Multimode Communication System Used in Local ...

Multimode Communication System Used in Local Area Network(LAN) Kaikai Xu and David Cheng

Xingfa Huang

Department of Electrical Engineering California State University, Fullerton Fullerton, CA, USA [email protected]

National Labs of Analog Integrated Circuits SISC, CETC, Chongqing Chongqing , China [email protected]

Abstract—The demand for ever faster data transmission rate (a few Gb/s up to 100Gb/s) has attracted considerable interest in the development of high-capacity optical data links for short-haul local-area network (LAN). In this paper, vertical-cavity surface-emitting laser (VCSEL), Multimode Fiber with high bandwidth, and Spatial Analyzer will be discussed. Finally, a WDM optical data link for low-cost LAN applications will be introduced. In addition, the misalignment between fiber and receiver will be demonstrated. Keywords-Vertical-cavity surface-emitting laser (VCSEL); Multimode Fiber; Spatial Analyzer; WDM; Misalignment;

I.

INTRODUCTION

The vertical-cavity surface-emitting laser(VCSEL) was developed in the 1990s, several decades after the edge-emitting laser. Its structure has several unique characteristics. One is that the beam is circular, the same shape as the fiber. This match improves fiber coupling performance. VCSELs have short cavity lengths, which tend to decrease response times. The result is that VCSELs can be modulated at very high speeds. VCSELs operating in the visible spectrum are appropriate as source for plastic-fiber systems. VCSELs are often selected as sources in datacom(LAN) networks operating at 850nm. Applications include the high-speed Gigabit Ethernet. These 850nm VCSELs can be operated up to 10Gb/s. Their low cost and high efficiency make them attractive in applications requiring large numbers of transmitter. For multimode fibers, it can enjoy the benefit of more relaxed alignment tolerances and therefore simplified packaging. Traditionally, due to modal dispersion, the bandwidth-length of a multimode fiber is much smaller than a single-mode fiber. However, new high-bandwidth multimode fibers, with bandwidth-length products on order of magnitude higher than standard multimode fibers, have recently been introduced. Furthermore, these advanced fibers are optimized for 850nm, where low-cost VCSELs are readily available. To use the plenty bandwidth of multimode fiber efficiently, wavelength division multiplexing(WDM) is also introduced in this paper. Here, we choose: 820nm, 840nm, 860nm, and 880nm.

To keep enough power being received by photo-detector, the coupling method between fiber and receiver is discussed. II.

LINK COMPONENTS

A. 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 ⎟ ηi I ⎜ r , t ⎟ N ⎜ r , t ⎟ 2 ⎞ ⎛ → ⎞ Leff 2 ⎛ → ⎞ I l (N , T ) ⎝ ⎠= ⎝ ⎠ − ⎝ ⎠ − G⎛ → ∇ N⎜ r,t ⎟ − ⎜ r , t ⎟ S (t )ψ ⎜ r ⎟ + ∂t q τn q ⎝ ⎠ ⎝ ⎠ τn ⎝ ⎠

S (t ) β 1 1 ∂S (t ) ⎛→⎞ ⎛→ ⎞ ⎛→ ⎞ =− + ⋅ ⋅ ∫ N ⎜ r , t ⎟dv + ∫ G⎜ r , t ⎟ S (t )ψ ⎜ r ⎟dv τ p τn V V ⎝ ⎠ VV ⎝ ⎠ ∂t ⎝ ⎠

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, ψ is normalized transverse mode profile, T is the device temperature, G is the gain, thermal leakage current,

I l is the

η I is the current-injection efficiency,

τ n is the carrier lifetime, Leff is the effective carrier diffusion length, and q is the electron charge. The power emitted by VCSEL can be derived from two equations above, as, Pout = k f S , in which k f = 0.015μW Fig.1 shows the emitting power depending on both drive current and temperature.

978-1-4244-4412-0/09/$25.00 ©2009 IEEE

For this WDM system, we use PRBS digital code and NRZ electrical signal as VCSEL’s deriving source. Each channel has a 5Gb/s digital signal. The wavelengths of light carrier in these 4 channels are: 820nm, 840nm, 860nm, and 880nm. The performance of this system is shown as follow,

Figure 1. Threshold current increases with temperature and power will be saturated at large driving current. The threshold current is 1.5mA at 20°C.(VCSEL with wavelength=820nm)

(a) Channel #1(820nm)

B. Multimode Fiber(GRIN) To usefully and effectively matching VCSEL with multimode, GRIN fiber is the first choice. Further, by keeping the change of refractive index in the core at a regular distribution (g=2), GRIN fiber support less mode than Step-Index(SI) multimode fiber. Traditionally equation about parabolic GRIN is: 2 ⎧ 2⎡ ⎛r⎞ ⎤ ⎪n ⎢1 − 2Δ⎜ ⎟ ⎥................0 ≤ r ≤ a n 2 (r ) = ⎨ 1 ⎣⎢ ⎝ a ⎠ ⎦⎥ ⎪ 2 ⎩ n1 [1 − 2Δ ]...............................r > a

(b) Channel #2(840nm)

For the fiber used in this experiment, core radius is a = 25μm , center(peak) refractive index is n1

= 1.4142 , Δ = 1%

Numerical Aperture NA = n1 2 Δ = 0.2 is applicable for transmitter-fiber coupling efficiency. III.

(c) Channel #3(860nm)

LINK PERFORMANCE

In this project, 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. A. Multimode WDM

(d) Channel #4(880nm) Figure 3. Spatial graphs of VCSELs output

From Fig.3, it is easy to see every graph includes 5 modes. In this design, we adjust VCSEL to distribute its totally emitting power into 5 portions equally, and every mode will own 1 portion. In other word, every mode in a VCSEL has the power equal to 20% of the total power of this laser. To demonstrate this point more actually, further analysis has been done.

Figure 2. Multimode 4 channels Multiplexing and Demultiplexing

(a)

(b) (a)

(c)

(d)

(b) Figure 5. spatial output from fiber. (a) coupling fiber and photodetector with alignment, (b) coupling fiber and photodetector with misalignment(offset 20um)

In this experiment, the photo-detector has a receiving area that is round with diameter 70um. And the misalignment 20um will disturb the expected power of photo-detector received. (e) Figure 4. Five modes of Channel#1(820nm), every mode owns a power 0.00417955W, which is 20% of the laser’s emitting power. (a) mode#1, (b) model#2, (c) model#3,(d) model#4, (e) mode$5.

B. Coupling Fiber and Photo-detector From scheme shown in Fig.2, we turn Channel#2-#4 off, and leave Channel#1 on. The phenomenon caused by coupling misalignment is,

REFERENCES [1]

Figure 6. Compared with Fig.5(b), this movement has effected receiving power. For(0,0), the spectrum out of the circle with radius 35um will be missed.

IV.

CONCLUSION

In this project, we have evaluated the characteristics of a VCSEL emitting at 820nm. For using the bandwidth-length provided by advanced multimode (MM) fiber, we choose (wavelength division multiplexing) WDM as the multiplex method. The modal spectrum at transmitters’ output and at fiber’s output are measured and discussed. For coupling between fiber and photo-detector, an excessive misalignment will reduce power received by the active region of photo-detector. The experiment results are similar to theoretical prediction, and identical to what we expected

Ning Guan, Katsuhiro Takenaga, Shoichiro Matsuo, and Kuniharu Himeno, “Multimode Fibers for Compensating Intermodal Dispersion of Graded-Index Multimode Fibers,” Journal of Lightwave Thechnology, vol 22, No.7, July 2004 [2] Syn-Yem Hu, Jack Ko, Eric R. Hegblom, and Larry A. Colden, “Multimode WDM Optical Data Links with Monolithically Integrated Multiple-Channel VCSEL and Photodetector Arrays,” IEEE Journal of Quantum Electronics, vol 34, No.8, August 1998 [3] M.García Larrodé, A. M. J. Koonen, “All-Fiber Full-Duplex Multimode Wavelength-Division-Multiplexing Network for Radio-Over-Multimode-Fiber Distribution of Broadband Wireless Services,” IEEE Transactions on Microwave Theory and Technique, vol 56, No.1, January 2008 [4] Lean, E, “Multimode Fiber Devices for Optical Fiber Links, Printing, and Display,” IEEE Transaction on Communication, Vol 26, Issue 7, pp 962-967, July 1978 [5] Christina Carlsson, Anders Larsson, and Arne Alping, “RF Transmission Over Multimode Fibers Using VCSELs—Comparing Standard and High-Bandwidth Multimode Fibers,” Journal of Lightwave Technology, Vol 22, No.7, July 2004 [6] Lewis B. Aronson, Brian E. Lemoff, Lisa A. Buckman, and David W. Dolfi, “ Low-Cost Multimode WDM for Local Area Networks up to 10Gb/s,” IEEE Photonics Technology Letter, Vol 10, No.10, October 1998 [7] Å Yorozu, J. S. Gustavsson, C. Carlsson, J. Vukusic, P. Modh, and A. Larsson, “High power single mode VCSEL’s for optical links and interconnects,” in Proc. Eur. Conf. Optical Communication(ECOC’03), Vol.6,2003, pp. 76-77 [8] .Morikun.J., Mena, P, Whitlock, B. K, Scarmozzino. R, “Multimode system simulation as an alternative to spreadsheet analysis for the study of Gb/s optical communication systems,” Conference Proceedings Lasers and Electro-Optics Society Annual Meeting-LEOS, v 1, 2003, p 154-155 [9] Wang, Jinfei, “Multimode fiber communication system based on mode group diversity multiplexing,” Zhongguo Jiguang/Chinese Journal of Lasers, v 35, n 12, December, 2008, p 1966-1969 [10] B.E.A.Saleh, M.C. Teich, “Fundamentals of Photonics 2nd Edition”, John Wiley & Sons, Inc, 2001 [11] D. Neamen, “Semiconductor Physics and Devices 3rd Edition”, McGraw Hill Higher Education, 2003