Wavelength-transparent nonlinear optical gate based on self-seeded gain modulation in folded tandem-SOA

In this paper, an all-optical nonlinear gate employing a folded tandem-SOA structure is proposed. With a partial reflection mirror for the amplified signal, we achieve a self-seeded gain modulation effect in the folded tandem-SOA, thus eliminating an external saturating source required for the conventional tandem-SOA optical gate. The performance analysis of the proposed device as a 2R regenerator / logic gates (NOR) shows excellent compatibility with the conventional structure, but in a highly integrated form with added benefit of wavelength transparency over wide spectral bandwidth (>100nm). Studies also have been carried out to investigate optimum operation condition of the device as a function of input wavelength and signal input power. ©2007 Optical Society of America OCIS codes: (130.3750) Logic devices; (200.3760) Logic-based optical processing; (200.4660) Optical logic; (250.5980) Semiconductor optical amplifiers References and links 1. J.C. Simon, “All optical regeneration,” ECOC, 469-467 (1998) 2. O. Leclerc, B. Lavigne, E. Balmefrezol, P. Brindel, L. Pierre, D. Rouvillain, and F. Seguineau, “Optical Regeneration at 40 Gb/s and Beyond,” IEEE J. Lightwave Technol. 21, 2779-2790 (2003) 3. A. Hamie, A. Sharaiha, M. Guegan, “Demonstration of an all-optical logic OR gate using gain saturation in an SOA,” Micorwave Opt. Technol. Lett. 39, 39-42 (2003) 4. S.H. Kim, J.H. Kim, B.G. Yu, Y.T. Byun, Y.M. Jeon, S. Lee, D.H. Woo, “All-optical NAND gate using cross-gain modulation in semiconductor optical amplifiers,” Electron. Lett. 41, 1027-1028 (2005) 5. G. Contestabile, R. Proietti, N. Calabretta, L. Giorgi and E. Ciaramella, “Evidence of noise compression by cross gain compression in SOAs,” OFC, JThB29 (2006) 6. A.D. Ellis, A.E. Kelly, D. Nesset, D. Pitcher, D.G. Moodie and R. Kashyap, “Error free 100Gbit/s wavelength conversion using grating assisted cross-gain modulation in 2mm long semiconductor amplifier,” Electron Lett. 34, 1958-1959 (1998) 7. T. Durhuus, B. Mikkelsen, C. Joergensen, S. L. Danielsen, and K. E. Stubkjaer, “All-optical wavelength conversion by semiconductor optical amplifiers,” IEEE J. Lightwave Technol., 14, 942-954 (1996) 8. X. Zhang, Y. Wang, J. Sun, D. Liu, D. Huang, “All-optical AND gate at 10 Gbit/s based on cascaded singleport-coupled SOAs,” Opt.Express, 12, 361-366 (2004) 9. A. Hamié, A. Sharaiha, M. Guégan, and B. Pucel, “All-optical logic NOR gate using two-cascaded semiconductor optical amplifiers,” IEEE Photonics Technol. Lett. 14, 1439-1441(2002) 10. G. Contestabile, R. Proietti, N. Calabretta, E. Ciaramella, “All optical regeneration by cross gain compression in semiconductor amplifiers,” ECOC, 3, 415-416 (2005) 11. Y. J. Jung, P. Kim, J. Park, N. Park, “Integral equation approach for the analysis of high-power semiconductor optical amplifiers,” Opt. Express 14, 2398-2403 (2006) 12. M. G. Davis and R. F. O’Dowd, "A transfer matrix method based large-signal dynamic model for multielectrode DFB lasers," IEEE J. Quantum Electron. 30, 2458-2466 (1994) 13. H. Lee, H. Yoon, Y. Kim, and J. Jeong, "Theoretical study of frequency chirping and extinction ratio of wavelength-converted optical signals by XGM and XPM using SOA’s," IEEE J. Quantum Electron. 35, 1213-1219 (1999) #78193 $15.00 USD Received 19 Dec 2006; revised 5 Apr 2007; accepted 5 Apr 2007; published 9 Apr 2007 (C) 2007 OSA 16 Apr 2007 / Vol. 15, No. 8 / OPTICS EXPRESS 4929


Introduction
Non-linear optical devices, one of the promising network elements for future all-optical 2R regenerated transmission systems and optical signal processing, have been investigated in depth under various configurations in the past [1][2][3][4][5].For such functionality, the high nonlinearity of Semiconductor Optical Amplifiers (SOA) is of perfect match, and has been widely exploited under various architectures.In particular, SOAs utilizing gain modulation, with its insensitivity to phase and simplicity for the device implementation, have been successfully utilized to realize wavelength conversion [6,7] or all optical logic gates [3,4,8,9].The most recent, and significant development in this category includes the cascaded tandem-SOA structures [5,9,10], which provide much sharper nonlinear response curves than single SOA based devices.Optical NOR-gate with Extinction Ratio (ER) enhancement has been demonstrated using cascaded SOAs [9].Utilizing also similar principles, ER enhancement and noise compression of the signal was also successfully achieved [5,10].
In this paper, we propose a novel all optical gate structure, which utilizes self-seeded gain modulation effects in folded tandem-SOA.Instead of the conventional tandem-SOA approach, which uses an extra external continuous wave (CW) source for the generation of the XGM (Cross gain modulation) effect, we use a self-seeded, gain modulation effect of the signal, by employing a partial reflection mirror at the output.Comparison of suggested structure with the CW-assisted tandem-SOA optical gate shows that compatible performance factors can be achieved with much simpler device structure, enabling higher level of integration of the device at a reduced cost ; also with the added critical benefit of wavelength transparency in the signal regeneration.Investigations on the optimum operating conditions of the device, as a function of input signal wavelength (>100nm) and input power have been carried out.Figure 1 illustrates the basic concept of; A) the CW-assisted tandem-SOA all-optical gate [9], and B) the folded tandem-SOA all-optical gate -proposed in this paper.Serially connected two SOAs (SOA1, SOA2 of gain G 1 , G 2 ) takes input signal P in at the input port of SOA1 as well as saturating signal at the output port of SOA2 (for the case A, from external CW source; for the case B, it is amplified and reflected input signal rG 1 G 2 P in , where r is a mirror reflectance).Stating the operation principle for CW-assisted tandem SOA structure (case A); CW saturating tone counter-propagating the SOA2 experiences XGM effects from the input signal G 1 P in , and provides inverted patterns of P ~in at the mid-stage output port (P o2 ).Meanwhile, at this stage, same magnitude of P o2 (∝ P ~in ) at the same time entering the first stage SOA1 also interacts with P in through the XGM effect, resulting further enhancement in the extinction ratio of the signal at the output port of the SOA1 (P o1 ~ G 1 P in ).Note that these resulting highly non-linear transfer characteristics (e.g., between P in and P o1 , P in and P o2 ) of tandem SOA structures can be effectively utilized to achieve all-optical regeneration [5,10] or all-optical NOR gate [9].Now understanding that; 1) usually the flight time of the signal can be neglected in the reasonable device operation speed, and that 2) the signal power at the output port of SOA2 is highly saturated (almost constant, irrespectively of the symbol type or delay of P in ), it is meaningful to consider the reflected output power of SOA2 as an alternative of external CW source, to similarly obtain the nonlinear transfer function which was observed in the case A).Case B) of Fig. 1 shows such a folded tandem SOA structure, which utilizes a self-seeded gain modulation effect between the signal and counter-propagating, amplified reflected signal.

Static Domain Analysis: Extinction Ratio
To begin, the performance of the folded tandem-SOA optical gate was analyzed in the static domain, considering both forward / backward propagating signals (both at 1549.2nm).At the output port of SOA2, a mirror of reflectance r (r = P r /P i ) was assumed.Fast and highly accurate analysis was made utilizing the integral equation formalism of SOA [11].For the parameters of SOA1 (SOA2), device length of 250μm, current of 500mA, confinement factor of 0.4, material gain coefficient of 2.7e×10 16 cm 2 , refractive index of 3.7, and material scattering loss of 34cm -1 were used.
In Fig. 2, we present the signal power distribution curves in folded tandem-SOA for different input power levels (P in = -6dBm, -2dBm, and 2dBm, labeled as 'A', 'B', and 'C' in the Fig. r = 0.5%).The output power P o1 measured at the signal extraction point (3dB-coupler in the middle, see Fig. 1 and 2) were -1.9dBm, 6.8dBm and 16.9dBm, with corresponding gain values (G 1 ) of 4.1dB, 8.8dB and 14.9dB respectively.As shown, the mid-stage gain G 1 of tandem-SOA measured at the output of SOA1 increased with the input signal strength, with its highly nonlinear saturation behavior -thus providing the enhancement in the signal extinction ratio.In terms of static extinction ratio, as much as 10.8dB of net ER gain was achieved with 8dB of input ER (-6 ~ 2dBm) and 18.8dB of output ER (-1.9 ~ 16.9dBm).

SOA1 Length
Fig. 2. Power distributions in the folded tandem-SOA, at different input signal (-6dBm, -2dBm, and 2dBm).Note that, the injected input signal propagates from right to left, and the output signal is extracted by a 3dB-coupler located in the middle.At the end of SOA2 (z=0), the amplified signal is reflected from the mirror (starting from -5 ~ 0dBm range in the Fig. ) Important to note is that the reflected power at the SOA2 output point was almost identical (ΔP ~ 1dB) from the saturated operation of tandem-SOA, irrespective of the input signal strength (ΔP = 8dB); thus virtually acting as a continuous wave source irrespectively of the symbol type or delay of P in .Further, in this regard, with the adjustment of the mirror reflectance, it was also possible to change the optimum operation point of the device (equivalent to the CW power adjustment in [5,10]).In Fig. 3, we plot the output power P o1 from the SOA1, as a function of input signal power P in at different mirror reflectivity (r = 4%, 2%, 0.5% and0.1%).The obtained optimum input power P in for the maximum ER enhancement was 7dBm with 4%, 4dBm with 2%, -1.7dBm with r = 0.5%, and -8.5dBm with r = 0.1%, respectively.Finer adjustment of optimum input power for ER enhancement should be possible with the change in mirror reflectance, or with the tunable MEMS mirror.
As mentioned earlier, one of the critical / inherent benefit of the proposed device lies in the wavelength transparency during the process of signal regeneration, without the need of external sources or wavelength tuning of it.In order to find out the operating bandwidth of the device, input signals at different wavelength (1450nm, 1500nm, 1550nm, all with -1.7dBm) have been tested.As can be seen in Fig. 4, almost identical numbers in ER enhancement was observed for all the signal wavelength, well exceeding 100nm, with the corresponding optimum mirror reflectivity (0.5%, 0.7%, 1.6% for 1550, 1500, 1450nm signal, covering from the gain peak wavelength to half-maximum gain wavelength).

Time Domain Analysis: 2R Regeneration, NOR gate
For the transient analysis of tandem-SOA, we plot in Fig 5 traces of P in , P o1 and P o2 , calculated with the transfer matrix method [12,13] (r = 0.5%, and CW tone = -1.7dBm).
In contrast to the case of CW-assisted tandem SOA, the slight reduction in the SOA2 output power (P o2 ), and signature of mirror reflection (or delay of the signal pattern) at the rising / falling edges of the pulse were observed.For the output of SOA1 (P o1 ), about 19dB of dynamic ER (-2dBm to 17dBm) was achieved, consistent with the number from static analysis.
To further investigate the impact of ER enhancement from the system perspective (as a 2R regenerator), the Q factor of the input / output signal of the device was also compared.10Gbps, 127 bit super-Gaussian PRBS input signal was used.Also assumed was ASE noise of -13.64dBm/0.1nm,with an OSNR of 14.64dB for 1dBm of average signal input power.Before the receiver, a 2nd order Butterworth optical filter of 90GHz 3dB bandwidth, and a 2nd order Butterworth electrical filter of 7.5GHz bandwidth were used.Assuming 10% of timing jitter from the decision circuitry, calculated Q-factors before / after the regeneration were 4.06 and 7.01, respectively.Significant improvement in Q value over 3 (or in terms of BER, 9 orders of improvement) was achieved from the 2R operation of the device.Worth to note, the mirrored spike effect in Fig. 5 (as a signature of signal delay in the device, combined with the reflections from mirror or facets) were filtered out after the receiver electrical filter, not seriously affecting the overall Q factor values.For the 2R operation detailing the eye-opening / Q-factor changes on the input signal power variations, eye-diagrams and Q-factors before / after the 2R regeneration are illustrated in Fig. 6 and 7 respectively.As can be seen in Fig. 6 (c), a well-defined, wide eye was obtained after the 2R regeneration at the optimum input power.As expected, increased level of noise was introduced from the '1' level (Fig. 6b) or '0' level (Fig. 6d) when the input bias was off-optimal point.Depending on the input Q-factor (Q in = 4, 5, 6), Q factor improvement of 3 ~ 6, for 1 ~ 3dB input signal strength window were obtained (Fig. 7).
Finally, the device functionality as a NOR gate was investigated.In Fig. 8, we plot the SOA2 output P o2 against two input signals (P A in , P B in ), which were simultaneously coupled to the SOA1 input port.As shown, the perfect operation of NOR function was obtained, with a negligible amount of amplitude variation and time delay (associated with SOA gain recovery time, especially in the rising edge of the pulse.Similar tendency can be found in Fig. 5).

Conclusion
Exploiting the highly saturated nature of tandem-SOA gain, in this paper we proposed and also demonstrated that a self-seeded gain modulation effect can be created, with the simple addition of low-reflectivity mirror; constituting a much simpler structure of folded tandem-SOA all-optical logic gate with inherent wavelength transparency.Performance compatibility of the device with the prior arts has been demonstrated in terms of various functionality, such as 2R regenerator and NOR gate.With the highly integrated structure and reduced cost, without the introduction of external saturating tone and added transparency in wavelength, the proposed structure should benefit the realization of the all optical gate, such as a nonlinear optical gate for 2R regeneration and NOR gate in the all-optical, wavelength routed network.

Fig. 4 .
Fig. 4. Wavelength dependency of output curve at fixed input power (left Fig. mirror reflectivity optimized for best ER enhancement).Gain spectrum of SOA (right.Input 0dBm)

Fig. 7 .Fig. 8 .
Fig. 7. Q factors after the regeneration, plotted as a function of P in at different input signal qualities (Q in = 4, 5, 6)