Abstract
The generation of high-repetition-rate optical frequency combs with an ultra-broad, coherent and smooth spectrum is important for many applications in optical communications, radio-frequency photonics and optical arbitrary waveform generation. Usually, nonlinear broadening techniques of comb-based sources do not provide the required flatness over the whole available bandwidth. Here we present a 10-GHz ultra-broadband flat-topped optical frequency comb (> 3.64-THz or 28 nm bandwidth with ~365 spectral lines within 3.5-dB power variation) covering the entire C-band. The key enabling point is the development of a pre-shaping-free directly generated Gaussian comb-based 10-GHz pulse train to seed a highly nonlinear fiber with normal dispersion profile. The combination of the temporal characteristics of the seed pulses with the nonlinear device allows the pulses to enter into the optical wave-breaking regime, thus achieving a smooth flat-topped comb spectral envelope. To further illustrate the high spectral coherence of the comb, we demonstrate high-quality pedestal-free short pulse compression to the transform-limited duration.
Highlights
Optical frequency combs with a high degree of coherence, excellent spectral flatness, stability, as well as a broad bandwidth at high repetition rates are desirable for various applications such as wavelength division multiplexing [1], optical orthogonal frequency division multiplexing [2], and optical arbitrary waveform generation [3]
Prior investigations in spectral broadening of high-rep-rate temporal pulse trains from actively mode-locked lasers [16,17,18,19,20] or electro-optic frequency comb generator [21] showed a high degree of flatness with broad bandwidth when sech [17,18], parabolic [18,19] or Gaussian [20,21] profile pulses are launched into an highly nonlinear fibers (HNLF) operating in the normal dispersion regime
The characteristics of this comb source are promising for the realization of high-performance microwave photonic filters [30], where the ultimate limit in the achievable time-bandwidth product is directly proportional to the number of comb lines
Summary
Optical frequency combs with a high degree of coherence, excellent spectral flatness, stability, as well as a broad bandwidth at high repetition rates are desirable for various applications such as wavelength division multiplexing [1], optical orthogonal frequency division multiplexing [2], and optical arbitrary waveform generation [3]. Prior investigations in spectral broadening of high-rep-rate temporal pulse trains from actively mode-locked lasers [16,17,18,19,20] or electro-optic frequency comb generator [21] showed a high degree of flatness with broad bandwidth when sech [17,18], parabolic [18,19] or Gaussian [20,21] profile pulses are launched into an HNLF operating in the normal dispersion regime. To illustrate the high-degree of stability and coherence of the source, we demonstrate pulse compression to the transform-limited duration
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