SBrT 2000 - XVIII Simpósio Brasileiro de Telecomunicações, 3 a 6 de Setembro, 2000, Gramado-RS SIGNAL GENERATION BY MIXING MODULATED OPTICAL CARRIERS IN FEED-BACKED SEMICONDUCTOR OPTICAL AMPLIFIERS A. C. Bordonalli, J. A. Guimarães, J. L. Benitez, E. Conforti and C. M. Gallep Departamento de Microonda e Óptica - Faculdade de Engenharia Elétrica e de Computação Universidade Estadual de Campinas – UNICAMP Caixa Postal 6101, CEP: 13083-970, Campinas, SP – Brazil ABSTRACT The gain-crossed property of semiconductor optical amplifiers is experimentally investigated. It was observed that optically feed- backed saturated amplifiers could achieve similar performance as that of cascaded amplifiers, offering up to 10 dB gain to two 8-nm-appart modulated optical carriers. Due to saturation, the amplifier gain mechanism is altered, leading to a mixing effect that causes the frequency components of the optical signals to be exchanged. For optical carriers modulated at 250 MHz and 400 MHz, this mixing effect produced, after photodetection, up and down-converted electronic components at 150 and 650 MHz, respectively. 1. INTRODUCTION Recently, photonic generation of RF, microwave and millimeter- wave signals has been widely studied. This technique uses the optical fiber as the transmission medium for considerably stable carriers that are generated in a wide range of RF and microwave frequencies, with applications on mobile communication [1][2], satellite communications [3], and subcarrier multiplexed systems [4]. Techniques such as optical phase-lock loop [5][6], injection locking [7] and mode-locking [8] have demonstrated the feasibility of the optoelectronic conversion and generation of RF and microwave carriers. Nevertheless, some of the techniques mentioned above are considerably complex and require accurate and tedious optical and/or electrical designs. A simple scheme for signal generation proposed in [9] was modified to observe the effects of cascaded semiconductor optical amplifiers (SOA) on the mixing of intensity modulated optical carriers [10]. It was experimentally shown that cascaded SOAs were able to mix two distinctly modulated optical carries separated by 8 nm. As expected, a spectrum analyzer showed that the photocurrent produced after filtering out one of the optical carriers had harmonic mixing contents corresponding to the sum and difference of the individual optical carrier modulation frequencies. The experiment was conducted for optical carriers modulated at 250 and 400 MHz. It was possible to produce, after photodetection, up and down-converted electronic components at 150 and 650 MHz. It was observed that the use of additional SOAs tends to improve the power level of the up and down converted frequency components in comparison with a 1-SOA experiment due to SOA stronger saturation. However, the use of cascaded SOAs may still be a premature option due to the actual device costs and the limits imposed on the maximum allowed coupling power. In spite of simple, the cascaded SOA technique for RF and microwave carrier generation requires a careful design in order to guarantee acceptable power levels for the up and/or down converted signals with low cost and device safekeeping. In this paper, an alternative approach to the cascaded SOA scheme for RF and microwave signal generation is presented. To minimize the number of SOAs used, an optically feed-backed SOA (FB-SOA) set-up is proposed. In this way, the optical signals will be re-injected into the SOA after one trip, ensuring a deeper saturation state for each device, and, therefore, higher power levels for the up and down converted frequency components. In order to compare the two SOA signal generation schemes, a 2-SOA cascaded experiment was reproduced using the same optical carrier modulation frequencies as in [10]. Then, the optically feed-backed SOA experiment was implemented using only one semiconductor optical amplifier. In this particular case, it was possible to observe that the FB-SOA approach can offer a performance similar to that of the cascaded scheme. However, it is believed that the FB-SOA configuration is able to offer better results than the cascaded experiment as some possible modifications in the experimental set-up can allow up to 9 dB more optical power injected into the SOA and 3 dB more optical power coupled into the photodetector. 2. THEORY AND EXPERIMENTS Fig. 1 illustrates the mixing mechanism for two amplitude- modulated optical carriers that are being simultaneously coupled into a SOA. One of the carriers is generated by a transmitter laser (TO) and is modulated at a frequency f TO . The other is produced by a local oscillator laser (LO) and modulated at a frequency f LO . If the coupled optical power is enough to deeply saturate the semiconductor optical amplifier, the carrier population in the gain region of the SOA decreases significantly, resulting in a considerable reduction of the amplifier gain. However, since the carrier dynamics within the SOA are very fast, the saturation condition causes the SOA gain to become sensitive to any variations that the photon number inside the SOA cavity can suffer. In this way, the SOA responds in tune with the fluctuations of the coupled optical power originated from the optical intensity modulation. Therefore, the optical gain is simultaneously modulated at both optical carrier modulation frequencies f TO and f LO in such a way that the SOA becomes unable to distinguish and separate the optical carriers and their own harmonic contents from each other. As a result, the modulated gain induces both carriers to be modulated at f TO and f LO at the SOA output. This effect, known as cross-gain saturation, is the main responsible for the SOA mixing characteristics.