تجزیه و تحلیل عملکرد از یک روش مدولاسیون جانبی نوری واحد با نسبت موزون نوری حامل به جانبی

In this work, we report an all-optical approach to tune the optical carrier-to-sideband ratio (OCSR) of the typical optical single sideband (OSSB) modulation signals. The key component is a broadband polarization-maintaining fiber Bragg grating (PMF-FBG), which can be fabricated using a chirped grating phase mask. Firstly, we employed this PMF-FBG to reflect two subcarriers (carrier and sideband) of the typical OSSB modulation signals with orthogonal polarization, simultaneously. Since chirped fiber gratings have broadband reflection bandwidth, it will be easy to align the target subcarriers to the grating's reflection bandwidth. Then, a linear polarizer is connected with its principal axes aligned at 45° relative to the fast axis of the PMF-FBG. By doing this, the OCSR is strictly dependent on the injected polarization direction at the PMF-FBG. In other words, the OCSR's tunablity can be realized by tuning a polarization controller placed in front of the grating. Thus continuous tunability of the OCSR can be done. To investigate this technique, a more normal case is considered, with multi tones as the RF source (N denotes the number of RF tones). It is found that when OCSR=N (in value) is satisfied, best receiver sensitivity can be predicted. Highlights ► An all-optical approach is proposed to tune the OCSR. ► It is characterized by OCSR tunability continuously and also low attenuation. ► A broadband PMF-FBG is employed, which is cheap and easy to fabricate. ► Multi RF tones modulation is considered. The optimum OCSR is found as OCSR=N.

Radio over fiber (RoF) is a promising technology for the next generation wireless communication networks [1] and [2]. A RoF link uses optical fiber to distribute high capacity wireless signals, such as ultrawideband and millimeter–wave between one central station (CS) and several base stations (BSs). One popular modulation technique used in RoF links is optical double sideband (ODSB) modulation, which can be realized by direct modulation of semiconductor laser or external modulation via electro-optic modulators. However, due to the chromatic dispersion in fiber links, the converted RF signals will experience dispersion-induced power fading [3], which makes long-distance fiber transmission unpredictable. To solve this problem, adaptable dispersion compensators were used [4], [5], [6] and [7]. The fading can be removed by using midway optical phase conjugation [4], pre-compensation in electrical domain with phase shifting [5], carrier phase-shifted double sideband modulation [6] and mixed polarization modulation [7]. However, a practical RoF network with hundreds of BSs needs hundreds of adaptable dispersion compensators since the dispersion between the CS and every BS are not identical. It will be costful to construct and maintain such a network. Optical single sideband (OSSB) modulation with an optimum optical carrier-to-sideband ratio (OCSR) is a good solution to improve modulation efficiency, receiver sensitivity, and remove fiber dispersion-induced power fading. So far, some techniques have been reported [8], [9], [10], [11] and [12]. In [8], an optical carrier-suppressed SSB modulation was proposed using a hyperfine blocking filter based on a virtually imaged phase-array (VIPA). It was found that the hyperfine blocking filter can be tuned for simultaneous OSSB and carrier suppression up to ∼30 dB lower than the sideband. However, this technique suffers from high insertion loss (more than 13 dB) and high power penalty caused by the blocking filtering effects. In [9], fiber grating was first proposed to improve link performance. By suppressing optical carrier via a narrow-band FBG, the OCSR can be decreased and receiver sensitivity can be improved. However, such an approach suffers from poor OCSR tunability. In [10], triangular chirped fiber Bragg gratings (CFBG) were used to filter out one sideband from the ODSB signals and balance the OCSR simultaneously. Also, its OCSR cannot be tuned continuously. Then in [11], a triple-arm Mach-Zehnder modulator is used to generate OSSB signal. The OCSR can be tuned by varying the DC-voltage applied to one arm which is without RF modulation. But the OCSR cannot reach 0 dB at small signal modulation. Later in [12], an OSSB modulation approach with tunable OCSR using an dual parallel Mach-Zehnder modulator, was reported. By changing the applied bias voltages the OCSR can be tuned continuously. However, it needs careful adjustment of the bias voltage and an expensive modulator. In this work, we report an all-optical approach to tune the OCSR of typical OSSB modulation signals. Unlike the previous reported ones [11] and [12], which rely on active bias control of modulator and also expensive modulators, this work uses polarization-maintaining fiber Bragg grating (PMF-FBG) as the key component, which is cheap and easy to fabricate. Besides, since OSSB modulators are already commercially available, approaches using fiber gratings [9] and [10] seem to be more attractive. Also differ from the previous works [8], [9] and [10], this approach is characterized by its continuously tunability of the OCSR and also low attenuation (or loss). In this work, a more normal case is considered, with multi tones as the RF source. It is found that when OCSR=N (in value) is satisfied (N denotes the number of RF tones), best receiver sensitivity can be predicted. Numerical analysis and some simulations are carried out to investigate the mechanism. Good agreement between the theory and the simulation has been found.

In summary, an OSSB modulation approach with tunable OCSR has been proposed and analyzed by the theory and simulation. The whole approach is based on a PMF-PBG. By tuning a polarization controller placed in front of the grating, continuously tunability of OCSR can be obtained. It also found that the OCSR should be properly tuned to satisfy the optimum value OCSR=N (in value), where N denotes the number of RF tones. Using commercially used software, the mechanism of this technique has been verified. Since we only use fiber gratings as the key component, this approach is simpler and easier than the previously reported ones.