Microwave Photonic Filters Based On Semiconductor Optical Amplifier

Using the photonic technology to realize the microwave and millimiter-wave processing has attracted considerable attention in recent years. Compared with traditional electronics-based microwave circuits, microwave photonic processing functions provide advantages such as low loss, light weight, broad bandwidth, good tunability, immunity to electromagnetic interference. Furthermore, it can remove so-called electronic bottleneck. It has many potentional ampliations in ultra-bandwidth wireless mobile communication, array phase radar, sensors of microwave and millimeter-wave, microwave and millimeter-wave signal processing. It provides the possibility of processing microwave and millimeter wave signals directly in the optical domain without the need of optoelectronic conversion process. In addition, they have the benefit of being inherently compatible with fiber based transmission system and can be incorporated into the optical fiber network. Semiconductor optical amplifier (SOA) can be widely applied in all-optical signal processing, and the SOA also exibit some advantages while used in microwave photonic filters, it can realize some microwave photonic filter with some characteristics which other components cannot realize. Supported by the National Basic Research Program of China under Grant No.2006CB302805, we have done some theoretical and experimental researches on microwave photonic filter based on SOAs, and some original results have been demonstrated(1) Operation principle and operation conditions for microwave photonic filters with negative coefficients or high Q and the notch filters with flat passband are summarized and analyzed. Based on the cross-gain modulation (XGM) effect of the amplified spontaneous emission spectrum (ASE) of the SOA, we have realized the notch filter with flat passband in optical domain. In this scheme, a detuned optical filter connected after the SOA is used to extract out the converted signal.(2) Operation principle and operation conditions of the high Q filter are analyzed and described. We realized the microwave filter with the Q of 193 based on an active loop with SOA and optical filter, and the rejection ratio is about 15 dB. However, we obtained a higher Q of 543, rejection ratio of 40 dB using an improved structure with ASE in SOA, and it exibits very stable operation ability.(3) Operation principle and operation conditions of the filter with cascaded structure are analyzed and described. Based on cascaded SOA loop and delay interferometer, we have realized the high Q filter with Q of 386 and rejection ratio of 15 dB. Furthermore, based on cascaded delay interferometer and SOA loop in which XGM of ASE was exploited, we obtained the Q of 1268 and rejection ratio of 30 dB. We have also demonstrated an ultrahigh filter scheme based on two cascadedâ…¡R filters. In this scheme, interference can be avoided with wavelength conversion based on the XGM of the ASE spectrum of a SOA in oneâ…¡R filter. The free spectral range (FSR) of the frontâ…¡R filter is designed to be different from that of the back one, thus the FSR of the cascaded filter is the least common multiple of that of eachâ…¡R filter. Due to vernier effect, only those frequency components that can match bothâ…¡R filters can be filtered out, which significantly increases the FSR and the Q of the cascaded filter. And owing to the cascaded structure, the peaks of the frequency response of the cascaded filter are sharpened along the leading and trailing edges. Thus the 3 dB bandwidth of the cascaded filter is reduced. Finally, the microwave photonic filter with a Q of 3338 and a rejection ratio of about 40 dB was demonstrated.(4) We have realized all-optical SOA-based microwave filter with passband and stopband interchanged based on the realization of the filter with negative coefficients and high Q filter using SOA and optical filter. And we have also realized all-optical filter for simultaneous implementation of microwave bandpass and notch responses based on SOA by adjusting the SOA current and the detuned optical filter. Finally, we have realized the transversal filter employing the characteristic of broad spectrum of SOA and array waveguide gratings. The filter with two taps, three taps and four taps have been demonstrated.