

[2] R. Soref, "The past, present, and future of silicon photonics," Selected Topics in Quantum Electronics, IEEE Journal of, vol. 12, no. 6, pp. 1678-1687, March 2007. [Online]. Available: http://dx.doi.org/10.1109/JSTQE.2006.883151

[3] Y. Kang, H.-D. Liu, M. Morse, M. J. Paniccia, M. Zadka, S. Litski, G. Sarid, A. Pauchard, Y.-H. Kuo, H.-W. Chen, W. S. Zaoui, J. E. Bowers, A. Beling, D. C. McIntosh, X. Zheng, and J. C. Campbell, "Monolithic germanium / silicon avalanche photodiodes with 340 Ghz gain - bandwidth product," Nature Photonics, vol. 3, no. 1, pp. 59-63, December , 2008.

[Online]. Available: http://dx.doi.org/10.1038/nphoton.2008.247

[4] M.J.Paniccia, V.Krutul, R.Jones, O.Cohen, J.E.Bowers, A.W.Fang, H.Park, "A Hybrid Silicon Laser: Silicon photonics technology for future tera-scale computing." Intel white paper, September 18, 2006. [Online].Available:download.intel.com/research/platform/sp/hl_wp1.pdf

[5] Alexander W. Fang, Hyundai Park, Ying-hao Kuo, Richard Jones, Oded Cohen, Di Liang, Omri Raday, Matthew N. Sysak, Mario J. Paniccia, J.E. Bowers. "Hybrid silicon evenescent devices," Materials Today, vol. 10, no. 7-8, pp. 28-35, July-August 2007. [Online]. Available: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.111.8222&rep=rep1&type=pdf

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[7] Ying-hao Kuo, Hui-Wen Chen, John E. Bowers, "High speed hybrid silicon evanescent electroabsorption modulator," Opt. Express.

vol.16. pp. 9936-9941, 2008. [Online]. Available: http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-13-9936

[8] "Optical Modulators Tutorial - Electroabsorption Modulator (EAM) and Lithium Niobate Mach-Zehnder Modulator (MZ Modulator) ", July 12, 2009.[Online].Available: http://www.fiberoptics4sale.com/wordpress/optical-modulators-tutorial-electroabsorption-modulator-eam-and-lithium-niobate-mach-zehnder-modulator-modulator-mz-modulator/

[9] Matthew N. Sysak, J.E. Bowers, A. W. Fang, H. Park, O. Cohen, R. Jones, M. J. Paniccia, O. Raday. "Experimental and theoretical thermal analysis of a Hybrid Silicon Evanescent Laser, " Optics Express, vol. 15, no. 23, pp. 15041 – 15046, 2007. [Online]. Available: http://escholarship.org/uc/item/4wm546vn

[10] J. Bowers, H. Park, Y. Kuo, A. W. Fang, R. Jones, M. J. Paniccia, O. Cohen, O. Raday, " Integrated Optical Amplifiers on Silicon Waveguides," in Integrated Photonics and Nanophotonics Research and Applications, OSA Technical Digest (CD) (Optical Society of America, 2007), paper ITuG1. [Online]. Available: http://www.opticsinfobase.org/abstract.cfm?URI=IPNRA-2007-ITuG1

[11] H.Park, “Silicon Photonics optical buffers” Ph.D. dissertation, University of California, Santa Barbara, USA, 2008.

[12] Hui-Wen Chen, Yinghao Kuo, J. E. Bowers, "Hybrid silicon modulators," Chin. Opt. Lett. vol. 7,pp. 280-285, 2009. [Online]. http://www.opticsinfobase.org/col/abstract.cfm?URI=col-7-4-280

[13] Mike Salib, Ling Liao, Richard Jones, Mike Morse, Ansheng Liu, Dean Samara-Rubio, Drew Alduino, Mario Paniccia, "Silicon photonics," Intel technology Journal, vol. 8, no. 2, pp. 143-159, May 2004. [Online]. Available: http://developer.intel.com/technology/itj/index.htm

[14] Y.Kuo, H.Park, A.W. Fang, J.E. Bowers. "High speed data amplification using hybrid silicon evanescent amplifier, "in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper CTuII1. [Online]. Available: http://www.opticsinfobase.org/abstract.cfm?URI=CLEO-2007-CTuII1M. Young, The Technical Writer's Handbook. Mill Valley, CA: University Science, 1989.

[15] R. Schatz, E. Berglind, and L. Gillner, “Parameter extraction from DFB lasers by means of a simple expression for the spontaneous emission spectrum,” IEEE Photonics Technology Letters, Vol. 16, No. 10 (1994)

Figure

Fig. 1. Concept of a future integrated terabit silicon optical Tx containing 25 hybrid
Fig. 3. The schematic diagram of MZM (Shizuoka Univ. Japan)
Fig. 8. Tx – Rx of integrated photonic components on a single silicon platform using pre – amplified photo detector
Fig. 9. (1)  calculated two dimensional mode profiles with different silicon waveguide widths of (a) 1.0 μm, (b) 1.25 μm and (c) 2.0 μm

References

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