WM5.pdf a C2934_1.pdf
Mode coupling: Why POF Supports 40Gbps
Arup Polley, Kasyapa Balemarthy, Stephen E. Ralph
School of Electrical and Computer Engineering, Georgia Institute of Technology 777 Atlantic Drive, Atlanta, Georgia 30332-0269 Tel: [phone]; Fax: [phone] Email: [e-mail]
Abstract: We demonstrate experimentally and numerically that mode-coupling in graded index plastic optical fiber enables 40Gbps over 200m in the presence of dramatic refractive index errors.
2007 Optical Society of America
OCIS codes: 060.2270, 060.2330
1. Introduction Plastic optical fiber (POF) is fast emerging as a medium for high-bandwidth, short reach links [1]. Impulse response, differential modal delay (DMD) and bit-error-rate measurements have all shown that 200m graded index POF (GI-POF) links are capable of 40Gbps performance [2]. Previous observations of the spatial evolution of the mode power distribution [3,4] have sought to establish the relative contributions of mode coupling (MC) and differential modal attenuation (DMA) in GI-POF [5]. Here, we focus on the temporal response of the POF and together with a comprehensive multimode fiber (MMF) model [6] demonstrate that very small DMD of GI-POF can be explained by the large mode coupling strength [3,4]. Furthermore, we also demonstrate for the first time, that large deviations in the refractive index profile which produce unacceptably large DMD in glass MMF are dramatically reduced with strong mode coupling typically found in GI-POF. Thus we establish the basis for 40Gbps capability in POF and quantify the strength of coupling needed to improve bandwidth in both POF and glass MMF. 2. Modeling and results The MMF model includes a mode solver and a split-step implementation of the mode coupling theory applicable for MC arising from random perturbation in the MMF [7]. The mode solver provides the transverse mode profiles, the group delays of the propagating modes in fiber with arbitrary size and arbitrary refractive index profiles. We examine a pure -profile, although not optimum profile [8], to demonstrate sensitivity to index profile irregularities. Experimentally and numerically the initial mode power distributions correspond to single mode fiber (SMF) excitation. The mode coupling coefficients (MCC) among different mode groups are computed for a range of mode coupling strength parameter [6]. An effective MCC, given by the average MCC of all mode groups is used as a figure of merit although it is known that the mode coupling increases with mode number. The mode coupling length (MCL) is the distance required to reach a near steady-state mode power distribution [7]. The highest order mode group consists of leaky modes and a modal attenuation is assumed for them. The measured temporal response of 200m 50m core GI-POF is depicted in Fig. 1a. Numerically computed responses using the MMF model without coupling and with typical POF coupling are shown in Fig 1b. and 1c. In both cases the excitation, 800nm 16ps FWHM, is scanned across the core diameter. The receiver is a commercial 50m MMF detector followed by a digital sampling scope with a net bandwidth of 25GHz. The DMD, the maximum temporal width (at 25% of peak power) between all offsets is measured to be 66 ps with only 2ps maximum delay between peaks [Fig. 1a]. This response is suitable for 40Gbps links.
Experimental
Numerical
Numerical
Fig. 1. (a) Experimentally measured DMD of 200 m 50 m core POF with DMD spread of 66 ps (25% points denoted by dashed line). Numerically generated DMD of 200 m 50 m core POF with = 2.1 with (b) no mode coupling and (c) mode coupling length of 30 m resulting in DMD spread of (b) 367 ps and (c) 97 ps respectively.
OSA 1-55752-834-9
a WM5.pdf C2934_1.pdf
Impulse responses of 200m 50m core GI-POF are numerically computed where a normalized refractive index delta of 1.5% is used with cladding refractive index of 1.34 and varying from 1.9 to 2.1. The mode coupling coefficient is varied over a wide range of 0.01 to 103 m-1. A DMD of 367ps is obtained for POF with non-ideal a = 2.1 and no mode coupling [Fig. 1b]. This DMD, ~2 ns/km, is representative of the worst allowed for standard grade FDDI fiber which only supports 160MHz-km. Uusing a mode coupling strength of 5m-1 results in a DMD of 97ps with only 16ps maximum delay difference between peaks [Fig. 1c]. The corresponding mode coupling length is 30m which is close to the reported mode coupling lengths of GI-POF [3,4]. Furthermore the separately measured mode coupling strength [9] has been shown to be at least as strong as 5m-1. Thus POF is typically in the strongly coupled regime. The effect of the MCC on the DMD has a near threshold like behavior [Fig. 2]. A MCC below 0.5 m-1 has little practical impact on the DMD yet the near minimum DMD is reached with an MCC of 10 or greater. Figure 2 demonstrates that even large deviations in index profile can be tolerated if sufficiently strong mode coupling is present. The DMD is similarly insensitive to the more local index deviations typically found in MMF including center dip and peaks and alpha kinks. There is little to be gained by increasing the MMC beyond that needed to minimize the DMD since mode coupling results in increasing power transfer to leaky modes and therefore, increases loss. We show the effect of increasing MCC on the net loss for a DMA of 0.3dB/m in Fig. 2.
Fig. 2.DMD and loss vs. mode coupling strength for 200 m 50 m POF with different
Fig. 3. DMD vs mode coupling for different attenuation rate of the leaky mode for 200 m 50 m POF with = 2.1
We have examined the effect of differential modal attenuation on DMD since high DMA may also limit DMD by shedding power for modes with large relative delay. We explore a range of DMA of 0, 0.3, 3 dB/m. A DMA of 0.3dB/m for the highest mode group produces the net loss of 7dB which is close to the modal attenuation of 200m GI-POF. Figure 3 shows that changing the DMA does not appreciably change the DMD and confirms that the mode coupling is the primary factor in enhancing the bandwidth of POF. However net loss scales linearly with the attenuation rate of the leaky modes. 3. Conclusion We demonstrate using experimental results and numerical model that in the strong mode coupling regime large variation in the refractive index profile can be tolerated while keeping the DMD small. We conclude that strong mode coupling and low attenuation rate of the leaky modes is the key to design low-loss, high-bandwidth MMF. 4. Ac...