Fiber optics frequency comb enabled linear optical sampling with operation wavelength range extension

Data rate measurement method with selectable range in linear optical sampling

Linear optical sampling (LOS) is one of the most powerful techniques for high-speed signal monitoring. To measure the data-rate of signal under test (SUT) in optical sampling, multi-frequency sampling (MFS) was proposed. However, the measurable data-rate range of the existing method based on MFS is limited, which makes it very difficult to measure the data-rate of high-speed signals. To solve the above problem, a range selectable data-rate measurement method based on MFS in LOS is proposed in this paper. Through this method, the measurable data-rate range can be selected to match the data-rate range of SUT and the data-rate of SUT can be measured precisely, independently of the modulation format. What's more, the sampling order can be judged using the discriminant in the proposed method, which is key for plotting eye diagrams with correct time information. We experimentally measure the baud-rates of PDM-QPSK signal from 800 MBaud to 40.8 GBaud in different ranges and judge the sampling orders. The relative error of measured baud-rate is less than 0.17% while the error vector magnitude (EVM) is less than 0.38. Compared with the existing method, under the same sampling cost, our proposed method realizes the selectivity of the measurable data-rate range and the judgment of sampling order, greatly extends the measurable data-rate range of SUT. Hence, the data-rate measurement method with selectable range has great potential for high-speed signal data-rate monitoring.

Fast linear optical sampling with high repetition-frequency using fiber delay lines

Linear optical sampling (LOS) is one of the most promising techniques for optical modulation analyzers. The LOS system generally adopts a mode-locked fiber laser (MFL) to generate an ultra-stable optical pulse to realize under-sampling for signal under test (SUT). However, it is challenging for MFL to produce a high-repetition-frequency pulse, making more measurement errors of conventional LOS technology, especially for high-speed signals. This paper proposes a dual-pulse mixing (DPM) based LOS system to increase the repetition frequency using fiber delay lines with the multiplied optical pulse. We propose the pulse location and peak extraction algorithms to compensate the time bias and amplitude bias in the DPM-based LOS system, which significantly improves the measurement speed and range. The experiment results show that the DPM-based LOS system can increase the number of sampling points twice compared with the conventional LOS within the same sampling time window. Furthermore, the proposed DPM-based LOS system can achieve less error vector magnitude with a reduction of 9.1% compared with the conventional LOS. Hence, the proposed DPM-based LOS system has great potential for high-speed signal processing.

Linear optical sampling enabled soliton nonlinear frequency spectrum classification

Nonlinear Fourier transform (NFT) is a powerful tool for characterizing optical soliton dynamics, which, however, suffers from fundamental limitations that ultra-wide bandwidth photodetectors and ultra-high sampling rate analog-to-digital converters should be used when accessing the full-field information of an ultrafast optical pulse. Herein, we report on the experimental demonstration of the linear optical sampling (LOS) enabled nonlinear frequency spectrum classification of ultrashort optical pulses, which could break this limitation. Instead of traditional coherent detection, the LOS overcomes the ultra-wide bandwidth constraint of commercially available optoelectrical devices. By finely adjusting the repetition rate difference between the soliton to be characterized and the sampling pulsed source, a 55.56-TSa/s equivalent sampling rate arising in the LOS can be secured, where only 400-MHz balanced photodetectors and 5-GSa/s analog-to-digital converter are used. Meanwhile, according to the nonlinear frequency spectrum calculated from the accurate full-field information, the promising concept of soliton distillation has been experimentally verified for the first time. The LOS-enabled NFT technique provides an alternative and efficient characterization tool for ultrafast fiber lasers, which facilities comprehensive insight into soliton dynamics.

Focal shift of an axisymmetric Bessel-Gaussian beam under Airy mixing modulation

In this paper, the focusing characteristics of Bessel-Gaussian beams are studied by means of vector diffraction theory. The vector field distribution of the axisymmetric Bessel-Gaussian beam of a cylindrical vector is derived by calculating and adding Airy mixing modulation to the Bessel-Gaussian beam. It is found that with a series of regular focusing change characteristics, the focusing presents strong stability of the optical chain structure, and the number of optical chain links can be adjusted. At the same time, it is pointed out that in the case of a tightly focused helically polarized beam, the polarization in the focal region is not uniform, but there was a similar horizontal shift in focus. Finally, the relevant practical application scenarios are briefly introduced. The correlation focus shift conversion can be widely used in electronic acceleration, optical sampling and operation, and biological imaging.

Peak-power-clamping in an all-polarization-maintaining Q-switched mode-locking fiber laser

We report the peak-power-clamping (PPC) effect in a polarization-maintaining (PM) Q-switched mode locking fiber laser. The laser cavity with a compact and stable all-PM fiber configuration can clearly demonstrate three different output states including normal Q-switching, PPC Q-switching, and PPC Q-switched mode-locking (QML) with the increasing pump power. To the best of our knowledge, it is the first time that PPC effect is successfully obtained and analyzed from the Q-switching to QML. This research extends the theory of PPC in pulsed lasers and reveals the potential to achieve ultra-high pulse energy.