مدولاتور با گیرندگی بالا فوق العاده خطی مبتنی بر MMI و تجزیه و تحلیل عملکرد آن
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Publisher : Elsevier - Science Direct (الزویر - ساینس دایرکت)
Journal : Optics & Laser Technology, Volume 53, December 2013, Pages 33–39
An electric optical modulator including a 1×2 multimode interference (MMI) coupler and an integrated Mach–Zehnder Interferometer (MZI) modulator that consists of a microring and a Phase Modulator (PM) is suggested in this paper. Such a modulator can achieve large output RF gain and high linearity performance at the same time. Moreover, due to the application of MMI coupler, traditional direct current (DC) bias circuits can be omitted and the modulator's deterioration caused by the phase error can be decreased a lot, which leads to reduced device complexity and increased device stability. The presented numerical and simulation results confirm the advantages of the newly proposed modulator over conventional modulators (MZI, RAMZI and IMPACC). Further analysis manifests that the proposed modulator has good tolerance in two aspects. Firstly, the modulator has an inherent ability to mitigate the detrimental effects of microring waveguide loss on slope efficiency and linearity by simply adjusting the external RF power split ratio. Secondly, the application of the MMI device increases the modulator's tolerance for the phase deviation caused by some environmental factors.
Radio over Fiber (RoF) or Radio over Free Space Optics (RoFSO) is well-established technique for the distribution of wireless communication signals in the future , , , ,  and . One key technique of RoF/RoFSO systems is the design of the electro-optic modulator which should maintain both high linearity and large output RF gain performance simultaneously in order to decrease high order distortion (such as the 3rd order inter-modulation distortion) and increase the ratio of signal to noise, especially, the instance of several RF signals modulating an optical carrier. Several linear electro-optic modulators have been presented by using different device configurations such as SOA-based modulators , MZI-based modulators , , , ,  and , and Microring-based modulators . Among these configurations, the MZI-based modulators are studied a lot and present high performance to some degree. The dual-signal MZI-based modulators  comprise of one modulator with two optical signals coupled to each arm respectively. The amplitudes and phases of the two input signals must be properly matched by using the predetermined values of the RF amplitude phase signal. The ring-assisted MZI-based modulators (RAMZI) ,  and  have been developed in recent years. The RAMZI couples microring(s) in the arm(s) of the MZI. This design can provide higher spurious free dynamic range (SFDR) performance but at the cost of a limited linearity range and stricter coefficient(s) control. The IMPACC couples a phase modulator on one arm of RAMZI and the modulating signals are fed onto the microring and the phase modulator according to an adjustable ratio. When the microring waveguide loss is taken into consideration, this design can maintain high SFDR but at the cost of the strict control of the direct current (DC) bias circuits  or a strict relationship between the lengths of the two arms of MZI . As far as we know, a lot of schemes proposed earlier mainly focus on the linearity improvement, and they seldom have found a simple and compact application to realize both high output RF gain and linearity performance simultaneously. Actually, the high output RF gain performance of a modulator is desirable for analog links and necessary to be fulfilled. Some previous methods to improve RF gain performance require long arm lengths of MZI arms, which will unfortunately decrease the bandwidth of the device . In this paper, we propose a novel and compact modulator composed by a Phase Modulator and Ring-Assisted MZI based on a 1×2 MMI coupler (MMI-PMRAMZI). Due to the exact split ratio and stable phase relations between the outputs, MMI couplers have been extensively researched as splitting elements ever since they were firstly suggested . Since the relative phase between the two arms of MZI is provided by MMI coupler instead of the DC bias circuits, the complexity of the integrated MZI is reduced and the stability is also improved. Furthermore, as a potential structure which shows ultra-linearity, microring has been used in many applications to improve linearity. In addition, we apply a novel combination of microring and PM, which is able to provide both ultra-linear phase modulation and high slope efficiency simultaneously, thus effectively enhancing the sublinearity and RF gain performance of MZI. Moreover, by properly adjusting the external RF ratio applied on the microring and PM, MMI-PMRAMZI shows the ability to recover optimal SFDR value regardless of microring waveguide loss. This paper is organized as follows. In Section 2, the theoretical model of the proposed modulator MMI-PMRAMZI is presented. Section 3 discusses and compares the device performances of MMI-PMRAMZI, MZI, RAMZI and IMPACC. In this section, two linearity criteria (linearity range m and SFDR) and output RF gain performance are addressed. Section 4 compares the performances of RAMZI, IMPACC and MMI-PMRAMZI when microring loss and the DC biased circuits' shift from the normal operation point are taken into consideration, respectively. Section 5 discusses the importance of the relative phase and the proposed modulator's advantages. Section 6 gives a conclusion.
نتیجه گیری انگلیسی
In this paper, a novel linearized electrooptic intensity modulator in RoF/RoFSO systems is proposed and analyzed theoretically. By the use of MMI, it is shown that this design exhibits some simplicity with no requirements for precise and complicated external control and avoids the degradation caused by the DC bias circuits. Besides, unlike previous modulators which solely focus on linearity, this design has an ability of maintaining ultra-linearity (SFDR value is 133.5 dB) and high output RF gain (1.1 and 1.96 times larger than MZI and RAMZI, respectively) simultaneously. Moreover, we show that this modulator has an inherent property to mitigate the detrimental effects of microring waveguide loss on slope efficiency and linearity by simply adjusting the external RF power split ratio. Furthermore, the application of MMI can increases the tolerance for phase shift caused by some environmental factors.