Plastic Fiber Optics Simulations

G. Kahanda (Ph.D. Student) and Syed V. Ahamed (Mentor) Graduate Center, City University of New York 365, Fifth Avenue, New York City, NY 10014

Abstract systems. The strands carry light in the visible (450-600 Plastic fiber optics media is viable for small networks. nm wavelength) band. The signal attenuation in the older The current applications include data distribution in PMMA POFs can be range between 90 to 100 dB/km in aircraft and automobiles. Plastic fiber systems being an the 550 nm range. The attenuation can be as high as 450- order of magnitude less expensive than silica fiber optic 500 times that for glass fibers (0.16-0.2 dB at 1550 m) systems and being more robust have applications in other and for this reason, the viable fiber length is traditional data communication systems. In this paper, correspondingly shorter. For the older perflorinated (PF) we present the simulation results for possible applications POF the attenuation can be range from about 25 to 75 dB for a high-rise office buildings and premises distribution in the 1000 nm range, and about 140-370 times that of architectures. The signal to noise ratio for optical rates glass fiber. up to 2.5 Gb/s and for link lengths up to 5 km are For the very short distances (up to 100 m), and lower reported, even though the signal level becomes too low rates (less than 1.5 Gb/s), the inexpensive LED’s will for the PIN diodes. The quality of components affects the perform satisfactorily, even though laser quality source performance dramatically and their impact is reported. become necessary for longer distances and higher data Experimental results are scarce in the literature for either rates. Loss of optical signal is the major concern. The of the two (PMMA and PF) categories of commercial receiver sensitivity and noise characteristics become plastic fibers. The simulations are focused for the PMMA crucial as the distances and data rates are enhanced plastic fiber to offer a window and envision other considerably. Details are presented in Sections III and IV. applications for this new breed of optical fibers. 2. Enhancement of FS* Simulation Platform Keywords: Plastic Fiber Optics, Simulations, Data-Rates, Ranges, Applications. 1. Introduction The FS* simulation platform [2, 3] is generic and provides the degradation of the energy in optical signal as a series of pulses traverse the FO communication systems. A fiber optics simulation platform [1] for silica fibers Six nodes (encoder, optical source, filter, fiber, pin diode was presented during late 1980s. Plastic fiber optic and photodiode) are available to determine the component technology for data communication was at a distant effects. The configuration of the system is shown in possibility while the simulation methodology, design Figure 1. strategy and optimization techniques for silica fibers were being perfected during early 1990s. More recently, The transfer function of each of the components is plastic optic fibers (POF) techniques are feasible for a embedded in the fiber computer aided design (CAD) number of applications and other small local area package. The simulation methodology consists of (a) networks such as the premises distribution systems or generating a pseudo random pulse sequence of arbitrary PDS discussed in [1]. The scientific methodologies for length, (b) generating the spectral components of the plastic fiber optics have not been explored to the extent sequence, (c) cascading the transfer functions of each of that is currently available for silica fibers. In this paper, the components in the spectral domain and (d) finally the simulation platform for silica fibers optic systems is recovering the signal in the time domain. The time extended to accommodate the POF environments. Data domain signals constitute the conventional eye diagrams rates, ranges, and the optimization techniques for POFs that contain the optical signal and the effect of each can be embedded in the simulation platform reported. component as the data traverses the fiber optic system. The simulation software is tightly coupled to component Plastic fiber optic systems are lighter, less expensive databases and the characteristics of commercial and easier to terminate. They are more robust (core components may be may be used in the simulations to diameter is about 20 times that of glass) than silica fiber build a optimal fiber optic system from the components as (ii) component mismatch is reflected as a degradation of they become available. the SNR due to the mismatch, The earlier simulation platform is enhanced to yield (iii) fiber optic cross-talk effects in wave length division four additional sets of results as follows: multiplex (WDM) systems can now be computed as (i) computation of the signal to noise ratio (SNR) is length, and data rate are optimized, and finally automatic as the computations take place, (iv) temperature sensitivity of the fiber can be studied by changing the fiber characteristics.

Figure 1. Simulation and CAD Platform for Fiber Optic point-to-point systems.

3. SIMULATION RESULTS Two eye diagrams (EDs) for a 50 m, PMMA fiber carrying data at 1 Gb/s and at Node 2 and Node 6 are A large number of results can be generated by altering shown in Figure 2 a and b. The data is binary and the the different encoding algorithms, design strategies, input pulse occupies full pulse duration. A LED source component characteristics, transmission methodologies, and other high grade (Gr 1) components were used to channel crosstalk, etc. In order to highlight the additional focus on fiber performance. The SNRs are presented in possible applications of the POFs, we present a subset of Table 1 for different lengths and rates. simulations dealing with conventional eye diagrams, and When the length in increased, the signal amplitude at the effects of data rates and distances (fiber length) on the the receiver becomes very low for the PIN diode even SNRs at the six test points (1 to 6) shown in Figure 1. though the eye diagrams appear attractive,. With customized PIN diodes (perhaps cascaded or stacked) and 3.1 ED and SNRs for PMMA POF length around 300-500 m the system is likely to become viable for small PDS and small LANs.

(a) After Source (b) After Detector

Figure 2. Eye Diagrams for 50 m, PMMA fiber carrying data at 1 Gb/s

Table -1 Binary code, Full-Width Pulses, High quality LED, and PMMA Fiber Fiber Length Rate Signal to Noise Ratio (SNR) in dB (m) (Gb/s) Gr = Grade Encoder Source Filter Gr 1 LED’s (Gr 1) (Gr 1) 50 1.75 394 43.55 36.21 50 2.0 394 37.32 30.92 100 1.75 394 43.55 36.21 100 2.0 394 37.32 30.92

3.2 Effect of Fiber Length and Data Rates

The signal level degradation become noticeable as the cable makeup is radically different from the fiber optic length is increased, even though the wave shape does not cables. deteriorate precipitously. Similar results are observed in For these reasons, the strategy for design optimization the copper-based digital distribution systems [4]. of the POF systems is significantly different from the However, the POF systems offer numerous features over established standards and practices for the copper-based the copper media in the loop subscriber systems [4]. The subscriber systems or the transoceanic silica glass fiber loss of signal strength is significant in both systems but optic systems. However, the strategies for signal recovery the environment is vastly different. Optical systems do may have some bearing to the possible applications of the not suffer from crosstalk effects. The digital loop carriers POF systems and need further exploration. systems (with the copper media [5]), constantly monitor In order to establish a base line for the possible the signal level throughout the loop plant and recover the application of POF in the premises distribution systems, signal before the noise interference can cause increase bit we present the levels of wave shape degradation as the error rates [6]. The data rates are also very low in the T length and bit rates are increased in the POF environment. (American based T1, T1C, and T2 systems) and the E An extensive set of similar results for the development of (European based E1 and E2) loop carrier systems and the the digital subscriber loop (DSL) were published in 1987 before the commercial deployment of the DSL by the length approaches 500 m for PMMF and about 1.2 km of telephone companies almost fifteen years later. Figures 3 the PF types of POF cables. The optical band also differs a through d, display the degradation of the eye diagrams for the two type of the POF. As new POFs become at 2 Gb/s over 100 m of the PMMA POF. The pulse width available these limits will also vary significantly. is reduced to 50 percent of the width in Figures 3a to 3d The simulation techniques have been validated by to enhance the SNR at the received data. Laser sources comparison with the early experimental results published become essential to achieve the SNR’s reported in Table [7] in the late 1980s. These results are from a silica glass 2. LED sources can cause considerable degradation in the fiber optics test bed. Experimental results from tests on eye diagram and leave behind a trail of optical energy POF cables are currently not available in the public signal after each transition. domain. The basic equations for the signal attenuation and The SNR is acceptable for most commercial dispersion are assumed uniform for both the glass and applications. However, the signal attenuation becomes plastic fibers. excessive for the reliable data recovery when the fiber

(a) Encoder (b) After Filter

(c) After Fiber (d) After Detector

Figure 3. Eye Diagrams for 100m at 2 Gb/s (Half-width pulses) of the PMMA POF. The attenuation through the fiber is acceptable but the PIN and APD noise is evident in (d).

The results in Table 2 indicate that the POF by itself However, as the length increases considerably, the does not degrade the signal significantly, even though the intrinsic noise in the PIN diode and the detector drown the attenuation can become significant. The basic equations signal. In the simulations, the laser source is of superior that govern attenuation and dispersion for the silica glass quality and it does not contribute to the channel noise. fiber and the plastic fiber are assumed to be the same.

Table 2. Binary code, Half-Width Pulses, Perfect Laser Source, and PMMA Fiber

Fiber Rate Signal to Noise Ratio (SNR) in dB Length (Gb/s) Encoder Source Filter Fiber PIN Detector Comments (m) (Gr 1) (Laser) (Gr Type (Gr (Gr 1) Gr 1 is Best Grade 1) PMMA 1) in current databases. 50 1.0 394 394 135 129.8 40.17 40.12 Good 50 1.25 394 394 135 132.0 40.17 40.12 Good 50 2.0 394 394 135 131.6 40.17 40.17 Good 100 1.0 394 394 135 129.9 40.38 40.23 Acceptable 100 2.0 394 394 135 129.9 40.38 40.23 Acceptable 100 2.5 394 394 135 128.1 40.38 40.23 Acceptable 100 5.0 394 394 135 124.9 40.40 40.20 Acceptable (?) 500 1.0 394 394 135 126.1 40.39 40.28 Acceptable (?) 500 5.0 394 394 135 159.9 40.29 40.26 Acceptable (?) 500 10.0 394 394 135 146 40.39 40.28 Low Signal Level (LSL) Acceptable (?) 1000* 1.0 394 394 135 124.5 40.56 40.18 (LSL), Acceptable ED 1000* 2.0 394 394 135 130.2 40.55 40.18 (LSL) Acceptable ED 1000* 5.0 394 394 135 62.50 40.14 40.23 (LSL) Acceptable ED 5000* 1.0 394 394 135 71.80 40.6 40.53 (VLSL) Low Signal 5000* 5.0 394 394 135 85.57 40.58 39.98 NG, Very Low Signal 5000* 10.0 394 394 135 64.29 40.57 40.57 NG, Very Low Signal

* The optical signal strength gets low as the fiber length is increased. The noise level in the receiver and the sensitivity of the timing recovery systems will influence the SNRs and the actual bit-error-rate (BER) in the POFs. Generally, it is necessary that the data is recovered before the integrated power due to all interference(s) start to approach a given percentage level of the received power. The exact value of percentage ratio depends on the BER requirement from the system. At very high rates, the SNR at the output of 500 m fiber (rows 9 and 10) is higher than at the end of the filter. We suspect that this is because the fiber acts as a filter in its own right and it can smooth out the sharpness at the peak of the received optical energy. The SNR computations become somewhat unpredictable when the value is over 120 dB.

4. DISCUSSION OF RESULTS We thank Professor Alfred Levine and Professor Mike Kress at the Graduate Center of the City University of The SNR can be enhanced by using half-width pulses. New York for their academic perspectives and also thank This observation is valid for the silica glass fibers. The Professor Victor Lawrence for his prior research and best architecture for the POF systems consists of a half development perspective at Lucent Technologies in pulse width modulator of binary data with laser sources Holmdel, New Jersey. and good quality receiver (PIN and (simple or avalanche photo detectors)) components. The cost will be References correspondingly higher. 1. S.V. Ahamed and V.B. Lawrence, “Design and The most adverse combination of components will be Engineering of Intelligent Communication Systems”, to use LEDs with poor quality of receiver components. Kluwer Academic Publishers, Boston, 1997, Springer- The cost will be low but the performance to cost ratio Verlag, New York, 2005. may be satisfactory for short length, low data rate 2. D. G. Duff, "Computer-Aided Design of Digital applications. The design optimization strategies for a Lightwave Systems", IEEE Journal of Selected Areas in variety of applications (such as medical centers, hospital, Communications, Vol. SAC-2, No. 1, pp. 171-185, Jan. small campus networks, and premises distribution 1984, also see S. V. Ahamed, "A Computer Aided Design systems) still remain to be explored. Environment for the Local Lightwave Communication In the traditional copper-based loop and premises Systems", paper presented at the IEEE, Fifth International distribution systems [1], the effect of reduced pulse width Workshop on Integrated Electronics and Photonics in had been observed and reported in Ref [5]. However, Communication, October 21-23, 1987, Research Triangle, since the copper media is used within the wire-pair North Carolina. bundled in the cables, the crosstalk (due to sharper data 3. A. Elrefaie, M. Romeiser, "Computer Simulation of pulses) also gets increased thus adversely effecting data Single-mode Fiber Systems", Optical Fiber transmission in other wire pairs and other digital carrier Communications Conference, Feb. '86, pp. 54-56. systems. 4. S. V. Ahamed, "Computer Based Optimization The wavelength division multiplexing (WDM) Techniques for the Design of HDSL" International alternative for POF systems has not been studied even Communications Conference New Orleans, May 1-5, though the simulation platform is capable of providing all 1994. Proc. of ICC '94. the necessary results. It is not evident if the WDM 5. S.V. Ahamed 1982, "Simulation and Design Studies of techniques are viable and/or desirable for POFs. Digital Subscriber Lines,” Bell System Technical Journal, Vol. 61, No.6, July-August, 1003-1077. 5. CONCLUSIONS 6. S.V. Ahamed, P.P. Bohn, and N.L. Gottfried 1981, “A Tutorial on Two-Wire Digital Transmission in the Loop Plant,” IEEE Transactions on Communications, COMM, The design optimization strategy for plastic optical fibers needs to be evolved much like the design strategy Vol. 29, 1554-1564. for the silica optical fibers or the digital subscriber lines. 7. R. A. Linke “High Capacity Coherent Lightwave Theoretical studies coupled to an intelligent and Systems” Journal of Lightwave Technology, Vol. 6, pp automated computer aided design [8] platform will 1750-1769, 1988. Also see and J. Gimlet and Nim provide the basis for standardization of the possible Chueng “Dispersion Penalty Analysis for LED/Single architectures and the ideal characteristics for the Mode Fiber Transmission Systems,” Journal of components. Lightwave Technology, Vol 4, pp 1381-1392, 1986. A study of possible applications, architectures, and 8. S. V. Ahamed, "Interoperability of Multiple Databases components characteristics will provide the POF industry for the Design and Simulation of High-speed Digital to mature as a viable contender in a targeted segment of Subscriber Lines", IEEE CS International Conference data communications market. on Data Engineering, RIDE-IMS'93, Third International Workshop on Research Issues in Data Engineering- Interoperability in Multiple database Systems, Vienna, Acknowledgements Austria, April 19-20, 1993, Proc. RIDE-IMS, pp. 262- 267.