RESUMEN
We experimentally demonstrate, for the first time to our knowledge, a microwave fractional Hilbert transformer in a few-mode fiber using a transversal filtering approach. The filter taps are provided by a tunable true-time delay line that is realized by exploiting the spatial dimension of a dispersion-engineered double-clad step-index few-mode fiber. Both the fractional order and operational bandwidth of the fractional Hilbert transformer can be continuously tuned by adjusting the tap coefficients and varying the operational optical wavelength, respectively. The magnitude and phase response for different fractional orders, ranging from 0.17 to 1.00 that correspond to phase shifts of 15° to 90°, are measured. Operational bandwidths of 7.4 to 10.6 GHz are demonstrated for a classical Hilbert transformer. Real-time temporal fractional Hilbert transform of a Gaussian-like pulse is also performed. Our results are in good agreement with theory, validating the viability of our approach for implementation of microwave fractional Hilbert transformers.
RESUMEN
A novel microwave frequency measurement scheme using a heterogeneous multicore fiber (MCF) is experimentally demonstrated. The inherently different relative group delays among the cores of a heterogeneous 7-core MCF are used to realize two individual 2-tap microwave filters with different free spectral ranges (FSRs). The ratio of the frequency response traces of these two filters is used to establish an amplitude comparison function (ACF). Furthermore, by varying the operational wavelength, the relative group delays between the cores and consequently the FSRs of the filters are tuned and different ACF curves are obtained. The complementary information provided by these different ACFs allows us to estimate the unknown frequency with an improved accuracy, over a broad measurement range. In our experiments, a measurement error of ±71 MHz is achieved over a frequency range of 0.5-40 GHz. The proposed scheme offers flexibility and compactness, thanks to the parallelism provided by the MCF.
RESUMEN
We present a novel double-clad step-index few-mode fiber that operates as a five-sampled tunable true-time delay line. The unique feature of this design lies in its particular modal chromatic dispersion behavior, which varies in constant incremental steps among adjacent groups of modes. This property, which to the best of our knowledge has not been reported in any other few-mode fiber to date, is the key to tunable operation of radiofrequency signal processing functionalities implemented in few-mode fibers. The performance of the designed true-time delay line is theoretically evaluated for two different microwave photonics applications, namely tunable signal filtering and optical beamforming networks for phased array antennas. In the 35-nm optical wavelength tuning range of the C-band, the free spectral range of the microwave filter and the beam-pointing angle in the phased array antenna can be continuously tuned from 12.4 up to 57 GHz and 12.6° up to 90°, respectively.
RESUMEN
A short piece of commercial-grade SMF-28 optical fiber is pumped with a 680 ps high-peak power green laser. Red Stokes and blue anti-Stokes beams are generated spontaneously from vacuum noise in different modes in the fiber via intermodal four-wave mixing. Detailed experimental and theoretical analyses are performed and are in reasonable agreement. The large spectral shifts from the pump protect the Stokes and anti-Stokes from contamination by spontaneous Raman scattering noise. This work highlights the predictive power and limitations of a theoretical model to explain the experimental results for a process that relies on the amplification of quantum vacuum energy over more than 11 orders of magnitude.
RESUMEN
We propose an all-fiber mode-locking device, which operates based on nonlinear switching in a novel two-concentric-core fiber structure. The design is particularly attractive given the ease of fabrication and coupling to other components in a mode-locked fiber laser cavity. The nonlinear switching in this coupler is studied, and the relative power transmission is obtained. The analysis shows that this nonlinear switch is practical for mode-locking fiber lasers and is forgiving to fabrication errors.
RESUMEN
The main differences in nonlinear switching behavior between multicore versus multimode waveguide couplers are highlighted. By gradually decreasing the separation between the two cores of a dual-core waveguide and interpolating from a multicore to a multimode scenario, the role of the linear coupling, self-phase modulation, cross-phase modulation, and four-wave mixing terms are explored, and the key reasons are identified behind higher switching power requirements and lower switching quality in multimode nonlinear couplers.