IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 21, NOVEMBER 1, 2006
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Performance of All-Optical Multicasting Via Dual-Stage XGM in SOA for Grid Networking Bernard H. L. Lee, Member, IEEE, Romli Mohamad, Member, IEEE, and Kaharudin Dimyati, Member, IEEE
Abstract—The letter presents a study on the performance of an all-optical multicasting technique utilizing the nonlinearity of semiconductor optical amplifier (SOA) technology for all-optical grid computing network. The technique not only has the capability to optically control the degree of multicasting but also performs all-optical switching simultaneously. Experimental results show the design’s capability to multicast an incoming 10-Gb/s optical signal onto 16 outgoing signals using cross-gain modulation in a single SOA. A second SOA is also included in the design as a 2R regenerator. Index Terms—Distributed computing, grid networking, multicasting, optical switching, wavelength conversion.
I. INTRODUCTION
Fig. 1. Conceptual setup of multicasting capable wavelength converter.
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HE GRID [1], [2] is an infrastructure that has the potential to integrate networking, communication, computation, and information that will provide a virtual global platform for computation and data management. In essence, the grid becomes the ultimate network for all forms of network traffic. In the context of grid computing, the need of high-speed multicasting is emerging especially in the field of ultrahigh resolution computing graphics and data cloning for multisite processing and storage. This realistically can only be achieved by having multicasting done optically in the physical layer. Conventionbroadcast-and-select that ally, optical multicasting is a 1 : uses optical splitters to broadcast the signal and optical gates to perform the selection. This technique, however, suffers from high optical splitting loss because the user is unable to dynamically control the level of splitting which is needed, thus causing a majority of the multicast data to be wastefully dropped. In light of the short-comings of the conventional multicasting techniques, many high-speed all-optical multicasting techniques such as four-wave mixing (FWM) [3], [4], cross-phase modulation [5], nonlinear polarization switching [6], and the usage of periodically poled lithiun niobate [7] were introduced. Nevertheless, they are highly complex and sensitive to polarization changes. Therefore, the authors propose a dynamically controllable multicasting technique using cross-gain modulation (XGM) in semiconductor optical amplifiers (SOAs). This technique has the ability to control the level of multicasting Manuscript received May 24, 2006; revised August 29, 2006. This work was supported by Telekom Malaysia. B. H. L. Lee and R. Mohamad are with the Photonics Technology Program, TM Research and Development Sdn Bhd, Serdang 43400, Selangor, Malaysia (e-mail:
[email protected]). K. Dimyati is with the Electrical Engineering Department, University of Malaya, Kuala Lumpur 50603, Malaysia (e-mail:
[email protected]). Color versions of Figs. 2–6 are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2006.884752
as needed (e.g., 1 : 2, 1 : 5, etc.) so optical signals no longer need to be wastefully dropped like in conventional broadcast-and-select techniques and it is a robust technique since XGM using multiquantum-well (MQW) SOAs is polarization insensitive. Furthermore, XGM offers regenerative properties to the multicast signals as discussed later in the letter. The layout of this letter will be as follows: Section II will explain the theory and the design of the multicast technique followed by the experimental setup and results in Section III. Finally, the conclusion is in Section IV. II. THEORY The principle of XGM is based upon the gain variation of the SOA after it reaches saturation point and it has found to have been in various applications. In [8], the material gain is defined as (1) where is the material gain coefficient, is the unsaturated is the saturation material gain, is the input intensity, and intensity. The net gain is reduced by 3 dB when the input power is equal to the saturation power. While maintaining the SOA at saturation, the gain of the probe signal varies inversely with the power of the input signal. Thus, the data of the input signal is then inversely transferred to the probe signal. The operational principle of the XGM is shown in Fig. 1. XGM is used to perform a 1 : 1 wavelength conversion as depicted in Fig. 1 or even a 1 : wavelength conversion in order to achieve multicasting. This is possible due to the mechanism of XGM, which allows the device to copy a single input signal onto the probe signal by varying the total gain of the SOA. This technique is effective for single or multiple wavelength probe signals, regardless of the number or the
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 21, NOVEMBER 1, 2006
Fig. 2. Experimental setup of multiwavelength XGM.
wavelengths of continuous-wave (CW) signals coupled into the probe signal. Furthermore, this technique allows easy selection of the wavelengths to be coupled into the SOA due to its large bandwidth (approximately 50 nm) which also allows fast tunability ( ns) equal to the time needed to switch on a laser or to tune a tunable laser. Nevertheless, the number of wavelengths that can be coupled into the probe signal is limited. This is because saturation in the multiwavelength XGM is achieved not by a single wavelength but instead through the aggregated wavelengths coupled into the probe signal. Since at saturation the gain of SOA reduces with the increment of the total input power, the input power of each wavelength is maintained. Thus, an increase in the number of wavelengths will in return decrease the power of each wavelength at the output of the SOA, and this will then require further amplification [amplification degrades optical signal-to-noise ratio (OSNR)] or regeneration. III. EXPERIMENTAL RESULTS The experiment was setup based on Fig. 2 using two 1-mmlong MQW SOAs, one for multicasting and another for regeneration. A 10-Gb/s pseudorandom bit sequence optical signal and a probe signal consisting of multiple wavelengths from a bank of lasers coupled together using an optical coupler were injected into the first SOA. XGM was performed at the first SOA where the input signal was copied onto a maximum of 16 different probe wavelengths, thus achieving 1-to-16 multicasting before the -factor degrades below six (bit-error rate (BER) ). An arrayed wavegide grating (AWG) is used to route the multicasting signal to their designated ports depending on their wavelengths. An AWG was chosen as the wavelength router not only because it is a passive optical wavelength router but also it is fully nonblocking [9]. Measurements of the worst converted signal were taken and it shows that the -factor of the converted signals’ proportionally decreases as the number of probe wavelengths increases. The best results show that the -factor drops below six after four wavelengths were coupled into the probe signal. The observation initially suggests that this technique is not feasible in a wavelength-division-multiplexing system. Further investigation into the sources of the degradation has revealed clear degrading factors in this technique. One of the effects of the SOA nonlinearity is the FWM [3], [4] by-products seen here in Fig. 3. The by-products of the FWM effect interfere with the original wavelengths in the probe signal if it falls onto the same wavelength, and induce intraband crosstalk that are unable to be filtered out. A second source of degradation is the OSNR degradation experienced by the converted signal as the number of wavelengths
Fig. 3. Sources of degradation in multiwavelength XGM.
increases. At the optimum operating point, both the total input signal and the probe signal’s power level have to be kept constant at a specific value in order to obtain the best conversion results. Since the absolute power of the probe signal is kept constant, increasing the number of wavelengths aggregated onto the probe signal will in turn reduce the power of the individual wavelengths. Reduction of the input power will directly reduce the output OSNRs of the converted signals since an SOA induces ASE noise. Furthermore, as the amplitude of the individual probe wavelength decreases so does its eye opening. The single-pass gain of an SOA is proportional to its active region length. Thus, increasing the length of the SOA will increase the gain and eventually allow more probe wavelengths to be inserted. Nevertheless, the 3-dB bandwidth of the SOA will decrease as its length increases thus reducing the operational bandwidth range. Although the FWM products are inevitable, techniques to avoid the interference have to be applied. Since the wavelength of the FWM products can be predicted, interference can be avoided by the careful selection of those CW wavelengths that do not fall onto the same wavelengths as the FWM products. Another alternative is that of increasing the channel spacing in order to minimize the FWM products. The amplitude of the FWM by-product decreases as the channel spacing increases. Nevertheless, this method is not bandwidth efficient since most of the spectrum is unused. These measures ensure that the FWM by-products do not interfere with the performance of the converted signals. A vast improvement can be observed in such cases (Fig. 4). By doing so, this technique is able to transfer the data from one input signal onto a maximum of 16 wavelengths with a -factor of better than six (equivalent to BER of less ). The eye diagram of one of the 16 multicast than optical signals after the second SOA which operates as the regenerator is shown in Fig. 5 and the -factor measurements of the multicast signals before and after the second SOA is shown in Fig. 6. The second SOA gives an average improvement of a factor of five to the multicast signals from the first SOA. This is due to the 2R regenerative effect of XGM [8].
LEE et al.: PERFORMANCE OF ALL-OPTICAL MULTICASTING VIA DUAL-STAGE XGM IN SOA
Fig. 4. Performance of multiwavelength XGM with variable spacing CW wavelengths.
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generate the signals routed by the wavelength router. Although this technique has been proven to perform 1-to-1 wavelength conversion at bit rates of up to 100 Gb/s [10], there have not been any results investigating the technique performing multicasting at such high bit rates found in the literature. Therefore, the XGM technique is likely to face the problem of maximum operational bit rate although lengthening the SOA chip will provide a temporary solution. A further advantage of the proposed architecture is of course the absence of large splitting losses. The number of copies to be multicast and their destinations are controlled by the number and wavelengths of probe signals used in the multiwavelength XGM. Since a passive wavelength router is used, routing complexity is minimized and ASE noise can be reduced. IV. CONCLUSION
Fig. 5. Eye diagram of 1-to-16 multicast optical signal after second SOA-based wavelength converter.
We have introduced and investigated an all-optical multicasting technique capable of performing dynamic wavelength routing and packet duplication simultaneously at 10 Gb/s and up to 1-to-16 multicast using a single SOA. XGM being a direct wavelength conversion technique allows fast wavelength tuning. A second SOA provided a 2R regenerative effect to improve the -factor of the multicast signals by a factor of five. Thus, this technique is a feasible approach towards all-optical multicasting which is essential for grid networking. REFERENCES
Fig. 6.
Q-factor measurements of multicast signals.
Experimental results with bit rates of 10 Gb/s have proven the feasibility of this architecture to perform all-optical packet multicasting. Although the signal quality deteriorates after the multiwavelength XGM, the basic operations of the architecture depicted in Fig. 2 which shows the additional XGM stage at the output of the wavelength router is able to improve the signal’s -factor by approximately a factor of five to six. This is due to the fact that the second-stage XGM has the ability to 2R re-
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