High-performance Analog Photonic Links Based On Polarization Modulation

Analog photonic links (APLs) have attracted significant interest in various applications, such as radar systems, wireless communications, and phased array antennas, thanks to the advantages in terms of low insertion loss, broadband, light weight, and immunity to electromagnetic interference as compared with its electronic counterpart. However, chromatic dispersion (CD) of the optical fiber would lead to spectrally periodical power fading because a conventional intensity or phase modulated APL employs double sideband (DSB) modulation. In addition, due to the sinusoidal transfer function of the intensity modulator, the higher-order harmonics and intermodulation components are generated, which reduces the spur-free dynamic range (SFDR) of the APLs. Especially, the third-order intermodulation distortion (IMD3) components are most undesirable because they always locate in the signal band, which can not be removed by an electrical filter. To reduce the power fading, one method is to use optical single sideband (SSB) modulation, but a wideband and wavelength-tunable optical SSB modulator is not easily achievable. The modulation chirp in a directly modulated laser diode (LD) is used to reduce the CD effect in the APLs. However, the frequency chirp in the LD is fixed, so the operation is effective only to APLs with a certain length of fiber or signals at a certain frequency. Recently, a carrier phase-shifted DSB modulation to compensate the dispersion-induced power fading by a dual-parallel Mach-Zehnder modulator (DPMZM) was proposed. The approach can be operated for signals at any frequency, but one of the sub-MZM in the DPMZM must be biased at the minimum transmission point (MITP). The key limitation associated with the operation of an MZM at the MITP is the bias drifting problem, which reduces the system stability. Several linearization approaches have been proposed to suppress the IMD3, such as electronic predistortion, feedforward, digital signal processing, postcompensation, DPMZM and incoherent combination. The electronic methods based on predistortion, feedforward and digital signal processing are limited by the electronic bottleneck, so the bandwidth is small. The postcompensator should be able to perform line-by-line processing, which is bulky and expensive. DPMZM requires precise control of the three DC biases, which is complex and unstable.To solve these problems, we propose two kinds of APLs based on polarization modulation which are easy to operate and make the system stable.In the third section, a novel APL based on an integratable modulator consisting of a polarization modulator (PolM) and a polarizer is proposed and demonstrated. The modulator can perform simultaneously amplitude modulation and phase modulation. The ratio between the two modulations can be adjusted by changing the polarization state of the optical signal introduced to the modulator, which is then used to compensate the dispersion-induced power fading in the APL. An experiment is carried out. The maximum point of the frequency response can be shifted to any frequency of interest. The proposed APL has a SFDR improvement as large as12.5dB as compared with a conventional IM-based link with20-km single mode fiber (SMF).In the fourth section, we propose and investigate a novel APL with large SFDR based on cascaded polarization modulators (PolMs). Analytical analysis and numerical simulation are performed. A reduction of the IMD3up to37.9dB and an improvement of the SFDR as large as13.2dB as compared with the conventional intensity-modulated link is achieved.