Photonic technologies have received considerable attention for enhancement of radio-frequency (RF) electrical systems, including high-frequency analog signal transmission, control of phased arrays, analog-to-digital conversion, and signal processing. Although the potential of radio-frequency photonics for implementation of tunable electrical filters over broad RF bandwidths has been much discussed, realization of programmable filters with highly selective filter lineshapes and rapid reconfigurability has faced significant challenges. A new approach for RF photonic filters based on frequency combs offers a potential route to simultaneous high stopband attenuation, fast tunability, and bandwidth reconfiguration. In one configuration tuning of the RF passband frequency is demonstrated with unprecedented (~40 ns) speed by controlling the optical delay between combs. In a second, fixed filter configuration, cascaded four-wave mixing simultaneously broadens and smoothes comb spectra, resulting in Gaussian RF filter lineshapes exhibiting extremely high (>60 dB) main lobe to sidelobe suppression ratio and (>70 dB) stopband attenuation.Optical frequency combs, generated via self-referenced and stabilized mode-locked lasers, have enabled revolutionary progress in precision optical frequency synthesis and metrology 1-4 . Optical combs are also of tremendous interest for other applications 5 , such as multi-wavelength coherent lightwave communications 6-8 , optical arbitrary waveform generation 9-11 , generation of low-phasenoise 12 or agile ultrabroadband microwaves 13 , and signal processing 6,14 . For these purposes, in which higher pulse repetition rates are desired and only moderate frequency stability is required, comb sources based on strong electro-optic modulation of a continuous-wave laser have seen substantial attention [15][16] . Here we report significant advances in RF photonic filters enabled by the ability to rapidly tune the timing of the comb and shape its power spectrum.
We demonstrate a scheme based on a cascade of lithium niobate intensity and phase modulators driven by specially tailored radio frequency waveforms to generate an optical frequency comb with very high spectral flatness. In this work we demonstrate a 10 GHz comb with 38 comb lines within a spectral power variation below 1-dB. The number of comb lines that can be generated is limited by the power handling capability of the phase modulator, and this can be scaled without compromising the spectral flatness. Furthermore, the spectral phase of the generated combs in our scheme is almost purely quadratic which, as we will demonstrate, allows for high quality pulse compression using only single mode fiber. OCIS codes: (060.0060) Fiber optics and optical communications; (060.5060) Phase modulation; (060.5625) Radio frequency photonics; (120.3940) Metrology; (320.5540
High power fiber lasers operating at the 1.5micron wavelength region have attractive features like eye-safety and atmospheric transparency, and cascaded Raman fiber lasers offer a convenient method to obtain high power sources at these wavelengths. A limitation to power scaling however has been the lower conversion efficiency of these lasers. We recently introduced a high efficiency architecture for high power cascaded Raman fiber lasers applicable for 1.5micron fiber lasers. Here we demonstrate further power scaling using this new architecture. Using numerical simulations we identify the ideal operating conditions for the new architecture. We demonstrate a high efficiency 1480nm cascaded Raman fiber laser with an output power of 301 W, comparable to record power levels achieved with rare-earth doped fiber lasers in the 1.5 micron wavelength region.
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