10-GHz Broadband Optical Frequency Comb Generation at 1550/1310 nm
Optical frequency combs (OFCs) have ultra-short optical pulses with a fixed repetition rate in the time domain, which can be used to generate extremely high peak power. Similarly, in the frequency domain, OFCs have comb teeth with equal frequency intervals which can coherently connect optical frequency to microwave frequency. OFCs have been widely used in the fields of optical frequency measurement, gas detection, astronomical spectroscopy, and calibration of basic physical constants. The inventors of OFCs, John L. Hall of the National Institute of Standards and Technology in USA and Theodor W. Hänsch of the Max Planck Society in Germany, won the Nobel Prize in Physics in 2005 . The mode-locked OFCs based on titanium-doped sapphire or fiber both have long resonant cavities, and the repetition rate of which are usually below GHz. OFCs with repetition rate higher than 10 GHz have important applications in the fields of astronomical planetary detection, optical communication and arbitrary waveform generation, which have become a new research hotspot. The traditional femtosecond laser can directly or indirectly obtain high-repetition optical frequency combs by reducing the cavity length or using high-finesse Fabry-Pérot cavity filtering. However, the corresponding control system is extremely complicated and accuracy-critical. The use of a continuous laser to pump a high-quality factor optical microcavity with whispering-gallery mode can produce OFCs with a very high repetition rate, but the repetition rate can only be adjusted in a very small range through changing refractive index, which caused by some effects such as thermal effect.
Fig. 1 (a) Optical spectra of the generated flat-topped OFCs with10 GHz repetition rate at 26.5 dBm pump power. (b) Optical spectra with a dispersive wave centered around 1310 nm from a fluorotellurite fiber under different launched powers of the femtosecond laser
This paper aims at solving the problems that the traditional femtosecond laser based on titanium-doped sapphire or femtosecond fiber lasersare difficult to generate OFCs with high repetition rate, and the OFCs from whispering-gallery microcavity cannot achieve widely tunable range of the repetition rate. They obtain optical frequency combs with high repetition frequencies (≥10 GHz) and widely tunable repetition rate around 1550 nm and 1310 nm, and solve the problem that high repetition-rate pulse cannot realize wide spectral coverage due to the limitation of the peak power of the pulse. When pumped pulses with repetition rate of 10 GHz near the zero dispersion wavelength in the normal dispersion region of a highly nonlinear fiber, they generate OFCs with the number of the teeth exceeds 500 based on the self-phase modulation effect around 1550 nm with a flatness of 5 dB as shown in Fig. 1a. Also, 291 fs pulse trains with 10 GHz repetition rate are obtained at 18 dBm pump power without complicated pulse shaping methods. At the same time, this work generates highly coherent dispersion waves at 1310 nm using the obtained picosecond and femtosecond pulses to pump in the anomalous dispersion region of fluorotellurate and silica high-nonlinear optical fibers, respectively, as shown in Fig. 1b. Meanwhile, this paper shows the influence of the peak power and the fiber dispersion on spectral coverage, flatness and center wavelength of dispersion wave as well as the spectrum broadening of OFCs when the repetition rate is up to 20 GHz. It has a significant instructional meaning on the generation and the spectral broadening of the OFCs with high repetition rate.
About The Group
In the past ten years, Prof. Yongzhen Huang's group has been systematically studying the mode and lasing characteristics of the microcavity semiconductor lasers that suitable for integration and related applications. Pioneered the research of polygonal microcavity semiconductor lasers, they have systematically studied the dynamic characteristics of whispering-gallery-microcavity lasers connected directly with an output waveguide and their integration, and the dynamic characteristics of optical-injection microcavity lasers. They have designed and fabricated the stable lasing dual-mode microcavity laser with adjustable wavelength spacing, and applied it to the generation of the THz wave based on the beat frequency and the generation of the tunable optical frequency comb based on the four-wave mixing. And also, they have developed a new kind of hybrid square-rectangular coupled lasing, which realizes high-speed optical storage based on the saturable absorption and mode competition between the modes. Prof. Huang is currently responsible for the National Natural Science Foundation of China with title of "Basic research on the semiconductor laser and multifunctional photon-integration" and the National Fundamental Scientific Instrument Development Project with "Study and fabrication of a tunable optical frequency comb system based on the dual-wavelength microcavity laser and the nonlinear fiber". Prof. Huang has won the Outstanding Graduate Instructor Award of the Chinese Academy of Sciences many times, and one of his students has won 100 Excellent Doctoral Dissertations of China in 2008, and four of them have won the Outstanding Doctoral Dissertations of the Chinese Academy of Sciences.
Han J Y, Huang Y L, Wu J L, Li Z R, Yang Y D et al. 10-GHz broadband optical frequency comb generation at 1550/1310 nm. Opto-Electron Adv 3, 190033 (2020).