Abstract
We utilize adaptive optimization to enhance the spectral broadening of an amplified electro-optic frequency comb with a 25 GHz repetition rate in a highly nonlinear fiber and subsequently generate sub-picosecond pulses. The spectral phase of the comb is adaptively optimized by a Fourier pulse shaper in a closed control loop with the HNLF output spectrum as the process variable to be optimized. Enhanced spectral broadening also increases the stimulated Brillouin scattering threshold allowing increased power scaling and thereby boosting the bandwidth by a factor of more than 13 times over the initial comb. System versatility to varying conditions is demonstrated by achieving consistent bandwidth enhancement (nearly or more than 100 lines) in varying operating conditions that distort the temporal profile of the comb. In all cases, the optimization yields a near transform limited pulse that enters the nonlinear fiber. Sub-picosecond pulse generation is achieved with a short length of single mode fiber post the nonlinear fiber.
Highlights
Optical frequency combs (OFC) form vital infrastructure in high bandwidth communications where they meet the increased data demands through wave length division multiplexing (WDM) [1], and orthogonal frequency division multiplexed (OFDM) super channels [2] in a compact and efficient manner
Wide-band combs can be generated by four-wave mixing of lasers in a highly non-linear fiber (HNLF) [6,17] but requires the HNLF to be strained [17] to overcome stimulated Brillouin scattering (SBS)
The repetition rate of the comb is directly determined by the spacing between lasers which is coupled to the extent of spectral broadening
Summary
Optical frequency combs (OFC) form vital infrastructure in high bandwidth communications where they meet the increased data demands through wave length division multiplexing (WDM) [1], and orthogonal frequency division multiplexed (OFDM) super channels [2] in a compact and efficient manner. BW at a given power can be enhanced by optimizing the temporal profile of the pulse entering HNLF for enhanced self-phase modulation (SPM) This would allow further power scaling of the system due to better distribution of the power across the comb increasing the maximal achievable BW. In [25], spectral phase of transform limited gaussian pulses from a mode locked laser-based frequency comb was optimized with evolutionary strategy algorithms to increase BW by defining the phase perturbations with a weighted Chebyshev polynomial sum. The optimal pulse shape in each system is a complicated function of system parameters such as power, initial spectral phase of comb, modulator driving conditions, length and dispersion profile (sign, magnitude of various orders of dispersion) of various fibers following the Fourier pulse shaper. The demonstrated technique to generate high repetition rate pulses can be implemented at a wide variety of wavelengths and repetition rates with equivalent components to potentially achieve high ablation rate of tissues and material targets with minimal thermal damage by operating in the so called ablation cooled regime [29]
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