Generation of linearized optical single sideband signal for broadband radio over fiber systems
Radio over fiber (RoF) technique shows advantages in- cluding low loss, high bandwidth, and immunity against gelectromagnetic interference. In most RoF systems, in- put radio frequency (RF) signals are carried on optical carriers via intensity modulation (IM) and recovered after transmission using direct detection (DD) by photodetec- tors (PDs). Generally, LiNbO3 Mach-Zehnder modula- tors (MZMs)[1] are utilized for IM/DD links. However, harmonics and intermodulation products are induced due to the sinusoidal transfer characteristic of MZM, which limits the dynamic range of the RoF systems. In addition, a MZM is typically biased at quadrature to low even-order distortion, resulting in a high residual optical-carrier power that may saturate the PD and in- duce nonlinearity through fiber transmission[2]. Also, the high optical-carrier power increases the intensity noise and shot noise that limit the system dynamic range[3]. To solve the above problems, several techniques have been proposed to mitigate carrier-induced noise and lin- earize the transfer function of the modulator. Those techniques to suppress optical carrier include optical car- rier filtering[4], low biasing of a MZM[5], and class-AB microwave photonics links[6]. Linearization is realized by designing linearized modulators[7,8] and high-linearity PDs[9]. Recently, coherent links are proposed to achieve dynamic range enhancement for better sensitivity[10]. Carrier-suppressed modulation in a coherent system has been demonstrated[11]. However, compared with IM/DD links, higher laser power and lower modulation depth are required to achieve large dynamic range in that scheme. In this letter, we propose a method to realize large dynamic range by using an unbalanced dual parallel Mach-Zehnder modulator (DPMZM) for linearization. The DPMZM has a structure similar to that used in IM/DD links[12,13]. However, as shown in Fig. 1, the modulator in our scheme is biased at its transmission null and driven by a two-tone RF signal, which gener- ates a suppressed carrier (SC) double-sideband (DSB) signal without high-order components (point A). The signal is then filtered by a following optical bandpass fil- ter (OBPF) to obtain an optical single sideband (OSSB) signal (point B), which can be coherently detected by adding an unmodulated optical carrier (point C) at the transmitter or mixing with an optical carrier provided by local oscillator before beating at the receiver (point D). The second-order terms disappear as the SC modulation is symmetric when the modulator is biased at the null. The third-order intermodulation (IMD3) is reduced by using the linearized DPMZM. In addition, by adjusting the power ratio of the first-order sidebands to the un- modulated optical carrier, a higher fundamental signal power after detection is obtained without increasing the noise floor. The structure of the proposed DPMZM is shown in Fig. 2. The splitting ratios of the input optical power and the input RF power are 1: 2 and 1:
2, respec- tively, and the bias conditions are properly chosen. The outputs of two modulators are coherently combined and IMD3 reduction is realized in optical field. The val- ues of
and in our scheme are set differently, and it will be proved in the following analysis that they are effective to reduce the IMD3 distortion. Both the primary and secondary modulators are biased at V and the phase shift between the two modulators is set to 180◦. As a result, a linearized SC-DSB signal can be obtained in optical domain. In order to optimize the values of and for the unbalanced DPMZM, we firstly undertake two-tone analysis.


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