The carrier-free phase-retrieval (CF-PR) receiver can reconstruct the optical field information only from two de-correlated intensity measurements without the involvement of a continuous-wave optical carrier. Here, we propose a digital subcarrier multiplexing (DSM)-enabled CF-PR receiver with hardware-efficient and modulation format-transparent merits. By numerically retrieving the optical field information of 56 GBaud DSM signals with QPSK/16QAM/32QAM modulation after 80-km standard single-mode fiber (SSMF) transmission, we identify that the DSM enabled CF-PR receiver is beneficial in reducing the implementation complexity of the CF-PR process, in comparison with the traditional single-carrier counterpart, because the lower symbol rate of each subcarrier is helpful in reducing the implementation complexity of multiple chromatic dispersion compensations and emulations during the PR iteration. Moreover, the DSM-enabled CF-PR receiver is verified to be robust toward various transmission imperfections, including transmitter-side laser linewidth and its wavelength drift, receiver-side time skew, and amplitude imbalance between two intensity tributaries. Finally, the superiority of the DSM-enabled CF-PR receiver is experimentally verified by recovering the optical field information of 25 GBaud 16QAM signals, after 40-km SSMF transmission for the first time. Thus, the DSM-enabled CF-PR receiver is promising for high-capacity photonic interconnection with direct detection.
KEYWORDS: Modulation, Polarization, Digital signal processing, Phase shift keying, Phase modulation, Modulators, Single mode fibers, Electro optics, Radio optics, Palladium
An image-rejection multi-band frequency down-conversion scheme is proposed and demonstrated based on photonic sampling. In this scheme, two radio-frequency (RF) signal replicas with a quadrature phase difference are sampled by two ultra-short optical pulse trains in orthometric polarization via the linear electro-optic modulation in a dual-polarization dual-drive Mach-Zehnder modulator (DP-DDMZM). After being polarization demultiplexed, the two sampled optical pulses are separated from each other and detected by using two photodetectors (PDs), respectively. Hence, the RF signals in multiple frequency bands are simultaneously down-converted to the intermediate-frequency (IF) band. Finally, the two IF signals are combined by using an electrical 90° hybrid coupler to eliminate the image-frequency components. In the experiment, an ultra-short optical pulse train with a repetition frequency of 8 GHz is generated by employing an electrooptic modulation-based time lens with the chirp compensation, and is used to achieve photonic sampling. The experimental results indicate that image-rejection down-conversion is achieved for the input signal in the frequency range of 6 GHz to 27 GHz, where the image rejection ratio is larger than 58 dB.
We developed a distributed refractive index (RI) sensor based on high performance optical frequency-domain reflectometry (OFDR) by simply bending a piece of standard single mode fiber (SMF) in a U shape. In the U-bent region, cladding modes are excited, which can reach to the boundary of the SMF to sense external RI variation. The cladding modes are then coupled back to the core mode and interfere with the fundamental mode. Thus, the fundamental mode can carry the varied RI information, and distributed index sensing is achieved by measuring the wavelength shifts of the local Rayleigh backscattered spectra. Thanks to the high signal SNR of OFDR, that compensating the bending induced loss, the proposed sensor can be bent in a small bending radius so that a high sensitivity of RI could be achieved. In the experiment, index sensitivity of 39.08 nm/RIU is achieved by imposing a bending radius of 4 mm, when the RI ranges from 1.3330 to 1.3773. Additionally, the proposed sensor maintains buffer coating intact, which boosts its practicability and application flexibility.
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