Due to their excellent properties, optical frequency combs (OFCs) have been utilized in numerous fields recently, including optical communication, spectroscopy, and microwave signal processing. The features of OFCs, such as broad spectral coverage, flat comb lines, and adjustable frequency spacing, remain the most challenging and desirable aspects to address. The bandwidth of traditional electro-optical modulation-based OFCs is limited to a few tens of nanometers. Combining highly nonlinear fiber with effective nonlinear parametric mixing is the most attractive method for spectral broadening and has been widely studied. However, there is still room for improvement in the spectral range of the demonstrated OFC. To extend the spectral coverage, we propose and experimentally demonstrate a femtosecond all-fiber ultra-wideband electro-optical frequency comb seeded from a 12.5 GHz electro-optically modulated pulse and highly nonlinear fiber, using joint time-frequency pulse reshaping technology. We have realized an OFC bandwidth with a 10 dB power variation over 145 nm, encompassing more than 1450 comb tones and covering the most-used S, C, and L bands. This result enhances the potential of such OFCs in multiband optical fiber communications.

The all-fiber ultra-wideband electro-optical frequency comb proposed in this study is mainly based on an electro-optical seed frequency comb and highly nonlinear fiber, complemented by joint time-frequency pulse reshaping technology. This configuration consists of three modules: the seed comb module, the joint time-frequency pulse reshaping module, and the nonlinear broadening module. The seed comb module primarily utilizes electro-optical modulation to generate the seed comb, which is composed of electro-optical intensity modulator, phase modulator, and their respective driving modules. The intensity modulator and phase modulator collaboratively control the flatness and width of the frequency comb, respectively. The second module reshapes the pulse in both the time and frequency domains. In the time domain, a nonlinear-optical loop mirror is employed to suppress the pedestal and parasitic sidelobes resulting from pulse compression. In the frequency domain, noise and pedestal components in the low-power spectrum are first reduced through filtering, optimizing the pulse shape. Subsequently, precise dispersion control ensures balance across all modules. The joint time-frequency pulse reshaping method achieves high shaping efficiency with minimal module count. The final module amplifies the pulse peak power and broadens the OFC spectrum using highly nonlinear fiber via parametric mixing. Through meticulous system configuration optimization, electro-optical frequency comb generation has been realized.

The results of the generated comb are shown in Fig. 7 with different resolutions. At a resolution of 0.02 nm, under the condition of 10 dB flatness, the optical frequency comb coverage exceeds 145 nm, meaning the number of carriers exceeds 1450 [Fig. 7(a)]. Meanwhile, the detailed spectral parts covering different bands are shown in Fig. 7(c). These show spectra of more than 350 tones in the C-band within 6 dB flatness, 200 tones in the S-band within 2 dB flatness, and 450 tones in the L-band within 5 dB flatness. At a resolution of 0.2 nm, the calculated optical signal-to-noise ratio (OSNR) is greater than 45 dB, as shown in Fig. 7(b). In addition, the measured average power of the comb is over 2.3 W, corresponding to an average power per comb line of roughly 0 dB. Since the zero-dispersion wavelength of the highly nonlinear fiber (HNLF) used is near 1675 nm, the OSNR in the long-wave part is higher. Moreover, due to the dominant self-phase modulation effect in the nonlinear spectrum broadening process, the spectral flatness, especially in the central regions, should be further improved. Using a polarization-maintaining all-fiber design, including the shaping module, the HNLF, and a high-power optical amplifier, is a better choice to strengthen the flatness of the generated electro-optical frequency comb.

An all-fiber ultra-wideband electro-optical frequency comb covering the most used S, C and L bands is proposed and experimentally implemented in this study. Based on the electro-optical modulated seed optical comb with a center wavelength of 1.5 μm and a frequency spacing of 12.5 GHz, this optical frequency comb generation is realized using joint time-frequency pulse reshaping technology to optimize the femtosecond pulse after compression. This includes frequency domain amplitude control and precise compensation of dispersion. As a result, the pulse pedestal and parasitic sidelobes are well suppressed. This demonstration has achieved a frequency comb output with a 10 dB power variation over 145 nm, corresponding to more than 1450 comb tones. Meanwhile, the measured average power of the comb is over 2.3 W, which corresponds to an average power per line of roughly 0 dB. This power level of a single-frequency comb is sufficient for optical signal transmission. Therefore, the experimental results show that the proposed all-fiber ultra-wideband electro-optical frequency comb has the potential for next-generation ultra-large multiband optical fiber communication and ultra-fast parallel signal processing, among other applications.