10-GHz broadband optical frequency comb generation at 1550/1310 nm
1 Introduction
Passively mode-locked laser frequency combs with typical sub-GHz repetition frequency and sub-100-fs optical pulses have revolutionized optical frequency metrology and precision timekeeping by providing an equidistant set of absolute reference lines that span in excess of an octave1. For applications such as calibration of astronomical spectrographs2, 3, optical arbitrary waveform generation4, 5, microwave signal processing6, and coherent communications7, higher repetition frequency comb (frep ≥ 10 GHz) has become a more effective implement. However, the passively mode-locked laser with higher repetition rate requires a shorter physical cavity in the optical domain, which needs complex design and operation8. Consequently, the tolerance required to control such a cavity length decreases to a scale that is difficult to be physically realized. In order to increase the line spacing of passively mode-locked laser frequency combs, Fabry-Pérot cavities are used to act as periodic, high resolution spectral filters, which require servo control and increase the complexity of the system2.
Actively mode-locked laser9, 10 and electro-optic modulation11-13 can directly generate optical frequency combs (OFCs) at a higher repetition rate, whose repetition rate is no longer limited by the cavity length but determined by the modulation frequency. However, the achievable optical bandwidth generated by the external modulation is limited by the electro-optic modulation efficiency and the ability of the modulator to carry radio frequency power9, 12, 14. Meanwhile, pulses with higher repetition rate will result in lower peak power at a limited average optical power, which prevents the generation of broadband flat OFC15. In order to overcome the limitations above, HNLF was applied to broaden high-repetition rate OFC. Nonlinear generation of ultra-flat broadened spectrum was achieved with adaptive pulse shaping4, 16, and 10 GHz flat OFCs was obtained using a wavelength selective element after the HNLF17. By applying complicated stress to HNLF, a flat frequency comb can be generated using a parametric gain18-20. Recently, resonant electro-optic frequency comb has been extensively studied for their high electro-optic conversion efficiency and the ability to easily generate broadband OFC21, 22. Although the optical bandwidth of 80 nm has been achieved, the spectral flatness needs to be further improved.
In this paper, we study the broadband OFC generation with a repetition rate of 10 GHz covering 1550 nm and 1310 nm by using two types of HNLFs with different dispersion at the pump wavelength of 1550 nm. Firstly, we theoretically and experimentally confirm that picosecond pulses with high repetition rates can generate broadband flat OFC and ultrashort pulses at 1550 nm by SPM effect at relatively low pump power without using any optical filter components for controlling. Then, by adjusting the power of pump light and the ZDW of HNLFs, we generate dispersive waves around 1310 nm with sufficient tunability and good coherence with the pump wavelength of 1550 nm.
2 Results and discussion
2.1 Generation of OFCs and ultra-short pulses
The frequency interval of the optical comb depends on the repetition rate of the seed pulse (
Fig. 1. The 10 GHz OFC and a 2.3-ps pulse generated from a mode-locked laser with (a) OFC spectrum, and (b) reconstructed temporal pulse profile (blue solid curve) and Gaussian fitting curve (red dotted curve).
For achieving spectral broadening, the 2.3-ps pulse is amplified by an Erbium-Ytterbium co-doping fiber amplifier (EYDFA) and fed into a 500 m HNLF with the group velocity dispersion (GVD) of ~0.293 ps2/km (see supplementary material), nonlinear coefficient of ~10.8 (W∙km)−1 and fiber attenuation of ~0.2 dB/km at 1550 nm, which is used to induce SPM effect for generating broadband OFCs. Schematic diagrams of the experimental setup for OFC and ultra-short pulse is shown in the selected section of Fig. S3(b) in supplementary material. Optical spectra after propagation through the 500 m HNLF are plotted in
Fig. 2. Optical spectra after propagation in 500 m HNLF. (a ) The experimental results when the input optical power into the HNLF is 15 dBm, 17 dBm, 19 dBm, 20 dBm, respectively. (b ) The simulation results when the input optical power is 14 dBm, 16 dBm, 18 dBm, 19 dBm, respectively.
where A is the envelope of the pulse electric field and ω0 is the center frequency of the input pulse. The fiber loss, dispersion, and nonlinear coefficient are α, β, and γ, respectively, and R(t) is the response function of the silica fiber. The 4th-order Runge-Kutta algorithms are used in the simulation, and as high as 4th order dispersion was taken into account in the calculation. The numerical results as shown in
To show the quality of the generated OFC, the whole broadened frequency comb at 20 dBm pump power is detected by a 50 GHz photodiode (PD) with a responsivity of 0.7 A/W. The beat note microwave signal is analyzed by an electrical spectrum analyzer (ESA), as shown in
Fig. 3. (a ) RF spectrum from the broadened frequency comb at 20 dBm pump power (RBW=200 kHz and VBW=50 kHz). (b ) Reconstructed temporal pulse profile with a FWHM duration of 291 fs after transmitting a 4 m SMF at 20 dBm pump power (blue solid curve) and Gaussian fitting curve (red dotted curve).
2.2 Generation of broadband OFCs using silica HNLFs
To further investigate the spectral broadening, the pump power is increased to 26.5 dBm. Schematic diagram of the experimental setup for the generation of broadband OFCs is shown in Fig. S3(a) in Supplementary Information. The 10 GHz flat-topped OFC with 43 nm bandwidth and more than 500 spectral lines within 5 dB power variation is generated without any optical filter, as shown in
Fig. 4. Optical spectra of the generated flat-topped OFC.
(a ) The 10 GHz repetition rate at 26.5 dBm pump power. (b ) The 18.5 GHz repetition rate at 25.5 dBm pump power.
Next, 1310 nm broadband OFC is generated using the 2.3 ps pulse at the center wavelength of 1550 nm with higher power. Schematic diagrams of the experimental setup for the generation of broadband OFC is shown in Fig. S3(a) in Supplementary Information. It is important to pump at anomalous dispersion region of the HNLF to achieve the broadband SC spectrum and to support soliton propagation27. A 500 m HNLF is used with the dispersion of ~ -0.496 ps2/km, the nonlinear coefficient of ~10 (W∙km)−1 and the fiber attenuation of ~0.762 dB/km at 1550 nm. As shown in
Fig. 5. (a ) Experimental supercontinuum spectra designed to produce a dispersive wave centered around 1310 nm. (b ) RF spectra from the generated 1310 nm dispersion wave at 32 dBm pump power (RBW=200 kHz and VBW=50 kHz).
3 Generation of broadband OFCs using fluorotellurite fibers
In order to reduce the effect of the modulation instability on the coherence of the optical frequency comb, we fabricated and used fluorotellurite fibers with a high nonlinear coefficient and a short length. Very recently, fluorotellurite fibers with a high nonlinear coefficient, good water resistance and high transition temperature, became the promising nonlinear medium for high power mid-infrared SC generation (frep ≤100 MHz) for their rich spectral broadening mechanism such as self-phase modulation, soliton fission, soliton self-frequency shift, and dispersive wave generation29, 30. In the experiment, fluorotellurite fibers were fabricated by rod-in-tube method, and the compositions with a large refractive index difference of the core and the cladding layers are 70TeO2-20BaF2-10Y2O3 (TBY) and 33AlF3-11MgF2- 17CaF2-8SrF2-9BaF2-12YF3-10TeO2 (AMCSBYT)31, respectively. Insets in
Fig. 6. The calculated dispersions of the fibers with different sizes. Insets, scanning electron microscope images of the fibers with the diameters of 3.7 μm, 3.3 μm and 3.1 μm, respectively.
To clarify the potential of the designed fluorotellurite fibers for broadband optical frequency comb generation at the pump wavelength of 1550 nm with 10 GHz pulses, we perform the following experiments. Schematic diagram of the experimental setup for the generation of broadband OFC is shown in Fig. S3(b) in supplementary information. The pump laser is the ultrashort pulses with a pulse width less than 300 fs and the repetition rate of 10 GHz which we have generated from 2.3 ps pulses as shown in
Fig. 7. (a ) Optical spectra with a dispersive wave centered around 1310 nm from a fluorotellurite fiber under different launched powers of the femtosecond laser. (b ) Optical spectra with tunable dispersive waves ranging from 1150 nm to 1310 nm from fluorotellurite fibers 1, 2, 3 with ZDWs at 1358 nm, 1409 nm and 1452 nm, respectively.
The influence of different ZDWs on dispersion waves is studied at fixed average pump power at 5.96 W, as shown in
In addition, the generalized nonlinear Schrödinger equation (1) is solved numerically to investigate the spectral broadening mechanism. High order (as high as 8th) dispersion was taken into account. The Raman response function coefficients in Eq. (2) are fR = 0.064, τ1 = 7.2 fs and τ2 = 59.3 fs, respectively.
Light source parameters are set as follows: operating wavelength of 1550 nm, pulse width of 800 fs, repetition rate of 10 GHz, and pulse peak power of 387 W.
Fig. 8. (a ) The simulated and measured SC from the fluorotellurite fiber with the ZDW of 1452 nm and the peak pump power of 387 W. (b ) RF spectrum from the generated 1310 nm dispersion wave in the fluorotellurite fiber (RBW=200 kHz and VBW=50 kHz).
4 Conclusions
In conclusion, we have generated broadband OFCs with good coherence based on a 10 GHz picosecond pulse. The key significance of the proposed approach is that the strong SPM effect and dispersion wave could generate broadband OFCs in 1550/1310 nm. In the normal dispersion region of a HNLF, 10 GHz flat-topped OFCs with 43 nm bandwidth within 5 dB power variation are generated by SPM without any optical filter. At the same time, we obtain microwave signal with 75 dB SMSR and achieve 291 fs ultra-narrow pulse. Experimental results are well consistent with the simulated spectra using generalized nonlinear Schrödinger equation. In the abnormal dispersion region of the HNLF, the dispersive waves around 1310 nm are generated with tunability range more than 40 nm and the corresponding RF beating wave with a SMSR of 26 dB. At the same time, the highly coherent dispersive waves at the wavelength from 1100 to 1400 nm with a 70 dB SMSR for beating wave have been generated in the fluorotellurite fibers with different ZDWs. We expect that the generated comb lines can serve as the wavelength-division multiplexing source for multiple wavelength channels in optical communication network.
5 Acknowledgements
We are grateful for financial supports from the National Natural Science Foundation of China (Grant No. 61527823) and the National Key R & D Program of China (Grant No. 2017YFB0405301).
6 Author contributions
Junyuan Han and Yali Huang contributed equally to this work. All authors commented on the manuscript.
7 Competing interests
The authors declare no competing financial interests.
8 Supplementary information
[5] S T Cundiff, A M Weiner. Optical arbitrary waveform generation. Nat Photonics, 2010, 4: 760-766.
[24] Agrawal G P.
Article Outline
Junyuan Han, Yali Huang, Jiliang Wu, Zhenrui Li, Yuede Yang, Jinlong Xiao, Daming Zhang, Guanshi Qin, Yongzhen Huang. 10-GHz broadband optical frequency comb generation at 1550/1310 nm[J]. Opto-Electronic Advances, 2020, 3(7): 07190033.