KEYWORDS: Mode locking, Microwave radiation, Pulse signals, Microsoft Foundation Class Library, Interference (communication), Harmonic generation, Oscillators, Frequency combs, Education and training, Optoelectronics
A harmonically mode-locked optoelectronic oscillator (HML-OEO) based on injection locking is proposed to generate microwave frequency combs (MFCs) with a high supermode noise suppression ratio (SNSR) and high frequency stability. N th -order harmonic mode locking is achieved via frequency mixing of the oscillation signal and an intermediate-frequency (IF) signal with a frequency interval equal to N times the free spectral range (FSR) in the loop. The supermode noise of the HML-OEO is suppressed by the injection of a single-tone microwave signal, which is aligned with the frequency of one longitudinal mode, in order to enhance the gain competitiveness of one group of longitudinal modes. Furthermore, the correlation of the adjacent pulses can be improved by the injection-locked effect. In the numerical simulation, a microwave frequency comb with a center frequency of 10 GHz and a SNSR of 81.1 dB is generated under the condition of 3 rd -order harmonic mode locking, by injecting the microwave signal with a power of −40 dBm. Furthermore, the correlation of adjacent pulses generated in the proposed HML-OEO is superior to that before injection locking.
High-power and high-energy ultrafast optical vortex beams have recently gained considerable interest for practical applications in laser-based material processing. Here, we investigate the parabolic pulse amplification of linearly polarized optical vortex beams with controllable topological charges of l = ±1 utilizing a few-mode large mode area polarization-maintaining Yb-doped fiber (PM-YDF) amplifier system. It yields a maximum average output power of approximately ~14.8 W at a repetition rate of approximately ~3.7 MHz, corresponding to a single pulse energy of approximately ~4 μJ. The output optical vortex pulses can be externally de-chirped to approximately ~200 fs with a peak power of approximately ~11.2 MW.
KEYWORDS: Modulation, Digital signal processing, Quadrature amplitude modulation, Signal intensity, Forward error correction, Data transmission, Single mode fibers, Signal processing
We propose a flexible bandwidth compression scheme for Intensity-Modulation and Direct-Detection (IM-DD) transmissions employing the Super-Nyquist Subcarrier Modulation (SN-SCM) signal. After transmitting a 25 Gbaud 16QAM SN-SCM signal over 2 km Standard Single-Mode Fiber (SSMF), 14.7% Spectral Efficiency (SE) enhancement is guaranteed with the help of the Decision-Feedback-Equalization with Multiple-Input-Multiple-Output structure (DFE_MIMO) for Inter-Subcarrier-Interference (ICI) mitigation.
Here we propose and demonstrate an all-normal-dispersion fiber laser operating within a gain-managed nonlinear amplification (GMNA) evolution regime. This laser configuration is capable of supporting the generation of stable, highenergy, linearly chirped ultrashort pulses with a nonlinear phase shift exceeding 50π. Utilizing a 5-meter dual-clad Ybdoped fiber with a core/cladding diameter of 30/250 μm as the gain medium, the laser directly generates ~1.4 ps pulses with an average power of 2.9 W at a repetition rate of 16.6 MHz, corresponding to a pulse energy of 174 nJ. The output spectrum exhibits 10-dB bandwidth of ~58 nm, spanning from 1043nm-1101nm. These output pulses can be externally compressed to ~68 fs using a pair of transmission gratings with a compression efficiency of ~83%, resulting in a peak power of ~1.8 MW. The beam quality factor (M2) was measured to ~1.2.
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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