The results of the development of a 6-channel microwave photonic beamforming system (BFS) for phased array antenna in receiving mode are presented. BFS incorporate DWDM technology based components of analogue fiber-optic transmission links with external modulation in RF range 0.1-18 GHz and specially manufactured chirped fiber Bragg grating. A number of BFS transmitters have six optical carriers separated with 100 GHz step (ITU grid, DWDM standard in C-band). The beamforming is realized by introducing inter-channel time delays when microwave modulated optical carriers interact with chirped fiber Bragg grating (СFBG) in reflection mode, throughout the total operating spectral range of 6-channel BFS (6x100 GHz). The used СFBG has 2 cm length. The results of measuring the S-parameters of BFS 6 channels are given, which enable to synthesize the far-field pattern of the phased array antenna with photonic BFS under investigation. Far-field patterns of four channel linear phased antenna array integrated with photonic beamformer model have been measured and compared with calculated patterns taking into account amplitude and phase errors arising in beamformer channels.
We report on the transverse mode selection in an all-fiber CW Raman laser based on a multimode graded-index fiber directly pumped by multimode laser diodes. Selection properties of special fiber Bragg gratings inscribed by UV CW or IR femtosecond radiation in the 100-μm core of graded-index fiber are experimentally compared. It is also theoretically explained why the better fundamental mode selection occurs in the femtosecond fiber Bragg grating inscribed in the fiber with lower core diameter. Fibers with core diameter of 62.5, 85 and 100 um are compared in the experiment. With core enlargement, the output power and slope efficiency increase sufficiently (from 47% to 84%) at the expense of slight beam-quality parameter increase (M<sup>2</sup> =1.3-3).
The results on tilted fiber Bragg gratings (TFBGs) inscription using the method of transverse scanning of the fiber core by a femtosecond laser beam is reported in this paper. As an example, TFBGs consisting of unidirectional and bi-directional grating planes and having a tilt angle up to 9° are created. It is shown that different transverse mode groups of the fiber cladding can be excited with the created structures. The corresponding resonant dips reach the amplitude up to 30 dB that indicates the inscription method efficiency.
We report on the first all-fiber CW Raman laser based on a multimode graded-index fiber directly pumped by multimode laser diodes. A joint action of Raman clean-up effect and mode-selection properties of special fiber Bragg gratings inscribed in the central part of fiber core, results in high-efficiency conversion of a multimode (M<sup>2</sup>~26) pump at 915 nm into a high-quality output beam at 954 nm. Fibers with core diameter of 62.5, 85 and 100 um are compared. With core enlargement, the output power and slope efficiency increase sufficiently (from 47% to 84%) at the expense of slight beam-quality parameter reduction (M<sup>2</sup>=1.3-3).
Femtosecond-pulse modification of the refractive index in transparent materials enables the inscription of fiber Bragg gratings with new features and extended capabilities. In this study we present the results of fiber Bragg gratings inscription in Corning 62.5/125 multimode graded index fiber with IR femtosecond laser pulses. The specifics of point-by-point inscription including single and multiple Bragg grating inscription in limited fiber segment as well as different transverse modes excitation/suppression is discussed. Multimode fiber Bragg gratings inscribed with femtosecond radiation are investigated for the first time directly in the Raman fiber laser cavity.