Modal Characteristics of Coupled Resonator Vertical Cavity Laser Diode

Transcription

1 Advances in Optoelectronic Materials (AOM) Volume 4, 06 doi: /aom Modal Characteristics of Coupled Resonator Vertical Cavity Laser Diode Kaikai Xu *, David Cheng, Xingfa Huang 3 State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, China Department of Electrical Engineering, College of Engineering and Computer Science, California State University, Fullerton, CA Sichuan Solid-State Circuit Research Institute, Chongqing, China * kaikaix@uestc.edu.cn Abstract Current local area network (LAN) bandwidth demands of > Gb/s are pushing the limit of the bandwidth distance product of multimode fiber. In order to overcome these limitations, a low-cost, compact wavelength-division-multiplexing (WDM) scheme is demonstrated. Aggregation bit rate of 5 Gb/s are achieved through 00 m of 6.5/5-µm multimode fiber which are used at 80, 840, 860, 880 nm with each operating at a bit rate of 5 Gb/s. This technology provides a practical solution for reaching bit rates up to 0 Gb/s in the LAN. The possible angular misalignment effects in fiber-optic coupling and alignment process is also identified and evaluated for the development of low-cost fiber-optic component manufacturing technologies. It is further demonstrated the design and implementation of a novel multiple-wavelength optical data link for low-cost multi-mode wavelength-division multiplexing (WDM) local-area network applications. Keywords Vertical-Cavity Laser; Integrated Optoelectronics; Laser Arrays; Wavelength-Division Multplexing; Misalignment Introduction Vertical-cavity surface-emitting lasers (VCSELs) are an established technology for short-distance optical networks and are being considered for future microwave link application at 850-nm wavelengths []. Compared with the edge-emitting lasers, the VCSEL is with much less fabrication cost and higher fiber coupling performance. Since VCSEL s circular beam is in good agreement with the fiber s shape, the light from VCSEL emitting from the transceiver module can be coupled into the fiber ends efficiently []. Multimode graded index fiber is known as the main fiber technology for optical communication applications, since it provides sufficient bandwidth for most applications without the critical tolerances of single mode fiber. On the other hand, multimode fiber is the predominant fiber in LAN s due to its low-cost installation and maintenance, the availability of low-cost LED transceivers, and the initial low bandwidth requirements [3]. Emergency for high-capacity telecommunication link and speed limitation of single wavelength links has resulted in an extraordinary increase in use of the wavelength-division multiplexing (WDM) in advanced light-wave networks [4]. This paper extends contribution of [5] by presenting a detailed assessment of the WDM optical data link for lowcost LAN applications. This link utilizes a monolithically integrated multiple-wavelength VCSEL array to transmit multiple channels of information simultaneously via a single multimode fiber. In addition, we examine the dependence of the misalignment between the fiber and the photo-detector, since the measurement of fiber position is one of the most important consideration in automating the fusion splicing of optical fibers. Thus, the main objective of the proposed work includes: () to study the modal dispersion characteristics and cutoff condition of a doubly clad fiber first and then increase the number of inner cladding to observe the effect of number of cladding on the guided modes; () to diminish the effects of angular misalignments on fiber for locating the optimal coupling position.

2 Advances in Optoelectronic Materials (AOM) Volume 4, 06 Multimode Devices and Subsystems Vertical-Cavity Surface-Emitting Laser The rate equations of this semiconductor laser are based on the following single-mode spatially dependent equations: N r, t ii r, t N r, t L eff Il N, T G r, t S t r N r, t t q q n S t S t t V V n N r, t dv G r, t S t r dv () p n V V I is the spatially dependent injection current, N is the carrier density scaled by the effective active layer volume V, S is total photon number, is normalized transverse mode profile, T is the device temperature, G is the gain, I l is the thermal leakage current, I is the current-injection efficiency, n is the carrier lifetime, carrier diffusion length, and q is the electron charge [6]. Fig. shows the schematic of the laser Power emitted by the VCSEL can be derived from two equations above, as, where k f is a constant that equals the value of 0.05 µw. () L eff is the effective Pout k f S (3) Typically, LEDs used in fiber communications produces power levels of several microwatts in the fiber. For low drive current, the output power is a linear function of current, as shown in Fig.. As the drive current becomes larger, eventually the output power saturates. The linearity of the diode refers to the linearity of the curve that relates the output power to the drive current. Threshold current increases with temperature and power tends towards saturation at large driving current. Threshold current is ~.5 ma at 0 C for the VCSEL device with emitting wavelength of 80 nm. FIG. DEVICE STRUCTURE FOR THE VCSEL SHOWING ELECTRICAL BIASING FIG. OUTPUT POWER VERSUS DRIVE CURRENT FOR THE LASER SOURCE Graded-Index Multimode Fiber A fiber with a parabolic (or almost-parabolic) variation in the index profile distribution is a grade-index fiber (GRIN). One mathematical mode for the radial index profile is

3 Advances in Optoelectronic Materials (AOM) Volume 4, 06 n r n n r r a a n r a (4) where n is the index of refraction at the center, is the fractional change in the index of refraction from the center to the edge of the core, r is the radial distance, and a is the core radius. In the system, a is of 5 µm, n is of.44 for wavelength aournd 850 nm, and is of %. Numerical Aperture NA n 0. is applicable for transmitter-fiber coupling efficiency. In fact, a typical graded-index multimode fiber (MMF) has an outside diameter of 5 or 40 µm with a fractional change of the index of refraction of -% induced by varying the doping level of impurities radially. Subsystem In the subsystem, we use optical devices mentioned above to construct and analyze a WDM multimode system. The spatial graphs at every important step are given. The misalignment between fiber and photo-detector is tested. For the WDM system, we use PRBS digital code and NRZ electrical signal as VCSEL s deriving source. Each channel has a 5 Gb/s digital signal. An illustration of the low-cost WDM transceiver module that has been under development is shown in Fig.3. There are four discrete VCSEL s at nominal wavelengths of 80 nm, 840 nm, 860 nm, and 880 nm. The light from the VCSEL s emits vertically from the transceiver module, and then is combined into the MMF using a 4: waveguide combiner. Correspondingly, the received light is de-multiplexed using a :4 polymer waveguide splitter which divides the light equally among the four outputs. The description of the charateristics of polymer waveguide splitter needs to be further elevated. The light from the VCSEL s emits vertically from the transceiver module. A 45 angle on the end of the polymer waveguide multiplexer reflects the light into the waveguides of the multiplexer. The light is combined into a MMF using a 4 : waveguide combiner structure as shown. The input waveguides of the 4 : combiner are chosen to be smaller than the output waveguides to achieve a loss as low as. db. It should be noted that a 4 : combiner using singlemode waveguides would incur a minimum loss of 6 db. The received light is demultiplexed using a : 4 polymer waveguide splitter which divides the light equally among the four outputs. Tiny dielectric interference filters are used to select each wavelength. Splitting before filtering introduces a fundamental 6-dB loss for a four-channel system. The measured insertion loss of the : 4 splitter is approximately 8.5 db per channel [7]. FIG. 3 CONFIGURATION FOR SIMULATION: MULTIMODE FOUR CHANNELS MULTIPLEXING AND DE-MULTIPLEXING For the present simulations, we use the oxide-confined 840-nm VCSEL with an oxide aperature diameter that is good agreement with the MMF core. More detailed information on the VCSEL characteristics is shown in Fig. 4 and Fig. 5. 3

4 Advances in Optoelectronic Materials (AOM) Volume 4, 06 FIG. 4 SPATIAL GRAPHS OF VCSELs OUTPUT: (a) CHANNEL #(80 nm); (b) CHANNEL #(840 nm) FIG. 5 SPATIAL GRAPHS OF VCSELs OUTPUT: (a) CHANNEL #3(860 nm); (b) CHANNEL #4(880 nm) Since the GRIN waveguide is different from the standard circular step-index fibers by having three types of cladding, the guided mode fields decay exponentially in the radial direction. The V-parameter for GRIN is defined as 4

5 Advances in Optoelectronic Materials (AOM) Volume 4, 06 V n ka an (5) The number of mode N in a multi-mode graded-index fiber is approximately given as [8] g 4a n N g g V g (6) Hence, the analytical study of the modal characteristics, cutoff condition and dispersion curves of circular optical waveguide having various cladding layers can be achieved. Fig. 6 and Fig. 7 present the modal dispersion characteristics and cutoff condition of a doubly clad fiber when the number of inner cladding two to three and then three to five, which is compared with the cut-off condition for single mode propagation. FIG. 6 FIVE MODES OF CHANNEL#(80nm), EVERY MODE OWNS A POWER mw, WHICH IS 0% OF THE LASER S EMITTING POWER. (a) MODE#, (b) MODEL#, (c) MODEL#3,(d) MODEL#4. FIG. 7 FIVE MODES OF CHANNEL#(80 nm), EVERY MODE OWNS A POWER mw, WHICH IS 0% OF THE LASER S EMITTING POWER: 5 MODES ARE TOTALLY INCLUDED. The VCSEL is adjusted to distribute its totally emitting power into 5 portions equally, and every mode will own portion. In other words, every mode in a VCSEL has the power equal to 0% of the total power of this laser. To 5

6 Advances in Optoelectronic Materials (AOM) Volume 4, 06 demonstrate this point more actually, further analysis has been done. The analytical study of the modal characteristics and cutoff condition for optical waveguide are discussed in more detail in [9], [0]. Influence of The Diameter of a Fiber on Measurement Accuracy Our discussion has assumed that the diameter of a MMF is 5 µm. However, in reality, unevenness occurring in the manufacturing process may reduce the accuracy of measuring the axial misalignment of a fiber. Hence, the discuss on how the diameter of a fiber can have an influence on measurement accuracy, using a simulation calculation, is required. In Fig. 3, only Channel# is on to obtain the maximum value of light intensity and the bright line central position from the light intensity distribution in the range of ~5 µm, a measuring range in the direction of the optical axis for the MMF with diameter of ~5 µm. The emission from the output end of the fiber is automatically derived using interferometric and fourier transformation. The simulated light intensity distribution in the y axis is shown in Fig. 8(a) where the observation plane position is in the range of -5 µm < x < 5 µm. The light coming from the front-end of the fiber is coupled onto the PD (photo-detector) by holding the post-end of the fiber over the PD. The deviation in vertical direction of the central axis between the MMF post-end and the photo-detector reduces the coupling efficiency of light beam to the detector system, and the offset is accordingly shown in Fig. 8(b). FIG. 8 SPATIAL OUTPUT FROM FIBER: (a) DEPENDENCE OF LIGHT INTENSITY ON OBSERVATION PLANE POSITION; (b) DEPENDENCE OF OFFSET POSITION ON OBSERVATION PLANE POSITION The phenomenon caused by coupling misalignment clearly shows that the photo-detector has a receiving area that is round with diameter 70 µm. And the misalignment 0 µm will disturb the expected power of photo-detector received. The respective effects of offsets on the coupling efficiency is obtained from the spatial distribution received by the photo-detector shown in Fig. 9. It is evident that the normalized coupling efficiency decrease with the increase of offsets, which is consistent with previous results [], []. FIG. 9 THE OFFSET HAS EFFECTED RECEIVING POWER. FOR THE COORDINATION OF (0, 0), THE SPECTRUM OUT OF THE CIRCLE WITH RADIUS 35 µm WILL BE MISSED 6

7 Advances in Optoelectronic Materials (AOM) Volume 4, 06 More useful approaches on increasing spatial light to multimode bier coupling efficiency is given in [3]. Finally, it is clearly from above consideration that the sensitivity of coupling with reference to the misalignments can be easily estimated, which leads to more convenience for the design of such types of couplers. Other factors such as optical time-domain reflectory (OTDR) and manufacturing cost are also important in the evaluation of the system [4]. Conclusions We report the design and implementation of a novel multiple-wavelength optical data link for low-cost multimode wavelength-division multiplexing (WDM) local-area network applications. This link utilizes a monolithically integrated multiple-wavelength vertical-cavity laser array and a narrow-band resonant-cavity photo-detector array to transmit multi-channels of data simultaneously via the MMF. In order to get higher coupling efficiency, the axis of the post-end of MMF to photo-detector coupling device must be aligned to avoid any offsets. Coupling of light from emitter to PD is achieved with considering the effect of misalignment, and the technology is demonstrated to be a major step toward industry width acceptance of optical interconnects. As the number of PDs increases with the deployment of wavelength division multiplexing to further improve the aggregated systems bandwidth, the interconnection bottleneck between the PD array and receiver electronics become increasingly problematic that should be solved in future work. REFERENCES [] P. Pepeljugoski, D. Kuchta, Y. Kwark, P. Pleunis, and G. Kuyt, 5.6 Gb/s trasmission over km of next generation mutlimode fiber, IEEE Photon. Technol. Lett., vol. 4, no. 5, pp , 00 [] D. Grasso, D. Serkland, G. Peake, K. Geib, and K. Choquette, Direct modulation characterisitcs of composite resonator vertical-cavity lasers, IEEE J. Quantum. Electron., vol. 4, no., pp , 006 [3] R. Lasky, U. Osterberg, and D. Stigliani, Optoelectronics for Data Communications, New York: Academic, 995, p. [4] G. Keiser, A review WDM technology and applications, Opt. Fib. Tech., vol. 5, no., pp. 3-39, 999 [5] K. Xu, D. Cheng, and X. Huang, Multimode communication system used in local area network, in Proc. Symposium on Photonics and Optoelectronics (SOPO 009), pp. -4, 009 [6] K. Choquette and K. Geib, Vertical-Cavity Surface-Emitting Lasers, C. Wilmsen, H. Temkin, and L. Coldren, Eds. New York: Cambridge Univ. Press, 999, pp [7] L. Aronson, B. Lemoff, L. Buckman, and D. Dolfi, Low-cost multimode WDM for local networks up to 0 Gb/s, IEEE Photon. Tech. Lett., vol 0, no. 0, pp , 998 [8] A. Cherin, Introduction to Optical Fiber, New York: McGraw-Hill, 983 [9] Y. Prajapati, V. Singh, and J. Saini, Modal analysis of a super-elliptical Bragg waveguide with a small number of periodic cladding layers based on a very simple analytical technology, Optik, vol. 0, no., pp. 4-9, 009 [0] J. Maurya, Y. Prajapati, V. Singh, and J. Saini, Effect of cladding layer on the mode of circular optical waveguides, Optik, vol. 4, no., pp , 03 [] S. Gangopadhyay and S. Sarkar, Misalignment considerations in laser diode to single-mode fibre excitation via hyperbolic lens on the fibre tip, Opt. Comm., vol. 46, no., pp , 998 [] Z. Tang, R. Zhang, and F. Shi, Effects of angular misalignments on fiber-optic alignment automation, Opt. Comm., vol. 96, no., pp , 00 [3] Z. Luo, P. Jiang, H. Yang, W. He, H. Zhou, and J. Wang, The impact of the off-axis for the spatial light to multimode fiber coupling system, Optik, vol. 4, no. 4, pp , 03 [4] G. Ribordy, N. Gisin, O. Guinnard, D. Stucki, M. Wegmuller, and H. Zginden, Photon counting at telecom wavelengths with commercial InGaAs/InP avalanche photodiodes: current performance, J. Mod. Opt., vol. 5, no. 9, pp , 004 7

8 Advances in Optoelectronic Materials (AOM) Volume 4, 06 Kaikai Xu received the B.S. degree from the University of Electronic Science and Technology of China (UESTC), Chengdu, China, the M.S. degree from the California State University at Fullerton (CSUF), and the Ph.D. degree from the University of California at Irvine (UCI) and the California Institute for Telecommunications and Information Technology (Calit), in June 006, May 009, June 04, respectively. Since September of 04, he has been appointed as a Distinguished Associate Professor at the University of Electronic Science and Technology of China (UESTC), Chengdu, China. Since December of 04, he has been appointed as a Distinguished Professor of Sichuan Province, China (i.e., the member of the 000 Plan of Sichuan Province, China). Dr. Xu is a member of IEEE, OSA, SPIE, and American Physical Society. His honor includes: Wiley Science Advisor, ICPEAC 03 travel award, Outstanding JAP Author, 03 CIOMP-OSA International Summer Session travel award, IONS-NA 7 travel award, Chunhui Plan grant awarded by the Ministry of Education of the People s Republic of China, and the First Place prize in the th International System-on-Chip (SOC) Conference IEEE Orange County and Western Digital Student Design Contest. His research interest includes optoelectronic devices, CMOS IC design, Silicon Photonics and the application of these for optoelectronic integrated-circuits (OEICs) systems. Until now, he has published 0+ peer-reviewed international journal and conference papers and book chapters as the first author/independent author. His international research team is supported by the Natural Research Foundation of South Africa, Calit of UCI, Natural Science Foundation of China, and the French government. Dr. Xu is a reviewer for the IEEE Journal of Selected Topics in Quantum Electronics and OSA Optics Express. He is also the Technical Program Committee (TCP) member/chair for several international conferences focusing on optoelectronics, such as OECC 05, POEM 05, and SOPO 05. He also serves on the Editorial Board for several international journals, such as "Optoelectronics Journal" published by the Hans Publisher, Newark, Delaware, USA, "Optics and Photonics Journal", International Journal of Materials, Mechanics and Manufacturing, and American Journal of Optics and Photonics. David Cheng received his B.S. from National Taiwan University, Taiwan and his M.S. and Ph.D. degrees in Solid State Physics from the University of California, Irvine, in 976. His teaching and research interests are in the areas of semiconductor devices, optic-electronics, electro-optics and lightwave technology, fiber-optic communications, modeling, simulation and system analysis. Dr. Cheng is also active in his professional societies. He has served as Chairman of the Institute of Electrical and Electronic Engineers (IEEE) Orange County Section (4,000 members) and Coordinator of IEEE Greater Los Angeles Council (4,000 members). He has been the President of the Asian Faculty and Staff Association, CSUF, since 994. Dr. Cheng has been awarded a prestigious IEEE Millennium Medal for his outstanding professional achievements and services. He is also honored by Hunan University, China with a professorship for his outstanding contributions to his profession. At present, he is the chair of the Department of Electrical Engineering, CSUF. 8